Donor–acceptor polymers with a regioregularly incorporated thieno[3,4-b]thiophene segment as a π-bridge for organic photovoltaic devices

Synthetic Metals 211 (2016) 75–83

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Donor–acceptor polymers with a regioregularly incorporated thieno [3,4-b]thiophene segment as a p-bridge for organic photovoltaic devices Honggi Kim, Hyungjin Lee, Youngjun Jeong, Ju-Un Park, Donghyun Seo, Hyojung Heo, Donghwa Lee, Yumi Ahn, Youngu Lee* Department of Energy Systems Engineering, DGIST, 333, Techno Jungang Daero, Hyeonpung-Myeon, Dalseong-Gun, Daegu 711-873, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Article history: Received 6 August 2015 Received in revised form 17 October 2015 Accepted 16 November 2015 Available online xxx

A series of donor-acceptor (D–A) polymers with regioregularly incorporated a thieno[3,4-b]thiophene as a p-bridge between electron donor and electron acceptor segments were successfully synthesized for the first time. The optical, thermal, and electrochemical properties of the D–A polymers with the thieno[3,4b]thiophene as a p-bridge were characterized by UV/vis spectroscopy, thermogravimetric analysis, and cyclic voltammetry measurements. They exhibited low energy bandgap because of extended p-conjugation length and increased electron density through conjugated polymer backbones. Moreover, they showed suitable HOMO and LUMO energy levels for photovoltaic applications. The bulk heterojunction organic photovoltaic devices based on the D–A polymers with the regioregularly incorporated thieno[3,4-b]thiophene segment exhibited about 10% higher power conversion efficiency (PCE) than the D–A polymer with thienyl segment as a p-bridge due to enhanced light harvesting ability and excellent nanoscale network. This study implies that the D–A polymers with the regioregularly incorporated thieno[3,4-b]thiophene segment as a p-bridge can achieve high PCE through structural modification of the polymers. ã 2015 Elsevier B.V. All rights reserved.

Keywords: Organic photovoltaic devices Donor–acceptor polymer Thieno[3,4-b]thiophene Low bandgap

1. Introduction Organic photovoltaic devices (OPVs) have attracted considerable attention because of their advantages of light weight, flexibility, low costs of materials, and low fabrication cost [1–4]. The bulk heterojunction (BHJ) for solar cell devices is composed of a phase-separated blend of electron-donors such as p-type conjugated polymers and electron-acceptors such as n-type fullerene materials [5–8]. Recently, the OPVs showed power conversion efficiency (PCE) up to 11.5% [9,10]. However, it is still necessary to develop new types of electron-donor polymers to achieve high PCE in device performance. The electron-donor polymers for OPVs need to possess several characteristics such as harvesting photons in a wide range of spectrum, efficient exciton dissociation, high hole mobility, suitable HOMO and LUMO energy levels, and good miscibility with the electron-acceptor materials [11–15]. To achieve these characteristics for electron-donor polymers, various donor–acceptor (D–A) types of copolymers

* Corresponding author. Fax: +82 53 785 6409. E-mail address: [email protected] (Y. Lee). http://dx.doi.org/10.1016/j.synthmet.2015.11.016 0379-6779/ ã 2015 Elsevier B.V. All rights reserved.

which contain electron-rich and electron-deficient segments alternatively have been synthesized and characterized [16–19]. The most commonly employed acceptor unit for D–A types of copolymers is the di-2-thienyl-2,1,3-benzothiadiazole (DTBT). Since the 2,1,3-benzothiadiazole (BT) segment has strong electron accepting ability, DTBT has been popular in synthesizing low bandgap D–A polymers [20–22]. Two flanking thienyl segments in the DTBT act as a p-bridge between the electron donor segment and the BT segment, leading to enhanced electron density through conjugated polymer backbones. In addition, they can reduce the severe steric hindrance between the electron donor segment and the BT segment to obtain more planar structure, leading to low energy bandgap. However, even though the DTBT is conjugated with strong electron donating segments for D–A polymers, the energy bandgap of the resulting D–A polymers is not low enough because the flanking thienyl segment in the DTBT can only provide limited electron density to the conjugated polymer backbone. [23] Thus, it is necessary to replace the flanking thienyl segment of the DTBT with a new electron-rich molecular structure to achieve low energy bandgap D–A polymers for photovoltaic applications. A thieno[3,4-b]thiophene (TT) segment has been utilized as a conjugated molecular backbone because it possesses interesting

