Introduction

In recent decades, small π-conjugated organic molecules or polymers have received substantial attention due to their semiconducting nature and processability, enabling printable electronic devices, such as organic light-emitting diodes, organic thin-film transistors and organic photovoltaics.1, 2, 3 Among organic photovoltaics, polymer solar cells (PSCs) are one of the applications of π-conjugated polymers, and they use a blended film of a p-type semiconducting polymer and an n-type fullerene derivative as a photoelectric conversion layer.4 For example, a combination of regioregular poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester is commonly used as the photoactive layer of a PSC, called a bulk-heterojunction system, showing a 3–5% power conversion efficiency (PCE).5, 6, 7 Recently, the PCE has been dramatically improved to 10% by the development of p-type semiconducting polymers having an extended absorption wavelength.8 This breakthrough is attributed to the development of a strategy in molecular design, which is the donor–acceptor alternative structure.9, 10, 11 The copolymerization of electron-rich (donor) and electron-deficient (acceptor) units enables tuning of the energy levels of the high occupied molecular orbital (HOMO) and the low unoccupied molecular orbital (LUMO) of the π-conjugated polymer. Therefore, when this polymer is applied to a PSC, a high built-in potential and a LUMO offset between the p-type and n-type materials should be achieved, maintaining an extended absorption wavelength to afford a high PCE.12

1,3,4-Thiadiazole (TDz) is a promising candidate as an electron-deficient unit having two electron-withdrawing imine (C=N) groups. In addition, TDz is a small unit compared with thiophene because of the lack of hydrogen atoms at the 3- and 4-positions.13, 14, 15, 16, 17, 18 The compact structure provides a great advantage in suppressing the torsion of the polymer backbone and interchain steric hindrance, achieving a high PCE in the PSC device due to the high hole mobility of the polymer and the low contact resistance at the polymer/electrode interface. We previously reported that the TDz-containing polymer poly[2,5-thiophene-alt-5′,5′-(2′′,5′′-bis(3′,4′-dihexylthien-2′-yl)-1′′,3′′,4′′-thiadiazole)] (P1, Figure 1a) showed a high hole mobility (8.81 × 10−2 cm−2 V−1 s−1) in an organic thin-film transistor device.17 By using a grazing incident wide-angle X-ray scattering (GIWAXS) measurement, it was demonstrated that P1 formed densely stacked crystalline domains with a short π-π stacking distance (3.51 Å), and the π-plane of the polymers lies parallel to the substrate (face-on orientation).19 Considering the relationship between the PSC performance and the microstructure of P1, the simple structure of a TDz unit is beneficial for improving PSC performance. In addition, several TDz-containing polymers consisting of different donor units, such as benzodithiophene (P2), dithienosilole (P3), dithienopyrrole (P4) and thieno[3,4-b]thiophene (P5) were synthesized and applied to PSC devices (Figure 1a).18 The best performance was achieved in a PSC using P5 with 2.8% PCE, whereas other polymers provide substantially lower PCEs (<1%) due to low short-circuit current densities (Jscs) and fill factors (FFs). The GIWAXS measurement revealed that the π-π stacking distance (dπ-π) is a dominant factor for PCE, and a compact donor unit provides a lower dπ-π than a bulky donor containing a fused ring, such as benzodithiophene and dithienopyrrole.19

Figure 1
figure 1

Chemical structures of TDz-containing polymers described in (a) previous reports and (b) this study.

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Based on these findings, in this study, we report the synthesis, the thermal and optoelectronic properties, as well as the morphology of a wide variety of new TDz-containing polymers with thiophene-based donor units, such as thiophene (PTDzTh), selenophene (PTDzSe), thieno[3,4-b]thiophene (PTDzTT), 3,3′-didodecyl-2,2′-bithiophene (PTDzBTh) and (E)-1,2-di-(3-dodecylthiophene)vinylene (PTDzTV), as depicted in Figure 1b, to clarify the relationships among the chemical structures, properties and crystalline morphology. Concerning their rigid structures, the solubilizing groups are extended from n-hexyl to n-dodecyl groups, causing a smaller increase in dπ-π than that of branched alkyl side chains.20, 21

Materials and Methods

Materials

All of the reagents were used as received unless otherwise stated. Tetrahydrofuran (99.5%, Wako Chemicals Co., Ltd) was refluxed over sodium benzophenone under nitrogen for 2 h and then distilled immediately before use. 2,5-Bis(trimethylstannyl)thiophene was recrystallized before use. 3,4-Didodecylthiophene,22 2,5-bis(trimethylstannyl)selenophene,23 2,5-bis(trimethylstannyl)thieno[3,2-b]thiophene,24 5,5′-bis(trimethylstannyl)-3,3′-didodecyl-2,2′-bithiophene25 and (E)-1,2-di(3-dodecyl-5-trimethylstannylthiophene)vinylene26 were synthesized according to previously described methods.

