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Geo Seminar F 2007


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Title: Geo Seminar F 2007

Geo Seminar F 2007
Solar Energy Conversion Current Trends
Two top reasons for Energy Alternatives
  • 1. Global Warming
  • Greenhouse gas changes since the industrial
    revolution are believed to be closely related to
    the change in climate that has been observed
    (Intergovernmental Panel on Climate Change
    (IPCC), 2007)
  • 2.Energy Independence National Security

Global WarmingThe NOAA Annual Greenhouse Gas
Index (AGGI)
  • Global averages of the concentrations of the
    major, well-mixed, long-lived greenhouse gases -
    carbon dioxide, methane, nitrous oxide, CFC-12
    and CFC-11 from the NOAA global flask sampling
    network since 1978. These gases account for about
    97 of the direct radiative forcing by long-lived
    greenhouse gases since 1750. The remaining 3 is
    contributed by an assortment of 10 minor halogen
  • (NOAA Earth System Research Laboratory, 2007)

World Carbon Dioxide Emissions by Region,
2001-2025(Million Metric Tons of Carbon
  • World carbon dioxide emissions expected to
    increase by 1.9 percent annually between 2001 and
  • Much of the increase in these emissions is
    expected to occur in the developing world where
    emerging economies, such as China and India, fuel
    economic development with fossil energy.
    Developing countries emissions are expected to
    grow above the world average at 2.7 percent
    annually between 2001 and 2025 and surpass
    emissions of industrialized countries near 2018.
  • (US DOE Energy Information Administration, 2004)

Solar Potential
  • The Sun energy drives the planet powers oceanic
    and atmospheric currents, the water cycle,
    typhoons, hurricanes, and tornadoes
  • Sun continuously delivers to Earth 1.2  105
    terawatts of power which dwarfs every other
    energy source, renewable or nonrenewable.
  • Human civilization produces and uses energy
    currently at rate of about 13 TW
  • The San Francisco earthquake of 1906, with
    magnitude 7.8, released an estimated 1017 joules
    of energy, the amount the Sun delivers to Earth
    in one second.
  • Earth's ultimate recoverable resource of oil,
    estimated at 3 trillion barrels, contains
    1.7  1022 joules of energy, which the Sun
    supplies to Earth in 1.5 days.
  • The amount of energy humans use annually, about
    4.6  1020 joules, is delivered to Earth by the
    Sun in one hour.
  • Source (Crabtree Lewis, 2007)

USAConcentrated Solar Power Resource Potential
  • (US DOE Energy Information Administration, 2007a)

Solar Versatility
  • Sunlight can be converted into
  • electricity by exciting electrons in a solar
  • chemical fuel via natural photosynthesis in green
    plants or artificial photosynthesis in
    human-engineered systems.
  • heat for direct use or further conversion to
    electricity (concentrated or unconcentrated)
    (Lewis Crabtree, 2005)
  • All three link seamlessly with existing energy
  • Image source (Crabtree Lewis, 2007)

Solar VersatilityAdded bonus
  • Stardust spacecraft (NASA image) in flight.
    Photovoltaic systems were an important power
    source for that mission. Solar cells have not
    only enabled exploration of space, the solar
    system, and the Earth in great detail, they also
    have enabled the emergence of the
    telecommunications industry. (US Department of
    Energy, 2006)

Solar vs. Fossil
  • Fossil fuels
  • Account for 80 - 85 of our energy consumption
  • are finite
  • distributed unevenly
  • When combusted produce greenhouse gases and
    harmful environmental pollutants
  • Solar energy
  • effectively inexhaustible
  • Widely available and unrestricted by geopolitical
  • Environmentally friendly, especially hen directly
    used no threat to health or climate
  • Theoretically enormous supply
  • However fossil fuels still much cheaper than
  • fossil-fuel are concentrated sources of energy,
    whereas the Sun distributes photons uniformly
    over Earth at a more modest energy density.
  • The use of biomass as fuel is limited by the
    production capacity of the available land and
  • (Crabtree Lewis, 2007)

  • Despite the enormous energy flux supplied by the
    Sun, the three conversion routes supply only a
    tiny fraction of our current and future energy
  • Solar electricity, at between 5 and 10 times the
    cost of electricity from fossil fuels, supplies
    just 0.015 of the world's electricity demand.
  • Solar fuel, in the form of biomass, accounts for
    approximately 11 of world fuel use, but the
    majority of that is harvested unsustainably.
  • Solar heat provides 0.3 of the energy used for
    heating space and water.
  • Source (Crabtree Lewis, 2007)

