Strain-engineered Si-doped Cs₃Bi₂I₉ perovskite for high-performance MIM capacitors: a DFT study

This study examines the energy storage potential of strain engineered Si doped Cs₃Bi₂I₉ perovskites using density functional theory. Key electronic and electromechanical parameters-band gap, born effective charge, polarization, piezoelectricity, and leakage suppression-were evaluated for intrinsic and strained systems. Undoped Cs₃Bi₂I₉ exhibits a wide 3.3 eV band gap and low polarization, making it suitable as a stable insulating material. Introducing 0.25 mol% Si narrows the band gap and introduces beneficial defects that enhance the dielectric constant and capacitance, with BEC analysis revealing strong local polarization around Si atoms and increased anisotropic stiffness. Under 0.10% strain, the 0.25 mol% Si doped system achieves polarization nearing 1C m^-2, strong out of plane piezoelectricity, and reduced leakage-properties ideal for flexible electronics and energy storage devices. In contrast, 0.50 mol% Si under strain shows excessive polarization and higher leakage due to larger lattice distortion. This work provides the first demonstration that Si-doping synergistically combined with strain-engineering can unlock high-κ dielectric behavior and enhanced polarization in Cs₃Bi₂I₉, establishing a new, lead-free perovskite platform for next-generation MIM capacitors.