Solar Energy Materials & Solar Cells 94 (2010) 2386–2394

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Solar Energy Materials & Solar Cells

0927-02

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/solmat

Poly(3-hexylthiophene): Functionalized single-walled carbon nanotubes:(6,6)-phenyl-C61-butyric acid methyl ester composites for photovoltaiccell at ambient condition

Rajiv K. Singh a,n, Jitendra Kumar b, Amit Kumar a, Vikram Kumar a, Rama Kant c, Ramadhar Singh a

a National Physical Laboratory (Council of Scientific and Industrial Research), Dr. K.S. Krishnan Marg, New Delhi 110012, Indiab Metals and Ceramics Division, University of Dayton Research Institute, Dayton, OH 45469-0171, USAc Department of Chemistry, University of Delhi, New Delhi 110007, India

a r t i c l e i n f o

Article history:

Received 8 March 2010

Received in revised form

19 August 2010

Accepted 22 August 2010Available online 17 September 2010

Keywords:

Poly(3-hexylthiophene)

Functionalized single-walled carbon

nanotubes

P3HT:FSWCNT:PCBM composite

Organic photovoltaic device

48/$ - see front matter & 2010 Elsevier B.V. A

016/j.solmat.2010.08.023

esponding author. Tel.: +91 11 4560 8207; fa

ail address: rajivsingh@mail.nplindia.ernet.in

a b s t r a c t

We report the synthesis and characterization of nonhygroscopic composites of poly(3-hexylthiophene):

functionalized single-walled carbon nanotubes:(6,6)-phenyl-C61-butyric acid methyl ester (P3HT:

FSWCNT:PCBM) for photovoltaic applications. The composite films have been characterized for their

structural, electronic, photo-physical and photovoltaic properties. Fourier transform infrared (FT-IR)

investigation suggests that the nanotubes can induce structural changes in P3HT matrix. The

homogeneous dispersion of nanotubes in P3HT and its self-arranged matrix in P3HT:PCBM are evident

from scanning electron microscopy (SEM). Ultraviolet–visible (UV–vis) spectrum indicates the betterment

of P3HT chain stacking by addition of nanotubes, which is further confirmed by transmission electron

microscopy (TEM). The small-angle X-ray scattering (SAXS) was used to determine the bulk

microstructure of the polymer composite. The photovoltaic cells have been fabricated using the

aforementioned photoactive composite and tested at ambient conditions. The comparison of the current

density–voltage (J–V) characteristics of photovoltaic cells in light and dark conditions, with and without

modified nanotubes, shows that the latter gives better photovoltaic properties. A photovoltaic cell using

modified nanotubes exhibit a photo-conversion efficiency of �1.8%. The addition of FSWCNT in

P3HT:PCBM composite enhances the conjugation length of P3HT:FSWCNT:PCBM composite, which in

turn enhances its absorption capacity of solar energy radiation.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

In the quest for finding alternative solar energy generationdevices in place of expensive silicon photovoltaic cells, organicphotovoltaic (OPV) cells have been proving themselves the front-runner [1–3]. Among the promising aspects of OPV, low cost ofmaterials, easy and inexpensive large area device fabricationmethodologies, considerable flexibility to modify the materialchemistries and hence easy tuning of device parameters are majorattractions for both academics and industrial laboratories [4,5].Further, finding of ultra-high charge transfer between polymer(electron donor (D)) and fullerene (C60) (electron acceptor (A)) hasgiven impetus to the attempt for fabrication of efficient OPV [1].Considerable efforts have been made to find best D–A pair in orderto create and transport charges to respective charge collectingelectrodes and generate electrical energy from solar radiationefficiently [1–12]. Among available materials, regioregular

ll rights reserved.

x: +91 11 4560 9310.

(R.K. Singh).

poly(3-hexylthiophene) (rr-P3HT):fullerene derivative; (6,6)-phe-nyl-C61-butyric acid methyl ester (PCBM) composite has beenfound to be a potential option [11–13]. Energy conversionefficiency for the rr-P3HT:PCBM bulk heterojunction OPV has beenreported over 7% [14]. Although the electron affinity of C60 isgreater than PCBM, advantage of using PCBM is its high solubilityin organic solvent as compared to its unsubstituted counterpart C60

that gives the former advantage over the latter in makinghomogeneous D–A bulk heterojunction. In order to achieve highperformance OPV, a rr-P3HT:PCBM in the weight percentage ratioof 1:1 is required. The high percentage of PCBM results intoconsiderable lowering in its solar energy absorption, which lowersthe overall cell efficiency.

