CHAPTER IV EXPLORING POLYMER:INORGANIC NANOPARTICLE ... · organic layer by hot-wire chemical vapor...

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CCHHAAPPTTEERR IIVV

71

Summary

In this chapter, results of study, regarding polymer:inorganic

nanoparticle blend morphology control by employing mixed solvent

has been discussed. It is explained why a specific mixed solvent

composition leads to appropriate film formation of composite

(Polymer:Inorganic nanoparticle) during spin coating. As a typical

case, P3HT:TiO2 blend film formations have been discussed by taking

chloroform as good solvent for P3HT while, ethanol, methanol and 2-

propanol as solvents to disperse TiO2 nanoparticles. Blend solution

and film morphology have been characterized by UV-visible

absorption spectroscopy, optical microscopy, scanning electron

microscopy, atomic force microscopy, and time resolved PL

spectroscopy. The reasons for film formation with varying degree of

phase separation have been explained by simulating the solvent

evaporation dynamics during film drying process. It has been shown

that, for the formation of finely intermixed blend films, good solvents

used for both the solute components must maintain a constant ratio

throughout the drying process. If the content of any one solvent

decreases in the course of film solidification larger aggregates of

individual phases are obtained. To further control the blend

morphology, controlling the cooling rate after thermal annealing has

been employed. If the content of any one solvent, in the mixed

solvent is less, finite solubility of the solute puts a limit on the film

thickness. To increase the film thickness, multiple layers have been

deposited and the charge transfer and morphology change has been

studied. Device results by using P3HT:TiO2 blends as the photoactive

layer have also been discussed. Synthesis and characterization of

nanocrystalline TiO2 powder by combustion method have been

discussed in the end.

Chapter-IV: Exploring Polymer:Inorganic ---- 72

IIVV--11:: IINNTTRROODDUUCCTTIIOONN

Polymer-inorganic nanoparticle composites have wide range of applications in magnetic,

electrical and photonic systems [1-5]. Such composites provide attractive means to

combine the merits of organic and inorganic materials that a single-phase material

cannot provide. Control of the blend morphology at the nanoscale/microscale is critical

for optimizing the properties of the composites [6-8].

IIVV--22:: LLIITTEERRAATTUURREE RREEIIVVIIEEWW

IV-2.1: REVIEW OF POLYMER:INORGANIC NANOPARTICLE BLEND MORPHOLOGY

CONTROL

There are numerous studies which are devoted to the morphology control of polymer-

inorganic nanoparticle composites for various applications. Janssen et al. prepared

MDMO-PPV:TiO2 bulk heterojunction by using a dry tetrahydrofuran solution containing

MDMO-PPV and titanium iso-propoxide (a precursor for TiO2) and spin coated on a

substrate to obtain a mixed film with a thickness of about 50-70 nm [9]. Subsequent

conversion of the precursor in dark, via hydrolysis in air results in the formation of a TiO2

phase in the polymer film. As a consequence of the presence of TiO2, the photoactive

film becomes resistant to scratching and can no longer be wiped off the substrate.

Furthermore the polymer in the film no longer dissolves in organic solvents like toluene.

However the conversion of Ti(OC3H7)4 was only 65%. Since no thermal annealing step

was used due to the presence of polymer, the obtained TiO2 was amorphous. This limited

the efficiency of the best device with Jsc = 0.6 mA/cm2, Voc = 520 mV and FF = 0.42 for 20

% TiO2 in the bulk heterojunction under AM1.5 condition. Janssen et al. also used a blend

of P3HT and ZnO nanoparticle as the photoactive layer and achieved 1.6 % PCE [10]. The

results indicated that nanoscopic mixing of ZnO and P3HT occurs, but does not occur

throughout the entire film and, consequently, the coarse mixing and higher roughness of

the films limited higher efficiencies. Jenny Nelson et al. [11] prepared bulk heterojunction

from Ligand capped TiO2 nanorods and P3HT. The nanorods of length ∼ 20 nm and

diameter ∼ 7 nm capped with TOPO and Z907 was used, but the efficiency was limited by

the lower charge transfer due to the presence of capping agent. Low current densities

0.10 mA/cm2 and 0.17 mA/cm2 were obtained with overall power conversion efficiencies

of 0.04 % and 0.07 % respectively with TOPO capped and Z907 capped nanoparticles


respectively. Garry Rumbles et al. [12] have obtained 3.2 % efficiency using low band

polymer PCPDTBT and ligand capped CdSe nanoparticle blend as the photoactive layer.

Wei-Fang Su [13] improved the charge mobility in TiO2 nanorods by ripening and by

Boron doping. Improved charge mobility in these nanorods resulted in 1.31 times and

1.79 times efficiency improvement. This work emphasizes the importance of

nanoparticles towards achieving higher power conversion efficiencies. Williams et al.

investigated the organic-inorganic hybrid solar cell with a p-i-n stack structure [14]. The p-

layer was a spin coated film of PEDOT:PSS [poly (3, 4-ethylenedioxythiophene) poly

(styrenesulfonate)]. The i-layer was hydrogenated amorphous silicon (a-Si:H), and the n-

layer was microcrystalline silicon (c-Si). The inorganic layers were deposited on top of the

organic layer by hot-wire chemical vapor deposition technique at 2000

There are reports where mixed solvents have been used to prepare blend films for

various applications [18, 19]. However, reports regarding the effect of mixed solvent

component properties and their initial composition for spin coating are not available.

