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EFFICIENCY ENHANCEMENT OF
POLYMER FULLERENE SOLAR CELLS
Martijn Lenes
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Efficiency enhancement of polymer fullerene solar cellsMartijn LenesPhD thesisUniversity of Groningen, The Netherlands
Zernike Institute PhD thesis series 2009-13ISSN 1570-1530ISBN 978-90-367-4016-6ISBN 978-90-367-4057-9 (digital version)
This research forms part of the research programme of the DutchPolymer Institute (DPI), Technology Area Functional PolymerSystems, DPI project #524
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RIJKSUNIVERSITEIT GRONINGEN
EFFICIENCY ENHANCEMENT OF
POLYMER FULLERENE SOLAR CELLS
Proefschrift
ter verkrijging van het doctoraat in de Wiskunde en Natuurwetenschappen
aan de Rijksuniversiteit Groningenop gezag van de
Rector Magnificus, dr. F. Zwarts,in het openbaar te verdedigen op
vrijdag 23 oktober 2009om 16:15 uur
door
Martijn Lenes
geboren op 12 augustus 1977te Hoogezand-Sappemeer
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Promotor: Prof. dr. ir. P. W. M. Blom
Beoordelingscommissie: Prof. dr. J. C. HummelenProf. dr. L.D.A. SiebbelesProf. dr. ir. P. Heremans
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Dedicated to the memory of
Bert de Boer (1973 – 2009)
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CONTENTS
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Table of Contents
CHAPTER 1..................................................................................................................... 9 1.1 Introduction: Solar Energy ............................................................................ . 10 1.2 Organic Semiconductors ................................................................................ 12 1.3 Charge generation in organic solar cells .......................................................... 13 1.4 Device fabrication and characterization techniques ......................................... 15
1.4.1 Solar cell fabrication .................................................................................. 15 1.4.2 Solar cell Characterization ......................................................................... 15 1.4.3 Characterization techniques ........................................................................ 17 1.4.4 Photocurrent measurements and modeling .................................................. 20
1.5 Optimization of energy levels in a donor acceptor system ............................... 22 1.5.1 Multijunction solar cells ............................................................................. 24
1.6 Aim and scope of this thesis ........................................................................... 26 REFERENCES ............................................................................................................... 28
CHAPTER 2................................................................................................................... 31 2.1 Introduction ................................ ................................................................... 32 2.2 Space-charge limited photocurrents ................................................................ 33 2.3 Device Fabrication and measurements ............................................................ 35 2.4 Device Simulations ........................................................................................ 40 2.5 Optical Considerations ................................................................................... 43 2.6 Conclusions ................................................................................................... 45 REFERENCES ............................................................................................................... 46
CHAPTER 3................................................................................................................... 49 3.1 Introduction ................................ ................................................................... 50 3.2 Parameters governing the charge dissociation ................................................. 51 3.3 Single Carrier Devices.................................................................................... 53 3.4 PEO-PPV:PCB-EH Solar Cells ...................................................................... 55 3.5 Device Simulations ........................................................................................ 56 3.6 Conclusions ................................................................................................... 59 REFERENCES ............................................................................................................... 60
CHAPTER 4................................................................................................................... 63 4.1 Introduction ................................ ................................................................... 64 4.2 Charge transport in pristine PCPDTBT films .................................................. 65 4.3 Charge transport in PCPDTBT:PCBM blends ................................................. 66 4.4 PCPDTBT:PCBM Solar Cells ........................................................................ 68 4.5 Device Simulations and Discussion ................................................................ 72 4.6 Conclusions ................................................................................................... 76 REFERENCES ............................................................................................................... 77
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CHAPTER 5................................................................................................................... 79 5.1 Introduction ................................ ................................................................... 80 5.2 The bisadduct analogue of PCBM .................................................................. 81
5.2.1 P3HT:bisPCBM solar cells......................................................................... 82 5.3 Other higher adduct fullerenes ........................................................................ 85
5.3.1 Solar cells based on higher adduct fullerenes .............................................. 89 5.3.2 Device simulations using charge trapping ................................................... 93
5.4 Conclusions ................................................................................................... 96
PUBLICATIONS .................................................................................................................... 97
SUMMARY ............................................................................................................................ 99
SAMENVATTING ................................................................................................................ 102
ACKNOWLEDGMENTS ....................................................................................................... 105
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Chapter 1
Introduction to organic solar cells
Abstract
Renewable energy sources are heavily sought after these days, both from an
environmental and from a cost point of view. Even though all forms of renewable
energy are indirectly powered by the sun, using the photovoltaic effect is probably
the most straightforward way of harnessing energy from the incoming solar
irradiation. Next to the traditional inorganic silicon-based photovoltaics, organic
solar cells are considered as a viable candidate for a large-area, flexible, and more importantly, low-cost energy source. In this introductory chapter the electro-
optical processes and current status of this type of devices is discussed. After
assessing critical loss mechanisms, several strategies for improving the efficiency
are discussed. Finally an overview of this thesis is given in which some of the
discussed strategies are investigated.
REFERENCES
R. Kroon, M. Lenes, J. C. Hummelen
P.W.M. Blom and B. de BoerPolymer Review, 2008, 3, 531.
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1.1 Introduction: Solar Energy
When searching for an alternative source of energy, the vast amount of energy
(1.75 1017
J) the earth receives from the sun is guaranteed to draw attention. In
fact, with the total energy usage in 2003 being 4.4 1020
J per year, less then onehour is needed to fulfill this demand.
1Nevertheless, harnessing this source of
energy in a cost effective way is not an easy task. Several technologies can be
employed; first of all sunlight can be converted to thermal energy which can
subsequently be used for hot water, heating or conversion into electrical energy.
Alternatively, sunlight can be converted directly to electrical energy using the
photovoltaic effect. The field of photovoltaics is at the moment dominated by
inorganic, silicon based, solar cells. Due to the large availability of the used
material and extensive knowledge from the microelectronics industry, crystallinesilicon solar cells currently have a 90% market share.
2The main drawback of this
type of devices is the high purity needed for proper device operation. The energy,
and thus costs, needed in the fabrication process limits its usefulness as an
alternative energy source. Nevertheless silicon based solar cells are already almost
at par with consumer electric grid prices in southern Europe.3Next generation solar
cell technology is focusing on two directions; on one side high efficiency, or on the
other side a moderate efficiency combined with low cost.4
For high efficienciesmultilayer cells are investigated, often in combination with solar concentrators in
order to get the most out of these very efficient but also expensive solar cells, oftenbased on III-V semiconductors. For low cost cells, thin film technologies are
employed in order to reduce material usage, which can be either amorphous and
microcrystalline silicon as well as other inorganic compounds such as cadmium
telluride (CdTe) or copper indium gallium selenide (CIGS).
Organic solar cells are a relatively new route towards a large area, low cost
energy source.5
Organic materials can be solution processable allowing for low
cost deposition techniques such as spin coating, doctor blading, inkjet printing and
ultimately roll to roll fabrication.6
Due to the almost infinite modification of the
molecular structure it is possible to tune the chemical and physical properties of the
materials allowing for great flexibility in design. The high absorption coefficient of
organic materials allows organic solar cells to absorb most of the light in extremely
(~100 nm) thin layers reducing material usage significantly. On the down sideorganic materials are often highly disordered, limiting charge carrier transport and
device efficiency. Organic solar cells exist in roughly 3 types; dye-sensitized, small
molecule-, and polymer- based devices.The dye-sensitized solar cell was first introduced by O‟Reagan and Grätzel in
1991 and consists of a nanoporous titanium oxide (TiO2) layer.7
The TiO2 material
is covered with a Ruthenium dye which, after the absorption of light, injects an
electron into the TiO2 after which the electrons can be collected at an electrode. An
electrolyte regenerates the dye and is responsible for the hole transportation to a
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counter electrode. The main disadvantage of DSC‟s is the use of the liquid
electrolyte, which causes stability problems. Replacing the liquid electrolyte with asolid up to now results in lower efficiencies yet considerable progress is made in
this direction.8
Small molecule solar cells are fabricated by thermal evaporation of a donor and
acceptor material in either a double layer structure9
or a bulk heterojunction similarto polymer solar cells.
10Advantage of small molecule cells is the large control of
the deposition enabling for instance combinations of bilayer and bulk
heterojunctions. On the downside the vacuum based deposition does not comply
with the concept of a low cost and high throughput fabrication technique and is
difficult to be applied to large areas.
Polymer solar cells are based on π conjugated polymers as electron donors.Modification of the molecular structure allows one to tailor chemical and physical
properties and have resulted in a number of well performing materials with
different band gaps and energy levels such as P3HT,11
PCPDTBT,12
PF10TBT13
and
pBBTDPP2.14
As an acceptor either another semiconducting polymer,15,16
inorganic
materials17
or fullerenes18
can be used. Historically fullerenes have resulted in
superior device performances yet the other options still remain a viable alternative.
In this thesis solar cells based on a blend of a polymer and a fullerene areinvestigated.
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1.2 Organic Semiconductors
The field of polymer electronics emerged in 1977 by the serendipous discoveryof a chemically doped polymer which exhibited a high conductivity,
19,20a
remarkable feature considering the traditional usage of polymers as insulating
material for metal components. While initial research focused on creating highly
conductive materials, attention was soon shifted towards the semiconducting
properties of conjugated materials. Conjugated materials, which can be either
polymers or small molecules, consist of an alternation of single and double carbon-
carbon bonds. Since each carbon atom is bound to only three neighboring atoms,one electron is left in a pz orbital. The mutual overlap between these pz orbitals
results in the formation of π bonds along the backbone. These delocalized πelectrons are the origin of the intrinsic semiconductor behavior of conjugated
materials. The charge transport in organic semiconductors is fundamentally
different from traditional inorganic physics where the concept of band conduction
and free charge carriers is used. Instead, carriers move through the material from
one localized state to another, a process called hopping.21
Due to the energetic and
spatial disorder in the materials this hopping process results in a relatively low
charge carrier mobility. Hence, organic semiconductor devices are not meant to
compete with high speed applications such as silicon computer chips but
applications should be found in combining ease of processing (and the associated
low cost) with moderate performances. Typical examples are field effect transistorsfor identification tags or sensors, light emitting diodes for large area lighting or
displays, memory devices and solar cells. Alternatively the option to process these
materials on plastic substrates offers opportunities for flexible devices such as
rolable displays.
Figure 1.1: Chemical structure of polyacelylene, the most general form of a conjugated polymer.
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1.3 Charge generation in organic solar cells
In contrast to inorganic photovoltaic devices, organic solar cells produce aneutral mobile excited state (exciton) after the absorption of light instead of free
charge carriers. Since the binding energy of this exciton is typically 0.2-0.8 eV,22
much higher than the thermal voltage, a single component organic solar cell will
result in extremely low efficiencies. In order to separate the excitons into free
charge carriers a donor-acceptor (D-A) system can be employed.9
When an exciton
reaches the donor/acceptor interface, the electron will transfer to the material with
the larger electron affinity and the hole will be accepted by the material with thelower ionization potential. Due to the low exciton diffusion lengths of typical 1 – 10
nm in polymeric materials23,24
a simple bilayer structure will result in low
efficiencies, since only photons absorbed within this distance from the
donor/acceptor interface will contribute to the device current.25
A drastic increase
in the generated photocurrent can be achieved by employing an interpenetrating
network of donor and acceptor materials.15,18
Ideally, in this so-called bulk
heterojunction (BHJ) all absorbed photons will be in the vicinity of a donor
acceptor interface and can contribute to the generated photocurrent.
Figure 1: Charge generation in a polymer:fullerene bulk heterojunction solar cell: a) absorption of aphoton resulting in an exciton, b) diffusion of the exciton towards the donor acceptor interface, c)electron transfer from donor to acceptor, d) dissociation of the bound electron hole pair into free
carriers, e) transport of free carriers towards the electrodes, f) collection at the electrodes. Loss
mechanisms are indicated by 1) non absorbed photons, 2) exciton decay, 3) geminate recombinationof the bound pair, 4) bimolecular recombination.
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The complete process starting from an absorbed photon and ending up with
charges collected at the electrodes is depicted in Fig. 1.1: First a photon is absorbedby the donor material (a) after which an exciton is created. This exciton diffuses
towards a donor/acceptor interface (b) where the electron is transferred to the
acceptor material (c). Even though the hole and electron are now on different
materials they are still strongly bound by Coulomb interaction and need to bedissociated into free carriers (d) after which they are transported through the two
respective phases (e) and can be collected at the electrodes (f). During each of the
above-mentioned processes energy can be lost resulting in various loss
mechanisms. First of all, not all photons are absorbed by the active layer, not only
due to limitations of the bandgap but also due to the often limited thickness of the
active layer (1). Secondly, excitons will decay when created too far from the D-Ainterface (2). After electron transfer, geminate recombination of the bound electron
hole pair can occur (3) as well as bimolecular recombination (4) of free charge
carriers during transport to the electrodes.
Figure 1.2: Schematic layout of a polymer:fullerene bulk heterojunction solar cell.
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1.4 Device fabrication and characterization techniques
1.4.1 Solar cell fabrication
In figure 1.2 the basic structure of a solar cell fabricated in this thesis is shown.
We start with a glass substrate, prepatterned with a layer of indium tin oxide (ITO)
which acts as a transparent electrode. In order to reduce the roughness of the ITO athin layer of poly (3,4-ethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS)
is spincoated on top of the substrate. Besides reducing the roughness of the ITO the
PEDOT:PSS provides a proper (high) work function and allows for a good wettingof the active layer. After a short baking step to remove all the water from the
PEDOT:PSS layer, the substrates are moved into an inert atmosphere for further
processing. Now the blend of polymer and fullerene is spincoated on top of thesubstrate. The devices are completed by thermal evaporation of a thin low work
function material (either samarium or lithium-fluoride) and a 100 nm thick
aluminum top contact. The top contacts are deposited through a shadow mask in
order to define the active layer. After completion of the devices they are transferredin a nitrogen filled container to the measurement setup which is also under inert
atmosphere.
1.4.2 Solar cell Characterization
The solar cell performance and electrical characteristics are determined by
measuring the current density to voltage ( J-V ) characteristics, both in dark andunder illumination. Figure 1.3 shows the typical J-V characteristics of a polymer:
fullerene solar cell under illumination. From the J-V curve four parameters can be
deduced. The current density under illumination at zero applied bias is called the
short circuit current density ( J sc), when the current density under illumination is
zero the cell is at the open circuit voltage (V oc) and the fill factor (FF ) relates the
maximum power the cell can deliver, to the open circuit voltage and short circuitcurrent density:
ocsc
mppmpp
V J V J FF
The power conversion efficiency is now determined by dividing the maximum
power point (Pmpp) by the incoming light power Pin:
in
ocsc
in
mpp
P
FF V J
P
P
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-0.2 0.0 0.2 0.4 0.6-120
-80
-40
0
40
80
120
160
MPP
JMPP
VMPP
JSC
Under Illumination
In Dark
J [ A / m 2 ]
V [V]
VOC
FF =
Figure 1.3: Typical current-voltage characteristics of a polymer:fullerene solar cell showing the V oc,,
J SC , FF and maximum power point ( MPP)
The power conversion efficiency has to be determined under standard test
conditions (STC) which includes the temperature of the solar cell (25 C), anillumination intensity of 1000 W/m
2and a spectral distribution of the illumination
source (air mass 1.5 or AM1.5).26
Since the spectrum of the used illumination
source is in general not the same as the AM1.5 solar spectrum the mismatch factor
( M ) for the measurement has to be determined using
27
„
where E R(λ) and E S(λ) are the AM1.5 solar spectrum and spectrum of the used
illumination source and S R(λ) and ST (λ) are the spectral responses of a reference cell
and the tested cell, respectively. For this purpose we use a silicon solar cell of
which the spectral response is determined at the Energy Centrum of the
Netherlands (ECN). In order to determine the spectral response of the tested cell
Incident photon-to-current efficiency (IPCE) measurements are performed. In thismeasurement the cell under test is illuminated by monochromatic light and the ratio
between the generated photocurrent (at short circuit conditions) and the incident
photon flux at that particular wavelength is determined (for the whole range of the
solar spectrum). Figure 1.4 shows an example of such and IPCE, also known as
External Quantum Efficiency (EQE) measurement, which besides determining the
mismatch factor of the measurement, is also very useful for analyzing loss
mechanisms in solar cells.
)()(
)()(
)()(
)()(
T R
T S
RS
R R
S E
S E
S E
S E M
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400 500 600 700 800 900 10000
10
20
30
40
50
60
70
80
90
100
E . Q . E .
[ % ]
Wavelength [nm]
Figure 1.4: External Quantum Efficiency (EQE), also known as Incident Photon-to-Current
Efficiency (IPCE) of a polymer:fullerene solar cell, measured at short circuit conditions.
1.4.3 Characterization techniques
Above it is shown that the efficiency of a solar cell is represented by its maximum
power point ( MPP). By itself the MPP does not contain much information on theworking of a solar cell, but it can be expressed as the product of short circuit
current ( J sc), open circuit voltage (V oc) and fill factor (FF ). Very generally, the V oc
is governed by the HOMO of the donor and the LUMO of the acceptor,28,29
the Jsc
depends on the photon absorption of the active layer and the FF is determined bythe (balanced) charge transport and recombination properties of the materials.
30In
reality these guidelines will only apply for optimized devices and even then are
only first approximations. Below some considerations are given when analyzing
the J-V curves of a solar cell and which experimental techniques can be used to
asses the potential of a solar cell.
The V oc of a polymer fullerene solar cell can be described by the following
relationship:31
in which q is the elementary charge, P the dissociation probability of a boundelectron – hole pair into free charge carriers, G M the generation rate of bound
electron-hole pairs, γ the Langevin recombination constant, N c the effective density
of states, k the Boltzmann constant, and T is the temperature. However, this
relation is only valid when the electrodes form ohmic contact with the HOMO of
the donor and the LUMO of the acceptor. If this is not the case the V oc will be
M
c
ocPG
N P
q
kT --V
2)1(lnLUMO(A)HOMO(D)
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limited to the difference in workfunction of the electrodes. This is often observed
when aluminium is used as cathode without a lithium fluoride (LiF) or other lowworkfunction interlayer, reducing the voltage significantly. On the other hand an
ill-defined cathode interface can give rise to a distinct S shape of the J-V curve
around the open circuit voltage resulting in low fill factor.32
Furthermore, a low
amount of photogenerated charges as well as recombination with charge traps, willresult in a lower V oc.
33
The current generated by a solar cell is ultimately governed by the amount of
absorbed photons. Because of the low exciton diffusion lengths in the donors23,24
a
bulk heterojunction is employed to harvest all the excitons.15,18
The domain size of
donor and acceptor thus plays a very important role in the actual short circuit
current measured in a device. In fact, the control of this morphology is the mostdifficult and most investigated part of the solar cell fabrication. Typically a large
range of solvents, polymer:fullerene ratios, annealing effects and additives are
required to induce the correct morphology. When domain sizes are too large,
excitons will be lost due to exciton decay. Photophysical studies can be employed
to see whether all excitons are able to reach an interface. However, too small
domain sizes can induce an enhanced recombination of charge carriers. Also the
donor and acceptor domains need to have a percolated pathway towards anode andcathode, respectively, in order for charges to be collected. A range of morphology
imaging tools including transmission electron microscopy (TEM), selected area
electron diffraction (SAED), scanning electron microscopy (SEM), scanning probe
microscopy (SPM) and atomic force microscopy (AFM) can be used for
characterization of the active layer morphology.34 Even when all of the generated excitons reach an interface this does not
automatically imply that all charges are actually converted into free charge carriers.Due to the low dielectric constant of the polymer and fullerene, the electron and
hole are coulombically bound at the interface and need to be dissociated into free
carriers by an electric field. Plotting the photocurrent as a function of effective field
can be used to determine the dissociation efficiency of a device.35,36
For
MDMO:PPV:PCBM blends the dissociation efficiency was shown to be only 60%
at short circuit conditions.35
Spectroscopic evidence for the occurrence of the (field
dependent) dissociation of a bound state at the donor-acceptor interface has been
found in both polymer:polymer and polymer:fullerene blends.37
The fill factor of a device depends in a complicated way on charge
dissociation, charge carrier transport and recombination processes.30
A good hole
and electron transport capability is of vital importance for proper device operation.
