Elemental Selenium as a Wide Bandgap Photoabsorber Appropriate for Tandem Integration with Silicon or CIGS

Tandem solar cells featuring Si or Cu(In,Ga)Se2 as the low bandgap photoabsorber (Eg ≈ 1.1 eV) would likely be of higher commercial relevance than the ideal combination of 0.9 and 1.6 eV. However, to match silicon in a tandem device the ideal top-cell photoabsorber should have a bandgap close to 1.75 eV, and only few candidates in this range have shown high photovoltaic performance. We suggest elemental selenium as a candidate due to its wide bandgap of 1.95 eV in its trigonal phase, its inexpensive single-element composition, its high absorption coefficient in the visible region (α > 105 cm-1), and its low melting point ~220C making it process-compatible for low-cost monolithic integration with most bottom cell material candidates. We have successfully fabricated single-junction selenium solar cells with efficiencies exceeding 5%, and we are currently benchmarking the stack design and fabrication of a monolithically integrated tandem device. We are using a variety of techniques to characterize each layer of the device including PES, XRD, Cryo-PL, THz spec., etc. and we are collaborating closely with computational theoreticians doing DFT calculations on our system. We aim to use the Se/Si-tandem to charge redox species in an integrated solar-driven redox-flow battery, as well as a parallel project in which the CO2 reduction reaction is studied photoelectrochemically.