Lead-based perovskite solar cells (PSCs) have emerged as a promising next generation of solar cells, as their power conversion efficiency (PCE) has been rapidly improved over the last few years, exceeding 25.2 % for single cells and 29.1 % for perovskite/Si tandem cells (Figure 1). Lead is an essential element in PSCs to achieve the highest efficiency owing to the superior light absorption and charge generation properties; PCE of PSCs without lead-based light absorber has shown cut in half. However, the presence of lead, facing the its potential negative environmental, health, and safety (EHS) effects, could cause consumers’ concerns in PSC commercialization. There is a great need to design the lead management strategy including minimizing potential toxic materials, e.g. lead, leakage and recycling/reusing approaches degraded PSCs and lead-containing pollutants to valuable materials (cover glass, transparent conductive oxide, transport layer, lead iodine, etc.).
In our recent study, we observed and investigated that bio- and eco-friendly hydroxyapatite (Ca10(PO4)6(OH)2, HAP) inorganic nanoparticles could adsorb the lead ions (Pb2+) from non-aqueous solvents, not even aqueous solution. Also, this research has revealed the change of adsorption mechanism of lead by HAP from ion-exchange under the aqueous condition to surface charge attraction under non-aqueous conditions. In general, non-aqueous solvents have been used for preparing lead-based perovskite precursor for fabricating PSCs – it would be valuable technique to purify them directly at below-ppm levels and realized cost effectiveness. From this perspective, more strongly negatively charged HAP composites by decorating iron (Fe) on HAP surface enhance the electrostatic interaction on the HAP surface and enable an increase in the adsorption density of Pb2+ ions with higher ionic strength. In this study, after purification using our designed HAP composites, the residual Pb concentration in pollutant dropped below 15 ppb of Pb, meeting the standards of the U.S. Environmental Protection Agency (EPA).
In addition, we have figured out a facile strategy to reach zero-Pb-pollutant policy in PSC market; Pb-based law materials such as PbI2, that are main chemical for perovskite, were recollected by Pb-adsorbed HAP composites using solubility difference between HAP and PbI2. HAP particles generally can be fully dissolved in acidic solution, but PbI2 shows low solubility in water, even acid solution. We confirmed that the recycling yield of Pb raw materials from Pb containing pollutant is reached up to 99.97% using this reprecipitation process, and recycled PbI2 is comparable in quality to commercial PbI2. Thus, the Pb management process in our study enables a zero-lead-emission process for future lead-based perovskite manufacturing.
Our main contribution is the intended pathway for zero-Pb emission, which offers a practical solution to deal with lead-containing pollutant from PSCs by using bio- and eco-friendly adsorbents. Furthermore, we expect that our approach can provide renewable energy technology to relieve the concerns on lead-related environmental issues as well as the methodology for recycling and managing lead from waste devices.
The full article, Sustainable lead management in halide perovskite solar cells, via https://www.nature.com/articles/s41893-020-0586-6.