When hole and electron transport are unbalanced a build up of space-charge results
in a square root dependence of the photocurrent on voltage, resulting in low fill
factors. Even a difference in hole and electron mobility of only one order of
magnitude can influence the device performance, which imposes limitations on the
active layer thickness in order to avoid space-charge problems. Light intensity
dependent measurements can provide information on which type of recombination,geminate or bimolecular, is dominant and whether space-charge problems play a
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role. Several type of experimental test beds can be employed to determine the
charge carrier mobilities in the materials such as field-effect transistor (FET),space-charge limited current (SCLC) measurements, photo induced charge carrier
extraction in a linearly increasing voltage (photo-CELIV), or time-of-flight
measurements (TOF). For solar cells, field-effect transistor measurements are less
useful due to the much higher charge carrier density in these types of devices,which strongly influences the mobility.
38Due to the large variety of measurement
techniques the comparison of values reported is often difficult. Furthermore, it is of
importance to determine the charge carrier properties in the polymer:fullerene BHJ
as it is fabricated in the actual solar cell, since blending a polymer with a fullerene
can have very different effects on the charge carrier properties. For instance, in
MDMO:PPV a 200 fold increase of the hole mobility is observed when blended ina 1:4 weight ratio with PCBM whereas for P3HT a decrease in mobility is observed
upon blending, only to be recovered by thermal or solvent annealing. In this thesis,
besides the standard J-V measurements, the determination of the charge carrier
mobilities, and modelling of the photocurrent are used as main tools for analysing
polymer:fullerene solar cells. These characterisation techniques will be discussed
in further detail below.
1.1.1 Single carrier devices
As explained above, in order to understand the device operation of polymer
solar cells it is crucial to determine the charge carrier mobility of the individual
components of the solar cell. In this thesis charge carrier mobilities are determinedusing the SCLC method. The advantages of this method are that the charge carrier
mobilities are determined in an identical geometry as the solar cell itself, and thatprocessing conditions are almost identical. Furthermore, both pristine materials as
well as the blends actually used in the solar cell can be investigated. Blending a
polymer with a fullerene can have very different effects on the charge carrier
properties.
In order to measure the charge carrier mobility of holes or electrons the
transport of the other charge carrier needs to be suppressed. This is done by
choosing appropriate electrode materials. One can either fabricate a double carrier
device similar to the solar cell, make a hole-only device by suppressing the electron
injection using a palladium (Pd) cathode, or suppress the hole injection by using an
aluminum oxide anode and thus make an electron-only device. Since the light
emission from solar cells is very low it is difficult to determine whether only one
carrier is injected. From experiments on the pure materials it is found that injection
from the “wrong” contact can be ruled out. Furthermore no difference in extractioncapabilities was observed. In its most basic form the SCLC is now given by
3
2
int0
8
9
L
V J r
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where 0 r is the dielectric constant of the material, L is the device layer thickness,V int is the internal field of the device (applied voltage V A corrected for the built in
voltage V bi and voltage drop over the connections V rs) and is the mobility of the
material. Previous studies have shown the charge carrier mobility in organic
materials to be dependent on both the electric field and the charge carrier density in
the device. In this thesis only the field dependence of the charge mobility is taken
into account using a stretched exponential dependence:
))(exp()(),( 0 E T T T E
where 0(T) is the zero field mobility and (T) describes the field activation.
Justification for neglecting the density dependence are the relatively low chargecarrier concentrations during device operation and the relatively low amount of
disorder in the used materials compared to for instance organic light-emitting
diodes.
1.4.4 Photocurrent measurements and modeling
In section 1.3.1 the basic characterization of an organic solar cell wasintroduced. From the fourth quadrant of the J-V measurement the characteristic
values J sc, V oc, FF and the maximum power point can be determined. For analyzingthe physical processes inside the solar cell however this representation is not very
adequate. Much more information can be obtained by plotting the photocurrent as a
function of the electric field inside the device.35
For this the photocurrent
J ph= J L− J D, where J L and J D are the current density under illumination and in dark,respectively, is plotted as a function of effective applied voltage V 0-V A. Here V 0 is
the compensation voltage defined as J ph (V 0) =0 and V A is the applied bias. Figure
1.5 shows a typical example of the photocurrent of a polymer:fullerene solar cell.
Two different regimes can be identified; for small effective voltages thephotocurrent increases linearly, which has been shown to be caused by a
competition between drift and diffusion currents.39
At higher voltages the
photocurrent gradually increases until it saturates to a maximum value. This
gradual increase has been attributed to the field- and temperature dependentdissociation of bound electron hole pairs at the donor-acceptor interface. The Braun
Onsager model of geminate recombination has been shown to adequately describe
this dissociation of bound electron hole pairs in a number of polymer:fullerenesystems. A typical fit of the photocurrent using this model is also shown in figure
1.5 showing the nice agreement of the model at high voltages. In order to fully
describe the photocurrent and the standard J-V characteristics, a numerical program
developed by Jan Anton Koster is used.36
This program, which includes drift and
diffusion of charge carriers, the effect of space-charge on the electric field,
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bimolecular recombination, and a field- and temperature dependent generation
rates of free charge carriers, has been shown to consistently describe a number of polymer:fullerene systems, including their temperature and illumination
dependence. A fit of the complete voltage regime of the photocurrent using the
simulation program is shown in figure 1.5.
Besides the increase of the photocurrent at reverse bias, spectroscopicexperiments have indicated the existence of a charge transfer state at the donor-
acceptor interface. There exists however a discrepancy between the lifetime of this
state, as determined from the Braun model used here (micro to milli-seconds), and
the spectroscopically lifetime (nanoseconds). Recently is was suggested to take
into account the local mobility of charge carriers at the interface, instead of
macroscopic determined mobilities when using the Braun model. Decay ratesdetermined in this way do agree with spectroscopic decay rates.
0.1 1 10
10
100
Data
Simulations
Braun model
J
p h
[ A / m 2 ]
V0-V [V]
Figure 1.5: Photocurrent density Jph as a function of effective applied voltage V0-V. The symbolsrepresent the experimental data, the dotted line a fit of the photocurrent at high reverse bias using the
Braun model and the solid line a fit over the whole voltage regime using the numerical simulationprogram.
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1.5 Optimization of energy levels in a donor acceptor system
Above we have discussed the use of a donor-acceptor (D-A) system in order toseparate excitons into free carriers. Unfortunately, during the transfer of the
electron from the LUMO of the donor to the LUMO of the acceptor, energy is
inevitably lost. This loss in energy is manifested in the low open circuit voltage of
a D-A BHJ solar cell compared to the bandgap of the absorber. The open circuit
voltage is ultimately limited by the difference between the HOMO of the donor and
the LUMO of the acceptor. This means that the energy offset between donor and
acceptor LUMO enables electron transfer but also, inevitably, results in a loss of V oc.
Figure 1.3: Energy diagram of a P3HT:PCBM solar cell (a) and strategies to reduce the loss of energy
during electron transfer by (b) reducing the LUMO of the donor (c) reducing the LUMO and HOMO
of the donor and d) raising the LUMO of the acceptor.
In Figure 1.3 the energy diagram for the P3HT:PCBM system is shown. What
is striking is the LUMO-LUMO offset which is much larger than the 0.3 – 0.5 eV
necessary for electron transfer to occur. This results in P3HT:PCBM cells havingan open circuit voltage of only 0.6 V, much smaller compared to the bandgap of
P3HT of 2 eV. It is the reduction of this excess LUMO-LUMO offset where a large
increase in device efficiency can be obtained.1 40
What is assumed here is that reducing the LUMO-LUMO offset does not reduce the
charge separation at the donor-acceptor interface. Recent experiments have hinted at thefact that the large offset in P3HT:PCBM is necessary for the efficient generation of free
charge carriers.
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To reduce the offset three strategies can be employed. Firstly, the LUMO of the
donor can be lowered resulting in so called small (or sometimes called low)bandgap donors. By decreasing the bandgap of the donor the absorption of the
donor material is extended towards higher wavelengths. In figure 1.6 the solar
spectrum is shown together with the integrated photon flux versus wavelength.
Since the absorption of P3HT is limited to 650 nm it is clear that a reduction in thedonor bandgap can result in a large increase of the amount of absorbed photons and
hence device current.
Figure 1.6: Photon flux as function of wavelength. The percentage of the total photon flux and thecorresponding maximum current is displayed at the x-axis.
Alternatively, both LUMO and HOMO level of the donor can be lowered. In
this case the bandgap of the donor remains constant and the device gains in
efficiency due to a larger V oc. An advantage of this strategy is the expected air
stability once the HOMO of the polymer is lower then 5.4eV.
41
One has to take intoaccount however, that for a very deep HOMO the workfunction of PEDOT:PSS
might not be adequate anymore, limiting the V oc.
As a third option, the LUMO-LUMO offset can be reduced by raising the
LUMO of the acceptor, were again the device gains in efficiency due to an
enhancement of the open circuit voltage. All these three strategies are illustrated in
Figure 1.5.
Which of the above-mentioned strategies, and thus which donor bandgap, is
optimal is still under debate and depends on the models used to predict efficiencies
and the restrictions made on the materials used. When calculating the increase in
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current generated by narrowing the bandgap of the donor for instance, one has to
take into account that not all photons with larger energy than the donor bandgap areabsorbed. Koster et al have calculated the increase in absorption by taking the
absorption profile of P3HT and shifting this absorption in energy to account for a
narrowing of the bandgap. Combined with realistic values for the fill factor and
charge dissociation, Koster et al predict an efficiency of 6.6% for a donor bandgapof 1.5 eV at which point the LUMO-LUMO offset is reduced to 0.5 eV (in
combination with [60]PCBM as the acceptor). Further narrowing of the bandgap
will have to be realized by raising the HOMO of the polymer which will result in a
lower V oc and no increase in efficiency is expected. Note, however, that very small
bandgaps may still have their use in infrared photo detectors and tandem or multi-
junction solar cells. As a second step Koster et al calculated the increase inefficiency when P3HT is taken as donor and the LUMO of the acceptor is raised up
to the 0.5 eV offset. For this strategy the predicted maximum efficiency was found
to be more than 8%, showing the great potential of energy level alignment at the
acceptor side. If one now allows both donor and acceptor LUMO to vary the
optimal bandgap can be determined. It was shown that this optimal bandgap is in
fact not small, as is usually asumed, but reaches a maximum around 1.9 eV.
1.5.1 Multijunction solar cells
In the discussion above only single layer cells are considered. When this
constraint is lifted even higher efficiencies can be obtained as is demonstrated inthe field of inorganic photovoltaics. Here multijunction solar cells have shown
efficiencies of over 40 percent,42
higher than the theoretic limit for single junctions
of 31 percent.43
For single junction solar cells there exists an optimal bandgap dueto a competition between a high voltage and a high current. When enlarging the
bandgap the voltage is raised yet the amount of photons which can be absorbed
decreases and vica versa. Multiple junctions enable photons with high energy to be
absorbed by a wide bandgap material and photons with low energy by a small
bandgap material. In this way the thermalisation losses, due to photons having a
larger energy compared to the bandgap of the absorber, can be diminished.
For polymer solar cells the realization of a tandem or multijunction
architecture is difficult to achieve, especially concerning the middle electrode(s).44
Since polymer solar cells are made of intrinsic semiconductors the work function
of this electrode should match the energy level of the acceptor on one side and that
of the donor on the other. Furthermore, the electrode should function as a good
recombination layer, needs to be optically transparent, and last but not least has to
protect each active layer from the deposition of the next one. Neverthelessconsiderable progress is made in the fabrication and the design of tandem and
multijunction cells. The fact that more well-performing materials, with proper
energy level alignments, are needed for both single and mulitjunction cells, is
reflected in the best performing polymer multijunction cell so far. In this tandem
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cell the sub cell which absorbs the low wavelength part of the spectrum generates a
lower open circuit voltage compared to the cell which absorbs the high wavelength,exactly opposite to the general design rule for multijunction cells.
45
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1.6 Aim and scope of this thesis
In the last decade the field of polymer:fullerene solar cells has seen an
impressive improvement of both device efficiency and understanding of the
physical processes governing these type of devices. The switch to chlorobenzene as
a solvent for spincoating MDMO-PPV:PCBM layers led to the first reasonable
performing device, achieving an efficiency of 2.5%. Due to the relatively largeavailability of the used materials there was now a model system for the community
which made a thorough investigation of the device physics possible. The MDMO-
PPV:PCBM system turned out to be a rather peculiar system in which the addition
of a fullerene to the polymer results in an increase of the hole mobility of the
polymer by a factor 200, which turned out to be essential for device operation and
explains the high amount of fullerene used in the optimized cell. Furthermore, thedissociation of the bound electron hole pair at the donor acceptor interface was
shown to be a significant limiting factor for PPV based devices.
Devices based on MDMO-PPV:PCBM blends typically have an active layer
thickness of 100 nm at which still a significant portion of light is not absorbed. Inchapter 2 of this thesis the origin of the need for such a relatively thin active layer
is investigated. It is shown that the decrease in fill factor which, from a device
point of view, is the origin for the decreasing efficiency upon increasing the active
layer thickness, is due to a combination of space-charge effects, a decreasing
dissociation efficiency and charge recombination.In chapter 3 a new glycol substituted PPV is investigated which has a higher
permittivity compared to normal PPVs. The aim here is to increase the abovementioned low dissociation efficiency of PPV based devices, which is strongly
dependent on the average permittivity of the active layer blend. Due to a significant
lower hole mobility of the polymer and morphology problems devices based on
this new polymer did not show improved power conversion efficiencies compared
to the model MDMO-PPV system. Nevertheless, an increase in dissociation
efficiency from 60 to 72% was observed for the enhanced permittivity polymerindicating the importance of the average permittivity in polymer:fullerene devices.
After the initial success of MDMO-PPV, poly(3 -hexylthiophene) emerged as a
new model material for polymer:fullerene devices. Thermal and solvent annealing
steps improved the charge mobility of the hole transport considerably, resulting ina balanced transport of both holes and electrons. Due to this balanced transport
much thicker active layers can be used without sacrifice of the fill factor, resulting
in a significantly enhanced device performance. Again, wide (commercial)
availability of the polymer allowed extensive research into the device physics and
optimization of the material system. With internal quantum efficiencies as high as
90% it is clear new materials are needed in order to further increase the device
efficiency. Using the knowledge from the MDMO-PPV and P3HT system a more
focused approach can now be taken when searching for new systems, especially
concerning the energy level optimization as discussed in the previous section.
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One of the approaches towards higher efficiencies is reducing the bandgap of
the donor polymer in order to increase the light harvesting of polymer:fullerenesolar cells. One of the most promising materials following this route is PCPDTBT.
In chapter 4 the charge transport and photogeneration of this material blended with
PCBM is investigated. Despite an almost balanced transport the photocurrent
shows a square root dependence on effective voltage. It is shown that this squareroot dependence does not stem from an unbalance in mobilities, as seen in other
polymer solar cells, but from an enhanced recombination of the bound electron
hole pair. This enhanced recombination is likely due to a too close intermixing of
polymer and fullerene.
In chapter 5 another route towards efficiency enhancement is investigated.
Instead of lowering the LUMO of the donor now the LUMO of the acceptor israised allowing for a very direct enhancement of the efficiency due to a larger open
circuit voltage. In order to achieve this raising of the LUMO level, the bisadduct
analog of PCBM was used. The additional functionalisation of the fullerene cage
leads to the saturation of one more double bond raising the LUMO level of the
molecule significantly. It is shown that despite the additional functionalisation and
the increased disorder introduced (due to having a multitude of isomers of the
molecule), replacing PCBM with bisPCBM results in only a very slightlydecreased photogeneration and transport properties. Combined with a significantly
enhanced open circuit voltage a power conversion efficiency of 4.5% was
achieved, which is among the highest reported for polymer:fullerene solar cells. In
the second part of chapter 5, bis- and trisadduct analogs of other fullerenes are
investigated. It is shown that the existence of multiple isomers leads to shallowtrapping for single carriers devices which do not affect the device operation of the
solar cells itself.
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REFERENCES
[1] N. S. Lewis, MRS Bull. 2007, 32, 808.\
[2] A. Slaoui, R. T. Collins, MRS Bull. 2007, 32, 211.
[3] M. Šúri, T. A. Huld, E. D. Dunlop, H. A. Ossenbrink, Solar Energy, 2007, 81,1295.
[4] M. A. Green, Physica E: Low-dimensional Systems and Nanostructures, 2002 14,65.
[5] C. J. Brabec, N. S. Sariciftci, J. C. Hummelen, Adv. Funct. Mater. 2001, 11, 15.
[6] C. J. Brabec, J.D Durrant, MRS Bul. 2008, 33, 607.
[7] B.O‟reagan, M. Gratzel, Nature, 1991, 353, 737.
[8] U. Bach, D. Lupo, P. Comte, J. E. Moser, F. Weissörtel, J. Salbeck, H. Spreitzer,M. Grätzel, Nature 1998, 395, 583.
[9] C. W. Tang, Appl. Phys. Lett . 1986, 48, 183
[10] J. Xue, B. P. Rand, S. Uchida, S. R. Forrest, Adv. Mater . 2005, 17 , 66
[11] F. Padinger, R. S. Rittberger, N. S. Sariciftci, Adv. Funct . Mater . 2003, 13, 85.
[12] D. Muhlbacher, M. Scharber, M. Morana, Z. Zhu, D. Waller, R. Gaudiana, C.Brabec, Adv. Mater. 2006, 18, 2884.
[13] L. H. Slooff, S. C. Veenstra, J. M. Kroon, D. J. D. Moet, J. S. Sweelssen, M. M.Koetse, Appl. Phys. Lett. 2007, 90, 143506.
[14] M. M. Wienk, M. Turbiez, J. Gilot, R. A. J. Janssen, Adv. Mater . 2008 20, 2556
[15] J. J. M. Halls, C. A. Walsh, N. C. Greenham, E. A. Marseglia, R. H. Friend, S. C.Moratti, A. B. Holmes, Nature 1995, 376, 498.
[16] C. R. McNeill, A. Abrusci, J. Zaumseil, R. Wilson, M. J. McKiernan, J. H.Burroughes, J. J. M. Halls, N. C. Greenham, R. H. Friend, Appl. Phys. Lett . 2007,
90, 193506.
[17] D. J. D. Moet, L. J. A. Koster, B. de Boer, P. W. M. Blom, Chemistry of Materials,2007, 19, 5856.
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[18] G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Science 1995, 270, 1789.
[19] C. K. Chiang, C. R. Fisher, Y. W. Park, A. J. Heeger, H. Shirakawa, E. J. Louis, S.
C. Gau, A. G. MacDiarmid, Phys. Rev. Lett . 1977, 39, 1098.
[20] H. Shirakawa, E. J. Louis, A. G. MacDiarmid, C. K. Chiang, A. J. Heeger J. Chem.Soc. Chem. Commun. 1977, 578.
[21] D. Hertel, H. Bassler, ChemPhysChem 2008, 9, 666.
