Investigation of Surface Potential on CdTe/CdS p-n Heterojunction with Kelvin Probe Force Microscopy

CdTe/CdS photovoltaic (PV) modules have become the lowest-cost producer of solar electricity, despite working at lower efficiency (up to 16.5%) than crystalline silicon cells [1]. Polycrystalline CdTe cells are more efficient comparing to their single crystalline counterparts. However, the charge transport mechanisms responsible of this effect are not clear [2]. In this communication, we report a series of Kelvin probe Force Microscopy (KFM) measurements, suitable to observe the topography and the distribution of the Contact Potential Difference (CPD) inside the structure of the device: CdTe/CdS/ITO/Glass. The sample is prepared by mechanical polishing in order to decrease the roughness after the cleaving process. In order to have a better understanding of the charge transport inside the solar cell and to vary the Fermi level pinning effect[3], different bias are applied to the sample. The CPD variations with different bias on cross-section in the dark and under illumination condition are presented on figure 1(a) and (b) respectively. In both cases, we observe the reverse bias widens the CdTe/CdS depletion region. Under illumination, electron-hole pairs are generated at the interface and modify the electric field inside the material at the same position. Moreover, the surface chemical composition has been investigated by Nano-Auger Electron Spectroscopy (AES) to confirm the boundary line of different layers as shown in figure 1(a)(b), which are assumed from the corresponding topography measurement. All the detailed results will be discussed in this presentation. (a) (b) (c) FIGURE 1. Cross section characterization on CdTe/CdS solar cell (a) CPD as measured by KFM with different bias under dark condition, electric field between the distance of 2um and 3um are indicated (b) under illumination (c) Nano-Auger analysis on surface chemical composites.