Synergetic Alkoxy Side-Chain and Chlorine-Contained End Group Strategy toward High Performance Ultra-Narrow Bandgap Small Molecule Acceptors

Ultra-narrow bandgap (ultra-NBG) small molecule acceptors (SMAs) show great potential in organic solar cells (OSCs) due to the extended near-infrared (NIR) absorption. In this work, a synergetic alkoxy side-chain and chlorine-contained end group strategy is employed to achieve A-DA'D-A type ultra-NBG SMAs by introducing alkoxy chains with oxygen atom at the second position into the thiophene beta position as well as replacing the F atoms with Cl atoms in the end group. As a result, the heptacyclic BZO-4F shows a redshifted absorption onset (960 nm) compared with Y11 (932 nm) without oxygen atoms in the side chains. Then, the fluorinated end groups are substituted with the chlorinated ones to synthesize BZO-4Cl. The absorption onset of BZO-4Cl is further redshifted to 990 nm, corresponding to an optical ultra-NBG of 1.25 eV. When blending with the polymer donor PBDB-T, the binary devices based on PBDB-T:BZO-4F and PBDB-T:BZO-4Cl deliver power conversion efficiencies (PCEs) over 12%. Furthermore, ternary devices with the addition of BZ4F-O-1 into PBDB-T:BZO-4Cl system achieve the optimal PCE of 15.51%. This work proposes a synergetic alkoxy side-chain and chlorine-contained end group strategy to achieve A-DA'D-A type ultra-NBG SMAs, which is important for future molecular design