Fabrication of Polymer Solar Cells

The polymer photovoltaic (PV) cells in this study contained an active layer of copolymers (P1-P5) blended with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) in solid films, which was sandwiched between a transparent indium tin oxide (ITO) anode and a metal

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cathode (Ca). Prior to the device fabrication, ITO-coated glass substrates (1.5×1.5 cm²) were ultrasonically cleaned in detergent, deionized water, acetone, and isopropyl alcohol. After routine solvent cleaning, the substrates were treated with UV ozone for 15 min. Then, a modified ITO surface was obtained by spin-coating a layer of poly(ethylene dioxythiophene):polystyrenesulfonate (PEDOT:PSS) (~30 nm). After baking at 130°C for one hour, the substrates were transferred to a nitrogen-filled glove box. Subsequently, on the top of PEDOT:PSS layer, the active layer was prepared by spin coating from blended solutions of polymers P1-P5:PCBM (with 1:1 w/w) with a spin rate ca. 1500 rpm, and the thickness of the active layer was typically ca. 80 nm. Initially, the blended solutions were prepared by dissolving both polymers and PCBM in 1,2 dichlorobenzene (DCB)(20 mg/1 ml), followed by continuous stirring for 12 h at 50°C. In the slow-growth approach, blended polymers in solid films were kept in the liquid phase after spin-coating by using the solvent with a high boiling point (such as a glass petri dish) and allowed to dry the solvent slowly. Finally, a calcium layer (30 nm) and a subsequent aluminum layer (100 nm) were thermally evaporated through a shadow mask at a pressure below 6×10^-6 Torr to have the active device area of 0.12 cm². The solar cell testing was done inside a glove box under simulated AM 1.5G irradiation (100 mW/cm²) using a Xenon lamp based solar simulator (Thermal Oriel 1000W). The light intensity was calibrated by a mono-silicon photodiode with KG-5 color filter (Hamamatsu). The external quantum efficiency (EQE) action spectra were obtained at short-circuit condition. The light source was a 450 W Xe lamp (Oriel Instrument, model 6266) equipped with a water-based IR filter (Oriel Instrument, model 6123NS). The light output from the monochromator (Oriel Instrument, model 74100) was focused on the photovoltaic cell under test.

68 3.2.4 Fabrication of Hole- and Electron-Only Devices

The hole- and electron-only devices in this study contain polymer blend films of P1-P5:PCBM (1:1 wt.) sandwiched between transparent ITO anode and cathode (MoO3 and Ca, respectively). The devices have been prepared following the same procedure as fabrication of BHJ devices, except that Ca was replaced with MoO3 ( = 5.3 eV) in the hole-only devices, and the PEDOT:PSS layer was replaced with Cs2CO3 ( = 2.9 eV) in the electron-only devices. The MoO₃ was thermally evaporated to a thickness of 20 nm and then capped with 50 nm of Al on the top of the active layer in the hole-only devices and Cs2CO3 was thermally evaporated with a thickness of approximately 2 nm on the transparent ITO in the electron-only devices. For both devices, annealing of the active layer was performed at 130°C for 20 min.

3.2.5 Synthesis of Monomers and Polymers

4-(Nonan-5-yl)-4H-dithieno[3,2-b:2',3'-d]pyrrole (1)

3,3’-Dibromo-2,2’-bithiophene (1 g, 3.08 mmol), t-BuONa (0.71 g, 7.40 mmol), Pd2dba3

(0.071 g, 0.007 mmol), and BINAP (0.288 g, 0.046 mmol) were dissolved in dry toluene (15 ml).

The solution was purged with N₂ for 30 min. Nonan-5-amine (0.88 g, 6.17mmol) was added via a syringe, and the mixture was stirred at 110°C under N2 for 12 h. After cooling, water was added to quench the reaction, and the solution was extracted twice with diethyl ether. The combined organic layer was dried over MgSO₄, and the solvent was removed by rotary evaporation. The crude product was purified by column chromatography using silica, hexane as an eluent to give a white solid (0.69 g, 73.3 %). ¹H NMR (300 MHz, CDCl3, ): 7.10 (d, J = 5.4

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Hz, 2H), 7.02 (d, J = 5.4 Hz, 2H), 4.26-4.16 (m, 1H), 1.96-1.82 (m, 2H), 1.81-1.32 (m, 2H), 1.14-1.05 (m, 8H), 0.88 (t, J = 7.2 Hz, 6H).

