High throughput combinatorial screening of Cu-Zn-Sn-S thin film libraries for the application of Cu2ZnSnS4 photovoltaic cells

摘要

The naturally occurring mineral of Cu2ZnSnS4 (CZTS) is a promising alternative absorber layer for thin film based photovoltaic devices. It has the remarkable advantage that it consists of abundant, inexpensive and non-toxic elements compared to its crystallographically related and highly successful counterparts: the Cu(In,Ga)(S,Se)2 (CIGSSe) and CuIn(S, Se)2 (CISSe) materialsystems. Therefore, there is real commercial potential for reduced material costs and improved device efficiencies. A two-stage high throughput combinatorial process for the fabrication of Cu-Zn-Sn-S thin film libraries is presented, which consists of either sequentially stacking or co-depositing Cu,Sn and Zn precursor layers by DC magnetron sputtering followed by a sulphurisation process. Sputtering conditions and target-substrate geometry aredeveloped to give compositionally graded Cu-Zn-Sn precursor layers spanning a wide spatial region around the point of stoichiometry. Conversion into Cu-Zn-Sn-S libraries is achieved by thermally evaporating a uniform layer of sulphurdirectly onto the metal alloy and annealing the sample at 500 °C in a furnace.Effects of the precursor composition on the structural properties of the films prior to the incorporation of sulphur are investigated. The sulphurised librariesare then studied by Scanning electron microscopy (SEM), X-ray diffraction(XRD) and Raman spectroscopy as a function of composition, to assess the effects on morphology and phase formation. Observations of changes in lattice parameters and crystallinity are clear. The opto-electronic and electrical properties of the CZTS film libraries are measured using photoconductivity and hot point probe techniques, respectively. Changes in the band gap and conductivity type are studied as a function of atomic ratios. Based on high performing compositions, devices have been fabricated with the highest achieving cell at 1.26 %. The observations are discussed in the context of the particular compositions and synthesis conditions, and recommendations are made for further work.