Despite being the most plentiful, free, and sustainable energy source, traditional photovoltaics (PVs) are still more expensive than fossil fuels in most places (Gratzel, 2014; Green et al., 2014). This highlights the critical need for the rapid advancement of PVs of the subsequent group that are both extremely competent and suitably priced. In this context, perovskite (PVK) solar cells (PSCs) have developed as a revolutionary thin-film PV technology, rapidly reigniting research into the development of PVs (Liu et al., 2013; Jeon et al., 2014; Correa-Baena et al., 2017). The power conversion efficiency (PCE) of PSCs has climbed recklessly now up to 23% which has been shown in (Kojima et al., 2009; National Center for Photovoltaics, 2024).

The major light absorbers in PSCs are PVK-type materials. Since the earliest days of PSC research, lead-based organic-inorganic halide PVKs (OIHPs) (Kim et al., 2012; Zhou et al., 2016), with a typical chemical formula of ABX₃ (Kim et al., 2012; Zhou et al., 2016), have been the most explored PVKs. PVK absorber materials now include “low-dimensional” and chalcogenide PVKs (PCP) (Snaith, 2013; Cao et al., 2015; Tsai et al., 2016; Xiao et al., 2018). Perovskites are a class of extremely effective solar cells based on Pb halides. The most popular type is Pb-based organic-inorganic halide perovskites, which have the general formula ABX₃. Here A can be CH₃NH₃ + (MA+) or HC(NH₂)₂ + (FA+); B is Pb²⁺; and X can be I, Br or Cl (shown in Figure 1). These perovskites are widely studied because of their high efficacy and low rate. They can be used in different forms, including thin films, quantum dots, and nanorods. However, the stability and degradation of perovskites are important issues that need.

Despite current research into alternate PVK materials, the B-site positive ion in modern PSCs is still lead (Fabini, 2015). If all the electricity in the United States were to be generated by PSCs using the most well-deliberated OIHP, the annual consumption of lead would be 160 tons (Fabini, 2015). Eliminating Pb from PSCs is the only long-term solution to the Pb-toxicity problem, even though PSCs may be managed and regulated to decrease environmental Pb discharge. The quantity of harmful components permitted in consumer or domestic niche applications, such as portable PVs, is extremely low (Hailegnaw et al., 2015). The band alignment between the perovskite material and the selective materials of n-type and p-type is crucial for effective charge extraction. In particular, the electron transport layer’s conduction band edge should be lower than the perovskites, while the hole transport layer’s valence band edge should be higher. This relationship is illustrated in Figure 2.

United States, Occupational Safety and Health Administration (OSHA), for instance, classifies lead and its compounds as very dangerous and has established a legally acceptable exposure limit of 0.05 mg/L for general industry (Levin et al., 1997). Due to the high expense of creating Pb-based PSCs and the need to invest heavily in protecting workers’ health from the metal’s narcosis and eye/nose/throat irritation. The necessity for organic positive ions to cover the “A-site” in the PVK assembly is another major issue with existing lead-based bulk OIHPs. These organic compounds have a mild interaction with the metal-halide octahedra at PSC circumstances when the inorganic cations are present (Brunetti et al., 2016).

Although PVKs include ammonia functional groups in their crystal structure, the organic species inside them are more hygroscopic, making them more susceptible to deterioration when exposed to air (Leijtens et al., 2015; Rong et al., 2015). Lead and chemical instability of PSCs based on lead-containing bulk OIHPs are the key difficulties. To solve these issues, researchers must identify novel PVK options that are innocuous and firm while hitherto possessing sufficient PCE as an alternative to the currently employed lead-based PSCs. From this viewpoint, this paper primarily emphasizes the critical need to pinpoint the historical roots of the current PVK light-absorber materials’ inherent toxicity and instability, and then address approaches to designing new environmentally friendly, stable PVKs. Future synthesis of novel lead-free PVK compounds is discussed to round up the paper.

Getting the basic stuff out, synthesis/processing of cells, cell assembly, utilization, and decommissioning of cells are the usual stages in the life span of a PSC panel (Volans, 1987; Babayigit et al., 2016), as illustrated in Figure 3A. Dangerous PVK-formed species will be released during all these phases, but it is too soon to do a full life cycle analysis of the PSC technology. Consequences like as land and water degradation are inevitable, as is the introduction of these toxins into the food chain, which ultimately reaches human people (Florence et al., 1988). Possible catastrophic incidents during PSC production, transit, storage, and use, such as fire or floods, provide additional environmental concerns (Dauvalter, 1955). As a result, Pb’s indirect toxicity has far-reaching effects on human health and the natural world. In addition, PVKs may be detrimental to ecosystems and people through a variety of different processes, such as acidification and nanotoxicity (see Figure 3B).

