**1. Introduction**

PSCs with organometal (Pb) halide perovskites as photo-absorber showed rapid development in terms of PCEs from 3.8 to 23.3% [1–7]. The typically used Pb-based perovskites possess several appealing advantages such as broadband absorption range, long diffusion length, low exciton binding energy, and high-charge-carrier mobility [2, 3, 8–14]. However, intrinsic toxicity of Pb-based perovskites is a serious issue for both human and environment [15–19]. In this context, the replacement of Pb element in PSCs is extremely important for economical clean energy conversion devices which would benefit mankind in future endeavor.

Organic-inorganic trihalide perovskite is generally represented by ABX3 (A = CH3NH3 + (MA), CH(NH2)2 + (FA), Cs+ ; B = Cu2+, Pb2+, Sn2+, Ge2+, Bi3+, Sb3+; X = I<sup>−</sup> , Cl<sup>−</sup> , Br<sup>−</sup> ). **Figure 1** illustrates the typical cubic perovskite structure with basic octahedron (BX6) unit. It has been witnessed that structure distortions determine the physical/electrical properties of ABX3 perovskite [20]. For example, Goldschmidt's tolerance prediction can be used for the dimensional evaluation of a perovskite as follows:

$$\mathbf{t} = \frac{(\mathbf{r}\_A \star \mathbf{r}\_X)}{\sqrt{2(\mathbf{r}\_B \star \mathbf{r}\_X)}} \tag{1}$$

**Figure 1.**

*Basic crystal structure of perovskite semiconductor, where A, B, and X represent (CH3NH3 + , CH(NH2)2 + , Cs+ ), (Cu2+, Pb2+, Sn2+, Ge2+, Bi3+, Sb3+), and (I<sup>−</sup>, Cl<sup>−</sup>, Br<sup>−</sup>), respectively.*

where *r*A, *r*B, and *r*X denote the ionic radii of A, B, and X, respectively. If the value of "*t*" results in the range between 0.813 and 1.107, then it is considered as a high-symmetry cubic three-dimensional (3D) perovskite, while two-dimensional (2D), one-dimensional (1D), and zero-dimensional (0D) perovskites are formed when "t" gives other values than the abovementioned range [21, 22]. The structural dimensionality approach is considered one of the essential factors because different dimensions of perovskite influence the kinetics of charge carriers. Nonetheless, this evaluation is not enough to be applied to all perovskite semiconductors. Therefore the probe for electronic dimensionality is equally important [23]. For example, perovskite materials with low electronic dimensionality but high structural dimensionality have less promises as light absorbers because of the barrier to isotropic current flow, large effective masses of holes/electrons, and deeper defect states.

The excellent performance of Pb-based perovskites is mainly due to high structural symmetry and strong antibonding coupling between Pb and I [24]. In a similar way, Cu2+, Ge2+, Bi3+, Sb3+, and Sn2+ with *ns*<sup>2</sup> lone pairs could be used with halides to obtain octahedral structure; therefore, they are investigated as alternatives to the toxic Pb element [15, 25, 26]. Herein, we will introduce the Pb-free perovskites from previously reported theoretical calculations and experimental studies.
