A crystalline low-bandgap polymer comprising dithienosilole and thieno[3,4-c] pyrrole-4,6-dione units for bulk heterojunction solar cells

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A crystalline low-bandgap polymer comprising dithienosilole and thieno[3,4-c]

pyrrole-4,6-dione units for bulk heterojunction solar cells

Mao-Chuan Yuan, Yi-Jen Chou, Chia-Min Chen, Chang-Lung Hsu, Kung-Hwa Wei

*

Department of Materials Science and Engineering, National Chiao Tung University, 300 Hsinchu, Taiwan

a r t i c l e i n f o

Article history:

Received 30 January 2011 Received in revised form 20 April 2011

Accepted 26 April 2011 Available online 5 May 2011 Keywords:

Crystalline conjugated polymer Thieno[3,4-c]pyrrole-4,6-dione Polymer solar cells

a b s t r a c t

We have used Stille coupling polymerization to synthesize a new low-bandgap conjugated polymer, PDTSTPD, that consists of an electron-rich dithieno[3,2-b:20_,30_{-d]-silole (DTS) unit and an}

electron-deﬁcient thieno[3,4-c]pyrrole-4,6-dione (TPD) moiety. The polymer exhibited an excellent thermal stability, crystalline characteristics, a broad spectral absorption, and a deep highest occupied molecular orbital (HOMO) energy level, resulting from combination of the rigid TPD and the coplanar DTS units in the polymer backbone. Moreover, the presence of the silicon atoms along the polymer chain ensured PDTSTPD having strong interchain stacking and good hole mobility. An optimal device incorporating the PDTSTPD:PC71BM blend at a weight ratio of 1:1 provided a power conversion efﬁciency of 3.42%.

1. Introduction

Interest in polymer solar cells (PSCs), which exhibitflexibility and solution-processability, has advanced dramatically because of their fascinating potential for low-cost, large-area production[1,2,3]. PSCs featuring bulk heterojunctions (BHJ) configurations, where the photoactive layers ordinarily consist of electron-donating polymers and electron-accepting fullerene derivatives, have been investigated extensively in recent years [4,5,6,7]. Many studies of PSCs have focused on the development of donoreacceptor (DeA) conjugated polymers because of their tunable optical/electronic properties and ambipolar charge transporting properties [8,9,10,11,12,13,14,15]. Specifically, the DeA design is a powerful strategy for narrowing the optical bandgap of polymers and thereby allowing greater harvest-ing of photons. Several low-bandgap DeA polymers have promisharvest-ing potential for use in PSC applications. For example, polymers con-taining rigid electron-donating carbazole [16,17,18,19,20], benzo [1,2-b:4,5-b0]dithiophene (BDT)[21,22,23,24,25], and dithieno[3,2-b:20,30-d]silole (DTS)[26,27,28,29]moieties, when conjugated with various electron-withdrawing units, can provide power conversion efficiencies (PCEs) of up to 7% after systematic optimization.

Several features are necessary when designing an efﬁcient low-bandgap polymer: a narrow low-bandgap for enhanced photon har-vesting; a low-lying highest occupied molecular orbital (HOMO) energy level to ensure high open-circuit voltages (Voc); sufﬁcient

lowest occupied molecular orbital (LUMO) offsets for efﬁcient charge dissociation; and crystalline characteristics to ensure good charge transport. Therefore, the selection of suitable electron acceptors and donors must be made by considering their intrinsic properties.

The thieno[3,4-c]pyrrole-4,6-dione (TPD) moiety is an attractive material because its rigid, fused, strongly electron-withdrawing structure can increase the thermal stability, enhance the poly-meric chain interactions, narrow the optical bandgap, and lower the HOMO energy level when incorporated in a polymeric back-bone. Notably, DeA polymers containing TPD moieties have dis-played good PCEs of 3e6% when applied in PSCs[30,31,32,33,34]. In addition, we recently reported efﬁcient PSCs based on the DeA polymer PBTTPD, which featured the electron-donating bithio-phene units and the electron-deﬁcient TPD moieties in its main chain and, therefore, high crystallinity and a low-lying HOMO energy level; these PSCs exhibited good hole mobility, high values of Voc, and, when optimized, an excellent PCE of 4.7%[35].

