Schulte, J.; Harbauer, K.; Ellmer, K.: Toward efficient Cu(In,Ga)Se2 solar cells prepared by reactive magnetron co-sputtering from metallic targets in an Ar:H2Se atmosphere. Progress in Photovoltaics 23 (2015), p. 1793-1805
In this paper, we show that a reactive co-sputtering process using metallic CuGa and In targets; an Ar:H2Se atmosphere is well suited for the deposition of photoactive Cu(In,Ga)Se2 (CIGSe) absorber layers for thin-film solar cells in a single process step. The achievement of single-phase and well-crystallized layers is thereby no major problem if a sufficiently high H2Se content and substrate temperatures in the range of 400–500 °C are used. However, in order to achieve the desired Cupoor film stoichiometry, which is crucial for the device performance, it has to be considered already that, at moderate substrate temperatures in the range of 400–500 °C, indium has a strong tendency to re-evaporate from the film surface if the film composition is Cu-poor. If excess indium is supplied, this effect can lead to a self-adjustment of the film composition. This allows a very wide process window in a one-stage process concerning the supply ratio from the two targets of [Cu]/([In] + [Ga])supply≈0.35–0.8. However, the maximum efficiencies achievable with such a process are limited to 11.7% because an adequate Cu-poor composition can only be achieved with significant Cu-poor conditions, which allow only a low material quality. By using an improved process with an intermediate Cu-rich composition and a final Cupoor stage, the absorber quality could be significantly improved; efficiencies of up to 14.3% have been achieved with CIGSe films prepared on Na-doped Mo back contacts.