Synthesis and Photovoltaic Performance of Low-Bandgap Polymers

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Macromolecules 2011, 44, 502–511 DOI: 10.1021/ma102173a

Synthesis and Photovoltaic Performance of Low-Bandgap Polymers on the Basis of 9,9-Dialkyl-3,6-dialkyloxysilafluorene Jae-Kyu Jin,† Jong-Kil Choi,† Bum-Joon Kim,† Hyun-Bum Kang,† Sung-Cheol Yoon,‡ Hong You,§ and Hee-Tae Jung*,† †

Department of Chemical & Biomolecular Engineering, KAIST, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Korea, ‡Advanced Materials Division, KRICT, Sinseongno 19, Yuseong-gu, Daejeon 305-600, Korea, and §R&D Center, SK Energy, 140-1 Wonchon-dong, Yuseong-gu, Daejeon 305-712, Korea Received September 24, 2010; Revised Manuscript Received November 25, 2010

ABSTRACT: A new series of polysilafluorene-type low-bandgap polymers containing 3,6-dialkyloxy-9,9dialkylsilafluorene and 4,7-di-2-thienyl-2,1,3-benzothiadiazole units has been synthesized. UV/vis absorption spectroscopy and grazing incident X-ray diffraction results showed that the alkoxy moiety on the silafluorene unit broadens the absorption band of the polymers because of its electron-donating property, enabling more efficient harvesting of photons from the solar spectrum. Furthermore, the silicon atoms of the polymers lead to a highly ordered structure, which is essential for high charge-carrier mobility. In addition, high molecular weight polymers can be prepared by using long octyloxy/hexyl solubilizing groups. The blend of new poly[2,7-(3,6-dioctyloxy-9,9-dihexylsilafluorene)-alt-5,5-(40 ,70 -di-2-thienyl-20 ,10 ,30 -benzothiadiazole)] (P4H) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) exhibited a power-conversion efficiency of 4.05% with an open-circuit voltage of 0.67 V, a short-circuit current density of 11.1 mA cm-2, and a fill factor of 54.3% under simulated 100 mW cm-2 air mass 1.5 global (AM1.5G) illumination.

Introduction The organic photovoltaic (OPV) cell has emerged as a promising candidate renewable energy source because of its low processing cost, lightweight and portability.1,2 The best known OPV devices consist of a mixture of conjugated polymers (donor) and fullerene derivatives (acceptor) between two electrodes as a single bulk-heterojunction (BHJ) active layer. On absorption of a photon, an electron in the highest occupied molecular orbital (HOMO) is excited to the lowest unoccupied molecular orbital (LUMO). This bound exciton migrates to an electron donor/ electron acceptor interface and then dissociates into an electronhole pair. After dissociation, each charge carrier moves to the anode and cathode to produce electricity. Thus, the conjugated polymers acting as the donor must have a low bandgap energy if they are to absorb a large portion of photons from the solar spectrum. Furthermore, the hole mobilities of the conjugated polymers must be high enough to extract holes to the anode from the interface of donor and acceptor before the hole and electron undergo recombination. In addition, a morphologically interpenetrating network with the electron acceptor is preferred to generate efficient exciton dissociation.3-5 Several low-bandgap polymers as BHJ donors have been developed to enhance the light-absorbing ability, charge-carrier mobility and charge dissociation. Although the blend system of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) has dominated for the past several years,3,4 there have been strong demands for the development of alternative donor materials to achieve high-performance BHJ devices. These include poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(40 ,70 -di-2-thienyl-20 , 10 ,30 -benzothiodiazole)] (PFODBT),6-9 poly[N-9-heptadecanyl2,7-carbazole-alt-5,5-(40 ,70 -di-2-thienyl-20 ,10 ,30 -benzothiadiazole)] (PCDTBT),10 poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b; *Corresponding author. E-mail: [email protected]. pubs.acs.org/Macromolecules

Published on Web 01/06/2011

3,4-b2]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTTBT),11 poly[9,9-dioctyl-2,7-dibenzosilole-alt-5,5-(40 ,70 -di-2-thienyl-20 ,10 , 30 -benzothiadiazole)] (PSiFDBT),12,13 and poly[4,40 -bis(2-ethylhexyl)dithieno[3,2-b:20 ,30 -d]silole]-2,6-diyl-alt-4,7-bis(2-thienyl)2,1,3-benzothiadiazole)-5,50 -diyl] (PSBTBT).14-18 Among the conjugated polymers, silicon-containing conjugated polymers such as PSBTBT have attracted considerable attention in recent years because of their high charge-carrier mobilities. For example, it has been demonstrated that the substitution of a carbon atom at the 5-position of the conjugate PCPDTBT by a silicon atom greatly improved cell efficiency, which may be attributed to the unique electronic properties of the silicon atom, such as a lowlying σ* orbital.14,16,17 In addition, changes in the molecular architectures induce highly ordered structures because of the strong tendency of silicon-containing molecules to stack on each other, generating high charge-carrier mobility. To date, however, these silicon-containing polymers have suffered from poor solubility because of the strong intermolecular stacking, causing difficulties in obtaining high molecular weights (Mn > 25 kDa) and in preparing uniform thin films.14 Indeed, it is well-known that a high molecular weight conjugated polymer is desirable for high power-conversion efficiency (PCE) because the absorption band and charge-carrier mobility are known to be strongly dependent on the molecular weight.19-21 Also, a much broader absorption band of the silicon-containing conjugated polymers is required to collect more photons from the incoming solar light.7,12,13 In the present work, we synthesized a new series of silafluorenebased polymer donors having ethyl/octyloxy, hexyl/hexyloxy, and hexyl/octyloxy solubilizing groups. We show that the alkoxy groups on the silafluorene unit not only broaden the spectral absorption ranges but also significantly improve solubility. It was found that the length of the solubilizing alkoxy groups strongly affects the photovoltaic performance: A polymer donor (P4H) having hexyl/octyloxy groups exhibits good solubility, which enables preparation of a high molecular weight polymer r 2011 American Chemical Society

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Macromolecules, Vol. 44, No. 3, 2011

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Scheme 1. Molecular Structures and Synthetic Routes of the Monomersa

a Key: (i) HBr, acetonitrile, NaNO2, reflux; (ii) NaOH, EtOH, reflux; (iii) I2, KIO3, H2SO4, acetic acid, 90 °C; (iv) 1.6 M n-BuLi, THF, -100 °C; (v) TMP, 1.6 M n-BuLi, SnMe3Cl, THF, -78 °C.

