Contact Wetting Angle as a Characterization Technique for Processing CdTe/CdS Solar Cells

摘要

The feasibility of measuring contact wetting angles to characterize processing induced changes to thin film semiconductors in CdTe/CdS solar cells is evaluated. The contact angles of water and formamide are used to determine the polar and dispersive surface energies of the thin films using two analysis methods. Changes in surface energies resulting from processing are correlated to changes in surface chemistry and structure detected by glancing incidence X-ray diffraction (GIXRD), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Surface energies are evaluated for sputtered In_2O_3: SnO_2, chemical surface-deposited CdS, and physical vapor-deposited (PVD) CdTe thin films under as-deposited and treated conditions. Treatments include thermal anneal in air, argon, and CdCl_2 ambient as well as surface etching. Indium tin oxide (ITO) and CdS films exhibit increased polar surface energy corresponding to enhanced crystallization of surfaces resulting from processing and increasing CdS growth temperature. Native oxidation of PVD CdTe (lll)-oriented film surfaces occurs rapidly and is readily detected by changes in contact angle. Surface energies of PVD (lll)-oriented CdTe stored under various humidities prior to processing are energetically similar due to native oxidation. The polar energy of CdTe surfaces is affected by the addition or removal of crystalline surface oxides during film processing.