A Vinylene-Linked Benzo[1,2-b:4,5-b’]dithiophene-2,1,3 ... · TGA, DSC and electrochemistry....

A Vinylene-Linked Benzo[1,2-b:4,5-b’]dithiophene-2,1,3-BenzothiadiazoleLow-Bandgap Polymer

Alessandro Abbotto,1 Mirko Seri,2 Milind S. Dangate,1 Filippo De Angelis,3

Norberto Manfredi,1 Edoardo Mosconi,3 Margherita Bolognesi,4 Riccardo Ruffo,1

Matteo M. Salamone,1 Michele Muccini1

1Department of Materials Science, Milano-Bicocca Solar Energy Research Center—MIB-Solar,

University of Milano-Bicocca, Via Cozzi 53, Milano I-20125, Italy2Consiglio Nazionale delle Ricerche, Istituto per lo Studio dei Materiali Nanostrutturati (CNR-ISMN),

Via P. Gobetti101, Bologna I-40129, Italy3Consiglio Nazionale delle Ricerche, Istituto CNR di Scienze e Tecnologie Molecolari (CNR-Italy),

Via Elce di Sotto 8, Perugia I-06213, Italy4Laboratory MIST E-R, Via P. Gobetti101, Bologna I-40129, Italy

Correspondence to: A. Abbotto (E-mail: [email protected]) or R. Ruffo (E-mail: [email protected])

Received 14 January 2012; accepted 8 March 2012; published online 30 March 2012

DOI: 10.1002/pola.26046

ABSTRACT: A new heteroarylene-vinylene donor–acceptor

polymer P(BDT-V-BTD) with reduced bandgap has been syn-

thesized and its photophysical, electronic and photovoltaic

properties investigated both experimentally and theoretically.

The structure of the polymer comprises an unprecedented

combination of a strong donor (4,8-dialkoxy-benzo[1,2-b:4,5-

b’]dithiophene, BDT), a strong acceptor (2,1,3-benzothiadia-

zole, BTD) and a vinylene spacer. The new polymer was

obtained by a metal-catalyzed cross-coupling Stille reaction

and fully characterized by NMR, UV–vis absorption, GPC,

TGA, DSC and electrochemistry. Detailed ab initio computa-

tions with solvation effects have been performed for the

monomer and model oligomers. The electrochemical investi-

gation has ascertained the ambipolar character of the poly-

mer and energetic values of HOMO, LUMO and bandgap

matching materials-design rules for optimized organic photo-

voltaic devices. The HOMO and LUMO energies are consis-

tently lower than those of previous heteroarylene-vinylene

polymer while the introduction of the vinylene spacer

afforded lower bandgaps compared to the analogous system

P(BDT-BTD) with no spacer between the aromatic rings.

These superior properties should allow for enhanced photo-

voltages and photocurrents in photovoltaic devices in combi-

nation with PCBM. Preliminary photovoltaic investigation

afforded relatively modest power conversion efficiencies of

0.74% (AM 1.5G, 100 mW/cm2), albeit higher than that of pre-

vious heteroarylene-vinylene polymers and comparable to

that of P(BDT-BTD). VC 2012 Wiley Periodicals, Inc. J Polym

Sci Part A: Polym Chem 50: 2829–2840, 2012

KEYWORDS: bulk heterojunction solar cells; conducting poly-

mers; conjugated polymers; donor–acceptor polymers; electro-

chemistry; heteroaromatics; heteroatom-containing polymers;

low bandgap polymers; polyaromatics; quantum chemistry

INTRODUCTION Low-bandgap p-conjugated semiconductingpolymers are attracting an increasing interest in a number ofmaterials science fields, including electrochromics, organictransistors and organic photovoltaics (OPV).1–4 In bulk heter-ojunction (BHJ) OPV devices a conjugated polymer, acting asthe donor, transfers an electron from the sunlight promotedexcited state to a fullerene derivative,5 typically PC61BM([6,6]-phenyl-C61-butyric acid methyl ester) or PC71BM([6,6]-phenyl-C71-butyric acid methyl ester), acting as theacceptor.6,7 The separated electron-hole pair is then trans-ported and collected at the electrodes. In order to achievehigh power conversion efficiencies (PCEs) an efficient sun-

light harvesting is needed to yield high short-circuit currentdensities (Jsc). This aspect necessarily requires the use oflow bandgap conjugated polymers as the donors, able to effi-ciently absorb up to the visible lower energy portion of thesolar spectrum. Recent design rules have established thatthe best donor polymers should have a bandgap energy inthe range of 1.2–1.7 eV.8 Further material-design rulesrequire an energy of the lowest unoccupied molecular orbital(LUMO) of the polymer higher than that of the LUMOPCBM,with a minimum offset of 0.3 eV, which corresponds to avalue of �4.0 eV (or higher) assuming �4.3 eV forLUMOPCBM, and, accordingly, an energy of the Highest

Additional Supporting Information may be found in the online version of this article.

VC 2012 Wiley Periodicals, Inc.

WWW.MATERIALSVIEWS.COM JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 2829–2840 2829

JOURNAL OFPOLYMER SCIENCE WWW.POLYMERCHEMISTRY.ORG ARTICLE

Occupied Molecular Orbital (HOMO) of the polymer rangingfrom �5.2 to �5.7 eV.

Spectral engineering in semiconducting polymers is an im-portant tool in order to achieve large OPV efficiencies.9 Avery common strategy to low-bandgap polymers is based onthe donor–acceptor approach, where p electron-rich donor(D) and p electron-poor acceptor (A) fragments are alter-nated along the p-conjugated backbone.10 This derives fromthe fact that D groups raise the HOMO energy and concomi-tantly A groups lower the LUMO energy, with the globaleffect of decreasing the polymer bandgap. In the last yearsresearchers have used several D and A groups, includingsophisticated systems.11 In particular, our attention wasattracted by two highly efficient fragments, 4,8-dialkoxy-benzo[1,2-b:4,5-b’]dithiophene (BDT) as a donor group and2,1,3-benzothiadiazole (BTD) as an acceptor group (Fig. 1).

The donor BDT group has been recently used in a significantnumber of highly efficient polymers for BHJ devices,12

including some of the present record polymers with overallconversion efficiencies approaching 8%, PTB7 and PBDTTT.13

The acceptor BTD ring has lately received great attentionthanks to its electron-withdrawing strength and ability toeffectively lower the LUMO energy.14 Again, the use of BTDpermitted to build a new polymer with top-ranked PCE(6.1%) and a remarkable internal quantum efficiencyapproaching unity.15

Furthermore, the use of the donor BDT and/or acceptor BTDled to polymers with optimal HOMO, LUMO, and bandgapenergies, closely matching those of the ideal conjugated poly-mer in OPV devices, showing that these polycyclic heteroaro-matic rings are very effective constituting units for thedesign of performing p-conjugated polymers for BHJ cells.Nevertheless, very few reports have described the simultane-ous use of both fragments in the same polymeric back-bone.16 Surprisingly, in none of these cases vinylene spacerswere used but an aryl–aryl bonding was always present,with the BDT and BTD groups either directly bonded to eachother or alternated with thienyl spacers. The most simplecombination, the polymer P(BDT-BTD), was investigated byHou et al., Yang and coworkers (Fig. 2).16(a) An electrochemi-cal bandgap of 1.9 eV and a PCE of 0.90 in combination withPC61BM was reported.

