PhotoVoltaic Cells

What is Solar Energy?
• 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.
Advantages and Disadvantages
• Advantages
• All chemical and radioactive polluting byproducts of the
thermonuclear reactions remain behind on the sun, while
only pure radiant energy reaches the Earth.
• 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
• Sun does not shine consistently.
• 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.
Solar Energy to Heat Living Spaces
 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.
Solar Energy to Heat Water
• 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.
Photovoltaics
Photo+voltaic = convert light to electricity
Solar Cells Background
• 1839 - French physicist A. E. Becquerel first recognized the
photovoltaic effect.
• 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
Driven by Space Applications in
Early Days
How does it work
The heart of a photovoltaic system is a solid-state device called

a solar cell.
Energy 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.
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.
Silicon
Shared electrons
Si Si Si Si
Si
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.
Intrinsic 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.
Extrinsic Semiconductor, n-type Doping
Conducting band, Ec
Si Si Si
Extra
Ed ~ 0.05 eV
Electron
Si As Si Eg = 1.1 eV
Si Si - Si
Valence band, 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).
Extrinsic Semiconductor, p-type doping
Conducting band, Ec
Si Si Si
Hole
Eg = 1.1 eV
Si B Si
Ea ~ 0.05 eV
Si Si - Si
Valence band, Ev
Electron
 Doping silicon with group III elements can creates empty holes in the
conduction band — positive charge carriers (p-type), B-(acceptor).
p-n Junction (p-n diode)
p n
I
V
i R O F
depletion layer
p n p n
V<0 - + 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.
p-n Junction (p-n diode)
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.
How Solar Cells Work
p - + n
hv > Eg - +
- +
- +
- +
 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
Fill Factor, FF = (VmpImp)/VocIsc
Current Density (mA/cm2)
20 Efficiency,  = (VocIscFF)/Pin
hv > Eg
10
Dark
0
Voc
-10 FF
Jmp
-20 Jsc Vmp
Light
-30
0.0 0.2 0.4 0.6 0.8 1.0
Voltage (volts)
• Efficiency,  = (VocIscFF)/Pin Voc  Eg, Isc  # of absorbed photons

• Decrease Eg, absorb more of the spectrum
• But not without sacrificing output voltage
Cost vs. Efficiency Tradeoff
Efficiency  1/2
Small Grain
Large Grain
and/or
Single
Polycrystalline
Crystals
Solids
d d
Long d Long d
High  Low 
High Cost Lower Cost
 decreases as grain size (and cost) decreases
Cost/Efficiency of Photovoltaic Technology
Costs are modules per peak W; $0.35-$1.5/kW-hr

First Generation
– Single Junction Silicon Cells
89.6% of 2007 Production

45.2% Single Crystal Si
42.2% Multi-crystal SI
• Limit efficiency 31%

• Single crystal silicon - 16-19%
efficiency
• Multi-crystal silicon - 14-15%
efficiency Silicon Cell Average Efficiency
• Best efficiency by SunPower Inc 22%
Second Generation
– Thin Film Cells
CdTe 4.7% & CIGS 0.5% of 2007

Production
• 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.
• CdTe – 8 – 11% efficiency (18%

demonstrated)
• CIGS – 7-11% efficiency (20%
demonstrated)
Third Generation
– Multi-junction Cells
• 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)
Main Application Areas – Off-grid
Space
Water Telecom
Pumping
Solar Home Systems

Main Application Areas
Grid Connected
Commercial Building
Systems (50 kW)
Residential Home PV Power Plants

Systems (2-8 kW) ( > 100 kW)
Future Energy Mix
Future Generation
– Printable Cells
Solution Processible Semiconductor

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

Nanotechnology Solar Cell Design
Interpenetrating Nanostructured Networks
glass
transparent electrode
-
- 100 nm
+
metal electrode
Dye Sensitized Solar Cell

PhotoVoltaic Cells

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What is Solar Energy?

• Originates with the

 Proper design of a building is for it to act as a solar

• 1883 - first solar cell built, by Charles Fritts, coated

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

a conductor a semiconductor an insulator

 Semiconductor is interest because their conductivity can be readily modulated

 Silicon is group IV element – with 4 electrons in their valence shell.

 Doping silicon lattice with group V elements can creates extra

Reverse bias Forward bias

 A p-n junction is a junction formed by combining p-type and n-type

 A p-n junction is the basic device component for many

• Efficiency,  = (VocIscFF)/Pin Voc  Eg, Isc  # of absorbed photons

Costs are modules per peak W; $0.35-$1.5/kW-hr

89.6% of 2007 Production

• Limit efficiency 31%

CdTe 4.7% & CIGS 0.5% of 2007

• New materials and processes to improve

• CdTe – 8 – 11% efficiency (18%

• Enhance poor electrical performance while maintaining very low

Solar Home Systems

Residential Home PV Power Plants

Solution Processible Semiconductor

Trans-polyacetylene (t-PA) Polythiophene (PT) Polypyrrole (PPY)

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