Tin halide perovskites (THPs) have emerged as promising lead-free candidates for eco-friendly perovskite solar cells, but their photovoltaic performance still lags behind that of lead-based counterparts due to poor thin-film quality. Constructing two-dimensional/three-dimensional (2D/3D) heterostructures can effectively regulate crystallization and suppress defect formation for developing high-quality polycrystalline THP thin films.1,2 Here, we introduced colloidal chemistry engineering strategies to synchronize the nucleation kinetics of 2D and 3D crystalline phases for the growth of homogeneous 2D/3D THP heterostructures.3 Consequently, the optimised devices deliver an excellent power conversion efficiency of 17.13% (certified 16.65%) and exhibit stable operation under continuous one-sun illumination for over 1,500 h in nitrogen without encapsulation. This study offers new insights into colloidal chemistry and crystallisation regulation of mixed-dimensional heterostructures, paving the way for high-performance lead-free perovskite photovoltaics.