USA Solar Thermal and Photovoltaic Collector
Manufacturing Activities 2006
Data For 2006 Report Released October 2007
  • (Energy Information Administration (EIA), 2007a)

The Role of Renewable Energy Consumption in the
Nation's Energy Supply, 2005
Data for 2005Release Date July 2007
  • (US DOE Energy Information Administration, 2007a)

Projections World Marketed Energy Use by
FuelType, 1980-2030
  • International Energy Outlook 2007 Report (US DOE
    Energy Information Administration, 2007c)

Global energy in perspective
  • World Energy consumption by fuel source in quads
    (1 quad 1.06 1018 joules) for the years 1999
    and 2020 (estimate) for various fuel sources.
  • Source (Moniz Kenderdine, 2002)

Missing Link Conversion Efficiency
  • The best commercial solar cells based on
    single-crystal silicon are about 18 efficient.
  • Laboratory solar cells based on cheaper dye
    sensitization of oxide semiconductors are
    typically less than 10 efficient
  • Laboratory solar cells based on even cheaper
    organic materials are 25 efficient.
  • The cheapest solar electricity comes not from
    photovoltaics but from conventional induction
    generators powered by steam engines driven by
    solar heat, with efficiencies of 20 on average
    and 30 for the best systems.
  • (Crabtree Lewis, 2007)

Missing Link Conversion Efficiency
  • The utilization gap between solar energy's
    potential and our use of it can be overcome by
    raising the efficiency of the conversion
    processes, which are all well below their
    theoretical limits. (Crabtree Lewis, 2007)

Conversion to ElectricitySolar Cells
  • (Wikipedia, 2007a)

P-N Junction
  • A p-n junction in thermal equilibrium with zero
    bias voltage applied. Electrons and holes
    concentration are reported respectively with blue
    and red lines. Gray regions are charge neutral.
    Light red zone is positively charged. Light blue
    zone is negatively charged. The electric field is
    shown on the bottom, the electrostatic force on
    electrons and holes and the direction in which
    the diffusion tends to move electrons and holes.
    (Wikipedia, 2007d)

Bandgap Energies of Semiconductors and Light
  • When light shines on crystalline silicon, photons
    with a certain level of energy can free electrons
    in the semiconductor material from their atomic
    bonds to produce an electric current.
  • This level of energy, known as the "bandgap
    energy," is the amount of energy required to
    dislodge an electron from its covalent bond and
    allow it to become part of an electrical circuit.
    To free an electron, the energy of a photon must
    be at least as great as the bandgap energy.
  • However, photons with energy gt the bandgap energy
    will expend that extra amount as heat when
    freeing electrons. (US Department of Energy,
  • Image from (US Department of Energy, 2006)

Solar Cell Efficiency
  • William Shockley and Hans Queisser established a
    theorethical efficiency limit of 31 for the
    performance of solar cells (Shockley Queisser,
  • The analysis was based on four assumptions
  • illumination with unconcentrated sunlight,
  • a single pn junction,
  • one electronhole pair excited per incoming
    photon, and
  • thermal relaxation of the electronhole pair
    energy in excess of the bandgap.
  • The efficiency limit of 31 for those conditions
    still a research goal.
  • The best single-crystal Si cells have achieved
    25 efficiency in the laboratory and about 18 in
    commercial practice. (Crabtree Lewis, 2007)
  • Cheaper solar cells made from other materials
    operate at significantly lower efficiency
    (Crabtree Lewis, 2007)

The three generations of solar cells.
  • First-generation - based on expensive silicon
    wafers 85 of the current commercial market.
  • Second-generation - based on thin films of
    materials such as amorphous silicon,
    nanocrystalline silicon, cadmium telluride, or
    copper indium selenide. The materials are less
    expensive, but research is needed to raise the
    cells' efficiency.
  • Third-generation - the research goal a dramatic
    increase in efficiency that maintains the cost
    advantage of second-generation materials. Their
    design may make use of carrier multiplication,
    hot electron extraction, multiple junctions,
    sunlight concentration, or new materials.
  • Goal Achieving high efficiency from inexpensive
    materials with so-called third-generation cells
  • The horizontal axis represents the cost of the
    solar module only it must be approximately
    doubled to include the costs of packaging and
    mounting. Dotted lines indicate the cost per watt
    of peak power.