C. These hybrid

devices exhibited power conversion efficiencies as large as 2.1 %. Ren et al. [15]

demonstrated quantum dot-based hybrid solar cells with improved electronic interaction

between donor and acceptor components, resulting in significant improvement in short-

circuit current and open-circuit voltage. The PCE of 4.1 % was achieved. Günes et al. [16]

investigated solution-processed bilayer heterojunction hybrid solar cells, using size-

quantized PbS nanoparticles and poly (3-hexylthiophene) (P3HT). Schmidt et al. [17] have

prepared composite films of multiwall carbon nanotubes (MWCNTs) in a poly (methyl

methacrylate) (PMMA) matrix, without using surfactant for MWCNTs. However,

nanotube aggregation during spin coating limited its homogeneous and uniform

distribution in PMMA.

IV-2.2: REVIEW OF MODELLING STUDY

Li et al. adopted the coarse-grained molecular dynamics, to study the dispersion and

aggregation mechanisms of spherical NPs in polymer melts [20]. There are several reports

regarding the polymer chain swelling induced by dispersed nanoparticles [21-23]. Tanaka

et al. studied the pattern evolution in an unstable binary liquid mixture containing glass

particles [24]. They found that the pattern evolution is dominated by the dynamic

interplay between phase separation and wetting. The mobility of the particles

significantly affects both the coarsening dynamics, and the final morphology. Also the


spatial and shape pinning effects of particles significantly modify the coarsening dynamics

of domains.

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WWOORRKK

One of the difficulties in polymer: nanoparticle/nanorod blend film formation during spin

coating is that, both polymer and nanoparticles may not have a common solvent, where

polymer can be soluble and simultaneously the solvent is capable of dispersing the

nanoparticles effectively. By coating the nanoparticles with particular surfactant of

choice, to enhance charge transfer between polymer and nanoparticles for electrically

conductive composites, it may not be dispersible in the same solvent suitable for

polymer. Use of available surfactants may hinder the charge transfer [18, 21]. One of the

strategies to overcome this difficulty can be to use a solvent combination that is capable

of stabilizing the nanoparticles and dissolving the polymer. Understanding the effect of

mixed solvent properties and their composition on the film morphology is very important

so that newer composites could be prepared by the simple spin coating technique.

Annealing the blend films is another method by which morphology is generally precisely

controlled in blend films. Upon heating the blends, above the glass transition

temperature of P3HT, the P3HT matrix becomes soft and also the blend components gain

higher mobility, and therefore the components can reorganize to thermally stable

morphology at that particular temperature. By rapidly cooling the blend to room

temperature the higher temperature state can freeze, and therefore, controlling the

cooling rate after thermal annealing for the polymer:nanoparticle blends can be another

method for fine morphology control of the blend films. For forming blend films of larger

thickness by spin coating, finite solubility of the components is a limit. When a mixed

solvent is used for blend film formation, the initial ratio of the solvents needed for film

formation depends upon the solute system used. If the solvent content used for a

particular solute is in lesser percentage, the amount of that solute, to be used in the

mixed solvent is limited due to its finite dispersibility/solubility. Therefore, spin coating

the solution multiple times is an option for obtaining thicker films.

In this chapter, we discuss the following:


a) Polymer: inorganic nanoparticle blend morphology control by employing mixed

solvents. As a typical case, P3HT:TiO2 (P-25) have been discussed by taking

chloroform as good solvent for P3HT while, ethanol, methanol and 2-propanol as

solvents to disperse TiO2

b) To investigate the reasons for formation of films with varying phase separation,

solvent evaporation dynamics during film drying has been studied by simulating

the mixed solvent evaporation process. Devices have been prepared by using

P3HT:TiO

nanoparticles.

2

c) Effect of controlling the cooling rate, after thermal annealing, on the morphology

and charge transfer has been studied.

blends as the photoactive layer, and the results have been discussed.

d) Multiple coatings to increase the film thickness have been discussed.

e) Synthesis of nanocrystalline TiO2

powder has been discussed.

IIVV--44:: EEXXPPEERRIIMMEENNTTAALL DDEETTAAIILLSS

IV-4.1: BLEND FILM FORMATION

To study the P3HT aggregation in mixed solvent, 0.015 mg P3HT (purchased from Rieke

metals, average molecular weight-50,000 g/mole) per millilitre of CF was used. TiO2

nanoparticle of highly uniform particle size ∼ 25 nm was purchased from Degussa,

Germany (P-25). In order to spin cast the P3HT: TiO2 blend films (thickness: ∼200 nm)

separate solutions for P3HT and TiO2 were prepared and finally added to achieve a

concentration of 3.5 mg/ml P3HT and equal volume of TiO2. The TiO2

IV-4.2: TIO

solution was

prepared by 5 minute bath sonication followed by 20 minute probe sonication. Films

were coated on corning glass at spin speed of 1500 rotations per minute for 30 seconds.

2

Nanosized Titania was obtained by the combustion of aqueous solutions containing

stoichiometric amounts of TiO(NO

POWDER SYNTHESIS BY SOLUTION COMBUSTION METHOD

3)2 (Titanyl nitrate) and glycine (C2H5NO2) as fuel.

