Cu2ZnSnS4 thin films solar cells: material and device characterization

Cu2ZnSnS4 (CZTS) quaternary compound has attracted much attention in the last years as new abundant, low cost and non-toxic material, with desirable properties for thin film photovoltaic (PV) applications. In this work, CZTS thin films were grown using two different processes, based on vacuum deposition of precursors, followed by a heat treatment in sulphur atmosphere. The precursors were deposited using two different approaches: (i) electron-beam evaporation of multiple stacks made of ZnS, Sn and Cu and (ii) co-sputtering deposition of the three binary sulphides CuS, SnS and ZnS. All the materials were characterized both as isolated films and as absorber layer in solar cells, produced using the typical structure Mo/CZTS/CdS/i:Zno/AZO. Both growth processes were found to give good quality kesterite films, showing CZTS as the main phase, large grains and suitable properties for PV application, but higher homogeneity and stoichiometry control were achieved using the co-sputtering route. A detailed investigation on CZTS optical properties, microstructure, intrinsic defect density and their correlation with the material composition is presented. A strong effect of the tin content on the bandgap energy, sub-gap absorption coefficient, crystalline domain and grain size is shown and a model based on the increase of the intrinsic defect density induced by a reduced tin content is proposed. These studies suggested a correlation between the increase of the bandgap energy and the improvement of the material quality, which was also confirmed by the performances of the final devices. CZTS thin films were then assembled into the solar cells and their properties as absorber layer were optimized by varying both composition and thickness. CZTS samples produced from stacked evaporated precursors allowed achieving a maximum efficiency of 3.2%, but reproducibility limits of the evaporation process made difficult to obtain further and rapid efficiency improvements. The co-sputtering route was demonstrated to be a more successful strategy, assuring a fine-control of the film composition with good process reproducibility. A fast improvement of solar cell efficiency was obtained using this approach and a maximum efficiency of 5.7% was achieved. The relationship between the absorber layer stoichiometry and the device performances was investigated: the effect of the Zn enrichment and a possible influence of the Cu/Sn ratio on the device performances are discussed. Investigation on CZTS/CdS and CZTS/MoS2 interfaces revealed that the optimization of both buffer-layer and back-contact technology is a primary need for further improvement of CZTS solar cells.