Sn-based PSCs, for instance, have been shown to attain PCEs of 10%, making them a promising green option for PSCs. Recent research (Babayigit et al., 2016), however, shows that in Sn-based PVK materials, oxidation may proceed rapidly in ambient or aqueous settings, resulting in the creation of hydroiodic acid (Babayigit et al., 2016; Abate, 2017).

Zebrafish study shows that Strontium Iodide is more intensely lethal when compared to Lead Iodide (Figure 2C). The greater acidification effects of Sn2+ compared to lead ions are largely to blame for this. Nanoscale lead-based and lead-free PVKs for PSCs and optoelectronics are expected (Im et al., 2014; Jellicoe et al., 2016). Nanoscale materials harm cells and biological systems, so, probably, nanoscale PVK materials are also dangerous.

The utilization of a mesoporous layer in the mesoporous architecture facilitates the expeditious extraction of photoinduced electrons from the perovskite material. This results in a reduction of the electron transport distance and eliminates the need for a high level of crystal quality to achieve effective light absorption (Tétreault et al., 2010). Nevertheless, in comparison to other arrangements, mesoporous perovskite solar cells often exhibit a reduced V_oc (Kang et al., 2016) and diminished light absorption beyond 720 nm wavelengths (Chen M. et al., 2016). The need for a perovskite overlayer to avoid mesoporous layer-HTL contact might cause short circuits (Yan et al., 2016). Moreover, there is an ongoing dispute over the role of the mesoporous layer, especially considering the remarkable efficiencies demonstrated by two-dimensional perovskite solar cell (PSC) devices. The highest recorded efficiency, as reported in reference (Fu et al., 2014), is at 20.7%. Titanium dioxide (TiO₂) is used as a mesoporous layer due to its broadband gap energy of 3.5 eV, chemical and thermal stability, photodegradation resistance, non-toxicity, and cost-effectiveness (Haruyama et al., 2015; Sabba et al., 2015). Figure 4 shows that mesoporous layer thickness affects perovskite polycrystal penetration into TiO₂ pores.

In a study conducted by authors (Wang et al., 2016), it was observed that a mesoporous layer of TiO₂ with a thickness ranging from 260 nm to 440 nm adequately filled the pores of mesoporous TiO₂, as depicted in Figure 5. These results suggest that between these bounds lies the sweet spot for maximizing light absorption while minimizing recombination due to route length. When testing the solar efficiency of each component, it was discovered that the mesoporous TiO₂ device, although thinner, performed well Figure 6. One possible explanation is that the higher electron density in TiO₂ improves charge transfer and collecting efficiency (Wu et al., 2017).

Nanoscale PVKs may be harmful due to their fibrous structure (Xiao Z. et al., 2015; Ju et al., 2017a; Ju et al., 2018), and radical species group (Ming et al., 2016). A meaningful evaluation of the toxicity of PVK compounds, both those already in use and those that have yet to be discovered, necessitates the prompt construction and implementation of a systematic system of biological investigations. Despite the potential importance of this avenue for PSC research, nothing has been done thus far.

Figure 7 shows that water, light, heat, and oxygen are the most detrimental to the stability of a PVK material. A wide range of processes, including polymorphic transformation, hydration, ion transport (Ke et al., 2017a), breakdown, and oxidation, are responsible for the degradation of OIHPs by these agents. While Pb-based OIHP deterioration has been extensively investigated in recent years, our knowledge of Pb-free PVKs is still in its infancy. The stability problem of upcoming PVK materials may be much more complicated than that of Pb-based OIHPs, according to certain studies in the literature.

Many decay pathways may be active simultaneously. CsSnI₃, a lead-free candidate PVK material with a 1.3 eV optical bandgap and decent carrier mobility, is one such example (Yin et al., 2017). CsSnI3 PVK is thermally stable because of its inorganic composition and robust covalent bonding for its lattice assembly (Volonakis et al., 2017). The “black” phase of g-CsSnI₃ rapidly undergoes a “yellow” polymorph transformation when exposed to air (Saparov et al., 2015; Slavney et al., 2016). Nevertheless, oxygen may quickly oxidize Sn (II) in CsSnI³ to Sn (IV), turning it into Cs₂SnI₆ (Giustino and Snaith, 2016).

