Enhanced Photovoltaic Performance of Amorphous Copolymers

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Enhanced Photovoltaic Performance of Amorphous Copolymers Based on Dithienosilole and Dioxocycloalkene-annelated Thiophene Jianming Huang, Yutaka Ie, Makoto Karakawa, Masahiko Saito, Itaru Osaka, and Yoshio Aso Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm503117j • Publication Date (Web): 01 Dec 2014 Downloaded from http://pubs.acs.org on December 11, 2014

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Chemistry of Materials

Jianming Huang,† Yutaka Ie,*,†,‡ Makoto Karakawa,† Masahiko Saito,§ Itaru Osaka,‡,║ and Yoshio Aso*,† †

The Institute of Scientific and Industrial Research (ISIR), Osaka University, 8-1, Mihogaoka, Ibaraki, Osaka 567-0047, Japan ‡ Japan Science and Technology Agency (JST)-PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 333-0012, Japan §

Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, HigashiHiroshima, Hiroshima 739-8527, Japan ║

Center for Emergent Matter Science (CEMS), RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0111, Japan

ABSTRACT: Organic photovoltaics (OPVs) have attracted considerable attention due to their potential for generating renewable energy. The power conversion efficiency (PCE) of the OPVs largely depends on the organic semiconducting materials. Thus, the elucidation of structure-property-OPV performance relationships is important for the rational improvement of OPVs. Here, lowbandgap copolymers comprising dithieno[3,2-b:2',3'-d]silole as a donor unit and dialkyl-substituted naphtho[2,3-c]thiophene-4,9dione as an acceptor unit were synthesized to investigate the influence of the polymer molecular weight and the alkyl chain length in the acceptor unit on the polymer properties and photovoltaic performance. All the prepared copolymers are amorphous in the solid state. Both the increase of polymer molecular weight and variation of the alkyl side chains in the acceptor unit subtly affected molecular properties. However, these structural modifications showed significant impact on the photovoltaic performance in bulk hetero-junction (BHJ) solar cells based on copolymer/[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), with PCEs that range between 2.35 and 5.21%. Furthermore, the optimization of thin-film fabrication by use of a ternary solvent system led to the appearance of improved morphology accompanied by subtly ordered states of the copolymer in the BHJ films, and hence, improved carrier mobility and charge-separation efficiency. Consequently, the BHJ solar cell can achieve a PCE of 7.85%, which is the highest performance among the amorphous copolymers in the conventional device structure. This result highlights the importance of fine-tuning both the molecular structure and device fabrication in the construction of high-performance organic photovoltaics based on amorphous copolymers and PC71BM.

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Organic photovoltaics (OPVs) have been intensively investigated over the last decade because of their potentially advantageous lightness, flexibility, low cost, and ease of processing, which highlight their promise as candidates for nextgeneration renewable energy sources.1-4 The establishment of the bulk hetero-junction (BHJ) concept, which comprises a blend of a -conjugated polymer as a donor and a methanofullerene derivative (represented by [6,6]-phenyl-C71butyric acid methyl ester (PC71BM)) as an acceptor in the organic active layer, has spurred OPV studies. 5,6 To improve power conversion efficiencies (PCEs) through the tuning of molecular properties, -conjugated polymers should fulfill the following requirements: a reduced band gap to broaden light absorption to the long-wavelength region and a low-lying highest occupied molecular orbital (HOMO) energy level to increase the open-circuit voltage (VOC).7 Therefore, the development of donor-acceptor (D-A) copolymers, which consist of alternating electron-rich and electron-deficient units, has become a straightforward approach.8-11 Significant progress has been achieved not only in the molecular design, but also the optimization of devices such as inverted12,13-15 and tandem16,17-

structures. Accordingly, the PCEs of BHJ-type polymerbased OPVs have rapidly increased in recent years, reportedly surpassing more than 8%.13-15,17-30 Regardless of such developments, the control of the morphology of BHJ films is still not well understood.31-34 In fact, the precise control of film morphology and microstructure for amorphous polymer/PC71BM-based BHJ films is still challenging,30,35 although several reports have suggested that the selection of solvent additives24,25,36,37 and alkyl side chains26,27,38-41 in the polymer is effective for obtaining high PCEs in crystalline polymer/PC71BM systems21-29. We also believe that amorphous polymer/PC71BM systems have the potential for higher PCEs with the proper manipulation of morphology. We previously reported that dihexyl-substituted naphtho[2,3-c]thiophene-4,9-dione (C6) can function as an electron-deficient unit in electron-transporting organic field-effect transistor (OFET) materials.42 When the C6 unit is copolymerized with the electron-donating dithieno[3,2-b:2',3'-d]silole (DTS) unit,43-46 the resulting copolymer (DTS-C6) in combination with PC71BM affords a high VOC of 0.90 V (Figure 1), which reflects the low-lying HOMO energy level of DTSC6.47 Nevertheless, the PCE was limited to ~5% mainly by the moderate short-circuit current (JSC) and fill factor (FF). This is

