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 2020 is already rivalling the best of Si solar cells.[1] 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.[2] 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 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. 

We also systematically investigate the impact of CsCl/MACl additive upon the morphology, grain size, crystal structure, defects, optical properties, and PV performance of FA-based PSCs. We demonstrate that the CsCl/MACl addition has a great potential to stabilize the cubic FAPbI3 with a 2×2×2 supercell expansion and the Im  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   planes. In addition, the optimized PSCs with 10 mol% CsCl exhibit the best device performance with a PCE of 21.98 %. These results prove that the incorporation of CsCl into the perovskite precursor mixture is an extremely effective method to obtain high-quality film formation, resulting in a highly efficient and stable PSC device.

1. NREL. Best Research-Cell Efficiency Chart, (accessed 3 April 2021).

2. Duong, T. et al. Adv. Energy Mater. 2017, 7 (14), 1700228.

Zoom link:

https://anu.zoom.us/j/84586588853?pwd=T2tqV01VM0RodzJOcHdVb3FxSWdKdz09
Password: 980579