Semitransparent Selenium Solar Cells as a Top Cell for Tandem Photovoltaics

Trigonal selenium (Se) is an elemental, direct‐bandgap (1.95 eV) semiconductor with a low processing temperature, which could be a suitable top absorber for tandem solar cell applications. For incorporation in tandem architectures, both sides of the Se cell should be semitransparent. However, all reported Se solar cells have metallic back contacts. To demonstrate the potential feasibility of Se as a wide‐bandgap absorber for tandems, herein, bifacial single‐junction selenium solar cells with device areas above 0.4 cm 2 are reported. When illuminating through the n‐type contact, the bifacial cell power conversion efficiency (PCE) is 5.2%, similar to a standard monofacial cell. The efficiency is lower (2.7%) when illuminating through the p‐type contact, which is attributed to low carrier diffusion lengths and lifetimes in selenium. This suggests the necessity to invert the typical single‐junction device structure when incorporating it into a tandem device.