The ShockleyQueisser limit can be exceeded by
violating one or more of its premises
  • Violating Premise 1 illumination with
    unconcentrated sunlight,
  • Concentrating sunlight allows for a greater
    contribution from multi-photon processes that
    contribution increases the theoretical efficiency
    limit to 41 for a single-junction cell with
    thermal relaxation. (Crabtree Lewis, 2007)
  • Image source (Wikipedia, 2007b)

The ShockleyQueisser limit can be exceeded by
violating one or more of its premises
  • Violating Premise 2 a single pn junction,
  • Multi junction cells A cell with a single pn
    junction captures only a fraction of the solar
    spectrum photons with energies less than the
    bandgap are not captured, and photons with
    energies greater than the bandgap have their
    excess energy lost to thermal relaxation. Stacked
    cells with different bandgaps capture a greater
    fraction of the solar spectrum the efficiency
    limit is 43 for two junctions illuminated with
    unconcentrated sunlight, 49 for three junctions,
    and 66 for infinitely many junctions. (Crabtree
    Lewis, 2007)

A multijunction device is a stack of individual
single-junction cells in descending order of
bandgap (Eg). The top cell captures the
high-energy photons and passes the rest of the
photons on to be absorbed by lower-bandgap cells.
(US Department of Energy, 2006)
The ShockleyQueisser limit can be exceeded by
violating one or more of its premises
  • Violating Premise 3 one electronhole pair
    excited per incoming photon
  • Carrier multiplication is a quantum-dot
    phenomenon that results in multiple electronhole
    pairs for a single incident photon.
  • Carrier multiplication was discussed by Arthur
    Nozik in 2002 and observed by Richard Schaller
    and Victor Klimov two years later.
  • In bulk-semiconductor solar cells, when an
    incident photon excites a single electronhole
    pair, the electronhole pair energy in excess of
    the bandgap is likely to be lost to thermal
    relaxation, whereas in some nanocrystals most of
    the excess energy can appear as additional
    electronhole pairs. If the nanocrystals can be
    incorporated into a solar cell, the extra pairs
    could be tapped off as enhanced photocurrent,
    which would increase the efficiency of the cell.
  • Nanocrystals of lead selenide, lead sulfide, or
    cadmium selenide generate as many as seven
    electrons per incoming photon, which suggests
    that efficient solar cells might be made with
    such nanocrystals.
  • Significant obstacles impede implementation of
    this method. We cannot attach wires to
    nanocrystals the way we do to bulk
    semiconductors collecting the electrons from
    billions of tiny dots and putting them all into
    one current lead is a problem in nanoscale
    engineering that no one has solved yet. A second
    challenge is separating the electrons from the
    holes, the job normally done by the space charge
    at the pn junction in bulk solar cells. Those
    obstacles must be overcome before practical
    quantum-dot cells can be constructed.
  • Cited (Crabtree Lewis, 2007)

The ShockleyQueisser limit can be exceeded by
violating one or more of its premises
  • Violating Premise 4 thermal relaxation of the
    electronhole pair energy in excess of the
  • Hot-electron extraction provides way to increase
    the efficiency of nanocrystal-based solar cells
    by tapping off energetic electrons and holes
    before they have time to thermally relax. (Nozik,
  • Femtosecond laser and x-ray techniques can
    provide the necessary understanding of the
    ultrafast decay processes in bulk semiconductors
    and their modification in nanoscale geometries
    that will enable the use of hot-electron
    phenomena in next-generation solar cells.
    (Crabtree Lewis, 2007)

Other cell technologiesThin film technologies
  • The various thin-film technologies currently
    being developed reduce the amount (or mass) of
    light absorbing material required in creating a
    solar cell. This can lead to reduced processing
    costs from that of bulk materials (in the case of
    silicon thin films) but also tends to reduce
    energy conversion efficiency, although many
    multi-layer thin films have efficiencies above
    those of bulk silicon wafers. (Wikipedia, 2007c)
  • Thin-film cells offer advantages beyond cost,
    including pliability, and potential integration
    with preexisting buildings and infrastructure.
    (Crabtree Lewis, 2007)
  • Novel conducting polymers enable solar cells that
    are flexible, inexpensive, and versatile. The new
    materials can be coated or printed onto flexible
    or rigid surfaces. (Image courtesy of Konarka
    Technologies.) (Crabtree Lewis, 2007)

Triple Junction Thin Film Cell
  • (United Solar Systems Corp., 2004)

