A Stable Chalcogenide Top Cell for Silicon-Based Tandem Photovoltaic Devices

Solar photovoltaics has become the most important renewable energy technology worldwide. It is forecast that 2.35TWp will be deployed by 2027. The extraordinary growth of solar over the past decade has been driven by cost reductions. However, it is now clear that the efficiency of single junction solar cells is approaching its limit. The next step change in solar photovoltaics will be the development of tandem cells capable of large-scale manufacturing. Perovskite-on-silicon tandem cells have been developed and are now commercially available. However, concerns remain about the long-term stability of the perovskite top cell especially for application in large scale utilities. Here we propose to develop cadmium selenide as a photo-absorber as an inorganic and stable top cell. Cadmium selenide is an n-type semiconductor with a bandgap of 1.72 eV which is ideal for use with a silicon bottom cell. The bandgap is similar to a perovskite. Cadmium selenide is already used as a precursor layer to form an CdSeTe alloy at the front of cadmium telluride solar cells. The challenge will be to maximise the photoactivity of cadmium selenide and to develop suitable contact electrodes to extract the charge carriers. The objective is to create a 4-terminal and then a 2-terminal tandem solar cell on silicon with a conversion efficiency exceeding 25%. This will establish proof of principle. The materials used in cell fabrication will be sustainable and the stability of the devices will be proven using accelerated environmental laboratory tests and real outdoor trials at a number of locations. A cadmium selenide-on-silicon tandem will de-risk the possible stability issues with perovskites but will also open up new application areas where high irradiance and high temperatures are common.