**3. Ge-, Bi-, Sb-, and Cu-based perovskites**

Although Sn-based PSC has achieved PCE over 8%, the oxidation of Sn and degradation of the perovskite still need to be addressed. In this context, other metal halide perovskites such as Ge, Bi, Sb, and Cu are considered as potential candidates to replace Pb element in the perovskite crystal. As Ge 4 s has higher orbital energy compared to Pb 6 s and Sn 5 s, thus Ge-based perovskites should exhibit smaller bandgaps. However, in practice the Ge-based perovskites such as CsGeI3, MAGeI3, and FAGeI3 displayed larger bandgaps (i.e., 1.63, 2.0, and 2.35 eV) than that of CH3NH3SnI3 (1.30 eV) and CH3NH3PbI3 (1.55 eV) [52]. The difference between experimental data and what we expected from high orbital energy is mainly due to the structural distortion of [GeI6] octahedral as the small ionic radius (0.73 Å) of Ge2+ substituting the bigger ionic radius of Pb2+ (1.19 Å) or Sn2+ (1.02 Å) [53]. Additionally, the Ge-based perovskites crystallize in polar space groups [54]. The Ge2+ cation cannot sustain at the center of octahedron and forms three long Ge▬I bonds (2.73–2.77 Å) and three short Ge-I bonds (3.26–3.58 Å) [53]. This means that the Ge cation cannot maintain

the desirable octahedral crystal structure which may be one of the reasons for poorer device performance (the best PCE up to 0.2% so far) as compared to Pbor Sn-based perovskite [52].

Furthermore, Bi and Sb can form A3B2X9 perovskite structure such as A3Bi2X9 and A3Sb2X9 [55, 56]. These perovskites typically form low-dimensional perovskites with two polymorphs. Devices fabricated with Cs3Bi2I9, (MA)3Bi2I9, and MA3Bi2I9 − xClx (hexagonal phase and *P*63/mm(194) space group) delivered PCE of 1, 0.33, and 0.38%, respectively [35]. The highest performance in the case of Cs3Bi2I9 perovskite was attributed to the low non-radiative recombination. The A cation has also a crucial role in the formation of Cs3Sb2I9 perovskite. For instance, the mixture of Cs<sup>+</sup> and MA+ cations forms dimer phases [57], which is structurally different phase from rubidium (Rb) as the A cation. The DFT analysis about the dimer and layer forms of A3Sb2I9 (A = Cs or Rb) indicated much preference of Rb-based systems (with formation energy of 0.25 eV) as layered phase compared to Cs-based perovskite (formation energy of 0.1 eV). Thus the Rb3Sb2I9 perovskite in PSC showed a PCE of 0.66% [57]. Recently, our research group fabricated Cu-based hybrid materials denoted as (MA)2CuX (where X = Cl4, I2Cl2, or Br2Cl2) to replace Pb [15]. It was revealed that chlorine (Cl<sup>−</sup>) in the perovskite crystal has a critical role in the stabilization of the as-prepared materials. The corresponding PCEs of the devices are depicted in **Figure 5**.

#### **Figure 5.**

*SEM micrographs of (MA)2CuCl4, (MA)2CuI2Cl2, and (MA)2CuBr2Cl2 with their chemical structures, respectively. Schematic illustration of the as-fabricated devices along with photovoltaic parameter table. Reprinted with permission from [35].*

**7**

*Quest for Lead-Free Perovskite-Based Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.83360*

It is concluded that the nontoxic elements should form octahedral crystal structure with halides. Besides the bandgaps, structural/electronic dimensionality, crystal defects, carrier motilities, and high-quality film formation of Pb-free perovskites are equally important. Although Sn is widely used to replace Pb in perovskites, it oxidizes from Sn2+ to Sn4+ and leads to high charge recombination and poor device performance. In addition, the fast crystallization of Sn-based perovskites forms pores in their thin films that deteriorate the solar cell performance. Thus, additives and additional solvent such as SnF2, SnCl2, SnF2-pyrazine complex, SnI2(DMSO)x complex, and HPA can be optimized to inhibit the oxidation reaction and control film growth rate. Another severe effect found in the Sn-based device is the hysteresis that is mostly associated with the imbalance charge transport and defects. In this context, mitigation of ion migration within perovskite and interfacial engineering by using suitable charge-transporting materials can reduce the hysteresis effect. The Ge-based perovskites exhibit larger bandgaps which can deliver high open-circuit voltage compared to Sn-based perovskite. However, they also show severe oxidation. Alternatively, perovskite structure with trivalent Bi3+/Sb3+ cations can be designed. Copper is also a promising alternative to Pb in mixed halide perovskites where the Cu reduction can be decreased and the material stability as well as the perovskite crystallization can be enhanced by manipulating the halide ions. Replacing the toxic Pb in perovskite crystals with environmentally friendly, nontoxic, earth-abundant, and cost-effective materials such as transition metals (Fe2+, Zn2+, Cu2+, etc.) is an important target for sustainable energy perspectives.

This work is supported partially by the National Natural Science Foundation of China (Grant nos. 51772096), Natural Science Foundation of Beijing Municipality (L172036), Joint Funds of the Equipment Pre-Research and Ministry of Education (6141A020225), Par-Eu Scholars Program, Science and Technology Beijing 100 Leading Talent Training Project, Beijing Municipal Science and Technology Project (Z161100002616039), Fundamental Research Funds for the Central Universities (2016JQ01, 2017ZZD02), and NCEPU "Double First-Class" Graduate Talent

**4. Summary and future outlook**

**Acknowledgements**

Cultivation Program.

**Conflict of interest**

The authors declare no conflict of interest.
