High Open Circuit Voltage Solution-Processed Tandem Organic

Article pubs.acs.org/cm

High Open Circuit Voltage Solution-Processed Tandem Organic Photovoltaic Cells Employing a Bottom Cell Using a New Medium Band Gap Semiconducting Polymer Ji-Hoon Kim,† Chang Eun Song,‡ Hee Un Kim,† Andrew C. Grimsdale,§ Sang-Jin Moon,∥ Won Suk Shin,∥ Si Kyung Choi,*,‡ and Do-Hoon Hwang*,† †

Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan, 609-735, Korea Department of Materials Science and Engineering KAIST, Daejeon, 305-701, Korea § Energy Research Institute @ NTU (ERI@N), Nanyang Technological University, 50 Nanyang Drive, 637553, Singapore ∥ Energy Materials Research Center, Korea Research Institute of Chemical Technology, Daejeon 305-343, Korea ⊥ Korea Research Institute of Chemical Technology, 100 Jang-dong, Yuseong-gu, Daejeon, 305-343, Korea ‡

S Supporting Information *

ABSTRACT: Two donor−acceptor (D−A) copolymers, based on the donor unit TIPS substituted benzodithiophene (TIPSBDT) and the acceptor quinoxaline-based units with or without fluorine substitution (PTIPSBDTDTQX and PTIPSBDT-DFDTQX), were designed and synthesized as a donor material for bulk-heterojunction (BHJ) photovoltaic cells. The introduction of F atoms with high electron affinity to be quinoxailine moieties is effective in further lowering both the HOMO and LUMO energy levels of PTIPSBDTDFDTQX to attain higher open-circuit voltage (Voc). Single junction photovoltaic cells were fabricated, and the polymers:PC71BM active layer morphology was optimized by adding 1,8-diiodooctane (DIO) as an additive. In a single layer photovoltaic device, they showed power conversion efficiencies (PCEs) of 2−6%. The solution process inverted tandem photovoltaic cells, in which two photovoltaic cells with different absorption characteristics are linked to use a wider range of the solar spectrum, were fabricated with each layer processed from solution with the use of BHJ materials comprising semiconducting polymers and fullerene derivatives. We first report here on the design of PTIPSBDT-DFDTQX equivalent poly(3-hexylthiophene), the current medium band gap polymer of choice, which thus is a viable candidate for use in the highly efficient bottom layer in inverted tandem cells. KEYWORDS: organic photovoltaic cells, medium band gap polymer, inverted tandem polymer solar cells

1. INTRODUCTION Bulk heterojunction (BHJ) organic photovoltaic cells (OPVs) based on conjugated oligomers and polymers as electron donating materials have drawn great attention as a promising technology for renewable energy in recent years, because of their advantages of a simple device structure, low cost, light weight, and easy large area fabrication on flexible substrates.1 The key aim of research into OPVs is increasing the performance of the devices, in particular the power conversion efficiency (PCE). While organic devices are unlikely to ever match the performance of commercially available silicon-based systems, which is around 15−20%, their lower production costs would make them potentially viable commercially at lower efficiencies of around 10%, which current theoretical models suggest is attainable.1b Efforts to achieve this include optimizing all aspects of the devices including device architecture, but for synthetic chemists the emphasis is on developing new acceptor materials2 and new conjugated polymer donors.3 Currently, most research is focused on the design and synthesis of © XXXX American Chemical Society

polymers with a low band gap (1.3−1.5 eV) to enable efficient absorption over most of the solar spectrum, with other factors to be considered in molecular design being a deep highest occupied molecular orbital (HOMO; −5.4 eV or lower) to roduce a high open circuit voltage (Voc) and improve stability toward oxidation, a deep lowest unoccupied molecular orbital (LUMO) to enable efficient donation to the electron acceptor (ca. −3.9 eV is suitable for donation to fullerene acceptors such as PCBM). Such polymers typically display strong light absorption spectra up to 600−900 nm and usually afford an enhanced short-circuit current density (Jsc). However, medium band gap polymers (1.7−2.0 eV band gap), which have been somewhat overlooked recently, can still play a major role in developing efficient OPVs. For example, a medium band gap polymer usually produces a higher open circuit voltage (Voc) Received: May 8, 2013 Revised: June 7, 2013

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dx.doi.org/10.1021/cm401527b | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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Scheme 1. Synthesis of Polymers PTIPSBDT-DTQX and PTIPSBDT-DFDTQX

cell requires either thermal or solvent annealing to reach maximum performance, a time-consuming process which is not conductive to roll-to-roll high throughput manufacturing. The best PCE of tandem cells employing the P3HT:PCBM bottom cell was 6.7% with a V oc of 0.63 V. 4b Recently the P3HT:IC60BA active system was used as a bottom cell component to increase the Voc of the tandem cell. The PCE of the tandem cell using the P3HT:IC60BA bottom cell reached 9.5% with a Voc of 0.84 V.7b In this regard, development of medium band gap polymers with high photovoltaic efficiency would be desirable in addition to high performance low band gap polymers. In this study, we report a new medium band gap polymer design, and its application both in high efficiency single junction BHJ OPVs and in new medium band gap polymerbased BHJ tandem OPVs, with both active layers processed from solution. Polymers containing the benzo[1,2-b:4,5-b′]dithiophene unit (BDT) have exhibited importance as a class of highly soluble, air-stable, high-performance p-type organic semiconductors.9 Li and co-workers reported a low band gap conjugated polymer, PBDTTT-TIPS, composed of alternating BDT units substituted with triisopropylsilyl (TIPS) groups which showed significant potential in OPVs, with enhanced mobilities, a high Voc value, and PCEs surpassing 4.33%, attributable to efficient cofacial π−π stacking.10,11 In this study, we synthesized a TIPS-substituted BDT (TIPSBDT) unit as the electron-donating part in a conjugated polymer. Among numerous electron-withdrawing groups, the use of fluorine has been proven to be effective in lowering the HOMO energy level and resulting in a higher Voc and improved performance in polymers containing fluorinated thienothiophenes,8,13 benzothiadiazoles,12 and benzotriazole.14 Chou and co-workers recently reported a new medium band gap polymer, PBDTTFQ, composed of 4,8-dialkyloxy-substituted BDT and a fluorinated quinoxaline derivative, and the single junction OPVs fabricated using the polymer showed a significantly high PCE of

