Compositional optimization of photovoltaic grade Cu₂ZnSnS ₄ films grown by pneumatic spray pyrolysis

We report on the compositional optimization of thin film deposition of Cu₂ZnSnS₄ (CZTS) onto Mo/glass substrates for low-cost solar cell applications, using pneumatic spray pyrolysis technique in air ambient. The influence of both, the starting-solution precursor's concentrations (mainly copper chloride and zinc acetate) and the further annealing conditions on the properties of the CZTS have been investigated. Before and after annealing, the films were analyzed with regards to its compositional, structural, morphological and electrical properties. Globally it was found that Cu poor and Zn rich conditions are needed to obtain the desired properties on the CZTS films, minimizing the probability for secondary phase occurrence. Solar cell prototypes were fabricated using a classical CZTS/CdS/i-ZnO/ZnO:Al structure and characterized, correlating their optoelectronic properties with the chemico-physical, structural and electrical characteristics of the layers. Selective oxidation of Zn containing phases is identified as the major drawback of the technique, and its elimination as the most important challenge to improve the device's performance.