Recent progress in hybrid perovskite solar cells through scanning tunneling microscopy and spectroscopy

Currently, sustainable renewable energy sources are urgently required to fulfill the cumulative energy needs of the world's 7.8 billion population, since the conventional coal and fossil fuels will be exhausted soon. Photovoltaic devices are a direct and efficient means to produce a huge amount of energy to meet these energy targets. In particular, hybrid-perovskite-based photovoltaic devices merit special attention not only due to their exceptional efficiency for generating appreciable energy but also their tunable band gaps and the ease of device fabrication. However, the commercialization of such devices suffers from the instability of the compositional materials. The cause of instability is the perovskite's structure and its morphology at the sub-molecular level; thereby revealing and eliminating these instabilities are a striking challenge. To address this issue, scanning tunneling microscopy/spectroscopy (STM/STS) presents a comprehensive method to allow the visualization of the morphology and electronic structure of materials at atomic-level resolution. Here, we review the recent developments of perovskite-based solar cells (PSCs), the STM/STS analysis of photoactive halide/hybrid and oxide materials, and the real-time STM/STS investigation of electronic structures with defects and traps that are believed to mainly affect device performances. The detailed STM/STS analysis can facilitate a better understanding of the properties of materials at the nanoscale. This informative study may hold great promise to advance the development of stable PSCs under atmospheric conditions.