Lone pair localization governs ferroelectric stability and excitonic properties in lead free halide perovskites.

Lead free halide perovskites are promising materials for sustainable optoelectronic technologies, but achieving simultaneous control of optical absorption and ferroelectric stability remains challenging. Using CsGeX[Formula: see text] (X = Cl, Br, I) as a model system, we establish a unified microscopic framework that links exciton binding, dielectric screening, and spontaneous polarization to the stereochemically active Ge [Formula: see text] lone pair. First principles many body calculations combined with polarization analysis and real space charge mapping reveal that electronic asymmetry rather than lattice tetragonality primarily governs ferroelectric strength across the halide series. Partial Rb substitution acts as a form of chemical pressure that enhances polarization and stabilizes the ferroelectric state while preserving visible light absorption and avoiding mid gap states. In contrast, hydrostatic compression provides a reversible route to tune exciton binding and band gap energies. We show that lone pair localization serves as a transferable descriptor connecting charge redistribution, dielectric response, and excitonic confinement. These findings establish lone pair engineering as a general strategy for designing multifunctional lead free perovskites with coupled optical and ferroelectric functionalities.