A method for making a heterojunction photovoltaic device (200) is provided for converting solar radiation to photocurrent and photovoltage with improved efficiency. The method and apparatus include an improved window layer (230) having an increased oxygen (140) concentration with higher optical bandgap and photo to dark conductivity ratio. The improved photovoltaic device (200) is made using a deposition method which incorporates the use of a gas mixture of an inert gas (115) and a predetermined amount of oxygen (140), deposited at or near room temperature. Window layers contemplated by the present invention include, but are not limited to, cadmium sulfide (CdS) and various alloys of zinc cadmium sulfide (Zn.sub.xCd.sub.1-xS). To further increase the efficiency of the resultant photovoltaic device (200), deposition parameters are controlled and monitored to improve the deposited window layer (230).
 The United States Government has rights in this invention under Contract No. DE-AC36-99G010337 between the United States Department of Energy and the National Renewable Energy Laboratory, operated for U.S. Department of Energy by the Midwest Research Institute.