The post-treatment effects on open circuit voltages and device performances in a high efficiency all-small-molecule organic solar cell

Post-treatment is a widely used approache to improve the device performance of all-small-molecule organic solar cells (ASM-OSCs), which leads to a balanced miscibility and crystallinity of the active layers. However, compared to as-cast devices, their efficiency is notably improved but normally accompanied with big open circuit voltage (V-oc) loss after thermal annealing (TA) or solvent vapor annealing (SVA), which limits further breakthrough of the device performance and lacks investigation. In this manuscript, we design a novel molecule BSCl with a deep HOMO energy level and a good molecular assembling ability. Using IDIC-4Cl as an acceptor and combining TA and SVA, a PCE of 13.03% was achieved. The device improvements, film formation and V-oc variations during the post-treatment methods were deeply investigated via Grazing incidence wide angle X-ray scattering and energy loss characterization. The characterization results illustrated that the decreased V-oc during TA should be mainly due to the decreased E-CT accompanied with a larger Delta Vnon-rad, which is further ascribed to their upshifts in the HOMO energy levels. The V-oc could be partly recovered after further SVA. Our results signify the importance of the shifts in the band-gap, E-CT, and ultimately the energy levels during the post-treatment methods, providing useful guidance on high efficiency molecule optimization and a deep understanding on the energy loss investigation and film formation via device optimization.