Effect of Na incorporation on the growth and properties of CdTe/CdS devices

Sodium is known to enhance p-type doping in copper indium diselenide (CIS)-based devices fabricated on soda-lime glass substrates, and similar amounts of Na are present in commercial cadmium telluride (CdTe) devices. We present the results on the effects of Na incorporation on the properties of CdTe/CdS solar cells prepared on borosilicate glass substrates. A NaF layer 10 to 30 nm thick was incorporated at either the CdS/CdTe interface or on the CdTe surface, as a source of Na. CdTe layers were deposited by close-spaced sublimation at substrate temperatures from 425°C (LT) to 620°C (HT), followed by heat-treatment in the presence of CdCl₂ vapor. Atomic force microscopy analysis showed that the samples with NaF at the CdS/CdTe interface deposited in He ambient have larger grains with a sub-grain structure that disappears after CdCl₂ heat treatment accompanied by an increase in grain size. Samples deposited in O₂ ambient have smaller grains without a sub-grain structure. For samples with NaF deposited on the CdTe surface, LT samples with CdCl₂ heat treatment showed a morphology similar to samples without NaF layers; but samples heat-treated in He ambient at 500°C prior to CdCl₂ treatment showed a different microstructure with platelets on the surface. HT samples with a NaF layer at the CdTe surface showed an additional layer at the surface with both types of heat treatments, indicating that NaF does not react readily for these samples. In addition to the discussion of the effect of Na incorporation on the material properties, we will include preliminary data on the effect of Na incorporation at the interface or surface on device properties.