Despite the very large variety of D-A polymers, relatively fewexamples of vinylene-linked conjugated polymers for OPV,

where D and A are heteroaromatic fragments separated by avinylene (V) spacer, have been described. We believe thatthis event, mostly due to the more stringent synthetic issuesand to the lack of convenient commercial precursors, repre-sents an important limitation in the field of semiconductingpolymers for OPV in view of the significant advantagesthat the vinylene spacer might offer such as promoted co-planarity of adjacent aromatic units, extended p-conjugation,enhanced intermolecular p-stacking, and, accordingly,reduced bandgaps.17 Swager and coworkers have pioneeredthe use of V-linked D-A polymers with donor alkoxybenzenerings18 as alternatives to conventional poly(p-phenyleneviny-lene) (PPV) systems.19 Naso and coworkers have investi-gated several fluorinated PPVs.20 Reynolds and coworkershave recently reported a vinylene-linked BTD-based polymerwith a low bandgap energy (1.7 eV) but still modest OPVefficiencies (0.3%).21 Our group has recently described twoV-linked D-A polymers where A was a pyridine ring with dif-ferent substitution patterns and D either an electron-rich3,4-ethylenedioxithiophene (EDOT),22 or a pyrrole moiety.23

The energetic characterization of the EDOT polymers P(2,5-Py-V-EDOT) and P(2,6-Py-V-EDOT) revealed HOMO (�5.1/�5.0 eV), LUMO (�3.4 eV), and narrow bandgap (1.6/1.7eV) energies fitting materials-design rules for optimized OPV.However, as for Reynolds’s case, photovoltaic devices in com-bination with PC71BM afforded a relatively modest PCE of�0.5% (AM 1.5G, 100 mW/cm2), which was mostly attrib-uted to the low molecular-weight of the polymers accessiblevia the chemical route.

We present here the first example of a vinylene-linked BDT-BTD low-bandgap polymer P(BDT-V-BTD) (Fig. 3). The poly-mer has been synthesized via metal catalyzed poly-couplingreaction and fully characterized in its optical, electrochemi-cal, charge carrier mobility, and photovoltaic properties.

FIGURE 1 Structure of donor BDT and acceptor BTD constitut-

ing units of the investigated D-A polymers.

FIGURE 2 Structure of polymer P(BDT-BTD).

FIGURE 3 Structure of the polymer P(BDT-V-BTD) investigated

in this work.

ARTICLE WWW.POLYMERCHEMISTRY.ORGJOURNAL OF

POLYMER SCIENCE

2830 JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 2829–2840

DFT/TDDFT computations have been performed for modeloligomers to analyze their optical and electronic properties.

EXPERIMENTAL

Chemical and Spectroscopic Characterization1H NMR spectra were recorded on a Bruker AMX-500 instru-ment operating at 500.13 MHz. All reagents were obtainedfrom commercial suppliers at the highest purity grade andused without further purification. Anhydrous toluene waspurchased from Sigma Aldrich and used as received in anargon-filled glove-box. Absorption spectra were recorded ona V-570 Jasco spectrophotomer. Transition temperatureswere determined by differential scanning calorimetry (DSC)using a Mettler Toledo DSC 821 instrument with a heatingand cooling rate of 15 �C/min under nitrogen. Thermogravi-metric analyses (TGA) were performed with a Mettler ToledoTGA/DSC STARe system at a heating rate of 10 �C/min undernitrogen. Gel permeation chromatography (GPC) analyseswere recorded on a Waters 1515 separation module usingpolystyrene as a standard and THF as an eluant.

Synthesis of P(BDT-V-BTD)In a glove-box filled with argon ([O2] < 1 ppm) a mixture of4,7-bis((E)-2-bromovinyl)benzo[c][1,2,5]thiadiazole (2)21 (265mg, 0.77 mmol) and 2,6-bis(trimethyltin)-4,8-dioctyloxy-benzo[1,2-b:4,5-b’]dithiophene (1)12(f) (Scheme 1) (590 mg,0.77 mmol) in toluene (40 mL) was put in a schlenk flask andstirred for 40 min. Then Pd(PPh3)4 (88 mg, 0.077 mmol) wasadded to the flask and the mixture stirred for another 40 min.The flask was tightly closed, removed from the glove-box, andstirred for 18 h at 110 �C under an argon atmosphere. Aftercooling down to room temperature MeOH (80 mL) was addedto the reaction mixture. The formed precipitate was collectedby filtration into a Soxhlet thimble and extracted with n-hex-ane and CHCl3. The solid, obtained by removing the solventfrom the CHCl3 fraction under reduced pressure at T below40 �C, was dried under vacuum for 1 day to give the polymer(200 mg, 0.32 mmol, 42%) as a violet solid. 1H NMR(C2D2Cl4): d 8.5 – 6.5 (8H, m, aromatic and vinylene protons),4.3 (4H, broad, OCH2), 1.9 (4H, broad, OCH2CH2), 1.8–1.2(20H, m, remaining CH2), 0.9 (6H, broad, CH3).

Electrochemical CharacterizationThe electrochemical characterization was performed by dif-ferential pulsed voltammetry (DPV) and cyclic voltammetry

(CV) in a two compartment three electrode cell assembled inan Argon filled glove box ([O2] <1 ppm) using an EG&GPARSTAT 2263 potentiostat/galvanostat. A gold disc, a Ptflag, and a Ag/AgCl wire were used as working, counter, andpseudoreference electrode, respectively. The electrolyte wasa 0.1 M solution of tetrabutylammonium perchlorate (Fluka,electrochemical grade, =99.0%) in anhydrous dichloro-methane (Sigma Aldrich >99.8%). The working electrodedisc was well polished with an 0.1 lm alumina suspension,sonicated for 15 min in deionized water and washed with 2-propanol before use. The pseudoreference electrode was cali-brated either by adding ferrocene (0.5 mM) to test solution(reductive DPV) or externally by a 0.5 mM solution of ferro-cene in the electrolyte (in absence of the polymer). Bothcalibrations provided the same result; the correspondingcathodic peak potentials differed for less than 5 mV.

Computational InvestigationAll the calculations have been performed with the Gaussian03 program package.24 3-21G* and 6-31G* basis sets wereused for geometry optimizations. A 6-31G* basis set wereused for single point energy calculations. For all calculationswe used a polarizable continuum model (PCM) to describesolvation effects.25(a) For the optical properties, in previouspapers on similar highly conjugated systems,26,27 we found aconsiderable red-shift of the lowest TDDFT excitation energycalculated by the B3LYP functional,25(b) compared to the ex-perimental absorption maxima. This shift was as large as0.6–0.8 eV and was related to the inaccurate description ofcharge-transfer excited states in highly delocalized sys-tems.26–28 Moreover, further red-shifts were calculated byintroducing solvation effects. To overcome this limitation weused here the B3LYP functional for geometry optimizationsand the MPW1K functional,29 containing 42% of Hartree–Fock exchange, for TDDFT excitation energies.27 Theincreased amount of Hartree–Fock exchanges ensures a cor-rection of the self interaction error typical of conventionalfunctionals and also improves the long-range exchangebehavior. The MPW1K excitation energies of the investigatedpolymers are in excellent agreement with the experimentalvalues. As far as the electrochemical properties are con-cerned, we proceeded to evaluate the energy of HOMO andLUMO of the investigated polymers applying the verticalapproximation due to the Koopmans theorem, that is, by tak-ing the negative of the HOMO and LUMO single particle

SCHEME 1 Synthesis of the polymer P(BDT-V-BTD).



eigenvalues at the B3LYP/6-31G* level in solution. Also inthis case we are able to nicely reproduce the measuredenergy values of HOMO and LUMO, with a deviation fromthe experimental data in the range of 0.1–0.2 eV.

