Solar energy originates from the thermonuclear fusion reactions of the sun. It represents the entire electromagnetic radiation that reaches Earth and has powered life for millions of years. Solar energy can be used to heat living spaces and water through solar collectors that absorb sunlight and storage systems. Photovoltaics directly convert sunlight into electricity using solar cells made of semiconducting materials like silicon that generate electron-hole pairs when struck by photons. The p-n junction in solar cells allows current to flow in one direction, producing electricity from sunlight.

1. Solar Energy:
The Ultimate
Renewable Resource
Xiangfeng Duan

2. What is Solar Energy?What is Solar Energy?
 Originates with theOriginates with the
thermonuclear fusionthermonuclear fusion
reactions occurring in thereactions occurring in the
sun.sun.
 Represents the entireRepresents the entire
electromagnetic radiationelectromagnetic radiation
(visible light, infrared,(visible light, infrared,
ultraviolet, x-rays, and radioultraviolet, x-rays, and radio
waves).waves).
 Radiant energy from the sunRadiant energy from the sun
has powered life on Earth forhas powered life on Earth for
many millions of years.many millions of years.

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

4. Solar Energy toSolar Energy to Heat Living SpacesHeat Living Spaces
 Proper design of a building is for it to act as a solarProper design of a building is for it to act as a solar
collector and storage unit. This is achieved throughcollector and storage unit. This is achieved through
three elements: insulation, collection, and storage.three elements: insulation, collection, and storage.

5. Solar Energy to Heat WaterSolar Energy to Heat Water
 A flat-plate collector is usedA flat-plate collector is used
to absorb the sun’s energy toto absorb the sun’s energy to
heat the water.heat the water.
 The water circulatesThe water circulates
throughout the closed systemthroughout the closed system
due to convection currents.due to convection currents.
 Tanks of hot water are usedTanks of hot water are used
as storage.as storage.

6. PhotovoltaicsPhotovoltaics
PhotoPhoto++voltaicvoltaic = convert= convert lightlight toto electricityelectricity

7. Solar Cells BackgroundSolar Cells Background
 1839 - French physicist A. E. Becquerel first recognized the photovoltaic1839 - French physicist A. E. Becquerel first recognized the photovoltaic
effect.effect.
 1883 - first solar cell built, by Charles Fritts, coated semiconductor1883 - first solar cell built, by Charles Fritts, coated semiconductor
selenium with an extremely thin layer of gold to form the junctions.selenium with an extremely thin layer of gold to form the junctions.
 1954 - Bell Laboratories, experimenting with semiconductors, accidentally1954 - Bell Laboratories, experimenting with semiconductors, accidentally
found that silicon doped with certain impurities was very sensitive to light.found that silicon doped with certain impurities was very sensitive to light.
Daryl Chapin, Calvin Fuller and Gerald Pearson, invented the firstDaryl Chapin, Calvin Fuller and Gerald Pearson, invented the first
practical device for converting sunlight into useful electrical power.practical device for converting sunlight into useful electrical power.
Resulted in the production of the first practical solar cells with a sunlightResulted in the production of the first practical solar cells with a sunlight
energy conversion efficiency of around 6%.energy conversion efficiency of around 6%.
 1958 - First spacecraft to use solar panels was US satellite Vanguard 11958 - First spacecraft to use solar panels was US satellite Vanguard 1
http://en.wikipedia.org/wiki/Solar_cell

8. Driven by Space Applications inDriven by Space Applications in
Early DaysEarly Days

9. The heart of a photovoltaic system is a solid-state device called aThe heart of a photovoltaic system is a solid-state device called a
solar cell.solar cell.
How does it workHow does it work

10. Energy Band Formation in SolidEnergy Band Formation in Solid
 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.

11. Energy Band Diagram of a Conductor,Energy Band Diagram of a Conductor,
Semiconductor, and InsulatorSemiconductor, 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.

12. SiliconSilicon
Si
Si
Si
Si
Si
-
Si Si Si
Si
SiSi
Si
Si
Si
Shared electrons
 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.

