The impact of processing on the optical absorption onset of CdTe thin-films and solar cells

We critically examine how two processing steps commonly applied in the preparation of cadmium telluride (CdTe)-based solar cells, i.e., the cadmium chloride treatment and the subsequent stepwise bromine/methanol wet etching process, impact the structural and optical properties of polycrystalline CdTe thin-films. In particular, drawing upon a conjuncture of photothermal deflection spectroscopy and spectroscopic ellipsometry experimental results, we determine the spectral dependence of the optical absorption coefficient, alpha (E), over the photon energy range from 1.1 to 2.0eV for samples of rf sputtered (RFS) and close space sublimation (CSS) CdTe. The impact of these processing steps on shaping the grazing incidence x-ray diffraction pattern is also examined. We extend the analysis to devices through interpretation of the spectral dependence of the external quantum efficiency associated with two cadmium chloride treated CdTe-based solar cells. The cells are comparably prepared with the exception of the absorber, one by RFS and the other by CSS. Through the use of our results for the thin-film CdTe optical functions and a model for the solar cell multilayer structure, we simulate the resultant external quantum efficiency spectrum. Through a critical contrast with the corresponding solar cell acquired experimental results, we glean insights into the carrier trapping and recombination processes that occur within the two types of CdTe absorbers.

Automated summary

This study critically examines the effects of cadmium chloride treatment and bromine/methanol wet etching on the structural and optical properties of polycrystalline CdTe thin-films. Experimental results were used to determine the spectral dependence of the optical absorption coefficient and to analyze the impact on the grazing incidence x-ray diffraction pattern. The study also extended to analyzing the external quantum efficiency of CdTe-based solar cells treated with cadmium chloride, providing insights into carrier trapping and recombination processes in different types of CdTe absorbers.