SUMMARY
The nucleation site plays a critical role in achieving the full coverage of perovskite film at both the macroscopic and microscopic scales, and it is systematically investigated for the first time in this study. The results show that under natural conditions, the incomplete coverage of perovskite film is due to both heterogeneous nucleation and homogeneous nucleation. The established concentration field and temperature field in the precursor solution show that there are two preferential nucleation sites, i.e., the upper surface of the precursor solution (homogeneous nucleation) and the surface of the substrate (heterogeneous nucleation). The nucleation sites are tuned by decreasing the drying pressure from the atmosphere to 3000 Pa, and then to 100 Pa, and then the microstructures of the perovskite films change from an incomplete coverage state to a monolayer full coverage state, and then to a bilayer full coverage state. At last, when the full coverage perovskite films are assembled into perovskite solar cells, the photovoltaic performance of the monolayer perovskite solar cells is slightly greater than that of the bilayer perovskite solar cells. The electrochemical characterization shows that there is more restrained internal recombination of the monolayer perovskite solar cells compared with bilayer perovskite solar cells.