A Versatile Optical Model Applied to CdTe and CdSe1–y Tey Alloys: Sensitivity to Film Composition and Relative Defect Density

A parametric optical model is developed for use in analyzing spectroscopic ellipsometry (SE) measurements of CdTe and $CdSe_{1-y}Te_{y}$ alloy films. Through fitting to measurements of a series of CdCl2 treated alloys spanning the full range of compositions, a database describing the optical response of $CdSe_{1-y}Te_{y}$ as a function of y is compiled. More specifically, spectral features are mathematically related to y, including bandgap-bowing. Additionally, the parametric model has the capability of quantitatively describing defect-induced sub-bandgap absorption through the determination of an Urbach energy (Eu). The accuracy of the SE-determined Eu is verified through complementary photothermal deflection spectroscopy measurements. For instance, Eu describing a CdTe film is measured to decrease from 42 to 20 meV after undergoing CdCl2 treatment. External quantum efficiency simulations of full solar cell structures making use of this database can be compared to actual measurements of devices to identify the source of performance gains and or losses as well as the composition of alloyed material within such devices.