Synthesis

3,4-Didodecyl-2-thiophenecarboxylic acid (2)

To a tetrahydrofuran solution (60 ml) of 3,4-didodecylthiophene (1; 8.71 g, 20.7 mmol) a nBuLi solution (Kanto Chemical Co., Inc., Tokyo, Japan) in hexane (2.67 m × 8.54 ml=22.8 mmol) was added dropwise at 0 °C. The solution was then stirred at room temperature for 1 h. After cooling the solution to 0 °C, CO2 was bubbled through the reaction mixture for 2 h. The solution was warmed to room temperature followed by quenching with water, and the product was extracted with ether. The ether solution was neutralized with HCl(aq) and then washed with water and brine. After the solution was dried over MgSO4 and filtered, the filtrate was evaporated under reduced pressure. The crude product was purified by flash silica gel column chromatography using hexane/ethyl acetate (9/1, v/v) as an eluent to afford 2 as a white solid (6.16 g, 64%). 1H nuclear magnetic resonance (NMR) (400 MHz, CDCl3, δ, p.p.m., 25 °C): 7.17 (s, 1H), 2.92 (t, 2H), 2.52 (t, 2H), 1.19−1.63 (m, 40H), 0.88 (t, 6H). 13C NMR (100 MHz, CDCl3, δ, p.p.m., 25 °C): 168.3, 152.0, 144.4, 127.7, 126.2, 32.0, 30.4, 30.0, 29.9, 29.8, 29.7, 29.6, 29.5, 28.8, 27.9, 22.8, 14.2. IR, ν (cm−1): 1651 (C=O stretching), 1464 (C−O−H bending), 1286 (C−O stretching). Anal. Calcd. for C29H52O2S (%): C, 74.94, H, 11.28. Found (%): C, 74.37, H, 11.83.

N,N′-Bis(3,4-didodecylthien-2-ylcarbonyl)hydrazine (3)

To a dry N-methylpyrrolidone solution (20 ml) of 2 (4.00 g, 8.65 mmol) was added dropwise at 0 °C thionyl chloride (0.652 ml, 9.01 mmol). The solution was then stirred at room temperature for 1.5 h. Triethylamine (1.30 ml, 12.9 mmol) and hydrazine anhydrous (0.136 ml, 4.29 mmol) were added and stirred overnight at 50 °C. The reaction was quenched with water, and the product was extracted with ether. The combined organic layer was washed with water and brine. After the solution was dried over MgSO4 and filtered, the filtrate was evaporated under reduced pressure. The crude product was purified by flash silica gel column chromatography using hexane/ethyl acetate (9/1, v/v) as an eluent to afford 3 as a yellow solid (3.44 g, 86%). 1H NMR (400 MHz, CDCl3, δ, p.p.m., 25 °C): 8.49 (s, 2H), 7.02 (s, 2H), 2.88 (t, 4H), 2.52 (t, 4H), 1.19−1.66 (m, 80H), 0.90 (m, 12H). 13C NMR (100 MHz, CDCl3, δ, p.p.m., 25 °C): 160.7, 148.4, 144.4, 126.8, 123.1, 32.0, 30.7, 30.0, 29.8, 29.8, 29.7, 29.6, 29.5, 28.8, 28.1, 22.8, 14.2. IR, ν (cm−1): 3175 (N−H stretching), 1586 (C=O stretching). MS m/z: Calcd. for C58H104N2Na1O2S2, 947.74, found 947.75.

2,5-Bis(3′,4′-didodecylthien-2′-yl)-1,3,4-thiadiazole (4)

A toluene solution (80 ml) of 3 (3.34 g, 3.61 mmol) and 2,4-bis(4-methoxyphenyl)-1,3,2,4-dithiadiphosphetane-2,4-dithione (Lawesson’s reagent, 1.75 g, 4.33 mmol) was refluxed for 3 h. The resulting solution was extracted with CHCl3, and the organic phase was washed with NaOH(aq), water and brine. After the solution was dried over MgSO4 and filtered, the filtrate was evaporated under reduced pressure. The filtrate was then purified by flash silica gel column chromatography using hexane/ethyl acetate (19/1, v/v) as an eluent to afford 4 as a yellow solid (2.08 g, 63%). 1H NMR (400 MHz, CDCl3, δ, p.p.m., 25 °C): 7.07 (s, 2H), 2.88 (t, 4H), 2.55 (t, 4H), 1.22−1.69 (m, 80H), 0.88 (m, 12H). 13C NMR (100 MHz, CDCl3, δ, p.p.m., 25 °C): 160.5, 143.5, 143.2, 127.1, 33.6, 33.5, 32.0, 30.2, 30.0, 29.7, 29.4, 29.1, 28.6, 28.5, 22.8, 14.1. IR, ν (cm−1): 1548 (C=N stretching). MS m/z: Calcd. for C58H103N2S3, 923.73, found 923.73.

