Photovoltaic performances of two alternating polymers having meta-octyloxy-phenyl modified dithieno[3,2-f:2′,3′-h]quinoxaline unit

Two donor-acceptor (D-A)-type alternative conjugated polymers, namely Z2 and Z3, based on meta-octyloxy-phenyl modified dithieno[3,2-f:2′,3′-h]quinoxaline segment as the electron-deficient unit, thiophene and 2,3-dioctylthienyl substituted benzo[1,2-b:4,5-b']dithiophene as the electron-giving segment were synthesized, which were used as donors to fabricate polymer solar cells (PSCs) with a small molecule electron-acceptor material i-IEICO-4F. With wide-bandgaps of 1.95 eV for Z2 and 2.02 eV for Z3, both of the two polymers provide complementary absorption with i-IEICO-4F. Moreover, Z3 shows deepened molecular frontier orbitals compared to Z2. Polymer solar cells based on Z2 and i-IEICO-4F acquired a moderate efficiency of 6.46%. While the device of Z3:i-IEICO-4F possessed exhibited better charge acquisition and generation properties, suppressed charge recombination probability, higher and more balanced hole and electron mobilities, and superior molecular orientation and crystallinity, thus achieving simultaneously improved short-circuit current density (J_SC, 16.76 mA cm⁻²), V_OC (0.936 V), fill factor (FF, 53.93%), and the resultant high PCE (8.46%). This work is of great significance for obtaining highly efficient PSCs with alkyoxy-phenyl modified dithieno[3,2-f:2′,3′-h]quinoxaline unit as the electron-attractive fused-ring core in wide bandgap polymers.