Theoretical Insights on Pb-Free Rhombohedral CsGeI<inf>3</inf> over Cubic CsMX<inf>3</inf> (M-: Pb, Sn, Ge, and X-: Cl, Br, I) Perovskite-Based Single-Junction Solar Cell with Efficiency &gt;30%

Herein, density functional theory-based comparative studies on cubic CsMX3 (M-: Pb, Sn, Ge; X-: Cl, Br, I) and rhombohedral CsGeI3 perovskites are reported. The structural stability indicator analysis shows that Ge-based perovskites may exhibit small rhombohedral distortion led by lone pair (4 (Formula presented.)) electrons on off-centering Ge atom. The role of lone pair on Ge atom in rhombohedral distortion is corroborated by bond length, electronegativity, and charge density distributions. Nearly agreeable bandgap ((Formula presented.)) values are noticed for cubic CsPbX3 (X-: Cl, Br, I) and rhombohedral CsGeI3, and the corresponding values are 3.04, 2.30, 1.72, and 1.45 eV, respectively. The effect of Jishi et al. reparameteried Tran–Blaha-modified Becke–Johnson exchange-correlation (XC) potential is discussed in context of optoelectronic properties. The changes in bandgap are attributed to the lifting of electronic degeneracy either by spin-orbit coupling or symmetry lowering in rhombohedral distortion. The rhombohedral CsGeI3 is thus further emphasized to account its ferroelectricity with computed total polarization ((Formula presented.)) around 32.75 μC cm−2. The suitable (Formula presented.) (1.45 eV), high absorption (≈105 cm−1), tolerable reflectivity (19%), and carrier effective masses (0.61 and 0.27) enable rhombohedral CsGeI3 (superior 30.5% spectroscopy-limited maximum efficiency [SLME] at 1 μm thickness) to outperform conventional CsPbI3 (27.6% SLME at 1 μm) and others which may attract PV community to further address its underexplored promise.