2,5-Bis(5′-bromo-3′,4′-didodecylthien-2′-yl)-1,3,4-thiadiazole (5)

To 4 (1.79 g, 1.94 mmol) in CHCl3 (20 ml) N-bromosuccinimide (1.22 g, 6.79 mmol) was added, and the solution was stirred overnight. The reaction was quenched with water, and the product was extracted with ether. After the solution was dried over MgSO4 and filtered, the filtrate was condensed under reduced pressure. The crude product was then purified by flash silica gel column chromatography using hexane/ethyl acetate (24/1, v/v) as an eluent to afford 5 as a yellow solid (1.92 g, 91%). 1H NMR (400 MHz, CDCl3, δ, p.p.m., 25 °C): 2.85 (t, 4H), 2.57 (t, 4H), 1.19−1.59 (m, 80H), 0.88 (m, 12H). 13C NMR (100 MHz, CDCl3, δ, p.p.m., 25 °C): 159.1, 143.4, 143.1, 127.4, 114.5, 32.1, 30.2, 30.1, 29.8, 29.5, 29.1, 29.0, 28.6, 28.5, 22.8, 14.2. IR, ν (cm−1): 1546 (C=N stretching). Calcd. for C58H100N2S3 (%): C, 64.42, H, 9.32, N, 2.59. Found (%): C, 64.49, H, 9.57, N, 2.38.

Poly[2,5-thiophene-alt-5′,5′-(2′′,5′′-bis(3′,4′-didodecylthien-2′-yl)-1′′,3′′,4′′-thiadiazole)] (PTDzTh)

To 5 (0.211 g, 0.195 mmol) and 2,5-bis(trimethylstannyl)thiophene (0.0799 g, 0.195 mmol) Pd2(dba)3 (10.2 mg, 0.0111 mmol) and tri(o-tolyl)phosphine (29.8 mg, 0.0979 mmol) in toluene (6.2 ml) in nitrogen was added, which was deoxygenated by bubbling with dry nitrogen for 15 min. The solution was refluxed for 2 days. 2-(Tributylstannyl)thiophene (0.013 ml, 0.039 mmol) was then added and stirred for 1 h. Subsequently, 2-bromothiophene (0.0057 ml, 0.059 mmol) was added and stirred for 1 h. After quenching the reaction with HCl(aq) and neutralizing with NaHCO3(aq), the product was extracted with CHCl3. The CHCl3 solution was washed with KF(aq) and was condensed under reduced pressure. Next, the solution was passed through a silica gel column using CHCl3 as an eluent and poured into water/methanol to precipitate the polymer. The polymer was purified by Soxhlet extraction using methanol, acetone, acetone/hexane (1/1, v/v) and CHCl3. After evaporating the CHCl3 solution, the polymer was freeze dried from its absolute benzene solution to afford PTDzTh as a red solid (0.128 g, 65%). Size exclusion chromatography (SEC): molecular weight (Mn)=26 600 g mol−1, dispersity (Đ)=1.77. 1H NMR (400 MHz, CDCl3, δ, p.p.m., 25 °C): 7.23 (s, 2H), 2.91 (s, 4H), 2.80 (s, 4H), 1.21−1.70 (m, 80H), 0.87 (t, 12H). IR, ν (cm−1): 1551 (C=N stretching). Anal. Calcd. for C62H104N2S3 (%): C, 74.04, H, 10.42, N, 2.69, S, 12.75. Found (%): C, 73.76, H, 10.15, N, 2.68, S, 12.72.

