CdTe material, which has high absorption coefficiency and optimum bandgap as well as stability, has been widely used as an absorber of thin film solar cells.
In general, a pure CdTe thin film contains Cd vacancies of lower concentration, with composition ratio of Te/Cd<1 and low p-type conductivity. However, due to the self-compensation of Cd vacancy, it is difficult to obtain a p-type CdTe semiconductor thin film. In addition, a CdTe polycrystalline thin film with high grain boundary density and large surface area particles has high boundary activity and a large number of defects, therefore resulting in high grain boundary barrier and many recombination centers which lower the photo-generated carrier density to decrease photoelectrical properties. In order to prepare good semiconductor materials, it is necessary to improve the conductivity and structural properties by doping donor or acceptor-like impurity.
Kim Hyon Chol, a researcher at the Faculty of Electronics, has analyzed the dependence of the microstructure and properties of thin films on the Te rich content and heat treatment of CdTe:Te films, to obtain the following results.
As Te rich content increases, diffraction intensity becomes stronger, which gets the highest at Te rich content of 5%, indicating that crystallinity of a film is improved and the defects and stress become less. At high Te rich contents (7, 10%), the grain growth does not change with Te rich content.
The incorporation of impurity Te atoms into a CdTe lattice affects lattice structure and preferential growth of crystal whereas extra Te atoms enter into grain boundaries, which is unable to make Te rich content affect the crystal growth of a thin film.
For the sample annealed in air after CdCl2 treatment, (111) peak has a significant preferential orientation and increase of each peaks indicates grain growth and good crystallinity.
Performance parameters of solar cells increase and then decrease with increase in Te rich content, and maximum efficiency is attained at Te content of 5%.
If further information is needed, please refer to his paper “The effect of Te-doping and heat treatment on the structural properties of CdTe absorber layer for CdS/CdTe solar cell” in “optical materials” (SCI).
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