Titanyl nitrate TiO(NO3)2 was produced by the controlled hydrolysis of Ti(i-OPr)4 (Sigma-

Aldrich) under ice cold condition followed by treatment with HNO3

Ti[OCH(CH

in accordance with

the following reactions:

3)2]4 + 3H2O TiO(OH)2 + 4C3H7

TiO(OH)

OH

2 + 2HNO3 TiO(NO3)2 + 2H2O


The valences of the elements C, H, and Ti were taken +4, +1, and +4, respectively whereas

the element oxygen was considered as an oxidizing element with a valency –2. The

valency of nitrogen was considered to be zero [25-28]. The amount of initial reactant

materials viz. titanyl nitrate (TiO(NO3)2) and glycine (C2H5O2N) were taken such that the

total oxidizing and reducing valency of the oxidizer and the fuel served as a numerical

coefficient for the stoichiometric balance so that the equivalence ratio was equal to

unity. This leads to maximum release of energy during the exothermic combustion

reaction. The initial mixture of oxidizer and fuel was kept for stirring at 700C until the

water is evaporated and a viscous solution was formed. The viscous solution was then

kept in a preheated furnace at 3500

9TiO(NO

C. Once the combustion is initiated, red spark appears

due to exothermic reaction from one end of the mixture and propagated steadily to the

other end in a self sustained manner. Combustion reaction of titanyl nitrate and glycine is

characterized by following balanced reaction:

3)2 + 10 C2H5NO2 9TiO2 +14N2 ↑+20CO2 ↑+ 25H2

Synthesized nanocrystalline powder was characterized by x-ray diffraction and

transmission electron microscopy.

O↑

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Figure 1 show the absorption spectra of P3HT in CF solution and by adding different

concentrations of MT in CF. For solution containing less than 15 % MT in CF (v/v) the

absorption spectra overlaps with the P3HT spectra (peak at ∼ 450 nm and no additional

shoulder peaks) [29, 30]. This indicates that P3HT is completely soluble in above mixed

solvent. At 15 % MT content, a shoulder begins to appear at 610 nm, the intensity of

which increases with rise in MT content. At 17.5 % and 20 % MT, shoulder peaks at 550

nm and 510 nm respectively begins to appear. With further increase in MT content,

absorption peak red-shifts, as generally observed for P3HT films. All these are indications

of increase in effective conjugation length and increased inter-chain interaction due to

increased local order in P3HT. Change in spectra became feeble when MT content is

increased beyond 30 %. From the above discussion, it can be summarized that, with 15 %

MT content, the CF: MT solvent begins to turn into poor solvent for P3HT, the extent of

which increases with further MT addition, and above ∼ 30 % MT content, P3HT attains

IV-5.1: EFFECT OF CO-SOLVENT ON THE SOLUBILITY OF P3HT IN CHLOROFORM


almost saturation level of dissolution in the mixed solvent.

Figure 1: Absorption spectra of P3HT in CF and by adding different contents of MT

This behaviour can be explained by using the Hansen Solubility Parameters (HSP) of the

solvents and polymer P3HT [31, 32]. For CF and MT the total Hildebrand value ∂ t,

dispersion force component ∂ d, polar component ∂ p and hydrogen bonding component

∂h of HSP, along with ∂ d, ∂p, ∂h value are shown in Table 1 [31]. For P3HT ∂d, ∂p, ∂h

and

the interaction radius R of P3HT are 19.1, 3.9, 6.4 and 6.4 respectively [33].

Table 1: Hansen solubility parameters of pure solvents

For a given solvent, if the distance from the centre of polymer solubility sphere Ds-p is less

than the polymer interaction radius, it will generally dissolve the polymer. Ds-p

𝐷𝐷𝑠𝑠−𝑝𝑝 = �4�∂𝑑𝑑𝑠𝑠 − ∂𝑑𝑑𝑝𝑝 �2

+ �∂𝑝𝑝𝑠𝑠 − ∂𝑝𝑝𝑝𝑝 �2

+ �∂ℎ𝑠𝑠 − ∂ℎ𝑝𝑝�2�

12� .....IV.1

can be

calculated by using the relation,

Where ∂xs, and ∂ xp

Solvent

, denote the Hansen component parameter for solvent and polymer

respectively. For a mixed solvent the respective parameters can be linearly added

according to the particular volume fraction

Hansen solubility parameters(for pure solvent)

𝛛𝛛 𝛛𝛛t 𝛛𝛛d 𝛛𝛛p Dh s-p

Chloroform 19.0 17.8 3.1 5.7 2.8

Ethanol 26.5 15.8 8.8 19.4 15.4

Methanol 29.6 15.1 12.3 22.3 19.7

2-propanol 23.5 15.8 6.1 16.4 12.2


𝜕𝜕 = ∑ 𝑥𝑥𝑖𝑖𝜕𝜕𝑖𝑖𝑖𝑖 .....IV.2

Where 𝑥𝑥𝑖𝑖 is the volume fraction of ith

𝜕𝜕𝑡𝑡2 = 𝜕𝜕𝑑𝑑2 + 𝜕𝜕𝑝𝑝2 + 𝜕𝜕ℎ2 .....IV.3

solvent. The total Hildebrand parameter for the

mixed solvent is calculated by

Ds-p value for CF and MT are 2.8 and 19.7 respectively, which shows that CF is a good

solvent while MT is a poor solvent for P3HT. For 15 % MT content, where P3HT

aggregation initiates, the total Hildebrand value and Ds-p deviates from pure CF value to

19.7 and 3.9 respectively (Table 2). This trend continues with increasing MT until at 31 %

content the Ds-p

To study the effect of MT on the P3HT solubility in CF in the presence of TiO

value becomes greater than the interaction radius of P3HT where it

attains almost saturation level of dissolution in the mixed solvent. This matches closely

with the experimental result where above 30 % MT the P3HT absorption spectra showed

feeble changes.