(Mn, 63.9 kDa; Mw, 160 kDa), resulting in high photovoltaic performance. On the other hand, a polymer with short ethyl/octyloxy groups (P2) has relatively poor solubility with low molecular weight (Mn, 13.6 kDa; Mw, 17.9 kDa), leading to poor photovoltaic performance. As a result, a high PCE of 4.05% with opencircuit voltage (Voc) = 0.67 V, short-circuit current density (Jsc) = 11.1 mA cm-2, and fill factor (FF) = 54.3% under 100 mW cm-2 AM1.5G was achieved using the P4H:PC71BM system, which results from efficient light harvesting, high charge-carrier mobility and efficient charge dissociation because of the desirable phase separation between P4H and PC71BM. Results and Discussion Synthesis. The chemical structures and synthetic routes of the monomers are depicted in Scheme 1. A series of silafluorene monomers (M1, M2, and M3) having ethyl/octyloxy, hexyl/ hexyloxy, and hexyl/octyloxy groups was prepared using the Sandmeyer reaction, alkylation of the hydroxy substituents, iodination, selective lithiation of the 2,20 -iodo substituents and subsequent cyclization with a dialkyldihalosilane.22 4,40 Dibromobiphenyl-3,30 -diol (2) was prepared from 4,40 -diaminobiphenyl-3,30 -diol (1) with a high yield of 87%. Then the hydroxyl groups of 4,40 -dibromobiphenyl-3,30 -diol (2) were alkylated with 1-bromohexane or 1-bromooctane in the presence of sodium hydroxide to afford 4,40 -dibromo-3,30 bis(hexyloxy)biphenyl (5) or 4,40 -dibromo-3,30 -bis(octyloxy)biphenyl (3), respectively. The iodination of 4,40 -dibromo-3, 30 -bis(octyloxy)biphenyl (3) and 4,40 -dibromo-3,30 -bis(hexyloxy)biphenyl (5) using I2/KIO3 gave 4,40 -dibromo-2,20 -diiodo5,50 -bis(octyloxy)biphenyl (4), and 4,40 -dibromo-5,50 -bis(hexyloxy)-2,20 -diiodobiphenyl (6), respectively. Then, the iodo groups on 4,40 -dibromo-2,20 -diiodo-5,50 -bis(octyloxy)biphenyl (4) were selectively lithiated by adding n-butyllithium dropwise

at a reaction temperature of -100 °C and subsequently reacted with dichlorodiethylsilane and dichlorodihexylsilane to afford 2,7-dibromo-9,9-diethyl-3,6-bis(octyloxy)-9silafluorene (M1) and 2,7-dibromo-9,9-dihexyl-3,6-bis(octyloxy)9-silafluorene (M3), respectively. 2,7-Dibromo-9,9-dihexyl3,6-bis(hexyloxy)-9-silafluorene (M2) was prepared under the same reaction conditions using 4,40 -dibromo-5,50 -bis(hexyloxy)-2,20 -diiodobiphenyl (6) and dichlorodihexylsilane. 4,7-Di(20 -trimethylstannylthiophen-50 -yl)-2,1,3-benzothiadiazole (M4) was prepared by reaction with 4,7-bis(thien-2-yl)-2,1,3benzothiadiazole (10) and SnMe3Cl.23 The chemical structures of the compounds were identified by 1H NMR, 13C NMR, FD-TOF-MS and elemental analysis (see the Experimental Section). The Stille coupling reaction was used to prepare the polymers (Scheme 2). The Stille polymerization was carried out in toluene at 100 °C for 24 h using Pd2(dba)3 and P(o-tolyl)3 as catalysts. As the polymerization proceeded, the color of the reaction solution changed from orange to dark brown and the viscosity gradually increased. The polymer was obtained by precipitation of the reaction solution in methanol, treatment with an aqueous solution of sodium diethyldithiocarbamate trihydrate and Soxhlet extraction with methanol, acetone and hexane. With the combinations of monomers of M2/M5/M6 (P1), M1/M4 (P2), M2/M4 (P3), and M3/M4 (P4), polymers having different solubilizing groups and molecular weights were prepared. P1 and P3 have hexyl groups as substituents at the silicon atom and hexyloxy groups on the silafluorene moiety. P1 has the same building blocks as those of P3; however, in contrast to P3, it has an irregular backbone structure because M5 and M6 were used instead of M4 (Scheme 2). P2 has substituents of ethyl/octyloxy groups while P4 has hexyl/octyloxy side groups.

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Table 1. Molecular Weight and Optical and Electrochemical Properties of Polymersa polymer

Mn (Da)

Mw (Da)

PDI

λmax (nm)

Egopt (eV)

LUMO (eV)

HOMO (eV)

P1 76 600 127 800 1.67 462, 601 1.69 -3.63 -5.48 P2 13 600 17 900 1.32 413, 576 1.82 -3.54 -5.29 P3 30 700 67 200 2.19 433, 642 1.75 P3H 102 300 228 900 2.24 430, 649 1.75 -3.56 -5.26 P4 19 000 33 900 1.78 426, 638 1.75 P4H 63 900 160 300 2.51 428, 644 1.75 -3.57 -5.28 PFODBT 31 200 93 900 3.01 394, 567 1.91 -3.59 -5.61 a Egopt was determined from the onset of the absorption edge and Egec was determined by cyclic voltammetry measurements.

Egec (eV) 1.85 1.75 1.70 1.71 2.02

Scheme 2. Chemical Structures of the Polymersa

Figure 1. Normalized absorption spectra of P1, P2, P3, P3H, P4, P4H, and PFODBT films. The films were spin-coated from o-DCB solutions and annealed at 140 °C (5 min) under N2 atmosphere.

a

Key: (i) Pd2(dba)3, P(o-tolyl)3, toluene, 100 °C.

In addition, the well-known D-A-type polymer PFODBT was prepared to use as a reference. The molecular weights and polydispersity indexes (PDI) of the polymers are summarized in Table 1. P2, having ethyl/octyloxy solubilizing groups, showed a lack of solubility; thus, the molecular weight was reduced by adjusting the mole ratio of M1/M4 to 1:0.95 and the resulting molecular weight (Mn) of the hot chlorobenzene-soluble part of polymer was 7.42 kDa with PDI of 1.30. On the other hand, P1, P3 and P4 have much higher solubilities because of the longer solubilizing groups than those of P2 and the chlorobenzene-soluble part showed much higher molecular weights (Mn) of 76.6, 30.7, and 19.0 kDa, respectively. Furthermore, the molecular weight of P3 and P4 could be further increased by adjusting the reaction conditions (1.5 times larger amount of catalyst was used). The high molecular weight polymers P3H and P4H showed Mn values of 102.3 and 63.9 kDa, respectively (P3H and P4H have the same molecular structure as P3 and P4, respectively). UV/Vis Absorption Spectra. Figure 1 shows the normalized UV/vis absorption spectra of P1, P2, P3, P3H, P4, P4H,

