Bulky electron donating side chain enhances the open-circuit voltage of benzodithiophene photovoltaic polymers

Abstract

Herein, a series of random donor copolymers were reported, by incorporating different contents of electron-rich 4,8-bis(9,9-bis(2-ethylhexyl)-9H-fluoren-2-yl)benzo[1,2-b:4,5-b']dithiophene (BDTF) units into PBDB-T (poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione))]) polymer to fine-tune the highest occupied molecular orbital (HOMO) energy level. As we expected, downshifted HOMO of corresponding copolymers was observed with increased the ratios of BDTF unit on PBDB-T polymer, causing by the synergy of weak electron-donating ability of the fluorene side chains and the enhanced dihedral angles. Notably, compared with PBDB-T, the HOMO energy level of P91 containing 10% BDTF was shifted down by 0.07 eV. Among them, the devices based on P55:ITIC reached a high open-circuit voltage (VOC) of 1.00 V with incorporation of 50% of BDTF units. The result indicated that the BDTF unit, as an excellent building unit in donor materials, could play a crucial role to realize high-VOC polymer solar cells.