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Journal of Photochemistry and Photobiology A: Chemistry 247 (2012) 52– 62

Contents lists available at SciVerse ScienceDirect

Journal of Photochemistry and Photobiology A:Chemistry

journa l h o me pag e: www.elsev ier .com/ locate / jphotochem

ynthesis of conjugated perylene diimide-based copolymer with,5′-bis(4-aminophenyl)-2-2′-bifuryl moiety as an active material for organichotovoltaics

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

Nanomaterials & Solar Energy Conversion Lab, Department of Chemistry, National Institute of Technology, Tiruchirappalli 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, Australia

r t i c l e i n f o

rticle history:eceived 26 December 2011eceived in revised form 6 July 2012ccepted 17 July 2012vailable online 30 August 2012

eywords:

a b s t r a c t

In this study, we have synthesized conjugated n-type perylene tetracarboxylic diimide-based copolymerwith 5,5′-bis(4-aminophenyl)-2-2′-bifuryl segment (PTCDI-PFDA) as an active material for the fabricationof bulk heterojunction photovoltaic device of type ITO/PEDOT:PSS/rr-P3HT:PTCDI-PFDA/Al. The synthe-sized n-type copolymer (an oligomer with a relatively low molecular weight) was characterized by 1HNMR, gel permeation chromatography and thermo gravimetric analysis. The electronic and structuralproperties of the copolymer were investigated by UV–vis and fluorescence spectroscopy. The copoly-

onjugated copolymerulk heterojunctionolar cellFT calculations

mer exhibited an optical band gap of about 1.59 eV. The HOMO and LUMO energy levels were calculatedusing cyclic voltammetry and density functional theory (DFT). The resulting copolymer was experimen-tally found to possess low-lying HOMO (ca. −5.24 eV) and high-lying LUMO (ca. −3.9 eV) energy levels.The fabricated device (D1) delivered an efficiency (�) of about 0.34% with a high open circuit voltage(Voc = 0.92 V), current density (Jsc = 2.83 mA/cm2), and fill factor (FF = 13%) under 100 mW/cm2 white light.

. Introduction

As an alternative to the mainstream silicon solar cells, a world-ide search is on for new types of solar cells that can be produced

t low cost [1–4]. Organic photovoltaics (PVs) based on conjugatedolymers have the potential to harness solar energy in a cost-ffective way because of their virtue of lightweight, flexibility andasy fabrication [1,5]. Consequently, the efficiencies of conjugatedolymer-based PV cells received a major boost with implementa-ion of the bulk heterojunction (BHJ) concept consisting of electrononor and acceptor molecules in a disordered bicontinuous inter-enetrating network [4,6]. The distributed donor/acceptor BHJ

n a nanoscale facilitates effective dissociation of photoinducedtrongly-bound excitons (electron–hole pairs) into free chargescross the active layer while maintaining exciton diffusion length of

∗ 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 © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jphotochem.2012.07.019

© 2012 Elsevier B.V. All rights reserved.

conjugated polymer ∼10 nm at the donor/acceptor interface [7,8].Simultaneously, the sufficient percolation pathways of donors andacceptors allow the charge carriers to migrate towards the elec-trodes via different phases. Thus, a successful BHJ polymer solardevice fabricated to date consists of blended regioregular 2,5-diyl-poly(3-hexylthiophene) (rr-P3HT) as a donor and a derivative offullerene ([6,6]-phenyl-C16-butyric acid methyl ester; PCBM) as anacceptor and delivers a bottle-neck in achieving high power conver-sion efficiency values (4–5%) [9,10]. The reason is rr-P3HT polymerabsorbs up to 22% of the influx photons of solar spectrum fol-lowed by fullerene-based materials (PCBM). However, BHJ devicesmade from PCBM are generally less efficient with some excep-tions [11]. This is because, there are some drawbacks of PCBM inelectronic and optoelectronic device applications, which are prob-ably due to their weak molar absorption coefficient at the visibleregion and the possibility of phase separation from the polymerdonors. In addition, fullerene (PCBM) derivatives may lead to (i)unmatched energy levels yielding a low open circuit voltage (Voc)

and (ii) the formation of micrometer-sized PCBM phase-segregateddomains resulting in low charge carrier mobility which leads to alow short circuit current (Jsc) and a fill factor (FF). Consequently,it is important to pursuit for an alternative approach by replacing

nd Ph

ta(oetpoloeoer

tdlosts(�oioPrTPm

2

2

RssecgepdS5h

2

trtppaeoN

2

b

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

he fullerene-based materials with non-fullerene acceptor materi-ls such as low molecular weight perylene tetracarboxylic diimidePTCDIs) derivatives or its polymers since they represent a classf highly thermostable n-type semiconductors with relatively highlectron affinity and high charge carrier mobility for efficient pho-ovoltaic devices [12,13]. Apparently, they can absorb maximumhoton flux density efficiently in the visible and near-IR regionf the solar spectrum leading to high quantum yields of photo-uminescence and also possess an appropriate frontier molecularrbitals alignment for improving light-harvesting properties andfficient ultrafast exciton dissociation at the D–A interface. More-ver, their electronic and optical properties can be manipulatedasily by chemical functionalization to meet the high-performanceequirements [14].

