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Copyright © 2005 SRI International
Clean Solar Energy
The impact of nanoscale
science on solar energy
production
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Sources: http://www1.eere.energy.gov/solar/pv_systems.html
http://thomashawk.com/hello/209/1017/1024/Staring%20at%20the%20Sun.jpg
What is Solar Energy?
• Energy produced by
the sun
• Clean, renewable
source of energy
• Harnessed by solar
collection methods
such as solar cells
• Converted into
usable energy such
as electricity
Photovoltaic (solar)
panel
Set of solar panels
Sun and electrical
power lines
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Energy from the Sun is Abundant
• Solar power
systems
installed in
the areas
defined by
the dark
disks could
meet the
world's
current total
energy
demand
Source: http://www.ez2c.de/ml/solar_land_area/
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20 TW
Sources: http://energy.cr.usgs.gov/energy/stats_ctry/Stat1.html
http://www.census.gov/main/www/popclock.html
Current U.S. Energy Demand
• The US consumes
~25% of the
world’s energy, but
has only 4.5% of
its population
• Solar cells would
need to cover an
area comparable to
the size of Texas to
meet US energy
demand today
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5
20 TW
180,000 TW
of sunlight
hit the earth
every day
Sources: http://energy.cr.usgs.gov/energy/stats_ctry/Stat1.html
http://www.census.gov/main/www/popclock.html
Projected U.S. Energy Demand in 2050
• To meet
projected U.S.
demand in 2050,
solar cells would
need to cover
approximately
one third of the
U.S.
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Solar Panel Use Today
• Large companies like
Google, Walmart, and
Microsoft use solar energy
to partially power some of
their facilities
Solar panels on Microsoft building
Solar panels being tested
on Walmart store
Sources: http://i.n.com.com/i/ne/p/2006/IMG_5396_550x367.jpg
http://www.solarwall.de/assets/images/Walmart_SW.jpg
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Sources: http://www.elp.uji.es/juan_home/images/solar_cell/flex1.JPG
http://www.wisconsun.org/images/siemen_cell.jpg, http://en.wikipedia.org/wiki/Solar_cell
Silicon-based
solar cell
Dye-sensitized
solar cell
Photovoltaic Solar Cells
• Generate electricity
directly from sunlight
• 2 Main types:
Single-crystal silicon
(traditional)
• Widespread
• Expensive to manufacture
Dye-sensitized (nano)
• Newer, less proven
• Inexpensive to manufacture
• Flexible
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Sources: http://www.econedlink.org/lessons/EM189/images/cartoon_tv.gif
http://www.torpedowire.com/solar.htm,
http://www.uoregon.edu/~stiedeke/a3/assignment03/a3/assignment_images/cartoon-sun.jpg
Solar Cells are Converters of Energy…
• Solar cells are devices
that take light energy as
input and convert it into
electrical energy
Light energy
Solar cell -
converts light
energy to
electricity
Electrical energy
(carried through wires)
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Source: http://ebiomedia.com/prod/cyclops/images/image004.jpg
…But Not All Energy is Converted
• Like chloroplasts in plants, solar cells can only
absorb specific wavelengths of light.
• In both, light that isn’t absorbed is either transmitted
through or reflected back.
• Whether a certain wavelength of lights gets
absorbed depends on its energy.
Chlorophyll molecules
absorb blue and red light,
but reflect green light
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A Little Background on Light
• Different colors of light have different wavelengths
and different energies
Source: http://www.mhhe.com/physsci/astronomy/arny/instructor/graphics/ch03/0305.html
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Absorption of Light by 
Atoms
Sources:  http://members.aol.com/WSRNet/tut/absorbu.htm, http://csep10.phys.utk.edu/astr162/lect/light/absorption.html
Single electron
transition in an
isolated atom
• Absorption occurs only when the energy of
the light equals the energy of transition of
an electron
Light
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• Molecules have
multiple atoms
bonded together
• More energy states
in molecules than
atoms
• More electron
“jumps” possible -
light with a range of
frequencies are
absorbed
Source: Image adapted from http://www.3dchem.com/molecules.asp?ID=135#
Absorption of Light by 
Molecules
Smallest ΔE possible
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• Electrons can
jump between
“bands”
• Incident light
with energy ≥
than the “band
gap” energy can
be used to excite
the electrons
Source: Image adapted from http://www.3dchem.com/molecules.asp?ID=135#
Absorption of Light by 
Ionic Compounds
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So What Does this Mean for Solar Cells?
• In dye-sensitized solar
cells…
Talk about highest
occupied molecular orbital
(HOMO) and lowest
unoccupied molecular
orbital (LUMO)
Source: Original Images
• In single-crystal silicon
solar cells…
Talk about conduction
band” (excited states)
and “valence band”
(ground states)
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A Closer Look at Solar Cells
• How do traditional,
silicon-based solar
cells and newer,
dye-sensitized
solar cells work?
• What are the
advantages and
disadvantages of
each type of cell?
Sources: http://www.norfolksolar.co.uk/img/system.gif
http://www.powerlight.com/newsletters/news_issue/3/newsletter_industry.htm
Silicon-based
solar cell
Dye-sensitized
solar cell
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Source: http://nanosense.org/activities/cleanenergy/solarcellanimation.html
How a Silicon-Based Solar Cell Works
• A positive hole
is left in the
electron’s place
• This separation of
electrons and holes
creates a voltage
and a current
• Light with energy greater than the band gap energy
of Si is absorbed
• Energy is given to an electron in the crystal lattice
• The energy excites the electron; it is free to move
Click image to launch animation
(requires web access)
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Silicon-Based Solar Cell Attributes
• Expensive
Made in high vacuum at high
heat
High manufacturing costs
• Need TLC
Fragile, rigid, thick
• Long return on investment
Takes 4 years to produce
energy savings equivalent to
cost of production
Source: http://www.powerhousekids.com/stellent2/groups/public/documents/pub/phk_ee_re_001505-2.gif
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How a Dye-Sensitized Cell Works
Click image to launch animation
(requires web access)
• Light with high enough energy excites electrons in
dye molecules
• Excited electrons
infused into
semiconducting TiO
2
,
transported out of cell
• Positive holes left in
dye molecules
• Separation of excited
electrons and “holes”
creates a voltage
Source: http://nanosense.org/activities/cleanenergy/solarcellanimation.html
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Source: http://www.imo.uhasselt.be/polytech/images/zonnecel1.jpg
Dye-Sensitized Solar Cells
• Relatively inexpensive
Made in non-vacuum setting
mainly at room temperature
Relatively simple manufacturing
process
• Need little TLC
Thin, lightweight, flexible
• Short return on investment
Takes approx 3 months to
produce energy savings
equivalent to cost of production
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Sources: http://www.imo.uhasselt.be/polytech/images/zonnecel1.jpg
http://www.norfolksolar.co.uk/img/system.gif
Dye-Sensitized and Silicon-based
Solar Cells Compared
• Dye-Sensitized
Relatively inexpensive
Need little TLC
Short return on
investment
• Traditional
Expensive
Need TLC
Long return on
investment
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