The Role of Grain Boundaries in Organic–Inorganic Hybrid Perovskite Solar Cells and its Current Enhancement Strategies: A Review

Grain boundaries (GBs) in perovskite polycrystalline films are the most sensitive place for the formation of the defect states and the accumulation of impurities. Thus, abundant works have been carried out to explore their properties and then try to solve the induced problems. Currently, two important issues remain. First, the role of GBs in charge carrier dynamics is unclear due to their component complexity/defect tolerance nature and the insufficiency in testing accuracy. Some works conclude that GBs are benign, while others consider GBs as carrier recombination centers. Things for sure are the deterioration in ion transport and perovskite decomposition. Second, to solve the known hazards of GBs, a lot of additives have been added to anchoring ions and passivate defects. But in most of those works, GBs and perovskite surfaces are treated in the same manner ignoring the fact that GB is essentially a homogeneous junction in a narrow and slender space, while surface is a heterogeneous junction with a stratified structure. In this review, we focus on works insight into GBs and additives for them. Additionally, we also discuss the prospects of the maturity of GB exploration toward upscaling the manufacture of perovskite photovoltaic and related optoelectronic devices.