Phase Evolution for Efficient Tin Perovskite Light-Emitting Diodes.

Tin perovskites are promising lead-free alternatives for near-infrared light-emitting diodes (LEDs), but their performance is limited by defects from uncontrolled crystallization. Three-dimensional (3D)/two-dimensional (2D) heterostructuring has emerged as a promising route to mitigate this issue, yet these films undergo phase evolution during annealing, and the optimal end point remains unclear. Here we investigate this phase evolution and identify the as-spun state, stabilized by the chiral additive trans-4-hydroxy-l-proline (THP), as the optimal configuration, delivering a photoluminescence quantum efficiency of ∼50% and a peak external quantum efficiency of 15.8%. We reveal that THP promotes the vertically oriented n = 1 2D phase preferentially distributed along grain boundaries that passivate defects, while annealing drives their cascade conversion to n = 2 and n > 2 phases, progressively degrading performance. This work establishes phase-evolution control as a design principle for high-performance tin perovskite optoelectronics.