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Journal of Photochemistry and Photobiology A: Chemistry 262 (2013) 34– 44

Contents lists available at SciVerse ScienceDirect

Journal of Photochemistry and Photobiology A:Chemistry

journa l h om epa ge: www.elsev ier .com/ locate / jphotochem

onjugated polymer based on oligobenzo[c]thiophene with low-lying HOMOnergy level as potential donor for bulk heterojunction solar cells

ichael Ruby Raja, Sambandam Anandana,∗, Rajadurai Vijay Solomonb,onnambalam Venuvanalingamb,∗, S. Sundar Kumar Iyerc,∗, Muthupandian Ashokkumard,e,∗∗

Nanomaterials & Solar Energy Conversion Lab, Department of Chemistry, National Institute of Technology, Trichy 620 015, IndiaSchool of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, IndiaDepartment of Electrical Engineering, Indian Institute of Technology Kanpur, Kanpur 208 016, IndiaSchool of Chemistry, University of Melbourne, Vic 3010, AustraliaDepartment of Chemistry, King Abdulaziz University, Jeddah, Saudi Arabia

a r t i c l e i n f o

rticle history:eceived 3 February 2013eceived in revised form 22 March 2013ccepted 9 April 2013vailable online 25 April 2013

eywords:onjugated polymerulk heterojunctionolar cellsFT calculations

a b s t r a c t

After the discovery of photoinduced charge transfer in conjugated polymers, organic pho-tovoltaic solar cells have been extensively studied due to many advantages that includeflexibility and low-cost. In this context, we have synthesized a new class of conjugatedpolymer, poly(2-(4-{1-cyano-2-[5-(3-thiophen-2-yl-benzo[c]thiophen-1-yl)-thiophen-2-yl]-vinyl}-2,5-bis-hexyloxy-phenyl)-but-2-enenitrile) (CN-PTBTBPB) for the fabrication of low cost organic solarcells. The electron donor properties of CN-PTBTBPB polymer was incorporated by means of facileKnoevenagel condensation process followed by a chemical oxidative polymerization method using oligo-5-(1-benzo[c]thiophen-3-yl)thiophene-2-carboxaldehyde with 1,4-bis(cyanomethyl) benzene linkages.The polymer was characterized by 1H NMR and GPC techniques. The electronic and structural prop-erties of the polymer were evaluated by UV–vis spectroscopy, fluorescence spectroscopy, and thermogravimetric analysis. In order to establish the energy diagram of the prepared polymer, the energy gap

between HOMO–LUMO was evaluated using electrochemical measurements and Density Functional The-ory (DFT). The resulting CN-PTBTBPB polymer was experimentally found to possess low-lying HOMO(about −5.84 eV) and high-lying LUMO (about −3.87 eV) energy levels. This polymer exhibited a rela-tively wide optical band gap of ∼1.95 eV in the solid state. Using this polymer, organic photovoltaic cellswere fabricated in open air of type ITO/PEDOT:PSS/CN-PTBTBPB:PCBM/Al showed promising photovoltaic properties.

. Introduction

The development of conjugated polymer-based organic photo-oltaic devices offers a potentially inexpensive way to producelectricity from sunlight. They receive increasing attention as aenewable energy source owing to their unique advantages: light-eight, low-fabrication cost, ease of processing, the possibility to

abricate mechanically flexible devices, large area coverage, andersatility in manipulating chemical structures [1–5]. This devel-

pment is due to the discovery of photoinduced electron transferrocesses that take place in conducting polymers which con-ist of donors and acceptors providing a molecular approach to

∗ Corresponding authors.∗∗ Corresponding author. Tel.: +61 3 83447090.

E-mail addresses: sanand@nitt.edu (S. Anandan), venuvanalingam@yahoo.comP. Venuvanalingam), sskiyer@iitk.ac.in (S.S.K. Iyer), masho@unimelb.edu.auM. Ashokkumar).

010-6030/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jphotochem.2013.04.013

© 2013 Elsevier B.V. All rights reserved.

high-efficiency photovoltaic conversion processes [6]. Thus themajority of efficient organic solar cells make use of an interpen-etrating network of donor and acceptor molecules. These so-calledbulk heterojunction (BHJ) layers exhibit nanoscale mixing of thetwo components ensuring quantitative charge generation at theextended donor–acceptor interface, whereas the bicontinuousnature allows effective charge collection [5,7,8]. Conceptually, thefirst BHJ polymer solar cell consisted of poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene (MDMO-PPV, band gap∼2.2 eV) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)with a power conversion efficiency (PCE) of 2.9% [9]. At present,the best BHJ polymer solar cell reported to date, made of regioreg-ular 2,5-diyl-poly(3-hexylthiophene) (rr-P3HT, band gap ∼1.9 eV)as a donor and PCBM [10,11] as an acceptor, has shown a highperformance exceeding 6% [12–14]. Here, rr-P3HT polymer mainly

harvests high energy photons with wavelengths below 650 nm,while the wavelength of the maximum solar photon flux densityis located at approximately 700 nm which corresponds to a lowenergy of 1.77 eV [8]. This is the barrier that must be overcome to

nd Ph

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pjepbfeacmtrlwgbH(rp[mbnptPptopavlhbdb

b1wtvInbbtt

2

2

Rss

M.R. Raj et al. / Journal of Photochemistry a

evelop a high efficiency BHJ solar cell. Therefore, the new strate-ies to design new conjugated donor polymer is desirable to have

narrow band gap to extend their light absorption into the near-nfrared region, which can efficiently capture a larger fraction ofncoming sun light.

To date, many researchers have reported on low-band gapolymers based on the alternating “donor–acceptor (D–A)” con-

ugated polymer approach [15,16]. These systems consist oflectron-rich and electron-deficient moieties along a �-conjugatedolymer backbone. Their electronic and optical properties coulde efficiently manipulated through intramolecular charge trans-er (ICT) and conjugated length, which contributes to a betterxtended light absorption and higher absorption coefficient forchieving high performance organic bulk heterojunction solarells [17]. Among the reported alternating D–A conjugated poly-ers, recent developments on low-band-gap polymers based on

he incorporation of electron-donating properties of thiopheneing system (donor) at the bis-(cyano-2-thienylvinylene) pheny-ene precursors (acceptor) for organic solar cell applications are

ell reviewed [18–23]. Further, there are several low-band-ap polymers, such as (i) soluble dioxythiophenecyanovinyleneased polymers, e.g. Th-CN-PPV (Eg = 1.8 eV, PCE = 0.01%), PProDOT-x2:CN-PPV (Eg = 1.7 eV, PCE = 0.1%) and PBProDOT-Hx2:CNV

Eg = 1.5 eV, PCE = 0.2%) [24], (ii) four analogues of the nar-ow band gap poly(3,4-propylenedioxythiophenedialkyl)cyano--phenylenevinylene (PProDOTR2:CNPPV) based polymer family25], (iii) bis(1-cyano-2-thienylvinylene)phenylene based poly-

ers (Eg = 1.72 eV, PCE = 0.14%) [26], (iv) novel alternating lowand gap phenylenevinylene copolymers with cyanovinylene 4-itrophenyls (PCE = 4%) [27] have shown a better maximumhoton flux harvesting in a broader spectrum and efficient pho-ogeneration of charge-separated states when blending withCBM. To effectively reduce the optical band gap of cyano-henylenevinylene (CN-PPV)-based derivative polymers to lesshan 2 eV, the introduction of more rigid planar electron-richligothiophene units with lower aromatic system have been incor-orated into the main chain. This allows the construction oflternating D–A conjugated segments at the dithienylethyleneinylene linkages in order to fine tune the HOMO and LUMO energyevels of a conjugated polymer [28]. However, these approachesave led to devices showing efficiencies much lower than the wide-and-gap counterparts because of the mismatch of energy levels ofonor polymers, lower absorption coefficient and phase separationetween donor and acceptor system.

