Download Amorphous SC and Solar Cells

Amorphous Semiconductors
And
Solar Cells
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Amorphous Semiconductors are used in
many applications, including:
•Solar Cells
•Thin Film Displays
•Electrophotography
•Switching devices
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Thin-Film Transistor (TFT) LCD Displays
•Liquid Crystal Displays Developed by RCA Laboratories in 1968
•Work by acting as a “light valve” either blocking light or allowing it to pass
•An electric field is applied to alter the properties of each Liquid Crystal Cell
(LCC) to change each pixel’s light absorption properties
•Colors are added through filtering process
•Modern Laptops produce virtually unlimited colors at very high resolution
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Thin-Film Transistor (TFT) LCD Displays
•Light comes from behind – either LED or
fluorescent source
•Beam of light is polarized, then goes through
TFT matrix, which decides which pixels should
be “on” or “off”
•If “on”, molecules in LCC will align in a single
direction, allowing light to pass
Path of light through a TFT LCD
In an TFT display, each LCC is stimulated
by a dedicated thin-film transistor matrix,
with one transistor at each pixel.
•Color filters block all wavelengths of light
except those within the range of the pixel.
Areas between pixels are printed black to
increase contrast.
•Exiting light passes through another polarizer
to sharpen image and eliminate glare
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LCD Addressing Modes
Three types of addressing have emerged
since LCD became the display medium in
1971:
•Direct
•Multiplex
•Active Matrix
In Direct, one signal
controls many
segments. Useful
for numeric displays,
e.g., watches and
calculators
Wires in Multiplex
are shared through
a matrix wiring
scheme, allowing
separate signals to
be delivered to each
pixel
Active matrix allows
charge storage,
enabling pixels to
refresh enabling
real-time video on
large screens
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Manufacturing and Display
Configurations
•Photolithography used to lay insulators, transistors and
conductors down on a glass substrate – the lower glass
in an LCD
•TFT displays require a transistor and capacitor for each
pixel
•For highest fidelity, RGB is replaced by GRGB and RGB
Delta Displays
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Three switch technologies: Amorphous
Silicon (a-Si), Polycrystalline Silicon )pSi) and Single Crystal Silicon (x-Si)
Amorphous Silicon is the standard for
TFT LCDs because they have:
•Good Color
•Good Grayscale Reproduction
•Fast Response
Advantages: An a-Si TFT
production process requires only 4
basic lithography steps, and
produces good quality large
screens – low cost
Mobility
TFT Process
(cm^2/V sec)
A-Si
0.3-0.7
conventional
p-Si
6
eximer p-Si
329
singe crystal
Si (x-Si)
600
Disadvantage: Because a-Si has
low mobility, a capacitor must be
added to each pixel
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Polycrystalline Silicon
Advantage:
•Adding only two process steps, NMOS
and PMOS transistors can be formed
•Meet requirements for HDTV displays
Disadvantage:
•Requires higher process temperatures
than a-Si – 600oC softens most types of
glass
a-Si junction
p-Si junction
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Breakthrough Technology
The eximer laser annealing process is capable of recrystallizing p-Si
film, increasing its mobility 660 times. This is possible because
polycrystalline Si absorbs UV light. The absorbed energy raises the
temperature of the p-Si film, thus annealing it.
The eximer laser process allows a cheaper and more conventional
glass to be used as a substrate, reducing production costs for the
mass production of p-Si TFTs.
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Photovoltaics
•Photoelectric effect discovered by
Edmund Bequerel in 1830
•Albert Einstein received the Nobel
Prize for describing the nature of light
and the photoelectric effect in 1905
•Bell Laboratories made the first
photovoltaic module in 1954. The
space industry in the 1960s and the
energy crisis in the 1970s spurred
further photovoltaic development
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Photovoltaics – Operating
Principles
Photovoltaics, also known as Solar
Cells are semiconductors, typically
Silicon
A solar cell uses junctions of an n-type
semiconductor (freely moving
electrons) with a p-type semiconductor
(freely moving holes) which creates a
type of diode that is in electric
equilibrium in the dark
Photons (electromagnetic
radiation) from the sun free
electrons and holes, causing a
DC current to flow from the n- to
the p-type material
Several cells are placed in series in
modules to achieve higher voltages
and power
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Two Photovoltaic Cell Types
•Single crystal or polycrystalline cells – use “doped” crystals for
making the cells, much like computer chips
•This is the most common technology
•Crystalline cells are expensive but last many years with little
degradation
•Silicon is the most common material, although others are under
development, such as Gallium Arsenide and Indium Selenide
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Improving Solar Cell Efficiency
•The energy of a photon is E = hn
•Electrons are elevated to the conduction
band if the frequency of the light equals or
exceeds the band gap energy
This means that light at a lower frequencies
do no work
•To get around this, cells with different band
gap energies are assembled into
multijunction cells
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Multi-junction Solar Cells
•The stack at right is a multijunction
with descending order of band gap
energy, Eg.
