Tailoring the thermoelectric properties of sol-gel grown CZTS/ITO thin films by controlling the secondary phases

In this work, we have demonstrated the effect of secondary phases on the thermoelectric properties of Cu2ZnSnS4 thin films (TF). The samples were grown on ITO substrate by chemical solution method and sulfurized in tube furnace having constant annealing temperature (500 degrees C) for different time duration (20, 40, 60 and 80 min). The temperature dependent (25-100 degrees C) thermoelectric properties (Seebeck coefficient, electrical conductivity and power factor) were measured using Seebeck effect and Hall effect. It was found that the value of Seebeck coefficient was increased from 70 to 753 mu V/degrees C as the sulpurization time decreased from 80 to 20 min. The enhancement in Seebeck coefficient with decreasing sulpurized time duration is related with emergence of secondary phases which cause the filtering of slow carriers at the interface of secondary phases. The presence of secondary phases in the grown samples was confirmed by various characterization techniques. Raman spectroscopy data suggested CZTS/ITO sample sulpurized for 80 min has less secondary phase but as the sulpurized time duration decreased to 20 min, secondary phases such as Cu2S, SnS, SnS2 and ZnS were emerged and enhanced the thermoelectric properties. The XRD measurements were also verified that CZTS/ITO sample sulpurized for 20 min has poor crystal quality as compared to other samples due to the presence of secondary phases. SEM and UV/Visible spectroscopy measurements were also performed to support our argument.