In-Situ growth of cadmium telluride nanocrystals in poly(3-hexylthiophene)matrix for photovoltaic application

Mohd Taukeer Khan,1,2,3 Amarjeet Kaur,3 S. K. Dhawan,1,a) and Suresh Chand2,b)

1Polymeric and Soft Materials Section, National Physical Laboratory, (CSIR), Dr. K. S. Krishnan Road,New Delhi-110 012, India2Organic and Hybrid Solar Cell Group, National Physical Laboratory, (CSIR), Dr. K. S. Krishnan Road,New Delhi-110 012, India3Department of Physics and Astrophysics, University of Delhi, Delhi-110 007, India

(Received 30 April 2011; accepted 17 July 2011; published online 29 August 2011)

In the present study, nanocrystals of cadmium telluride (CdTe) have been directly synthesized in

poly(3-hexylthiophene) (P3HT) matrix without use of any surfactant. In situ synthesis of

nanoparticles in polymer matrix improves the polymer-nanoparticles interface, which facilitates

efficient electronic interaction between them. Spectral results suggest that CdTe nanocrystals are

bound with P3HT via dipole-dipole interaction and form a charge transfer complex. Structural and

morphological studies reveal that CdTe works as transport media along/between the polymer

chains, which facilitate percolation pathways for charge transport. Therefore, enhancement in

current density has been observed for the bulk heterojunction (BHJ) device of P3HT-CdTe

nanocomposites blended with PCBM. An open circuit voltage (VOC) of 0.80 V was obtained from

the BHJ device due to the increase in the energy level offset between the donor and acceptor. This

new photovoltaic element could provide a new nanoscale criterion for the investigation of

photoinduced energy/charge transport in organic-inorganic interfaces. VC 2011 American Institute ofPhysics. [doi:10.1063/1.3626464]

I. INTRODUCTION

Organic-inorganic hybrid nanocomposites are potential

systems for organic photovoltaic devices1–4 because it

includes the desirable characteristics of organic and inor-

ganic components within a single composite. They have

advantage of tunability of photophysical properties of the

quantum dots (QDs) and retained the polymer properties like

solution processing, fabrication of devices on large and flexi-

ble substrates.5–9 Due to different electron affinities of QDs

and polymer, a built in potential is generated at polymer-

QDs interfaces which assist the charge transfer between

polymer and QDs. Up to now, the organic-inorganic hybrid

thin film solar cells have shown power conversion efficiency

of �3% (Ref. 10). In conventional synthesis of QDs (CdTe,

CdSe), they were capped with organic aliphatic ligands, such

as tri-n-octylphosphine oxide (TOPO) or oleic acid. It has

been shown that when the QDs are capped with organic

ligands, they hinder the efficient electron transfer from the

photoexcited polymer to the nanoparticles.11 To remove the

organic ligands, polymer-nanoparticles were treated with

pyridine. However, pyridine is an immiscible solvent for the

polymer and flocculation of the P3HT chains in an excess of

pyridine may lead to the large-scale phase separation result-

ing in poor photovoltaic device performance.12 To overcome

the effects of the capping ligands on charge transport we

have directly synthesized uncapped nanoparticles inside the

polymer matrix. The in situ growth of the nanocrystals in

polymer templates controls the dispersion of the inorganic

phase in organic phase, thus ensuring a large, distributed sur-

face area for charge separation. Moreover, nanocrystals are

uniformly distributed to the entire device thickness and thus

contains a built in percolation pathway for transport of

charge carriers to the respective electrodes.

In surfactant-assisted synthesis, nanoparticles growth is

controlled by electrostatic interactions of the surfactant func-

tional group and steric hindrance of surfactant side alkyl

chains. P3HT provides a combination of both effects as it

contains an electron donating sulfur functionality, a potential

anchorage for the nucleation, and growth of nanoparticles

along with steric hindrance due to long hexyl side

chains.13,14 This new photovoltaic element could provide a

new nanoscale criterion for the investigation of photoinduced

energy/charge transport in organic-inorganic interfaces.

Direct synthesis of CdS nanorods13 and CdSe nanopar-

ticles14 in P3HT matrix, and also PbS nanorods in poly(2-

methoxy-5-(2-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-

PPV),15 have been reported previously. As CdTe have opti-

mal bandgap for solar cells and absorb higher amount of so-

lar radiation compared to the CdSe, CdS, PbS nanocrystals,

therefore replacement of theses nanocrystals with CdTe

would enable these hybrid devices to further enhancement in

power conversion efficiency.

The present investigation reports the synthesis of CdTe

nanocrystals in P3HT matrix with three different combina-

tions of P3HT and CdTe (P3HT1.5, P3HT11 and P3HT12).

These P3HT-CdTe nanocomposites can be dissolved in all

common solvents for the polymer, from which thin films can

be readily cast. Structural and morphological study revealed

that CdTe nanoparticles have been successfully synthesized

in P3HT matrix. Optical measurements of nanocomposites

a)Electronic mail: skdhawan@mail.nplindia.ernet.in.b)Electronic mail: schand@mail.nplindia.ernet.in.

0021-8979/2011/110(4)/044509/7/$30.00 VC 2011 American Institute of Physics110, 044509-1

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films show that photoinduced charge separation occurs at the

P3HT-CdTe interfaces, indicating this is a promising

approach for the fabrication of efficient organic-inorganic

hybrid photovoltaic devices. Photovoltaic performance of

P3HT:PCBM as well as P3HT-CdTe:PCBM have been

investigated in device configuration viz. indium tin oxide

(ITO)/poly(3,4-ethylendioxythiophene)-poly(styrene sulfonate)

(PEDOT:PSS)/P3HT:PCBM/Al and ITO/PEDOT:PSS/

P3HT-CdTe:PCBM/Al, respectively. These devices are des-

ignated as device A and device B, respectively. Based on

these investigations very interesting and important results

have been found wherein the JSC and VOC of device B have

increased. Improvement in JSCis due to enhancement of solar

absorption and the formation of charge transfer complex

(CTC) which reduces the defect states and barrier height at

the polymer-nanoparticles interfacial boundaries. The enhance-

ment in VOC is explained by the increase in the energy level

offset between the LUMO of the acceptor and the HOMO of

the donor. The fundamental facet revealed in situ synthesis of

P3HT-CdTe nanocomposites via surfactant-free method,

which, to our best knowledge, is designed for the first time

and its optical, electrical study shows that a P3HT-CdTe

nanocomposite is highly plausible for solar cell application.

II. EXPERIMENTAL METHODS

A. Synthesis

P3HT has been synthesized by chemical oxidative cou-

pling method as described previously.16 In situ growth of

CdTe nanoparticles in P3HT matrix was carried out as sche-

matically illustrated in Fig. 1. In a typical synthesis of

P3HT11, 50 mg of P3HT has been dissolved in 10 ml of

chlorobenzene to which 0.2 mmol of cadmium acetate dihy-

drade in chlorobenzene has been added. The reaction mix-

ture has been heated for 2 hrs at 160 �C. Tellurium precursor

has been made by treating 0.4 mmol of tellurium powder

(Acros Organics) in trioctylphosphine (TOP) (Sigma

Aldrich, USA), at 160 �C for 2 hrs under argon flow. The tel-

lurium precursor has been then injected in to the P3HT-Cd

solution and the resultant bright orange reaction mixture was

allowed to react for 2 hrs at 160 �C under argon atmosphere.

Growth of CdTe NCs has been completed when the solution

color turned to black. On the completion of the reaction,

unreacted cadmium acetate and precursor of tellurium have

been removed by treating nanocomposites with hexane. The

reaction mixture was separated by centrifugation and dried

in vacuum at 80 �C. Similarly, other compositions of P3HT

containing different mole ratios of Cd-acetate were synthe-

sized and are designated as P3HT1.5 for 0.1 mmol and

P3HT12 for 0.4 mmol having Te precursor in the ratios of

0.2 mmol for P3HT1.5 and 0.8 mmol for P3HT12. The syn-

theses of different P3HT-CdTe compositions have been also

carried out at 220 �C using the same procedure discussed

above.

B. Characterization

For optical and scanning electron microscope (SEM)

studies, P3HT and P3HT-CdTe nanocomposites were dis-

solved in chlorobenzene and thin films of these solutions

were deposited on glass substrates by spin casting at 1500

rpm for 120 s, and annealed at 120 �C for 30 min. Absorption

spectra were recorded by Shimadzu UV-1601 spectropho-

tometer. Photoluminescence measurement was carried out at

room temperature. The samples were excited with the wave-

length of 510 nm optical beam and the photoluminescence

(PL) signal was detected with the Perkin Elmer LF 55 having

Xenon source spectrophotometer (in the wavelength region

of 530–850 nm). Fourier transform infrared spectroscopy

(FTIR) spectra were recorded on Nicolet 5700 in transmis-

sion mode, wavenumber range 400–4000 cm�1 with a reso-

lution of 4 cm�1 performing 32 scans. Samples for high-

resolution transmission electron microscopy (HRTEM) were

prepared by putting a drop of nanocomposites solutions on

carbon grids and images were taken using a Tecnai G2 F30

S-Twin instrument operated at an accelerating voltage of

300 kV, having a point resolution of 0.2 nm and a lattice

resolution of 0.14 nm.

C. Solar cells fabrication and testing

For the fabrication of device A and device B, ITO (sheet

resistance �18 X/cm2) substrates have been carefully cleaned

in ultrasonic baths of acetone and isopropyl alcohol and dried

at 120 �C for 60 min in vacuum. Prior to use, substrate have

been treated with oxygen plasma.

PEDOT: PSS (Sigma Aldrich, USA) layers were spin-

coated at 2000 rpm for 2 min. onto the ITO substrate and

cured at 120 �C for 60 min in vacuum. P3HT:PCBM and

P3HT11:PCBM both have been taken in the ratio of 1:0.8

with a concentration of 1 wt. % in chlorobenzene. The solu-

tion containing P3HT plus PCBM was designated as solution

A and other containing P3HT-CdTe nanocomposite plus

PCBM were designated as solution B. The chlorobenzene so-

lution A and B have been spin casted at 1500 rpm for 2 min

on the top of PEDOT:PSS layer in an inert atmosphere, fol-

lowed by annealing at 130 �C for 30 min. Finally, Aluminum

(Al) contacts 150 nm has been applied via evaporation

through a shadow mask at 2� 10�6 Torr. The device active

area has been taken �0.1 cm2 for all the devices discussed in

this work. The J-V characteristics of device A and device

FIG. 1. (Color online) Proposed mechanism for in situ growth of the CdTe

nanoparticles in the P3HT matrix. (a) P3HT was synthesized by chemical

oxidative polymerization route. (b) Schematic of Cd2þ ions were assumed to

be coupled with the unpaired S along the P3HT planar chain network. (c)

Schematic diagram of P3HT capped CdTe nanoparticles after reaction of

TOPTe with Cd2þ ions coupled P3HT.

044509-2 Khan et al. J. Appl. Phys. 110, 044509 (2011)

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B have been performed in the dark and under halogen lamp

illumination with irradiance of 80 mWcm�2, using a Keithley

2400 Source-Measure unit, interfaced with a computer.

III. RESULTS AND DISCUSSION

A. Morphological study

High resolution transmission electron micrograph

(HRTEM) images and electron diffraction (insets) patterns

of the synthesized P3HT-CdTe nanocomposites P3HT1.5,

P3HT11, and P3HT12 at 160 �C are shown in Fig. 2. Trans-

mission electron micrograph images reveal that ratio of

P3HT and cadmium acetate plays a significant role in con-

trolling the size and shape of the nanocomposites. A differ-

ence in contrast at different places indicates that the CdTe

nanocrystals are capped by P3HT. It is evident from the Figs

2(a) and 2(b) that at low CdTe concentration the P3HT ma-

trix shows more binding with CdTe nanocrystals and forma-

tion of even nanorods structure of P3HT-CdTe as seen by

enlarged image [Fig. 2(b)]. However as the CdTe concentra-

tion increase Figs. 2(c) and 2(e) the binding between CdTe

and P3HT reduces and saw the precipitation of CdTe nano-

crystals appear rather than percolated network. The optimum

percolation and interaction between P3HT and CdTe take

place in P3HT11 as shown in Figs. 2(c) and 2(d), where the

nanorods formation as well as individual CdTe precipitation

has been suppressed. Hence, further device investigation has

been carried out in P3HT11. However, this interaction

between polymer and nanoparticles indicates that nanocom-

posites have potential for the charge transfer at polymer-

nanoparticles interfaces, which results in the PL quenching

and the improvement of short circuit current density. The

mechanism of this interaction has revealed that the sulfur

atom of P3HT can interact with the CdTe nanoparticles by

dipole-dipole interaction and CdTe nanocrystals have been

deposited uniformly and compactly on or in-between the

P3HT chains to form nanoparticles as suggested in Fig. 1(c).

The selected area electron diffraction patterns of P3HT1.5,

P3HT11 and P3HT12 are shown in the inset of Figs. 2(b),

2(d) and 2(f), respectively, which confirmed the high crystal-

linity of the CdTe.

HRTEM images of the synthesized P3HT-CdTe nano-

composites P3HT1.5, P3HT11, and P3HT12 at 220 �C are

shown in Fig. 3. In the present case nanorod formation of

P3HT-CdTe is absent due decrease in the bonding between

P3HT and CdTe and nanocrystals shows better crystallinity.

Moreover, the particle size in the present case is large com-

pared with that of above, due to at higher temperature aggre-

gation of particle take place. Therefore further studies have

FIG. 2. HRTEM images and electron diffraction (ED; insets) patterns of

(a)–(b) P3HT1.5, (c)–(d) P3HT11 and (e)–(f) P3HT12 nanocomposites syn-

thesized at 160 �C. Bar scale 20 nm for panels (a), (c), and (e) and 5 nm for

(b) (d) and (f).

FIG. 3. HRTEM images and electron diffraction (ED; insets) patterns of

(a)–(b) P3HT1.5, (c)–(d) P3HT11 and (e)–(f) P3HT12 nanocomposites syn-

thesized at 220 �C.

044509-3 Khan et al. J. Appl. Phys. 110, 044509 (2011)

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been carried out on P3HT-CdTe hybrid system synthesized

at 160 �C.

The surface morphology of the of the pristine P3HT and

P3HT-CdTe nanocomposite films have been examined by

SEM images. Figure 4(a) shows the nearly flat surface mor-

phology of pristine P3HT film. The SEM images with differ-

ent P3HT and CdTe compositions (P3HT1.5, P3HT11,

P3HT12) have been shown in Fig. 4(b)–(d). At low concen-

tration of CdTe (P3HT1.5), the nanocrystals aggregate to

form sludge like structure due to binding between P3HT and

CdTe as shown in Fig. 4(b). However, with increase of the

CdTe concentration [Fig. 4(c)], the binding between CdTe

and P3HT reduces leading to the formation of multifoliated

leaf like structures. On the contrary, further increase in CdTe

concentration multifoliated leaf like structures reduces lead-

ing to precipitation of CdTe nanocrystals (as evident from

the difference in brightness) shown in Fig. 4(d).

B. Spectral study

The success of formation of P3HT-CdTe nanocompo-

sites have been confirmed by the Fourier transform infra-red

(FT-IR) spectra as shown in Fig. 5. Strong absorption bands

of P3HT at 2953, 2920 and 2854 cm�1 have been assigned

to the asymmetric C–H stretching vibrations in –CH3, –CH2,

and the symmetric C–H stretching vibration in –CH2 respec-

tively. They are ascribed to the alkyl-side chains. The bands

at 1456, 1377 cm�1 are due to the thiophene ring stretching

and methyl deformation respectively. The C-C vibrations

appear at 1260 cm�1. The characteristic C-S band stretching

has been observed at 1111 cm�1 while absorption band at

822 cm�1 and 725 cm�1 have been assigned to the aromatic

C-H out-of plane stretching and methyl rocking respectively.

