Effect of rapid thermal processing conditions on the properties of Cu2ZnSnS4 thin films and solar cell performance

In the present work, we have studied the effect of several sulphurization conditions on the properties of Cu2ZnSnS4 thin films obtained through rapid thermal processing (RTP) of multi-period precursors with 8 periods of Zn/SnS2/CuS. In this study we varied the heating rate, the maximum sulphurization temperature, the time at maximum temperature and the amount of evaporated sulphur. The samples were characterized through scanning electron microscopy, energy dispersive spectroscopy, Raman scattering spectroscopy, X-ray diffraction, photoluminescence and I-V measurements. We have observed that at heating rates above 0.5 °C/s the samples delaminated severely. As a result further tests were carried out at 0.2 °C/s heating rate. The morphological studies revealed that the samples sulphurized at higher temperatures, shorter times and higher amount of evaporated sulphur exhibited larger grain sizes. The structural analysis based on Raman scattering and XRD did not lead to a clear distinction between the samples. Photoluminescence spectroscopy studies showed an asymmetric broad band characteristic of CZTS, which occurs in the range of 1.0-1.4 eV and a second band, on the high energy side of the previous one, peaking at around 1.41 eV. The intensity of this latter band varies from sample to sample revealing substantial differences in their optical properties. This band appears to originate either from the surface of the absorber or from the CdS layer and has a clear correlation with cell efficiency. The higher the intensity of this band the lower the cell efficiency, presumably due to the increase in recombination resulting from CZTS surface decomposition and eventually from the CdS with modified optoelectronic properties. The cell results hint toward a detrimental effect of long sulphurization times and a positive effect of higher sulphur vapour pressure and higher sulphurization temperature. Solar cell efficiencies improved with increased grain size in the absorber layer. The highest cell efficiency obtained in this study was 3.1%.