Fuse the π-Bridge to Acceptor Moiety of Donor-π-Acceptor Conjugated Polymer: Enabling an All-Round Enhancement in Photovoltaic Parameters of Nonfullerene Organic Solar Cells

The D-pi-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-pi-A polymers mostly suffered from upward highest occupied molecular orbital (HOMO) energy levels, leading to inferior open-circuit voltage (V-OC) and efficiencies in the fabricated solar cells. Herein, we explored a new approach in response to this issue via the strategy of pi-bridge fusion to A moiety. As a result, the medium band gap D-pi-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 pi-bridge fusion design witness an all-round improvement in photovoltaic parameters including the focused V-OC, short-circuit current density (J(SC)), 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 V-OC of 0.94 V, J(SC) 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%, V-OC of 0.77 V, J(SC) of 14.54 mA cm(-2), and FF of 0.66 and even the reported classical fluorinated-BTz based polymer J51 (V-OC 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.