Comparison of thiophene- and selenophene-bridged donor–acceptor low band-gap copolymers used in bulk-heterojunction organic photovoltaics†

Hung-Yang Chen,abc Shih-Chieh Yeh,a Chin-Ti Chen*a and Chao-Tsen Chen*c

Received 11th June 2012, Accepted 28th August 2012

DOI: 10.1039/c2jm33735e

We report a detailed comparison of absorption spectroscopy, electrochemistry, DFT calculations, field-

effect charge mobility, as well as organic photovoltaic characteristics between thiophene- and

selenophene-bridged donor–acceptor low-band-gap copolymers. In these copolymers, a significant

reduction of the band-gap energy was observed for selenophene-bridged copolymers by UV-visible

absorption spectroscopy and cyclic voltammetry. Field-effect charge mobility studies reveal that the

enhanced hole mobility of the selenophene-bridged copolymers hinges on the solubilising alkyl side

chain of the copolymers. Both cyclic voltammetry experiments and theoretical calculations showed that

the decreased band-gap energy is mainly due to the lowering of the LUMO energy level, and the raising

of the HOMO energy level is just a secondary cause. These results are reflected in a significant increase

of the short circuit current density (JSC) but a slight decrease of the open circuit voltage (VOC) of their

bulk-heterojunction organic photovoltaics (BHJ OPVs), of which the electron donor materials are a

selenophene-bridged donor–acceptor copolymer: poly{9-dodecyl-9H-carbazole-alt-5,6-

bis(dodecyloxy)-4,7-di(selenophen-2-yl) benzo[c][1,2,5]-thiadiazole} (pCzSe) or poly{4,8-bis(2-

ethylhexyloxy)benzo[1,2-b;4,5-b0]dithiophene-alt-5,6-bis(dodecyloxy)-4,7-di(selenophen-2-yl)benzo[c]

[1,2,5]-thiadiazole} (pBDTSe), or a thiophene-bridged donor–acceptor copolymer: poly{9-dodecyl-9H-

carbazole-alt-5,6-bis(dodecyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole} (pCzS) or poly

{4,8-bis(2-ethylhexyloxy)benzo[1,2-b;4,5-b0]dithiophene-alt-5,6-bis(dodecyloxy)-4,7-di(thiophen-2-yl)

benzo[c][1,2,5]-thiadiazole} (pBDTS); the electron acceptor material is [6,6]-phenyl-C61-butyric acid

methyl ester (PCBM). Judging from our device data, the potential Se–Se interactions of the

selenophene-bridged donor–acceptor copolymers, which is presumably beneficial for the fill factor (FF)

of BHJ OPVs, is rather susceptible to the device fabrication conditions.

1. Introduction

In the last two decades, polymer-based organic photovoltaics

(OPVs) have attracted intensive studies in both academy and

industry due to their potential value in the global solar energy

market.1 The so-called bulk-heterojunction (BHJ) has been

proven to be the most successful device structure for OPVs so

far.2 Using p-conjugated polymers as the electron donors and

soluble fullerene (such as PCBM) as the electron acceptor is most

common in BHJ OPVs.2–10 Many recent research activities have

focused on improving polymer donor materials toward low

band-gap energy (Eg), the energy difference between the HOMO

and LUMO levels. Low band-gap polymers have an advantage

of more light harvesting of influx photons in matching the solar

spectrum and hence a larger short-circuit current density (JSC) of

OPVs. On the other hand, the low lying HOMO energy level of

the polymer donor material is important to keep the high open-

circuit voltage (VOC) of OPVs. Many experimental or theoretical

studies have demonstrated that there is a linear relation between

the VOC and the energy difference between the LUMO level of

the electron accepting material and the HOMO level of the

electron donating material.8 It is not uncommon that a con-

flicting situation may take place in optimizing the JSC andVOC of

OPVs, if the small Eg of the electron donating polymer material is

managed by elevating its HOMO level. Here, we want to report

that replacing the thiophene p-conjugated bridge of the donor–

acceptor copolymers with selenophene is an effective approach to

increase JSC without the comprise of much VOC of BHJ OPVs.

Thiophene-containing polymers are among the best investi-

gated materials as electron donors in BHJ OPVs due to their

aInstitute of Chemistry, Academia Sinica, Taipei, Taiwan 11529, Republicof China. E-mail: cchen@chem.sinica.edu.tw; Fax: +886 2 27831237; Tel:+886 2 27898542bNano Science and Technology Program, TIGP, Academia Sinica, Taipei,Taiwan 11529, Republic of ChinacDepartment of Chemistry, National Taiwan University, Taipei, Taiwan11617, Republic of China† Electronic supplementary information (ESI) available: The draincurrent–voltage plots and transistor transfer characteristics ofcopolymers, details of computational study, 1H NMR spectra ofpolymers. See DOI: 10.1039/c2jm33735e

This journal is ª The Royal Society of Chemistry 2012 J. Mater. Chem., 2012, 22, 21549–21559 | 21549

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p-conjugated character and incredible synthetic versatility. The

current state-of-the-art thiophene-containing polymers utilized

in BHJ OPVs as electron donors have achieved power conversion

efficiencies (PCE) of over 7%.5–10 Selenophene is the chalcoge-

nophene homologue with chemical and physical properties

similar to those of thiophene. Several possible advantages of

selenophene over thiophene would be expected for application in

organic electronics.11–13 For examples, increased conductivity of

organic conductors has been found by the replacement of the

sulfur atom by the selenium atom, such as tetrathiafulvalene

(TTF) and tetraselenafulvalene (TSF) derivatives.14,15 This is due

to the intermolecular Se–Se interactions, which would facilitate

intermolecular charge transfer. Similarly, it has been shown that

selenophene-based polymers have better conductivity and charge

mobility than thiophene-based polymers due to the larger

p-overlap of the larger p-orbitals of selenium atoms.16

Furthermore, both theoretical studies and experimental evidence

indicate that selenophene-based polymers have a lower Eg.17,18

This is due to the more pronounced quinoidal character found

for selenophene than thiophene, which facilitates the p-conju-gated connection to other structural moieties, and hence

lengthens the p-conjugation along the polymer chain.19,20 Since

the ionization potential of oligoselenophenes is around 4.6 eV,

which is only 0.05 eV lower than the calculated ionization

potential for oligothiophenes, it is not enough to account for the

0.18 eV difference of Eg.11 In other words, the heteroatom of the

polychalcogenophene alters the energy level of the material more

on the LUMO than the HOMO.

Various thiophene- or selenophene-containing materials,

including small molecules,21,22 oligomers,23 and polymers,24–30

have been synthesized for organic field-effect transistors

(OFETs) and similar or enhanced charge mobility has been

observed for most selenophene-containing materials. However,

materials containing selenophene designed for light harvesting

are still rare.31–42 Just a couple of reports regarding thiophene-

and selenophene-bridged donor–acceptor copolymers used for

BHJ OPVs have been published very recently.41,42 In this work, a

series of thiophene- and selenophene-bridged donor–acceptor

low band-gap copolymers have been designed and synthesized.

Two different electron donor moieties, carbazole and benzo[1,2-

b:4,5-b0]dithiophene, were chosen to combine with a common

acceptor moiety, benzo[c][1,2,5]benzothiadiazole, resulting in

two pairs of copolymer couples pCzS–pCzSe and pBDTS–

pBDTSe (see Fig. 1). A detailed comparison of absorption

spectroscopy, electrochemistry, DFT calculations, field-effect

charge mobility, as well as the OPV characteristics between these

thiophene- and selenophene-bridged donor–acceptor copoly-

mers is presented herein.

2. Results and discussion

2.1 Copolymer design and synthesis

In order to obtain high molecular weight copolymers with good

solubility, all monomers, Cz, BDT, BTS, and BTSe (Scheme 1),

were anchored with long and/or branched solubilizing alkyl

chains in their designs. One kind of alkyl chain and two kinds of

alkoxy substituent were utilized in the electron-donating

and electron-accepting units, and they are N-dodecylcarbazole,

4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b0]dithiophene, and 5,6-

bis(dodecyloxy)benzo[c][1,2,5]thiadiazole.

