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 exceeding 20%. 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-pi-Core-pi-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 pi-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 pi-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.