[22] V.I. Arkhipov, H. Bassler, Phys. Status Solidi A 2004, 201, 1152
[23] D. E. Markov, E. Amsterdam,P. W. M. Blom, A. B. Sieval, J. C. Hummelen, J.
Phys. Chem. A 2005, 109, 5266.
[24] D. E. Markov, C Tanase, P. W. M. Blom, J Wildeman, Phys. Rev. B. 2005, 72,
045217.
[25] M. Granstrom, K. Petritsch, A. C. Arias, A. Lux, M. R. Andersson, R. H. Friend, Nature 1998, 395, 257.
[26] IEC-904-3, IEC Standard
[27] J. M. Kroon, M. M. Wienk, W. J. H. Verhees, J. C. Hummelen, Thin Solid Films 2002, 403, 223.
[28] C. J. Brabec, C. Winder, N. S. Sariciftci, J. C. Hummelen, A. Dhanabalan, P. A.van Hal, and R. A. J. Janssen, Adv. Funct. Mater. 2002, 12, 709.
[29] L. J. A. Koster, V. D. Mihailetchi, R. Ramaker, and P. W. M. Blom, Appl. Phys.
Lett. 2005, 86 , 123509.
[30] P. W. M. Blom, V. D. Mihailetchi, L. J. A. Koster, D. E. Markov, Adv. Mater. 2007, 19, 1551.
[31] L. J. A. Koster, V. D. Mihailetchi, R. Ramaker, P. W. M. Blom, P.W.M. Appl.Phys. Lett. 2005, 86, 123509.
[32] D. Gupta, M Bag, K. S Narayana, Appl. Phys. Lett. 2008, 92, 093301.
[33] M. M. Mandoc, F. B. Kooistra, J. C. Hummelen, B. de Boer, P. W. M. Blom, Appl.
Phys. Lett. 2007, 91, 263505.
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[35] V. D. Mihailetchi, L. J. A. Koster, J. C. Hummelen, P. W.M. Blom, Phys. Rev.
Lett. 2004, 93, 216601.
[36] L. J. A Koster, E. C. P. Smits, V. D. Mihailetchi, P. W. M. Blom, Phys Rev. B 2005, 72, 085205.
[37] D Veldman, O Ipek, S. C. J. Meskers, J. Sweelssen, M.M. Koetse, S.C. Veenstra,
J.M. Kroon, S.S. van Bavel, J. Loos, R.A.J. Janssen, . J.A.C.S., 2008, 130, 7721
[38] C. Tanase, E. J. Meijer, P. W. M. Blom, D. M. de Leeuw , Phys. Rev. Lett, 2003
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[39] R. Sokel, R. C. Hughes, J. Appl. Phys. 1982, 53, 7414.
[40] T. M. Clarke, A. M. Ballantyne, J. Nelson, D. D. C. Bradley, J. R. Durrant, Adv.
Mater. 2008, 42, 4029.
[41] J. Locklin, M. M. Ling, A. Sung, M. E. Roberts, and Z. Bao: Adv. Mater . 2006, 18
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[42] M. A. Green, K. Emery, Y. Hishikawa, W. Warta, Prog. Photovolt: Res. Appl. 2008, 16 , 61.
[43] W. Shockley, H. J. Queisser, J. Appl. Phys. 1961, 32, 510.
[44] A. Hadipour, B. de Boer, P. W. M. Blom, Adv. Funct. Mater. 2008, 18, 169.
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Science 2007, 317, 222.
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Chapter 2
Increasing the active layer thickness of
polymer:fullerene solar cells
Abstract
Solar cells based on poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-
phenylenevinylene] (MDMO-PPV) as electron donor and [6,6]-phenyl C 61 butyric
acid methyl ester (PCBM) as electron acceptor have for many years been the
working horse of the field of polymer photovoltaics. A striking feature of these
solar cells is that at a device thickness of 100 nm at the polymers absorption
maximum only 60% of the incident light is absorbed. From a light harvesting point
of view, an increase in the active layer thickness is thus expected to significantly
enhance the generated photocurrent and hence efficiency. Experimentally however,
the efficiency lowers when using active layers beyond 100 nm, due to a decrease in
fill factor. This decrease has been attributed to an increasing series resistance,
although its physical meaning is not clear for solar cells where charge carriers are
generated throughout the device. In this chapter the origin of this decreasing fill
factor is investigated. It is demonstrated that the formation of space-charge, and
charge recombination puts a limit to the active layer thickness. At the end of the
chapter the effect of optical interference effects on the results are discussed.
REFERENCES
M. Lenes, L.J.A. Koster
V.D. Mihailetchi and P.W.M. Blom
Applied Physics Letters, 88, 243502 (2006)
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2.1 Introduction
Although outperformed by poly(3-hexylthiophene) (P3HT) today, bulk
heterojunction solar cells based on poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-
phenylenevinylene] (MDMO-PPV) as electron donor and [6,6]-phenyl-C61-butyric
acid methyl ester (PCBM) as electron acceptor have been one of the most studied
systems in this field.1
Typically these devices have an active layer of around 100nm achieving power conversion efficiencies of up to 2.5% under AM1.5
illumination.2
At this active layer thickness at the polymers absorption maximum
only 60 percent of the incoming light is absorbed. It is evident that increasing the
active layer thickness will result in an increased absorption in the device
accommodating larger photocurrents. In spite of this increased absorption it is
found that upon increasing the active layer thickness beyond typically 100 nm theoverall power conversion efficiency does not increase, mainly due to a decreasing
fill factor. This decreasing fill factor has been attributed to an increasing series
resistance, although its physical meaning for solar cells, where charge carriers are
generated throughout the device is not clear.3,4
Furthermore it is expected thatcharge recombination will play an important role in thicker devices since the
charge carriers need to travel a larger distance to be collected at the contacts.3,5,6
Previous work by Mihailetchi et. al.7
has shown that a large unbalance in
charge transport in donor and acceptor leads to space-charge effects in polymer
bulk heterojunction solar cells. In their work a difference between hole andelectron mobility of 2 to 3 orders lead to a completely space-charge dominated
photocurrent resulting in fill factors of only 42%. For MDMO-PVV:PCBM devicesalso a difference in hole and mobilities is observed, but here the difference is only
one order of magnitude.8
Since the fill factor of a 100 nm MDMO-PPV:PCBM
device is typically 60% space-charge effects do not play a role here. The main
question is now whether this still holds for active layer thicknesses beyond 100 nm.
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2.2 Space-charge limited photocurrents
Since the hole mobility in polymers is in general smaller than the electronmobility in fullerenes under illumination, holes will accumulate in the solar cell.
This will in turn result in a change of the electric field inside the device. In the
region near the anode the electric field will be increased, enhancing the extraction
of holes. It has been shown that at sufficiently high intensities and mobility
differences a space-charge limited regime governs the photocurrent described by
the following equation7,9
V qG J hr ph
25.0
0
75.0
8
9)( (2.1)
where G is the generation rate of free carriers and μ is the mobility of the slowest
carrier, holes in this case. As can be seen from equation 2.1 a space-charge limited
photocurrent is characterized by a square-root dependence on voltage and a three
quarter dependence on generation rate and thus intensity. Because the photocurrent
depends on the square-root of the applied voltage a purely space-charge limited
device will have a maximum fill factor of 42%, which is considerably lower thanthe 60% for standard MDMO-PPV:PCBM devices. Since the mobility difference in
MDMO-PPV:PCBM devices is of only one order of magnitude, space-chargeeffects do not play a role in standard devices. Here the photocurrent density at short
circuit and reverse bias is governed by10,11
LT E qG J ph ),((2.2)
where q is the elementary charge, G(E,T) is the field,- and generation dependent
rate of free carriers and L the active layer thickness. Therefore, increasing the
active layer thickness will in general result in a higher photocurrent due to the
increase in absorption. Since the space-charge limited photocurrent is independent
on device thickness it is expected that upon increasing the active layer thickness at
some point a transition will occur from a non space-charge limited to a space-charge limited device as illustrated in figure 2.1. Here, for the sake of simplicity, aconstant generation rate G is assumed. Two regimes in the photocurrent can be
identified; regime I where drift- and diffusion currents compete and which varies
linearly with voltage, and regime II where the photocurrent saturates. If now the
active layer thickness is increased the photocurrent will increase and at some point
will intersect the, thickness independent, space-charge limit. Now a new regime
appears in the photocurrent, characterized by a square-root dependence on voltage.
Therefore, this simplified picture predicts a strong decrease in fill factor whenincreasing the active layer thickness, even when charge recombination does not
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play a role. At which point this occurs and whether the field dependence of the
generation rate and charge recombination plays a role has to be determined bymodelling of the actual photocurrents.
Figure 2.1: Illustration of the transition from a non space-charge limited (upper figure) to a space-charge limited (lower figure) device by increasing the device thickness. Since the space-charge limit
is independent on device thickness and increasing the active layer will in general result in a higherphotocurrent at some point the photocurrent will intersect the space-charge limit resulting in a square-root dependence on voltage (regime II in the lower figure). When the device is below the space-
charge limit only two regimes can be distinguished, regime I where drift- and diffusion currentscompete and which varies linearly with voltage and regime II where the photocurrent saturates. Fielddependent motilities, field dependent generation rate and charge recombination are ignored in thissimplified picture.
Jph
V
P h o
t o c u r r e n t
V0-V
Jph
=qGL
S p a c
e C h a
r g e L i m i
t
Jph
V2
Jph
V
Jph
=qGL
V0-V
P h o t o c u r r e n t
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2.3 Device Fabrication and measurements
Solar cells were made using the standard fabrication technique from a blend of
MDMO:PPV and PCBM in a 1:4 weight ratio spin cast from chlorobenzene. By
varying the concentration of the blend solution and spin procedure samples were
made with active layer thicknesses of 128 to 368 nm. After fabrication the devices
were measured in a nitrogen atmosphere under illumination of a white lighthalogen lamp calibrated by a silicon diode. To obtain light intensity dependent
measurements a set of neutral density filter was used, yielding an intensity
variation of two orders of magnitude
100 150 200 250 300 350 400
0.74
0.75
0.76
0.77
0.78
0.79
0.80
0.81
100 150 200 250 300 350 400
40
45
50
55
60
65
100 150 200 250 300 350 4001.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
2.3
2.4
100 150 200 250 300 350 400
40
45
50
55
60
O p e n C i r c u i t V o l t a g e [ V ]
Active Layer Thickness (nm)
F i l l F a c t o r [ % ]
Active Layer Thickness (nm)
E f f i c i e n c y [ % ]
Active Layer Thickness (nm)
S h o r t C i r c u i t C u r r e n t [ A / m 2 ]
Active Layer Thickness (nm)
Figure 2.2: Open circuit voltage, short circuit current, fill factor, and overall power conversionefficiency as a function of active layer thickness under 1kW/m2 illumination.
Figure 2.2 shows the open circuit voltage (V oc), short circuit current ( J sc), fillfactor (FF ) and overall power conversion efficiency (η) of the fabricated devices as
a function of active layer thickness. As expected the open circuit does not vary as a
function of active layer thickness, the short circuit increases as a result of an
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increase in absorption and the fill factor decreases. Due to an increasing short
circuit current being countered by a decreasing fill factor the overall powerconversion efficiency stays approximately constant. To understand the decrease in
fill factor the photocurrents of a thin (128 nm) and thick (368 nm) have been
studied in more detail including their illumination intensity dependence.
In figure 2.3 the photocurrent density J ph= J L− J D, where J L and J D are thecurrent density under illumination and in dark, respectively of a 128 nm device is
shown as a function of effective applied voltage V 0-V A. Here, V 0 is the
compensation voltage defined as J ph (V 0) =0 and V A is the applied bias. Also shown
is the space-charge limit calculated from Equation 2.1 using µh=3×10-8
m2 /Vs and
G=1.9×1027
m-3
s-1
. This device is clearly not space-charge limited and two regimes
can be distinguished. For voltages close to V 0 the photocurrent scales linearly witheffective applied voltage due to a combination of between drift- and diffusion
currents. With increasing applied voltage (V 0-V A>0.1 V) the photocurrent saturates
to J ph=qG(E,T)L
Figure 2.3: Photocurrent density J ph= J L−J D versus effective applied voltage V 0-V A of a deviceconsisting of an active layer of 128 nm. Also shown is the predicted space-charge limit using equation1.1 with µh=3×10-8m2 /Vs and G=1.9×1027 m-3s-1. The photocurrent is below the predicted space-charge limit.
0.01 0.1 1 10
10
100
Measurement
Space Charge Limit
J p h
[ A / m 2 ]
V0-V [V]
128 nm Device
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Figure 2.4: Photocurrent density J ph= J L− J D versus effective applied voltage V 0-V A of a deviceconsisting of an active layer of 368 nm. Also shown is the predicted space-charge limit using equation1.1 with µh=3×10-8m2 /Vs and G=0.9×1027 m-3s-1. In this case the photocurrent density follows thespace-charge limit with a square-root dependence on applied voltage.
The photocurrent density of a thick (368 nm active layer) device is shown in
Figure 2.4 together with the predicted space-charge limited using µh=3×10-8
m2 /Vs
and G=0.9×1027
m-3
s-1
. For this active layer thickness the photocurrent intersects
the space-charge limit and three regimes appear. Like in the 128 nm device the
photocurrent is linear for small applied voltages (V 0-V A<0.1 V). For 0.3V<V 0-
V A<0.7 the photocurrent density shows a square-root behavior typical for a space-
charge limited photocurrent followed again by a saturation of the photocurrent. The
fill factor of this device will be strongly reduced due to the occurrence of the
square root dependence of the photocurrent.
0.1 1 10
10
100
Measurement
Space Charge Limit
J p h
[ A / m 2 ]
V0-V [V]
368nm Device
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Figure 2.5: Intensity dependent measurements performed on a thin (128 nm top) and thick (368 nmbottom) device. For the thin device the fit of α remains close to unity whereas for the thick device αapproaches the theoretic value of ¾ in the space-charge regime.
To check whether the 368 nm device is truly space-charge limited, illumination
intensity dependent measurements were performed. Figure 2.5 shows the intensity
dependence of the photocurrent for both devices at various voltages including fits
of J ph I α. For a pure space-charge limited photocurrent one expects a value of ¾
for α whereas one expects a linear (α=1) dependence in a normal device.12
For the
100 1000
1
10
100
1000
1
10
100
1
10
100
1000
100 1000
1
10
100
S = 0.90
S = 0.95
Jph
@ V0-V= 0.1 V
Incident Light Power [W/m2]
J
p h
[ A / m 2 ]
128 nm Device
Jph
@ V0-V= 1V
368 nm Device
S = 0.94
Jph
@ V0-V= 0.2V
J p h
[ A / m 2 ]
S = 0.83
Jph
@ V0-V= 5V
Incident Light Power [W/m
2
]
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thin device α ranges from 0.9 in the linear regime to 0.95 in the saturated regime
indicating that space-charge effects play almost no role here. For the thick devicehowever α=0.83 at V 0-V A=0.2 V, approaching the theoretical value of ¾ for the
pure space-charge dominated regime. Again the intensity dependence becomes
almost linear in the saturated regime. Hence we can conclude that the square-root
behavior of the thick device occurs due to space-charge effects.
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2.4 Device Simulations
In order to gain insights into the loss mechanisms in thick and thin
polymer:fullerene bulk heterojunction solar cells fits have been performed using
the numerical program described in chapter 1. In figure 2.6 the result of the fits of
the 128 nm and 368 nm device are shown. Figure 2.7 shows the calculated
potential through both devices in dark and under illumination at maximum powerpoint. Where for the 128 nm device both potentials are equal, for the 368 nm
device the potential under illumination is altered. As predicted for a space-charge
limited device, the electric field near the anode is increased due to a build-up of
holes and the electric field near the cathode is decreased.
Figure 2.6: Simulations of a MDMO-PPV:PCBM device made with a 128 nm (a) and 368 nm (b)
active layer.
0.01 0.1 1 10
10
100
0.01 0.1 1 10
10
Measurement
Fit
(b)
J p h
[ A / m 2 ]
V0-V
A[V]
(a)
Measurement
Fit
J
p h
[ A / m 2 ]
V0-V
A[V]
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Figure 2.7: Simulated potential (anode right, cathode left) through a thin (128 nm, top) and thick (368
nm, bottom) device in dark and under illumination. For the thick device the potential is clearlychanged upon illumination due to space-charge effects.
The simulations allow us to address various loss mechanisms individually.
Table 2.1 lists the current density under illumination ( J L), the dissociation
probability (<P>) and recombination losses at short circuit (V sc=0) and maximum
power point (V mpp=0.653 and 0.50 for the 128 nm and 368 nm device,
respectively). The average dissociation efficiency decreases from 61% to 45%
when going from a 128 nm to a 368 nm device and it decreases to 40% at
maximum power point. This can be attributed to the decrease in electric field due
-50 0 50 100 150 200 250 300 350 400
-0.4
-0.3
-0.2
-0.1
0.00.1
0.2
0.3
0.4
-20 0 20 40 60 80 100 120 140
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
Under illumination
In Dark
Under illumination
In Dark
P o t e n t i a l [ V
]
x [nm]
Potential in 368 nm Device at VMpp
=0.50V
x [nm]
P o t e n t i a l [ V ]
Potential in 128 nm Device at VMpp
=0.65V
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to the increase in layer thickness. Secondly, the recombination losses increase for
the 368 nm devices. At short circuit conditions losses are still small but atmaximum power point 35% of the generated charges are lost due to recombination.
Hence besides space-charge effects also the decreasing dissociation probability and
charge recombination play an important role in thick polymer:fullerene bulk
heterojunction solar cells. It should be noted however that it is not possible toexactly determine these processes individually since they are interrelated. Due to
space-charge effects the electric field is reduced in a large part of the device
leading to a lower dissociation probability and increase in carrier transit times.
Therefore the increase in recombination not only originates from an increase in the
distance carriers need to travel to reach the electrode, but is also amplified by
space-charge effects.
V [V] J L [A/m2] <P> [%] rec.
loss.[%]
128 nm device
368 nm device
V sc= 0
V mpp= 0.653V sc= 0
V mpp= 0.50
29.0
19.5
59.8
37.7
61
52
45
40
2
14
9
35
Table 2.1: An overview of voltage, current density, average dissociation probability, and relative
number of free carriers lost due to recombination at short circuit (SC) and maximum power point(MPP) for a 128 nm and a 368 nm device.
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2.5 Optical Considerations
In the simulations described thus far, optical effects have been ignored. Due tothe relatively thin layers compared to the wavelength of the incident light, optical
interference effects are known to play an important role in organic solar cells.13,14,15
When increasing the active layer thickness the number of absorbed photons does
not increase gradually but goes through a series of maxima and minima. In our
simulations we have accommodated for this fact by determining the saturated
photocurrent at each device thickness, which is directly related to the number of
absorbed photons in the devices using equation 2.2. In this way we do not have touse optical interference modelling to determine the total amount of absorbed
photons.
Due to optical interference, however, the absorption profile through the
active layer is inhomogeneously distributed (see figure 2.8). In the above described
modelling a constant generation profile is assumed. One can imagine that an
inhomogeneous profile will alter the operation of a solar cell, especially when the
electric field itself is non-uniform as is the case in thick MDMO-PPV:PCBM cells.
0 100 200 3000
1x1027
2x1027
3x1027
4x1027
Optical Profile
Avarage
E x c i t o n g e n e r a t i o n r a t e [ # / m 3 s ]
Distance from Al electrode [nm]
Figure. 2.8. Simulated exciton generation rate profile (solid line) and its average (dashed line) as afunction of distance x from the cathode.
