Optimizing Monolayers for High-Efficiency Methylammonium-Free Perovskite Solar Cells

Self-assembled monolayers (SAMs) have been widely applied to perovskite solar cells (PSCs) due to their transparency, tunability, selectivity, and diversity. While numerous studies have focused on synthesizing various new types of SAMs to enhance charge transport, there remains a need for further research focused on reducing clusters of SAMs and improving the film coverage. Achieving uniform SAMs and ensuring intimate contact with the perovskite film are processes that require deeper investigation. In this study, we employ additive engineering to introduce a micromolecule, 4-mercaptophenylacetic acid (4MA), into [4-(3,6-diphenyl-9H-carbazol-9-yl) butyl] phosphonic acid (Ph-4PACz). The incorporation of 4MA can not only prevent the aggregation of Ph-4PACz through a competitive mechanism, leading to improved film coverage and uniformity, but also passivate the perovskite layer and reduce charge recombination in PSCs. As a result, more homogeneous and higher-quality perovskite films are achieved on both the surface and the bottom of the perovskite layer. Moreover, 4MA also optimizes the alignment of the energy band and enhances the carrier transfer. These combined effects contribute to a high power conversion efficiency of 23.8% in methylammonium-free PSCs. Our study demonstrates the potential of additive engineering in SAMs and presents a method for producing uniform SAMs, which is highly advantageous for industrial applications.