Plasmonic assisted optical absorption

enhancement in thin films

a theoretical perspective

Aparajita Mandal

JRF, department of ERU

Indian Association for the Cultivation of Science

Outline

Plasmonics:

• Simulation Model

• Goal of the present work

• Results and Interpretations

• Conclusion

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Plasmonics

Localized Surface plasmon:

The stationary oscillation of conduction electrons confined on a nano sized metal boundary under the excitation of electromagnetic field.

Plasmonics

Particle size dependence

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• Plasmonics: Particle Size dependence

Electrostatics

-

--

++

++

-Phase retardation across the volume

Electrodynamics

---

+++ +

-

+

-

Dipolar mode

Higher order mode

R<< λ

R ≈λ

Advantages

• Local field enhancement (Light can be confined to a volume which is much smaller the wavelength).

• Enhanced light scattering

• Tunability over the optical properties (such as resonance frequency, forward or backward scattering, bandwidth) of the nano particles by controlling the size, shape and dielectric environment.

( )eff g

Mie Theory

Overall scattering or absorption cross section σ scat/abs for a single NP as a sum of the contribution from all the normal modes supported by the particles.

Where σ nscat/abs is the contribution of the nth normal mode.

/ /

1

scat abs scat abs

n

n

Theoretically infinite number of plasmon modes is possible….

For very small particles , contribution of only dipolar (DP) mode (n = 1 ) is effective.

As particle size increases, higher order modes (e.g. n = 2 corresponds to quadrupole and so on) gradually start to dominate the extinction spectra.

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Effects of higher order modes on extinction spectra

Higher order modes introduce additional peaks at shorter wavelengths thandipolar peak.

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For an array of MNPs

• Electromagnetic coupling exists between MNPs, strength of which isdetermined by the interparticle seperation.

+ +

- -

+

-

+

-Nature of interaction is dependent on polarization of incident light

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Effect of inter-particle coupling on extinction spectra

• Redshift or blue shift of resonance wavelength.

• spectral overlap between different modes.

• Nature of shift depends on the increase or decrease in the restoringforce of oscillating electron cloud.

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Absorption Enhancement in thin films

• In thin film solar cells such as amorphous silicon (a-Si:H) solar cells theintrinsic layer need to be thin ( 300nm) in order to reduce the light

induced degradation.

• This thickness restriction results in reduced absorption of the solarspectrum and low short circuit current of such solar cells.

• Plasmonics is a promising way of enhancing light absorption within thethin absorber layer.

Goal of the Present work

To determine the overall contribution on the optical absorption enahncement within anhydrogenated amorphous silicon film (a-Si:H) of thickness 200nm of front patterned AgMNPs having size of the order of visible wavelength.

Going beyond dipolar contribution on absorption : Ag MNPs are large enough tosupport higher order plasmon modes.

To investigate how coupling between the MNPs modifies the higher order modes andthereby influencing the absorption.

Usefulness of large MNPs.

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Understanding the role of Spacer layer thickness for front patterning.

Plane wave

illumination

a-Si:H

SiO2 Ag MNPs

2R D

Ag nano particles array for enhancing light absorption within the a-Si:H thin film

Parameters: R, D, surface coverage, SiO2 thickness, shape and type of metal nano particles (Ag, Au or Al).

Structure used for simulation using Finite Difference Time Domain (FDTD) Method that solves Maxwell’s curl equations of electromagnetism

Simulation Model

Plane wave

illumination

PML

PML

PBC PBC

PBC

a-Si:H

SiO2Ag MNPs

FDTD simulation span

PBC

Periodic Boundary Condition (PBC)models the periodicity of the MNPs inX and Y directions.

It also takes into account interparticlecoupling effect.

Perfectly Matched Layer (PML)prevents any non physical reflectionsfrom Z direction.

Schematic of the structure simulated using FDTD method

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Results

Figure 2. For the array of Ag nano particles with radii R and

constant array period of 350 nm. Covergae factor changes

from 31 % to 58 %.

Fraction of light absorbed (Pabs (λ)) with respect to wavelength (λ)

Figure 1. For the array of Ag nano particles with radii R

and constant surface coverage of 31 %.

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Parameter that makes the difference is inter particle coupling

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Interpretations

Increases the surface coverage from 31% to 58% increases the electromagneticcoupling between the L-MNPs .

