Transcript PV Panels and P N Junctions

PV Panels and P N Junctions
How PV Panels work
Or
An Introduction to the World of
Microelctronics
What are PV Panels?
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PV Panels are thin layers of Silicon that
have been designed to convert solar
energy into electricity.
Silicon is a semi conductor.
Semi conductors are elements that
under certain conditions conduct
electricity and under others do not.
Periodic Table of Elements
Silicon basics
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Second most
abundant element
on Earth.
Elemental silicon
transmits more than
95% of all
wavelengths of
infrared light
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The essentials
Name: silicon
Symbol: Si
Atomic number: 14
Atomic weight:
28.0855 (3)
Group number: 14
Period number: 3
Complete Valence Bands
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All elements want to have their outermost
band complete, or no missing electrons.
For all elements with the exception of
Hydrogen and Helium that outermost band
will require 8 electrons.
Due to the Heisenberg Uncertainty Principle
the atoms are able to share electrons in their
outermost shells.
Crystal Lattices
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Silicon is solid at room temperature and
its atoms are arranged in a lattice type
of structure.
Since Silicon has 4 electrons in its outer
band it is looking to share or borrow 4
electrons from another atom or atoms
so that it has its complete outer band of
8 electrons.
Silicon Crystals
Silicon Crystals Multicolored
Silicon Crystals and Light
Can That Really Happen?
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This will happen but only under specific
conditions (Si is a semi conductor).
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This EHP (electron hole pair) can be
caused by light or heat to name a few.
In Si the bad gap is 1.12 eV.
At room temperature there will be 145
trillion EHPs at any one time.
Types of Doping
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Impurities are added on purpose to increase
electron mobility or flow.
B, In are Group III dopants. (green)
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These elements have only 3 electrons in their
outer band so a hole is created. P-type Si.
P, As, Sb are Group V dopants. (blue)
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These elements have 5 electrons in their outer
shells so a electron is free to move. N-type Si.
Doped Silicon Crystals
Types of Electrical Flow
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Current is the measure of the flow of
electrons.
There are two ways that electricity can
flow.
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Conduction band electron current
Valence band hole current.
PN Junctions
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When P-type Si is placed next to N-type
Si you have what is called a PN
junction.
PN junctions are at the heart of things
like PV panels and electrical diodes.
The band gaps in N-type and P-type Si
are different however due to the
presence of different dopants.
Band Gaps
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Different dopants mean that there are
different band gaps.
Boron Doped
Phosphorus Doped
PN Junctions
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When the two Si types are placed next
to one another a depletion region is
formed.
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The depletion region is created when the
free electrons from the N-type side rush
over to fill the holes left in the P-type side.
The depletion regions have no free
carriers and therefore resist electron
flow.
Depletion Regions
Depletion Regions
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If the size of the depletion region is
“large” enough then the electrons will
not be able to cross the gap.
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Think of it like the Grand Canyon vs. rain
water rushing towards the storm drain.
No one* would consider jumping the Grand
Canyon, but a rain water stream most can
handle.
The same is true of electrons.
Biasing the Junctions
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If the junction is biased (a voltage is
applied to either side) then current will
flow. This is where the photons come in
to play in the PV panels.
Applying the voltage changes the
electric Grand Canyon into that small
storm water stream.
Biased PN Junctions
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By applying a positive
voltage to the P-type
side and a negative
voltage to the N-type
side the depletion
region shrinks
dramatically.
This is because more
free carriers are added
to the system.
Silicon Crystals and Light
Sunlight and PV panels
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The photons that are raining down on us
from the sun are little packets of energy.
There are some packets which just happen to
be at exactly 1.12 eV.
The photons from the sun can therefore be
harnessed to increase the number of free
carriers in the Si.
More free carriers means the junction is
biased and current can flow across the gap.