Moisture from the air may also penetrate CsSnI₃ PVK thin films, where it can form hydrates and break down the material into metal halides. The PCE of PSCs based on CsSnI3 may drop precipitously due to a combination of these degrading processes. Due to their extreme instability in the ambient environment, they can decay in a matter of minutes if not enclosed.

It is difficult to examine these pathways in isolation, but doing so is essential if we are to solve the PSC instability problem once and for all. In addition, there are several other Pb-free PVK possibilities whose stability and deterioration have been poorly researched (Xiao et al., 2017a; Xiao et al., 2017b; Yang et al., 2017), including CsGeI₃, CsSnxGe1-xI₃, Cs2TiI₆-xBrx, and Cs₂AgBiBrI₆. Research into their resistance to the major environmental variables (humidity, light, heat, oxygen) and possible breakdown mechanisms is promising. New, Pb-free, stable PVKs for PSCs can be designed using the information gleaned by studying the stability of these developing PVKs.

Theoretical simulations screen the enormous number of PVK family compounds and derivatives to find safe and stable candidate PVKs. There are typically two phases to such materials screening processes. Finding potential elements to substitute for Pb in current Pb-based OIHPs is the first step in solving the toxicity problem. Substitutes for PVKs in PV applications must have many of the same fundamental electrical, conveyance, and ocular features. Group IV elements Sn₂₊ and Ge²⁺ are often cited in the literature as suitable lead ion substitutes. Lead-free Sn- and germanium-based PVKs for PV applications have emerged from this logic. Sn-based PSCs have PCE approaching 10% (Ju et al., 2017b), even though Sn2+ may be quickly oxidized to Sn⁴⁺, lowering performance. Potential Pb-free PVK candidates can also be identified using an electrical structure-based approach.

Pb-based OIHPs have high PCEs because the Pb lone-pair 6s orbital has a strong antibonding interaction with the I 5p orbital, allowing for longer carrier lifetimes and diffusion lengths (Zhang et al., 2017). Several researchers have focused on PVKs made from non-traditional metals because of the presence of lone-pair ns2 (McMeekin et al., 2016; Nakajima and Sawada, 2017; Ali et al., 2018a). New compounds having antibonding contact between orbitals around the valence band maximum can likewise exhibit band-edge behaviour like lead-based PVKs. Skutterudite structure has been proposed, for instance (Dai et al., 2017). Unlike lead-based PVKs, IrSb₃ possesses a band-edge feature indicative of p-p* antibonding interaction. The crystal structure for PVK materials provides another angle from which to hunt for promising Pb-free PVK options.

Replace Pb2+ with an aliovalent metal cation and the resulting PVK structure will have a different chemical formula from the usual AB(II/III/V) X3/X6/X9 (Zhao et al., 2017a; Zhao et al., 2017b). The electrical structure of the compounds will be altered because of this structural alteration (Sakai et al., 2017). Figure 8A shows how crystal structure and chemical composition can be used to create novel PVK-type compounds with favourable electrical structures for PV applications.

To accurately anticipate the stability of PVKs, a theory-experiment integrated method is required because of the complexity of the problem (Pang et al., 2016; Sun and Yin, 2017) To get a general idea of the stability of the PVK phase, a common empirical rule is to utilize Goldchmidt’s tolerance factor (t). Cubic assemblies are suggested by fits in the 0.9% t% one range for PVKs, whereas orthorhombic assemblies are suggested by fits in the 0.71%–0.9% t% one range. Other configurations include the hexagonal assembly, for t% 0.71 or t R 1. Figure 8B displays a flow chart for showing non-hazardous metal-based PVK intrants with PV constancy. This technique logically combines theory and experiment. Figure 8A depicts the first step of the process, which involves identifying a suitable metal-free PVK that does not include lead. Although the organic A-site positive ion is intrinsically unstable, positive ions such as caesium ions (Castelli et al., 2012; Korbel et al., 2016) are employed as replacements (Schmidt et al., 2017; Takahashi et al., 2018) due to their sturdier ionic interaction through unknown negative ions.

