A study of impurities in some CdS/CdTe photovoltaic cells prepared by wet-chemical methods using secondary ion mass spectrometry and X-ray photoelectron spectroscopy

Abstract

Quantitative Secondary Ion Mass Spectrometry has been used to determine the elemental profiles and concentrations of isotopes ¹²C, ¹⁶O, ³⁴S and ³⁵Cl within n-CdS/p-CdTe thin film photovoltaic cells. Chemical Bath Deposition (CBD) was used to deposit the CdS window layers. The annealing process induces the formation of a chloride-rich surface layer on CdS as evidenced from X-ray photoelectron spectroscopy measurements. The carbon impurity in heterostructures appears to influence the chloride-promoted recrystallisation of CdTe. High concentrations of ¹⁶O, of the order 10²⁰–10²¹ atoms cm^–3 throughout the cells, are consistent with the formation of oxide material in the post-deposition thermal processing. Isotopic profiles for ¹²C, ³⁴S and ³⁵Cl have similar maxima (≈10¹⁹ atoms cm^–3) but concentrate at the CdS–CdTe interface. The relatively high tolerance to high concentrations of impurities in our cells suggests that wet chemical methods may have great potential in the fabrication of large area/low cost devices.