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partially due to the low number-average molecular weight (Mn) of DTS-C6 (19.5 kg mol–1, hereinafter designated DTSC6-L), since Mn is known to have an influence on the morphology and optoelectronic properties. 28,29,48-53 The copolymer side chains are also known to have an important role in determining OPV performance.26,27,38-41 With these results and precedents in mind, we endeavored to fine tune (1) the synthesis to obtain a higher Mn and (2) the alkyl chain components in the naphtho[2,3-c]thiophene-4,9-dione (C) unit, as well as (3) its thin-film fabrication in blends with PC71BM. In this contribution, we develop four copolymers (DTS-C4, DTS-C6, DTS-CEH, and DTS-C8) and study the impact of these structural variations on copolymer properties and performance in BHJ-type OPVs (Figure 1). We found that the copolymers DTS-C6 and DTS-CEH showed improved photovoltaic performance in conventional devices with PCEs over 5%. Furthermore, by controlling the film morphology using a ternary solvent system, we successfully demonstrated that the PCE of DTS-C6/PC71BM solar cells further increased to 7.85%. It should be mentioned that, although these copolymers are amorphous in the solid state, the thin-film fabrication technique used in this study promoted the formation of a proper morphology with subtly ordered copolymers in the BHJ films, leading to dramatically improved JSC and FF values. These results give us an important clue for improving the photovoltaic performance of amorphous copolymer systems.

Figure 1. Chemical structures investigated in this study.

General Information. Column chromatography was performed on silica gel, KANTO Chemical silica gel 60N (40–50 μm) or neutral alumina (Merck Aluminum oxide 90 standardized). Thin-layer chromatography plates were visualized with UV light. Reverse-phase preparative liquid chromatography (LC) was performed on a JASCO LC system equipped with Phenomenex 00F-4377-P0-AX using acetonitrile/acetone (1/3) as an eluent at 8 mL min–1. Preparative GPC was performed on a Japan Analytical Industry LC-918 equipped with JAI-GEL 1H/2H. Analytical GPC was performed on a Hitachi HighTechnologies Corporation L-2420/L-2130 equipped with a Shodex K-803L. 1H and 13C NMR spectra were recorded on a JEOL ECS-400 or a JEOL ECA-600 spectrometer in CDCl3 with tetramethylsilane (TMS) as an internal standard. Data are reported as follows: chemical shift in ppm (), multiplicity (s = singlet, t = triplet, m = multiplet, br = broad), coupling constant (Hz), and integration. Mass spectra were obtained on a Shimadzu GCMS-QP-5050 or a Shimadzu AXIMA-TOF. Thermal properties were measured under nitrogen at a heating rate of 10 °C min–1 with a Shimadzu DSC-60 or a Shimadzu TGA-50. UV-vis spectra were recorded on a Shimadzu UV3600 spectrophotometer. All spectra were obtained in spectrograde solvents. Ionization potentials were measured using a Riken Keiki Co. Ltd. AC-3. The measurements were per-

