Quest for Lead-Free Perovskite-Based Solar Cells

Today, the perovskite solar cells (PSCs) are showing excellent potentials in terms of simple processing, abundance of materials, and architectural integration, as well as very promising device’s power conversion efficiencies (PCEs), rocketed from 3.8% in 2009 to 23.3% in 2018. However, the toxic lead (Pb) element containing the chemical composition of typically used organic-inorganic halide perovskites hinders the practical applications of PSCs. This chapter starts with a general discus-sion on the perovskite crystal structure along with the serious efforts focused on Pb replacement in these devices. Section 2 will elaborate the fundamental features of tin (Sn)-based perovskites together with their performance in the PSCs. Other alternative elements, such as copper (Cu), germanium (Ge), bismuth (Bi), and antimony (Sb), will be discussed in Section 3. The end will summarize the chal-lenges and opportunities based on the chapter contents.

Organic-inorganic trihalide perovskite is generally represented by ABX 3 (A = CH 3 NH 3 + (MA), CH(NH 2 ) 2 + (FA), Cs + ; B = Cu 2+ , Pb 2+ , Sn 2+ , Ge 2+ , Bi 3+ , Sb 3+ ; X = I − , Cl − , Br − ). Figure 1 illustrates the typical cubic perovskite structure with basic octahedron (BX 6 ) unit. It has been witnessed that structure distortions determine the physical/electrical properties of ABX 3 perovskite [20]. For example, Goldschmidt's tolerance prediction can be used for the dimensional evaluation of a perovskite as follows: t = ( r A + r X ) _______ √ _______ 2 ( r B + r X (1) A Guide to Small-Scale Energy Harvesting Techniques 2 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, Cu 2+ , Ge 2+ , Bi 3+ , Sb 3+ , and Sn 2+ with ns 2 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.

Sn-based perovskites
Tin (Sn) element has been widely used as an alternative to Pb, since both occur in group IVA of the periodic table with similar ionic radii (Pb: 1.49 Å and Sn: 1.35 Å). Therefore, Pb substitution by Sn would cause no obvious lattice distortion in perovskites [27]. The intrinsic instability that results in the decomposition of unstable products such as SnI 2 and HI (acidifier) and toxicologically inactive oxygenated Sn precipitates are still remained the toxicity issues in the Sn-based perovskites [16]. However, benefiting from the stability of PSCs based on Sn perovskites and easy cleaning of Sn from the human body compared to Pb, Sn-based perovskites could be a better choice than perovskite with Pb cation. Density functional theory (DFT) and GW approximation are typically used for the structural and electrical properties of Sn-based perovskites [28,29]. For instance, the theoretical-experimental calculations about the bandgap of MASnI 3 showed  a range of 1-1.3 eV, charge mobility of 1.6 cm 2 V −1 s −1 , and diffusion length up to 30 nm (Figure 2A) [29,30,32,33]. Additionally, SnF 2 -doped MASnI 3 showed tenfold larger carrier lifetime with diffusion length of 500 nm ( Figure 2B) [31]. Replacing MASnI 3 by FASnI 3 results in a desirable bandgap of 1.41 eV and minimized oxidation of Sn 2+ to Sn 4+ [34,35]. It is also reported that replacement of "A" cation by Cs + in ASnX 3 yields higher charge mobility, lower exciton binding energy, and large optical absorption coefficient compared to conventional MAPbI 3 [36,37]. However, plasticity of the tin-halide-tin angle and Cs + cation migration [38] showed CsSnX 3 a phase transitional perovskite with respect to temperature [39].
Since Sn-based perovskites possess low crystallization barrier and high solubility in solvents, their thin films can be fabricated at low temperature in the PSCs. For example, a high crystalline MASnI 3 -perovskite thin film has been prepared from a transitional SnI 2 •3DMSO intermediate phase ( Figure 3A) [40]. This high-quality perovskite film formation in a hole-selective layer-free PSCs resulted in a photocurrent of 21 mA cm −2 . In order to obtain low doping level in MASnI 3perovskite thin film, a low-temperature vapor-assisted solution process was employed ( Figure 3B), where the excess of Sn 2+ compounds due to Sn(OH) 2 and SnO resulted in low hole-doping level [41,44]. Furthermore, anti-solvent dripping process was used to control fast crystallization and fabricate pinhole-free thin films of Sn-based perovskites. The diethyl ether dripping on FASnI 3 and chlorobenzene on (FA) 0.75 (MA) 0.25 SnI 3 enabled the as-prepared PSCs to obtain efficiency of 6.2 and 8.1%, respectively (Figure 3C and D) [42,43].
It is reported that the oxidation of Sn-based perovskites (self-doping from Sn 2+ to Sn 4+ ) leads to carrier recombination and poor device performance. In this context, specific amount of SnF 2 can be used as an inhibitor for Sn 4+ . The large quantity of SnF 2 may generate phase separation such as plate-like aggregates on the perovskite film surface. Here, the strong binding affinity in SnF 2 -pyrazine complex was helpful to inhibit the phase separation caused by excess SnF 2 ( Figure 4A) [45]. Furthermore, the built-in potential can also be optimized through SnF 2 and thus high open-circuit voltage of the device by energetic landscape alignments [46] as illustrated in (Figure 4B). Besides SnF 2 , other additives such as SnBr 2 , SnI 2 , and SnCl 2 have also been explored where SnCl 2 exhibits the highest stability by inhibiting the decomposition/oxidation [34,47]. Moreover, the employment of hypophosphorous acid in CsSnIBr 2 showed seed-like perovskite where Sn 2+ oxidation was significantly reduced [48]. As a result, the charge recombination rate was decreased by fourfold compared to the control devices. The as-prepared cells displayed excellent oxygen-moisture stability at ambient conditions and thermal stability in vacuum environment.
The inclusion of large ammonium cations for tuning the dimensionality or generating massive Schottky defects has also been tested in Sn-based perovskites to optimize the device performance. For example, a different ratio of phenylethylammonium (PEA) as a cation can yield perovskites with two-dimensional, threedimensional, and three-dimensional-two-dimensional-mixed dimensionalities.
The high-quality thin film of the mixed PEA-FA perovskite (20% of PEA) delivered a stable device efficiency of 5.9% [49]. The composition of ethylenediammonium (en) and formamidinium (FA) in FASnI 3 /MASnI 3 /CsSnI 3 crystal indicates no effect on the dimensionality of the perovskite because the size of en is too large for the unit cell cage and it can remove a certain {SnI} + species. Here, the point defects are recognized as the Schottky defects. However, the optimized devices based on {en} MASnI 3 and {en}FASnI 3 showed efficiency of 6.6 and 7.1%, respectively [50,51].