4-(Heptadecan-9-yl)-2,6-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-4H-dithieno[3,2-b:2',3'-d]pyrrole (2)

Compound 1 (0.5 g, 1.63 mmol) was dissolved in 50 ml of dry THF, and the solution was cooled down to -78ºC under nitrogen protection. Then, 0.65 ml of n-BuLi (2.5M in Hexane, 3.26 mmol) was added, and the solution was warmed up to room temperature for 30 min and cooled again to -78ºC. 2-Isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1.0 ml, 4.90 mmol) was rapidly injected into the solution by a syringe, and the resulting mixture was stirred at -78ºC for 1 h and left to stir overnight at room temperature. The resulting mixture was quenched with H₂O and extracted with DCM. The DCM extracts were washed with saturated brine and dried with MgSO4. The solvent was removed by rotary evaporator and the product was further purified by column chromatography on silica using a mixture of hexane and DCM (4:1) as an eluent to yield a white solid (0.49 g, 53.8%). ¹H NMR (300 MHz, CDCl3, ): 7.50 (s, 2H), 4.20 (m, 1H), 2.05-1.97 (m, 2H), 1.86-1.78 (m, 2H), 1.37 (s, 24H), 0.84 (t, J = 6.0 Hz, 6H). ¹³C NMR (75 MHz, CDCl3, ): 147.78, 121.51, 120.68, 84.31, 60.26, 34.93, 29.07, 25.02, 22.60, 14.13. MS (EI):

m/z [M⁺] 557.42 calcd m/z [M⁺] 557.0. Anal. calcd. for C29H45B2NO4S2: C, 62.49, H, 8.14, N, 2.51; Found: C 62.22, H 8.01, N 2.54.

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4,4'-dihexyl-5,5'-bis(4-hexylthiophen-2-yl)-2,2'-bithiazole (3)

To a solution of monomer M2 (1 g, 2.02 mmol) in THF (30 ml), 2-(4-hexylthiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1.78 g, 6.06 mmol), Pd(PPh3)4 (35 mg) and 10 mL of 2 M aqueous K2CO3 solution were added. The reaction mixture was purged with N2 and refluxed for 24 h. The reaction mixture was cooled to room temperature and extracted with DCM followed by washing with water and brine, and dried by anhydrous MgSO4. The organic fraction was concentrated by rotary evaporation and pure product was obtained by recrystallization from methanol as a yellow solid (1.09 g, 80.7%). ¹H NMR (300 MHz, CDCl3, ): 7.02 (s, 2H), 6.95 (s, 2H), 2.92 (t, J = 6.0 Hz, 4H), 2.61 (t, J = 6.0 Hz, 4H), 1.36-1.25 (m, 32H), 0.88 (t, J = 6.0 Hz, 12H).

5,5'-bis(5-bromo-4-hexylthiophen-2-yl)-4,4'-dihexyl-2,2'-bithiazole (M5)

At room temperature, NBS (0.80 g, 4.48 mmol) was added portion wise to a solution of compound 3 (1.0 g, 1.49 mmol) in 25 ml THF. After 30 min, water was added and the crude compound was extracted with DCM. The organic layer was washed by water and brine, and dried by anhydrous MgSO4. The organic fraction was concentrated by rotary evaporation and pure product was obtained by recrystallization from methanol as an orange solid (0.98 g, 79.7%).

1H NMR (300 MHz, CDCl₃, ): 6.87 (s, 2H), 2.87 (t, J = 6.0 Hz, 4H), 2.57 (t, J = 6.0 Hz, 4H), 1.36-1.25 (m, 32H), 0.89 (t, J = 6.0 Hz, 12H). ¹³C NMR (75 MHz, CDCl3, ): 157.97, 154.98, 143.09, 132.62, 128.52, 122.52, 110.23, 31.82, 30.56, 29.80, 29.35, 22.80, 14.30. MS (EI): m/z [M⁺] 658.60 calcd. m/z [M⁺] 659.0. Anal. calcd. for C38H54Br2N2S4: C, 55.19; H, 6.58; N, 3.39.

Found: C, 55.12; H, 6.50; N, 3.30.