To fulfill the deficiencies of PCBM as a source of electron acceptorand transporter, single-walled carbon nanotubes (SWCNT) andnanorods [15–20] may be employed as it is better electronicconductor and because of its high aspect ratio the quantity requiredfor making efficient bulk heterojunction is very low as compared toPCBM, which lowers the overall cost of the OPV device. Recently, ared shift in p�pn band of P3HT is reported [21] in UV–vis spectrumof pristine P3HT-CNT composite. But the main obstacle in using

R.K. Singh et al. / Solar Energy Materials & Solar Cells 94 (2010) 2386–2394 2387

SWCNT as electron acceptor material for OPV lies in its inability todisperse homogeneously into the polymer matrix. In order todisperse SWCNT in the polymer matrix, it has to be dissolved in asolvent in which polymer also gets dissolved and this can beachieved by attaching functional groups to SWCNT backbone[22,23].

The functionalization of SWCNT is an attempt to unfoldnanotubes from its as-synthesized bundled form and hence makinga more intractable material having larger exposed surface area soas to get a homogeneous dispersion in the desired polymer matrixand forming the best possible D–A charge transfer complex. Thefunctionalized SWCNT (FSWCNT) has shown a remarkable effect onphoto-physical properties of P3HT:FSWCNT composite by formingan efficient D–A charge transfer complex, which enhances thecomposite conductivity manifold. This is attributed to the homo-geneous dispersion of FSWCNT in pristine P3HT matrix [22]. Thephotoluminescence (PL) studies have shown significant groundlevel interaction between the p-conjugated backbone of P3HT andFSWCNT. Six order enhancement in conductivity by addition ofonly �4% FSWCNT in the self-assembled matrix of pristineP3HT:FSWCNT has been reported [22]. However, films of pristineP3HT:FSWCNT self-assembled matrix have shown low thermalstability and hygroscopicity as revealed by its thermo-gravimetricanalysis (TGA). Furthermore, it has been observed that thecomposite is highly corrosive towards metal electrodes perhapsdue to its high water retention capacity. TGA investigations haverevealed [22] that the hygroscopic nature of pristine P3HT:FSWCNTcomposite is associated with excess addition of polar side groupssuch as �OH, �COOH, �SO3H, etc., which attracts moisture atroom temperature in ambient conditions. In order to modify thedeficiency of our previous work [22], which may prove vital inachieving significant photovoltaic cell efficiency, we have changedthe extent of addition of polar side groups to nanotubes backbone inthe present investigation. In spite of tremendous efforts in bothfabrication and understanding of OPV, the operation stability hasreached its lifetime up to a year [24,25]. These degradation issuesshowed that the OPV topic is not fully understood and remainscomplicated [24–31]. This has given us an impetus to fabricate anddemonstrate a polymeric photovoltaic device at ambient conditions.

Here we report the synthesis and characterization of a nonhy-groscopic photoactive composite; P3HT:FSWCNT:PCBM, which givespower conversion efficiency (Z) �1.8% at ambient condition. Thecurrent density–voltage (J–V) characteristics of photovoltaic cells;P3HT:FSWCNT, P3HT:PCBM, and P3HT:FSWCNT:PCBM compositeshave been evaluated under dark and standard light source atambient conditions and discussed in detail. The intent of the presentinvestigation is to evolve a process to develop a low-cost polymericphotovoltaic device.

Scheme 1. Synthesis and purification process

2. Experimental

2.1. Synthesis of P3HT

Pristine P3HT polymer was synthesized by chemical oxidativepolymerization at low temperature, �228 K, in our laboratory[32–34]. Drop-wise addition of 3-hexylthiophene monomer(0.1 M) into ferric chloride (FeCl3) (0.4 M) suspension in chloro-form (CHCl3) kept at�228 K in an inert N2 atmosphere for over24 h, results into synthesis of P3HT. The details have beendescribed in our previous works [32–34]. Synthesized P3HT wasprecipitated from reaction mixture by adding copious amount ofmethanol. Repeated purification was performed by methanol anddeionized water to remove methanol soluble oligomers andexcess of oxidant impurity. The pristine P3HT polymer wasobtained after removing the intercalated oxidant impurity frompolymer matrix by solvent extraction treatment with liquidammonia (NH3) for chloride ion and ethylenediaminetetraaceticacid (EDTA) for iron ion. The molecular weights: Mw¼32,658;Mn¼29,298 of pristine P3HT determined by the GPC techniquegive a polydispersity (PD) index of 1.11. The schematic ofsynthesis and purification process is given in Scheme 1.