and PFODBT films. The optical bandgaps of the polymers were calculated from the onset of absorption and are listed in Table 1. All of the polymer films except P1 show absorption spectra typical of D-A alternating polymers; specifically, the spectra contain two distinct absorption peaks, one at short wavelength (λmax ∼384-433 nm) and another at long wavelength (λmax ∼567-649 nm). The absorption peak in the short wavelength can be identified with a delocalized excitonic π-π* transition and the long wavelength absorption band can be attributed to a localized transition between donor-acceptor charge transfer states.24,25 However, P1 shows a weak absorption peak at 601 nm and a strong absorption peak at 462 nm, which correspond to the randomly distributed DBT and silafluorene segments on the polymer backbone, respectively (Figure 1). The optical bandgap of PFODBT, used as a reference material, was ∼1.90 eV, which is consistent with the reported value.8 However, the λmax of our PFODBT was slightly red-shifted with respect to the literature value, from 555 nm to ∼567 nm, perhaps because of the high molecular weight of the PFODBT used in this study (Mn: 31.2 kDa) as compared with that in the previous report (Mn: 8.0 kDa).8 P2, P3, P3H, P4, and P4H are significantly redshifted compared with PFODBT (Figure 1b): P2 (λmax ∼ 576 nm), P3 (λmax ∼ 642 nm), P3H (λmax ∼ 649 nm), P4 (λmax ∼ 638 nm), and P4H (λmax ∼ 644 nm). Therefore, the present findings clearly indicate that the electron-donating property of the alkoxy groups significantly reduces the bandgap energy of the polymers. Moreover, the steric hindrance effect of the alkoxy groups is not so strong as to greatly alter the planarity along the polymer backbone, considering that both PSiFDBT and PFODBT have λmax values ranging from 560 to 565 nm.12,13 Furthermore, the absorption peaks of P3, P3H, P4, and P4H are much broader than that of P2, suggesting stronger intermolecular interactions in P3, P3H, P4, and P4H.17 In other words, such broad absorption spectrum of P3, P3H, P4, and P4H might be attributed to a favorable molecular conformation for chain packing or aggregation,

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Figure 2. J-V characteristics of photovoltaic devices under AM1.5G solar simulated light with an intensity of 100 mW cm-2. The device structure is ITO/PEDOT:PSS/polymer:PCBM/Al. Key: (a) photovoltaic device performances of P1, P2, P3, and P4; (b) molecular weight effect on device performance of P2 (Mn: 13.6 kDa), P3 (Mn: 30.7 kDa), P3H (Mn: 102.3 kDa), P4 (Mn: 19.0 kDa), P4H (Mn: 63.9 kDa); (c) effect of annealing on the J-V characteristics of P2, P3H, and P4H devices; (d) comparison of the photovoltaic performances [a P3HT:PC61BM = 1:0.8 w/w; b PFODBT: PC61BM=1:4 w/w; c P4H:PC61BM=1:3 w/w; d P4H:PC71BM=1:2.5 w/w]. Table 2. Performance of the photovoltaic devices polymer

polymer/PCBM ratio (w/w)

Jsc (mA cm-2)

Voc (V)

FF (%)

PCE (%)

5.04 4.68 6.23 6.75 5.35 8.39 10.7 11.1 9.04 5.49

0.81 0.61 0.65 0.63 0.68 0.63 0.62 0.67 0.60 0.85

55.2 41.2 46.8 55.2 41.7 61.0 49.0 54.3 56.2 39.6

2.26 1.17 1.89 2.34 1.51 3.22 3.24 4.05 3.06 1.86

P1 1:3 (PC61BM) P2 1:3 (PC61BM) P3 1:3 (PC61BM) P3H 1:3 (PC61BM) P4 1:3 (PC61BM) P4H 1:3 (PC61BM) P4H 1:3 (PC71BM) P4H 1:2.5 (PC71BM) 1:0.8 (PC61BM) P3HTa PFODBT 1:4 (PC61BM) a P3HT (grade: 4002-EE) was obtained from Rieke Metals, Inc.

compared to P2. In fact, P2 may have a large torsion angle along the polymer chain because of the significantly unsymmetrical length of alkyl and alkoxy side chain or some other factors in molecular structure.26 In addition, we cannot rule out any possibilities that the greater degree of conjugation as a result of their higher molecular weights of P3, P3H, P4, and P4H, affects the red-shifted absorption band.6,8,12,13 Therefore, the light absorbing abilities of the hexyl/hexyloxy and hexyl/octyloxy side groups are much more desirable than those of the ethyl/octyloxy side groups. Electrochemical Properties. To investigate the positions of the HOMO and LUMO levels of the polysilafluorene-type low-bandgap polymers, cyclic voltammetry (CV) measurements were carried out. CV results (Table 1) showed that the LUMO levels of P1, P2, P3H, P4H, and PFODBT are in the range -3.54 to -3.63 eV because they have the same electron-accepting segment (DBT). In contrast, the HOMO

levels of P1, P2, P3H, and P4H are significantly increased compared with that of PFODBT (-5.61 eV) as a result of the alkoxy groups. The HOMO levels of P2, P3H, and P4H are -5.29, -5.26, and -5.28 eV, respectively, while P1 shows a lower HOMO level of -5.48 eV, possibly due to the irregular backbone structure. As a result of the increased HOMO level, the bandgap energies become much smaller than that of PFODBT (2.02 eV), with values ranging from 1.69 to 1.82 eV. The Voc values of devices fabricated from P1, P2, P3H, P4H, and PFODBT were estimated by using a semiempirical equation relating Voc and the HOMO level of the donor.27 The Voc values of P1 and PFODBT were calculated to be 0.88 and 1.01 V, respectively. P2, P3H, and P4H were estimated to have Voc values of 0.69, 0.66, and 0.68 V, respectively, which are similar because of the similar HOMO levels of these polymers. These calculated Voc results agree well with our findings on the performances of the photovoltaic devices (Figure 2).

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Figure 3. Absorption spectra and external quantum efficiency (EQE) and absorption spectrum of P4H:PC71BM and P4H:PC61BM (1:3 w/w).

