Correlation of Local Isomerization Induced Lateral and Terminal Torsions with Performance and Stability of Organic Photovoltaics

Organic photovoltaics (OPVs) have achieved great progress in recent years due to delicately designed non-fullerene acceptors (NFAs). Compared with tailoring of the aromatic heterocycles on the NFA backbone, the incorporation of conjugated side-groups is a cost-effective way to improve the photoelectrical properties of NFAs. However, the modifications of side-groups also need to consider their effects on device stability since the molecular planarity changes induced by side-groups are related to the NFA aggregation and the evolution of the blend morphology under stresses. Herein, a new class of NFAs with localisomerized conjugated side-groups are developed and the impact of local isomerization on their geometries and device performance/stability are systematically investigated. The device based on one of the isomers with balanced side- and terminal-group torsion angles can deliver an impressive power conversion efficiency (PCE) of 18.5%, with a low energy loss (0.528 V) and an excellent photo- and thermal stability. A similar approach can also be applied to another polymer donor to achieve an even higher PCE of 18.8%, which is among the highest efficiencies obtained for binary OPVs. This work demonstrates the effectiveness of applying local isomerization to fine-tune the side-group steric effect and non-covalent interactions between side-group and backbone, therefore improving both photovoltaic performance and stability of fused ring NFA-based OPVs.
Funding Agencies|APRC Grant of the City University of Hong Kong; Innovation and Technology Commission of Hong Kong; Environment and Ecology Bureau of Hong Kong; Research Grants Council of Hong Kong [9380086, 9610508]; US Office of Naval Research [GHP/018/20SZ, MRP/040/21X]; Shen zhen Science and Technology Program [202020164]; Guangdong Major Project of Basic and Applied Basic Research [11307621, 11316422, N00014-20-1-2191]; Guangdong-Hong Kong-Macao Joint Laboratory of Optoelectronic and Magnetic Functional Materials [C6023-19GF]; Office of Science, Office of Basic Energy Sciences, of the US Department of Energy [SGDX20201103095412040]; [2019B030302007]; [2019B121205002]; [DE-AC02-05CH11231]