Investigation of Structural, Chemical, and Electrical Properties of CdTe/Back Contact Interface by TEM and XPS

CdTe solar cell back contact preparation usually includes a chemical etching process which helps to obtain a Te-rich p-doped CdTe surface. In this work we compared the influence of two different etching solutions [nitric–phosphoric (NP) and nitric–acetic acid (NA)] on the CdTe surface. Transmission electron microscopy indicated that a Te-rich layer was formed on the surface of polycrystalline CdTe films after the etching process. The layer thickness was 80 nm and 10 nm for NP and NA etching solutions, respectively. In addition, the images showed that the influence of the etching solution was preferentially along the grain boundaries. The chemical properties of the etched CdTe surface were studied by using x-ray photoelectron spectroscopy. The nitric–phosphoric acid yielded a relatively thicker Te-rich layer on the CdTe surface. On the other hand, the J–V properties of the solar cells prepared using nitric–acetic acid showed no rollover behavior, indicating improved back contact. The solar cells prepared with the NA and NP etching processes yielded >10% solar cell efficiency. The CdTe solar cell homogeneity was improved by the NA etching method.