Synthesis and characterization of quinoxaline-based polymers for bulk-heterojunction polymer solar cells

A series of quinoxaline (Qx)-based copolymers, poly[2,7-(9,9-bis(2- ethylhexyl)dibenzosilole)-alt-5,5-(5′,8′-di-2-thienyl-2, 3-bis(4-octyloxyl)phenyl)quinoxaline] (P1), poly[4,8-bis(2-ethylhexyloxy) benzo[1,2-b:4,5-b′]dithiophene-alt-5,5-(5′,8′-di-2-thienyl-2, 3-bis(4-octyloxyl)phenyl)quinoxaline] (P2), and poly[4,4′-bis(2- ethylhexyl)-dithieno[3,2-b:2′,3′-d]silole-alt-5,5-(5′, 8′-di-2-thienyl-2,3-bis(4-octyloxyl)phenyl)quinoxaline] (P3), were synthesized and characterized for use in polymer solar cells (PSCs). We describe the effects of the various donor segments on the optical, electrochemical, field-effect carrier mobilities, and photovoltaic characteristics of the resulting Qx-based copolymers. The results indicated that the donor units in the copolymers significantly influenced the band gap, electronic energy levels, carrier mobilities, and photovoltaic properties of the copolymers. The band gaps of the copolymers were 1.71-2.03 eV. Under optimized conditions, the Qx-based polymers showed power conversion efficiencies for the PSCs of 0.87-2.15% under AM 1.5 illumination (100 mW/cm²). Among the studied Qx-based copolymers, P2, which contained a benzo[1,2-b:4,5-b′]dithiophene unit, showed a power conversion efficiency of 2.15% with a short circuit current of 7.06 mA/cm², an open-circuit voltage of 0.67 V, and a fill factor of 0.46, under AM 1.5 illumination (100 mW/cm²).