A study of rare-earth doped silicon based films as a luminescent downshifting layer for cadmium telluride photovoltaics

Abstract

The peak efficiency range for CdTe solar cells is between 500-700nm; however efficiencies are limited at wavelengths shorter than 500nm due to the fact that at higher energies, most photons are absorbed in the CdS layer of the module and cannot contribute to the cell current. This means that incident photons with higher energies are ‘wasted’ as they are not efficiently absorbed by the cell. Luminescent downshifting (LDS) is a third-generation photovoltaic technology in which an external layer applied to the front surface of the cell absorbs high energy photons and re-emits them towards the cell at energies where they are more efficiently absorbed, thus avoiding front surface loss mechanisms. This research project investigates the use of cerium and terbium co-doped silicon oxide thin films grown using electron cyclotron resonance plasma enhanced chemical vapour deposition (ECR PECVD) as a luminescent down-shifting layer. Post-deposition annealing in a quartz tube furnace caused the formation of cerium disilicate (Ce2Si2O7) nanocrystallites, which were found to strongly absorb incident light at wavelengths below 360 nm and efficiently sensitize Tb3+ ions in the film for re-emission. The effect of annealing time and sample composition on physical and optical properties was studied. Film compositions were determined through Rutherford backscattering spectrometry, revealing an incremental increase in rare earth concentration. Photoluminescence measurements showed a distinct Tb3+ peak around 550nm, which is close to the ideal efficiency wavelength for CdTe photovoltaics. Variable Angle Spectroscopic Ellipsometry measurements were used to determine the index of refraction of as-deposited and annealed films. UV-Visible absorption spectroscopy and transmission ellipsometry measurements showed a sharp increase in absorption around 400nm confirming wide separation between absorption and emission bands. When LDS films were coupled with thin film CdTe, subsequent absorption spectroscopy and transmission measurements showed stronger absorption at short wavelengths, as anticipated.