In nanocomposites of P3HT-CdTe, the intensity of peaks

corresponding to C-S bond and aromatic C-H out-of plane

stretching decreases. Also a shift by 25 cm�1 (from 1110 to

1135 cm�1), to the higher energy region of C-S characteristic

band has been observed in P3HT-CdTe, indicating the

enhancement of the C-S bond energy. Moreover, the charac-

teristic band of thiophene ring shows a redshift from 822 to

816 cm�1 with the increase of concentration of CdTe in

polymer matrix. These findings suggest additional intermo-

lecular interaction between polymer and nanocrystals which

arises due to strong dipole-dipole interaction between the

Cd2þ ions and S atoms as shown in Fig. 1(b).

The normalized UV-vis spectra of the P3HT and P3HT-

CdTe nanocomposites films are shown in Fig. 6(a). The max-

imum absorption of pristine P3HT films has been observed

at 510 nm which corresponds to the p-p* transition of the

conjugated chain in the P3HT (Ref. 17 and 18). For the

P3HT-CdTe composite films, the absorption spectrum has

been broader compared to that of pristine P3HT. The broad-

ness in absorption spectra indicates the presence of CdTe

nanocrystals in polymer matrix.14 Maximum absorption for

P3HT1.5 and P3HT11 have been red shifted to 515 nm and

518 nm, respectively. Redshift in P3HT-CdTe nanocompo-

sites suggest the formation of charge transfer states in P3HT-

CdTe nanocomposites resulting in partial electron transfer

from P3HT to CdTe (Ref. 19). On further increase of the

amount of CdTe in P3HT (P3HT12) there is a blueshift in

absorption spectra compared to that of P3HT1.5 and

P3HT11, which is observed at 514 nm. This means at higher

concentration of CdTe in P3HT, there is smaller shift in

absorption spectra. The smaller shift in absorption at high

concentration of CdTe in P3HT compared to that of low con-

centration of CdTe (P3HT1.5, P3HT11) is due to weak inter-

action between polymer-nanocrystals.

The photoluminescence quenching can be used as a

powerful tool for evaluation of charge transfer from the

excited polymer to the nanoparticles.20,21 Once the photo-

generated excitons are dissociated, the probability for recom-

bination should be significantly reduced. In Fig. 6(b), we

compared PL spectra of pristine P3HT films with that of dif-

ferent nanocomposites P3HT-CdTe films. The P3HT and

P3HT-CdTe nanocomposites exhibited emission maximum

around 660 nm. PL intensity of the nanocomposite film hasFIG. 4. SEM micrograph of spin casted thin films of panels (a) P3HT, (b)

P3HT1.5, (c) P3HT11 and (d) P3HT12 annealed at 120 �C for 30 mins.

FIG. 5. (Color online) FTIR spectra of P3HT and P3HT-CdTe

nanocomposites.

044509-4 Khan et al. J. Appl. Phys. 110, 044509 (2011)

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been significantly reduced as compared with the value of the

P3HT film and the reductions of PL intensity have been

increased with the increase of the CdTe concentration in

polymer. Reduced PL intensity of the composites relative to

the pristine P3HT indicates that charge transfer, thereby

exciton dissociation at interface of CdTe and P3HT occurred

(Fig. 7) (Ref. 22). This PL quenching experiment provides

us with good evidence that the nanoparticles will be able to

transfer their excited state hole to the polymer. Charge trans-

fer take place in conjugated polymer-semiconductor nano-

crystals composites at the interface where the P3HT with a

higher electron affinity (�3.37 eV) transferred electron onto

CdTe with relatively lower electron affinity (�3.71). In con-

version, the polymer absorb the solar photons (charge gener-

ation), the electron is transferred to the CdTe nanocrystals

and the hole potentially can transfer to the polymer (charge

separation). This is a well known effect of the ultrafast elec-

tron transfer from the donor to acceptor, and it is expected to

increase the exciton dissociation efficiency in photovoltaic

devices.23,24 Moreover, the PL spectra of P3HT-PCBM and

P3HT11-PCBM are also shown in Fig. 6(b). On incorpora-

tion of PCBM in P3HT and P3HT11 (P3HT-CdTe), the PL

spectra has been quenched relative to the P3HT and

P3HT11. The PL quenching upon addition of PCBM in

P3HT and P3HT11 further confirm the electron transfer

from P3HT to CdTe or PCBM and CdTe to PCBM. The QY

of P3HT decreases from initially 26% to 11% on incorpora-

tion of CdTe nanocrystals in to the P3HT matrix. The P3HT,

P3HT1.5, P3HT11, P3HT12 shows the QY of 26%, 20%,

17%, 14%, respectively. Reduction in QY of polymer/nano-

crystal composites compared to that of pristine P3HT, imply-

ing that a large amount of singlet excitons are not able to

radiate onto ground state and they dissociate at the polymer/

nanocrystals interface as suggested in Fig. 7.

C. P3HT-CdTe:PCBM solar cells

Figure 8(a) shows the J-V characteristics of device A

and B under AM 1.5 illuminations with an intensity of 80

mWcm�2. The performance of device A showed a short-cir-

cuit photocurrent (JSC) of 2.25 mAcm�2, an open-circuit

voltage (VOC) of 0.58 V, a fill factor (FF) of 0.44, and a

power conversion efficiency (PCE) of 0.72%. However, insitu growth of CdTe nanocrystals in P3HT matrix (device

B), the PCE value increased up to 0.79% thereby improving

the Jsc to 3.88 mAcm�2, VOC of 0.80 V, while FF diminish-

ing to 0.32.

The increase in the value of JSC of device B can be

understood in terms of host (P3HT) and guest (CdTe) charge

transfer type interaction. In fact, there are various possibil-

ities by which CdTe can interact with host P3HT. It can ei-

ther go structurally into P3HT main chain or forms donor

acceptor charge transfer complex (CTCs) or form molecular

aggregates. However, the enhancement in JSC in P3HT11

nanocomposites indicates that CTCs formation between the

host and guest may be the dominant mechanism of interac-

tion between the two. This suggested mechanism is indeed

supported by the PL quenching in P3HT-CdTe nanocompo-

sites, decrease in QY and energy levels of different materials

used shown in Fig. 7. On incident of light, both P3HT and

CdTe absorb light and generate excitons. Here, electron

affinities of P3HT, CdTe and PCBM are 3.37 eV, 3.71 eV

and 4.2, respectively, hence it is energetically favorable for

electron transfer from P3HT to CdTe or PCBM and CdTe to

PCBM or hole injection from CdTe to P3HT as indicated by

arrows in Fig. 7 (Ref. 25).

Moreover, enhancement in JSC may also results in

improvement in light absorption in P3HT-CdTe composites

FIG. 6. (Color online) (a) Normalized

absorption spectra of P3HT and P3HT-

CdTe nanocomposites films synthesized

at different ratio of P3HT and CdTe pre-

cipitated with hexane. (b) Photolumines-

cence spectra of P3HT, P3HT-CdTe

nanocomposites, P3HT-PCBM and P3HT-

CdTe-PCBM films after excitation by radi-

ation of 510 nm wavelengths.

FIG. 7. (Color online) Schematic illustration of the energy diagram of con-

figuration of device B. The P3HT, CdTe and PCBM have HOMO levels at

5.27, 5.48 and 6.0 eV while LUMO levels at 3.37, 3.71 and 4.2 eV, respec-

tively for facilitating the charge transfer at the P3HT-CdTe nanocomposites

and PCBM interface. The arrows indicate the expected charge transfer in

process P3HT-CdTe-PCBM.

044509-5 Khan et al. J. Appl. Phys. 110, 044509 (2011)

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compared to that of pristine P3HT. As in composite device

P3HT and CdTe both absorb light compared to that of

P3HT: PCBM device where PCBM contribution is very

small, hence light harvesting is more in hybrid system so

that number of exciton generated upon incident of light is

increases and as a results current density increases.

Figure 8(b) shows the J-V characteristics of P3HT and

P3HT-CdTe (P3HT11) nanocomposites thin films in hole

only device configuration viz. ITO/PEDOT:PSS/P3HT/Au

and ITO/PEDOT:PSS/P3HT-CdTe/Au. The nature of J-Vcharacteristic of composites thin film is different from that of

pristine P3HT. In case of composites film the hole current

has been observed to be more than that in pristine P3HT.

The enhancement in hole current in P3HT-CdTe composites

compared to that of pristine P3HT can be understood in

terms of host (P3HT) and guest (CdTe) charge transfer type

interaction. In the composites film the CdTe nanocrystals are

bound with P3HT via dipole-dipole interaction and form a

CTC. The charge carriers which had to jump from one chain

to another to transport through P3HT are now assisted by the

CdTe nanocrystals. The calculated value of activation energy

of localized states has been found to be 52 meV for P3HT

and 11 meV for P3HT-CdTe (Ref. 26). As activation energy

in P3HT-CdTe is lower compared to the pristine P3HT, the

CdTe nanocrystals support transportation of holes which

improves their mobility and results into enhancement in the

hole current.

The enhancement in VOC in device B can be understood

in terms of lower HOMO level of CdTe compared to P3HT

(Fig. 7). VOC is correlated with the energy difference

between the HOMO of the donor polymer and the LUMO of

the acceptor.27,28 Clearly, a lower HOMO energy level pro-

vides a higher open circuit voltage (Voc). The measured dif-

ference (0.21 eV) of the HOMO energy levels between

P3HT and CdTe almost completely translated into the

observed difference in Voc (�0.22 V).

The cells suffered from low fill factors which may be

caused by low shunting and a high series resistance.29–31 The

presence of polymer or nanocrystal pathways that connect

the anode to the cathode is a source of current leakage or

electrical shorts, depending on the conductivity of the path-

way.32 The incorporation of CdTe nanocrystals into a

P3HT–PCBM matrix results enhancement in photoconduc-

tivity of the active layer.33 Thus increased photoconductivity

of the active layer is responsible for the decreasing fill factor

and change of I-V shape of device B from device A. The

addition of one hole-blocking layer at cathode and another

electron-blocking layer at anode can prevent the polymer

and nanocrystal from shorting the two electrodes under illu-

mination. This and other similar approaches aimed at

increasing FF in hybrid PV cells are currently under investi-

gation within our laboratory.

IV. CONCLUSIONS

In conclusion, the successful synthesis of CdTe nanopar-

ticles in a P3HT matrix without need of any surfactant has

been demonstrated. BHJ solar cells device of P3HT-CdTe:

PCBM have been fabricated and these results were compared

with those of the P3HT:PCBM. Incorporation of CdTe in

P3HT:PCBM device increases the JSC and VOC. Increase in

JSC is due to CTCs formation between the host (P3HT) and

guest (CdTe QDs) duly supported by UV-vis absorption and

PL quenching studies and decrease of QY. Improvement in

VOC is due to the increase in the energy level offset between

the LUMO of the acceptor and the HOMO of the donor. Fur-

ther improvement can be achieved by controlling over the

morphology of the photoactive layer, improving the contacts

between photoactive layer and cathode and reducing the cur-

rent leakage by introducing the electron and hole blocking

layers before respective electrodes.

ACKNOWLEDGMENTS

The authors would like to thank Director NPL for his

keen interest in work. We sincerely thank Dr. Ritu Srivas-

tava, Mr. Neeraj Chaudhary and Dr. Kuldeep Singh all from

NPL, for their cooperation and useful discussions. Our spe-

cial thanks to Mr. K.N. Sood for recording the SEM micro-

graph and to Dr. Renu Pasricha for recording HRTEM

images. The authors are also thankful to Dr. Avadesh Prasad

from Department of Physics and Astrophysics, University of

Delhi for his useful suggestions. One of us (M.T.K.) is thank-

ful to CSIR, New Delhi, India, for the award of Senior

Research Fellowship.

1Y. Y. Lin, T. H. Chu, S. S. Li, C. H. Chuang, C. H. Chang, W. F. Su, C. P.

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FIG. 8. (Color online) (a) J-V curves

obtained from device A and device B

under AM 1.5 illuminations at irradia-

tion intensity of 80 mW/cm2 (b) J-V

characteristics of pristine and P3HT-

CdTe nanocomposites films in hole only

device configuration viz. ITO/

PEDOT:PSS/P3HT or P3HT-CdTe/Au

at room temperature in dark.

044509-6 Khan et al. J. Appl. Phys. 110, 044509 (2011)

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044509-7 Khan et al. J. Appl. Phys. 110, 044509 (2011)

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Hole transport mechanism in organic/inorganic hybrid system basedon in-situ grown cadmium telluride nanocrystals in poly(3-hexylthiophene)

Mohd Taukeer Khan,1,2 Amarjeet Kaur,2 S. K. Dhawan,1,a) and Suresh Chand1,b)

1Polymeric and Soft Materials Section, National Physical Laboratory, (CSIR), Dr. K. S. Krishnan Road,New Delhi-110 012, India2Department of Physics & Astrophysics, University of Delhi, Delhi-110 007, India

(Received 29 March 2011; accepted 21 April 2011; published online 9 June 2011)

The present manuscript demonstrates the hole transport mechanism in an organic/inorganic hybrid

system based on in-situ grown cadmium telluride (CdTe) nanocrystals in a poly(3-hexylthiophene)

(P3HT) matrix. The increase of hole current in the hybrid system is correlated with the formation of

a host-guest (P3HT-CdTe) charge transfer complex duly supported by photoluminescence

quenching. The hole transport mechanism in P3HT is governed by a space charge limited current

with temperature, carrier density, and field dependent mobility. Incorporation of CdTe nanocrystals

in a polymer matrix results in enhancement in the value of trap density Hb from 2.8� 1018 to

5.0� 1018 cm�3 and reduction in activation energies from 52 meV to 11 meV. At high trap density,

trap potential wells start overlapping; this results in decrease of activation energies.VC 2011 American Institute of Physics. [doi:10.1063/1.3594647]

I. INTRODUCTION

Conjugated polymer poly(3-hexylthiophene) (P3HT) is

well known for its application in solar cells as an electron

donor material in combination with n-type inorganic nano-

crystals, which results in better electron transportation and

improved efficiency.1–6 In comparison to the all organic

electron donor:acceptor devices, this hybrid approach of the

photo-active medium is supposed to be the better option as

they possess the desirable characteristics of both organic

(absorption) and inorganic (transport) systems. Up to now,

the organic-inorganic hybrid thin film solar cells have exhib-

ited a power conversion efficiency of up to g� 3%,7 which

is relatively small compared to the P3HT:PCBM system

(g� 6%).8 The lower g in hybrid system is because of an

inadequate charge transfer between polymer-nanocrystals

and the poor nanoscale morphology of the composites film.

In a conventional route, nanocrystals are synthesized through

the use of surface ligands. The incorporation of such nano-

crystals with the polymer would form an insulating interface,

which hinders the electron transfer from the photo-excited

polymer to the nanoparticles.9 The in-situ growth of the

nanocrystals in polymer templates controls the dispersion of

the inorganic phase in organic thus providing a large distrib-

uted surface area for charge separation. Moreover, nanopar-

ticles are uniformly dispersed in the entire device thickness

and thus contain a built in percolation pathway for transport

of charge carriers to the respective electrodes.

The main elements used in hybrid solar cells are P3HT

(donor) and CdSe nanocrystals (acceptor). As CdTe absorb a

higher amount of solar radiation compared to the cadmium

Selenide (CdSe), replacement of CdSe with CdTe therefore

would enable these hybrid devices to be further enhanced in

power conversion efficiency. This new photovoltaic element,

i.e., P3HT incorporated with in-situ grown CdTe nanocrys-

tals, could provide a new nanoscale criterion for the investi-

gation of photoinduced energy/charge transport in organic/

inorganic interfaces. The improved performance of hybrid

solar cells is expected because of the improved charge trans-

fer at the organic/inorganic interfaces10 and enhanced solar

radiation absorption. There are practically no discussions

available on P3HT incorporated with CdTe nanocrystals

related to the study on its charge transport.

