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 (Voc) 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 Voc 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 Voc during TA should be mainly due to the decreased ECT accompanied with a larger ΔVnon-rad, which is further ascribed to their upshifts in the HOMO energy levels. The Voc could be partly recovered after further SVA. Our results signify the importance of the shifts in the band-gap, ECT, 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.
- This article is part of the themed collection: Celebrating Tobin Marks’ 75th Birthday