Regulation of film morphology and vertical phase separation for inverted organic solar cells via a volatile solvent additive

Abstract

The additives play a vital role in both photovoltaic performance and device stability of organic solar cells (OSCs). Although solvent additives have been extensively utilized in conventional OSC structures to adjust film morphology and regulate crystallization behavior of photoactive materials, their attempt and specific roles in inverted OSCs remain rarely explored. Herein, a kind of halogen-free and volatile solvent additive methyl benzoate (MB) is selected for optimization of the inverted OSCs. It is found that MB could produce distinct positive interaction with L8-BO acceptors, leading to enhanced molecular crystallization and appropriate microstructure in PM6:L8-BO active layers. In addition, the MB-processed PM6:L8-BO films exhibit an optimized vertical phase distribution driven by differences of miscibility between components. The improved horizontal and vertical morphology facilitates charge transport and suppresses charge recombination in the resultant inverted OSCs. Consequently, the power conversion efficiency (PCE) increases from 15.88 % to 17.08 %. Furthermore, the volatile MB with a low boiling point and high vapor pressure could prevent residual in active layers and avoid thermal degradation of OSCs. Benefiting from synergistic effects of the positive volatility and improved acceptor crystallinity, the MB-processed devices demonstrate enhanced thermal stability compared to the control devices. This work highlights the potential of volatile solvent additives for fabricating efficient and stable inverted OSC devices.