absorption enhancement,inter particle coupling,localised surface plasmon,metal nano particles,mie theory,plasmonics,quadrupolar modes,silver,spacer layer, amorphous silicon

1. Plasmonic assisted optical absorption
enhancement in thin films
a theoretical perspective
Aparajita Mandal
JRF, department of ERU
Indian Association for the Cultivation of Science

2. ICSEP
19-21 dec. 2012

Outline
Plasmonics:
• Goal of the present work
• Simulation Model
• Results and Interpretations
• Conclusion

3. Plasmonics
Localized Surface plasmon:
The stationary oscillation of conduction electrons confined
on a nano sized metal boundary under the excitation of
electromagnetic field.

4. ICSEP
19-21 dec. 2012
Plasmonics
Particle size dependence

5. • Plasmonics: Particle Size dependence
Electrostatics
+
++ ++
Dipolar mode
R<< λ
-- - - Phase retardation
across the volume
R ≈λ
-
-
+
+
+
+
--
Electrodynamics
Higher order
mode

6. Advantages
• Local field enhancement (Light can be confined to a volume which is much
smaller the wavelength).
•
Enhanced light scattering
( eff   g )
• 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.

7. ICSEP
19-21 dec. 2012
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.

scat / abs
 scat / abs
  n
n 1
Where σ n scat/abs is the contribution of the nth normal mode.
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.

8. ICSEP
19-21 dec. 2012
Effects of higher order modes on extinction spectra

Higher order modes introduce additional peaks at shorter wavelengths than
dipolar peak.

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19-21 dec. 2012
•
For an array of MNPs
Electromagnetic coupling exists between MNPs, strength of which is
determined by the interparticle seperation.
+
+
-
+
-
-
+
Nature of interaction is dependent on polarization of incident light

10. ICSEP
19-21 dec. 2012
Effect of inter-particle coupling on extinction
spectra
•
Redshift or blue shift of resonance wavelength.
•
Nature of shift depends on the increase or decrease in the restoring
force of oscillating electron cloud.
•
spectral overlap between different modes.

11. Absorption Enhancement in thin films
•
In thin film solar cells such as amorphous silicon (a-Si:H) solar cells the
intrinsic layer need to be thin ( 300nm) in order to reduce the light
induced degradation.
• This thickness restriction results in reduced absorption of the solar
spectrum and low short circuit current of such solar cells.
•
Plasmonics is a promising way of enhancing light absorption within the
thin absorber layer.

12. ICSEP
19-21 dec. 2012
Goal of the Present work
 To determine the overall contribution on the optical absorption enahncement within an
hydrogenated amorphous silicon film (a-Si:H) of thickness 200nm of front patterned Ag
MNPs having size of the order of visible wavelength.
 Going beyond dipolar contribution on absorption : Ag MNPs are large enough to
support higher order plasmon modes.
 To investigate how coupling between the MNPs modifies the higher order modes and
thereby influencing the absorption.
 Understanding the role of Spacer layer thickness for front patterning.
 Usefulness of large MNPs.

13. Ag nano particles array for enhancing light absorption within the aSi:H thin film
Plane wave
illumination
Ag MNPs
2R
D
SiO2
a-Si:H
Structure used for simulation using Finite Difference Time Domain (FDTD)
Method that solves Maxwell’s curl equations of electromagnetism
Parameters: R, D, surface coverage, SiO2 thickness, shape and type of
metal nano particles (Ag, Au or Al).

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19-21 dec. 2012
FDTD simulation span
Simulation Model
PML
PBC
 Periodic Boundary Condition (PBC)
models the periodicity of the MNPs in
X and Y directions.
PBC
PBC
PBC
It also takes into account interparticle
coupling effect.
Plane wave
illumination
Ag MNPs
SiO2
a-Si:H
 Perfectly Matched Layer (PML)
prevents any non physical reflections
from Z direction.
PML
Schematic of the structure simulated using
FDTD method

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19-21 dec. 2012
Results
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 %.
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 %.

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19-21 dec. 2012
Parameter that makes the difference is
inter particle coupling

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19-21 dec. 2012

Interpretations
Increases the surface coverage from 31% to 58% increases the electromagnetic
coupling between the L-MNPs .
 As a result, the distinct DP and QP peak in the scattering spectra combines together
resulting in a cooperative spectral property [1] that produces a single and broad spectral
absorption 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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19-21 dec. 2012
Results
Total Integrated absorption
To comprehend the optical absorption due to AM1.5G solar
radiation, we have calculated the total integrated absorption as,
P 
T
800
 I    P   d 
abs
400
I (λ) describes the irradiance of AM1.5G at λ.

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19-21 dec. 2012
Observations
 We observe a second maxima (PST) in PT.
If we consider PT for a wide range of R
from 40 nm -200 nm, there will be two
peaks : One occuring for smaller R that
have dipole dominated extinction spectra
[2] , another (PST) for larger R
supporting higher order modes .
 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)
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.

20. ICSEP
19-21 dec. 2012
Results
Absorption enhancement over the visible spectrum
Absorption enhancement,     
Enhancement throughout broad spectrum
Pabs   
Pb ( )
Enhancement within narrow wavelengths band
ɳ˂1 is represented as black regions

21. Role of SiO2 (spacer layer) thickness
SiO2
Optimum Optimum
Total
thickness Radius
Period Integrated
(nm)
(nm)
(nm)
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:
•
•
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].
t =0
t = 2 nm
t = 20 nm

22. Choice of metal:
Comparison between Ag, Au and Al
Absorption Enhancement
1.4
1.2
Al
Au
Ag
1.0
0.8
0.6
400
450
500
550
600
650
700
750
wavelength (nm)
 Proper choice of metal depending on the absorption enhancement in the desired
wavelength range can be made.

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19-21 dec. 2012
Conclusion
•
There is significant contribution of quadrupolar mode in optical
absorption.
•
Using the size characteristics of dipolar and quadrupolar plasmon modes
and their dependence on the array period, it is possible to tailor position
of absorption peaks.
•
The enhancement may be tuned to occur either over a broad spectral
range or as a narrow band as demanded for the device application.
•
Aluminum would be a promising candidate in plasmonic solar cell
applications as it can provide efficient absorption enhancement in
the visible wavelength range.

24. 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.

25. ICSEP
19-21 dec. 2012
Thank you for your kind attention

26. 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 aSi: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.