The D-π-A conjugated polymers with a benzotriazole (BTz) unit as the A moiety have been intensively investigated as donor materials in nonfullerene solar cells. However, these BTz even the fluorinated-BTz constructed D-π-A polymers mostly suffered from upward highest occupied molecular orbital (HOMO) energy levels, leading to inferior open-circuit voltage (VOC) and efficiencies in the fabricated solar cells. Herein, we explored a new approach in response to this issue via the strategy of π-bridge fusion to A moiety. As a result, the medium band gap D-π-A polymer PY2 was evolved into wide band gap D-A polymer PY1 with fused-DTBTz as the new A moiety, accompanied with a greatly declined HOMO energy level by 0.26 eV, a remarkable blue-shifted absorption onset by about 51 nm, and concurrently moderately enhanced face-on stacking orientations in neat polymer and donor/acceptor blend films. The synergetic optimizations in energy level, absorption characteristic and molecular stacking feature via the π-bridge fusion design witness an all-round improvement in photovoltaic parameters including the focused VOC, short-circuit current density (JSC), and fill factor (FF), with narrow band gap ITIC as the acceptor material. Specifically, the PY1-based solar cells produce an optimal power conversion efficiency (PCE) of 12.49%, with superior VOC of 0.94 V, JSC of 18.46 mA cm-2, and FF of 0.72, significantly surpassing those of PY2-based optimal device with a PCE of 7.39%, VOC of 0.77 V, JSC of 14.54 mA cm-2, and FF of 0.66 and even the reported classical fluorinated-BTz based polymer J51 (VOC of 0.82 V, PCE of 9.26%). Promisingly, there is a huge room for improvement in photovoltaic properties with rational fluorination or chlorination of the fused-DTBTz unit or the D moiety of the D-A polymers.
Keywords: conjugated polymers; donor−π−acceptor; organic solar cells; photovoltaic properties; π-bridge fusion.