than a low band gap polymer, because of its deeper HOMO energy level. Moreover, broad band gap polymers also have an important application in tandem OPVs, which involve two (or more) stacked cells with different active layers, each absorbing different parts of the solar spectrum.4 Typically, for a doublejunction cell, such a tandem structure consists of a bottom cell with a medium band gap material, an interconnecting layer, and a top cell with a low band gap material. This multijunction/ tandem structure can reduce the thermal loss of photonic energy during the photon-to-electron conversion process. It also helps to maximize the Voc, as the tandem cell voltage is determined by the sum of the individual cell voltages, and the medium band gap material in the bottom cell, which is responsible for the absorption of high-energy photons, provides a higher Voc than the low band gap material. Therefore, by adopting polymers with matched absorption spectra, a tandem photovoltaic cell can effectively utilize the photonic energy and optimize Voc, which leads to a higher PCE than can be obtained from either individual cell. From theoretical modeling, it has been calculated that a tandem cell with a cell using a medium band gap material of 1.7−1.75 eV and a cell using a low band gap material of 1.1−1.5 eV might attain a PCE of 14%.1b To date, the most commonly used medium band gap material in such cells has been regioregular poly(3-hexylthiophene) (P3HT). P3HT has been the most widely studied of all OPV materials, especially in conjunction with fullerene acceptors such as PCBM.5 The PCE of P3HT based devices is typically in the range of 3−4% when it is blended with PCBM, but it can be improved to over 5% by device modifications or to 7% by using other fullerene derivatives such as IC60BA.6 By contrast many single cells using other donors have been reported with efficiencies of over 7%,1c,7 with the best cells reportedly being well over 8%.8 Currently, most tandem OPVs utilize P3HT as the component to harvest solar radiation from 300 to 600 nm. However, this cell suffers from a low Voc of only 0.6 V due to its shallow HOMO energy level. Second, the P3HT-based BHJ B



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8.0%.15 Herein, we synthesized two medium band gap conjugated polymers composed of TIPSBDT and quinoxaline (DTQX) or fluorinated quinoxaline (DFDTQX) derivatives. We expected that the TIPSBDT unit could further lower the HOMO energy level of the resulting polymer by combination with the fluorinated quinoxaline acceptor part. PTIPSBDTDTQX ({poly{4,8-bis(triisopropylsilylethynyl)benzo[1,2-b:4,5b′]dithiophene-alt-5,5-(5′,8′-di-2-thienyl-2,3-bis(4-octyloxyl)phenyl)quinoxaline}) and PTIPSBDT-DFDTQX (poly{4,8bis(triisopropylsilylethynyl)benzo[1,2-b:4,5-b′]dithiophene-alt6,7-difluoro-5,5-(5′,8′-di-2-thienyl-2,3-bis(4-octyloxyl)phenyl)quinoxaline}) were synthesized as medium band gap conjugated polymers for OPVs. These alternating copolymers, PTIPSBDT-DTQX and PTIPSBDT-DFDTQX, show excellent photovoltaic performance in single junction devices with PCEs over 6%. When the high Voc material PTIPSBDT-DFDTQX was used as an active layer in the bottom cell of solutionprocessed inverted tandem photovoltaic cells, PCEs of over 7.4% were achieved. The chemical structures of PTIPSBDTDTQX and PTIPSBDT-DFDTQX are shown in Scheme 1. (Synthetic routes to the monomers and polymers are shown in Schemes S1 and S2.)

Table 1. Average Molecular Weights and Thermal Properties of the Synthesized Polymers polymer PTIPSBDTDTQX PTIPSBDTDFDTQX

polymer yield (%)

Mna (g/mol)

Mwa (g/mol)

PDIa

Tdb (°C)

55

30 000

80 000

2.7

397

54

21 000

42 000

1.9

432

a Mn, Mw, and PDI of the polymers were determined by gel permeation chromatography using polystyrene standards in CHCl3. bTemperature at 5% weight loss at a heating rate of 10 °C/min under nitrogen.

showed their maximum absorptions at 349, 414, and 541 nm and 348, 419, and 548 nm, respectively, whereas those of the corresponding polymer films moved to the longer wavelength region and showed maximum absorptions at 356 and 575 nm and 350 and 582 nm, respectively. The red shift in the absorption spectra of the polymer films (Figure 1b) compared to the solution spectra (Figure 1a) is a commonly observed phenomenon in conjugated systems due to aggregation of the polymer main chains and interchain interactions in the solid film. In particular, there are shoulder peaks in the long wavelength region of the PTIPSBDT-DFDTQX film absorption spectra, indicating that there exist some ordered structures in the polymer films, which should benefit the charge mobility and photovoltaic performance of the polymer. 17 The absorption of the PTIPSBDT-DFDTQX film (∼45 nm) is only red-shifted a little compared to that of its solution, which reveals that either strong aggregation exists in the solution for the fluorinated polymer or reduced interactions exist in the solid state (Figure S4). As compared to the absorption spectrum of P3HT (Eg ∼ 1.9 eV, Figure 1b), which is the most commonly used bottom layer material in tandem cells, the absorption in the short wavelength region of the spectrum from 350 to 450 nm is much higher, owing to absorption by the TIPSBDT units above 450 nm, making these materials much better at harvesting high energy photons. Optical band gap energies (Egopt) of the polymer thin films were determined for PTIPSBDT-DTQX and PTIPSBDT-DFDTQX by measuring their UV−visible absorption onsets in thin films to be 1.81 and 1.85 eV, respectively. To compare the relative absorption strength of the PTIPSBDT-DTQX and PTIPSBDT-DFDTQX, we measured the absorption coefficients of the two polymer films. The measured absorption coefficient of PTIPSBDTDFDTQX (8.5 × 104 cm−1) was higher than that of PTIPSBDT-DTQX (5.6 × 104 cm−1), as shown in Figure 1c. This result is consistent with that of the previously reported similar polymer system.14 2.3. Electrochemical Properties. The HOMO and LUMO energy levels of PTIPSBDT-DTQX and PTIPSBDTDFDTQX were determined by cyclic voltammetry (CV) with a platinum plate as the counterelectrode and Ag/Ag+ as a reference electrode in anhydrous acetonitrile with 0.1 M tetrabutylammonium tetrafluoroborate (TBABF4) at a scan rate of 50 mV/s.18a The HOMO energy values were calculated using the equation EHOMO = −(Eonset,ox + 4.71) eV, where Eonset,ox is the onset oxidation potential versus Ag/Ag+. The reduction potential of PC71BM was also measured by CV. The measured LUMO energy level of PC71BM was −3.90 eV. The cyclic voltammograms of the polymer films and PC71BM solution are shown in the Supporting Information. The polymer structure and corresponding energy levels are shown in Figure 1d. The onset of oxidation for PTIPSBDT-DTQX

