Supplementary Dataset Supporting the Main Article Entitled:Lone pair localization governs ferroelectric stability and excitonic properties in lead free halide perovskites

This study investigates the microscopic coupling between excitonic properties and ferroelectric stability in lead free halide perovskites using CsGeX3 (X = Cl, Br, I) as a model system. Using first principles density functional theory combined with GW and Bethe–Salpeter equation calculations, we establish a unified framework linking exciton binding energy, dielectric screening, and spontaneous polarization to the stereochemically active Ge 4s2 lone pair. We demonstrate that electronic asymmetry driven by lone pair localization governs ferroelectric strength across the halide series. Partial Rb substitution acts as chemical pressure, enhancing polarization while preserving visible light absorption and avoiding mid gap states. Hydrostatic strain provides an additional route to tune band gaps and excitonic binding. The results identify lone pair localization as a transferable descriptor connecting charge redistribution, dielectric response, and excitonic confinement, providing design principles for multifunctional lead free optoelectronic materials.本研究以CsGeX3（X = Cl、Br、I）为模型体系，探究无铅卤化物钙钛矿中激子特性与铁电稳定性之间的微观耦合机制。借助结合了GW方法与贝特-萨尔皮特方程（Bethe–Salpeter equation）计算的第一性原理密度泛函理论，本研究构建了一套统一的理论框架，将激子结合能、介电屏蔽效应与自发极化与具有立体化学活性的Ge 4s²孤对电子关联起来。研究表明，由孤对电子局域化驱动的电子不对称性，主导了该卤化物系列的铁电极化强度。部分铷取代可充当化学压力，在保留可见光吸收能力且避免产生带隙中间态的同时，增强铁电极化强度。静水应变则为调控带隙与激子结合能提供了额外途径。本研究结果证实，孤对电子局域化可作为一种可迁移的描述符，关联电荷再分布、介电响应与激子限域效应，为多功能无铅光电材料的设计提供了指导原则。