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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])supply0.35-0.8. 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. [ABSTRACT FROM AUTHOR]

By simultaneous sputtering from metallic CuGa and In targets in an Ar:H 2 Se atmosphere onto heated substrates, single phase and well crystallized Cu(In,Ga)Se 2 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. [ABSTRACT FROM AUTHOR]

Abstract: Energy dispersive X-ray diffraction (EDXRD) has been used to observe in situ the formation of Cu(In,Ga)S2 (CIGS) films during reactive magnetron co-sputtering. By simultaneous sputtering from a CuGa and an In target in an Ar:H2S atmosphere the CIGS films, which can be used as absorber layers for thin film solar cells, can be grown in a single step without any post-treatments. By carefully analyzing the in situ experiments, several effects have been found which should be considered during designing the recipe for an absorber deposition process in order to achieve CIGS films with the desired stoichiometry and good crystallinity. For instance substrate temperatures below 380°C should be used in order to achieve a sufficient condensation of In atoms which is needed to avoid the formation of intermetallic Cu–Ga phases. Since, on the other hand, high substrate temperatures are necessary in order to grow large crystallites, a process which starts at a low temperature and has temperature ramp up later on is possible because In which is bound in the chalcopyrite phase evaporates only for much higher temperatures. These results demonstrate the opportunities of EDXRD for the real-time analysis of the growth process of thin films which can provide information not obtainable by any ex situ characterization method. [Copyright &y& Elsevier]