Download Photoelectric effect explained

Photoelectric effect explained
• But now, a behavior of light was observed that fit
Planck’s energy packet idea.
• So electromagnetic radiation appears to behave
as if it is both a wave and a particle.
• In fact, you can think of light as discrete wave
packets-packets of waves which, depending upon
the measurement you make, sometimes exhibit
particle behavior and sometimes exhibit wave
behavior.
• Einstein won the Nobel prize for his explanation
of the photoelectric effect.
Semi conductors
• Devices which have conductive properties in
between a conductor and an insulator.
• Normally, the outer (valence) electrons are
tightly bound to the nucleus and cannot
move.
• If one or all of them could be freed up, then
the material can conduct electricity
• Silicon is an example of a semi-conductor.
Silicon
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Element 14 in the periodic table
Very common element (sand, glass composed of it)
8th most common element in the universe
Its 4 outer valence electrons are normal tightly bound
in the crystal structure.
• However, when exposed to light, the outer electrons
can break free via the photoelectric effect and conduct
electricity.
• For silicon, the maximum wavelength to produce the
photoelectric effect is 1.12 microns. 77% of sunlight is
at wavelengths lower than this.
But its not quite this simple
• You also need to produce a voltage within the silicon to
drive the current.
• So the silicon must be combined with another material.
This process is called doping.
• 2 types of doping: P and N
– If you replace one of the silicon atoms in the crystal lattice
with a material that has 5 valence electrons, only 4 are
need to bond to the lattice structure, so one remains free.
The doped semi conductor has an excess of electrons and
is called an N type semiconductor.
– Doping elements can be arsenic, antimony or phosphorus.
P-types
• If you dope with an element with only 3 valence
electrons, there is a vacancy, or hole left where the 4th
electron should be.
• If the hole becomes occupied by an electron from a
neighbor atom, the hole moves through the
semiconductor. This acts like a current with positive
charge flowing through the semi conductor, so it
appears to have a net positive charge
• Called a P-type semiconductor.
• Doping elements could be boron, aluminum, or indium
Creating the solar cell
• To create the solar cell, bring a p-type silicon into
contact with an n-type silicon.
• The interface is called a p-n junction.
• Electrons will diffuse from the n material to the p
material to fill the holes in the p material. This
leaves a hole in the n material.
• So the n-material ends up with an excess positive
charge and the p material ends up with an excess
negative charge.
• This creates an electric field across the junction.
Current in the solar cell
• Any free electrons in the junction will move
towards the n –type material and any holes
will move toward the p -type material .
• Now sunlight will cause the photoelectric
effect to occur in the junction. Thus free
electrons and holes are created in the junction
and will move as described above.
• Current flows!
Solar Cells
• Typically 2 inches in diameter and 1/16 of an inch
thick
• Produces 0.5 volts, so they are grouped together
to produce higher voltages. These groups can
then be connected to produce even more output.
• In 1883 the first solar cell was built by Charles
Fritts. He coated the semiconductor selenium
with an extremely thin layer of gold to form the
junctions. The device was only around 1%
efficient.
Generations of Solar cells
• First generation
– large-area, high quality and single junction devices.
– involve high energy and labor inputs which prevent
any significant progress in reducing production costs.
– They are approaching the theoretical limiting
efficiency of 33%
– achieve cost parity with fossil fuel energy generation
after a payback period of 5-7 years.
– Cost is not likely to get lower than $1/W.
Generations of Solar cells
• Second generation-Thin Film Cells
– made by depositing one or more thin layers (thin film)
of photovoltaic material on a substrate.
– thickness range of such a layer varies from a few
nanometers to tens of micrometers.
– Involve different methods of deposition:
• Chemical Vapor deposition the wafer (substrate) is exposed
to one or more volatile precursors, which react and/or
decompose on the substrate surface to produce the desired
deposit. Frequently, volatile by-products are also produced,
which are removed by gas flow through the reaction
chamber.
Thin Film deposition techniques
• Electroplating
– electrical current is used to reduce cations (positively
charged ions) of a desired material from a solution
and coat a conductive object with a thin layer of the
material.
• Ultrasonic nozzle
– spray nozzle that utilizes a high (20 kHz to 50 kHz)
frequency vibration to produce a narrow drop size
distribution and low velocity spray over the wafer
• These cells are low cost, but also low efficiency
The Third Generation
• Also called advanced thin-film photovoltaic
cell
• range of novel alternatives to "first
generation” and "second generation” cells.
• more advanced version of the thin-film cell.
Third generation alternatives
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non-semiconductor technologies (including polymer cells and biomimetics)
quantum dot technologies
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also known as nanocrystals, are a special class semiconductors. which are crystals composed
of specific periodic table groups. Size is small, ranging from 2-10 nanometers (10-50 atoms) in
diameter.
tandem/multi-junction cells
– multijunction device is a stack of individual single-junction cells
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hot-carrier cells
– Reduce energy losses from the absorption of photons in the lattice
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upconversion and downconversion technologies
– Put a substance in front of the cell that converts low energy photons to higher energy ones or
higher energy photons to lower energy ones that the solar cells can convert to electricity.
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solar thermal technologies, such as thermophotonics(TPX)
– A TPX system consists of a light-emitting diode (LED) (though other types of emitters are
conceivable), a photovoltaic (PV) cell, an optical coupling between the two, and an electronic
control circuit. The LED is heated to a temperature higher than the PV temperature by an
external heat source. If power is applied to the LED, , an increased number of electron-hole
pairs (EHPs) are created.These EHPs can then recombine radiatively so that the LED emits light
at a rate higher than the thermal radiation rate ("superthermal" emission). This light is then
delivered to the cooler PV cell over the optical coupling and converted to electricity.
Efficiency and cost factors
• Average cost per peak watt is $1.00-$3.00. Coal fired
plant is $1.00/watt.
• Efficiency is not great.
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Recall, 77% of the incident sunlight can be used by the cell.
43% goes into heating the crystal.
Remaining efficiency is temperature dependent
Average efficiency of a silicon solar cell is 14-17%
• The second and third generation technologies
discussed are designed to increase these efficiency
numbers and reduce manufacturing costs
Novel approaches
• UA astronomer Roger Angel
• Uses cheap mirrors to focus
sunlight on 3rd generation solar
cells (triple junction cells) which
handle concentrated light
• $1.00 per watt achievablecompetitive with coal plants
• Potential: 1 solar farm 100 miles
on a side could provide electricity
to the whole nation
• Does not have to be all in one
place
Solar Cooling
• Consider a refrigeration system with no moving parts.
– Heat the coolant (say ammonia gas dissolved in water) and force it via a
generator into an evaporator chamber where it expands into a gas and cools.
Move it to a condenser and cool it back to a liquid and repeat the process.
• These systems actually have existed for a number of years, refrigerators in
the 1950s were sold with this technology (gas powered and there was/is a
danger of CO emissions).
• Energy to heat the coolant and drive it through the system comes from
burning fuel or a solar cell to provide electricity to do the heating.
– Need what is called a concentrating collector (lens or other system to
concentrate more light on the solar cell).
• Ideally, you could do this with a flat plate collector system, though you do
not obtain as much cooling.
• Devices are not widely used, due to the intermittency of sunlight