Transparent conducting oxides (TCOs) have been used in several applications for three or four decades but, despite the huge volume of experience in the field, there remain many unanswered questions at both applied and fundamental levels. The reason that these issues have not been addressed, or only superficially so, is largely because the performance of the TCOs has been adequate to meet the demands of most applications considered thus far . For the purposes of the present document, the applications of concern are photovoltaic cells and modules, but the broader range of applications includes transparent electrode materials for both electrochromic cells and for liquid crystal display devices