2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterization of the Polymers. The two polymers PTIPSBDT-DTQX and PTIPSBDTDFDTQX were synthesized by Stille cross coupling polymerization of the BDT-bistin with the dibromoquinoxalines in the presence of a Pd(PPh3)4 catalyst. While Stille coupling often gives lower molar masses than Suzuki coupling it was used in this case because of the potential lability of the silyl groups under the strongly basic Suzuki coupling conditions. All of the polymers display good solubility in common organic solvents such as chloroform, chlorobenzene, and toluene and formed uniform thin films by spin-casting. Molecular weights and the polydispersity index (PDI) of the polymers were determined by gel permeation chromatography (GPC) analysis with calibration against polystyrene standards. The number-average molecular weights (M n ) of PTIPSBDT-DTQX and PTIPSBDT-DFDTQX were found to be 30 000 and 21 000 g/mol, respectively, with polydispersity indices (PDI) of 2.7 and 1.9. These molar masses, while satisfactory, may be capable of improvement by techniques such as the use of microwaves.16 High molar masses and low polydispersity are generally thought to improve OPV performance by enhancing charge transport. The thermal stability of the polymers was investigated by thermogravimetric analysis (TGA; see Figure. S1). The TGA analysis reveals that the onset temperatures for 5% weight loss (Td) of PTIPSBDT-DTQX and PTIPSBDT-DFDTQX are 397 and 432 °C, respectively. The molecular weights, PDI, and Td of the polymers are summarized in Table 1. 2.2. Optical Properties. Figure 1a and b show the UV− visible absorption spectra of PTIPSBDT-DTQX and PTIPSBDT-DFDTQX in dilute chloroform solution and as thin films, respectively. The absorption spectra of the polymers showed two major absorption bands. The peaks seen in the shorter wavelength region below 500 nm could be attributed to the π−π transition in the conjugated main chains, and the second absorption band in the longer wavelength region at around 570−700 nm can be attributed to the strong internal charge-transfer (ICT) interaction between the electrondonating and electron-accepting units in the polymers. The PTIPSBDT-DTQX and PTIPSBDT-DFDTQX solutions C



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Figure 1. UV−visible absorption spectra of (a) the polymers in chloroform solution, (b) the polymer and P3HT thin films, and (c) the absorption coefficients of the polymer thin films. (d) Energy band diagram of the polymers, P3HT, PC71BM, ITO, and Al electrodes.

and PTIPSBDT-DFDTQX occurred at 0.77 and 0.91 V, corresponding to ionization potential values of −5.48 and −5.62 eV, respectively. The fluorinated polymer (PTIPSBDTDFDTQX) exhibits a relatively low-lying HOMO energy level of −5.62 eV, which is in an ideal range to ensure better stability and can be anticipated to produce a higher Voc, in accordance with the known linear correlation of Voc with the difference between the HOMO energy level of the donor polymer and the LUMO energy level of an acceptor.18b Indeed, the introduction of electronegative F atoms lowers both the HOMO and LUMO energy levels in these quinoxaline-based polymers, the result of which agrees with the previous reports on the effect of fluorination on other acceptor units.8,14 In addition, both materials display HOMO energy levels at least 0.5 eV lower than that of P3HT (−5.1 eV), implying that a higher Voc could be obtained than that of the P3HT-based devices (∼0.6 V). The UV−visible absorption properties, optical band gaps, and HOMO/LUMO energy levels of the polymers are summarized in Table 2. 2.4. Organic Thin Film Transistors (OTFTs) Characteristics of the Polymer Thin Films. To measure the field-effect hole mobility of the polymers, organic thin film transistors

(OTFTs) were fabricated on a silicon wafer using a bottom contact geometry (channel length L = 12 μm and width W = 120 μm) under a nitrogen atmosphere. The device fabrication process is described in detail in the Experimental Section. The high charge carrier mobility (at least 10−3 cm2 V−1 s−1) in the active polymer is considered necessary to reduce the photocurrent loss and obtain high performance in OPV devices.19 Figure 2 shows the transfer curves for the polymers. The TFTs

Table 2. Optical and Electrochemical Properties of the Synthesized Polymers λmax (nm)

λedge (nm)

b

film

film

349, 414, 356, 541 575 348, 419, 350, 548 582

λmax,abs (nm)

polymers PTIPSBDTDTQX PTIPSBDTDFDTQX

solution

a

optical Egopt (eV)c

HOMO (eV)

LUMO (eV)

684

1.81

−5.48

−3.66

669

1.85

−5.62

−3.75

b

1 × 10−5 M in anhydrous chloroform. bPolymer film spin-cast on a quartz plate from a solution in chloroform at 1500 rpm for 30 s. c Calculated from the absorption band edge of the copolymer films, Eg = 1240/λedge a

Figure 2. Transfer characteristics of OTFTs fabricated using the copolymers as the active layer at a constant source-drain voltage of −80 V. (a) PTIPSBDT-DTQX and (b) PTIPSBDT-DFDTQX. D



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Figure 3. (a) J−V characteristics of polymer/PC71BM (1:2, w/w) with and without DIO as a processing additive, (b) when using different IFL regular single junction OPVs under AM 1.5G illumination (100 mW/cm2). (c) J−V characteristics of polymer/PC71BM (1:2, w/w with DIO) when using different IFL regular single junction OPVs in the dark and (d) EQEs of the corresponding devices.

Table 3. Comparison of the Photovoltaic Properties of the OPVs Based on Polymer:PC71BM without or with a DIO Additive and Different Top Electrode under the Illumination of AM 1.5 G, 100 mW cm−2 cell type

polymers PTIPSBDT- DTQX

regular single cell

PTIPSBDTDFDTQX

D/A ratio

top electrode

DIO [3 vol %]

1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2

Al Al Ca/Al PFN/Al Al Al Ca/Al PFN/Al

no yes yes yes no yes yes yes

Voc [V]a 0.72 0.77 0.81 0.79 0.89 0.88 0.91 0.91

± ± ± ± ± ± ± ±

0.002 0.004 0.008 0.004 0.008 0.01 0.08 0.01

Jsc [mA/cm2]a 3.63 3.81 4.74 4.67 5.49 10.03 10.18 10.61

± ± ± ± ± ± ± ±

0.002 0.009 0.005 0.110 0.04 0.07 0.06 0.04

FFa 0.30 0.30 0.42 0.42 0.37 0.50 0.54 0.62

± ± ± ± ± ± ± ±

0.002 0.005 0.005 0.005 0.002 0.007 0.012 0.009

PCE [%]a 0.79 0.90 1.55 1.56 1.84 4.52 5.05 6.08

± ± ± ± ± ± ± ±

0.008 (0.80) 0.04 (0.94) 0.04 (1.59) 0.04 (1.60) 0.02 (1.87) 0.03 (4.55) 0.07 (5.12) 0.04 (6.12)

a

Photovoltaic properties of copolymers/PC71BM-based devices spin-coated from a chloroform solution for polymers. Only the optimized recipes were considered for the estimation of the average PCE. Data have been averaged on 10 devices. The performance of the best device is given in parentheses.