The objective of this paper is to study the effect of

in-situ incorporation of CdTe nanocrystals on the hole trans-

port in P3HT conjugated system. For this, we have studied

current density versus voltage (J–V) characteristics of pure

P3HT as well as P3HT-CdTe nanocomposite thin films in a

hole-only device configuration as a function of temperature.

Based on these investigations, we have found very interest-

ing and important results wherein the incorporation of CdTe

nanocrystals in P3HT matrix results in the enhancement in

the hole current and change the transport mechanism from

the mobility model to the trap model.

II. THEORY

A. Exponential distribution of trap states

Due to the low mobility of charge carriers in organic

semiconductors, the injected carrier forms a space charge.

This space charge creates a field that opposes the applied

bias and thus decreases the voltage drop across junction; as a

result, space charge limited currents (SCLCs) have been pro-

posed as the dominant conduction mechanism in organic

semiconductors.11,12 Ohmic conduction can be described by

J ¼ qnlV

d; (1)

where q is the elementary charge, d is the thickness of the

film, l is the carrier mobility, and n is the carrier density.

Pure SCLC with no traps is given by Child’s law13

a)Electronic mail: skdhawan@mail.nplindia.ernet.b)Electronic mail: schand@mail.nplindia.ernet.in.

0021-8979/2011/109(11)/114509/5/$30.00 VC 2011 American Institute of Physics109, 114509-1

JOURNAL OF APPLIED PHYSICS 109, 114509 (2011)

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j ¼ 9

8ere0l

V2

d3; (2)

where ere0 is the dielectric permittivity.

SCLC theory with an exponential trap distribution pro-

poses that the space charge that limits conduction is stored in

the traps. The exponential distribution of traps in energy and

space is described as14

NðEÞ ¼ Hb

Etexp � E

Et

� �; (3)

where N(E) is the distribution function of hole trap density at

an energy level E above the valence bandedge, Hb is the total

trap density at the edge of valence band. Et is the characteris-

tic trap energy that is often expressed in terms of the charac-

teristic temperature of trap distribution TC as Et¼ kBTC and

l¼Et/kBT¼TC/T. where kB is the Boltzmann constant. The

parameter l determines the distribution of traps in the forbid-

den gap.

In case of exponential distribution of traps, assuming

that the trapped hole carrier density (pt) >> free hole carrier

density (p) and using continuity equation and boundary con-

dition for current density (J) and applied voltage (V) as

J ¼ qlpðxÞFðxÞ; (4)

V ¼ð

FðxÞdx: (5)

The expression for J is given by

J ¼ q1�llNv2lþ 1

lþ 1

� �lþ1 l

lþ 1

ere0

Hb

� �l Vlþ1

d2lþ1; (6)

where F(x) is the electric field inside the film and Nv is the

effective density of states. From Eq. (6), the slope of the cur-

rent-voltage characteristics on a log-log plot is lþ 1. There-

fore from the slopes on the log–log plots of current density

versus voltage, one can extract the trap energy width Et.

B. Unified mobility model

This model is based on percolation in a variable range

hopping (VRH) system with an exponential distribution of

localized states.15–17 Percolation is the term used for move-

ment of charge carriers through a random network of

obstacles. Consider a square lattice, where each site is ran-

domly occupied or empty. Occupied sites are assumed to be

electrical conductors while the empty sites represent insula-

tors, and the electrical current can flow between nearest

neighbor conductor sites. Percolation paths are the most opti-

mal paths for current and transport of charge carriers, which

are governed by the hopping of charge carriers between these

conducting sites. The system can be described as a random

resistor network,18 a system made up of individual discon-

nected clusters of conducting sites the average size of which

is dependent on a reference conductance G. The conductance

between sites is given by

G ¼ G0 exp �sij

� �(7)

with

sij ¼ 2arij þEj � Ei

�� ��þ Ei � EFj j þ Ej � EF

�� ��2kBT

: (8)

All conductive pathways between sites with Gij < G are

electrical insulators, while conductive pathways between

sites with Gij � G are electrical conductors. At some critical

conductance in between, therefore, a threshold conductance

GC exists where for the first time, electrical current can per-

colate from one edge to the other.

A bond is defined as a link between two sites that have a

conductance Gij � G. The average number of bonds B is

equal to the density of bonds; (Nb) divided by the density of

sites that form bonds, (Ns) in the material. Critical bond

number BC is the average number of bonds per site for which

threshold percolation occurs. The onset of percolation is

determined by calculating the critical average number of

bonds per site17

BðG ¼ GCÞ ¼ BC ¼Nb

NS: (9)

Vissenberg and Matters16,19,20 set the critical bond number

to Bc 5 2.8. The total density of bonds is given by

Nb ¼ 4pð ð ð

r2ijgðEiÞgðEjÞhðsc � sijÞdEidEjdrij: (10)

The density of sites Ns

Ns ¼ð

gðEÞhðsckBT � E� EFj jÞdE: (11)

At a low carrier concentration, exponential density of states

in amorphous organic semiconductors is given by:17,20

gðEÞ ¼N0

kBT0

expE

kBT0

� �

0;E > 0

8<: ; �1 < E � 0; (12)

where No is the total density of states (molecular density) per

unit volume and To is a characteristic temperature that deter-

mines the width of the exponential distribution.

Combining Eqs. (8) to (12), the expression for Bc

BC ¼ pN0

T0

2aT

� �3

expEmax

kBT0

� �; (13)

where Emax ¼ EF þ sCkBT is the maximum energy that par-

ticipates in bond formation. According to the percolation

theory, the conductivity of the system can be expressed as

r ¼ r0 exp½�sC�; (14)

where r0 is the prefactor and sc is the critical exponent of the

critical conductance when percolation first occurs (when

B 5 Bc). Using Eqs. (13) and (14), we get

r ¼ r0

T0

T

� �4sin p TT0

BC 2að Þ3

p

24

35

T0=T

: (15)

114509-2 Khan et al. J. Appl. Phys. 109, 114509 (2011)

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The conductivity can be converted into mobility by dividing

by e. p, where e is the electronic charge and p is the carrier

density:21,22

lðT; p;FÞ ¼ r0

q

T0

T

� �4 sin p TT0

BC 2að Þ3

24

35

T0=T

pT0Tð Þ�1: (16)

The average charge carrier density as a function of the

applied bias voltage V is given by13

pðVÞ ¼ 0:75e0erV

qd2

� �: (17)

III. EXPERIMENTAL

CdTe nanocrystals (� 5 nm) have been grown in-situ in

a P3HT matrix. In a typical synthesis of P3HT11, 50 mg

P3HT has been dissolved in 10 ml chlorobenzene. Cadmium

acetate dihydrate, 0.2 mmol (0.1 mmol for P3HT1.5, 0.4

mmol for P3HT12, and 0.6 mmol for P3HT13), was added in

P3HT-chlorobenzene solution. The reaction mixture was

heated for 2 hr at 160 �C. A tellurium precursor was made by

treating 0.4 mmol (0.2 mmol for P3HT1.5 or 0.8 mmol for

P3HT12, and 1.2 mmol for P3HT13) tellurium powder

(Acros organics) in trioctylphosphine (TOP) (Sigma Aldrich,

USA), at 160 �C for 2 hr under argon flow. The tellurium

precursor so obtained has then injected into the P3HT-Cd so-

lution, and the resultant bright orange reaction mixture was

allowed to react for 2 hr at 160 �C under argon atmosphere.

Growth of CdTe NCs has been completed when the solution

color turned black. After completion of the reaction, the

unreacted Cd-acetate and Te precursor were been removed

by treating nanocomposites with hexane. The reaction mix-

ture was separated by centrifugation and dried in vacuum at

80 �C.

The effect of CdTe nanocrystals on the hole conduction

in P3HT has been carried out at different temperatures in

hole only device configuration. For this purpose, the hole

conduction mechanisms of P3HT as well as P3HT-CdTe

nanocomposites thin films have been investigated in device

configuration viz. indium tin oxide (ITO)/poly(3,4-ethylen-

dioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS)/

P3HT/Au and ITO/PEDOT:PSS/P3HT-CdTe/Au, respec-

tively. These devices are designated as device A and device

B, respectively. Work functions of Au and ITO are close to

the highest occupied molecular-orbital (HOMO) of P3HT

and CdTe as well as far below the lowest unoccupied molec-

ular-orbital (LUMO) energy level (Fig. 2(a)); as a result, in

the Au: ITO device, the transport is dominated by holes. For

the preparation of devices A and B, ITO (sheet resistance,

�18 X/cm2) substrates have been carefully cleaned in ultra-

sonic baths of acetone and isopropyl alcohol and dried at

120 �C for 60 min in vacuum. PEDOT:PSS layers have been

spin-coated at 2000 rpm for 2 min onto the ITO substrate

and cured at 120 �C for 60 min in vacuum. P3HT and P3HT-

CdTe both have been taken with a concentration of 20 mg/

ml in chlorobenzene. Thin films of P3HT and P3HT-CdTe

have been spin casted in inert atmosphere, followed by

annealing at 120 �C for 30 min. Finally, gold (Au) contact,

200 nm, has been applied via evaporation through a shadow

mask at 2� 10�6 Torr. The device active areas have been

taken �0.1 cm2 for all the devices discussed in this work.

J–V characteristics of the devices have been measured with

Keithley 2400 Source-Measure unit, interfaced with a

computer.

IV. RESULTS AND DISCUSSION

Figure 1(a) shows the transmission electron micrograph

(TEM) of P3HT-CdTe nanocomposite. Nanocrystals of

CdTe have been found in between the polymers chain. The

inset of Fig. 1(a) shows the high resolution TEM (HRTEM)

and electron diffraction pattern of CdTe nanocrystals in

P3HT matrix. In Fig. 1(b), we compared photoluminescence

(PL) spectra of pristine P3HT with that of different nano-

composites of P3HT-CdTe. The PL intensity of the nano-

composites has been significantly reduced as compared to

the value of pristine P3HT, and the reduction of PL intensity

has been increased with the increase of the CdTe concentra-

tion in polymer. The reason for the photoluminescence

quenching in composites may be due to the p–p interaction

of P3HT with CdTe,23 forming additional decaying paths of

the excited electrons through the CdTe (see Fig. 2(a)).

Reduced PL intensity of the composites relative to the refer-

ence P3HT, indicates the host-guest charge transfer complex

formation and, thereby, the occurrence of exciton dissocia-

tion at interface between P3HT and CdTe-NCs.24 This PL

quenching experiment provides good evidence that the nano-

particles will be able to transfer their excited state hole to the

polymer.

Figure 2 shows the J–V characteristics of device A

measured in the temperature range of 290-150 K. On

FIG. 1. (Color online) (a) TEM images of above syn-

thesized CdTe nanocrystals in P3HT matrix (Bar scale

10 nm). Inset shows electron diffraction patterns of

P3HT-CdTe nanocomposites. (b) Photoluminescence

spectra of P3HT and different P3HT-CdTe nanocompo-

sites films after excitation by radiation of 375 nm

wavelengths.

114509-3 Khan et al. J. Appl. Phys. 109, 114509 (2011)

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lowering the temperature, the current has been found to

decrease. In organic semiconductors, charge transport is gov-

erned by hopping of a carrier from site to site of an empty

density of states. The thermal energy helps to cross the ener-

getic barrier between two adjacent sites. This implies that

the charge transport in organic semiconductor is thermally

activated. Therefore on lowering the temperature, the current

has been found to decrease.

At low applied bias, the J–V characteristics showed ohmic

behavior [Eq. (1)] as injected carriers are negligible compared

to that of the applied bias.25 At moderate field, the injected

carrier density becomes so high that the field due to the car-

riers dominates the applied bias. At this point, the J–V charac-

teristics may switch to pure SCLC and follow Eq. (2). On

further enhancement of field, the quasi-Fermi level intersects

an exponential trap distribution, and characteristics will begin

to follow Eq. (6). The hole mobility up to this field is constant

and also independent of the hole density. The fit of the J–Vcharacteristics of the P3HT device using the Eq. (6) is poor at

high applied bias where current density deviates strongly as

expected from Eq. (6). This discrepancy has been analyzed by

unified mobility model given by Eq. (16). This model accounts

the influence of temperature, carrier density, and applied field

on the carrier mobility.17 The solid curves in Fig. (2) have

been obtained by combining Eqs. (6) and (16) using a com-

puter program. The value of different parameters for solid

curves are; d¼ 110 nm, er¼ 3, e0¼ 8.85� 10�14 F/cm,

Hb¼ 2.8� 1018 cm�3, Nv¼ 1� 1019 cm�3, TC¼ 400 K,

T0¼ 325 K, r0¼ 4� 104 S/m, a�1¼ 1.12 A, and Bc¼ 2.8.

Figure 3 shows the J-V characteristics of device B meas-

ured at different temperatures. Interestingly the nature of

P3HT-CdTe composite thin film is different than that of pris-

tine P3HT. In case of composite film, the hole current has

been observed to be more than that in pristine P3HT at all

temperatures. The inset of Fig. 3 shows the comparison of

J–V characteristics of devices A and B at 150 K. The compo-

sites exhibited an S-shaped characteristic, and the rate of

reduction of current with temperature is low compared to that

in pristine P3HT. We tried to fit the experimental data with

mobility model. The data did not show agreement with the

mobility model for single set of parameter values. On the

other hand, the comparison of experimental data with Eq. (6)

showed a good agreement with same value of parameters at

different temperatures. Solid curves in Fig. (3) represent the

plot of Eq. (6) at respective temperatures. The values of pa-

rameters used in the calculations are; Hb¼ 5.0� 1018 cm�3,

Nv¼ 6.0� 1018 cm�3, l¼ 6.0� 10�5 cm2 V�1 s�1, d¼ 110

nm, and Tc¼ 400 K. For the characteristics measured at 250

K, 220 K, 195 K, 175 K, and 150 K, the agreement was

obtained for l¼ 7.8� 10�5, 1.16� 10�4, 2.4� 10�4,

3.55� 10�4, 7.5� 10�4 cm2 V�1 s�1, respectively.

The enhancement in current density in P3HT-CdTe thin

film can be understood in terms of host (P3HT) and guest

(CdTe) charge transfer type interaction. In fact, there are var-

ious possibilities by which CdTe can interact with host

P3HT. It can either go structurally into P3HT main chain or

form donor/acceptor charge transfer complexes (CTCs) or

form molecular aggregates. However, the enhancement in Jin device B indicates that CTCs formation between the host

FIG. 3. (Color online) Experimental (symbols) and calculated (solid lines)

J-V characteristics of device B at different temperature in hole only device

configuration viz. ITO/PEDOT:PSS/P3HT-CdTe/Au. The inset shows the

comparison of J–V characteristics of device A and B at 150 K.

FIG. 2. (Color online) (a) Schematic

illustration of the energy diagram of

configuration of device B. The P3HT,

CdTe have HOMO levels at 5.27, 5.48

while LUMO levels at 3.37, 3.71,

respectively for facilitating the charge

transfer at the P3HT-CdTe. (b) Experi-

mental (symbols) and calculated (solid

lines) J–V characteristic of P3HT thin

films at different temperatures in hole

only device configuration viz. ITO/

PEDOT:PSS/P3HT/Au.

114509-4 Khan et al. J. Appl. Phys. 109, 114509 (2011)

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and guest may be the dominant mechanism of interaction

between the two, duly supported by photoluminescence

quenching [Fig. 1(b)]. In the composites film, the CdTe

nanocrystals are bound with P3HT via dipole-dipole interac-

tion and form a CTC. The charge carriers, which had to

jump from one chain to another to transport through P3HT,

are now assisted by the CdTe nanocrystals. The calculated

value of activation energy of localized states has been found

to be 52 meV for P3HT and 11 meV for P3HT-CdTe. As

activation energy in P3HT-CdTe is lower compared to the

pristine P3HT, the CdTe nanocrystals support transportation

of holes; this improves their mobility and results into

enhancement in the current.

The change of mobility from field dependent in P3HT to

field independent in the P3HT-CdTe thin film can be

explained on the basis of increase of trap density (Hb) and

reduction in activation energy. Usually, an electric field

raises the mobility because it lowers the activation barriers.

