Simulation of Native Oxide‐Passivated CsSn_0.5Ge_0.5I₃ Highly Stable Lead‐Free Inorganic Perovskite Solar Cell

Abstract

The rapid evolution of efficiency from 3% in 2009 to 25% today [1,2] has made perovskite a potential alternative to costly Si and GaAs photovoltaic material. The remarkable optical absorption properties, wide-bandgap tunability, high diffusion length, defects tolerance, and solution-based benign fabrication have made perovskite a significant photovoltaic (PV) material. However, on the other hand, Pb presence and phase stability are concerning challenges associated with perovskite. [3] Replacing Pb with the same group smaller element, Sn is seen in MASnX 3 , FASnX 3 , and inorganic CsSnX 3 . The performance of MASnI3 is 7.1%, [2] and FA-based SnX 3 is about 14.3%; [4] similarly, the CsSnX 3 have crossed 10%. [5] Sn replacement might have solved the Pb toxicity issue but it is still plagued with stability issues. The rapid oxidation of Sn þ2 to Sn þ4 is an unresolved issue in high-efficiency CsSnI 3 perovskites. [6] It is consistently reported in the literature that the Sn readily goes under oxidation from Sn þ2 to Sn þ4 , which rapidly degrades and further aggravates the stability issues of Sn-based perovskite. [6] Group 14 elements of Pb, Sn, Cd, and Ge all form narrow-bandgap perovskite; among these, only Ge fulfils the criteria of Earth abundance and nontoxicity. Some reports in the literature exist about the partial substitution of Ge in Snbased perovskites, such as MASnGeI 3 , CsSnGeI 3 etc. The mixed inorganic perovskite, thus resulting out of Sn-Ge systems, has demonstrated higher stability than Sn alone at the B site in perovskite. [7] Ge, Sn, and Pb have increasing ionic radii and similar electronic configurations with ns 2 lone pair whose activity increases with reducing ion size. The probability of a hole appearing in the valence band is higher in the Sn/Ge-based perovskite than in the Pb-based perovskite. The hybridization of the 5s and 4s bands of Sn and Ge with the 5p state of halide in the valance band causes the generation of holes. [8] Pure Ge-based perovskites such as MAGeI 3 and CsGeI 3 have exhibited remarkably high ionic conductivity and similar optical properties as MAPbI 3 with high absorption and weak reflectance. [9,10] The imaginary part of the dielectric function and absorption coefficient show a blueshift in Ge-based perovskite compared to Pb-based perovskite, implying the strong absorption of the visible region in Ge perovskite. [11] The negative free energy of formation for MAGeI 3 is similar to MAPbI 3 , suggesting similar stability of the two perovskite phases. The order of the stability of Pb-, Sn-, Ge-based perovskite is summarized as MAPbI 3 > MAGeI 3 > MASnI 3 > MAGeCl 3 >MAGeBr 3 by Rossella et al. and [6] Sun et al., [9] where the stability of MAGeI 3 is comparable with MAPbI 3 . Ge is lighter when compared with both Pb and Sn, enabling Ge-based perovskite to have higher power density than later. [7] The stability order for Cs-based perovskite (CsGeI 3 ) is CsSn 0.5 Ge 0.5 I 3 > CsSnI 3. The CsSn x Ge 1-x I 3 perovskite becomes relatively stable due to forming a Ge-rich, 5 nm-...