Structural and Chemical Characterization of Multi-cation Mixed-halide Perovskite

Abstract

Perovskite solar cells (PSCs) have demonstrated a breakthrough in power conversion efficiency (PCE), making them the fastest developing solar cell technology in history. The highest efficiency of PSCs up to 25.5% reported in 2021 is already rivalling the best of Si solar cells. However, the commercialization of PSCs faces two major challenges: the instability of the perovskite layer and the toxicity of lead. Furthermore, the exact mechanism behind their tremendous increase in efficiency and outstanding optical properties remains unclear. Especially, the atomic structure and microstructure of hybrid metal halide perovskite layers are also not well understood yet. Recently, several inorganic cations (Rb, Cs) have been incorporated into the mixed-halide perovskite materials to improve the stability of PSCs. Nonetheless, the underlying mechanism of how those cations affect the microstructure and stability of perovskite materials are still unclear. In this study, we use low-dose electron microscopy and atomic structure simulations to identify the microstructure, crystal structure and defects of the multi-cation mixed-halide perovskite. We demonstrate that Cs+ cations were uniformly incorporated throughout the perovskite layer, while Rb+ cations are segregated as a discrete Rb-rich phase at the grain boundary. Our electron diffraction studies show the coexistence of cubic and tetragonal structure in the perovskite film at room temperature. We use a combination of low-dose electron microscopy studies and atomic structure simulations to identify a common twinning structure in both cubic and tetragonal phases of the multi-cation mixed-halide perovskite. We also present clear evidence of {111} twins in the cubic structure which are equivalent to {011} twins in the tetragonal structure. Then, a unique way for differentiating between the tetragonal and cubic forms of these twins is developed. Our systematic electron diffraction analyses and simulations demonstrate unequivocally that the same twinning structure is present in both phases, notably with a perfectly coherent {111} twin boundary in the cubic phase and a semi-coherent {011} twin boundary in the tetragonal phase. We propose that the twin boundary is a key nucleation region for phase segregation which acts as a potential well or barrier to electrons and holes depending on its composition and hence bandgap. Moreover, we systematically investigate the impact of MACl/CsCl additive upon the morphology, grain size, crystal structure, defects, optical properties, and PV performance of FA-based PSCs. We demonstrate that the MACl/CsCl addition has a great potential to stabilize the cubic FAPbI3 with a 2x2x2 supercell expansion and the Im3 space group. The morphology and crystal structure of the nanotwins (NTs) and stacking faults (SFs) of the perovskite films are studied. A detailed analysis employing electron diffraction patterns shows that both the NTs and SFs are on the cubic {111} planes. In addition, the optimized PSCs with 10 mol% CsCl exhibit the best device performance with a PCE of 21.98%. Finally, we investigate the key role of halide anions in additives using CsX (X = I, Br, and Cl) on the microstructure, crystal structure, structural defects, optoelectronic properties and photovoltaic parameters of FAPbI3 PSCs. Among the additives tested, Cs doped FAPbI3 perovskite film with Cl anion showed the highest PL intensity, longest PL lifetime and highest PCE compared to these film with I and Br anions. The morphology and crystallography of {111} NTs and SFs are studied using TEM and SAED. The microstructure, crystallography and atomic structure model of two twin boundaries intersecting with an angle of around 70.5 degrees are presented. We also systematically investigate the degradation pathways and their underlying mechanism for Cs doped FAPbI3 perovskite under ambient conditions.