2.5. Regular Single Layer BHJ OPVs Performance and Film Morphology. To demonstrate the potential application of the two conjugated polymers PTIPSBDT-DTQX and PTIPSBDT-DFDTQX in OPVs, we used PC71BM as an electron acceptor, and device configurations of ITO/ PEDOT:PSS/polymer:PC71BM/IFL/Al, where the interfacial layer (IFL) was variously Ca, PFN, or TiOx. The device fabrication process is described in detail in the Experimental Section. The performance of OPVs was strongly affected by the processing parameters, such as the choice of solvent, blend ratio of the polymer and PC71BM, the use of additives, and the choice of the interfacial layer. We investigated the performance of the OPV materials under a variety of conditions (see the Supporting Information). The active layers were spin-coated from a chloroform solution of the donor polymers and acceptor. 1,8-Diiodooctane (DIO, 3 vol %) was used as a processing additive to optimize the morphology of the active layer. The fabricated devices showed the highest short-circuit

of the polymers were found to exhibit typical p-channel TFT characteristics. The TFT mobilities were calculated in the saturation region using the following equation: Ids = (WC i /2L)μ(Vgs − Vth)2

where Ids is the drain-source current in the saturated region, W and L are the channel width and length, respectively, μ is the field-effect mobility, Ci is the capacitance per unit area of the insulating layer (SiO2, 300 nm), and Vgs and Vth are the gate and threshold voltages, respectively.20 The OTFTs fabricated using PTIPSBDT-DTQX and PTIPSBDT-DFDTQX showed hole mobilities of 6.0 × 10−5 and 1.2 × 10−3 cm2 V−1 s−1, respectively. The higher mobility measured for PTIPSBDTDFDTQX compared with PTIPSBDT-DTQX was understood to result from the higher degree of order between the polymer chains and better π−π stacking in the former, as previously inferred from the UV−visible results.21 E



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currents and power-conversion efficiencies at a donor to accept composition ratio of 1:2. Figure 3a and b show the current−voltage (J−V) characteristics of the OPVs based on the copolymer/PC71BM (1:2 w/w) variously with and without a DIO processing additive, without an interfacial layer, with Ca, or with PFN, under illumination of AM 1.5G, 100 mW/cm2, and the photovoltaic parameters of the devices are summarized in Table 3. The devices prepared from polymer:PC71BM blends (without IFL) of PTIPSBDTDTQX and PTIPSBDT-DFDTQX exhibited open-circuit voltage (Voc) of 0.73 and 0.90 V, respectively, the values being related to the difference between the HOMO energy level of the polymer and the LUMO energy level of PCBM. We infer that the fluorinated polymer PTIPSBDT-DFDTQX (−5.62 eV) exhibited a Voc 0.2 V higher than that of PTIPSBDT-DTQX (−5.48 eV), because of its lower-lying HOMO energy level. The short-circuit current density (Jsc) of the devices incorporating PTIPSBDT-DTQX and PTIPSBDTDFDTQX were 3.65 and 5.48 mA/cm2 , respectively. Furthermore, when we incorporated 3 vol % DIO into the 1:2 (w/w) polymer:PC71BM blends, the devices based on PTIPSBDT-DTQX and PTIPSBDT-DFDTQX exhibited slightly increased values of Jsc of 3.89 and 10.03 mA/cm2, respectively, resulting in increased PCEs. The Jsc of OPV is affected by many factors, including the absorption strength of the active layer and the charge carrier mobility. These results agreed well with the OTFT measurements, so that as could be expected from the hole mobilities of the polymers (6.0 × 10−5 cm2 V−1 s−1 for PTIPSBDT-DTQX and 1.2 × 10−3 cm2 V−1 s−1 for PTIPSBDT-DFDTQX) and the stronger absorption peaks of PTIPSBDT-DFDTQX compared to PTIPSBDT-DTQX, the fluorinated polymer PTIPSBDT-DFDTQX exhibited a higher Jsc than the PTIPSBDT-DTQX. The higher mobility of PTIPSBDT-DFDTQX is expected to contribute, in part, not just to the higher photocurrent but also to higher FFs and overall better device performance. To evaluate the photoresponse of PTIPSBDT-DFDTQX and calibrate the Jsc data, external quantum efficiencies (EQE) of the devices using DIO were measured. (Figure 3d). The device showed a relatively high photoconversion efficiency over the whole wavelength range of 300−700 nm, with monochromatic EQE values around 60%. The Jsc calculated by integrating the EQE curve with an AM 1.5G reference spectrum is within 5% error compared to the corresponding Jsc obtained from the J−V curve. The PCEs improved from 0.81 to 0.93% for the PTIPSBDTDTQX:PC71BM (1:2) device and from 1.87 to 4.55% for the PTIPSBDT-DFDTQX:PC71BM (1:2) device upon the addition of DIO, suggesting that the additive had a much greater influence on the ordering of the latter than the former. Confirmation of the morphology of the polymer/PC71BM active layers was also investigated by transmission electron microscopy (TEM) for the blend films prepared with and without the DIO additive. The dark regions in the TEM images as shown in Figure 4 confirm the presence of large PC71BM domains with sizes far larger than typical exciton diffusion lengths in polymers (∼10 nm). Consequently, poor exciton dissociation and low current density are expected. In contrast, significantly more homogeneous morphologies are found for PTIPSBDT-DTQX/PC71BM and PTIPSBDT-DFDTQX/ PC71BM films processed with DIO. As can be seen in Figure 4 (a+3% DIO, c+3% DIO), the PTIPSBDT-DFDTQX/ PC71BM (1:2) films showed a homogeneous and more

Figure 4. TEM images of polymer/PC71BM blends (1:2 weight ratio) cast from chloroform or chloroform with 3 vol % DIO. (a) PTIPSBDT-DTQX/PC71BM (1:2) without DIO (100 nm scale); (b) PTIPSBDT-DTQX/PC71BM (1:2) without DIO (200 nm scale); (a+3% DIO) PTIPSBDT-DTQX/PC71BM (1:2) with DIO (100 nm scale); (b+3% DIO) PTIPSBDT-DTBTz/PC71BM (1:2) with DIO (200 nm scale); (c) PTIPSBDT-DFDTQX/PC71BM (1:2) without DIO (100 nm scale); (d) PTIPSBDT-DTQX/PC71BM (1:2) without DIO (200 nm scale); (c+3% DIO) PTIPSBDT-DFDTQX/PC71BM (1:2) with DIO (100 nm scale); (d+3% DIO) PTIPSBDTDFDTBTz/PC71BM (1:2) with DIO (200 nm scale).

interconnected network of polymer-PC71BM domains than the corresponding PTIPSBDT-DTQX/PC71BM (1:2) active layers. By contrast, a relatively high level of aggregation was observed in the PTIPSBDT-DTQX/PC71BM (1:2) film, which reduced the area for charge separation, leading to a low photocurrent. Interestingly, as can be seen in Figure 4 (d+3% DIO), the formation of ordering has been markedly improved in PTIPSBDT-DFDTQX/PC71BM. It is evident that the DIO additive has promoted the demixing of the polymer and PCBM and has induced the formation of nanofibrillar structures, leading to the final morphology as shown in Figure 4 (d+3% DIO). These results agreed well with the trends in Jsc and FF for the OPV measurements. The well-ordered nanofibrillar morphology of PTIPSBDT-DFDTQX should result in good charge carrier transporting characteristics, so the higher mobility of PTIPSBDT-DFDTQX is expected to result in higher FFs and better device performance.22 Previous studies have reported improvements in the photovoltaic performance of polymer-OPVs upon introduction of various IFL materials. Typically, we made two different types of photovoltaic devices with IFLs with structures ITO/ PEDOT:PSS/polymer:PC 7 1 BM/Ca/Al and ITO/PEDOT:PSS/polymer:PC71BM/PFN/Al. The active layers of the device using PFN were spin coated from chloroform solution, while the PFN interfacial layer was spin coated from a methanol solution in which the active layers are only sparingly soluble. The device PTIPSBDT-DTQX:PC71BM (1:2 with F



