1. IV th International Conference on Advances in Energy Research
10-12 December 2013 @ IIT Bombay, Mumbai
Tapered Silicon Nanopillars for enhanced
performance thin film solar cells
Avra Kundu, Sonali Das, S. M. Hossain, Swapan K. Datta
and Hiranmay Saha*
DST SOLAR HUB
Centre of Excellence for Green Energy and Sensor Systems
Bengal Engineering and Science University,
Shibpur, Howrah
ICAER, IIT-B, 10 Dec 2013
Prof. Avra Kundu, CEGESS, BESU

2. Contents
Introduction
Nanotexturing in solar cells
Tapered silicon nanopillars
Design
Optimization
Realization
Characterization
Conclusions
ICAER, IIT-B, 10 Dec 2013
Prof. Avra Kundu, CEGESS, BESU

3. Introduction
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Prof. Avra Kundu, CEGESS, BESU

4. Solar Cell Structure and basic operation
Injection of light into
the cell (Photons)
Absorption of photons
and creation of EHPs
Collection of EHPs for
flowing in external circuit
ICAER, IIT-B, 10 Dec 2013
Prof. Avra Kundu, CEGESS, BESU

5. Foremost important criterion
Reduction of Reflectance
Now, for bare Silicon substrate
Light
nair=1.00
nSi=3.45
ICAER, IIT-B, 10 Dec 2013
Prof. Avra Kundu, CEGESS, BESU

6. Conventionally, for maximum photon injection
Graded multi layer AR coatings on Si
 Better phase matching between the layers
 Reflectance minimized
Produce broadband response
Complex structure
Adds up cost for large scale fabrication
n0
n1
n2
n3
n4
ns>n4>n3>n2>n1>n0
ns
ICAER, IIT-B, 10 Dec 2013
Prof. Avra Kundu, CEGESS, BESU

7. Conventionally, for maximum photon injection
Micro-texturization on Si (formation of hillocks with an height in range
of 5-10µm.
 Light trapping by multiple reflections
Produce broadband response
Reduces reflection in the range of 13-15%
Not suitable for thin- film solar cells
ICAER, IIT-B, 10 Dec 2013
Prof. Avra Kundu, CEGESS, BESU

8. What do we do??
We turn to nature (Bio mimetics)
(a) Image of the insect sample used in this study: the chestnut leafminer Cameraria
ohridella. (b) SEM image giving an overview of the head morphology.
(c)and(d)IIT-B, 10 Dec 2013 of the corneal surface, revealing the corneal nipples.
Prof. Avra Kundu, CEGESS, BESU
ICAER, Higher magnifications

9. A closer look
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Prof. Avra Kundu, CEGESS, BESU

10. The mechanism
If D<λ the refractive index takes up an effective value between
Silicon and air as the structure cannot be resolved by light. (SWSs)
Transition between the air-material interface thus appears as a
continuous boundary and we can fool light.
Also, for H/λ~0.4, reflectance reaches 0%.
ICAER, IIT-B, 10 Dec 2013
Prof. Avra Kundu, CEGESS, BESU

11. The mechanism (contd.)
The graded zone shows the transition of wavelength between these two
extremes without any reflection because of the gradual change in density and
refractive index (“n”) with depth.
ICAER, IIT-B, 10 Dec 2013
Prof. Avra Kundu, CEGESS, BESU

12. A novel idea:
Nanotexturing in solar cells
ICAER, IIT-B, 10 Dec 2013
Prof. Avra Kundu, CEGESS, BESU

13. Simulation Model
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Prof. Avra Kundu, CEGESS, BESU

14. Choosing the base diameter
In order to form a radial junction in the tapered nanopillar, a
junction depth of around 200nm has been considered.
Therefore for a junction depth of 200nm on either side of the
nanopillar, the minimum base diameter of the nanopillar
with some separation between junctions (~100nm) comes to
about 500nm.
ICAER, IIT-B, 10 Dec 2013
Prof. Avra Kundu, CEGESS, BESU