2.2. Functionalization of single-walled carbon nanotubes

The chemical functionalization of SWCNT was performed inorder to make SWCNT partially soluble in organic polar solvents.This route will not only assist in preparation of the composite, butalso open new prospects in aligning and forming better moleculardevices, on nano-scale architectures. The first step of functionaliza-tion of SWCNT involves reactions of SWCNT with different oxidants,known as acidic treatment. It resulted in the opening and shorteningof nanotubes with insertion of new functional groups such ascarboxyl, hydroxyl, sulphonic groups, etc. [22]. With functionaliza-tion of these groups, the solubilization of SWCNT in differentsolvents gets considerably improved.

Single-walled carbon nanotubes (SWCNT) were purchased fromSigma-Aldrich and used for functionalization with no prior treat-ment. The procured SWCNT have diameter �1.5–2 nm and length�2–5 mm and were in bundled form. The functionalization wascarried out with a view to obtain nonhygroscopic functionalizedSWCNT (FSWCNT) by varying the weight of SWCNT and volume ofacid mixture. The suspended raw SWCNT in a concentratedsulphuric acid/nitric acid mixture (3:1 v/v) were sonicated for�3 h. The acid-treated nanotubes, after filtration, were thoroughlywashed with dilute aqueous solution (0.5 M) of sodium hydroxide(NaOH) and then with deionized water through porcelain gouch

to obtain pristine poly(3-hexylthiophene)

R.K. Singh et al. / Solar Energy Materials & Solar Cells 94 (2010) 2386–23942388

(filter) attached with vacuum suction system till the filtrate becamecolorless. The filtered FSWCNT were dried at �353 K for 2 h and theresultant residue was weighed and kept in desiccators. The stepsinvolved in the functionalization of SWCNT are given in Scheme 2.

2.3. Synthesis of P3HT:FSWCNT composite

The weighed amount of FSWCNT powder (obtained as shown inScheme 2) was taken in sampling bottle and chlorobenzene(C6H5Cl) was added to it. After that the reaction mixture wassonicated for 24 h. The solution of FSWCNT in C6H5Cl was allowedto stand for 12 h, so that suspended FSWCNT gets settled down andonly solubilized FWSCNT in C6H5Cl remains. The solution ofFWSCNT in C6H5Cl was then carefully pipetted out and weighedamount of synthesized P3HT was added to it. The solution wassonicated for 15 min to mix the two components thoroughly.

H2SO4:HNO3

SWCNT

Sonicated f orSTEP 1

HOOCHO3S

COOHSO3H

SO3HCOOH

Acidic FSWCNT

HSTEP 2

Aqueo

Scheme 2. The chemical functionaliz

Scheme 3. Fabrication of P3HT:FSWCNT bulk heterojunction as

A part of the solution was taken for FT-IR spectroscopy and partwas used to cast freestanding films on quartz substrate for SEM.UV–vis spectroscopy was performed on spin-coated film over ITO,and a part of solution was used for fabricating thin film of activelayer for making photovoltaic cells. The process is illustrated inScheme 3.

2.4. Synthesis of P3HT:PCBM composite

The pristine P3HT and PCBM were taken in equal weight inseparate beakers and equal volume of solvent (chlorobenzene orchloroform) was added to each of them. After shaking a completeclear solution of both was obtained. In order to make theP3HT:PCBM composite, equal volume of both P3HT and PCBMsolutions were mixed. The freshly prepared solution was used forfabricating active layer for OPV device.

HOOC

HO3S

COOHSO3H

SO3HCOOH

(3:1 v/v)

Acidic FSWCNT

3 hrs

2O HO3S

COOHSO3HCOOH

FSWCNT

HO3S

COOHSO3HCOOH

FSWCNT

us NaOH

(Aqueous solution)

Filtered and dried at 353 K for 2 hrs

(Powder form)

ation and purification of SWCNT.

a photoactive layer for OPV device and freestanding films.

