Side chains and backbone structures influence on 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT)-based low-bandgap conjugated copolymers for organic photovoltaics

Abstract

Five 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT)-based conjugated copolymers with controlled molecular weight were synthesized to explore their optical, energy level and photovoltaic properties. By tuning the positions of hexyl side chains on DTBT unit, the DTBT-fluorene copolymers exhibited very different aggregation properties, leading to 60 nm bathochromic shift in their absorptions and the corresponding power conversion efficiencies (PCEs) value of photovoltaic cells varied from 0.38%, 0.69% to 2.47%. Different copolymerization units, fluorene, carbazole and phenothiazine were also investigated. The polymer based on phenothiazine exhibited lower PCE value due to much lower molecular weight owing to its poor solubility, although phenothiazine units were expected to be a better electron donor. Compared with the fluorene-based polymer, the carbazole-DTBT copolymer showed higher short circuit current density (J_sc) and PCE value due to its better intermolecular stacking.

Keywords

4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT) / conjugated polymers / low-bandgap / organic photovoltaics

Introduction

Organic photovoltaics (OPVs) are a promising sustainable energy source with unique advantages, such as low-cost, light weight, and flexibility [1-4]. Bulk heterojunction active layer was usually a blend of an electron-donating conjugated polymer and an electron-accepting fullerene derivative, such as [6,6]-phenyl-C₆₁-bulyric acid methyl ester (PC₆₀BM) [5,6] or [6,6]-phenyl-C₇₁-bulyric acid methyl ester (PC₇₀BM) [7-10]. Poly (3-hexylthiophene) (P3HT) has been intensively investigated as the electron donor in organic/polymer photovoltaic cells with the power conversion efficiency (PCE) up to 6.5% [11-15]. In the past few years, a lot of works were focused on the developing the low-bandgap donor polymers to further increase the OPVs performance [16-28]. The variable structures of low-bandgap conjugated polymers provided a possibility toward increasing the short circuit current density (J_sc) and the open circuit voltage (V_oc) [29-34] by modifying their highest occupied molecular orbital (HOMO) levels. 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT) has been comprehensively studied as building blocks in low-bandgap copolymers [35-41]. The thiophenes in the DTBT units could reduce the bandgap for the extending of the donor-acceptor conjugation and increasing of the planarity, as well as improve hole-transporting properties of the resulting copolymers [42]. poly{ [(2,7-(9-(2’-ethylhexyl)-9-hexyl-fluorene)-alt-[5,5-(-(4,7-di-2-thienyl-2,1,3-benzothiadiazole))}(PFDTBT) was introduced into OPVs as the electron donor polymer and exhibited a relatively high PCE of 2.2% when it was blended with PC₆₀BM. The solubility of PFDTBT was very poor with the number average molecular weight (M_n) of 4800, which limited the PCE of the corresponding devices [43]. Then, docecyl side chains were introduced onto the 9-positions of fluorene units to enhance the solubility of the copolymer poly{[(2,7-(9,9-didodecyl-fluorene)-alt-[5,5-(-(4,7-di-2-thienyl-2,1,3-benzothiadiazole))} (DiD-PFDTBT) and a higher M_n (31,000) was achieved. However, the distinctly steric hindrance of the side chains decreased the π-π stacking and subsequently reduced the charge mobility of DiD-PFDTBT, leading to a relatively lower PCE (1.4%) than that of PFDTBT [44]. Unbulky side chains with methyl groups located on the third and seventh carbon of octyl were applied to the fluorene units to prepare poly{[2,7-(9,9-bis-(3,7-dimethyl-octyl)-fluorene)]-alt-[5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)]} Bis(DMO-PFDTBT), which not only increased the solubility of the polymer but also decreased the steric effect between polymer chains. The resulting polymer exhibited a PCE up to 4.5% when it was blended with PC₇₀BM [45].