To see if these effects play a role in our case an optical model has been used to
calculate the generation profile of excitons in MDMO:PPV-PCBM solar cells.16
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Figure 2.9 shows simulations of the IV curve of a 368 nm device using either the
average or the generation profile shown in figure 2.8. It is clear that using anaverage profile results in a slight underestimation of the simulated IV curve due to
a majority of excitons being generated near the anode (where the field in the device
is enhanced) in the real device. For layer thicknesses up to 250 nm no significant
difference between an average or a generation profile was seen in the simulations.16
-0,2 0,0 0,2 0,4 0,6
-60
-40
-20
0
20
Avarage
Generation Profile
J L
[ A / m 2 ]
V [V]
Figure. 2.9. Simulated IV curves using an average or a generation profile. Due to inhomogenities in
both electric field and generation profile the average profile leads to a slight underestimation of thesimulated IV curve.
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2.6 Conclusions
Increasing the active layer thickness of MDMO-PPV:PCBM bulk heterojunction solar cells does not result in a higher power conversion efficiency,
because the increase in short circuit current is cancelled by a decrease in fill factor.
Using intensity dependent measurements and simulations of the photocurrent it is
shown that a difference of one order of magnitude in the electron and hole mobility
poses a limit on the active layer thickness of around 100 nm. When increasing the
active layer beyond that point a space-charge limited regime appears in the
photocurrent limiting the fill factor of the device. Secondly, the dissociationprobability is decreased and charge recombination is increased in thicker samples,
both by space-charge effects and by an increase in the distance carriers need to
traverse. Furthermore, optical simulations show that for relatively thin active layers
assuming a uniform generation rate is a valid approximation. The way to overcome
the limitation on the active layer thickness is to enhance the transport of the slowest
charge carriers, in this case the photogenerated holes in the MDMO-PPV. State of
the art P3HT devices have been shown to maintain high fill factors for active layers
beyond 350 nm at which almost all of the incoming light is absorbed. These high
fill factors have been attributed to a balanced charge transport in the blend.17,18
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REFERENCES
[1] C.J. Brabec , N.S. Sariciftci and J.C. Hummelen, Adv. Fuct. Mater. 11, 15, (2001).
[2] S. E. Shaheen, C. J. Brabec, N. S. Sariciftci, F. Padinger, T. Fromherz, J. C.
Hummelen, Appl. Phys. Lett. 78, 84, (2001).
[3] P. Schilinsky, C. Waldauf, J. Hauch, and C. J. Brabec, J. Appl. Phys. 95, 2816,
(2004).
[4] I. Riedel, J. Parisi, V. Dyakonov, L. Lutsen, D. Vanderzande, and C. J. Brabec, ,
Adv. Funct. Mater. 14, 38, (2004).
[5] I. Riedel, V. Dyakonov, Phys. Status Solidi A. 201, 1332, (2004).
[6] W. Ma, C. Yang, X. Gong, K. Lee and A.J. Heeger, Adv. Funct. Mat. 15, 1617,(2005).
[7] V. D. Mihailetchi, J. Wildeman, and P. W. M. Blom, Phys. Rev. Lett. 94, 126602,(2005).
[8] V. D. Mihailetchi, L. J. A. Koster, P. W. M. Blom, C. Melzer, B. de Boer, J. K. J.
van Duren, R. A. J. Janssen, Adv. Funct. Mater. 15, 795, (2005).
[9] A. M. Goodman and A. Rose, J. Appl. Phys. 42, 2823, (1971).
[10] V. D. Mihailetchi, L. J. A. Koster, J. C. Hummelen, and P. W. M. Blom, Phys.Rev. Lett. 93, 216601, (2004).
[11] L. J. A. Koster, E. C. P. Smits, V. D. Mihailetchi, and P. W. M. Blom, Phys. Rev.B, 72, 085205 (2005).
[12] L. J. A. Koster, V. D. Mihailetchi, H. Xie, and P. W. M. Blom, Appl. Phys. Lett.87, 203502 (2005).
[13] H. Hoppe, N. Arnold, N. S. Sariciftci, and D. Meissner, Sol. Energy Mater.Sol. Cells, 80, 105 (2003).
[14] H. Hoppe, N. Arnold, D. Meissner, and N. S. Sariciftci, Thin Solid Films, 451-452,589 (2004).
[15] N. K. Persson, H. Arwin, and O. Inganäs, J. Appl. Phys. 97, 034503 (2005).
[16] J.D. Kotlarski,P. W. M. Blom,L. J. A. Koster, M. Lenes and L. H. Slooff, J. Appl.
Phys. 103, 084502 (2008).
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[17] V. D. Mihailetchi, H. Xie, B. de Boer, L. J. A. Koster, P. W. M. Blom, Adv. Funct.
Mater. 16, 699, (2006).
[18] V. D. Mihailetchi, H. Xie, L. J. A. Koster, B. de Boer, L. M. Popescu, J. C.
Hummelen, P. W. M. Blom, Appl. Phys. Lett. 89, 012107, (2006)
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Chapter 3
Charge dissociation in polymer:fullerene
bulk heterojunction solar cells with enhanced permittivity
Abstract
The dissociation efficiency of bound electron-hole pairs at the donor-acceptor
interface in bulk heterojunction solar cells is partly limited due to the low
dielectric constant of the polymer:fullerene blend. In this chapter the photocurrent
generation in blends consisting of a fullerene derivative and an oligo(oxyethylene)
substituted poly(p-phenylenevinylene) derivative with an enhanced relative permittivity of 4 is investigated. It is demonstrated that in spite of the relatively low
hole mobility of the glycol substituted PPV the increase of the spatially averaged
permittivity leads to an enhanced charge dissociation of 72% at short circuit
conditions for these polymer:fullerene blends.
REFERENCES
M. Lenes, F. B. Kooistra, J.C. Hummelen
I. Van Severen, L. Lutsen
D. Vanderzande, T. J. Cleij and P. W.M. Blom J. Appl. Phys, 2008 104, 114517
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3.1 Introduction
An important difference between semiconductors used in inorganic and organic
solar cells is the much lower permittivity of the latter. As a result, strongly bound
excitons are created after absorption of light instead of free charge carriers. To
overcome this problem a donor-acceptor system is used in which the electron
transfers from the donor to the acceptor material. However, electron-hole (e-h)pairs generated in this way, are still strongly bound by Coulomb interaction and
need to be dissociated into free carriers in order to be collected at the electrodes.1,2
The occurrence of such an interfacial geminate charge pair has been
spectroscopically observed for polymer:polymer systems 3,4 and recently also for
polymer:fullerene blends.5
In addition, a strong indication for the existence of a
bound e-h pair in polymer:fullerene blends is the field- and temperaturedependence of the photocurrent at reverse bias. At sufficiently high reverse bias all
bound e-h pairs are dissociated, leading to a saturated photocurrent that is field-
and temperature independent.2
As a result, the saturated photocurrent is a direct
measure for the amount of photons absorbed in the blend.6
Typically, thedissociation efficiency in poly(2-methoxy-5-(3´,7´-dimethoxyloctyloxy)-p-
phenylene vinylene) (MDMO-PPV) and [6,6]-phenyl-C61-butyric acid methyl ester
(PCBM) bulk heterojunction (BHJ) solar cells is only 60% at short circuit
conditions, representing a major loss mechanism in these devices.2,7
In this chapter
an oligo(oxyethylene) substituted PPV derivative (Figure 3.1) with an enhancedrelative permittivity of 4 is investigated to see whether the dissociation efficiency
in PPV based solar cells can be enhanced.
Figure 3.1: Chemical structure of poly[2-methoxy-5-(triethoxymethoxy)-1,4-phenylene vinylene](PEO-PPV) and phenyl-C61-butyric acid 2-ethylhexyl ester (PCB-EH) being the donor and acceptormaterial used in this chapter, respectively.
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3.2 Parameters governing the charge dissociation
The field- and temperature dependent process of charge dissociation of thebound e-h pair in polymer:fullerene solar cells can be described using Onsager‟stheory of ion pair dissociation.
8This e-h pair can dissociate into free carriers by a
rate constant k D given by:
1801831
4
3)(
432 /
3
bbbbe
ak E k
kT E
R D B (3.1)
with E B being the electron-hole pair binding energy, )8 /( 223T k E qb B , E is
the field strength and ε=ε0εr the permittivity.1
Free carriers generated in this way
can recombine back to the bound state by Langevin recombination with a rate k R9
),min(k R he
q(3.2)
where it has been pointed out that the recombination strength is dominated by the
slowest carrier mobility in bulk heterojunction solar cells.10
Finally, the bound state
can decay to the ground state with a rate k F . The model predicts the probability thatfree charge carriers will be produced at a particular field ( E ), temperature (T ), and
donor-acceptor separation radius (a):
F D
D
k E k
E k E T aP
)(
)(),,( (3.3)
From the above-mentioned equations, it is clear that the charge dissociation is
governed by four relevant parameters, viz. the charge carrier mobility μ, the
permittivity of the blend ε, the initial separation distance a, and the decay rate k F .
For the MDMO-PPV:PCBM system it has been shown to be vital to take into
account the overall relative permittivity of the blend when describing the chargedissociation.
11The relative permittivity εr of MDMO-PPV is 2.1,
12whereas the εr
of PCBM amounts to 4. As a result, when going from a 1:1 to a 1:4
polymer:fullerene weight ratio the increase in photocurrent is shown to originate
not only from an increase in the hole mobility, but also from an increase of the
average permittivity, due to the loading of more PCBM. An increase in the
permittivity of the donor polymer is therefore expected to enhance the dissociationefficiency and thus the efficiency of the solar cells. In this study an
oligo(oxyethylene) substituted PPV derivative with εr = 4 is used to study the effect
of an enhanced permittivity of the blend on the photogeneration.
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0,1 1 10
0,6
0,8
1
2,0 2,5 3,0 3,5 4,00,60
0,65
0,70
0,75
r=4
D i s s o
c i a t i o n E f f i c i e n c y
V0-V [V]
r=2 D
i s s o c i a t i o n
a t S C
Permittivity
Figure 3.2: Calculated dissociation efficiency versus effective applied voltage with a polymerpermittivity ranging from 2 to 4. The dissociation efficiency at short circuit conditions is indicated inthe inset.
In figure 3.2 the expected increase in dissociation efficiency is shown. Here we
start with calculating the dissociation using the Braun model and input parameters
as determined in ref 7. The permittivity of the polymer donor material is nowraised until the value of 4, taking into account the 1:4 polymer:fullerene weight
ratio in the blend. From the calculations we expect a significant increase in
dissociation efficiency at short circuit conditions to 73%.
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3.3 Single Carrier Devices
Figure 3.1 shows the chemical structure of poly[2-methoxy-5-(triethoxymethoxy)-1,4-phenylene vinylene] (PEO-PPV), the donor material used
in this chapter. The synthesis and characterization have been previously reported.13
Furthermore, previous studies using impedance measurements have shown the
relative permittivity of the material to be equal to 4.14
Also shown in figure 3.1 is
the chemical structure of phenyl-C61
-butyric acid 2-ethylhexyl ester (PCB-EH), the
acceptor used here. PCB-EH is used instead of PCBM to provide a better mixing of
donor and acceptor. The charge transport in pristine polymer films was investigatedby sandwiching a layer of PEO-PPV between a layer of indium tin oxide (ITO)
covered with ~70 nm of poly(3,4ethylenedioxythiophene)/poly(styrenesulfonate)(PEDOT:PSS) and a palladium (Pd, 20 nm)/gold (Au, 80 nm) top electrode. The
high work function of Pd prevents electron injection and only holes flow through
the device. Figure 3.3 shows the J-V characteristics of such a PEO-PPV hole-only
device, corrected for the built-in voltage (V bi) and the series resistance of the
electrodes. The J-V curve is fitted to a space-charge limited current (SCLC),
yielding a hole mobility of 1.8 x 10-10
m2 /Vs. The observed hole mobility is
comparable to hole mobilities reported for pristine MDMO-PPV films.15
ForMDMO-PPV, however, blending the polymer in a 1:4 weight ratio with PCBM
results in a dramatic increase in hole mobility of more than two orders of
magnitude,16 which turns out to be essential for device operation.2,7
Figure 3.3: Current density versus voltage, corrected for built-in voltage and series resistance of PEO-PPV hole only device. Data (symbols) is fitted (solid line) using a space-charge limited current.
As a next step the hole transport in a PEO-PPV:PCB-EH blend (1:4 weightratio) is investigated , also using a Pd top electrode to prevent electron injection
into the PCB-EH. In figure 3.4 the J-V characteristics of such a hole only device
are shown. The determined hole mobility of 4 x 10-11
m2 /Vs indicates that an
enhancement of the hole mobility, as seen in blends of MDMO-PPV, does not
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occur for blends of PEO-PPV and PCB-EH. As was shown in chapter 2 a low hole
mobility is likely to lead to the formation of space-charges as well as a lowdissociation efficiency and is expected to limit the performance of the PEO-
PPV:PCB-EH solar cells. To separately illustrate the effect of a low charge carrier
mobility on the charge dissociation efficiency, figure 3.5 shows calculations of the
Braun model using parameters for the MDMO-PPV:PCBM system;7
a=1.25x10-9m, k f =1x105
s-1
r =3.4 and h=4x10-8
m2 /Vs and identical parameters
but now with the lower hole mobility of PEO-PPV, 4x10-11
m2 /Vs. The calculated
charge dissociation at short-circuit is reduced from 62% to only 22% by lowering
the hole mobility to such a low value. Furthermore, the formation of space-charges17
is expected to limit the fill factor.
Figure 3.4: Current density versus voltage, corrected for built-in voltage and series resistance of PEO-PPV:PCB-EH hole only device. Data (symbols) is fitted (solid line) using a space-charge limitedcurrent..
Figure 3.5: Dissociation efficiency for an MDMO-PPV:PCBM solar cell as determined in ref 7 (solid
line), together with a calculated dissociation efficiency for a similar system but with a lower hole
mobility of 4x10-11 m2 /Vs. The dissociation efficiency at short circuit conditions (as indicated by thevertical solid line) drops from 62% to 22% due to the lower hole mobility.
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3.4 PEO-PPV:PCB-EH Solar Cells
Polymer:fullerene bulk heterojunction solar cells were fabricated by spincoating a layer of PEO-PPV:PCB-EH in a 1:4 weight ratio from chlorobenzene on
top of PEDOT:PSS covered ITO. As a cathode, 1 nm of lithium fluoride and 100
nm of aluminium was evaporated. Figure 3.6 shows the J-V characteristics of a
PEO-PPV:PCB-EH solar cell measured under illumination of a white light halogen
lamp set at 1000W/m2. Due to the low charge carrier mobility the optimal device
thickness is only 68 nm,18
resulting in a low short circuit current of 13.8 A/m2.
Combined with a fill factor of 52% and an open circuit voltage of 0.67 V theestimated overall power efficiency is 0.5% which is considerably lower compared
to the model system MDMO-PPV-PCBM. The main reason for this lower
efficiency is the low short circuit current which is likely to be caused by an
unfavourable large domain formation of polymer and fullerene, leading to a loss of
excitons.
Figure 3.6: Current density versus voltage characteristics for a PEO-PPV:PCB-EH solar cell under
illumination of a 1000 W/m2 halogen lamp.
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3.5 Device Simulations
Even though the power conversion efficiency is much lower as compared toMDMO-PPV:PCBM the devices can still be used to study the effect of the raised
permittivity on the charge dissociation. For this, the photocurrent density is plotted
as a function of effective applied voltage V 0-V A, as is shown in figure 3.7. The
previously mentioned Braun model can only be applied to describe the
photocurrent at relatively high effective voltages where the photocurrent is in the
saturated regime and is governed by the dissociation of bound electron-hole pairs.2
To fully describe the photocurrent the numerical program was used.7
Inputparameters for the numerical program were identical to the above described Braun
model i.e., the charge carrier mobilities including their field dependence, the
average permittivity εr , separation distance a and decay rate k f . Since the charge
carrier mobility has been determined using hole only diodes and the electron
mobility of the fullerene is known19
only a and k f were used as fitting parameters.
Both separation distance a and decay rate k f have a different effect on the fits of the
photocurrent. The distance a determines at which voltage the dissociation saturates
whereas the decay rate k f determines how fast the dissociation drops when the field
in the device is lower. Indicated in figure 3.7 is a fit to the experimental
photocurrent using our numerical program with input parameters εr =4, h=4x10-11
m2 /Vs, a=1.5x10
-9m, e=2.5x10
-7m
2 /Vs and k f =4x10
4s
-1. Note that the value to
which the photocurrent saturates is considerably lower compared to normalPPV:PCBM cells. This is due to the in paragraph 3.4 mentioned coursemorphology which leads to a loss of excitons. The amount of lost excitons can in
principle be calculated by comparing the saturated photocurrent with the amount of
absorbed photons. For the analysis of the dissociation efficiency these lost photons
do not play an important role.
In figure 3.8, using the same input parameters, a fit using the Braun model
is shown. From these figures one can see that for voltages V 0-V >3 the photocurrent
is dominated by the field dependent dissociation of bound electron hole pairs and
that bimolecular recombination and space-charge effects do not play a role.
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Figure 3.7: Experimental photocurrent density Jph as a function of effective applied voltage V 0−VA under 1kW/m2 illumination for a PEO-PPV:PCB-EH solar cell. Circles indicate experimental data,
solid line fit of the photocurrent.
This allows one to directly determine the dissociation efficiency by comparing
the generated photocurrent at short circuit with the saturated photocurrent at a large
reverse bias resulting in a dissociation efficiency of no less than 72%. Thus, we canconclude that, even when the charge carrier mobility is significantly lower as
compared to MDMO:PPV, the charge dissociation at short circuit is increased by
using a high permittivity polymer. Above we calculated the dissociation efficiency
for materials with a hole mobility of 4x10-11
m2
/Vs and normal permittivity to beonly 22%. The origin of the observed enhanced dissociation efficiency in the PEO-
PPV:PCB-EH blend is due to two effects, as shown in figure 3.8; First of all, the
initial separation distance of charges is enlarged from 1.25 x10-9
to 1.5 x 10-9
m and
the decay rate k f is lowered from 1x105
s-1
to 4x104
s-1
. Using these parameters we
predict the dissociation efficiency to be 45%. The second effect is the direct effectof the higher relative permittivity of PEO-PPV of 4, raising the dissociation
efficiency even more to the measured 72%. With these parameters, but now
combined with the MDMO:PPV hole mobility of h=4x10-8
m2 /Vs, the calculated
dissociation is as high as 78%. Combined with an increase in fill factor, our
numerical program predicts a power conversion efficiency of 3.5% to be possible
for a PPV-type polymer with a relative permittivity of 4.
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Figure 3.8: Dissociation efficiency as a function of effective voltage calculated for various values of
permittivity of the polymer ε, initial separation distance a and decay rate k F . In all cases the holemobility is taken to be the measured value of 4x10 -11 m2 /Vs. Starting from a low dissociation of 22%at short circuit conditions (indicated by the vertical solid line) the dissociation efficiency is increasedfirst to 45% by an improved separation distance and decay rate and subsequently to the measuredvalue of 72% by increasing the relative permittivity of the polymer from 2 to 4. The measuredphotocurrent is indicated as a reference (symbols).
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3.6 Conclusions
To conclude, an oligo(oxyethylene) substituted PPV derivative was used tostudy the effect of an enhanced permittivity on the dissociation efficiency in
polymer:fullerene bulk heterojunction solar cells. Besides the permittivity, also the
charge carrier mobility, separation distance, and decay rate are important factors
determining the charge dissociation. Despite a low hole mobility of 4x10-11
m2 /Vs
in the blend of PEO-PPV and PCB-EH, a dissociation efficiency of 72% was
observed. It was shown that the effect of a higher relative permittivity is twofold.