Numerous perylene tetracarboxylic diimide (PTCDIs) deriva-ives have been widely used as acceptor materials in BHJ solarevices [15–20]. However, our approach is to synthesize pery-

ene tetracarboxylic diimide (PTCDIs)-based copolymer composedf conjugated segments as a component of the main chain and/oride chain because introduction of such components may increasehe electron accepting nature of perylene tetracarboxylic diimidekeleton. Hence, we combine perylene tetracarboxylic diimidePTCDI) with 5,5′-bis(4-aminophenyl)-2-2′-bifuryl (it consists of-conjugated units, i.e., furyl rings containing oxygen atoms) inrder to prepare a new kind of highly conjugated electron accept-ng copolymer (PTCDI-PFDA) with a good solubility in chlorinatedrganic solvents and a moderate solubility in THF. The synthesizedTCDI-PFDA copolymer shows significant absorption in the visibleegion of the solar spectrum and high electron affinity (EA = 3.9 eV).he EA value (LUMO level) of PTCDI-PFDA is higher than that ofCBM which indicates that it can be used as an alternative n-typeaterial in BHJ solar cells.

. 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 practical organic chemistry, 5thdition. Na2SO4 was used as the drying agent in all the purifi-ation procedures. High purity (99.99%) nitrogen and argonases were used to maintain an inert atmosphere. Poly-(3,4-thylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS),erylenetetracarboxylic diimide (PTCDI) and regioregular 2,5-iyl-poly(3-hexylthiophene) (rr-P3HT) were purchased fromigma–Aldrich and used without purification. Starting material,5′-bis(4-aminophenyl)-2-2′-bifuryl (PFDA) was synthesized at aigher yield by following the available literature method [21].

.1.1. Characterization methods1H NMR spectrum was recorded on BRUKER 400 MHz spec-

rometer using CDCl3 as the solvent with chemical shifts reportedelative to the TMS as the internal standard. Molecular weights ofhe copolymer were determined from calibrated curve based onolystyrene standards recorded on a gel permeation chromatogra-hy (GPC) at a flow rate of 1 ml/min o-dichlorobenzene solutiont 40 ◦C. Elemental analysis was performed with a Vario EL IIIlemental analyzer. Electrochemical measurements were carriedut using computer controlled AUTOLAB-potentiostat/galvanostat,etherlands.

.1.2. Photophysical measurementThe absorption and emission spectra were recorded on a dou-

le beam ultraviolet–visible spectrophotometer (Model: T90+, PG

otobiology A: Chemistry 247 (2012) 52– 62 53

Instruments Ltd, UK) and Shimadzu spectrofluorometer (RF-5301PC), respectively. The samples were degassed using pure argon gasfor 15 min prior to each experiment. Fluorescence lifetime mea-surements were carried out in a picosecond time correlated singlephoton counting (TCSPC) spectrometer with a tunable Ti-sapphirelaser (TSUNAMI, Spectra physics, USA) as the excitation source of450–530 nm excitation wavelength. The fluorescence decay curveswere analyzed using the software provided by IBH (DAS-6).

2.1.3. ElectrochemistryElectrochemical measurements were recorded in a conventional

three-electrode system using Autolab electrochemical analyzer. Aplatinum wire counter electrode, a glassy carbon disc working elec-trode (3 mm) while and Ag/AgCl reference electrode were usedin the cell. Before initiating the experiments, pure argon gas waspurged through the solution for 15 min in order to remove dissolvedoxygen. The cyclic voltammogram of the copolymer was obtainedby dissolving 5 mM of the solution in acetonitrile containing 0.1 Mtetrabutylammonium hexafluorophosphate as supporting elec-trolyte. Later, ferrocene (Fc) was added to the mixture as an internalstandard in order to calibrate the potential of the quasi referenceelectrode (Ag/AgCl electrode). The HOMO and LUMO energies ofcopolymer were determined based on a value of −4.8 eV for Fc/Fc+

with respect to the zero vacuum level.

2.1.4. Film morphologyTopographic and phase images of blended polymer (active

material) were measured using a Nanoscope IV Digital instrument(Veeco innova Model: DiSPM Lab version.5.01/6.0), operated in tap-ping mode with commercially available Si cantilevers (Micromash,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.

2.2. Density functional theory

All calculations were performed using Gaussian 09 programmepackage [22]. The polymer was optimized at B3LYP/6-31G(d) level.It has been already shown that B3LYP/6-31g(d) gives decent groundstate structures of conjugated polymers [23]. The frontier molec-ular orbital energies were determined from the optimized groundstate geometry. Excitation energies and oscillator strengths wereestimated in TDDFT framework where at least 10 electronic exci-tations (singles) were computed at B3LYP/6-31G(d) level and thisfunction is already known for reproducing various experimentallyobserved properties of polymer systems [24].

