Cross-section potential analysis of CdTe/CdS solar cells by kelvin probe force microscopy

The spatial distribution of the electric potential in working polycrystalline CdTe/CdS thin film solar cell cross-sections is analyzed by Kelvin probe force microscopy (KPFM). Measurements are carried out in air ambient, in tapping mode, with morphological and surface electric potential resolution of 50 nm. The paper describes the method of sample preparation, theory and application of the KPFM measurement, and analysis of CdTe/CdS thin film devices at different illumination bias. The voltage difference measured by KPFM across the junction agrees with the external VOC measured under identical illumination conditions. The gradient of potential is found to develop within the CdTe layer (at the CdTe/CdS interface) and extends halfway through the CdTe and is consistent with independent measurements of the depletion width. Variations in the spatial distribution of the electric potential along the junction occur at dimensions consistent with CdTe grain size, suggesting different electrical properties. Additionally, the presence of a weak back contact barrier is also observed. Maximum contrast in electric potential is found by measuring immediately after mechanical polishing with an aqueous suspension of 0.02 μm alumina. The paper discusses interpretation of results with respect to variations in electron work function, conductivity and the influence of surface states. Since the method does not require complete contacts, the electrical operation of CdTe/CdS cells at different stages of completion can be analyzed.