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electrical and optical properties [24–27]. It is well-known that the TT segment possesses a higher electron density than the thienyl segment [28–31]. Moreover, it can provide more possible positions for further modification of electronic properties and solubility of conjugated polymers with minimal steric hindrance. Thus, a series of poly(thieno[3,4-b]thiophene)benzothiophene (PTB) copolymers comprised of TT and benzo[1,2-b:4,5-b0 ]dithiophene (BDT) segments such as PTB7, PBDTTT-C, and PBDTTT-C-T have been developed for high-efficiency OPVs [26–32]. However, it is very difficult to incorporate the TT segment as a p-bridge between the electron donor and electron acceptor segments because of synthetic difficulty of a mono-brominated TT segment. In addition, since the TT segment has an asymmetric molecular structure, the flanking TT segment with the BT unit basically possess random structural regioregularity. Therefore, it is crucial to control the orientation of the mono-brominated TT segment during the coupling reaction with the BT unit. Recently, we reported synthesis of a regioregular p-type copolymer comprised of TT-BDT repeating units and perfectly controlled TT orientation using kinetically controlled mono-bromination on the 6-position of thieno[3,4-b] thiophene segment and the Stille coupling reaction [28]. Therefore, it is possible to replace the flanking thienyl segment of the DTBT with the regioregularly controlled TT segment to achieve new low bandgap D–A polymers for OPVs. In this paper, we describe the synthesis and characterization of a series of D–A polymers with a TT segment as a p-bridge between the electron donor and electron acceptor segments. The optical, thermal, and electrochemical properties of the synthesized polymers were characterized by UV/vis spectroscopy, thermogravimetric analysis (TGA), and cyclic voltammetry (CV) measurements. The D–A polymers containing the TT segment exhibited low energy bandgaps because of extended p-conjugation length and increased electron density through polymer backbones. Moreover, they showed suitable HOMO and LUMO energy levels for photovoltaic applications. The bulk heterojunction organic photovoltaic devices based on the D–A polymers with the regioregularly incorporated TT segment exhibited about 10% higher power conversion efficiency (PCE) than the polymer with thienyl segment as a p-bridge due to enhanced light harvesting ability and excellent nanoscale network. This study implies that the D–A polymers containing the TT segment as a p-bridge can achieve high PCE through structural modification of the polymers. 2. Experimental 2.1. Materials and synthesis All reagents were obtained from Sigma–Aldrich, ACROS, and TCI and used without further purification. Thieno[3,4-b]thiophene-2carboxylic acid 2-ethyl-hexyl ester (1) and monomer 2 were synthesized according to literature procedures [20,32]. All reactions were carried out under nitrogen. 1H NMR and 13C NMR spectra were measured by NMR spectrometer (AVANCE 3, Bruker) using tetramethylsilane as internal reference. CDCl3 was used as a solvent for NMR analysis. 2.1.1. 6-Bromo-thieno[3,4-b]thiophene-2-carboxylic acid 2-ethylhexyl ester (2) Thieno[3,4-b]thiophene-2-carboxylic acid 2-ethyl-hexyl ester (1) (1.49 g, 5.03 mmol) was added into a round flask with DMF (10 mL). A solution of NBS (0.90 g, 5.03 mmol) in DMF (10 mL) was added dropwise to the reaction mixture and stirred for 30 min. The reaction mixture was poured into deionized (DI) water and extracted with ethyl acetate several times. The organic phase was dried over anhydrous sodium sulfate. The solvent was removed under vacuum. The residue was purified by column

