What Is Selenium?

Selenium is a nonmetallic chemical element with atomic number 34 and the symbol Se, situated between sulfur and tellurium in Group 16 of the periodic table. It is a semiconductor whose electrical conductivity increases markedly under illumination, a property called photoconductivity that gives selenium a central place in the history and practice of electronic devices. The element was discovered by Swedish chemist Jöns Jacob Berzelius in 1817 as a contaminant in sulfuric acid production and was later found to exhibit photoconductive behavior in 1873, which initiated a century of investigation into solid-state light-to-electricity conversion.

Selenium exists in several allotropic forms. Gray trigonal selenium, the most thermodynamically stable form, consists of helical chains of atoms arranged in a hexagonal crystal structure. Amorphous selenium is a glassy solid that can be deposited as a thin film and is more practically important for most device applications than the crystalline form. Red and black allotropes also exist but are less technically significant. The element draws on solid-state physics, chemistry, and materials science, and its properties bridge those of metals and insulators in ways that make it useful across a range of electronic and optoelectronic applications.

Electronic and Optical Properties

Selenium behaves as a p-type semiconductor with a band gap of approximately 1.9 eV in its crystalline trigonal form, shifting to roughly 2.0 to 2.2 eV in the amorphous state. This wide band gap places its optical absorption edge in the visible spectrum, making it sensitive to blue and green light while transmitting red and infrared. The photoconductivity of selenium is several orders of magnitude higher in illuminated conditions than in the dark, enabling it to function as a light-controlled switch in photovoltaic and imaging devices. The article on selenium as a critical chemical element in nano and quantum physics reviews how selenium's electronic structure underpins its optical sensitivity and discusses quantum confinement effects in selenium nanocrystals and nanorods.

Photovoltaics and Thin-Film Solar Cells

Selenium was used to fabricate the world's first solid-state photovoltaic cell in 1883, making it the oldest material with a documented solar energy conversion history. Interest in selenium photovoltaics declined when silicon emerged as a more efficient alternative, but the material has regained research attention for applications where its spectral properties are advantageous. Its band gap near 1.9 eV is well matched to indoor lighting spectra, and selenium-based solar cells have demonstrated efficiencies exceeding 20% under standard indoor illumination conditions. Recent research on selenium thin-film photovoltaics reports that ordering the one-dimensional chain structure of trigonal selenium significantly improves charge transport and conversion efficiency, with certified outdoor efficiencies reaching 10% in optimized devices.

Xerography and Imaging

Amorphous selenium became the dominant photoreceptor material in electrophotography, the process underlying laser printers and photocopiers, through most of the twentieth century. In a xerographic drum, a uniform electrostatic charge is deposited on the selenium layer; light reflected from a document selectively discharges the illuminated areas, leaving a latent charge image that attracts toner particles. Selenium alloys with arsenic and tellurium were developed to extend photosensitivity across the full visible range and improve mechanical durability. The photoconductor's combination of high dark resistivity and rapid photoresponse made it technically superior to earlier organic and inorganic alternatives. Research on amorphous selenium-based xerographic photoreceptors characterizes the charge transport properties that determine image resolution and drum longevity.

Applications

Selenium has applications in a wide range of disciplines, including:

• Thin-film solar cells and indoor photovoltaics
• X-ray flat-panel detectors for digital radiography and mammography
• Electrophotographic drums in laser printers and photocopiers
• Glass and ceramics manufacturing as a decolorizing and coloring agent
• Semiconductor device fabrication as a trace dopant
• Biological and nutritional research as an essential trace element