Photoelectrowetting is a modification of the wetting properties of a surface (typically a hydrophobic surface) using incident light. Whereas ordinary electrowetting is observed in surfaces consisting of a liquid/insulator/conductor stack, photoelectrowetting can be observed by replacing the conductor with a semiconductor to form a liquid/insulator/semiconductor stack. This has electrical and optical properties similar to the metal/insulator/semiconductor stack used in metal-oxide-semiconductor field effect transistors (MOSFETs) and charge-coupled devices (CCDs). Replacing the conductor with a semiconductor results in asymmetrical electrowetting behavior (in terms of voltage polarity), depending on the semiconductor doping type and density.
Incident light above the semiconductor's band gap creates photo-induced carriers via electron-hole pair generation in the depletion region of the underlying semiconductor. This leads to a modification of the capacitance of the insulator/semiconductor stack, resulting in a modification of the contact angle of a liquid droplet resting on the surface of the stack in a continuous way. The photoelectrowetting effect can be interpreted by a modification of the Young-Lippmann equation.