Anisotropic Strain-Induced Centrosymmetry Breaking in Cubic Formamidinium Lead Iodide (α-FAPbI₃) Thin Films

Organic–inorganic halide perovskites have emerged as promising materials for next-generation optoelectronic devices due to their exceptional photophysical properties. Among them, α-formamidinium lead tri-iodide (α-FAPbI₃) with a cubic symmetry (space group of (Formula presented.)) has garnered attention as a potential absorber in solar cells for its narrow bandgap and superior stability. The fundamental mechanisms governing its high performance have yet to be fully elucidated. In this study, we demonstrate that centrosymmetry breaking in [001] preferentially oriented α-FAPbI₃ thin films (POF) arises from inevitable anisotropic strain during film formation. Using circular polarization-dependent pump-probe transient absorption spectroscopy, we observe Rashba-type band splitting exclusively in POF, indicating symmetry breaking. Angle-dependent X-ray diffraction and photoluminescence (PL) reveal significant residual stress in POF compared to randomly oriented films (ROF), confirming strain-induced lattice distortion. Furthermore, time-resolved PL and microwave conductivity measurements reveal top-back inhomogeneous carrier dynamics and anisotropic charge carrier mobility, supporting the presence of the anisotropic strain-induced symmetry breaking. Our findings provide direct experimental evidence that inevitable strain in POF induces static Rashba effects, offering new insights into strain engineering for high-performance perovskite optoelectronics and potential quantum applications.