Full donor-type conjugated polymers consisting of alkoxy- or alkylselenophene-substituted benzodithiophene and thiophene units for organic photovoltaic devices

Full donor-type conjugated polymers containing benzodithiophene and thiophene derivative units were synthesized as electron donors for organic photovoltaic devices. The alkoxy-substituted benzo[1,2-b:4,5-b′]dithiophene (BDT) monomer, 2,6-bis(trimethyltin)-4,8-di(2-ethylhexyloxyl)benzo[1,2-b:4,5-b′]dithiophene, was polymerized with 2,5-dibromothiophene through a Pd(0)-catalyzed Stille coupling reaction. To enhance the interchain interactions between polymers chains, an alkylselenophene-substituted BDT derivative was newly synthesized, and copolymerized with the same counter monomer parts. The two newly synthesized polymers were characterized for use in organic photovoltaic devices as electron donors. Measured optical band gap energies of the polymers were 2.10 and 1.96 eV, depending on polymer structure. Field-effect transistors were fabricated using the polymers to measure their hole mobilities, which ranged from 10−3 to 10−5 cm2 V−1 s−1 depending on the polymer structure. Bulk heterojunction organic photovoltaic cells were fabricated using conjugated polymers as electron donors and a [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) as an electron acceptor. One fabricated device showed a power conversion efficiency of 2.73%, an open-circuit voltage of 0.72 V, a short-circuit current of 7.73 mA cm−2, and a fill factor of 0.46, under air mass (AM) 1.5 global (1.5 G) illumination conditions (100 mW cm−2).