In this study, we prepared a coplanar, electron-rich DTS donor unit featuring a silicon atom at its bridging 5-position; ordinarily, the presence of the silicon atom in the polymer backbone enhanced the interchain packing and carrier transporting ability of a polymer

[27,28,36]. Furthermore, based on the several advantageous prop-erties of the electron-deﬁcient TPD moiety, we prepared a new low-bandgap DeA conjugated polymer PDTSTPD, in which the TPD moiety was conjugated with the electron-rich DTS unit to provide crystalline characteristics, a narrow optical bandgap, and a deep HOMO energy leveldall desirable properties for application in PSCs.

* Corresponding author. Tel.: þ886 35 731871; fax: þ886 35 724727. E-mail address:[email protected](K.-H. Wei).

Contents lists available atScienceDirect

Polymer

j o u r n a l h o m e p a g e : w w w . e l s e v ie r . c o m / l o c a t e / p o l y m e r

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stirred at this temperature for 20 min and then warmed to room temperature for 2 h. After cooling again to78_{C, trimethylstannyl}

chloride (1 M in hexane, 7.5 mL, 7.5 mmol) was added and then the mixture was warmed gradually to room temperature and stirred overnight. The mixture was poured into water and extracted with Et2O (3 40 mL). The combined organic layers were washed thrice

with water (100 mL), dried (MgSO4), and concentrated under

reduced pressure to afford M2 (1.66 g, 88%) as a yellow liquid.1H NMR (Fig. S1) (300 MHz, CDCl3),

d

(ppm): 7.08 (s, 1H), 1.22e1.44 (m,

24H), 0.84e0.90 (m, 10H), 0.37 (s, 18H). MS (m/z): [M]þcalcd for C30H54S2SiSn2, 744.1; found, 744.

2.1.2. PolymerPDTSTPD

M1 (110 mg, 0.260 mmol), M2 (193.5 mg, 0.260 mmol), and tri(o-tolyl)phosphine [P(o-Tol)3; 6.3 mg, 8.0 mol%] were dissolved in dry

chlorobenzene (CB, 3 mL) and deoxygenated with N2for 15 min.

Tris(dibenzylideneacetone)dipalladium [Pd2(dba)3; 4.8 mg, 2.0 mol

%] was added and the solution was again deoxygenated for 15 min. The reaction mixture was heated at 120C for 48 h under N2and

then 2-tributylstannylthiophene (0.16 mL) and 2-bromothiophene (0.05 mL), the end-capping units, were added individually to the solution, with subsequent heating for 6 h each time. After cooling to room temperature, the solution was added dropwise into MeOH (100 mL). The crude polymer was collected, dissolved in hot CB, ﬁltered through a 0.5-

m

m poly(tetraﬂuoroethylene) (PTFE) ﬁlter, and reprecipitated in MeOH. The solid was washed with acetone and CHCl3in a Soxhlet apparatus. The CHCl3solution was concentrated

and then added dropwise into MeOH. Finally, the polymer was collected and dried under vacuum to give PDTSTPD (125 mg, 70.5%).

1_{H NMR (}_{Fig. S2}_{) (500 MHz, C}

2D2Cl4),

d

(ppm): 8.01 (s, 2H), 3.67 (br,

2H), 1.99 (br, 1H), 0.94e1.43 (m, 48H). Anal. calcd for C38H53NO2S3Si:

C, 67.11; H, 7.85; N, 2.06; found: C, 67.80; H, 7.93; N, 1.73. 2.2. Measurements and characterization

1_{H NMR spectra were recorded using a Varian UNITY 500-MHz}

spectrometer. Thermogravimetric analysis (TGA) was performed using a TA Instruments Q500; the thermal stabilities of the samples

were determined under a N2 atmosphere by measuring their

weight losses while heating at a rate of 20C min1. Differential scanning calorimetry (DSC) was performed using a PerkineElmer Pyris 1 unit operated at heating and cooling rates of 20 and 40C min1, respectively. Size exclusion chromatography (SEC) was performed using a Waters chromatography unit interfaced with a Waters 1515 differential refractometer; polystyrene was the standard; the temperature of the system was set at 45C and THF was the eluant. UVeVis spectra of dilute DCB solutions (1 105M) were recorded at ca. 25 and 55C using a Hitachi U-4100 spec-trophotometer. Solid ﬁlm for UVeVis analysis was obtained by