Fabrication and Characterization of PhotovoltaicCells and OFETsPhotovoltaic devices were fabricated with the standardarchitecture ITO/PEDOT:PSS/P(BDT-V-BTD):PCBM/LiF/Al. In-dium-Tin Oxide (ITO) covered glass substrates (sheet resist-ance 10 X/square were cleaned sequentially by ultrasonictreatment in deionized water, acetone and isopropyl alcohol,then dried and placed in a Oxygen/UV plasma chambercleaner for 10 mins. A poly(3,4-ethylenedioxythiophene) /poly(styrenesulfonate) (PEDOT:PSS) (Baytron P) layer about80 nm thick was spin-coated at 2000 rpm (60 s) onto thesubstrates, followed by annealing at 150 �C for 30 mins.PC61BM and PC71BM were bought from American DyeSource. Two blended solutions of P(BDT-V-BTD) and PC61BMin dry CHCl3 were prepared, with polymer concentration of7 mg/mL and PC61BM concentrations of 7 and 10.5 mg/mL,leading to solutions with polymer: fullerene weight ratios of1:1 and 1:1.5, respectively. Another solution was preparedwith P(BDT-V-BTD) (7 mg/mL) and PC71BM (10.5 mg/mL)in dry CHCl3 leading to a solution with P(BDT-V-BTD):PC71BM weight ratio of 1:1.5. Blend solutions werekept in an ultrasonic bath for 2 hours, then spin-coated inair at 1500 rpm for 60 s onto glass/ITO/PEDOT:PSS sub-strates (active layer thickness 50 nm). LiF and Al cathodes(0.6 nm and 70 nm thick) were deposited sequentiallythrough a mask on top of the active layer in a vacuumchamber at a pressure of �10�6 Torr. The active area of thedevice was 6 mm2. Current density-voltage curves wererecorded with a Keithley 236 source-meter unit by illuminat-ing a single device through a mask with simulated AM 1.5Girradiation (100 mW/cm2) from an Abet Technologies Sun2000 Solar Simulator. All devices were tested in oxygen andwater free environment inside a glove box filled with nitro-gen. The thicknesses of the films were measured by a profi-lometer. Atomic force microscopy (AFM) images were takenwith a Solver Pro (NTMDT) scanning probe microscope intapping mode. Absorption of thin films was measured usinga JASCO spectrometer.

External Quantum Efficiency (EQE) was measured with ahome built system on encapsulated devices: monochromaticlight was obtained with a Xenon arc lamp from Lot-Oriel(300 Watt power) coupled with a Spectra-Pro monochroma-tor. The photocurrent produced by the device passedthrough a calibrated resistance (51 Ohms) and the Voltagesignal was collected after the resistance with a Merlin Lock-In Digital Amplifier. Signal was pulsed by means of an opticalchopper (around 300 Hz frequency). Internal Quantum Effi-ciency (IQE) was calculated starting from the EQE andabsorption spectra, taking into account the followingassumptions: (i) 7% of incident light is reflected at the air/glass interface, (ii) the Al contact is considered a perfect mir-ror, therefore the fraction of light absorbed by the activelayer (A%) is calculated from the doubled absorption spec-

trum (Abs) of the active layer, as A% ¼ 1–10�(2*Abs). ThusIQE ¼ EQE/A%.

Bottom-gate/top-contact OFETs were fabricated on hexa-methyldisilazane (HMDS)-treated, p-doped Silicon waferscovered with a SiO2 dielectric layer. Trimethylsilation of theSiO2 surface was done by exposure to HMDS vapors in air-free, room temperature reaction. P(BDT-V-BTD):PC61BM 1:1.5(w/w) or P(BDT-V-BTD) only films were prepared in thesame condition used for the OPVs active layers preparation.To complete the OFET devices, 50 nm of Au was thermallyevaporated over the semiconducting layer through a shadowmask in a vacuum chamber at a pressure of �10�6 Torr toyield the source and drain electrodes (channel length ¼1000 lm and channel width ¼ 70 lm).

RESULTS AND DISCUSSION

Synthesis, Chemical Characterization, andOptical PropertiesThe polymer P(BDT-V-BTD) was obtained according toScheme 1 by a Stille poly-cross-coupling reaction catalyzedby Pd(PPh3)4 starting from the bis-trimethyltin derivative 1and the bis-2-bromovinyl BTD derivative 2.

Polymerization proceeded in good yields producing materialswith moderate number-average molecular weights. The poly-mer was precipitated in methanol, collected by filtration,and purified by Soxhlet extraction. Low-molecular-weightoligomers were removed by extraction with n-hexane. Thesoluble higher-molecular-weight fraction was extractedwith chloroform with a significant portion of solid residue,not soluble in common organic solvents, remaining in theSoxhlet thimble.

The new polymer was characterized by 1H NMR and UV–visspectroscopy, GPC, DSC, and TGA. Figure 4 shows the 1HNMR spectrum of P(BDT-V-BTD). Table 1 summarizes themain structural, optical, and thermal properties of the poly-mer along with those of our previously reported vinyleneEDOT-pyridine polymers P(2,6-Py-V-EDOT) and P(2,6-Py-V-EDOT),22 and of the reference polymer P(BDT-BTD) contain-ing the same donor and acceptor heteroaromatic units butwithout the vinylene spacer.16(a)

Figure 5 shows the UV–vis spectra in solution and as thinfilms of P(BDT-V-BTD). A second polymer, where the twolinear octyl chains of the BDT unit were replaced by two 2-ethylhexyl branched chains, was prepared starting fromthe corresponding bis-trimethyltin precursor.16(b) Althoughphotophysical properties were similar to that of the octylderivative, solubility and film processability unexpectedlyresulted poorer and no significant photovoltaic responsein OPV cells in combination with PC61BM or PC71BM wasmeasured. For these reasons the 2-ethylhexyl substitutedpolymer was no further considered in this work.

The GPC data revealed molecular-weights which are some-what smaller than P(BDT-BTD) but higher than previousvinylene polymers P(2,6-Py-V-EDOT) and P(2,5-Py-V-EDOT).A considerable bathochromic effect due to the introduction


POLYMER SCIENCE


of the new donor and acceptor fragments has been observed,which in turn yielded smaller optical bandgap energies(from 1.9–2.0 to 1.7 eV), in agreement with that observedfor P(BDT-BTD).

When measured as thin film, the absorption spectrum ofP(BDT-V-BTD) is broadened in the low-energy portion, likelydue to aggregated networks from p-p interactions in thesolid state arising also from the vinylene-induced increasedplanarity. This results in lower absorption onsets and opticalbandgaps (1.5 eV), again as similarly found for P(BDT-BTD).A thermogravimetric (TGA) and differential scanning calo-rimetry (DSC) analysis was carried out in order to determinethe thermal stability and transitions of the new polymer. Thepolymer exhibited a reasonable thermal stability with a deg-

radation temperature above 300 �C and higher than that ofthe reference polymers (Table 1). The thermal stability istherefore satisfactory for use in OPVs and other optoelec-tronic devices. The DSC scan revealed no obvious thermaltransitions in the temperature range from 25 to 400 �C.

Electrochemical CharacterizationThe polymer powder was dissolved (0.5 mM) in an electro-lyte solution of 0.1 M tetrabutylammonium perchlorate(TBAClO4) in CH2Cl2. Cyclic voltammetries (CVs) performedin the whole potential range between 0.7 and �1.8 Vshowed several redox processes (Fig. 6). Oligomer oxidationand reduction processes lied at potential above 0.2 andbelow �1.5 V, respectively, and the corresponding peak posi-tions and currents are stable upon cycling. Before both

FIGURE 4 1H NMR spectrum of P(BDT-V-BTD) in 1,1,2,2-tetrachloroethane-d2 (6.0 ppm from TMS) (number of protons refer to the

repeat unit).

