Low-Temperature Fabrication of Phase-Pure α-FAPbI₃ Films by Cation Exchange from Two-Dimensional Perovskites for Solar Cell Applications

Abstract

α-FAPbI3-based perovskite solar cells have recently attracted increasing attention as a result of an ideal bandgap and longer exciton lifetime of FAPbI3 compared to perovskites with other compositions. However, in a traditional fabrication method, the α-FAPbI3 films were usually obtained by a direct phase transition from δ to α phase at a high annealing temperature, leading to low quality with poor crystallinity and numerous defects. The formation and stabilization of phase-pure, material-pure, high-quality α-FAPbI3 films remain challenging. In this work, a FA vapor-assisted cation-exchange pathway from low-dimensional perovskites to three-dimensional α-FAPbI3 was built, through which phase-pure and material-pure α-FAPbI3 films were achieved at 100 °C below the temperature of thermodynamic δ-to-α phase transition (∼150 °C). Through an in-depth study, the cation-exchange pathway was found to have a low reaction barrier directly toward α-FAPbI3 and suppress the formation of δ-FAPbI3, leading to high-quality α-FAPbI3 with high orientation and few trap states at a low annealing temperature. Consequently, small-area devices and large-area modules with as-prepared α-FAPbI3 films were achieved with improved performance, showing great potential for further study and application.

Low‐Temperature‐Processed Stable Perovskite Solar Cells and Modules: A Comprehensive Review

“…reported a FA vapor‐assisted cation‐exchange pathway to achieve the phase transition from δ‐to‐α phase at 100 °C. [ 162 ] Notably, Hui et al. stabilized the α‐FAPbI 3 based on a vertically aligned lead iodide thin film grown from an IL, methylamine formate.…”

“…[161] Similarly, Cheng et al reported a FA vapor-assisted cation-exchange pathway to achieve the phase transition from δ-to-α phase at 100 °C. [162] Notably, Hui et al stabilized the α-FAPbI 3 based on a vertically aligned lead iodide thin film grown from an IL, methylamine formate. [163] The realized solar cell demonstrated a PCE of 24.1% and superior stability retaining 80% and 90% of their original efficiencies for of 500 h at 85 °C and constant light stress, respectively.…”

A review on theoretical studies of structural and optoelectronic properties ofFA‐based perovskite materials with a focus onFAPbI3

“…These low‐dimensional perovskites not only provide environment stability but also are useful for phase stability of FA‐based perovskites. For instance, Cheng et al reported that the high annealing temperature needed for δ‐to‐α transformation leads to low quality with poor crystallinity and numerous defects in FAPbI 3 perovskite materials 135 . Using DFT calculation, they showed that the transition from the δ phase to α‐phase is an endergonic process and goes through the distortion of the crystal lattices, which needs energy input to reconstruct the octahedron.…”

2021 Pioneers in Energy Research: Vivian Yam

“…Wong and co-workers report the realization of using the inductive effect stemming from the electron-withdrawing characters of the F substituent and heteroarene to modulate the physical properties of a parent donor molecule DTCPB for vacuum-processed organic photovoltaics. Zheng and co-workers present a new low-temperature method for fabricating phase-pure and material-pure α-FAPbI 3 films from two-dimensional perovskites for solar cell applications. Li and co-workers report a type of ternary all-polymer solar cell that uses low-cost polymer donor PTQ 10 as the third component (the second polymer donor) in the PBDB-T:PTPBT-ET 0.1 system, achieving a power conversion efficiency (PCE) of over 14.5%.…”