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formed under N2 conditions. The irradiation power was 10 nW cm–2. Elemental analyses were performed on a Perkin Elmer LS-50B by the Elemental Analysis Section of Comprehensive Analysis Center (CAC), ISIR, Osaka University. The surface structures of the deposited organic film were observed by atomic force microscopy (Shimadzu, SPM9600), and the film crystallinity was evaluated by an X-ray diffractometer (Rigaku, SmartLab). X-ray diffraction patterns were obtained using the Bragg-Brentano geometry with Cu K radiation as an X-ray source with an acceleration voltage of 45 kV and a beam current of 200 mA. The scanning mode was set to scans between 3°–30° with scanning steps of 0.01°. The GIXD measurements were conducted at the SPring-8 on beamline BL46XU. The spin-coated film on the PEDOT:PSS coated ITO glass was irradiated at a fixed incident angle of 0.12° through a Huber diffractometer, with an X-ray energy of 12.39 keV ( = 1Å). GIXD patterns were recorded with a twodimensional image detector (Pilatus 300K). Preparation of Materials. All reactions were carried out under a nitrogen atmosphere. Solvents of the highest purity grade were used as received. Unless stated otherwise, all reagents were purchased from commercial sources and used without purification. Compound 1 was prepared by reported procedures, and the 1H NMR spectrum of this compound was in agreement with that previously reported.47 Synthesis of Br-C4-Br. Compound 1 (0.80 g, 2.42 mmol) was placed in a two-necked round-bottomed flask, dissolved with CH2Cl2 (10 mL), and degassed with nitrogen. Anhydrous aluminum trichloride (AlCl3) (1.30 g, 9.70 mmol) was added at 0 °C. A solution of o-dibutylbenzene (690 mg, 3.63 mmol) in CH2Cl2 (5 mL) was then added dropwise to the stirred mixture at 0 °C. After stirring the reaction mixture at 0 °C for 2 h, it was poured into ice and extracted with chloroform (CF). The resulting organic layer was washed with water and dried over Na2SO4. After removal of the solvent under reduced pressure, the residue was purified by column chromatography on silica gel (hexane/ethyl acetate = 10/1) to yield Br-C4-Br (0.82 g, 75%). Preparative GPC using CF as eluent was performed for further purification. White solid; mp 132–133 °C; 1H NMR (CDCl3, δ): 8.04 (s, 2H), 2.74 (t, 4H, J = 7.8 Hz), 1.63-1.56 (m, 4H), 1.44-1.39 (m, 4H), 0.95 (t, 6H, J = 7.1 Hz); 13C NMR (CDCl3, δ): 178.0, 148.6, 133.1, 132.3, 128.2, 120.6, 32.8, 32.7, 22.8, 14.1; MS (EI) m/z 484 (M+); Anal. calcd for C20H20Br2O2S: C 49.61, H 4.16; found: C 49.50, H 4.12. Synthesis of Br-CEH-Br. Br-CEH-Br was synthesized from 1 and o-(2-ethylhexyl)benzene with a yield of 65% by following the procedure used for the preparation of Br-C4-Br. White solid; mp 120–121 °C; 1H NMR (CDCl3, δ): 8.02 (s, 2H), 2.66 (t, 4H, J = 6.4 Hz), 1.63 (m, 2H), 1.31-1.24 (m, 16H), 0.890.83 (m, 12H); 13C NMR (CDCl3, δ): 178.1, 148.4, 133.2, 132.0, 129.2, 120.6, 40.6, 37.5, 32.6, 28.9, 25.7, 23.1, 14.2, 11.0; MS (EI) m/z 596 (M+); Anal. calcd for C28H36Br2O2S: C 56.38, H 6.08; found: C 56.00, H, 6.05. Synthesis of Br-C8-Br. Br-C8-Br was synthesized from 1 and o-dioctylbenzene with a yield of 80% by following the procedure used for the preparation of Br-C4-Br. White solid; mp 130–131 °C; 1H NMR (CDCl3, δ): 8.04 (s, 2H), 2.72 (t, 4H, J = 7.8 Hz), 1.63-1.55 (m, 4H), 1.38-1.26 (m, 20H), 0.87 (m, 6H); 13C NMR (CDCl3, δ): 178.0, 148.6, 133.2, 132.3, 128.2, 120.6, 33.0, 31.9, 30.7, 29.7, 29.5, 29.3, 22.8, 14.2; MS (EI) m/z 596 (M+); Anal. calcd for C28H36Br2O2S: C 56.38, H 6.08; Found: C 56.20, H, 6.09.


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Synthesis of DTS-C4. DTS-Sn (260 mg, 0.35 mmol), Br-C4Br (190 mg, 0.35 mmol), Pd2(dba)3·CHCl3 (7 mg, 7 mol), and P(o-tolyl)3 (9 mg, 28 mol) were placed in a microtube and dissolved with toluene (2 mL). The mixture was then reacted in a microwave reactor at 180 °C for 40 min, and then cooled to room temperature. The polymer solution was precipitated into methanol (100 mL) at room temperature. The resulting purple-black solid was filtered into a Soxhlet thimble, and extracted with methanol, hexane, and CF until the wash from each extraction became colorless. When there was no solid remaining in the thimble, the CF fraction was concentrated, and chlorobenzene was added (5 mL). The copolymer solution was then poured into methanol at room temperature. The obtained precipitate was filtered, and dried under vacuum to yield DTS-C4 (92 mg, 86%). Black solid; 1H NMR (1,1,2,2tetrachloroethane-d2, 130 °C, δ) 8.20 (br, 2H), 7.95 (br, 2H), 2.84 (br, 4H), 1.76 (br, 4H), 1.55-1.05 (m, 16H), 0.90 (br, 18H); GPC (CF at 40 °C): Mn = 41.5 kg mol–1, molecularweight dispersity (DM) = 2.0. Synthesis of DTS-C6. DTS-C6 was synthesized from DTSSn and Br-C4-Br with a yield of 90% by following the procedure used for the preparation of DTS-C4. Black solid; 1H NMR (1,1,2,2-tetrachloroethane-d2, 130 °C, δ) 8.20 (br, 2H), 7.95 (br, 2H), 2.85 (br, 4H), 1.76 (br, 4H), 1.56-1.10 (m, 34H), 0.91 (br, 18H); GPC (CF at 40 °C): Mn = 43.7 kg mol–1, DM = 1.6. Synthesis of DTS-CEH. DTS-CEH was synthesized from DTS-Sn and Br-CEH-Br with a yield of 77% by following the procedure used for the preparation of DTS-C4. Black solid; 1H NMR (1,1,2,2-tetrachloroethane-d2, 130 °C, δ) 8.20 (br, 2H), 7.95 (br, 2H), 2.85 (br, 4H), 1.76 (br, 4H), 1.58-1.14 (m, 42H), 0.93 (br, 18H); GPC (CF at 40 °C): Mn = 53.4 kg mol–1, DM = 1.7. Synthesis of DTS-C8. DTS-C8 was synthesized from DTSSn and Br-C8-Br with a yield of 91% by following the procedure used for the preparation of DTS-C4. Black solid; 1H NMR (1,1,2,2-tetrachloroethane-d2, 130 °C, δ) 8.20 (br, 2H), 7.95 (br, 2H), 2.84 (br, 4H), 1.77 (br, 4H), 1.60-1.15 (m, 42H), 0.93 (br, 18H); GPC (CF at 40 °C): Mn = 46.3 kg mol–1, DM = 2.0. Space-charge-limited current measurements. Hole-only and electron-only devices were prepared with the structures ITO/PEDOT:PSS/active layer/Au and ITO/TiO x/active layer/Ca/Au, respectively. The active layers were prepared from 10 mg mL–1 solutions of materials. The carrier mobilities of these devices were calculated by the following equation: J = 9V2d3, where , and d are the dielectric constant of the active layer, the permittivity of free space, the carrier mobility, and the measured thickness of the active layer, respectively. We used the values of  = 3 and  = 8.8 × 10–12. Photovoltaic device fabrication and evaluation. Organic photovoltaic devices were prepared with the structure ITO/PEDOT:PSS/active layer/Ca/Al. ITO-coated glass substrates were first cleaned by ultrasonication in toluene, acetone, H2O, and 2-propanol for 10 min, respectively, followed by O2 plasma treatment for 10 min. ITO-coated glass substrates were then activated by ozone treatment for 1 h. PEDOT:PSS was spin-coated on the ITO surface at 3000 rpm for 1 min and dried at 135 °C for 10 min. Under these conditions, the thickness of the PEDOT:PSS is ca. 30 nm. The active layers were then prepared by spin-coating on the ITO/PEDOT:PSS electrode at 800–1200 rpm for 2 min in a glove box. Except for

