Implications of Crystallization Temperatures of Organic Small Molecules in Optimizing Nonfullerene Solar Cell Performance

Crystallization behaviors of both small-molecule donors and acceptors are critical yet complicated factors that determine the microstructure of all small-molecule organic solar cells. To achieve desired photovoltaic performance, it is of vital importance to elucidate the role of key crystallization parameters of each component in optimizing the morphology of blend films. To this end, four binary blends based on crystalline donors with various crystallization temperatures (namely DRTT-R, DRTT-T, DRTT-2T, and DRTT-TT) and a weakly crystalline acceptor N3 were selected. We determined the crystallization onset temperature (T-c,T-onset) and aggregation transition temperature of the small molecules in neat/blend films by temperature-dependent X-ray diffraction and UV-vis measurements. Based on the detailed analysis of molecular crystallization, film morphology evolution, and device performance, a strong correlation between the T-c,T- onset of photoactive materials and optimal thermal annealing (TA) conditions for device performance was established across these systems. Guided by this finding, a two-step TA approach was used to realize DRTT-T:N3 blend films with highly ordered molecular packing and appropriate phase separation morphology, thereby yielding a high power conversion efficiency of 13.21%. Our study demonstrates that using simple crystallization parameters, that is, T-c,T-onset, can rationalize the annealing protocols, which is instructive for promoting the performance of all small-molecule electronic blends.