Photovoltaic Properties. To investigate the photovoltaic characteristics of the polymers, photovoltaic devices with a structure of ITO/PEDOT:PSS (35 nm)/polymer:PCBM (100130 nm)/Al (150 nm) were fabricated. The current densityvoltage (J-V) curves of the devices measured under simulated illumination by AM1.5G (100 mW cm-2) are shown in Figure 2, and their photovoltaic performances are summarized in Table 2. P1, P2, P3, and P4 exhibit PCEs of 2.26%, 0.97%, 1.78%, and 1.51%, respectively. We found that the molecular weight of the polymers strongly affects the photovoltaic performance. Regardless of the length of the solubilizing alkoxy groups, the PCEs of P2 (Mn: 13.6 kDa), P3 (Mn: 30.7 kDa), and P4 (Mn: 19.0 kDa) are strongly influenced by the polymer molecular weight. In addition, it is noteworthy that P1 (Mn: 76.6 kDa) having irregular polymer backbone structure shows the highest PCE of 2.26% exhibiting Jsc comparable to those of the other polymers, which is due to the high Voc of P1. This result clearly shows that the PCEs of D-A-type polymers are strongly affected by the molecular weight of the polymer donors, similar to previous results for P3HT/PCBM systems.3,19,28 To further confirm the molecular weight dependence, the photovoltaic performances of low molecular weight polymers (P3 and P4) and high molecular weight polymers (P3H and P4H) were compared (Figure 2b). P3H (Mn: 102.3 kDa) and P4H (Mn: 63.9 kDa) have the same molecular structure as P3 and P4, respectively, but have different molecular weights. Interestingly, P4H exhibits a much higher PCE of 3.22% than P4 (PCE: 1.51%) and P3H exhibits a PCE of 2.34% while its low molecular weight counterpart P3 exhibits a PCE of 1.89%, mainly because of the improved Jsc as well as FF. From these results it becomes evident that molecular weight control is very important for efficient photovoltaic performance. However, P3H shows a lower PCE of 2.34% than P4H in spite of its high molecular weight, indicating that too high a molecular weight could have an adverse effect on the PCE. On the basis of our results, to obtain a high PCE, the molecular weight of the polymer should be high enough but not too high, because too high a molecular weight polymer may not form nanoscale interpenetrating networks due to poor solubility. In Figure 2d, the J-V characteristics of devices fabricated from P4H, P3HT (4002-EE, Rieke Metals, Inc.) and PFODBT are plotted to compare their performances. PFODBT: PC61BM (1:4 w/w) and P3HT:PC61BM (1:0.8 w/w), which are used as references, show PCEs of 1.86% and 3.06%, respectively. Although P4H:PC61BM (1:3 w/w) has a lower Voc (0.63 V) than PFODBT, P4H:PC61BM (1:3 w/w) exhibits a

much higher PCE of 3.22% because of the significantly improved Jsc (8.39 mA cm-2). In addition, when P4H: PC71BM (1:3 w/w) was used, the device showed an increased Jsc of 10.7 mA cm-2 because of the much stronger absorption of PC71BM (Figure 3a) and the PCE improved to 3.24%. As shown in Figure 3, P4H-based devices absorb a wide range of visible light. The absorption spectra and external quantum efficiencies (EQEs) of P4H:PC71BM and P4H:PC61BM (1:3 w/w) devices exhibit well-matched profiles. The EQEs of P4H:PC61BM and P4H:PC71BM start to increase steeply around 740 nm and reach 46.9% at 550 nm and 59.0% at 540 nm, respectively, and high EQEs are maintained over a wide range of wavelengths, 360 to 650 nm, showing average values of 52% and 40%, respectively. Furthermore, when the blending ratio of P4H:PC71BM was adjusted to 1:2.5 w/w and the annealing temperature of the spin-cast P4H:PC71BM was changed to 70 °C from 50 °C, Jsc, Voc, and FF increased to 11.1 mA cm-2, 0.67 V, and 54.3%, respectively, and as a result, the PCE was greatly improved to 4.05% (Table 2). Morphology. By observing the morphologies of blend films, we found that the degree of phase separation between the conjugated polymers (electron donor) and PCBM (electron acceptor) plays a significant role in determining the device efficiency. The nanoscale morphologies of polymer:PC61BM films were investigated using noncontact atomic force microscopy (AFM). As shown in Figure 4, distinctly different morphologies of polymer:PCBM blend films were observed in their phase images, indicating that the active layer morphology is strongly affected by the solubility of the polymer. As mentioned, the nanoscale morphology of the active layer is a very important factor for efficient exciton dissociation because if the diffusion length of the photogenerated exciton (∼10 nm) is too small, it is easy for recombination to occur before reaching the interface between the polymer and PCBM. PFODBT:PC61BM and P1:PC61BM contain a number of aggregates and the large grain size can be clearly observed in the phase images (Figure 4, parts a and d). In contrast, P4H: PC61BM has a well-connected percolated network for both the polymer and PC61BM phases with a significantly smaller phase domain (Figure 4c). The long polymer chain of P4H, with its high molecular weight, seems to favor the formation of well-percolated networks by bridging the neighboring crystalline domains, providing a connected pathway for charge carriers.20 In addition, long side chains enable high solubility of polymer and lead to an increase in the interspacing between the polymer chains. Thus, it is expected that PCBM molecules easily diffuse between polymer chains giving large interfacial area while P2 and PFODBT exhibit large aggregates

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Figure 4. AFM phase images of polymer:PCBM films: (a) P2:PC61BM (1:3 w/w) film; (b) P3H:PC61BM (1:3 w/w) film; (c) P4H:PC61BM (1:3 w/w) film; (d) PFODBT:PC61BM (1:4 w/w) film. The insets show AFM topology images.

due to their unsatisfactory solubility in solvent. From these results, we conclude that the favorable morphology of the P4H-based device is the major reason for its high PCE. Grazing Incident X-ray Diffraction. To study the effect of molecular structure on the crystallographic structure, onedimensional plots of out-of-plane grazing incident X-ray diffraction (GIXRD) of as-spun and annealed polymer:PCBM films on PEDOT:PSS/ITO/glass substrates were examined (Figure 5). In the figure, the insets are the corresponding twodimensional (2-D) images of annealed polymer:PCBM films. (In this study, we carried out GIXRD measurements of polymer:PCBM films without Al electrode. Indeed, there is a possibility that morphologies of the active layer of OPV devices are different between the devices in the presence and in the absence of Al electrode.29) P1:PC61BM and PFODBT: PC61BM exhibit a rather weak (100) peak at qz equal to 0.2143 and 0.2057 A˚-1 (the corresponding d100 spacings are calculated to be 29.3 and 30.5 A˚), respectively, and the peak intensities remain almost unchanged after annealing (Figure 5, parts a and f). On the other hand, P2:PC61BM, P3H:PC61BM, P4H:PC61BM, and P4H:PC71BM films are shown to have a highly ordered molecular structure oriented normal to the

substrate, showing strong (100) peaks at qz equal to 0.3129 (d100 = 20.1 A˚), 0.2868 (d100 = 21.9 A˚), 0.2723 (d100 = 23.1 A˚), and 0.2636 A˚-1 (d100 = 23.8 A˚), respectively, (Figure 5b-e) and, after thermal annealing, the out-of-plane X-ray reflection intensities of P3H:PC61BM, P4H:PC61BM, and P4H: PC71BM increased significantly and the d100 spacings were slightly reduced to 21.3, 23.1, and 23.6 A˚, respectively, indicating enhanced interchain packing. This result suggests that the changes in crystallographic structure of the polymer thin film after thermal annealing, such as enhanced interchain packing, seem to be one of the reasons for the improved PCE. (In addition, thermal annealing is known to increase PCE because of morphological changes and strengthened adhesion between the polymer:PCBM layer and Al.30-32) By thermal annealing at 140 °C for 5 min, the PCEs of P2: PC61BM, P3H:PC61BM, and P4H:PC61BM were improved from 0.97, 1.78, and 2.51% to 1.17, 2.34, and 3.22%, respectively, exhibiting significant increases in Voc, FF, and Jsc.17,33-35 (Figure 2c) The GIXRD results also clearly show that the d100 spacing is directly related to the length of the alkyl groups, and the silicon-containing polymers P2, P3H, and P4H, despite their long solubilizing groups, have long-range