Poly[2,5-selenophene-alt-5′,5′-(2′′,5′′-bis(3′,4′-didodecylthien-2′-yl)-1′′,3′′,4′′-thiadiazole)] (PTDzSe)

To 5 (0.200 g, 0.185 mmol) and 2,5-bis(trimethylstannyl)selenophene (0.0846 g, 0.182 mmol) Pd2(dba)3 (10.9 mg, 0.0119 mmol) and tri(o-tolyl)phosphine (60.0 mg, 0.197 mmol) in toluene (8.0 ml) in nitrogen was added, which was deoxygenated by bubbling with dry nitrogen for 15 min. The solution was refluxed for 2 days. 2-(Tributylstannyl)thiophene (0.012 ml, 0.037 mmol) was then added and stirred for 1 h. Subsequently, 2-bromothiophene (0.0054 ml, 0.056 mmol) was added and stirred for 1 h. After quenching the reaction with HCl(aq) and neutralizing with NaHCO3(aq), the product was extracted with CHCl3. The CHCl3 solution was washed with KF(aq) and was condensed under reduced pressure. Then, the solution was passed through a silica gel column using CHCl3 as an eluent and poured into water/methanol to precipitate the polymer. The polymer was purified by Soxhlet extraction using methanol and CHCl3. After evaporating the CHCl3 solution, the polymer was freeze dried from its absolute benzene solution to afford PTDzSe as a red solid (0.134 g, 72%). SEC: Mn=17 800 g mol−1, Đ=2.10. 1H NMR (400 MHz, CDCl3, δ, p.p.m., 25 °C): 7.39 (s, 2H), 2.91 (s, 4H), 2.77 (s, 4H), 1.68–1.20 (m, 80H), 0.88 (t, 12H). IR, ν (cm−1): 1550 (C=N stretching). Anal. Calcd. for C62H104N2S3Se (%): C, 70.74, H, 9.96, N, 2.66. Found (%): C, 70.75, H, 9.96, N, 2.57.

Poly[2,5-thieno[3,2-b]thiophene-alt-5′,5′-(2′′,5′′-bis(3′,4′-didodecylthien-2′-yl)-1′′,3′′,4′′-thiadiazole)] (PTDzTT)

To 5 (0.206 g, 0.191 mmol) and 2,5-bis(trimethylstannyl)thieno[3,2-b]thiophene (0.0890 g, 0.191 mmol) Pd2(dba)3 (11.8 mg, 0.0129 mmol) and tri(o-tolyl)phosphine (35.4 mg, 0.0955 mmol) in toluene (6.2 ml) in nitrogen was added, which was deoxygenated by bubbling with dry nitrogen for 15 min. The solution was refluxed for 2 days. 2-(Tributylstannyl)thiophene (0.012 ml, 0.037 mmol) was then added and stirred for 1 h. Then, 2-bromothiophene (0.0055 ml, 0.057 mmol) was added and stirred for 1 h. After quenching the reaction with HCl(aq) and neutralizing with NaHCO3(aq), the product was extracted with CHCl3 and o-dichlorobenzene (o-DCB). The CHCl3 and o-DCB solution was washed with KF(aq) and was condensed under reduced pressure. Next, the solution was passed through a silica gel column using CHCl3 as an eluent and poured into water/methanol to precipitate the polymer. The polymer was purified by Soxhlet extraction using methanol, acetone, hexane and CHCl3. After evaporating the CHCl3 solution, the polymer was freeze dried from its absolute benzene solution to afford PTDzTT as a red solid (0.155 g, 75%). SEC: Mn=21 100 g mol−1, Đ=1.78. 1H NMR (400 MHz, CDCl3, δ, p.p.m., 25 °C): 7.39 (s, 2H), 2.93 (s, 4H), 2.81 (s, 4H), 1.68–1.21 (m, 80H), 0.87 (t, 12H). IR, ν (cm−1): 1543 (C=N stretching). Anal. Calcd. for C64H104N2S5 (%): C, 72.39, H, 9.87, N, 2.64, S, 15.10. Found (%): C, 72.30, H, 9.59, N, 2.52, S, 15.40.

Poly[5,5′-(3,3′-didodecyl-2,2′-bithiophene)-alt-5′′,5′′-(2′′′,5′′′-bis(3′′,4′′-didodecylthien-2′′-yl)-1′′′,3′′′,4′′′-thiadiazole)] (PTDzBTh)

