Xiangfeng Duan

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Originates with the thermonuclear fusion reactions occurring in the sun.
Represents the entire electromagnetic radiation
(visible light, infrared, ultraviolet, x-rays, and radio waves).
Radiant energy from the sun has powered life on Earth for many millions of years.

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Advantages
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All chemical and radioactive polluting byproducts of the thermonuclear reactions remain behind on the sun, while only pure radiant energy reaches the Earth.
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Energy reaching the earth is incredible. By one calculation, 30 days of sunshine striking the Earth have the energy equivalent of the total of all the planet’s fossil fuels, both used and unused!
Disadvantages
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Sun does not shine consistently.
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Solar energy is a diffuse source. To harness it, we must concentrate it into an amount and form that we can use, such as heat and electricity.
Addressed by approaching the problem through:
1) collection, 2) conversion, 3) storage.

 Proper design of a building is for it to act as a solar collector and storage unit. This is achieved through three elements: insulation, collection, and storage.

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A flat-plate collector is used to absorb the sun’s energy to heat the water.
The water circulates throughout the closed system due to convection currents.
Tanks of hot water are used as storage.

 Photo+voltaic = convert light to electricity

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1839 - French physicist A. E. Becquerel first recognized the photovoltaic effect.
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1883 - first solar cell built, by Charles Fritts, coated semiconductor selenium with an extremely thin layer of gold to form the junctions.
1954 - Bell Laboratories, experimenting with semiconductors, accidentally found that silicon doped with certain impurities was very sensitive to light.
Daryl Chapin, Calvin Fuller and Gerald Pearson, invented the first practical device for converting sunlight into useful electrical power.
Resulted in the production of the first practical solar cells with a sunlight energy conversion efficiency of around 6%.
1958 - First spacecraft to use solar panels was US satellite Vanguard 1 http://en.wikipedia.org/wiki/Solar_cell

The heart of a photovoltaic system is a solid-state device called a solar cell.

 Each isolated atom has discrete energy level, with two electrons of opposite spin occupying a state.
 When atoms are brought into close contact, these energy levels split.
 If there are a large number of atoms, the discrete energy levels form a
“continuous” band.

Energy Band Diagram of a Conductor,
Semiconductor, and Insulator a conductor a semiconductor an insulator
 Semiconductor is interest because their conductivity can be readily modulated
(by impurity doping or electrical potential), offering a pathway to control electronic circuits.

Shared electrons
Si
Si Si
Si Si Si
Si
Si
-
Si
 Silicon is group IV element – with 4 electrons in their valence shell.
 When silicon atoms are brought together, each atom forms covalent bond with 4 silicon atoms in a tetrahedron geometry.

 At 0 ºK, each electron is in its lowest energy state so each covalent bond position is filled. If a small electric field is applied to the material, no electrons will move because they are bound to their individual atoms.
=> At 0 ºK, silicon is an insulator.
 As temperature increases, the valence electrons gain thermal energy. If a valence electron gains enough energy (Eg), it may break its covalent bond and move away from its original position. This electron is free to move within the crystal.
 Conductor Eg <0.1eV, many electrons can be thermally excited at room temperature.
 Semiconductor Eg ~1eV, a few electrons can be excited (e.g. 1/billion)
 Insulator, Eg >3-5eV, essentially no electron can be thermally excited at room temperature.

Si
Si
Si
As
Si
Si
Extra
Electron
Conducting band, E c
E g
= 1.1 eV
E d
~ 0.05 eV
Si Si
-
Si
Valence band, E v
 Doping silicon lattice with group V elements can creates extra electrons in the conduction band — negative charge carriers (n-type), As- donor .
 Doping concentration #/cm 3 (10 16 /cm 3 ~ 1/million).

