Bär, M.; Ennaoui, A.; Klaer, J.; Kropp, T.; Sáez-Araoz, R.; Lehmann, S.; Grimm, A.; Lauermann, I.; Loreck, Ch.; Sokoll, St.; Schock, H.-W.; Fischer, Ch.-H.; Lux-Steiner, M.C.; Jung, Ch.: Intermixing at the heterointerface between ZnS/Zn(S,O) bilayer buffer and CuInS2 thin film solar cell absorber. Journal of Applied Physics 100 (2006), p. 064911/1-9
10.1063/1.2345034
Abstract:
The application of Zn-compounds as buffer layers was recently extended to wide-gap CuInS2 (¿CIS¿) based thin-film solar cells. Using a new chemical deposition route for the buffer preparation aiming at the deposition of a single-layer, nominal ZnS buffer without the need for any toxic reactants such as, e.g. hydrazine has helped to achieve a similar efficiency as respective CdS-buffered reference devices. After identifying the deposited Zn-compound, as ZnS/Zn(S,O) bi-layer buffer in former investigations [M. Bär, A. Ennaoui, J. Klaer, T. Kropp, R. Sáez-Araoz, N. Allsop, I. Lauermann, H.-W. Schock, and M.C. Lux-Steiner, ¿Formation of a ZnS/Zn(S,O) bilayer buffer on CuInS2 thin film solar cell absorbers by chemical bath deposition¿, J. Appl. Phys., accepted.], this time the focus lies on potential diffusion/intermixing processes at the buffer/absorber interface possibly, clarifying the effect of the heat-treatment, which drastically enhances the device performance of respective final solar cells. The interface formation was investigated by x-ray photoelectron and x-ray excited Auger electron spectroscopy. In addition, photoelectron spectroscopy (PES) measurements were also conducted using tuneable monochromatized synchrotron radiation in order to gain depth-resolved information. The buffer side of the buffer/absorber heterointerface were investigated by means of the characterization of Zn(S,O)/ZnS/CIS structures where the ZnS/Zn(S,O) bi-layer buffer was deposited successively by different deposition times. In order to make the (in terms of PES information depth) deeply buried absorber side of the buffer/absorber heterointerface accessible for characterization, in these cases the buffer layer was etched away by dilute HClaq. We found that while (out-leached) Cu from the absorber layer forms together with the educts in the chemical bath a (Zn(1-Z),Cu2Z)S-like interlayer between buffer and absorber, Zn is incorporated in the uppermost region of the absorber. Both effects are strongly enhanced by postannealing the Zn(S,O)/ZnS/CIS samples. However, it was determined that the major fraction of the Cu and Zn can be found quite close to the heterointerface in the buffer and absorber layer, respectively. Due to this limited (in the range of one monolayer) spatial extent, these ¿diffusion¿ mechanisms were rather interpreted as a CBD induced and heat-treatment promoted Cu-Zn ion exchange at the buffer/absorber interface. Possible impacts of this intermixing on the performance of the final solar cell devices will also be discussed.