chromatography on silica gel using methylene chloride and hexane (1:2) as an eluent to obtain pure compound 2 (0.79 g, 41.8%) as oil. 1 H NMR (CDCl3, 400 MHz): 7.54 (s, 1H), 7.23 (s, 1H), 4.25-4.23 (m, 2H), 1.73–1.68 (m, 1H), 1.48–1.30 (m, 8H), 0.76–0.89 (m, 6H). 13C NMR (CDCl3, 100 MHz): 162.84; 145.89; 140.80; 138.88; 122.36; 112.56; 102.78; 68.22; 38.85; 30.49; 28.96; 23.93; 22.97; 14.06; 11.08. HRMS (m/z): calcd for C15H19BrO2S2, 376.00; found, 376.10 [M]+. 2.1.2. 4,7-Bis(2-(2-ethyl-hexyl ester)-thieno[3,4-b]thiophene-6-yl)benzo[1,2,5]thiadiazole (3) 2,1,3-benzothiadiazole-4,5-bis(boronic acid pinacol ester) (0.33 g, 0.84 mmol), tetrakis (triphenylphosphine) palladium(0) (0.05 g, 0.04 mmol) and compound 2 (0.79 g, 2.10 mmol) were dissolved in toluene (20 mL) with a solution of potassium carbonate (0.70 g, 5.04 mmol) in DI water (10 mL) and ethanol (10 mL). The mixture was refluxed and stirred overnight at 110  C under nitrogen. The mixture was cooled to room temperature, and then the organic phase was dried over anhydrous sodium sulfate, solvent was removed under vacuum. The residue was purified by column chromatography on silica gel using methylene chloride and hexane (1:2) as an eluent to obtain pure compound 3 (0.40 g, 65.7%) as red solid. 1H NMR (CDCl3, 400 MHz): 8.12 (d, 2H), 8.05 (s, 2H), 7.53 (s, 2H), 4.28–4.27 (m, 4H), 1.75–1.64 (m, 2H), 1.50–1.25 (m, 16H), 0.96–0.88 (m, 12H). 13C NMR (CDCl3, 100 MHz): 162.99; 152.27; 143.17; 140.88; 140.57; 129.81; 126.68; 125.64; 124.21; 114.41; 68.19; 38.84; 30.50; 28.95; 23.97; 23.00; 14.10; 11.11. HRMS (m/z): calcd for C38H40N2O4S5, 724.16; found, 724.30 [M]+. Anal. Calcd for: C36H40N2O4S5: C, 59.64; H, 5.56; N, 3.86; S, 22.11. Found: C, 59.96; H, 5.52; N, 3.82; S, 22.04. 2.1.3. Monomer 1 Compound 3 (0.40 g, 0.55 mmol) was added into a round flask with chloroform (10 mL). A solution of N-bromosuccinimde (NBS) (0.22 g, 1.21 mmol) in chloroform (10 mL) was added dropwise to the reaction mixture and stirred for 10 min. The reaction mixture was poured into DI water and extracted with ethyl acetate several times. The organic phase was dried over anhydrous sodium sulfate and solvent was removed under vacuum. The residue was purified by column chromatography on silica gel using methylene chloride and hexane (1:1) as an eluent to obtain pure monomer 1 (0.37 g, 76.2%) as deep blue solid. 1H NMR (CDCl3, 400 MHz): 8.10 (d, 2H), 7.96 (s, 2H), 4.31–4.25 (m, 4H), 1.77–1.72 (m, 2H), 1.56–1.35 (m, 16H), 0.99–0.88 (m, 12H). 13C NMR (CDCl3, 100 MHz): 162.61; 151.88; 142.53; 142.24; 141.18; 131.08; 126.13; 125.11; 124.79; 102.46; 68.44; 38.87; 30.50; 28.99; 23.97; 23.02; 14.11; 11.11. HRMS (m/z): calcd for C38H38Br2N2O4S5, 881.98; found, 882.20 [M]+. Anal. Calcd for: C36H38Br2N2O4S5: C, 48.98; H, 4.34; N, 3.17; S, 18.16. Found: C, 49.08; H, 4.40; N, 3.05; S, 17.67. 2.1.4. General method of the Stille polycondensation reaction for PBTTBT and PBDTBT PBTTBT and PBDTBT were prepared by the Stille polycondensation reaction with monomers 1 and 2, and same equivalent of 2,6-bis(trimethyltin)-4,8-bis(2-ethylhexyloxy) benzo[1,2-b:4,5-b0 ] di-thiophene, and tetrakis(triphenylphosphine) palladium(0) (5 mol%) in toluene (10 mL), respectively. The reaction mixture was stirred and refluxed for 24 h at 110  C. The mixture was cooled to room temperature. Then, toluene was removed under vacuum. To remove byproducts and oligomers, soxhlet extractions were used with n-hexane and methanol and pure polymers were obtained by drying extracted solution from chloroform under vacuum. PBTTBT Mw: 20.6 kDa, Mn: 29.8 kDa, PDI: 1.47. 1H NMR (CDCl3, 400 MHz): 7.72–6.83 (br, 6H), 4.62–3.82 (br, 8H), 2.10–0.75 (m, 60H), 1.48–1.30 (m, 16H), 0.96–0.88 (m, 12H). FT-IR (ATR, cm1):