CF/1,8-diiodooctane (DIO) processing, the thickness of the active layer was ca. 80 nm. Ca and Al electrodes were evaporated on the top of the active layer through a shadow mask to define the active area of the devices (0.09 cm2) under a vacuum of 10–5 Pa to a thickness of 100 nm, determined by a quartz crystal monitor. After sealing the device from the air, the photovoltaic characteristics were measured in air under simulated AM 1.5G solar irradiation (100 mW cm–2) (SAN-EI ELECTRIC, XES-301S). The current-voltage characteristics of the photovoltaic devices were measured by using a Keithley 2400 source meter. The external quantum efficiency (EQE) spectra were measured with a Soma Optics Ltd. S-9240. The thickness of the active layer was determined by a KLA Tencor Alpha-step IQ.

The general synthetic route for the copolymers was reported previously (Scheme 1).47 4,4-Bis(2-ethylhexyl)-2,6bis(trimethylstannyl)-4H-dithieno[3,2-b:2',3'-d]silole (DTS-Sn) was obtained according to a published procedure.44 However, to guarantee the stoichiometry of DTS-Sn relative to the dibromo counterpart during the copolymerization,54-56 the material was further purified by reverse-phase preparative LC using acetonitrile/acetone (1/3) as eluent. 1H NMR spectra of DTSSn before and after purification were shown in the Supporting Information (SI). The new acceptor units Br-Cx-Br (x = 4, 8, and EH) were prepared by Friedel-Crafts acylation of the corresponding dialkylbenzene with 2,5-dibromo-3,4thiophenedicarbonyl dichloride (1). Finally, the alternating copolymers were built up via a Stille coupling reaction between DTS-Sn and Br-Cx-Br in the presence of Pd2(dba)3·CHCl3 and P(o-tolyl)3 in toluene at 180 °C for 40 min under microwave irradiation. After removal of the lowmolecular-weight compounds via Soxhlet extraction using methanol and hexane, the high-molecular-weight copolymers were extracted with CF. Details of the synthetic procedures and characterization of all the new compounds can be found in the Experimental Section. Analytical GPC using a polystyrene standard with CF as eluent determined the Mns to be 41.5, 43.7, 53.4, and 46.3 kg mol–1 for DTS-C4, DTS-C6, DTS-CEH, and DTS-C8, with DM of 2.0, 1.6, 1.7, and 2.0, respectively (Table 1 and Figure S1 in the SI). It should be mentioned that, under this synthetic protocol, the Mn of DTS-C6 nearly doubles compared to DTS-C6-L,47 and all the copolymers have Mn values greater than 40 kg mol–1. The coupling reaction between DTSSn and Br-C6-Br in toluene under conventional heating condition (120 °C, 48 h) gave comparable Mn (43.7 kg mol–1) and DM (1.6). As a result, the apparent difference of synthetic condition between DTS-C6 and DTS-C6-L is the purity of DTSSn. Thus, we considered that the improved purity of DTS-Sn

Scheme 1. Synthesis of target copolymers.



