Transparent conducting oxides (TCOs) are an increasingly important component ofphotovoltaic (PV) devices, where they act as electrode elements, structural templates,and diffusion barriers, and their work function controls the open - circuit device voltage.They are employed in applications that range from crystalline - Si heterojunction withintrinsic thin layer (HIT) cells to organic PV polymer solar cells. The desirable charac -teristics of TCO ma te rials that are common to all PV technologies are similar to therequirements for TCOs for flat - panel display applications and include high optical trans -missivity across a wide spectrum and low resistivity. Additionally, TCOs for ter res trial PVapplications must use low - cost ma te rials, and some may require device - technology -specific properties. We review the fundamentals of TCOs and the matrix of TCOproperties and proc essing as they apply to current and future PV technologies.