With incorporation of a thiophene or selenophene spacer in

these copolymers, steric hindrance between the side chains of the

donor and acceptor monomers, which is more pronounced

between benzene-based monomers such as carbazoles (Cz) and

benzo[c][1,2,5]thiadiazoles (BT), can be minimized. This mini-

mized steric hindrance, enabling a more planar and extended

p-conjugation, and thus a smaller band-gap energy.

The synthetic procedure for the corresponding monomers used

in the polymerization reaction is shown in Scheme 2. For the Cz

electron donating monomer, the boron ester group was func-

tionalized at the 2- and 7-positions of carbazole in order to

extend the electron p-conjugation of the polymer and it

(compound 1) was synthesized from 4,40-dibromo-1,10-biphenyl

according to the reported procedures.43 For another electron

donating monomer, BDT, it was synthesized from compound 2

and the trimethyltin attachment was readily achieved according

to reported procedures.44,45 Two solubilising dodecyloxyl chains

attached to the electron-accepting BT unit were in place from the

very beginning of the BT synthesis and the detailed synthetic

procedures can be found in the literature reported by Janssen

et al.46 In our synthetic design, the p-conjugating bridge, thio-

phene or selenophene, was first attached to the electron-accept-

ing BT unit instead of the electron-donating unit. They were

synthesized from 2-stannylated thiophene or 2-stannylated sele-

nophene by a Stille cross-coupling reaction with compound 3 to

give 4S and 4Se, which were then subjected to bromination to

afford BTS and BTSe, respectively.

Copolymers were prepared by conventional Suzuki or Stille

cross-coupling with high isolated yields of 87–93% (Scheme 1).

For the copolymers pCzS and pCzSe, the polymerization reac-

tion was readily achieved by Suzuki cross-coupling. For the

copolymers pBDTS and pBDTSe, the Stille cross-coupling

reaction was chosen to facilitate the copolymerization between

BTS or BTSe and BDT. All four copolymers were purified by

Soxhlet extraction with a washing solvent sequence of methanol,

hexane, and finally chloroform. The copolymers pCzS, pCzSe,

Fig. 1 Chemical structures of thiophene- and selenophene-bridged

donor–acceptor copolymers. Scheme 1 Synthesis of copolymers.

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pBDTS, and pBDTSe are readily dissolved in common organic

solvents like chloroform, tetrahydrofuran, chlorobenzene, and

1,2-dichlorobenzne at room temperature.

2.2 Physical and thermal properties

Table 1 summarizes the characterization results of all copolymers

prepared herein. All copolymers exhibit high polymerization

yields (87–93%) and high molecular weights (Mw ! 94.1–112.3

kg mol"1), even after a series of Soxhlet solvent purifications.

This can be attributed to their good solubility by adding long and

branched alkyl chains both on the donor and acceptor moiety of

the copolymers. It has been reported in the literature that solu-

bilizing alkyl chains have a very significant influence on the

polymerization yields and molecular weights of donor–acceptor

copolymers.47,48 Heteroatom substitution on these donor–

acceptor copolymers only slightly affects the polymerization

yields and molecular weights, which is due to the same polymer

backbone being shared by the S- and Se-containing copolymers.

All copolymers have thermal decomposition temperatures (Td)

over 300 #C (Fig. 2), indicating the good thermal stability of these

copolymers. In addition, it can be found that similar Td values

were observed for copolymers with the same polymer backbone

(pCzS vs. pCzSe or pBDTS vs. pBDTSe). We ascribe this to the

thermal decomposition starting with the soft part (alkyl chains)

of the copolymers, which is the same for the same type of

copolymer.

2.3 Spectroscopic properties

Both UV-visible absorption and photoluminescence (PL) spectra

were significantly affected by the heteroatom substitution of these

copolymers (Fig. 3 and 4). Either in solution or as thin film, an

obvious red-shifted UV-visible absorption was observed for

pCzSe or pBDTSe compared with pCzS or pBDTS, respectively.

Such red-shifted absorption happens to both absorption bands at

short and long wavelengths, 390–440 and 530–645 nm, respec-

tively. The detailed spectroscopic data are summarized in Table 2.

Vision-wise, these four copolymers are highly colorful and

they are quite distinctive from each other (Fig. 3). Owing to the

aggregation between copolymer chains in the solid state, both

pCzS–pCzSe and pBDTS–pBDTSe couples exhibit a red-shifted

absorption in their thin film states compared with their dilute

chloroform solutions.

Their Eg values were calculated from the onset absorption edge

of the thin film samples. pCzSe (Eg ! 1.86 eV) exhibits a reduced

band-gap energy of 0.1 eV when compared with that of pCzS

(Eg ! 1.96 eV). The same reduced band-gap energy was observed

for pBDTS (Eg ! 1.81 eV) and pBDTSe (Eg ! 1.71 eV) as well.

This result demonstrates a similar effect of narrowing band-gap

energy in the two types of donor–acceptor copolymer when the

thiophene p-conjugation bridge between the donor and acceptor

moieties is replaced by selenophene. The reduction of the band-

gap energy can be mainly attributed to the more electron rich and

more quinoidal character of selenophene than thiophene. Poly-

mers with a smaller band-gap energy are advantageous for

sunlight harvesting of OPVs.

2.4 Energy level determination

The energy levels (HOMO and LUMO) of the electron donating

material are crucial parameters governing the overall perfor-

mance of BHJ OPVs. The HOMO of the electron donating

material is closely related to theVOC of BHJ OPVs.8 On the other

hand, the LUMO–LUMO energy offset (at least 0.3 eV for P3HT

exciton)49 between the donor and acceptor materials is

Scheme 2 Synthesis of donor moieties, Cz and BDT, and chalcogeno-

phene-attached acceptors, BTS and BTSe.

Table 1 Polymerization results and thermal properties of copolymers

Copolymer Yielda (%) Mnb (kg mol"1) Mw

b (kg mol"1) PDI (Mw/Mn) Tdc (#C)

pCzS 93 66.1 102.9 1.56 342pCzSe 88 33.9 94.1 2.78 337pBDTS 90 62.7 106.7 1.70 312pBDTSe 87 63.7 112.3 1.76 311

a Yield after purification by Soxhlet extraction. b Determined by GPC in tetrahydrofuran (THF) at 40 #C using polystyrene as standard. c Thedecomposition temperature corresponding to a 5% weight loss determined by TGA at a heating rate of 10 #C min"1.

Fig. 2 Thermal decomposition diagrams for S- and Se-based

copolymers.

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responsible for the dissociation of excitons, which enables the

passing of the electron from donor to acceptor and sustaining the

JSC of BHJ OPVs. Cyclic voltammetry (CV) was used to analyze

the impact of heteroatom substitution on the energy levels of the

donor–acceptor copolymers. Fig. 5 shows electrochemical cyclic

voltammograms of the copolymer couples pCzS–pCzSe and

pBDTS–pBDTSe. For all copolymers, both oxidation and

reduction potentials were measured separately and were esti-

mated from their onset potentials in the cyclic voltammograms.

The HOMO and LUMO energy levels were calculated from their

onset potentials using the ferrocene–ferrocenium redox couple

(0.4 V vs.Ag–Ag+ in the study herein) as the reference to vacuum

level (4.8 eV).50

As shown in Fig. 5 and Table 3, the selenium-containing

copolymers (pCzSe and pBDTSe) are oxidized slightly easier than

their sulfur analogues (pCzS and pBDTS) by 0.04 and 0.03 V,

respectively. On the other hand, a greater decrease of reduction

potential (0.06 and 0.07 V) was observed for pCzS–pCzSe and

pBDTS–pBDTSe, respectively. These results show that the

substitution of sulfur with selenium in these donor–acceptor

copolymers has a more significant impact on the LUMO than the

HOMO energy level. Therefore, lowering the LUMO energy level

of these copolymers is the major cause of the reduction of band-

gap energy with the change of heteroatom (S to Se). In addition,

the difference in band-gap energy acquired from electrochemical

measurements for sulfur- and selenium-containing copolymers is

0.10 eV for both copolymer couples, which is consistent with that

obtained from the optical band-gap. This also demonstrates the

same magnitude of band-gap energy reduction happens to both

copolymers when the bridged thiophene is replaced by

Fig. 3 UV-visible absorption spectra of copolymers pCzS–pCzSe (top)

and pBDTS–pBDTSe (bottom) in chloroform solution and as thin films.

The photos shown on the right-hand side display the visual colors of the

copolymers in dilute chloroform solution and as thin films.

