Reactive sputtering of thin Cu2S films for application in solar cells

Thin layers of cuprous sulphide were deposited by reactive r.f. sputtering; the target was pure copper and the sputtering gas was an ArH₂S mixture. We describe here how the composition of the films and their stoichiometry can be derived accurately both from X-ray diffraction and the optical reflection and transmission spectra. Measurement of the electrical resistivity can be used as a quick qualitative identification method.The application of these characterization methods to our sputtered films indicates that the crucial parameters to be controlled are the total pressure of the sputtering gas and, in particular, the partial pressure of the H₂S. Too low partial pressures of H₂S result in the presence of copper precipitations in the Cu₂S film, whereas too high H₂S partial pressures result in the presence of copper-deficient Cu_xS phases; there is an intermediate range of H₂S partial pressures in which pure chalcocite films (Cu₂S) are obtained. When these films were sputtered onto evaporated CdS layers, we obtained Cu₂S/CdS solar cells with a total area efficiency of above 4%.