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

Si Nanowire based Solar Cell

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Si Nanowire Based Solar Cells

Si Nanowire Based Solar CellsSubmitted 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 loadIt supplies DC powerSize 10x10 cm :size of a CDThickness is in fractions of mmMetal pattern is to make electrical contacts.

Solar Cells are:SafeCleanDurableReliableQuietInstallable anywhere

Energy conversion in a Solar CellLight is shoneElectrons are knockedoutElectrons and holesmove in oppositedirectionsElectrical 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 DrawbackNot 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:Inner P-regionIntrinsic or pure SiOuter N-region

Si Nanowire PropertiesIncreased Surface Area:Very narrow pointed structures.Diameter in nanometersLength in micrometers.Greater area made of p-n junctions is exposed to sunlightIncreases absorptivity.Reduced size

Si Nanowire PropertiesReduced ReflectivityEfficient electron transportReduced recombinationLight 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 diametersSiNWs grown vertical, perpendicular to the surface of substrateElectrons strikes the surfaceDistance of hole/electron travel is minimizedDistance is the order of nanometers

Fabrication Of Nanowires

Techniques Of FabricationSpontaneous GrowthEvaporation CondensationDissolution CondensationVLS MethodStress induced recrystallization

Template Based SynthesisElectrochemical DepositionColloidal DispersionLithography (top down method)Electro spinning

Fabrication: VLS Growth of NanowireVLS stands for Vapour-Liquid-SolidIt is a method for growth of 1-D structures like Nanowires from chemical vapor depositionThe 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 wireIt is driven in the presence of a catalyst whose presence accelerates the reaction without taking part in it.

Basic Principle: VLS MethodA 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 dropletIt precipitated between liquid and substrate interfaceSteps of VLS Growth Mechanism

Si Nanowires using VLS MechanismA 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 growthThe 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 DeviceNW Core Diameter: 80-100 nm.Shell Thickness: 400 nm.Shell layer thick enough to completely cover the coreReduces 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 OutlookSolar 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 technologyThe 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