In this study, we report on the synthesis of a broadand gap �-conjugated polymer (CN-PTBTBPB) consisting of,3-di(thiophene-2-yl)benzo[c]thiophene donor unit incorporatedith bis-(cyano-2-thienylvinylene) phenylene precursors in order

o achieve high power conversion efficiency by designing a photo-oltaic device [ITO/PEDOT:PSS/CN-PTBTBPB:PCBM (1:1, w/w)/Al].n addition, we have suggested that incorporated electron-donatingature of 1,3-di(thiophene-2-yl)benzo[c]thiophene moiety into theis-(cyano-2-thienylvinylene) phenylene linkage-based polymerrings different degrees of intramolecular charge transfer (ICT) tohe �-conjugated system and hence may provide a means to fineune HOMO and LUMO energy levels.

. Experimental

.1. Materials

All chemicals were obtained from Sigma–Aldrich (USA) andankem (India) and were used without further purification. Allolvents were used after distillation and stored over molecularieves as described in Vogel’s text of practical organic chemistry,

otobiology A: Chemistry 262 (2013) 34– 44 35

5th edition. Na2SO4 was used as the drying agent in all thepurification procedures. High purity (99.99%) nitrogen and argongases were used to maintain an inert atmosphere. Poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)was purchased from Sigma–Aldrich and used without purifica-tion. Starting materials, 1,4-biscyanomethyl-2-5-bis[hexyloxy]benzene (1) [26,29] and 5-(1-benzo[c]thiophen-3-yl)thiophene-2-carboxaldehyde (2) were synthesized by following the availableliterature methods [30,31].

2.1.1. Material synthesisThe synthetic routes of monomer and polymer were outlined in

Scheme 1(a) and (b).Synthesis of monomer: (2Z,2′Z)-2,2′-(2,5-bis(hexyloxy)-

1,4-phenylene)bis(3-(5-(3-(thiophen-2-yl)benzo[c]thiophen-1-yl)thiophen-2-yl)acrylonitrile) (M1)

The monomer (M1) was prepared through Knoevenagel con-densation of 1,4-biscyanomethyl-2-5-bis[hexyloxy]benzene (1,1 g, 2.8 mmol) with oilgo-5-(1-benzo[c]thiophen-3-yl)thiophene-2-carboxaldehyde (2, 2.7 g, 8 mmol) dissolved in 20 ml of freshlydistilled methanol in a dried three neck 100 ml round bottomflask. Potassium tert-butoxide (2.6 g, 27 mmol) was added as a solidunder nitrogen atmosphere and this mixture is stirred for 20–24 hat reflux temperature (70 ◦C). Upon cooling, 50 ml of water wasadded, and the mixture was extracted three times with 40 ml ofCH2Cl2. The organic layer was dried over anhydrous Na2SO4 andthe solvent was evaporated. The product was obtained as stickyreddish violet semisolid. After purification by column chromatog-raphy, (silica gel 60–120 mesh, initially used the ratio of 20:80 mlCHCl3: petroleum ether and then finally used 50:50 ml) the stickyviolet semisolid (0.4 g, yield: 68%) was obtained. 1H NMR, 500 MHz(CDCl3, ı): 8.0–7.7 (brs, 2H), 7.53(d, 2H, J = 8.5 Hz), 7.35–7.27 (m,8H), 7.11 (m, 8H), 6.6 (s, 2H), 4.12 (q, 4H), 2.36 (m, 4H), 1.61 (t,4H), 1.44–0.93 (m, 8H), 0.89–0.82 (m, 6H) ppm. ESI (MS): m/z: 806[[M+2]+ – (2 × −C6H13)]. Anal. Calcd. for C56H48N2O2S6: C, 69.10;H, 4.97; N, 2.88; S, 19.77. Found: C, 68.95; H, 6.13; N, 2.61; S, 19.49.

2.2. Polymerization of the monomer

The polymer was prepared by chemical oxidation of themonomer using anhydrous FeCl3 (Sigma–Aldrich, 99.99%) basedon the Sugimoto et al. procedure [32]. Anhydrous FeCl3 (4 mmol)was suspended in 20 mL of dried CHCl3 under a pure nitrogenatmosphere, to which a solution of the monomer (1 mmol) in20 mL of dry CHCl3 was added drop-wise under vigorous stir-ring. The mixture was stirred for 24 h at room temperature undera nitrogen atmosphere. The dark solution was poured into dis-tilled methanol (50 ml), and the black precipitate was filtered ona membrane filter. The precipitate was then immersed into a10 ml of methanol containing 1%, v/v solution of hydrazine andstirred for 24 h at room temperature. Furthermore, traces of Fe(III)salts, oligomers, unreacted monomers and low molecular weightfractions of polymers were removed by Soxhlet extraction usinga mixture of methanol:acetone (1:1) for 48 h. The soluble frac-tion containing polymer (CN-PTBTBPB) was collected by extractionusing dried CHCl3 for 24 h. Finally, chloroform solution was distilledunder vacuum to yield the polymer as shining dark violet powder.The obtained polymer yield is 70%. 1H NMR, 400 MHz (CDCl3, ı):7.44(brs, 2H), 7.26–6.88 (m, 16H), 6.50 (brs, 2H), 4.05–3.98 (m,4H), 2.26–2.17 (m, 4H), 1.31–1.16 (br, 12H), 0.78–0.76 (m, 6H)

ppm. GPC analysis: Mn = 128 kg/mol, Mw = 146 kg/mol; PDI = 1.14.Anal. Calcd. for C56H46N2O2S6: C, 69.10; H, 4.69; N, 2.91; S,19.80. Found: C, 68.10; H, 6.24; N, 2.31; S, 19.29. UV–vis (CHCl3):�max (ε × M−1 cm−1): 526 nm (62,242), 489 nm (47,153), 454 nm

36 M.R. Raj et al. / Journal of Photochemistry and Photobiology A: Chemistry 262 (2013) 34– 44

react

(o

2

ttpppatC

2

bIPfspl4w

2

tAafttCpbLstorl

Scheme 1. (a) Synthesis of monomer (M1) by Knoevenagel condensation

33,950), 400 nm (70,730), 375 nm (99,965), 357 nm (103,738). Flu-rescence in CHCl3 (�ex = 490 nm): 562 nm, 532 nm.

.3. Characterization methods

1H NMR spectra were recorded on BRUKER 400 MHz spectrome-er using CDCl3 as the solvent with chemical shifts reported relativeo the TMS as the internal standard. Molecular weights of the pre-ared polymers were determined from calibrated curve based onolystyrene standards recorded on a Gel Permeation Chromatogra-hy (GPC) at a flow rate of 1 ml/min THF solution at 40 ◦C. Elementalnalysis was performed with a Vario EL III elemental analyzer. Elec-rochemical measurements were carried out using CHI-650C modelH instruments, USA.

.4. Photophysical measurement

The absorption and emission spectra were recorded on a dou-le beam ultraviolet-visible spectrophotometer (Model: T90+, PG

nstruments Ltd., UK) and Shimadzu spectrofluorometer (RF-5301C), respectively. The samples were degassed using pure argon gasor 15 min prior to each experiment. Fluorescence lifetime mea-urements were carried out in a picosecond time correlated singlehoton counting (TCSPC) spectrometer with a tuneable Ti-sapphire

aser (TSUNAMI, Spectra physics, USA) as the excitation source of50–550 nm excitation wavelength. The fluorescence decay curvesere analyzed using the software provided by IBH (DAS-6).

