Minimizing Environmental Risks of Lead Halide Perovskite Solar Cells by Zhang Hui from Nanjing Tech University in collaboration with Antonio Abate, Michael Grätzel, and Nam-Gyu Park.
- Pollution and public acceptance concerns: Lead ions present in lead halide perovskites can potentially leak into the environment from broken solar cells, posing harmful effects. Strict regulations on lead usage worldwide have prompted the development of environmentally friendly and cost-effective strategies for recycling end-of-life products.
- Lead immobilization strategies: The focus is on converting water-soluble lead ions into insoluble, nonbioavailable, and nontransportable forms to suppress lead leakage in the event of device damage. Chemical approaches such as grain isolation, lead complexation, structure integration, and adsorption of leaked lead are analyzed based on their feasibility in minimizing lead leakage. The aim is to achieve effective lead-chelating capability without significantly impacting device performance, production cost, and recycling processes.
- Establishing standard lead-leakage tests and mathematical models: The need for standardized lead-leakage testing and related mathematical models is emphasized to reliably assess the potential environmental risk of perovskite optoelectronics. This will contribute to determining the effectiveness of lead immobilization methods and provide guidance for sustainable development in perovskite optoelectronic technology.
The toxicity issue of lead in perovskite solar cells has raised concerns among people. The use of lead poses a threat to the environment and human health. While lead naturally exists in the Earth’s crust, human activities over the past centuries, such as mining and the use of lead in gasoline, paint, and electronic products, have increased the risks associated with lead exposure. To mitigate these risks, the use of lead is tightly regulated, and specific restrictions have been implemented. However, existing legislation does not explicitly mention lead-based electronic products based on perovskite, highlighting the urgent need to assess the risks of these materials to ensure the safety and sustainability of perovskite-based electronic products. Recently, a research team from Nanjing Tech University collaborated with Antonio Abate, Michael Grätzel, and Nam-Gyu Park to analyze the chemical approaches for immobilizing lead in perovskite solar cells, aiming to minimize lead leakage and reduce its potential environmental impact. Their research emphasizes the establishment of standardized lead leakage testing and related mathematical models to reliably assess the potential environmental risks of perovskite photovoltaic technology.
Perovskite solar cells hold great promise for widespread applications. However, public concerns exist due to the potential environmental hazards of lead ion leakage that may occur in these cells. Strict global regulations on lead usage have prompted the search for environmentally friendly and economically viable strategies for the disposal and recycling of products containing lead. To address this issue, the research team explored the chemical methods for immobilizing Pb2+ ions in perovskite solar cells and evaluated the feasibility of these methods in suppressing lead leakage.
The research team, led by Zhang Hui from Nanjing Tech University, collaborated with Antonio Abate, Michael Grätzel, and Nam-Gyu Park to study the effectiveness of immobilizing lead ions through chemical approaches, including grain encapsulation, lead chelation, structural integration, and chemical adsorption. Grain encapsulation is a strategy that converts water-soluble lead ions into insoluble, non-bioavailable, and non-transferable forms under a wide range of pH and temperature conditions, thus inhibiting lead leakage. Additionally, the lead chelation method involves the formation of lead-additive complexes through rational additive engineering to reduce the solubility of lead compounds. The structural integration approach enhances the stability of the perovskite structure by increasing the binding strength between constituent elements and interfacial cohesion, preventing water dissolution and lead leakage. Furthermore, the research team explored the incorporation of adsorbents into external encapsulation layers to enhance lead adsorption capacity. They emphasized the importance of establishing standardized lead leakage testing and related mathematical models to reliably assess the potential environmental risks of perovskite photovoltaic technology.
These research findings demonstrate the feasibility and effectiveness of chemical methods for immobilizing lead ions in overcoming lead leakage issues. However, trade-offs need to be made for different immobilization strategies while ensuring device performance. Further research and testing are necessary to achieve the sustainable development and application of perovskite solar cells.
point1：The difficulty of replacing lead in perovskite devices
In addressing the toxicity issue of perovskite, a key question is whether it is possible to achieve excellent optoelectronic performance in lead-free perovskite without the presence of lead. Although some progress has been made in this regard, lead-free perovskite solar cells still exhibit significantly lower power conversion efficiency and stability compared to lead-containing perovskite photovoltaic cells. This is because lead-containing perovskites possess a unique orbital hybridization configuration that contributes to their outstanding optoelectronic properties. Therefore, researchers have attempted to replace lead with other metals that have similar orbital configurations, with tin (Sn)-based perovskites being the most extensively studied alternative.