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Figure 5. Out-of-plane GIXRD profiles of polymer films coated on PEDOT:PSS/ITO/glass. Insets are corresponding 2-D images of annealed polymer:PCBM films: (a) P1:PC61BM (1:3 w/w) film; (b) P2:PC61BM (1:3 w/w) film; (c) P3H:PC61BM (1:3 w/w) film; (d) P4H:PC61BM (1:3 w/w) film; (e) P4H:PC71BM (1:3 w/w) film; (f) PFODBT:PC61BM (1:4 w/w) film.

ordered structures even when mixed with PCBM, which is beneficial for high charge-carrier mobility.The d100 spacing is increased as the length of the solubilizing groups of the polymer is increased; P2:PC61BM (P2 has ethyl/octyloxy groups) has d100 spacing of 19.5 A˚, while P4H:PC61BM, which has the longest solubilizing groups of hexyl/octyloxy substituents, exhibits d100 spacing of 23.1 A˚. However, in spite of the long octyloxy solubilizing groups, P4H:PC61BM has a much smaller d100 spacing than PFODBT:PC61BM, with a stronger peak intensity, indicating a more ordered structure and smaller interchain distance. These findings partially explain the much higher hole mobility of P4H compared to PFODBT. According to space-charge limited current (SCLC) hole mobility measurements, the hole mobility of PFODBT is 1.2 10-5 cm2 V-1 s-1. P1 also exhibits a rather low hole mobility of 1.1 10-5 cm2 V-1 s-1, probably because of the irregular backbone structure, which might hinder the stacking of polymer backbones. On the other hand, P2, P3H, and P4H still have about 2 to 18 times higher hole mobilities than PFODBT, with values of 2.2 10-4, 2.1 10-5, and 4.5 10-5 cm2 V-1 s-1, respectively. In addition, upon blending with PCBM, the high molecular weight polymers P3H and P4H might be more efficient in charge transport because high molecular weight polymers can interconnect neighboring crystalline domains, enabling easy transport of the charge carrier through the connected pathway. Therefore, the OPVs based on P3H and P4H could be expected to have higher Jsc and FF as a result of a high hole-transporting ability and nanoscale interpenetrated network with PCBM. Conclusion We have prepared a series of new low-bandgap polymers containing 9,9-dialkyl-3,6-dialkyloxysilafluorene and 4,7-di-2thienyl-2,1,3-benzothiadiazole units. These polymers showed a broadened absorption band compared with poly[2,7-(9,9-dioctylfluorene)-alt-5,5-(40 ,70 -di-2-thienyl-20 ,10 ,30 -benzothiodiazole)] (PFODBT) because of the alkoxy groups; as a result, they show

better overlap with the solar spectrum, enabling efficient photon harvesting. Interestingly, poly[2,7-(3,6-dioctyloxy-9,9-dihexylsilafluorene)-alt-5,5-(40 ,70 -di-2-thienyl-20 ,10 ,30 -benzothiadiazole)] (P4H) exhibited good solubility in spite of its high molecular weight because of its sufficiently long solubilizing groups. As a consequence, P4H:PCBM exhibited high photovoltaic performance mainly because of the high Jsc and FF resulting from favorable P4H:PCBM blend morphology. P4H exhibited a high PCE of 3.22% when PC61BM was used as the electron acceptor. Furthermore, when PC71BM was used, the PCE greatly improved to 4.05% with a Voc of 0.67 V, a Jsc of 11.1 mA cm-2 and an FF of 54.3% under 100 mW cm-2 AM1.5G. Therefore, we conclude that P4H is a promising polymer donor for light harvesting and that the PCE could be further improved by precise optimization of the fabrication procedures and device structure. Experimental Section Materials. 4,7-Dibromo-2,1,3-benzothiadiazole (M5) and 2,7-dibromo-9,9-dioctylfluorene (M7) were purchased from Aldrich and used after purification with column chromatography. All of the other chemicals were obtained from Aldrich and TCI and used without further purification. 2,5-Bis(trimethylstannyl)thiophene (M6) and 4,7-bis(5-(trimethylstannyl)thiophen2-yl)-2,1,3-benzothiadiazole (M4) were prepared following the reported procedures with modification.23 Preparation of 4,40 -Dibromobiphenyl-3,30 -diol (2). 4,40 -Diaminobiphenyl-3,30 -diol (1) (15.00 g, 69.37 mmol) was added to a mixture of hydrobromic acid (40 M, 60 mL) and water (240 mL) and acetonitrile (240 mL), and dissolved by heating the reaction mixture to 90 °C. The reaction mixture was cooled to 0 °C in an ice bath, then sodium nitrite (12.21 g, 176.9 mmol) in water (20 mL) and cuprous bromide (22.10 g, 154 mmol) in hydrobromic acid (240 mL) were added successively with vigorous stirring. The reaction mixture was warmed to room temperature and refluxed. After 12 h of reaction, the reaction mixture was washed with deionized water and extracted with chloroform. The product was obtained as a white solid by column chromatography and recrystallized twice from a dichloromethane/n-hexane mixture.

