Enriching bottom acceptors for high-performance inkjet-printed organic solar cells

The formation of gradient vertical phase separation structure through layer-by-layer (LbL) deposition is favorable for enhancing carrier transport and suppressing carrier recombination in the organic solar cells (OSCs). However, it is difficult to control the enrichment of the donor or acceptor at the interface, especially in the inverted OSCs. Furthermore, the underlying dynamic mechanism of donor and acceptor enrichment during the LbL process, as well as its impact on the carrier transport and recombination in OSCs remains unclear. In this work, we demonstrated the vertical phase separation formation in the inverted OSCs during LbL printing follows a dissolution and re-solidification paradigm, with re-solidification being the predominant step. By regulating the printing temperature of the donor layer, vertical phase separation structures with different enrichment of bottom acceptors were successfully fabricated. Based on these structures, the relationship between the enrichment thickness of bottom acceptors and the carrier transport and recombination dynamics was investigated. It demonstrated that the increase in the deposition temperature led to the bottom acceptor enrichment thickness increased approx. 10 nm. Consequently, the bimolecular recombination coefficient was reduced from 1.54 × 10^-12 to 4.54 × 10^-13 cm³ s⁻¹, and the charge transfer was enhanced, leading to the improvement of fill factor (FF) from 66.33% to 72.14% and power conversion efficiency (PCE) from 12.63% to 14.28%. This study demonstrated controllable enriching of the bottom acceptor through temperature control of the upper donor layer by LbL printing is an effective strategy to suppress the bimolecular recombination in the OSCs.