A DFT Study on Tunable Optoelectronic Properties and Stability of (2-CEA)-Based Perovskites for Solar Cells

Perovskite solar cells (PSCs) have emerged as promising candidates for next-generation photovoltaics, yet their commercial viability is hindered by concerns over lead toxicity and stability. Herein, we systematically investigate the structural, electronic, optical, and thermodynamic properties of lead-free perovskites with the general formula (2-CEA)2BBr4–xIx (B = Sn, Ge; x = 0–4) and (2-CEA)2PbBr4 using density functional theory. The results reveal that 11 perovskite systems exhibit remarkable thermodynamic stability with negative formation enthalpies and sustained structural integrity during ab initio molecular dynamics simulations at 300 K. Substitution of Pb with Sn or Ge substantially reduces the bandgap, and it shows a further decreasing trend with increasing iodine doping concentration. Electronic structure analyses reveal that Sn/Ge substitution and iodine doping increase bond covalency and orbital hybridization, which collectively contribute to the narrowing of the bandgap. Optical property calculations demonstrate that both Sn- and Ge-based systems extend the spectral absorption range and enhance the absorption coefficient across the visible and near-ultraviolet regions. Notably, (2-CEA)2SnI4 and (2-CEA)2GeI4 exhibit the highest dielectric responses and strongest visible absorption. Our computational study not only validates the promising optoelectronic performance of lead-free (2-CEA)2BBr4–xIx (B = Sn, Ge; x = 0–4) perovskites but also provides theoretical guidance for the rational design of efficient, stable, and environmentally friendly PSC materials.