TABLE 1 Structural and Optical Properties of P(BDT-V-BTD) in CHCl3 Solution and as a Thin Film and Comparison with Reference

Polymers

Polymer

Mw

(kg mol�1)aMn

(kg mol�1)b PDIc

CHCl3solution Film

Tg

(�C)dTd

(�C)ekmax

(nm)

konset(nm)

Egapopt

(eV)fkmax

(nm)

konset(nm)

Egapopt

(eV)f

P(BDT-V-BTD) 12.3 7.2 1.7 560 730 1.7 561 850 1.5 – 330

P(2,6-Py-V-EDOT)g 5.2 5.0 1.0 353 650 1.9 364 760 1.6 110 240

P(2,5-Py-V-EDOT)g 2.3 2.3 1.0 415 615 2.0 424 770 1.6 – 265

P(BDT-BTD)h 31 18 1.7 591i 730i 1.7i 595 850 1.5 – 270

a Weight-average molecular weight obtained by GPC.b Number-average molar mass obtained by GPC.c Polydispersity index (PDI ¼ Mw/Mn).d Optical bandgap (calculated on the low energetic edge of the absorp-

tion spectrum).

e Glass transition temperature determined by DSC.f Decomposition temperature determined by TGA (5% weight loss).g From ref. 22.h From ref. 16(a).i Data in THF.



processes, sharp charge-trapping-like peaks were observedthe current and potential of which shifted cycle by cycle. Thereversible redox process around �1.0 V was observed onlyafter the first scan. The process increased upon cycling andis likely due to the redox system of coupled chains whichgrow at high oxidative potentials (>0.5 V).

To better understand the electrochemical behavior we sepa-rately explored the two potential regions wherein oligomersoxidation and reduction take place (Fig. 7). Upon doing thisthe charge trapping peaks were no longer visible and thecurrent relative to the intermediate potential redox processaccordingly did not increase. HOMO-LUMO energy valueswere determined by Differential Pulsed Voltammetry (Fig. 8).Since both peaks showed irreversible features compared tothe peak of the internal standard (ferrocene), in this specificcase energy values were estimated by the onsets of the ris-ing currents. The onsets were calculated by taking the inter-cepts between peak tangent and current baseline. The

results are collected in Table 2 along with those of the refer-ence polymers.

As expected, the stronger acceptor character of the BTD moi-ety decreased both the HOMO and LUMO energies comparedto previous vinylene-linked polymers, with the polymerbeing harder to oxidize and easier to reduce than the pyri-dine derivatives. In contrast, the different energetic valueswith respect to P(BDT-BTD) are more difficult to explain,being the acceptor and donor components identical. Indeed,we found that if the data for P(BDT-BTD)16(a) are recali-brated (the pseudo-reference Ag electrode recalibrated vs.ferrocene) the oxidation/reduction onsets lie in a potentialregion very similar to the vinylene-linked polymer. Thus, therecalculated HOMO/LUMO levels of the P(BDT-BTD) wereactually in excellent agreement with those of P(BDT-V-BTD).

DFT/TDDFT CalculationsTo gain insight into the structural, electronic, and opticalproperties of the investigated systems, we performed DFT/

FIGURE 5 Absorption spectra of P(BDT-V-BTD) measured in

CHCl3 solution (solid line) and as thin film (dashed line).

FIGURE 6 Full range CV of P(BDT-V-BTD) in 0.1 M TBAPF6

CH2Cl2 solution at 20 mV/s.

FIGURE 7 CVs of P(BDT-V-BTD) in 0.1 M TBAPF6 CH2Cl2 solu-

tion at 20 mV/s performed in different potential ranges.

FIGURE 8 DPVs of P(BDT-V-BTD) in 0.1 M TBAPF6 CH2Cl2 solu-

tion at 10 mV/s.


POLYMER SCIENCE


TDDFT calculations, including solvation effects, on the mono-mers BDT-V-BTD and BDT-BTD and on their selected oligom-ers (BDT-V-BTD)n and (BDT-BTD)n, with the aim to simulatethe properties of the polymeric species. To reduce the com-putational overhead we replaced the n-octyl and n-dodecylchains of the monomers BDT-V-BTD and BDT-BTD with amethyl (ACH3) substituent, thus looking at the P(Me-BDT-V-BTD) and P(Me-BDT-BTD) models. For the characterizationof the optical and electrochemical properties of this type ofpolymers we followed the same approach developed in ourprevious work.22 In particular, the theoretical study of apolymeric systems can be performed via two differentapproaches: (i) by considering the infinite polymer as a peri-odic system, and (ii) by analyzing extended systems withincreasing but finite dimension. Here, we chose the second

approach, where the polymer is studied with a growing upapproach starting from the principal monomeric unit. Such‘‘cluster’’ approach allows us to exploit all the computationalmachinery developed for isolated systems,30 including thecalculation of excited states using TDDFT and solvationeffects in a simple yet effective way, by means of PCM.25(a)

Geometry optimization in vacuum of the monomers was per-formed by the B3LYP functional using both 3-21G* and 6-31G* basis sets, finding minimal differences between the twodata sets. Thus, for the larger system, we used the 3-21G*basis set, allowing the description of the extended oligomericsystems, whereby a large number of atoms, and thus a largeassociated computational overhead, needs to be considered.The optimized geometries of representative systems areshown in Figure 9. The optimized structures are planar in all

TABLE 2 Electrochemical Properties of P(BDT-V-BTD) in 0.1 M TBAPF6 CH2Cl2 Solution and Comparison with Reference Polymersa

Polymer Eonsetox (V) HOMO (eV) Eonset

red (V) LUMO (eV) EgapEC (eV)b

P(BDT-V-BTD) 0.30 �5.5 �1.50 �3.7 1.8

P(2,6-Py-V-EDOT)c �0.23 �5.0 �1.80 �3.4 1.6

P(2,5-Py-V-EDOT)c �0.11 �5.1 �1.78 �3.4 1.7

P(BDT-BTD)d �5.1(�5.6)e �3.2 (�3.7)e 1.9

a All potentials are reported vs. Fc/Fcþ and HOMO and LUMO energies

are derived from the electrochemical data based on the assumption

that the Fc/Fcþ redox couple is 5.2 eV relative to vacuum.b Electrochemical bandgap, obtained from the difference between the

reduction and the oxidation potential onset (or LUMO and HOMO

energies).

c From ref. 21.d From ref. 16(a).e HOMO and LUMO recalculated from ref. 16(a) by considering that the

Fc/Fcþ redox couple is 5.2 eV relative to vacuum.

FIGURE 9 Optimized geometries of (Me-BDT-V-BTD)n and (Me-BDT-BTD)n monomers (n ¼ 1) and oligomers (n ¼ 3, 5).



cases, apart from the methoxy groups which lie outside theplane.

We performed TDDFT excited state calculations (Table 3) onthe optimized structures of the monomers and oligomers upto five monomeric units (n ¼ 5). In our previous work wefound that B3LYP provides accurate ground state geome-tries,27 whereas the MPW1K functional,29 which has anincreased percentage of Hartree–Fock exchange, provides ex-citation energies in much closer agreement with experimen-tal values.27

Both approaches have their merits, with Koopmans theoremoffering a simple but approximate computational procedure,requiring only a calculation on the neutral species. The cal-culation of Gibbs free energies, on the other hand, is accu-rate but computationally very demanding, requiring calcula-tion of geometries and vibrational frequencies in vacuo andgeometries in solution, for the neutral, cation, and anion spe-cies. In our previous work we have tested both approachesfor the P(2,6-Py-V-EDOT) and P(2,5-Py-V-EDOT) monomersand trimers,22 the latter being the largest systems for whichcalculation of Gibbs free energies was possible within a rea-sonable computer overhead.

