Selenium is a good conductor of electricity in presence of light. This phenomenon is called photoconductivity. How does this phenomenon occur? Are there any other elements or compounds in which this phenomenon is noted?
A very handy diagram and explanation of photoconductivity is shown below, from the American Physical Society page Viewpoint: Holey Intrinsic Photoconductivity:
The caption explains:
Excitation process leading to photoconductivity in a condensed matter system (upper panel) and its cold-atom counterpart (lower panel). In the condensed matter system, incoming laser light ($hv$) excites an electron to the valence band, leaving a positively charged hole behind. In the cold-atom analogy, the optical lattice is tuned to allow an atom to move into the excited band, also leaving a hole behind.
From the Wikipedia page Photoconductivity, states that some 'classic' examples of photoconductive materials (asides from selenium) include:
Some more examples exist in the paper Organic photoconductive materials: recent trends and developments include the chemical classes:
- Azo pigments
- Perylene pigments
User15489(Santiago)'s answer explains the phenomenon of photo-conductivity in a general way. My answer typically describes the photoconductivity of selenium.
The following is an excerpt from the book Weller, M.; Overton, T.; Rourke, J.; Armstrong, F. Inorganic Chemistry, 6th ed:- (see link)
Selenium exhibit both photovoltaic character, where light is converted directly into electricity, and photoconductive character. The photoconductivity of grey selenium arises from the ability of incident light to excite electron across its reasonably small band gap (2.6 eV in the crystalline form, 1.8 eV in the amorphous form). These property make selenium useful in the prdouction of photocells and exposure meters for photographic use, as well as solar cells.