Vacuum-Assisted Deposition of Highly Hydrophobic Self-Assembled Monolayer for High-Efficiency Perovskite Solar Cells

Recently, various carbazole-based self-assembled monolayers (SAMs) have been investigated for use in the hole transport layer (HTL) of perovskite solar cells (PSCs). In particular, [4-(3,6-dimethyl-9H-carbazol-9-yl)butyl] phosphonic acid (Me-4PACz) is attracting attention as an HTL for high-efficiency PSC due to its potential for high open-circuit voltage (V_OC) and fill factor (FF). However, Me-4PACz has strong hydrophobicity due to its methyl group (–CH₃) and long alkyl chain (C4H₈), which makes it difficult to deposit a high-quality perovskite layer on top of conventionally coated Me-4PACz. In this study, to overcome these limitations, a new SAM deposition method based on a vacuum-assisted deposition (VD) process is attempted. As a result, a uniform perovskite layer is deposited on top of Me-4PACz through a very simple VD process, achieving 20.31% efficiency, which is not only close to the highest efficiency among Me-4PACz-based PSCs, but also improved long-term stability of the device. It is believed that the findings of this study can be a cornerstone for using SAM-based HTL with high hydrophobicity in the future.