2.1 Materials
All starting materials were purchased from Sigma-Aldrich or Tokyo Chemical Industry. All solvents were purified by passage under N2 atmosphere. 5,10-Bis(2-decyltetradecyl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-5,10-dihydroindolo[3,2-b]indole and (E)-1,2-bis(5-bromoselenophen-2-yl)ethene were synthesized via published literature procedures [12]. Material used as the hole transport layer (HTL) was supplied by CLEVIOS™ with poly (3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS), AI4083. Solvent of CB (chlorobenzene), CN (1-chloronaphthalene), DIO (1, 8-diiodooctane) and IPA (2-propanol) was used in Sigma Aldrich. PC70BM was purchased from an EM Index company in Korea. Titanium (IV) isopropoxide was supplied by Sigma Aldrich.
2.1.1 Synthesis of (E)-1,2-bis(5-bromoselenophen-2-yl)ethene
Under N2, (E)-1,2-di(selenophen-2-yl)ethene (1 g, 3.495 mmol), N-bromosuccinimide (1.49 g, 8.388 mmol), and DMF (25 mL) were combined and stirred for 3 h at room temperature. The solution was then diluted with H2O and extracted with dichloromethane. The organic extract was dried over MgSO4 and evaporated. The crude product was column chromatographed with hexane as the eluent and then crystallized from ether and methanol. Greenish white crystal, yield: 1.21 g (78.06%), 1H NMR (CDCl3, 300 MHz): δ (ppm) = 7.155 (2H, d, J = 1.4 Hz), 6.899 (2H, d, J = 1.4 Hz), 6.774 (1H, s).
2.1.2 Synthesis of 3,9-dibromodibenzo[b,f][1,5]diazocine-6,12(5H,11H)-dione (1)
Methyl 2-amino-4-bromobenzoate (30 g, 198 mmol) was dissolved in anhydrous THF (800 mL). Then, NaH (12.8 g, 533 mmol, 60% dispersion in mineral oil) was slowly added at room temperature. Next, the solution was stirred at 90 °C for 12 h. The solution was cooled to 25 °C and then poured slowly into a 2 M HCl anhydrous solution and ice to produce a solid. The precipitated product was collected by filtration, washed with distilled water (250 mL), and washed with methanol (50 mL) to afford 3,9-dibromodibenzo[b,f][1,5]diazocine-6,12(5H,11H)-dione (1). White solid (crude), 1H NMR MR (DMSO-d6, 500 MHz): δ (ppm) = 167.90, 135.55, 134.25, 134.04, 131.06, 128.49, 120.42, 67.49, HRMS-EI+ (m/z): Found: [M]+ 393.8955; C14H8Br2N2O2 requires [M]+ 393.8953.
2.1.3 Synthesis of sodium (5E,11E)-3,9-dibromodibenzo[b,f][1,5]diazocine-6,12-bis(olate) (2)
Under N2, 1 (50 g, 126 mmol) was dissolved into methanol (500 mL). Sodium t-butoxide (26.7 g, 278 mmol) was added slowly to the solution at room temperature, and the solution was stirred at 80 °C for 12 h. Finally, the methanol was removed by evaporation and the precipitated product was collected by filtration and washed with Et2O (50 mL) to afford sodium (5E,11E)-3,9-dibromodibenzo[b,f][1,5]diazocine-6,12-bis(olate) (2). White solid (crude), 1H NMR (DMSO-d6, 300 MHz): δ (ppm) = 6.85 (4H, s), 6.77 (2H, s), 13C NMR (DMSO-d6, 500 MHz): δ (ppm) = 170.87, 149.59, 135.39, 128.99, 126.46, 123.92, 119.99.
