If one is asked to tell what the hottest photovoltaic material is in recent years, the answer will mostly be perovskites, or metal halide perovskites. The extremely high power conversion efficiencies and solution processability make perovskite photovoltaics a promising solar energy technology. There has been a worldwide upsurge of research interest in this field, and a number of companies are competing to commercialize this solar technology. However, are perovskite photovoltaics ready for commercialization? This turns out to not to be an easy question to answer. Although both efficiencies and stabilities of perovskite solar cells have been significantly improved in the past ten years, the toxic lead (Pb) involved in lead halide perovskites, once leaking out of broken perovskite modules under extreme weather conditions like hail and storm, will cause serious contamination on soil and underground water. The perovskite PV community is always challenged by such a concern, and whether Pb toxicity issue can be well-addressed or not, probably determines the fate of perovskite photovoltaics, or will curb their applications, as most countries or regions still have strict regulations on lead contamination. Replacing lead using other metals always results in lower efficiencies or poorer stability, while sophisticated encapsulations make perovskite photovoltaics blows away its key advantage of being low cost. A solution to this problem is urgently needed before the commercialization of perovskite solar technology.
The researchers in perovskite photovoltaic community have started to pay attention to such a critical issue. Recently, coating lead-adsorbing materials onto the two sides of perovskite solar devices has been reported to reduce the lead leakage when perovskite devices were damaged under extreme weather conditions like hail and storm. A lead-adsorbing layer on the front glass surface can indeed adsorb lead ions that leak out of broken modules through cracks, but it suffers from several outstanding drawbacks that may limit its application. First, the long-term exposure of the lead adsorbing layer to rainwater containing various metal cations, like Pb2+, Cd2+, Ca2+, or Mg2+, can saturate and defunctionalize the lead-adsorbing layers. Besides, the surface lead-adsorbing layer can be damaged by UV radiation in sunlight, or cleaning processes to remove sand or dust from perovskite solar modules. Regarding these, the surface coating layer on glass, besides showing excellent lead-adsorbing capacity, should also have multiple functions or properties of scratching resistance, UV stability, etc, which greatly limits the choices of available materials.
An ultimate and sustainable solution to this problem was demonstrated by a research group in University of North Carolina at Chapel Hill who are working on stabilizing and scaling-up perovskite solar devices. Huang and his colleagues are naturally interested in solving this long-standing challenge so that perovskites PV technology can be a real impact to a clean and sustainable energy solution. Inspired by the water purification industry, Dr. Shangshang Chen, the first author of this work, chose a low-cost and abundant cation exchange resin as the lead-adsorbing materials to minimize the lead leakage from damaged perovskite modules. Different from the previous strategy of coating lead-adsorbing materials onto the front surface of glass substrates that is vulnerable to the contamination caused by rain or dust, they embedded the mesoporous lead-adsorbing resins as a scaffold for perovskite layers. Such a new device structure, on the on hand, can well get rid of the aforementioned contamination issue; on the other hand, the mesoporous resin scaffold was found to have no detrimental effects on film crystallinity and device performance. Most importantly, the mesoporous resins in perovskite layers are more effective in reducing lead leakage in simulated water dripping tests than that coated on the front surface of glass substrates. Further coating another resin layer onto the top of metal electrode is able to reduce the lead leakage concentration to 11.9 ppb under the simulated heavy rainfall condition (50 mm h-1).
To learn more about this story, please refer to the paper by Chen et al. “Preventing lead leakage with built-in resin layers for sustainable perovskite solar cells”, Nat. Sustainability (2021), https://doi.org/10.1038/s41893-021-00701-x.