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2921.4 ( CH3,  CH2 ), 2855.8 (CH3,  CH2 ), 1707.7 (C¼O), 1240.7 (C O), 1182.2 (C O), 882.6 (¼CH), 809.3 (¼CH). Anal. Calcd for: (C62H74N2O6S7)n: C, 63.77; H,6.39; N, 2.40; S, 19.22. Found: C, 63.72; H, 6.39; N, 2.23; S, 18.81. PBDTBT FT-IR (ATR, cm1): 2920.3(CH3,  CH2 ), 2854.6 ( CH3,  CH2), 1427.2 ( CH3,  CH2 ), 1358.2, 1230.6 (CO), 821.9 (¼CH). Anal. Calcd for: (C40H42N2O2S5)n: C, 64.65; H,5.70; N, 3.77; S, 21.58. Found: C, 63.83; H, 5.66; N, 3.65; S, 21.17. 2.1.5. General method of the Suzuki polycondensation reaction for PFTTBT and PFDTBT PFTTBT and PFDTBT were prepared by the Suzuki polycondensation reaction with monomers 1 and 2, and same equivalent of 9,9-dihexylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester, tetrakis(triphenylphosphine) palladium(0) (5 mol%) in toluene (10 mL). A solution of sodium carbonate in water (5 mL) and ethanol (5 mL) were added into a round flask. The mixture was refluxed and stirred for 24 h at 110  C under nitrogen. The mixture was cooled to room temperature. The mixture was poured into DI water and extracted with chloroform. The organic phase was dried over anhydrous sodium sulfate and solvent was removed under vacuum. To remove byproducts and oligomers, soxhlet extractions were used with n-hexane and methanol and pure polymers were obtained by drying extracted solution from chloroform under vacuum. PFTTBT Mw: 24.3 kDa, Mn: 45.8 kDa, PDI: 1.88. 1H NMR (CDCl3, 400 MHz): 7.97–6.93 (br, 10H), 4.68–4.14 (br, 4H), 2.22–0.77 (m, 56H). FT-IR (ATR, cm1): 2921.6 (CH3,  CH2 ), 2852.9 ( CH3,  CH2), 1706.7 (C¼O), 1439.5 ( CH3,  CH2), 1238.6 (CO), 1186.1 (C O), 808.7 (¼CH). Anal. Calcd for: (C61H70N2O4S5)n: C, 69.41; H,6.68; N, 2.65; S, 15.19. Found: C, 68.26; H, 6.72; N, 2.36; S, 15.18. PFDTBT Mw: 3.6 kDa, Mn: 3.8 kDa, PDI: 1.06. 1H NMR (CDCl3, 400 MHz): 8.15–6.81 (12H, m), 2.17–1.99 (m, 4H), 1.68–1.55 (br, 2H), 1.25–1.11 (br, 11H), 0.79–0.76 (m, 9H). FT-IR (ATR, cm1): 2923.1( CH3,  CH2 ), 2850.8 ( CH3, CH2 ), 1465.1 ( CH3,  CH2), 1443. 6 ( CH3, CH2 ), 878.0 (¼C H), 828.8 (¼CH). Anal. Calcd for: (C39H38N2S7)n: C, 74.24; H,6.07; N, 4.44; S, 15.25. Found: C, 72.95; H, 6.05; N, 4.33; S, 15.69.

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2.2. Material characterizations The Fourier transform infrared spectroscopy (FT-IR) spectra were measured by FT-IR spectrometer (Continuum, Thermo Scientific). Elemental analyses were performed by a elemental analyzer (Vario MICRI cube, Elementar). Thermal properties of the polymers were measured by using a TGA instrument (TG8120, Rigaku). The analyses were carried out at rate of 10  C min1 from ambient temperature to 500  C under nitrogen. Molecular weights of the polymers were measured by using a gel permeation chromatography (GPC) (e2695, Waters) with tetrahydrofuran (THF) as an eluent. The molecular weights and polydispersities were estimated using polystyrene standards as calibrant. The absorption spectra were measured by using a UV/vis spectrophotometer (CARY 5000, Agilent). The absorption spectra of the polymer solutions were measured in chloroform in quartz cuvettes and films were measured by the thin films on glass substrates. CV measurements were carried out on a CV instrument (VMP3, BioLogic) with a glassy carbon electrode, Pt electrode, and Ag/AgCl electrode as working electrode, counter electrode, and reference electrode, respectively, in acetonitrile solution of 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6). The surface morphology measurements were performed by using an atomic force microscope (AFM) (NX10, Park systems). 2.3. Fabrication and characterization of organic photovoltaic devices An Indium Tin Oxide (ITO) coated glass was cleaned by ultrasonic treatment in acetone, DI water, and isopropyl alcohol and dried by using nitrogen gas. The cleaned ITO coated glass was treated in an UV-ozone chamber for 20 min and immediately spincoated with a ZnO solution. The ZnO layer was formed by sol–gel method. The sol–gel derived ZnO was prepared by dissolving zinc acetate dihydrate (Zn(CH3COO)22H2O, 1 g) and ethanolamine (NH2CH2CH2OH, 0.28 g) in 2-methoxyethanol (CH3OCH2CH2OH, 10 mL) under vigorous stirring for 24 h in the air. The ZnO coated ITO glass was annealed on a hot plate for 1 h at 200  C in air. The polymer and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), which were blended in different weight ratio, were dissolved in

Scheme 1. Molecular structures and synthesis of monomers 1 and 2.

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chlorobenzene. Then, the polymer:PC71BM solution was spincoated on top of the ZnO layer and dried for 60 min at room temperature and then annealed for 10 min at 160  C. Finally, an anode layer composed of a MoO3 layer (10 nm) and an Ag layer (100 nm) was deposited by thermal evaporation with the shadow mask in a high vacuum thermal evaporator (<107 Torr). Therefore, we fabricated the inverted type OPVs with a ITO/ZnO (40 nm)/ polymer:PC71BM/MoO3 (10 nm)/Ag (100 nm) configuration. Thickness of each layer was measured using a scanning electron microscope (SU8020, Hitachi). The current–voltage (J–V) characteristics were measured using a solar simulator (K-3300, McScience) in the air without an encapsulation of the cell. One sun light intensity (100 mW/cm2) was calibrated by using a c-Si photodiode (K801S-K13, McScience) as a standard. 3. Results and discussion 3.1. Polymer synthesis and characterization The general synthetic routes toward monomers and polymers are outlined in Schemes 1 and 2. Synthesis of an alkylester substituted TT (1) was carried out as previously reported [20]. Since compound 1 has an asymmetric molecular structure, we carried out selective mono-bromination on 6-position of compound 1 to obtain compound 2. It is noteworthy that the monobrominated TT (2) was isolated successfully in pure form. The mono-brominated compound 2 was coupled with 2,1,3-benzothiadiazole-4,7-bis(boronic acid pinacol ester) to afford compound 3 by the Suzuki coupling reaction. Then, compound 3 was dibrominated to afford the electron accepting monomer 1. The selective mono-bromination of the TT makes it possible to

