Cd free buffer layers for solar cells

CdS layers obtained by chemical bath deposition (CBD) are usually used as buffer layer in chalcogenide based thin film solar cells. However, if good results are obtained, cadmium is toxic for the environment while the break of the physical vapour deposition (PVD) cycle, needed for CBD-CdS, constitutes a bottleneck in the process. Therefore many attempts are under way, all over the world, to obtain Cd free buffer layers. In the present work CuAlX₂ (X = Se, Te) and In₂S₃ have been obtained by vacuum deposition and annealing treatment. Their properties are compared to those of CdS buffer layer. The CuAlX₂ (X = Se, Te) and In₂S₃ films are crystallized in the chalcopyrite and the cubic structure respectively. The measured band gaps are 2.4 eV, 2.7 eV and 2.9 eV for CuAlTe₂, CuAlSe₂ and In₂S₃ respectively. While In₂S₃ is systematically n-type, a Zn treatment allows n-type films of CuAlX₂ to be obtained. The films are resistive (10^-6 Ω^-1cm^-1 < σ < 10^-3 Ω^-1 cm^-1). All these properties made CuAlX₂ (X = Se, Te) and In₂S₃ good candidates as new buffer layers. A first attempt, using Mo/CuInSe₂ /In₂S₃/ZnO cell, has been performed; the efficiency obtained is 8 %