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High Performance Wide Bandgap and Stable Perovskite-on-Silicon Tandem Solar Cells

Periodic Reporting for period 1 - POSITS (High Performance Wide Bandgap and Stable Perovskite-on-Silicon Tandem Solar Cells)

Berichtszeitraum: 2017-06-01 bis 2019-05-31

The world is moving toward a low-carbon future. The EU Commission’s 2015 Paris climate agreement vowed to keep global average temperature to well below 2°C above pre-industrial levels and undertake rapid global emission reductions. Generation of clean energy such as photovoltaics is one of the most promising solutions. Novel organic inorganic perovskite materials for solar cells have drawn vigorous scientific interest over the past few years due to their potential low cost, high charge carrier mobility, absorption coefficient and efficiency. The two top scientific journals, Nature and Science, selected perovskites as one of the biggest scientific breakthroughs in 2013. As perovskites are still in the early development stages, barriers need to be overcome before they can be commercialized. This interdisciplinary project draws from chemistry, physics and engineering with the aim of developing stable high bandgap mixed-cation/halide perovskite devices. Both photo- and moisture stability will be carefully studied and improved upon leading to the ultimate goal of fabricating a stable monolithic perovskite/silicon tandem solar cell with an efficiency >30%. Monolithic perovskite/silicon tandem technology has the potential to revolutionize the photovoltaics industry by greatly decreasing cost through improved efficiency making clean energy more competitive against fossil fuels. The project will be conducted at EPFL PV-Lab, a dedicated photovoltaic research centre with over 30 years of experience and close links with Swiss and European research institutes and industries. EPFL is a highly-ranked European engineering institution with world-class facilities, industrial connections and technological parks. The fellow Dr. Terry Chien-Jen Yang will be joining PV-Lab to bring unique nano-material and analytical expertise as well as collaborations from the prestigious School of Photovoltaic and Renewable Energy Engineering, UNSW in Sydney, Australia.
The work was carried out by the fellow T.C.J. Yang of the beneficiary, PV-Lab, EPFL at the Institute of Microengineering in Neuchatel, Switzerland during the period (1st June 2017 to 31st May 2019). The project results were in-line with the objectives, milestones and deliverables with some slight deviations. Project results established that photoinduced halide phase segregation is a serious issue that needs to be tackled immediately. Many research groups elsewhere are now focusing efforts to mitigate this detrimental phenomenon. For the final objective, it was shown that it is possible to create stable current-matching 2-terminal perovskite/Si tandem solar cells on textured silicon cells. However, the efficiency target of >30% is still very difficult to obtain mostly due to the photoinduced halide phase segregation phenomenon, which limits the open-circuit voltage of the higher bandgap (optimally ~1.7 eV) perovskite top cell beyond the state-of-the-art. Another key area to progress the perovskite/Si technology is to move onto upscalable deposition techniques for conformal coatings on top of textured silicon solar cells, which means the elimination of the solution spin-coating step mainly used in the first half of the project.
In summary some key points of the impact of the POSITS project as a result of the efforts of the fellow Dr. T.C.J. Yang and colleagues from PV-Lab, CSEM and other groups within and outside of EPFL for the perovskite and perovskite/Si tandem solar cells field include:
• The review of various perovskite materials that could potentially improve on stability and reduce photoinduced halide phase segregation.
• The access to optical data, including complex refractive indices of CsxFA1-xPbI3-yBry perovskites with optical bandgaps from 1.5 to 1.8 eV, for researchers working on modelling/simulations.
• The demonstration that lead-halide perovskites can have strong intolerance to certain impurities such as Bismuth.
• Temperature dependent PL of perovskites and the freezing out of halide migration at low temperatures showing that it is a diffusion related process.
• The modelling of diffusion mechanisms related to the photoinduced halide phase segregation effect.
• High photoluminescence quantum yield as measured by absolute photoluminescence can be reached for bare planar perovskite films on fused silica glass.
• The use of the 2-step hybrid deposition technique to deposit conformal perovskite film on top of textured silicon solar cells.
• The importance of using textured rather than flat silicon bottom cells due to the gain in short-circuit current. This is where we believe the perovskite/Si field should move towards in the future.
POSITS Research Summary Figures