Not only a direct enhancement of the charge dissociation is observed, but also theseparation distance and decay rate are improved upon increasing the relative
permittivity. Therefore, it is concluded that enhancing the relative permittivity of
the polymer can be very beneficial for the device operation of polymer:fullerene
solar cells.
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REFERENCES
[1] C. L. Braun, J. Chem. Phys. 80, 4157 (1984).
[2] V. D. Mihailetchi, L. J. A. Koster, J. C. Hummelen, and P. W. M. Blom, Phys.Rev. Lett. 93, 216601 (2004).
[3] C. Yin, T. Kietzke, D. Neher and H.H. Horhold, Appl. Phys. Lett 80, 092117
(2007).
[4] A.C. Morteani, P. Sreearunothai, L.M. Herz, R.H. Friend and C. Silva, Phys. Rev.Lett. 92, 247402 (2004).
[5] D. Veldman, O. Ipek, S. C. J. Meskers, J. Sweelssen, M. M. Koetse, S. C.
Veenstra, J. M. Kroon, S. S. van Bavel, J. Loos, R. A. J. Janssen, J.Am. Chem.Soc. 130, 7721 (2008).
[6] J.D. Kotlarski, P.W.M. Blom, L.J.A. Koster, M. Lenes and L.H. Slooff, J. Appl.Phys. 103, 084502 (2008).
[7] L. J. A. Koster, E. C. P. Smits, V. D. Mihailetchi, and P. W. M. Blom, Phys. Rev.B, 72, 085205 (2005).
[8] L. Onsager, Phys. Rev. 54, 554 (1938)
[9] P. Langevin, Ann. Chim. Phys. 28, 433 (1903).
[10] L.J.A. Koster, V.D. Mihailetchi and P.W.M. Blom, Appl. Phys. Lett. 88, 052104
(2006).
[11] V.D. Mihailetchi, L.J.A. Koster, P.W.M. Blom C. Melzer, B. de Boer, J.K.J. vanDuren and R.A.J. Janssen, Adv. Func. Mat. 15, 795 (2005).
[12] H.C.F. Martens, H.B. Brom and P.W.M. Blom, Phys. Rev. B, 60, R8489 (1999).
[13] I. Van Severen, M. Breselge, S. Fourier, P. Adriaensens, J. Manca, L. Lutsen, T.J.
Cleij and D. Vanderzande, Macromol. Chem. Physic. 208, 196, (2007).
[14] M. Breselge, I. van Severen, L. Lutsen, P. Adriaensens, J. Manca, D. Vanderzandeand T. Cleij, Thin Solid Films, 511, 328 (2006).
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[15] P.W.M. Blom, M.J.M de Jong and M.G. van Munster Phys. Rev. B, 55, R656(1997).
[16] C. Melzer, E.J. Koop, V.D. Mihailetchi and P.W.M. Blom, Adv. Fuc. Mat. 14, 865(2004).
[17] V. D. Mihailetchi, J. Wildeman, and P. W. M. Blom, Phys. Rev. Lett. 94, 126602(2005).
[18] M. Lenes, L.J.A. Koster, V.D. Mihailetchi and P.W.M. Blom Appl. Phys. Lett. 88,
243502 (2006).
[19] V.D. Mihailetchi, J.K.J. van Duren, P.W.M. Blom, J.C. Hummelen, R.A.J. Janssen,J.M. Kroon, M.T. Rispens, W.J.H. Verhees, M.M. Wienk, Adv. Func. Mat. 13, 43(2003)
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Chapter 4
Recombination-limited photocurrents in
small bandgap polymer:fullerene solar cells
Abstract
The charge transport and photogeneration in solar cells based on the low bandgap
conjugated polymer, poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-
b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and a
methanofullerene is studied. The efficiency of the solar cells is limited by a
relatively low fill factor that contradicts with the observed good and balanced
charge transport in these blends. Intensity dependent measurements display a
recombination limited photocurrent, characterized by a square root dependence on
effective applied voltage, a linear dependence on light intensity, and a constant
saturation voltage. Numerical simulations show that the origin of the
recombination limited photocurrent stems from the short lifetime of the bound
electron-hole pairs at the donor-acceptor interface.
REFERENCES
M. Lenes, M. Morana
C. J. Brabec and P. W. M. Blom
Adv. Funct. Mat. 19 , 1106 (2009)
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4.1 Introduction
As discussed in chapter 1 the large offset between donor and acceptor LUMO
of P3HT and PCBM results in a significant loss of energy. One way to overcome
this problem is by decreasing the LUMO level of the polymer creating so called
small bandgap donors. Due to the lowering of the donor bandgap the absorption is
expanded towards higher wavelengths, allowing more photons to be absorbed, evenwhen one takes into account that not all photons above the bandgap are absorbed.
Besides an expected enhanced efficiency compared to P3HT:PCBM when used in
single active layers, small bandgap donors are also desired for multijunction solar
cells or infrared photodetectors.1
One route towards small bandgap polymers is by coupling electron donor and
acceptor units together in a polymer. Most of the polymers created using this route,however, have resulted in significantly inferior performances compared to solar
cells based on P3HT. The reason for the low performance is mainly due to the poor
carrier transport in these polymers, resulting in low fill factors and quantum
efficiencies.2,3,4,5,6,7,8,9
One of the most promising devices following this approachare based on poly [2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-
b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT), reaching power
conversion efficiencies of up to 3.2% when combined with [6,6]-phenyl-C 71-
butyric acid methyl ester ([70]PCBM).10
In spite of the increased absorption the
power efficiency is still lower than the state-of-the-art P3HT:PCBM cells, of whichefficiencies have been reported of more than 5%.11
The efficiency is mainly limited
by a low fill factor (FF ) of only 40%. In chapter 2 it has been demonstrated that astrongly unbalanced charge transport leads to space-charge limited photocurrents,
characterized by a square-root dependence on applied voltage.12
This dependence
limits the fill factor to about 40%. Remarkably, measurements performed on
PCPDTBT-based field effect transistors resulted in hole mobilities of the polymer
as high as 2×10 – 6
m2V
– 1s – 1
. Even though field-effect mobilities are quantitatively
difficult to relate to charge carrier mobilities in actual solar cells, due to the muchlower charge carrier densities in the latter devices,
13the high field-effect mobilities
clearly indicate that the quality of the hole transport in PCPDTBT must be very
good.14
Combined with the electron transport properties of intrinsic PCBM films,
which already have been investigated in great detail,15
a balanced transport istherefore expected. Consequently, the origin of the reduced fill factors and external
quantum efficiencies in these blends is not clear. In this chapter the charge
transport and photogeneration of PCPDTBT:PCBM solar cells is studied to gain
more insight into the loss mechanisms in this type of devices.
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4.2 Charge transport in pristine PCPDTBT films
As mentioned above, even though field-effect mobilities give a valuable insightinto the quality of the charge carrier transport, ideally one would like to measure
the charge carrier mobility in a device geometry similar to the actual solar cell.
Here the charge transport is studied in a vertical device geometry, similar to solar
cells. By choosing suitable top and bottom contacts one can either inject both
charge carriers, or choose to block one carrier and measure either the hole or
electron current. The transport through these single carrier devices is modeled with
a space-charge limited current (SCLC). Figure 4.1 shows the measured J-V characteristics of a hole only device of PCPDTBT. From the J-V measurements the
zero-field mobility is determined to be 5.5x10-8
m2 /Vs. This mobility is about a
factor of 30 lower than the earlier reported field-effect mobility, due to the density
dependence of the mobility. However, the observed hole mobility of 5.5x10-8
m2 /Vs for PCPDTBT is about a factor of 2-3 larger than the mobility obtained from
diodes based on pristine regio-regular P3HT.16
As a result in its pristine form
PCPDTBT is at least as good a hole transporter as regio-regular P3HT.
Since the electron transport in polymer:fullerene blends occurs through the
fullerene phase, electron transport through the polymer is of no importance for the
device operation of organic solar cells. Nevertheless, also the electron transport
through pristine PCPDTBT films is studied as shown in Figure 4.1. As reported
previously,10 the polymer also shows signs of electron transport. In fact, theobserved electron mobility of 4x10
-9m
2 /Vs is only one order of magnitude lower
than the hole mobility. Interestingly, the electron transport in the pristine material
exhibits normal SCLC behavior where polymers usually show a much stronger
voltage dependence due to charge trapping.17
0,0 0,4 0,8 1,2 1,6 2,0 2,41E-3
0,01
0,1
1
10
100
1000
Hole Only Device
Electron Only Device
J [ A / m 2 ]
V-Vres
-Vbi
Figure 4.1: J-V characteristics, corrected for built-in voltage and series resistance, of a PCPDTBT
hole and electron only device. Data are fitted (solid line) with a space-charge limited current using afield dependent mobility resulting in a hole mobility of 5.5 x 10 -8 m2 /Vs and electron mobility of 4x10-9 m2 /Vs, respectively.
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4.3 Charge transport in PCPDTBT:PCBM blends
Blending a polymer with a fullerene often significantly alters the charge carriertransport in both polymer and fullerene compared to the pristine case. For instance,
in the case of MDMO-PPV, a 200 fold increase in hole mobility is observed when
blending the polymer with PCBM.18
On the other side, blending P3HT with PCBM
results in a reduced hole transport, only to be recovered by thermal or solvent
annealing.19,20
Therefore, single carrier measurements on the actual blend used in
the solar cell are needed to relate the charge carrier transport to the solar cell
performance. Blends of PCPDTBT and PCBM were prepared in a 1 to 4 weightratio, which was reported to be optimal.
10The charge transport is determined in
single carrier devices as described above for pristine polymer films. Figure 4.2
shows the J-V characteristic of a hole and electron only device of a
PCPDTBT:PCBM blend with a weight ratio of (1:4). The determined hole mobility
of PCPDTBT in the blend of 3x10-8
m2 /Vs almost equals the hole mobility in
pristine films. This indicates that the hole transport in the polymer is not altered by
blending it with PCBM. Furthermore, the determined hole mobility is equal to hole
mobilities reported in MDMO-PPV:PCBM (1:4) blends18
and P3HT:PCBM (1:1)
blends after annealing.16
The determined electron mobility of 7x10-8
m2 /Vs is
slightly lower than values reported for MDMO-PPV:PCBM and P3HT:PCBM
blends, that typically amount to 1.0x10-7
-2.0x10-7
m2 /Vs.
18,19,20Similar electron and
hole mobility values were found by ambipolar transport studies on OFETs at highfullerene loadings.
21
0,0 0,7 1,4
0,1
1
10
100
1000
10000
Hole Only DeviceElectron Only Device
J
[ A / m 2 ]
V-Vbi-V
rs
Figure 4.2: J-V characteristics, corrected for built-in voltage and series resistance, of a
PCPDTBT:PCBM hole- and electron-only device with a weight ratio of (1:4). Data are fitted with a
space-charge limited current using a field-dependent mobility, resulting in a hole mobility in the
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blend of 3x10-8 m2 /Vs and an electron mobility of 7x10-8 m2 /Vs.
The single carrier measurements presented here demonstrate that in the blendsthe hole and electron mobilities are balanced and closely match the mobilities
reported for MDMO-PPV and P3HT based blends. It is therefore highly unlikely
that the relatively low quantum efficiencies and fill factors are a consequence of
unbalanced transport or too low charge carrier mobilities and more investigation of the solar cells is needed.
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4.4 PCPDTBT:PCBM Solar Cells
The inset of Figure 4.3 shows the current vs. voltage ( J-V ) curve at roomtemperature of a typical PCPDTBT:PCBM solar cell made in this study. The
external quantum efficiency (see figure 4.4) has been determined at ECN in Petten
to estimate the correct short circuit current under AM 1.5 illumination and thus the
mismatch factor of our measurements. Efficiencies of 2.2% are obtained which is
somewhat lower than the 2.7% reported previously for PCPDTBT:[60]PCBM.10
As
reported previously the power conversion efficiency is limited by a low external
quantum efficiency (<35%) and fill factor (40%).
-0.2 0.0 0.2 0.4 0.6
-60
-40
-20
0
0.1 1 10
10
100
J L
[ A / m 2 ]
V [V]
J p h
[ A / m 2 ]
V0-V
Figure 4.3: Photocurrent of a PCPDTBT:PCBM solar cell versus effective applied voltage. The black line indicates a square root dependence. Inset: J-V characteristics of a PCPDTBT:PCBM solar cell.
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400 500 600 700 800 9000
5
10
15
20
25
30
35
40
45
E Q E [ % ]
Wavelength [nm]
Figure 4.4: External Quantum Efficiency of a PCPDTBT:PCBM solar cell.
For studying the device physics it is very useful to plot the photocurrent of asolar cell as a function of effective applied voltage. The photocurrent density is
defined as J ph=J L−J D, where J L and J D are the current density under illumination
and in dark, respectively, and the effective applied voltage as V eff =V 0−V A. Here V 0
is the compensation voltage defined as J ph(V0)=0 and V A is the applied bias. The
photocurrent versus effective applied voltage of a PCPDTBT:PCBM solar cell isalso shown in figure 4.3. It is clear that at large reverse bias the photocurrentsaturates, at which point all generated electron-hole pairs are dissociated and
collected at the electrodes, which indicates that the mean electron and hole drift
lengths we(h) = e(h)τ e(h) E are equal to, or larger than the sample thickness L and no
recombination occurs.22
The photocurrent shows a sharp decrease at lower effective
applied voltages, resulting in a rather low short circuit current and low fill factor.
Furthermore, a square root dependence of the photocurrent as a function of
effective voltage is observed, as is indicated by the black line. The origin of such a
square root dependence of the photocurrent has been explained by Goodman and
Rose in 1971.22
If the mean electron or hole (or both) drift length becomes smaller
than L, recombination of charge carriers becomes considerable. If there is also a
difference between hole and electron drift length, a non uniform electric field will
occur across the devices, which will give rise to a square-root dependentphotocurrent:
V qG J eheh ph )()( (4.1)
with G the generation rate of free charge carriers. Here a low mobility or shortlifetime of the free carriers, due to recombination or trapping, limits the
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photocurrent. Additionally, at high light intensities the build up of space-charges
(which is the origin of the non-uniform electric field) reaches a fundamental limit.In this limit the maximum electrostatically allowed photocurrent is limited by the
mobility of the slowest charge carrier and is given by
V qG J hr ph
25.0
0
75.0
8
9)( (4.2)
which again has a square-root dependence on voltage. The latter has been
experimentally demonstrated in a system where the charge carrier mobilities are
heavily unbalanced.12 The way to distinguish between these two physically distinct
cases is by light-intensity dependent measurements. Where in the first(recombination limited) case the photocurrent scales linearly with light intensity,
in the second (the space-charge limited) case it scales with a ¾ power law
dependence. Furthermore, the point at which the square root regime forms a
transition into the saturation regime, the saturation voltage V sat , is either
independent on light intensity (recombination-limited) or scales with a one half
power on light intensity (space-charge limited case). From figure 4.5 it is clear that
with decreasing light-intensity V sat is not changing, as expected for arecombination-limited photocurrent. Furthermore, in Fig. 4.6 it is shown that in the
square-root regime the photocurrent is linearly scaling with light-intensity. As a
result the photocurrent observed for PCPDTBT:PCBM devices clearly shows thefingerprints of a recombination-limited photocurrent. Since the mobilities of the
charge carriers in the device are known we can estimate the lifetime using equation
(2), resulting in a lifetime of ~ 10-7
s. This value, estimated under the assumption
that the dominant limitation comes from the hole transport, may slightly change if
electron transport is considered as well.
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0.1 1 10
10
100
J p h
[ A / m 2 ]
V0-V [V]
Vsat
Figure 4.5: Photocurrent of a PCPDTBT:PCBM solar cell versus effective applied voltage at differentintensities varying over more then 1 order of magnitude. Solid lines indicate square root andsaturation regimes as a guide for the eye where Vsat indicates the saturation voltage.
1000
10
100
Veff=0.3 S=1.01V
eff=0.7 S=1.02
Veff
=4.0 S=1.04
J p h
[ A / m 2 ]
Intensity [W/m2
]
Figure 4.6: Intensity dependence of the photocurrent at different effective voltages. The slope (S)
determined from the linear fit (solid lines) to the experimental data is indicated in the figure.
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4.5 Device Simulations and Discussion
In a polymer:fullerene solar cell the photogenerated excitons dissociate at thedonor-acceptor interface via an ultrafast electron transfer from the donor to the
acceptor. However, the ultrafast electron transfer to the acceptor does not directly
result in free carriers, but in a bound electron-hole pair (due to the Coulomb
attraction between the carriers). This pair also needs to be dissociated, assisted by
temperature and by the internal electric field, before it decays to the ground state.20
As proposed by Braun, this bound pair is metastable, enabling multiple
dissociations and being revived by the recombination of free charge carriers viaLangevin recombination.
23Finally, the free carriers are transported to the
electrodes, a process governed by charge carrier mobility. In the above mentioned
Goodman and Rose model a direct generation of free carriers (from now on calledGGR) is assumed. In a polymer:fullerene solar cell, however, the amount of
generated free carriers will not only depend on the amount of generated bound
electron-hole pairs (G B), but also on their dissociation probability (P). In that case
the generation rate of bound pairs G B is proportional to the incident light intensity
and is taken as a measure for the amount of absorbed photons (assuming that all
generated excitons dissociate at the donor-acceptor interface). As a result, when
equation 4.1 is applied to an organic solar cell the calculated lifetime can only be
considered as an effective lifetime (τ eff ). This can be seen more clearly when one
considers the device at open circuit voltage: Since no charges are extracted ( J ph=0)there is an equilibrium between the generation and recombination of free charge
carriers in the device, given by:
eff
GR
nG (4.3)
with n=p being the free electron/hole density and GGR the recombination rate of
free carriers. Thus, if τ eff is small, indicating lots of recombination, also the freecarrier density will be small for a given generation rate of free carriers GGR. When
the formation and dissociation of bound electron-hole pairs as an intermediate step
is taken into account the amount of free carriers that are generated will be given by
PG B and hence one can state that
nPG B
(4.4)
where τ is now the true lifetime of free charge carriers as given by Langevin
recombination.
One can also say that
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eff
B
n
P
nG (4.5)
Thus, when the bound-pair generation rate G B is taken as a measure for the amount
of generated charges, as has been done in device modeling, the effective lifetime
τeff can be small either due to a small life time τ of the free carriers or due to a low
dissociation probability P of the bound pairs.
0.1 1 10
10
100
0.1 1 10
10
100
Data
Simulations
J p h
[ A / m 2 ]
V0-V [V]
Dissociation Probability
DissociationProbability[%]
Figure 4.7: Simulation of the photocurrent at different intensities. Symbols represent measurement,solid line fit to the data, dotted line calculated dissociation efficiency
In order to disentangle the effects of P and τ on τ eff we performed device
simulations using a numerical program which solves Poisson‟s equation and thecontinuity equations, including diffusion, space-charge effects and chargedissociation of bound electron-hole pairs.