In organic semiconductors, most of the charge carriers are

trapped in localized states. An applied field gives rise to the

accumulation of charge in the region of the semiconducting

layer. As these charges are accumulated, (i) spatial overlap

between the trap potential increases that lower the activation

barriers26 and (ii) only a fraction of total charge carriers are

required to fill all the traps, the remaining carriers will on av-

erage require less activation energy to hop away to a neigh-

boring site. This results in a higher mobility with increasing

field. Incorporation of CdTe nanocrystals in the P3HT matrix

simultaneously enhances the value of trap density from

2.8� 1018 to 5.0� 1018 cm�3 and produces extrinsic charge

carriers.27 At high trap density, the trap potential wells over-

lap; this results in decreasing activation energies (from 52

meV to 11 meV). Furthermore, increase in the charge carrier

density on incorporation of CdTe nanocrystals in P3HT ma-

trix results in only partial filling of carriers even in deeper

intrinsic states; this leads to an upward shift of the Fermi

level to the effective transport level and concomitant

increase of the jump rate. This implies that even at low field

larger numbers of free charge carriers are available for trans-

port and hence the mobility in P3HT-CdTe films is inde-

pendent of applied field.

V. CONCLUSIONS

In conclusion, on in-situ incorporation of CdTe-NCs

into P3HT matrix, a pronounced change in the transport

mechanism and enhancement in hole current of the hybrid

system have been observed. Hole transport in pristine P3HT

has been observed to follow the unified mobility model,

whereas the hybrid film exhibited agreement with the trap

conduction mechanism, and mobility is field independent.

This change of the conduction mechanism is an important

finding and has been attributed to the enhancement in the

overlapping of traps potential wells; this results in a decrease

in activation energies. Our results constitute important pro-

gress for the use of inorganic nanocrystals in polymer for the

large area solution processed hybrid photovoltaic cells.

ACKNOWLEDGMENTS

Authors would like to thank Director NPL for his keen

interest in their work. We sincerely thank Dr. Ritu Srivas-

tava, Dr. Rajeev Singh, Dr. Pankaj Kumar, and Dr. Kuldeep

Singh, all from NPL, for their cooperation and useful discus-

sions. M.T.K. is thankful to CSIR, India, for the award of

Senior Research Fellowship.

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Memory effect in cadmium telluride quantum dots doped ferroelectricliquid crystals

A. Kumar,1 J. Prakash,2 Mohd Taukeer Khan,3 S. K. Dhawan,3 and A. M. Biradar1,a�

1Liquid Crystal Group, National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi 110012, India2Instrument Design Development Centre, Indian Institute of Technology Delhi, Hauz Khas,New Delhi 110016, India3Conducting Polymer Group, National Physical Laboratory, Dr. K. S. Krishnan Road,New Delhi 110012, India

�Received 17 August 2010; accepted 5 September 2010; published online 21 October 2010�

A pronounced memory effect has been observed in cadmium telluride quantum dots �CdTe-QDs�doped ferroelectric liquid crystals �FLCs� by using dielectric and electro-optical methods. Thememory effect has been attributed to the charge storage on the CdTe-QDs upon the application ofdc bias across the sample cell. The FLC molecules remain in the switched state in vicinity of thecharge stored on QDs even after removal of bias. It has been observed that the memory effectdepends on doping concentrations of CdTe-QDs and the FLC material used. © 2010 AmericanInstitute of Physics. �doi:10.1063/1.3495780�

Ferroelectric liquid crystals �FLCs� have been employedin various promising applications, such as flat panel displays,spatial light modulators, optical antennas, etc., due to theirimportant characteristic features such as good optical con-trast, fast response, low threshold voltage, and memory ef-fect. The metal nanoparticles �NPs� have been doped intoliquid crystals �LCs� to observe various interesting phenom-ena, such as nonvolatile memory effect,1 enhancedphotoluminescence,2 enhanced electrical conductivity,3 andinduced LC alignments.4 The doping of NPs has improvedthe electro-optical properties such as good contrast and lowpower operation of LC based display devices.5,6 The semi-conducting quantum dots �QDs� have attracted a great dealof interest in the scientific community for their promisingapplications such as next generation photonic devices, QDdisplays, and biomedical imaging.7–10 Apart from them, vari-ous studies have been carried out by researchers for utilizingthe QDs in the realization of nonvolatile memory devices.Nassiopoulou et al.11 have observed a large shift in thecapacitance-voltage �C-V� hysteresis of the metal-oxide-semiconductor �MOS� structures containing germaniumQDs. The memory effect has been observed in a MOS struc-ture containing zirconium �Zr� nanocrystals embedded inZrO2 dielectric layer by Lee et al.12 The high frequency C-Vcharacteristics of the structures with silicon NPs have shownhysteresis indicating the charging of NPs with electrons/holes due to charge carrier tunneling from the substratethrough the thin oxide at positive/negative biases.13 Chargeretention properties of QDs have been studied14,15 and foundthat the charge carriers stored on QDs could persist over timescales exceeding seconds or even hours.16

On the other hand, the anisotropic ordering of LCs canimpart order onto the nanosized guest particles and henceresearchers have utilized this property to organize QDs in theform of their self-assembly.17 Hirst et al.18 studied themechanism to organize QDs by using anisotropic LC me-dium and explored the possibility for the fabrication of mul-

tifunctional switchable devices. However, the effect of QDson the properties of LC materials is rarely reported. Theunderstanding of the interaction between QDs and LCs is achallenging area of research for its utilization to achieve con-trolled self-assembly of QDs and improved electro-opticalcharacteristics of LCs.

In this paper, we observed the effect of cadmium tellu-ride �CdTe�-QDs on the electro-optical properties of FLCs. Ithas been observed that the doping of CdTe-QDs in variousFLC materials favors pronounced memory effect. The ob-served memory state does not come back to its original �un-switched� state instantaneously but it retains in that state fora remarkable time. We also observed that retention ofmemory state depends on the doping concentration of QDsand FLC materials.

The CdTe-QDs have been synthesized in the form ofP3HT �Poly-3�hexylthiophene��-CdTe nanocomposites. TheP3HT polymer is used for capping to prevent the agglomera-tion of QDs. The high resolution transmission electron mi-croscopy �HRTEM� images and electron diffraction patternof the synthesized P3HT-CdTe nanocomposites have beenshown in Fig. 1. Highly resolved HRTEM image has alsobeen given in the inset of Fig. 1�a�. The typical size of CdTe-QDs varies between 2–7 nm in diameter. The sample cellsfor the present study were prepared using indium tin oxidecoated glass plates. The desired �squared� electrode area was0.45�0.45 cm2. The thickness of the cell was maintainedby using �4 �m thick Mylar spacers. The FLC materials

a�Author to whom correspondence should be addressed. Electronic mail:abiradar@mail.nplindia.ernet.in.

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FIG. 1. �Color online� �a� HRTEM image with scale bar: 20 nm �the insetshows highly magnified image with scale bar: 5 nm �b� electron diffractionpattern of P3HT/CdTe-QDs nanocomposites.

APPLIED PHYSICS LETTERS 97, 163113 �2010�

0003-6951/2010/97�16�/163113/3/$30.00 © 2010 American Institute of Physics97, 163113-1

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�pure and CdTe-QDs doped� were filled into these cells bycapillary action above their respective isotropic tempera-tures. The homogeneous alignment of FLC cells has beenachieved by using rubbed polyimide technique. The phasesequences of FLC materials used are as follows:

cryt . ↔7 °C

SmC� ↔59.5 °C

SmA� ↔60 °C

iso . �LAHS19�

cryt . ↔1 °C

SmC� ↔58 °C

SmA� ↔64 °C

iso . �LAHS18�

cryt . ↔−14 °C

SmC� ↔60.5 °C

SmA� ↔64 °C

iso . �FLC 6304�

cryt . ↔?

SmC� ↔60.8 °C

SmA� ↔86.8 °C

N ↔100.8 °C

iso . �KCFLC 7S�

The dielectric measurements of pure and CdTe-QDs dopedFLC materials were performed using an impedance analyzer�Wayne Kerr, 6540A, U.K. � in the frequency range 20 Hz–1MHz with measuring voltage of 0.5 V. The optical micro-graphs were taken using polarizing optical microscope �CarlZeiss, Germany�.

Figure 2 shows the optical micrographs of bright anddark states of pure and �4 wt % CdTe-QDs doped FLC�LAHS19�. It can be seen clearly from Fig. 2, that the dopingof CdTe-QDs does not perturb the alignment of LAHS19material remarkably. However, the presence of CdTe-QDs inthe material can be clearly seen in the form of light scatteringcenters in the dark state of CdTe-QDs doped LAHS19 �Fig.2�d��. It is found that the dispersion of spherical particlessuch as cadmium sulphide QDs into the LCs typically intro-duce random surfaces that can disrupt the uniform LCalignment.17 It has also been observed that the presence oflarge colloidal particles in LCs is usually associated with adefect in the texture.18 The memory effect in CdTe-QDsdoped various FLC material has been observed using dielec-tric measurements. Figure 3 shows the behavior of dielectricpermittivity ���� with frequency at different dc biases ofCdTe-QDs doped FLCs. The behavior of memory in pureFLC materials has been shown in insets of figures. All FLCmaterials used exhibit SmC� phase at room temperature. InSmC� phase, the FLC molecules are arranged in a helicoidalmanner and show higher value of �� due to the contributionof Goldstone mode �GM� which occurs due to phase fluctua-tion of FLC director. The application of sufficient bias acrossthe cell suppresses the GM contribution �due to unwinding of

helicoidal structure� and results in the lower and static valueof ��. On the removal of bias, FLC molecules attain theirhelicoidal structure again due to restoring forces and ��reaches its original value. If somehow, FLC molecules retaintheir switched state �unwound state� even after removal ofbias then it means the material exhibits memory. Figure 3�a�shows the occurrence of dielectric memory in CdTe-QDsdoped LAHS19 material. It can be seen clearly from figurethat the switched state has been retained for CdTe-QDsdoped LAHS19 even after removal of bias whereas it re-stored its original state instantaneously in case of pureLAHS19 �inset of Fig. 3�a��. Similarly, Figs. 3�b�–3�d� depictthe occurrence of dielectric memory in CdTe-QDs dopedLAHS18, FLC6304, and KCFLC 7S materials, respectively.It is evident from Figs. 3�b�–3�d� that all FLC materials showdielectric memory by doping CdTe-QDs whereas pure coun-terparts do not show the same as can be seen from the insets�of Figs. 3�b� and 3�d��. The memory effect in pure FLC6304 has been studied and found that it does not showmemory effect by dielectric measurements.1 It has been dem-onstrated earlier that surface stabilization �SS� leads to thebistability and memory effect in FLCs.19 To exclude the pos-sibility of SS, we used either DHFLC material �which haveultrashort pitch� or FLC material keeping cell thicknessgreater than pitch value. This means that by doping CdTe-QDs in the conventional FLCs, one can get a memory stateeven in the thicker non-SS FLC geometries �Fig. 3�d��.

It has been observed that the dielectric memory persistedover time scales exceeding minutes. The value of �� CdTe-QDs doped LAHS19 material does not reach its original�0 V� value instantaneously but takes some remarkable time.The value of �� after 4 min was found almost 50% of itsoriginal value indicating that almost half of the FLC mol-ecules were still in switched state even after 4 min of re-moval of bias. The retention of memory state for other FLCs/CdTe-QDs composites has also been observed. It has beenfound that the retention of memory state follows the sametrend in case of FLCs/CdTe-QDs as CdTe-QDs dopedLAHS19. However, the retention time is found to be depen-dent on the concentration of CdTe-QDs and the FLC mate-rials used.

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FIG. 2. �Color online� Polarizing optical micrographs of �a� bright state, �b�dark state of pure and �c� bright state, �d� dark state of �4 wt % CdTe-QDsdoped LAHS19 material. !" !# !$ !% !&

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FIG. 3. �Color online� Behavior of dielectric permittivity ���� at different dcbiases of CdTe-QDs doped �a� LAHS19, �b� LAHS18, and �c� FLC 6304 in�4 �m thick cells, and �d� KCFLC 7S in �10 �m thick cell at roomtemperature.

163113-2 Kumar et al. Appl. Phys. Lett. 97, 163113 �2010�

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The occurrence of memory effect can be understood bytaking the interaction of FLC molecules with CdTe-QDs intoaccount. The operation of a memory device in a junctionlikeCdS nanocomposites/conducting polymer poly�2-methoxy-5-�2-ethylhexyloxy�-1,4-phenylene-vinylene� heterostructureembedded in polyvinyl alcohol �PVA� matrix has been dem-onstrated by studying C-V characteristics, and attributed thismemory behavior to the trapping, storage, and emission ofholes in the CdS nanoneedles embedded in PVA matrix.20

The phenomena such as QD memory21 and electricalbistability22 in CdSe-QDs based devices due to their chargetrapping and storage characteristics have also been demon-strated. The CdTe-QDs used for this study have been grownin situ in the matrix of conducting polymer P3HT, i.e., theCdTe-QDs are capped with P3HT. The applications of dcbias across the CdTe-QDs doped FLCs sample cells inducecharge transfer from FLC molecules to CdTe-QDs. TheP3HT is a conducting polymer and hence it provided a lowresistive path to the charges in reaching toward CdTe-QDs.The charge stored on CdTe-QDs can retain for several min-utes and hence it does not remove instantaneously after re-moval of bias. The FLC molecules nearby these CdTe-QDsretain their switched state in the effect of charge stored onthe CdTe-QDs which has been resulted in the form of pro-nounced dielectric memory. It is found that the charge storedon the CdTe-QDs has been released in a slower fashion andtaken upto 5 min duration to discharge completely in case of�4 wt % CdTe-QDs doped LAHS19 material. Moreover,the charge retention time on CdTe-QDs is found to dependon the FLC material used. The maximum charge retentiontime of �10 min has been observed in case of �4 wt %CdTe-QDs doped LAHS18 material which has slightly dif-ferent composition than that of LAHS19 whereas in case ofKCFLC 7S/CdTe-QDs composite, �� reached its originalvalue �0 V state� in comparatively shorter duration��1 min�. However, to understand the exact nature of inter-actions between FLC molecules and QDs in achieving im-proved electro-optical properties of FLCs are still a challeng-ing task to be investigated.

A pronounced memory effect in FLCs/CdTe-QDs com-posites has been observed. The observed memory effect hasbeen attributed to conducting polymer mediated chargetransfer from FLC molecules to CdTe-QDs. It is found thatthe stored charges on CdTe-QDs do not remove instanta-neously after removal of bias but they take several minutesto discharge completely. The retention of memory state is

found to be dependent on the concentration of CdTe-QDsand the FLC material used.

The authors sincerely thank Professor R. C. Budhani, theDirector, National Physical Laboratory, for continuous en-couragement and interest in this work. We sincerely thankDr. Poonam Silotia of Delhi University for useful discus-sions. The authors �A.K. and J.P.� are thankful to UniversityGrant Commission �UGC� and Council of Scientific and In-dustrial Research �CSIR�, New Delhi for providing financialassistance.

