The Interfacial Reaction at ITO Back Contact in Kesterite CZTSSe Bifacial Solar Cells

The synthesis route based on co-electroplating of copper, zinc, tin, and chalcogen precursor plus post-chalcogenization demonstrates the tremendous potential to realize industrial manufacture of earth-abundant kesterite materials for sustainable photovoltaics. Exploration of appropriate annealing temperature is significant to gain insight into the crystallization of kesterite solar materials on the back contacts based on transparent conducting oxides in bifacial device. The Cu₂ZnSn(S_x, Se_1-x)₄ (CZTSSe) absorber films have been fabricated by post-selenizing co-electroplated metal-sulfide precursors on ITO substrate at 500, 525, and 550°C. Experimental proof, including electron microscopies, X-ray diffraction, optical transmission/reflection spectra, polarized Raman, and IR techniques, is presented for the interfacial reaction between the ITO back contact and CZTSSe absorber. This reaction contributes to substitutional diffusion of In into CZTSSe (CZTISSe) to a considerable extent and formation of a SnO₂ interfacial layer when the temperature is higher than 500°C. In incorporation does not much change the optical absorption, band gap, and phonon spectra of CZTSSe; whereas, it leads to lattice expansion more or less. The bifacial kesterite solar devices are successfully fabricated, and the device performance is analyzed and discussed.