Silicon Nanowire based Solar Cells International Congress On Renewable Energy ICORE 2010 2 nd...

Silicon Nanowire based Solar Cells

International Congress On Renewable EnergyICORE 2010

2nd December, 2010.Pragya Singh

Pratul K Singh


ICORE - 2010, Pragya Singh, Pratul K Singh

Outline:

•Light Trapping in Cells•Solar Cell•Energy Conversion in Solar Cell•Solar Cells Everywhere•Planar Silicon Solar Cell•Nanowires- Properties•Silicon Nanowires- Properties•Fabrication•Techniques for SiNW Deposition•Experiment at the SSN Research Centre

Light Trapping in Cells:


Sun already provides all the energy needed to support life.

So the Challenge?

Light Trapping in Cells:


One to One conversion:Blue photon = 2 times more energy than the Red Photon Both produce 1 electron each.

Effective light energy utilized?



Outline:


Solar Cell• Cell is thin Si wafer

• Size 10x10 cm : size of a CD

• Thickness is in fractions of mm

• Metal pattern is to make electrical contacts.




Outline:


Energy Conversion in Solar Cell

Light is shone Electrons are knocked out Electrons and holes move in opposite directions Electrical output is generated between the contacts.




Outline:


Solar Cells Everywhere

Solar cells :SafeCleanQuietDurableReliableInstallable anywhere


The Main Catch


Material Cost

Thickness of material Purity

Fabrication Cost



Outline:


Planar Silicon Solar Cells:


Thickness for an efficient light absorption

High purity to avoid recombining

High Reflectance

High Recombination Rate.

Sliced into Nano-scale


Diameters from 1 to 50nm

Nano scale Silicon has a color difference

Quantum Confinement Nano sized Silicon shows:

Physical Optical Electronics properties change Electrons occupy different energy levels

Conductance can be Improved


Bulk Silicon atom:TetravalentTend to achieve StabilityBond with 4 other atoms

Silicon Nanowire:TetravalentTend to achieve stabilityCling with atmospheric OxygenSilica is formed


Prevention of Silica:

Conductivity increases 10 timesPrevention at high temperatures – 700CHigh Vaccum

Conductance can be Improved



Outline:


– No Lattice Mismatch.

– Flexibility to create heterostructures.

– Broad range of materials.

– Integration of compound semiconductor based optoelectronic devices with silicon based microelectronics.


Nanowires- Properties



Outline:


Recombination:

• Poor efficiency may be due to recombination within the bulk silicon element.

• Photon strikes the p-n junction in bulk silicon, Produces an electron-hole pair.

• Electron and hole must travel along the wire to produce current.


Silicon Nanowires- Properties

Recombination:

Tendency to recombine with other oppositely charged charge carrier

Resulting in heat generation rather than electrical energy



Reduced Recombination in SiNW:Small diameters. SiNWs grown vertical, perpendicular to the surface of the substrate.Electrons strikes on the surface.Distance of hole/electron travel is minimized. Distance is of the order of nanometers.



Light Trapping:

Light falling on the substrate gets reflected and once again gets absorbed by silicon nanowires.



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

Increased absorptivity

Reduced reflectivity

Efficient electron transport

Reduced Recombination



Tiny PV Cells:

Composed of 3 layers: – Inner P region

– Intrinsic or pure silicon

– Outer N region



Photon strikes the outer shellElectron-hole pair is created Travels in the radial direction towards the P layer

(core) before recombination.




Outline:


Fabrication

• Catalyst Particles :

• The catalyst must be inert to the reaction products (during CVD nanowire growth).

• Gold

• Aluminum

• Tin

• Indium

• Gallium




Outline:


Techniques for SiNW DepositionTechniques:

Supercritical-Fluid-Based and Solution-Based Growth Techniques

Molecular Beam EpitaxyLaser AblationSilicon Monoxide EvaporationPECVD




Outline:


Experiment at the SSN Research Centre


Si-NW Growth Principle


Si-NW Deposition Process Steps


• Silicon (4 inch) n-type Substrate Cleaned

• Chromium layer of 10.7nm deposited by EBE Method

• Gold Layer of 2.6nm deposited by EBE Method

• The sample introduced in the PECVD chamber

• Heated to 580 degrees to form Nano particles

• Silane introduced with hydrogen for 30 minutes at various temperatures

SiNW – Deposition Experiment• Catalyst Nano Particles:

– Gold Film Layer (2.6nm) Deposited on Silicon Substrate by EBE

– Heated at 580 degrees to form Nano Particles

– Figure1 SEM image of Nano particles of Gold

Figure 1. SEM Image of Gold Nano Particles on Silicon Substrate


SiNW – Deposition ExperimentPECVD system.

Temperature : 380 degrees

Pressure in mTorr : 500mT

Time of Deposition : 30 minutes

Gases Used : Silane, Hydrogen and Argon


Si NW Comparison

Si Nano Wire at SSNRC Literature Survey Reference: IRAM, Saclay Institute of Matter and Radiation


Thank You


Silicon Nanowire based Solar Cells International Congress On Renewable Energy ICORE 2010 2 nd...

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