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The energy problem

The world uses about 13 TW of power today.

We probably need to generate ~ 30 TW of power in 2050.

Within the next 10 years, the amount of oil we can extracteach year will probably start to decline.

If we do not dramatically reduce our emissions of carbondioxide, the average temperature of the planet will probablyrise by several degrees.

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Our options

Burn fossil fuels and sequester the carbon

Nuclear

Renewable energy

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Why solar cells are likely to provide a significant

fraction of our power

We need ~ 30 TW of power, the sun gives us 120,000 TW.

Solar cells are safe and have few non-desirableenvironmental impacts.

Using solar cells instead of burning coal to generateelectricity is a much easier way to reduce carbon emissionsthan replacing gasoline in vehicles.

Solar cells provide electricity exactly when we need it themost.

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Conventional p-n junction photovoltaic (solar)cell

e-

usable photo-voltage (qV)

Energy

e-

Jenny Nelson, The Physics of Solar Cells, 2003.

n-typep-type

max

= 32%heat loss

heat loss

h

h+

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Polycrystalline silicon solar cells

12 % efficiency$350/m2

Cost ($/W)Cell $2.50Making the module $1.00Inverter $0.50

Installation $4-5.00TOTAL $8-9.00

DOE numbers

Average cost of PV cell electricity: $.27/kW-hr

Todays grid electricity: $0.06/kW-hr

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Single crystal cells

Dick Swanson will show you how SunPower makes 21 % efficient cells.

What is the potential for using less silicon and reducing the costs?

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Multijunction cellsGrid

p

p

p

p

p

n

nn

++

++

p

n

p

p

nn

++

++

p

n

pn

++

++

Ge Substrate (0.67 eV)

GaAs (1.42 eV)

GaInP (1.90 eV)

Al InP

AlGaInP

GaInP

GaInP

n GaAs

GaAs:N:Bi (1.05 eV)

GaAs:N:Bi

GaAs:N:Bi

nn

SpectroLab has achieved 37 % efficiency

Costs are estimated at $50,000/m2, soconcentrators must be used.

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Quantum Dot Solar Cells

Energy levels are quantized. Electrons do not rapidly give upthere energy to phonons.

Art Nozik, Inorganic Chemistry, 44 (2005) p. 6893.

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The cheapest option

Efficiency: 0.3 %

We dont have theland and water toprovide the world

with energy thisway.

Can we artificiallyimprove theefficiency?

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Thin Film Cells

CIGS (CuInGaSe2)World record: 19.5 %

StableIs there enough In available?

amorphous SiWorld record: 12.1 %not completely stable

CdTeWorld record: 16.5 %

StableCd is toxic

A thin film of semiconductor isdeposited by low cost methods.

Less material is used.

Cells can be flexible.

Chris Eberspacher will tell you how Nanosolar prints CIGS cells.

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5 % efficient organic cells can be deposited fromsolution

P. Fairley, IEEE Spectrum. Jan. 2004 p.28