2.2.1. Photovoltaic device fabrication and characterizationFor solar cell device fabrication, indium tin oxide (ITO) coated

conducting glass substrates (15–20 � cm−2) were used as anodeand aluminum as cathode. First, ITO substrates were patterned inisolated configuration, followed by ozonization for 90 min and thencleaned with detergent, ultrasonicated for 20 min each in deionizedwater, acetone, and isopropanol and finally dried in oven at 100 ◦Cfor 1 h. The thickness and roughness of ITO were measured to be 110and 10 nm, respectively, using a Tencor Alpha Step 500 thicknessprofilometer. A hole transporting layer of PEDOT:PSS was spin-coated on the ITO substrate at 1000 rpm for 1 min and was annealedat 100 ◦C for 30 min on a hot plate under vacuum. A blend mix-ture consisting of rr-P3HT and PTCDI-PFDA was made in anhydrous

chlorobenzene (1:1 weight ratio at a concentration of (5 mg/ml)). Inorder to attain homogeneity, the solution was stirred for 5 h at 60 ◦Cand then filtered using PTFE filter (0.2 �m pore size). Then the abovemixture was spin-coated on top of the PEDOT: PSS layer at 800 rpm

54 M.R. Raj et al. / Journal of Photochemistry and Photobiology A: Chemistry 247 (2012) 52– 62

(b) fa

fvPmw(twwPSwUcoKwueascs

3

a

3

00a5eanldaAt4w

4.1. Characterization of PTCDI-PFDA copolymer

The synthesized copolymer has a low molecular weight (molec-ular weight = 1831 (g/mol), Mn = 810 (g/mol), Mw = 855 (g/mol))

O

O

O

O

O

O

PTCDI

O OH2N NH2

PFDA

+

m-Cresol

Isoquinoline180oC

15-20 h

N

O

N

O

O

O

O O

Fig. 1. (a) Schematic diagram of BHJ solar cell and

or 1 min and the resulting films were dried at 100 ◦C under highacuum for 30 min. The thickness of the PEDOT:PSS (25 nm) and rr-3HT:PTCDI-PFDA (80 nm) layers were measured using the aboveentioned profilometer after the surface of the films had been cutith a diamond cutter to expose the substrate. Aluminum layer

100 nm) was deposited on the polymer films by thermal evapora-ion at a pressure of less than 5 × 10−6 mbar to form the cathode.e fabricated a polymer solar cell device in open air conditionith the typical structure as follows: ITO/PEDOT-PSS(25 nm)/rr-

3HT:PTCDI-PFDA(80 nm) (1:1,w/w)/Al(100 nm) (D1) (Fig. 1a).uch fabricated device consists of 4 isolated cells per substrateith an active area of 20 mm2 (Fig. 1b). After encapsulation byV-curing glue, the electrical characterization (such as short cir-uit photocurrent (Isc), open circuit voltage (Voc) and fill factor (FF))f the encapsulated devices was performed in air ambient using aeithley 4200 semiconductor characterization system and an Orielhite light solar simulator operating at an intensity of 100 mW/cm2

sing AM 1.5D filter. The incident photon to current conversionfficiency (IPCE) as a function of wavelength was measured withn Oriel 300 W Xe Arc lamp in combination with an Oriel Corner-tone 260¼ monochromator. The number of incident photons wasalculated for each wavelength using a calibrated monocrystallineilicon diode as reference.

. Material synthesis

The synthetic route of conjugated perylene diimide-basedcceptor copolymer was outlined in Scheme 1.

.1. Synthesis of copolymer via polycondensation

A mixture of perylenetetracarboxylic diimide (PTCDI) (0.1 g,.25 mmol) and 5,5′-bis(4-aminophenyl)-2-2′-bifuryl (PFDA,.08 g, 0.25 mmol) was dissolved in 20 ml m-cresol and a catalyticmount of isoquinoline (0.05 g) was taken in a dried three-neck0 ml round bottom flask. The mixture was slowly heated withffective stirring for 1 h until the temperature reached 180–200 ◦Cnd then maintained at the same temperature for 15–20 h underitrogen atmosphere. The progress of the reaction was fol-

owed until the residual perylene diimide (PTCDI) was no longeretectable by thin layer chromatography using a mixture of ethylcetate:chloroform (1:4) and a drop of methanol as an eluent.

fter completion of the reaction, the reaction mixture was cooled

o room temperature. Then the reaction mixture was dissolved in0 ml of tert-butyl methyl ether (TBMB) and the reaction mixtureas poured into a mixture of hexane and 2 N HCl (50 ml:50 ml)

bricated bulk heterojunction solar device model.

for precipitation. The resulting precipitate was collected in G4funnel by filtration, and then washed thoroughly with water anda mixture of TBMB:hexane (1:1) (3× 20 ml). The residual PTCDIwas not detected by thin layer chromatography. Purification wascarried out by Soxhlet extraction with mixture of CHCl3:CH3OH(90:10%) for 10–15 h to afford the pure product as a brownishred solid, 0.264 g (yield: 78%). 1H NMR (400 MHz, CDCl3) ı (ppm):7.55–7.50 (d, 4H, J = 10.8 Hz, Ph-H), 7.35 (d, 4H, Ph-H), 7.26 (m, 2H,Furyl-H), 7.10 (m, 2H, Furyl-H), 5.80 (m, 4H, Ph-H), 5.35 (m, 4H, Ph-H). Elemental Analysis Calculated for PTCDI-PFDA (C44H20N2O6):C, 78.33; H, 3.10; N, 4.15; Found: C, 77.63; H, 3.04; N, 4.21. GPCanalysis: molecular weight = 1831 (g/mol), Mn = 810 (g/mol),Mw = 855 (g/mol); PDI = 1.05. UV–vis data (in o-dichlorobenzene):�max (ε × M−1 cm−1): 579 nm (4000), 532 nm (10,480), 495 nm(8160), 373 nm (5720) and 319 nm (9080). Fluorescence data (ino-dichlorobenzene at �ex = 495 nm): �em (nm): 580, 540.

