Enhanced charge separation and prolonged carrier lifetime in mixed Sn–Ge halide perovskite enabled by spontaneous symmetry breaking and moderate disorder

Abstract

Constructing metal halide perovskites with intrinsic ferroelectric properties represents an effective strategy to enhance the performance of perovskite-based optoelectronic devices. For the most common ferroelectric hybrid organic–inorganic halide perovskites, the specific orientation of A-site organic cations is crucial for the formation of ferroelectricity. However, this orientation is highly susceptible to random fluctuations with temperature changes, which in turn leads to unstable ferroelectric properties. Herein, we present an innovative strategy of constructing intrinsic ferroelectric halide perovskites by alloying B-site metals in all-inorganic halide perovskites, elucidated through first-principles calculations. Our findings reveal that robust ferroelectricity can be achieved through the ordered arrangement of germanium (Ge) and tin (Sn) along the (110) direction. Specifically, ferroelectric CsSn_0.5Ge_0.5I₃ maintains a direct bandgap of 1.38 eV and exhibits enhanced thermal stability due to its reduced structural thermal fluctuations and compact inorganic sublattice. Further simulations of ferroelectric CsSn_0.5Ge_0.5I₃ reveal that the decreased structural thermal fluctuations can suppress low-frequency mode electron–phonon vibrational couplings and accelerate the decoherence process, thereby reducing non-radiative recombination channels. Meanwhile, in the ferroelectric Sn–Ge alloying system, the conduction band minimum is found to be more heavily composed of Ge atomic orbitals compared to its nonferroelectric counterparts. The enhanced contribution of Ge atomic orbitals to the conduction bands not only modifies the band structure but also significantly enhances the electron–hole separation, thereby effectively suppressing the non-radiative recombination rate. The correlations we have established between perovskite composition and properties offer valuable insights and guidance for future explorations and developments in the field of ferroelectric perovskite applications.