Cu₂ZnSnS₄ thin-films generated from a single solution based precursor: the effect of Na and Sb doping

A new solution based route for depositing Cu2ZnSnS4 (CZTS) thin films is described, focusing on the effects of Sb and Na co-doping. X-ray diffraction and Raman spectroscopy confirm formation of the kesterite phase with a measurable improvement in crystallinity upon doping. A sharp band gap absorption edge at 1.4 eV is determined from diffuse reflectance measurements, whilst improvement in the photoluminescence yield and sharpening of the band-to-band emission spectra are observed in the presence of Na and Sb. The performance of devices with the configuration: glass/Mo/CZTS/CdS/i-ZnO/ ZnO:Al/Ni-Al and total area of 0.5 cm2 is reported. Analysis of over 200 cells shows that introduction of Na and Sb leads to an increase of the average power conversion efficiency from 3.2±0.6 to 5.2±0.3%. The best cell with efficiency of 5.7% is obtained upon Na and Sb doping, featuring 14.9 mA cm 2 short-circuit current, 610 mV open circuit voltage and 63% fill factor under simulated AM 1.5 illumination. This performance ranks among the highest in pure sulphide CZTS cells. We propose that the improvement in crystallinity and cell performance is linked to the formation of alkali antimony chalcogenides flux during the annealing step, in addition to Sb and Na decreasing disorder in specific lattice positions of the CZTS unit cell.