Extrinsic Semiconductor, p-type doping
Conducting band, E c
Si Si Si
Hole
E g
= 1.1 eV
Si B
Si
E a
~ 0.05 eV
Si Si
-
Si
Valence band, E v
Electron
 Doping silicon with group III elements can creates empty holes in the conduction band — positive charge carriers (p-type), B-(acceptor).

p
n
I
V
V<0 i depletion layer p
+ n
R O F p n
V>0
V>0 V<0
Reverse bias Forward bias
 A p-n junction is a junction formed by combining p-type and n-type semiconductors together in very close contact.
 In p-n junction, the current is only allowed to flow along one direction from p-type to n-type materials.

 Solar Cells
 Light-emitting Diodes
 Diode Lasers
 Photodetectors
 Transistors
 A p-n junction is the basic device component for many functional electronic devices listed above.

hv > E g
p -
-
-
-
-
+
+
+
+
+ n
 Photons in sunlight hit the solar panel and are absorbed by semiconducting materials to create electron hole pairs.
 Electrons (negatively charged) are knocked loose from their atoms, allowing them to flow through the material to produce electricity.

The Impact of Band Gap on Efficiency
30
20
10
0
-10
-20
-30
Fill Factor, FF = (V mp
I mp
Efficiency,

= (V oc
I sc
)/V oc
FF)/P in
I sc
Dark
J sc
J mp
Light
FF
0.0
0.2
0.4
0.6
Voltage (volts)
V mp
0.8
V oc
1.0
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Efficiency,  = (V oc
I sc
FF)/P in
V oc

E g
, I sc

# of absorbed photons
Decrease E g
, absorb more of the spectrum
But not without sacrificing output voltage hv > E g

Efficiency
 t
1/2
Large Grain
Single
Crystals
Small Grain and/or
Polycrystalline
Solids
Long d
High t d
High Cost d
Long d
Low t
Lower Cost t decreases as grain size (and cost) decreases

Cost/Efficiency of Photovoltaic Technology
Costs are modules per peak W; $0.35-$1.5/kW-hr

– Single Junction Silicon Cells
89.6% of 2007 Production
45.2% Single Crystal Si
42.2% Multi-crystal SI
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Limit efficiency 31%
Single crystal silicon - 16-19% efficiency
Multi-crystal silicon - 14-15% efficiency
Best efficiency by SunPower Inc 22%
Silicon Cell Average Efficiency

– Thin Film Cells
CdTe 4.7% & CIGS 0.5% of 2007 Production
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New materials and processes to improve efficiency and reduce cost.
Thin film cells use about 1% of the expensive semiconductors compared to First Generation cells.
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CdTe – 8 – 11% efficiency (18% demonstrated)
CIGS – 7-11% efficiency (20% demonstrated)

– Multi-junction Cells
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Enhance poor electrical performance while maintaining very low production costs.
Current research is targeting conversion efficiencies of 30-60% while retaining low cost materials and manufacturing techniques.
Multi-junction cells – 30% efficiency (40-43% demonstrated)

Water
Pumping
Space
Solar Home Systems
Telecom

Residential Home
Systems (2-8 kW)
Commercial Building
Systems (50 kW)
PV Power Plants
( > 100 kW)

Renewable Energy Consumption in the US Energy Supply, 2007 http://www.eia.doe.gov/cneaf/solar.renewables/page/trends/highlight1.html

Top 10 produce 53% of world total
Q-Cells, SolarWorld - Germany
Sharp, Kyocera, Sharp, Sanyo –
Japan
Suntech, Yingli, JA Solar – China
Motech - Taiwan

2550 ¢
25
20
15
(in the U.S. in 2002)
10
5
1-4
¢
2.35.0 ¢ 6-8
¢
0
Coal Gas Oil
5-7
¢
6-7
¢
Wind Nuclear Solar
Cost

– Printable Cells
Solution Processible Semiconductor
Organic Cell
Nanostructured Cell

Organic Photovoltaics Convert Sunlight into
Electrical Power.
n
Trans-polyacetylene (t-PA)
S n
Polythiophene (PT)
N
H n
Polypyrrole (PPY)

Interpenetrating Nanostructured Networks
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