Fig. 4 Normalized photoluminescence spectra of all copolymers in

chloroform.

Table 2 UV-visible absorption and photoluminescence data of allcopolymers

UV-visible absorptionPL

CHCl3Film

CHCl3

lmax (nm)lmax

(nm)lonset(nm) Eopt

ga (eV)

lmax

(nm)

pCzS 530 578 627 1.96 624pCzSe 552 596 658 1.86 644pBDTS 579 594 684 1.81 664pBDTSe 599 644 726 1.71 686

a Eoptg is the band-gap energy calculated from the intersection of the

tangent on the lowest energetic edge of the copolymer thin filmabsorption spectrum with the baseline by Eopt

g ! 1240/lonset.

Fig. 5 Cyclic voltammograms of copolymer couples pCzS–pCzSe (top)

and pBDTS–pBDTSe (bottom). The ferrocene–ferrocenium redox couple

is also shown as a standard ("4.8 eV vs. vacuum level) in each plot.

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selenophene. However, a disparity was observed for the band-gap

energy measured from spectroscopic and electrochemical

methods. This is due to the energy barrier at the interface between

the polymer thin film and the electrode surface.51 Also, the

difference can be ascribed to the exciton binding energy that

differentiates the transport band-gap (electrochemical method)

and optical band-gap (spectroscopic method).52

Both spectroscopic and electrochemical methods showed a

meaningful reduction of Eg for the selenophene-bridged

copolymers. Thiophene and selenophene mainly play a role as a

‘‘p-conjugation bridge’’ in extending p-conjugation and elimi-

nating steric hindrance in these donor–acceptor copolymers. The

extension of p-conjugation and elimination of steric hindrance

decrease Eg, and the magnitude of the decreased Eg is closely

related to the quinoidal character of the bridging moiety along

the donor–acceptor copolymer chain.19 According to the esti-

mated aromatic stabilization energy (ASE),20 which is 18.6 and

16.7 kcal mol"1 for thiophene and selenophene, respectively, it is

consistent with the fact that there is a more quinoidal character in

selenophene- than in thiophene-containing copolymers.11

Heeney et al. have reported that poly(3-hexylselenophene)

(P3HS) shows a reduction in Eg when compared with poly-

(3-hexylthiophene) (P3HT), principally due to the lowering of

the polymer LUMO, with the HOMO being almost unaffected.53

Based on a computational study of P3HT, it has been reported

that the HOMO of thiophene-based polymers has no orbital

coefficient on the sulphur heteroatom, whilst the LUMO has a

significant contribution from the sulphur heteroatom.54

Although P3HT and P3HS are not donor–acceptor type

copolymers like those studied herein, computational studies have

also been found to be adequate for probing the electron density

distribution of the HOMO–LUMO in donor–acceptor p-conjugated copolymers.55 Therefore, it the following section, the

influence of heteroatom substitution on the HOMO–LUMO of

these copolymers is investigated by a computational study in

order to get an in-depth understanding of the relationship

between Eg and the HOMO–LUMO of pCzS–pCzSe and

pBDTS–pBDTSe copolymers.

2.5 Computational studies

A theoretical model of D–p–A–p–D–p–A–p, a dimer-like (D–

p–A–p)2 segment, is set up for the computational study due to its

resemblance to the copolymer structure and its computational

simplicity as well. In simulating the copolymer chain structure,

the centered donor (D) or acceptor (A) are connected to each

other, or peripheral A and D, respectively, through a p-conju-gation bridge, either thiophene or selenophene. In addition, we

limit the long alkyl chains of the copolymer to ethyl groups for

computational simplicity. The replacement of long alkyl chains

with shorter ones does not affect the optimized structure of the

p-conjugated molecules but speeds up the computational process

significantly. The experimental details of the computational

method are described in the ESI.†

Table 3 summarizes the calculation results of the dimer-like

model of copolymers and the comparisons with experimental

data. Although the predicted energy levels of these copolymers

are not quite the same as those obtained from experimental CV

data, certain correlations between the experimental results and

theoretical calculations are found to be consistent with each

other. For instances, DFT accurately predicts deeper lying

HOMO levels and a larger Eg for the sulphur-containing

copolymers (pCzS and pBDTS) compared to the selenium-con-

taining ones (pCzSe and pBDTSe). Moreover, DFT predicts that

the replacement of sulphur by selenium in these donor–acceptor

copolymers has a more significant impact on the LUMO than the

HOMO energy level. Specifically, the heteroatom Se substitution

results in an increase of the HOMO energy levels of 0.02 and 0.03

eV for the pCzS–pCzSe and pBDTS–pBDTSe couples, respec-

tively, which are much smaller than the 0.06 and 0.09 eV lowering

of the LUMO energy levels, respectively. This computational

result provides further confirmation of the influence of the

heteroatom substitution on the HOMO–LUMO energy levels

found by experimental methods.

The distinct electronic influence of heteroatom substitution on

theHOMOandLUMOof these copolymers canbeobserved from

their electron density distribution in the p-conjugated copoly-

mers. As shown in Fig. 6, it can be clearly seen that the HOMO

electron density mostly spans over the central p–A–p–D–pmoiety, except the heteroatomsulphur (yellow color) of thiophene

or selenium (orange color) of selenophene on the p-conjugationbridge. There is an electron density nodal plane across the sulphur

of thiophene or selenium of selenophene. Such an electron density

nodal plane accounts for the small HOMO energy level difference

between the thiophene- and selenophene-bridged copolymers. On

the other hand, the electron density of the LUMO is mainly

concentrated on p–A–p on the left-hand side, and p–A–p on the

right-hand side of the D–p–A–p–D–p–A–p model structure

Table 3 Electrochemical data and related energy levels of copolymers

Cyclic voltammetry DFT calculations

Eoxonset

(V)ECVHOMO

(eV)DECV

HOMOa

(eV)Eredonset

(V)DECV

LUMO

(eV)DECV

LUMOb

(eV)ECVg

c

(eV)DEcal

HOMOd

(eV)DEcal

LUMO

(eV)DEcal

HOMOe

(eV)Ecalg

(eV)DEcal

LUMOf

(eV)Ecal.g

(eV)

pCzS 0.99 "5.390.04

"1.38 "3.020.06

2.370.10

"4.790.02

"2.450.06

2.34pCzSe 0.95 "5.35 "1.32 "3.08 2.27 "4.77 "2.51 2.26pBDTS 0.92 "5.32

0.03"1.14 "3.26

0.072.06

0.10"4.79

0.03"2.62

0.092.17

pBDTSe 0.89 "5.29 "1.07 "3.33 1.96 "4.76 "2.71 2.05

a DECVLUMO is the HOMO energy level difference of pCzS–pCzSe and pBDTS–pBDTSe. b DECV

LUMO is the LUMO energy level difference of pCzS–pCzSeand pBDTS–pBDTSe. c ECV

g is the energy band gap calculated from the energy level difference between the HOMO and LUMO obtained from cyclicvoltammetry. d DECV

g is the difference of band-gaps between pCzS–pCzSe and pBDTS–pBDTSe copolymer couples. e DEcalHOMO is the HOMO energy

level difference of pCzS–pCzSe and pBDTS–pBDTSe. f DEcalLUMO is the LUMO energy level difference of pCzS–pCzSe and pBDTS–pBDTSe.

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shown in Fig. 6. Different from those of the HOMO, most of the

sulphur atoms in thiophene or selenium atoms in selenophene are

covered by electron density in the LUMO of all four model

structures. Our computational results indicate that the hetero-

atom of thep-conjugation bridge thiophene or selenophene plays

a more significant role in the LUMO than in the HOMO of these

donor–acceptor p-conjugated copolymers. These data also

suggest that the thiophene–selenophene spacer behaves like a part

of the acceptor unit, in addition to just a simple p-conjugationextending unit. Our computational results provide insight into the

origin of reducing Eg for selenophene-bridged donor–acceptor

copolymers. Basically, our computational results are similar to

what was found for P3HT and P3HS previously,53,54 although

P3HT and P3HS are not donor–acceptor copolymers like the four

copolymers studied herein.

