L16 Solar Cells

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Nanoparticle Enhanced ThinFilm Solar Cells


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Solar Cells

Solar cells convert visible light to

electricity. It is one of the clean sources of energy.

In theory a 100 square mile area coveredwith solar panels would be sufficient forthe energy needs of the US.

Solar cells are based on the photovoltaiceffect.


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Photovoltaic Effect

Discovered in 1839 as a light induced

current between two metal electrodes. When photons are absorbed in a material,

some electrons gain enough energy tobecome free and contribute to current.

If the light is incident on a p-n junction, theabsorbed photon creates an electron-holepair.


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p-n J unction Solar Cells Most commonly made of crystalline silicon

by diffusing an n-type layer into a p-typesubstrate (or vice versa).

When a p-n junction is formed, near thejunction, the extra electrons from the n-side are attracted to the extra holes fromthe p-side.

As the two sides rush towards each other,a neutral depletion layer is formed with abuilt-in electric field that opposes anyfurther charge movement.

When light is incident on the junction,each absorbed photon creates anelectron-hole pair.

The electrons and holes are separated bythe built-in electric field at the junction.

The electrons move to the n-side and theholes to the p-side and create a potentialdifference.

If the device is connected to an externalload, a current will flow.

+ + + + + +_ _ _ _ _ _

n

p

n

p-

+

n

p

I

V

-

+

V-

+


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Current Voltage Characteristics

mmmVIP =

Since the solar cell isessentially a diode, it has theI-V characteristics of one

I

V

Im

Vm

RL

h

IL

RL

IS

V

I

Overall solar cell efficiency:c

m

EA

P=

,where E is solar irradiance in W/m2

under standard conditions andAc is thesurface area of the solar cell in m2.


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Efficiency in p-n J unction Cells

Practical efficienciesare around 15%.

Solar cell efficiency isaffected by manyfactors:

Reflectance of thesurface

Band gap of the cellmaterial vs. incidentphoton energy

Collection efficiency offree electrons

Internal deviceresistance

Sibandgap

Typical c-Si Solar Cell Structure


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Comparison of Efficiencies


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Thin Film Solar Cells

While many technicaldifficulties with crystalline

cells can be overcome ormitigated, cost is still a majorfactor.

Thin film solar cells can becheaper to manufacture.

The main materials used inthin film solar cells areamorphous Silicon, CdTeand CIS/CGIS.

Thicknesses are ~1 micron.

Efficiencies are < 10%.


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Enhanced Light Scattering with

Nanoparticles One issue with thin film cells is the

reduced absorption of sunlight dueto the small thickness of the film.

One group in Australia (ANU) isusing silver nanoparticles toenhance absorption. (Opt. Exp. 16,21793 (2008)

A thin (~100nm) film of silver is

deposited on the oxidized surfaceof a thin silicon film solar cell byvacuum evaporation.

Subsequent heating at 200 Ccreates silver nanoparticles

through bubbling. The silver nanoparticles can

scatter visible light very efficientlyand create a trapped mode for theincident light.


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Nanoparticles and Quantum Wells

Another approach is to combinea quantum well (QW) withnanoparticles. (APPLIEDPHYSICS LETTERS 93,091107 2008)

A quantum well is created whena lower band gap material is

sandwiched between twohigher gap barriers. The lower band gap increases

light absorption but also trapsthe carriers.

Using the nanoparticlesenhanced scattering property,fewer QWs can be used.


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Low Cost CIGS Manufacturing

A company called ISET has patented a low-cost, non-vacuumapproach to the deposition of the CIGS absorber layer.

This patented process uses water-based inks made from tinynanoparticles of copper, indium and gallium oxides as precursormaterials.

The proprietary ink formulation allows us to fix the composition ofprecursor oxides molecularly and thus by gas-solid processing,

they accurately and uniformly control the composition of thesemiconductor over large areas.


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Dye Sensitized Solar Cells

A photon is absorbed bythe dye and excites anelectron in the dye.

The electron is theninjected into the wires or

particles.

Travels to the electrode,which is connected to an

external circuit.

The dye is regeneratedby the electrolyte.


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Enhanced Light Absorption with

Nanoparticles Because c-Si has a band gap in the NIR, most of the

energy of the visible and UV photons are wasted as heat

when the photon is absorbed. Depositing thin layers with different band gaps will

improve efficiency but increase cost. Creating nanocrystalline layers of porous silicon is a

convenient way of achieving this goal. When crystals are confined to nanometer size, the band

gap of the material becomes wider.

Porous silicon can be created by electrochemical etchingof a c-Si surface and produce ~2 nm size particles withband gaps in the visible or near UV. (IEEE Photonics

Technology Letters 16, 1927-1929 (2004))


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Organic Photovoltaics

Unlike p-n junction devices, solar cells made out oforganic molecules are made out of electron donor and

acceptor materials. When an electron-hole pair is created in a polymer

chain, it is strongly bound.

In order to create a current, this pair has to bedissociated. The acceptor material is for this purpose.


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Enhancing Organics PVs with

Nanoparticles Nanoparticles can be

more efficient asacceptors.

The process, called

charge transfer, canincrease efficiency.

WFU Nanotech isvery active in thisarea.


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Another Polymer Example

Using an aluminum oxide templateconsisting of nanometer sizedhoneycomb array the polymer P3HT isdrawn through the openings throughvacuum and capillary forces.

The structure is then backfilled with C60which acts as the electron acceptor.

The aluminum oxide allows the P3HTchains to align themselves in a stackedarray which increases conductivity.

The honeycomb structure increases thesurface area which helps with electrontransfer.

While reported efficencies are low (~1%) the method is cheap and scalable.

Ref: Adv. Funct. Mater. 2010, 20, 540

545.

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L16 Solar Cells