24Relevant parameters for the simulation
program are the charge carrier mobilities, including their field and/or densitydependence, dielectric constant , separation distance a and the decay rate of the
bound electron-hole pairs k f . Since the charge carrier mobilities are measured and
the dielectric constant is known only a and k f are used as fitting parameters. Figure
4.7 shows the fit of the simulation program using a = 2.1x10-9
m and k f = 1.7x107
s-
1. Using the same fit parameters we can fit all measured light intensities. When the
calculated dissociation probability is compared with the measured and simulatedphotocurrent (as is indicated in figure 4.7) it is clear that the strong field
dependence of the photocurrent for effective voltages > 0.4 V originates from the
field dependent dissociation of the bound electron-hole pairs. What is striking in
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the device simulations is the high value of k f needed to fit the data. As an
indication, for MDMO-PPV:PCBM and P3HT:PCBM cells a value of ~ 104
isfound. This indicates that the solar cells are limited by a high decay-rate, and thus
short lifetime, of the bound electron-hole pair. Recently Hwang et. al. have shown
experimental evidence of such an intermediate charge transfer state with a short
lifetime using photoinduced absorption spectra.25
To indicate the strong effect thisdecay rate has on the performance of the solar cells, simulations with a decreasing
k f have been performed up to the point at which the decay rate is equal to P3HT
and MDMO-PPV values (see figure 4.8). Upon lowering of the decay rate the
typical square root behavior disappears and the photocurrent becomes significantly
less field dependent, manifesting itself in a greatly increased short circuit current
and fill factor. The simulations indicate that when k f for the PCPDTBT:PCBMdevices would be as low as for the P3HT:PCBM cells an efficiency of ~ 7% can be
achieved. This demonstrates the potential of these low band gap polymer:fullerene
devices, when the increased recombination of the bound pairs can be prevented.
-2.0 -1.5 -1.0 -0.5 0.0 0.5
-120
-100
-80
-60
-40
-20
0
0.1 1 10
10
100
DataFit
J L
[ A / m 2 ]
Voltage [V]
P h o t o c u r r e
n t [ A / m 2 ]
V0-V
decreasing kf
Figure 4.8: Simulation of photocurrent for different values of the decay rate k f starting from thedetermined decay rate of 1.7x107 s-1 and decreasing one order at a time until the value of 1.7x10 4 s-1
typical for normal polymer:fullerene systems. Inset: Simulation of the current under illumination for
these values of k f .
Above, we have shown that PCPDTBT:PCBM solar cells show signs of a
recombination limited photocurrent as predicted by Goodman and Rose. Using our
numerical simulation program we are able to show that the low dissociation
probability is the cause of the low effective lifetime of the free carriers τ eff . We
show the lifetime of the bound electron-hole pair to be significantly shorter
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compared to other polymer:fullerene systems. Moreover, the effective lifetime
predicted using equation (4.2) ~ 10-7
s, matches the lifetime of the bound pair 1/ k f .(k f .= 1.7x10
7s
-1) predicted by the simulation model.
Therefore, we can conclude that the decrease of the photocurrent at low
effective voltages, and hence low fill factor of the device, is due to a short lifetime
of the bound electron-hole pairs. In earlier work an almost complete intermixing of the PCPDTBT polymer with PCBM at the molecular level was reported.
26When
donor and acceptor are too closely intermixed carriers can end up being trapped in
dead ends and will not dissociate fully into free carriers leading to a large decay
rate and hence small effective lifetime. This hypothesis seems to be confirmed by
recent results on PCPDTBT:PCBM solar cells by Peet et al.27
Here they show that
the addition of alkanedithiols to the solution results in a dramatic increase in deviceperformance. It is shown that adding alkanedithiol results in larger phase separation
of donor and acceptor which in turn results in a much higher fill factor and external
quantum efficiency. Apparently the larger phase separation results in an increase of
the effective lifetime of the charge carriers, such that the device is no longer
recombination limited, as predicted by the simulations.
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4.6 Conclusions
The charge transport and photogeneration in PCPDTBT:PCBM solar cells isstudied to gain insight into the loss mechanisms in these devices. The hole
transport in the polymer phase has been shown to be unaffected upon blending with
fullerenes, with a mobility of 5.5 x 10-8
m2 /Vs. The electron mobility of PCBM in
the blend has been determined to be 7x10-8
m2 /Vs, which is slightly lower than the
pristine value for PCBM. Thus the electron and hole transport are almost balanced
and the mobilities are sufficiently high to reach high fill factors and efficiencies.
Nevertheless, the fill factor of PCPDTBT:PCBM solar cells is relatively low,originating from a square root regime in the photocurrent as a function of effective
voltage. Where in chapter 2 this square root dependence was shown to be due to an
unbalance in charge carrier mobilities this is not the case here. The photocurrent is
shown to be recombination limited, characterized by a square-root dependence on
effective applied voltage, a linear dependence on light intensity and a constant
saturation voltage. Simulations of the photocurrent show that the solar cells are
limited by a short lifetime of bound electron-hole pairs. It is suggested that this
short lifetime is due to an unfavorable morphology where donor and acceptor are
too intimately mixed.
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REFERENCES
[1] A. Hadipour, B. de Boer and P. W. M. Blom, , Adv. Funct. Mater., 18, 169, (2008).
[2] S. E. Shaheen, D. Vangeneugden, R. Kiebooms, D. Vanderzande, T. Fromherz, F.
Padinger, C. J. Brabec, N. S. Sariciftci, Synth. Met. 2001, 121, 1583.
[3] C. Winder, G. Matt, J. C. Hummelen, R. A. J. Janssen, N. S. Sariciftci, C. J.
Brabec, Thin Solid Films 2002, 403 – 404, 373.
[4] A. Dhanabalan, J. K. J. van Duren, P. A. van Hal, J. L. J. van Dongen, R. A. J.
Janssen, Adv. Funct. Mater. 2001, 11, 255.
[5] A. P. Smith, R. R. Smith, B. E. Taylor, M. F. Durstock, Chem. Mater. 2004, 16 ,
4687.
[6] X. Wang, E. Perzon, F. Oswald, F. Langa, S. Admassie, M. R. Andersson, O.Inganäs, Adv. Funct. Mater. 2005, 15, 1665.
[7] X. Wang, E. Perzon, J. L. Delgado, P. de la Cruz, F. Zhang, F. Langa, M.
Andersson, O. Inganäs, Appl. Phys. Lett. 2004, 85, 5081.
[8] F. Zhang, E. Perzon, X. Wang, W. Mammo, M. R. Andersson, O. Inganäs, Adv.Funct. Mater. 2005, 15, 745.
[9] L. M. Campos, A. Tontcheva, S. Günes, G. Sonmez, H. Neugebauer, N. S.
Sariciftci, F. Wudl, Chem. Mater. 2005, 17 , 4031.
[10] D. Mühlbacher, M. Scharber, M. Morana, Z. Zhu, D. Waller, R. Gaudiana, C.
Brabec, Adv. Mater. 2006, 18, 2884.
[11] M. D. Irwin, D. B. Buchholz, A. W. Hains, R. P. H. Chang, T. J. Marks, PNAS2008, 8, 2783
[12] V. D. Mihailetchi, J. Wildeman, P.W. M. Blom, Phys. Rev. Lett. 2005, 94, 126602
[13] C. Tanase, P. W. M. Blom, and D. M. de Leeuw , Phys. Rev. B 2004 70, 193202.
[14] M. Morana, P. Koers, C. Waldauf, M. Koppe, D. Muehlbacher, P. Denk, M.Scharber, D. Waller, C. Brabec, Adv. Funct. Mater. 2007, 17 , 3274.
[15] V. D. Mihailetchi, J. K. J. van Duren, P. W. M. Blom, J. C. Hummelen, R. A. J.Janssen, J. M. Kroon, M. T. Rispens, W. J. H. Verhees, M. M. Wienk, Adv. Funct.
Mater. 2003, 13, 43.
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[16] V. D. Mihailetchi, H. Xie, B. de Boer, L. J. A. Koster, P. W. M. Blom, Adv. Funct.
Mater 2006 , 16 , 699.
[17] M. M. Mandoc, B. de Boer, P. W. M. Blom, Physical Review B, 2007, 75, 193202
[18] V. D. Mihailetchi, L. J. A. Koster, P. W. M. Blom, C. Melzer, B. de Boer, J. K. J.van Duren, R. A. J. Janssen, Adv. Funct. Mater. 2005, 15, 795.
[19] V.D. Mihailetchi, H. Xie, L.J.A. Koster, B. de Boer, L.M. Popescu, J.C.Hummelen, P.W.M. Blom, , Appl. Phys. Lett. 2006, 89, 012107
[20] V. D. Mihailetchi, L. J. Koster, J. C. Hummelen, P. W. Blom, Phys. Rev. Lett.2004, 93, 216 601.
[21] M. Morana, M. Wegscheider, A. Bonanni, N. Kopidakis, S. Shaheen, M. Scharber,
Z. Zhu, D. Waller, R. Gaudiana and C. J. Brabec, Adv. Funct. Mat , 2008, 18, 1757
[22] A. M. Goodman, A. Rose, J. Appl. Phys. 1971, 42, 2823.
[23] C. L. Braun, J. Chem. Phys. 1984, 80, 4157.
[24] L. J. A. Koster, E. C. P. Smits, V. D. Mihailetchi, P. W. M. Blom, Phys. Rev. B
2005, 72, 085 205.
[25] I.W. Hwang, C. Soci, D. Moses, Z. Zhu, D. Waller, R. Gaudiana, C. J. Brabec, A.J.
Heeger Adv. Mater. 2007, 19, 2307.
[26] Z Zhu, D Waller, R. Gaudiana, M. Morana, D. Muhlbacher, M.Scharber,C.J.Brabec, Macromolecules 2007 , 40, 1981.
[27] J. Peet, J. Y. Kim,, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger, G. C. Bazan,
Nature Materials 2007, 6, 497.
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Chapter 5
Higher adduct fullerenes for enhanced open
circuit voltage and efficiency in polymer solar cells Abstract
The bisadduct analog of PCBM, bisPCBM, is investigated which has a significant
lower electron affinity as compared to the standard acceptor PCBM. By this raise
of the LUMO level the energy loss in the electron transfer from donor to acceptor
material is reduced, manifesting itself as an increase of the open circuit voltage of
polymer:fullerene bulk heterojunction solar cells. Maintaining high currents and
fill factors an externally verified power conversion efficiency of 4.5% is achieved
for a P3HT:bisPCBM solar cell, 20% higher as compared to the efficiencies of
P3HT:PCBM cells, clearly showing bisPCBM to be the superior acceptor
compared to standard PCBM. Next to bisPCBM, other higher adduct fullerenes
are investigated, including C 70 and thienyl based materials. It is shown that theoccurrence of a multitude of different isomers results in a decrease in charge
carrier transport in single carrier devices for some of the materials. Surprisingly,
the solar cell characteristics are very similar for all materials. This apparent
discrepancy is explained by a significant amount of shallow trapping occurring in
the fullerene phase which does not hamper the solar cell performance due the
filling of these shallow traps during illumination. Furthermore, the trisadduct
analogue of [60]PCBM is investigated which, despite an even further increase in
open circuit voltage, results in a significantly reduced device performance due to a
strong deterioration of the electron mobility in the fullerene phase.
REFERENCES
M. Lenes, G. A. H. Wetzelaer, F. B. Kooistra, S. C. Veenstra,J. C. Hummelen, P. W. M. Blom
Adv. Mater . 2008, 20, 2116
M. Lenes, S. W. Shelton, A. B. Sieval, D. F. KronholmJ. C. Hummelen, P. W. M. Blom
Adv. Funct. Mat. Published Online
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5.1 Introduction
In the first chapter of this thesis the prototypical polymer:fullerene system
P3HT:PCBM was introduced. With a record efficiency of 5.4%1
this material
system is approaching efficiencies warranting large scale commercialization.
Already significant research effort is put into developing large area technologies
employing these materials.2
In order for this research effort to be successful it iscrucial that materials are available on a relatively large scale. As discussed in the
introductory chapter, however, solar cells based on P3HT and PCBM are nearing
their maximum performance. Three strategies to improve the performance beyond
that of P3HT:PCBM solar cells are given. Firstly, small bandgap polymers such as
the ones used in the previous chapter can be used to enhance the light absorption of
the solar cell. Secondly, polymers with lower HOMO and LUMO levels can beused in order to increase the open circuit voltage. Lastly, acceptors with higher
LUMO levels can be employed to raise the open circuit voltage. In this chapter the
third strategy is employed. Thus far, fullerenes have always been the acceptor of
choice when making polymer solar cells. Even though polymer n-type materialshave a large potential due to the additional absorption in the acceptor, so far
efficiencies have been moderate due to problems with charge trapping,
dissociation, and phase separation.3,4
Hybrid solar cells combine polymers with
inorganic nanoparticles and are also considered to have great potential, but so far
stay behind in performance.
5
Therefore, fullerenes with higher LUMO levels arehighly desired. Changing the substituent of PCBM has shown to result in a slightly
higher Voc however, the amount of enhancement using this method is limited.6
Other fullerenes as reported thus far have not resulted in a significant improvement
compared to PCBM.7,8
In this chapter higher adduct fullerenes are investigated as a
candidate for acceptors in polymer solar cells.
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5.2 The bisadduct analogue of PCBM
First we introduce bisPCBM, which is the bisadduct analogue of [60]PCBM, asa new fullerene based n-type semiconductor material. BisPCBM is normally
obtained as a side product in the preparation of PCBM.9
The material used is
obtained by standard chromatographic separation from the other reaction products.
The material consists of a number of regio-isomers. The general structure of these
isomers (with the second addend at various positions on the fullerene cage) is
depicted in Fig. 5.1. BisPCBM has a substantially higher LUMO than PCBM,10
as
can be seen by cyclo voltametric (CV) comparison of bisPCBM and PCBM (Fig.5.1). An increase of the LUMO level of ~ 100 meV was found, raising the LUMO
to 3.7 eV below the vacuum level. Here the pure isomeric mixture of bisadducts
(free of monoadduct and higher adducts) was used. The bisadduct isomer mixture
is made up of a minimum of 17 isomers, as indicated by LC-MS traces. The 1H-
NMR data further indicate that the bisadducts consist of very complex mixture of
isomers, showing at least 17 methoxy resonance signals. First, layers of pristine
bisPCBM were investigated to see whether the additional functionalization of the
fullerene, and the fact that the material is made up out of a mixture of isomers,
have any negative side effects on the charge transport properties.
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-4
-3
-2
-1
0
1
2
3
4
5
6
O
O
O
O
PCBM
bis PCBM
C u r r e n t [ A ]
Voltage [V]
Figure 5.1: Cyclic Voltametry measurement performed on PCBM (solid line) and bisPCBM (dashedline). Experimental conditions: V vs Fc/Fc+, Bu 4NPF6 (0.1 M) as the supporting electrolyte,ODCB/acetonitrile (4/1) as the solvent, 10 mV/s scan rate. The inset shows the generalized chemical
structure of the bisPCBM regio-isomers (i.e. the bottom addend is attached in a cyclopropane mannerat various [6,6] positions, relative to the top one).
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The electron transport through the fullerene was measured by sandwiching a
layer of bisPCBM between a layer of indium tin oxide (ITO) covered with ~70 nmof poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and a
samarium(5 nm)/aluminium(100 nm) top electrode. Since the work function of
PEDOT:PSS (5.2 eV) is significantly lower than the HOMO of bisPCBM (6.1 eV),
hole injection into the fullerene can be neglected and only electrons flow atforward bias. Figure 5.2 shows the J-V characteristic of a bisPCBM electron only
device with a thickness of 182 nm, with the applied voltage corrected for the built-
in voltage and series resistance of the contact. The transport through these single
carrier devices is space-charge limited, resulting in a low-field electron mobility of
7 x 10-8
m2 /Vs. Even though the measured electron mobility for bisPCBM is lower
compared to values reported for normal PCBM (2 x 10-7
m2
/Vs), measured underthe same conditions,
11the observed electron mobility is still expected to result in a
balanced charge transport when combined with P3HT.
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1
100
1000
Fit
Data
J [ A
/ m 2 ]
V-Vbi-V
rs
Figure 5.2: Current density versus voltage, corrected for built in voltage and series resistance of abisPCBM electron only device. Data (symbols) is fitted (solid line) using a space-charge limited
current with a field dependent mobility.
5.2.1 P3HT:bisPCBM solar cells
Next, bisPCBM was used as an acceptor in a polymer:fullerene solar cellsusing the solvent annealing technique.
12P3HT and bisPCBM were dissolved in
1,2-dichlorobenzene (oDCB) by stirring the mixture for 2 days. The blend was spin
cast on top of ITO covered with PEDOT:PSS and left to dry in a closed petri dish
for 48 hours. After the solvent annealing a short (5 minute) thermal annealing step
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was done at 110o
C. To finish the devices a samarium(5 nm)/aluminium(100 nm)
top contact was evaporated. Since bisPCBM has a lower electron mobility andhigher molecular weight compared to normal PCBM, also a different optimal
composition of the blend was anticipated. Indeed, optimization showed a polymer
fullerene weight ratio of (1:1.2) to give the highest efficiencies. The optimal active
layer thickness for P3HT:bisPCBM was found to be ~ 250-300 nm. Afterfabrication the samples were evaluated and the best cells were shipped inside a
nitrogen filled container to the Energy research Centre of the Netherlands (ECN),
to accurately determine the device performance. As a reference, P3HT cells with
normal PCBM in a 1:1 weight ratio were made with the same fabrication
procedure. The optimal thickness of these cells was somewhat higher than for
bisPCBM, around 350 nm.
400 500 600 700 800 900 10000
10
20
30
40
50
60
70
80
bis PCBM
PCBM
E . Q . E .
[ % ]
Wavelength [nm]
Figure 5.3: External quantum efficiency of a P3HT:PCBM and P3HT:bisPCBM solar cell.
Figure 5.3 shows the external quantum efficiency (EQE) determined atECN for P3HT:bisPCBM and P3HT:PCBM solar cells. Even though similar in
shape normal PCBM devices result in slightly higher external quantum
efficiencies, probably due to a thicker active layer. From the EQE measurements
the short circuit current density under AM 1.5 conditions was estimated to be 96
A/m2
for P3HT:bisPCBM versus 104 A/m2
for P3HT:PCBM. Figure 5.4 shows the
J-V characteristics of the cells measured using a halogen lamp with a light output
equivalent to an AM1.5 light source with an intensity of 1.16 kW/m2. The open
circuit voltage of the P3HT:bisPCBM cell amounted to 0.73 V, which is 0.15 V
higher than the cell with P3HT:PCBM. As predicted by the EQE measurements the
short circuit current is only slightly lower for P3HT:bisPCBM. Due to the
enhanced V oc, bisPCBM is clearly the superior acceptor in combination with P3HT.
In order to accurately quote efficiencies, calibrated measurements are needed. Our
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best cell was measured under a 1000 W/m2, simulated AM1.5 illumination from a
WXS-300S-50 solar simulator (WACOM Electric Co). These externally verifiedmeasurements resulted in an open circuit voltage of 0.724 V, fill factor of 68% and
a short circuit current of 91.4 A/m2. The resulting power conversion efficiency
amounts to 4.5% for the P3HT:bisPCBM solar cell with an active area of 0.16 cm2.
Devices with larger active areas of 1 cm2
showed a small decrease in fill factor to62%, resulting in efficiencies of 4.1%. The discrepancy between the calculated
short circuit current from the EQE measurements and the AM 1.5 current is
probably due to the absence of a bias illumination during the EQE measurement.
The efficiency of 4.5% is about a factor 1.2 larger as compared to the efficiencies
of our best P3HT:PCBM cells of 3.8%. This improvement is entirely due to the
increase of Voc.
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8150
100
50
0
50
100
150
bis PCBM
PCBM
C u r r e n t
D e n s i t y [ A / m 2 ]
Voltage [V]
Figure 5.4: Current density versus voltage of P3HT:PCBM and P3HT:bisPCBM solar cells underillumination of a halogen lamp with an intensity equivalent to 1.16 sun.
