Periodic Reporting for period 2 - PERTPV (Perovskite Thin-film Photovoltaics (PERTPV))
Reporting period: 2019-10-01 to 2021-09-30
There has been significant advancement with combining perovskite with silicon cells, to deliver a “tandem” junction cell with much higher efficiency than either sub-cell. This “perovskite-on-silicon” technology has already been certified at closet to 30% efficiency for 1cm2 cells, demonstrating a clear path toward moving well beyond the efficiencies achievable with today’s terrestrial PV technologies. Furthermore, PerTPV partner Oxford PV is ramping up towards volume manufacturing, illustrating a high technology readiness for this tandem technology. Although the perovskite-on-silicon approach is likely to deliver the first perovskite PV products, it restricts the manufacturing and module format to wafer based and welds the perovskite technology to wafer based silicon, which is considered to have a relatively high embodied energy.The PerTPV project aimed to deliver perovskite-on-perovskite thin-film tandem cells, on both rigid glass and flexible substrates.The PerTPV consortium consists of the leading academic groups in perovskite PV research, commercial research institutes, and three commercial partners at appropriately complementary stages in the value chain (technology driver, materials supplier and equipment supplier). In addition to the consortium’s ambitious target to surpass 30% power conversion efficiency in a thin film all-perovskite tandem cell and delivering a stable module technology, it also performed full life cycle analysis to ensure a safe means to undertake mass deployment and recycling of the perovskite PV modules.
Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far
The project was divided into 5 technical work packages (WPs); WP1. Materials advancement, WP2. Single junction devices, WP3.Tandem devices, WP4. Environmental health, safety and end-of-life recycling and WP5. Modules and product assessment.The first period of the project has been spent with focussed effort on WP1 and WP2. In the tandem cells, the perovskite solar absorber materials are required to absorb specific regions of the solar spectrum, so that when the cells are stacked on top of each other, their absorptions are complementary. This is the basis for enhancing the efficiency in a tandem cell.In WP2 we identified precise ionic compositions of the perovskite solar absorber materials required for these cells and demonstrated improved efficiency of the single junction devices. We also demonstrated routes to significantly enhance the long term stability of the perovskite solar cells under combined heat and light, and demonstrated encouraging stability for the low band gap tin-based perovskite absorbers. In WP3, we designed the tandem device stacks and identified the materials required for all the layers, including the critical “recombination layer” which is located between the front and rear cell in the tandem device stack. In the second period, we realised highly efficient tandem cells. In parallel to our activity advancing the cell technology an materials choice, we have been developing processing routes which are industrially compatible.This includes the development of “clean” inks, which for instance do not rely on toxic solvents, as well as the optimisation of large area coating methodologies via for example, graver printing on flexible substrates or thermal evaporation. In WP4, we made assessments of the life cycle analysis of different methodologies for the production of perovskite PV modules and developed an end of life recycling protocol. The much talked about presence of lead in the perovskite absorber layer, has only a very small contribution to the international reference life cycle data. The main drivers are energy consumed in the production of materials and devices. In WP5, we have advanced the thin film module technology. For module production, uniform deposition of all the layers is made over the entire substrate and then the module is scribed into thin “cells” (order of 1cm in width) and interconnected electronically. The ratio between the width of the cells and the region of interconnect between the cells gives the so-called geometric fill factor of the module. We have improved the laser scribing technology within PertPV to minimise this interconnection region to below 300 micrometres, which will lead to over 95% geometric fill factor. We successfully integrated the tandem cells into modules.
Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)
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.