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Effect of cadmium sulphide quantum dot processing and post thermal annealing onP3HT/PCBM photovoltaic device

M. Taukeer Khan a,b, Ranoo Bhargav a, Amarjeet Kaur b, S.K. Dhawan a,⁎, S. Chand a,⁎a Polymeric and Soft Materials Section, National Physical Laboratory (CSIR), Dr. K. S. Krishnan Road, New Delhi 110012, Indiab Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India

a b s t r a c ta r t i c l e i n f o

Available online 20 August 2010

Keywords:P3HTQuantum dotsCharge transfer complexSolar cells

The present study demonstrates the effect on photovoltaic performance of poly(3-hexylthiophene) (P3HT)on doping of cadmium sulphide (CdS) quantum dots (QDs). The P3HT/CdS nanocomposite shows a 10 nmblue shift in the UV–vis absorption relative to the pristine P3HT. The blue shift in the absorption of the P3HT/CdS nanocomposite can be assigned to the quantum confinement effect from the CdS nanoparticles.Significant PL quenching was observed for the nanocomposite films, attributed to additional decaying pathsof the excited electrons through the CdS. Solar cell performance of pure P3HT and dispersed with CdS QDshave been studied in the device configuration viz indium tin oxide (ITO)/poly(3,4-ethylendioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS)/P3HT:PCBM/Al and ITO/PEDOT:PSS/ P3HT:CdS:PCBM/Al, respectively.Incorporation of CdS QDs in the P3HT matrix results in the enhancement in the device efficiency (x) of thesolar cell from 0.45 to 0.87%. Postproduction thermal annealing at 150 °C for 30 min improves deviceperformance due to enhancement in the device parameters like FF, VOC and improvement in contact betweenactive layer and Al.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Poly(3-hexylthiophene) (P3HT) is the potential polymer whichfinds its application in organic thin film transistors (OTFT) [1–3], andorganic photovoltaic (OPV) [4–8] devices owing to its higher regio-regularity and mobility as compared to other conjugated polymersbeing used for these devices. Recent trends in OPV devices is towardsthe use of hybrid organic/inorganic nanocomposites comprising of aconjugated polymer incorporated/embedded with nanoparticles andquantum dots [9–13]. There are investigations available on P3HTincorporated with quantum dots/nanoparticles of cadmium selenide(CdSe) relating to the study on its charge transport [14,15], and devicefabrication possibilities [16,17]. The preparation methods of CdSe QDsutilize expensive raw materials such as organic phosphines, octade-cene, and aliphatic amine [18]. Environmentally, organic phosphineligands should be avoided because of their high toxicity, which wouldincrease the control cost of chemical pollution [19]. If the productioncost of QDs could be decreased greatly through deploying cheap rawmaterials with lower toxicity and decreasing reaction temperatures,large-scale preparation and practical application of QDs would beaccessible. In the present study, we have used CdS QDs which aresynthesized by economic raw materials like cadmium nitrate and

sodium sulphide without using any highly toxic organic phosphenes.The synthesis of quantum dots was carried out at room temperaturewhile CdSe and CdTe are synthesized at a relatively highertemperature (at 300 °C). This initiated broad exploration on thelarge-scale preparation of low-cost QDs through employing cheap,environmentally benign raw materials and relatively lower reactiontemperatures.

In the present paper we are combining CdS nanoparticles withpoly(3-hexylthiophene) (P3HT). Incorporation of nanoparticles withpolymer enables us to utilize the properties of both semiconductorpolymer (solution processability, flexibility) and nanoparticles (mo-bility). The fundamental question that we are attempting to address inthis paper is whether the introduction of semiconductor nanoparticlesinto a polymer matrix causes any noticeable improvement ordeterioration of device efficiency. For this we have studied currentdensity vs. voltage (J–V) characteristics of pure P3HT as well asdispersed with CdS QDs in the device configuration viz indium tinoxide (ITO)/poly(3,4-ethylendioxythiophene)-poly(styrene sulfo-nate) (PEDOT:PSS, 80 nm)/ P3HT:PCBM/Al and ITO/PEDOT:PSS/P3HT:CdS:PCBM/Au, respectively. Here PEDOT:PSS was used forhole collection and Al electrode was used for electron collection. Onincorporation of CdS QDs in the P3HT matrix, we have found veryinteresting and important results wherein the power conversionefficiency increased from 0.45% to 0.87% by improving the shortcircuitphotocurrent (Jsc), open-circuit voltage (VOC) and fill factor (FF).These effect have been explained on the basis of the formation ofcharge transfer complex (CTC) between the host (P3HT) and guest

Thin Solid Films 519 (2010) 1007–1011

⁎ Corresponding authors. Tel.: +91 11 4560 9401; fax: +91 11 25726938.E-mail addresses: skdhawan@mail.nplindia.ernet (S.K. Dhawan),

schand@mail.nplindia.ernet.in (S. Chand).

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(CdS QDs) duly supported by UV–vis absorption and PL quenchingstudies. We have also studied the effect of post thermal annealing ondevice performance and found improved efficiency of devices afterthermal treatment at 150 °C for 30 min due to improved nanoscalemorphology, increased crystallinity and improved contact to theelectron-collecting electrode.

2. Experimental section

2.1. Materials

3-Hexylthiophene, 1-dodecanethiol and ferric chloride (FeCl3)were purchased from Sigma Aldrich and used as received. Sodiumsulphide fused flakes (Na2S), was purchased from Fisher Scientific,and cadmium nitrate (Cd(NO3)2.4H2O) was purchased from ThomasBaker. HPLC grade chloroform (CHCl3), hexane and methanol werepurchased from MERCK. All solvents were freshly distilled prior touse.

2.2. Synthesis of poly(3-hexylthiophene)

The polymerization of P3HT was done by the oxidative couplingmethod [20–22]. This method is based on the use of a Lewis acid suchas ferric chloride (FeCl3) to initiate the polymerization. The reactionmixture was kept stirred under a nitrogen environment at−40 °C for24 h. After polymerization, the mixture color turned into green andsubsequently washed with methanol and deionized water to removeundesired impurities like unprocessed oxidant, monomer andoligomers. This polymer contains residual iron salts and chlorine asan impurity. To get a pristine polymer, the chloroform (CHCl3)solution of the polymer was subsequently treated with an aqueoussolution of ethylene diamine tetra-acetic acid (EDTA), ammoniasolution at 62 °C. After this treatment, polymer was dried in vacuumat 60 °C for 2 h and kept in vacuum.

2.3. Synthesis of cadmium sulphide (CdS) quantum dots

CdS nanoparticles were synthesized according to a previousliterature [23,24].Two hexane solutions of Aerosol OT (AOT) (0.2 M,50 ml) were prepared. An aqueous solution of cadmium nitrate Cd(NO3)2.4H2O (0.4 M) was added to one hexane solution, while anaqueous solution of Na2S (0.4 M) was added to the other solution inorder to achieve a [H2O]/[AOT] ratio of 6 for both solutions. Thesolutions were stirred for 3 h. The micellar solution containing Cd(NO3)2 was then added slowly to the micelle solution containing Na2Sat room temperature under a nitrogen atmosphere. CdS nanoparticleswere obtained after the solution was stirred for 3 h. 1-Decanethiol(DT) molecules (4.3 mmol) were added to a hexane solution of CdSnanoparticles (1.5 M). This solution was stirred for 5 h, and methanolwas subsequently added in order to remove the AOT molecules. Afterthe methanol phase was removed, the hexane phase was evaporated.The residual solution was then dropped into a large volume ofmethanol, and the resultant yellow precipitate was filtered off using a0.2-μm membrane filter, yielding purified DT-protected CdSnanoparticles.

2.4. Fabrication and measurement of device

In our experiments, the ratio of P3HT:PCBM was taken 1:0.8 andP3HT:CdS:PCBM was taken 1:1:0.8 with a concentration of 1 wt.% inchlorobenzene. Two solutions of P3HT were prepared in chloroben-zene and in one of them CdS was added and sonicated for 4 h in orderto well disperse CdS in P3HT. PCBM solution in chlorobenzene wasadded in above solutions and the mixed solution was ultrasonicatedfor 2 h. The solution containing P3HT plus PCBM was designated assolution A and other containing P3HT plus CdS and PCBM was

designated as solution B. For preparation of device A and device B, theITO-coated glass substrate was first cleaned with detergent, ultra-sonicated in acetone, trichloroethylene and isopropyl alcohol, andsubsequently dried in an vacuum oven. Highly conducting PEDOT:PSS(Aldrich USA) was spin casted at 4000 rpm for 2 min. The substratewas dried for 10 min at 150 °C in vacuum and thenmoved into a glovebox for spin casting the photoactive layer. The chlorobenzene solutionA and B was then spin casted at 1500 rpm for 2 min on the top of thePEDOT:PSS layer. Subsequently 120 nm Al film was deposited on topof the active layer. Thermal annealing was carried out by directlyplacing the completed device at 150 °C in a vacuum oven. Theperformance of the devices has been studied by their J–V character-istics in the dark and under halogen lamp illumination with irradianceof 80 mWcm−2, using a Keithley 2400 Source-Measure unit, inter-faced with a computer.

2.5. Structural and optical properties analysis

The TEM sample of CdS was prepared by drying a droplet of the CdSnanoparticles dispersed in deionised water on a carbon grid while theTEM sample of P3HT/CdSwas prepared by drying a droplet of the abovehybrid solution on an uncoated grid. High resolution transmissionelectronmicroscopy (HRTEM) imageswere obtainedwith an apparatusequipped with a HAADF detector (image size: 2014×2014 pixels; scantimes: 5–20 s; camera length: 200 mm). Absorption spectra were taken

Fig. 1. XRD spectra of CdS QDs, P3HT and P3HT/CdS nanocomposite films.

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on Shimadzu UV-1601 spectrophotometer. Photoluminescence (PL)spectra were recorded on a Perkin Elmer LF 55 with Xenon sourcespectrophotometer. XRD of the polymer film was taken in RINT2000 Rigaku make X-ray diffractometer (40 KV, 30 mA, λ=1.54059 A,Cu-Kα1). The HRTEM apparatus is equipped with a HAADF detector(image size: 2014×2014 pixels; scan times: 5–20 s; camera length:200 mm). An energy-dispersive X-ray spectrometer attached to theTecnai G2 F30was used to perform elemental analyses at spots selectedin the HAADF-STEM images. Image processing was performed usingDigital Micrograph software (Gatan).

3. Result and discussion

3.1. Regio-regularity and molecular weight of P3HT

A comparative study of the photophysical properties of the P3HTpolymer synthesized for the present investigationwith those reportedearlier for P3HT synthesized by various routes [25] shows the qualityof P3HT. The reported band for aromatic C–H out of plain vibration isat 820 cm−1, which is the characteristics of 2,5-disubstituted-3-hexylthiophene for rr-P3HTwhereas the corresponding band for rdm-P3HT occurs at 827 cm−1 [26]. The observed band for aromatic C–Hout of plain vibration at 822 cm−1 [Fig. 1(a)] of pristine P3HT of thepresent work, confirms the synthesis of P3HT having rr-P3HT likechain conformation. Regio-regular P3HT has solid-state absorptionsranging from λmax=520–530 nm and for regio-random P3HT to 480–500 nm [25,26]. The absorption spectrum of our P3HT film has anabsorption peak at 526 nmwith an edge at 606 nmwhich confirms itsregio-regularity. 1H NMR study also confirms the regio-regularity ofP3HT (δ=6.977). Molecular weights of P3HT of the present work are:Mw=39,278 and Mn=27,298 with a PD index of 1.44.

3.2. Structural characterization

3.2.1. XRD analysisFig. 1 shows XRD patterns for pure P3HT, CdS/P3HT nanocompo-

site and CdS powder. In the CdS XRD spectra, three broad peaks at2θ=27°, 44° and 52° belong to the (111), (220) and (311) planesrespectively of cubic CdS. The XRD peaks are broadened due to thesmall size of QDs. The average crystallite size determined from theDebye–Scherrer formula d=0.9λ/β cosθ is estimated to be about2.33 nm, where λ is the wavelength of the X-rays used, β is the fullwidth at half maximum and θ is the angle of reflection. The strong firstorder reflection, (100), of P3HT observed at 2 angle 5.45°,corresponds to interlayer spacing 16.4 Å. The second order reflection(200) of P3HT, observed at 2 angle 10.86°, corresponds to interlayerspacing 8.402 Å. In comparison, XRD data of CdS/P3HT shows that the2θ values match the (1 0 0), (1 0 0), (1 1 1), (2 2 0) and (3 1 1) planes.The appearances of few additional peaks in composites are attributedto the presence of QDs in the P3HT matrix.

3.2.2. HRTEM imagesHRTEM images of CdS QDs and P3HT/CdS nanocomposite are

shown in Fig. 2(a–c) and (d–f), respectively. It is seen from Fig. 2(a)that the size of the QDs ranges from 5 to 6 nm and their shape isspherical. Also it is seen from Fig. 2(b) on a higher resolution thatthere exist (1 1 1), (2 2 0) and (3 1 1) planes of cubic CdS havinginterplanar spacing 3.36, 2.06 and 1.76 Å, respectively. This formationof different planes is explicitly confirmed by the diffraction patternshown in Fig. 2(c). Further Fig. 2(d) and (e) shows that QDs are evenlydistributed within the P3HT and form uniform hybrid nanocompositefilms. Different planes of CdS QDs in the P3HTmatrix are shown by thediffraction pattern in Fig. 2(e).

Fig. 2. High resolution TEM images of (a) CdS nanoparticles in the range of 5–6 nm, (b) lattice resolution of cubic CdS QDs, (c) diffraction image of CdS QDs, (d–e) CdS nanoparticlesdispersed in poly(3-hexylthiophene) matrix and (f) diffraction image of CdS QDs in the P3HT matrix.

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3.3. Optical study

3.3.1. UV–vis absorption spectraUV–vis absorption spectra of P3HT and P3HT/CdS nanocomposite

solution in chloroform and of thin films are shown in Fig. 3(a). Regio-regular P3HT have solid-state absorptions ranging from λmax=520–530 nm and solution absorption ranging 442–456 nm. The absorptionspectrum of our P3HT solution has an absorption peak at 448 nm andthin films show absorption at 526 nm which confirms its regio-regularity. Strong absorption band at 448 nm for P3HT is attributed tothe excitation of electrons in the π-conjugated system. The P3HT/CdSnanocomposite shows that absorption band at 438 nm is 10 nm blueshifted relative to the pristine P3HT solution. The blue shift inabsorption of the P3HT/CdS nanocomposite can be assigned to thequantum confinement effect from the CdS nanoparticles [27–29].Maximum absorption intensity in the nanocomposite is lower due toscattering caused by the nanoparticles in the P3HT matrix. As shownin Fig. 1(a) CdS quantum dots show a broad absorption from 290 to700 nm with a maximum absorption peak at 292 nm and an edge at440 nm. The absorption spectrum of the P3HT film has an absorptionpeak at 526 nm with an edge at 606 nm, and exhibited a red shiftcompared with that of a comparable solution, indicating that it iseasier for the P3HT to form a planar orientation in the solid film. Theabsorption spectrum of the P3HT/CdS thin film also exhibits a 15 nmblue shift relative to P3HT, indicating that the CdS nanoparticles in thefilm also have a quantum confinement effect.

3.3.2. Photoluminescence spectraPhotoluminescence quenching in a bulk heterojunction is a useful

indication of the degree of success of exciton dissociation andefficiency of charge transfer between the donor–acceptor compositematerials [30,31]. P3HT has a photoluminescence property, [32,33]and the photoluminescence spectra of P3HT and P3HT/CdS solution inCHCl3 at excitation wavelength 448 nm, are presented in Fig. 3(b).Significant PL quenching was observed for the nanocompositesolution. The PL intensity of the composite solution is significantlyreduced compared with the value of the P3HT in Fig. 3(b). Thisindicates that charge transfer, thereby exciton dissociation at interfacebetween CdS and P3HT, has taken place. Higher exciton dissociationefficiency accounts for higher device performance. For an excitationwavelength of 448 nm the solution show an emission at 674 and668 nm for P3HT and P3HT/CdS respectively. The reason for thephotoluminescence quenching of P3HT/CdS may be due to the π–πinteraction of P3HT with CdS [34,35], forming additional decayingpaths of the excited electrons through the CdS. The small blue shift(4 nm) in the nanocomposite emission spectra indicates that theground state energy level is more stable in the nanocomposite thanthat of pristine P3HT. This may be possible through the resonancestability of π clouds of P3HT and CdS through π–π interaction.

Fig. 3. (a) UV–visible absorption spectra of P3HT and P3HT/CdS QD nanocompositefilms and solution in the chloroform (b) photoluminescence spectra of P3HT and P3HT/CdS QD nanocomposite solution in chloroform at the excitation wavelength 448 nm.