2.1.4 Synthesis of (5E,11E)-3,9-dibromo-6,12-dichlorodibenzo[b,f][1,5]diazocine (3)
Under N2, dried 2 (20 g, 45.5 mmol) was dissolved into CB (500 mL). Phosphorus oxychloride (21 mL, 227.5 mmol) and phosphorus pentachloride (28.4 g, 136.5 mmol) were poured slowly to the solution. Then, the solution was added dropwise to anhydrous pyridine (1.8 mL, 22.7 mmol) at room temperature. The reaction mixture was stirred at 170 °C for 2 h. After cooling to room temperature, CB was removed by distillation. The mixture was cooled to room temperature and poured slowly over sodium hydroxide and ice to produce a solid, and then the reaction mixture was neutralized to ~ pH 9. The reaction mixture was extracted with ethyl acetate, and the organic layer was dried with anhydrous MgSO4 and the solvent removed. The crude product was purified with column chromatography with dichloromethane and hexane (1:1) and recrystallized with acetone. After drying, (5E,11E)-3,9-dibromo-6,12-dichlorodibenzo[b,f][1,5]diazocine (3) was obtained. White, yield: 11.8 g (60%), 1H NMR (DMSO-d6, 300 MHz): δ (ppm) = 7.50–7.43 (4H, m), 7.33–7.32 (2H, m), 13C NMR (DMSO-d6, 500 MHz): δ (ppm) = 156.30, 146.06, 129.63, 129.40, 125.97, 124.79, 124.74, HRMS-EI+ (m/z): Found: [M]+ 429.8276; C14H6Br2Cl2N2 requires [M]+ 429.8275.
2.1.5 Synthesis of 2,7-dibromo-5,10-dihydroindolo[3,2-b]indole (4)
Compound 3 (10 g, 23.1 mmol) was dissolved into purified THF (46 mL), and zinc powder (18.1 g, 227.2 mmol) was poured into the mixed solution. Then, trifluoroacetic acid (41 mL, 554.4 mmol) was added dropwise at room temperature. The mixture was stirred at room temperature for 12 h and filtered to remove the zinc powder. The reaction mixture was extracted with ethyl acetate, and the organic phase was dried with MgSO4 and concentrated under reduced pressure. The crude product was washed with methanol, affording 2,7-dibromo-5,10-dihydroindolo[3,2-b]indole (4). White solid, yield: 5.0 g (60%), 1H NMR (DMSO-d6, 300 MHz): δ (ppm) = 11.35 (2H, s), 7.71 (1H, s), 7.69 (1H, d, J = 1.92 Hz), 7.24 (1H, d, J = 1.70 Hz), 7.21 (1H, d, J = 1.70 Hz), 13C NMR (DMSO-d6, 500 MHz): δ (ppm) = 168.73, 136.46, 132.74, 130.98, 130.67, 128.73, 123.62, HRMS-EI+ (m/z): Found: [M]+ 361.9048; C14H8Br2N2 requires [M]+ 363.9034.
2.1.6 Synthesis of 2,7-dibromo-5,10-bis(2-decyltetradecyl)-5,10-dihydroindolo[3,2-b]indole (5)
Compound 4 (5 g, 13.7 mmol) was dissolved into purified THF (300 mL) under N2. Then, NaH (2.0 g, 82.2 mmol, 60% dispersion in mineral oil) was slowly added, and the reaction mixture was stirred at 70 °C for 30 min at atmosphere. 11-(Bromomethyl)tricosane (17.2 g, 41.1 mmol) was added to the reaction mixture and stirred at 100 °C for 24 h. After completion, the reaction was cooled to room temperature. The reaction mixture was extracted with ethyl acetate, and the organic phase was dried with MgSO4 and concentrated under reduced pressure. The mixed compound was purified by column chromatography with n-hexane and recrystallized with dichloromethane and acetone. After drying, 2,7-dibromo-5,10-bis(2-decyltetradecyl)-5,10-dihydroindolo[3,2-b]indole (5) was obtained. White, yield: 8.55 g (60%), 1H NMR (CD2Cl2, 300 MHZ): δ (ppm) = 7.73 (2H, d, J = 8.4 Hz), 7.63 (2H, d, J = 1.5 Hz), 7.28 (2H, dd, J = 1.5, 1.8 Hz), 4.34 (4H, d, J = 9 Hz), 2.19–2.16 (4H, br), 1.29–1.20 (80H, br), 0.91 (12H, t, J = 6.78, 6.78 Hz), 13C NMR (CDCl3, 500 MHz): δ (ppm) = 141.60, 126.12, 121.33, 118.78, 115.29, 113.16, 113.05, 50.02, 38.64, 31.93, 31.62, 29.91, 29.67, 29.61, 29.53, 29.39, 29.35, 26.45, 22.71, 14.14, HRMS-FAB+ (m/z): Found: [M]+ 1034.6669; C62H104Br2N2 requires [M]+ 1034.6566.