2 (P3HT:TiO2

volume ratio 1:1), absorption spectra was again recorded (Figure 2(a)).

Figure 2: (a) Absorption spectra of P3HT in chloroform and by adding MT in the presence of TiO2

, (b) change in solution colour with different percentage of MT in CF

With the addition of TiO2, the onset of P3HT aggregation takes place at 10% MT content

compared to 15% content when TiO2 was not used. This shows that with the addition of

TiO2 quality of the solvent becomes poorer for P3HT [23, 24]. Explanation of this effect is

given later in the manuscript. Inset shows the colour of P3HT: TiO2 solution with different

concentration of MT in CF. Change in solution colour also indicates the P3HT aggregation

level (figure 2(b)). With the use of TiO2 in the mixed solution, the P3HT aggregation

(a) (b)


initiates at 12.5 % and 15 % content of ET and 2P respectively and the ∂ t value is

consistently around 19.4. For preparing the films higher concentration of P3HT and TiO2

Similar experiments were carried out by replacing MT with ET and 2P. Figure 3 show the

UV visible spectra of the solutions containing ET and 2P in place of MT. The results are

summarized in Table 2.

was used. Changes in solution colour indicate that above study is valid for such higher

concentration also.

Figure 3: Absorption spectra of P3HT in CF and by adding different contents of (a) ET, (b)ET in the presence of TiO2, (c) 2Pand, (d) 2P in the presence of TiO

2

It consistently shows that the point where aggregation of P3HT begins the total

Hildebrand parameter is around 19.7, while the point where no more changes are

observed in absorption peak the Ds-p values are greater than or very close to the

interaction radius of P3HT.With the use of TiO2 in the mixed solution, the P3HT

aggregation initiates at 12.5 % and 15 % content of ET and 2P respectively and the ∂ t

value is consistently around 19.4. With ET and 2P in place of MT, the TiO2 aggregation

Figure 3:

(a) (b)

(c) (d)


was observed with 2.5 % and 7.5 % content respectively, while with 5 % and 10 % content

respectively solution appeared homogeneous with no TiO2

aggregates.

Table 2: HSP and Ds-p of mixed solvents (various co-solvents with chloroform) at various co-solvent content, with and without TiO2

is shown

It is now clear that, beyond a particular content of the co-solvent, aggregation of P3HT

starts. Extent of P3HT aggregation increases with increasing poor solvent content, and at

a particular volume ratio it attains almost saturation level of dissolution.

IV-5.2: EFFECT OF CHLOROFORM ON THE DISPERSION OF TIO2

To study the effect of chloroform on TiO

IN CO-SOLVENT

2 dispersion in the mixed solvent, the probe

sonicated TiO2

solution was kept for sedimentation. Figure 4 show the photographs

taken after 10 minutes.

Figure 4: P-25 dispersed in chloroform with different percentage of methanol (MT)

With the addition of 2.5 % or less MT, sedimentation was observed, and the TiO2

aggregates were visible through the naked eye also. With 5 % MT and above no

sedimentation was observed and the solution appeared clear. This shows better

dispersion of the nanoparticles in the mixed solvent with increased quantity of MT. With

the addition of P3HT in the solution, TiO2

Co-solvent

aggregates were visible with 2.5 % MT or less

content, and no significant changes were observed than that observed without P3HT.

Without TiO 2 With TiO2

Peak at 610 nm Feeble change beyond this Peak at 610 nm

Vol. % ∂ Dt Vol. % s-p D Ds-p Vol. % s-p ∂t

Ethanol 20 19.8 4.0 40 21.0 6.5 12.5 19.4

Methanol 15 19.7 3.9 30 20.1 6.3 10 19.4

2-propanol 25 19.6 4.1 50 20.6 6.6 15 19.3


At this stage it seems that 2P is the best additive in CF to prepare good P3HT:TiO

IV-5.3: BLEND FILM FORMATION AND MODELLING

2

blend

films. This is because initial larger percentage of 2P, can be incorporated in the solution

without P3HT aggregation. On the contrary it was found that good blend films were not

formed by using CF:2P mixture. In fact good films with varying phase separation of the

component solutes were formed using CF:MT and CF:ET mixtures. The morphology

evolution in the blend films using different solvent combinations can be explained by

studying the mixed solvent evaporation dynamics during spin coating. Accurate

evaporation rates are not important for us and the ratio of the evaporation rates of the

solvent components is sufficient to explain the coating behaviour. The evaporation rate

of CF, MT, ET and 2P is 10.00k, 5.52k, 4.27k and 3.26k respectively in unit cm/s [34]. Here

k is a constant. Few other parameters of the solvents and solutes are shown in Table 3.