synthesize regioregular D–A polymers incorporating the TT segment as a p-bridge between the electron donor and electron acceptor segments. The electron accepting monomer 1 was copolymerized with electron donating benzodithiophene and fluorene monomers to afford PBTTBT and PFTTBT using the Stille and Suzuki polycondensation reactions, respectively. To compare the effect of incorporation of the TT segment as a p-bridge into D– A polymers, we also synthesized PFDTBT and PBDTBT as control polymers by carrying out the Suzuki and Stille polycondensation reactions with electron accepting monomer 2 and electron donating fluorene and benzodithiophene monomers. Therefore, four different polymers (PBTTBT, PFTTBT, PFDTBT, and PBDTBT) with the structural variation were synthesized. The chemical structures of the polymers were characterized with 1H-NMR and IR spectroscopies. PBTTBT, PFTTBT, and PFDTBT are readily soluble in organic solvents such as chloroform, THF, chlorobenzene, and dichlorobenzene. However, PBDTBT exhibits limited solubility in organic solvents due to the lack of side chains on a thiophene segment. The number-average and weight-average molecular weights (Mn, Mw) and polydispersity index (PDI) of the polymers were measured by the GPC against polystyrene standards by using THF as an eluent. Table 1 summarizes number-average and weightTable 1 Molecular weights and thermal properties of PBTTBT, PFTTBT, and PFDTBT. Polymer

Mn (kg/mol)

Mw (kg/mol)

PDI

Td ( C)

PBTTBT PFTTBT PFDTBT

20.6 24.3 3.6

29.8 45.8 3.8

1.47 1.88 1.06

328 365 407

Scheme 2. Molecular structures and synthesis of PBTTBT, PFTTBT, PFDTBT, and PBDTBT.

H. Kim et al. / Synthetic Metals 211 (2016) 75–83

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average molecular weights and PDI of the polymers. PBTTBT and PFTTBT have Mn of 20.6–24.3 kg/mol and Mw of 29.8–45.8 kg/mol with PDI of 1.47–1.88. Both polymers have similar molecular weight, thereby minimizing the potential complication from the influence of different molecular weights. However, PFDTBT shows relatively low Mn and Mw values, which is attributed to ineffective polymerization caused by rigid molecular structure and limited solubility [34–36]. This result clearly implies that the incorporation of the alkyl ester substituted TT segment into the conjugated polymer backbone can successfully increase molecular weight of the D–A polymers. The thermal stabilities of the polymers were characterized by TGA under nitrogen. As shown in Fig. 1, the 5% weight loss temperatures of the polymers are shown from 330 to 408  C, indicating that all of the polymers possess high thermal stability. The change of donating unit and introduction of the TT segment slightly influence thermal stability of the polymers.

optical bandgap of 1.17 eV. PFTTBT shows maximum absorption wavelength at 690 nm with an onset absorption wavelength at 943 nm, corresponding to an optical bandgap of 1.31 eV. On the other hand, PFDTBT shows maximum absorption wavelength at 554 nm with an onset absorption wavelength at 632 nm, corresponding to an optical bandgap of 1.96 eV. These results clearly indicate that introduction of the TT segment in a D–A polymer increases electron density through molecular backbones, resulting in low optical bandgap. In addition, it is observed that the intramolecular charge transfer in PBTTBT is much stronger than PFTTBT because benzodithiophene segment possesses a stronger electron donating ability than the fluorene segment [37]. Moreover, the benzodithiophene segment might form a quinoidal structure with the TT segment, leading to low optical bandgap [30].

3.2. Optical properties of polymers

Cyclic voltammetry (CV) measurement was used to investigate electrochemical properties of both polymers such as the energy levels of HOMO and LUMO as well as electrochemical bandgap. Fig. 3 shows cyclic voltammograms of PBTTBT, PFTTBT, and PBDTBT thin films. The redox potential of ferrocene/ferrocenium (Fc/Fc+) was measured at 0.42 eV to the Ag/AgCl electrode. It is assumed that the redox potential of Fc/Fc+ has an energy level of 4.80 eV. Therefore, the HOMO and LUMO energy levels of the polymers are calculated according to the following Eq. (1).
HOMOðeVÞ ¼ 4:80eV  Eox onset  0:42eV ðeVÞ

