The EU is transitioning to a clean, affordable, sustainable and competitive energy system. Renewable energies such as solar power play a key role in driving this transition and reducing the EU’s dependence on fossil fuels. To maximise the renewable share of energy supply in building and urban street sectors, photovoltaics (PV) must be further deployed. “When possible, deployment of solar panels into constructed surfaces is preferred, to avoid the artificialising of open fields or land use/reduction of natural spaces. However, the integration of PV modules is not always an aesthetic success, resulting in social acceptance issues from both decision-makers and/or the public,” notes Frédéric Sauvage, research director at the French National Centre for Scientific Research (CNRS) in France and project coordinator. This is because PV technologies are opaque or semi-transparent and can only be installed on roofs or in non- or semi-transparent facades. Enter the EU-funded IMPRESSIVE project, which took on the challenge of developing transparent PV cells that can be integrated on a large scale, as windows in buildings, for instance. “Our aim was to develop PV technology that is totally non-intrusive: aesthetically pleasing, totally transparent and colourless, while being efficient and low-cost,” adds Sauvage.
Large-bandgap perovskite and dye-sensitised solar cells
IMPRESSIVE’s intention was to have the PV technology selectively convert light beyond the human eye’s sensitiveness. To achieve this, the project developed two absorbers: an efficient UV absorber based on perovskite solar cells; and a near-infrared (NIR) absorber based on dye-sensitised solar cells. These paved the way for a fully transparent and colourless PV tandem display. “When combined, these technologies convert the sun’s energy with a power conversion efficiency (PCE) of 14 % and a level of average visible transmittance (AVT) greater than 55 %. The separate technology allows attainment of a PCE up to 8 % and an AVT level up to 80 % which correspond to the same appearance as that of a double-glazing window,” confirms Sauvage.
A window producing a fully decarbonised electricity
IMPRESSIVE demonstrated that semi-transparent UV perovskite can reach more than 10 % PCE with an AVT of approximately 60 %. “We showed, through our specific composition and device architecture, the possibility to pass the IEC61646 accelerated ageing test protocol, which is an important bottleneck solved towards industrialisation,” highlights Sauvage. In parallel, the CNRS has patented a new family of new NIR-selective sensitisers affording higher performance and stability and reaching excellent transparency when integrated into specifically optimised dye-sensitised solar cells. “Our PV window allows the reduction of air conditioning during summer and heating during winter in buildings and domestic houses, owing to the internal technology which has specific absorption in one part of the NIR (800-1000 nm) and reflectivity in the other NIR region (> 1000 nm). We offer a window producing fully decarbonised electricity by making use of sunlight,” concludes Sauvage. Compared to currently produced PV technology, which requires high-quality single crystals including multiple purification steps under vacuum and/or high temperatures (> 1000 °C), IMPRESSIVE’s technology is solution process and thus eco-friendlier as regards to energy payback time, less than 3 months, and CO2 emissions. Towards replacing passive windows by electricity-producing windows, the project is actively working to continue improving the performance of the two technologies and to bring the maturity of this clear PV technology towards industrialisation.
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