ICRESD07_14

Revue des Energies Renouvelables ICRESD-07 Tlemcen (2007) 83 90

83

Modelling and Simulation of organic bulk heterojunction solar cells

A. Cheknane1*, T. Aernouts2 et M. Merad Boudia3

1 Laboratoire dEtude et Dveloppement des Matriaux Semi-conducteurs et Dilectriques,

Universit Amar Telidji, Laghouat, Algrie 2

Polymer and Molecular Electronics, Interuniversity Micro-Electronics, Centre IMEC, Kapeldreef 75, B-3001 Leuven, Belgique

3 Unit de Recherche Matriaux & Energies Renouvelables,

Universit Abou Bakr Belkad, B.P. 119, Tlemcen, Algrie

Abstract - The characteristics of organic bulk heterojunction solar cells have been investigated and modelled. A composite of poly(2-methoxy-5-(39,79-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) and (6,6)-phenyl-C61- butyric acid methyl ester (PCBM) is used as photo-active material, sandwiched between a transparent Indium Tin Oxide (ITO)-electrode and an Al backside contact. This study aims to show the electrical effect of an extra interfacial layer of poly (3, 4-ethylenedioxythiophene)/(poly(styrenesulfonate)( PEDOT/PSS) on top of the ITO-electrode. Current-voltage characteristics of devices with and without this additional interfacial layer are compared and modeled by an equivalent electrical diagram. The simulation results clearly demonstrated that the current-voltage characteristics of the bulk heterojunction solar cell are affected by the presence of the PEDOT/PSS layer. Therefore, the solar cell is described, with sound accuracy, by single or two diode-equivalent models indicating the effects of the interfacial layer and of the thickness of the active layer on the photovoltaic performance of the studied devices. The theoretical results are in good agreement with the experimental observations. Keywords: Organic solar cell - Bulk heterojunction - Current-voltage characteristic - Modelling - Simulation.

1. INTRODUCTION

Organic solar cells based on conjugated polymers are promising candidates for a cheap and flexible alternative to inorganic devices. The most recent and efficient polymer solar cells fabricated today are based on the concept of bulk heterojunction [1]. The discovery of ultrafast, photo-induced charge transfer from a conjugated polymer to Buckminsterfullerene (C60) [1-2], strongly determines the choice for an MDMOPPV/PCBM blend as the active layer in such a photovoltaic cell [2]. So, much effort has been directed to the achievement of an ideal bulk heterojunction; for instance, improved charge collection is achieved because of the existence of a bi-continuous network along which charges can travel trough the organic layer towards their respective contacts [2]. It is the object of this work to study the electrical effect of an extra interfacial layer of poly (3, 4-ethylenedioxythiophene)/(poly(styrenesulfonate) (PEDOT/PSS) on top of the ITO-electrode. Furthermore, current-voltage (I-V) characteristics of devices with and without this additional interfacial layer are compared and modelled by an equivalent electrical diagram. I-V curves are measured under simulated solar illumination (AM1.5-spectrum, 100 mW/cm2). A numerical procedure is described to calculate the parameters of the equivalent circuit of a solar cell defining the difference between experimental characteristics and the theoretical model.

2. EXPERIMENTAL SETUP Typical polymer-based devices, such as photovoltaic cells or polymer light emitting diodes,

are fabricated in a sandwich structure consisting of a conjugated polymer film spin coated onto one electrode with a second electrode evaporated on top. In the experiments described here, for * [email protected]

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the standard devices, (Fig. 1) blends of MDMO-PPV: PCBM (1:4wyw) were spin coated from a chlorobenzenesolution w7x onto ITO coated glass. Afterwards, an Al-electrode was evaporated on top through a shadow mask, resulting in cells with an active area of 0.02 cm2. Keeping the devices in an inert nitrogen atmosphere, JV-characteristics were obtained with an HP4145B semiconductor parameter analyser. Under these circumstances illumination could only be provided by a halogen lamp. Some devices were taken out of the nitrogen box to measure them under simulated solar illumination (AM1.5-spectrum, 100 mW/cm2). Devices with an extra 140-nm thick PEDOT/PSS layer, spin-coated on top of ITO before depositing the active layer, were also fabricated. Thickness variations of the active layer were established by applying different spin speeds during the deposition of the organic blend for different cells. Measuring the thickness of the active layer was done as a final step using a Dektak profilometer.

Fig. 1: Bulk donor-acceptor heterojunction solar cell, also with

the chemical structure of the different organic materials used, [2]

3. THEORY A major difference between inorganic and organic semiconductors is the exciton binding

energy [3]. It is smaller for Si ( meV25 , 2.41kJ/mol) than for organic counterparts ( meV300= , 28.95 kJ/mol). Therefore, the exciton is more stable in organics, than in inorganics. Organic solar cell efficiency, , is typically influenced by the following parameters [4]:

a) photon absorption A , b) generation of excitons, c) exciton diffusion diff , d) Hole-electron separation (exciton dissociation) TC , e) Carrier transport towards the electrodes tr , f) Charge collection at respective electrodes.