4. Results and discussion

PTCDI-PFDA

Scheme 1. Synthesis of alternating conjugated PTCDI-PFDA copolymer.

M.R. Raj et al. / Journal of Photochemistry and Photobiology A: Chemistry 247 (2012) 52– 62 55

Table 1Polymerization results for PTCDI-PFDA copolymer.

Polymer M.wt (g/mol)a Mn (g/mol)a Mw (g/mol)a Mz (g/mol)a PDI Yieldb

PTCDI-PFDA 1831 810 855 914 1.05 78%

rds).

adpws7ttTtttcstPadPra

4

cddti3asr(atapovotgtamsti3sraPic

a Determined from GPC analysis (eluent: o-dichlorobenzene; polystyrene standab Isolated polymer yield upon polymerization.

nd polydispersity index (PDI: 1.05) as determined by GPC in o-ichlorobenzene solvent relative to narrow low molecular weightolystyrene standards and the data are summarized in Table 1,hich indicate that the copolymer may be an oligomer. 1H NMR

pectrum of the PTCDI-PFDA displayed a broad up field signal at.54–7.35 ppm assigned to the eight perylene protons, whereashe bifuryl group protons and aromatic phenyl protons gave mul-iplets in the range of 7.26, 7.10, and 5.80–5.35 ppm, respectively.hus, the assigned chemical shifts are found to be consistent withhe anticipated copolymer structure as shown in Scheme 1. Thehermal property of PTCDI-PFDA copolymer was investigated byhermal gravimetric analysis (TGA). TGA curve of the resultingopolymer in Fig. S1 (see supporting information) shows a two-tage weight loss. The first stage weight loss that is stable upo 100–280 ◦C can be assigned to aromatic ring degradation ofFDA unit. The second stage weight loss takes place at 370 ◦Cnd continues to 650 ◦C and above, which is attributed to theecomposition of PTCDI-PFDA back bone [25]. It suggests that theTCDI-PFDA copolymer is thermally stable up to 650 ◦C. Thus, theesulting copolymer is an excellent candidate for optoelectronicpplications [26].

.1.1. Photophysical propertiesThe normalized UV–vis absorption and molar extinction

oefficient spectra of copolymer PTCDI-PFDA were acquired in o-ichlorobenzene (1 × 10−5 M) (Fig. 2a and b) and the observedata are collated in Table 2. The PTCDI-PFDA exhibits two dis-inct absorption bands: one broad, featureless absorption bandn the UV region (330–430 nm) with two sub-bands one at73 nm (ε = 5720 cm−1) and the other at 358 nm (ε = 6360 cm−1) arettributed to the �–�* transfer of the entire �-conjugated PFDAystem. The longer wavelength absorption band is in the visibleegion (450–800 nm) with three strong sub-bands at ca. 464 nmε = 5560 cm−1), 495 nm (ε = 8160 cm−1), 532 nm (ε = 10,480 cm−1),nd a broad hump at 579 nm (ε = 4000 cm−1) which is assignedo vibronic structure of the perylene diimide moiety. The peakt 464 nm originates due to S0–S2 electronic transition of a per-endicular dipole moment. The peak at 495 nm and 532 nm areriginated from the electronic transitions from S0 to 1- and 0-ibronic states of S1, respectively [27,28]. The low energetic edgef the absorption spectrum of the resulting copolymer in solu-ion appeared at 780 nm, which corresponds to an optical bandap of Eopt

g = 1.59 eV. Upon excitation of PTCDI-PFDA at 495 nm,wo distinct emission bands at 540 nm and 580 nm and a humpppears at 630 nm (Fig. 2c). It can be seen that the resulting copoly-er emits mostly an intense greenish yellow color. The excitation

pectrum of the PTCDI-PFDA resembles to that of absorption spec-rum, which indicates that the fluorescence may be the mirrormage relative to the S0–S1 absorption, with a Stokes shift of about9 nm for the 0 → 0 transition. This reflects that most of the S1tates only involved the inter-conversion processes before undergoadiative relaxation. Time-resolved photoluminescence recorded

t �max = 530 nm (Fig. 2d) shows that the fluorescence decay ofTCDI-PFDA copolymer. This longer decay time may be due tontramolecular charge transfer (ICT) over entire conjugation ofopolymer skeleton.

4.1.2. Electrochemical propertiesThe electrochemical properties of PTCDI-PFDA were studied

using cyclic voltammetry measurements to determine the ion-ization potentials (HOMO/LUMO levels) for understanding thecharge injection processes. The PTCDI-PFDA copolymer shows tworeversible reduction and one irreversible oxidation at a moder-ately high reduction and oxidation potential respectively as shownin Fig. 3a. Two reversible reduction peaks were observed at thecathodic region – the first one corresponds to a one-electron pro-cess, which is related to the formation of the perylene diimideradical monoanion (PDI–PDI•−) and the second reduction peaks arecorresponding to a two-electron processes, which can be assignedto the formation of the perylene diimide dianion (PDI−–PDI−) [29].The broad two reduction peaks in the negative region of CV curve isindicative of perylene diimide molecule accepting three electronsdue to PDI bearing electron-rich carbonyl groups that generatemonoanion radical and dianion forms of perylene diimide [30].The PTCDI-PFDA copolymer also exhibits two irreversible oxida-tion peak were located at range of 0.20–1.14 V in the anodic region,which can be assigned to the electron-pushing effect of oxygenand nitrogen heteroatom attached to the bifuryl ring and phenylring, respectively [30d]. Based on the standard redox potentialof ferrocene/ferrocenium ion (Fc/Fc+), which is assumed to havean absolute energy level of −4.8 eV relative to vacuum level [31]and from the measured first oxidation potential for PTCDI-PFDA(0.06 V), we evaluate the HOMO and LUMO energy levels as well aselectrochemical band gap (Eg,ec) using the following equations:

HOMO (eV) = −e(Eonsetox + 4.74) (eV)

LUMO (eV) = −e(Eonsetred + 4.74) (eV)

Eg,ec = Eonsetox − Eonset

red (eV)

where Eonsetox and Eonset

red are the oxidation and reduction potentialsrelative to Ag/Ag+ electrode. The estimated redox potentials andthe HOMO and LUMO energy levels are summarized in Table 2.The first onset oxidation potential of PTCDI-PFDA is +0.50 V andreduction potential is −0.83 V vs Ag/Ag+ electrode. The highestoccupied molecular orbital (HOMO) and lowest unoccupied molec-ular orbital (LUMO) energy levels of PTCDI-PFDA copolymer werealso estimated to be −5.24 eV and −3.9 eV, respectively. The deter-mined electrochemical band gap is (Eg,ec) 1.34 eV. It is worth notingthat the determined electrochemical band gap and the estimatedoptical band gap of PTCDI-PFDA (Eopt

g = 1.59 eV) are relatively ingood agreement for the resulting copolymer. The estimated HOMO(ca. −5.24 eV) energy level of PTCDI-PFDA was relatively low-lyingcompared to that of rr-P3HT (ca. −5.0 eV), which ensures betteroxidation stability. A complete picture of the band energy diagramof PTCDI-PFDA copolymer (Fig. 3b) was constructed based on elec-trochemical study which shows low-lying HOMO energy level (ca.−5.24 eV) of the copolymer is relatively below the air stable to oxi-dation threshold (i.e. ca. −5.2 eV) [32]. Further, the estimated LUMOenergy level was energetically located on the threshold value of anideal donor polymer as well as above the work function (around

4.2 eV) of aluminum metal electrode, which is efficient to facili-tate an effective charge transfer to aluminum metal contact. It isimportant to note that the electron acceptor PTCDI-PFDA shows agreater LUMO band offset (ca. 1.0 eV) than exciton binding energy,

56 M.R. Raj et al. / Journal of Photochemistry and Photobiology A: Chemistry 247 (2012) 52– 62

300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d A

bs

orb

an

ce

(a

.u.)

Wavelength(nm)

a

300 400 500 600 700 8000.0

2.0x103

4.0x103

6.0x103

8.0x103

1.0x104

1.2x104

x M

-1C

m-1

Wavlength(nm)

b

300 400 500 600 7000

200

400

600

Wavelength(nm)

Flu

ore

se

nc

e In

ten

sit

y (

a.u

.)

EmissionExcitation

0

100

200

Flu

ore

sen

ce In

ten

sity

(a.u

.)

c

10 20 30 40 50

101

102

103

104

Co

un

ts

Time (ns)

d

F wavc of PTs -resol

esttfhaelcL(

TO

ig. 2. (a) Normalized absorbance spectrum of PTCDI-PFDA copolymer (excitationoefficient spectrum of PTCDI-PFDA copolymer. (c) Fluorescence emission spectrumpectrum (�max

ex = 528 nm detected with emission wavelength of 565 nm). (d) Time

stimated to be 0.4–0.5 eV [33,34] or �E of 0.3 eV [34b]. It can beeen that the LUMO band offset (ca. 1.0 eV) of PTCDI-PFDA betweenhe LUMO of donor (rr-P3HT, ca. 2.9 eV) and the LUMO of accep-or PTCDI-PFDA (ca. 3.9 eV) have been shown sufficient drivingorce for photoinduced efficient charge separation. Apparently, itas been proposed that the higher theoretical open circuit volt-ge (Voc) of the device is found to be linearly dependent on HOMOnergy level of electron donor (ED) polymer and LUMO energy

evel of electron acceptor (EA) polymer [35–37]. Thus, PTCDI-PFDAopolymer with high-lying LUMO energy level with respect toUMO of rr-P3HT is expected to obtain a high open circuit potentialVoc) in the polymer PV cells. Thus, the solar cell device based on

able 2ptical properties, electrochemical properties, and electronic energy levels of PTCDI-PFD

Polymer �max(nm)/(εmax, M−1 cm−1) �edgea �max

ex Eoptg (e

PTCDI-PFDA 373 (5770) 780 540 1.59

464 (5560) 580495 (8160) 630

532 (10,480)

a Onset point from absorption spectrum.b Optical band gap.c Onset point from oxidation potential.d HOMO level from onset point of oxidation potential.e Onset point from reduction potential.f LUMO level from onset point from reduction potential.g Electrochemically estimated energy band gap.

elength of 490 nm) in concentration of 1 × 10−5 M in o-DCB. (b) Molar extinctionCDI-PFDA: emission spectrum (�em = 580 nm when excited at 495 nm) − excitationved decay kinetics of PTCDI-PFDA copolymer with 530 ± 2 nm pulsed excitation.

rr-P3HT/PTCDI-PFDA copolymer system showed Voc of 0.92 Vdespite of the HOMO–LUMO energy difference of 1.1 eV.