2.1.7 Synthesis of 2,7-DHI
Compound 5 (3.5 g, 3.37 mmol) was dissolved into anhydrous DMF (200 mL) under N2. Then, potassium acetate (4.3 g, 28.6 mmol), bis(pinacolato)diboron (4.2 g, 16.87 mmol), and Pd(dppf)2Cl2 (0.25 g, 0.34 mmol) were added, and the reaction mixture was stirred at 90 °C for 12 h. After completion, DMF was removed by distillation. The reactant was extracted with dichloromethane, and the organic phase was dried with MgSO4 and concentrated under reduced pressure. The mixture was purified by column chromatography (dichloromethane:hexane = 1:2) and recrystallized with dichloromethane and acetone. After drying, 2,7-DHI was obtained. White, yield: 2.6 g (68%), 1H NMR (CD2Cl2, 300 MHz): δ (ppm) = 7.94 (2H, s), 7.88 (2H, d, J = 8.1 Hz), 7.57 (2H, d, J = 8.1 Hz), 4.45 (1H, d, J = 7.50 Hz), 2.28–2.27 (2H, br), 1.14–1.20 (104H, br), 0.91 (12H, t, J = 6.76, 6.76 Hz), 13C NMR (CD2Cl2, 500 MHz): δ (ppm) = 140.99, 127.27, 123.75, 117.33, 116.60, 116.10, 83.51, 49.64, 38.77, 31.91, 31.57, 29.91, 29.66, 29.59, 29.52, 29.36, 29.33, 26.44, 24.72, 22.68, 22.68, 13.87, HRMS-FAB+ (m/z): Found: [M]+ 1131.0029; C62H104Br2N2 requires [M]+ 1131.0060.
2.2 Instrumentation
The 1H NMR spectra were recorded using a Bruker DRX 300 MHz spectrometer. Mass spectra were measured using a JEOL JMS-700. The thermal analysis measurements were performed using a thermogravimetric analyser (TGA) (TGA 2050, TA Instruments) under N2, and the samples were heated at 10 °C/min. Differential scanning calorimetry (DSC) was conducted under N2 using a TA Instruments 2100 DSC. The samples were heated at 10 °C/min from 0 to 350 °C. UV–vis absorption spectra were measured using a PerkinElmer LAMBDA-900 UV/vis/IR spectrophotometer. Cyclic voltammograms of materials were recorded on an Epsilon E3 at room temperature in a 0.1 M solution of tetrabutylammonium perchlorate (Bu4NClO4) in acetonitrile under N2 at a scan rate of 50 mV/s. A Pt wire was used as the counter electrode and an Ag/AgCl electrode was used as the reference electrode.
