NOVELTY - An electrode layer (120), a barrier layer, a copper layer, and an indium layer are sequentially formed on the surface region of a transparent substrate (110), to form a multilayered structure. The structure is thermally treated (200) in an environment containing a sulfur bearing species (210), and a copper-indium disulfide material (220) is formed. The interface region is maintained free from metal disulfide layer. A window layer is formed overlying the copper-indium disulfide material, to obtain a thin film photovoltaic device.    USE - Formation of bifacial thin film photovoltaic device (claimed) e.g. solar cell.    ADVANTAGE - The method enables manufacture of thin film photovoltaic devices having energy-conversion efficiency of 10% or more under standard test conditions. The device can be configured for a tandem or other multilevel cell arrangement.    DETAILED DESCRIPTION - An electrode layer is formed overlying the surface region of a transparent substrate. A barrier layer is formed overlying the electrode layer to form an interface region. A copper layer is formed overlying the barrier layer, and an indium layer is formed overlying the copper layer to form a multilayered structure. The multilayered structure is thermally treated in an environment containing a sulfur bearing species, and a copper-indium disulfide material is formed. The copper-indium disulfide material comprises a copper-to-indium atomic ratio of 1.35:1-1.6:1. The interface region is maintained free from metal disulfide layer having a semiconductor characteristic which is different from the copper-indium disulfide material. A window layer is formed overlying the copper-indium disulfide material, to obtain a thin film photovoltaic device. An INDEPENDENT CLAIM is included for thin film photovoltaic device.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the formation of thin film photovoltaic device.    Substrate (110)    Electrode layer (120)    Thermal-treatment process (200)    Sulfur bearing species (210)    Copper-indium disulfide material (220)