201 icaer ppt_sonali_v1

IV th International Conference on Advances in Energy Research
10-12 December 2013 @ IIT Bombay, Mumbai

Mixture of metal and dielectric nanoparticles for
improved performance of silicon solar cell
Sonali Das, Prasenjit Dey, Avra Kundu, S. M. Hossain, H. Saha,
Swapan K. Datta

DST SOLAR HUB
Centre of Excellence for Green Energy and Sensor Systems
Bengal Engineering and Science University,
Shibpur, Howrah

ICAER, IIT-B, 11 Dec 2013

Sonali Das, CEGESS, BESU

Contents
 Towards high efficiency solar cell
 Nanoparticles – A Brief Review

 Choice of Nanoparticles
 Objectives
 Design, Simulation, Optimizations

 Experiments carried out
 Conclusion



Requirements for high efficiency silicon solar
cells



Requirement 1
Key requirement 1
Maximized injection of photons into the cell by designing an antireflection coating at the
front surface which reduces reflection coefficient without significant loss of energy due to
Joule heating.

Bare Silicon is highly
reflective

Avg. reflection ~30%


A single transparent dielectric
layer as Anti reflective coating
on TOP surface of Silicon
Reduce reflection to ~18%


Requirement 2
Key requirement 2

absorption

of

injected photons with better
collection by silicon

Fraction of injected photon that is absorbed

Maximized

1.0
0.9
0.8
0.7
0.6
0.5
0.4
200m

0.3

20m
2m

0.2

1m

0.1
0.0
300

400

500

600

700

800

900

1000

1100

Wavelength (nm)

Light absorption in silicon solar cell becomes critical as the thickness of an absorber layer is
decreased to reduce cost.

To compensate for lower light absorption in such physically thin devices, we have to
incorporate light-trapping schemes in order to increase their optical thickness.



Texturization – fulfilling both requirements

Textured on the TOP
surface of Silicon
Reduce reflection to ~13%

Textured front surface
of Silicon with ARC
Light absorption due to
path length enhancement

Reduce reflection to ~3-4%

Texture dimensions are of the order of 1 – 10 µm. Such large-scale geometries are
not suitable for thin-film cells ( 1-2 µm ).
It increases minority carrier surface recombination due to greater surface area
reducing the collection efficiency of the photo-generated carriers.
Novel approaches are needed for photon injection management and light trapping
without texturing in both thick and thin silicon solar cells.



Recently, nanoparticles have been proposed as an
alternative method to reduce reflection and achieve light
trapping in silicon solar cells

H.A.Atwater and A.Polman , Plasmonics for improved photovoltaic devices , Nature Materials , 9 , 205 – 213 ( 2010 )



Nanoparticles (NPs)
A Brief Review



Schematic Representation of a NP
Direction of
incident light

Cext=Csca+Cabs

R
Cext
Csca
Cabs
T

Csca (Scattering Cross- section):The area, on which if the radiation is incident,

will

Scattering and the power scattered by the particle
scatter the same power as absorption cross-sections depend on
Cabs (Absorption Cross- section):The area, on which if the radiation is incident, will
polarizability
absorb the same power as the power absorbed by the particle



Polarizability
α is the polarizability of the particle, given by

 p /  m 1
  3V
 p / m  2

V is the particle volume , εp is the dielectric function
of the particle and εm is the dielectric function of
the embedding medium

Resonant enhancement happens when |2 εm + εp | is minimum
 At the plasmon resonance frequency, polarizibility becomes maximum.
 Scattering becomes maximum well exceeding the geometrical cross section
of the particle at the plasmon resonance.



Attributes of NPs needed for their
incorporation in Si solar cell in Si Solar Cell
Features of NPs needed for their incorporation
Efficient scattering in the 300nm-1100nm wavelength region

Absorption leading to joule heating should be minimized

Csca


Cabs


Choice of Nanoparticles



Choice of metal nanoparticles

Simulated optical extinction (black lines), scattering (blue lines) and absorption
(red lines) efficiencies of 2013 diameter metal spheres in air Das, CEGESS, BESU
Sonali .
ICAER, IIT-B, 11 Dec 100 nm

Choice of dielectric nanoparticles
Silica (1.46)
d=100nm

Silicon nitride (2.05)
d=100nm

Silica Nanoparticles can be easily realized by
well known Stober technique
Titanium dioxide (2.62)
d=100nm



Key parameters
Key parameters to be monitored for achieving high efficiency
solar cell
Injection of incident photons
Absorption of injected photons: Path length enhancement
Collection of electron hole pairs from absorbed photons



Injection into substrate
Silica NPs (100nm radius)

Ag NPs (100nm radius)