To 5 (0.200 g, 0.185 mmol) and 5,5′-bis(trimethylstannyl)-3,3′-didodecyl-2,2′-bithiophene (0.153 g, 0.185 mmol) Pd2(dba)3 (12.2 mg, 0.0133 mmol) and tri(o-tolyl)phosphine (38.6 mg, 0.127 mmol) in toluene (6.2 ml) in nitrogen was added, which was deoxygenated by bubbling with dry nitrogen for 15 min. The solution was refluxed for 2 days. 2-(Tributylstannyl)thiophene (0.012 ml, 0.037 mmol) was added and stirred for 1 h. Subsequently, 2-bromothiophene (0.0054 ml, 0.056 mmol) was added and stirred for 1 h. After quenching the reaction with HCl(aq) and neutralizing with NaHCO3(aq), the product was extracted with CHCl3. The CHCl3 solution was washed with KF(aq) and was condensed under reduced pressure. Then, the solution was passed through a silica gel column using CHCl3 as an eluent and poured into water/methanol to precipitate the polymer. The polymer was purified by Soxhlet extraction using methanol, acetone, acetone/hexane (1/1, v/v) and CHCl3. After evaporating CHCl3, the polymer was freeze dried from its absolute benzene solution to afford PTDzBTh as a red solid (0.105 g, 39%). SEC: Mn=27 400 g mol−1, Đ=1.51. 1H NMR (400 MHz, CDCl3, δ, p.p.m., 25 °C): 7.13 (s, 2H), 2.91 (s, 4H), 2.78 (s, 4H), 2.59 (s, 4H), 1.68–1.20 (m, 120H), 1.10–0.82 (m, 18H). IR, ν (cm−1): 1518 (C=N stretching). Anal. Calcd for C90H154N2S5 (%): C, 75.88, H, 10.90, N, 1.97, S, 11.25. Found (%): C, 75.97, H, 11.42, N, 1.88, S, 11.59.

Poly[5,5′-((E)-1,2-di(3-dodecylthiophene)vinylene)-alt-5′′,5′′-(2′′′,5′′′-bis(3′′,4′′-didodecylthien-2′′-yl)-1′′′,3′′′,4′′′-thiadiazole)] (PTDzTV)

To 5 (0.201 g, 0.186 mmol) and (E)-1,2-di(3-dodecyl-5-trimethylstannylthiophene)vinylene (0.159 g, 0.186 mmol) Pd2(dba)3 (12.0 mg, 0.0131 mmol) and tri(o-tolyl)phosphine (29.9 mg, 0.0982 mmol) in toluene (7.0 ml) in nitrogen was added, which was deoxygenated by bubbling with dry nitrogen for 15 min. The solution was refluxed for 2 days. 2-(Tributylstannyl)thiophene (0.012 ml, 0.037 mmol) was then added and stirred for 1 h. Next, 2-bromothiophene (0.0054 ml, 0.056 mmol) was added and stirred for 1 h. Subsequently, the solution was poured into water/methanol to precipitate the polymer. The polymer was purified by Soxhlet extraction using methanol, acetone, hexane and CHCl3. After evaporating CHCl3, the polymer was poured into methanol to afford PTDzTV as a purple solid (0.150 g, 55%). SEC: Mn=26 600 g mol−1, Đ=1.64. 1H NMR (400 MHz, CDCl3, δ, p.p.m., 25 °C): 7.01 (s, 2H), 7.00 (s, 2H), 2.90 (s, 4H), 2.79 (s, 4H), 2.68 (s,4H), 1.72–1.19 (m, 130H), 0.97–0.81 (m, 18H). IR, ν (cm−1): 1531 (C=N stretching). Anal. Calcd. for C92H156N2S5 (%): C, 76.18, H, 10.84, N, 1.93, S, 11.05. Found (%): C, 76.55, H, 11.18, N, 1.76, S, 10.60.

Measurements

Molecular weights and Ð were measured by SEC on a (Jasco GULLIVER 1500, Jasco, Co., Tokyo, Japan) equipped with a pump, an absorbance detector (ultraviolet (UV), λ=254 nm) and three polystyrene gel columns based on a conventional calibration curve using polystyrene standards. CHCl3 (40 °C) was used as a carrier solvent at a flow rate of 1.0 ml min−1. 1H and 13C NMR spectra were recorded on a JEOL JNM-ECX400 (JEOL, Ltd., Tokyo, Japan) in chloroform-d calibrated to tetramethylsilane as an internal standard (δH 0.00). Fourier-transform infrared spectroscopy (FT-IR) spectra were obtained on a Horiba FT-720 spectrometer (Horiba, Ltd., Kyoto, Japan). Thermal analysis was performed on a Seiko EXSTAR 6000 TG/DTA 6300 thermal analyzer (Seiko Instruments Inc., Chiba, Japan) at a heating rate of 10 °C min−1 for thermogravimetry, and differential scanning calorimetry (DSC) was performed on Seiko DSC 6300 (Seiko Instruments Inc.) connected to a cooling system at a heating rate of 10 °C min−1. UV–visible absorption spectra were recorded using a Hitachi U-4100 spectrophotometer (Hitachi High-Tech Science Co., Tokyo, Japan). For the thin-film spectra, polymers were dissolved in CHCl3 and then filtered through a 0.45-μm pore size polytetrafluoroethylene (PTFE) membrane syringe filter, followed by drop casting onto a quartz substrate. HOMO and LUMO levels in the solid films were recorded by a Riken Keiki AC-3 photoelectron yield spectrometer (Riken Keiki Co., Ltd., Tokyo, Japan), and the UV–visible absorption edge was obtained from the absorption spectra of the films. GIWAXS measurements were conducted at beamline BL46XU of SPring-8, Hyogo, Japan. The sample films were prepared by drop casting the polymer solutions (10 mg ml−1 in CHCl3) on the Si substrates, followed by air drying for 1 h. The samples were irradiated at a fixed incident angle on the order of 0.12° through a Huber diffractometer (HUBER Diffraktionstechnik GmbH & Co. KG, Rimsting, Germany) with an X-ray energy of 12.398 keV (X-ray wavelength, λ=0.10 nm), and the GIWAXS patterns were recorded with a two-dimensional image detector (Pilatus 300 K, DECTRIS, Baden, Switzerland) with a sample-to-detector distance of 174 mm.

