Chemistry 115

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Chemistry 115 Lecture 29 Outline Chapter
12 Semconductors and Solar Energy Reading for
today pg 460-465, pg 477 479
(nanotechnology) Recitation Exam
Review FINAL EXAM THURSDAY, MAY 1, 320
PM Elliot Hall of Music
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Solar Energy
solar radiation hitting Earths surface (for
midday sun) 925 W/m2 (power energy per
time, 1 W 1 J/s)

• Solar energy available in U.S. in 1 year is about
  10 times total energy available in ALL known oil
  reserves on the planet.

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Solar Energy

• Obviously cant use it all
• How much could we use?
• How is solar energy captured and used?
• How does the cost per Watt compare (today) to
  oil-based energy?

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Using Solar Energy

• Passive
• Allowing solar radiation to be captured as heat
  such as heating water or bricks or other heat
  sinks. Heat is then released as needed.
• Active
• Allowing solar radiation to strike a solar cell
  which converts it to an electric current. Store
  the electricity in batteries to use later or use
  it do work (example, powering a motor)

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Passive Solar Panels

• Solar Thermal panels the original solar
  panels
• Used to heat water

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Passive Solar Home Design
Heat absorbing material which will radiate heat
back out at night. Example brick or certain
tiles.
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Solar Cells (Photovoltaics)

• Overview of how they work.
• To understand how solar cells work, we must look
  at the electronic energy levels of solids, at the
  atomic level

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What do you think?

• From your reading
• What is true about the energy levels in solids?
• Electrons can freely conduct electricity in the
  solid as long as it has a regular crystal
  lattice.
• Insulators, metals and semiconductors are
  distinguished by their crystal lattice structure.
• Semiconductors consist of material that is p-type
  on one side and n-type on the other, where
  electrons can move across the junction.
• The energy levels of the individual atoms in the
  solid combine and form continuous bands (rather
  than separate orbitals).

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Metals, Semiconductors, Insulators
Are defined in terms of their electrical
conductivity. Metallic conductor a substance
that conducts electricity by the movement of
electrons and whose resistivity increases with
increasing temperature. Sodium, copper,
silver, gold, iron, aluminum, mercury, ReO3,
V2O3, PbMo6S8, YBa2Cu3O7
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Metals, Semiconductors, Insulators
Semiconductor a substance that conducts
electricity by the movement of electrons and
whose resistivity decreases with increasing
temperature. Si, Ge, GaAs, GaP,
(Si1-xGax)Px, Ti2O3, La2CuO4
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Metals, Semiconductors, Insulators
Insulator a substance with an extremely high
resistance that conducts only under extremely
severe conditions. Glass, polyethylene, diamond,
cellulose, NaCl, TiO2, NH4Cl, KNO3
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Metals, Semiconductors, Insulators
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Orbital Overlap in Metals
Where are the electrons in linear Lin
molecules? Nucleus
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Orbital Energies, Band Theory
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Filling of Energy Levels
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Allowed Electron Energies
2p 2s
1s
Li Li2 Li3
Lin
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Metals
Metallic conductors are characterized by a
partially filled band. Electrons which occupy a
partially filled band are mobile and move under
the influence of a potential.
Li 1s22s1
Be 1s22s2
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Insulators
Insulators are characterized by a filled band
separated from empty band by a large gap
(forbidden zone). Electrons which occupy a
filled band are not mobile.
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Extrinsic and Intrinsic Semiconductors

• Intrinsic semiconductors the pure, crystalline
  material has semiconducting properties (such as
  pure Si or Ge)
• Extrinsic semiconductors
• The material is doped with a small amount of
  impurity of a similar material (about 1 in 106 to
  1 in 108 atoms)
• Decreases Eg of intrinsic material.
• p-type doped with element with fewer electrons.
  Provides positive charge carriers (holes).
• n-type doped with element with more electrons.
  Provides negative charge carriers (electrons).

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Semiconductors
Intrinsic semiconductors are characterized by a
filled band separated from empty band by a gap
which is small enough that electrons can be
promoted across the gap by thermal energy. Thus
there are a few mobile electrons in both bands.
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N-Type p-Type Semiconductors (Extrinsic)
Si doped with B or Al
Si doped with P or As
p-type n-type
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Conductivity
Electrons move toward positive electrode.
Holes move toward negative electrode.

