Elemental Se: fundamentals and its optoelectronic applications

Elemental selenium (Se), as the oldest photovoltaic material, has many advantages such as a direct fundamental band gap, earth-abundance, simple composition, nontoxicity, and a low melting point for optoelectronic applications. Both its crystalline and amorphous forms have been studied as an active layer in solar cells and detectors. In this review, the fundamental properties of Se, including the crystal structure, and optical and electronic properties, have been revisited. Then we separately summarize the advantages, progress and status of utilizing crystalline Se (c-Se) in solar cells and amorphous Se (a-Se) in detectors. Additionally, we briefly discuss the potential and remaining issues of c-Se solar cells as a top cell in tandem photovoltaic devices. We hope that this review could be helpful for researchers involved in the study of Se for optoelectronic applications.