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DIO) with a Ca/Al cathode showed a PCE of 1.59%, a Jsc of 4.78 mA/cm2, a Voc of 0.82 V, and an FF of 0.40, while PTIPSBDT-DFDTQX:PC71BM (1:2 with DIO) with a Ca/Al cathode displayed a high PCE of 5.12%, a Jsc of 10.25 mA/cm2, a Voc of 0.91 V, and an FF of 0.55. The PFN IFL devices fabricated using PTIPSBDT-DFDTQX showed increased Jsc and FF. PTIPSBDT-DTQX:PC71BM (1:2 with DIO) with PFN/Al bilayer cathode exhibited a PCE of 1.60%, a Jsc of 4.76 mA/cm2, a Voc of 0.80 V, and an FF of 0.42, while PTIPSBDTDFDTQX:PC71BM (1:2 with DIO) with the PFN/Al bilayer cathode exhibited a PCE of 6.12%, a Jsc of 10.64 mA/cm2, a Voc of 0.91 V, and an FF of 0.63. Figure 3c shows the current−voltage (J−V) characteristics of the regular single junction OPVs based on the PTIPSBDTDFDTQX:PC71BM (1:2 w/w with DIO) when using different IFLs in the dark. In comparison with those with only Al cathodes, a bilayer cathode, either a Ca/Al cathode or a PFN/ Al, can obviously improve Jsc and FF. This result could be explained by the measured shunt resistance (Rsh) and series resistance (Rs) values of the devices under dark conditions. The measured Rsh values of the Al, Ca/Al, and PFN/Al cathode devices were 6.42 × 104, 2.52 × 103, and 1.90 × 106 Ω cm2, respectively. On the other hand, the Rs of the PFN/Al cathode device (2.38 Ω cm2) was much smaller than of that of the Al cathode device (13.41 Ω cm2). These higher Rsh and lower Rs values caused the higher Jsc and FF of the PFN/Al cathode compared to the devices with an Al cathode. This result could be explained by the PFN establishing better interfacial contacts by decreasing the series resistance, resulting in enhanced electron collection at the cathode and decreasing the possibility of hole−electron recombination in the active layer.23 The photovoltaic performance of the OPVs (devices using different solvent, additive, and/or TiOx) is summarized in the Supporting Information. 2.6. Inverted Tandem Photovoltaic Cells Performance. The versatile photovoltaic applications of PTIPSBDTDFDTQX are demonstrated by its use in solution-processed inverted tandem photovoltaic cells. As mentioned above, the tandem photovoltaic cell architecture, which consists of a bottom cell with a medium band gap material, charge recombination layer, and a top cell with a low band gap material, is an effective way to harvest a broader part of the solar spectrum and make more efficient use of the photonic energy than the single junction structure.4 We now first report here a highly efficient solution-processed inverted tandem cell device fabricated using PTIPSBDT-DFDTQX in place of P3HT as the bottom cell donor material. Figure 6a shows the UV−visible absorption spectra of PTIPSBDT-DFDTQX and poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5b′]dithiophene-alt-4,7-((2-ethylhexyl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyrid-ine] (PBPT-8) in chloroform solution and in the solid state. Recently, Ma and co-workers reported the low bandgap polymer PBPT-8.24 Although PBPT-8 exhibited a low band gap of 1.56 eV and broad light absorption in the range 300−800 nm, conventional OPVs using it showed only a moderate PCE of around 3.5%. Significantly higher PCEs of over 5% were obtained from inverted OPVs using PBPT-8/PC71BM. Accordingly, in this study, we used PBPT-8 as the top cell low band gap material in solution-processed inverted tandem OPVs. The overlap of the absorption spectra of the low band gap polymer (PBPT-8) and PTIPSBDTDFDTQX is small, and these materials cover the solar spectrum from 350 to 800 nm complementarily, indicating a good match

for the tandem structure. As shown in Figure 5a and b, the organic tandem photovoltaic cells fabricated for this study have

Figure 5. Inverted tandem photovoltaic device. (a) Device structure of the inverted tandem photovoltaic device. (b) Chemical structures of PTIPSBDT-DFDTQX, PC71BM, and low band gap polymer (PBPT8).

an inverted architecture with a conducting polymer layer of PEDOT:PSS, coated with ethoxylated polyethlyenimine (PEIE) as the charge recombination layer.25 A PTIPSBDTDFDTQX:PC71BM (1:2 weight ratio with 3 vol % DIO) blend and PBPT-8:PC71BM (1:2 weight ratio with 3 vol % DIO) blend were employed as the bottom and the top photoactive layers, respectively. ITO coated with PEIE was used as the electron-collecting electrode of the bottom cell. Starting with the deposition of the first PEIE layer, the bottom photoactive layer, the recombination bilayer, and the top photoactive layer were all spin-coated sequentially from solution, to produce a device structure of ITO/PEIE/PTIPSBDTDFDTQX:PC71BM/PEDOT:PSS/PBPT-8:PC71BM/MoO3/ Ag. The performance of the solution-processed inverted tandem OPVs was strongly affected by the processing parameters, such as the choice of annealing temperature, thickness of bottom or top cells, and different electron acceptor (PC61BM or PC71BM). We investigated the performance of OPVs fabricated under a variety of conditions (see the Supporting Information). The highest power conversion efficiency was observed in the optimized device fabricated with 90 and 80 nm thickness of the active layer in the bottom and top cells respectively, an annealing temperature of 80 °C, and PC71BM as the electron acceptor in both bottom and top cells. The device fabrication and testing process is described in detail in the Experimental Section. The J−V characteristics of the tandem cells and the performance parameters are shown in Figure 6b. The tandem device with a PTIPSBDT-DFDTQXbased bottom layer gave a PCE of 7.40%, with a Voc of 1.52 V, a Jsc of 7.80 mA/cm2, and a FF of 0.62. All of the devices showed a Voc of 1.52 V, which is equal to the sum of the bottom cell and top cell; in addition, a high Jsc of 7.80 mA/cm2 and a FF above 0.62 were achieved, leading to high PCEs for the inverted tandem devices. The tandem cell shows a higher Voc and FF than the average for either of the individual cells, which is in accordance with the results from other tandem cells. It should G



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Figure 6. (a) UV−visible spectra of the donor of bottom layer and donor of top layer. (b) J−V characteristics of the tandem photovoltaic cells used in the PTIPSBDT-DFDTQX-based bottom layer.

Figure 7. Photovoltaic parameters of the inverted tandem photovoltaic devices under AM 1.5G solar illumination. (a) Open-circuit voltage, (b) short-circuit current density, (c) fill factor, and (d) power conversion efficiency as a function of average values.