13. Intrinsic SemiconductorIntrinsic Semiconductor
 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.

14. Extrinsic Semiconductor, n-type DopingExtrinsic Semiconductor, n-type Doping
Electron
-
Si Si Si
Si
SiSi
Si
Si
As
Extra
Valence band, Ev
Eg = 1.1 eV
Conducting band, Ec
Ed ~ 0.05 eV
 Doping silicon lattice with group V elements can creates extra
electrons in the conduction band — negative charge carriers (n-type), As-
donor.
 Doping concentration #/cm3
(1016
/cm3
~ 1/million).

15. Valence band, Ev
Eg = 1.1 eV
Conducting band, Ec
Ea ~ 0.05 eV
Electron
-
Si Si Si
Si
SiSi
Si
Si
B
Hole
 Doping silicon with group III elements can creates empty holes in the
conduction band — positive charge carriers (p-type), B-(acceptor).
Extrinsic Semiconductor, p-type dopingExtrinsic Semiconductor, p-type doping

16. V
I
R O F
p n
p n
V>0 V<0
Reverse bias Forward bias
p-n Junction (p-n diode)p-n Junction (p-n diode)
 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.
i
p n
V<0 V>0
depletion layer
- +

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

18. How Solar Cells WorkHow Solar Cells Work
 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.
p n
- +
- +
- +
- +
- +
hv > Eg

19. The Impact of Band Gap on EfficiencyThe Impact of Band Gap on Efficiency
 Efficiency,Efficiency, ηη = (V= (VococIIscscFF)/PFF)/Pinin VVococ ∝∝ EEgg, I, Iscsc ∝∝ # of absorbed photons# of absorbed photons
 Decrease EDecrease Egg, absorb more of the spectrum, absorb more of the spectrum
 But not without sacrificing output voltageBut not without sacrificing output voltage
hv > Eg
-30
-20
-10
0
10
20
30
CurrentDensity(mA/cm2
)
1.00.80.60.40.20.0
Voltage (volts)
Jsc
Voc
FF
Dark
Light
Jmp
Vmp
Fill Factor, FF = (VmpImp)/VocIsc
Efficiency, η = (VocIscFF)/Pin

20. Cost vs. Efficiency TradeoffCost vs. Efficiency Tradeoff
Efficiency ∝ τ1/2
Long d
High τ
High Cost
d
Long d
Low τ
Lower Cost
d
τ decreases as grain size (and cost) decreases
Large Grain
Single
Crystals
Small Grain
and/or
Polycrystalline
Solids

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

22. 89.6% of 2007 Production89.6% of 2007 Production
45.2% Single Crystal Si45.2% Single Crystal Si
42.2% Multi-crystal SI42.2% Multi-crystal SI
 Limit efficiency 31%Limit efficiency 31%
 Single crystal silicon - 16-19%Single crystal silicon - 16-19%
efficiencyefficiency
 Multi-crystal silicon - 14-15%Multi-crystal silicon - 14-15%
efficiencyefficiency
 Best efficiency by SunPower Inc 22%Best efficiency by SunPower Inc 22%
Silicon Cell Average Efficiency
First GenerationFirst Generation
–– Single Junction Silicon CellsSingle Junction Silicon Cells

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

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

25. Solar Home Systems
Space
Water
Pumping
Telecom
Main Application Areas – Off-grid

26. Residential Home
Systems (2-8 kW)
PV Power Plants
( > 100 kW)
Commercial Building
Systems (50 kW)
Main Application Areas
Grid Connected

27. Future Energy MixFuture Energy Mix

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

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

30. (in the U.S. in 2002)
1-4
¢
2.3-5.0 ¢ 6-8
¢
5-7
¢
Production Cost of ElectricityProduction Cost of Electricity
0
5
10
15
20
25
C o al G a s O il W ind Nucle ar S o la r
C o s t
6-7
¢
25-50 ¢
Cost,¢/kW-hr

31. Future GenerationFuture Generation
–– Printable CellsPrintable Cells
Organic Cell
Nanostructured Cell
Solution Processible Semiconductor

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

33. Nanotechnology Solar Cell Design

34. Interpenetrating Nanostructured Networks
-
metal electrode
transparent electrode
glass
+
- 100 nm
---
metal electrode
transparent electrode
glass
+
- 100 nm

35. Dye Sensitized Solar Cell