Characterization of etching procedure in preparation of CdTe solar cells

An etching procedure for forming a low resistance contact to polycrystalline CdTe thin films in CdS/CdTe solar cells was studied. The etching solution used was a mixture of HNO₃, H₃PO₄ and H₂0. X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS) and electric measurements revealed that the etching results in a formation of crystalline tellurium on the film surface, thereby increasing substantially the conductivity of the surface layer. The total process was found to consist of three steps: (i) immediately after an immersion into the etching solution there was a certain induction period with no discernible changes, (ii) a subsequent reaction step during which poorly crystallized elemental tellurium was formed, gaseous byproducts liberated and the surface changed its colour, and (iii) after taking out of the etching solution the tellurium crystallized causing a strong decrease in the sheet resistance. In situ XRD and electric measurements were carried out to follow the third step. The chemical aspects of the three steps as well as their contributions to the reproducibility and control of the overall etching procedure have been considered.