Modern Processing and Insights on Selenium Solar Cells: The World's First Photovoltaic Device

The first solid-state solar cells, fabricated approximate to 140 years ago, were based on selenium; these early studies initiated the modern research on photovoltaic materials. Selenium shows high absorption coefficient and mobility, making it an attractive absorber for high bandgap thin film solar cells. Moreover, the simplicity of a single element absorber, its low-temperature processing, and intrinsic environmental stability enable the utilization of selenium in extremely cheap and scalable solar cells. In this paper, a detailed study of selenium solar cell fabrication is presented, and the key factors that affect the selenium film morphology and the resulting device efficiency are presented. Specifically, the crystallization process from amorphous film into functional crystalline device is studied. The importance of controlling the process is shown, and methods to align the growth orientation are suggested. Finally, the crystallization process under illumination, which has general importance for the fabrication of thin film photovoltaics, is investigated. Specifically for selenium, the illumination significantly improves the film morphology and leads to device efficiency of 5.2%, with open-circuit voltage of 0.911 V, short-circuit current density of 10.2 mA cm(-2), and fill factor of 55.0%. These findings form a solid foundation for future improvements of the photovoltaic material and device architecture.