Table 3: Solvent and solute properties used in simulation

During spin coating, the applied solution on the rotating substrate gets uniformly

distributed within a fraction of second [35]. The amount of CF and MT evaporation

through the drying liquid film is a function of both evaporation rates of the component

solvents, as well as of the exposed surface area of each component. It is true that

evaporation rate of solvents will be different from the stated values during spin coating.

However the ratio of their evaporation rates can be considered unchanged. The ratio of

each solvent component evaporated in small time ∆t can be given by

Amount of CF evaporation in time ∆tAmount of MT evapor ation in time ∆t

= 10.00k×[volume of CF ]2

3� ×∆t

5.52k×[volume of MT ]2

3� ×∆t .....IV.4

P3HT:TiO2

Solvent

blend films with various initial concentrations of MT in CF were coated. The

Evaporation rate (ether = 1)

Density (g/cm3

Evaporation Rate ) (after conversion in cm/s)

Chloroform 1.9 1.48 10.00k

Ethanol 8.3 0.79 4.27k

Methanol 6.3 0.79 5.52k

2-propanol 11.0 0.785 3.26k

P3HT --- 1.1 ---

TiO --- 2 3.8 ---


Scanning electron microscope (SEM) images of the films are shown in figure 5. With

increased quantity of MT in the mixed solvent domain coarsening of each phase reduces

considerably, i. e. films of varying level of phase separation are obtained by changing the

initial MT content in the mixed solvent. However at hundreds of micrometer scale no

features were observed.

Figure 5: SEM images of P3HT: TiO2

blend films prepared with different concentrations of MT in CF

In each case, the time evolution of MT and CF content in the drying film during spin

coating was derived through simulation. The results are shown in figure 6. The total

volume of initial mixed solvent is normalized to 100. The results in general show that, due

to solvent evaporation during film drying process, the quantity of both CF and MT

reduces in the spinning film. Additionally the percentage of MT compared to CF in the

mixed solvent reduces as the film drying process proceeds. With 5 % MT in the initial

solvent, the MT content reduces to 0.15 % at the instant when solubility limit of P3HT in

CF (50 mg/ml) is crossed; i. e. when the film is considerably wet. Due to the low MT

content at instants, when the film is considerably wet, TiO2 aggregation takes place and

large domains of TiO2 can be observed in the resulting film. With increasing MT content,


however, the TiO2 aggregate size decreases. With 7.5 % and 10 % MT in the initial solvent

the percentage of MT reduces to 1.8 % and 4.4 % respectively when solubility limit of

P3HT is crossed. Due to availability of more and more MT in those successive cases,

possibility of TiO2 aggregation reduces. This is clearly reflected in SEM images. With 10 %

MT, the resulting film has considerably small TiO2

domain size and the two phases are

finely intermixed. At 10 % MT the P3HT aggregation initiates, however the extent of

aggregation is less.

Figure 6: Simulation results show the variation of MT and CF in drying films and the percentage of co-solvent. (a), (b), and (c) show variation with 5 %, 7.5 %, and 10 % MT, respectively, in CF


The images of blend films with 5 %, 7.5 % and 10 % ET are also shown in figure 7. Finely

intermixed phases were obtained with 7.5 % ET content in the initial mixed solvent.

Simulation result in figure 8 show that due to the low evaporation rate of ET, the

percentage of ET remains nearly constant throughout the coating process with 7.5 % ET

in the initial solution. At the point where P3HT solubility in CF is crossed the ET content

increases slightly to 7.7 %. Almost constant ratio of the two solvents during film drying

prevents individual phase aggregation to a great extent. Inset in the image with 7.5 % ET

content (10 % volume of TiO2 w. r. t. P3HT) shows pits around the TiO2 aggregates. This

confirms the fact that, ET is mainly concentrated around the TiO2 aggregates. Due to this,

local concentration of ET around the TiO2 aggregates can increase to such an extent that,

P3HT aggregation takes place. This may be the reason why P3HT aggregation initiates

with lower content of co-solvent compared to that without TiO2

. This is again supported

with the fact that P3HT aggregation initiates with same co-solvent content in low as well

as high concentrations (as observed from the colour change of solution).

Figure 7: SEM images of P3HT: TiO2

blend films prepared with different concentrations of ET in CF. Inset in the image with 10.0% ET content shows optical microscope image

With 5% ET in the initial solvent also the ET content reduces to 2.7 % when solubility limit


of P3HT is crossed. In this case also, finely intermixed phases are obtained. However,

signs of TiO2 aggregation were also observed, compared to the case with initially 7.5 %

ET. With same quantity of MT (5 %) in the initial solvent, however, large aggregates of

TiO2

were obtained in the resulting film. This shows that in addition to initial quantity of

co-solvent in the solution, the quantity of it during the film drying process (as decided by

evaporation rates of the component solvents) is an extremely important parameter

which determines the phase separation.