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Table 1 Characteristics and properties of copolymers.

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Copolymers

Mn/kg mol–1

DM

Td/°C

max/nm a

Egopt/eV b

EHOMO/eV c

ELUMO/eV d

μh/cm2 V–1 s–1

DTS-C4

41.5

2.0

412

605

1.63

-5.33

-3.70

9.9 × 10–5

DTS-C6

42.8

1.7

412

610

1.62

-5.25

-3.63

1.2 × 10–4

DTS-CEH

53.4

1.7

410

613

1.62

-5.27

-3.65

1.1 × 10–4

DTS-C8

46.3

2.0

410

612

1.64

-5.27

-3.63

1.3 × 10–4

a

In film. b Determined by the onset of the UV-vis absorption spectrum in the film. c Determined by PESA. d ELUMO = EHOMO + Egopt.

leads to increase the molecular weight of copolymers. The copolymers exhibited good solubility in chlorinated solvents such as CF, chlorobenzene, and o-dichlorobenzene (o-DCB). Thermogravimetric analysis (TGA) showed that all of the copolymers have good thermal stability, with 5%-weight-loss temperatures (Td) of over 400 °C (Figure S2(a), Table 1). Differential scanning calorimetry (DSC) analysis showed no obvious exothermic or endothermic transitions between 50 and 300 °C for any of the copolymers at a heating rate of 10 °C min–1 (Figure S2(b)). These DSC profiles along with that of the corresponding DTS-C6-L47 and their X-ray diffraction results (see below) indicate that the copolymers based on DTS and Cx units intrinsically adopt amorphous morphological behaviors,57 regardless of the molecular weight and alkyl chain substituents. The UV-vis absorption spectra of DTS-C4, DTS-C6, DTSCEH, and DTS-C8 in CF solution and as films on a quartz plate are shown in Figure 2. The corresponding spectroscopic data in the films are summarized in Table 1. The copolymers show one weak shoulder (400–500 nm) and one main absorption band (550–750 nm), which are attributed to π–π* transitions and intramolecular charge transfer between the donor and acceptor units of the copolymers, respectively.47 As shown in Figure S3, the absorption spectrum of DTS-C6 in CF solution was almost superimposable on that of DTS-C6-L, indicating that the backbone length of DTS-C6 exceeds the effective conjugation length of this copolymer. The absorption spectra of the copolymer films were red-shifted compared to those in solution (Figure 2). The shoulder at around 680 nm might imply the appearance of interchain π–π interactions. These copolymers possess similar spectroscopic shapes and peaks even as films, indicating that the alkyl groups in the acceptor units have little influence on the interchain associations in the solid state. The optical HOMO–LUMO energy gaps (Egopt) of these copolymers, extracted from the film absorption onsets, are ca. 1.62–1.64 eV. To determine the HOMO energy levels (EHOMO) of these copolymers in the solid state, we performed photoelectron spectroscopy in air (PESA). From the onsets of the PESA results (Figure S4), the ionization potential of DTS-C6 was nearly at the same level as DTS-C6-L (–5.19 eV) under the same measurement conditions,47 and the EHOMO values for the copolymers were determined to be in a narrow range between –5.25 and – 5.33 eV. Based on the EHOMO and Egopt values, the LUMO energy levels (ELUMO) were calculated to be between –3.63 and – 3.70 eV. These values are listed in Table 1. We previously determined the hole mobility of DTS-C6-L using OFET devices.47 However, to align the carrier-transport direction with the OPV devices, the hole mobilities of the copolymers in this study were measured by the space-chargelimited currents (SCLC) technique, with the hole-only device

[ITO/PEDOT:PSS/copolymer/Au].58 The current density (J)– voltage (V) characteristics of these devices are shown in Figure S5. As summarized in Table 1, the SCLC hole mobilities (μh) of these copolymers vary within the narrow range between 9.9 × 10–5 and 1.3 × 10–4 cm2 V–2 s–1. These results indicate that the copolymers have good hole mobilities, irrespective of the differences in the alkyl groups in the acceptor units. X-ray diffraction (XRD) measurements of the copolymer films on PEDOT:PSS-coated ITO glass did not show distinct diffraction patterns (Figure 3), indicative of the amorphous nature of the solid state.

Figure 2. UV-vis absorption spectra of DTS-C4, DTS-C6, DTSCEH, and DTS-C8 in CF solution (dashed line) and as films (solid line).

Figure 3. X-ray diffractograms of DTS-C4, DTS-C6, DTS-CEH, and DTS-C8 films on ITO/PEDOT:PSS substrates.


