Potential lead toxicity and leakage issues on lead halide perovskite photovoltaics
Recently, lead halide perovskite solar cells have become a promising next-generation photovoltaics candidate for large-scale application to realize low-cost renewable electricity generation.
Although perovskite solar cells have tremendous advantages such as high photovoltaic performance, low cost and facile solution-based fabrication, the issues involving lead could be one of the main obstacles for its commercialization and large-scale applications.
Lead has been widely used in photovoltaics industry, yielding its environmental and health issues of vital importance because of the widespread application of photovoltaics.
When the solar cell panels especially perovskite solar cells are damaged, lead would possibly leak into the surrounding environment, causing air, soil and groundwater contamination.
Therefore, lots of research efforts have been put into evaluating the lead toxicity and potential leakage issues, as well as studying the encapsulation of lead to deal with leakage issue during fire hazard and precipitation in photovoltaics. In this review, we summarize the latest progress on investigating the lead safety issue on photovoltaics, especially lead halide perovskite solar cells, and the corresponding solutions.
We also outlook the future development towards solving the lead safety issues from different aspects.
The photovoltaics (PV) industry has grown at unprecedented rates over the last two decades, with hundreds of thousands of PV systems installed worldwide. The cumulative installed electricity generation from photovoltaic technology has increased rapidly from only 805 GW h in 2000 to 549,833 GW h in 2018, while the annual and accumulative installed electricity capacities in 2019 were up to 97 GW and 578 GW, respectively (Padoan et al., 2019).
Compared with conventional energy technologies, PV is considered to be more environmental-friendly. It has been analyzed that, PV technology can help to reduce around 90% of the pollution associated with greenhouse gas emission, criteria pollutant emission, as well as emissions originated from heavy metals and radioactive species (Fthenakis et al., 2008).
Although the PV is generally considered as a form of green energy, the usage of lead in PV cannot be neglected with the fast development of photovoltaics due to the potential environment risks. Over the last decade, perovskite solar cells (PSCs) have become the most promising next-generation photovoltaic technology because of its unprecedentedly superb properties such as suitable bandgap, high absorption coefficient, well-balanced charge transfer, long carrier diffusion length and easy solution processing method (Cheng et al., 2021, Green et al., 2014).
Halide perovskite crystals have the general chemical formula of ABX3, where A sites are organic or inorganic cations like methylammonium (MA, CH3NH3+), formamidinium (FA, NHCHNH3+) and Cs+, B sites are divalent metal cations like Pb2+ and Sn2+, and X sites are halogen anions like Cl-, Br- and I-.
The latest certified efficiency of laboratory-scale (~ 0.1 cm2) PSCs has achieved 25.5% in 2020, exceeding that of other commercialized solar cells such as multi-crystalline silicon (mc-Si, 23.3%), thin-film solar cells of cadmium telluride (CdTe, 22.1%) and copper indium gallium selenide (CIGS, 23.4%) (Green et al., 2020, Kojima et al., 2009, NREL, 2021). Nevertheless, there is still much potential on efficiency improvement. In fact, the efficiency of single-junction PSCs is expected to approach 30% toward the theoretical value in the next decade by light management and defect control (Ma and Park, 2020, Park, 2019). Therefore, the emerging perovskite solar cell is becoming a more and more popular, potentially a game changer in the photovoltaics industry.
It usually requires at least 20-year operational lifetime with less than 10% efficiency drop in performance for marketable photovoltaic technology (Grancini et al., 2017). To improve both intrinsic stability and extrinsic environmental stability of perovskite, a lot of efforts have been devoted to compositional engineering, interface engineering, as well as developing all-inorganic perovskites and encapsulation methods, and thereby significant successes have been achieved (Boyd et al., 2019). To date, some perovskite solar cells and modules are capable of reaching an equal of 10-year lifetime, while the effective worktime of the most stable PSCs was estimated to 87 years, achieved by inorganic cation tuning and using FA-based perovskites (He et al., 2020, Turren-Cruz et al., 2018).
However, the lead toxicity of PSCs has become an emerging environmental issue for its practical application owing to the inclusion of Pb2+ (Mallick and Visoly-Fisher, 2021, Ravi et al., 2020, Sheikh et al., 2021).
Compared with great successes achieved to improve PSCs device stability, there is still a long way to go on exploring effective ways to address lead toxicity issue of PSCs.