In general, organic solar cells produce reasonable open-circuitvoltages (Voc). If the fabricated solar cell contains a single layer of aconjugated polymer, the Voc scales with the work function differ-ence between electrodes and thus follow the metal-insulator-metalmodel [38a]. However, if the fabricated solar cell contains both theelectron and hole accepting polymers, the Voc also scales linearly

with the work function difference with an additional contributiondepending on the light intensity [38b]. Such a contribution is dueto the accumulation of charge carriers at the organic/organic inter-face giving rise to a diffusion current, which must be compensated

A copolymer.

V)b Eonsetox (V)c/HOMO (eV)d Eonset

red(V)e/LUMO (eV)f Eec

g (eV)g

+0.5/−5.24 −0.83/−3.90 1.34

M.R. Raj et al. / Journal of Photochemistry and Photobiology A: Chemistry 247 (2012) 52– 62 57

Fig. 3. (a) Cyclic voltammetric curve of PTCDI-PFDA copolymer in ACN solution containing 0.1 M n-Bu4NPF6 as electrolyte. The measurement was carried at a glassy carbond ith fera

brtovtgdeae

4

tmst

isc electrode against Ag/Ag+ as a reference. Each measurement was calibrated wccepting PTCDI-PFDA (N1) copolymer.

y a drift current at open circuit. For the solar cell device based onr-P3HT/PCBM copolymer system showed Voc of 0.62 V despite ofhe HOMO–LUMO energy difference of 1.0 eV. For cells with non-hmic contacts, the observed Voc is in agreement with the expectedalue however for the ohmic contacts, the measured value is lowerhan the predicted value, which may possibly be due to the ener-etic disorder of the charge transport levels. In the case of solar cellevice based on rr-P3HT/PTCDI-PFDA copolymer system, such annergetic disorder is less compared to rr-P3HT/PCBM copolymernd hence the observed Voc (0.92 V) is nearer to the HOMO–LUMOnergy difference (1.1 eV).

.1.3. Computational analysisDensity functional theory (DFT) has become an outstanding tool

o predict a variety of ground-state properties of small and largeolecules. Therefore we performed DFT calculations to under-

tand the structural and electronic properties of this polymer athe molecular level. All the calculations have been carried out

rocene at scan rate of 100 mV/s. (b) Band energy diagram for donor rr-P3HT and

at B3LYP/6-31g(d) level. The optimized geometry of PTCDI-PFDAcopolymer is given along with the important bond lengths in Fig. 4.The perylene and furyl units have a dihedral angle of 109◦, imposingasymmetry on the whole molecule. From the computed results, it isclear that all the C C bond lengths fall between 1.37 and 1.49 A. Thisreveals that the entire C C bonds in this copolymer backbone arepartial double bonds. This is an excellent evidence for the compre-hensive �-delocalization throughout the molecule. Fig. 5 shows thefrontier molecular orbitals (HOMO, HOMO−1, HOMO−2, HOMO−3,and LUMO, LUMO+1, LUMO+2 and LUMO+3) and the correspondingenergy level diagram. The calculated HOMO and LUMO energy lev-els of ground-state optimized geometry of PTCDI-PFDA were foundto be −4.94 eV and −3.51 eV, respectively, which corroborate withelectrochemically determined HOMO and LUMO energy levels of

PTCDI-PFDA (see Table 2). In addition, the estimated band gap andHOMO–LUMO energy levels of PTCDI-PFDA are in good agreementwith the both spectral and electrochemical analysis. From Fig. 5, itis interesting to note that the HOMO is evenly localized on the furyl

58 M.R. Raj et al. / Journal of Photochemistry and Photobiology A: Chemistry 247 (2012) 52– 62

groun

rtLdctHia

i

Fig. 4. B3LYP/6-31g(d) optimized

ing unit alone where as the LUMO is mainly lying on the peryleneetracarboxylic diimide segment. On the other hand, HOMO−1 andUMO+1 are predominantly localized on perylene tetracarboxyliciimide unit of the copolymer. Similarly HOMO−2 and LUMO+3 areoncentrated on the furyl rings and LUMO+2 lies on the peryleneetracarboxylic diimide unit as depicted in Fig. 4. This indicates thatOMO → LUMO and HOMO → LUMO+1 transitions are arising from

ntramolecular charge-transfer (ICT) and the HOMO−1 → LUMO+1bsorptions are mainly of � → �* nature in this copolymer.

The compositions of various molecular orbitals by consider-ng furyl ring unit and perylene tetracarboxylic diimide unit as

Fig. 5. Computed frontier molecular orbitals an

d state geometry of PTCDI-PFDA.

two segments are listed in Table 3. As evident from the molecu-lar orbital diagrams, the contributions of the furyl ring unit andperylene unit are around 98% and 2%, respectively towards theHOMO level. But LUMO is completely delocalized over perylene unit(100%). similarly HOMO−1 and LUMO+1 are also completely spreadover perylene unit. LUMO+2 and LUMO+3 are getting contribu-tions mainly from perylene and furyl ring units and other segments

contribute a little.

The low lying excited states were computed through time-dependent DFT calculations (TDDFT) in an effort to rationalizethe nature of electronic transitions, contributing configurations

d its energy level diagram of PTCDI-PFDA.