2.6 Field-effect charge mobility properties

To understand the influence of the heteroatom substitution on

the charge transporting properties of the materials, the charge

mobility characteristics of these sulphur- and selenium-con-

taining copolymers were examined in their organic field-effect

transistors (OFETs). The detailed fabrication process and

testing of the bottom-gate, top-contact copolymer OFETs can

be found in the Experimental section. The current–voltage

output and transfer characteristics of the four copolymer

OFETs can be found in the ESI.† The copolymer OFET testing

results indicate that all four copolymers are hole transporting

with no sign of electron transport, although the electrical

characterization of the OFETs was conducted in an ambient air

environment. From the data summarized in Table 4, the

OFETs based on pCzSe show an average hole mobility (mavg)

of 9.6 $ 10"3 cm2 V"1 s"1, which is about one order of

magnitude higher than that of pCzS (mavg ! 8.0 $ 10"4 cm2

V"1 s"1). However, pBDTSe exhibits a hole mobility of 3.0 $10"4 cm2 V"1 s"1, which is comparable with or somewhat

smaller than that of pBDTS (mavg ! 6.8 $ 10"4 cm2 V"1 s"1).

The enhanced mobility in pCzSe-based OFETs may be

attributed to the increased inter-polymer chain contact by Se–Se

interactions. Similar Se–Se interactions are somehow inhibited

due to the more sterically demanding 2-ethylhexyl side chains

that prevent the pBDTSe copolymer chain from close contact.

Our inference above is consistent with the fact that branched

alkyl side chains (such as 2-ethylhexyl) are better than longer but

linear alkyl side chains (such as dodecyl) in enhancing polymer

solubility.56

2.7 Photovoltaic performance

The comparison of thiophene and selenophene-bridged lowband-

gap donor–acceptor copolymers applied for BHJ OPVs is pre-

sented in this section. These copolymers are designed to function

as electron donors and PCBM is employed as the electron

acceptor. Fine-tuning the thickness of the active layer is necessary

in the performance optimization of BHJ OPVs. Although it is

advantageous for light absorption by increasing the thickness of

the active layer, the optimum active layer thickness is limited by

Fig. 6 Electron density distribution of copolymer D–p–A–p–D–p–A–p model: HOMOs (left images) and LUMOs (right images).

Table 4 Field-effect performances of the four copolymers

Copolymer mavg (cm2 V"1 s"1) mmax (cm

2 V"1 s"1) Ion/Ioff VT (V)

pCzS 8.0 $ 10"4 9.0 $ 10"4 1.1 $ 104 11.4pCzSe 9.6 $ 10"3 1.1 $ 10"2 4.0 $ 104 "34.3pBDTS 6.8 $ 10"4 8.6 $ 10"4 5.3 $ 103 "21.3pBDTSe 3.0 $ 10"4 3.1 $ 10"4 5.7 $ 102 "35.2

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the low charge carrier mobility and short diffusion length or

lifetime of photo-generated excitons in organic materials.19,57 In

order to achieve a fair comparison between the thiophene- and

selenophene-bridged donor–acceptor copolymers in BHJ OPVs,

devices were fabricated with varied concentrations and spin-

coating rates to achieve the optimal thickness.

Table 5 summarizes the photovoltaic results of S- and Se-based

BHJ OPVs fabricated under different conditions, and some of

the results are displayed in Fig. 7. It can be clearly seen that the

OPVs based on Se-containing copolymers (pCzSe and pBDTSe)

exhibit significantly higher JSC than those of the S-containing

copolymers (pCzS and pBDTS). For examples, a maximum JSCof 9.54 mA cm"2 can be achieved for pCzSe (device Se5), which is

significantly larger than the maximum JSC that can be obtained

for pCzS (6.66 mA cm"2, device S2). A similar trend in JSC can

also be found between the pBDTS (maximum JSC ! 7.38 mA

m"2, device S10) and pBDTSe (maximum JSC ! 8.62 mA m"2,

device Se10) OPVs. Furthermore, the magnitude of the enhanced

JSC of the pCzSe–pCzS couple is higher than that of the

pBDTSe–pBDTS couple. This can be explained by the much

increased hole mobility of the pCzSe–pCzS couple than the

pBDTSe–pBDTS couple. It is known that the hole mobility of

the donor polymer is responsible for the transportation of posi-

tive charges (holes) dissociated from excitons. Hence, the

enhanced JSC values of the pCzSe OPVs are not only due to

increased sunlight absorption, but also due to the increased

charge transport ability of the pCzSe copolymer.

The higher JSC for the selenophene-containing OPVs can be

attributed to their longer absorption wavelength, i.e., more influx

photons from the solar simulator are absorbed in the seleno-

phene-containing OPVs compared with the thiophene-contain-

ing OPVs. Such higher JSC values also get verified from their

incident photon-to-current efficiency (IPCE) spectra. As shown

in the bottom part of Fig. 7, the Se-based copolymers (pCzSe and

pBDTSe) exhibit broader photocurrent wavelengths and/or more

enhanced photocurrents relative to the S-based copolymers

(pCzS and pBDTS).

Table 5 Photovoltaic performances (VOC: open circuit voltage, JSC: short circuit current density, FF: fill factor, PCE: power conversion efficiency,RSH:shunt resistance, RS: series resistance) of four copolymers with different device fabrication conditions

Devicea mg mL"1Spin rate(rpm) VOC (V) JSC (mA cm"2) FF (%) PCE (%) RSH (U cm2) RS (U cm2)

pCzS S1 5.0 1000 0.75 4.59 66.8 2.30 1650 8.58S2 10.0 600 0.83 6.66 59.3 3.28 1600 13.46S3 10.0 1000 0.83 5.23 69.4 3.01 2580 7.70S4 10.0 1600 0.84 4.62 72.2 2.80 3320 7.19S5 15.0 600 0.82 6.44 54.9 2.90 650 9.89S6 15.0 1000 0.83 6.58 56.8 3.10 860 9.37

pCzSe Se1 5.0 1000 0.79 5.62 72.3 3.21 3210 6.00Se2 10.0 600 0.76 8.84 52.5 3.52 460 7.91Se3 10.0 1000 0.78 6.22 65.4 3.17 1690 7.00Se4 10.0 1600 0.79 6.55 66.9 3.46 1750 6.23Se5 15.0 600 0.77 9.54 41.0 3.01 300 14.36Se6 15.0 1000 0.80 7.44 36.1 2.15 320 16.96

pBDTS S7 8.0 700 0.73 7.37 60.3 3.24 1450 9.17S8 8.0 1000 0.72 6.97 53.6 2.69 740 19.38S9 8.0 2000 0.70 3.93 62.2 1.71 1200 10.09S10 10.0 600 0.72 7.38 56.8 3.02 980 9.32S11 10.0 800 0.71 6.96 58.8 2.91 1490 6.33

pBDTSe Se7 8.0 700 0.65 7.47 52.6 2.55 550 13.35Se8 8.0 1000 0.66 6.39 50.7 2.14 680 24.93Se9 8.0 2000 0.63 3.47 56.2 1.23 1360 7.35Se10 10.0 600 0.65 8.62 54.5 3.05 760 8.19Se11 10.0 800 0.65 8.46 51.3 2.82 800 8.03

a For pCzS and pCzSe, the device structure is ITO/PEDOT:PSS/pCzS or pCzSe: PCBM (1 : 4)/Ca (20 nm)/Al (100 nm); and for pBDTS and pBDTSe,the device structure is ITO/PEDOT:PSS/pBDTS or pBDTSe: PCBM (1 : 2)/Ca (20 nm)/Al (100 nm).

Fig. 7 J–V curves (top) and IPCE spectra (bottom) of pCzS (device S2),

pCzSe (device Se2), pBDTS (device S7) and pBDTSe (device Se10) OPVs.

The parentheses denote the device number in Table 5.

