Theoretical insights into a potential lead-free hybrid perovskite: substituting Pb²⁺with Ge²⁺

Abstract

In recent years, perovskite solar cells have been considerably developed, however the lead in the absorber MAPbI3 is a potential threat to the environment. To explore potential alternatives, the structural and electronic properties of MAGeX3 (X = Cl, Br, I) were investigated using different density functional theory methods, including GGA-PBE, PBE-SOC, HSE06 and HSE-SOC. The results implied that MAGeI3 exhibits an analogous band gap, substantial stability, remarkable optical properties, and significant hole and electron conductive behavior compared with the so far widely used absorber MAPbI3. Moreover, the calculations revealed that the energy splitting resulting from the spin-orbit coupling is evident on Pb, moderate on Ge, I and Br, and negligible on Cl. Our work not only sheds some light on screening novel absorbers for perovskite solar cells but also deepens the understanding of these functional materials.

Progress on lead-free metal halide perovskites for photovoltaic applications: a review

“…In comparison to Pb 2+ , the divalent germanium cation (Ge 2+ ) is in the same oxidation state but exhibits a lower electronegativity, a more covalent character and an ionic radius (73 pm) lower than Pb 2+ (119 pm) [51, 52]. Nevertheless, Goldschmidt tolerance factor calculations support the formation of germanium halide perovskites, as shown for CH 3 NH 3 GeX 3 (X = Cl, Br, I) compounds with tolerance factor values of 1.005 (CH 3 NH 3 GeCl 3 ), 0.988 (CH 3 NH 3 GeBr 3 ), and 0.965 (CH 3 NH 3 GeI 3 ), which coincide with t values reported for the ideal perovskite structure (0.97 <  t  < 1.03) [107, 108]. …”

“…Theoretical considerations using density functional theory (DFT) methods show that germanium halide perovskites have high absorption coefficients as well as similar absorption spectra and carrier transport properties as the lead analogues [33, 42, 107, 109]. First-principle calculations of CsGeX 3 (X = Cl, Br, I) perovskites show that the band gaps depend on the halide ion, i.e.…”

“…Moreover, mixed halide germanium perovskites such as Cs 2 GeCl 2 I 4 , Cs 2 GeBr 2 I 4 , and Cs 2 GeI 2 Br 4 were predicted to be promising direct band gap semiconductors [109]. Sun et al extended the theoretical investigations to hybrid germanium halide perovskites, namely to CH 3 NH 3 GeX 3 (X = Cl, Br, I) compounds [107]. The calculated band gaps based on PBE (Perdew–Burke–Ernzerhof) functionals were found to show a similar trend as for the cesium-based compounds, i.e.…”

“…CH 3 NH 3 GeCl 3 (3.76 eV) > CH 3 NH 3 GeBr 3 (2.81 eV) > CH 3 NH 3 GeI 3 (1.61 eV) [107] and the band gaps of the iodide-based compounds are similar to the lead analogues CsPbI 3 (1.73 eV) and CH 3 NH 3 PbI 3 (1.57 eV) [110]. …”

Current Advancements in Material Research and Techniques Focusing on Lead-free Perovskite Solar Cells

“…51 The mixed halide Ge perovskite with the form of MAGeX 3 (X = I, Br, Cl) was discussed. The calculated tolerance factors (TFs) of MAGeX 3 are 0.965 for I, 0.988 for Br, and 1.005 for Cl, while the TF of MAPbI 3 was calculated to be 0.834.…”