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Omni-directional Broadband Antireflection coating for solar cells using Indium Tin Oxide (ITO) based Fractal Structures

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Title
Omni-directional Broadband Antireflection coating for solar cells using Indium Tin Oxide (ITO) based Fractal Structures
Author
Won, Sang-Hyuk
Advisor
Choi, Kyoung-Jin
Issue Date
2015-08
Publisher
Graduate School of UNIST
Abstract
Solar cells have extrinsic loss from reflection of the front surface which is one such loss mechanism and has been managed in the past with the usage of planar antireflection coatings. But these coatings are each limited to a single wavelength of light and do not account for varying incident angles of the incoming light. Various nanostructures made from wet or dry etching, solution processes have shown the capability to restrict reflection for differing wavelengths and angles of incidence. Especially Graded Refractive Index nanostructures based on Indium Tin Oxide (ITO) were modeled and show a broadband, multi angled reflectance decrease due to an effective grading of the refractive index. Indium Tin Oxide (ITO) is advantageous for its ease of production, large amount, conventional process and refractive index matching to the air source region and silicon substrate. In this study we have demonstrated Indium Tin Oxide (ITO) Fractal structures which has Graded Refractive Index (GRIN) profile for Omni-directional Broadband anti-reflection coating to silicon solar cells. Indium Tin Oxide (ITO) Graded Refractive Index Fractal structures were deposited on silicon surfaces through self-catalyst vapor liquid solid(VLS) growth mechanism using conventional Oblique Angle Deposition (OAD) with computational ? controlled servo assisted electron beam evaporator while elevating substrate temperature below 200℃. We introduced two types of silicon solar cells. The first one has polished planar surfaces and second one has pyramid structure commonly used for industrial production. Graded Refractive Index (GRIN) Fractal structures were successfully applied to planar silicon surface and exhibits both superior optical and electrical properties. With GRIN structure height 1um, planar solar cell`s efficiency was increased 48.9% due to reduced series resistance and reflectance while maintaining shunt resistance at normal incident angle 90˚. But in the omni-directional point of view due to reduced angular reflectance up to 70˚ below 10%, 2um GRIN structure reveals the best annual power density enhancement 73.9% compared to 68.2%. Furthermore we investigated conventional pyramid textured silicon surfaces which has self ? induced oblique angle, Indium Tin Oxide (ITO) GRIN structures were deposited onto pyramid without tilting servo (normal flux incident angle 90˚). The deposited GRIN 1um exhibits total reflection in 300 to 1100nm wavelength length region below 5% that can enhance its annual power density 44% compared to the non-coating textured silicon solar cell. Still the surface passivation issues which is important role in crystalline silicon solar cell efficiency is remained. These issues are not discussed in this study but will be investigated further. Our results in achieving increasing annual power density using Graded refractive index anti-reflection coating is greatly encouraging as regards the development of solar cell industry.
Description
Department of Materials Science Engineering
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