The most promising PVK candidates may be identified by combining A with B-site replacement. This method has been used to effectively anticipate and synthesise compounds like CsSnI₃, Cs₂AgBiBr₆, and Cs₂TiBr₆, although there may be additional PVK options that are less toxic and more stable (Lee et al., 2012; Burschka et al., 2013; Pilania et al., 2016; Li Z. et al., 2018). Several alternatives to B-site ions in PVK structures, including monovalent metals and trivalent metals (Xiao et al., 2014; Chen et al., 2018a; Ávila et al., 2017; Xiao J. et al., 2015; Elumalai et al., 2016), have been proposed. Substituting a tetravalence metal for the B-site ion stabilizes vacancy-order double PVKs (Kalyanasundaram and Grätzel, 1998; Leijtens et al., 2013). The same atoms in various valence states can substitute the B-site ion in electronic double PVKs (Xin et al., 2011).

Using Figure 8B’s manufacturing cycle, first, validate new compounds’ intrinsic or thermodynamic stabilities using “density functional theory (DFT)” based design to guarantee they have the requisite PV-related electronic/optical properties. As shown in Figure 3, when the PVKs have been synthesized, they are put through a series of experiments to determine how stable or degradable they are in the presence of various external environmental stimuli.

DFT-based mechanistic research complements experimental studies (Bi et al., 2013; Leijtens et al., 2014). These parallel theory-experiment studies (Son et al., 2014; Zuo et al., 2015) show how to modify PVK crystal structure/composition to find more stable PVK candidates. As can be seen in Figure 4B, the entire stability-screening procedure considers the crucial electrical assembly and the PVKs’ photovoltaic properties.

High amount simulated ingredients strategy can anticipate lead-free PVK ingredients for PV utilizing computational quantum-mechanical, thermodynamic, database development, and intelligent data-mining methods. Although useful, these calculations cannot replace a full material simulation, and they often only solve a portion of the design issue. The DFT-computed descriptors are useful for screening candidate materials and determining their important features (Agresti et al., 2016).

Pauling’s principles (Ball et al., 2013), are computational descriptors for intrinsic stability. Bandgap and effective mass can approximate light absorption and carrier mobility. The basic components of a material can provide an approximation of its cheap cost and nontoxicity. Descriptors screen candidates with a PVK structure and its modifications for materials with the right attributes (Ball et al., 2013).

High-throughput computational design is computationally expensive due to the huge conformation space and the enormous number of acceptable descriptors for essential properties, many of which are derived by more precise functional modelling (Wojciechowski et al., 2014). In divergence, a good machine learning model may be taught using existing data or data derived through computations. To find novel candidate materials with desirable qualities, this approach may be applied to the periodic table, yielding insights, and guiding experimental design (Niu et al., 2015). PVK stability and bandgap have been estimated in previous works using a variety of techniques based on the identification of pertinent properties (Christians et al., 2014; Qin et al., 2014; Jeon et al., 2015).

Using additives and adjustments to the solar cell fabrication process, lead-free PSCs may be made more efficient and stable. One or more of the following goals can be accomplished with the introduction of additives: control of oxidation, reduction of vacancies, alteration of the optical bandgap, increase of the fill factor (FF), or improvement of efficiency. The utilisation of appropriate techniques in fabrication can successfully address issues such as inadequate crystallisation, unfavourable morphology, and undesirable defects. This is crucial in achieving uniform and defect-free perovskite films without any pinholes (Ke et al., 2017c).

Recent advancements in the field of PSCs have focused on using different treatments, introducing hole transport materials, and including chiral compounds to enhance their efficiency and open-circuit voltage (Voc). These strategies will be further explored in the following discussion.

For PSCs to be extensively employed, breakthrough PVK materials that are extremely effective in light of electrical change, non-hazardous, and firm must be researched. The quest for novel PVK materials is anticipated to receive significantly expanded R&D funding during the next few years. While ongoing research towards this objective appears to develop rather randomly, we propose a reasonable roadmap that may help speed up R&D in this field. This road map is the first step toward understanding existing and future PVK noxiousness/dilapidation processes and advocates for more standardized experimental methodologies. After a comprehensive understanding of the toxicity/degradation pathways is achieved, innovative eco-friendly PVKs may be created that are particularly resistant to environmental stresses (using complementing theory-experiment techniques). Significant challenges are predicted for the future of lead-free stable PVKs thin-film manufacturing experiments in this paper. The latter is an essential step toward efficient and timely manufacturing of useful products. There are also promising new avenues for research into synthesis and processing that this opens. We anticipate that in the future, efficient and environmentally friendly PSCs will be realized because of this type of integrated scientific and technical R&D.