Spin coated forphotovoltaic device

P3HT + FSWCNT + PCBMSonicated for 15min

P3HT:FSWCNT:PCBM(Active material)( CHCl3/ C6H5Cl )

Scheme 4. Fabrication of P3HT:FSWCNT:PCBM bulk heterojunction as photo-

active layer for organic photovoltaic device.

R.K. Singh et al. / Solar Energy Materials & Solar Cells 94 (2010) 2386–2394 2389

2.5. Synthesis of P3HT:FSWCNT:PCBM composite

A 1:1 ratio of P3HT:PCBM was prepared by mixing equalvolume of solution of same strength of PCBM and pristine P3HT ineither C6H5Cl or CHCl3 as solvent. Weighed amount of pristineP3HT and PCBM (10 or 20 mg) in 1:1 ratio was added to thefreshly prepared FSWCNT solution in desired solvent to obtainP3HT:FSWCNT:PCBM solution. The solution was then heated at�353 K to ensure complete dissolution. After cooling to roomtemperature, spin coating of the P3HT:FSWCNT:PCBM solutionwas performed to get thin photoactive layer (Scheme 4).

2.6. Preparation of photovoltaic cells

A series of cells differing in their compositions and thicknesseswere fabricated at ambient conditions, on glass/ITO substrates. Thesubstrates were cleaned in an ultrasonic bath with water andacetone repeatedly. Cleaned ITO was dried using N2 gas for 10 min.A thin intermediate layer of PEDOT:PSS was spin coated over thecleaned and dried ITO substrate, which collects holes fromphotoactive layer and transfer it to ITO layer efficiently. The glass/ITO/PEDOT:PSS layer was subsequently dried for 15 min at�373 K. Subsequently, photoactive layers (e.g. P3HT:PCBM, P3HT:FSWCNT and P3HT:FSWCNT:PCBM) were spin coated over glass/ITO/PEDOT:PSS layer at ambient conditions and thus makingpartial devices with structure comprising glass/ITO/PEDOT:PSS/P3HT:PCBM, glass/ITO/PEDOT:PSS/P3HT:FSWCNT and glass/ITO/PEDOT:PSS/P3HT:FSWCNT:PCBM. The deposited photoactive layerwas dried and annealed at �353 K at an ambient thermal condi-tion in furnace for 10 min to remove any traces of solvent. Anotherpurpose of drying and annealing of photoactive layer is for betterpolymeric chains packing and to enhance the self-assembly ofpolymeric chains leading to improved device performance. Finally,the fabrication of photovoltaic devices were completed bydepositing thin cathode layer (Al electrode) by thermal depositionof pure Al metal over photoactive layer and thus making completedevice structure glass/ITO/PEDOT:PSS/P3HT:PCBM/Al, glass/ITO/PEDOT:PSS/P3HT:FSWCNT/Al and glass/ITO/PEDOT:PSS/P3HT:FSWCNT:PCBM/Al. The thermal deposition of Al electrode wasperformed in such a way that the resultant device active surfacearea becomes �4 mm2.

2.7. Measurements

Nicolet 5700 FT-IR spectrometer (4000–400 cm�1) was used tostudy the FT-IR spectra of pristine P3HT, FSWCNT and P3HT:FSWCNT composite in CHCl3 solution. UV–vis spectra of P3HT,P3HT:PCBM and P3HT:FSWCNT:PCBM composite thin films formedby spin coating at 1000 rpm on ITO have been recorded on PerkinElmer Cambda 25 UV–vis spectrometer. Leo 440 scanning electronmicroscope was used to investigate surface evolution of the films ofpristine P3HT, P3HT:FSWCNT and P3HT: FSWCNT:PCBM compo-site. The microstructural characterization was carried out utilizing

a high-resolution transmission electron microscope Tecnai G2 F30S-Twin (FEI; Super Twin lens with Cs¼1.2 mm) instrumentoperating at an accelerating voltage at 300 kV, having a pointresolution of 0.2 nm, and with a lattice resolution of 0.14 nm. SAXSmeasurement was conducted at room temperature using Bruker-AXS NanoSTAR instrument. The instrument is equipped with anX-ray tube (Cu Ka radiation operated at 45 kV/35 mA), cross-coupled Gobel mirrors, three-pinhole collimation, evacuated beampath, and a 2D gas-detector (HISTAR) [35]. The accessible scatteringrange of the instrument can be varied by selecting differentdistances between the sample and the detector of 42.2 and66.2 cm. The modulus of the scattering vector is q¼4psin y/l, inwhich y is scattering angle and l is X-ray wavelength. SAXS datawere recorded in the q¼0.007–0.21 A�1 range, i.e., 2y¼0.1–31.Dark and illuminated current density–voltage (J–V) characteristicsof OPV devices have been obtained using calibrated tungstenilluminated source (�80 mW cm�2) attached to the Keithleymodel 4200 semiconductor characterization system with pream-plifier. All the measurements were carried out at ambientconditions.