.5. Electrochemical analysis

Electrochemical measurements were recorded in a conventionalhree-electrode system using CHI-650C electrochemical analyzer.

platinum wire was used as counter electrode; a platinum discs working electrode (3 mm) while Ag/AgCl electrode purchasedrom BASi (USA) was used as the quasi reference. Before ini-iating the experiments, pure argon gas was purged throughhe solution for 15 min in order to remove dissolved oxygen.yclic voltammagrams were obtained by dissolving 2 mM of theolymer in acetonitrile:chloroform (2:3) containing 0.1 M tetra-utylammonium hexafluorophosphate as a supporting electrolyte.ater, ferrocene (Fc) was added to the mixture as an internaltandard in order to calibrate the potential of quasi reference elec-

rode (Ag/AgCl electrode). The HOMO and LUMO energy valuesf polymer were determined based on a value of −4.8 eV for fer-ocene/ferrocenium ion (Fc/Fc+) with respect to the zero vacuumevel.

ion. (b) Synthesis of CN-PTBTBPB by chemical oxidative polymerization.

2.6. Film morphology

Topographic and phase images of blend polymer (active mate-rial) were measured using the Nanoscope IV Digital instrument(Veeco innova Model: DiSPM Lab version.5.01/6.0), operated intapping mode with commercially available Si cantilevers (Micro-mash, resonant frequency (Fo) of 313–346 kHz) with a typical springconstant (K) of 20–80 N/m in a non-contact mode. Images werecollected continuously with the scan rate of 0.50 Hz in air. Sev-eral locations of the film were examined to ensure uniformity andreproducibility.

2.7. Density Functional Theory

All calculations were performed using Gaussian 09 programmepackage [33]. The polymer was optimized at hybrid exchange-correlation B3LYP functional in conjugation with 6-31g(d) basis set.It has been already shown that B3LYP/6-31g(d) gives decent groundstate structures of conjugated polymers [34]. The frontier molecularorbital energies were determined from the optimized ground stategeometry. In order to gain further insights into the excitation prop-erty of this polymer, TDDFT (Time-Dependant Density FunctionalTheory) calculations have been employed at the CAM-B3LYP/6-31g(d) level, the functional which has already known for reproducingvarious experimental excited state properties of polymer systems[35].

2.8. Photovoltaic device fabrication and characterization

For solar cell device fabrication, indium tin oxide (ITO)-coatedconducting glass substrates (15–20 � cm−2) were used as anodeand aluminium as cathode. First, ITO substrates were patterned inisolated configuration, followed by ozonization for 90 min and thencleaned with detergent, ultrasonicated in deionized water, acetone,and isopropanol for 20 min in each solvent and finally dried inoven at 100 ◦C for 1 h. The thickness and roughness of ITO were110 and 10 nm, respectively, measured using a Tencor Alpha Step500 thickness profilometer. A hole transporting layer of PEDOT:PSSwas spin-coated on the ITO substrate at 1000 rpm for 1 min andwas annealed at 100 ◦C for 30 min on a hot plate under vacuum.A blend mixture consists of CN-PTBTBPB and PCBM was made inan anhydrous chlorobenzene (1:1 weight ratio at a concentration

of (5 mg/mL)). In order to attain homogeneity, the solution wasstirred for 5 h at 60 ◦C and then filtered using PTFE filter (0.2 �mpore size). Then the above mixture was spin-coated on top of thePEDOT: PSS layer at 800 rpm for 1 min and the resulting films were

M.R. Raj et al. / Journal of Photochemistry and Ph

Fs

dPmftp5c((stcfKwutwwoc

3

3

o5tmMw

ig. 1. (a) Schematic diagram of BHJ solar cells. (b) Fabricated bulk heterojunctionolar device model.

ried at 100 ◦C under high vacuum for 30 min. The thickness of theEDOT:PSS (25 nm) and CN-PTBTBPB: PCBM (80 nm) layers wereeasured using the above mentioned profilometer after the sur-

ace of the films had been cut with a diamond cutter to exposehe substrate. Aluminium layer (100 nm) was deposited on theolymer films by thermal evaporation at a pressure of less than

× 10−6 mbar to form the cathode. We fabricated a polymer solarell device with the typical structure as follows: ITO/PEDOT:PSS25 nm)/CN-PTBTBPB:PCBM (80 nm) (1:1, w/w)/Al (100 nm) (D1)Fig. 1a). Such fabricated device consisted of 4 isolated devices perubstrate with an active area of 20 mm2 (Fig. 1b). After encapsula-ion by UV-curing glue, the electrical characterization (such as shortircuit photocurrent density (Jsc), open circuit voltage (Voc) and fillactor (ff)) of the encapsulated devices was performed in air using aeithley 4200 semiconductor characterization system and an Orielhite light solar simulator operating at an intensity of 100 mW/cm2

nder AM 1.5 G (air mass 1.5 global) condition. The incident photono current conversion efficiency (IPCE) as a function of wavelengthas measured with an Oriel 300 W Xe Arc lamp in combinationith an Oriel Cornerstone 2601/4 monochromator. The number

f incident photons was calculated for each wavelength using aalibrated monocrystalline silicon diode as reference.

. Results and discussion

.1. Characterization of CN-PTBTBPB

In this study, polymer CN-PTBTBPB was synthesized by chemicalxidative polymerization of monomers 1,4-biscyanomethyl-2--bis[hexyloxy]benzene (1) with 5-(1-benzo[c]thiophen-3-yl)-

hiophene-2-carboxaldehyde (2) (Scheme 1). The resulting poly-

er has relatively high molecular weights (Mn = 128 kg/mol,w = 146 kg/mol) and the lowest polydispersity (PDI = 1.14) whichas determined by Gel Permeation Chromatography (GPC) in THF

otobiology A: Chemistry 262 (2013) 34– 44 37

relative to narrow low molecular weight polystyrene standardand the data are summarized in Table 1. The observed highmolecular weight of the resulting polymer is in relatively goodagreement with a previously reported chemical oxidative polymer-ization results by Colladet et al. [26]. The molecular structure ofthe synthesized polymer was confirmed by 1H NMR and elemen-tal analysis. 1H NMR spectrum of the polymer shows the peaksof alkyl side chains at 0.76–2.26 ppm and the methyleneoxy pro-tons ( CH2O ) appear at 3.97–4.04 ppm. Further, the aromaticphenyl ring protons appear at 6.50 ppm and the vinyl protons adja-cent to the phenyl rings are appearing at 7.44 ppm. The protonsof 1.3-di(thiophene-2-yl) ring of benzo[c]thiophene moiety adja-cent to the oligobenzo[c]thiophene rings and the aromatic phenylprotons on oligobenzo[c]thiophene system are found in the rangeof 6.87–7.29 ppm. Finally, the aliphatic protons resonated at 0.77,1.17, 1.31 and 2.22 ppm respectively. Thus, from the 1H NMR spec-trum (Fig. S2, Supporting information), the assigned peaks arefound consistent with the proposed structure.

The thermal property of CN-PTBTBPB polymer was investigatedby means of thermal gravimetric analysis (Fig. S3, Supporting infor-mation). The CN-PTBTBPB polymer showed three stages weightloss, first weight loss at 235 ◦C and the second weight loss takesplace at 260 ◦C then stable up to 350 ◦C. The third weight lossappears at 420 ◦C and stable up to 700 ◦C, which can be attributedto decomposition of the conjugated polymer back bone. It indicatesthat CN-PTBTBPB is thermally stable up to 400 ◦C and thus CN-PTBTBPB is a good potential candidate material for optoelectronicapplications.