4. MODELING AND SIMULATION In the literature one can find different equivalent circuits used in modelling an organic solar

cell. Through this section, we review the most important models.

4.1 Three-diode model

B. Mazhari [5] proposed a solar cell model for organic solar cells, as shown in figure 2. intshR

represents loss due to polaron recombination and intsR models charge extraction to the electrodes. Diodes 1D and 2D are ideal diodes that represent short circuit under forward bias and open circuit under reverse bias, respectively.

4.2 Single-diode model Under illumination, the cell may be represented by an equivalent circuit, based on a single-

diode model, as shown in Figure 3. It is described as a current source in parallel with the junction as shown in equation (1):

ICRESD2007: Modelling and simulation of organic bulk heterojunction solar cells

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Fig. 2: A simplified equivalent circuit two diode model for

organic solar cells in which losses are modelled in terms of resistances

Fig. 3: Equivalent scheme of a real PV cell under

illumination, based on a single-diode model

( )

=+

+ 1RIV

TkneexpII

RV

RR1I ssL

shsh

s (1)

Where LI originates from the charge generated by illumination; sI is the saturation current under reverse bias; LR is the external circuit resistance; sR and shR are respectively series resistance and shunt resistance; n n is the ideality factor.

Series resistance of a given PV solar cell depends on resistivities of the semiconductor bulk, the metal electrodes, and the metal/semiconductor interface. Similarly, in an organic solar cell, the series resistance depends on the resistivities of the organic material(s), the metal electrodes and the metal/organic interface [6]. The series and shunt resistances are linked as shown in equation (2):

sh

sscLsc R

RIII = (2)

In order to modelize an organic solar cell, Jain and Kapoor [7] presented a new single-diode based approach using Lambert W-function. The model is shown in Figure 4.

Fig. 4: Equivalent circuit for an organic solar cell (Lambert W-function approach)

4.3 Two-diode model The new simplified equivalent circuit, of a two diode model solar cell, may be described by

the lumped parameter equivalent circuit of Figure 5 [8-9].

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Fig. 5: Equivalent circuit of a two-diode model solar cell

For a given incident light intensity, at a given temperature, the implicit VI relationship is shown in equation (3):

( ){ }[ ] ( ){ }[ ]1R.IVBexpI1R.IVBexpIR

R.IVII s202s101sh

sph +++= (3)

Where TkqB1 = and nBB 12 = phI is the photocurrent; q is the electronic charge; k is Boltzmanns constant, T is the

temperature; sR is the series resistance; shR is the shunt resistance and n is the diode ideality factor; 01I represents the electronic conduction phenomena in the quasi neutral region of the junction, such as diffusion, recombination and the drift effect [8]; 02I corresponds to the carrier recombination via deep levels in the space-charge region of the junction [10]. The reverse saturation current 02I is generally 3 to 7 orders of magnitude larger than 01I ; the diode quality factor n equals 2 for the approximation corresponding to the Shockley-Read-Hall recombination current in the space-charge region [8, 11]; n is also a fit parameter that is more than 2.

The experimental study of the temperature dependence of the two terms should help to verify their different origins. Such studies have been published under dark conditions [8, 12].

It is of great interest to determine the temperature dependence of parameters 01I and 02I while the cell is operating as a generator under illumination. It should be noted that: the illumination does not cause a simple translation of current-voltage characteristic along the current axis [8, 13], the series resistance effect is non linear and the injection level goes from a low to a high level.

Differentiation of equation (3) while considering of the values:

scII0sh ld

VdR=

= (4)

And

C0VV0s ld

VdR=

= (5)

Then

( ) ( )ssc2022ssc1011s0sh

shR.I.Bexp.I.BR.I.Bexp.I.B

RR1

1R

= (6)

And

( ) ( )oc2022oc1011sh

0ssV.Bexp.I.BV.Bexp.I.B

R1

1RR++

= (7)

0sR and 0shR are the experimental values of the dynamic resistance which express the behaviour of the VI curve around the open-circuit voltage and around the short-circuit points.

For ocVV = and scII = , we deduce from equation (3) that:


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( )[ ] ( )[ ]1V.Bexp.I1V.Bexp.IRVI oc202oc101

sh

ocph ++= (8)

( ) ( ) ( )[ ]( ) ( )oc2ssc2

oc1ssc101sh

scshsoc

02 V.BexpR.I.Bexp

V.BexpR.I.Bexp.IR

I.RRVI

+= (9)

The value of the saturation current 01I is found to be the root of the equation ( ) 0IG 01 = , with:

( ) ( ) mp01 I.P.MIIG = (10) Where Imp is the experimental value and ( )P.MI is the value calculated from equation (3)

for maximum power. The basic flow of the program needed to resolve the double exponential equation is outlined in Figure 6.

