Organic-inorganic hybrid perovskites (OIHPs) have attracted great research interests as promising materials for the next generation photovoltaic energy harvesting, electro-optic detection, and all-optical conversion. Their remarkable properties are underpinned by hybrid perovskite atomic structures.  

Understanding the atomic structure and structural instability of organic-inorganic hybrid perovskites is the key to appreciate their remarkable photoelectric properties and understand failure mechanism.  

However, atomic imaging of OIHPs by electron microscopy is challenging due to the extreme beam-sensitivity. In fact, so far, the damage-free pristine structure of CH₃NH₃PbI₃ (MAPbI₃) has never been captured at the atomic scale. 

Recently, Dr. WANG Xiao’s group at the Shenzhen Institute of Advanced Technology (SIAT) of the Chinese Academy of Sciences collaborated with Dr. ZHAO Jinjin, Dr. LI Jiangyu and Dr. GAO Peng ’s team from Shijiazhuang Tiedao University, Southern University of Science and Technology and Peking University respectively, adopted low-dose imaging by direct-detection electron-counting camera, successfully imaged the atomic structure of perovskite MAPbI₃ and discovered the degradation pathway of MAPbI₃ in detail.  

Their study was published in Nature Communications. 

The researchers investigated the atomic structure via an imaging technique using a negative value of the spherical-aberration coefficient. As the dose increased, the intensity of MA⁺ was decreased with the formation of V_MA-. At a certain dose, the intensity kept constant, indicating a relatively stable intermediate phase, MA_0.5PbI₃, which was verified by further TEM analysis and molecular dynamic simulation.  

Consistent with the CL measurement, DFT calculations showed that the band gap will increases as the density of MA vacancy increases, which provided a potentially new strategy to tune the bandgap in constructing tandem solar cell and facilitated multiwave electroluminescence emission, adjusting various color luminescence under increasing bias voltage. With the discovery of the intermediate, the researcher further investigated the atomic-scale decomposition pathway of MAPbI₃. At the first stage, the V_MA- formed and the intermediate phase MA_0.5PbI₃ emerged. Consequent diffusion of Pb²⁺ and I^- into  V^-_MA and the [PbI₆]^4- octahedron slipping from corner sharing to edge sharing makes the structure gradually evolve to PbI₂.   

This work can be used to guide the future TEM characterizations, enrich the understandings of the degradation mechanism and optimization strategies, and provide atomic-scale insights into understanding its fundamental properties. 

Fig. 1 Atomic-imaging of the loss of MA⁺ and intermediate phase. (Image by SIAT)

Fig. 2 Atomic-scale imaging of the decomposition pathway. (Image by SIAT)