Hole-Transport-Underlayer-Induced Crystallization Management of Two-Dimensional Perovskites for High-Performance Inverted Solar Cells

Currently, intensive research is being conducted on Ruddlesden–Popper-type two-dimensional (2D) perovskite-based solar cells because of the advantages such as excellent power conversion efficiency (PCE) and desirable environmental stability. A well-crystallized perovskite film plays a very important role in enhancing the performance of perovskite solar cells (PSCs). In this study, we investigated the effect of the wetting property of the underlying hole-transport layer (HTL) on the crystallization of 2D perovskite films. Three HTL layers with varied hydrophobicities, including hydrophilic poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), moderate-hydrophobic poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) (PTAA), and high-hydrophobic poly(3-hexylthiophene-2,5-diyl) (P3HT), were used to fabricate 2D PSCs, and the quality of the resulting perovskite films was examined. We found that moderate-hydrophobic PTAA resulted in a compact and uniform perovskite morphology with enhanced crystallinity. By contrast, the perovskite film formed on hydrophilic PEDOT:PSS showed a rough morphology with overlapping perovskite planes. This is because the hydrophobic PTAA surface could suppress the heterogeneous nucleation of perovskite and facilitate the boundary mobility of perovskite grains. Therefore, a high PCE value of 12.3% was achieved for the PTAA-based PSCs, which is higher than that of the PSCs using PEDOT:PSS (10.5%). However, the perovskite film formed on the highly hydrophobic P3HT showed a low surface coverage, resulting in an even lower PCE of 8.3%. This might be due to the higher hydrophobicity of P3HT, which leads to a low adhesion force with the precursor. The 2D PSCs using PTAA also show better long-term and thermal stability than the PEDOT:PSS-based case, with a suppressed PCE degradation from 26 to 17%. Our results indicate that using moderate-hydrophobic PTAA as the HTL is beneficial for fabricating efficient 2D PSCs with higher stability.