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(Color online) (a) Schematic illustration (not to scale) of the KPFM analysis on the CIGS absorber surface with a GGI ratio of 0.32. The topography (linear line fit) and CPD (raw data) images are plotted in the middle and right column, respectively. (b) Schematic illustration of the KPFM analysis on the untreated cross section of a solar cell fabricated with the same CIGS absorber. Due to geometrical constraints, the sample has to be tilted and the layer thickness of ZnO appears to be enlarged. Nonetheless, the ZnO and CIGS layers can be easily identified in the topography (parabolic line fit) and in the CPD image (raw data). The dashed line in the CPD image marks the line section to be analyzed in Fig. 3(a).
(Color online) (a) CPD image over the CIGS/CdS/ZnO structure from a cross section of a solar cell with a GGI of 0.32. (b) The CPD line profile indicated by the dashed line in (a). (c) The relative vacuum energy derived by mirroring the CPD curve in (b). (d) The relative conduction band energy derived from (c) using electron affinities of individual materials from the literature (see Refs. 21 and 22). Positions 1, 2, and 3 denote the interfaces p-CIGS/OVC, OVC/CdS, and CdS/i-ZnO, respectively.
(Color online) (a) CPD line profile along the dashed line marked in Fig. 1(b). The solid and dashed curves in the inset show the bias spectroscopy curves on CIGS and ZnO, respectively. The contact potential difference ΔCPD between these two layers is determined via the difference between the minima of these two spectroscopy curves. (b) Applying this measurement principle, we recorded ΔCPD values for all four samples. Repeating this procedure with five different cantilevers (CL1 to CL5), the results reveal an offset between individual cantilevers, but the tendency (dashed line) is the same for all of them. (c) The Fermi energy curve E F(CIGS) (dashed-dotted line) derived by mirroring the dashed line in (b) shows direct evidence for a Fermi energy shifting. The position of E F(CIGS) to other energy levels is shown only qualitatively.
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