AgNO3) reference electrode; ferrocene/ferrocenium ion (Fc/Fcþ)

was used as the internal standard (0.09 V). The onset potentials were determined from the intersection of two tangents drawn at the rising and background currents of the cyclic voltammogram. HOMO and LUMO energy levels were estimated relative to the energy level of the ferrocene reference (4.8 eV below vacuum level)

[21,38]. The X-ray diffraction pattern of the pristine polymer thin ﬁlm was measured using a Bruker D8 high-resolution X-ray diffractometer operated in grazing-incidence mode. Topographic

and phase images of the polymer:PCBM ﬁlms (surface area:

5 5

m

m2) were obtained using a Digital Nanoscope III atomic force microscope operated in the tapping mode under ambient condi-tions. The thickness of the active layer of device was measured using a Veeco Dektak 150 surface proﬁler. Transmission electron microscopy (TEM) images of the copolymer:PC71BM ﬁlms were

recorded using a FEI T12 TEM operating at 120 keV. 2.3. Fabrication and characterization of photovoltaic devices

Indium tin oxide (ITO)-coated glass substrates were cleaned stepwise in detergent, water, acetone, and isopropyl alcohol (ultrasonication; 20 min each) and then dried in an oven for 1 h; subsequently, the substrates were treated with UV ozone for 30 min prior to use. A thin layer (ca. 20 nm) of poly(ethy-lenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS, Baytron P VP AI 4083) was spin-coated (5000 rpm) onto the ITO substrates. After baking at 140 C for 20 min in air, the substrates were

Fig. 1. TGA thermogram of the polymer PDTSTPD, recorded at a heating rate of 20C min1under a N2atmosphere.

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transferred to a N2-ﬁlled glovebox. The polymer and PC71BM were

co-dissolved in dichlorobenzene (DCB) at various weight ratios, but with aﬁxed total concentration (30 mg mL1_{). The blend solution}

was stirred continuously for 12 h at 60C,ﬁltered through a PTFE ﬁlter (0.2

m

m), the photoactive layer was obtained by spin coating the blend solution onto the ITO/PEDOT:PSS surface at 800 rpm for 60 s with further special treatment for 20 min at 90C. The thick-nesses of the photoactive layers were ca. 85e95 nm. Finally, an Al (100 nm) layer was thermally evaporated through a shadow mask under a vacuum of less than 1106_{torr. The effective layer area of}

one cell was 0.04 cm2. The current densityevoltage (JeV) charac-teristics were measured using a Keithley 2400 source-meter. The photocurrent was measured under simulated AM 1.5 G illumination

at 100 mW cm2using a Xe lampebased Newport 66902 150 W

solar simulator. A calibrated silicon photodiode with a KG-5filter was employed to check the illumination intensity. External quantum efficiency (EQE) was measured using an SRF50 system (Optosolar, Germany). A calibrated mono-silicon diode exhibiting a response at 300e900 nm was used as a reference. For hole mobility measurements, the hole only devices were fabricated with struc-tures of ITO/PEDOT:PSS/polymer/Au and ITO/PEDOT:PSS/poly-mer:PC71BM/Au. The hole mobility was determined byfitting the

dark JeV curve into the space-charge-limited current (SCLC) method

[11,39], based on the equation

J ¼ 9₈

3

0

3

r

m

h V2 L3

where

3

0is the permittivity of free space,

3

ris the dielectric constant

of the material,

m

his the hole mobility, V is the voltage drop across

the device, and L is the thickness of active layer. 3. Results and discussion

3.1. Synthesis and characterization of the polymer

Scheme 1displays the synthetic routes that we used to prepare the monomers and the polymer. The TPD-based polymer PDTSTPD

was prepared through Stille polymerization using the corre-sponding monomers M1/M2 and Pd2(dba)3/P(o-Tol)3as the

cata-lyst; the polymer comprised the DTS and TPD units. PDTSTPD had a number-average molecular weight (Mn) of 12 kg mol1, with

a polydispersity of 2.0, as determined through gel permeation chromatography (GPC) using polystyrene standards. The polymer was readily soluble in THF, CHCl3, CB, and DCB. We investigated the

