Fine-tuning central extended unit symmetry via atom-level asymmetric molecular design enables efficient binary organic solar cells

Abstract

The central unit plays a significant role in Y-type acceptor-based organic solar cells (OSCs). However, acceptors featuring an asymmetric central unit are rare, and their structural properties as well as interactions with donors remain unclear. In this work, we propose an atom-level asymmetric molecular design strategy to develop and synthesize two asymmetric acceptors, CH-Bzq and CH-Bzq-Br, alongside a control acceptor, CH-PHE, which has a symmetric structure. Theoretical calculations and experimental results demonstrate that subtle variations in the atom-level chemical structure effectively regulate molecular dipole moments, packing behavior, and active layer morphology, ultimately influencing device performance. Notably, due to favorable phase separation, improved charge carrier dynamics, and superior morphology, the PM6:CH-Bzq-Br-based binary device achieves an impressive power conversion efficiency (PCE) of 19.42%. Remarkably, when the green solvent ortho-xylene (o-xy) was used for processing, an outstanding PCE of 16.08% was achieved in a module. Our work highlights the significant potential of atom-level asymmetric molecular design for fine-tuning active layer nanomorphology, a crucial factor in the development of high performance OSCs.