Colossal grain growth in Cd(Se,Te) thin films and their subsequent use in CdTe epitaxy by close-spaced sublimation

Many technologies deposit thin films on inexpensive substrates, resulting in small grains due to classic nucleation and grain growth theory. For example, state-of-the-art solar cells are made by depositing CdSeTe and CdTe layers on inexpensive glass coated with nanocrystalline transparent conducting oxides (TCOs), like SnO2. Characteristically, the grain size of these films is on the order of the film thickness, i.e. a few microns. CdTe small-grain films have poor electro-optical properties and require CdCl2 passivation which fails to fully passivate grain boundaries, causes carrier compensation, and prevents implementing other II-VI alloys and materials to improve performance. Here, we present a method to increase grain size to 1 mm in CdSexTe1-x thin films deposited on glass/TCO substrates without CdCl2 treatment. The colossal grain growth is driven by mechanisms distinct from classic nucleation, grain growth, and Ostwald ripening and only occurs at low selenium content (x similar to 0.1). We also demonstrate how these films can serve as templates for subsequent large-grain epitaxy of other compositions like CdTe, again without exposure to CdCl2. The results open new paths for thin film solar cell technology, and thin film devices in general.