Si Nanowire based Solar Cell

Si Nanowire Based Solar Cells

Submitted By:Urvi Sharma (13ESKEC086)

Submitted to:Mr. Rahul Pandey

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.

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

Energy conversion in a Solar Cell

• Light is shone

• Electrons are knockedout

• Electrons and holesmove in oppositedirections

• Electrical output isgenerated between thecontacts

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.

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.

Nanowires

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

Si Nanowire

Tiny PV cells:

Composed of three layers:

i. Inner P-region

ii. Intrinsic or pure Si

iii. Outer N-region

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

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.

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.

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

Fabrication Of Nanowires

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

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.

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

• 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

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

• 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

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

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.

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

Si Nanowire Based Solar Cell

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

Benefits of Nanowire geometry

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

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.

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.

Thank You