Homogeneous 2D/3D heterostructured tin halide perovskite photovoltaics.

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 THP thin films. However, the high aggregation barrier prevents large 2D perovskite colloids from forming stable clusters, making 2D THPs nucleate more slowly than their 3D analogues. Such distinct nucleation kinetics cause undesirable 2D/3D phase segregation that compromises both photovoltaic performance and device durability. Here we introduce small inorganic caesium cations to partially replace bulky organic cations in the electrical double layers of 2D THP colloids, reducing the colloid size to lower their aggregation barrier. The reduced electrostatic repulsion promotes the coagulation of 2D and 3D THP colloids in the precursor solution, synchronizing their nucleation kinetics for the growth of 2D/3D heterostructured THP thin films with a homogeneous microstructure and markedly reduced trap states. Consequently, the caesium-incorporated THP solar cells 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 the colloidal chemistry and crystallization engineering of mixed-dimensional heterostructures, paving the way for high-performance lead-free perovskite photovoltaics.