Organic Photovoltaic Cells Based on Nonhalogenated Polymer Donors and Nonhalogenated A-DA′D-A-Type Nonfullerene Acceptors with High VOC and Low Nonradiative Voltage Loss

Compared with other all-inorganic/organic-inorganic hybrid solar cells, the large voltage loss (V_loss) of organic photovoltaic (OPV) cells, especially the nonradiative voltage loss (ΔV_nonrad), limited the further improvement of performance. Although A-DA'D-A-type Y-series nonfullerene acceptors (NFAs) largely improve the power conversion efficiencies (PCEs) to 18%, the open-circuit voltage (V_OC) of this kind of material was still restricted to below 1.0 V. Herein, we designed and synthesized a narrow bandgap (E_g = 1.41 eV) acceptor BTA77 with an A-DA'D-A-type backbone containing a nonhalogenated terminal group to achieve high electroluminescence efficiency and high V_OC. Combined with the nonhalogenated polymer PBDB-T with a conjugated thiophene side chain, BTA77 realized a V_OC of 0.944 V, a V_loss of 0.552 V, and a PCE of 13.75%, which is one of the highest PCEs based on nonhalogenated A-DA'D-A-type acceptors with V_OC > 0.9 V. After further blending with the nonhalogenated donor polymer PBT1-C with a conjugated phenyl side chain, the V_OC increases to 1.021 V with a super low ΔV_nonrad of 0.14 V owing to the greatly improved electroluminescence external quantum efficiency (EQE_EL) of 4.42 × 10^-3. Our results indicate that there is still a large room to decrease the ΔV_nonrad and increase V_OC by synergistic molecular engineering of p-type polymers and n-type small molecules.