We have identified ideal perovskite compositions for the wide, middle and low band gap perovskites.Key advancements have been made in the long term stability of both the Pb based wide band gap perovskites and the low band gap Pb:Sn perovskite absorber materials and solar cell devices. We have devised new tandem solar cell device architectures which we have theorised to result in efficiency well beyond the present state-of-the-art. The practical challenges of fabricating tandem cells meant that the absolute efficiency was not as high as targeted, but significant learning was gained which will lead to the delivery of significant performance improvements beyond the lifetime of PerTPV. Our LCA studies revealed that the presence of Pb in the perovskite absorber material only contributes a very small amount to the overall environmental and toxicological impact,as judged by the international reference life cycle data analysis. Environmental fate studies have also revealed that rapid and efficient sequestration of Pb into the soil, will result in less transport as well as lower bioavailable share, decreasing its adverse environmental effects, in the worst-case scenario event of Pb leaching into the environment.Real leaching tests on modules showed no Pb leaching over 9 months outside for undamaged modules, and less than 1% of the Pb in the modules leached over 9 months for completely cracked up modules.This very small leaching of Pb should be put into context with the worst case scenario, where we assume 100% of the Pb leaches out into 2.5cm thick top soil trench beneath the module. In this worst-case scenario, the total concentration of lead in the soil remains considerably below the threshold for concern. Since a very small fraction of this lead would be bioavailable, and less than 1% of this lead would actually leach into the soil in the case of a real cracked module left in the field for 9 months, the possible negative environmental impact of the leaching of Pb into the environment during the use phase is negligible.Concerning mini-module development, we have reduced the interconnection region between the cells, which are defined by laser etching, resulting in an increase in geometric fill factor, current density and efficiency in the mini-modules. Single junction minimodules with efficiencies of over 20% were realised.Tandem modules with ~30% relative drop in performance with respect to the small cells was also realised. In summary, we are on track to deliver a game-changing thin-film technology, which is highly efficient, stable and manufacturable. Further challenges still remain with increasing the reproducibility, stability and efficiency of the low band gap perovskite, which presently limits the performance of the tandem cells.Future work should include producing the low band gap perovskite via scalable manufacturing methodologies, such as thermal evaporation, further enhancing the stability of the low band gap perovskites, and then scaling up the manufacturing processes for both the single junction and tandem thin-film calls and modules.
