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Anti-reflection coatings for silicon solar cells

Daniel Nilsen WrightDepartment for Solar Energy

Institute for Energy Technology

2008-01-16

Outline• IFE group• Introduction• Anti-reflective coatings

• Principles• Deposition Techniques

• PECVD• PECVD systems• Parameter dependence

• Optimizing performance• Weighting • Simulation results

• Conclusion

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Solar Energy at IFE• 11 Scientific Researchers

• 2 post doctorates

• 6 Ph.D. student

• 2 Technicians

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The New PV Laboratory

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Introduction• Solar cell efficiency is given mainly by the cell’s

ability to:1. Convert incoming photons to electron-hole pairs2. Avoid the electron and hole recombining before they reach

the solar cell contacts

• The anti-reflective coating (ARC) increases solar cell performance by reducing the amount of reflected photons

• Some ARC films passivate recombination sites on the surface and in the bulk of Si

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Principles of ARCs

filmin light ofVelocity in vacuumlight ofVelocity

=n

Si

ARCMet

al

cont

act

4λ

=nd

• Refractive Index, • Slows down light

• Extinction Coefficient, k• Light absorbing properties of film

• Optical thickness of film = Refractive Index x film thickness • Lowest reflection occurs at destructive interference• Bare Si substrate R > 30%• With ARC R< 10%

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Types of ARC films• Titanium Oxide, TiO2

• Atmospheric Pressure Chemical Vapor Deposition

• Indium Tin Oxide, ITO• Transparent conducting oxide

• Silicon Oxide, SiO2• Thermal oxidation• High temperature• Passivates surface

• Silicon Nitride, SiNx:H• Plasma Enhance Chemical Vapor Deposition (PECVD)• Low Temperature• Passivates Surface and Bulk

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PECVD• Versatile technique for depositing films at low

temperature• By varying deposition parameters, films with different

properties can be realized• PECVD SiNx:H is the most common ARC for mc-Si

solar cells• Many PECVD techniques exist:

• Parallel Plate (direct)• Microwave excited (remote)• Expanding thermal plasma (remote)• Inductively coupled plasma (remote)

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PECVD• SiH4 + N2O

• SiOxNy:H• n = 1.46 – 1.9

• SiH4 + NH3 / N2• SiNx:H• n = 1.85 – 3.3

• SiH4• a-Si:H• n = 4.2

• Doping of a-Si:H RF

Plasma

Pumping and pressure controller

Gas inlet with flow controllers

Substrates

HeatersAnd

temperaturecontrollers

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PECVD• Vary gas flow ratio

Wright, D.N., E.S. Marstein, and A. Holt, EFFECT OF ANNEALING ON PECVD SILICON NITRIDE FILMS in 21st European Photovoltaic Solar Energy Conference and Exhibition. 2006. Dresden, Germany.

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Optimization by simulation• Prerequisites:

• ñ = n + ik dispersion relationships

• Use Bruggemann’s Effective Medium Approximation

• Solar spectrum• A model• Time

• Optical optimisation• Iterate ñ and film thickness• Calculate T, R and A

R

A

T Si

ARC

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Solar spectrum

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Single Layer SiNx:H

Non-encapsulated Encapsulated

To be published in Solar Energy Materials and Solar Cells

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Double Layer

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Indium Tin Oxide

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Conclusion• IFE’s Solar Energy Department is growing and

getting a very nice new laboratoy.• Anti-reflective coatings are necessary to avoid optical

losses and increase the current of the solar cell• PECVD is a versatile technique for applying ARCs at

low temperature• Optical simulations can give valuable insight into

optimum parameters for ARCs

Thank you