To overcome the poor solubility of DTBT-based polymers, side chains were also introduced onto thiophenes of DTBT units. It was found that the side chains on the DTBT units not only affected their molecular weight, solubility and geometry of the corresponding polymers, but also determined their absorptions, energy levels, and film morphologies. Octyl groups were introduced into thiophenes at 3- or 4-positions of the DTBT units respectively to investigate their influence. The substituted on 4-positions of thiophenes exhibited a less steric hindrance than that on 3-positions. The corresponding polymer of 4-positions showed a triple PCE value of that with octyl substituted on 3-positions [8]. Charge mobility of the DTBT based low-bandgap polymers were also depend on the alkyl chains positions on thiophene and the polymers with substitutes on 4-positions provided higher mobility for its stronger intermolecular stacking than that on 3-positions [46].

To improve the charge mobility of the DTBT based polymer, nitrogen atoms were introduced into the 9-positions of fluorenes which provided carbazole-based polymer. Poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-ben-zothiadiazole)] (PCDTBT) was investigated as the donor material for its fully aromatic, which provided a better chemical and environmental stability. PCDTBT showed a relatively high PCE value of 3.6% when blended with PC₆₀BM [47], and the PCE could be further improved to 6.1% by using the titanium oxide (TiO_x) layer as an optical spacer and hole blocker when the PC₇₀BM was chosen as the electron acceptor [48].

According with the results of P3HT, molecular weight and polydispersity index (PDI) of the donor polymers are one of the key factors for the corresponding OPVs performances [49-53]. All the above studies of the DTBT-based polymers included both chemical structure and molecular effects. To obtain a clear outline how the chemical structures determined the device performances, it is urgent to exclude the influence of molecular weight when tuning the side chains and copolymerization units in the DTBT-based polymers, which will help to provide a general procedure for materials optimization. In this paper, five DTBT-based copolymers with comparable repeated units were prepared with varying copolymerization segments and hexyl side chains on thiophenes of DTBT units (Scheme 1). 3,7-dimethyl-octyls were chosen as the side chains on fluorenes or carbazoles for the copolymers without side chains substituted on DTBT to improve their solubility. All the absorptions, energy levels and the morphologies of resulting polymers, as well as their photovoltaic properties were studied with similar repeating units in the polymer backbone. Copolymer based on phenothiazine and DTBT were also employed. However, its molecular weight is much lower than the other four polymers for its poor solubility.

Experimental section

Materials

All reagents and solvents, unless otherwise specified, were obtained from Sigma-Aldrich or TCI chemical Co., and were used as received. Anhydrous tetrahydrofuran (THF) was distilled over sodium/diphenylketone under N₂ prior to use. All manipulations involving air-sensitive reagents were performed under an atmosphere of dry nitrogen. The 2,2'-(9,9-dioctyl-9H-fluorene-2,7-diyl)bis(4,4,5,5- tetramethyl-1,-3,2-dioxaborolane)(1) [54], 4,7-bis(5-bromothiophen-2-yl)benzo-2,1,3-thiadiazole (2) [54], 4,7-bis(5-bromo-3-hexylthiophen-2-yl)benzo-2,1,3-thiadiazole (4) [55], 2,6,12,16-tetramethylheptadecan-9-ol [56], P2 [56], and P3 [45] were synthesized according to literature procedures.

Synthesis of the monomers

2,6,12,16-tetramethylheptadecan-9-yl 4-methylbenzenesulfonate (8)

P-toluenesulfonyl chloride (11.20 g, 58.75 mmol) in CH₂Cl₂ (50 mL) was added to the stirred solution of 2,6,12,16-tetramethylheptadecan-9-ol (12.18 g, 39.16 mmol), Et₃N (97.9 mmol, 13.75 mL) and Me₃N•HCl (3.74 g, 39.16 mmol) in CH₂Cl₂ (250 mL) in a 500 mL flask at 0-5°C, the mixture was stirred overnight. The organic phase was washed with water and brine, dried over magnesium sulfate and concentrated under reduced pressure. The crude product was purified by silica-gel column chromatography (hexanes/ethyl acetate= 10/1) to obtain colorless oil (15.6 g, yield: 81%). ¹H NMR (400 MHz, CDCl₃, ppm) δ: 7.79 (d, J = 8.1 Hz, 2H), 7.31 (d, J = 8.0 Hz, 2H), 4.51 (qt, J = 5.8 Hz, 1H), 2.43 (s, 3H), 1.55-1.49 (m, 4H), 1.12-1.09 (br, 32H), 0.86 (t, J = 6.6 Hz, 6H); ¹³C NMR (400 MHz, CDCl₃, ppm): 144.24, 134.92, 129.61, 127.73, 85.30, 39.27, 36.97, 32.52, 31.74, 31.55, 27.94, 24.65, 22.64, 21.57, 19.42; m/z[m⁺] 483.6 ; Calcd: 484.0.