Table 4. Characteristics of PTIPSBDT-DFDTQX-Based BHJ Photovoltaic Cells (Inverted Single and Tandem) under the Illumination of AM 1.5G, 100mW cm−2 cell inverted tandem cells

top cell bottom cellb tandem cellc

b

active layer PBPT-8: PC71BM PTIPSBDT -DFDTQX:PC71BM

ratio

thickness [nm]

1:2 1:2

80 90

Voca [V]

Jsca [mA/cm2]

FFa

PCEa [%]

0.71 0.81

13.59 9.34

0.57 0.68

5.55 5.20

1.51 ± 0.014

7.60 ± 0.147

0.63 ± 0.0079

7.20 ± 0.144 (7.40)

a

Photovoltaic properties of copolymers/PC71BM-based devices spin-coated from a chloroform solution for polymers. Only the optimized recipes were considered for the estimation of the average PCE. Data have been averaged over 10 devices. The performance of the best device is given in parentheses. bITO/PEIE/Polymer:PC71BM/MoO3/Ag configuration. cITO/PEIE/PTIPSBDT-DFDTQX:PC71BM/PEDOT:PSS/PEIE/PBPT8:PC71BM/MoO3/Ag configuration.

requires matching of the photocurrents of the two component cells. Figure 7 shows the average performance parameters of the inverted tandem OPVs devices. The 12 fabricated devices exhibited on average a PCE of 7.21%, a Jsc of 7.59 mA/cm2, a Voc of 1.51 V, and a FF of 0.63 (Table S12). In our tandem cells, this has not yet been realized, and we anticipate that with further device improvement a PCE of over

be noted that the difference in FF between tandem and single cells was not as big as between the regular single junction devices (0.68 for bottom cells and 0.62 for tandem cell) because the FF of a tandem device is determined by the combined behavior of the two subcells. When the photocurrents from both cells are almost equal, the FF will be the average of the two subcells. The optimization of tandem cells H



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for 2 h under a N2 atmosphere to produce nonpolar and smooth surfaces onto which the polymers could be spin-coated. The polymers were dissolved to a concentration of 0.5 wt % in chlorobenzene. Films of the organic semiconductors were spin-coated at 1000 rpm for 50 s to a thickness of 60 nm. All device fabrication procedures and measurements were carried out in the air at room temperature. Device Fabrication. Regular Structure Single Cell. In this study, the devices were fabricated with the structure ITO/ PEDOT:PSS/polymer:PC71BM/IFL/Al. The procedure for cleaning the ITO surface included sonication and rinsing in distilled water, methanol, and acetone. The hole-transporting PEDOT:PSS layer (45 nm) was spin-coated onto each ITO anode from a solution purchased from Heraeus (Clevios P VP AI4083). Each polymer:PC71BM solution was then spin-coated onto the PEDOT:PSS layer. The polymer solution for spin-coating was prepared by dissolving the polymer (8 mg/mL) in 97% chloroform/3% 1,8-diiodooctane (DIO). Calcium and aluminum contacts were formed by vacuum deposition at pressures below 3 × 10−6 Torr, providing an active area of 0.09 cm2. The PFN OPV devices were fabricated with a structure of ITO/ PEDOT:PSS/polymer:PC71BM/PFN/Al. The PFN solution in methanol was spin-coated on the top of the obtained active layer at 2000 rpm for 30s to form a thin interlayer of 5 nm. The thickness of the active layer was measured by using a KLA Tencor Alpha-step IQ surface profilometer with an accuracy of ±1 nm. The current density− voltage (J−V) characteristics of all the polymer photovoltaic cells were determined by illuminating the cells with simulated solar light (AM 1.5G) with an intensity of 100 mW/cm2 using an Oriel 1000 W solar simulator. Electronic data were recorded using a Keithley 236 sourcemeasure unit, and all characterizations were carried out in an ambient environment. The illumination intensity used was calibrated by employing a standard Si photodiode detector from PV measurements Inc., which was calibrated at the National Renewable Energy Laboratory (NREL). The external quantum efficiency (EQE) was measured as a function of the wavelength in the wavelength range 360 to 800 nm using a halogen lamp as the light source, and the calibration was performed by using a silicon reference photodiode. Measurement was carried out after masking all but the active cell area of the fabricated device. All the characterization steps were carried out in an ambient laboratory atmosphere. Inverted Tandem Cells. The device architecture of the tandem photovoltaic cell is shown in Figure 5a. The precleaned ITO substrates were treated with UV−ozone. The PEIE solution was spin coated onto ITO substrates, at a speed of 5000 rpm for 1 min and with an acceleration of 1000 rpm/s, and annealed at 120 °C for 10 min on a hot plate in ambient air. The thickness of PEIE was estimated to be 10 nm. Then, the substrates were transferred into a N2-filled glovebox. The bottom layer of PTIPSBDT-DFDTQX:PC71BM (1:2 weight ratio in chloroform with 3 vo l% DIO) was prepared by spin coating at a speed of 1000 rpm for 30 s. The PTIPSBDT-DFDTQX device was solvent-annealed for 1 h at ambient temperature in the glovebox. To directly coat the hydrophilic PEDOT:PSS onto the hydrophobic active layer, the spin-casting of the PEDOT:PSS layer from Clevios P VP AI4083 diluted with an equal volume of 2-propanol and 0.2 wt % polyoxyethylene-6-tridecylether (Aldrich) was conducted. After this, PEDOT:PSS was coated with PEIE following the same conditions as previously described. The top active layer of PBPT-8:PC71BM (1:2 weight ratio in chloroform with 3 vol % DIO) was deposited by spincoating at 1000 rpm for 20s. The thickness of PBPT-8:PC71BM was estimated to be 80 nm. The device fabrication was completed by thermal evaporation of 10 nm MoO3 and 100 nm Ag as the anode under a vacuum at a base pressure of 3 × 10−6 Torr. The effective area of the device was measured to be 0.09 cm2. Synthesis of Monomers and Polymers. Synthesis of 4,8Bis(triisopropylsilylethynyl)-benzo[1,2-b:4,5-b′]dithiophene (A). To an oven-dried 500 mL round-bottom flask equipped with a stir bar and cooled to 0 °C under the protection of argon was added THF (50 mL) and 6.7 mL of triisopropylsilyl acetylene (29.9 mmol), followed by the dropwise addition of 19.3 mL of n-BuLi (32.7 mmol, 1.6 M solution in hexanes). This mixture was stirred for 2 h, then THF (120 mL) and benzo[1,2-b:4,5-b′]dithiophene-4,8-dione (5 g, 13.7 mmol) were

8% should be attainable. Indeed, the model developed by Konarka suggests that the theoretical maximum for these cells may be over 10%.1b The photovoltaic performance of the inverted tandem device is summarized in Table 4.

