Molecular order manipulation with dual additives suppressing trap density in non-fullerene acceptors enables efficient bilayer organic solar cells†

Abstract

The order of molecular aggregation at the donor–acceptor interface strongly affects the charge generation and extraction properties, determining the performance of organic electronic devices. Herein, we focused on bilayer organic solar cells and selected a combination of solid- and solvent-additives, namely 1-chloronaphthalene (CN) and trans-bis(dimesitylboron)stilbene (BBS), to tune the acceptor's molecular arrangement in the bilayer active layer structure. When CN alone was added, the molecular orientation in the acceptor film changed from face-on to edge-on, and the crystallinity of the thin film significantly increased owing to the J-aggregation of the acceptor. While dual additives were used, a flocculent morphology was attained, leading to further increased crystallinity and an improved order of molecular aggregation, thus reducing the trap states in the acceptor layer. As a result, using dual additives resulted in decreased trap-assisted charge recombination and enhanced charge extraction, hence an excellent fill factor and optimum efficiency of 19.32%. The findings elucidate that morphology optimization using dual additives can strengthen the molecular arrangement order, which is a practical approach for high-performing bilayer organic solar cells.