M.R. Raj et al. / Journal of Photochemistry and Photobiology A: Chemistry 247 (2012) 52– 62 59

-0.5 0.0 0.5 1.0 1.5 2.0

-4

-3

-2

-1

0

1

2

Cu

rren

t d

en

sit

y ( m

A c

m-2) Voltage(V)

Dark

AM 1.5D condition

b

-1.0 -0.5 0.0 0.5 1.0 1.5

10-4

10-3

10-2

10-1

100

101

Cu

rren

t d

en

sit

y (

mA

cm

-2)

Voltage(V)

Dark

AM 1.5D condition

a

Fig. 6. J–V characteristics of (a) ITO/PEDOT:PSS(25 nm)/rr-P3HT:PTCDI-PFDA

Table 3Composition of FMOs of PTCDI-PFDA.

MOs Perylene unit Furyl ring unit

HOMO−3 0% 100%HOMO−2 6% 94%HOMO−1 100% 0%HOMO 2% 98%LUMO 100% 0%LUMO+1 100% 0%

ttseiialerws(iprj

TTPc

(Fig. 6) of four isolated cells individually by masking otherthree cells. Also provided is a linear curve under illuminationfor comparison in Fig. 6. For the [ITO/PEDOT:PSS(25 nm)/rr-P3HT:PTCDI-PFDA(80 nm) (1:1,w/w)/Al(100 nm); D1] device, an

16

LUMO+2 91% 9%LUMO+3 11% 89%

o the transitions and charge transfer probability. The details ofhe electronic transitions, namely excitation energies, oscillatortrength and contributing configurations for the most probablelectronic transitions of PTCDI-PFDA are summarized in Table 4. Its interesting to note that most of the prominent bands are due tontramolecular charge transfer (ICTs) and some � → �* excitationsre also observed. The calculated results show that the transitionocated at 510 nm with larger oscillator strength corresponds to thexperimentally observed band at 532 nm. Singly excited configu-ations HOMO−1 → LUMO (99%) mainly contributes to this statehich corresponds to � → �* excitations. Meanwhile ICT bands

uch as 499 nm and 443 nm are arising from HOMO−2 → LUMO97%) and HOMO → LUMO+1 (99%) transitions with very weakntensity (f = 0.0003 and f = 0.0001) respectively. But an intense

eak is observed at 384 nm with high intensity (f = 1.2735) whichesults from HOMO → LUMO+3 (99%). These results suggest thatudicious substitution of different electron-donating moiety onto

able 4he most probable vertical excitation energies and oscillator strengths of PTCDI-FDA molecule in gas phase and their corresponding contributing singly excitedonfigurations computed at B3LYP/6-31G(d) level.

�abs (nm) f �E (eV) Assignment

510 0.9710 2.4321 HOMO−1 → LUMO (99%)499 0.0003 2.4831 HOMO−2 → LUMO (97%)443 0.0001 2.7972 HOMO → LUMO+1 (99%)424 0.1367 2.9239 HOMO → LUMO+2 (99%)405 0.0002 3.0616 HOMO−4 → LUMO (98%)400 0.0001 3.0992 HOMO−7 → LUMO (42%)

HOMO−5 → LUMO (52%)394 0.0001 3.1492 HOMO−6 → LUMO (93%)

HOMO−6 → LUMO+1 (4%)384 1.2735 3.2270 HOMO → LUMO+3 (99%)

(80 nm) (1:1 wt%)/Al(100 nm), (b) linear curve of D1 at same condition.

the symmetrical and asymmetrical bay position of the perylenediimide core induces the positive shift in the LUMO energy levels(i.e., shift in the negative potential region), but has no significantchange in the HOMO energy levels and this leads to a tremen-dously reduced HOMO–LUMO energy gaps. These results are alsoin accordance with the experimental findings discussed earlier.

4.1.4. Photovoltaic device performanceIn order to evaluate the photovoltaic properties, typical

BHJ devices were made by sandwiching a mixture of as syn-thesized acceptor (PTCDI-PFDA) with electron donor (rr-P3HT)between PEDOT:PSS coated ITO and Al as hole and electroncollecting electrodes, respectively. The BHJ device structure isrepresented as follows: [ITO/PEDOT:PSS(25 nm)/rr-P3HT:PTCDI-PFDA(80 nm) (1:1,w/w)/Al(100 nm);D1]. In order to examine thedevice performance, the current density (J)–voltage (V) char-acteristics have been measured in the dark and under AM1.5G illumination (100 mW/cm2) from a calibrated solar sim-ulator with simultaneous recording of their J–V characteristics

400 450 500 550 600 650 7000

2

4

6

8

10

12

14

IPC

E(%

)

Wavelength(nm)

Fig. 7. IPCE curve of the PSC device (D1) based on the copolymer PTCDI-PFDA.

60 M.R. Raj et al. / Journal of Photochemistry and Photobiology A: Chemistry 247 (2012) 52– 62

F t%) bl ◦

r

odode

Pii(flgLlaprp[gtmtstesoaaHtcIb(

TP1

ig. 8. AFM tapping mode images (2.4 �m × 2.4 �m) of rr-P3HT:PTCDI-PFDA (1:1 wr-P3HT:PTCDI-PFDA (1:1 wt%) blend film.

pen circuit voltage (Voc) of about 0.92 V, a short circuit currentensity (Jsc) of about 2.83 mA/cm2, and a fill factor (FF) of 13% werebtained (Table 5). It should be pointed out that these photovoltaicevices have no output potential at zero current in the dark asxpected.