Article The final product was obtained as a white solid (20.56 g, 60.14 mmol; yield: 87%). 1H NMR (600 MHz, CDCl3, δ ppm): 7.49 (d, 2H, ArH), 7.19 (s, 2H, ArH), 6.97 (d, 2H, ArH), 5.60 (s, 2H, OH). 13C NMR (150 MHz, CDCl3, δ ppm): 152.50, 141.15, 132.30, 120.38, 114.50, 109.77. FD-MS (m/z): calcd for C12H8Br2O2, 341.89; found, 341.97. Anal. Calcd for C12H8Br2O2: C, 41.90; H, 2.34; Br, 46.46; O, 9.30. Found: C, 41.3; H, 2.30. Preparation of 4,40 -Dibromo-3,30 -bis(octyloxy)biphenyl (3). Sodium hydroxide (14.04 g, 351.0 mmol) was added to a solution of 4,40 -dibromobiphenyl-3,30 -diol (2) (20.00 g, 58.50 mmol) in ethanol (600 mL). The solution was heated to reflux and 1-bromooctane (67.41 g, 351.0 mmol) was added dropwise. After reaction for 24 h, the reaction mixture was cooled to room temperature, washed with deionized water several times and extracted with dichloromethane. Purification through column chromatography gave the product as a white solid (yield: 74%). 1 H NMR (600 MHz, CDCl3, δ ppm): 7.56 (d, 2H, ArH), 7.016.96 (m, 4H, ArH), 4.06 (t, 4H, OCH2), 1.89-1.82 (m, 4H, CH2), 1.54-1.28 (m, 20H, CH2), 0.90-0.86 (m, 6H, CH3). 13C NMR (150 MHz, CDCl3, δ ppm): 155.76, 141.18, 133.45, 120.28, 111.97, 111.86, 69.35, 31.83, 29.79, 29.29, 29.28, 29.12, 25.00, 22.65, 14.09. FD-MS (m/z): calcd for C28H40Br2O2, 566.14; found, 566.25. Anal. Calcd for C28H40Br2O2: C, 59.16; H, 7.09; Br, 28.11; O, 5.63. Found: C, 59.3; H, 7.41. Preparation of 4,40 -Dibromo-2,20 -diiodo-5,50 -bis(octyloxy)biphenyl (4). 20% H2SO4 (30 mL), KIO3 (1.66 g, 7.77 mmol), and I2 (4.93 g, 19.4 mmol) were added to a solution of 4,40 dibromo-3,30 -bis(octyloxy)biphenyl (3) (10.00 g, 17.66 mmol) in acetic acid (300 mL), and the mixture was heated to 90 °C and left for 12 h. The reaction mixture was cooled to room temperature, extracted with chloroform and washed with aqueous Na2S2O3. After evaporation of chloroform, the product (13.2 g, 16.1 mmol) was obtained as a colorless solid by purification by column chromatography (yield: 91%). 1H NMR (600 MHz, CDCl3, δ ppm): 8.03 (s, 2H, ArH), 6.70 (s, 2H, ArH), 4.01-3.95 (m, 4H, OCH2), 1.86-1.79 (m, 4H, CH2), 1.50-1.28 (m, 20H, CH2), 0.90-0.86 (m, CH3). 13C NMR (150 MHz, CDCl3, δ ppm): 155.57, 148.01, 141.94, 114.31, 112.83, 87.19, 69.47, 31.76, 29.23, 29.17, 28.88, 25.88, 22.63, 14.09. FD-MS (m/z): calcd for C28H38Br2I2O2, 817.93; found, 818.02. Anal. Calcd for C28H38Br2I2O2: C, 41.00; H, 4.67; Br, 19.48; I, 30.94; O, 3.90. Found: C, 41.3; H, 4.70. Preparation of 2,7-Dibromo-9,9-diethyl-3,6-bis(octyloxy)silafluorene (M1). 4,40 -Dibromo-2,20 -diiodo-5,50 -bis(octyloxy)biphenyl (4) (6.5 g, 7.95 mmol) was dissolved in 100 mL of THF and cooled to -100 °C. Then, 1.6 M n-butyllithium solution in hexane (9.98 mL, 16.0 mmol) was slowly added and the temperature was maintained below -100 °C for 1 h. Then dichlorodiethylsilane (1.31 g, 8.42 mmol) was added dropwise at -100 °C. After reaction for 1 h at this temperature, the reaction mixture was warmed to room temperature and stirred for 24 h. The organic layer was extracted with diethyl ether, washed with deionized water several times and dried over Na2SO4. After removal of solvent with a rotary evaporator, the product (2.62 g, 4.03 mmol) was obtained by column chromatography with n-hexane (yield: 51%). 1H NMR (600 MHz, CDCl3, δ ppm): 7.69 (s, 2H, ArH), 7.23 (s, 2H, ArH), 4.15 (t, 4H, OCH2), 1.55-1.26 (m, 20H, CH2), 0.97-0.85 (m, 16H, CH2, CH3). 13C NMR (150 MHz, CDCl3, δ ppm): 153.37, 144.70, 133.44, 126.51, 108.53, 102.09, 27.90, 25.41, 25.33, 25.28, 22.15, 18.76, 10.19, 3.54. FD-MS (m/z): calcd for C32H48Br2O2Si, 650.18; found, 650.31. Anal. Calcd for C32H48Br2O2Si: C, 58.89; H, 7.41; Br, 24.49; O, 4.90; Si, 4.30. Found: C, 58.9; H, 7.44. Preparation of 4,40 -Dibromo-3,30 -bis(hexyloxy)biphenyl (5). Compound 5 was prepared following the same procedure as was used for compound 3, using 1-bromohexane instead of 1-bromooctane. 1H NMR (600 MHz, CDCl3, δ ppm): 7.56 (d, 2H, ArH), 7.01-6.96 (m, 4H, ArH), 4.08 (t, 4H, OCH2), 1.88-1.83 (m, 4H, CH2), 1.54-1.51 (m, 4H, CH2), 1.38-1.34 (m, 8H, CH2), 0.920.90 (m, 6H, CH3). 13C NMR (150 MHz, CDCl3, δ ppm): 155.79,