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5.3 Other higher adduct fullerenes
In the previous paragraph, the bisadduct analog of PCBM was introduced as anew acceptor for use in polymer solar cells. Next, other higher adducts analogues
were investigated in order to see whether the concept can be extended to other
fullerenes. Furthermore, the charge transport of the various higher adducts was
studied in more detail to explain the peculiar feature that the bisadduct mixture of
isomers can be used to produce high performance solar cell. It is noted that in this
follow-up study the device performance is somewhat lower compared to the one
described above. Furthermore, a larger difference in short circuit current betweenmono and bisadducts is observed. The reason for this behaviour is the different
fabrication technique used here, based on chloroform as solvent and thermal
annealing. This fabrication technique is chosen above spin coating from
orthodichlorobenzene and solvent annealing, due to the much larger spread in
device performance of the latter, making a comparison of single carrier devices and
solar cells difficult. Furthermore, other than accounting for the difference in weight
ratio, all cells were fabricated using identical procedures and no optimisation was
done for the individual materials.
Figure 5.5: Materials used in this chapter. From top left to bottom right, regioregular poly[3-hexylthiophene] (P3HT), [60]PCBM, and highly generalized structures for the isomeric mixtures of
the bisadducts bis[60]PCBM, bis[70]PCBM, bis[60]ThCBM, bis[70]ThCBM, and the trisadducttris[60]PCBM.
S
C6H13
n
O
O
O
O
O
O
O
OS
O
O
S
O
OS
O
O
S
O
O
O
O
O
O
O
O
OO
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Due to its more asymmetric shape the C70 based [70]PCBM has a higher
absorption coefficient compared to [60]PCBM which has shown to be useful forcomplementing the absorption of small bandgap polymers.
13The thienyl based
[6,6]-thienyl-C61-butyric acid methyl ester (ThCBM) has been developed to
provide a better conformity between polymer and fullerene in P3HT based
devices.14
The generalized chemical structures of these materials are shown infigure 5.5. All fullerenes were synthesised according to a procedure reported in our
previous work. Next to the standard p-type polymer P3HT and n-type molecule
[60]PCBM, the bisadduct analogues of [60]PCBM, [70]PCBM, [60]ThCBM,
[70]ThCBM and the trisadduct analogue of [60]PCBM have been investigated. In
order to study the effect of the additional fuctionalisation and the fact that a
mixture of isomers is used, electron transport measurements have been performedon blends of P3HT and fullerenes.
In figure 5.6 the J-V characteristics of electron-only devices (using an AlOx
electrode) of all P3HT:fullerene blends at room temperature are shown. The device
currents of the bisadducts are all lower compared to P3HT-PCBM blends where the
biggest difference occurs for the blend based on [70]ThCBM. For the trisadduct the
electron current is even further decreased by 3 orders of magnitude. A possible
explanation for this drop in device current can be an increase in disorder in thefullerene phase, due to the presence of a multitude of isomers of the fullerenes. In
order to determine the amount of disorder in the materials the temperature
dependence of the zero-field mobility as determined from the various electron only
devices is studied. According to the Gaussian disorder model this temperature
dependence is governed by the width of a Gaussian density of states σ following15
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0.0 0.5 1.0 1.5 2.0 2.5 3.0
1E-4
1E-3
0.01
0.1
1
10
100
1000
10000
100000
[60]PCBM trisPCBM
bis[60]PCBM bis[60]ThCBM
bis[70]PCBM bis[70]ThCBM
J
[ A / m 2 ]
V [V]
Figure 5.6: J-V characteristics of P3HT:methanofullerene blend electron single carrier devices.
eaE
T k T k B B
278.05
3exp
2 / 32
(5.3)
where μ∞ is the mobility as the temperature goes to infinity, a is the intersite
spacing and k B is Bolzmann‟s constant. Figure 5.7 shows the temperature
dependence of the zero-field mobility as determined from the electron onlydevices. According to Equation 5.3 the amount of disorder σ can be calculated
from the slope of the (log) mobility versus 1/T 2. For PCBM a σ of 68 meV is
determined, which agrees with the previously reported value.11
For the bisadducts
the magnitude of the disorder is significantly larger as given in the inset of figure5.7. Note that for tris[60]PCBM σ could not be determined. At low temperatures
the electron current decreased below the leakage current of the devices due to local
shorts, so that the electron mobility could not be measured in this material.
Looking at the room temperature zero-field mobilities, as compared to PCBM,
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0.000010 0.000012 0.000014 0.000016 0.000018 0.000020 0.000022 0.000024
1E-15
1E-14
1E-13
1E-12
1E-11
1E-10
1E-9
1E-8
1E-7
0
[ m 2 / V s ]
1/T2
PCBM 68
bis[60]PCBM 94
bis[70]PCBM 100
bis[60]ThCBM 88
bis[70]ThCBM 129
trisPCBM
[meV]
Figure 5.7: Temperature dependence of the zero field mobility of P3HT:fullerene single carrierdevices. The Gaussian disorder model is used to determine the disorder parameter σ for the variousmethanofullerenes in the blend.
a decrease in mobility of up to 2 orders of magnitude is seen for the bisadducts, and
an even higher decrease for the trisadduct. It is expected that the device
performance of solar cells based on these materials will suffer significantly from
the much lower mobility due to space-charge formation, additional recombinationlosses and a lower dissociation probability of the bound electron-hole pairs.
16,17,18
Using the numerical program, we have performed simulations in order to analyze
the effect such a lowering of the mobility has on the device performance. In figure
5.8 the J-V characteristics of a P3HT:PCBM reference device is shown. Using theelectron mobility for PCBM determined above and typical simulation parameters
as previously reported,19 the J-V characteristics are described adequately. After
accounting for the increase in V oc of the bisadducts and trisadducts the J-V
characteristic is then calculated using the mobilities determined using the electron-
only devices. As expected, due tothe lower electron mobilities the calculated short
circuit current and fill factor decrease dramatically resulting in a predicted drop inefficiency of up to 60% for bis[70]ThCBM.
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-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
-120
-100
-80
-60
-40
-20
0
20
Data
PCBM
bis[60]ThCBM
bis[60]PCBM
bis[70]PCBM
bis[70]ThCBM
trsiPCBM
J L [ A / m 2 ]
V [V]
Figure 5.8: Predictions of solar cell characteristics for all fullerenes. First the standard P3HT:PCBM J-V curve (symbols) is fitted using the numerical program (solid line). Next the J-V curve for theother fullerenes is calculated taking into account the lower mobility as determined in figure 3 and theincrease in Voc of the bis and tris adducts.
5.3.1 Solar cells based on higher adduct fullerenes
Next, bulk heterojunction solar cells were fabricated using the methanofullerenesintroduced above. Figure 5.9 shows the J-V characteristics of all solar cells at room
temperature under simulated AM1.5 illumination. As discussed in the first part of
this chapter, the raised LUMO level of the bisadducts result in a significantenhancement of the open circuit voltage of the devices. What is very surprising
however, is that all bisadducts show an almost identical device performance in
contradiction to the predicted performance shown in Fig. 5.8. Apparently, the
lower electron currents as seen in the electron-only devices are not at all reflectedin the performance of the solar cells. For the trisadduct however, despite the even
further enhanced open circuit voltage(which is among the highest reported for a
P3HT based device)20
the power conversion efficiency drops dramatically as
predicted from the deteriorated electron transport. Another remarkable feature, as
can be seen in Figure 5.10, is that a difference in device performance of the solar
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cells between monoadducts and bisadducts reappears when cooling the samples
below room temperature. Furthermore, the solar cells which show a strongertemperature dependence of their performance are those which gave low device
currents in the electron-only devices. These observations strongly suggest that the
transport in the fullerene phase is hampered by a large amount of shallow trapping.
When shallow traps are present the J-V characteristics at low voltages are alsodescribed by a quadratic dependence on voltage, given by
-0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
-100
-80
-60
-40
-20
0
PCBM
bis[60]PCBM
bis[70]PCBM
bis[60]ThCBM
bis[70]ThCBM
trisPCBM
J L
[ A / m 2 ]
V [V]
Figure 5.9: J-V characteristics of P3HT:fullerene solar cells under illumination of a simulated AM1.5irradiation with an equivalent of 1.4kW/m2.
(5.1)
with
(5.2)
and N c the effective density of states, N t the amount of traps and E t the trap-depth. In this case θμ represents an effective mobility, that contains the ratio
of free and trapped charges. The relatively low electron-only currents for a
3
2
int0
8
9
L
V J r
T k
E
N
N
B
t
t
c exp
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number of bisadducts are in that case due to the fact that many electrons are
immobile because they are trapped in shallow traps. For the solar cells,
during illumination a number of trap states will be filled, leading to an
enhanced transport and the device operation then approaches the one of the
trap-free PCBM device. Such an illumination dependent transport has
recently been observed in n-type polymers..21
Further evidence for chargetrapping in solar cells can be obtained from the intensity dependence of the
open circuit voltage of the devices.22
For trap-free polymer:fullerene solarcells, when plotting the Voc versus the natural logarithm of the light
intensity, the slope of the Voc follows S=(k BT/q), where k B is the Bolzmann
constant, T is the temperature and q is the elementary charge. In the case of recombination with trapped charges, however, the intensity dependence of
the V oc is enhanced. Figure 5.11 shows the Voc dependence on light intensityfor our devices. Again, the fullerenes which exhibit lower electron currents
in the electron only devices and a stronger temperature dependence in the
solar cells, show a larger dependence of the Voc versus light intensity.
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300 280 260 240 220 2000
4
8
12
16
20
24
28
32
36
40
300 280 260 240 220 2000.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
300 280 260 240 220 2000
10
20
30
40
50
60
70
80
90
300 280 260 240 220 2000.3
0.4
0.5
0.6
PCBM
bis[60]PCBM
bis[70]PCBM
bis[60]ThCBM
bis[70]ThCBM
M
a x i m u m P o w e r P i n t [ W / m 2 ]
Temperature [K]
PCBM
bis[60]PCBM
bis[70]PCBM
bis[60]ThCBM
bis[70]ThCBM
O p e n C i r c u i t V o l t a g e [ V ]
Temperature [K]
PCBM
bis[60]PCBM
bis[70]PCBM
bis[60]ThCBM
bis[70]ThCBM
PCBM
bis[60]PCBM
bis[70]PCBM
bis[60]ThCBM
bis[70]ThCBM
S h o r t C i r c u i t C u r r e n t [ A / m 2 ]
Temperature [K]
F i l l F a c t o r
Temperature [K]
Figure 5.10: Solar cell parameters; maximum power point (MPP), short circuit current density (J sc),
open circuit voltage (Voc), and fill factor (FF) of P3HT:fullerene solar cells as a function of
temperature.
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7,38906 20,08554 54,59815 148,41316 403,42879 1096,63316 2980,95799
0,45
0,50
0,55
0,60
0,65
0,70
0,75
0,80
0,85
0,90
0,95
PCBM
bis[60]PCBM
bis[70]PCBM
bis[60]ThCBM
bis[70]ThCBM
tris[60]PCBM
1.28
1.30
1.30
1.40
1.55
1.57
V o c
[ V ]
Intensity [W/m2]
slope [kT/q]
Figure 5.11: Open circuit voltage versus the natural logarithm of the intensity of P3HT:fullerene solarcells. The slope of the Voc vs. intensity in units of [kT/q] is given in the legend.
5.3.2 Device simulations using charge trapping
In order to quantitatively describe the electron only devices and solar cells the
effect of charge trapping on the simulations is incorporated. Figure 5.12 shows the J-V characteristics of a P3HT:bis[70]PCBM electron-only device on a double log
scale. When modelling these electron currents, it is assumed that the mobility of
bis[70]PCBM is identical to reference [60]PCBM. We note that field effect
transport studies have shown the mobility of [60]PCBM and [70]PCBM to be equal
within experimental error.23
Next, we introduce shallow traps that are exponentially
distributed in energy. We observe that a relatively narrow distribution in energy, asexpected for (random) disorder, gives better results than only a discrete trap level.
The width of the distribution is governed by a trap temperature T trap=340K.22
For
such an exponential trap distribution the effective number of traps has been shown
to vary with temperature with exp{-[1/2(σ 2
/kT]/kT trap}. Using this temperaturedependence of the effective number of traps (assuming σ to be 68meV asdetermined from the PCBM devices) we can describe the whole temperature range.
For the simulations of the photocurrent of the P3HT:PCBM andP3HT:bis[70]PCBM solar cells in figure 5.12, we again start with our description
of the P3HT:PCBM reference device. Using the same set of parameters, we
subsequently add the enhanced open circuit voltage, and the trap distribution as
determined from the electron-only device of the P3HT:bis[70]PCBM blend. As canbe seen in figure 5.13, the incorporated trap distribution indeed does not lower the
device performance and we can describe the J-V characteristics adequately. As a
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result the occurrence of shallow traps simultaneously explains the reduced electron
currents and the relatively good solar cell performance, due to filling of these trapsunder illumination. The nature of the shallow trapping is likely to be one or more
specific bisadduct isomers with lower lying LUMO‟s. Although we expect that
certain single isomers of bisadducts can show improved performance, the fact that
the mixture of isomers can be used as such, and that it still results in a properdevice operation is a great benefit for commercialisation of polymer:fullerene solar
cells.
0.1 1
0.01
0.1
1
10
100
1000
T [K]
295
270
250
230
210
J [ A / m 2 ]
V-Vbi-V
rs
Figure 5.12: J-V characteristics, corrected for built-in voltage and series resistance of a
P3HT:bis[70]PCBM electron single carrier at different temperatures. The J-V curves are fitted usingPCBM mobilities and an exponential trap distribution.
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-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
-120
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-80
-60
-40
-20
0
20
PCBM Databis[70]PCBM Data
PCBM Fit
bis[70]PCBM Fit
J L
[ A / m 2 ]
V [V]
Figure 5.13: J-V characteristics of a P3HT:PCBM and P3HT:bis[70]PCBM solar cell. The J-V curve
(symbols) are fitted with our numerical program where for the P3HT:bis[70]PCBM cell anexponential trap distribution is introduced.
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5.4 Conclusions
A novel type of fullerene, bisPCBM, with a higher LUMO level compared to thatof PCBM, is used in order to minimize the energy loss in the electron transfer from
the donor to the acceptor material in bulk heterojunction solar cells. The additional
functionalization of the fullerene cage in bisPCBM was shown to have little
negative influence on the charge-carrier properties of the fullerene. As predicted,
the higher LUMO resulted in a significantly enhanced open-circuit voltage when
used in combination with P3HT, while maintaining a high short-circuit current and
fill factor. An externally verified power-conversion efficiency of 4.5% wasreported for a P3HT:bisPCBM solar cell. We showed that the bisadduct isomer
mixture, free of monoadduct and higher adducts, can be used without further
separation of the individual isomers, resulting in an enhanced cell performance
compared to that of PCBM. Furthermore, several other higher adduct fullerenes are
investigated in combination with P3HT. At first sight the higher adduct fullerenes
show signs of an enhanced disorder, reflected by a reduced current in electron-only
devices. Such an enhanced disorder however does not comply with the temperature
and intensity dependence of the solar cells. Instead, a substantial amount of shallow
trapping is likely to be the cause of the reduced currents in the electron-only
devices. Under illumination these trap states are filled and normal solar cell
operation is observed. An exponential trap distribution has been shown to
adequately describe both electron only and solar cell. The nature of the shallowtrapping is likely to be specific bisadduct isomers with lower lying LUMO‟s. Thefact that the mixture of isomers can be used as such, and still results in a proper
device operation is a great benefit for commercialisation of polymer:fullerene solar
cells. The trisadduct analogue of PCBM however, despite leading to a high open
circuit voltage of 813 mV, results in a significantly reduced device performancedue to a deterioration of the charge transport in the fullerene.
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[1] M. D. Irwin, D. B. Buchholz, A. W. Hains, R. P. H. Chang, T. J. Marks, PNAS,2008, 8, 2783.
[2] C. J. Brabec, J.D Durrant MRS Bul. 2008, 33, 607.
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[5] D. J. D. Moet, L. J. A. Koster, B. de Boer, P. W. M. Blom, Chemistry of Materials,2007, 19, 5856.
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[17] V. D. Mihailetchi, L. J. Koster, J. C. Hummelen, P. W. Blom, Phys. Rev. Lett .2004, 93, 216 601.
[18] M. Lenes, L. J. A. Koster, V. D. Mihailetchi, P. W. M. Blom, Appl. Phys. Lett .2006, 88, 243502.
[19] V. D. Mihailetchi, H. Xie, B. de Boer, L. J. A. Koster, P. W. M. Blom, Adv. Funct.
Mater 2006, 16 , 699.
[20] R. B. Ross, C. M. Cardona, D. M. Guldi, S. G. Sankaranarayanan, M. O. Reese, N.
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PUBLICATIONS
M. Lenes, S. W. Shelton, A. B. Sieval, D. F. Kronholm, J. C. Hummelen and P.
W. M. Blom ”Electron trapping in higher adduct fullerene- based solar cells” Adv. Funct. Mat. Published online (DOI: 10.1002/adfm.200900459)
D. J. D. Moet, M. Lenes, J. D. Kotlarski, S. C. Veenstra, J. Sweelssen, M. M.
Koetse, B. de Boer, P. W. M. Blom “Impact of molecular weight on charge
carrier dissociation in solar cells from a polyfluorene derivative” Org. Electronics, 2009, 10, 1275
M. Lenes, M. Morana, C. J. Brabec, P. W. M. Blom “Recombination-limited
photocurrents in low bandgap polymer:fullerene solar cells” Adv. Funct. Mat.
2009, 19, 1106
M. Lenes, F. B. Kooistra, J. C. Hummelen, I. van Severen, L. Lutsen, D.
Vanderzande, T. J. Cleij, P. W. M. Blom “Char ge dissociation inpolymer:fullerene bulk heterojunction solar cells with enhanced permittivity” J.
Appl. Phys. 2009, 104, 114517.
R. Kroon, M. Lenes, J. C. Hummelen, P. W. M. Blom, B. de Boer “Small bandgap polymers for organic solar cells” Polymer Reviews, 2008, 48, 531
M. Lenes, G. A. H. Wetzelaer, F. B. Kooistra, S. C. Veenstra, J. C. Hummelen,
P. W. M. Blom “Fullerene Bisadducts for Enhanced Open-Circuit Voltages and
Efficiencies in Polymer Solar Cells” Adv. Mat. 2008, 20, 2116
J. D. Kotlarski, P. W. M. Blom, L. J. A. Koster, M. Lenes, L. H. Slooff
“Combined optical and electrical modelling of polymer : fullerene bulk heterojunction solar cells” J. Appl. Phys. 2008, 103, 084502.
M. Lenes, L. J. A. Koster, V. D. Mihailetchi, P. W. M. Blom “Thicknessdependence of the efficiency of polymer : fullerene bulk heterojunction solar
cells” Appl. Phys. Lett. 2006, 88, 243502.
S. Steudel, S. De Vusser, K. Myny, M. Lenes, J. Genoe, P. Heremans
“Comparison of organic diode structures regarding high-frequency rectificationbehavior in radio-frequency identification tags” J. Appl. Phys. 2006, 99, 114519.
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L. J. A. Koster, V. D. Mihailetchi, M. Lenes, P. W. M. Blom “PerformanceImprovement of Polymer: Fullerene Solar Cells Due to Balanced Charge
Transport”, Organic Photovoltaics. Materials, Device Physics, and
Manufacturing Technologies. Edited by C.J.Brabec, U. Scherf, and V.
Dyakonov, WILEY-VCH, Weinheim, ISBN: 978-3-527-31675-5.