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Regarding diffusion and dissociation of copolymer-based

excitons, wavelength dependent IPCE reveals insightful infor-

mation. The photocurrent contribution from the second

absorption band around 420–430 nm of pCzSe is very

pronounced, surpassing the photocurrent contribution from the

first absorption band at longer wavelengths of 500–600 nm. A

similar enhanced IPCE can be found for the second absorption

band around 410–420 nm of pBDTSe. Such wavelength depen-

dent IPCE enhancement is not proportional to the absorption

intensity at the long and short wavelength of pCzSe or pBDTSe

shown in Fig. 3. Since such IPCE enhancement at the second

absorption band does not happen to pCzS or pBDTS, we may

make the following two inferences. Firstly, the excitons gener-

ated in the selenophene–p-conjugation bridged copolymers,

particularly with the second absorption wavelength 410–430 nm,

may be more readily separated into electrons and holes than the

thiophene–p-conjugation bridged ones. Secondly, the short

wavelength-generated excitons of pCzSe or pBDTSe may be

more efficient for exciton diffusion than the longer wavelength-

generated excitons of the respective copolymer. Our two infer-

ences are directly derived from the fundamental principle of

IPCE, also known as the external quantum efficiency (EQE) of

OPVs, documented in the literature.58

Concerning the HOMO energy level of the copolymers and the

VOC of the OPVs, the correlation among the four copolymers

studied herein is rather clear. For most of the device data listed in

Table 5, the VOC of the thiophene-bridged copolymer OPVs is

slightly larger than those of the selenophene-bridged copolymer

OPVs. This is in general consistent with the thiophene-bridged

copolymers having slightly deeper HOMO energy levels, as

determined experimentally. With the results obtained herein, it is

fair to say that there is an advantage of selenophene rather than

thiophene in donor–acceptor copolymer BHJ OPVs, i.e., what it

gains from increased JSC is more than what it loses from

decreased VOC.

For the fill factor (FF) aspect of OPVs, it seems to be more

sensitive to the device fabrication conditions than the effect of

heteroatom substitution of the copolymers studied herein. Most

selenophene-bridge-containing OPVs exhibit slightly lower FF

values than the thiophene-bridge-containing OPVs when the

devices are fabricated under similar conditions, for example,

device S2-6 vs. Se2-6 or device S7-11 vs. Se7-11 in Table 5. This

can be mainly attributed to the poor RSH of the devices (see data

in Table 5). Poor RSH usually indicates more current leakage

pathways due to charge recombination or trapping. In terms of

the interdigitated charge-transporting nano-morphology, the

selenophene p-conjugation bridged copolymers do not form as

good a thin film with PCBM as the thiophene p-conjugationbridged copolymers do. However, the selenophene-containing

pCzSe devices (Se1–Se6) exhibit the widest range of FF variation

(36.1–72.3%) among the four copolymer-based OPVs. This could

be a good indication that the potential Se–Se interactions of the

selenophene-containing copolymers depend on the device fabri-

cation conditions.

3. Conclusions

In conclusion, this work has demonstrated a detailed comparison

between thiophene- and selenophene-bridged donor–acceptor

copolymers in spectroscopic, thermal, electrochemical, field-

effect charge transporting as well as photovoltaic properties.

Significant red-shifted UV-visible absorption and photo-

luminescence were observed for the selenophene-bridged

copolymers. The reduction of the band-gap energy by the

replacement of thiophene with selenophene is more due to the

lowering of the LUMO energy level than the raising of

the HOMO energy level of the copolymers. The photovoltaic

results obtained for the selenophene-bridged copolymer BHJ

OPVs show a higher JSC than that of their thiophene-bridged

copolymer counterparts, mostly due to the enhanced photocur-

rent, responsive to the exciton dissociation or diffusion at short

wavelengths and sunlight absorption at long wavelengths. The

contribution of the selenophene with higher hole mobility to the

larger JSC of the BHJ OPVs is structure (solubilizing side chain of

the copolymers) dependent. Although the selenophene-bridged

donor–acceptor copolymer OPVs exhibit a higher JSC, which is

beneficial for the PCE of OPVs, such an advantage is partially

offset by a smaller VOC and FF. The inferior VOC and FF are

ascribed to the higher HOMO energy level and the less appro-

priate nano-morphology of the PCBM blended thin film (and

hence smaller RSH), respectively.

4. Experimental

4.1 General characterization methods

1H and 13C NMR spectra were recorded on a Bruker AV-400

MHz or AV-500 MHz Fourier transform spectrometer at room

temperature. Elemental analyses (on a Perkin-Elmer 2400 CHN

elemental analyzer), electron ionization (EI), or fast atom

bombardment (FAB) mass spectroscopy (MS) were performed

by the Elemental Analyses and Mass Spectroscopic Laboratory,

respectively, in-house service of the Institute of Chemistry,

Academic Sinica. The number- and weight-average molecular

weights of the polymers were determined by gel permeation

chromatography (GPC) on a Waters GPC-1515 with a 2414

refractive index detector, using THF as the eluent and poly-

styrene as the standard at 40 #C. UV-vis absorption spectra were

recorded on a Hewlett-Packard 8453 diode array spectropho-

tometer. Photoluminescence (PL) spectra were recorded on a

Hitachi fluorescence spectrophotometer F-4500. Thermal

decomposition temperatures (Td’s) of the copolymers were

measured by thermogravimetric analysis (TGA) using Perkin-

Elmer TGA-7 analyzer systems. Cyclic voltammetry (CV) was

used to study the electrochemical properties of the copolymers.

The copolymer thin film coated on a platinum electrode was

studied in an anhydrous and nitrogen-saturated solution of

0.10 M tetrabutylammonium hexafluorophosphate (Bu4NPF6)

acetonitrile solution at a scan rate of 50 mV s"1. Ag–Ag+ (0.1 M

of AgNO3 in acetonitrile) was used as reference electrode. For

calibration, the redox potential of ferrocene–ferrocenium (Fc–

Fc+) was measured under the same conditions. Under these

conditions, the onset oxidation potential of ferrocene

(4ox, Ferrocene) was 0.40 V versus Ag–Ag+. It is assumed that the

redox potential of Fc–Fc+ has an absolute energy level of "4.8

eV to vacuum.59 The energy levels of the highest occupied

molecular orbital (EHOMO) and the lowest unoccupied molecular

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orbital (ELUMO) were then calculated according to the following

equations:

EHOMO ! "[(4ox " 4ox, Ferrocene) + 4.8)] (eV)

ELUMO ! "[(4red " 4ox, Ferrocene) + 4.8)] (eV)

where 4ox and 4red are the onset oxidation and reduction

potentials vs. Ag–Ag+ of the copolymer, respectively.

4.2 Synthesis

Materials. All chemicals were purchased from Aldrich, Alfa

Aesar, Acros, and TCI Chemical Co., and they were used

without further purification. Solvents such as dichloromethane

(CH2Cl2), chlorobenzene, terrahydrofuran (THF), N,N-dime-

thylformamide (DMF) and toluene were distilled after drying

with appropriate drying agents. The dried solvents were stored

over 4 !A molecular sieves before use. 2,7-Dibromo-9-dodecyl-

9H-carbazole (1),60 4,8-di(2-ethylhexyloxy)benzol[1,2-b:4,5-b0]

dithiophene (2),61 2,6-bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)

benzo[1,2-b:4,5-b0]dithiophene (BDT)45 and 4,7-dibromo-5,6-

bis(dodecyloxy)benzo[c][1,2,5]thiadiazole (3)46 were synthesized

following procedures reported in the literature. [6,6]-Phenyl-C61-

butyric acid methyl ester (PCBM) and poly(3,4-ethyl-

enedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) were

purchased from UR and H. C. Starck, respectively. They were

used as received.

9-Dodecyl-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-

9H-carbazole (Cz). To a solution of compound 1 (0.99 g, 2.00

mmol) in dried tetrahydrofuran (30 mL) in a flame-dried 150 mL

flask at "78 #C was added dropwise 1.70 mL (4.20 mmol) of n-

butyllithium (2.5M in hexane). The mixture was stirred at"78 #C

for 1 hour and 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxabor-

olane (0.90 mL, 0.819 g, 4.4 mmol) was added rapidly to the

solution. After one additional hour at "78 #C, the resulting

mixture was warmed to room temperature and stirred for 16

hours. The mixture was poured into water, extracted with diethyl

ether four times and dried over magnesium sulfate. After removal

of the magnesium sulfate, the solvent was evaporated under

reduced pressure, and the residue was purified by passing through

a silica gel flash column using hexane : ethylacetate (10 : 1) as the

eluent. The product was then recrystallized in ethanol to obtain a

white solid (0.85 g, 72%). 1H NMR (400 MHz, CDCl3): d (ppm)

8.09 (d, 2H, J ! 7.6 Hz), 7.86 (s, 2H), 7.66 (d, 2H, J ! 7.6 Hz),

4.36 (t, 2H, J ! 7.2 Hz), 1.83–1.90 (m, 2H), 1.38 (s, 24H), 1.18–

1.30 (m, 18H), 0.85 (t, 3H, J ! 7.2 Hz). 13C NMR (400 MHz,

CDCl3): d (ppm) 140.4, 125.1, 124.8, 120.0, 115.2, 83.8, 42.9, 31.9,

29.6, 29.5, 29.4, 29.3, 29.2, 27.1, 24.9, 22.7, 14.1.