Fig. 6: Flow chart of the program for the resolution, of the double exponential equation (n = 2)

5. RESULTS AND DISCUSSION

The simulated VJ plots, together with experimental counterparts, are represented in Figures 7-14 with or without an extra interfacial layer of poly (3, 4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) on top of the ITO-electrode. The current-voltage characteristics of devices with and without this additional interfacial layer are compared and modelled by an equivalent electrical diagram. The plotted curves show that the single-diode model provides a very precise description of the VJ characteristics of the device with additional interfacial layer with different active layer thicknesses: 140 nm, 190 nm, 210 nm and 420 nm.

However, the characteristics VJ curves, for standard cell without an interfacial PEDOT/PSS layer with active layer thicknesses: 210 nm, 220 nm, 310 nm, and 370 nm, show clearly that the tested solar cells can be described with the two diode equivalent model.

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Fig. 7: Characteristics JV- curves for standard cell without an interfacial PEDOT/PSS layer.

The active layer thickness = 370 nm





Fig. 10: Characteristics JV- curves for standard cell without an interfacial PEDOT/PSS layer. The active layer

thickness=210 nm

Fig. 11: Characteristics JV- curves for

standard cell with an interfacial PEDOT/PSS layer. The active layer thickness = 420 nm

Fig. 12: Characteristics JV- curves for standard cell with an interfacial PEDOT/ PSS layer. The active layer thickness = 210 nm


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Fig. 13: Characteristics JV- curves for

standard cell with an interfacial PEDOT/PSS layer. The active layer thickness = 190 nm

Fig. 14: Characteristics JV- curves for standard cell with an interfacial PEDOT/PSS layer. The active layer thickness = 140 nm

6. CONCLUSION In summary, the simulation illustrates clearly that the extra interfacial layer of poly

(3,4ethylenedioxythiophene)/(poly(styrenesulfonate) (PEDOT/PSS) on top of the ITO-electrode of organic bulk heterojunction solar cells affects strongly the electrical characteristics of the device. We have shown that the studied device can be described by single or two equivalent models demonstrating the effects of the interfacial layer and of the thickness of the active layer on the photovoltaic performance. A methodology for extracting electrical parameters, for both models, has been outlined and illustrated trough simulation results.

From the results reported here, it is clear that the single-diode model provides a very precise description of the J-V characteristics of the device with additional interfacial layer; whereas the estimated parameters proved that the two-diode equivalent model described consistently the tested solar cells without additional interfacial layer.

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[2] T. Aernouts, W. Geens, J. Poortmans, P. Heremans, S. Borghs and R. Mertens, Extraction of Bulk and Contact Components of the Series Resistances in Organic Bulk Donor-Acceptor Heterojunctions, Thin Solid Films Vol. 403/404, pp. 297 - 301, 2002.

[3] M. Burgelman and B. Minnaert, Including Excitons in Semiconductor Solar Cell Modelling, Thin Solid Films, Vol. 511/512, pp. 214 218, 2006.

[4] A. Moliton and J.M. Nunzi, How to Model the Behaviour of Organic Photovoltaic Cells, Polym Int., Vol. 55, pp. 583 600, 2006.

[5] B. Mazhari, An Improved Solar Cell Circuit Organic Solar Cells, Solar Energy Materials & Solar Cells, Vol. 90, pp. 1021 1033, 2006.

[6] J.M. Nunzi, Organic Materials and Devices for Photovoltaic Applications, ERT Cellules Solaires Photovoltaques Plastiques, Labo POMA, Angers, France, pp. 197 224, 2002.

[7] A. Jain and A. Kapoor, A New Approach to Study Organic Solar Cell Using Lambert W-Function, Solar Energy Materials & Solar Cells, Vol. 86, pp. 197 205, 2005.

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[8] J.P. Charles, Caractrisation I-V et Fonctionnement des Photopiles Solaires, Thse de Doctorat s Sciences, Universit des Sciences et Techniques du Languedoc, Juillet 1984.

[9] G.L. Araujo, E. Sanchez and M. Marti, Determination of the Two-Exponential Solar Cell Equation Parameters from Empirical Data, Solar Cells, Vol. 5, pp. 199 204, 1982.

[10] C.J. Brabec, J.A. Hauch, P. Schilinsky and C. Waldauf, Theme Article Production Aspects of Organic Photovoltaics and Their Impact on the Commercialization of Devices, Mrs Bull., Vol. 30, p. 50, 2005.

[11] J.P. Charles, A. Haddi, A. Maouad, H. Bakhtiar, A. Zerga, A. Hoffmann and P. Mialhe, La Jonction, du Solaire la Microlectronique, Rev. Energ. Ren., Vol. 3, pp. 1 16, 2000.

[12] M. Wolf, G.T. Noel and R.J. Stirn, IEEE Trans. on Electron Dev., ED-24, Vol. 4, pp. 419 428, 1977.

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