Figure 8: Simulation results show the variation of ET and CF in drying films and the percentage of ET. (a), (b), and (c) show variation with 5 %, 7.5 %, and 10 % ET content respectively in CF


With initial 10% ET however low quality rough films (roughness visible to naked eye)

were obtained. Simulation results show that with such high quantity of ET in the initial

solvent, ET quantity increases rapidly during the course of film drying process. At the

point where solubility limit of P3HT is crossed, ET content increases to 14.7 % and beyond

that, ET content increases to the point where the mixed solvent becomes completely

poor solvent for P3HT, before complete film drying. In fact, in this case, CF evaporates

prior to ET.

Using 2P in place of MT, however, it was expected that blend films could be prepared

with initial larger percentage of 2P, as more quantity of 2P can be added to P3HT: CF

before P3HT aggregation. However, too low evaporation rate of 2P was the limit here and

with 10 % 2P content in the initial solution, 2P content rises to 25.1 % at the point where

solubility of P3HT is crossed and thereafter 2P content rises rapidly and reaches 100 %

mark before complete film solidification (Figure 9(a)). Resultant film was rough and very

similar to that obtained with 10 % ET content (Figure 9(b)). Reducing the content of 2P to

7.5 % it was found that TiO2 was not dispersible in the mixed solvent and the TiO2

aggregates were visible in the solution.

Figure 9: For 10 % 2P in CF (a) Optical microscope image of P3HT:TiO2

blend films, (b) simulation result

IV-5.4:

To obtain larger film thickness, the concentration of solution may be increased. However,

if the content of any one solvent, in the mixed solvent system is less, finite solubility of

the solute puts a limit on the film thickness. Moreover, due to the presence of other

solvent (poor solvent) the solubility of a particular solute or both the solutes may further

MORPHOLOGY CONTROL BY CONTROLLING THE RATE OF COOLING

AFTER THERMAL ANNEALING (STUDY WITH SINGLE AND DOUBLE LAYERS)

(a) (b)


decrease. To increase the film thickness, multiple layers of the film may be deposited.

However, common solvent for the already coated film and the coating solution, can

partially wipe off the already existing film, which leads to decrease in film thickness than

expected. Inspite of this fact, we have been able to obtain higher thickness of the

resultant films, by multiple depositions. The thickness of film obtained by single coating

was ∼ 200 nm, while after second coating the thickness increases to ∼ 275 nm. To further

control the blend morphology, controlling the cooling rate after thermal annealing has

been employed. Samples were annealed at 1400

C for 15 minutes and the rapidly cooled

sample was brought to room temperature in 30 seconds time by keeping it over a plate

kept at lower temperature. The slow cooled sample was brought to room temperature in

1 hour time.

Figure 10: Time decay components from TCSPC analysis

Figure 10 show the TRPL study results of the as coated blend films (single layer and

double layers), and after thermal annealing. The PL decay was fitted with double

exponential function and the primary decay components are tabulated in Table 4.


Table 4: Fitted parameters of TRPL decay of different films

In pristine P3HT film, the PL decays with characteristic average lifetime of 9747.26 fs. In

blend films however the PL decays rapidly, compared to pristine P3HT film. In the as

coated blend films the characteristic average life time is 7851.1 fs. In the film where

cooling was performed rapidly to room temperature the characteristics lifetime is

5629.53 fs, while for the film cooled slowly to room temperature the lifetime is 6550 fs.

A lower average lifetime for rapidly cooled film indicates more contact area between the

two phases [36-38]. Upon slow cooling the life time is greater and therefore an indication

of decreased area between the two. Thus there is a better intermixing of the two phases

due to rapid cooling after annealing compared to slow cooling. It is well known that the

enthalpy forces can drive components from the bulk to the surface; the component with

the lowest surface energy tends to migrate to the surface, thereby minimizing the total

energy of the system. Heating P3HT above its glass transition temperature, the heavy

TiO2 domains are well mixed in the soft and lighter P3HT matrix. Upon rapid cooling, the

intermixed high entropy state of the blend is preserved which disables the slow surface

segregation of the TiO2 domains. Effectively, the TiO2 domains may move closer to the

P3HT-substrate interface. On slow cooling the TiO2

Film

domains get sufficient time to diffuse

to the air film interface, which finally leads their surface segregation. In the case of

τ1 A(fs) τ1 2 A(ps) τ2 av (fs)

Pristine P3HT 1016.4 0.14 11288 0.85 9747.26

Blend(Single layer)

As coated 786.1 0.18 9362.4 0.82 7851.1

Annealed (Rapid cooling) 712.96 0.2169 6992.2 0.783 5629.53

Annealed (Slow cooling) 902.27 0.2268 8472.5 0.77315 6550.51

Blend (double layer)

As coated 827.5 0.1635 12682 0.8364 10742.52

Annealed (Rapid cooling) 784.9 0.2272 7071.4 0.7727 5642.399

Annealed (Slow cooling) 823.56 0.231 7189.2 0.7689 5718.706


double layer the characteristic average lifetime is 10742.52 fs, which reduces to 5642.399

fs and 5718.706 fs for rapidly cooled and slowly cooled films respectively. The

characteristics 10742.52 fs lifetime for the as deposited double layer is higher than that

of pristine P3HT film. The reason for this behaviour cannot be explained from the present

data and more study is required to confirm the fact. However, it can be concluded that,

rapid cooling of the blend film results in better interface area between the two phases,

which is beneficial for application in solar cell.

The AFM topography images for the single layer and double layer blend film before and

after annealing is shown in figure 12.