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 CsGeI 3 , MAGeI 3 , and FAGeI 3 displayed larger bandgaps (i.e., 1.63, 2.0, and 2.35 eV) than that of CH 3 NH 3 SnI 3 (1.30 eV) and CH 3 NH 3 PbI 3 (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 [GeI 6 ] octahedral as the small ionic radius (0.73 Å) of Ge 2+ substituting the bigger ionic radius of Pb 2+ (1.19 Å) or Sn 2+ (1.02 Å) [53]. Additionally, the Ge-based perovskites crystallize in polar space groups [54]. The Ge 2+ 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  [45,46]. Reprinted with permission from [35]. 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 A 3 B 2 X 9 perovskite structure such as A 3 Bi 2 X 9 and A 3 Sb 2 X 9 [55,56]. These perovskites typically form low-dimensional perovskites with two polymorphs. Devices fabricated with Cs 3 Bi 2 I 9 , (MA) 3 Bi 2 I 9 , and MA 3 Bi 2 I 9 − x Cl x (hexagonal phase and P63/mm(194) space group) delivered PCE of 1, 0.33, and 0.38%, respectively [35]. The highest performance in the case of Cs 3 Bi 2 I 9 perovskite was attributed to the low non-radiative recombination. The A cation has also a crucial role in the formation of Cs 3 Sb 2 I 9 perovskite. For instance, the mixture of Cs + 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 A 3 Sb 2 I 9 (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 Rb 3 Sb 2 I 9 perovskite in PSC showed a PCE of 0.66% [57]. Recently, our research group fabricated Cu-based hybrid materials denoted as (MA) 2 CuX (where X = Cl 4 , I 2 Cl 2 , or Br 2 Cl 2 ) to replace Pb [15]. It was revealed that chlorine (Cl − ) 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.

Summary and future outlook
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 Sn 2+ to Sn 4+ 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 SnF 2 , SnCl 2 , SnF 2 -pyrazine complex, SnI 2 (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 Bi 3+ /Sb 3+ 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 (Fe 2+ , Zn 2+ , Cu 2+ , etc.) is an important target for sustainable energy perspectives.