**2. Origin of lead-free perovskite solar cells**

Later a revolutionary report by Kojima et al. [33], and a successive breakthrough by Kim et al. and Lee et al. [34, 35] showed that lead-based perovskite solar cells have significantly transmuted the field of photovoltaics. Different generations exhibit perovskite solar cells' extraordinarily high power conversion efficiency because of high absorption coefficient [36] and defect tolerance [37] and to the long exciton diffusion length [38] of perovskite materials.

This noteworthy improvement in PSC solar cells restricted by critical intrinsic device instability of PSCs [39, 40] and toxic to the environment restricted for breakthrough outdoor application [41–46]. According to the World Health Organization (WHO), report Pb through metabolism, and children are at a particularly high risk of Pb poisoning the human body cannot purge [47, 48]. Furthermore, Pb can simply spread into the air, water, and soil [49, 50].

More recently, the scientific community has been pointed for Pb-material replacements that have been reported promising efficiency progress [51–53]. **Figure 3a** bar graph shows the recent the highest certified power conversion efficiency (PCE) of the different types of photovoltaics, perovskite solar cells (PSC, 29.1%), organic solar cells (OSC, 17.4%), dye-sanitized solar cells (DSC, 13.8%), CIGS (23.4%), CdTe (22.1%), polycrystalline siliconcells (PCSSC, 22.3%), and monocrystalline silicon solar cells (MCSSC, 26.7%). **Figure 3b** shows that reported PCEs of Pb-based, Bi-based, and Sn-based PSCs from the preliminary stage of development to date. Sn-based PSCs (Sn-PSCs) have thus far shown the greatest prospects though there are fewer reports on Sn-PSCs compared with those on Pb-PSCs, the PCE and stability of Sn-based PSCs have been enhanced quickly. For example, Sn-based perovskites such as cesium tin iodide (CsSnI3), formamidinium tin iodide (FASnI3), and methylammonium tin iodide (MASnI3), have direct band gaps of approximately 1.3 eV, 1.41, and 1.20 eV, respectively, which are narrower than Pb-based perovskites [31, 54].

#### **Figure 3.**

*(a) Highest certified PCEs of the different types of photovoltaics. (b) Reported PCEs of Pb-based, Bi-based, and Sn-based PSCs.*
