**3.2 Bismuth-based perovskite**

Bi is a nontoxic and has the comparable properties to Pb and has satisfactory tolerance factor rule and enhanced the stability. It has unique properties such as 0D dimensionality, indirect band gaps, and mobilities (**Figure 3a**) [60, 61]. According to Park report [63], Bi-based perovskite A3Bi2I9 (A to be Cs and MA) as a photovoltaic absorber has estimated band of A3Bi2I9 to be ca. 2.1 eV for MA and 2.2 eV for Cs, and the exciton binding energy as 70 meV (while Pb perovskite is at 2550 meV). The reported photovoltaic parameters as (for CS3Bi2I9 PCE =1.09%, FF = 0.6, Voc = 0.85 V, Jsc = 2.15mAcm<sup>2</sup> , 0.12%, for MA3Bi2I9 FF = 0.33, Voc = 0.68 V, Jsc = 0.52mAcm<sup>2</sup> ). This low efficiency because of the number of reasons, such as excess reactant residue, extra band-gap states, poor morphology, and interface contact. Therefore, the Bibased perovskite is ideal as solar cell absorbers due to low mobilities and good stability [62, 63] as shown in **Table 2**.
