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

Thin films (0.02-2.0 mum) of CdTe were electrodeposited from an acidic electrolyte containing a high concentration of Cd2+ and a low concentration of TeO2 using chemically prepared US 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 Na2SO3), 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 degreesC resulted in conversion to p-type with formation of a heterojunction with the CdS. In the thinnest structures studied, photocurrent spectroscopy showed that the US film remained photoactive after heat treatment, and a clear US response could also be seen in the electroabsorbance spectra. Heat treatment of CdS\CdTe structures with thicker CdTe films (> 0.2 mum) resulted in a formation of a photo-inactive US 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 hand gap of the CdTe, and this is attributed to CdTe1-xSx alloy formation (x = 0.05-0.07). (C) 2002 Elsevier Science B.V. All rights reserved.