Dr. Majid Gharghi Profile

Dr. Majid Gharghi

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Proceedings Article | 10 September 2008 Paper

Design of anti-reflection coating for spherical silicon photovoltaic devices

M. Gharghi, S. Sivoththaman

Proceedings Volume 7045, 704509 (2008) https://doi.org/10.1117/12.795698

KEYWORDS: Spherical lenses, Reflectivity, Silicon, Reflection, Optical spheres, Silicon solar cells, Refractive index, Photovoltaics, Antireflective coatings, Chemical vapor deposition

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Spherical silicon photovoltaic devices are bonded to flexible substrates to produce light-weight flexible solar modules. In order to maximize the conversion efficiency, the optical loss must be minimized. The concept of conventional anti reflection coating (ARC) does not directly apply to the spherical device due to different geometry. The optimum design of the ARC must maximize the optical power transmission from air to the Si crystal bulk. In addition to the refractive index and the thickness of the ARC, the power distribution on the exposed spherical surface, incidence angle dependent reflection, and multiple reflections at the spherical air-ARC and ARC-silicon interfaces also influence the ARC design. The effects of the spherical shape on the variations of the reflection are analyzed. It is shown that the optimum design is essentially different from the conventional ARC with uniform quarter-wavelength thickness. It is required that the design compensates the effect of variation of the incidence angle across the spherical surface. To achieve this, the thickness should have a zenith-angle dependence. Chemical vapor deposition techniques can potentially be employed for the deposition of the designed films.

Proceedings Article | 21 September 2007 Paper

Graded silicon based PECVD thin film for photovoltaic applications

M. Gharghi, S. Sivoththaman

Proceedings Volume 6674, 66740A (2007) https://doi.org/10.1117/12.735091

KEYWORDS: Refractive index, Silicon, Silicon films, Amorphous silicon, Plasma, Absorption, Plasma enhanced chemical vapor deposition, Reflectivity, Hydrogen, Reflection

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Silicon based thin film alloys are deposited using plasma enhanced chemical vapor deposition (PECVD) with silane, ammonia, and nitrous oxide as precursors with different partial pressure ratios. Numerous deposition conditions have been considered to produce films with a wide range of refractive indices. The optical properties of the films are mostly affected by hydrogen content and stoichiometry, which are characterized by means of Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS) respectively. The results of spectroscopic ellipsometry measurement of the refractive index are correlated with stoichiometry extracted using XPS to enable the prediction of optical properties from process conditions. Based on the film characterization results, a graded index film is deposited to minimize the reflection loss. The optical properties of the film to be used as anti-reflection coating (ARC), i.e. the transmittance and reflectance, are measured using an optical spectrophotometer. In spite of the optical absorption in the high refractive index part of the film, it is shown that by employing a very thin layer of amorphous silicon, it is possible to reduce reflection below conventional graded index films consisting of silicon oxynitride, and still maintain the transmittance required for solar cell applications.

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