3.2. Photophysical properties

The normalized UV–vis absorption spectrum and molar extinc-tion coefficient spectrum of polymer CN-PTBTBPB (both in solutionmedium and as well as thin film prepared via drop-cast method)were acquired in dilute chloroform solution (1 × 10−5 M) (Fig. 2(a)and (b)) and also the data are presented in Table 2. Thepolymer in chloroform solution exhibits two distinct absorp-tion bands: one broad absorption band in the UV region(300–400 nm) with band maxima at 375, 398 (shoulder peak)and 428 nm, which is probably attributed to the �–�* electrontransition of the entire close analogues of the �-conjugated 1,3-di(thiophene-2-yl)benzo[c]thiophene system and the other longerwavelength absorption bands in the visible region (450–530 nm)with absorption maxima at 456 nm, 489 nm and 526 nm canbe assigned to the intramolecular charge transfer (ICT) between1,3-di(thiophene-2-yl)benzo[c]thiophene conjugated system and1,4-biscyanomethy-l-2-5-bis[hexyloxy]benzene segment and theincreased conjugation length of the polymer backbone. Whereasthe absorption spectrum of the polymer CN-PTBTBPB thin film(�max = 530 nm) is red-shifted by 4 nm with respect to theirsolution-state spectrum. Such a red shift of the polymer in thinfilm is a consequence of intermolecular �–� stacking interac-tion in the solid state. Further, in order to identify the solventeffects on absorption spectrum of CN-PTBTBPB polymer, the UV–visabsorption spectra of CN-PTBTBPB polymer were also acquired innon-polar and polar solvents such as chlorobenzene, tetrahydro-furan (THF) and methanol (Fig. 2(a)). In non-polar solvents such aschlorobenzene and THF, the polymer CN-PTBTBPB exhibits strongabsorption maxima at 538 nm and 526 nm which may be due to dif-ferent ICT interactions. (The absorption maxima were successfullydeconvoluted into three distinct bands (see Fig. S4 in SI) at 482 nm,542 nm and 578 nm in chlorobenzene and at 455 nm, 491 nm and

540 nm in THF.) Whereas in the polar solvent (methanol), the poly-mer CN-PTBTBPB exhibits broad band shoulder in the region of400–600 nm, which was deconvoluted into three broad absorp-tion bands at 404 nm, 488 nm and 505 nm, which indicates that

38 M.R. Raj et al. / Journal of Photochemistry and Photobiology A: Chemistry 262 (2013) 34– 44

Table 1Polymerization results for CN-PTBTBPB polymer.

Polymer Mn (kg/mol)a Mw (kg/mol)a Mz (kg/mol)a PDI Yieldsb

CN-PTBTBPB 128 146 162 1.14 70%

ttbtostbowwBwv

Facww

a Determined from GPC (eluent: THF; polystyrene standards).b Isolated polymer yield upon polymerization.

he polar solvent significantly affects the absorption properties ofhe polymer. In addition, the reason for the origin of the observedroad band at 550–620 nm in Fig. 2a is due to either the coexis-ence of an intrachain molecules or a kind of aggregation statesf polymer appeared in solution. Generally, such aggregated stateshow weaker vibronic bands [36]. Furthermore, this polymer solu-ion upon excited at 490 nm exhibits two distinct intense emissionands one at 532 nm and the other as shoulder peak at 562 nm inrange region (Fig. 2(c)). Under excitation at the other absorptionavelengths (450 and 535 nm), the exhibited fluorescence spectraere almost identical to that obtained under excitation at 510 nm.

oth the excitation and fluorescence bands show a fine structureith two peaks and one shoulder. The excitation spectrum has a

ibronic structure with two peaks at 480 nm and 516 nm and one

ig. 2. (a) Normalized UV–vis absorption spectra of polymer CN-PTBTBPB in different snd in addition thin films prepared via drop-cast method in dilute chloroform solution

hloroform solution (1 × 10−5 M). (c) Fluorescence emission spectrum of CN-PTBTBPB polyhen excited at 490 nm) and excitation spectrum (�max

emi= 516 nm detected with emission

ith 530 ± 2 nm pulsed excitation.

shoulder at 450 nm, while fluorescence peaks appears at 532 nmand 562 nm. It is important to note that almost the mirror symmetrybetween the excitation and emission spectra is observed, althoughthe relative intensity of fluorescence peaks differs slightly from thatof the excitation peaks. The excitation and fluorescence bands arereminiscent of those of oligobenzo[c]thiophene analogues. Thus,it confirms that the resulting polymer emits mostly the intenseblue colour. In general, for polyaromatic molecules in solution, theexcitation spectrum of a single fluorescent species (pure emittingspecies) in dilute solution is identical to the absorption spec-trum. However in our system, there is a discrepancy between

the absorption and the excitation spectra of the polymer in solu-tion indicating the presence of small amounts of non-fluorescentspecies. Time-resolved fluorescence recorded at the emission

olvents (CHCl3, chlorobenzene, THF and methanol) of concentration (1 × 10−5 M)(1 × 10−5 M). (b) Molar extinction coefficient spectrum of CN-PTBTBPB polymer inmer in chloroform solution (1 × 10−5 M), where emission spectrum (�max

emi= 562 nm

wavelength of 560 nm). (d) Time-resolved emission decay of CN-PTBTBPB polymer

M.R. Raj et al. / Journal of Photochemistry and Photobiology A: Chemistry 262 (2013) 34– 44 39

Table 2Optical properties, electrochemical properties, and electronic energy levels of CN-PTBTBPB polymer.

Polymer �max

(nm)/(εmax

M−1 cm−1)

�edge (nm)a �edge (nm)b Eoptg (eV)c/Eopt

g (eV)d Eonsetox (V)e/HOMO (eV)f Eonset

red(V)g/LUMO (eV)h Eg,ec (eV)k

CN-PTBTBPB 357 (103,738)375 (99,965)489 (47,153)526 (62,242)

629 635 1.97/1.95 +1.12/−5.84 −0.85/−3.87 1.97

a Onset point from absorption spectrum.b Onset point from absorption spectrum in solid state.c Optical band gap in chloroform solution.d Optical band gap in polymer thin film.e Onset point from oxidation potential.f HOMO level derived from onset point of oxidation potential.

m(rds

3

itapTbfaa(ee

H

L

E

Fd

g Onset point from reduction potential.h LUMO level derived from onset point of reduction potential.k Electrochemically determined energy band gap.

aximum of �max = 530 nm (Fig. 2(d)) shows that the residualcharge transfer) emission of CN-PTBTBPB polymer in dilute chlo-oform has a long lifetime component from 6 ns to 46 ns. This longerecay time may be due to intramolecular charge transfer (ICT) emis-ion taking place over the entire conjugated polymer system.

