Selenium – A non-toxic and stable material for future tandem solar cells

Objective

Clean energy is crucial for a carbon neutral European continent, and wind, hydropower, geothermal, biomass, and solar energy conversion will generate hundreds of billions economic activity in the coming years. Among these, photovoltaics (PV) plays a crucial part in electricity generation directly transformed from sunlight. Recently, the efficiency of the dominant PV technology, Si, has reached over 26%, which is near the 29.4% theoretical efficiency limit for single-junction solar cells. In order to overcome this limit, the tandem concept that minimizes thermalization losses of photo-excited carriers has been successfully proven with multi-junction cells, where different band gaps are stacked in series. Over the past decades, the main challenge has been the lack of an efficient, long-term stable, low cost, and process compatible top sub-cell. Recently, selenium (Se) became an attractive option because of its suitable high bandgap , feasible process at low temperature, and reported efficiency. This proposal (SeNTASC) aims at developing selenium absorber photovoltaic (Se PV) top subcell stacking with high-efficiency Cu(In,Ga)Se2 (CIGS) bottom subcell toward long-term stable and high-efficiency tandem solar cells by implementing novel approaches in buffer layer bandgap modulation, advanced hole-selective metal oxide layer modification, and finally achieving an inverted bifacial Se PV. The above-mentioned strategies will lead to ~10 % efficiency with open circuit voltage over 1 Volt with bandgap around 1.95 eV in an inverted superstrate configuration. Reaching these objectives, Se will be established as an excellent candidate for a top subcell in a tandem with high-quality CIGS solar cells. In this configuration, a maximum theoretical efficiency near 40% is expected.