thermal behavior of the polymer through TGA and DSC analyses. The TGA curve (Fig. 1) revealed that the polymer exhibited an excellent thermal stability, with 5%-weight-loss temperatures (Td)

greater than 450C.Fig. 2displays the thermal transition of the polymer in DSC analysis;Table 1summarizes the related data. In the endothermic trace, PDTSTPD exhibited a melting point at 332C, but glass transition was not detectable. In contrast to the exothermic trace, the polymer exhibited a distinct crystallization point at 307C.Table 1summarizes the molecular weight, poly-dispersity, and thermal properties of the polymer. We confirmed the crystalline characteristic from the grazing-incidence X-ray diffraction pattern of the polymer thin film (Fig. 3). PDTSTPD exhibited sharp diffraction peaks at 4.1 which we assign to its (100) crystal plane; the d-spacing of 21.5 Å correspond to the interchain separation defined by its alkyl side chains. In addition,

Fig. 3. X-ray diffraction pattern of the pristine PDTSTPDﬁlm and PDTSTPD:PC71BM

blends at various weight ratios (w/w).

Fig. 4. UVeVis absorption spectra of PDTSTPD in dilute DCB (1 105M) solutions, recorded individually at 25 and 55C, and as solidﬁlm.

Fig. 2. DSC trace of the polymer PDTSTPD, recorded at a heating rate of 20C min1 and a cooling rate of 40C min1under a N2atmosphere.

Table 1

Molecular weights and thermal properties of the polymers.

Polymer Mn(104) PDI Tc(C) Tm(C) Td(C)

PDTSTPD 1.2 2.0 307 332 465

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3.2. Photophysical properties

Fig. 4presents absorption spectra of the polymer, recorded in dilute DCB solutions and as solidﬁlm; the spectra of the solutions were recorded at both ca. 25 and 55C.Table 2summarizes the spectral data. In solution at 25C, PDTSTPD exhibited the absorp-tion maximum at 611 nm, which we assign to the internal charge transfer (ICT) interaction between the electron-accepting TPD and electron-donating DTS units. Moreover, a signiﬁcant vibronic shoulder appeared at 665 nm while the shoulder diminished when we heated the solutions at 55C, implying that a certain degree of

p

e

p

aggregation was already in effect in the solution states[28]. The absorption spectra of the polymer in the solid state was similar to their corresponding solution spectra, with only slight red-shifts of 3 nm of their absorption maxima, meaning some intermolec-ular interactions were already existent in its solution[31]. More-over, the intensity of the vibronic shoulder of the polymer in the solid state was increased signiﬁcantly relative to that in solution, indicating that much stronger interchain

p

e

p

stacking occurred in the solid state. The optical bandgap (Eoptg ) of PDTSTPD, estimated

from the onset of the absorption in its solidﬁlm, was 1.70 eV; the value is smaller than that of the polymer PBTTPD (1.82 eV) because the presence of the more coplanar and electron-rich DTS donor unit in the main chain extended the range of spectral absorption. 3.3. Electrochemical properties

We used CV to investigate the redox behavior of the polymer and obtain its HOMO and LUMO energy levels.Fig. 5presents the

essentially from oxidation of the electron-donating DTS unit. In the reduction trace, the polymer had a reductive onset potential of1.65 V, which we attributed to the reduction of the electron-deﬁcient TPD moiety. On the basis of the onset potentials, we estimated the HOMO and LUMO energy levels relative to the energy level of the ferrocene reference (4.8 eV below vacuum level). Accordingly, the HOMO and LUMO energy levels of PDTSTPD were 5.46 and 3.15 eV, respectively. This deep HOMO energy level was due to the presence of the strongly electron-withdrawing TPD moiety in its polymer backbone; moreover, the value (5.46 eV) was lower than 5.2 eV, suggesting its good stability against oxidization in air [16,40]. The LUMO energy level of PDTSTPD was greater than that of PC71BM (4.2 eV)[41]; thus,

a suf_{ﬁcient LUMO offset existed for efﬁcient charge dissociation in} the active layer[4,41]. The electrochemical bandgap of the polymer, estimated from the difference between its onset potential for oxidation and reduction, was _{2.31 eV; the value is somewhat} larger than its optical bandgap (1.70 eV). Such similar difference between the electrochemical and optical bandgap has been observed in studies of other DeA polymers[26,31,42], presumably resulting from the interface barrier for charge injection[42,43]. 3.4. Photovoltaic properties

