Mixed Self-Assembled Hole-Transport Monolayer Enables Simultaneous Improvement of Efficiency and Stability of Perovskite Solar Cells

As one of the interface engineering methods for realizing high-performance perovskite solar cells (PSCs), self-assembled monolayers (SAMs) with hole-transport properties have recently been applied as an effective way to reduce energy losses at the hole-transport layer/perovskite interface, especially in PSCs with p–i–n structure. However, there are still limitations in implementing PSC with high efficiency and high stability due to the inherent weaknesses of single SAMs. Herein, it is demonstrated that a mixed self-assembled hole-transport monolayer with an appropriate mixture of [2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (MeO-2PACz) and [4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid (Me-4PACz) enables simultaneous improvement of efficiency and stability of PSCs. In the mixed SAM, MeO-2PACz maintains favorable wettability to produce high-quality films, while the deep highest occupied molecular orbital of Me-4PACz optimizes the energy level for efficient charge transfer, resulting in improved PSC performance. Encouragingly, Me-4PACz mitigates the stability issues of MeO-2PACz, producing mixed SAM-based PSCs with excellent stability. These PSCs achieve up to 20.63% efficiency and exhibit excellent thermal long-term stability, retaining 90% and 80% of their initial efficiency after approximately 1400 and 2100 h at 65 °C in an N₂ atmosphere. These findings suggest the potential of mixed SAM approaches for the realization of high-performance PSCs.