Investigation of the emissivity of graphite when it is coated with cadmium telluride (CdTe)

Abstract

Today, thin film CdTe solar cells are used commercially in many applications. Advances in this technology can aid in meeting increased energy demands. Research is focused on increasing the efficiency of solar cells. By creating more value with minimum environmental impact, Cadmium Telluride (CdTe) solar cells promise to have the potential to become one of the leading eco efficient energy solutions available in the photovoltaic (PV) market today. CdTe PV has been commercialized at the GW/year level. Many deposition systems for CdTe PV employ graphite structures and they get coated with CdTe during operation. The objective of this thesis is to investigate how the emissivity of the Photovoltaic production tool using graphite surfaces is affected when coated with CdTe at the operating conditions. According to the published literature graphite has an emissivity of 0.65-0.95 and CdTe has an emissivity ranging between 0.2 - 0.8 depending upon the radiation and wavelength range. The current study was done in three steps. In the first step, the reflection and emissivity of Graphite coated with CdTe was calculated using Kirchhoff's Law of Radiation. In the second step, graphite-CdTe layer stack was simulated using thin film optical software and reflection data for different thicknesses of CdTe coated graphite was calculated. In the third step, the Advanced Research Deposition system in the Materials Engineering Laboratory was used to deposit CdTe on graphite substrates using a Close Space Sublimation deposition process. The graphite substrates were made from material similar to those used in these applications. The substrates were purified to remove impurities. The CdTe thickness was measured using a Taylor Hobson Profilometer. Rate of heating and cooling of graphite was measured and compared to the rate of cooling of graphite substrates coated with CdTe to evaluate the emissivity. Results in all the three steps showed that CdTe coating had a negligible effect on the emissivity of graphite at the conditions simulating the production environment.