Excitons and Solar EneExcitons and Solar EnergyrgyShuhua LiangShuhua Liang

UTK Physics departmentUTK Physics department

Energy ProblemEnergy Problem• Full list • Rank Company Revenues Profits ($ millions)• 1 Exxon Mobil 339,938.0 36,130.0• 2 Wal-Mart Stores 315,654.0 11,231.0• 3 Royal Dutch Shell 306,731.0 25,311.0• 4 BP 267,600.0 22,341.0• 5 General Motors 192,604.0 -10,567.0• 6 Chevron 189,481.0 14,099.0• 7 DaimlerChrysler 186,106.3 3,536.3• 8 Toyota Motor 185,805.0 12,119.6• 9 Ford Motor 177,210.0 2,024.0• 10 ConocoPhillips 166,683.0 13,529.0• (from FORTUNE top 500 companies 2006)

Solar EnergySolar Energy

• (from GCEP)

How to get energy from sun How to get energy from sun light?light?

• Use electron!

(by shuhua)

Energy band theoryEnergy band theory• The problem of single electron

approximation• In experiment people find structure

in energy gap.

Excition and binding energyExcition and binding energy2

_ 2_ _2Binding Energy

ground state radius

eEn a

-= e

• Binding energy

• Reduced mass

_ _ 0

1 1 1

reduced electron hole

electronground state radius

reduced

M M M

Ma a

M

= +

æ ö÷ç ÷= eç ÷ç ÷çè ø

2

_ 2_ _2Binding Energy

ground state radius

eEn a

-= e

How to generate Current?How to generate Current?• Use two different type of material to separate the exciton (electron-hole)

The first generation Si Solar CeThe first generation Si Solar Cellll

The disadvantage of Si cell The disadvantage of Si cell • Expensive $565.00 Weight: 28.00 lbs

• Low efficiency (around 16%)

Second generation solar cell Second generation solar cell 1)multiple junction photovoltaic cells

:high efficiency :high cost(from Sonny X. Li ) 2)Thin film :low efficiency :low cost

Third Generation Third Generation • Quantum Dot• Size: 2-10 nm• Accommodate 5-1000electrons• 2Dor 3D free electron model (depend on the shape)• Structure: core&shell(from Evident Technologies )

How does it work?How does it work?• Larger and heavier

quantum dots are at bottom

How does it work?How does it work?

Why Quantum Dot is good?Why Quantum Dot is good?• High efficiency due

to one photon can excite several electrons

• Energy gap decide the min frequency of light

• Can control the volt and current

Quantum Dot can select Quantum Dot can select frequencyfrequency

• Technically we can design the size of quantum dot to design the gap width.

• (from google)

Thank YouThank You