Cathodoluminescence Analysis of Grain Boundaries and Grain Interiors in Thin-Film CdTe

We used low-temperature cathodoluminescence (CL) spectrum imaging (CLSI) with nanoscale spatial resolution to examine charge-carrier recombination and defects at grain boundaries (GBs) and grain interiors (GIs) in as-deposited and CdCl2-treated CdTe thin films. Supporting time-resolved photoluminescence, T = 4 K photoluminescence, secondary ion mass spectrometry, and electron backscatter diffraction measurements were conducted on the same films. Color-coded maps of the luminescence transition energies (photon energy maps) were used to analyze the qualitative characteristics of the CLSI data. We applied an image analysis algorithm to the pixels in grayscale CL intensity images to compare the luminescence intensities and spectra at the GIs and GBs quantitatively and with statistical relevance. Our results show that GBs in as-deposited films are active recombination centers and are thus harmful to solar cell operation. CL GB defect contrast is quantifiably reduced for the CdCl2-treated film, which is direct evidence of passivation of deep GB core states resulting from the treatment. However, the CdCl2 treatment is not a perfect fix for GB recombination, and GB recombination may still be limiting performance in CdCl2-treated devices.