Chalcogenides for solar applications
In its “Strategic Research Agenda for Photovoltaic Solar Energy Technology”[1], the European Union has set a target for solar generated electricity to become competitive with conventional electricity generation by 2020–2030. Second generation thin-film chalcogenide materials are among the most promising candidates for large-scale PV manufacturing and are quickly becoming commercial products. These materials offer stable and efficient (above 10%) photovoltaic modules fabricated by scalable thin-film technologies and cell efficiencies above 20 % (CIGS). CIGSe thin films were deposited by RF magnetron sputtering at room temperature from a single quaternary target on various substrates producing stoichiometric films. Cu2ZnSnS4 thin films were fabricated using a novel two stage process incorporating RF magnetron sputtering from a single target and then annealed in a rapid thermal processor without any toxic gases or reactive sulphur. GaLaS thin films were deposited by a patented high throughput PVD MBE System. The effect of the annealing temperature on the sulphur in the films was investigated. The XRD phase analysis confirms the preference of GLS to oxidise to form a more stable compound. The films were also analysed by ellipsometry, EDX and Raman. CuSbS2 thin films were fabricated using two techniques. Firstly, using an atmospheric pressure chemical vapour deposition system. CuSbSx was successfully deposited on soda lime substrates at a very fast rate of around 35 µm in 1 hour. For RF sputtering two and single stage depositions were carried out. By two stages, the films were annealed at various temperatures (250° to 325°) and times (5 to 60 min). The annealing was done without any toxic gases or reactive sulphur. Single phase CuSbS2 films with good adhesion, good crystallinity and with no apparent sulphur loss can be achieved. By a single stage process, CuSbS2 crystals were made in situ by depositing at 200° and 250°. The grain sizes were measured by SEM imaging, and grains above 1 µm can be fabricated. The effect of etching with bromine and HCl was also investigated. The cells made showed a high current density using CuSbSx films (up to 1-2 mA/cm2).