Small Energy Loss and Broad Energy Levels Offsets Lead to Efficient Ternary Polymer Solar Cells from a Blend of Two Fullerene-Free Small Molecules as Electron Acceptors

Ternary polymer solar cells (PSCs) are fabricated that consisted of a blend of IEICO-4F and 3TT-FIC as electron acceptors and PBDB-T as an electron donor. The power conversion efficiency (PCE) of ternary PSCs is increased from 9.9% to 11.22% via the blend of IEICO-4F and 3TT-FIC as electron acceptors (the ratio of IEICO-4F:3TT-FIC is 6:4), which enhances the open-circuit voltage (V-OC) from 0.71 to 0.78 V. The main contribution of the blended acceptor is its broad energy level offset between the donor and acceptor (from 1.26 to 1.30 eV) due to the enhanced lowest unoccupied molecular orbital energy levels of the blended acceptor. Meanwhile, the small energy losses of the ternary photoactive layer (reduced from 0.55 to 0.52 eV) are simultaneously due to the more efficient exciton dissociation and the suppressed charge carrier recombination. This work provides an effective strategy for improving the photovoltaic performance of binary PSCs.