As a matter of fact, we found both approaches to beadequate to describe the investigated systems, especiallyconsidering the somewhat uncertain conversion between rel-

ative measured electrochemical potentials and absolute val-ues referred to the vacuum. In this work we thus evaluatedthe oxidation and reduction potentials by taking the negativeof the HOMO and LUMO single particle eigenvalues. We thenadopted the selected hybrid level of theory to extended ourinvestigations to the whole oligomer series, from the mono-mer (n ¼ 1) to the pentamer (n ¼ 5) (Table 3). Using a lin-ear fit31 and plotting the HOMO, LUMO and excitation ener-gies of the oligomers against 1/n we can extrapolate thevalues of the infinite polymer as the intercept with ordinateaxes (Fig. 10 and Table 3).

The calculated extrapolated values of the absorption energiesin solution (Table 3, fit n ¼ 1–5) reproduced the experimen-tal absorption onsets, located at about 1.7 eV for both sys-tems. The computed B3LYP/6-31G* HOMO and LUMO levelsof (Me-BDT-V-BTD)n and (Me-BDT-BTD)n are listed in Table 3for the oligomers series (n ¼ 1–5) (see Fig. 11 for isodensitycontour plots).

We notice that the calculated single particle HOMO andLUMO values are in excellent agreement with the experimen-tal estimates (Table 3). In particular for P(Me-BDT-V-BTD)the calculated HOMO and LUMO energies are �4.7 and �3.1eV, respectively, reproducing the experimental optical gap(1.7 eV) and the electrochemical measurements. For theP(Me-BDT-BTD) polymer, the calculated HOMO and LUMO

TABLE 3 Calculated HOMO,a LUMO,a and Excitationb Energies of (Me-BDT-V-BTD)n and (Me-BDT-BTD)n

N

(Me-BDT-V-BTD)n (Me-BDT-BTD)n

Exc. En (eV) HOMO (eV) LUMO (eV) Egap (eV) Exc. En (eV) HOMO (eV) LUMO (eV) Egap (eV)

1 2.56 (2.63) �5.21 (�5.13) �2.69 (�2.68) 2.52 (2.45) 2.91 (2.88) �5.39 (�5.27) �2.63 (�2.62) 2.76 (2.65)

2 2.18 (2.21) �4.98 (�4.90) �2.87 (�2.85) 2.11 (2.05) 2.36 (2.35) �5.21 (�5.09) �2.90 (�2.86) 2.31 (2.23)

3 2.07 (2.07) �4.91 (�4.83) �2.94 (�2.92) 1.97 (1.91) 2.21 (2.18) �5.13 (�5.02) �3.00 (�2.97) 2.13 (2.05)

4 2.01 (2.00) �4.79 (�4.79) �2.95 (�2.95) 1.84 (1.84) 2.14 (2.10) �5.10 (�4.98) �3.05 (�3.02) 2.05 (1.96)

5 1.99 (1.97) �4.86 (�4.78) �2.99 (�2.96) 1.87 (1.82) 2.11 (2.06) �5.08 (�4.96) �3.07 (�3.04) 2.01 (1.92)

Fit 1.80 (1.83) �4.73 (�4.68) �3.06 (�3.03) 1.67 (1.65) 1.88 (1.84) �5.00 (�4.89) �3.18 (�3.14) 1.82 (1.75)

a TDDFT excitation energies calculated on the B3LYP/3-21G* optimized

geometries with the MPW1K functional.

b HOMO and LUMO energies in THF (in vacuo in parentheses) calculated

at the B3LYP/6-31G* level on the B3LYP/3-21G* optimized geometries.

FIGURE 10 Calculated trends of lowest excitation energies (left), and HOMO and LUMO values (right) for P(Me-BDT-V-BTD)

(squares) and P(Me-BDT-BTD) (diamonds).


POLYMER SCIENCE


energies are �5.0 and �3.2 eV, respectively, showing a calcu-lated HOMO-LUMO difference of 1.8 eV in excellent agree-ment with the optical and electrochemical data shown inTables 1 and 2, respectively.

Photovoltaic PropertiesThe use of P(BDT-V-BTD) in combination with fullerenederivatives (PC61BM and PC71BM) as donor and acceptormaterials in BHJ OPV devices was investigated. The standardOPV configuration glass/ITO/PEDOT:PSS/donor:acceptor/LiF/Al was used. Devices fabrication details are reported inthe ‘‘Experimental’’ section. Figure 12 shows the characteris-tic current density-voltage (J-V) plots of the most representa-tive solar cells with different donor:acceptor weight ratiosand different acceptors (PC61BM or PC71BM), measuredunder standard AM 1.5G illumination.

The best performing device yielded a PCE of 0.74% withcurrent density (JSC), open circuit voltage (Voc), and fill factor(FF) of 2.69 mA cm�2, 0.68 V, and 41%, respectively (Table4). The Voc values measured for the P(BDT-V-BTD) based

devices are almost independent on the blend compositionand on the acceptor (PC61BM or PC71BM). The mean Vocvalue of 0.68 V well matches with the Voc reported for thesolar cell based on the analogous single bonded P(BDT-BTD)donor material (see Table 4), in agreement with the similarmolecular structure and the HOMO energy levels of the twopolymers (Tables 2 and 3). On the other hand, within allP(BDT-V-BTD) based solar cells, the measured FF and JSC didnot exceed 45% and 2.69 mA cm�2, respectively. This couldbe due to unfavorable thin-film electrical and/or morphologi-cal characteristics hampering the PCEs of the devices, de-spite the optimization of the polymer optoelectronicproperties.

Many efforts to improve the device performances were car-ried out by following a variety of strategies. In general, thevariation of the donor:acceptor blend ratios is well known toaffect significantly the photovoltaic parameters. By increasingthe polymer amount in the blend films, going from 1:1 to1.2:1 P(BDT-V-BTD):PC61BM weight ratio, corresponding toblend films with an excess of polymer, we observed a slightdecrease of the overall OPV performance (PCE going from0.58 to 0.52, mainly due to decreased Jsc and FF) (Table 4).On the opposite, by increasing the PC61BM content, from 1:1to 1:1.5 D:A weight ratio, a raise of the JSC (from 2.34 to2.69 mA cm�2) and FF (from 35 to 41%) were obtained,leading to a PCE of 0.74%. This result could be likelyascribed to enhanced charge separation and transport proc-esses in the active blend due to a more favorable self-organi-zation of the interpenetrating donor and acceptor phases. Afurther increase of the PC61BM content led to reduced JSC,and consequently PCEs (Table 4), confirming the bestdonor:acceptor blend weight ratio of 1:1.5.

It is well known that thermal and solvent annealing of theactive layer or of complete BHJ OPV devices could lead toimproved blend film nano-morphology, electrical propertiesand device performances.32 We found that thin film thermalannealing in the range 60–140 �C, and for different times,resulted in film degradation, adversely affecting photovoltaicparameters. Moreover, different solvents (e.g., toluene, chlor-obenzene, and ortho-dichlorobenzene) were used to spin-

FIGURE 11 Isodensity plots of HOMOs and LUMOs for P(Me-

BDT-V-BTD) and P(Me-BDT-BTD) monomers and trimers.

FIGURE 12 J-V curves, under illumination, of optimized P(BDT-

V-BTD): PCBM solar cells.