5,6-Bis(dodecyloxy)-4,7-di(selenophen-2-yl)benzo[c][1,2,5]thia-diazole (4Se). In a 150 mL flame-dried two-neck round-bottom

flask with a condenser, compound 3 (2.12 g, 3.20 mmol), tribu-

tyl(selenophen-2-yl)stannane (3.50 g, 8.33 mmol), PdCl2(PPh3)2(0.09 g, 0.13 mmol) and dried tetrahydrofuran (50 mL) were

added. The reaction mixture was heated to reflux for 3 days. The

reaction mixture was then cooled to room temperature and the

solvent was evaporated. The crude product was purified with

silica gel flash column chromatography using hex-

ane : dichloromethane (10 : 1) as eluent to obtain a deep orange

solid (1.56 g, 64%). 1H NMR (400 MHz, CDCl3): d (ppm) 8.82

(d, 2H, J ! 4.0 Hz), 8.19 (d, 2H, J ! 5.2 Hz), 7.45–7.48 (m, 2H),

4.12 (t, 4H, J ! 7.2 Hz), 1.90–1.97 (m, 4H), 1.20–1.50 (m, 36H),

0.87 (t, 6H, J ! 6.0 Hz). 13C NMR (400 MHz, CDCl3): d (ppm)

151.5, 150.6, 138.3, 133.2, 132.8, 129.6, 119.2, 74.5, 31.9, 30.6,

29.7, 29.6, 29.5, 29.4, 25.9, 22.7, 14.1. HRMS (m/z): [M+] calcd

for C38H56O2N2SSe2 764.2393; found 764.2382. Anal. found

(Calcd): C, 59.93 (59.83); H, 7.67 (7.40); N 3.65 (3.67)%.

5,6-Bis(dodecyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadia-zole (4S). Compound 4S was synthesized according to the same

procedure as 4Se. Yield: 70%. 1H NMR (400 MHz, CDCl3): d

(ppm) 8.44 (d, 2H, J ! 3.6 Hz), 7.48 (d, 2H, J ! 5.6 Hz), 7.19–

7.22 (m, 2H), 4.09 (t, 4H, J ! 6.8 Hz), 1.84–1.93 (m, 4H), 1.20–

1.42 (m, 36H), 0.90 (t, 6H, J ! 7.2 Hz).

4,7-Bis(5-bromoselenophen-2-yl)-5,6-bis(dodecyloxy)benzo[c][1,2,5]thiadiazole (BTSe). Compound 4Se (0.61 g, 0.80 mmol)

was dissolved in 20 mL chloroform : acetic acid (1 : 1) in a two-

neck round-bottom flask. Then NBS (0.31 g, 1.76 mmol) was

added at room temperature and reacted for 16 hours. The reac-

tion mixture was then washed with deionized water, NaOH

aqueous solution and deionized water to a pH near 7. A red-

orange solid was obtained by removing solvent, followed by

purification with silica gel flash column chromatography using

hexane : dichloromethane (10 : 1) as eluent (0.70 g, 95%). 1H

NMR (400 MHz, CDCl3): d (ppm) 8.64 (d, 2H, J ! 4.0 Hz), 7.37

(d, 2H, J ! 4.4 Hz), 4.13 (t, 4H, J ! 7.2 Hz), 1.91–1.98 (m, 4H),

1.20–1.49 (m, 36H), 0.86 (t, 6H, J ! 6.0 Hz). 13C NMR (400

MHz, CDCl3): d (ppm) 151.1, 150.1, 140.1, 133.1, 132.8, 120.1,

118.7, 74.8, 31.9, 30.5, 29.7, 29.6, 29.5, 29.4, 25.9, 22.7, 14.1.

HRMS (m/z): [M+] calcd for C38H54O2N2Br2SSe2 920.0603;

found 920.0599. Anal. found (Calcd): C, 49.82 (49.57); H, 6.03

(5.91); N 3.04 (3.04)%.

4,7-Bis(5-bromothiophen-2-yl)-5,6-bis(dodecyloxy)benzo[c][1,2,5]thiadiazole (BTS). Compound BTS was synthesized according to

the same procedure as that of BTSe. Yield: 93%. 1H NMR (400

MHz, CDCl3): d (ppm) 8.34 (d, 2H, J ! 4.0 Hz), 7.15 (d, 2H, J !4.0 Hz), 4.10 (t, 4H, J! 7.2 Hz), 1.88–1.95 (m, 4H), 1.19–1.48 (m,

36H), 0.86 (t, 6H, J ! 6.4 Hz). 13C NMR (400 MHz, CDCl3): d

(ppm) 151.5, 150.4, 135.7, 131.0, 129.7, 117.0, 115.5, 74.6, 31.9,

30.3, 29.7, 29.6, 29.5, 29.4, 25.9, 22.7, 14.1. HRMS (m/z): [M+]

calcd for C38H54O2N2Br2S3 824.1714; found 824.1716. Anal.

found (Calcd): C, 55.20 (55.13); H, 6.58 (6.60); N 3.39 (3.37)%.

General synthetic procedures for copolymers: poly{9-dodecyl-

9H-carbazole-alt-5,6-bis(dodecyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole} (pCzS) and poly{9-dodecyl-9H-carbazole-

alt-5,6-bis(dodecyloxy)-4,7-di(selenophen-2-yl)benzo[c][1,2,5]-thiadiazole} (pCzSe). The copolymers pCzS and pCzSe were

synthesized via Suzuki cross-coupling polymerization. A repre-

sentative procedure is as follows. To a 25 mL two-neck round

bottom flask, Cz (73.4 mg, 0.125 mmol), BTS (103.4 mg, 0.125

mmol) or BTSe (115.1 mg, 0.125 mmol), dry toluene (10.0 mL),

and 1.0 mL of 20% aqueous tetraethylammonium hydroxide,

Et4NOH(aq), were added. The reaction mixture was

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deoxygenated through three freeze–pump–thaw cycles. Then,

Pd2dba3 (4.6 mg, 0.005 mmol) and p(o-tol)3 (6.1 mg, 0.02 mmol)

were added under nitrogen and the reaction mixture was vigor-

ously stirred at 90–95 #C for 72 hours. After cooling to room

temperature, the mixture was poured into methanol–water

(10 : 1, 200 mL). The precipitate was filtered through a Soxhlet

thimble, which was then subjected to Soxhlet extraction with

methanol, hexane, and chloroform. The chloroform fraction was

concentrated, re-precipitated with methanol, filtered, and then

finally dried in a vacuum oven. pCzS (116 mg, 90%). Anal. found

(Calcd): C, 74.53 (74.28); H, 8.66 (8.75); N 4.11 (4.19)%. GPC (40#C using THF as eluent): Mw ! 102.9 kg mol"1, Mn ! 66.1 kg

mol"1, PDI ! 1.56. pCzSe (120 mg, 87%). Anal. found (Calcd):

C, 67.79 (67.92); H, 7.68 (8.00); N 3.83 (3.83)%. GPC (40 #C

using THF as eluent):Mw ! 94.1 kg mol"1,Mn ! 33.9 kg mol"1,

PDI ! 2.78. 1H NMR spectra of pCzS and pCzSe were acquired

(Fig. S3 and S4 in the ESI†).

