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 CdSe_xTe_1−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 ∼ 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.