Tin has a similar ionic radius (118 pm) to lead (119 pm) and features a lone pair in its 5s orbital and empty 5p orbitals, with effective nuclear charges (Zeff) of 10.63 and 9.10, respectively. However, Sn2+ ions tend to be oxidized to Sn4+ (with a standard reduction potential of E0 = 0.15 V for Sn2+/Sn4+, compared to E0 = 1.67 V for Pb2+/Pb4+). This tendency may be attributed to the absence of lanthanide elements, resulting in a smaller Zeff for the 5s lone pair electrons of Sn ions compared to the 6s lone pair electrons in lead ions. Consequently, the presence of Sn4+ generated within the perovskite film unexpectedly leads to high defect density, thereby reducing the optoelectronic performance. Additionally, SnI2 is believed to exhibit higher acute toxicity compared to PbI2.
Apart from tin, another IV group element with the same valence electron configuration is germanium (Ge). However, due to the smaller ionic radius of germanium ions (73 pm) and higher oxidation tendency (with an E0 = 0 V for Ge2+/Ge4+), germanium-based perovskites exhibit poorer optoelectronic characteristics and stability.
In the quest for stable lead-free perovskite materials, researchers have also explored other compounds, including ns2 elements containing Bi3+ and Sb3+. However, the crystal structures formed by these compounds possess relatively wide bandgaps and poor charge transfer ability, limiting their optoelectronic properties.
Currently, lead remains the most promising element in terms of the optoelectronic performance, thermodynamic properties, and environmental stability of perovskite crystals (see Table in Box 1).
Box 1: Typical properties of lead and other substitute ions, as well as compounds of halide perovskites containing these ions
O: Feasible; X: Not feasible. Data sourced from the referenced literature.
Point 2: Environmental Impacts of PSCs
To assess the environmental impacts of PSCs, a life cycle assessment approach has been employed, considering all stages from the extraction, purification, and preparation of lead-related raw materials to the manufacturing, installation, maintenance of PSCs, and end-of-life disposal. The assessment of PSCs’ life cycle has yielded some positive conclusions, suggesting that PSCs are more sustainable compared to other technologies such as commercial silicon solar cells. However, the leakage of lead in PSCs remains a concerning issue. Once installed, a significant portion of the panel’s lifespan is subject to uncontrolled atmospheric conditions, and panel damage can result in lead dissolution and diffusion. Potential exposure concentrations can be determined through life cycle analysis and leaching studies, but the impact on human health or the environment depends on the amount of organically bioavailable total lead and the presence of toxicity concerns in the bioavailable fraction.
In soil, the bioavailability of lead depends on the form of lead in water, soil chemistry factors such as ionic strength, pH value, natural organic matter, and soil types such as clay, loam, etc. The organic cations in perovskites can alter the pH of the soil and affect plant uptake of lead.
Figure 1: Pathways of Lead Leakage in PSCs and Assessment of Potential Environmental Impacts
Therefore, when evaluating environmental or human health risks, the form of lead, chemical transformations, and surrounding chemical matrices should be considered. The weekly lead intake (LWI) by humans is considered as a health indicator of lead exposure, with the Food and Agriculture Organization of the United Nations setting the upper limit at 0.025 mg/kg. By assuming that all lead in damaged PSC panels will leak and enter the environment within a limited time frame, LWI levels can be estimated under different percentages of dispersion and environmental diffusion. The scheme presented in Figure 1 is calculated based on considering different potential scenarios to estimate the potential levels of LWI. From these results, it can be inferred that only a small fraction of the total lead may pose a risk to humans, as in many cases, LWI would be higher than the estimated levels for humans 3000-5000 years ago and the discontinued adult LWI limit set in 2010.
