Photonics for Photovoltaics.pdf

7/27/2019 Photonics for Photovoltaics.pdf

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Photonics for Photovoltaics

Prof. D.M. Bagnall*,

M. Banakar, S.A. Boden, T.L. Temple, D.N.R. Payne, R.S.A. Sesuraj,

A. Asadollahbaik and M.A. Rind


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Dark clouds gather over China's once-booming solar

industryChina's push into solar energy was supposed to be a proud example of how thecountry was advancing into hi-tech manufacturing. But now the whole sector ison the brink of bankruptcy.

Solar power, along with biotechnology and aerospace, was declared a "strategic emerging industry" and was given grants and lo w-cost

loans.Photo: Getty Images


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There is a large diversity of technologies...!

C-Si mC-Si CIGS CdTe/CdS a-Si p-Si Multi-junction Si Dye-sensitised Polymer Nano-rod

Antireflection/ Light- trapping can benefit all device types


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Overview: ten commandments (observations)

1) Real devices must have an A/R scheme and light-trapping is good for all devices

2) Most A/R schemes are textures (and play a part in light-trapping)

3) The Yablonovich limit is quite impressive

4) All layer thicknesses should be optimised for minimum reflection

5) We dont yet have the computational power we need for Finite element

modelling of real devices.6) Plasmonics might (or might not) allow us to make better solar cells

7) Plasmonic properties depend on metal, dielectric environment, size, shape

8) It is difficult to avoid unwanted absorption in metal nanoparticles

9) Scattering, absorption in metal nanoparticles is modified by the photonic

environment10) There are alternatives to plasmonics - wavelength-scale features

(11) Wondrous things are possible but are not always practical )


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(1) Real devices must have an A/R scheme& light-trapping is good for all devices


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Improved light-trapping provides opportunity to:

Increase efficiency by increasing ISC

Reduce absorber thickness

may increase carrier collection

decrease requirement on diffusion length

decrease material usage

increase fabrication throughput


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Light-trapping has two key themes

taking more near the bandgap (C-Si, a-Si) narrow band

ultra-thin devices (a-Si, n-Si) - broadband

PVCDROM


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(2) Most A/R schemes are textured (andplay a part in light-trapping)


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Micron-scale texturing inverted pyramids

Sub-wavelength texturing

m0th-eyes

textured TCOs

mC-surfaces

Mie scatterers

Plasmonics (?)

A/R techniques are nearlyalways light-trapping

(and vice versa)


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Boden & Bagnall, APL 93 (2008)

Moth-eye texturing can providebroadband (wavelength and angle)

low reflectance

Exact shape of features is hugelyimportant

Polycrystalline texturing often hassame effect

Modelling random textured surfacesis a big challenge (effective mediumtheory isnt good enough)


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Broadband AR effects can be obtained (with scattering thrown in) but

Small particles: Strong, narrow resonance. Mainly absorbing

Large particles: Weak, broad resonance. Mainly scattering

Hard to avoid short-wavelength losses

What about plasmonics for AR?

In principle, we could exploit forward scattering and should be broadband

[Temple and Bagnall, Progress in Photovoltaics, 2012]


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Enhancement and loss mechanisms

Scatteringtowards

semiconductor

Near-fieldenhancement

Carrierinjection

Scattering

away fromsemiconductor

Parasitic

absorption

e-


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(3) All layer thicknesses should beoptimised for minimum reflection


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Transfer-matrix methodology can provide analyticalsolutions for simple multi-layered systems

But interfaces are seldom sharp

Modelling for (ellipsometry, reflection spectrometry) useeffective medium approximations

We really need to use finite-element techniques (FDTD)


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(4)The Yablonovich limit is quiteimpressive


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A perfect Lambertian reflector allows a path-length

enhancement of4n2d (around 50x enhancement for silicon)

This is theYablonovith or Ergodic limit

[Yablonovitch J. Opt. Soc. Am. 1982)


18/43David Payne (PVSAT6), Ahktar Rind (PVSAT8)

Detector Angle ()

Wavelength(nm)

0 20 40 60 80

500

550

600

650

700

750

800

RelativeIntensity(Lo

g10

Scale)

-4

-3.5

-3

-2.5

-2

Textured TCOs arent perfect Lambertians

Practical limit on roughness as increasingroughness often degrades deviceperformance

x20 enhancements are realistic

Absorption coefficients are dropping byorders of magnitude as you approach the

band edge


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(5)We dont yet have the computational

power we need for Finite elementmodelling of real devices


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In principle large area, high-resolution AFM images of each

interface could allow reasonableapproximation

But nano-scale features areimportant and avoiding

diffraction requires large areas


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(6) Plasmonics might (or might not)allow us to make better solar cells

(more scattering, flatter?)


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Our focus (at Southampton)


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(7) Plasmonic resonances are dependent onthe metal, dielectric environment, size, shape


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Surrounding medium

The higher the refractive index the better?

Tune the peak position by increasing the refractive index of the surrounding

medium


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Surrounding medium

Increasing the refractive index means that the maximum totalscattering is achieved at smaller radii.

However, increasing the refractive index reduces the maximum totalscattering and increases absorption.


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Shape: anisotropy

Tuning by shape results instrong, narrow resonances.

Polarization dependency is offsetby extremely high extinction

efficiency

Selectively target wavelengthrange to scatter, or combinemultiple particle types forbroadband scattering.

Reduced effect of higher-ordermodes and interband absorption.

More complex design andfabrication than spheres.


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(8) It is difficult to avoid unwantedabsorption in metal nanoparticles


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Radiative efficiency overview

Ag sphere

N=1.550 nm radius


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30[Temple and Bagnall, Progress in Photovoltaics, 2012]


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31[Temple and Bagnall, Progress in Photovoltaics, 2012]

Ag: any position, any materialAu: rear of any device

Al: a-Si onlyCu: rear thick c-Si


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(9) Plasmonic properties are modifiedby the photonic environment


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Plasmonic mirror

plasmonic mirror should reflect all light

maximise diffuse reflection, minimise absorption

at the moment our experiments dont include silicon (adding silicon will

make a big difference)

The plasmonic mirror


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FIG. 1. (a) Cross- section SEM image of the fabricated plasmonic mirror. Image taken at atilt of 36o to show the nanodisc array (b) Measured total and diffuse reflectance spectrafor random arrays of 100nm Ag nanodiscs at 6%surface coverage, on varying thicknessesof SiO2 on a Ag mirror.

Sesuraj and Bagnall, rejected APL (2012)(!)


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Simulated spectra of the scattered power for Ag nanodiscs of 100nm, 150nm and 200nm diameteron 110nm SiO2- on- Ag mirror (b) Optical characterisation results and (c- e) SEM images for

plasmonic mirror samples with 100nm, 150nm and 200nm nanodiscs at 6%surface coverage, on110nm SiO2- on- mirror substrate


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(10) There are alternatives to plasmonics? -Wavelength-scale features


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40Banakar, PVSAT8

Nanowires are:

Anti-reflective

Light-trapping

Scattering

Photonic bandgap

Bulk-junctions


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41Naughton et al, Phys. Status Solidi (2010)


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(11) Wondrous things are possible(but are they practical?)


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Fraunhofer: Ralf B. Wehrspohn, Johannes pping,Thomas Beckers and Reinhard Carius (SPIE 2011)

Documents

Photonics for Photovoltaics.pdf