Perovskite solar cells (PSCs) have demonstrated a breakthrough in power conversion efficiency, making them the fastest developing solar cell technology in history. The highest efficiency of PSCs up to 25.2% reported in 2020 is already rivalling the best of Si solar cells.[1] However, one of the obstacles commonly seen in PSCs is the poor stability under ambient conditions such as light, moisture, air, etc. To improve the stability of PSCs, several inorganic cations (such as Pb, Cs) have been incorporated into the mixed halide perovskite materials.[2, 3] Nonetheless, the underlying mechanism of how those cations affect the morphology, structure, composition is unknown, and factors affecting the stability of perovskite materials are still unclear.

In this study, the chemical composition was determined combining both energy dispersive X-ray spectrometry analyses in scanning transmission electron microscopy mode and focused ion beam with time of flight secondary ion mass spectrometry. Moreover, we use a combination of low-dose transmission electron microscopy techniques 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 visibility of forbidden reflections in the electron diffraction patterns. Furthermore, the coexistence of cubic and tetragonal structure in the perovskite film at room temperature is also revealed. We also present clear evidence of {111} twins in the cubic structure and the equivalent {011} twins in the tetragonal structure using selected area electron diffraction patterns. Finally, we develop a unique way for differentiating between the tetragonal and cubic forms of these twins. The insights presented in this study are highly beneficial for the research and development of the multi-cation mixed-halide perovskite materials.

1. NREL. Best Research-Cell Efficiency Chart, (accessed 4 Jun 2020).

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

3. Saliba, M. et al. Science 2016, 354 (6309), 206-209.