M. Lenes, V. D. Mihailetchi, L. J. A. Koster, and P. W. M. Blom, “Space-
charge formation in thick MDMO-PPV:PCBM solar cells”, Proceedings of SPIE
6192, 120 (2006).
D. Jarzab; F. Cordella, M. Lenes, F. B. Kooistra, P. W. M. Blom, J. C.Hummelen, M. A. Loi “Charge Transfer Dynamics in Polymer-Fullerene Blends
for Efficient Solar Cells" Submitted for Publication.
M. Kuik, H. T. Nicolai, M. Lenes, M. Lu, P. W. M. Blom “Introducing trap-
assisted recombination in polymer light emitting diodes” Manuscript in
preparation.
H. Diliën, A. Palmaerts, M. Lenes, B. de Boer, P. W. M. Blom, T. J. Cleij, L.
Lutsen, D. Vanderzande “Soluble poly(thienylene vinylene) derivatives for photovoltaic applications” Manuscript in preparation.
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SUMMARY
The main goal in organic photovoltaics is the development of a large-area, flexible,
and most importantly, a low-cost energy source. The materials used in this thesis,conjugated polymers and fullerene derivatives, can be made soluble enabling low
temperature processing techniques such as spin-coating, doctor blading and ideally
roll-to-roll fabrication (think of solar cells being printed at high speed, similar to
newspapers). On the downside, the inherently disordered nature of the used
materials and processing conditions leads to devices with inferior charge carrier
mobilities compared to their inorganic counterparts such as silicon. This in turn
places the research field of organic electronics in the area of low frequency low
performance devices. The balance between production cost, device lifetime and
efficiency will in the end determine the viability of organic solar cells.
In this thesis that latter part of this balance between cost, lifetime and
efficiency is investigated. In the first introductory chapter the current
understanding of the working principles of polymer fullerenes bulkheterojunction
(BHJ) solar cells is discussed. Using this information, the main loss mechanisms in
this type of devices are identified. It is shown that the intrinsic low mobility of the
polymer and fullerene does in fact not significantly limit the device performance,
as long as they are well balanced. This is reflected by the high (~90%) internal
quantum efficiencies achieved in, for instance, devices made from a polythiophene
(P3HT) and a methanofullerene (PCBM). Nevertheless these devices only achieve
power conversion efficiencies of typically 4%. The question then arises whichprocesses are responsible for the efficiency losses. It is shown that a significant
amount of energy is lost due to misalignment of the energy levels of the used
materials. In polymer solar cells a donor-acceptor (D-A) system is used in order to
separate excitons into free carriers. Unfortunately, during the transfer of the
electron from the lowest unoccupied molecular orbital (LUMO) of the donor to theLUMO of the acceptor, energy is inevitably lost. This loss in energy is manifested
in the low open circuit voltage of a D-A BHJ solar cell compared to the bandgap of
the absorber. Three strategies are suggested in this thesis to reduce this offset,
either resulting in an expected increase in the amount of absorbed light or an
increase of the output voltage of the solar cell.
The second and third chapter of this thesis focuses on poly(p-phenylene
vinylene) (PPV) type polymers, a class of materials heavily used and studied inpolymer electronics. The switch to chlorobenzene as a solvent for spincoating
MDMO-PPV:PCBM layers led to the first reasonable performing device, achieving
an efficiency of 2.5%. Devices based on MDMO-PPV:PCBM blends typically
have an active layer thickness of 100 nm at which still a significant portion of light
is not absorbed. In chapter 2 of this thesis the origin of the use of such a relatively
thin active layer is investigated. It is shown that the decrease in fill factor which,from a device point of view, is the origin for the decreasing efficiency upon
increasing the active layer thickness, is due to a combination of space-charge
effects, a decreasing dissociation efficiency and charge recombination.
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Besides the thin active layers used in PPV solar cells, another loss
mechanism can be identified. The dissociation of a bound electron hole pair at thedonor acceptor interface has been shown to be a significant limiting factor for PPV
based devices. In chapter three a new glycol substituted PPV is investigated, which
has a higher permittivity compared to normal PPV‟s. The aim here is to increasethe above mentioned low dissociation efficiency of PPV based devices, which isstrongly dependent on the average permittivity of the active layer blend. Due to a
significant lower hole mobility of the polymer and morphology problems, devices
based on this new polymer did not show the expected improved power conversion
efficiencies compared to the model MDMO-PPV system. Nevertheless, an increase
in dissociation efficiency from 60 to 72% was observed for the enhanced
permittivity polymer, indicating the importance of the average permittivity inpolymer:fullerene devices.
As mentioned above, optimizing the LUMO level offset between donor and
acceptor is one of the most forward ways of increasing device performance. By
lowering the LUMO of the donor in theory more light can be absorbed resulting in
an increased device current. One of the most promising materials following this
route is PCPDTBT. In chapter 4 the charge transport and photogeneration of this
material blended with PCBM is investigated. Despite an almost balanced transportthe photocurrent shows a square root dependence on effective voltage. It is shown
that this square root dependence does not stem from an unbalance in mobilities as
is seen in chapter 2, but from an enhanced recombination of the bound electron
hole pair. This enhanced recombination is likely due to a too close intermixing of
polymer and fullerene.In chapter 5 another route towards efficiency enhancement is investigated.
Instead of lowering the LUMO of the donor now the LUMO of the acceptor israised allowing for a very direct enhancement of the efficiency due to a larger open
circuit voltage. In order to achieve this raising of the LUMO level, the bisadduct
analog of PCBM was used. The additional functionalisation of the fullerene cage
leads to a saturation of the double bonds, raising the LUMO level of the molecule
significantly. It is shown that, despite the additional functionalisation that increases
the disorder (due to having a multitude of isomers of the molecule), replacing
PCBM with bisPCBM results in only a very slightly decreased photogeneration
and transport properties. Combined with a significantly enhanced open circuit
voltage a power conversion efficiency of 4.5% was achieved, a relative increase of
20% compared to PCBM and among the highest reported for P3HT based
polymer:fullerene solar cells. In the second part of chapter 5, bis- and trisadduct
analogs of other fullerenes are investigated. It is shown that the existence of
multiple isomers leads to shallow trapping for single carriers devices, which do not
affect the device operation of the solar cells itself.
In conclusion, existing device models were used to identify limiting factors
for the power conversion efficiency of polymer solar cells. Several strategies are
employed in order to reduce these limits. In some cases the proposedimprovements worked as expected, only to be countered by unexpected side effects
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(chapter 3). In other cases new physical phenomena were observed (a lifetime
limited photocurrent, chapter 4). A breakthrough in performance is described inchapter 5, where the proposed raising of the LUMO level of the acceptor resulted
in an increased open circuit voltage and no negative side effects and hence a
significant improved efficiency, exactly as predicted.
With the results presented in this thesis, another step is made towards theunderstanding of the device physics of polymer solar cells, and higher efficiencies
are achieved. Maybe it is time to focus on the other side of the balance, to actually
make the devices low-cost and long living.
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Het voornaamste doel van de organische fotovoltaïsche technologie is het
ontwikkelen van een grote schaal, flexibele en vooral goedkope energiebron. De
materialen beschreven in dit proefschrift, geconjugeerde polymeren en fullereen
derivaten, kunnen oplosbaar gemaakt worden en zijn hierdoor geschikt voor lage
temperatuur processen zoals spin coaten, doctor blading en idialiter roll-to-roll
fabricatie (denk hierbij aan zonnecellen die als kranten op hoge snelheid gedrukt
worden). Helaas geldt ook voor deze materialen dat ze inherent wanordelijk zijn,
mede door deze fabricage technieken, wat leidt tot lage ladingsdrager mobiliteiten
vergeleken met hun inorganische tegenhangers zoals silicium. Dit plaatst het
vakgebied van de organische halfgeleiders in een gebied van lage frequentie, lage
performance toepassingen. Uiteindelijk zal de balans tussen kosten, levensduur en
efficiëntie de waarde van de organische zonnecel moeten bepalen.
Deze thesis zal het laatste onderdeel van deze balans, de efficiëntie,behandelen. In het inleidende eerste hoofdstuk wordt de huidige stand van kennis
met betrekking tot de principes van polymere fullerene bulk heterojunctie (BHJ)
zonnecellen behandeld. Met deze informatie worden de voornaamste
verliesprocessen geïdentificeerd. Er wordt aangetoond dat de lage ladingsdrager
mobiliteiten in feite de efficiëntie niet significant beïnvloed, mits ze gebalanceerd
zijn. Dit komt naar voren in de hoge interne efficiëntie gehaald in bijvoorbeeld
zonnecellen gemaakt van een mix van polythiophene (P3HT) en een
methanofullerene (PCBM). Desalniettemin behalen dit soort devices slechts eenefficiëntie van rond de 4%. De vraag is dan welke processen er dan
verantwoordelijk zijn voor het energieverlies. Er wordt getoond dat een significant
deel van de energie verloren gaat door een onvoordelige afstand tussen de
energieniveau‟s van de gebruikte materialen. In polymere zonnecellen wordt
gebruik gemaakt van een zogeheten donor-acceptor systeem om excitonen in vrijeladingsdragers te om te zetten. Helaas gaat er tijdens de overdracht van een
electron van de laagste ongevulde moleculaire orbitaal (LUMO) van de donor naar
die van de acceptor energie verloren. Dit verlies in energie manifesteert zich in de
lage openklemspanning in vergelijking met de bandgap van het absorberendmateriaal. Drie strategiën om dit verlies van energie te verminderen worden
voorgesteld, waarvan ofwel wordt verwacht dat de hoeveelheid geabsorbeerd licht
wordt verhoogd, ofwel een verhoging van het geleverde voltage wordt verhoogd.Het tweede en derde hoofdstuk van deze thesis richten zich op poly(p-
phenylene vinylene) (PPV) type polymeren, een veel gebruikte en bestudeerde
klasse materialen in de polymere electronica. Door gebruik te maken van
chlorobenzeen als oplosmiddel tijdens het spincoaten van MDMO-PPV:PCBMlagen zijn in het verleden de eerste relatief efficiënte devices gemaakt met een
efficiëntie van 2.5%. Zonnecellen gebaseerd op mengsels van deze twee materialen
hebben typisch een actieve laag van 100nm dik, welke nog een significant deel van
het zonlicht doorlaat. In hoofdstuk twee wordt de reden voor het gebruik van een
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dergelijk dunne actieve laag bestudeerd. De daling van de vulfactor, wat vanuit een
device oogpunt de oorzaak van de daling in efficiëntie is, blijkt veroorzaakt dooreen combinatie van ruimtelading effecten, een verminderde dissociatie efficiëntie
en ladings recombinatie.
Buiten de dunne actieve laag in PPV zonnecellen kan nog een ander
verliesmechanisme aangeduid worden. Aangetoond is dat de dissociatie vangebonden electron-gat paren aan het donor acceptor interface een belangrijk
verliespad is in PPV gebaseerde zonnecellen. In hoofdstuk drie wordt een nieuw
glycol gesubstitueerde PPV gebruikt met een hogere diëlectrische constante in
vergelijking met normale PPV‟s. Het doel is nu de bovengenoemde dissociatieefficiëntie te verhogen, welke sterk afhankelijk is van de gemiddelde dielectrische
constante van de actieve laag. Door een significant lagere gatenmobiliteit enmorfologie-problemen gaven de zonncellen gebaseerd op dit polymeer niet de
verwachte winst in efficiëntie. Desalniettemin werd een verhoging van de
dissociatie efficiëntie van 60 naar 72% geobserveerd voor het nieuwe polymeer,
wat het belang aangeeft van de gemiddelde dielectrische constante op de werking
van een polymere zonnecel.
Zoals hierboven beschreven, is het optimaliseren van de LUMO energie
niveau‟s van donor en acceptor één van de meest directe manieren om de deviceefficiëntie te verhogen. Door het LUMO energieniveau te verlagen kan in theorie
meer licht door het polymeer worden geabsorbeerd, wat leidt tot een hogere device
stroom. Eén van de meest veelbelovende materialen in die categorie is PCPDTBT.
In hoofdstuk vier wordt het ladingstransport en photogeneratie van dit materiaal
gemixed met PCBM onderzocht. Ondanks een gebalanceerd transport vertoont dephotostroom een wortelafhankelijkheid ten opzichte van het effectieve voltage. Er
wordt aangetoond dat deze wortelafhankelijkheid niet komt door eenongebalanceerd transport, zoals beschreven in hoofdstuk twee, maar door een
verhoogde recombinatie van het electron gat paar. Deze verhoogde recombinatie
wordt waarschijnlijk veroorzaakt door een te fijne mix van polymeer en fullereen.
In hoofdstuk vijf wordt een andere strategie om de device efficiëntie van
een polymere zonnecel te verhogen onderzocht. In plaats van het verlagen van de
LUMO van de donor wordt nu de LUMO van de acceptor verhoogd, wat tot een
zeer directe verhoging van de efficiëntie leidt door middel van een hogere
openklemspanning. Het verhogen van de LUMO van het fullereen wordt
bewerkstelligd door gebruik te maken van de bisadduct analoog van PCBM. De
additionele functionalisering van het fullereen leidt tot een saturatie van het aantal
dubbele bindingen wat het LUMO energieniveau van het molecuul aanzienlijk
verhoogt. Er wordt aangetoond dat, ondanks de extra functionalisatie, welke de
wanorde in het systeem verhoogd (doordat er nu meerdere isomeren van het
molecuul bestaan) het vervangen van PCBM met bisPCBM slechts in zeer kleine
mate de kortsluitstroom en transport van de zonnecel vermindert. Gecombineerd
met een significant verhoogde openklemspanning werd een efficiëntie van 4,5%
behaald, relatief 20% hoger in vergelijking met PCBM, en één van de hoogstgerapporteerde efficiënties voor een P3HT gebaseerde zonnecel. In het tweede deel
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van hoofdtuk vijf worden bis,- en trisadduct analogen van andere fullerenen
onderzocht. Er wordt aangetoond dat het bestaan van meerdere isomeren leidt totondiepe vangstcentra voor het electronentransport die echter de zonnecel niet
beperken tijdens zijn werking.
Tot besluit; bestaande device modellen zijn gebruikt om limiterende
factoren voor de efficiëntie van polymere zonncellen de identificeren.Verschillende strategien worden behandeld om deze limiten op te heffen. In
sommige gevallen werkten de voorgestelde veranderingen als verwacht echter
werden verbeteringen gecompenseerd door onverwachte bijverschijnselen van de
veranderingen (hoofdstuk drie). In andere gevallen werden nieuwe fysische
fenomenen geobserveerd zoals een de levensduur begrensde fotostroom in
hoofdstuk vier. Een doorbraak in performance is beschreven in hoofdstuk vijf,waar de voorgestelde verhoging van het LUMO energie niveau van de acceptor
resulteerde in een verhoogde openklemspanning zonder daarbij in te boeten op
andere vlakken. De behaalde efficiëntie steeg daardoor significant, precies als
voorspeld.
Met de resultaten in deze thesis is een volgende stap gemaakt richting het
begrijpen van de device fysica van polymere zonnecellen en werden hogere
efficiënties behaald. Misschien wordt het wel tijd om naar de andere kant van depolymere zonnecel te kijken, het daadwerkelijk goedkoop maken en de levensduur
van de devices.
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ACKNOWLEDGEMENTS
The work described in this thesis, and the actual appearing of this thesis, would not
have been possible without the aid and support of many people. First and foremost,
I would like to thank Paul Blom for his excellent guidance over the last 4 years.
You have shown me how science should be done, always having a clear vision of
the bigger picture, starting from the first experiments right up to the presenting of
results in journals and on conferences. Groningen is going to have a hard time
finding a proper replacement.
The work in this thesis is a direct continuation of the excellent work done byValentin Mihailetchi and Jan Anton Koster. Their results, guidance in
experimenting and modeling gave the beginning of my Ph D a jumpstart like noother and I will be the last to forget how lucky I was following up their projects.
The name of Kees Hummelen has been a continuous appearance during mytime in Groningen. As a supplier of state of the art materials, co-author, a member
of the reading committee, and company during conferences en meetings. Yet I will
mostly remember you as an example of an extremely passionate scientist and your
role as ambassador for our funny plastic solar cells. Besides Kees I would like to
thank all the other bucky‟s; Floris Kooistra, Alex Sieval and David Kronholm, who
made sure I never had any shortage of acceptor materials.Besides ample acceptor materials, I also did not lack donor materials. Without
the low bandgap polymer supplied by Konarka I would not have been able to attain
the results in chapter 4. I thank Christoph Brabec and Mauro Morana, not only for
providing the polymer, but also for the thorough discussions about the results and
comments on the AFM paper. Dirk Vanderzande and Thomas Cleij I thank for their
collaboration on the PEO-PPV project as well as the other experiments through the
exchange with Arne. Arne, we had a great time in Groningen. It was sometimeshard to keep up with your desires to grab a pintje and your eierballen have reached
a legendary status. Yet it was also very nice working so close with a chemist. I
hope company life will not wear you down too much.
The work presented here was part of project#524 of the Dutch PolymerInstitute. I thank John van Haare as project leader, Sjoerd Veenstra and Jan Kroon
from ECN, and all other people from within the cluster for their cooperation.
Where Paul can be seen as the front engine of the train called MEPOS, Minte
Mulder and Jan Harkema are the engines in the back making sure the whole bunchstays together. Any you manage to run a cleanroom in the meantime. Great job!
Renate, the same thing holds for you. Thank you for getting all the paperwork for
the thesis defense in order.
Paul Heremans and Laurens Siebbeles are acknowledged for their effort and
time to read my thesis and making corrections and suggestion. Paul Heremans I
also would like to thank for introducing me to the field of organic electronics
during my industrial internship some years ago.
I was lucky to be assigned to two valuable students. The record breaking solar
cells in this thesis were made by Gertjan Wetzelaer, and I have to constantly
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ACKNOWLEDGMENTS
remind myself he was only doing a “korte stage” at that time. I enjoyed your visitto Valencia and our Spanish lessons with “Helga”. Steve, you crack me up. I havenever heard of a person ending up in the wrong place, train, plane so often and still
remain absolutely calm. I hope you had a good time in Groningen, we definitely
were glad to have you over. Also from a scientific point of view, your experiments
resulted in some nice research.Herman and Kriszty, as roomies it was only natural for you to be my
paranimfen, you have been close to me all the time during my Ph D, we might as
well finish it that way. Kuik, you have come to be such a close friend to me during
these years that I asked you to be my best man last year. I think that pretty much
says enough. Hylke, without your and Herman‟s work the last 48 hours, I couldn‟t
even think of finishing the thesis in time. I hope things work out for you in theStates, anyways we will be working together in our own company in a few years.
Did you think of a great idea yet?
It has been said many times by Hylke: “MEPOS is the best group”. He wasright. Besides the people already mentioned here I thank Afshin, Auke, Johan,
Rene, Dorota, Fatemeh, Milo, Yuan, Irina, Francesco, Magda, Edsger, Date, Mark
Jan, Eek, Alex, Paul, Ilias, Jia, Andre, Fabrizio, Ronald, Dennis, Christina, Teunis,
Maria Antonietta for the great time in Groningen.Caroline, by correcting my Dutch summary you have officially helped
finishing this thesis. If I were to write a thesis covering the rest of my life you
would be the only co-author.
There is still one person missing in these acknowledgements. Even though not
officially involved in my project, Bert de Boer has played an important role duringmy Ph. D. He was always willing to discuss the more political issues of science;
career choices, how to read between the lines of a referee comment, who to put onyour paper as co-author, how to collaborate with partners, etc. and was a huge
driving force in the group. His passing away earlier this year is an immense loss,
both inside and outside the scientific community.
Martijn Lenes