Revealing the Universal Pressure‐Driven Behavior of Hybrid Halide Perovskites and Unique Optical Modifiability in Extremely Soft 2D Tin‐Based System

Abstract As promising photovoltaic materials, halide perovskites display large structural modifiability specific to their organic‐inorganic hybrid lattices, thus providing key to unlock many enhanced and novel physical properties. In particular, hybrid perovskites exhibit extraordinary functional responses to mechanical stimulation due to their soft lattices. However, a general pattern describing the evolution of perovskite properties under pressure is missing, rendering such research without theoretical guidance and further the optimization of optoelectronic performance. Here, a framework delineating the pressure‐dependent evolutions of lattice structure, bandgap, and photoluminescence (PL) across four distinct regions in all perovskites assuming 3D and 2D structures is unveiled, accrediting long‐range disorderness to be the common origin of bandgap blueshift, PL annihilation, and structural amorphization. Using such developed model as an instructional guideline, an optical bandgap and luminescent evolutions in quasi‐2D tin(II) iodide perovskites are revealed, where (C 4 H 9 NH 3 ) 2 (CH 3 NH 3 )Sn 2 I 7 (C 4 H 9 NH 3 + : butylammonium; CH 3 NH 3 + : methylammonium) is found to be the softest perovskite (bulk modulus ≈4.8 GPa) known so far. By meticulously choosing appropriate peak pressure ≈4 GPa, (C 4 H 9 NH 3 ) 2 (CH 3 NH 3 )Sn 2 I 7 shows irreversible defect healing (carrier lifetime prolongation from 7 to 22 ns) and permanent PL enhancement upon decompression to ambient condition, signifying practicality of the pressure‐driven behaviors unveiled in this work.