We investigated the photovoltaic properties of PDTSTPD in PSCs having the device structure ITO/PEDOT:PSS/polymer:PC71BM/Al,

where the active layers were spin-coated from blended DCB solu-tions. Devices prepared from the polymers and PC71BM at various

Fig. 5. Cyclic voltammogram of the polymer PDTSTPD as solidﬁlm. Fig. 6. JeV characteristics of PSCs incorporating PDTSTPD:PC71

BM blends at various weight ratios (w/w).

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weight ratios were systematically investigated; in some cases, we added a small amount of 1,8-diiodooctane (DIO; 4%, by volume relative to DCB) to optimize the morphologies of the blends.Fig. 6

presents the JeV curves of the devices incorporating various poly-mer:PC71BM blend ratios; the values of series resistance (Rs) and

shunt resistance (Rsh) of devices were estimated according to the

equations described in the literature[44];Table 3summarizes the corresponding values of Vocand short-circuit current densities (Jsc),

ﬁll factors (FFs), PCEs, Rs, and Rsh. The devices incorporating the

PDTSTPD:PC71BM blends at various weight ratio exhibited values

of Vocof 0.87e0.88 V, related to the difference between the HOMO

energy level of the polymer and the LUMO energy level of PC71BM

[45]. A device incorporating the PDTSTPD:PC71BM blend at

a weight ratio of 1:1 exhibited a PCE of 1.94%, with values of Voc, Jsc,

and FF of 0.87 V, 4.51 mA cm2, and 49.4%, respectively. When we

increased the amount of PC71BM to a weight ratio of 1:3 (w/w), the

values of Jsc, FF, and PCE of the device improved to 5.74 mA cm2,

58.7%, and 2.96%, respectively, because of the relatively smaller Rs

and larger Rsh. The values of Jsc and PCE decreased slightly to

5.25 mA cm2and 2.69%, respectively, upon increasing the amount of PC71BM further to a weight ratio of 1:4.Fig. 7andFig. S3(see

Supporting information) display the corresponding topographic and phase images of the PDTSTPD:PC71BM blends. We observed

a relatively smoother surface with smaller phase-separated domains for the blend at a weight ratio of 1:3, relative to those obtained at weight ratios of 1:1 and 1:2; as a result, better device performance was obtained for the former. Furthermore, when we incorporated DIO (4 vol%) into the 1:3 (w/w) PDTSTPD:PC71BM

blend, the device exhibited slightly increased values of Jscand FF of

6.60 mA cm2and 59.9%, respectively, resulting in an increased PCE

Fig. 7. Topograhic images of PDTSTPD:PC71BM blends at weight ratios of (a) 1:1, (b) 1:2, (c) 1:3, (d) 1:4, (e) 1:1 (processed with 4 vol% DIO), (f) 1:2 (processed with 4 vol% DIO) and

(g) 1:3 (processed with 4 vol% DIO).

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of 3.36%. In addition, the incorporation of 4% DIO (v/v) into the 1:1 (w/w) PDTSTPD:PC71BM blend increased the value of Jsc signi

ﬁ-cantly to 7.72 mA cm2, increased the FF slightly to 51.5%, and therefore enhanced the PCE to 3.42%, presumably because more-intimate mixing of PDTSTPD and PC71BM resulted in a lower Rs.

Fig. 8 shows the TEM images of PDTSTPD/PC71BM blend ﬁlms.