PSC fabrication and characterization

Chlorobenzene (CB) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) were purchased from Sigma-Aldrich Co. LLC. (Tokyo, Japan) and Luminescence Technology Corporation (Hsin-Chu, Taiwan, R.O.C), respectively. Glass substrates coated with transparent indium tin oxide (ITO) with a sheet resistivity of 15 ohm sq−1 and patterned with discrete 3-mm stripes were purchased from Luminescence Technology Corporation. The pre-patterned ITO-coated substrates were cleaned with detergent, deionized water, acetone and isopropanol by sonication and then treated in UV–ozone for 30 min before coating the layers. For the cells with a sandwiched device structure, 40-nm PEDOT:PSS (poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate)) (Baytron PVP AI 4083 (H.C. Starck Ltd., Tokyo, Japan)) was spin coated and annealed at 120 °C for 15 min on a hotplate in air. The PEDOT:PSS-coated substrates were transferred to a glove box, and the active layers of Polymer:PC71BM were spin coated from a solution of polymer:PC71BM (1:1 by weight) in CB. Calcium (Ca) and aluminum (Al) were subsequently deposited as a cathode using a shadow mask in the cells, which were thermally evaporated in a vacuum chamber with a base pressure of 1 × 10−6 Pa. The thicknesses and deposition rates were monitored by quartz crystal sensors. The active area of each cell was ~3 × 3 mm2, defined by the overlap of the ITO and Al. The fabricated cells were encapsulated in a nitrogen-filled glove box and then brought to ambient conditions for characterization of current density–voltage (J-V) using a CEP-2000 integrated system by Bunkoukeiki Co., Ltd, Tokyo, Japan. Light state J-V characteristics were measured under 100 mW cm−2 Air mass 1.5G (AM1.5G) simulated light.

Results and Discussion

Synthesis of TDz-containing polymers

The general synthetic route to the electron-deficient monomers and polymers is displayed in Scheme 1. The TDz-containing monomer 5 was synthesized according to the same procedure as that in previous reports, except for the change from n-hexyl groups to n-dodecyl groups.17 Dodecyl side chains are introduced into the polymers to improve the solubility because the previously reported polymer, P5, showed poor solubility, even in hot o-DCB. The polymers were synthesized by the Stille coupling polymerization between 5 and different distannyl compounds. After the Soxhlet extraction, all of the polymers with moderate molecular weights (Mn=17.8–32.9 kg mol−1) were obtained, which are summarized in Table 1. All of the polymers, except for PTDzTT, were successfully solubilized in tetrahydrofuran, CHCl3, hot CB and hot o-DCB, whereas PTDzTT is soluble in these hot solvents.

Table 1 Polymerization results and thermal properties
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Thermal properties

The thermal stability of the polymers was evaluated by TG/DTA (differential thermal analysis) measurements, and the data are summarized in Table 1. All of the polymers show nearly the same weight loss behavior with 5 wt% weight loss temperatures (Td5%) at ~400 °C (Figure 2a). This result indicates that all of the polymers have sufficient thermal stability for PSC fabrication. The DSC measurement revealed that the melting point (Tm) of PTDzTh, PTDzSe and PTDzTT is ~180 °C (Figures 2b and d). Compared with these values, the Tm values of PTDzBTh and PTDzTV are 90 and 154 °C (Figures 2e and f), respectively, are significantly lower due to the high content of dodecyl groups.

Figure 2
figure 2

(a) TGA curves and (b–f) DSC charts of TDz-containing polymers.

