DFT study of the effects of substitution of Pb by Ca, Co and Fe on the structural, electronic and optical properties of methylammonium and formamidinium lead chlorides

Hybrid organic-inorganic lead halide perovskites have emerged as promising materials for optoelectronics and solar cells, offering prospects of improved cell performance and/or low-cost manufacturing. However, concerns regarding their stability and the toxicity of lead to the environment has ignited experimental and theoretical search for possible replacement of lead by non-toxic elements.

Density functional theory (DFT) was used to investigate the relationship between the structure, electronic and optical properties of lead-free and lead-based chloride perovskites with general formula MAMCl₃ and FAMCl₃ where MA and FA represent the organic cations CH₃NH₃⁺and NH₂CHNH₂⁺respectively, and M denotes metal ion (M = Pb²⁺, Ca²⁺, Fe²⁺, Co²⁺). Possible metal substitutes for lead are investigated.

The results indicate that MAPbCl₃ and FAPbCl₃ perovskites are direct band materials with their gap energy very close to the experimentally observed values, indicating a good level of theory. The transition metal substituents are indirect band materials having lower band gap energies than the corresponding lead-based compounds due to the presence of subgap levels in proximity to the conduction band (CB). The direct and indirect band gap materials all show optical absorption spectra spanning the visible and near-UV regions, making them suitable absorber materials in photovoltaic devices. Additionally, the indirect materials, MACoCl₃ (band gap 2.32 eV) and FACoCl₃ (band gap 2.08 eV), have the smallest band gap energy values and show significant absorption in the visible region, thus, cobalt is the most suitable potential substitute for lead in the chloride perovskites. Additionally, these materials are good candidates for potential applications as photodiodes, photosensors, and photodetectors.