10-(2,6,12,16-tetramethylheptadecan-9-yl)-10H-phenothiazine (10)

The NaH (1.15 g, 48 mmol) was dispersed in the dry THF (100 mL) and was added into the solution of 10H-phenothiazine (4.42 g, 21.2 mmol) in the dry THF (150 mL). After the mixture was refluxed for 1 h with stirring, 2,6,12,16-tetramethylheptadecan-9-yl-4-methylbenzenesulfonate (15.6 g, 32 mmol) in 100 mL dry THF was dropped to the solution within 1 h, and the reaction was stirred for overnight under refluxing. The mixture was quenched with water (200 mL), and extracted three times with hexane (300 mL). The combined organic fraction was dried over magnesium sulfate, and the solvent was removed under reduced pressure. The crude product was purified by silica-gel column chromatography (hexane as eluent) resulting a yellow oil (6.3 g, yield: 61%). ¹H NMR (400 MHz, CDCl₃, ppm) δ: 7.09-7.05 (m, 4H), 6.91-6.81 (m, 4H), 3.58-3.54 (m, J = 5.8 Hz, 1H), 2.07 (m, 2H), 1.79 (m, 2H), 1.49-1.08 (m, 20H), 0.87-0.81 (m, J = 5.2 Hz, 18H); ¹³C NMR (400 MHz, CDCl₃, ppm): 145.49, 127.32, 126.87, 126.71, 122.44, 117.64, 64.89, 39.33, 37.36, 36.80, 34.66, 32.76, 32.01, 28.02, 24.77, 22.73,19.58; m/z[m⁺] 494.8; Calcd: 494.3.

3,7-dibromo-10-(2,6,12,16-tetramethylheptadecan-9-yl)-10H-phenothiazine (11)

The solution of N-bromosuccinimide (17.00 g, 95.5 mmol) in dry dimethylformamide (DMF) (50 mL) was added dropwise into the mixture of 10-(2,6,12,16-tetramethylheptadecan-9-yl)-10H-phenothiazine (19.66 g, 39.8 mmol) in dry DMF (50 mL), the reaction was stirred overnight at room temperature, quenched with water, extracted with CH₂Cl₂ three times (450 mL), washed with brine, and dried over magnesium sulfate. The solvent was removed under reduced pressure and the crude residue was purified by silica-gel column chromatography (hexane as eluent) to give yellow color oil (17.09 g, yield: 66%). ¹H NMR (400 MHz, CDCl₃, ppm) δ: 7.20-7.16 (m, 4H), 6.72 (d, J = 5.8 Hz, 2H), 3.47 (m, J = 5.5 Hz, 1H), 1.98 (m, 2H), 1.81 (m, 2H), 1.78-1.03 (br, 20H), 0.83 (br, J = 6.6 Hz, 18H); ¹³C NMR (400 MHz, CDCl₃, ppm): 144.30, 129.88, 129.57, 128.20, 118.76, 114.91, 65.32, 39.26, 37.26, 36.73, 34.59, 32.63, 31.91, 31.73, 27.98, 24.67,22.68, 19.97; m/z[m⁺] 651.2; Calcd: 651.2.