Fig. 4. J–V curves obtained from device A and device B under AM 1.5 illumination at anirradiation intensity of 80 mW/cm2, (a) devices without thermal annealing and(b) devices with postproduction heat treatment at 150 °C for 30 min.

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3.4. J–V characteristics

Fig. 4 shows the J–V characteristics of device A and B under AM 1.5illumination with an intensity of 80 mWcm−2. The performance ofdevice A showed a short-circuit photocurrent (Jsc) of 2.57 mAcm−2,an open-circuit voltage (VOC) of 0.45 V, a fill factor (FF) of 0.30, and apower conversion efficiency (PCE) of 0.45%.Whenwe incorporate CdSQDs in the P3HT matrix (device B), the PCE value increased up to0.87% by improving the Jsc of 4.65 mAcm−2, VOC of 0.45 V, and FF of0.32. The performance of devices A and B thermal annealing at 150 °Cfor 30 min are shown in Fig. 4(b). After thermal treatment device Adelivers an VOC of 0.58 V, a JSC of 2.26 mA/cm2, a FF of 0.45 and a deviceefficiency of 0.74%. Device B after thermal annealing gives an VOC of0.58 V, a JSC of 2.98 mA/cm2 and a FF of 0.44, resulting in an estimateddevice efficiency of 0.95 %. These data are summarized in Table 1.

The modulation of device parameters i.e. increase in the value ofVOC, JSC and FF in device B can be understood in terms of host (P3HT)and guest (CdS QDs) charge transfer type interaction. In fact there arevarious possibilities by which doped CdS can interact with host P3HT.It can either go structurally into the P3HT main chain or form donoracceptor charge transfer complex (CTC) or formmolecular aggregates.However, the enhancement in JSC in P3HT on CdS dispersion indicatesthat CTC formation between the host and guest may be the dominantmechanism of interaction between the two. This suggested mecha-nism is indeed supported by the UV–vis absorption and PL emissionstudies in pure P3HT and CdS dispersed P3HT as shown in Fig. 3(a)and (b), respectively. Fig. 3(a) shows a blue shift in the UV–visabsorption peak from 523 nm to 512 nm on incorporation of CdS QDsin the P3HT matrix which may be attributed to the CTC/quantumconfinement effect from the CdS nanoparticles [27]. Also small blueshift (4 nm) in the nanocomposite PL spectra indicates that during theCTC formation ground state energy level is more stable in thenanocomposite than that of pristine P3HT. This may be possiblethrough the resonance stability of the π clouds of P3HT and CdSthrough π–π interaction [28] as a result of CTC formation.

Similarly PL quenching seen in Fig 3(b) on CdS dispersion in P3HTis a direct evidence of CTC formation between the host and guest sincePL quenching is an indication of the degree of success of excitondissociation and efficiency of charge transfer between the donor–acceptor composite materials. The PL quenching in P3HT/CdS hasbeen attributed to the π–π* interaction of P3HT with CdS, formingadditional decaying paths of the excited electrons through CdS. To bemore precise during CTC formation CdS QDs may diffuse into theamorphous-crystalline boundaries of the P3HT polymer and intro-duces the conducting path thus reducing the defect states and barrierheight at these interfacial boundaries.

The increase in PCE (and the improved FF) after thermal treatmentimplies a significant decrease in the series resistance [36], thermallyinduced morphology modification, thermally induced crystallizationand improved transport across the interface between the bulkheterojunction material and aluminum (Al) electrode [5]. Theimproved nanoscale morphology results in a more efficient chargegeneration. The higher crystallinity and improved transport across the

interface result in a better charge collection at the electrodes withreduced series resistance and higher fill factor.

4. Conclusions

In conclusion, we have constructed and studied a bulk heterojunc-tion photovoltaic device that contains P3HT:CdS:PCBM and the resultsare compared with those of the P3HT:PCBM. Incorporation of CdS inP3HT:PCBM device increases the device performance due to CTCformation between the host (P3HT) and guest (CdS QDs) dulysupported by UV–vis absorption and PL quenching studies. Postpro-duction thermal annealing decreases the series resistance and improvesthe contact between the active layer and Al, resulting in enhanceddevice efficiency. Further improvement can be anticipated if bettercontrol over the morphology of the photoactive blend can be gained.

Acknowledgements

The authorsMTK is thankful to CSIR, NewDelhi, India, for the awardof Senior Research Fellowship. We sincerely thank, Dr. Ritu Srivastavaand Dr. Renu Paschricha for their cooperation and useful discussion.

References

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(2005) 2175.[8] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Nat. Mater. 4

(2005) 864.[9] T. Stoferle, U. Scherf, R.F. Mahrt, Nano Lett. 9 (2009) 453.

[10] W.U. Huynh, J.J. Dittmer, A.P. Alivisatos, Science 295 (2002) 2425.[11] S.D.Oosterhout,M.M.Wienk, S.S. vanBavel, R. Thiedmann, L.J.A. Koster, J.Gilot, J. Loos,

V. Schmidt, R.A.J. Janssen, Nat. Mater. 8 (2009) 818.[12] Y. Zhou, Y. Li, H. Zhong, J. Hou, Y. Ding, C. Yang, Y. Li, Nanotechnology 17 (2006)

4041.[13] W.J.E. Beek, M.M. Wienk, R.A.J. Janssen, Adv. Mater. 16 (2004) 1009.[14] K. Kumari, S. Chand, P. Kumar, S.N. Sharma, V.D. Vankar, V. Kumar, Appl. Phys. Lett.

92 (2008) 263504.[15] S. Bhattacharya, S. Malik, A.K. Nandi, A. Ghosh, J. Chem. Phys. 125 (2006) 174717.[16] J.Y. Kim, K. Lee, N.E. Coates, D. Moses, T.Q. Nguyen, M. Dante, A.J. Heeger, Science

317 (2007) 222.[17] L. Wang, Y.S. Liu, X. Jiang, D.H. Qin, Y. Cao, J. Phys. Chem. C 111 (2007) 9538.[18] C.R. Bullen, P. Mulvaney, Nano Lett. 4 (2004) 2303.[19] W.W. Yu, X.G. Peng, Angew. Chem. Int. Ed. 41 (2002) 2368.[20] R. Singh, J. Kumar, R.K. Singh, A. Kaur, K.N. Sood, R.C. Rastogi, Polymer 46 (2005)

9126.[21] S. Amou, O. Haba, K. Shirato, T. Hayakawa, M. Ueda, T. Takeuchi, M. Asai, J. Polym.

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41 (2006) 198.[29] R. Maity, U.N. Maiti, M.K. Mitra, K.K. Chattopadhyay, Physica E 33 (2006) 104.[30] J. Yu, D.H. Hu, P.F. Barbara, Science 289 (2000) 1327.[31] G. Yu, A.J. Heeger, J. Appl. Phys. 78 (1995) 4510.[32] B.K. Kuila, A.K. Nandi, Macromolecules 37 (2004) 8577.[33] B.K. Kuila, A.K. Nandi, J. Phys. Chem. B 110 (2006) 1621.[34] J. Xu, J. Wang, M. Mitchell, P. Mukherjee, M. Jeffries-EL, J.W. Petrich, Z. Lin, J. Am.

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1980, p. 56.

Table 1Performance of the P3HT/PCBM solar cell with and without CdS QD doping and thermalannealing.

Devices Voc (Volts) Jsc (mA/cm2) FF (%) Efficiency (%)

Device A 0.45 2.57 30.0 0.45Device B 0.45 4.65 32.0 0.87Device A annealed 0.58 2.26 45.0 0.74Device B annealed 0.58 2.98 43.99 0.95

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Synthetic Metals 160 (2010) 1530–1534

Contents lists available at ScienceDirect

Synthetic Metals

journa l homepage: www.e lsev ier .com/ locate /synmet

lectrical, optical and hole transport mechanism in thin films ofoly(3-octylthiophene-co-3-hexylthiophene): Synthesis and characterization

ohd Taukeer Khana,b, Manisha Bajpaia, Amarjeet Kaurb, S.K. Dhawana,∗, Suresh Chanda

Polymeric & Soft Materials Section, National Physical Laboratory (CSIR), Dr. K. S. Krishnan Road, New Delhi 110012, IndiaDepartment of Physics & Astrophysics, University of Delhi, Delhi 110007, India

r t i c l e i n f o

rticle history:eceived 11 March 2010eceived in revised form 23 April 2010

a b s t r a c t

The present study demonstrates the designing of copolymer poly(3-octylthiophene-co-3-hexylthiophene) (P3OT-HT) and study of the hole transport mechanism in it. Detailed structural,optical and thermal studies of P3OT-HT discuss its synthesis aspects. Current density–voltage charac-

ccepted 13 May 2010vailable online 9 June 2010

eywords:ole transport

teristics have been studied at different temperatures (290–110 K) to understand the mechanism of holetransport in P3OT-HT. It has been established that current density in P3OT-HT thin films is governed byspace charge limited conduction with traps distributed exponentially in energy and space. Hole mobilityis both temperature and electric field dependent arising due to substituent functional groups attached

obilitypace charge limited current3OT-HT

the polymer backbone.

. Introduction

Poly(3-hexylthiophene) P3HT and poly(3-octylthiophene)3OT are the conjugated polymers well known [1–6] to be used inolymer solar cells as electron donor materials. Owing to its highegioregularity and high mobility, P3HT is so far the best donoraterial in combination with [6,6]-phenyl C61 butyric acid methyl

ster (PCBM) as the electron acceptor material. Power conversionfficiency ∼6% have already been realized [7] in polymer solarells based on P3HT:PCBM donor:acceptor interpenetrating bulketero-junctions with suitable charge transport and collection

nterface layers. However, the application of conjugated polymer3OT in polymer solar cells is more in combination with CNTs8–11] rather than the PCBM, may be due to its energetic com-atibility with CNTs. However, there are hardly any significanteports in literature about P3OT:PCBM combination based solarells. It may be primarily due to lower [12] hole mobility in it asompared to the corresponding high mobility in P3HT [13]. Inhe present study we have enhance the mobility of P3OT withopolymerization of P3HT, which will give better results in com-ination with CNTs and nanoparticles. Copolymers are of special

nterest because of their controllable nanoscale domain sizes, and

ffer the opportunity to optimize and tailor electronic and opticalroperties. Moreover, it also exhibits the potential to observeovel phenomena (e.g., energy transfer, charge transfer) which areot feasible in homopolymer [14]. Previous study on copolymer

∗ Corresponding author. Tel.: +91 11 4560 9401/9202; fax: +91 11 25726938.E-mail address: skdhawan@mail.nplindia.ernet (S.K. Dhawan).

379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2010.05.016

© 2010 Elsevier B.V. All rights reserved.

of 3HT and 3OT also shows that [15] copolymer approach is theoptimum balance between a sufficient processibility of polymerfrom non-polar solvents like toluene and xylene and a high solidstate order. With a view to enhance hole mobility, we have copoly-merized 3-octylthiophene (3OT) with 3-hexylthiophene (3HT)and designed conjugated copolymer namely P3OT-HT (shownin Scheme 1), and studied the mechanism of hole transport init. Interestingly, we found that hole mobility in this copolymerlies between that of P3OT and P3HT. In this communication, wereport interesting results on the synthesis and self-assembly ofcopoly(3-alkylthiophenes) and enhancement in hole mobility oncopolymerization. Two compositions of the P3OT-HT have beendesigned. Current density versus voltage (J–V) characteristics ofP3OT-HT were studied in hole only device configuration indiumtin oxide ITO (� ∼ 4.8 eV)/poly(3,4-ethylendioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS, 80 nm, � ∼ 5.0 eV)/P3OT-HT(∼150 nm)/Au (� ∼ 5.2 eV) in the temperature range of 290–110 K.Current density in P3OT-HT thin films have been found to begoverned by space charge limited conduction (SCLC) with trapsdistributed exponentially in energy and space. It was observed thatthe hole mobility in P3OT-HT thin films depend on temperature aswell as electric field.

2. Experimental

2.1. Materials and measurements

3HT, 3OT, Ferric chloride (FeCl3) and PEDOT:PSS were pur-chased from Sigma–Aldrich, USA and used as received. HPLC gradeschloroform (CHCl3), acetonitrile and tetra-n-butylammonium-

M.T. Khan et al. / Synthetic Metals 160 (2010) 1530–1534 1531

n solid

tmb2cIawratotiataitssTrfitPac

2

wbit

SP

Fig. 1. UV–vis absorption spectra of all polymers (a) thi

etrafluoroborate (TBATFB) were purchased from MERCK, Ger-any. The sample for XRD and UV absorption were prepared

y spin-coating of polymer solution onto glass substrate at000 rpm for 60 s and annealed at 393 K for 30 min. The J–Vharacteristics of the copolymer device having configurationTO/PEDOT:PSS/P3OT-HT/Au was studied for determining temper-ture and field dependent hole mobility. A thin film of PEDOT:PSSas spin coated onto pre-cleaned and plasma treated ITO (sheet

esistance ∼ 18 �/�) coated glass substrates at 2000 rpm and curedt 393 K for 30 min in vacuum. Subsequently, homogeneous solu-ion was made by dissolving P3OT-HT in binary solvent solutionf chloroform and chlorobenzene in 1:1 ratio with material con-ent of 30 mg/ml. Copolymer P3OT-HT thin films were spin castedn an inert atmosphere on the PEDOT:PSS films at 2000 rpm,nd cured at 393 K for 15 min. On top of these films, Au elec-rodes ∼250 nm were deposited by vacuum thermal evaporationt 2 × 10−6 Torr. These hole only devices were sealed using UVrradiated epoxy resin to inhibit undesired atmospheric oxida-ion. Absorption spectra were recorded by Shimadzu UV-1601pectrophotometer. Thermogravemetric analysis and differentialcanning calorimetry measurement was performed on a METTLEROLEDO, TGA/SDTA 851e and DSC822e respectively with heatingate of 10 ◦C/min under nitrogen atmosphere. XRD of polymerlms were taken in RINT 2000 Rigaku powder X-ray diffractome-er (40 kV, 30 mA, � = 1.54059 A, Cu-K�1). The J–V characteristics of3OT-HT in the temperature range of 290–110 K was performed inliquid nitrogen cryostat, under a reduced pressure of 10−6 Torr

oupled with Keithley 2400 source meter unit interfaced with PC.

.2. Synthesis of polymer P3HT, P3OT and copolymer P3OT-HT

The polymerization and copolymerization of 3-alkylthiophenes

as done by oxidative coupling method [16,17]. This method is

ased on the use of a Lewis acid such as ferric chloride (FeCl3) tonitiate the polymerization. For the polymerization of 3HT and 3OT,he monomer to oxidant ratio was taken 1:4. Equal molar ratio

cheme 1. Scheme of copolymer copoly(3-octylthiophene-3-hexylthiophene)3OT-HT.

film on glass substrate and (b) solution in chloroform.

of 3-octylthiophene (3OT, 0.05 M) and 3-hexylthiophene (3HT,0.05 M) were added drop wise in ferric chloride (0.4 M) suspen-sion in chloroform. The reaction mixture was kept stirring undernitrogen environment at 233 K for 24 h. After polymerization,to remove undesired impurity like unreacted oxidant, monomerand oligomers, polymer was subsequently washed with methanoland deionised water and dried in vacuum at 350 K for 3 h. FeCl3entrapped in the polymer matrix was removed by successivetreatment with aqueous ammonia and disodium salt of ethylenediamine tetra-acetic acid (EDTA) [18] at 335 K and dried in vac-uum at 350 K. Above procedures were repeated several times tominimize the FeCl3 impurity present in the polymer matrix. Thinfilms of synthesized polymers were prepared by solution casting ofchloroform solution on flat glass substrate at room temperature.

