Phenothiazine-based self-assembled monolayers for efficient tin perovskite solar cells with Co-cations†

Abstract

We developed three self-assembled monolayer (SAM) molecules, PTz² (1), PTz (2) and PTzBr (3), and investigated the mixing of guanidinium (GA) and methylammonium (MA) cations at the A-site, alongside formamidinium (FA), to create mixed cations during the deposition of the tin perovskite layer onto phenothiazine-based SAM-coated ITO substrates using a two-step fabrication method. This study reveals the synergy between the larger ammonium-like GA cations and FA, resulting in the structure FA_0.75GA_0.25SnI₃ that inhibits moisture diffusion into the perovskite layer to provide ideal grain passivation. Consequently, the PTzBr (3) SAM-based device showed the best performance, achieving a power conversion efficiency of 7.8% and showing negligible hysteresis effects. Additionally, the PTzBr (3) device demonstrated remarkable long-term storage stability, retaining about 80% of its initial efficiency for over 4000 h without encapsulation, and remaining stable for 9 h under one-sun illumination. The thermal, morphological, optical, electrochemical, charge recombination, and single-crystal properties of the phenothiazine-based SAMs (1–3) were also investigated to understand the superior performance of the PTzBr (3) device.