Avoid Pitfalls in Identifying Perovskite Grain Size.

Perovskite grain size has been used as one of the indicators to evaluate the morphological quality of perovskite films. Large grain size is customarily regarded as an indicator of high photovoltaic performance because it is thought to result in superior charge carrier transport properties due to less carrier scattering by the grain boundaries (GBs). Consequently, the characterization of perovskite grain size has become routine in perovskite solar cell research, and large grain size is in general pursued. However, grain size estimation relying on the prevailing methods, such as scanning electron microscopy (SEM), can be viewed only as "apparent grain size", which is incomplete or possibly wrong for evaluating GB distribution. To avoid the pitfalls and advocate for accurate methodologies in identifying perovskite grain size, this Viewpoint highlights the pitfalls by demonstrating specific examples and then provides an appropriate platform to accurately evaluate perovskite grain information.