Highly Efficient CdS/CdTe Solar Cells Investigated by Cathodoluminescence Spectroscopy
Solid State Phenomena
Thin film solar cells consisting of a glass/ITO/CVD-CdS/CSS-CdTe/ Cu:C/Ag layer system with conversion efficiencies up to 14.4% and (5-8) µm thick CdTe layers as well as CdS(90 nm)/CdTe(6 µm) layers deposited on Si have been investigated by cathodoluminescence (CL) spectroscopy at 5 K. The spectral position of the CdTe exciton line was used to probe the interface region between the n-type CdS layer and CdTe grains at a cleaved edge of the solar cell and neighboring grains at the top of
... the top of uncovered CdTe layers. For substrate temperatures (T sub ) smaller than 610 o C during the CdTe deposition, the width of the intermixed region between the CdS layer and CdTe grains was found to be smaller than the lateral resolution of the CL (≈300 nm). For T sub as large as (630−650) o C, a decrease of the CdTe band gap appears to be present up to 0.6 µm from the CdS/CdTe interface. For the grain boundaries, however, a remarkable red shift of the exciton CL line is observed throughout the whole CdTe layer. A CdCl 2 treatment homogenizes the distribution of acceptor-like defects or impurities leading to an optimized p-conversion of the CdTe layer. CL spectra of the CdS window layer exhibit two broad bands centered at 1.72 (red) and 2.04 eV (yellow). Other authors found similar bands in CdS microcrystals and in single crystal CdS bombarded with electrons indicating that the yellow band can be assigned to Cd interstitials. In the solar cells, the yellow band is suppressed by the CdCl 2 treatment indicating a passivation or out-diffusion of Cd interstitials.