Full text loading...
KPFM measurements on the surface of the CIGSe absorber showing (a) topography and (b) CPD images. (c) CPD (blue solid lines) and topography (black dashed lines) line sections at three different grain boundaries marked in the images (a) and (b). According to the CPD line shape they are denoted as GBdip, GBstep, and GBpeak. The inset in the upper left corner shows a schematic of the sample alignment (not to scale).
KPFM measurements from an untreated cross section of the solar cell based on the same CIGSe absorber. The topography and CPD images (size: 6 × 4.8 μm2, resolution: 11.7 nm/step) are shown in (a) and (b), respectively. The ZnO, CIGSe, Mo layers can be easily distinguished from top to bottom. The expected position of the CdS layer is indicated on the left side of the images with a double line. The line section L1 is analyzed in Fig. 3. The inset on the upper left corner shows schematic of the tilted sample alignment (not to scale). (c) CPD (blue solid lines) and topography (black dashed lines) line sections at L2, L3, and L4 shown in (b), corresponding to the three types of grain boundaries GBdip, GBstep, and GBpeak, which are also observed on the absorber surface. However, the magnitudes of potential variations at GBdip and GBpeak-type grain boundaries on the cross section are significantly smaller compared to the absorber surface.
(a) Topography and CPD line profiles taken at the position L1 in (b) through the Mo/CIGSe/CdS/ZnO-structure. The potential variation caused by CdS (gray bar) is difficult to analyze, because this very thin layer is frequently covered by the ZnO overlayer after cleavage.18 Since the work function is defined for negatively charged electrons, the CPD distribution mirrors the distribution. Therefore, for a better comparison with the energy diagram in (b), an inverted coordinate for the CPD values is used. (b) A qualitative band diagram through this structure. As our sample is short circuited, Fermi energies in all the layers are at the same level. Therefore, the variation of the work function = Evac-EF reflects in the variation of the vacuum level Evac.
Article metrics loading...