Rasmus Nielsen
Selenium, the world’s oldest photovoltaic material, has experienced a renaissance in research over the past decade, with certified solar cell efficiencies climbing from the historical record of 5% to breaking the 10% barrier. Its wide bandgap makes it a particularly interesting candidate for tandem solar cells and indoor photovoltaic applications, yet despite steadily improving the carrier collection, devices consistently suffer from a substantial open-circuit voltage deficit. This review provides a critical analysis of the material properties and optoelectronic quality of state-of-the-art selenium thin films. Published results from independent groups are digitized and directly compared, collectively painting a comprehensive picture of the carrier dynamics, supported and contextualized by drift-diffusion simulations. Strategies for synthesizing and processing selenium thin films are also examined in detail, highlighting not only best practices but also the underlying crystal growth kinetics that ultimately govern material quality. Finally, a series of open questions and challenges is presented, spanning from fundamental materials science and atomic-scale defect physics to device-level engineering, providing a roadmap to unlock the intrinsic photovoltaic potential of selenium and guide the future development of higher-efficiency selenium solar cells.