10-(henicosan-11-yl)-3,7-bis(4,4,5,5-tetramethyl-1,3,2-diocaborolan-2-yl)-10-H-Phenothiazine (3)

N-butyllithium (16.0 mL, 30 mmol, 2.5 mol/L in hexane) was added dropwise to the solution of compound 11 (6.52 g, 10 mmol) in dried THF (150 mL) in a 250 mL flask at -78°C. The mixture was stirred at -78°C for 1 h and 2-iso-propoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (8.1 mL, 40 mmol) was added rapidly to the solution. After additional one hour at -78°C, the resulting mixture was warmed to room temperature and stirred overnight. The mixture was poured into water, extracted with diethyl ether four times and dried over magnesium sulfate. The solvent was removed under reduced pressure and the residue was purified by silica-gel column chromatography (hexane as eluent, the silica–gel pretreatment with Et₃N) to obtain the title product as a yellow powder solid. (2.1 g, yield: 30%). ¹HNMR (400 MHz, C₆D₆, ppm) δ: 7.98 (s, 2H), 7.92 (d, J = 8.4 Hz, 2H), 6.95 (d, J = 8.2 Hz, 2H), 3.67 (m, J = 6.5 Hz, 1H), 1.99 (br, 2H), 1.83 (br, 2H), 1.48-1.07 (br, 44H), 0.79-0.72 (br, J = 6.0 Hz, 18H); ¹³C NMR (400 MHz, C₆D₆, ppm): 147.66, 133.74, 133.68, 125.74, 116.67, 83.63, 64.78, 39.25, 37.35, 36.76, 34.61, 32.63, 31.75, 27.94, 24.79, 22.68, 19.99, 19.48; m/z[m⁺] 745.7; Calcd: 745.2.

2,7-dibromo-9-(2,6,12,16-tetramethylheptadecan-9-yl)-9H-carbazole (9)

A three neck flask fitted with an addition funnel and a magnetic stirring bar was charged with 2,7-dibromo-carbazole (5.0 g, 10 mmol) and dried THF (150 mL). When the carbazole was completely dissolved, NaH (1.47 g, 61.5 mmol) was added to the solution, and refluxed for 1 h with stirring. Then the solution of compound 8 (15.0 g, 30.8 mmol) in dried THF (50 mL) was dropped into the mixture within 1 h, and refluxed overnight with stirring. The mixture was poured into distilled water (400 mL), and the aqueous layer was extracted with hexane three times, the combined organic fraction was dried with over magnesium sulfate and the solvent was removed under reduced pressure. The crude product was purified by column chromatography (silica-gel, hexane as eluent) resulting a colorless oil (7.5 g, yield: 78%). ¹H NMR (400 MHz, CDCl₃, ppm) δ: 7.88 (t, J = 8.6 Hz, 2H), 7.69 (s, 1H), 7.52 (s, 1H), 7.27 (br, 2H), 4.34 (m, J = 5.5 Hz, 1H), 2.16 (br, 2H), 1.91 (br, 2H), 1.13 (br, 20H), 0.81 (br, 18H); ¹³C NMR (400 MHz, CDCl₃, ppm): 142.98, 139.44, 122.36, 121.47, 120.89, 119.89, 119.20, 114.51, 112.21, 57.48, 39.15, 36.73, 33.60, 32.54, 31.25, 30.66, 27.89, 19.64, 14.15; m/z[m⁺] 619.5; Calcd: 620.2.

2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9-(2,6,12,16-tetramethylheptadecan-9-yl)-9H-carbazole (2)

N-butyllithium (7.5 mL, 12 mmol, 2.5 mol/L in hexane) was added dropwise to a solution of compound 9 (3.0 g, 4.8 mmol) in dry THF (150 mL) in a 250 mL flask at -78°C .The mixture was stirred at -78°C for 1 h and 2-iso-propoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2.96 mL, 14.5 mmol) was added rapidly to the solution. After additional one hour at -78°C, the resulting was warmed to room temperature and stirred overnight. The mixture was poured into water, extracted with diethyl ether four times and dried over magnesium sulfate, the organic phase was removed under reduced pressure and the residue was purified by silica-gel column chromatography (CH₂Cl₂/hexane= 10:1 as eluent) to obtain a white solid (0.85 g, yield: 25%). ¹H NMR (400 MHz, CDCl₃, ppm) δ: 8.11 (br, 2H), 8.03 (s, 1H), 7.86 (s, 1H), 7.65 (d, J = 7.4 Hz, 2H), 4.63 (m, 1H), 2.31 (br, 2H), 1.96 (br, 2H), 1.41 (br, 24H), 1.14 (br, 20H), 0.79 (br, 18H); ¹³C NMR (400 MHz, CDCl₃, ppm): 141.99, 138.68, 126.02, 124.76, 124.72, 124.58, 119.97, 119.70, 118.20, 115.44, 56.83, 39.18, 36.72, 33.62, 31.40, 31.12, 27.86, 24.74, 22.55, 19.61, 14.22; m/z[m⁺] 713.6; Calcd: 714.2.

