**3.1 Ge-based perovskite**

Ge, is finest substitute for lead in the same group as Pb. For example, AGeI3 perovskite group, CsGeI3 has the narrowest band gap of about 1.6 eV (**Figure 2**), but MAGeI3 and FAGeI3 have direct band gaps of 1.9–2. 2 eV. The band gap (Eg) of the MAGeX3 perovskites estimated, and the Eg of MAGeI3 was 1.61 eV, in contrast to 2.81 eV and 3.76 eV for the Br and Cl anions, correspondingly. And high-quality CsSn0.5Ge0.5I3 perovskite films with a band gap of 1.5 eV solar cells reported an incredible PCE of 7.11% [56]. The germanium-based-perovskite instability due to from the tendency of Ge2+ to oxidize into Ge4+ [57]. The new result was achieved CsSn0.5Ge0.5 I3, which provided a PCE of 7.11% with improved stability concerning the CsSnI3 [58]. Furthermore, germanium in 0.75 MA0.25Sn1 xGexI3 was newly provided PCE of 7.9% [59] as shown in **Table 1**.


#### **Table 1.**

*Germanium halide perovskites PCEs in photovoltaic devices.*