General synthetic procedures for copolymers: poly{4,8-bis(2-

ethylhexyloxy)benzo[1,2-b;4,5-b0]dithiophene-alt-5,6-bis(dodecy-loxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole} (pBDTS)

and poly{4,8-bis(2-ethylhexyloxy)benzo[1,2-b;4,5-b0]dithiophene-alt-5,6-bis(dodecyloxy)-4,7-di(selenophen-2-yl)benzo[c][1,2,5]-thia-diazole} (pBDTSe). The copolymers pBDTS and pBDTSe were

synthesized via Stille cross-coupling polymerization. A repre-

sentative procedure is as follows. To a 25 mL two-neck round

bottom flask, BDT (96.5 mg, 0.125 mmol), BTS (103.4 mg,

0.125 mmol) or BTSe (115.1 mg, 0.125 mmol), and dry chlo-

robenzene (12.0 mL) were added. The reaction mixture was

deoxygenated through three freeze–pump–thaw cycles. Then,

Pd2dba3 (4.6 mg, 0.005 mmol) and p(o-tol)3 (6.1 mg, 0.02 mmol)

were added under nitrogen and the reaction mixture was reacted

for 72 hours at 120 #C. After cooling to room temperature, the

mixture was poured into methanol (200 mL). The precipitate

was filtered through a Soxhlet thimble, which was then sub-

jected to Soxhlet extraction with methanol, hexane, and chlo-

roform. The chloroform fraction was concentrated, re-

precipitated with methanol, filtered, and then finally dried in a

vacuum oven overnight. pBDTS (129 mg, 93%). Anal. found

(Calcd): C, 68.99 (69.02); H, 8.27 (8.33); N 2.44 (2.52)%. GPC

(40 #C using THF as eluent): Mw ! 106.7 kg mol"1, Mn ! 62.7

kg mol"1, PDI ! 1.70.pBDTSe (133 mg, 88%). Anal. found

(Calcd): C, 68.99 (69.02); H, 8.27 (8.33); N 2.44 (2.52)%. GPC

(40 #C using THF as eluent): Mw ! 112.3 kg mol"1, Mn ! 63.7

kg mol"1, PDI ! 1.76. 1H NMR spectra of pBDTS and

pBDTSe were acquired (Fig. S5 and S6 in the ESI†).

4.3 Device fabrication and testing

Photovoltaic devices were fabricated on patterned ITO glass

substrate. The active layer of each solar cell device is 0.04 cm2.

ITO glass substrates were sonicated sequentially in detergent,

deionized water, acetone, and isopropanol. The ITO substrates

were then baked on a hot plate at 150 #C for 10 minutes. A

PEDOT:PSS thin film was spin-coated (2000 rpm, 60 s) and

then baked at 150 #C for 10 minutes. Next, an active layer was

spin coated on top of the PEDOT:PSS layer from a 1,2-

dichlorobenzene (1,2-DCB) solution of the copolymer:PCBM

blends. Different concentrations and spin-coating speeds were

tested to optimize the best active layer thickness. Finally, 20 nm

of Ca and 80 nm of Al were deposited on top of the active layer

in a vacuum of about 8 $ 10"6 Torr to complete the photo-

voltaic device fabrication. All devices were encapsulated inside a

nitrogen-filled glove box.

The current density–voltage (J–V) characteristics of the BHJ

OPVs were measured in the dark and under AM 1.5G solar

illumination from a class A solar simulator (Oriel 300 W),

controlled by a programmable source meter (Keithley 2400). The

light intensity was calibrated using a Si photodiode (PVM 172;

area ! 3.981 cm2) from the National Renewable Energy Labo-

ratory. The power conversion efficiency (PCE) of the device was

calculated based on JSC, VOC, and FF, obtained from device

measurements that were conducted under 1-sun AM 1.5G solar

illumination. The series resistance (RS) and shunt resistance

(RSH) were estimated by taking the reciprocal from the slope of a

tangent to the J–V curve at the open- and short-circuit condi-

tions, respectively.

For the incident photon-to-electron conversion efficiency

(IPCE) measurements, an AM 1.5G solar simulator was used to

generate the bias light. A monochromator (Newport Model

74100), which was calibrated with a National Institute of Stan-

dards and Technology calibrated photodiode and chopped at 250

Hz, was used to select the wavelengths between 400 and 750 nm

for illuminating the OPV. The photocurrent from the OPV was

measured through a lock-in amplifier (Signal Recovery 7265),

which was in turn referenced to the chopper frequency. All

electrical measurements were carried out in air.

Organic field-effect transistors (OFETs) were fabricated with a

bottom-gate, top-contact structure. The channel width (W) and

length (L) of the transistors are 2 mm and 50 mm, respectively.

The device was built on a heavily doped Si wafer with a layer of

thermally grown SiO2 (%300 nm) with a capacitance of 12 nF

cm"2. The Si wafer functioned as the gate electrode while the

octyltrichlorosilane (OTS)-modified SiO2 layer acted as the gate

dielectric. The SiO2-covered Si wafer was cleaned and treated

with OTS prior to use. The OTS treatment was performed by

immersing the substrate in OTS solution (10 mL OTS in 40 mL

dried toluene) for 5 minutes, followed by rinsing with toluene,

and then air drying. The solution of S- or Se-containing copol-

ymer in 1,2-dichlorobenzene (10 mgmL"1) was filtered through a

0.45 mm syringe filter and spin-coated on the substrate at 1500

rpm for 60 s at room temperature in a glove box. Subsequently,

the gold (Au) source/drain electrode pairs (70 nm) were deposited

on top of the polymer thin film by high vacuum thermal evap-

oration through a shadow mask to define the channel length (L,

50 mm) and width (W, 2 mm).

The electrical measurements were carried out in air using a

semiconductor parameter analyzer (Keithley 2636). The satu-

ration mobility (msat) was extracted from the slope of the square

root of the drain current plot vs. VG from the following

equation:

ID;sat !W

2LC1mLsat&VG " VT'2

where ID,sat is the drain-to-source saturated current; W/L is the

channel width to length ratio; Ci is the capacitance of the insu-

lator per unit area (Ci! 12 nF cm"2), andVG andVT are the gate

voltage and threshold voltage, respectively.

21558 | J. Mater. Chem., 2012, 22, 21549–21559 This journal is ª The Royal Society of Chemistry 2012

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References

1 R. F. Service, Science, 2011, 332, 293.2 G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science,1995, 270, 1789.

3 C. Soci, I. W. Hwang, D. Moses, Z. Zhu, D. Waller, R. Gaudiana,C. J. Brabec and A. J. Heeger, Adv. Funct. Mater., 2007, 17, 632.

4 R.C.Coffin, J. Peet, J.Rogers andG.C.Bazan,Nat.Chem., 2009, 1, 657.5 H.-Y. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, L. Yu,Y. Wu and G. Li, Nat. Photonics, 2009, 3, 649.

6 Y. Liang, Z. Xu, J. Xia, S.-T. Tsai, Y. Wu, G. Li, C. Ray and L. Yu,Adv. Mater., 2010, 22, E135.

7 T.-Y. Chu, J. Lu, S. Beaupre, Y. Zhang, J.-R. M. Pouliot, S. Wakim,J. Zhou, M. Leclerc, Z. Li, J. Ding and Y. Tao, J. Am. Chem. Soc.,2011, 133, 4250.

8 S. C. Price, A. C. Stuart, L. Yang, H. Zhou andW. You, J. Am. Chem.Soc., 2011, 133, 4625.

9 M.-S. Su, C.-Y. Kuo, M.-C. Yuan, U.-S. Jeng, C.-J. Su andK.-H. Wei, Adv. Mater., 2011, 23, 3315.

10 H. Zhou, L. Yang, A. C. Stuart, S. C. Price, S. Liu and W. You,Angew. Chem., Int. Ed., 2011, 50, 2995.

11 A. Patra and M. Bendikov, J. Mater. Chem., 2010, 20, 422.12 S. Das and S. S. Zade, Chem. Commun., 2010, 46, 1168.13 A. Patra, Y. H. Wijsboom, G. Leitus and M. Bendikov, Chem.

Mater., 2011, 23, 896.14 M. R. Bryce, J. Mater. Chem., 1995, 5, 1481.15 H. Kobayashi, H. Cui and A. Kobayashi, Chem. Rev., 2004, 104,

5265.16 D. J. Crouch, P. J. Skabara, J. E. Lohr, J. J. W. McDouall,

M. Heeney, I. McCulloch, D. Sparrowe, M. Shkunov, S. J. Coles,P. N. Horton and M. B. Hursthouse, Chem. Mater., 2005, 17, 6567.