Synthesis of the polymer

Poly[(9,9-dioctyl-fluoren)-alt-4,7-bis(3-hexyl-thiophene-2-yl)benzo-2,1,3-thiadia-zole] (PFDTBT-in, P1) is presented in detail as a representative example

In a 50 mL flask, compound (1) (0.3213 g, 0.5 mmol), compound (4) (0.3132 g, 0.5 mmol), tris(dibenzylideneacetone)dipalladium (20 mg), and postassium carbonate (1.3891 g, 10 mmol) were dissolved in degassed toluene (5 mL) and degassed deionized water (5 mL). After 4 days at 80°C, the polymer was obtained and precipitated in methanol/water (20:1). The polymer was filtered and washed with ethyl acetate, hexane, dichloromethane, chloroform on Soxhlet apparatus. The chloroform fraction was dried and characterized (370 mg, yield: 84%). 1H NMR (400 MHz, CDCl3, ppm) δ: 7.75 (br, 6H), 7.65 (br, 2H), 7.43 (br, 2H), 2.77 (br, 4H), 2.08 (br, 4H), 1.75 (br, 4H), 1.36-1.11 (br, 36H), 0.87-0.73 (br, 12H).

Poly[(N-9'-2,6,12,-16-Tetramethyl-heptadecane-2,7-carbazole)-alt-5,5-(4',7'-di-2-thienyl-2',1',3′-benzothiadizaole)](TMHD-PCzDTBT, P4)

In a 50 mL flask, compound (2) (0.7137 g, 1 mmol), compound (5) (0.4582 g, 1 mmol), tris(dibenzylideneacetone)dipalladium (30 mg) and postassium carbonate (1.3891 g, 10 mmol) were dissolved in the mixture of degassed toluene (10 mL) and degassed deionized water (5 mL). After 50 h reaction at 80°C, the polymer was obtained and precipitated in methanol /water (20:1), filtered and washed on Soxhlet apparatus with ethyl acetate, hexane, dichloromethane, chloroform and o-dichlorobenzene. The o-dichlorobenzene fraction was dried and characterized (423 mg, yield: 49%). ¹H NMR (400 MHz, CDCl3, ppm) δ: 8.21 (br, 1H), 8.12 (br, 2H), 7.93 (br, 2H), 7.81 (br, 2H), 7.79 (br, 2H), 7.73 (br, 1H), 7.61 (br, 2H), 7.51 (br, 2H), 7.17 (br, 2H), 4.65 (br, 1H), 2.38 (br, 2H), 2.07 (br, 2H), 1.43-1.03 (br, 10H), 0.84-0.78 (br, 18H).

Poly[(N-10'-2,6,12,16-tetramethylheptadecane-3,7-phenothiazine)-alt-5,5-(4',7'-di-2-thienyl-2',1',3′-benzothiadizaole)] (TMHD-PPDTBT, P5)

In a 50 mL flask, compound (3) (0.3729 g, 0.5 mmol), compound (5) (0.2291 g, 0.5 mmol), tris(dibenzylideneacetone)dipalladium (20 mg), postassium carbonate (1.3891 g, 10 mmol) were dissolved in degassed toluene (10 mL) and degassed deionized water (5 mL). After 5 days at 80°C, the polymer was obtained and precipitated in methanol /water (20:1), The polymer was filtered and washed with ethyl acetate, hexane, dichloromethane, chloroform on Soxhlet apparatus. The chloroform fraction was dried and characterized (270 mg, yield: 70%). ¹H NMR (400 MHz, CDCl3, ppm) δ: 8.10 (br, 2H), 7.83 (br, 2H), 7.53 (br, 2H), 7.42 (br, 4H), 6.95 (br, 2H), 3.65 (br, 1H), 2.13 (br, 2H), 1.86 (br, 2H) 1.47-1.26 (br, 20H), 0.89-0.84 (br, 18H).