3. Results and discussion

3.1. FT-IR investigation

Fourier transform infrared (FT-IR) investigation of pristineP3HT (red line), P3HT:FSWCNT composite (green line) andFSWCNT in CHCl3 (violet line) is shown in Fig. 1. The interactionbetween FSWCNT and P3HT at molecular level is distinctivelyvisible from the broadening of the FT-IR peaks with loading ofFSWCNT. The change becomes more significant as the aromaticC–H stretching vibrations show a broad hump and three newpeaks arise in the region 1300–1000 cm�1. These peaks have beenassigned to C–C single bond stretching of FSWCNT. The new peakaround 1800–1700 cm�1 arises due to C¼O in conjugation withC¼C due to the addition of carboxylic group in the functionaliza-tion of SWCNT. The peaks arising due to P3HT remain visible, buta peak around 1500–1200 cm�1 arising due to C¼C stretchingoverlaps with FSWCNT to give a broad peak. The vibration bandsat 1454 and 1510 cm�1 are associated with symmetric andasymmetric ring stretching vibrations, respectively, and theirrelative ratio indicates the conjugation length.

3.2. Scanning electron microscopy (SEM)

The basis of formation of a heterostructure depends on theability of constituent materials to mix homogeneously andinterfaced/wrapped each other as much as possible. A scanningelectron microscope (SEM) was employed to investigate, atmicroscopic level, the extent of homogeneous dispersion ofelectron collecting and transporting carbon nanotubes andelectron donating semiconducting polymer. Fig. 2(a, b) showsthe microscopic surface morphology of pristine P3HT andP3HT:FSWCNT heterostructure obtained by SEM from which anidea can be made about the extent of nanotubes dispersion inP3HT matrix. P3HT exhibit flat surface morphology (Fig. 2(a)),which is a common feature of this type of polymer. It isinteresting to see the homogeneous dispersion of FSWCNT inP3HT matrix as shown in Fig. 2(b). In few regions, nanotubes arealso found to be slightly agglomerated and this may be due torebundling of nanotubes in due course of solvent evaporation. Thesize of homogeneously dispersed nanotubes domains is oftenextended up to �300 nm (Fig. 2(b)). This forms the basis of a largeextent of electron collection from P3HT polymeric chains and its

Fig. 1. FT-IR spectra of pristine P3HT (red line), P3HT:FSWCNT composite (green line) films and FSWCNT (violet line) in chlorobenzene solution exhibiting characteristic

bands. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

a b

c d

Fig. 2. Surface morphology of (a) pristine P3HT (b) homogeneously dispersed functionalized single-walled carbon nanotubes (FSWCNT) in P3HT matrix. Illustration of (c)

SEM image and (d) a representative sketch of P3HT:PCBM:FSWCNT charge transfer complex. The ( ) denotes polymer chain, (O) denotes the PCBM and ( ) denotes

the FSWCNT.

R.K. Singh et al. / Solar Energy Materials & Solar Cells 94 (2010) 2386–23942390

transport to positive electrode. Fig. 2(c) shows the granularmorphological evolution of P3HT:FSWCNT:PCBM film. It isimportant to note here that the size of individual granules issame as average length of carbon nanotubes (�2–3 mm). Thisindicates that the nanotubes help in binding the PCBM and P3HTmolecules tightly, which fulfills the criteria of a close interfacebetween acceptor and donor in order to make efficient charge

transfer between the two. It appears that the PCBM moleculesinteract with FSWCNT to form a weakly electrostatic attachmentor self-assembled P3HT:PCBM:FSWCNT complex (Fig. 2(c)). Simi-lar type of bonding has been reported [36] earlier. Fig. 2(d)illustrates the morphology and arrangement of the constituents ofP3HT:FSWCNT:PCBM composite. A sketch of P3HT (ropes), PCBM(circle) and FSWCNT (rod) has been presented in this figure. Thus,

BA

a b

c d

Fig. 3. High-resolution transmission electron micrograph of (a) P3HT, (b) solubilized FSWCNT, (c) P3HT:PCBM and (d) P3HT:FSWCNT:PCBM composite.

