Overview of advantages and fabrication of solar cells made from silicon nanowires. IT includes few slides of conventional solar cells and benefits of using silicon nanowire.

1. Si Nanowire Based Solar Cells
Submitted By:
Urvi Sharma
(13ESKEC086)
Submitted to:
Mr. Rahul Pandey

2. What is a Solar Cell ? • A device that converts solar
energy directly to electricity
by photovoltaic effect:
• It supplies voltage and
current to a resistive load
• It supplies DC power
• Size 10x10 cm :size of a CD
• Thickness is in fractions of
mm
• Metal pattern is to make
electrical contacts.

3. Solar Cells are:
• Safe
• Clean
• Durable
• Reliable
• Quiet
• Installable anywhere

4. Energy conversion in a Solar Cell
• Light is shone
• Electrons are knocked
out
• Electrons and holes
move in opposite
directions
• Electrical output is
generated between the
contacts

5. Current Technology: Photovoltaic Cells
Light in, electricity out !
• If energy of the incident
photons equals or exceeds
the band gap of the material,
then electrons move from
valence band to conduction
band.
• They are susceptible to
electric field and form
electricity.

6. The Drawback
• Not all energy is converted back.
• Solar cells can only absorb a certain wavelengths.
• Light that isn't absorbed is either reflected back or transmitted
through.
• Less efficient and high manufacturing cost.
• They do not work on cloudy days or low sunlight conditions.

7. Nanowires
Reference: Eric C Gamett Et Al, Annual Rev. Mater. Res. 2011, Nanowire Solar Cells

8. Si Nanowire
Tiny PV cells:
Composed of three layers:
i. Inner P-region
ii. Intrinsic or pure Si
iii. Outer N-region

9. Si Nanowire Properties
Increased Surface Area:
• Very narrow pointed structures.
• Diameter in nanometers
• Length in micrometers.
• Greater area made of p-n junctions is exposed to sunlight
• Increases absorptivity.
• Reduced size

10. Si Nanowire Properties
• Reduced Reflectivity
• Efficient electron transport
• Reduced recombination
• Light Trapping:
Light falling on the substrate gets reflected and once again gets
absorbed by silicon nanowires.
• No lattice Mismatch.

11. Si Nanowire Properties
Recombination:
• Poor efficiency is due to recombination within the bulk silicon element.
• Photon strikes the p-n junction in bulk silicon. Produces an electron-
hole pair.
• Electron an hole must travel along the wire to produce current.

12. Si Nanowire Properties
Reduced Recombination in SiNW:
• Small diameters
• SiNW’s grown vertical, perpendicular to the surface of substrate
• Electrons strikes the surface
• Distance of hole/electron travel is minimized
• Distance is the order of nanometers

13. Fabrication Of
Nanowires

14. Techniques Of Fabrication
● Spontaneous Growth
○ Evaporation Condensation
○ Dissolution Condensation
○ VLS Method
○ Stress induced
recrystallization
● Template Based Synthesis
○ Electrochemical Deposition
○ Colloidal Dispersion
● Lithography (top down method)
● Electro spinning

15. Fabrication: VLS Growth of Nanowire
• VLS stands for Vapour-Liquid-Solid
• It is a method for growth of 1-D structures like Nanowires from
chemical vapor deposition
• The name VLS mechanism reflects the pathway of silicon, which
coming from the vapor phase diffuses through the liquid droplet and
ends up as a solid Si wire
• It is driven in the presence of a catalyst whose presence
accelerates the reaction without taking part in it.

16. Basic Principle: VLS Method
• A foil or powder of group III metal
(Ga,In, Al) is heated in presence of
nitrogen or NH3 at temperature
suitable for vaporising of source
and dissociation of the nitride gas.
• Catalyst: transition metals like Fe,Ni
or Co and their oxides or noble
metals like Au and Ag
Reference: Selective growth of Si Nanowire, Lingling Ren, Hongmei Li and Liandi Ma, 2011

17. • Catalyst forms a liquid droplet by
itself.
• It acts as a trap for growth species.
• Growth species is evaporated first
then diffuses and dissolves into
liquid droplet
• It precipitated between liquid and
substrate interface
Steps of VLS Growth Mechanism

18. Si Nanowires using VLS Mechanism
• A thin film(1-10nm) off catalyst (Au or
Ag) is deposited onto a wafer
substrate(Si) by sputter deposition or
thermal evaporation or any other
suitable method.
• Then the growth metal is heated to
evaporate. The vapor is absorbed by
molten catalytic droplet which becomes
supersaturated and gets precipitated to
the bottom and forms AuSi.
Reference: Semiconductor Nanowire Growth and Integration, Next-Generation Electronics to Sustainable Energy, 2014, pp. 1-53

19. • As the melting point of AuSi is greater than 2500℃ which is much higher
than reaction temperature(1200℃) so it grows on Si substrate in a
hexagonal crystal structure.
Reference: Fabrication of Nanowire Tubes using VLS mechanism, Magnus Willander, QingXiang Zhao, and Omer Nur, 2007 Newsroom

20. Reference: Silicon nanowires as viewed through a scanning electron microscope, Nanotechweb.org.

21. Requirements for VLS growth
• The catalyst must be able to form liquid solution.
• The catalyst must be inert and should not react with the metal solid.
• For controlled unidirectional growth , the solid-liquid interface must be
well defined crystallographically.
• The interfacial energy plays a very important role.

22. Features of Fabricated Device
• NW Core Diameter: 80-100 nm.
• Shell Thickness: 400 nm.
• Shell layer thick enough to completely cover the core
• Reduces sheet resistance.
• Shell doping higher; Depletion region expected mainly in
crystalline NW core.
• Average wire density: 2*1018/cm2

23. Si Nanowire Based Solar Cell
Reference: Diagram of the silicon nanowire/P3HT photovoltaic cell structure, Portland State University, Nanoelcectronics, www.pdu.edx.com

24. Benefits of Nanowire geometry
Reference: Benefits of the nanowire geometry. Erik C. Garnett, Mark L. Brongersma, Yi Cui and Michael D. McGehee

25. Benefits of the nanowire geometry.
• Periodic arrays of nanowires with radial junctions maintain all the advantages
including
• reduced reflection,
• extreme light trapping,
• single-crystalline synthesis on nonepitaxial substrates.
• Axial junctions lose the radial charge separation benefit but keep the others.
• Substrate junctions lack the radial charge separation benefit and cannot be
removed from the substrate to be tested as single-nanowire solar cells.

26. Remaining Challenges And Future Outlook
• Solar cells are less expensive and use fewer natural resources.
• The radial junction nanowire geometry,opening up the possibility to
use a small amount of abundant,nontoxic,low-cost material to make
solar cells with performance close to that of current planar technology
• The ability to make single-crystalline nanowires on low-cost
substrates such as aluminum foil and to relax strain in subsequent
epitaxial layers removes two more major cost hurdles associated with
high-efficiency planar solar cells.

27. Thank You