Investigation of Optical and Electrochemical Properties of Benzene Based Solution Processable 2,1,3-Benzooxadiazole Comprising Polymers

Four novel solution processable benzooxadiazole containing alternating copolymers namely poly (5,6-bis(octyloxy)-4-(5-phenylthiophen-2-yl)-7-(thiophen-2-yl)benzo[c][1,2,5]oxadiazole)PBOTPh, poly (5,6-bis(octyloxy)-4-(5-phenylselenophen-2-yl)-7-(selenophen-2-yl)benzo[c][1,2,5]oxadiazole)PBOSPh, poly (4-(furan-2-yl)-5,6-bis(octyloxy)−7-(5-phenylfuran-2-yl)benzo[c][1,2,5]oxadiazole)PBOFPh and poly (5,6-bis(octyloxy)-4-(5-phenylthieno[3,2-b]thiophen-2-yl)-7-(thieno[3,2-b]thiophen-2-yl)benzo[c][1,2,5]oxadiazole) PBOTTPh incorporating 5,6-bis(octyloxy)benzo[c][1,2,5]oxadiazole as the acceptor moiety and benzene as the donor moiety were synthesized via Suzuki polycondensation reaction. Various electron donor moieties were used as π bridges in the polymer backbone to tailor the optoelectronic properties of the polymers. To explore the electrochemical and electrochromic properties and the photovoltaic properties of the copolymers, cyclic voltammetry, spectroelectrochemistry and kinetic studies were conducted. Thiophene, selenophene, furan and thienothiophene π bridges were incorporated into the polymer backbones. Thiophene and selenophene based alternating copolymers showed purple color in its neutral state whereas furan and thienothiophene bearing copolymers have pinkish purple and dark purple in their neutral states respectively. The optical band gaps of the polymers were calculated as 1.85, 1.76, 1.58, 1.79 eV for thiophene, selenophene, thienothiophene and furan based copolymers, respectively. Among all the alternating copolymers, the lowest optical band gap corresponds to thienothiophene comprising alternating copolymer which is 1.58 eV and red shifted in absorption due to the stronger donor character compared to other analogs. HOMO and LUMO levels were calculated from cyclic voltammetry studies using onsets of oxidation and reduction, respectively. HOMO levels were calculated as −5.96 eV for PBOTPh, −5.85 eV for PBOSPh, −5.63 eV for PBOTTPh and −5.74 eV for PBOFPh. LUMO levels were determined as −4.02 eV for PBOSPh, −4.03 eV for PBOTTPh and −3.87 eV for PBOFPh. Bulk heterojunction (BHJ) polymer solar cells based on selenophene containing polymer were fabricated. The device architecture was: ITO/PEDOT:PSS/PBOSPh:PC71BM/Ca/Al. After optimization studies, best results for 130 nm thickness, Polymer:PC71BM with 1:2 ratio, Voc, Jsc, FF, PCE (%) were found as 0.62 V, 4.53 mA cm−2, 0.58, 1.63% under illumination of AM 1.5 G 100 mW cm−2.