300 400 500 600 700 800

0.0

0.4

0.8

1.2

1.6

Abs

orpt

ion

Wavelength (nm)

P3HT P3HT:FSWCNT P3HT:PCBM P3HT:FSWCNT:PCBM

Fig. 4. UV–vis. spectra of P3HT (——), P3HT:FSWCNT (� � � � �), P3HT:PCBM

( � � � � � � � ) and P3HT:FSWCNT:PCBM (����) composite.

R.K. Singh et al. / Solar Energy Materials & Solar Cells 94 (2010) 2386–2394 2391

solubilization of nanotubes and its dispersion allow the laminarstacking in P3HT polymeric matrix. This suffices the importantcriterion of formation of a charge transferring heterojunctionmuch needed for developing a better photovoltaic device.

3.3. High-resolution transmission electron microscopy

The bright field high-resolution transmission electron micro-scope has been used to investigate the mesoscopic interactionbetween the FSWCNT and P3HT. Fig. 3(a) shows the TEM image ofpristine P3HT. This figure reveals that P3HT is a mixture ofamorphous and crystalline matrix. The crystallinity amounts tolaminar stacking of p–p conjugated backbone. Fig. 3(b) shows theTEM image of solubilized FSWCNT. The solubilized FSWCNT arearranged horizontal to one another. Fig. 3(c) shows the TEMimage of P3HT:PCBM matrix. The crystallinity of P3HT is notobserved in P3HT:PCBM matrix [37]. Fig. 3(d) shows the TEMimage of P3HT:FSWCNT:PCBM composite. The presence ofFSWCNT can be vividly observed. The section analysis of tworegions A and B (as shown in Fig. 3(d)) was performed to know theexact nature of this semi-crystalline composite. The crystallinityin section area A and B (Fig. 3(d)) of the composites has beeninvestigated using the inverse fast Fourier transform (IFFT)technique. Region A exhibits the fringe width (�0.71 nm)corresponding to P3HT polymer, whereas region B shows thepresence of fringe width corresponding to both P3HT (�0.83 nm)and FSWCNT (�1.20 nm).

3.4. Ultraviolet–visible (UV–vis) spectroscopy

Fig. 4 shows the UV–vis spectra of pristine P3HT, P3HT:PCBM,P3HT:FSWCNT and P3HT:FSWCNT:PCBM composites. The pristine

P3HT has an absorption band at 505 nm, which corresponds top–pn transition of its conjugated backbone. The prominent featureobserved in UV–vis spectrum of P3HT:PCBM composite is asubstantial shift in the absorption maxima (lmax). On the additionof PCBM, the lmax shows a blue shift (�488 nm) compared topristine P3HT (�505 nm). This blue shift can be due to theformation of interpenetrating bulk heterojunction network ofP3HT:PCBM; thus reducing the conjugation length of the P3HTbackbone. When FSWCNT is added to the P3HT polymer matrix,

R.K. Singh et al. / Solar Energy Materials & Solar Cells 94 (2010) 2386–23942392

then both P3HT: FSWCNT and P3HT: FSWCNT:PCBM compositesshow red shift, i.e.�513 and 515 nm, respectively, in comparisonto pristine P3HT (�505 nm). The interaction of pristine P3HT withFSWCNT, uncoils the P3HT conjugated backbone and henceincreases the conjugation length [21]. This interaction also bringsabout structural ordering in the matrix. There is a variation inabsorption intensity of P3HT, P3HT:FSWCNT, P3HT:PCBM andP3HT: FSWCNT:PCBM samples (Fig. 4). It is evident from thisfigure that the absorption intensity of P3HT:PCBM gets reduced to50% of pristine P3HT. This is due to the fact that only the P3HTpolymer is photoactive in the P3HT:PCBM composite; thus only50% of the matrix is actively absorbing the radiation. Moreover,mixing of PCBM in P3HT makes a very good interpenetratingnetwork of donor (P3HT) and acceptor (PCBM) bringing out thecoiling of P3HT resulting in the reduction of conjugation length ofP3HT. In P3HT:FSWCNT:PCBM composite, though the amount ofP3HT in the composite matrix is less as compared to pristineP3HT, its absorption is almost the same. This increase is due to theincrease in the conjugation length because of the presence ofFSWCNT. Thus, FSWCNT provide support for the polymer chainbackbone, to form a laminar orientation and better crytallizationas observed in the TEM investigation. This combined effect thusenhances the photon transition in the polymer composite.

