The present invention includes methods and apparatus therefrom for preparing thin films of doped semiconductors for radiation detector and photovoltaic applications, and particularly method and apparatus that increase dopants of alkali metals in Group IBIIIAVIA layers. In a particular aspect, the present invention includes a method of preparing a doped Group IBIIIAVIA absorber layer for a solar cell, with the absorber layer being formed by reaction, with a Group VIA material, of a metallic stack with a plurality of layers, in which each layer contains a concentration of an alkali metal selected from the group of Na, K and Li.