Because the electron scattering density of PC71BM is higher than

that of the conjugated polymer, the PDTSTPD domains appear as bright regions whereas the dark regions can attributed to PC71BM

domains. Fig. 8a and b exhibits of the island shape features of aggregated PC71BM domains (dark areas) in the blendﬁlms with

PDTSTPD/PC71BM weight ratios of 1:1 and 1:3, respectively that

were processed without additives. In contrast,Fig. 8c and d displays homogeneous morphology for the PDTSTPD/PC71BM blendﬁlms at

weight ratios of the 1:1 and 1:3 that were processed with 4 vol% DIO additive, indicating that the incorporation of 4% DIO (v/v) optimized the miscibility of polymer chains with PC71BM. Through

optimization of the blend morphology, such high values of Jscand

PCE were also possible for the PDTSTPD:PC71BM blend at a weight

ratio of 1:1.Fig. 9presents the EQE curves of the optimized devices based on the PDTSTPD:PC71BM blends at weight ratios of the 1:1

and 1:3 (w/w) (both processed with 4 vol% DIO). These devices exhibited broad EQE responses from 300 to 750 nm, which we attribute to the enlarged spectral absorptions of the polymer. The

transport in the devices. 4. Conclusions

We prepared a new low-bandgap polymer, PDTSTPD, by conjugating an electron-deﬁcient thieno[3,4-c]pyrrole-4,6-dione (TPD) moiety with an electron-rich dithieno[3,2-b:20,30-d]silole (DTS) unit. Because of the presence of TPD moieties, the polymer exhibited an excellent thermal stability, crystalline characteristics, a broad spectral absorption, and a low-lying HOMO energy lev-eldall features that are desirable for solar cell applications. Manipulating the compositions and modulating the morphologies of the blends allowed us to optimize devices based on these

poly-mer:PC71BM blends. An optimal device incorporating the

PDTSTPD:PC71BM blend at a weight ratio of 1:1 displayed a better

PCE of 3.42%. Acknowledgment

We thank the National Science Council, Taiwan, for _ﬁnancial support (NSC 98-2120-M-009-006).

Appendix. Supplementary data

Supplementary data related to this article can be found online at

doi:10.1016/j.polymer.2011.04.057. References

[1] Yu G, Gao J, Hummelen JC, Wudl F, Heeger AJ. Science 1995;270:1789e91. [2] Günes S, Neugebauer H, Sariciftci NS. Chem Rev 2007;107:1324e38. [3] Krebs FC, Jorgensen M, Norrman K, Hagemann O, Alstrup J, Nielsen TD, et al.

Sol Energy Mater Sol Cells 2009;93:422e41.

[4] Thompson BC, Fréchet JMJ. Angew Chem Int Ed 2008;47:58e77.

[5] Li G, Shrotriya V, Huang J, Yao Y, Moriarty T, Emery K, et al. Nat Mater 2005;4: 864e8.

[6] Li YF, Zou YP. Adv Mater 2008;20:2952e8.

[7] Chiu M-Y, Jeng U-S, Su C-H, Liang K-S, Wei K-H. Adv Mater 2008;20:2573e8. [8] Chang Y-T, Hsu S-L, Chen G-Y, Su M-H, Singh T-A, Diau EW-G, et al. Adv Funct

Mater 2008;18:2356e65.

[9] Huang F, Chen KS, Yip HL, Hau SK, Acton O, Zhang Y, et al. Am Chem Soc 2009; 131:13886e7.

[10] Zoombelt AP, Leenen MAM, Fonrodona M, Nicolas Y, Wienk MM, Janssen RAJ. Polymer 2009;50:4546e70.

[11] Chang Y-T, Hsu S-L, Su M-H, Wei K-H. Adv Mater 2009;21:2093e7. [12] Zhou E, Cong J, Tajima K, Hashimoto K. Chem Mater 2010;22:4890e5. [13] Li Y, Li H, Xu B, Li Z, Chen F, Feng D, et al. Polymer 2010;51:1786e95. [14] Inganäs O, Zhang F, Tvingstedt K, Andersson LM, Hellström S, Andersson MR.

Adv Mater 2010;22:E100e16.

[15] Zhao W, Cai W, Xu R, Yang W, Gong X, Wu H, et al. Polymer 2010;51: 3196e202.

[16] Blouin N, Michaud A, Gendron D, Wakim S, Blair E, Neagu-Plesu R, et al. J Am Chem Soc 2008;130:732e42.

[17] Park SH, Roy A, Beaupré S, Cho S, Coates N, Moon JS, et al. Nat Photonics 2009; 3:297e302.

Fig. 9. EQE curves of PSCs incorporating the PDTSTPD:PC71BM blends at weight ratios

of 1:1 and 1:3 (w/w) (both processed with 4 vol% DIO).

