Route towards low cost-high efficiency second generation solar cells: current status and perspectives

The most efficient thin film solar cells are based on Cu(In,Ga)(S,Se)₂ (CIGSSe) and CdTe compounds, known as second generation polycrystalline thin films. The challenge of these materials is to reduce the cost per watt of solar energy conversion, but they are actually formed by expensive and/or scanty elements in the earth’s crust such as In, Ga, Te and other that present toxicity issues like Cd. In the last years, new materials with properties of interest for photovoltaic applications and formed by non toxic and relatively abundant elements, have been suggested as alternatives to the main second generation solar cells based on CdTe and CIGSSe. Semiconductor compounds with kesterite structure (Cu₂ZnSn(S_xSe_1−x)₄, Cu₂ZnSnS₄, Cu₂ZnSnSe₄) and other like In₂S₃, all of them Cadmium-free have been proposed as new candidates for thin film solar cells. However, reported solar cell efficiencies for these compounds have not yet reached the expected values. In this work, we present a review of the limiting factors for achieving high efficiency in thin film solar cells, related to deposition methods as well as the different mechanisms that limit cell performance. Significant results in the processing of solar cells using some of these compounds and preliminary results of the In₂S₃ deposition with an overview to its use as buffer layer are presented.