In this work, we describe the development of a novel bath comprising lactic acid/sodium lactate pH buffer for electrodeposition of CuInSe2 (CIS) thin films, by which long-term bath stability was found to be improved and near-stoichiometric CIS films with smooth surface morphology were obtained. XRD and SEM studies reveal a dramatic improvement of the crystalline quality of CIS films after selenization treatments. Raman study confirms the absence of copper selenide phases in the CIS films. Optical study shows that the band gap energy of the obtained CIS film is 0.98 eV. The resultant photovoltaic device shows a conversion efficiency of 1.55%. (C) 2009 Elsevier B.V. All rights reserved.