The observed high open circuit voltage (Voc = 0.92 V) for rr-3HT:PTCDI-PFDA based device was indicative of covalentlyncorporated �-conjugated PFDA moiety that plays a key rolen high-lying HOMO and LUMO energy levels of the copolymerPTCDI-PFDA). This is in good agreement with the results obtainedrom CV and theoretical calculations. However, the correspondingower current density (Jsc = 2.83 mA/cm2) and FF (13%) values sug-est that there is a larger LUMO band offset (ca. 1.0 eV) between theUMO of donor (rr-P3HT) and the LUMO of acceptor (PTCDI-PFDA)eading to charge carrier transport losses during electron dissoci-tion at the D/A interface. It is very important to note here that thehotovoltaic performance of the PV device follows in a monotonicelation of the LUMO offset. That is, larger the LUMO offset of thehotoactive materials, will lower the power conversion efficiency35,39]. This reflects that the lower efficiency is caused by eithereminate recombination of the bound electron–hole pair or radia-ive recombination of free charge carriers at the D/A interface. This

ay supported by Marcus [40] that if the energy offset is too largehen slow down of forward electron transfer and thermal groundtate charge separation occurs without photoexcitation. Further,he low value of fill factor of the device is also attributed to thelectron traps present in the copolymer (PTCDI-PFDA) phase whichtrongly contributes to the trap-assisted recombination lossesf the system [41]. Therefore, there are considerable innovativepproaches towards the design of good electron transport char-cteristics of perylene diimide-based copolymer with higher lyingOMO and LUMO energy levels via chemical functionalization in

he bay position of perylene skeleton. The higher fill factor (FF)

an be further improved by optimizing the electrical design. ThePCE (%) of rr-P3HT:PTCDI-PFDA device shows a broader responseetween 400 and 660 nm with an efficiency of 14.4% at 515 nmFig. 7). Further, the photoaction spectrum of D1 shows an onset at

able 5hotovoltaic properties of the PSC based on rr-P3HT:PTCDI-PFDA (D1) solar cell (AM.5D, 100 mW/cm2) at ambient atmosphere.

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

D1 2.83 0.92 0.13 0.34

end film in chlorobenzene and annealed at 120 C. (b) The cross-sectional image for

about 650 nm (1.90 eV), which is in relatively close resemblance tothe optical band gap and coincides with the absorption spectrumof PTCDI-PFDA copolymer. In addition, the film morphology con-sisting of active composite layer also plays a very important rolein determining the power conversion efficiency in photovoltaicapplications. AFM images of rr-P3HT:PTCDI-PFDA blend filmsshows the uniform phase separation with roughness 2.04 nm asshown in Fig. 8 which supports the better free charge carriersbetween the donor/acceptor interface.

5. Conclusions

The specific intermixing of organic photovoltaic materials hasbeen chosen to simultaneously achieve both the dissociation ofphotoinduced excitons at a D/A interface and maximum absorp-tion in the visible region of the solar spectrum. We note that thetheoretical calculation of the HOMO and LUMO energy levels of thephotoactive components is most fundamental in studying organicsolar cells properties. A new acceptor copolymer (PTCDI-PFDA) inwhich high degree of �-conjugated PFDA unit incorporated withPTCDI moiety was synthesized and characterized by various meth-ods. The PTCDI-PFDA copolymer showed extended light absorptionfrom the near-infrared region and in addition it showed relativelyhigher absorption coefficient. The HOMO and LUMO energy levelsof PTCDI-PFDA copolymer were determined by both electrochem-ical study as well as theoretical calculations. The experimentaldata and theoretical calculations showed high-lying HOMO andlow-lying LUMO energy levels for PTCDI-PFDA which are sufficientdriving forces for harvesting maximum photon flux density in thevisible solar spectrum. The successfully lowered HOMO and LUMOenergy levels delivered high open circuit voltage (Voc = 0.92 V) forthe BHJ photovoltaic device fabricated from rr-P3HT:PTCDI-PFDA.However, as expected, the larger LUMO offset point of acceptorleads to either geminate recombination of the bound electron–holepair or radiative recombination of free charge carriers which ren-dered a lower short circuit current and much poor fill factor. The D1device delivered PCE of 0.34% in the case of blended rr-P3HT:PTCDI-PFDA device system.

Supporting information available

Thermal gravimetric analysis and DFT calculations (XYZ-coordinates) data of copolymer.

nd Ph

A

4attFst(

A

fj

R

[

[

[

[

[

[

[

[

[

[

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

cknowledgements

The author SA thanks DST, New Delhi (Ref. No. SR/S1/PC-9/2009) for the sanction of major research project. Also theuthors (SA and MA) thank DST, New Delhi and DIISR, Australia forhe sanction of INDIA-AUSTRALIA collaborative research fund forheir collaborative research. Author SA thanks DST for sanctioningIST (SR/FST/CSI-190/2008 dated 16th March 2009) and Nanomis-ion projects. RVS thanks University Grants Commission, India forhe financial support through Maulana Azad National FellowshipRef. No. F.40-17(C/M)/2009(SA-III/MANF)).

ppendix A. Supplementary data

Supplementary data associated with this article can beound, in the online version, at http://dx.doi.org/10.1016/.jphotochem.2012.07.019.

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