71 General Polymerization Procedure

The synthetic routes of polymers are shown in Figure 3.1. All polymerization steps were carried out through the palladium(0)-catalyzed Suzuki coupling reactions. In a 25 ml flame dried two-neck flask, compound 2, dibromo monomers (M1-M5), and (PPh3)4 Pd(0) (1.5-0.5 mol %) were dissolved in a mixture of toluene (10 ml) and aqueous 2 M K₂CO₃ (5 ml). The reaction mixture was vigorously stirred at 90°C for 3-4 days. After reaction, an excess of iodobenzene was added to the reaction then one hour later, an excess of phenylboronic acid was added and the reaction was refluxed overnight to complete the end-capping reaction. The polymer was purified by precipitation in methanol/water (10:1), filtered and washed on Soxhlet apparatus using hexane, acetone, and chloroform. The chloroform fraction was reduced to 40-50 ml under reduced pressure, precipitated in methanol/water (10:1, 500 ml), filtered through 0.45 μm nylon filters, and finally air-dried overnight.

P1

Following the general polymerization procedure, compound 2 (0.5 equiv.) and M1 (0.5 equiv.) were used in this polymerization, and the polymer was obtained as a black powder. Yield:

50%. ¹H NMR (300 MHz, CDCl3, ): 7.95 (s, 2H), 7.12 (s, 2H), 4.21 (m, 1H), 2.81 (br, 2H), 1.79-1.33 (br, 2H),1.15-1.03 (br, 8H), 0.86 (m, 6H). Anal. calcd. for C₂₃H₂₅N₃S₄: C, 58.56; H, 5.34; N, 8.91; S, 27.19; Found: C, 58.06; H, 5.13; N, 8.22.

P2

Following the general polymerization procedure, compound 2 (0.5 equiv.) and M2 (0.5 equiv.) were used in this polymerization, and the polymer was obtained as a black powder. Yield:

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66%. ¹H NMR (300 MHz, CDCl3, ): 7.20 (s, 2H), 4.21 (m, 1H), 3.03 (m, 4H), 2.95-1.74 (br, m, 6H), 1.60-1.15 (br, m, 22H), 0.90-0.83 (m, 12H). Anal. calcd. for C35H49N3S4: C, 65.68; H, 7.72;

N, 6.57; S, 20.04; Found: C, 65.23; H, 7.52; N, 6.14.

P3

Following the general polymerization procedure, compound 2 (0.5 equiv.) and M3 (0.5 equiv.) were used in this polymerization, and the polymer was obtained as a black powder. Yield:

62%. ¹H NMR (300 MHz, CDCl3, ): 7.95 (s, 2H), 7.20 (s, 4H), 4.21 (m, 1H), 2.93 (m, 4H), 2.95-1.70 (br, m, 6H), 1.61-1.11 (br, m, 22H), 0.90-0.83 (m, 12H). Anal. calcd. for C43H53N3S6: C, 64.21; H, 6.64; N, 5.22; S, 23.92; Found: C, 63.71; H, 6.64; N, 5.05.

P4

Following the general polymerization procedure, compound 2 (0.5 equiv.) and M4 (0.5 equiv.) were used in this polymerization, and the polymer was obtained as a black powder. Yield:

59 %. ¹H NMR (300 MHz, CDCl₃, ):7.30-7.13 (s, 6H), 4.23 (m, 1H), 3.03 (m, 4H), 2.95- 1.74 (br, m, 6H),1.61-1.13 (br, m, 22H), 0.90-0.83 (m, 12H). Anal. calcd. for C43H53N3S6: C, 64.21; H, 6.64; N, 5.22; S, 23.92; Found: C, 65.11; H, 6.34; N, 5.41.

P5

Following the general polymerization procedure, compound 2 (0.5 equiv.) and M5 (0.5 equiv.) were used in this polymerization, and the polymer was obtained as a dark red powder.

Yield: 61%. ¹H NMR (300 MHz, CDCl₃, ):7.20-6.91 (br, 4H), 4.21 (m, 1H), 3.03 (m, 10H),

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2.95-1.74 (br, m, 10H),1.81-1.15 (br, m, 32H), 0.90-0.83 (m, 18H). Anal. calcd. for C55H77N3S6: C, 67.92; H, 7.98; N, 4.32; S, 19.78; Found: C, 67.22; H, 7.46; N, 4.01.

Figure 3.1 Synthesis of M5 and polymers (P1-P5) ^a.

aReagents and conditions: (ii) Toluene ,t-BuONa, Pd2dba3,BINAP, Nona5-amine (ii) THF, n-BuLi, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, -78°C (iii) THF, 2-(4-hexylthiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, Pd(PPh₃)₄, 24 h (vi) NBS, THF, 30 min (v) Toluene, 2M K2CO3, Pd(PPh3)4, 90°C, 3-4 days.

74 3.3 Results and discussion