Designing High Performance Organic Donor Molecules for Photovoltaics

The advancement of non‐fullerene acceptors has rocketed the power conversion efficiency (PCE) of organic photovoltaic (OPV) devices to values reaching close to 21%. However, the development of complementary donor materials has not kept pace, posing a key challenge for further improving device performance. In this theoretical study, we combine density functional theory (DFT) with Marcus theory to systematically design and evaluate donor molecules with ‐A architectures. Our focus lies in tuning electronic and optical properties – such as frontier molecular orbital energies, and singlet and triplet excitation characteristics – toward more efficient charge generation when coupled to non‐fullerene acceptor Y6. In small donor systems featuring fused thiophene ‐bridges, we find two‐dimensional delocalization of the highest occupied molecular orbital (HOMO) across the backbone and the core's side chain, which enhances the transition dipole moment. Furthermore, while fused ‐bridges lead to relatively stable transition rate constants across various interfacial configurations, they exhibit a limited CT state manifold, which may impede efficient charge separation following excitation of the acceptor. These findings provide molecular design insights critical for next‐generation high‐performance all‐molecule OPV devices.