Selenization is an essential reaction for the production of the metal selenide absorption layer in compound semiconductor thin-film solar cells. Application of supercritical ethanol facilitates the conversion from a Cu-In film to a CuInSe2 film at a low temperature of 300 degrees C. In this study, we investigate the contribution of supercritical alcohols to the selenization reaction by employing some kinds of supercritical alcohol media with different reduction powers. We found that the high reduction power of ethanol is necessary for selenization without excess selenium deposition at low temperature. Moreover, H-1 NMR analysis revealed that at supercritical conditions, decomposition of diethyl selenide occurs and reactive selenium species with a short lifetime at process temperatures dictate the selenization reaction, whereas stable selenium species prevent the deposition of excess selenium. (C) 2013 Elsevier B.V. All rights reserved.