• Electron and hole move in opposite directions
  when conducting (in response to an applied
  voltage difference)

Electrodes
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Energy Required Eg

• Need to put in Eg of energy to induce a current
• In metals, this is essentially zero (just need to
  apply a potential difference). The electrons can
  travel freely at room temp
• In insulators, it is very large - no mobile
  electrons at room temp
• In semiconductors, it is in the IR/Visible range.
  A small number of electrons are mobile at RT

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Characteristics of Typical Semiconducting
Materials
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Integrated Circuits (computer chips)

• Put n-type and p-type material together to get a
  p-n rectifier, or diode
• Current can only move in one direction across the
  p-n junction
• Other combinations of p and n can be used to make
  other devices.
• Combinations can be printed on silicon make
  integrated, solid-state circuits.

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How an LED Works

• Light Emitting Diode
• p-n junction
• when an electron and hole get to the junction,
  they can combine and emit light. (This is
  actually the electron losing energy by falling
  into the hole.)
• Electricity in, light out

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• Light Emitting Diode when electron and hole get
  to junction, they can combine and emit light.
• Electricity in, light out

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How a Solar Cell Works

• Opposite of an LED. Apply the voltage in
  reverse.
• Light in, electricity out
• Solar panels are made out of hundreds of solar
  cells (photovoltaic cells) in series.

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What are the possibilities for solar power?

• Current solar cells operate between 20-30
  efficiency (i.e. 100 J in as sunlight, 20-30 J
  out as electricity)
• Cost is currently about 35/kWh (versus 2/kWh
  from fossil fuel-based electricity)
• Pollution-free during use.
• Grid connected solar electricity can be used
  locally, minimizing transmission losses and
  excess generated can be sold to power companies
  (to be used later.)
• Requires energy storage through battery systems
  (for times of no sun)
• Solar cells produce DC which must be converted to
  AC (for existing power grids). This incurs an
  energy loss of 4-12

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Think about it

• If solar cells are 33 efficient, what total area
  (in square miles) would need to be covered by
  solar panels to provide all of the power for the
  U.S. each year?
• In the US, we have 800X energy needed from the
  sun, so we can use US area/800 to find
• Answer 14,000 mi2 118mi x 118 mi.
• This is about 40 of land in IN.

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Vineyards Going Solar!...
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New Materials for Solar Power

• Traditional solar cells are brittle and bulky.
• New flexible materials are being developed and
  researched.

A silicon nanoparticle-based flexible solar
material.
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New Materials for Solar Power

• Others are organic and polymer-based

Pentacene-based solar cell developed by
researchers at Georgia Institute of Technology
Organic polymer-based solar cell material being
developed by the Air Force Research Labs
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New Materials for Solar Power

• Many uses for flexible solar cells

9.2 oz roll, 9 W (15.4 V at 0.6 A) Great for
hiking!
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New Materials for Solar Power

• Many uses for flexible solar cells

9.2 oz roll, 9 W (15.4 V at 0.6 A) Great for
hiking!
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But what about Hydrogen?
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But what about Hydrogen?

• Generation of hydrogen gas is also a viable
  solution if
• Hydrogen can be generated cheaply
• It can be stored safely
• And transported to where its needed

Several approaches towards hydrogen generation
include splitting water using special
nanoparticles, or teaching bacteria to make
hydrogen more efficiently.
As for storage, thats still a problem. An
alternative is to take CO2 and H2 and make
methanol.
CO2 3H2 ? CH3OH H2O
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Doped TiO2 Nanoparticle Electrodes for Solar-H2
ConversionRaftery Group Research

• Research Goal Develop low-cost, efficient TiO2
  materials with suitable properties for solar
  water splitting.
• Experiments Synthesis, characterization and
  evaluation of anion-doped TiO2 electrodes
  measuring H2 production under sunlight simulator.

TiO2 nanoparticles
Tetrabutylamonium hydroxide (TBN)
2 layers TiO2 films on FTO
2H2O (l) ? 2H2 (g) O2 (g)
Photoelectrochemical Cell
Pt electrode
h
Anode 2H2O 4h ? O2 4H
Cathode 4H 4e- ? 2H2
TiO2 electrode
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The Big Picture

• Energy bands result when large numbers of atoms
  combine to form a crystal.
• The spacing between the filled and unfilled bands
  is the bandgap with energy Eg.
• The size of Eg will determine the properties of
  the material.
• LEDs electricity in light out. Solar cell is
  reverse.
• Both use the p-n junction of extrinsic
  semiconductors.
• There is more than enough solar energy striking
  the planet to meet our planets energy demands,
  and much more than is available from fossil
  fuels.
• Researchers are developing materials and power
  distribution systems that will make solar power
  even easier and more user-friendly than it
  already is.

This could be you!