3.5. Small-angle X-ray scattering (SAXS)

The microstructure and domain spacing for P3HT:PCBM andP3HT:PCBM:FSWCNT composites were examined using the small-angle X-ray scattering (SAXS) technique. A representative SAXSpattern obtained for P3HT:PCBM and P3HT:PCBM:FSWCNT is shownin Fig. 5. The main peak for P3HT:PCBM and P3HT:PCBM:FSWCNTcomposites is centered at value for the scattering wave vector qcorresponding to �32 nm domain spacing [38]. The presence ofweak and broad peak of P3HT:PCBM composite centered atscattering wave vector q corresponding to �10–11 nm domainspacing is consistent with the width of the lamellae fibrils reportedearlier for various samples of P3HT and P3HT:C60 composites[38,39]. However, no higher-order reflection is observed in thespectrum of the composites of the present work as reported earlierfor copolymers of P3HT [38]. The main characteristic is the variationin the intensity count (log(I)) for the two samples subjected for SAXSmeasurement. The P3HT:PCBM:FWSCNT composite shows more

0.00 0.05 0.10 0.15 0.20 0.250.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

log

(I)

q(Å-1)

P3HT:PCBM:FSWCNT P3HT:PCBM

Fig. 5. SAXS spectra of P3HT:PCBM:FSWCNT (——) and P3HT:PCBM (–––)

composites at room temperature. Arrows show the position of principal reflections

corresponding to which domain spacing was calculated for both the samples.

intense peak than P3HT:PCBM composite, the reason of which couldbe attributed to the lamellae arrangement and better crystallinitydue to incorporation of FSWCNT in the P3HT:PCBM matrix.

3.6. Current density–voltage (J–V) characteristics

Fig. 6 shows the current density–voltage (J–V) characteristics oforganic photovoltaic device (OPV) made out of P3HT:FSWCNTcomposite, under dark and illuminated (�80 mW cm�2) conditions.The forward bias is defined as the positive voltage applied to the ITOelectrode. The dark current is considerably high in forward biascondition than in the reverse bias, indicating distinct diode behaviorof the present OPV device. The rectification ratio at 1 V is found to be�34. Under illumination, the OPV shows short-circuit current density(Jsc) �0.341 mA cm�2 and open-circuit voltage (Voc) �0.465 V. Thefill factor (FF) is calculated using equation FF¼(JmVm)/(JscVoc), where Jmand Vm are defined as current density and voltage of the J–V curve atmaximum power output, respectively. The typical FF value for thefabricated device is found to be 0.42 and the estimated value of powerconversion efficiency (Z) is �0.1%. The observed photovoltaicproperty of the present device is due to the formation of polymer–nanotube (P3HT:FSWCNT) bulk heterojunction, which acts as anactive layer. The polymer absorbs photons from solar radiations aboveits band edge, which forms electron–hole pairs (excitons). Then thedissociation of excitons into electron and hole takes place at polymer–nanotube interface and subsequently the electrons and holes aretransported to the respective electrodes through percolative network.Thus the formation of P3HT:FSWCNT bulk heterojunction is respon-sible for the generation of photocurrent and hence it is functioningas photovoltaic device. The power conversion efficiency of P3HT:FSWCNT fabricated at ambient conditions is low as compared to OPVmade out of P3HT:PCBM composite in an inert atmosphere [40]. Thepossible reason of the observed low-current density in the OPV madeout of P3HT:FSWCNT composite could be understood in terms ofrecombination of excitons in the polymer matrix and possibleenvironmental degradation of active layer. Since the extent ofnanotubes functionalization is low, even though composite containsappreciable amount of nanotubes (�5%), it was anticipated that thenumber of effective polymer/nanotube junctions will be low, whichconsequently leads to the high recombination of excitons resultinginto overall low device efficiency. The low extent of nanotubesfunctionalization results into rebundling of individual nanotubes andhence chance of formation of polymer–nanotubes heterojunction getslowered. The value of Jsc also depends on the efficient transport of

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(Acm

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Voltage (V)

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Fig. 6. The current density–voltage (J–V) characteristics of organic photovoltaic

device made of P3HT:FSWCNT composite under dark and illuminated (80 mW

cm�2) conditions.

