Characterisation of CdS ∣ CdTe heterojunctions by photocurrent spectroscopy and electrolyte electroreflectance/absorbance spectroscopy (EEA/EER)

Abstract

Thin films (0.02–2.0 μm) of CdTe were electrodeposited from an acidic electrolyte containing a high concentration of Cd 2+ and a low concentration of TeO 2 using chemically prepared CdS layers on fluorine-doped tin oxide coated glass as substrates. Characterisation of the as-deposited and thermally annealed CdTe ∣ CdS heterostructures by photocurrent spectroscopy was carried out using transparent stabilising electrolyte contacts (1.0 M Na 2SO 3), which allowed illumination from the CdTe or CdS (glass) sides. The same electrolyte was used for electrolyte electroabsorption and electroreflectance spectroscopy (EEA/EER). Comparison of the photocurrent spectra for the two illumination directions allowed detection of type conversion and junction formation arising from heat treatment. The as-deposited CdTe films were n-type, and heat treatment at 415 °C resulted in conversion to p-type with formation of a heterojunction with the CdS. In the thinnest structures studied, photocurrent spectroscopy showed that the CdS film remained photoactive after heat treatment, and a clear CdS response could also be seen in the electroabsorbance spectra. Heat treatment of CdS ∣ CdTe structures with thicker CdTe films (>0.2 μm) resulted in a formation of a photo-inactive CdS layer, which gave rise to the well-known loss of photoresponse in the blue that is characteristic of CdS ∣ CdTe solar cells. Electrolyte electroabsorbance and electroreflectance measurements showed that annealing changed the band gap of the CdTe, and this is attributed to CdTe 1− x S x alloy formation ( x=0.05–0.07).