3. Results and discussion

3.1. UV–vis absorption spectroscopy

UV–vis spectra of all polymer thin films are shown in Fig. 1(a). Inconjugated polymers, the extent of conjugation directly affects theobserved energy of the �–�* transition, which appears as the max-imum absorption [19]. The wavelength of maximum absorption(�max) in solid film of the P3OT, P3OT-HT and P3HT are observedat 511 nm, 512 nm and 518 nm, respectively. The blue shift ofthe absorption of the P3OT-HT film than that of P3HT has beenattributed to steric hindrance of octyl side chain attached to copoly-mer matrix which may be difficult to rotate compared to hexyl sidechain to form the more advantageous arrangement. Polymer filmalso shows an absorption shoulder at 600 nm, 595 nm and 598 nmfor P3OT, P3OT-HT and P3HT respectively are assigned to the 1Bu

vibronic sidebands [19] which confirm the interchain absorption inpolymer [20,21]. Fig. 1(b) shows the absorption spectra of all poly-mers in chloroform solution. The maximum absorption of P3OT,P3OT-HT and P3HT in chloroform appeared at 442 nm, 439 nm and443 nm respectively which has been attributed to HOMO �-LUMO�* transition [22]. The absorption spectra of polymers showed blueshift in solution compared with that of the solid films. The blue shiftin solution is attributed to coil like structure in solution whereassolid films have rod like structure. Coil like structure have shorteffective conjugation length compared to rod like structure withhigher conjugation length, this results in increase of �–� stackingand blue shift in solution phase.

3.2. Thermal studies

In order to investigate the thermal stability of polymers, ther-mogravimetric analysis (TGA) experiments were performed under

1532 M.T. Khan et al. / Synthetic Metals 160 (2010) 1530–1534

F nder nn

nf4gFapsm1so2

3

or5adrccg(1

space. For a case when the traps are distributed exponentially in

ig. 2. (a) TGA of P3OT, P3HT and copolymer P3OT-HT with scan rate 10 ◦C/min uitrogen atmosphere.

itrogen gas. As shown in Fig. 2(a), the onset point of weight lossor P3OT, P3OT-HT and P3HT are observed at 427 ◦C, 434 ◦C and40 ◦C respectively, possibly caused by decomposition of the endroups; indicating that the polymer have good thermal stability.rom above results it is concluded that long alkyl side group poly(3-lkylthiophenes) decompose earlier than short alky side groupoly(3-alkylthiophenes). Differential scanning calorimetry (DSC)can of the polymers are shown in Fig. 2(b). In the DSC of the copoly-er P3OT-HT, two melting transitions with endothermic peaks at

64 ◦C and 228 ◦C were observed. The observed two melting tran-itions are characteristic of its copolymer architecture composedf P3OT and P3HT, which have melting transitions at 186 ◦C and15 ◦C, respectively.

.3. XRD studies

Fig. 3 shows X-ray diffraction (XRD) pattern of solution cast filmsf the above polymers, precured at 120 ◦C. The strong first ordereflection, (1 0 0), of P3OT, P3HT and P3OT-HT are at 2� angle 4.24◦,.08◦, and 4.7◦, corresponding to interlayer spacing 20.83 Å, 17.38 Å,nd 18.786 Å respectively [23]. Observed intensity of copolymer isecreased compared to homopolymer P3HT and P3OT due to regio-andom structure (HT:HH 65:35) of copolymer P3HTOT which wasalculated by 1H NMR study in CDCl3. Regio-random structure of

opolymer is attributed to the random repeating of hexyl and octylroup attached to the polymer matrix. The second order reflection2 0 0) of P3OT, P3HT, and P3OT-HT are observed at 2� angle 8.62◦,0.52◦, and 9.48◦, corresponding to interlayer spacing 10.25 Å,

Fig. 3. XRD spectra of solution cast polymer films, annealed at 120 ◦C.

itrogen atmosphere (b) DSC scan of polymers with heating rate 10 ◦C/min under

8.40 Å, and 9.34 Å respectively. Observed dP3OT-HT values (18.786 Åand 9.34 Å) in the copolymer P3OT-HT are smaller than the P3OThomopolymer and larger than the P3HT homopolymer, suggestingpartial inter-digitation between the side chains and/or the occur-rence of tilting of the octyl chains in P3OT-HT. XRD study shows thatthe interlayer spacing increases with elongation of alkyl side chain.This shows that the stacks of planer thiophene main chain wereuniformly spaced by alkyl side chain. Copolymer P3OT-HT showstwo strong peaks at 2� angle 16.860◦, 14.04◦ which corresponds todifferent two d020 values of 5.254 Å and 6.303 Å respectively. The6.303 Å spacing is due to the interlayer stacking distance betweenP3OT in a layered packing structure (dP3OT), whereas the 5.254 Åspacing comes from the interlayer stacking distance betweenP3HT (dP3HT). These peaks confirm the formation of copolymerP3OT-HT.

3.4. Hole transport in thin film of P3OT-HT

Fig. 4 shows the J–V characteristics of P3OT-HT thin film inhole only configuration as mentioned above at different temper-atures in the range 290–110 K. These experimental results wereanalysed based on the theory of space charge limited conduction(SCLC) [24–27] with traps distributed exponentially in energy and

the energy space within the forbidden gap, distribution functionfor the hole trap density as a function of energy level E above thevalence band, and at a distance x from the injecting contact can be

Fig. 4. Experimental (symbols) and calculated [solid line using Eqs. (2) and (3)] J–Vcharacteristics of hole only device of copolymer P3OT-HT for the temperature range290–110 K.

M.T. Khan et al. / Synthetic Metals 160 (2010) 1530–1534 1533

F tivatiom enden(

w

h

wcao

∫Arbb

J

wmifioTd

ofiuti

�

wvirt(dnat

�

wc

ig. 5. (a) Experimental (symbols) and calculated [solid line using Eq. (4) with acobility versus temperature T. (b) The coefficient � (which described the field dep

5), using T0 = 500 K and ˇ = 6.9 × 10−5 eV/V1/2 cm1/2.

ritten as:

(E, x) = Hb

kBTCexp

−E

EtS(x) (1)

here Hb is the density of traps at the edge of valence band, Et isharacteristic trap energy and kB is the Boltzmann constant. Et islso often expressed in terms of the characteristics temperature TCf trap distribution Et = kBTC.

Density of trapped holes at position x is given by pt(x) =h(E, x)f (E)dE where f(E) is the Fermi-Dirac distribution function.

ssuming that the trapped hole carrier density (pt) » free hole car-ier density (p) and using continuity equation J = q�p(x)F(x) andoundary condition V =

∫F(x)dx the J–V characteristic is governed

y [27,28]:

= q1−l�Nv

(2l + 1l + 1

)l+1( l

l + 1εε0

Hb

)l V l+1

d2l+1(2)

here J is the current density, V is the applied voltage, q the is ele-entary charge, d is the thickness of the material film, � is the field

ndependent hole carrier mobility, F(x) is the electric field inside thelm, Nv is the effective density of states, ε is the dielectric constantf material, ε0 is permittivity of the free space and l = Et/kBT = TC/T.he parameter l determines the distribution of traps in the forbid-en gap.

When the experimental data in Fig. 4 has been analysed in termsf Eq. (2), it has been found that the theory fits up to intermediateelds and at high fields (corresponding to ≥6 V), the current grad-ally deviates from the above proposed theory and becomes largerhan as expected from Eq. (2). This discrepancy has been analyzedn terms of field dependent mobility [29–31]:

p(E, T) = �(0, T)exp(�(T)√

E) (3)

ith �(0,T) the hole mobility at zero field and �(T) the field acti-ation factor, which reflects the lowering of the hopping barriersn the direction of the applied electric field. The applied field givesise to increase of the charge carrier density. In order to describehe hole conduction in P3OT-HT at high fields, we combine the SCLCEq. (2)) with the field dependent mobility (Eq. (3)). Temperatureependence of zero field mobility is shown in Fig. 5(a) in an Arrhe-ius plot. They decrease with decrease of temperature. We observethermally activated behaviour of zero field mobility according to

he equation:

(0, T) = �0exp(

− �

kBT

)(4)

here � is the thermal activation energy and kB is the Boltzmannonstant.

n energy � = 0.21 eV and �0 = 3.6 × 10−5 cm2/Vs] Arrhenius plot of the zero fieldce of the mobility) as a function of temperature T. The solid line is according to Eq.

Temperature dependent high field J–V characteristics is under-stood in terms of the coefficient �(T). From the J–V curve (Fig. 4) andEqs. (2) and (3) we calculate the value of �(T) at each temperature.Fig. 5(b) shows the variation of �(T) as a function of temperature.The experimental results show that there is a linear dependenceaccording to equation [31]:

�(T) = ˇ(

1kBT

− 1kBT0

)(5)

with ˇ = 6.9 × 10−5 eV/V1/2 cm1/2, T0 = 500 K.Expressions (3)–(5) describe the Arrhenius dependence of the

mobility which arises if moving charges must hope over a coulombbarrier of height � in energy. In such a case, an electric field depen-dence arises because the barrier height is lowered electric field byan amount ˇ

√E.

The set of J–V characteristics as a function of temperaturecan be fully described by Eqs. (2)–(5) using the parame-ters Hb = 3.8 × 1018 cm−3, Nv = 3 × 1019 cm−3, ε = 3, ε0 = 8.85 × 10−14

F/cm, TC = 560 K, Et = 46 meV, d = 150 nm, �0 = 3.6 × 10−5 cm2/Vs,T0 = 500 K � = 21 eV and ˇ = 6.9 × 10−5 eV/V1/2 cm1/2.

A microscopic interpretation of this ubiquitous mobility is thatthe charge transport in disordered organic conductors is thoughtto proceed by means of hopping in a Gaussian site-energy distri-bution. This density of states (DOS) reflects the energetic disorderof hopping site due to fluctuation in conjugation lengths, struc-tural disorder [31,32]. Copolymerizing of P3OT and P3HT couldcreate regio-random structural due to random repetition of hexyland octyl unit and energetic disorder due to different energy levelsP3HT and P3OT. Due to these structural and energetic disorders incopolymer, the hole mobility is strongly dependent on temperatureand electric field. The introduction of the hexyl group into the P3OTmatrix can also lead to structural defects and hence increase of trapdensity, so that a fraction of the charges moving inside the P3OT-HTfilms are trapped thereby reducing the mobility. Thus copolymer-ization is expected to diminish the mobility and increase its electricfield dependence for hole.

It is thus explicitly established from above that hole transportin P3OT-HT copolymer thin films shows field and temperaturedependent mobility at higher fields with hole transport fittingparameters as ˇ = 6.9 × 10−5 eV/V1/2 cm1/2, �0 = 3.6 × 10−5 cm2/Vs,

T0 = 500 K and � = 21 eV, respectively. It is important to mentionhere that the hole mobility in P3OT-HT copolymer estimated asabove lies between that of P3HT and P3OT and hence it can findapplication in polymer solar cells with a new donor:acceptor com-bination.

1 c Meta

4

opcaa(umP

A

fMt

R

[

[

[

[

[

[

[

[

[

[

[

[

[[[

[

[

[[

534 M.T. Khan et al. / Syntheti

. Conclusions

In conclusion, we demonstrate the synthesis of a copolymerf 3-alkylthiophenes and establish the mechanism of hole trans-ort in it. It has been suggested that hole transport in P3OT-HTopolymer in governed by SCLC with traps distributed in energynd space and hole mobility being strongly dependent on temper-ture and electric field. The estimated value of zero field mobility�0) ∼ 3.6 × 10−5 cm2/Vs, is quite good and shows the promise ofsing P3OT-HT as a new donor material in the development of poly-er solar cells. Efforts are in progress to fabricate solar cells using

3OT-HT as donor with PCBM and even CNTs as acceptor materials.

cknowledgments

The authors MTK and MB are thankful to CSIR, New Delhi, India,or the award of Junior Research Fellowship. We sincerely thank Dr.

.N. Kamalasannan, Dr. Ritu Srivastava and Dr. Rajeev K. Singh forheir cooperation and useful discussion.

eferences

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OFFPRINT

Polymeric-nanoparticles–induced verticalalignment in ferroelectric liquid crystals

A. Kumar, J. Prakash, P. Goel, T. Khan, S. K. Dhawan, P.

Silotia and A. M. Biradar

EPL, 88 (2009) 26003

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October 2009

EPL, 88 (2009) 26003 www.epljournal.org

doi: 10.1209/0295-5075/88/26003

Polymeric-nanoparticles–induced vertical alignment

in ferroelectric liquid crystals

A. Kumar1, J. Prakash1(a), P. Goel1, T. Khan2, S. K. Dhawan2, P. Silotia3 and A. M. Biradar1(b)

1 Liquid Crystal Group, National Physical Laboratory - Dr. K. S. Krishnan Road, New Delhi-110012, India2 Conducting Polymer Group, National Physical Laboratory - Dr. K. S. Krishnan Road, New Delhi-110012, India3Department of Physics and Astrophysics, University of Delhi - Delhi-110007, India

received 13 July 2009; accepted in final form 5 October 2009published online 5 November 2009

PACS 61.30.-v – Liquid crystalsPACS 77.84.Nh – Liquids, emulsions and suspensions; liquid crystalsPACS 42.79.Kr – Display devices, liquid-crystal devices

Abstract – Here, we report the polymeric (copolymer of benzene and pentacene) nanoparticles(PNPs) induced vertical alignment in ferroelectric liquid crystals (FLCs). The nanoparticles usedin this study have been synthesized via chemical route method. The PNPs have been doped inFLC mixture. It has been observed that pentacene molecules (presented in PNPs used) prefer anupright orientation on the indium-tin-oxide–coated surfaces, which in turn provide assistance toalign FLC molecules vertically (or homeotropically). It has also been observed that the additionof PNPs into the FLC materials improves the electro-optical response. However, the transitiontemperatures of the PNPs-doped FLC materials have been lowered. These findings will provide afascinating tool to align FLC materials devoid of any surface treatment. Moreover, these studieswould be helpful in the realization of low threshold and faster liquid crystal display devices.