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141.21, 133.48, 120.32, 112.00, 111.89, 69.38, 31.53, 29.11, 25.69, 22.60, 14.03. FD-MS (m/z): calcd for C24H32Br2O2, 510.08; found, 510.18. Anal. Calcd for C24H32Br2O2: C, 56.27; H, 6.30; Br, 31.19; O, 6.25. Found: C, 56.0; H, 6.25. Preparation of 4,40 -Dibromo-5,50 -bis(hexyloxy)-2,20 -diiodobiphenyl (6). Compound 6 was prepared following the same procedure as for compound 4 but using compound 5 instead of compound 3. 1H NMR (600 MHz, CDCl3, δ ppm): 8.03 (s, 2H, ArH), 6.71 (s, 2H, ArH), 3.99 (m, 4H, OCH2), 1.84-1.81 (m, 4H, CH2), 1.50-1.47 (m, 4H, CH2), 1.35-1.32 (m, 8H, CH2), 0.91-0.89 (m, 6H, CH3). 13C NMR (150 MHz, CDCl3, δ ppm): 155.53, 147.98, 141.90, 114.29, 112.81, 87.21, 69.44, 31.43, 28.84, 25.54, 22.52, 13.99. FD-MS (m/z): calcd for C24H30Br2I2O2, 761.87; found, 761.97. Anal. Calcd for C24H30Br2I2O2: C, 37.72; H, 3.96; Br, 20.91; I, 33.22; O, 4.19. Found: C, 37.5; H, 3.93. Preparation of 2,7-Dibromo-9,9-dihexyl-3,6-bis(hexyloxy)silafluorene (M2). Compound M2 was prepared following the same procedure as for compound M1, using dichlorodihexylsilane instead of dichlorodiethylsilane. 1H NMR (600 MHz, CDCl3, δ ppm): 7.68 (s, 2H, ArH), 7.22 (s, 2H, ArH), 4.15 (t, 4H, OCH2), 1.91-1.86 (m, 4H, CH2), 1.57-1.36 (m, 4H, CH2), 1.30-1.17 (m, 24H, CH2), 0.93-0.82 (m, 16H, CH2, CH3). 13C NMR (150 MHz, CDCl3, δ ppm): 157.21, 148.43, 137.38, 131.13, 112.46, 106.02, 69.26, 33.00, 31.54, 31.33, 29.16, 25.73, 23.79, 22.61, 22.55, 14.06, 14.03, 12.39. FD-MS (m/z): calcd for C36H56Br2O2Si, 706.24; found, 706.31. Anal. Calcd for C36H56Br2O2Si: C, 61.01; H, 7.96; Br, 22.55; O, 4.51; Si, 3.96. Found: C, 60.8; H, 7.97. Preparation of 2,7-Dibromo-9,9-dihexyl-3,6-bis(octyloxy)silafluorene (M3). Compound M3 was prepared by the same procedure as described for compound M1 using dichlorodihexylsilane. 1H NMR (600 MHz, CDCl3, δ ppm): 7.68 (s, 2H, ArH), 7.23 (s, 2H, ArH), 4.14 (t, 4H, OCH2), 1.89-1.85 (m, 4H, CH2), 1.56-1.53 (m, 4H, CH2), 1.41-1.18 (m, 32H, CH2), 0.90-0.82 (m, 16H, CH2, CH3). 13C NMR (150 MHz, CDCl3, δ ppm): 157.47, 148.55, 137.50, 131.41, 112.77, 106.42, 69.53, 32.96, 31.86, 31.38, 29.37, 29.34, 29.27, 26.15, 23.86, 23.82, 22.68, 22.55, 14.05, 13.99, 12.53. FD-MS (m/z): calcd for C40H64Br2O2Si, 762.3042; found, 762.42. Preparation of 4,7-Di(2-trimethylstannylthiophen-5-yl)-2,1,3benzothiadiazole (M4). 1.6 M n-butyllithium solution in hexane (13.27 mL, 21.24 mmol) was added to a solution of 2,2,6,6tetramethylpiperidine (3.00 g, 21.2 mmol) in THF (60 mL), at a temperature of -78 °C. Next, the reaction mixture was kept at this temperature for 1 h and then warmed to room temperature. The reaction mixture was cooled to -78 °C again and a solution of compound 7 (2.45 g, 8.17 mmol) in THF (25 mL) was added dropwise. After reaction for 1 h at -78 °C, 1.0 M SnMe3Cl solution in hexane (21.67 mL, 21.24 mmol) was added. The reaction mixture was warmed to room temperature and stirred for 24 h. The organic layer was extracted with diethyl ether, washed with deionized water several times and dried over Na2SO4. After removal of solvent using a rotary evaporator, recrystallization from ether-ethanol gave the product (2.62 g, 4.03 mmol) as orange crystals (3.82 g, yield: 74%). 1H NMR (600 MHz, CDCl3, δ ppm): 8.17 (d, 2H, ArH), 7.85 (s, 2H, ArH), 7.29 (d, 2H, ArH), 0.43 (s, 18H, CH3). 13C NMR (150 MHz, CDCl3, δ ppm): 152.65, 145.04, 140.21, 136.08, 128.38, 125.82, -8.02. FD-MS (m/z): calcd for C20H24N2S3Sn2, 627.91; found, 628.04. Anal. Calcd for C20H24N2S3Sn2: C, 38.37; H, 3.86; N, 4.47; S, 15.37; Sn, 37.92. Found: C, 38.1; H, 3.82; N, 4.97; S, 13.9. Preparation of P1 Using M2, M5, and M6 in a Mole Ratio of 0.5:0.5:1 (P1). Compound M2 (0.255 g, 0.364 mmol), compound M5 (0.106 g, 0.364 mmol), and compound M6 (0.300 g, 0.728 mmol) were dissolved in toluene (25 mL). The solution was purged with N2 for 30 min, Pd2(dba)3 (0.033 g, 0.036 mmol) and P(o-tolyl)3 (0.044 g, 0.146 mmol) were added, and then slowly heated to 100 °C. After 24 h, the reaction mixture was poured into methanol (400 mL) and the dark precipitate was collected

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by filtration. The filtrate was dissolved in chlorobenzene (300 mL), an aqueous solution of sodium diethyldithiocarbamate trihydrate (6.0 g in 200 mL of deionized water) was added, and the mixture was stirred at 90 °C. After 12 h, the organic phase was washed with deionized water several times. The solution was concentrated, precipitated into methanol and collected by filtration. The filtrate was subjected to Soxhlet extraction with methanol, acetone, hexane, and chlorobenzene. The polymer solution in chlorobenzene was concentrated and slowly dropped into methanol. The polymer was obtained as a dark solid by filtration and drying under vacuum. GPC: Mn, 76 600; Mw, 127 800; PDI, 1.67. 1H NMR (600 MHz, CDCl3, δ ppm): 8.277.85 (m, 5H, ArH), 7.72-7.30 (m, 5H, ArH), 4.36-4.15 (m, 4H, OCH2), 2.10-1.87 (m, 4H, CH2), 1.73-1.20 (m, 28H, CH2), 1.07-0.80 (m, 16H, CH2, CH3). Preparation of Poly[2,7-(3,6-dioctyloxy-9,9-diethylsilafluorene)alt-5,5-(40 ,70 -di-2-thienyl-20 ,10 ,30 -benzothiadiazole)] using M1 and M4 in a mole ratio of 1:0.95 (P2). P2 was prepared following the same procedure as used in the preparation of P1. Compound M1 (0.347 g, 0.533 mmol) and compound M4 (0.318 g, 0.507 mmol) were used as monomers. GPC: Mn, 7420; Mw, 9670; PDI, 1.30. 1H NMR (600 MHz, CDCl3, δ ppm): 8.20-8.10 (m, 2H, ArH), 8.01-7.82 (m, 4H, ArH), 7.74-7.63 (m, 2H, ArH), 7.447.30 (m, 2H, ArH), 4.36-4.13 (m, 4H, OCH2), 2.09-1.87 (m, 4H, CH2), 1.72-1.24 (m, 20H, CH2), 1.11-0.84 (m, 16H, CH2, CH3). Preparation of Poly[2,7-(3,6-dihexyloxy-9,9-dihexylsilafluorene)alt-5,5-(40 ,70 -di-2-thienyl-20 ,10 ,30 -benzothiadiazole)] Using M2 and M4 in a mole ratio of 1:1 (P3, P3H). P3 was prepared following the same procedure as used in the preparation of P1. Compound M2 (0.380 g, 0.538 mmol) and compound M4 (0.338 g, 0.538 mmol) were used as monomers. GPC: Mn; 30,700, Mw; 67,200, PDI; 2.19. High molecular weight P3H was prepared using 1.5 times as much Pd2(dba)3 and P(o-tolyl)3 as were used in the preparation of P3. GPC: Mn, 102 300; Mw, 228 900, PDI; 2.24. 1H NMR (600 MHz, CDCl3, δ ppm): 8.20-8.15 (m, 2H, ArH), 8.017.89 (m, 4H, ArH), 7.73-7.67 (m, 2H, ArH), 7.43-7.36 (m, 2H, ArH), 4.36-4.15 (m, 4H, OCH2), 2.10-1.89 (m, 4H, CH2), 1.74-1.22 (m, 28H, CH2), 1.06-0.82 (m, 16H, CH2, CH3). Preparation of Poly[2,7-(3,6-dioctyloxy-9,9-dihexylsilafluorene)alt-5,5-(40 ,70 -di-2-thienyl-20 ,10 ,30 -benzothiadiazole)] Using M3 and M4 in a Mole Ratio of 1:1 (P4, P4H). P4 was prepared following the same procedure as used in the preparation of P1. Compound M3 (0.400 g, 0.525 mmol) and compound M4 (0.323 g, 0.525 mmol) were used as monomers. GPC: Mn, 19 000; Mw, 33 900; PDI, 1.78. High molecular weight P4H was prepared using 1.5 times as much Pd2(dba)3 and P(o-tolyl)3 as were used in the preparation of P4. GPC: Mn, 63 900; Mw, 160 300; PDI, 2.51. 1H NMR (600 MHz, CDCl3, δ ppm): 8.21-8.16 (m, 2H, ArH), 8.01-7.87 (m, 4H, ArH), 7.73-7.67 (m, 2H, ArH), 7.44-7.35 (m, 2H, ArH), 4.36-4.14 (m, 4H, OCH2), 2.09-1.88 (m, 4H, CH2), 1.72-1.21 (m, 36H, CH2), 1.06-0.82 (m, 16H, CH2, CH3). Preparation of Poly[2,7-(9,9-dioctylfluorene)-alt-5,5-(40 ,70 -di2-thienyl-20 ,10 ,30 -benzothiadiazole)] Using M7 and M4 in a Mole Ratio of 1:0.95 (PFODBT). PFODBT was prepared following the same procedure as used in the preparation of P1. Compound M7 (0.134 g, 0.245 mmol) and compound M4 (0.146 g, 0.233 mmol) were used as monomers. GPC: Mn, 31 200; Mw, 93 900; PDI, 3.01. 1H NMR (600 MHz, CDCl3, δ ppm): 8.16 (br, 2H, ArH), 7.92 (br, 2H, ArH), 7.74-7.58 (m, 6H, ArH), 7.47 (br, 2H, ArH), 2.13-1.97 (m, 4H, CH2), 1.50-0.71 (m, 30H, CH2, CH3). Characterization. NMR spectra were recorded on a Bruker Ultrashield Plus 600 spectrometer. Mass spectra were obtained using a Jeol JMS-T100 AccuTOF GC mass spectrometer. The number-average molecular weight (Mn) and weight-average molecular weight (Mw) were measured using a PL XT-20 Rapid high temperature GPC (Polymer Laboratories Ltd.). Polystyrene standards were used for calibration and TCB was used as an eluent. Elemental analysis was carried out using a FlashEA 1112 CHNS analyzer (Thermo Electron Corporation). Absorption

