Reactive magnetron co-sputtering of Cu(In,Ga)Se2 absorber layers by a 2-stage process: Role of substrate type and Na-doping

By simultaneous sputtering from metallic CuGa and In targets in an Ar:H2Se atmosphere onto heated substrates, single phase and well crystallized Cu(In,Ga)Se2 thin films can be directly deposited in a single process step. However, the preparation of Cu-poor films, which are needed for high solar cell efficiencies, is impeded by the re-evaporation of excess indium, which occurs readily at moderate substrate temperatures in the range of 400 °C to 500 °C. Therefore, a significant In-excess is necessary during the second deposition stage in order to transform the final film composition into the desired Cu-poor regime ([Cu]/([In] + [Ga]) < 0.95). Higher open circuit voltages and efficiencies are achieved for absorbers produced with an intermediate Cu-rich composition and/or by using Na containing precursor films (NaF or Mo:Na). A best cell efficiency of 12.9% is achieved, which demonstrates the high potential of the investigated reactive magnetron sputtering process for solar cell manufacturing, as it is well suited for large-area, industrial applications.