Investigation on improvements of TCO processing on CdTe thin film solar cells

Abstract

Cadmium Stannate (Cd2 SnO4) is a high performance TCO (transparent conducting oxide) that has been used in the fabrication of leading performance thin film CdTe solar cells. Several studies in this work were done to enhance the technical knowledgebase and industry processing of the Cd2 SnO4: mechanical film properties, tantalum doping, pulsed-DC compared to RF plasma deposition, and modeling of the sputter system in Fluent and Comsol. Mechanical properties, determined from nanoindentation measurements, are valuable for flexible substrate design and topcoats. It was found films annealed in oxygen rather than nitrogen increases elastic modulus by 16 per cent and hardness by 32 per cent, but film electrical and optical properties were degraded. Tantalum was also investigated as an n-type dopant in Cd2 SnO4 with the goal of removing the lab-standard CdS plate anneal, which is impractical for industrial manufacturing. Various doping levels, deposition temperatures, and post-anneals were done on sputtered films. Quantification of film quality is done using three properties: electrical, optical, and crystalline. These properties are measured with x-ray diffraction, Hall effect, spectrophotometer, and a profilometer to measure thickness and surface roughness. While 1-10 per cent Ta is not an effective dopant, a significant discovery is approximate film properties can be achieved with a 550 degrees C. undoped Cd2 SnO4 deposition with a standard anneal compared to the standard 20 degrees C. film with CdS anneal. This film may be competitive in industry as an improvement for the current ITO (SnO4 :F). Pulsed DC plasma sputtering was investigated to enhanceCd2 SnO4 film properties and is known to deposit faster than with RF for certain materials. It was found PDC improves film qualities at deposition temperatures of 300 degrees C. and above, compared to standard RF Cd2 SnO4 films. Even without any anneal, the 530 degrees C. PDC deposition has good properties compared to the RF 550 degrees C. While conductivity is not as high with PDC, the mobility of the film is outstanding. With added CdS anneals, deposition temperatures 300 degrees C. and above give improvements to the standard RF film. Deposition rates of these PDC films is increases 6-21 per cent over RF at corresponding temperatures. The sputter deposition was also modeled in Fluent (gas and surface reactions) and Comsol (plasma) in order to approximate deposition rates under various conditions and optimize chamber properties. The result is there is a solid base for a comprehensive model for the sputter deposition.