17 S. S. Zade and M. Bendikov, Org. Lett., 2006, 8, 5243.18 H. Pang, P. J. Skabara, S. Gordeyev, J. J. W. McDouall, S. J. Coles

and M. B. Hursthouse, Chem. Mater., 2007, 19, 301.19 R. Kroon, M. Lenes, J. C. Hummelen, P. W.M. Blom and B. de Boer,

Polym. Rev., 2008, 48, 531.20 J. M1ochowski and M. Giurg, in Aromaticity in Heterocyclic

Compounds, Springer, 2009, p. 289.21 Y. Kunugi, K. Takimiya, K. Yamane, K. Yamashita, Y. Aso and

T. Otsubo, Chem. Mater., 2003, 15, 6.22 Y. Kunugi, K. Takimiya, Y. Toyoshima, K. Yamashita, Y. Aso and

T. Otsubo, J. Mater. Chem., 2004, 14, 1367.23 N. S. Cho, S. K. Lee, J. H. Seo, M. Elbing, J. D. Azoulay, J. Park,

S. Cho, A. J. Heeger and G. C. Bazan, J. Mater. Chem., 2008, 18,4909.

24 D. J. Crouch, P. J. Skabara, J. E. Lohr, J. J. W. McDouall,M. Heeney, I. McCulloch, D. Sparrowe, M. Shkunov, S. J. Coles,P. N. Horton and M. B. Hursthouse, Chem. Mater., 2005, 17, 6567.

25 Y. M. Kim, E. Lim, I.-N. Kang, B.-J. Jung, J. Lee, B. W. Koo,L.-M. Do and H.-K. Shim, Macromolecules, 2006, 39, 4081.

26 D. J. Crouch, P. J. Skabara, M. Heeney, I. McCulloch, D. Sparrowe,S. J. Coles and M. B. Hursthouse, Macromol. Rapid Commun., 2008,29, 1839.

27 H. Kong, D. S. Chung, I.-N. Kang, J.-H. Park,M.-J. Park, I. H. Jung,C. E. Park and H.-K. Shim, J. Mater. Chem., 2009, 19, 3490.

28 M. Al-Hashimi, J. G. Labram, S. Watkins, M. Motevalli,T. D. Anthopoulos and M. Heeney, Org. Lett., 2010, 12, 5478.

29 Z. Chen, H. Lemke, S. Albert-Seifried, M. Caironi, M. M. Nielsen,M. Heeney, W. Zhang, I. McCulloch and H. Sirringhaus, Adv.Mater., 2010, 22, 2371.

30 S. P. Mishra, A. E. Javier, R. Zhang, J. Liu, J. A. Belot, I. Osaka andR. D. McCullough, J. Mater. Chem., 2011, 21, 1551.

31 A. M. Ballantyne, L. Chen, J. Nelson, D. D. C. Bradley, Y. Astuti,A. Maurano, C. G. Shuttle, J. R. Durrant, M. Heeney, W. Duffyand I. McCulloch, Adv. Mater., 2007, 19, 4544.

32 S. K. Lee, N. S. Cho, S. Cho, S.-J. Moon, J. K. Lee and G. C. Bazan,J. Polym. Sci., Part A: Polym. Chem., 2009, 47, 6873.

33 D. S. Chung, H. Kong, W. M. Yun, H. Cha, H.-K. Shim, Y.-H. Kimand C. E. Park, Org. Electron., 2010, 11, 899.

34 F. Qiao, X. Hou, Y. Lu, H. Chen, A. Liu, X. Hu and N. S. Choon,Sol. Energy Mater. Sol. Cells, 2010, 94, 442.

35 K. A. Mazzio, M. Yuan, K. Okamoto and C. K. Luscombe, ACSAppl. Mater. Interfaces, 2011, 3, 271.

36 J.-Y. Lee, S.-H. Kim, I.-S. Song and D.-K. Moon, J. Mater. Chem.,2011, 21, 16480.

37 S. Haid, A. Mishra, C. Uhrich, M. Pfeiffer and P. B!auerle, Chem.Mater., 2011, 23, 4435.

38 R. Xia, M. Al-Hashimi, W. C. Tsoi, M. Heeney, D. D. C. Bradleyand J. Nelson, Sol. Energy Mater. Sol. Cells, 2012, 96,112.

39 M. Nakano, H. Mori, S. Shinamura and K. Takimiya, Chem. Mater.,2012, 24, 190.

40 H. A. Saadeh, L. Lu, F. He, J. E. Bullock, W. Wang, B. Carsten andL. Yu, ACS Macro Lett., 2012, 1, 361.

41 W.-H. Lee, S. K. Son, K. Kim, S. K. Lee, W. S. Shin, S.-J. Moon andI.-N. Kang, Macromolecules, 2012, 45, 1303.

42 M. Shahid, R. S. Ashraf, Z. Huang, A. J. Kronemeijer, T. McCarthy-Ward, I. McCulloch, J. R. Durrant, H. Sirringhaus andM. Heeney, J.Mater. Chem., 2012, 22, 12817.

43 D. Patra, D. Sahu, H. Padhy, D. Kekuda, C.-W. Chu and H.-C. Lin,J. Polym. Sci., Part A: Polym. Chem., 2010, 48, 5479.

44 J. Hou, M.-H. Park, S. Zhang, Y. Yao, L.-M. Chen, J.-H. Li andY. Yang, Macromolecules, 2008, 41, 6012.

45 Y. Liang, D. Feng, Y.Wu, S.-T. Tsai, G. Li, C. Ray and L. Yu, J. Am.Chem. Soc., 2009, 131, 7792.

46 M. Helgesen, S. A. Gevorgyan, F. C. Krebs and R. A. J. Janssen,Chem. Mater., 2009, 21, 4669.

47 S. C. Price, A. C. Stuart andW. You,Macromolecules, 2010, 43, 4609.48 H. Zhou, L. Yang, S. Xiao, S. Liu and W. You, Macromolecules,

2010, 43, 811.49 C. Deibel, D. Mack, J. Gorenflot, D. Rauh and V. Dyakonov, Phys.

Rev. B: Condens. Matter Mater. Phys., 2010, 81, 085202.50 Y. Li, Y. Cao, J. Gao, D.Wang, G. Yu and A. J. Heeger, Synth.Met.,

1999, 99, 243.51 Z.-K. Chen, W. Huang, L.-H. Wang, E.-T. Kang, B. J. Chen,

C. S. Lee and S. T. Lee, Macromolecules, 2000, 33, 9015.52 S. W. Cho, L. F. J. Piper, A. DeMasi, A. R. H. Preston, K. E. Smith,

K. V. Chauhan, P. Sullivan, R. A. Hatton and T. S. Jones, J. Phys.Chem. C, 2010, 114, 1928.

53 M. Heeney, W. Zhang, D. J. Crouch, M. L. Chabinyc, S. Gordeyev,R. Hamilton, S. J. Higgins, I. McCulloch, P. J. Skabara, D. Sparroweand S. Tierney, Chem. Commun., 2007, 5061.

54 K. Oyaizu, T. Iwasaki, Y. Tsukahara and E. Tsuchida,Macromolecules, 2004, 37, 1257.

55 N. Blouin, A. Michaud, D. Gendron, S. Wakim, E. Blair, R. Neagu-Plesu, M. Belletete, G. Durocher, Y. Tao and M. Leclerc, J. Am.Chem. Soc., 2008, 130, 732.

56 T. Y. Chu, J. Lu, S. Beaupre, Y. Zhang, J. R. Pouliot, S. Wakim,J. Zhou, M. Leclerc, Z. Li, J. Ding and Y. Tao, J. Am. Chem. Soc.,2011, 133, 4250.

57 D. E. Markov, E. Amsterdam, P. W. M. Blom, A. B. Sieval andJ. C. Hummelen, J. Phys. Chem. A, 2005, 109, 5266.

58 P. Peumans, A. Yakimov and S. R. Forrest, J. Appl. Phys., 2003, 93,3693.

59 J. Pommerehne, H. Vestweber, W. Guss, R. F. Mahrt, H. Bassler,M. Porsch and J. Daub, Adv. Mater., 1995, 7, 551.

60 Y. Chen, W. Huang, C. Li and Z. Bo, Macromolecules, 2010, 43,10216.

61 G. Zhang, Y. Fu, Q. Zhang and Z. Xie, Chem. Commun., 2010, 46,4997.

This journal is ª The Royal Society of Chemistry 2012 J. Mater. Chem., 2012, 22, 21549–21559 | 21559

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