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

• Conjugated polymers containing fluorinated quinoxaline or triazole derivatives were synthesized.
• The synthesized polymers were used as p-type donor materials for OPVs.
• Bulk heterojunction organic photovoltaic cells were fabricated using the polymers.
• One of the fabricated devices showed a power conversion efficiency of 4.22%.

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.

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