The sulfurization of co-evaporated Cu2ZnSnSe4 (CZTSe) thin films is studied. In this work, a relationship between the Na concentration and the further S incorporation in the CZTSe absorber layer is found. Na is added by a NaF precursor layer before CZTSe co-evaporation and/or by diffusion from the soda-lime glass substrate.
Higher Na concentrations result in bigger grain sizes, higher S/(S + Se) atomic ratios and photovoltaic devices with higher VOC, as investigated by SEM, GIXRD, Raman and I-V measurements. Results using an alternative substrate as ceramic verify the importance of the control of the Na content, which alters the carrier concentrations and elements distribution with much higher S and Na concentrations at the surface in this particular case.
By using this methodology, it is possible to tune the kesterite band gap energy by the creation of S-gradient through the absorber thin film. This may provide a new approach for the control of Cu2ZnSn(S,Se)4 growth and development of high efficiency kesterite solar cells. Efficiencies of 5.5% and 6.4% are obtained using ceramic and SLG substrates respectively (without antireflection coatings and without grids).
de la Cueva, L., Sánchez, Y., Calvo-Barrio, L., Oliva, F., Izquierdo-Roca, V., Khelifi, S., Bertram, T., Merino, J.M., León, M. and Caballero, R., 2018. Sulfurization of co-evaporated Cu 2 ZnSnSe 4 thin film solar cells: The role of Na. Solar Energy Materials and Solar Cells, 186, pp.115-123.
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