The UV/vis absorption spectra of PBTTBT, PFTTBT, and PFDTBT in solution and thin films are shown in Fig. 2. The polymer solution is prepared in chloroform and the thin film is prepared by spincoating dichlorobenzene solution of the polymers on a glass substrate. As shown in Fig. 2(a), in the absorption solution, PBTTBT and PFTTBT show strong absorption peak around 450 nm presumably due to delocalized excitonic p–p* transition in the conjugated polymer backbones. The maximum absorption bands of the polymers are observed around 700 nm, which can be ascribed to the intramolecular charge transfer (ICT) between the donor and acceptor segments. As for the absorption spectra of PBTTBT, PFTTBT, and PFDTBT measured at the same concentration (0.01 mg/mL), the absorption intensity of PBTTBT is slightly higher than that of PFTTBT and PFDTBT (Fig. S8). As shown in Fig. 2(b), the thin films of the polymers exhibit red-shifted absorption spectra. Thus, the absorption spectra of the polymers in the thin films exhibit longer maximum absorption peak and onset wavelength, which can be attributed to effective intermolecular interaction between the conjugated polymer backbones. Especially, PFTTBT shows weak vibronic shoulder due to effective p–p stacking between the conjugated polymer backbones. Table 2 summarizes optical properties of the polymer solutions and films. PBTTBT exhibits a maximum absorption wavelength at 749 nm with an onset absorption wavelength at 1060 nm, corresponding to an

3.3. Electrochemical properties of polymers

  LUMOðeVÞ ¼ 4:80eV  Ered onset  0:42eV ðeVÞ

ð1Þ

Electrochemical properties of the polymers are summarized in Table 3. The oxidation onsets of PBTTBT, PFTTBT, and PFDTBT are 0.55, 0.90, and 1.24 eV, corresponding to HOMO energy levels of 4.93, 5.28, and 5.62 eV, respectively. The reduction onsets of the polymers are 0.80, 0.74, and, 0.78 eV, resulting in LUMO energy levels of 3.58, 3.64, and 3.60 eV, respectively. These results clearly verify that the HOMO energy level of the D–A polymer is mainly determined by electron rich benzodithiophene or fluorene segments. It is also found that the HOMO energy level of PFTTBT is 0.35 eV higher than PBTTBT because the fluorene segment in PFTTBT has lower electron donating ability than the benzodithiophene segment in PBTTBT. In addition, PFTTBT shows a relatively high-lying HOMO energy level compared to PFDTBT, which might be attributed to electron rich nature of the TT segment compared to the thienyl segment. On the other hand, the LUMO energy levels of PBTTBT, PFTTBT, and PFDTBT are almost identical, indicating that the LUMO energy levels of the polymers are mainly determined by electron deficient BT segment. The electrochemical bandgap values of the polymers calculated from CV measurements are 1.35, 1.64, and 2.02 eV, indicating that these values are in good agreement with the optical bandgap values. These results clearly demonstrate that the electrochemical properties of the polymers can be tuned by modifying the molecular structure of the D–A polymers. 3.4. Photovoltaic properties of polymers

Fig. 1. TGA plots of PBTTBT, PFTTBT, and PFDTBT with a heating rate of 10  C/min under nitrogen.

To examine the photovoltaic performance of the polymers as electron donors and investigate the influence of the TT segment of conjugated polymer backbones on the photovoltaic performance of the solar cell devices, the inverted bulk heterojunction (BHJ) organic photovoltaic (OPV) devices were fabricated with a structure of ITO/ZnO/polymer:PC71BM/MoO3/Ag. The active layers of polymer composites were spin-coated from polymer/PC71BM

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Fig. 2. UV/vis absorption spectra of PBTTBT, PFTTBT, and PFDTBT (a) in chloroform solution and (b) in thin films.

Table 2 Optical properties of PBTTBT, PFTTBT, and PFDTBT. Polymer

PBTTBT PFTTBT PFDTBT

Solution

Table 3 Electrochemical properties of PBTTBT, PFTTBT, and PFDTBT. Eg0pt (eV)

Film

lmax (nm)

lonset (nm)

lmax (nm)

lonset (nm)

726 639 528

1020 778 609

749 690 554

1060 943 632

1.17 1.31 1.96

solutions prepared in chlorobenzene and 1,8-diiodooctane (DIO, 3 v/v%). The active layer thickness was about 80 nm. Fig. 4 shows current density–voltage (J–V) characteristics of inverted BHJ OPVs with the polymers under AM 1.5 G illumination (100 mW/cm2). Representative characteristics of the solar cell devices such as open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF), and PCE are summarized in Table 4. The maximum PCEs of PBTTBT and PFTTBT were obtained when a 1:2 composite ratio of polymer and PC71BM were used. On the other hand, the maximum PCE of PFDTBT was obtained when a 1:3 composite ratio of polymer and PC71BM were used. The Voc of the BHJ solar cell devices increases from PBTTBT to PFTTBT to PFDTBT (from 0.54 to 0.78 to 0.92 V). The solar cell device based on PBTTBT turns out to have the lowest Voc because PBTTBT (4.93 eV) possesses a relatively high-lying HOMO energy level compared to PFTTBT (5.28 eV) and PFDTBT (5.62 eV). It is well-known that the Voc mainly depends on the difference between the HOMO energy level of the electron donor and the LUMO energy level of the electron acceptor. These results

