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Perovskite Thin-film Photovoltaics (PERTPV)

Periodic Reporting for period 1 - PERTPV (Perovskite Thin-film Photovoltaics (PERTPV))

Reporting period: 2018-04-01 to 2019-09-30

Solid state lead halide perovskites have recently emerged as the latest thin-film photovoltaic device class. High power conversion efficiencies (up to 25%) and stabilities (> 1000 hours at 80˚C under one sun illumination) have been obtained using lab scale processes and small area cells (<1cm2). The building blocks of the perovskite materials are very low cost and the ability to process into the final perovskite thin-film with low temperature fast processes. This makes these materials very cost efficient and promises to deliver a future PV technology with a levelled cost of electricity (LCOE) below that of existing mainstream PV.
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 28% 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 hence misses out the future promise of manufacturing via large area sheet-to-sheet or reel-to-reel coating, and weds the perovskite technology to wafer based silicon, which is considered to have a relatively high embodied energy.

The PerTPV project, aims to advance the perovskite thin-film PV technology to the next level by undertaking a “double pronged” drive on both performance (efficiency and stability) and the development of scalable device and module fabrication methodologies, compatible with high volume manufacturing. This project aims to deliver perovskite-on-perovskite 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 certifiably stable module technology, it will also perform full life cycle analysis and ensure a safe means to undertake mass deployment and recycling of the perovskite PV modules.
The project is divided into five 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 have identified precise ionic compositions of the perovskite solar absorber materials required for these cells and demonstrated improved efficiency of the single junction devices. We have 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 absorber.

In WP3, we have 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 will realise the highly efficient tandem cells. In parallel to our activity, 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 have made first assessments of the life cycle analysis of different methodologies for the production of perovskite PV modules and end of life recycling. 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 are now well set to integrate the tandem cells into modules, once the technology is transferred from WP3.
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. These will be reduced to practice in the second period.

We have followed two approaches for ink development and both are advancing well without the requirement for the use of dimethylformamide, which is a toxic solvent highly regulated in manufacturing.

Lifecycle analysis has confirmed the relatively low environmental impact of thin-film perovskite PV technology. It is also clear than 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.

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. In the next period we will progress the module development from single junction to tandem modules, and perform rigorous stability assessments upon encapsulated mini-modules.

In summary, we are on track to deliver a game-changing thin-film technology, which is highly efficient, stable and manufacturable. The second period will see effort shifting towards advancing the efficiency of the tandem cells, module development and product assessment.
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