Navigating the Growth‐Strain Trade‐Off in 2D Perovskite Solar Cells via Surface Self‐Templated Dual‐Anion Synergy

Two‐dimensional (2D) perovskites are promising candidates for solar cells. However, the processing strategies that promote large, oriented grains often concurrently induce tensile strain and microcracks, creating a critical growth‐strain trade‐off. Herein, we propose a surface self‐templated dual‐anion synergy by partially substituting iodide with low amounts of thiocyanate and chloride (2 mol%) in 2D (AA) 2 (MA) 3 Pb 4 I 13 perovskites ( n = 4), in which SCN anion spontaneously enriches at the top surface as a structure‐directing template that drives oriented grain growth but inevitably introduces tensile strain, and Cl anion concurrently segregates to the top surface as a strain buffer that alleviates this strain without sacrificing grain size. Systematic in situ and depth‐resolved analysis reveals that such synergy effectively navigates the growth‐strain trade‐off of 2D perovskites, yielding enlarged grains, strong vertical orientation, and a crack‐free morphology, which contributes to the suppression of non‐radiative recombination and the enhancement of charge transport in solar cells. As a result, the champion device with dual‐anion engineering achieves a remarkable efficiency of 19.11%, and retains over 95% of its initial efficiency after 1800 h under one‐sun irradiation. This work establishes a fundamental understanding of the growth‐strain dilemma in 2D perovskites, providing a universal design principle for high‐efficiency 2D perovskite solar cells.