TABLE 4 Photovoltaic Parameters of Optimized P(BDT-V-BTD):

PCBM-Based Devices, Compared to Reference Polymer P(BDT-

BTD) Solar Cell

Materials

D:A

Ratio

(wt/wt)

Jsc

(mA/cm2)

Voc

(V)

FF

(%)

PCE

(%)

P(BDT-V-BTD): PC61BM 1.2:1 2.20 0.70 34 0.52

P(BDT-V-BTD): PC61BM 1:1 2.34 0.70 35 0.58

P(BDT-V-BTD): PC61BM 1:1.5 2.69 0.68 41 0.74

P(BDT-V-BTD): PC71BM 1:1.5 2.05 0.71 45 0.66

P(BDT-V-BTD): PC61BM 1:1.8 2.24 0.68 38 0.58

P(BDT-BTD): PC61BMa 1:1 2.97 0.68 44 0.90

a Values taken from ref. 16(a).



cast the blends, but poor quality films were obtained as aconsequence of the limited polymer solubility. The mostmacroscopically homogeneous and electrically uniform filmswere here obtained from chloroform solutions, resulting inthe best OPV performances.

In order to increase the device photocurrent generation,strategies to improve the blend film light collection abilitywere carried out. The photocurrent generation, together withother parameters such as hole mobility, optimum annealingtemperature and ideal morphology, is also determined by theamount of photons absorbed by the active layer, which inturns depends on the onset wavelength of the polymer(Eopt

gap) and on the blend film absorbance.

A high polymer absorption molar coefficient (e) is obtainedin solution (emax ¼ 3.5 � 105 mol�1 cm�1 considering Mw ¼12.3 kg mol�1), which is in the same order of magnitude ofother well performing OPV polymers.33 Therefore, in addi-tion to the intrinsically high polymer molar absorption, theactive layer film thickness was increased with the aim toenhance the overall blend film absorbance. Indeed, in P(BDT-V-BTD):PC61BM based devices, increasing the film thicknessup to 50 nm led to an improvement in photocurrent genera-tion and overall efficiency. However, a further increase in theactive layer thickness, although leading to enhanced film ab-sorbance, did not afford improved JSC and PCE. Optimized50-nm thick active layer films showed absorbance valueslower than 0.3 over the visible spectral window (see Sup-porting Information Fig. 1). Therefore, it is likely that suchlow film absorption, limited by thickness (50 nm), besidesother morphological and electrical device features, is an im-portant factor limiting the photocurrent response of P(BDT-V-BTD) based devices. On the other hand, since the activelayers in optimized devices still have >50% transparency inthe visible region, their potential use as photoactive materi-als in semitransparent cells for tandem or stackable devicescould be considered in future works on P(BDT-V-BTD) baseddevices.34

An additional strategy to improve the photocurrent responseof P(BDT-V-BTD) based solar cells is based on the replace-ment of PC61BM with PC71BM as acceptor material in theactive layer, in order to exploit the superior light harvestingproperties of the latter in the visible region.35 Although anenhancement in the absorption spectrum of the P(BDT-V-BTD):PC71BM blend film was recorded in comparison withthe analogous PC61BM based film (Supporting InformationFig. 1), no significant improvements of PCE values wereobserved (Table 4). This result might be attributed to thepossible PC71BM aggregation tendency and its poor miscibil-ity with the polymer, resulting in lower thin-film qualitywith limited nanoscale morphology, phase separation and,thereby, inferior electrical properties compared to PC61BMbased devices.36

Hole mobility (lh) of P(BDT-V-BTD) based films was meas-ured in order to investigate the polymer charge carriertransport properties which, together with other parameterssuch as optical absorption, morphology and energetic levels

alignment, affect the performances of a donor material inBHJ OPV devices. We measured lh of the polymer in pristineand blended films (with PC61BM) in organic field effect tran-sistor (OFET) devices.37 The transfer plots of a top-contactOFET prepared with optimized P(BDT-V-BTD) film spun onSi/SiO2/HDMS showed, as expected, a p-type behavior withlh ¼ 6.3 � 10�5 cm2 V�1 s�1, in agreement with other lowband-gap polymers showing similar OFET hole mobilitiesand comparable or better photovoltaic performances com-pared to P(BDT-V-BTD).22,38 Moreover, OFET devices pre-pared with optimized P(BDT-V-BTD):PC61BM film exhibited alh value of 4.6 � 10�5 cm2 V�1 s�1, very similar to thatmeasured for the polymer alone, indicating that the presenceof PC61BM does not significantly affect the morphology andthe continuity of the polymer phase over the film surface.

Polymer carrier mobility is strictly correlated with molecularweight and nano-morphology. In particular, recent works onP3HT clearly confirmed the relationship between polymercharge carrier mobility, OPV performance, and molecularweight.39 In fact it was demonstrated that P3HT OFET mobil-ity is controlled by the probability of carriers to cross thelow-conductive disordered regions lying within the highly con-ductive crystalline domains. This probability is higher for highmolecular weight P3HT samples where molecular connectionsbridge the crystalline domains, while it is lower in low molec-ular weight P3HT films where the intercrystalline molecularconnections are missing. Therefore, it is possible that the rela-tively low molecular weight of P(BDT-V-BTD), which affectsthe polymer electrical and morphological characteristics, couldrepresent one of the critical factor that limits the photovoltaicperformance of the corresponding BHJ solar cells.

To get an insight into the micro- and nano-structural organi-zation of the polymer in the active layer, we investigated bytapping-mode AFM the morphological features of the P(BDT-V-BTD):PC61BM and P(BDT-V-BTD):PC71BM blend filmsaffording the best OPV performances (see SupportingInformation Fig. 2). The measurements revealed surfaceroughness (Root Mean Square, rms) of �1.3 and �1.5 nmfor the P(BDT-V-BTD):PC61BM and the P(BDT-V-BTD):PC71BMfilms, respectively, suggesting a smooth and homogeneoussurface of the blends. The two thin films have similar mor-phologies but different domain size, which depends on thenature of the acceptor in the blend. P(BDT-V-BTD):PC61BMfilm exhibits smaller features with respect to the P(BDT-V-BTD):PC71BM film, likely due to the lower aggregation tend-ency of PC61BM compared to PC71BM.36 This might also jus-tify the better performance of the PC61BM based devices.

To analyze the spectral response of the best performing solarcell, External Quantum Efficiency (EQE) of P(BDT-V-BTD):PC61BM 1:1.5 (wt/wt) based device was measured. The EQEspectrum (Fig. 13) shows the highest generated-electronsover incident-photons ratio, comprised within 0.15 and 0.17,in the spectral range between 450 and 650 nm, which corre-sponds to the lower-energy absorption band of the polymer.This indicates that photocurrent mainly arises from theabsorption of P(BDT-V-BTD) in the photoactive layer. The


POLYMER SCIENCE


short-circuit current density of the device inferred from con-voluting the EQE over the entire wavelength range (350–750nm) matches well, within the experimental error, with theJSC value obtained in the J-V curve measurement (2.53 and2.69 mA cm�2, respectively). An estimation of the InternalQuantum Efficiency (IQE) of the (BDT-V-BTD):PC61BM 1:1.5(w/w) device is also given in Figure 13. While IQE values in-ferior to unity over the entire spectral range reveal limitedphotocurrent generation capability of the device,40 the frac-tion of light absorbed by the active layer (A%, calculatedfrom Absorbance, see ‘‘Experimental’’ section) results as highas 0.8 in the range 450–650 nm, which is consistent withthe similarity of the IQE and EQE curves. Therefore, themodest photocurrent generation capability in the optimizedP(BDT-V-BTD):PC61BM devices has rather to be attributed tolimited polymer charge mobility than to poor active layerlight collection efficiency.

CONCLUSIONS

A new heteroarylene-vinylene donor–acceptor low bandgappolymer is presented. The design has been based on the un-precedented combination of strong donor (BDT) andacceptor (BTD) alternating units together with a vinylenespacer in order to exploit increased planarization along thep backbone and efficient p delocalization with respect to themost common approach in the literature presenting directaryl–aryl connection. The Stille metal-catalyzed cross-cou-pling reaction afforded polymeric species with larger molec-ular weights compared to previously reported heteroarylene-vinylene donor–acceptor polymers,22,23 though still nonopti-mal for best photovoltaic devices.