Measurements

¹H-NMR and ¹³CNMR spectra were recorded by Bruker-AF301 at 400 MHz. Mass spectra were carried out on an Agilent (1100 LC/MSD Trap). UV-visible absorption spectra were measured using a Shimadzu UV-VIS-NIR Spectrophotometer (UV-3600). Solid films of UV-vis spectra measurement were spin-coated on quatrz plate from o-dichlorobenzene solution. Thermogravimetric analysis (TGA) were undertaken using a PerkinElmer Instruments (Pyris1 TGA) under nitrogen atmosphere at a heating rate of 10°C/min. Cyclic voltammetry measurements were carried out in a conventional three electrode cell using a glassy carbon as working electrode in acetonitrile solution of 0.1 mol/L of tetrabutylammoniumhexafluorophosphate (Bu₄NPF₆) as the supporting electrolyte at scan rate of 50 mV/s. In each case, a glassy carbon working electrode were coated with a thin layer of polymer, a platinum wire as the counter electrode, and a sliver wire as the quasi-reference electrode were used, and Ag/AgNO₃ (0.1 mol/L) electrode were served as a reference electrode for all potentials quoted herein. The redox couple of ferrocene/ferrocenium ion (Fc/Fc⁺) was used as external standard. The corresponding HOMO levels were calculated using E_ox/onset for experiments in solid films. Polymer films on indium tin oxide (ITO) anode electrode were formed by spin-coating method. Atomic force microscopy (AFM) was obtained in tapping-mode using Veeco (DIMENSION 3100). Size chromatography was performed on Waters Alliance GPCV2000 with a refractive index detector. Columns: Water Styvagel GE*1, Water Styvagel HMW GE*2. The eluent was 1,2,4-trichlorobenzene. The working temperature was 135°C, and the resolution time was 2 h. The concentration of the sample was 0.5 mg/mL, which was filtered (filter: 0.2 μm) prior to the analysis. The molecular weight was calculated according to calibration with polystyrene standards. Devices characterization were carried out under AM 1.5 G irradiation with the intensity of 100 mW/cm² (Oriel 91160, 150 W) calibrated by a NREL certified standard silicon cell. Current versus potential (I-V) curves were recorded with a Keithley 2400 digital source meter. External quantum efficiencies (EQEs) were detected under monochromatic illumination (Oriel Cornerstone 260 1/4 m monochromatic equipped with Oriel 70613 NS QTH lamp), and the calibration of the incident light were performed with a monocrystalline silicon diode. The thickness of films were recorded by a profilometer (Alpha-Step 200, Tencor Instruments).

Fabrication of polymer photovoltaic cells

The structure of device was glass/ITO/PEDOT:PSS/Polymer:PCBM/Ca/Al. ITO-coated glass substrates were ultrasonically cleaned in detergent, deionized water, acetone, and isopropyl alcohol before the device fabrication. Then the substrates were treated with O₂ plasma for 5 min, and PEDOT:PSS (Baytron PVP Al 4083) was spin-coated onto ITO-coated glass substrate, followed by annealing at 150°C for 30 min. The thickness of the PEDOT:PSS layer was about 50 nm, as determined by a Dektak 6 M surface profilometer. Then, the active layer was prepared by spin coating from the solution of polymer and PC₆₀BM. The substrate was transfer into an evaporation chamber. A Ca layer (10 nm) was evaporated on the substrate under high vacuum (2×10^-4 Pa), and then Al electrode (100 nm) was evaporated on the Ca layer under high vacuum as well. Post device annealing was carried out inside a nitrogen-filled glove box. The current-voltage characteristics of the photovoltaic cells were measured using a Keithley 2400 I-V measurement system. The measurement were conducted under the irradiation of AM 1.5 G simulated solar light (100 mW/cm²), light intensity was adjusted by using a standard silicon photovoltaic cells with an optical filter.

