Halide perovskite semiconductors with extraordinary optoelectronic properties have been fascinatedly studied. Halide perovskite nanocrystals, single crystals, and thin films have been prepared for various fields, such as light emission, light detection, and light harvesting. High‐performance devices rely on high crystal quality determined by the nucleation and crystal growth process. Here, the fundamental understanding of the crystallization process driven by supersaturation of the solution is discussed and the methods for halide perovskite crystals are summarized. Supersaturation determines the proportion and the average Gibbs free energy changes for surface and volume molecular units involved in the spontaneous aggregation, which could be stable in the solution and induce homogeneous nucleation only when the solution exceeds a required minimum critical concentration (Cmin). Crystal growth and heterogeneous nucleation are thermodynamically easier than homogeneous nucleation due to the existent surfaces. Nanocrystals are mainly prepared via the nucleation‐dominated process by rapidly increasing the concentration over Cmin, single crystals are mainly prepared via the growth‐dominated process by keeping the concentration between solubility and Cmin, while thin films are mainly prepared by compromising the nucleation and growth processes to ensure compactness and grain sizes. Typical strategies for preparing these three forms of halide perovskites are also reviewed.

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Stable electron transport materials (ETMs) with fewer surface defects and proper energy level alignments with halide perovskite active layers are required for efficient perovskite solar cells (PSCs) with long-term durability. Here, two-dimensional van der Waals mixed valence tin oxides Sn 2 O 3 and Sn 3 O 4 are controllably synthesized and applied as ETMs for planar PSCs. The synthesized Sn 2 O 3 and Sn 3 O 4 have size of 5−20 nm and disperse well in water as stable colloids for months. Both Sn 2 O 3 and Sn 3 O 4 exhibit typical n-type semiconductor energy band structures, low trap density, and suitable energy level alignments with halide perovskites. Steady-state power conversion efficiencies (PCEs) of 22.36% and 21.83% are obtained for Sn 2 O 3 -based and Sn 3 O 4 -based planar PSCs. In addition, the half cells without hole transport materials and back electrodes show good UV-stability with average PCE of 99.0% and 95.7% for Sn 2 O 3 -based and Sn 3 O 4 -based devices remaining after 1000 h of ultraviolet soaking with an intensity of 70 mW cm −2 .