.3. Electrochemical properties

The electrochemical properties of CN-PTBTBPB were examinedn solution using cyclic voltammetry measurements to evaluateheir ionization potentials/electron affinities (HOMO/LUMO levels)nd to understand charge injection processes in the as synthesizedolymer and also for potential utility to fabricate solar cell devices.he polymer CN-PTBTBPB showed multiple quasi-reversible redoxehaviours (Fig. 3(a)). Based on the standard redox potential oferrocene/ferrocenium ion (Fc/Fc+), which is assumed to have anbsolute energy level of −4.8 eV relative to vacuum level [37]nd from the measured first oxidation potential for CN-PTBTBPBlocated at 0.12 V), we evaluate the HOMO and LUMO energy lev-ls as well as electrochemical band gap (Eg, ec) using the followingquations:

OMO (eV) = −e(Eonsetox + 4.72) (eV)

UMO (eV) = −e(Eonsetred + 4.72) (eV)

g,ec = Eonsetox − Eonset

red (eV)

ig. 3. (a) Cyclic voltammetric curve of CN-PTBTBPB in 0.1 M solution of n-Bu4NPF6 in

onating CN-PTBTBPB (P1) and acceptor PCBM.

where Eonsetox and Eonset

red are the oxidation and reduction poten-tials relative to Ag/Ag+ electrode. The electrochemical propertiesas well as the energy level parameters of polymer are listedin Table 2. The onset of CN-PTBTBPB oxidation is +1.12 V andreduction is −0.85 V vs. Ag/Ag+ electrode which correspond toHOMO level of −5.84 eV and a LUMO level of −3.87 eV. The rea-son for such low-lying HOMO energy level for CN-PTBTBPB maybe due to strong electron-donating effect of 1,3-di(thiophene-2-yl)benzo[c]thiophene analogues rendered by the presence ofelectron-rich sulphur and nitrogen heteroatoms. It reflects thatoligobenzo[c]thiophene analogues allow the modulation of energylevel (HOMO and LUMO energy level) of the polymer. It is also worthnoting that the calculated electrochemical band gap (Eg,ec = 1.97 eV)and the estimated optical band gap of CN-PTBTBPB are relativelyin good agreement for the resulting polymer. A complete pictureof the energy band diagram of CN-PTBTBPB polymer (Fig. 3(b))was constructed which shows low-lying HOMO energy level (ca.−5.84 eV) of the polymer is well below the air stable to oxidationthreshold (i.e. ca −5.2 eV) [21,38], which may generally yield a highopen-circuit potential (Voc) value in polymer solar cells since Voc islinearly correlated between the HOMO of the donor and the LUMOof acceptor. Furthermore, the relatively larger HOMO band offset ofabout ca. 0.66 eV between the HOMO of CN-PTBTBPB and the HOMOof PCBM. Also, the large LUMO band offset of about ca. 0.43 eV

between the LUMO of CN-PTBTBPB and the LUMO of PCBM alwaysguarantees for efficient electron dissociation at the donor/acceptorinterface in the polymer solar cells. Further, the LUMO energy level(ca.−3.87 eV) of the polymer (CN-PTBTBPB) somewhat raised above

ACN:CHCl3 (2:3) solution, at scan rate of 100 mV/s. (b) Energy band diagram for

40 M.R. Raj et al. / Journal of Photochemistry and Ph

toHlt

3

eboactbilt

Fig. 4. B3LYP/6-31g(d) optimized ground state geometry of CN-PTBTBPB.

he threshold value upon comparing with the LUMO energy levelf PCBM (ca. 4.0–4.3 eV) [25,39]. Thus, it assures that the low-lyingOMO (ca. −5.84 eV) energy level and LUMO (ca. −3.87 eV) energy

evel of CN-PTBTBPB polymer is a good candidate for sufficient pho-oinduced electron transfer to soluble fullerene acceptors [40].

.4. Theoretical calculations

With a view to understand the structural and electronic prop-rties of this polymer at the molecular level, DFT calculations haveeen carried out at B3LYP/6-31g(d) level. The optimized geometryf CN-PTBTBPB is given in Fig. 4. It shows CN-PTBTBPB is a planarnd symmetric molecule and the important bond lengths are indi-ated in Fig. 4. From the computed results, it is clear that excepthe arms of the CN-PTBTBPB, remaining C C bonds lengths fall

etween 1.37 and 1.47 A. This reveals that the all the C C bonds

n this polymer backbone are partial double bonds. This is an excel-ent evidence for the comprehensive �-delocalization throughouthe molecule. In order to characterize the optical transitions, it

Fig. 5. Computed frontier molecular orbitals and

otobiology A: Chemistry 262 (2013) 34– 44

is useful to examine the frontier molecular orbitals, since theseorbitals play a dominant role. Fig. 5 shows the frontier molecularorbitals (HOMO, HOMO-1, HOMO-2, and HOMO-3, LUMO, LUMO+1,LUMO+2 and LUM) +3) and the corresponding energy level diagramcomputed at B3LYP/6-31g(d) level. From the figure it is interest-ing to note that the HOMO and LUMOs are evenly localized onthe whole �-conjugated system. On the other hand, HOMO-1 andLUMO+1 are predominantly localized on either ends of the poly-mer. This indicates that HOMO → LUMO+1 and HOMO-1 → LUMOtransitions arise from intramolecular charge-transfer (ICT) and theother absorptions are mainly of � → �* nature of this polymer.The estimated band gaps for the polymer CN-PTBTBPB obtainedfrom the low energetic edge at 629 nm of the absorption spec-trum in chloroform solution and that of the thin film at 635 nm areEopt

g = 1.97 eV and Eoptg = 1.95 eV, respectively. The calculated band

gap from HOMO (ca. −4.77 eV) and LUMO (ca. −2.61 eV) energylevel of CN-PTBTBPB by DFT is found to be EDFT

g = 2.16 eV. In gen-eral, the calculated band gap is relatively good agreement withexperimentally obtained band gap.

The lowest excited states were calculated by means of time-dependent DFT calculations (TDDFT) at CAM-B3LYP/6-31g(d) inan effort to rationalize the nature of electronic transitions, thecontributing configurations to the transitions and charge transferprobability. The details of the electronic transitions namely excita-tion energies, oscillator strength, and contributing configurationsfor the first ten most probable electronic transitions of CN-PTBTBPBare summarized in Table 3. It is interesting to note that most of theprominent bands are due to � → �* excitations and some ICTs arealso observed. The calculated results show that transition locatedat 525 nm and with larger oscillator strength corresponds to theexperimentally observed band at 526 nm. Singly excited configura-tions HOMO → LUMO (72%) and HOMO-1 → LUMO+1 (22%) mainlycontribute to this state. Similarly, the 465 nm peak arises fromHOMO → LUMO+1 (46%) and HOMO-1 → LUMO (45%) transitions

and has lower oscillator strength weak intensity (f = 0.0139) but anintense peak is observed at 303 nm (f = 0.1594). This peak resultsfrom the transitions of HOMO-1 → LUMO+3 and HOMO-3 → LUMOwith 27 and 19% contributions, respectively. In general our

its energy level diagram of CN-PTBTBPB.

M.R. Raj et al. / Journal of Photochemistry and Photobiology A: Chemistry 262 (2013) 34– 44 41

Table 3The vertical excitation energies and oscillator strengths of CN-PTBTBPB moleculein gas phase and their corresponding contributing singly excited configurationscomputed at CAM-B3LYP/6-31G(d) level.

�abs (nm) f �E (eV) Assignment

525 3.0153 2.3627 HOMO → LUMO (72%)HOMO-1 → LUMO+1(22%)

465 0.0139 2.6065 HOMO → LUMO+1(46%)

380 0.0283 3.3662 HOMO → LUMO+2(32%)HOMO-2 → LUMO(31%)

356 0.0017 3.4847 HOMO-1 → LUMO(35%)HOMO-1 → LUMO+2(19%)

340 0.0334 3.6496 HOMO-2 → LUMO(22%)

339 0.0351 3.6521 HOMO → LUMO+3(20%)HOMO-2 → LUMO(18%)

313 0.0679 3.9610 HOMO-3 → LUMO(44%)HOMO → LUMO (7%)

303 0.1594 4.0930 HOMO-1 → LUMO+3(27%)HOMO-3 → LUMO(19%)

300 0.0025 4.1353 HOMO-4 → LUMO(32%)HOMO → LUMO+3(11%)

283 0.0008 4.3783 HOMO-7 → LUMO+1(43%)

co

teerittctc

3

hocasPeai

0.0 0.2 0.4 0.6 0.8 1.0

1E-6

1E-5

1E-4

1E-3

0.01

0.1

1

10

Cu

rre

nt

de

ns

ity

(m

Ac

m-2

)

Vol tage (V)

Dark

AM 1.5G condition

devices with in an uncertainty of ±5%.Generally, the thickness of the photoactive organic materials

coated is ∼80 nm showing extinction coefficients in the range of

350 40 0 45 0 50 0 55 0 60 0 65 0 70 0 75 0 80 00.0

0.4

0.8

1.2

1.6

CN-PTBTBPB:PCB M (1:1 w/w%)

CN-PTBTBPB:PCB M (1:1 w/w%)

Wavelength(nm)

Ab

so

rba

nc

e (

a.u

.)