Fig. 10. Dark JeV curves for hole-dominated carrier devices incorporating the pristine PDTSTPD ﬁlm, the 1:1 (w/w) PDTSTPD:PC71BM blend and the 1:1 (w/w)

(7)

[18] Qin R, Li W, Li C, Du C, Veit C, Schleiermacher HF, et al. J Am Chem Soc 2009; 131:14612e3.

[19] Chen G-Y, Lan S-C, Lin P-Y, Chu C-W, Wei K-H. J Polym Sci Part A Polym Chem 2010;48:4456e64.

[20] Hsu S-L, Chen C-M, Wei K-H. J Polym Sci Part A Polym Chem 2010;48: 5126e34.

[21] Liang Y, Feng D, Wu Y, Tsai S-T, Li G, Ray C, et al. J Am Chem Soc 2009;131: 7792e9.

[22] Chen H-Y, Hou J, Zhang S, Liang Y, Yang G, Yang Y, et al. Nat Photonics 2009;3: 649e53.

[23] Zhou H, Yang L, Price SC, Knight KJ, You W. Angew Chem Int Ed 2010;49:7992e5. [24] Yuan M-C, Chiu M-Y, Chiang C-M, Wei K-H. Macromolecules 2010;43:

6270e7.

[25] Zhang G, Fu Y, Xie Z, Zhang Q. Polymer 2011;52:415e21.

[26] Hou J, Chen H-Y, Zhang S, Li G, Yang Y. J Am Chem Soc 2008;130:16144e5. [27] Scharber MC, Koppe M, Gao J, Cordella F, Loi MA, Denk P, et al. Adv Mater

2010;22:367e70.

[28] Chen H-Y, Hou J, Hayden AE, Yang H, Houk KN, Yang Y. Adv Mater 2010;22:371e5. [29] Hoven CV, Dang XD, Cofﬁn RC, Peet J, Nguyen T-Q, Bazan GC. Adv Mater 2010;

22:E63e6.

[30] Zou Y, Najari A, Berrouard P, Beaupré S, Aïch BR, Tao Y, et al. J Am Chem Soc 2010;132:5330e1.

[31] Zhang Y, Hau SK, Yip H-L, Sun Y, Acton O, Jen AK-Y. Chem Mater 2010;22: 2696e8.

[32] Piliego C, Holcombe TW, Douglas JD, Woo CH, Beaujuge PM, Fréchet JMJ. J Am Chem Soc 2010;132:7595e7.

[33] Zhang G, Fu Y, Zhang Q, Xie Z. Chem Commun 2010;46:4997e9.

[34] Guo X, Xin H, Kim FS, Liyanage ADT, Jenekhe SA, Watson MD. Macromolecules 2011;44:269e77.

[35] Yuan M-C, Chiu M-Y, Liu S-P, Chen C-M, Wei K-H. Macromolecules 2010;43: 6936e8.

[36] Lu G, Usta H, Risko C, Wang L, Facchetti A, Ratner MA, et al. J Am Chem Soc 2008;130:7670e85.

[37] Welch GC, Cofﬁn R, Peet J, Bazan GC. J Am Chem Soc 2009;131:10802e3. [38] Pommerehne J, Vestweber H, Guss W, Mahrt RF, Bässler H, Porsch M, et al.

Adv Mater 1995;7:551e4.

[39] Melzer C, Koop EJ, Mihailetchi VD, Blom PWM. Adv Funct Mater 2004;14: 865e70.

[40] de Leeuw DM, Simenon MMJ, Brown AR, Einerhand REF. Synth Met 1997;87: 53e9.

[41] Lindgren LJ, Zhang F, Andersson M, Barrau S, Hellström S, Mammo W, et al. Chem Mater 2009;21:3491e502.

[42] Wu P-T, Kim FS, Champion RD, Jenekhe SA. Macromolecules 2008;41:7021e8. [43] Chen Z-K, Huang W, Wang L-H, Kang E-T, Chen BJ, Lee CS, et al.

Macromol-ecules 2000;33:9015e25.

[44] Moliton A, Nunzi J-M. Polym Int 2006;55:583e600.

[45] Scharber MC, Mühlbacher D, Koppe M, Denk P, Waldauf C, Heeger AJ, et al. Adv Mater 2006;18:789e94.