Isomerization of Noncovalently Conformational Lock in Nonfused Electron Acceptor toward Efficient Organic Solar Cells

The organic solar cell (OSC) is one of the most promising technologies for utilizing solar energy, and great progress has been made to achieve high-performance and low-cost OSCs through the design and synthesis of nonfused-core nonfullerene acceptors with intramolecular noncovalent interactions in the past few years. Herein, two isomeric nonfused electron acceptors, p-BCIC and o-BCIC, with different molecular shapes and symmetries are designed and synthesized, and the effect of the molecular symmetry is systematically investigated. p-BCIC is centrosymmetric with an S-shape molecular geometry, while o-BCIC is axisymmetric and possesses a C-shape molecular geometry. The molecular symmetry and shape exhibit trivial effects on the light absorption spectra, energy levels, and optical bandgaps of the two molecules but can influence the molecular packing, film morphology, and thus, device performance. Because of the diminished charge recombination, enhanced charge mobilities, and a favorable morphology, the o-BCIC-based solar cells exhibit an impressive power conversion efficiency of 11.6%, which outperforms the p-BCIC-based ones (10.4%). Our results demonstrate that the molecular packing and film morphology can be effectively tuned by regulating the symmetry of simple, nonfused electron acceptors through the isomerization of a noncovalently conformational lock.