0

10

20

30

IPC

E (%

)

Fig. 7. The absorbance and photoaction (IPCE) spectra of the CN-PTBTBPB:PCBM(1:1%, w/w) blend film.

Table 4Photovoltaic properties of the BHJ solar cell based on CN-PTBTBPB:PCBM (D1) solar

2

HOMO → LUMO+5(23%)

alculated results are in good agreement with experimentallybserved results.

The absorption wavelength increases with the introduc-ion of electron donating properties of functionalized oligob-nzo[c]thiophene analogues, which is in good agreement withxperimental normalized absorption spectrum in Fig. 2(a). Thiseflects in computed dipole moment also where the dipole moments slightly larger in the first excited state (1.0807 D) compared tohe ground state (0.9087 D). Consequently, the computed oscilla-or strength is lower due to the long-range charge shift and thisan be assigned to a combination of � bonding orbitals extendinghroughout the entire �-conjugated system and this may cause aascading effect in aiding the charge separation [41].

.5. Photovoltaic device performance

In order to investigate the photovoltaic properties, typical bulketerojunction (BHJ) devices were made by sandwiching a mixturef as synthesized donor (CN-PTBTBPB) with soluble fullerene-basedomponent (PCBM) between PEDOT:PSS coated ITO and Al, as holend electron collecting electrodes, respectively. The BHJ devicetructure is represented as follows: [ITO/PEDOT:PSS (25 nm)/CN-

TBTBPB:PCBM (80 nm) (1:1, w/w)/Al (100 nm); D1]. In order toxamine the device performance, the current density (J) – volt-ge (V) characteristics of the fabricated device have been measuredn the dark and under AM 1.5 G illumination (100 mW/cm2) from

Fig. 6. J–V characteristics of ITO/PEDOT:PSS/CN-PTBTBPB:PCBM (1:1 wt%)/Al (D1)device.

a calibrated solar simulator with simultaneous recording of theirsemi-logarithmic plot of current–voltage characteristics (Fig. 6) offour isolated cells individually by masking other three cells. Almostall the four cells show similar results within an uncertainty of ±5%.For the [ITO/PEDOT:PSS (25 nm)/CN-PTBTBPB:PCBM (80 nm) (1:1,w/w)/Al (100 nm); D1] device, we observed an open circuit voltage(Voc) of about 0.68 V, a short circuit current density (JSC) of about4.78 mA/cm2, and a fill factor (FF) of 0.41. The device performanceresults are tabulated in Table 4. It should be pointed out that thesephotovoltaic devices have no output potential at zero current inthe dark as expected. Reproducibility of the photovoltaic resultspresented above was confirmed by testing three such photovoltaic

cell (AM 1.5 G, 100 mW/cm ) at open air condition.

Device Jsc (mA/cm2) Voc (V) FF PCE (%)

D1 4.78 0.68 0.41 1.33

42 M.R. Raj et al. / Journal of Photochemistry and Photobiology A: Chemistry 262 (2013) 34– 44

F end filh

1ftfilaaapaJPtito3oamsatt

ttyclattdgd

dtsvdrdti

ig. 8. AFM tapping mode images (5 �m × 5 �m) of (a) CN-PTBTBPB:PCBM (1:1) bleight vs. distance) of CN-PTBTBPB:PCBM (1:1) blend film.

–2 × 105 cm−1, leading to sufficient light absorption, since the dif-usion length of excitons in the organic materials is typically inhe range of 5–10 nm [42]. Hence, most researchers have chosenlm thickness of 80 nm for the device fabrication with the active

ayer composition (donor polymer:PCBM (1:1%, w/w)) [43–46]. Inddition, enhancing the active layer composition (about 1:2, 1:3nd 1:4%, w/w) or thickness (about 180–200 nm) is one possiblepproach to harvest maximum light absorption resulting in higherhotovoltaic performance. In such a scenario, when increasing thective layer composition beyond 1:2% (w/w), leads to a decrease in

sc and overall PCE, which may be attributed to a disruption of theCBM clusters in the polymer matrix. This could adversely affecthe morphology, photoinduced charge transfers and an increasen series resistance in BHJ solar cells [47]. The photoaction spec-rum (IPCE) of CN-PTBTBPB:PCBM (D1) device in Fig. 7 shows annset at about 745 nm (1.66 eV) and light covers a wide range from30 nm to 800 nm. The IPCE (%) of D1 device exhibits a maximumf 31% at 490 nm. Upon viewing the action spectrum with that ofbsorption spectrum, the action spectrum of the device does notatch well with the absorption spectrum of the polymer. The pos-

ible reason is that most of the excitons generated by illuminationt the ITO/PEDOT:PSS/CN-PTBTBPB:PCBM device may either radia-ively recombine or form charge traps during exciton diffusion inhe polymer matrix upon interaction with the PCBM [48].

The characteristic high open circuit voltage (Voc = 0.68 V) forhe device (D1) using CN-PTBTBPB polymer was indicative ofhe fused electron-pushing properties of 1,3-di(thiophene-2-l)benzo[c]thiophene segment by benzene ring system throughyanovinylene linkage favour a key role in low-lying HOMO energyevel of the polymer (CN-PTBTBPB), which was relatively in goodgreement with the results from the electrochemical analysis andheoretical calculations. It can be seen that the considerable fill fac-or observed could be attributed to the possible limited excitonissociation efficiency in the device system. These findings sug-est that appropriate film morphology is crucial for improving theevice performance effectively.

Hence the study of film morphology using AFM on optimizedevice has provided deep insight into the significant difference inhe power conversion efficiency of CN-PTBTBPB:PCBM device ashown in Fig. 8(a). The CN-PTBTBPB:PCBM blend film showed aery smooth surface. The cross-sectional image (surface height vs.istance) of CN-PTBTBPB:PCBM (1:1) blend film showed low RMS

oughness around <2 nm (Fig. 8(b)). As a result, CN-PTBTBPB:PCBMevice shows a smooth surface morphology which is responsible forhe generation of free charge carriers to reach the donor/acceptornterface.

m in chlorobenzene and annealed at 120 ◦C. (b) The cross-sectional image (surface

4. Conclusions

In summary, we have successfully synthesized a low-lyingHOMO energy level conjugated (CN-PTBTBPB) polymer that con-sists of donor moiety (1,3-di(thiophene-2-yl)benzo[c]thiophene)incorporated at the bis-(cyano-2-thienylvinylene) phenylenelinkage. The incorporation of 1,3-di(thiophene-2-yl)benzo[c]thiophene moiety in conjugated polymer leads toalter the HOMO–LUMO energy levels was determined by exper-imental methods and DFT theoretical calculations. Due to suchenergy level alterations, the resulting polymer absorbs extendedlight up to the near-infrared region and shows a relatively higherabsorption coefficient. The fabricated solar device based onCN-PTBTBPB:PCBM exhibited the power conversion efficiency of1.33%.

