The microstructure of thin film CdTe absorber layers deposited by pulsed dc magnetron sputtering

High rate pulsed DC magnetron sputtering has been used to fabricate thin film CdTe photovoltaic devices. Fabrication of thin film photovoltaic devices using magnetron sputtering has the advantages of excellent coating uniformity and use of lower substrate temperatures during deposition. The CdTe and CdS layers were deposited from compound targets. The substrates were held at 200ºC during deposition, a process condition previously found to minimize the stress in the coatings. The devices were deposited using rotation to provide coatings with exceptionally uniformity. This enabled the CdTe absorber thickness to be reduced to ~1μm. The microstructure of the as-deposited material is dense and columnar. The cadmium chloride activation treatment results in an increase in grain size and it also removes the majority of planar defects. However, void formation has been observed which becomes more severe with increasing time of the activation process. In some cases the void formation leads to catastrophic failure and delamination at the CdS/CdTe junction. The microstructure of the films has been characterized before and after activation using a number of techniques including High Resolution Transmission Electron Microscopy of device cross-sections. The observations made have been correlated to device performance.