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Table 2 OPV performance based on copolymers/PC71BM blend films.

a

Blend films

thickness/nm

VOC/V

JSC/mA cm–2

FF

PCE/%

μh/cm2 V–1 s–1 a

DTS-C4:PC71BM

85

0.91

10.10

0.47

4.28 (4.01 ± 0.22) b

7.8 × 10–5

DTS-C6:PC71BM

82

0.90

10.59

0.55

5.21 (5.05 ± 0.13) b

1.0 × 10–4

DTS-CEH:PC71BM

80

0.94

9.46

0.58

5.15 (4.97 ± 0.18) b

1.1 × 10–4

DTS-C8:PC71BM

80

0.88

5.00

0.53

2.35 (2.28 ± 0.08) b

9.8 × 10–5

DTS-C6-L:PC71BM

80

0.90

10.05

0.50

4.55 (4.37 ± 0.11) b

3.4 × 10–5

ITO/PEDOT:PSS/blend films/Au. b The average and standard deviation of five devices are provided in parentheses, see the SI for details.

To investigate the photovoltaic performance of the copolymers, conventional BHJ solar cells were fabricated using PC71BM as the electron acceptor, with device structures consisting of glass/ITO/PEDOT:PSS/DTS-Cx (x = 4, 6, EH, and 8):PC71BM/Ca/Al. The fabrication conditions of the active layers were optimized by varying the blending ratio; the optimal weight ratio of all copolymer/PC71BM combinations was 1:2. Note that thermal annealing did not essentially improve the photovoltaic performance, probably due to the amorphous nature of the copolymers. The active layers were prepared by spin-coating under a nitrogen atmosphere from o-DCB solution without thermal annealing, and film thicknesses were controlled to ~80 nm. These devices were tested under air mass 1.5 global (AM 1.5 G) solar irradiation with an irradiation intensity of 100 mW cm–2. The best-performance J–V curves are shown in Figure 4(a) and Figure S6, and extracted device parameters are listed in Table 2. All the J–V curves and device parameters are summarized in Figure S7 and Tables S1-S5

Figure 4. (a) J–V curves and (b) EQE spectra of the DTSC4/PC71BM (red), DTS-C6/PC71BM (blue), DTS-CEH/PC71BM (green), and DTS-C8/PC71BM (black) OPV devices.

To investigate the influence of molecular weight, the photovoltaic properties of DTS-C6 and DTS-C6-L were first evaluated. With a film thickness of 80 nm, DTS-C6-L delivered a moderate PCE of 4.55%. In contrast, the DTS-C6-based device showed an improved PCE of 5.21% with a VOC of 0.90 V, JSC of 10.59 mA cm–2, and FF of 0.55 (Figure S6). The PCE improvement is mainly ascribed to the increased JSC and FF. To reveal the origin of the improvement in the DTS-C6/PC71BMbased OPVs, we utilized atomic force microscopy (AFM) and SCLC measurements. AFM images of these blend films showed that, with increasing Mn, the nanometer-scale aggregated domains were slightly enlarged, with average roughness (Ra) values of 1.8 nm for DTS-C6-L and 2.1 nm for DTS-C6 (Figure S8(a)(c)). The SCLC hole mobility revealed for the DTS-C6/PC71BM film was 1.0 × 10–4 cm2 V–2 s–1, which is almost one order of magnitude larger than that of the DTS-C6L/PC71BM film (3.4 × 10–5 cm2 V–2 s–1) (Figure S9). Thus, the

increased JSC and FF for the DTS-C6-based device are derived from the contribution of morphological change and increased hole mobility. A similar trend noting the influence of molecular weight on photovoltaic performance has been reported.28,29,48-53 Under the same fabrication conditions, DTS-CEH exhibited a comparable PCE of 5.15%. In contrast, DTS-C4 and DTS-C8 showed similarly high VOC values, but displayed lower overall efficiencies. This deterioration is due to the decreased FF for DTS-C4 and JSC for DTS-C8. As shown in Figure 4(b), the EQE spectra of these devices exhibited similar photoresponses, between 300 and 800 nm. However, as a reflection of its low JSC values, the DTS-C8-based device showed the lowest EQE value among the four devices. Given the similar molecular weights, UV-vis spectra, HOMO and LUMO energy levels, and hole mobilities among the four copolymers, the apparent differences in the photovoltaic performance are somewhat surprising. Since the JSC and FF are partly correlated to nanoscale film morphology and carrier mobility, AFM and SCLC measurements of these blend films were performed. As shown in Figure S8, the AFM height images showed significant differences in surface morphology, and the average roughness (Ra) was estimated to be 1.7 nm for DTS-C4, 3.0 nm for DTS-CEH, and 3.2 nm for DTS-C8. The relatively high alkyl chain density for DTS-C8 in comparison to the other copolymers may cause poor intermixing with PC 71BM, which would hamper efficient exciton dissociation and result in low JSC. As summarized in Figure S10 and Table 2, the SCLC h values of these blend films range between 7.8 × 10 –5 and 1.1 × 10–4 cm2 V–2 s–1. It is worth noting that these values do not largely decrease in comparison with those for the pure copolymer films (Table 1), which is also a good indication of the amorphous nature of these copolymers. Since the h and FF in OPV devices are known to be correlated, we attribute the low FF for DTS-C4 to its low h Recent extensive research has established several strategies for controlling the film morphology of blend films.37,59-64 Based on these reports, we attempted to optimize solvent additives for the DTS-C6/PC71BM and DTS-CEH/PC71BM blend films by screening, to further improve OPV performance. However, the DTS-CEH/PC71BM film resulted in less efficient performance (Figure S11, Tables S6, S7). On the other hand, the DTS-C6/PC71BM films showed significant improvements. The best-performance J–V curves for the DTS-C6/PC71BMbased OPVs are shown in Figure 5(a), and the extracted OPV parameters are listed in Table 3. All the J–V curves and device parameters are summarized in Figures S12, S13 and Tables S8-S10. Upon adding 3% (v/v) 1,8-diiodooctane (DIO)59,60 to the spin-coating solvent (o-DCB), the PCE was further increased to 5.56% with a VOC of 0.87 V, JSC of 9.88 mA cm–2,