Acknowledgements

The author S.A. thanks DST, New Delhi (Ref. No.SR/S1/PC-49/2009) for the sanction of major research project. Also theauthors (S.A. and M.A.) thank DST, New Delhi and DEST, Australiafor the sanction of INDIA-AUSTRALIAN collaborative research fundfor their collaborative research. Author S.A. thanks DST for sanc-tioning FIST (SR/FST/CSI-190/2008 dated 16th March 2009) andNanomission projects.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version, at http://dx.doi.org/10.1016/j.jphotochem.2013.04.013.

References

[1] Y.-J. Cheng, S.H. Yang, C.-S. Hsu, Synthesis of conjugated polymers for organicsolar cell applications, Chemical Reviews 109 (2009) 5868–5923.

[2] (a) B.C. Thompson, J.M.J. FrØchet, Polymer-Fulleren-Solarzellen, AngewandteChemie 120 (2008) 62–82, B.C. Thompson, J.M.J. FrØchet, Polymer–fullerenecomposite solar cells, Angewandte Chemie – International Edition 47 (2008),58–77;(b) J. Brabec, N.S. Sariciftci, J.C. Hummelen, Plastic solar cells, Advanced Func-tional Materials 11 (2011) 15–26.

[3] A.C. Arias, J.D. MacKenzie, I. McCulloch, J. Rivnay, A. Salleo, Materials andapplications for large area electronics: solution-based approaches, Chemical

Reviews 110 (2010) 3–24.

[4] (a) Y. Liang, D. Feng, Y. Wu, S.-T. Tsai, G. Li, C. Ray, L. Yu, Highlyefficient solar cell polymers developed via fine-tuning of structural andelectronic properties, Journal of the American Chemical Society 131 (2009)7792–7799;

nd Ph

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

M.R. Raj et al. / Journal of Photochemistry a

(b) Y. Liang, Y. Wu, D. Feng, S.-T. Tsai, H.-J. Son, G. Li, L. Yu, Development ofnew semiconducting polymers for high performance solar cells, Journal of theAmerican Chemical Society 131 (2009) 56–57.

[5] Y.F. Li, Molecular design of photovoltaic materials for polymer solar cells:toward suitable electronic energy levels and broad absorption, Accounts ofChemical Research 45 (2012) 723–733.

[6] N.S. Sariciftci, L. Smilowitz, A.J. Heeger, F. Wudl, Photoinduced electron trans-fer from a conducting polymer to buckminsterfullerene, Science 258 (1992)1474–1476.

[7] H. Hoppe, N.S. Sariciftci, Organic solar cells: an overview, Journal of MaterialsResearch 19 (2004) 1924–1945.

[8] K.M. Coakley, M.D. McGehee, Conjugated polymer photovoltaic cells, Chemistryof Materials 16 (2004) 4533–4542.

[9] T. Munters, T. Martens, L. Goris, V. Vrindts, J. Manca, L. Lutsen, W. De Ceun-inck, D. Vanderzande, L. De Schepper, J. Gelan, N.S. Sariciftci, C.J. Brabec,A comparison between state-of-the-art ‘gilch’ and ‘sulphinyl’ synthesisedMDMO-PPV/PCBM bulk hetero-junction solar cells, Thin Solid Films 403/404(2002) 247–251.

10] G. Li, V. Shrotriya, Y. Yao, J. Huang, Y. Yang, Manipulating regioregular poly(3-hexylthiophene): [6,6]-phenyl-C61-butyric acid methyl ester blends—routetowards high efficiency polymer solar cells, Journal of Materials Chemistry 17(2007) 3126–3140.

11] A.C. Mayer, S.R. Scully, B.E. Hardin, M.W. Rowell, M.D. McGehee, Polymer-basedsolar cells, Materials Today 10 (2007) 28–33.

12] (a) F. Padinger, R.S. Rittberger, N.S. Sariciftci, Effects of postproductiontreatment on plastic solar cells, Advanced Functional Materials 13 (2003)85–88;(b) W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Thermally stable,efficient polymer solar cells with nanoscale control of the interpene-trating network morphology, Advanced Functional Materials 15 (2005)1617–1622.

13] J. Hou, H.-Y. Chen, S. Zhang, G. Li, Y. Yang, Synthesis, characterization, andphotovoltaic properties of a low band gap polymer based on silole-containingpolythiophenes and 2,1,3-benzothiadiazole, Journal of the American ChemicalSociety 130 (2008) 16144–16145.

14] (a) E. Wang, L. Wang, L. Lan, C. Luo, W. Zhuang, J. Peng, Y. Cao, High-performancepolymer heterojunction solar cells of a polysilafluorene derivative, AppliedPhysics Letters 92 (2008) 033307–33313;(b) S.H. Park, A. Roy, S. Beaupre, S. Cho, N. Coates, J.S. Moon, D. Moses, M. Leclerc,K. Lee, A.J. Heeger, Bulk heterojunction solar cells with internal quantum effi-ciency approaching 100%, Nature Photonics 3 (2009) 297–303.

15] (a) E.E. Havinga, W. Ten Hoeve, H. Wynberg, Alternate donor–acceptor small-band-gap semiconducting polymers: polysquaraines and polycroconaines,Synthetic Metals 55 (1993) 299–306;(b) M.R. Raj, S. Anandan, R. Vijay Solomon, P. Venuvanalingam, S.S.K. Iyer, M.Ashokkumar, Synthesis of conjugated perylene diimide-based copolymer with5,5′-bis(4-aminophenyl)-2-2′-bifuryl moiety as an active material for organicphotovoltaics, Journal of Photochemistry and Photobiology A: Chemistry 247(2012) 52–62.

16] P.A. Troshin, O.A. Mukhacheva, O. Usluer, A.E. Goryachev, A.V. Akkuratov, D.K.Susarova, N.N. Dremova, S. Rathgeber, N.S. Sariciftci, V.F. Razumov, D.A.M.Egbe, Improved photovoltaic performance of PPV-based copolymers usingoptimized fullerene-based counterparts, Advanced Energy MaterialsAdvancedEnergy Materials 3 (2013) 161–166.

17] P.A. Troshin, O.A. Mukhacheva, A.E. Goryachev, N.N. Dremova, D.Voylov, C. Ulbricht, D.A.M. Egbe, N.S. Sariciftci, V.F. Razumov, Materialstructure–composite morphology–photovoltaic performance relationship fororganic bulk heterojunction solar cells, Chemical Communications 48 (2012)9477–9479.

18] K. Colladet, M. Nicolas, L. Goris, L. Lutsen, D. Vanderzande, Low-band gappolymers for photovoltaic applications, Thin Solid Films 451/452 (2004)7–11.

19] N.S. Cho, D.-H. Hwang, B.-J. Jung, E. Lim, J. Lee, H.-K. Shim, Synthesis,characterization, and electroluminescence of new conjugated polyfluorenederivatives containing various dyes as comonomers, Macromolecules 37 (2004)5265–5273.

20] B.C. Thompson, Y.-G. Kim, J.R. Reynolds, Spectral broadening in MEH-PPV:PCBM-based photovoltaic devices via blending with a narrow bandgap cyanovinylene–dioxythiophene polymer, Macromolecules 38 (2005)5359–5362.

21] J.H. Hou, Z. Tan, Y.J. He, C.H. Yang, Y.F. Li, Branched poly(thienylene vinylene)swith absorption spectra covering the whole visible region, Macromolecules 39(2006) 4657–4662.

22] L. Huo, J. Hou, C. He, M. Han, Y. Li, Synthesis, characterization andphotovoltaic properties of poly[1′ ,4′-bis-(thienyl-vinyl)]-2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene, Synthetic Metals 156 (2006)276–281.

23] Y. Fang, H. Huang, P. Jiang, H. Li, P. Shen, Z. Tian, B. Zhao, S. Tan, Phenyleneviny-lene copolymers of dihexylthienylbenzothiadiazole and triphenylamine ortetraphenylbenzidine: synthesis, characterization and photovoltaic properties,Journal of Materials Science 47 (2012) 5706–5714.