Jin et al. spectra of polymer films were recorded on a Jasco V-530 UVvis spectrometer and the optical bandgaps of the polymers were calculated from the onset of the absorption spectra. Cyclic voltammetry (CV) measurements were conducted on a CH Instruments 608B electrochemical analyzer at a scan rate of 50 mV s-1 with a 0.05 M solution of tetrabutylammonium hexafluorophosphate (Bu4NPF6) in acetonitrile. The oxidation and reduction potentials of polymer film coated on a Pt disk were measured using a Pt wire and a Ag wire as a counter electrode and a quasireference electrode, respectively. Under these conditions, the onset of oxidation and reduction potentials of the polymer thin films against the Ag quasireference electrode were measured and calibrated against the ferrocene/ferrocenium (Fc/Fcþ) redox couple. The HOMO and LUMO levels of the polymers were calculated by assuming the absolute energy level of Fc/Fcþ as -4.80 eV to vacuum.36 Grazing incident X-ray diffraction (GIXRD) with a synchrotron X-ray beam (4CII beamline, Pohang Accelerator Laboratory) was used to investigate the crystallographic structure of polymer thin film coated on the PEDOT:PSS. The wavelength of the X-rays was 1.3807 A˚, and the incident angle was fixed at 0.2°. Device Fabrication and Measurement. The patterned ITO glass substrates were cleaned by ultrasonication in acetone, methanol and deionized water, after which they were dried by N2 blowing and dry cleaned using UV-ozone for 10 min. The PEDOT:PSS (AI4083, H. C. Stack) was spin-coated on the cleaned ITO glass at a rate of 2,500 rpm and then the substrates were baked at 140 °C for 30 min in a N2 glovebox. The polymer solutions (0.7-1.5 wt %) were prepared by dissolving in o-DCB with mild heating (70 °C) for 24 h, PCBM was added to afford polymer:PCBM blend solutions and mixed further for 12 h. The polymer:PCBM solutions were spin-coated at a rate in the range of 700 to 2,200 rpm and the spin-coated films were dried at 70 to 90 °C for 30 min in a N2 glovebox. The thickness of the active layer was measured using an Alpha-Step IQ profilometer (KLATencor). The spin-coated substrates were transferred to a vacuum evaporation chamber for cathode deposition. Al (150 nm) was deposited on the active layer by thermal evaporation at a pressure below 1.0 10-6 Torr and the resulting devices had an active area of 0.102 cm2. Before measuring the photovoltaic performance, the devices were annealed at 140 °C for 5 min and the current-voltage behavior was recorded on a Keithley 236 Source Measurement unit under simulated 100 mW cm-2 light. A solar simulator (Newport) with a 300 W xenon lamp and AM1.5G air mass filter was used to evaluate the performance of the devices and the light intensity was carefully calibrated using a silicon photovoltaic reference cell (Bunkou Keiki Co., BS520). EQE was measured using a Solar Cell Spectral Response IPCE Measurement System (PV Measurements, Inc.). For SCLC hole-mobility measurements of the polymers, the devices (ITO/PEDOT:PSS/Polymer/Al) were fabricated following the same method and their current-voltage behavior was recorded using a Keithley 236 Source Measurement unit.36

Acknowledgment. This research was supported by the EEWS (Energy, Environment, Water, and Sustainability) program of KAIST and the WCU (World Class University) program through the Korea Science and Engineering Foundation funded by the Ministry of Education, Science and Technology (R32-2008-00010142-0). References and Notes (1) Mayer, A. C.; Scully, S. R.; Hardin, B. E.; Rowell, M. W.; McGehee, M. D. Mater Today 2007, 10, 28. (2) Brabec, C. J.; Durrant, J. R. MRS Bull. 2008, 33, 670. (3) Thompson, B. C.; Frechet, J. M. J. Angew. Chem., Int. Ed. 2008, 47, 58. (4) Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv. Mater. 2009, 21, 1323.

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Synthesis and Photovoltaic Performance of Low-Bandgap Polymers

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