Role of Cu on the electrical properties of CdTe/CdS solar cells: A cross-sectional conductive atomic force microscopy study

The authors have studied the electrical properties of CdTe/CdS solar cells using conductive atomic force microscopy (C-AFM) applied to cross sections of the device. This novel technique uses the sharp tip of an atomic force microscope to contact the sample and apply an electrical potential, allowing the study of device properties with spatial resolution second to none. The CdTe/CdS/SnO2/substrate structures were treated with CdCl2 and etched with bromine/methanol or nitric/phosphoric acid solution. Finally, a Cu-containing back contact was applied to the surface of the device. The C-AFM analysis showed the existence of high-conductivity regions in CdTe close to the film surface, while the regions close to the junction with CdS remain resistive. The width of the conductive area in general varied laterally and, occasionally, reached the junction in some spots, causing microshunts in the devices. By analyzing the fracture of the cross sections, they observed that the conductive areas are concentrated at grain-boundary regions, whereas the interior of the grains is resistive. Analysis of samples without a Cu-containing back contact revealed that the conductive areas are caused by the diffusion of Cu from the back contact into the CdTe. The conductive layers directly affect the current transport in the device and impose a minimum CdTe thickness to prevent short-circuit problems. (c) 2007 American Vacuum Society.