Optimization of functional layers for kesterite thin film solar cells

Among the most promising emerging photovoltaic (PV) technologies, we find kesterite absorber materials such as Cu2ZnSnS4 (CZTS) and Cu2ZnSn(S,Se)4 (CZTSSe). These materials can be deposited with low-cost methodologies, even on flexible substrates, and for this reason, they are suitable for use in Building Integrated Photovoltaic (BIPV) or Product Integrated Photovoltaic (PIPV). However, issues like inner defects, back surface recombination and a non-optimal band alignment with the toxic but conventionally used CdS buffer layer still limit the device performances. The use of an alternative material to CdS, such as ZnSnO (ZTO) and TiO2, could improve charge transport and make the devices more sustainable. In our works, the growth on CZTS of ZTO and TiO2 via Atomic Layer Deposition (ALD) was developed. Different stoichiometry, compositions and thicknesses were tested. The efficiencies reached for both CZTS/ZTO and CZTS/TiO2 are comparable to our CZTS/CdS control devices1). Here we report also on the optimization of the back contact to design the kesterite solar cells grown by wet method2) on flexible substrates. Among different choices, the high-purity Mo foil is one of the most interesting substrates, thanks to its high temperature resistance, high conductivity and lack of metallic impurities. The functionalization of this substrate has been performed. The efficiencies obtained are comparable to our devices on standard soda-lime glass rigid substrates.