Key Point 3: Strategies for Lead Fixation in PSCs
By encapsulating perovskite particles within hydrophobic organic materials (e.g., polystyrene), waterproof oxides (e.g., TiO2, SiO2, Al2O3), or insoluble lead salts (e.g., PbS, PbSO4, Pb(OH)2), the pathways for water ingress and ion efflux can be effectively blocked. The choice of a low-permeability covering material ensures strong hydrophobicity, high density, and complete coverage of perovskite grains. For example, in situ encapsulation of grain boundaries and surfaces can be achieved by introducing small molecule precursors either before or after perovskite crystallization, or by depositing hydrophobic molecules or functional salts (e.g., sulfonates, sulfates, sulfides) on top of the perovskite layer. Lead-containing perovskite films with well-controlled grain size distribution have demonstrated excellent water stability and potential applications as bioimaging scintillators, showing minimal cytotoxicity towards target organisms, suggesting reduced bioavailability. Additionally, the insertion of a waterproof layer into the internal or external encapsulation of PSCs can prevent water permeation. However, these methods may fail in the case of device damage. Although some self-healing properties can be conferred by mixing curable materials with sealants, the effectiveness of protection may be compromised as the curing of damaged sealants typically requires external stimuli such as UV radiation or heating.
- Lead Chelation
The strategy involves the addition of suitable additives that form low solubility complexes with lead ions (Pb2+), thereby reducing the solubility of lead compounds in perovskites. Typical additives should possess two electron-donating Lewis base functionalities (e.g., carbonyl, thiol, sulfonate, sulfide, porphyrin rings, crown ethers) that coordinate with the Lewis acidic Pb2+ ions through acid-base interactions. The hydrophobic backbone or side chains of the additives should contain hydrophobic moieties (e.g., long alkyl chains, fluorine groups, carbon nanotubes) to facilitate the precipitation of the formed complexes in water. Hence, the resulting complexes become hydrophobic after the chelation of ligands with Pb2+ ions. For example, the addition of carbon nanotubes grafted with polyacrylic acid (CNT-PAA) as additives in perovskite precursors effectively suppresses lead leakage in corresponding PSCs.
- Structural Integration
Integrating the perovskite structure within the device by improving the binding strength between constituent elements, interconnectivity within the bulk, and interfacial cohesion can increase the energy barriers for water permeation, structural collapse, and delamination, thereby enhancing the stability of the structure and preventing water dissolution and lead leakage. For instance, enhancing the interfacial/integrated bridging with strong coordination capability or dipole-dipole interactions has been demonstrated to be effective in counteracting crystal collapse and retarding lead release on the top surface of perovskite. However, this may be compromised in the case of device damage. Thus, the entire structure needs to be integrated, including the surface, bulk, and interfaces of the perovskite layer. The integration of perovskite grains can be achieved by introducing polymerizable monomers within the perovskite layer to construct perovskite/polymer matrices. For example, the addition of acrylamide monomers as additives in perovskite films allows the formation of polyamide through in situ polymerization, which undergoes transformation with over-coordinated Pb2+ ions on grain boundaries and perovskite surfaces, forming robust chelation structures within the deposited film. Moreover, polyamide readily forms hydrogels when exposed to water, further preventing Pb2+ dissolution and diffusion into the water. Additionally, the aggregation effect of monomers during polymerization can induce compressive strain within the perovskite layer, thereby increasing the activation energy for ion migration and the barrier for water permeation, improving the stability of the crystal under high humidity conditions. Furthermore, impregnating perovskite into rigid and mesoporous structures is also expected to prevent structural collapse.
- Lead Adsorption
Due to the direct correlation between the lead sequestration efficiency (SQE) and the density of adsorption sites, sufficient loading materials are required to ensure an adequate lead adsorption capacity. Therefore, implementing Pb adsorbents in the inner layers of the device may be insufficient as the ability for sequential removal is limited. Excessive insulating materials can reduce the electrode conductivity. Moreover, the thickness of the charge transport layer is typically only a few hundred/tens of nanometers, limiting the capacity to capture all Pb2+ in the perovskite film. A better approach is to embed lead adsorbents within the external encapsulation, avoiding limitations in the loading amount while maintaining device performance. For example, an excellent approach proposed by Li et al. involves depositing highly transparent Pb adsorbents on the top of the front glass without filtering the incident light and inserting a mixture of polymer sealants and Pb2+-binding materials between the rear electrode and the encapsulation lid. This chemical approach, where both sides exhibit significant lead adsorption capacity, can significantly reduce lead leakage by 96%. Furthermore, it is advisable to combine lead adsorbents with different activities under different temperature and pH conditions. For instance, lead adsorbents composed of phosphonic acid and methylene phosphonic acid groups can maintain a high lead sequestration efficiency (SQE) over a wide temperature range due to temperature-dependent deprotonation effects.
