Impurities in chemical bath deposited CdS films for Cu(In,Ga)Se-2 solar cells and their stability

The highest efficiencies for Cu(In,Ga)Se-2-based thin dim solar cells have been achieved with CdS films prepared by a solution growth method known as the chemical bath deposition (CBD) technique. The impurity content in such cadmium sulfide films has been examine. By means of secondary ion mass spectroscopy, Fourier transform infrared spectroscopy, Rutherford backscattering spectrometry, and x-ray photoelectron spectroscopy, we conclude that CBD grown CdS films contain similar to 11 atom percent (a/o) 0, similar to 5 a/o N as well as C and H. Unlike previously published results, cadmium carbonate (CdCO3) and water (H2O) are found to be the main oxygen-containing compounds. No clear evidence of cadmium hydroxide [Cd(OH)(2)] and/or oxide (CdO) is found. Carbon-nitrogen bonds originating from any pseudohalogenic ions [e.g., cyanamide (NCN2-) thiocyanate (SCN-), cyanate (OCN-), cyanide (CN-)] are identified as the main nitrogen impurity. To investigate the stability of the impurities, the films were subjected to air annealing (200 to 350 degrees C) as well as storage (1.5 month) In three different environments: desiccator, humidity of 79%, and at 100 degrees C. Air annealing results in a decrease of the water content, which disappears at 350 degrees C. On exposure to humidity, the water content increases somewhat. The CN bonds are very sensitive to all treatments, but do not totally decompose, not even at 350 degrees C. In contrast, CdCO3 is much more stable.