Polymer

Eonsetox (V)

HOMO (eV)

Eonsetred (V)

LUMO (eV)

Ehec (eV)

PBTTBT PFTTBT PFDTBT

0.55 0.90 1.24

4.93 5.28 5.62

0.80 0.74 0.78

3.58 3.64 3.60

1.35 1.64 2.02

clearly confirm that extended p-conjugation length and increased electron density by incorporation of the TT segment into the conjugated polymer backbone results in increase of HOMO energy level of the polymers. In the case of PBTTBT and PFTTBT, PFTTBT exhibits a relatively higher Voc than PBTTBT because the fluorene segment in PFTTBT possesses lower-lying HOMO energy level compared to the benzodithiophene segment in PBTTBT. In contrast, the Jsc of the BHJ solar cell devices decreases from PBTTBT to PFTTBT to PFDTBT. The highest Jsc of 7.40 mA/cm2 is observed in a solar cell device based on PBTTBT, which is 4.8 and 13.0% higher than PFTTBT (7.06 mA/cm2) and PFDTBT (6.55 mA/ cm2) based solar cell devices. This is mainly due to enhanced light harvesting ability by incorporation of the TT segment into the conjugated polymer backbone. Relatively high FF values of 44–46% are observed in solar cell devices based on PBTTBT and PFTTBT, which are much higher than PFDTBT based devices (37.0%). This result also implies that the TT segment in PBTTBT and PFTTBT improves film morphology of the polymer/PC71BM films. As a result, the solar cell device based on PFTTBT shows the best PCE of 2.46%, whereas PBTTBT and PFDTBT based devices show 1.85 and

Fig. 3. Cyclic voltammograms of (a) PBTTBT, (b) PFTTBT, and (c) PFDTBT thin films.

H. Kim et al. / Synthetic Metals 211 (2016) 75–83

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the Mott–Gurney law (Eq. (2)). [33] J¼

Fig. 4. J–V characteristics of BHJ solar cell devices based on PBTTBT/PC71BM, PFTTBT/PC71BM, and PFDTBT/PC71BM under the illumination of AM1.5 G, 100 mW/ cm2.

Table 4 Photovoltaic properties of BHJ solar cell devices based on PBTTBT/PC71BM, PFTTBT/ PC71BM, and PFDTBT/PC71BM under the Illumination of AM1.5 G, 100 mW/cm2. Polymer

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

PBTTBT PFTTBT PFDTBT

0.54 0.78 0.92

7.40 7.06 6.55

46.1 44.4 37.0

1.85 2.46 2.23

2.23%, respectively. The photovoltaic results clearly demonstrate that increasing electron density and light harvesting ability of conjugated D–A polymers, by introducing the TT segment into it, can significantly improve the device performance. 3.5. Charge transport and morphological properties of polymers To investigate how the TT segment affects the charge transport of the polymers, the space charge limited current (SCLC) measurements were used to evaluate the hole mobility of the polymers. The hole-only devices were fabricated with the structure ITO/PEDOT: PSS/polymer/Al. Hole mobilities of polymers were calculated from

9 V2 er e0 mh 3 8 L

ð2Þ

where e0 is the permittivity of free space, er is the dielectric constant of the polymer, mh is the hole mobility, V = Vappl  Vbi  Va (Vappl: applied bias; Vbi: built-in potential due to the difference in electrical contact work function; Va: voltage drop due to contact resistance and series resistance across the electrodes), and L is the thickness of polymer. As shown in Fig. 5, the average hole mobility values of PBTTBT, PFTTBT, and PFDTBT are found to be 9.66  105, 1.08  105, 1.39  104 cm2/Vs, respectively. These results clearly imply that PFDTBT has the highest hole mobility because of the strong intermolecular interaction and molecular packing between polymer backbones. It may be attributed to the lack of side chain on the thiophene segment between fluorene and BT segments. Therefore, although PFDTBT exhibits the large energy bandgap, it shows the moderate Jsc due to the improved charge carrier transport in the active layer. On the other hand, PBTTBT and PFTTBT exhibit relatively low hole mobilites because the bulky side chains on the TT segment tend to retard the charge transport between the polymer backbones. Especially, PFTTBT exhibits the lowest hole mobility, which is ascribed to the severe steric hindrance between fluorene and TT segments. It has been reported widely that the film morphology of the active layer has a great influence on exciton diffusion and dissociation as well as charge carrier transport and collection. The nanoscale morphologies of the polymer:PC71BM blended films with DIO additive were studied using tapping-mode atomic force microscopy (AFM). As shown in Fig. 6, the blended film of PFTTBT/ PC71BM shows evenly distributed domains with sizes of tens of nanometers and continuous interpenetrating networks without any observed large aggregates of either the donor or the acceptor. Moreover, it has a very low root mean square (RMS) surface roughness (<0.60 nm). These results clearly suggest that PFTTBT could be mixed with PC71BM at the molecular scale. It is wellknown that the ideal domain size in the active layer is on the order of tens of nanometers. The nanoscale network between PFTTBT and PC71BM could enable efficient charge separation as well as charge transport, leading to enhanced solar cell efficiency. On the other hand, the blended film of PBTTBT/PC71BM shows many aggregated domains with the size of over 100 nm originated from either the donor or the acceptor and relatively weak phase separation with a RMS surface roughness of 7.74 nm. This poor phase separation with a lot of domains could induce exciton recombination before reaching the donor–acceptor interface. Such poor miscibility in PBTTBT/PC71BM leads to inefficient exciton dissociation at interface and low charge carrier generation, resulting in a decrease of Jsc and FF values. As for the blended film of PFDTBT/PC71BM, although it has a low RMS surface roughness (<0.50 nm), large aggregates with diameter of over 1000 nm are still observed. They result in low solar cell efficiency because of inefficient diffusion of exciton to D–A interface. 4. Conclusion

