Ultra-narrow bandgap non-fullerene acceptors for organic solar cells with low energy loss

Abstract

Two ultra-narrow bandgap non-fullerene acceptors (NFAs), 4TO-T-4F and 4TO-Se-4F, were designed and synthesized for application in organic solar cells (OSCs). Thiophene and selenophene were used as π-spacers to extend the effective conjugated length via intramolecular noncovalent O⋯S or O⋯Se interactions and lower the bandgap. Both NFAs, 4TO-T-4F and 4TO-Se-4F, showed strong absorbance in the range of 700–1000 nm in the near-infrared region with an ultra-narrow bandgap of ∼1.30 and ∼1.27 eV, respectively. Using polymer PTB7-Th as the donor, which has complementary absorption and matched energy levels with these NFAs, the optimized device based on PTB7-Th:4TO-T-4F exhibited a PCE of 8.87%, with a low energy loss of 0.55 eV; and the PTB7-Th:4TO-Se-4F-based OSC device gave a PCE of 7.4%, with a low energy loss of 0.57 eV. Our results indicate that increasing the effective conjugate length of the molecular backbone, thus improving the electron-donating ability of the donor unit in NFAs via noncovalent O⋯S or O⋯Se interactions, is an efficient strategy to design and synthesize ultra-narrow bandgap NFAs for OSCs with low energy loss.