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(Acm

-2)

Voltage (V)

P3HT:PCBM (chlorobenzene) P3HT:PCBM (chloroform) P3HT:FSWCNT:PCBM

(chlorobenzene)

Fig. 7. The current density–voltage (J–V) characteristics under illumination at

80 mW cm�2 for different photovoltaic cells made out of pristine P3HT:PCBM

composite (in chloroform and chlorobenzene) and pristine P3HT:FSWCNT:PCBM

composite (in chlorobenzene).

Table 1Open-circuit voltage (Voc), photocurrent density (Jsc), fill factor (FF) and power

conversion efficiency (Z) of different organic photovoltaic devices fabricated at

ambient conditions for the present investigation.

Devices Voc (V) Jsc

(mA/cm2)FF g (%)

P3HT:FSWCNT (chlorobenzene) 0.465 0.34 0.42 0.1

P3HT:PCBM (chloroform) 0.516 2.80 0.40 0.7

P3HT:PCBM (chlorobenzene) 0.527 3.76 0.37 0.9

P3HT:FSWCNT:PCBM (chlorobenzene) 0.520 5.53 0.49 1.8

R.K. Singh et al. / Solar Energy Materials & Solar Cells 94 (2010) 2386–2394 2393

photo-generated electrons to Al electrode and any loss of electrons atcomposite–Al electrode interface due to reflection of electrons by Allayer. The effective transport of dissociated charges to the respectiveelectrodes depends primarily on formation of percolative network ofFSWCNT in polymer composite and loss of electrons at composite–Alinterface.

Fig. 7 shows the plots of photocurrent density (Jsc) versus voltage(V) of the photovoltaic cells fabricated and characterized in thepresent work. P3HT without acceptors shows a good photocurrentand is comparable to those reported [21] earlier for rr-P3HT. The cellsfabricated using P3HT:PCBM composite as photoactive layer showbetter result when chlorobenzene is used as solvent as compared tochloroform. As illustrated previously this is due to better assemblingof solar photon absorbing P3HT chains [3]. The solar cells fabricatedusing P3HT:FSWCNT:PCBM composite show best photovoltaic prop-erty out of all solar cells fabricated in the present investigation. Theimprovements in both Jsc and FF (Table 1) for this cell can beattributed to the enhanced chromophore density and hole transportby the FSWCNT network, which can assist in better chain assemblingand maximizing the surface area for collection of charges and providebetter conducting pathways to transport charges to the electrode.Enhanced hole collection can also be partly ascribed to thegeometrical field enhancement at the FSWCNT, which has beenreported to be as great as 5-fold as compared to other electronconductors [23]. The characteristic parameters of presently fabricatedsolar energy generation devices are given in Table 1.

4. Conclusions

The controlled functionalization of SWCNT enables to developthe nonhygroscopic composites with P3HT polymer. We have

fabricated different photovoltaic cells from the combination ofP3HT, PCBM and FSWCNT. The addition of nanotubes improvesthe photo-physical properties of photoactive materials by theirstructural improvement and interfacial contact among theconstituents. Incorporation of FSWCNT in the P3HT:PCBM matrixenhances the absorption intensity as well as charge collectionefficiency resulting into better device performance. The devicefabricated from P3HT:FSWCNT:PCBM composite shows the bestphoto-conversion efficiency (Z�1.8%), which is 18 times higherthan that observed for P3HT:FSWCNT (Z�0.1%) and twice thanthat of P3HT:PCBM (Z�0.9%).

Acknowledgements

The authors would like to thank Director, National PhysicalLaboratory, New Delhi, for permission to publish this work. Theauthors are grateful to Prof. C.N.R. Rao for SAXS measurement andencouragement. They also thank Drs S.K. Dhawan, Renu Pasricha,A.K. Srivastava, M. Deepa, Mr. K.N. Sood, all of National PhysicalLaboratory, New Delhi, and Neenu Varghese, JNCASR for theirhelp. Two of the authors (AK and RS) are thankful to the Council ofScientific & Industrial Research (CSIR), New Delhi, India, for theaward of Junior Research Fellowship and Emeritus ScientistFellowship, respectively.

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