As a result, the distinct DP and QP peak in the scattering spectra combines togetherresulting in a cooperative spectral property [1] that produces a single and broad spectralabsorption peak within the a-Si:H layer.

Redshift in figure 1 is associated with both the effect of changing R and D.

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To comprehend the optical absorption due to AM1.5G solarradiation, we have calculated the total integrated absorption as,

800

400

absT

P I P d

I (λ) describes the irradiance of AM1.5G at λ.

Results

Total Integrated absorption

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Observations

We observe a second maxima (PST) in PT.

If we consider PT for a wide range of Rfrom 40 nm -200 nm, there will be twopeaks : One occuring for smaller R thathave dipole dominated extinction spectra[2] , another (PS

T) for larger Rsupporting higher order modes .

We attribute the occurrence of PST as resulting from the optimum coupling

between the higher order modes of L-MNPs depending upon the values of D.

Position of PST with respect to R changes

with D.Total integrated absorption PT as a function of Ag nano particle radii

for array period (a) 350 nm (b) 380 nm (c) 400 nm (d) 450 nm.

PST indicates the second maxima due to higher order modes.

r denotes PT in bare a-Si:H (without MNPs)

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Results

Absorption enhancement over the visible spectrum

Absorption enhancement,

ɳ˂1 is represented as black regions

Enhancement throughout broad spectrum Enhancement within narrow wavelengths band

( )

abs

b

P

P

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Role of SiO2 (spacer layer) thickness

SiO2

thickness

(nm)

Optimum

Radius

(nm)

Optimum

Period

(nm)

Total

Integrated

Absorption

(Watt m-2)

0 75 400 191.6

2 65 350 201.58

20 65 400 196.14

For nano particles having fixed geometrical cross section, increase in SiO2 layer thickness:

t = 0 t = 2 nm t = 20 nm

• Increases the effective scattering cross section of the NPs.• Decreases the coupling fraction of scattered light into the a-Si:H layer.

Therefore, there exists an optimum spacer layer thickness [Table.1].

Choice of metal:Comparison between Ag, Au and Al

400 450 500 550 600 650 700 750

0.6

0.8

1.0

1.2

1.4

Absorp

tion

En

han

cem

en

t

wavelength (nm)

Al

Au

Ag

Proper choice of metal depending on the absorption enhancement in the desiredwavelength range can be made.

Conclusion

• There is significant contribution of quadrupolar mode in opticalabsorption.

• Using the size characteristics of dipolar and quadrupolar plasmon modesand their dependence on the array period, it is possible to tailor positionof absorption peaks.

• The enhancement may be tuned to occur either over a broad spectralrange or as a narrow band as demanded for the device application.

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• Aluminum would be a promising candidate in plasmonic solar cellapplications as it can provide efficient absorption enhancement inthe visible wavelength range.

Publications:

Jounal publication:

1. Aparajita Mandal and Partha Chaudhuri, Size and period optimization of front

pattered interactiong metal nanoparticles for maximizing absorption of solar

radiation in amorphous silicon thin films, J. Opt. 14 (2012) 065001 (6pp).

2. Aparajita Mandal, Partha Chaudhuri, Journal of Renewable and Sustainable

Energy 5, 031614, 2013.

3. Aparajita Mandal and Partha Chaudhuri, “Contribution of higher order plasmonic

modes on optical absorption enhancement in amorphous silicon thin films”,

Volume 300, Pages 77–84, 2013.

Conference Proceedings:

1. Aparajita Mandal and Partha Chaudhuri, Simulation of optical absorption

enhancement in amorphous silicon using front surface patterning with metal

nanostructures, Energy and Eco friendly Materials, December 12-16th,

Coimbatore, India, page 67-70, Macmillan Advanced Research Series.

Thank you for your kind attention

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Additional details…..

Scattering cross section ( ) with respect to wavelength of a single Ag nano particle in air as a function of radius . Dipolar (DP), Quadrupolar (QP) and Octupolar (OP) resonances are indicated.

Plasmonic contribution in optical absorption within a 200 nm thick a-Si:H layer as a function of wavelength for an array of Ag nano particles radii of (a) 135 nm and (b) 65 nm. Peaks in due to the dominating dipolar (DDP) and quadrupolar mode (DQP) is shown.