3. CONCLUSIONS In conclusion, we have successfully designed and synthesized a series of π-conjugated alternating copolymers composed of quinoxaline-based electron-accepting units and triisopropylsilylethynyl (TIPS)-substituted benzodithiophene electron-donating units by Stille cross-coupling polymerization for application in solution-processed tandem OPVs. The only structural difference between the two is that PTIPBDT-DFDTQX bears two fluorine atoms on the quinoxailine ring of the PTIPBDTDTQX. These new polymers showed medium band gaps and deeper HOMO energy levels than P3HT because of the strongly electron-accepting quinoxaline units, suggesting that they have good oxidative stability and can produce high Voc’s in photovoltaic devices made by combination with fullerene derivatives. Regular single junction devices achieved PCEs around 6% after optimization by use of additives and interfacial layers. Solution-processed Tandem OPVs based on PTIPSBDT-DFDTQX with PC71BM showed high power conversion efficiencies of up to 7.40%, matching the best P3HT-PCBM based tandem OPVs. We envision that this work will pave the way for future materials design to assist in producing efficient tandem polymer-based solar cells. 4. EXPERIMENTAL SECTION Materials. All starting organic compounds were purchased from Aldrich, Alfa Aesar, or TCI Korea and were used without further purification. Bis(triphenylphosphine)palladium(II)chloride and tetrakis(triphenylphosphine)palladium(0) were purchased from Strem. Indene-C60 bisadduct (IC60BA) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) were purchased from EM-index. Poly(3hexylthiophene) (P3HT) was purchased from Rieke Metals. Solvents were dried and purified by fractional distillation over sodium/ benzophenone and handled in a moisture-free atmosphere. Column chromatography was performed using silica gel (Merck, Kieselgel 60 63-200 MYM SC). PBPT-8 polymer was synthesized according to the literature procedure.24 The molar mass obtained (Mn/Mw = 30 000/ 48 000 g/mol) is similar to that reported in the literature. Measurements. 1H and 13C NMR spectra were recorded using a Varian Mercury Plus 300 MHz spectrometer, and the chemical shifts were recorded in units of parts per million with chloroform as the internal standard. The elemental analysis was operated using Vario Micro Cube in the Korea Basic Science Institute (Busan, Korea). The absorption spectra were measured using a JASCO JP/V-570 model. The molecular weights of the polymers were determined by gel permeation chromatography (GPC) analysis relative to a polystyrene standard using a Waters high-pressure GPC assembly (model M590). Thermal analyses were carried out on a Mettler Toledo TGA/SDTA 851e under a N2 atmosphere with a heating and cooling rate of 10 °C/ min. Cyclic voltammetry (CV) was performed on a CH Instruments Electrochemical Analyzer. The CV measurements were carried out in acetonitrile solutions containing 0.1 M tetrabutylammonium tetrafluoroborate (TBABF4) as the supporting electrolyte, using Ag/ AgNO3 as the reference electrode, a platinum wire as the counterelectrode, and a platinum working electrode. Fabrication of Field-Effect Transistors Devices. Organic thin film field-effect transistors (OTETs) were fabricated using a bottomcontact geometry device (channel length L = 12 μm and width W = 120 μm). The source and drain contacts consisted of gold (100 nm), and the dielectric was silicon oxide (SiO2) with a thickness of 300 nm. The SiO2 surface was cleaned, dried, and pretreated with a solution of 10.0 mM octyltrichlorosilane (OTS-8) in toluene at room temperature I



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Synthesis of 5,8-Bis(5-bromothiophen-2-yl)-6,7-difluoro-2,3-bis(4-(octyloxy)phenyl)quinoxaline (DFDTQX). Bromosuccinimide (0.60 g, 4.79 mmol) was added to a stirred solution of compound 4 (1.00 g, 1.92 mmol) in DMF (25 mL) in darkness. The mixture was stirred at room temperature for 4 h. A bright yellow solid precipitate was formed. The mixture was filtered and thoroughly washed with methanol. The solid was then washed once with cold diethyl ether and purified by flash chromatography to give 1.80 g (80%) of compound DFDTQX. 1H NMR (300 MHz, CDCl3, ppm): δ 7.80 (d, 2H), 7.66(d, 2H), 7.18 (d, 2H), 6.94 (d, 2H), 4.01 (t, 4H), 1.83 (m, 4H), 1.48−1.29 (m, 20H), 0.89 (t, 6H). 13C NMR (75 MHz, CDCl3, ppm): δ 161.8, 159.1, 158.4, 152.7, 150.3, 147.3, 130.1, 129.9, 129.2, 127.8, 119.5, 111.6, 70.1, 33.9, 32.2, 29.3, 26.2, 23.5, 13.5. Anal. Calcd for C44H46N2S2: C, 58.93; H, 5.17; N, 3.12; S, 7.15. Found: C, 60.10; H, 5.21; N, 3.51; S, 7.20. General Polymerization Procedure. All copolymers were synthesized by Stille coupling polymerization. The Stille coupling reaction was used to synthesize the copolymers shown in Scheme 1. The 2,6-bis(trimethyltin)-4,8-bis(triisopropylsilylethynyl)-benzo[1,2b:4,5-b′]dithiophene (TIPSBDT), 2,3-bis(4-(3,7-dimethyloctyloxy)phenyl)-5,8-bis(5′-bromo-dithien-2-yl)quinoxailne (DTQX), and 5,8bis(5-bromothiophen-2-yl)-6,7-difluoro-2,3-bis(4-(octyloxy)phenyl)quinoxaline (DFDTQX) monomers were synthesized according to the previous report. The reaction mixture of then tetrakis(triphenylphosphine)palladium in 12 mL of anhydrous toluene and 3 mL of DMF was stirred at 120 °C for two days, and then the excess amount of 2-bromothiophene and tripropyl(thiophen-2-yl)stannane, the end-capper, dissolved in 1 mL of anhydrous toluene were added and stirring continued for 12 h. Polymer purification involved the reaction mixture being cooled to approximately 50 °C and 200 mL of methanol being added slowly with vigorous stirring of the reaction mixture. The polymer fibers were collected by filtration and reprecipitation from methanol and acetone. The polymers were then purified further by washing for 2 days in a Soxhlet apparatus, with acetone used to remove oligomers and catalyst residues. Column chromatography with a chloroform solution was then performed on the polymer. The reprecipitation procedure in chloroform/methanol was then repeated several times. The resulting polymers were soluble in common organic solvents. Poly{4,8-bis(triisopropylsilylethynyl)benzo[1,2-b:4,5-b′]dithiophene-alt-5,5-(5′,8′-di-2-thienyl-2,3-bis(4-octyloxyl)phenyl)quinoxaline} (PTIPSBDT-DTQX). 2,6-Bis(trimethyltin)-4,8-bis(triisopropylsilylethynyl)-benzo[1,2-b:4,5-b′]dithiophene (TIPSBDT; 400 mg, 0.78 mmol) was mixed with 2,3-bis(4-(3,7-dimethyloctyloxy)phenyl)-5,8-bis(5′-bromo-dithien-2-yl)quinoxailne (DTQX; 392.8 mg, 1.0 equiv), tetrakis(triphenylphosphine)palladium (3.0 mg, 2.6 μmol), and toluene (12 mL) and DMF (3 mL) for this polymerization. Elem Anal. Calcd: C, 73.19; H, 7.72; N, 2.19; S, 10.02. Found: C, 72.53; H, 7.67; N, 3.01; S, 9.93. Poly{4,8-bis(triisopropylsilylethynyl)benzo[1,2-b:4,5-b′]dithiophene-alt-6,7-difluoro-5,5-(5′,8′-di-2-thienyl-2,3-bis(4octyloxyl)phenyl)quinoxaline} (PTIPSBDT-DFDTQX). 2,6-Bis(trimethyltin)-4,8-bis(triisopropylsilylethynyl)-benzo[1,2-b:4,5-b′]dithiophene (TIPSBDT; 400 mg, 0.78 mmol) was mixed with 5,8bis(5-bromothiophen-2-yl)-6,7-difluoro-2,3-bis(4-(octyloxy)phenyl)quinoxaline (DFDTQX; 409.2 mg, 1.0 equiv), tetrakis(triphenylphosphine)palladium (3.0 mg, 2.6 μmol), and toluene (12 mL) and DMF (3 mL) for this polymerization. Elem Anal. Calcd for C, 71.19; H, 7.35; N, 2.13; S, 9.75. Found: C, 70.25; H, 7.38; N, 2.83; S, 9.43.

