Incorporating a vertical BDT unit in conjugated polymers for drastically improving the open-circuit voltage of polymer solar cells

Abstract

In order to search for new wide band gap materials, vertical benzodithiophene (BDT) is designed as an electron donating unit to construct D–A type conjugated polymers. Two polymers PVB1 and PVB2 were synthesized by the Stille coupling reaction with benzothiadiazole (BT) and thieno[3,4-c]pyrrole-4,6-dione (TPD) as the accepting moieties, respectively. These polymers show wide optical band gaps of over 2.0 eV and low HOMO energy levels of below −5.5 eV. Polymer solar cell (PSC) devices were fabricated using the above polymers as donors and fullerene derivatives as acceptors. PSCs based on PVB1 and PC₇₁BM showed a power conversion efficiency (PCE) of 2.84% with a V_oc of 1.00 V, a J_sc of 6.80 mA cm⁻² and a FF of 0.42. And PSCs based on PVB2 and PC₇₁BM showed a PCE of 2.05% with a V_oc of 1.09 V, a J_sc of 5.33 mA cm⁻² and a FF of 0.35. Both polymers showed a high V_oc of over 1.0 V, which should be attributed to the deep-lying HOMO levels of the polymers. They would be potential candidates as wide band gap components to construct tandem solar cells.