Figure 11: AFM topography images of (a), (b), and (c) are the single layer films, while (d), (e) and (f) are the double layer films; Scale: 5μm x 5 μm

The RMS roughness in all the films was around 40-45 nm. A clear trend regarding the

change in rms roughness in single layer and double layer films, both before and after

thermal annealing was not observed. However it is clear from the images that the

domain size decreases in the film cooled rapidly after annealing. This effect was observed

both in single as well as double layer films. Also, the average height of the films slowly

cooled after annealing showed significant reduction in average height. More than 25 %

reduction in average height was observed in films cooled rapidly compared to that of

slowly cooled films. This is an indication of the fact that for rapidly coooled film the TiO2

particles move effectively towards substrate:P3HT interface.


III-5.5: DEVICE PERFORMANCE

Devices were fabricated by the method described in chapter II. Performance of the

devices prepared by using double layers was better compared to that using single layer.

Therefore devices prepared by double layers has been discussed. The J-V characteristics

were recorded at illumination intensity 50 mW/cm2. At higher intensities the

performance degraded. The current density in devices, both before (0.0227 mA/cm2) and

after (0.0298 mA/cm2) thermal annealing at 1400C for 15 minutes is low. The possible

reasons are discussed below. It can be seen from the J-V curves (figure 12), that the curve

under illumination is not an near parallel shift of the curve under dark. There exist a

crossing point of the two curves (under dark and under illumination) at around 0.74 V. At

this point the dark current and the current under illumination are equal. This means that

there is no photogenerated current at this point. This indicates that the contact potential

is equal to the externally applied electric field at this point. This shows that the

photocurrent is strongly field dependent. The increase in photocurrent with applied bias

indicates that there is not enough internal field to provide a driving force for extracting

the generated bound geminate electron-hole pair, and therefore the photocurrent

depends upon the external field. The bound geminate electron-hole pairs, are the pairs of

electron on the acceptor side and the hole at the donor side, bound by coulomb

attraction. The low internal field may be due to the fact that the thickness of the

photoactive layer (double coated films) is ∼ 275 nm. Due to the large thickness, the

internal field is lower for the same external voltage. This low field is not able to extract

the generated bound geminate electron-hole pairs and they recombine [39, 40]. This

recombination is again confirmed by the fact that the maximum photocurrent is not

reached at 0 Volts, i. e. under short circuit condition. In fact, the maximum photocurrent

is reached at more negative bias, corresponding to a higher internal field. This happens in

organic solar cells when the charge-pair dissociation is more difficult, e. g. if the active

layer is thicker, when for the same (external) voltage, the internal field becomes lower

(also at V = 0). These separated charge pairs can therefore recombine, due to the lack of

sufficient field to extract them, which leads to loss of the generated exciton. This

phenomenon therefore can lead to drastic reduction in photocurrent. In addition to this,

non-geminate recombination having a bimolecular kinetics can also reduce the

photocurrent. With the present data we were not able to differentitate between the two,


however the probability of non-germinate recombination is low, especially at low

intensity levels.

Figure 12: J-V characteristics of devices in dark and under illumination for double layer devices (a) unannealed active layer, (b) annealed active layer.

In addition to the above recombination processes, a major contribution to low

photocurrent is the large domain sizes in the blend film. It can be seen in the AFM

images, that the domain size of the double coated films is ∼ 100 nm. The TRPL results

have shown that the average exciton life times in the blend films is much larger than that

found for P3HT:PCBM blends (chapter-II). This indicates that the ultrafast charge transfer

in P3HT: TiO2 blends is less than that found in P3HT:PCBM blends (Chapter-II). The lower

lifetime τ1 in the blend films (before and after thermal annealing) are comparable and

are very close to that of pristine P3HT films (compared to P3HT:PCBM blend films). The

domain size in these blends is much larger than the exciton diffusion length in P3HT. Such


large domains can lead to loss of generated excitons, before it reaches the donor-

acceptor interface. Upon thermal annealing the P3HT:TiO2 interface improves by

decrease in cavities at the interface. Also, the improved contact at the molecular level

and enhanced quality of interface in the bulk-heterojunction results in the increase in

charge separation yield, and therefore exciton lifetime is less in annealed films. A

respectable open circuit voltage 0.63 V and 0.56 V was obtained for unannealed and

annealed blend layer. Upon annealing the open circuit volage decreased. Such effect has

earlier been observed for P3HT:PCBM devices. Before annealing the P3HT phase is in

disordered state, as chloroform with a high evaporation rate was used as the solvent.

Therefore it shows a large optical bandgap (chapter III). Upon annealing the absorption

redshifts and the optical bandgap decreases [41-43]. Due to this, the HOMO level of P3HT

shifts such that the effective difference between the HOMO of P3HT and the conduction

band of TiO2 decreases. This results in the decrease in the Voc

Regarding the dark J-V characteristics it was observed that it does not coincides the

origin. It intersects the voltage axis at positive voltage. This indicates that a forward

voltage is required to make the dark current zero. This is due to the large reverse current

in dark. This effect was more in unannealed devices, which indicates that the leakage

currents have reduced by annealing the blend. We feel that further study is required to

better understand the device performance.

upon thermal annealing.

