Challenges in the development of metal-halide perovskite single crystal solar cells

Metal halide perovskites (MHPs), as a new generation optoelectronic materials, have attracted a surge of attention due to their remarkable intrinsic properties. Despite tremendous advances in polycrystalline thin-films (TFs), the presence of grain boundaries serving as recombination centers severely affects the performance and stability of the devices. MHP single crystals (SCs) containing less grains or no grains are anticipated as promising candidates for optoelectronic applications by virtue of their longer carrier mobility and lifetime, low defect density, long diffusion lengths, and wide optical-absorption range that are expected to deliver superior optoelectronic performance. Whilst MHP SCs can surpass the limits of their TF counterparts, the exploration of MHP SC solar cells lags far behind. In this review, the challenges of the MHP SC solar cell development are considered to set some guidelines in this emerging research direction. In particular, the scientific and technical barriers are systematically discussed in terms of their harmful impacts on the material's functionality as the active layer for solar cell application. The correlation between these potential limiting factors and the device performance is fundamentally discussed. In addition, by analyzing the strategies thus far in mitigating these obstacles, some innovative insights are proposed to further improve the performance and practical applications of MHP SC solar cells.