Results and discussion

Synthesis and characterization of the polymers

The synthesis of all the monomers and polymers are outlined in Scheme 2. Polymerizations were carried out by a Suzuki polymerization using the Pd(PPh₃)₄ as catalyst at 80°C for varying days. The M_n and PDI of copolymers were determined by high temperature gel permeation chromatography (GPC) with dichlorobenzene as eluent and polystyrene as a standard, and shown in Table 1. The M_n of P5 is 8600 that is much lower than other four polymers, which can be attributed to its poor solubility in toluene and precipitated during polymerization. The decomposition temperatures of these copolymers were investigated by thermogravimetric analysis (TGA) and shown in Fig. 1. P1, P2 and P3 exhibited 5% decomposition temperature over 400°C. P4 and P5 showed relatively low decomposition temperature (T_d) about 330°C. Neither obvious glass transition nor melting transition was observed for all the copolymers, indicating that the amorphous nature of the obtain polymers.

Optical and electrochemical properties of the polymers

The UV-vis absorption spectra of polymer films were investigated, as shown in Fig. 2 and Table 1. All the copolymers exhibited two absorption peaks. The long wavelength absorption bands could be attributed to the intramolecular charge transfer (ICT). Compared with the ICT absorption of P1 at 486 nm, the absorption of P2 showed 44 nm red shift, which could be attributed to strong intermolecular stacking, indicating that alkyl side chains at the 4-positions of thiophenes displayed much less steric hindrance than at the 3-positions and leading to a red-shift absorption of P2. For the polymer without hexyl side chains on DTBT units (P3), its long wavelength absorption located at 550 nm and exhibited 18 nm red-shift than P2, indicating stronger intermolecular π-stacking in P3 film. According to the above absorption spectra comparison of P1, P2, and P3, it clearly showed that alkyl substituted and its positions on DTBT units displayed strong steric effect and determined the intermolecular stacking. For P4, in which fluorene units were replaced by carbazoles, showed maxima absorptions at 400 and 562 nm respectively. There was 12 nm red shift of long wavelength absorption from P3 to P4, which may be attributed a stronger ICT for the atom replacing or better intermolecular π-stacking. P5 exhibited two absorption peaks situated at 390 and 564 nm. The long wavelength absorption was almost the same as P4 although the sulfur atom in phenothiazine was expected to show a strong electron donating ability. The reason may be originated from less flatness in phenothiazine than carbazole, and resulted in decreased intermolecular interaction.

The electrochemical properties of the polymers were characterized by cyclic voltammetry (CV) at room temperature. The CV curves were recorded referenced to an Ag/Ag⁺ (0.01 mol/L of AgNO₃ in acetonitrile) electrode, which was calibrated against the ferrocene/ferrocenium (Fc/Fc⁺) redox couple (4.74 eV) below the vacuum level. The HOMO levels of the copolymers were obtained from the onset of oxidation potentials (E_ox,onset), as shown in Fig. 3. The LUMO levels of these five copolymers were about at -3.4~-3.5 eV, which was mainly dominated by the BT units (Table 1).

Calculated from CV, the energy gap of P2 was raised 0.12 eV than that of P1. The reason was that the alkyl chains were introduced at the 3-positions of the thienyl group, apparently reduced the electron delocalization and rendered a lower HOMO and higher LUMO energy levels. The HOMO level of P3 is -5.44 eV and its LUMO level was calculated from the bandgap results based on its absorption spectrum. P4 exhibited a similar HOMO level to P3, which indicated the carbazole units did not exhibit much stronger electron donating ability than fluorenes and the ICT absorption in the two copolymers were mainly contributed by the two thienyl groups in DTBT. So its 12 nm red-shift ICT absorption from P3 to P4 mainly originated from much stronger intermolecular aggregation in P4, which might result better π-π stacking and higher charge mobility. The HOMO and LUMO levels of P5 was about 0.2 eV higher than those of P4 for a stronger ICT from phenothiazine to 2,1,3-benzothiadiazole due to the existence of sulfur atoms. All the polymers expect for P5 had a relative low HOMO levels, indicating that the devices would have a relative large V_oc, which was proved by the results of the photovoltaic device performances.

