Record Efficiencies for Selenium Photovoltaics and Application to Indoor Solar Cells

Until recently, Selenium photovoltaics remained overlooked and stalled with a last efficiency record of 5% established more than 30 years ago. We recently demonstrated a 30% increase in efficiency and a simultaneous reduction in thickness by a factor of 5x. Despite the expectation for continued improvements, selenium, with a bandgap of ~1.9 eV, is not well matched for a single junction outdoor AM1.5 spectrum, but selenium matches the indoor light spectrum of LED's and CFL's perfectly. In indoor lighting conditions we have achieved >13% efficiency which is more efficient than typical commercial a-Si and c-Si in indoor lighting, while fabrication is vastly simpler. Unlike alternative promising technologies such as perovskites, Se is both stable in air and has minimal toxicity risks. In this paper we will highlight the improvements in device structure and annealing that led to the recent record efficiency, and summarize the key variables and needs of solar cells for indoor applications.