Copyright c© EPLA, 2009

The impact of nanotechnology on liquid crystal (LC)media has resulted in fascinating improvements in theoverall performance of LCs-based display and non-displayapplications. It has long been appreciated that the minuteaddition of nanoparticles (NPs) to LC materials hasimproved many special characteristics in the form offrequency modulation response [1], non-volatile memoryeffect [2], faster electro-optic response [3], low driving volt-age [4], enhanced photoluminescence [5] and reduced resid-ual dc [6]. For electro-optical devices based on LCs, thevertical (also called as homeotropic) alignment of LCshas been a challenging topic of research from both thefundamental and technological point of view for a longperiod of time. The vertically aligned LCs has been exten-sively used for liquid crystal displays (LCDs) such aslarge-area LCD televisions, information display devicesand digital displays in medical devices due to its unprece-dented contrast ratio and wide viewing angle charac-teristics [7]. The various techniques have been investi-gated to accomplish the vertical alignment of LCs by the

(a)Also at Instrument Design Development Center, IIT Delhi -

New Delhi-110016, India.(b)E-mail: abiradar@mail.nplindia.ernet.in

researchers around the world. It has been reported thatthe surfaces of the substrates such as glass, oxides, andmetals exhibits the ability to align LC molecules verticallyprovided some cleaning processes on the substrates [8].However, the vertical alignment in these cases showed apoor reproducibility and uniformity. The surface treat-ment of silanes and large alkyl side chain alcohols ashomeotropic coupling agents has been a most widelyused technique for homeotropic alignment of LCs [9].The oblique evaporation of silicon mono-oxide films onthe substrate surface has been used to align LCs verti-cally [10]. Hwang et al. have studied the vertical align-ment of LCs on an amorphous silicon oxide (a-SiOx) thinfilm and found this as the consequence of anisotropicinteractions (such as LC-LC interaction, Debye interac-tion, and London dispersion) between LCs and a-SiOxthin films [11]. The vertical alignment of LCs using fluo-rinated diamond like carbon thin films is also an exampleof such techniques [12]. Cheng et al. have explored thepossibility of favorable vertical alignment of LCs havinglarge negative dielectric anisotropy (∆ε) with a minuteaddition of the material possessing a longitudinal dipoleand therefore a positive ∆ε in the LCs [13]. The verti-cal alignment of LCs by incorporating NPs has emerged

26003-p1

A. Kumar et al.

as a fascinating field of research as it provides a bettertool to align LCs without the use of any alignmenttechniques. It has been shown that the NPs of polyhe-dral oligomeric silsesquioxane (POSS) can induce verticalalignment in nematic LCs having positive anisotropy [14].The electro-optical properties of POSS NPs-induced verti-cally aligned LC cells are very similar to the conventionalhomeotropic cells with alignment layers. It has also beendiscussed that the aligning capability induced by POSS isdue to adsorption of POSS on the inner surfaces of thesubstrates and highly influenced by doping concentrationof POSS [15]. Few studies have been reported the uprightorientation of pentacene molecules (C22H14), a promisingcandidate for organic electronics to fabricate organic thin-film transistors [16], photodiodes [17] and high-frequencyrectifiers [18], on the weakly interacting surfaces of thesubstrates such as glass, ITO etc. This property (havingupright orientation) of pentacene has attracted our atten-tion to use it for achieving homeotropic alignment in ferro-electric liquid crystals (FLCs) by a minute addition of NPshaving pentacene.In this letter, we report how the polymeric (copolymerof benzene and pentacene) nanoparticles (PNPs) help toinduce vertical alignment in FLCs. We have also studiedthe effect of these PNPs on the intrinsic parameters liketransition temperature, spontaneous polarization, thresh-old voltage, response time etc. of the FLC mixtures.The PNPs have been synthesized by the chemicalroute method and the typical size of the particlesprepared was between 80 and 120 nm. The proposedmolecular structure of synthesized PNPs is shown below:

n

The weight average molecular mass of the PNPsused is 1086–1314 depending on the n value whichremains 7–10. The PNPs show solubility in many organicsolvents like chlorobenzene, N-methyl-1-pyrollidine andchloroform etc. A few wt% of PNPs were dissolved inthe FLC mixtures through the chloroform solvent andsonicated in ultrasonic bath for 15min. The sonicatedmixture was baked at 65 ◦C for 3 h ensuring the completeevaporation of chloroform. The mixture of FLCs andPNPs has been introduced into the LC sample cells bymeans of capillary action at temperatures just above theisotropic temperatures of the respective FLC material.The LC sample cells were prepared using indium tin oxide(ITO)-coated glass substrates cleaned well with acetone.

(a) (b)

(d)(c)

Fig. 1: (Color online) Optical micrographs of (a) Felix 20,(b) Felix 17/100, (c) CS 1016, and (d) LAHS 19 at roomtemperature.

The cell gaps were maintained using 3.5µm thick Mylarspacers. The optical micrographs of the FLC materialshave been recorded using a polarizing optical microscope(Ax-40, Carl Zeiss, Germany) fitted with a CCD camera.The dielectric measurements have been carried out byusing impedance analyzer 6540A (Wayne Kerr, UK). Theprobe amplitude during the dielectric measurements waskept 0.5V. The material constants such as spontaneouspolarization, rotational viscosity, and reponse time havebeen determined by using an automatic liquid crystaltester (ALCT, Instec, USA). The probe amplitude andAC frequency during the determination of materialconstants were 8V and 10Hz, respectively.The phase sequences of the used FLC mixtures are asfollows:

cryst−8 ◦C←→ SmC∗

15−18 ◦C←→ SmA

75 ◦C←→ N

92−103 ◦C←→ Iso,

(Felix 20)

cryst−28 ◦C←→ SmC∗

73 ◦C←→ SmA

77 ◦C←→ N

84−87 ◦C←→ Iso,

(Felix 17/100)

cryst−21 ◦C←→ SmC∗

56 ◦C←→ SmA

67 ◦C←→ N

73 ◦C←→ Iso, (CS 1016)

cryst3−7 ◦C←→ SmC∗

60 ◦C←→ SmA

62.5 ◦C←→ Iso. (LAHS 19)

The optical micrographs of different FLC materialsdoped with PNPs have been shown in fig. 1. One canclearly see from the figure that first three FLC materialshave attained a vertical alignment. However, in the caseof LAHS 19, the vertical alignment has not been favoredmuch since the traces of planar (multidomain) alignmentcan be clearly seen in the optical micrograph (fig. 1(d)).The LAHS 19 is a special kind of FLCs named as deformed

26003-p2

Polymeric-nanoparticles–induced vertical alignment in ferroelectric liquid crystals

102

103

104

105

106

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5 (a)

ε '

Frequency (Hz)

0 V 1 V 2 V 4 V 6 V 8 V 10 V 12 V 0 V again

103

104

105

106

0.05

0.10

0.15

0.20(b)

tan δ

Frequency (Hz)

30 0C

32 0C

34 0C

36 0C

38 0C

40 0C

42 0C

44 0C

46 0C

48 0C

50 0C

52 0C

55 0C

Fig. 2: (Color online) Frequency dependence of (a) dielectric permittivity (ε′) for different bias voltages at room temperature,(b) dielectric loss factor (tan δ) measured in the temperature range 30 ◦C–55 ◦C in the homeotropically aligned cell of PNPs-doped Felix 17/100.

helix ferroelectric liquid crystal (DHFLC) having ultrashort pitch (∼ 0.3–0.7µm) and high value of spontaneouspolarization (∼ 90 nC/cm2). So, it is difficult to explainwhy PNPs induced vertical alignment is very poor inthe case of DHFLC. It might be due to denser packing(because of ultra short pitch) of the molecules of DHFLCwhich results in the poor dispersion of PNPs in the mate-rial and hence a moderate vertical alignment. The verti-cal alignment in FLC materials has also been confirmedby dielectric relaxation spectroscopy. Figure 2 shows thebehavior of dielectric permittivity (ε′) with frequency atdifferent applied voltages at room temperature (fig. 2(a))and of dielectric loss factor (tan δ) with frequency at differ-ent temperatures (fig. 2(b)) for FLC material Felix 17/100.It is clear from fig. 2(a) that the value of ε′ does notsuppress even at higher values of applied voltages suggest-ing the alignment to be homeotropic. In the case of thevertical alignment of the liquid crystals, the long molecu-lar axis is perpendicular to the substrates surface prevent-ing the Goldstone mode to take place and leaving theonly possibility of molecular rotation around their shortmolecular axis which gives the lower values of ε′. Also,on the application of bias, the ε′ value does not suppressremarkably for the vertically aligned LC cells. On the otherhand, in the case of homogeneous alignment of LCs, thelong molecular axis is parallel to the substrates surfacegiving the larger values of ε′ due to the Goldstone modewhich, on the application of bias, get suppressed to givelower values of ε′. We have also observed the behaviorof ε′ with frequency at different biases for FLC mixturesFelix 20 and found a similar behavior of ε′ with bias asthat of Felix 17/100. But the DHFLC material LAHS 19shows the presence of Goldstone mode due to poor verti-cal alignment. Figure 2(b) shows the behavior of tan δ forthe vertically aligned Felix 17/100 which shows beauti-ful relaxation peaks in sub-kHz frequency regimes that

(a) (b)

(c)

Fig. 3: (Color online) Optical micrographs of PNPs-doped Felix17/100 in the SmC∗ phase at (a) 0V, (b) 20V, and (c) 25V(which changes the orientation from homeotropic to planar) atroom temperature (30 ◦C).

correspond to the molecular relaxations due to the motionof molecules around their short axis.It is worth mentioning here that the vertically alignedcells can be transformed into homogeneous configurationby applying a high electric field across the cell. It isnoticeable that the transformation from homeotropicto homogeneous configuration has been favored in LCmaterials because of the negative dielectric anisotropy(∆ε). In materials having such transformation, the dipolemoment along the short molecular axis (µS) dominates.On the application of sufficient electric field acrossthe cell, the LC molecules attain orientation parallelto the substrate surface due to a stronger coupling ofµS with the electric field. Figure 3 shows the optical

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30 35 40 45 50 55

3.7

3.8

3.9

4.0

4.1

4.2

4.3

4.4

4.5 (a)

ε

ε′⊥ε′

ε′⊥ε′

'

Tempearture (0C)

30 35 40 45 50 553.2

3.4

3.6

3.8

4.0

4.2

4.4(b)

ε'

Temperature (0C)

Fig. 4: (Color online) A comparison of the dielectric permittivity (ε′⊥: planar state; ε′‖: homeotropic state) as a function of

temperature at frequency (a) 10 kHz and (b) 100 kHz. This figure estimates the dielectric anisotropy.

micrograph of the transformed (on the application ofelectric field) Felix 17/100 material under crossed polar-izers which exhibits multidomain nature due to randommolecular arrangement as the substrate surfaces wereuntreated. The application of such transformation (fromhomeotropic to homogeneous) can be used to calculatethe dielectric anisotropy of the FLC material using onlysingle sample cell. We have determined the parallel andperpendicular components of ε′ corresponding to verticaland homogeneous alignment at various frequencies (10and 100 kHz) for PNPs-doped FLC Felix 17/100. Thecomparison of parallel and perpendicular componentsε′ (ε′‖ and ε

′⊥) has been shown in the fig. 4.

The explanation why the PNPs have induced thevertical alignment in the FLCs lies in the nature ofthe interactions of pentacene molecules with differentsubstrate surfaces. The growth morphology of pentaceneat various surfaces has been investigated extensively byvarious groups around the world [19–21]. The molecularstructure of thin pentacene films grown on a Cu (110)surface has been investigated by using various experimen-tal techniques such as He atom scattering, low-energyelectron diffraction, thermal desorption spectroscopy,X-ray photoelectron spectroscopy, and X-ray absorptionspectroscopy [22] which reveal that the orientation ofpentacene molecules on the Cu (110) depends on the filmthickness of the pentacene. In the monolayer regime, themolecules form ordered structure with planar adsorptiongeometry, whereas for film thicknesses greater than 2 nm,subsequent growth proceeds with the stand-up orienta-tion of pentacene molecules. Zheng et al. found that themolecular orientations of pentacene thin-film structuresare strongly influenced by the metal substrates [23].They proposed that the pentacene molecules stand up onthe surface and form thin-film phase structure on silverwhereas grow in domains with molecules, either lying

flat or standing up, on the gold substrates. It has beenobserved that the deposition of pentacene molecules onSiO2 results in well-ordered monolayers as well as thickerfilms with the upright orientation of the molecules [24].Recently, Zhang et al. have investigated the dynamicgrowth process and morphology of pentacene on differentsubstrates [25]. They showed experimentally that thepentacene molecule is perpendicular to the silicon wafersurface with a slight tilt angle. However, they couldnot found any indication of perpendicular orientation ofthe pentacene molecules in the case of polymer-treatedITO-coated substrates. The roughness of the underlyingsubstrate also plays an important role in the growth oforganic film on it. The surfaces having the roughness ona molecular scale do not provide well-defined adsorptionsites for large organic molecules such as pentacene andhence an upright molecular geometry can then be favoredfor the first monolayer [24]. The precise determinationof the growth morphology of pentacene on differentsubstrate surfaces is still a very challenging problemto be investigated as it depends on a lot of factorsinvolved in the deposition process, deposition parameters,nature of substrates etc. We have used bare ITO-coatedglass substrates cleaned with acetone to assemble theLC sample cells and filled them with PNPs-dopedFLC materials above their isotropic temperatures. Weproposed that during the filling of PNPs-doped FLCmaterials, the PNPs near the ITO surfaces may form athin-film phase with stand-up orientation of the moleculesbecause the pentacene molecules grown on substrateswith low-surface energy tend to align vertically [18], whichworks as the template for the FLC molecules resultingin the uniform vertical alignment of FLC materials.However, this phenomenon of vertical alignment of FLCsby addition of PNPs is rather unclear and needs furtherinvestigations.

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Polymeric-nanoparticles–induced vertical alignment in ferroelectric liquid crystals

-1 0 1 2 3 4 5 6 7 8 9 10

0

4

8

12

16

20

24(a)

PS (

nC

/cm

2)

Applied Voltage (V)

0 1 2 3 4 5 6 7 8 9 10

0

100

200

300

400

PNPs doped Felix 17/100 Pure Felix 17/100

(b)

η (m

Pa

*s)

Applied Voltage (v)

0 1 2 3 4 5 6 7 8 90

20

40

60

80 (c)

τ R(x

10

-4)s

ec

Applied Voltage (V)

PNPs doped Felix 17/100 Pure Felix 17/100

Fig. 5: (Color online) Behavior of (a) spontaneous polarization(Ps), (b) rotational viscosity (η) and (c) response time (τR)with applied voltage for PNPs-doped Felix 17/100 in planarconfiguration at room temperature.

The influence of the PNPs on the physical parametersof the FLCs has also been taken into account. For this,we have determined various physical parameters in pure

102

103

104

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106

0.0

0.5

1.0

1.5

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2.5

3.0

tan δ

Frequency (Hz)

35 0C

40 0C

45 0C

50 0C

55 0C

57 0C

58 0C

60 0C

Fig. 6: (Color online) Frequency dependence of the dielec-tric loss factor (tan δ) measured in the temperature range30 ◦C–60 ◦C in the planar state of the cell of PNPs-doped Felix17/100.

and PNPs-doped Felix 17/100. The effect of PNPs onthe material constants of Felix 17/100 has been shownin fig. 5. It is clear from figs. 5(a) and (b) that thespontaneous polarization and rotational viscosity of thePNPs-doped Felix 17/100 material have been lowered.It is noticeable that the combined effect of reduction inthese parameters has resulted in the faster response of thePNPs-doped Felix 17/100 material as can be seen fromfig. 5(c). Also, the addition of PNPs is more advantageousas the value of threshold voltage of PNPs-doped Felix17/100 material has been lowered remarkably as reflectedfrom fig. 5.The addition of PNPs into FLCs has affected theorder of FLC molecules and hence produces a smallerorder which has been reflected in the reduction of theSmC∗-SmA transition temperature (TC) of PNPs-dopedFLC materials. It has been reported earlier that theaddition of POSS NPs into the nematic LC perturbed theorder parameter (S ) and produced a smaller S in POSS-doped nematic LC [14,26]. Figure 6 shows the behavior oftan δ with frequency at different temperatures for Felix17/100 FLC material in homogeneous configuration inorder to show the reduction of the SmC∗-SmA transitiontemperature (TC) of PNPs-doped FLC materials. One canclearly see that TC has been shifted from 73

◦C to 58 ◦C.The reduction in TC has also occurred in the case of otherFLC mixtures under investigations except Felix 20 forwhich the reduction in TC is very low (∼ 1%).The PNPs-induced vertical alignment of FLC moleculeshas been demonstrated. It has been proposed that PNPsinduce homeotropic alignment of FLC mixtures which is aconsequence of the surface interaction between the ITOand PNPs in such a fashion that the PNPs stand upon the ITO surface with tilted orientation. The upright

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orientation of pentacene molecules provides assistance toFLC molecules in attaining a uniform vertical alignment.We have also observed that the addition of PNPs into theFLC mixtures improves the physical parameters of theFLCs. The response time as well as threshold voltage hasbeen lowered in PNPs-doped FLC materials. The PNPs-induced vertical alignment of FLCs will certainly providea promising tool to align FLCs devoid of the use of anysurface treatment techniques as well as to achieve lowthreshold and faster liquid crystal display devices.

∗ ∗ ∗

The authors sincerely thank Dr. Vikram Kumar,Director of the National Physical Laboratory, for contin-uous encouragement and interest in this work. Wesincerely thank Dr. S. S. Bawa, Dr. I. Coondoo,Mr. A. Choudhary, Mr. G. Singh, Ms. A. Malikand Mr. T. Joshi for fruitful discussions. We are alsothankful to the Surface Science Group, NPL for providingultrasonication facility. The authors (AK and JP) arethankful to UGC, New Delhi and CSIR, New Delhi forfinancial assistance.

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