15. Optimization of top diameter
Variation of top diameter from 100nm to 500nm
the base diameter of 500nm, height of 500nm, period of 500nm.
 Reflectance is reduced for a nanopillar as its top diameter goes on decreasing.
 This is mainly due to the gradual grading of the refractive mismatch between silicon and air.
It is important to point out here that for a dtop =
0nm, for which the structure resembles that of a
cone has not been considered as such pointed
interfaces will lead to greater difficulty in forming
good front surface ohmic contacts with low
contact resistance
Therefore dtop = 100nm has been considered where an integrated reflectance of 1.9% over the
wavelength region of 300-1100nm has been obtained.
ICAER, IIT-B, 10 Dec 2013
Prof. Avra Kundu, CEGESS, BESU

16. Optimization of height
Variation of height of 500nm, 1000nm, 1500nm
dbase = 500nm, dtop = 100nm, period =500nm
It
is
observed
that
for
nanopillars having a height of
1500nm,
the
integrated
reflection is of the order of
0.265% making it a suitable
choice.
ICAER, IIT-B, 10 Dec 2013
Prof. Avra Kundu, CEGESS, BESU

17. Optimization of period
Variation of period of 500nm, 550nm, 600nm
dbase = 500nm, dtop = 100nm, h = 1500nm
It is seen that a minimum period of 500nm
gives an optimum reflection characteristic.
This corresponds to the fact that tapered
nanopillars are to be formed in close proximity
without any gap between the adjacent pillars.
This helps in a gradual grading in the
refractive index as well as the density between
air and bulk silicon to achieve a better optical
performance.
ICAER, IIT-B, 10 Dec 2013
Prof. Avra Kundu, CEGESS, BESU

18. Path length enhancement in the optimized
geometry
Optimized nanopillar
(dtop = 100nm, dbase = 500nm, h = 1500nm, p= 500nm)
It is seen that that the angular scattering from the nanopillars is very high (θmax = 420 at
500nm) which causes significant light trapping which becomes more prominent for thin solar
cells with back surface reflector structure provided by the back metal contact.
ICAER, IIT-B, 10 Dec 2013
Prof. Avra Kundu, CEGESS, BESU

19. Absorption in the optimized geometry
--
1500nm
Case 3
Cases
500nm
Case 2
Case 1
-0
2
4
6
8
10
20
12
2
Total number of absorbed photons (X10 /m /s)
ICAER, IIT-B, 10 Dec 2013
Prof. Avra Kundu, CEGESS, BESU

20. Creativity in midst of old:
Realization
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Prof. Avra Kundu, CEGESS, BESU

21. The good news…
With the advent of nanotechnology such SWSs
can now be realized
 Reactive Ion Etching in the nanoscale with suitable
masks
Nano- masks of Silica, Polystyrene balls etc.
 Selective etching through Alumina template
 Nano Imprint Lithography (NIL)
ICAER, IIT-B, 10 Dec 2013
Prof. Avra Kundu, CEGESS, BESU

22. The process
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23. Results
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24. Results (contd..)
The average reflection loss reduces
significantly (~5% over the spectral range
of 300-1100nm).
Bentham PVE 300
ICAER, IIT-B, 10 Dec 2013
Prof. Avra Kundu, CEGESS, BESU

25. Conclusions
 A significant reduction in reflection is achieved by nanotexturization of the
silicon surface.
 As the geometry is capable of realizing radial junctions, this structure will allow
100% collection of the generated carriers due to absorbed photons in the
nanopillars.
Further parametric optimization of the etching process is still ongoing for
obtaining the desired nanopillar geometry.
 A drastic improvement in reflection over bare Si solar cells and microtextured
solar cells has been achieved.
 The challenge of an increased surface recombination velocity due to an increase
in the surface area needs to be addressed.
ICAER, IIT-B, 10 Dec 2013
Prof. Avra Kundu, CEGESS, BESU

26. Acknowledgements
Department of Science and Technology
(DST)
for
providing
necessary
financial
support.
Prof. A.K. Barua and Prof. R. Bhattacharya.
All members of CEGESS
ICAER, IIT-B, 10 Dec 2013
Prof. Avra Kundu, CEGESS, BESU

27. THANK YOU
ICAER, IIT-B, 10 Dec 2013
Prof. Avra Kundu, CEGESS, BESU