Transcript
Page 1: Si Nanowire based Solar Cell

Si Nanowire Based Solar Cells

Submitted By:Urvi Sharma (13ESKEC086)

Submitted to:Mr. Rahul Pandey

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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.

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

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Energy conversion in a Solar Cell

• Light is shone

• Electrons are knockedout

• Electrons and holesmove in oppositedirections

• Electrical output isgenerated between thecontacts

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Current Technology: Photovoltaic CellsLight 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.

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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.

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Nanowires

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

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Si Nanowire

Tiny PV cells:

Composed of three layers:

i. Inner P-region

ii. Intrinsic or pure Si

iii. Outer N-region

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Si Nanowire PropertiesIncreased 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

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Si Nanowire Properties• Reduced Reflectivity

• Efficient electron transport

• Reduced recombination

• Light Trapping:Light falling on the substrate gets reflected and once again getsabsorbed by silicon nanowires.

• No lattice Mismatch.

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Si Nanowire PropertiesRecombination:

• 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.

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Si Nanowire PropertiesReduced 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

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Fabrication Of Nanowires

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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

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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.

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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

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• 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

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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

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• 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

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

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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.

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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 incrystalline NW core.

• Average wire density: 2*1018/cm2

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Si Nanowire Based Solar Cell

Reference: Diagram of the silicon nanowire/P3HT photovoltaic cell structure, Portland State University, Nanoelcectronics, www.pdu.edx.com

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Benefits of Nanowire geometry

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

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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.

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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.

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Thank You


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