Rapid fabrication of Cu₂ZnSnS₄:CdS graded interfaces via spray coating

Cu₂ZnSnS₄ (CZTS) is a non-toxic and cheap photovoltaic absorber material, however, many challenges still remain for the successful fabrication of these devices. In this study, CZTS was deposited with CdS, a commonly used buffer layer, using different spray coating procedures (i.e., two-step, single-step and graded deposition techniques), to demonstrate a facile and rapid fabrication technique of creating CZTS:CdS photovoltaic devices. Scanning electron microscopy (SEM) was used to determine the morphology of the deposited films, showing that the two-step, single-step and the graded deposition with 2 s of overlap time produced clear and defined layers. However, when the overlap time exceeded 4 s the layer became less defined, thicker and less dense, resulting in failed photovoltaic devices. This was attributed to the additional source of air during the deposition period, resulting in air being trapped by the deposited precursor, which therefore forms more foam-like functional layers. X-ray diffraction (XRD) analysis reflected this change in morphology, with graded samples above 4 s displaying no obvious CZTS peaks, suggesting an ineffective decomposition route. Depositions using a two-step, single-step and the graded deposition technique with 2 s of overlap time were shown to form functional photovoltaic devices, with a 2 s graded overlap performing three times better than the ungraded samples, resulting in an improved power conversion efficiency of 0.41%. It is important to highlight that the device prepared with a 2 s graded interface significantly increased the current peak performance of CZTS photovoltaic devices deposited using xanthate precursors.