Fig. 5. Hole (mh) carrier mobilities of PBTTBT, PFTTBT, and PFDTBT obtained by SCLC method.

We successfully synthesized a series of D–A polymers with the regioregulary incorporated TT segment as a p-bridge between the electron donor and electron acceptor segments for the first time. The optical, thermal, and electrochemical properties of the synthesized polymers were characterized by UV/vis spectroscopy, TGA, and CV measurements. It was found that the polymers containing the TT segment exhibited low bandgaps and suitable HOMO and LUMO energy levels for photovoltaic applications. The BHJ solar cell devices based on the D–A polymers with the TT

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Fig. 6. Tapping mode AFM topography images of the blend films of (a) PBTTBT/PC71BM, (b) PFTTBT/PC71BM, and (c) PFDTBT/PC71BM.

segment exhibited about 10% higher power conversion efficiency than the polymer with thienyl segment as a p-bridge due to enhanced light harvesting ability and excellent nanoscale network. This study implies that the D–A polymers containing the TT segment as a p-bridge can achieve high PCE through structural modification of the polymers. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future Planning (NRF2015R1A2A2A01003622, NRF-2015M1A2A2056216). This work was also supported by the Technology Innovation Program (10052802) funded by the Ministry of Trade, Industry & Energy (MI, Korea). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.synthmet. 2015.11.016. References [1] L. Dou, J. You, J. Yang, C.-C. Chen, Y. He, S. Murase, T. Moriarty, K. Emery, G. Li, Y. Yang, Tandem polymer solar cells featuring a spectrally matched low-bandgap polymer, Nat. Photonics 6 (2012) 180–185. [2] J.Y. Kim, K. Lee, N.E. Coates, D. Moses, T.-Q. Nguyen, M. Dante, A.J. Heeger, Efficient tandem polymer solar cells fabricated by all-solution processing, Science 317 (2007) 222–225. [3] G. Li, R. Zhu, Y. Yang, Polymer solar cells, Nat. Photonics 6 (2012) 153–161. [4] Y.-J. Cheng, S.-H. Yang, C.-S. Hsu, Synthesis of conjugated polymers for organic solar cell applications, Chem. Rev. 109 (2009) 5868–5923. [5] N.S. Sariciftci, L. Smilowitz, A.J. Heeger, F. Wudl, Photoinduced electron transfer from a conducting polymer to buckminsterfullerene, Science 258 (1992) 1474–1476. [6] S. Günes, H. Neugebauer, N.S. Sariciftci, Conjugated polymer-based organic solar cells, Chem. Rev. 107 (2007) 1324–1338. [7] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Highefficiency solution processable polymer photovoltaic cells by self-organization of polymer blends, Nat. Mater. 4 (2005) 864–868. [8] G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Polymer photovoltaic cells: enhanced efficiencies via a network of internal donor-acceptor heterojunctions, Science 270 (1995) 1789–1791. [9] Y. Liu, J. Zhao, Z. Li, C. Mu, W. Ma, H. Hu, K. Jiang, H. Lin, H. Ade, H. Yan, Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells, Nat. Commun. 5 (2014) 5293. [10] C. Liu, C. Yi, K. Wang, Y. Yang, R.S. Bhatta, M. Tsige, S. Xiao, X. Gong, Singlejunction polymer solar cells with over 10% efficiency by a novel twodimensional donor-acceptor conjugated copolymer, ACS Appl. Mater. Interface 7 (2015) 4928–4935. [11] C.J. Brabec, S. Gowrisanker, J.J.M. Halls, D. Laird, S. Jia, S.P. Williams, Polymerfullerene bulk-heterojunction solar cells, Adv. Mater. 22 (2010) 3839–3856.

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