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Optical and electronic properties

The UV–visible spectra of the as-cast polymer films are depicted in Figure 3. PTDzTh, PTDzSe and PTDzTT show an absorption maxima (λmax) at 511–520 nm and an onset of absorption spectra (λonset) at 588–596 nm, which are nearly the same as those of P1 (λmax=513 nm and λonset=591 nm).17 However, the λmax of PTDzBTh (483 nm) is substantially shorter compared with the others, despite having a similar λonset (596 nm). This difference indicates that in the solid film state of PTDzBTh, most of the polymer backbones are twisted to reduce the conjugation length due to a steric hindrance induced by the head-to-head structure of 3,3′-didodecyl-2,2′-bithiophene.25 This presumption is also supported by the absence of a shoulder peak attributed to an intermolecular packing structure. However, an approximate 30-nm red shift is found in the λmax and λonset for PTDzTV (547 and 629 nm), which would be preferable for PSC application. Because the steric hindrance in the donor units is reduced by the introduction of a vinylene group, it appears that the optical band gap (Egopt) is effectively decreased due to the extended conjugation length of the thienylene vinylene unit.

Figure 3
figure 3

UV–visible spectra of the as-cast polymer films on the quartz substrate.

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Photoelectron yield spectroscopy was performed for the as-cast polymer films to investigate the HOMO energy levels. LUMO energy levels were estimated from the calculated HOMO values and the Egopt, according to the equation LUMO=HOMO+Egopt. As summarized in Figure 4, all of the polymers have significantly deeper HOMO energy levels than that of regioregular poly(3-hexylthiophene) due to the strong electron deficiency of the TDz units, which would provide high open-circuit voltage (Voc) values in PSC devices. In addition, the highest HOMO and relatively lowest LUMO energy levels were found in the PTDzTV film among all of the polymers, suggesting that the π-conjugation between TDz and the thienylene vinylene units is better than the other combination due to the improved orbital mixing between the donor and acceptor units. The other polymers showed similar Egopt values, despite their different HOMO energy levels, which may indicate the spatial isolation of HOMOs and LUMOs in their main chain.

Figure 4
figure 4

Energy diagrams of rrP3HT and TDz-containing polymers.

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Crystalline structure

The PSC performance is strongly influenced by the microstructure of the polymer in the photoactive layer, such as the orientation, π-π stacking distance, and crystallinity, among others.27, 28, 29, 30, 31 To investigate these factors, GIWAXS measurements were performed for the pristine polymer films, and the obtained two-dimensional GIWAXS images are shown in Figure 5. In particular, π-conjugated polymers exhibit two types of scattering peaks; one peak is attributed to the lamellar spacing distance, which appears in a low q region (q=1–3 nm−1), and the other peak appears in a high q region (q=15–18 nm−1), corresponding to the π-π stacking distance. Because the scattering data in the high qz region is incorrect due to the distortion, we investigated the orientation of the polymer backbone from the lamellar spacing peaks.31, 32, 33 In Figure 6, the scattering intensity in a low q region is plotted as a function of the azimuth angle (φ). In this figure, the in-plane and out-of-plane profiles correspond to φ=0° and 90°, respectively. Note that the total reflection of light partially overlaps the scattering peak around φ=90°. Although the pristine films of PTDzBTh and PTDzTV exhibit no scattering peaks in the azimuth profile, PTDzTh and PTDzSe show broad scattering peaks in a low φ region, suggesting that the lamellar direction is parallel to the substrate (in-plane direction). Thus, assuming that the polymers form a typical π-stacked lamellar structure, the results indicate that their crystalline structure has a face-on rich orientation. However, a highly oriented edge-on crystalline structure was found in the pristine PTDzTT films. The difference in the orientation may be related to the polymer solubility in the cast solvent, CHCl3, in which highly soluble polymers (PTDzTh and PTDzSe) prefer to form face-on rich crystalline domains, and poorly soluble polymers (PTDzTT) tend to pack in an edge-on orientation. Similar results were reported for diketopyrrolopyrrole-based polymers by Zhang et al.;34 hence, the same phenomenon most likely occurred in the TDz-based polymers.

Figure 5
figure 5

Two-dimensional GIWAXS images of as-cast pristine polymer films.

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Figure 6
figure 6

Azimuth plots of the lamellar spacing peaks for the pristine films.