Photovoltaic characteristics of the polymer thin films

OPV devices were fabricated with the five polymers as the electron donors and the PC₆₀BM as the electron acceptor to explore the effect of the side chains and backbone on the properties of DTBT based copolymer photovoltaic cells. The structure of devices were ITO/PEDOT:PSS (50 nm)/polymer:PC₆₀BM/ Ca (10 nm) /Al (100 nm).

The solutions of polymer/PC₆₀BM were spin-coated on PEDOT:PSS modified ITO glass with of 1,2-dichlorobenzene as the solvent. No additive was applied to simplify the comparison. Ratios of the different polymer/PC₆₀BM had been optimized from 1:1, 1:2, 1:3, 1:4, and the best ratio was chosen as shown in Table 2. The thickness of the active layers, around 60-70 nm, was also optimized for all polymer films. The resulting films were dried under vacuum at room temperature and then annealed for 15 min. The optimized annealing temperatures for the five polymers were listed in Table 2. Then cathodes were deposited onto active layers at 2 × 10^-4 Pa with a metal mask. The J-V characteristics of photovoltaic cells were shown in Fig. 4 and the J_sc, fill factor (FF), and PCE of the devices were summarized in Table 2.

Compared with devices of P1, the devices of P2 showed doubled J_sc and PCE value, which is consistent with the result that P2 had stronger intermolecular aggregation than P1 for the different position of hexyl. And due to the same reason, P3 showed much higher J_sc and PCE than P2. When the fluorene units were replaced by carbazoles, P4 exhibited a little higher PCE than that of P3, which originated from the increased J_sc although its V_oc and FF were lower than those of P3. The increased J_sc in devices of P4 was contributed by its strong intermolecular stacking in films. P5 showed much lower PCE than P4 for the reasons of poor solubility and J_sc. All the devices had shown a relatively higher V_oc value from 0.76 to 0.87 V except for P5. According to the above results, the PCE of the five DTBT-based polymers were mainly determined by the J_sc of the devices, as shown in Table 2.

To study the contribution of polymer absorption to J_sc, the external quantum efficiency (EQE) of device based on the copolymers were measured under the illumination of monochromatic light as showed in Fig. 5. The maximum EQEs of P3 and P4 were much higher than those of P1 and P2, agreeing with the devices results.

Morphology

The nanoscale morphologies for polymer/PCBM films were studied using tapping-mode atomic force microscopy (AFM). Phase images (top) and surface topography (under) were taken for each film and shown in Fig. 6. Distinctly different morphologies of polymer:PCBM blend films were observed in their phase images, indicating that the active layer morphology was strongly affected by the structures of the polymers. The root mean square roughnesses (R_rms) of these five polymers were 0.26, 0.32, 0.49, 0.45 and 0.26 nm respectively, which indicated that the stronger intermolecular stacking would exhibit large size of phase separations and R_rms. This agreed with corresponding devices performances.

Conclusions

Five DTBT based copolymer with controlled M_n were synthesized and characterized to explore the influence of side chains and backbone structure on the properties of absorption spectrums, energy levels, and photovoltaic devices. Strong steric hindrance would be produced when side chains were introduced onto the thiophenes in DTBT units, which also exhibited position dependence when modifying the side chains from 3- to 4-positions. For the polymer without alkyl substituted on DTBT, the polymer showed stronger intermolecular stacking and lower bandgap. When the copolymerization units were changed from fluorene to carbazole, the red-shift ICT absorption in carbazole copolymer was mainly attributed to its better π-π stacking. And for all the five DTBT-based copolymers, the PCE of their corresponding devices mainly depended on their J_sc, which were determined by the intermolecular stacking of the donor polymers and had also been proved by their morphology results.

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