Figure 2: Lead Fixation Methods in PSCs
Key Point 4: Comparison of Lead Fixation Strategies in PSCs and Design of Lead Leakage Measurement Schemes
A systematic comparison was made among the four lead fixation strategies in terms of working mechanisms, protective effects, and their impact on device performance. It is noteworthy that internal lead fixation strategies (i.e., separation, chelation, integration) exhibit high selectivity and rapid response as Pb2+ ions are pre-protected before leakage. However, their lead sequestration efficiency (SQE) is relatively lower (approximately 60-80%). The lead fixation ability is related to the density of functional sites in the embedded additives, especially for chelation methods. However, most additives are insulating and, in some cases, light-absorbing, which can disrupt charge transport and photon capture, and the interaction between additives and Pb precursors can affect perovskite crystallization. Therefore, a trade-off may exist between PCE and SQE when the additive concentration exceeds the tolerance of perovskite materials. However, an appropriate amount of Pb-fixing additives can beneficially improve PCE and device lifetime, defined by the ratio of PCE and lifetime of the optimized device to the reference device, respectively. Grain encapsulation and chemical chelation methods may face challenges in lead recovery processes due to the inertness of grains and the insolubility of formed lead chelates, as lead recovery relies on the ease of extracting lead from the device. Additionally, the formation of a uniform coverage layer in the perovskite layer may be problematic in large-scale manufacturing as controlling the layer thickness is difficult, limiting the upscaling of PSCs. In these aspects, structural integration appears to be more promising, where the lead fixation ability is correlated with the structural stability of the additives rather than chelation sites, resulting in relatively high SQE (approximately 80%). In contrast, external implementation of lead adsorbents is more effective in suppressing lead leakage when SQE approaches 100%, as a large amount of material can be loaded without affecting device performance. However, this method still has some drawbacks that may reduce its effectiveness.
It should be noted that lead leakage in PSCs and its adsorption largely depend on the testing conditions, such as temperature, pH value, volume of exposed water, and modes of device damage. However, the reported SQE values in Table 2 were measured under completely different conditions. To quantitatively assess lead leakage in PSCs and compare the situations using different lead fixation techniques in various laboratories worldwide, it is necessary to establish a standardized lead leakage testing method supported by computational models. Furthermore, it is recommended to measure certain metrics, such as total leakage lead concentration (cLL), leakage rate (LR), and SQE, using standardized approaches, and simulate two exposure scenarios of perovskite under harsh weather conditions (acidic and heavy rain) as shown in Table 2 and Figure 3a (immersion and dripping). Additionally, lead leakage measurements should be performed using aged perovskite films rather than complete devices, with or without the presence of delamination sealants, to simulate the complete exposure of the perovskite layer to water. Furthermore, biological tests can be conducted to assess the impact of leaked lead on plant or animal growth.
Figure 3: Proposed Lead Leakage Measurement and Lead Fixation Device Structures
Research on lead-based PSCs has made rapid progress in terms of efficiency and stability. It is now time to further investigate how to implement this promising technology on a large-scale industrial level, considering sustainability, and avoid potential lead leakage throughout the long operational lifetime, from precursor preparation to solar panel deployment. Additionally, when deploying optoelectronic devices based on lead halide perovskites, in-depth occupational and local population risk assessments are needed to ensure prevention of lead leakage during operation and at the end of the device’s lifespan. This is not only a legal requirement but also an ethical obligation. Specific legislation regarding lead use can drive innovation in lead fixation and device recycling strategies. Moreover, emergency response plans should be established to reduce soil contamination by lead unintentionally emitted into the air during fire accidents. Furthermore, standardized testing should be conducted before PSCs are introduced to the market to assess potential risks of lead leakage.
Zhang, H., Lee, J. W., Nasti, G., et al. Lead immobilization for environmentally sustainable perovskite solar cells. Nature, 617, 687–695 (2023).