added. The mixture was stirred at room temperature for 24 h and then quenched with water. The resulting mixture was poured into saturated ammonium chloride solution and extracted with ethyl acetate. The organic layer was washed with brine and dried over anhydrous MgSO4. After the solvent being evaporated, the residue was dissolved in THF (120 mL), and then SnCl2·2H2O (15.3g, 68.1 mmol, in 50 mL 50% acetic acid) was added dropwise. The mixture was stirred at room temperature overnight and poured into water and extracted with EA. The organic layer was washed with sodium bicarbonate and brine and then dried over anhydrous MgSO4. Solvent was removed, and the crude product was purified with column chromatography on silica with hexane as the eluent, to yield 2 (3.6g, 48%) as a light green solid. 1H NMR (300 MHz, CDCl3, ppm): δ 7.61 (d, 2H), 7.56 (d, 2H), 1.23 (m, 42H). 13C NMR (75 MHz, CDCl3, ppm): δ 140.86, 138.51, 128.28, 123.14, 112.18, 102.63, 101.62, 18.78, 11.33. Anal. Calcd for C36H46S2Si2: C, 69.75; H, 8.41; S, 11.64. Found: C, 69.71; H, 8.40; S, 11.63. Synthesis of 2,6-Bis(trimethyltin)-4,8-bis(triisopropylsilylethynyl)benzo[1,2-b:4,5-b′]dithiophene (TIPSBDT). Under the protection of argon, 5.1 mL of n-BuLi (8.1 mmol, 1.6 M solution in hexanes) was added dropwise via syringe to compound A (1.5 g 2.7 mmol) and TMEDA (1.2 mL, 8.1 mmol) in THF (30 mL) which was cooled at −78 °C. After stirring at low temperature for 30 min, 10.8 mL of trimethyltinchloride (10.8 mmol, 1 M solution in THF) was added in one portion. The reaction mixture was allowed to warm to room temperature overnight. The reaction mixture was quenched with water, then concentrated via rotary evaporation. The residue was diluted with chloroform and washed with brine and water. The organic layer was dried over MgSO4, and concentrated via rotary evaporation. The crude compound was recrystallized with isopropyl alcohol to yield a pale yellow solid. (1.9g, 80%) 1H NMR (300 MHz, CDCl3, ppm): δ 7.69 (s, 2H), 1.23 (m, 42H). 0.47 (s, 18H). 13C NMR (75 MHz, CDCl3, ppm): δ 144.68, 143.51,139.10, 110.36, 103.33, 100.64, 19.06, 11.39, −8.3. Anal. Calcd for C38H62S2Si2Sn2: C, 52.06; H, 7.13; S, 7.32. Found: C, 52.09; H, 7.11; S, 7.29. Synthesis of 5,8-Dibromo-6,7-difluoro-2,3-bis(4-(octyloxy)phenyl)quinoxaline (3).26 3,6-Dibromo-4,5-difluorobenzene-1,2-diamine (1.00 g, 1.30 mmol) and compound 2 (1.55 g, 1.30 mmol) were dissolved in 60 mL of ethanol; then 20 mL of acetic acid was added, and the mixture was heated under reflux for one day. After evaporation of the solvent, the mixture was poured into water (100 mL) and extracted with CHCl3. After the reaction had finished, the reaction mixture was extracted three times with chloroform and brine. The organic layer was separated and dried with anhydrous magnesium sulfate, and then the solvent was removed by using a rotary evaporator. The crude product was precipitated in methanol. The product yield was 95% (1.3 g). 1H NMR (300 MHz, CDCl3, ppm): δ 7.64 (d, 4H), 6.89 (d, 4H), 3.98 (t, 4H), 1.79 (m, 4H), 1.45−1.29 (m, 20H), 0.88 (t, 6H). 13C NMR (75 MHz, CDCl3, ppm): δ 160.8, 159.9, 158.5, 140.9, 130.8, 124.9, 120.7, 104.5, 70.1, 39.1, 30.4, 29.8, 27.2, 22.7, 14.1. Anal. Calcd for C36H42N2: C, 59.03; H, 5.78; N, 3.82. Found: C, 58.88; H, 5.69; N, 4.01. Synthesis of 6,7-Difluoro-2,3-bis(4-(octyloxy)phenyl)-5,8-di(thiophen-2-yl)quinoxaline (4).27 Tripropyl(thiophen-2-yl)stannane (1.19 g, 4.85 mmol) was added to a stirred solution of compound 3 (1.00 g, 1.90 mmol) and bis(triphenylphosphine)palladium(II) dichloride (95 mg, 0.06 mmol) in toluene (50 mL). The mixture was refluxed overnight. The resulting mixture was extracted with ethyl acetate and brine. The organic layer was washed with sodium bicarbonate and brine and then dried over anhydrous MgSO4. Solvent was removed, and the crude product was purified with column chromatography on silica with hexane as the eluent, to yield compound 4 (1.5g, 58%) as a light yellow solid. 1H NMR (300 MHz, CDCl3, δ): 8.03 (d, 2H), 7.70 (d, 2H), 7.63 (d, 2H), 7.23(t, 2H), 6.91 (d, 2H), 4.01 (t, 4H), 1.80 (m, 4H), 1.46−1.29 (m, 20H), 0.88 (t, 6H). 13C NMR (75 MHz, CDCl3, δ): 160.1, 158.7, 154.7, 150.9, 147.5, 132.6, 131.2, 130.5, 129.8, 127.6, 125.2, 119.8, 70.1, 35.6, 30.5, 29.1, 26.3, 21.9, 15.8. Anal. Calcd for C44H48N2S2: C, 71.51; H, 6.55; N, 3.79; S, 8.68. Found: C, 70.49; H, 6.48; N, 3.99; S, 9.01.

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic routes of monomers, TGA measurements of polymers, GPC traces, UV−visible absorption spectra, J−V curves, and comparisons of the photovoltaic properties of the OPVs (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. J



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.-H.H.); sikchoi@kaist. ac.kr (S.K.C.). Author Contributions

J.-H.K. and C.E.S. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant (No. 2012055225) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Education, Science and Technology, Korea, New and Renewable Energy Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Ministry of Knowledge Economy (MKE; No. 20113010010030), and the National Science Foundation (NRF) grant funded by the Korean government (MEST) through GCRC-SOP (Grant No. 2011-0030668).

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