The electrochemical investigation has highlighted the ambi-polar character of the new polymer, with both p- and n-dop-ing processes being observable. In agreement with the intro-duction of a strong acceptor fragment in the polymericbackbone, HOMO and LUMO energies were significantlydown shifted by 0.3–0.5 V with respect to their counterparts

containing a pyridine ring as acceptor.22 This result is impor-tant in view of the expected higher photovoltage of OPVcells, being Voc proportional to the difference LUMOPCBM –HOMOpolymer.

8 On the other hand, the presence of the vinyl-ene spacer allowed for lower bandgap energies, in excellentagreement with the computational prediction, and improvedthermal stability compared to the corresponding speciesP(BDT-BTD) with direct bonding between the two heteroaro-matic units. In conclusion, the experimental and computa-tional studies have ascertained structural, photophysical, andelectronic properties matching optimal materials design rulesfor efficient donor polymers in OPV devices. Indeed, investi-gation of the photovoltaic properties in devices containingPCBM as the acceptor has shown higher photovoltages andphotocurrents compared to those of the previous heteroary-lene-vinylene polymers. The still nonoptimal thin-film nano-morphologies and hole mobility properties, likely due to therelatively low molecular weight, could account for the mod-est PCE lower than 1%, similar to those obtained for P(BDT-BTD). Efforts are under way to efficiently convert theencouraging properties of these heteroarylene-vinylene do-nor–acceptor polymers into more efficient OPV devices.

ACKNOWLEDGMENTS

The authors thank Fondazione Cariplo (Grant 2007-5085) for fi-nancial support. They are grateful to Alberto Bianchi for kind as-sistance in GPC measurements. Financial support from EUthrough projects FP7-ICT-248052 (PHOTO-FET) and FP7-ICT-287594 (SUNFLOWER), from Consorzio MISTE-R through projectFESR-Tecnopolo AMBIMAT, and from CNR through project EFORis acknowledged. F.D.A. and E.M. thank Fondazione Istituto Ital-iano di Tecnologia, Project SEED 2009 ‘‘HELYOS’’ for financial sup-port. They also thank G. Ruani for useful discussions.

REFERENCES AND NOTES

1 Facchetti, A. Chem. Mater. 2011, 23, 733–758.

2 Beaujuge, P. M.; Reynolds, J. R. Chem. Rev. 2010, 110, 268–320.

3 (a) Forrest, S. R.; Thompson, M. E. Chem. Rev. 2007, 107,

923–925; (b) Zaumseil, J.; Sirringhaus, H. Chem. Rev. 2007,

107, 1296–1323.

4 (a) Po, R.; Maggini, M.; Camaioni, N. J. Phys. Chem. C 2010,

114, 695–706; (b) Cheng, Y.-J.; Yang, S.-H.; Hsu, C.-S. Chem.

Rev. 2009, 109, 5868–5923; (c) Dennler, G.; Scharber, M. C.; Bra-

bec, C. J. Adv. Mater. 2009, 21, 1323–1338.

5 (a) Clarke, T. M.; Durrant, J. R. Chem. Rev. 2010, 110,

6736–6767; (b) Muccini, M.; Danieli, R.; Zamboni, R.; Taliani, C.;

Mohn, H.; Muller, W.; Meer, H. U. Chem. Phys. Lett.,1995, 245,

107; (c) Schlaich, H.; Muccini, M.; Feldmann, J.; Bassler, H.;

Gobel, E. O.; Zamboni, R.; Taliani, C.; Erxmeyer, J.; Weidinger,

A. Chem. Phys. Lett. 1995, 236, 135.

6 Brabec, C.; Dyakonov, V.; Parisi, J.; Sariciftci, N. S.; Organic

Photovoltaics; Springer-Verlag: Berlin, 2003.

7 (a) Thompson, B.; Fr�echet, J. M. J. Angew. Chem. Int. Ed. 2008,

47, 58–77; (b) Loi, M. A.; Toffanin, S.; Muccini, M.; Forster, M.;

Scherf, U.; Scharber, M. Adv. Funct. Mater. 2007, 17, 2111.

8 Scharber, M.; Muhlbacher, D.; Koppe, M.; Denk, P.; Waldauf,

C.; Heeger, A. J.; Brabec, C. J. Adv. Mater. 2006, 18, 789–794.

9 Beaujuge, P. M.; Amb, C. M.; Reynolds, J. R. Acc. Chem. Res.

2010, 43, 1396–1407.

FIGURE 13 EQE (white spots) and IQE (black squares) spectra

of optimized P(BDT-V-BTD): PC61BM, 1:1.5 (w/w) devices. The

absorption spectrum of the active layer (black line) is included

for comparison.



10 Havinga, E. E.; Hoeve, W.; Wynberg, H. Polym. Bull. 1992,

29, 119–126.

11 Chen, J.; Cao, Y. Acc. Chem. Res. 2009, 42, 1709–1718.

12 Some recent examples are: (a) Najari, A.; Beaupr�e, S.; Ber-

rouard, P.; Zou, Y.; Pouliot, J.-R.; Lepage-P�arusse, C.; Leclerc,

M. Adv. Funct. Mater. 2011, 21, 718–728; (b) Zhang, G.; Fu, Y.;

Zhang, Q.; Xie, Z. Chem. Commun. 2010, 46, 4997–4999; (c)

Piliego, C.; Holcombe, T. W.; Douglas, J. D.; Woo, C. H.; Beauj-

uge, P. M.; Frechet, J. M. J. J. Am. Chem. Soc. 2010, 132,

7595–7597; (d) Zhang, Y.; Hau, S. K.; Yip, H.-L.; Sun, Y.; Acton,

O.; Jen, A. K.-Y. Chem. Mater. 2010, 22, 2696–2698; (e) Zou, Y.;

Najari, A.; Berrouard, P.; Beaupr�e, S.; R�eda Aich, B.; Tao, Y.;

Leclerc, M. J. Am. Chem. Soc. 2010, 132, 5330–5331; (f) Huo, L.;

Hou, J.; Chen, H.-Y.; Zhang, S.; Jiang, Y.; Chen, T. L.; Yang, Y.

Macromolecules 2009, 42, 6564–6571.

13 (a) Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray,

C.; Yu, L. Adv. Mater. 2010, 22, E135–E138; (b) Chen, H.-Y.;

Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Wu, Y.;

Li, G. Nat. Photonics 2009, 3, 649–653.

14 (a) Lee, S. K.; Cho, J. M.; Goo, Y.; Shin, W. S.; Lee, J.-C.;

Lee, W.-H.; Kang, I.-N.; Shim, H.-K.; Moon, S.-J. Chem. Com-

mun. 2011, 47, 1791–1793; (b) Zhou, H.; Yang, L.; Liu, S.; You,

W. Macromolecules 2010, 43, 10390–10396; (c) Beaujuge, P. M.;

Subbiah, J.; Choudhury, K. R.; Ellinger, S.; McCarley, T. D.;

So, F.; Reynolds, J. R. Chem. Mater. 2010, 22, 2093–2106;

(d) Scharber, M. C.; Koppe, M.; Gao, J.; Cordella, F.; Loi, M. A.;

Denk, P.; Morana, M.; Egelhaaf, H.-J.; Forberich, K.; Dennler,

G.; Gaudiana, R.; Waller, D.; Zhu, Z.; Shi, X.; Brabec, C. J.

Adv. Mater. 2010, 22, 367–370; (e) Zhang, X.; Steckler, T. T.;

Dasari, R. R.; Ohira, S.; Potscavage, W. J., Jr.; Tiwari, S. P.;

Coppee, S.; Ellinger, S.; Barlow, S.; Bredas, J.-L.; Kippelen, B.;

Reynolds, J. R.; Marder, S. R. J. Mater. Chem. 2010, 20,

123–134; (f) Price, S. C.; Stuart, A. C.; You, W. Macromolecules

2010, 43, 797–804.

