Tuning Electronic and Structural Properties of Lead-Free Metal Halide Perovskites: A Comparative Study of 2D Ruddlesden-Popper and 3D Compositions

In recent decades, two-dimensional (2D) perovskites have emerged as promising semiconductors for next-generation photovoltaics, showing notable advancements in solar energy conversion. Herein, we explore the impact of alternative inorganic lattice BX-based compositions (B=Ge or Sn, X=Br or I) on the energy gap and stability. Our investigation encompasses BA₂Ma_n-1B_nX_3n+1 2D Ruddlesden-Popper perovskites (for n=1-5 layers) and 3D bulk (MA)BX₃ systems, employing first-principles calculations with spin-orbit coupling (SOC), DFT-1/2 quasiparticle, and D3 dispersion corrections. The study unveils how atoms with smaller ionic radii induce anisotropic internal and external distortions within the inorganic and organic lattices. Introducing the spacers in the low-layer regime reduces local distortions but widens band gaps. Our calculation protocol provides deeper insights into the physics and chemistry underlying 2D perovskite materials, paving the way for optimizing environmentally friendly alternatives that can efficiently replace with sustainable materials.