One approach is the development of lead-free perovskites by replacing toxic Pb with other environmental-friendly cations such as Sn2+, Ge2+, Mg2+, V2+, Mn2+, Ni2+, Zn2+ and Co2+, but their performance and stability are far behind the lead-based PSCs (Hoefler et al., 2017, Jena et al., 2019).
Among them, tin has been explored as a less toxic alternative because of their similar ionic radii (1.35 Å for Sn2+, 1.49 Å for Pb2+) and comparably good semiconductor characteristics (Jena et al., 2019, Ke et al., 2019). However, they are not as stable and efficient as Pb-based perovskites owing to the easy oxidation of Sn2+ to Sn4+ in ambient environment (Konstantakou and Stergiopoulos, 2017). So far, the highest efficiency of FASnI3 based PSCs was up to 14.81%, achieved by the introduction of 4-fluoro-phenethylammonium bromide based 2D tin-perovskite capping layer (Yu et al., 2021). To date, high efficiency PSCs were achieved by lead-containing perovskites (Min et al., 2021), and Pb-based PSCs would be more likely to be the first to enter the PV market for large-scale production (Lyu et al., 2017, Park and Zhu, 2020, Wang et al., 2019). Therefore, it is vital and necessary to assess the toxicity of Pb-based PSCs and find some ways to solve the problem before it goes to the market.
In this review, we summarize the use of lead in photovoltaics, especially its application and toxicity issue in lead halide perovskite solar cells. We also discuss the potential lead leakage related issues in midlife and end-of-life of Pb-based PSCs and summarize the corresponding solutions to manage lead loss, including designing fail-safe encapsulation and device structure and making cyclic utilization of lead in end-of-life PSCs. In the last, we also outlook the prospects of potential approaches for lead management of Pb-based PSCs.
Lead issue in photovoltaics
Lead is a toxic heavy metal and is generally found in bedrock and soils in the mineral form of ore galena (lead sulfide, PbS) with a natural concentration of 12–20 ppm, while the amounts in stream water and sea water are pretty low with 0.03 μg/L and 1.0 μg/L, respectively (van der Voet et al., 2013). Lead level in atmosphere had experienced an increase because of the use of leaded petrol, but it eventually dropped once lead was phased out of petrol and related products (Tsai and Hatfield, 2011
Lead leakage in PSCs
In general, lead in PSCs would leach from damaged encapsulation at three stages throughout their entire lifetime: (1) panel production, (2) panel use in midlife, and (3) end-of-life disposal of PSCs. Among three stages, the lead leakage mechanism and solutions in midlife and end-of-life stages will be the focus on our discussion. In midlife, the PSCs may suffer from mechanical load cycles such wind and snow loads and temperature changes which may cause microcracks or breakage to panels (IEA
Fail-safe encapsulation and device structure
Recent years, researchers started looking at the safety issue of the potential lead leakage in the case of damaged encapsulation and several solutions have been proposed, as shown in Fig. 6. One is to enhance the physical encapsulation to better protect the device, for example, a self-healing coating could heal itself if there is a scratch. The other is chemical approaches by introducing lead adsorbents in device structure. Even if the encapsulation is failed, lead can be captured by the
Summary and outlook
As discussed above, the overall lead content used in PSCs is actually pretty low.
However, considering the high toxicity of lead, the negative impacts on animal and human health, as well as on plant growth and the entire ecosystem are still non-negligible.
In the case of damaged encapsulation, the majority of lead in PSCs would be washed off into environment by rain. Furthermore, the improper disposal of end-of-life PSCs also causes lead leaching problems. Though catastrophic encapsulation
CRediT authorship contribution statement
Meng Ren: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Resources, Writing – original draft, Writing – review & editing. Xufang Qian: Conceptualization, Methodology, Resources, Supervision, Writing – review & editing. Yuetian Chen: Methodology, Supervision, Writing – review & editing. Tianfu Wang: Supervision, Writing – review & editing. Yixin Zhao: Conceptualization, Formal analysis, Methodology, Resources, Supervision, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
This work was supported by the National Natural Science Foundation of China Grant (22025505, 51861145101), Program of Shanghai Academic/Technology Research Leader (Grant no. 20XD1422200) and Cultivating Fund of Frontiers Science Center for Transformative Molecules (2019PT02).
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