•Junction materials can be mixed
(e.g., GaAs and Si) provided they
are dimensionally compatible to
tailor bandgap energy
•Multijunction solar cells have
reached efficiencies of up to 35%
Cell materials of interest include:
•Amorphous Silicon
•Copper Indium Diselenide
•Gallium Aresnide and
•Gallium Indium Phosphide
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Amorphous Silicon
•Amorphous materials have no long-range
crystalline order
•In 1974, researchers found that
photovoltaic devices could be made using
amorphous silicon by properly controlling
deposition and composition
•Amorphous silicon absorbs solar radiation
40 times more efficiently than singlecrystal silicon – a film 1-micron thick can
absorb 90% of the usable solar energy
•Amorphous silicon can be processed at
relatively low temperatures on low-cost
substrates making it very economical
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Amorphous Silicon
The lack of crystalline regularity in amorphous silicon results in “dangling
bonds”. Here, electrons recombine with holes. When amorphous silicon is
doped with small amounts of hydrogen (“hydrogenation”), the hydrogen
atoms combine chemically with the dangling bonds, permitting electrons to
move through the amorphous silicon
Cells are designed to have ultra-thin (0.008micron) p-type top layer, a thicker (0.5 to 1micron) intrinsic (middle) layer, a very thin
(0.02-micron) n-type bottom layer. The top
layer is so thin and transparent that most light
passes right through. The p- and n- layers
create an electric field across the entire
intrinsic region
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Solar Cell Processing Steps
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Solar Cell Efficiency
Power is the product of voltage and current: Vmax X Imax = Pmax
A solar cells energy conversion efficiency, (h or “eta”) is the
percentage of power converted (from absorbed light to electrical
energy) and collected, when a solar cell is connected to an
electrical circuit. It is calculated using the ratio of Pmax divided by
the input light irradiance under “standard” test conditions (E, in
W/m2) and the surface area of the solar cell (Ac in m2)
Pmax
h
E  Ac
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Economics of Solar Power
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Crystalline PV Cell Economics
•Total cost of conventional crystalline PV cells is about $500/m2
($50/sq.ft) of collector area
•The output of 1-m2 is 125 Watts, so, at a cost of $500/m2, this
corresponds to $4/Watt of electricity, not counting necessary
auxiliary components
•The lowest reported cost are $3/Watt for photovoltaic cells in
2002 (IEA). Crystalline silicon cells accounted for 80% of the total
worldwide in 2002.
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Photovoltaics - Economics
Efficiencies
•Efficiencies vary from 6%
for amorphous Si cells to up
to 35% for exotic GaAs or
InSe cells
•Efficiency is 14-16% in
commercially available mcSi cells
•Power distribution systems
include inverters to connect
to the grid – system
efficiencies are between 519%
Costs
•A GaAs or InSe cell delivers
4 times the electrical power
– at over 100 times the
cost!
•In 2005, photovoltaic
electricity cost $0.30 $0.60/kWh in the US.
Compare this to the
~$0.10/kWh from other
sources
•The payback period for
solar cell implementation
can be from 1 to 20 years.
A typical value is 5 years
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Alternative energy sources
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Solar Steam Plant – Four Corners, CA
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Optical Memory and Data Storage
•Use amorphous Chalcogen
(group VI elements, e.g. Se, S or
Te)
•Photo-induced phase transitions
between crystalline and
amorphous phases
•Photo-induced phase
transitions between crystalline
and amorphous phases or
reversible photostructural
changes in the amorphous
phase
Light induces cross-linking of
neighboring chains in Se. When a
photon is absorbed, an electron from
one of the non-bonding (lone-pair)
orbitals that form the top of the valence
band is transferred into the conduction
band, leaving the other electron
unpaired. This unpaired electron can,
through interaction with lone-pair
electrons of a neighboring chain, form
an additional bond cross-linking the two
chains. Inter-chain bonds strain glass,
rupturing bonds, resulting in defect
pairs to form with a dangling bond.
The dangling bonds recombine to form
a structure different from the original.
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An originally amorphous layer can be
locally crystallized when exposed to a laser
pulse. The difference in optical properties
between glass and crystal is the basis for
optical recording
Crystallization of a GST film by a long lowerintensity pulse and its amorphization by a short
higher-intensity pulse
Structure of a crystallized GST. The larger
white circles represent Te sites, smaller
black/white circles represent Sb/Ge sites, and
dashed circles represent Sb/Ge vacancies
GST: Ge-Sb-Te chalcogenide glasses
Because there is
little volume change
in GST between
glass and
crystalline phases,
little atomic
movement is
required, and it can
change shape very
quickly
The GST layer is
sandwiched between
two protection layers.
Each layer is ~20-100
nm thick
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