24] B.C. Thompson, Y.-G. Kim, T.D. McCarley, J.R. Reynolds, Soluble nar-row band gap and blue propylenedioxythiophene–cyanovinylene poly-mers as multifunctional materials for photovoltaic and electrochromicapplications, Journal of the American Chemical Society 128 (2006)12714–12725.

[

otobiology A: Chemistry 262 (2013) 34– 44 43

25] E.M. Galand, Y.-G. Kim, J.K. Mwaura, A.G. Jones, T.D. McCarley, V. Shrotriya,Y. Yang, J.R. Reynolds, Optimization of narrow band-gap propylene-dioxythiophene:cyanovinylene copolymers for optoelectronic applications,Macromolecules 39 (2006) 9132–9142.

26] K. Colladet, S. Fourier, T.J. Clej, L. Lutsen, J. Gelan, D. Vanderzande, L.H. Nguyen,H. Neugebauer, S. Sariciftci, A. Aguirre, G. Janssen, E. Goovaerts, Low band gapdonor–acceptor conjugated polymers toward organic solar cells applications,Macromolecules 40 (2007) 65–72.

27] (a) G.D. Sharma, P. Suresh, S.S. Sharma, Y.K. Vijay, J.A. Mikroyannidis,Effect of solvent and subsequent thermal annealing on the performance ofphenylenevinylene copolymer: PCBM solar cells, ACS Applied Materials & Inter-faces 2 (2010) 504–510;(b) J.A. Mikroyannidis, A.N. Kabanakis, P. Balraju, G.D. Sharma, Enhancedperformance of bulk heterojunction solar cells using novel alternatingphenylenevinylene copolymers of low band gap with cyanovinylene 4-nitrophenyls, Macromolecules 43 (2010) 5544–5553.

28] H.A. Ho, H. Brisset, P. Frere, J. Roncali, Electrogenerated small bandgap�-conjugated polymers derived from substituted dithienylethylenes, Jour-nal of the Chemical Society, Chemical Communications 1995 (1995)2309–2310.

29] H. Spreitzer, W. Kreuder, H. Becker, H. Schoo, R. Demandt, Aryl-substitutedpoly(p-arylene-vinylenes), process for their preparation and their use in elec-troluminescence components, US Patent No. 6,458,909.

30] A.K. Mohanakrishnan, M.V. Lakshmikantham, C. McDougal, M.P. Cav, J.W. Bald-win, R.M. Metzger, Studies in the dithienylbenzo[c]thiophene series, Journal ofOrganic Chemistry 63 (1998) 3105–3112.

31] P. Amaladass, J. Arul Clement, A.K. Mohanakrishnan, Synthesis and charac-terization of benzannelated thienyl oligomers, European Journal of OrganicChemistry 22 (2008) 3798–3810.

32] R. Sugimoto, S. Takeda, H.B. Gu, K. Yoshino, Preparation of soluble polythio-phene derivatives utilizing transition metal halides as catalysts and theirproperty, Chemistry Express 1 (1986) 635–638.

33] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato,X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada,M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,O. Kitao, H. Nakai, T. Vreven Jr., J.A. Montgomery, J.E. Peralta, F. Ogliaro, M.Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Nor-mand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N.Rega, N.J. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo,R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W.Ochterski, R.L. Martin, K.K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador,J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J.Cioslowski, D.J. Fox, Gaussian 09, revision 02, Gaussian, Inc., Wallingford, CT,2009.

34] B.C. Lin, C.P. Cheng, Z.P.M. Lao, Reorganization energies in the transports ofholes and electrons in organic amines in organic electroluminescence stud-ied by density functional theory, Journal of Physical Chemistry A 107 (2003)5241–5251.

35] T. Yanai, D.P. Tew, N.C. Handy, A new hybrid exchange-correlation functionalusing the Coulomb-attenuating method (CAM-B3LYP), Chemical Physics Let-ters 393 (2004) 51–57.

36] M.R. Raj, S. Ramkumar, S. Anandan, Photovoltaic studies on perylenediimide-based copolymers containing electronic push–pull chromophores, RSCAdvances 3 (2013) 5108–5120.

37] (a) J. Pommerehne, H. Vestweber, W. Guss, R.F. Mahrt, H. Bassler, M. Porsch, J.Daub, Efficient two layer leds on a polymer blend basis, Advanced Materials 7(1995) 551–554;(b) Y.H. Zhou, P. Peng, L. Han, W.J. Tian, Novel donor–acceptor moleculesas donors for bulk heterojunction solar cells, Synthetic Metals 157 (2007)502–507.

38] D.M. De Leeuw, M.M.J. Simenon, A.R. Brown, R.E.F. Einerhard, Stability of n-typedoped conducting polymers and consequences for polymeric microelectronicdevices, Synthetic Metals 87 (1997) 53–59.

39] Y. Li, L. Xue, H. Li, Z. Li, B. Xu, S. Wen, W. Tian, Macromolecules 42 (2009)4491–4499.

40] J.C. Hummelen, B.W. Knight, F. Lepec, F. Wudl, J. Yao, C.L. Wilkins, Preparationand characterization of fulleroid and methanofullerene derivatives, Journal ofOrganic Chemistry 60 (1995) 532–538.

41] M.-S. Tsai, Y.-C. Hsu, J.T. Lin, H.-C. Chen, C.-P. Hsu, Organic dyes containing 1H-phenanthro[9,10-d]imidazole conjugation for solar cells, Journal of PhysicalChemistry C 111 (2007) 18785–18793.

42] P.E. Shaw, A. Ruseckas, I.D.W. Samuel, Exciton diffusion measure-ments in poly(3-hexylthiophene), Advanced Materials 20 (2008)3516–3520.

43] Y. Lee, T.P. Russell, W.H. Jo, Synthesis and photovoltaic properties oflow-bandgap alternating copolymers consisting of 3-hexylthiophene and[1,2,5]thiadiazolo[3,4-g]quinoxaline derivatives, Organic Electronics 11 (2010)846–853.

44] N. Tore, E.A. Parlak, O. Usluer, D.A.M. Egbe, S.E. San, P. Aydogan, Effect of blendratio on poly(p-phenylene–ethynylene)-alt-poly(p-phenylene–vinylene)

polymer solar cell, Solar Energy Materials and Solar Cells 104 (2012)39–44.

45] Y. Liu, X. Wan, B. Yin, J. Zhou, G. Long, S. Yin, Y. Chen, Efficient solution processedbulk-heterojunction solar cells based a donor–acceptor oligothiophene, Journalof Materials Chemistry 20 (2010) 2464–2468.

4 and Ph

[

[

[48] J.J. Benson-Smith, L. Goris, K. Vandewal, K. Haenen, J.V. Manca, D. Vanderzande,

4 M.R. Raj et al. / Journal of Photochemistry

46] Y. Wu, Y. Jing, X. Guo, S. Zhang, M. Zhang, L. Huo, J. Hou, A thieno[3,4-f]isoindole-

5,7-dione based copolymer for polymer solar cells, Polymer Chemistry 4 (2013)536–541.

47] G. Namkoong, J. Kong, M. Samson, I.-W. Hwang, K. Lee, Active layer thicknesseffect on the recombination process of PCDTBT:PC71BM organic solar cells,Organic Electronics 14 (2013) 74–79.

otobiology A: Chemistry 262 (2013) 34– 44

D.D.C. Bradley, J. Nelson, Formation of a ground-state charge-transfer complexin polyfluorene//[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) blend filmsand its role in the function of polymer/PCBM solar cells, Advanced FunctionalMaterials 17 (2007) 451–457.