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Figure 5. (a) J–V curves and (b) EQE spectra of the DTS-C6/PC71BM OPV devices fabricated using o-DCB (blue), o-DCB/DIO (black), and o-DCB/CF/DIO (red). AFM height images of blend films fabricated using (c) o-DCB, (d) o-DCB/DIO, and (e) o-DCB/CF/DIO. 2D GIXD images of the DTS-C6/PC71BM films fabricated using (f) o-DCB, (g) o-DCB/DIO, and (h) o-DCB/CF/DIO.

and FF of 0.65. The improved efficiencies are the result of the increased FF (0.65 versus 0.55). When CF was used as the solvent instead of o-DCB, the device exhibited a VOC of 0.85 V, JSC of 15.54 mA cm–2, and FF of 0.41, and delivered a slightly decreased PCE of 5.42% (Figure S14). The resulting 190 nm film thickness under these fabrication conditions leads to improved light absorption while conductance is decreased; these can competitively affect the performance parameters and explain the increased Jsc and the decreased FF. In fact, the corresponding 90 nm thickness film showed the decreased Jsc (8.47 mA cm–2) and the increased FF (0.51) (Figure S14(a)). To maintain both the high FF (o-DCB/DIO) and high JSC (CF/DIO), a ternary solvent composition of o-DCB, CF, and DIO was then optimized.64 We found that the best volume ratio of o-DCB/CF was 4:1 with 3% DIO. Therefore, the OPV device based on DTS-C6/PC71BM (1:2 weight ratio) using oDCB/CF/DIO as the processing solvent exhibited the best PCE of 7.85%, with a VOC of 0.86, JSC of 14.39 mA cm–2, and FF of 0.64. The EQE spectrum of this device showed a high photore-

sponse between 300 and 800 nm with a maximum value of 70% at 500 nm (Figure 5(b)). The calculated JSC (14.66 mA cm–2) from the EQE spectrum was within 1.8% error, which verified the accuracy of the photovoltaic measurements. The internal quantum efficiency (IQE) of this device was estimated by the transmission absorption spectrum and EQE spectrum.36 The estimation includes the assumption that the Al electrode has the reflectivity of 90%.65 As shown in Figure S15, the IQE of this device was high, with the maximum values approaching 80%. The five devices showed an average PCE of 7.71% (Figure S13, Table S10). According to the height images of the blend films, the surfaces of o-DCB/DIO- and o-DCB/CF/DIOprocessed films tended to organize into larger domains with Ra values of 2.7 and 3.4 nm, respectively (Figure 5(c)–(e)). The series resistance (Rs) values of the OPV devices estimated from the slope of the J–V curve near zero current are summarized in Table 3. The value of Rs decreases with increasing FF, which implies an increase in the carrier mobility


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Table 3 OPV performance based on DTS-C6/PC71BM blend films. Processing

thickness

VOC

JSC

FF –2

Solvent

/nm

/V

/mA cm

o-DCB

80

0.90

10.59

o-DCB:DIO a

82

0.87

CF:DIO b

190

o-DCB:CF:DIO c

79

PCE

μh

Rs 2

2

–1 –1 d

/cm V s

μe /cm V–1 s–1 e 2

/%

/ cm

0.55

5.21 (5.13 ± 0.16) f

10.6

1.0 × 10–4

2.9 × 10–5

9.88

0.65

5.56 (5.41 ± 0.10) f

6.3

1.6 × 10–4

3.6 × 10–5

0.85

15.54

0.41

5.42 (5.19 ± 0.12) f

15.6

2.3 × 10–4

7.5 × 10–6

0.86

14.39

0.64

7.85 (7.71 ± 0.12) f

5.3

2.0 × 10–4

7.0 × 10–5

a

o-DCB:DIO (v/v) = 97:3. b CF:DIO (v/v) = 97:3. c o-DCB/CF (v/v) = 4:1 with 3% DIO. d ITO/PEDOT:PSS/blend films/Au. ITO/TiOx/blend films/Ca/Al. f The average and standard deviation of five devices are provided in parentheses, see the SI for details.