2.3 Polymerization of P-IDI-SVS
The polymer was synthesized by the Suzuki coupling reaction using palladium catalysts. 5,10-Bis(2-decyltetradecyl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-5,10-dihydroindolo[3,2-b]indole (0.3 g, 0.265 mmol) and (E)-1,2-bis(5-bromoselenophen-2-yl)ethene (SVS-Br) (0.117 g, 0.265 mmol) were dissolved into dry toluene (8 mL) and nitrogen-bubbled for 20 min in a Schlenk flask. Then, Pd2(dba)3 (0.003 g, 0.0039 mmol), P-(o-tol)3 (0.007 g, 0.0238 mmol), K3PO4 (0.225 g, 1.060 mmol), distilled water (3 mL), and Aliquat 336 (1 drop) were added, and the reaction mixture was stirred at 95 °C under N2 for 80 h. Then, phenyl boronic acid was added and stirred into the reaction mixture for 6 h, and bromobenzene was added and stirred for 6 h for end capping. After cooling down, the mixture was precipitated in methanol. The filtered polymer was purified and fractionated by Soxhlet using methanol, acetone, n-hexane, THF, and CF. Each of the THF, CF, and CB fractions were evaporated under reduced pressure, and the product was precipitated in methanol, filtered, and finally dried under high vacuum. The Soxhlet portions of the polymer were fractionated to investigate the molecular weight effect [10]. The number-average molecular weights (Mn) of the polymers were estimated by gel permeation chromatography using a polystyrene standard with THF and CF solvent. The THF and CF Soxhlet portions had Mn = 109 kDa with 1.015 of PDI and Mn = 128 kDa with 1.006 of PDI. Yield: 0.24 g (85.71%), 1H NMR (CDCl3, 500 MHz): δ (ppm) = 8.20–6.28 (12H, br), 4.42–3.37 (6H, br), 1.38–0.81 (92H, br).
2.4 Device fabrication
P-IDI-SVS:PC70BM organic solar cells based on bulk heterojunctions were fabricated on indium tin oxide (ITO) coated glass substrates. The glass substrate coated with ITO was washed with detergent and sonicated with distilled water, acetone and isopropyl alcohol (IPA) for 20 min. After washing, IPA is dried with nitrogen, placed in an oven and dried for more than 20 min. ITO was subjected to UV-ozone treatment for 15 min to change the surface properties of ITO from a hydrophobic surface to a hydrophilic surface. A hole transport layer of PEDOT:PSS (Al4083) was spin-coated on ITO (thickness: 40 nm or less). The PEDOT:PSS substrate was annealed in air at 140 °C for 10 min to remove the solvent. A P-IDI-SVS:PC70BM solution was prepared at a concentration of 20 mg/mL as a function of a donor:acceptor ratio (1:2 w/w) in CB with 1-chloronaphthalene (CN) 3 vol% and diiodooctane (DIO) 3 vol%. This solution was spin-casted on top of the PEDOT:PSS layer in an Ar-filled glove box after filtering. Then, a 10 nm or less TiOx layer as an electron transfer layer was formed on the photoactive layer by spin-coating. A 100 nm aluminium layer as a cathode electrode was then deposited under a pressure of 2.0 × 10–6 Torr through thermal evaporation. All devices were encapsulated using a resin and a cover glass.
2.5 Characterization
The current density–voltage (J–V) characteristics and impedance spectroscopy of the OPV were measured using a ZIVE SP1 and solar simulator with illuminated AM 1.5 Global conditions at an intensity of 100 mW/cm2 and a cell area of 0.15 cm2. Short-circuit current verification of JSC related to J–V curve by measuring incident photocurrent efficiency (IPCE) spectrum of solar cell after power calibration (ABET technologies, Inc., LS150, USA) using a monochromatic chromatograph (Dongwoo OPTRON Co., Ltd., MonoRa-500i, Korea). The surface morphology of the blend of P-IDI-SVS:PC70BM was observed by AFM (Park NX10) using non-contact mode. The PL spectra were measured by fluorescence spectroscopy (F-7000 fluorescence spectroscopy, HITACHI, Tokyo, Japan) for the quenching rate of the active layer of P-IDI-SVS:PC70BM. The excitation wavelength was 530 nm and the PMT voltage was 700 V. The time-correlated single photon counting (TCSPC) technique (XperRam Ultimate) under excitation from a 405 nm laser a 20 MHz repetition rate. The Raman Intensity were measured by Raman microscopy (Xperam200(Nanobase Inc.). The laser wavelength was 532 nm, and the power was 4 µW for each device. The magnification of the object lens was 50×. We calculated the average of ten datasets of Raman spectra in the same position for each sample.