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The in-plane and out-of-plane profiles are shown in Figure 7, and structural parameters such as the distances of lamellar spacing and π-π stacking (dlam, dπ-π), with their correlation length (Llam, Lπ-π) are summarized in Table 2. All of the polymers exhibit weak and broad scattering peaks in both of the pristine films, and especially, PTDzTh and PTDzSe do not show the higher order of scattering peaks. Therefore, it can be concluded that these five polymers have relatively low crystallinity in the as-cast pristine films. The dlam values of PTDzBTh (3.26 nm) and PTDzTV (3.14 nm) are nearly twice the size of an extended dodecyl group (1.52 nm),35 implying that they have little interdigitation in a lamellar spacing direction. PTDzTh and PTDzSe show lower dlam values (2.92, 2.85 nm) with lower Llam values (7.74, 7.42 nm), suggesting that their side chains interdigitate to disturb the π-π stacking of the polymer main chains. For PTDzTh and PTDzSe, significantly large dπ-π values (0.395, 0.402 nm) with low Lπ-π values (1.84, 1.91 nm) are found. PTDzTT also shows the smallest dlam value (2.74 nm), and the Llam value is relatively high (10.2 nm) among the polymers. The steric hindrance caused by the interdigitation appears to be small, which is supported by the highest Lπ-π value (5.12 nm). Thus, PTDzTT shows a good balance between rigidity of the main chain and solubility influenced by the side chains; thus, a highly crystalline order appears in the π-π stacking direction.

Figure 7
figure 7

One-dimensional GIWAXS profiles of the pristine polymer films in the (a) in-plane and (b) out-of-plane direction.

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Table 2 Structural parameters based on GIWAXS measurement
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PSC performance

To investigate the PSC performance of the synthesized polymers, the PSC devices were fabricated with the standard architecture of glass/ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al. The J-V characteristics of the PSCs are shown in Figure 8, and their parameters are summarized in Table 3. Most of the devices showed high Voc values and moderate FF values, except for those of PTDzBTh, whose low performance was most likely caused by the low film quality. The Voc values have a tendency to increase as the HOMO energy levels decrease; thus, the lowest HOMO level of the PTDzTT afforded the highest Voc of 0.965 V among the polymers. The Jsc values were affected by the Egopt values and the crystalline nature of the polymers. Comparing PTDzTh, PTDzSe and PTDzTT whose Egopt values are nearly the same, the narrower dπ-π and higher Lπ-π values provided slightly improved Jsc and FF. Therefore, PTDzTT afforded the second highest PCE of 0.412% among the polymers. However, the lowest Egopt polymer, PTDzTV, yielded a substantially higher Jsc value of 1.12 mAcm−2 than that of the highly crystalline PTDzTT (0.798 mAcm−2). Therefore, it can be concluded that the light absorption behavior of the polymers strongly affects the Jsc values, and the highly crystalline structure slightly improves the Jsc and FF values. However, the TDz polymers containing n-dodecyl side chains showed substantially lower performances than that of P1 having n-hexyl side chains, especially in Jscs, and lower shunt resistances, which were suggested by the shallow slopes of the J-V curves under the open-circuit condition. We believe that there are difficulties in the charge separation and charge collection processes. Our polymers have substantially longer side chains around the compact acceptor unit than P1, so that the PC71BM molecules could not approach the TDz units. Therefore, the charge preparation may be difficult due to the unsuitable intermolecular arrangement.36 In addition, researchers reported that compared with the polymers with branched side chains, those with linear side chains provide higher non-geminate and geminate recombination rates37 and can form aggregates with energetically unfavorable states for the charge dissociation.38 Therefore, we speculate that the replacement of the linear side chains placed around the acceptor unit with the branched side chains placed around the donor unit would improve the Jsc and shunt resistance.

Figure 8
figure 8

J-V characteristics of the devices (ITO/PEDOT:PSS/Polymer:PC71BM/Ca/Al) with different donors under 100 mW cm−2 illumination of an AM 1.5G solar spectrum.

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Table 3 The PSC performance of ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al
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Conclusion

Novel TDz-based conjugated polymers were synthesized, and their thermal/optoelectronic properties and polymer crystalline structures were investigated. The polymers show moderate molecular weights (Mn=17.8–32.9 kg mol−1) and sufficient thermal stability (Td5%~400 °C) for PSC fabrication. All of the polymers exhibit deeper HOMO energy levels (<−5.2 eV) compared with regioregular poly(3-hexylthiophene) due to the electron deficiency of the TDz units. In addition, PTDzTV shows the most extended absorption wavelength (λonset=629 nm) among the polymers due to the improved conjugation length. GIWAXS measurements revealed that PTDzTT shows the highest ordered crystalline structure in the π-π stacking direction due to a good balance between rigidity and solubility. PSCs using TDz-based polymers were fabricated to investigate the photovoltaic performance, and high Voc values up to 0.965 V were achieved because of the deep HOMO energy levels.

scheme 1

Synthetic routes for PTDzTh, PTDzSe, PTDzTT, PTDzBTh and PTDzTV.