in the o-DCB/CF/DIO-processed film. In fact, the SCLC h was increased with increasing FF values except for the thicker film from CF/DIO (Figure S16, Table 3). The SCLC electron mobilities (μe) evaluated with device structures comprising ITO/TiOx/blend films/Ca/Al also displayed an increase in the electron-transporting characteristics of the o-DCB/CF/DIOprocessed film (Figure S16, Table 3). To further investigate the film microstructures of the o-DCB, o-DCB/DIO, and o-DCB/CF/DIO films, two-dimensional grazing incidence X-ray diffraction (2D GIXD) was used. The 2D GIXD images and their profiles are shown in Figure 5(f)– (h) and Figure S17, respectively. In all the films, the characteristic peaks originating from PC71BM are clearly observable at 1.3 Å–1, and significant differences in the degree of crystallinity for PC71BM are not observed. In contrast, visible differences can be detected for DTS-C6. Thin films fabricated with o-DCB do not show the diffraction peaks derived from DTS-C6, implying the amorphous nature of the copolymer in this BHJ film. Interestingly, when the thin films were fabricated using oDCB/DIO, a weak diffraction is seen at 1.6 Å–1 towards the qz axis, which corresponds to the – stacking between DTS-C6 molecules. In addition, this peak becomes slightly more intense after o-DCB/CF/DIO processing, which indicates the improved ordering of DTS-C6 among the three fabrication conditions (Figure S17). Accordingly, we can conclude that the use of the ternary solvent in the amorphous DTSC6/PC71BM system generates larger domains accompanied by subtly ordered - stacking of DTS-C6 backbones, which may contribute to enhanced carrier mobility as well as chargeseparation efficiency, and lead to improved photovoltaic performance.

In summary, the synthesis of high-molecular-weight copolymers DTS-Cx (x = 4, 6, EH, and 8), with Mn values exceeding 40 kg mol–1, was accomplished by the use of pure DTS-Sn in palladium-catalyzed polymerization reactions. The intrinsic amorphous behavior of these copolymers was revealed by DSC and XRD measurements. Both the polymer molecular weight and alkyl side chain identity in the acceptor unit exert subtle influences on molecular properties. On the other hand, a noticeable effect between these structural variations and the photovoltaic performance was observed, which is correlated with film morphology and/or the carrier mobility of the blend films. Furthermore, as a result of the thin-film fabrication method using a ternary solvent system, the BHJ solar cell based on the DTS-C6/PC71BM combination achieved a PCE of 7.85%, which is the highest value reported for amorphous copolymers in a conventionally structured device.30,35 Important-

e

ly, this study clearly illustrates that even copolymers that favor an amorphous morphology in the solid state can achieve improved photovoltaic performance by fine-tuning of the film morphology. In other words, the establishment of novel fabrication techniques for the ordering against the amorphous copolymer in BHJ films will pave the way for the development of OPV. Further studies based on this concept will give us important insights into structure–property–OPV performance relationships and will accelerate advancement in BHJ solar cells.

Supporting Information. The results of analytical GPC, TGA, DSC, UV-vis, PESA, SCLC, OPV, IQE, GIXD, and NMR measurements as well as AFM images. This material is available free of charge via the Internet at http://pubs.acs.org.

*E-mail: [email protected]; [email protected]

This work was supported by a Grant-in-Aid for Scientific Research on Innovative Area from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), a Grant-in-Aid for Scientific Research and Exploratory Research from Japan Society for the Promotion of Science (JSPS), and by cooperative Research with Sumitomo Chemical Co., Ltd. We are thankful to Prof. Yasujiro Murata and Dr. Atsushi Wakamiya, Institute for Chemical Research (ICR) in Kyoto University for PESA measurements. This work was partially supported by the Collaborative Research Program of the Institute for Chemical Research, Kyoto University (grant 2012-55, 2013-9). We acknowledge Mr. Takeo Makino for synthesis support. The authors thank Dr. Tomoyuki Koganezawa for supporting the GIXD measurements. GIXD measurements were performed at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2014A1530). We also thank Dr. Keisuke Tajima in RIKEN for supporting IQE measurements. Thanks are extended to the CAC, ISIR, for assistance in obtaining elemental analyses.

GPC, gel-permeation chromatography; NMR, nuclear magnetic resonance; PEDOT:PSS, poly(3,4ethylenedioxythiophene:poly(styrenesulfonate); ITO, indium tin oxide; GIXD, grazing incidence X-ray diffraction; EI, electron ionization.



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Table of Contents (TOC)


9

Enhanced Photovoltaic Performance of Amorphous Copolymers

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