1.8

Inj

On
Si

with nano

/Inj

without nano

1.4
1.2

bare si
10%
20%
40%
60%
78%

Injwith nano/Injwithout nano

1.6

1.0
0.8
0.6
0.4

1.6
bare si
10%
20%
40%
60%
78%

1.4

1.2

1.0

0.2
0.0
300

400

500

600

700

800

900

1000

1100

0.8
300

wavelength (nm)

400

500

600

700

800

900

1000

1100

wavelength (nm)

For bare Si



Path length enhancement inside substrate
Ag NPs (100nm radius)

Silica NPs (100nm radius)

1.6

1.015

(Pa/Pt)nano/(Pa/Pt)bare

On
Si

1.4

(Pa/Pt)nano/(Pa/Pt)bare

bare si
10%
20%
40%
60%
78%

1.5

1.3
1.2
1.1
1.0
300

400

500

600

700

800

900

1000 1100

bare
10%
20%
40%
60%
78%

1.010

1.005

1.000
300

wavelength (nm)

400

500

600

700

800

900

1000 1100

wavelength (nm)

For bare Si



Summing it up…
Metal Nanoparticle

Dielectric Nanoparticle

Scattering Efficiency
Angular Scattering
Path length enhancement
inside substrate
Enhanced Photon
Absorption due to path
length enhancement
No ohmic losses
Phase matching
between layers


Therefore,
a judicious mixture of metal and dielectric
nanoparticles
may help us in utilizing the positive aspects of
each of the nanoparticles.



Objectives
To enhance
Injection of incident photons
Path length of injected photons : Absorption of injected photons
Collection of electron hole pairs

Optimization of
Material of nanoparticles(silver or silica)
Size of nanoparticles

Area Coverage of nanoparticles


Design Methodology
 Silver nanoparticles of varying size have been placed on top of silicon
substrate with different area coverage for obtaining the maximum
absorption of incident power inside silicon.

 After obtaining an optimum size and coverage of the silver nanoparticles,
the remaining bare surface of the silicon is covered with an optimum size of
silica nanoparticles for reducing the reflection loss even further.
Direction of incident light

Silicon



Simulation Model
Lumerical FDTD Solutions, www.lumerical.com

Pabs/Pinj is the most important term to be monitored
Pabs absorbed power within the given silicon block  Power Monitor 2 - Power Monitor 3


Pinj
injected light into silicon
Power Monitor 2



Analytical Calculations
From the simulations…

cos(av ) 

the path length of the oblique light
into the solar cell

W
 

Pabs () 

ln 1  
 Pinj () 

 
 with _ nano 



Subsequently,
Number _ of _ absorbed _ photons 

1100nm 

1  e

300nm




Wb

cos av

Wb



1  R bn e cos av

W


cos av
1 R
fn _ with _ nano R bn e




 T()N 0 ()d




Finally,

 ez  R e 2Wb  z 
bn
Generation _ Rate  G  , z   N 0 ()T()() 
 1  R bn R fn _ with _ nano e2z


EHPs 

1100nm J



300nm

ph

q


d 

1100nm








T()N0 ()IQE()d

300nm


Optimization: Silver Nanoparticles
 Size of nanoparticles
(radius: 10nm – 200nm)
 Area coverage of
nanoparticles (10%-50%)

To summarize, an increase in the collected EHPs is obtained for 15±5%
coverage of 100nm particles and 40±5% coverage of 50nm particles.


Number of electron- hole pairs collected (X 1021/m2/s)

Case Study
Case 1

100nm Silver: 20% coverage

Case 2

25nm Silica: Full coverage

Case 3

100nm Silver; 20% coverage
25nm Silica: Remaining

1.8

1.7


1.6

Case 4



1.5

Case 5

1.4
Case 1

Case 2

Case 3

Cases


Case 4

Case 5



Experiments carried out



Preparation of Silica Nanoparticles by modified
Stober technique
HYDROLYSIS
Hydrolysis of Tetra ethyl ortho- silicate
suspension _ in _ ethanol
Si(OC2 H5 )4  4H 2O  Si(OH )4  4C2 H5OH

pH _1112 _ NH3

POLYCONDENSATION
suspension _ in _ ethanol
Si(OH )4  SiO2 (sol )  2H 2O

pH _1112 _ NH3

CENTRIFUGATION AND WASHING IN PREPARED
MEDIUM 2-3 times

FESEM image of silica nanoparticles

DRYING OF CENTRIFUGED PARTICLES
at 500C for 5 hours
ULTRASONICATION in desired medium for final
COLLOIDAL SOLUTION



Size Variation of Silica Nanoparticles
It is observed that
the average particle size is
80nm for methanol,
300nm for ethanol and
500nm for propanol.

DLS by silica NPs prepared in different
alcohol media


As alcohol molecular weight increases from
methanol to propanol, the average particle
size increases from 80nm to 500 nm.
This can be attributed to a change in viscosity
or the polarity of the solvent caused by the
increased molecular weight of the alcohol.