The FF before thermal annealing was 0.24 and after annealing it increased to 0.28. This

increase is due to the reduced series resistance afer thermal annealing. Overall PCE after

annealing increased to 0.0093 % compared to 0.0068 % before thermal annealing.

IV-5.6: SYNTHESIS OF TIO2

Nanocrystalline TiO

POWDER AND COMPARISION WITH P-25

2 was synthesized for the study mentioned above. However the

obtained TiO2 was not dispersible in the solvents used. Therefore it was not used for the

study mentioned above. However, the results are discussed here. Figure 13 show the x-

ray diffraction (XRD) pattern of as synthesized titania which indicated crystalline anatase

TiO2 with average crystallize size equal to 5.5 nm. The crystalline nature of the as

synthesized titania powder indicates that during the exothermic combustion synthesis a

large quantity of heat was released, leading to the formation of crystalline TiO2. Also

after the combustion reaction at 3500C, the furnace was allowed to cool to room

temperature before the powder was taken out. During this period also the powder may


gain crystallinity [44]. The effect of the heat treatment on phase composition and

crystalline domain size was investigated in order to identify the best compromise

between improvement of crystallinity and crystallite size growth. The XRD pattern of

titania after heat treatment at 400°C, 5000C, and 800°C for 7 hour is also shown in figure

13. Upon heating at 400°C, the crystallinity of the anatase phase increased (with average

crystallite size 12.3 nm) indicated by the increase in peak intensity at 2θ value 23.4

degree. However upon heating at 500°C for 7 hours rutile phase peaks appeared clearly

in the XRD pattern [45-47]. The average crystallite size in this case (average of anatase

and rutile phase calculated from the peaks at 23.40 and 25.30 was 18.6 nm. The peak at

2θ value 25.30 (due to 101 plane) for anatase phase and peak at 27.40 (due to 110 plane)

corresponding to rutile phase was used to calculate the ratio of the mass fraction of

anatase and rutile phase in the TiO2

Anatase % = � 0.79 IAIR +0.79IA

� × 100 .....IV.5

powder [28].

Rutile % = � IRIR +0.79IA

� × 100 .....IV.6

The crystallite size was calculated by using the Scherrer relation as given in chapter-II.

The XRD pattern of commertially available TiO2

powder P-25, is also shown in figure for

comparision. The crystallite size and the anatase and rutile percent is tabulated below in

Table 5.

Figure 13: X-ray diffraction pattern of synthesized TiO2 powder and comparision with commertially available P-25.


Table 5: Properties of TiO2 powder calculated from the XRD patterns

Sample Crystallize size (nm) Rutile % Anatase %

As prepared 5.5 0 100

Annealed 4000 12.3 C for 7 h 0 100

Annealed 5000 18.6 C for 7 h 47.4 52.6

Annealed 8000 28.0 C for 7 h 100 0

P-25, Dugussa 23.5 14 86

P-25 has 86.0 % anatase phase and average crystallize size 23.5 nm. In the synthesized

TiO2 the content of rutile TiO2 increases upon annealing but, at the cost of anatase

phase. For the TiO2 powder heated at 5000C the peak intensity corresponding to anatase

phase (23.40) decreased while that of rutile phase (25.30) increased, compared to that

heated at 4000C. This indicates the decrease in anatase content with increase in

annealing temperature. Figure 14 show the TEM images of as prepared TiO2

nanoparticles, after annealing at 4000C and 8000C for 7 hours each, and that of P-25. The

images confirm the formation of nanosized TiO2 particles. The TiO2

powder synthesized

above was not dispersible in CF, MT, ET, or 2P. Therefore these were not used for studies.

It is therefore that P-25 was purchased from Degussa.

Figure 14: TEM images of the TiO2 nanoparticles, (a) as synthesized, heated at (b) 4000C, (c) 8000

C, for 7 hours each, and (d) P-25.

CCOONNCCLLUUSSIIOONNSS::

The following has been shown in the present chapter:

a) Why a specific mixed solvent composition leads to appropriate film formation of


composite (polymer:inorganic nanoparticle) during spin coating by using mixed

solvents. For the formation of finely intermixed blend films, good solvents used for

both the solute components must maintain a constant ratio throughout the drying

process. If the content of any one solvent decreases in the course of film

solidification larger aggregates of individual phases are obtained. By knowing the

evaporation rates of the component solvents and their HSP, exact quantities of

mixed solvents for appropriate film formation can be predicted through simulation.

The film formation has been mainly explained by studying the solvent evaporation

dynamics, and therefore this approach can be applied to suitable mixed solvent

composition for any polymer:nanoparticle system. We believe that, this study can

provide a guideline for choosing mixed-solvent components and predict their

specific ratio to form polymer-inorganic nanoparticle film with different degree of

phase separation by spin coating for varying applications.

b) The thickness of films limited by the solubility of the solute in the mixed solvent can

be increased by multiple coatings.

c) By controlling the cooling rate of the blend film the morphology of the polymer:

inorganic nanoparticle system can be controlled.

d) TiO2

was synthesized by solution combustion synthesis method. However, this

powder was not dispersible in CF, MT, ET or 2P. Therefore this was not used for

further studies.


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CHAPTER IV EXPLORING POLYMER:INORGANIC NANOPARTICLE ... · organic layer by hot-wire chemical vapor...

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