15 Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon,

J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Pho-

tonics 2009, 3, 297–302.

16 (a) Hou, J.; Park, M.-H.; Zhang, S.; Yao, Y.; Chen, L.-M.; Li,

J.-H.; Yang, Y. Macromolecules 2008, 41, 6012–6018; (b) Hou,

J.; Chen, H.-Y.; Zhang, S.; Yang, Y. J. Phys. Chem. C 2009, 113,

21202–21207; (c) Price, S. C.; Stuart, A. C.; You, W. Macromole-

cules 2010, 43, 4609–4612.

17 Jang, S.-Y.; Lim, B.; Yu, B.-K.; Kim, J.; Baeg, K.-J.; Khim, D.;

Kim, D.-Y. J. Mater. Chem. 2011, 21, 11822–11830.

18 Epstein, A. J.; Blatchford, J. W.; Wang, Y. Z.; Jessen, S. W.;

Gebler, D. D.; Gustafson, T. L.; Lin, L. B.; Wang, H.-L.; Park, Y.

W.; Swager, T. M.; MacDiarmid, A. G. Synth. Met. 1996, 78,

253–261.

19 Li, C.; Liu, M.; Pschirer, N. G.; Baumgarten, M.; Mullen, K.

Chem. Rev. 2010, 110, 6817–6855.

20 Farinola, G. M.; Babudri, F.; Cardone, A.; Omar, O. H.; Naso,

F. Pure Appl. Chem. 2008, 80, 1735–1746.

21 Mei, J.; Heston, N. C.; Vasilyeva, S. V.; Reynolds, J. R. Mac-

romolecules 2009, 42, 1482–1487.

22 Abbotto, A.; Calderon, E. H.; Dangate, M. S.; De Angelis, F.;

Manfredi, N.; Mari, C. M.; Marinzi, C.; Mosconi, E.; Muccini, M.;

Ruffo, R.; Seri, M. Macromolecules 2010, 43, 9698–9713.

23 Abbotto, A.; Herrera Calderon, E.; Manfredi, N.; Mari, C. M.;

Marinzi, C.; Ruffo, R. Synth. Met. 2011, 161, 763–769.

24 Gaussian 03, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.;

Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J.

A., Jr., Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.;

Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.;

Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada,

M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.;

Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.;

Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo,

C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;

Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P.

Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;

Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.;

Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Fores-

man, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cio-

slowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;

Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M.

A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M.

W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople,

J. A. Gaussian, Inc., Wallingford CT, 2004.

25 (a) Cossi, M.; Barone, V. J. Chem. Phys. 2001, 115, 4708; (b)

Becke, A. D. J. Chem. Phys. 1993, 98, 5648.

26 Kim, S.; Lee, J. K.; Kang, S. O.; Ko, J.; Yum, J.-H.; Fantacci,

S.; De Angelis, F.; Di Censo, D.; Nazeeruddin, M. K.; Graetzel,

M. J. Am. Chem. Soc. 2006, 128, 16701–16707.

27 (a) Pastore, M.; Mosconi, E.; De Angelis, F.; Graetzel, M. J. Phys.

Chem. C 2010, 114, 7205–7212; (b) Abbotto, A.; Manfredi, N.; Mar-

inzi, C.; Angelis, F. D.; Mosconi, E.; Yum, J.-H.; Xianxi, Z.; Nazeerud-

din,M. K.; Gratzel, M. Energy Environ. Sci. 2009, 2, 1094–1101.

28 Hagberg, D. P.; Yum, J.-H.; Lee, H.; De Angelis, F.; Mari-

nado, T.; Karlsson, K. M.; Humphry-Baker, R.; Sun, L.; Hagfeldt,

A.; Graetzel, M.; Nazeeruddin, M. K. J. Am. Chem. Soc. 2008,

130, 6259–6266.

29 Lynch, B. J.; Fast, P. L.; Harris, M.; Truhlar, D. J. J. Chem.

Phys. 1999, 110, 7650.

30 Zhang, X.; Steckler, T. T.; Dasari, R. R.; Ohira, S.; Potscav-

age, W. J., Jr.; Tiwari, S. P.; Coppee, S.; Ellinger, S.; Barlow, S.;

Bredas, J.-L.; Kippelen, B.; Reynolds, J. R.; Marder, S. R. J.

Mater. Chem. 2010, 20, 123–134.

31 Karsten, B. P.; Bijleveld, J. C.; Viani, L.; Cornil, J.; Giersch-

ner, J.; Janssen, R. A. J. J. Mater. Chem. 2009, 19, 5343–5350.

32 Moul�e, A. J.; Meerholz, K. Adv. Funct. Mater., 2009, 19, 1.

33 Coakley, M.; McGehee, M. D. Chem. Mater. 2004, 16, 4533.

34 Shrotriya, V.; Wu, E. H.; Li, G.; Yao, Y.; Yang, Y. Appl. Phys.

Lett. 2006, 88, 064104.

35 (a) Arbogast, J. W.; Foote, C. S. J. Am. Chem. Soc. 1991,

113, 8886; (b) Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.;

Knol, J.; Hummelen, J. C.; Van Hal, P. A.; Janssen, R. A.

Angew. Chem. Int. Ed. Engl. 2003, 42, 3371.

36 He, Y.; Li, Y. Phys. Chem. Chem. Phys. 2011, 13, 1970.

37 (a) Loser, S.; Bruns, C. J.; Miyauchi, H.; Ponce Ortiz, R.; Fac-

chetti, A.; Stupp, S. I.; Marks, T. J. J. Am. Chem. Soc. 2011,

133, 8142; (b) Gomes, H. L.; Stallinga, P.; Dinelli, F.; Murgia, M.;

Biscarini, F.; De Leeuw, D. M.; Muccini, M.; Mullen, K. Polym.

Adv. Technol. 2005, 16, 227; (c) Xu, Z.-X.; Roy, V. A. L.; Stal-

linga, P.; Muccini, M.; Toffanin, S.; Xiang, H.-F.; Che, C.-M.

Appl. Phys. Lett. 2007, 90, 223509.

38 Kim, J.; Kim, S. H.; Jung, I. H.; Jeong, E.; Xia, Y.; Cho, S.;

Hwang, I.-W.; Lee, K.; Suh, H.; Shim, H.-K.; Woo, H. Y. J. Mater.

Chem. 2010, 20, 1577.

39 (a) Coffin, R. C.; Peet, J.; Rogers, J.; Bazan, G. C. Nat. Chem.

2009, 1, 657; (b) Pingel, P.; Zen, A.; Abell�on, R. D.; Grozema, F.

C.; Siebbeles, L. D. A.; Neher, D. Adv. Funct. Mater. 2010, 20,

2286; (c) Tong, M.; Cho, S.; Rogers, J. T.; Schmidt, K.; Hsu, B.

B. Y.; Moses, R. D.; Coffin, C.; Kramer, E. J.; Bazan, G. C.;

Heeger, A. J. Adv. Funct. Mater. 2010, 20, 3959.

40 Burkhard, G. F.; Hoke, E. T.; Scully, S. R.; McGehee, M. D.

Nano Lett. 2009, 9, 4037.


POLYMER SCIENCE


A Vinylene-Linked Benzo[1,2-b:4,5-b’]dithiophene-2,1,3 ... · TGA, DSC and electrochemistry....

Documents

Transcript of A Vinylene-Linked Benzo[1,2-b:4,5-b’]dithiophene-2,1,3 ... · TGA, DSC and electrochemistry....