FTIR of Silica Nanoparticles
The FTIR spectra of the colloidal silica NPs
show prominent absorption
band arising from asymmetric vibration of
Si-O-Si at the wave number 1090 cm–1.
C-O bonding (1400-1800 cm–1) and Si-C
(2357 cm–1) are also observed due to bare
polished Si wafer itself (inset of Figure 1).

FTIR of prepared silica NPs in ethanol medium spin coated
on bare polished Si wafer measured by Shimadzu Solid Spec
3700 UV-VIS-NIR Spectrophotometer (Inset: FTIR of bare
polished Si wafer).



Silver Nanoparticles from Nanocomposix

FESEM image of silver nanoparticles


DLS by silver NPs of Nanocomposix


Nanoparticles’ Specification

Diameter

Silica Nanoparticle
300nm

Mass Concentration
Particle
Concentration
pH of the solution

17mg/ml
453.7E+09
particles/ml
11

Particle Surface
Solvent

Uncoated
Ethanol


Silver Nanoparticle
100nm
0.020mg/ml
3.7E+09 particles/ml

5.7
PVP
DI Water


Results
Experiments have been carried out by preparing a colloidal solution of 1:1::silver: silica nanopa
rticles. The mixture of the colloidal solution was then spin coated on the bare silicon surface.

FESEM image showing a mixture of silver and silica nanoparticles



Results (contd.)
Reflection measurements of
the samples have been done
using Bentham PVE 300
Photovoltaic Characterization
equipment.

It is seen that the average
reflectance of the bare
surface (~ 30 %) decreases to
a value of about 12%.



Results (contd.)
Further, cell has been fabricated with this coated wafer.
An enhancement in short-circuit current density of about 28% is obtained from a
baseline value of 14.5mA/cm2.



Conclusions
Maximum enhancement in absorption occurs with a mixture of 100nm radius silver
nanoparticles having 20% coverage along with 50nm radius silica nanoparticles
covering the remaining bare surface.
Experiments are currently underway to obtain the desired design coverage with
synthesized silver and silica nanoparticles.



Acknowledgement
Department of Science and Technology (DST) for
providing necessary financial support.
 All members of CEGESS



THANK
YOU


A new approach: Plasmonic nanoparticle
Oscillating Electromagnetic energy

e-


+


Why nanoparticles on the top

Backward Forward
Scattering Scattering

Path
length

Reduced Reflection
Increased Absorption near the Junction


Choice of nanoparticles
Metal
 High scattering efficiency
 Presence of dipolar
resonance
 Suffers from ohmic
dissipation and absorption loss

Dielectric
 Low scattering efficiency
 Absence of dipolar resonance but
presence of higher quadrupolar
modes
 No ohmic dissipation and
absorption loss

Metal nanoparticles can reduce reflection with increased photon injection when
applied on the top of bare Si.
But its not the case when applied on the top of an optimized AR layer because of the
loss of energy due to the metal absorption itself.

So, dielectric nanoparticles have been chosen.


Further for Ag NPs on nitride coated Si SCs...
1.4

(Tf)nano / (Tf) nitride

1.2

nitride on bare
10%
20%
30%
40%
50%
60%
70%
78%

100nm radius Ag NP

5%

1.0

0.8
0.6
0.4

0.2
0.0
300

400

500

600

700

800

900

1000

1100

wavelength (nm)

Enhancement of injection w.r.t the bare silicon
Degradation of injection w.r.t the nitride silicon

Metal (Ag) nanoparticles may not be beneficial for

enhancement in efficiency of ARC silicon solar cells .


Cross-sections and efficiencies of NP
For small spherical NPs (sizes<1/10 th the wavelength (λ) of light), the scattering,
absorption and extinction cross sections/efficiencies

1 2 4 2
2
Cext  Csca  Cabs
( ) 
Cabs 
Im( )
6 

C
Csca
Cabs
Qext  ext2
Qsca  2
Qabs  2
a
a
a
The dynamic depolarization effect becomes predominant for larger radius particles
where all the electrons do not oscillate in phase
Csca 

For large spherical NPs (sizes>1/10 th the wavelength (λ) of light), the scattering,
absorption and extinction efficiencies (by Mie Theory)
Qsca

2
 2
x



 (2n  1)(| a
n 1

n

|2  | bn |2 )

2
Qext  2
x

where an and bn are the Mie Coefficients, n is
the index running from 1 to ∞ and x is the
size parameter




 (2n  1) Re(a
n 1

n

 bn )

Qabs  Qext  Qsca

For infinite series, n is truncated to nmax.

nmax  x  4 x1/ 3  2


201 icaer ppt_sonali_v1

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