High open-circuit voltage organic photovoltaic cells fabricated using semiconducting copolymers consisting of bithiophene and fluorinated quinoxaline or triazole derivatives

Two new alternating copolymers consisting of bithiophene and fluorinated quinoxaline or fluorinated benzotriazole derivatives, namely, poly[{4,4′-bis(2-ethylhexyl)-2,2′-bithiophene-5,5′-diyl}-alt-{6,7-difluoro-5,5-(5,8-di-2-thienyl-2,3-bis(4-octyloxyl)phenyl) quinoxaline}] (PBT-DFDTQX) and poly[{4,4′-bis(2-ethylhexyl)-2,2′-bithiophene-5,5′-diyl}-alt-{4,7-bis(5-thiophen-2-yl)-5,6-difluoro-2-(heptadecan-9-yl)-2H-benzo[d][1,2,3]triazole}] (PBT-DFDTBTz), were synthesized by the Stille cross coupling reaction for application in organic photovoltaic cells. The optical band gaps of PBT-DFDTQX and PBT-DFDTBTz were measured to be 1.77 and 1.90 eV, respectively. The synthesized polymers showed relatively deep highest occupied molecular orbital (HOMO) energy levels (−5.52 eV for PBT-DFDTQX and −5.54 eV for PBT-DFDTBTz) owing to the strong electron accepting nature of fluorine. The polymers were used to fabricate bulk heterojunction photovoltaic devices with [6,6]-phenyl C71-butylic acid methyl ester (PC71BM) as the electron acceptor. Devices fabricated using PBT-DFDTQX and PBT-DFDTBTz showed the maximum power conversion efficiency (PCE) of 4.01 and 4.22%, respectively, with a high open-circuit voltage of over 0.95 V under AM 1.5G (100 mW/cm2) conditions.