Diffusion of Cu, In, and Ga in In2Se3/CuGaSe2/SnO2 thin film photovoltaic structures

This article is a study of copper, indium, and gallium interdiffusions in In2Se3/CuGaSe2/SnO2/glass thin film heterostructures annealed at different temperatures. The use of CuGaSe2 material in place of Cu(In,Ga)Se2 is only required by the indium diffusion studies. The CuGaSe2 layers were grown by close-spaced vapor transport for two types of sources having different grain sizes. The In2Se3 films were deposited by thermal evaporation. The heterostructures were annealed in vacuum at different temperatures and analyzed by secondary ion mass spectroscopy (SIMS). The copper, indium, and gallium SIMS concentration profiles show that the copper diffuses up to the In2Se3 film surface and that the indium can diffuse far away from the In2Se3/CuGaSe2 interface towards SnO2. The copper, indium, and gallium diffusions were studied and the interdiffusion parameters were computed. The simultaneous interdiffusions of copper and indium induces the formation of a p–n junction responsible for the photovoltaic effect of the Zn/In2Se3/CuGaSe2/SnO2/glass photocells, the SnO2 side being lighted. This hypothesis is supported by results carried out from electron beam induced current measurements, showing a notable shift of the junction from the In2Se3/CuGaSe2 interface through the CuGaSe2 layer in terms of the annealing temperature, resulting in an increasing of the photovoltages up to 650 mV.