Other cell technologies Dye-sensitized and
Organic solar cells
  • Dye-sensitized solar cells were introduced by
    Michael Grätzel and coworkers in 1991(O'Regan
    Grätzel, 1991)
  • Dye-sensitized solar cells effectively separate
    the two functions provided by silicon in a
    traditional cell design. Normally the silicon
    acts as both the source of photoelectrons, as
    well as providing the potential barrier to
    separate the charges and create a current. In the
    DSSc, the semiconductor is used solely for charge
    separation, the photoelectrons are provided from
    a separate photosensitive dye. Additionally the
    charge separation is not provided solely by the
    semiconductor, but works in concert with a third
    element of the cell, an electrolyte in contact
    with both. (Wikipedia, 2007a)
  • Organic/polymer solar cells are built from thin
    films (typically 100 nm) of organic
    semiconductors such as polymers and
    small-molecule compounds like polyphenylene
    vinylene, copper phthalocyanine (a blue or green
    organic pigment) and carbon fullerenes. Energy
    conversion efficiencies achieved to date using
    conductive polymers are low at 6 efficiency18
    for the best cells to date. However, these cells
    could be beneficial for some applications where
    mechanical flexibility and disposability are
    important. (Wikipedia, 2007c)

Conversion to ElectricitySolar Cells Types and
  • (Kazmerski et al., 2007)

Conversion to Fuels
  • Photosynthesis Natures way
  • An estimated 100 TW of solar energy go into
    photosynthesis, the production of sugars and
    starches from water and carbon dioxide.
  • Green plants convert sunlight into biomass with a
    typical yearly averaged efficiency of less than
    0.3 - Enough for plants to cover the Earth but
    too low to readily satisfy the human demand for
  • The early stages of photosynthesis are efficient
  • Two molecules of water are split to provide four
    protons and electrons for subsequent reactions,
    and an oxygen molecule is released into the
  • The inefficiency lies in the later stages, in
    which carbon dioxide is reduced to form the
    carbohydrates that plants use to grow roots,
    leaves, and stalks.
  • The research challenge is to make the overall
    conversion process between 10 and 100 times more
    efficient by improving or replacing the
    inefficient stages of photosynthesis.
  • Cited (Crabtree Lewis, 2007)

Conversion to Fuels
  • The metabolic pathways of plants have evolved for
    organisms' survival and reproduction, not for
    fuel production.
  • The efficient steps that are relevant for fuel
    production might conceivably be isolated and
    connected directly to one another to produce
    fuels such as H2, CH4, or alcohols.
  • Hybridizing nature in that way takes advantage of
    the elaborate molecular processes that biology
    has evolved and that are still beyond human
    reach, while eliminating the inefficient steps
    not needed for fuel production. (Crabtree
    Lewis, 2007)
  • There are three routes to improving the
    efficiency of photosynthesis-based solar fuel
  • breeding or genetically engineering plants to
    grow faster and produce more biomass,
  • connecting natural photosynthetic pathways in
    novel configurations to avoid the inefficient
    steps, and
  • using artificial bio-inspired nanoscale
    assemblies to produce fuel from water and CO2.
  • The first route - occupation of GMO industry
  • The second and third routes, which involve more
    direct manipulation of photosynthetic pathways,
    are still in their early stages of research.
    (Crabtree Lewis, 2007)

Artificial photosynthesis
  • Artificial photosynthesis - using inanimate
    components to convert sunlight into chemical
    fuel. (Gust et al., 2001)
  • Increased understanding of photosynthetic energy
    conversion and advances in chemical synthesis and
    instrumentation have made it possible to create
    artificial nanoscale devices and semibiological
    hybrids that carry out many of the functions of
    the natural process. Artificial light-harvesting
    antennas can be synthesized and linked to
    artificial reaction centers that convert
    excitation energy to chemical potential in the
    form of long-lived charge separation. (Gust et
    al., 2001)
  • Image (Crabtree Lewis, 2007)

Artificial photosynthesis
  • An artificial antennareaction-center complex
    that mimics the early stages of photosynthesis.
    The central hexaphenylbenzene core provides
    structure and rigidity for the surrounding wheel
    of five bis(phenylethynyl)anthracene antennas
    that gather light at 430475 nanometers. The
    energy is transferred to a porphyrin complex in
    110 picoseconds (orange arrows), where it
    excites an electron that is transferred to the
    fullerene acceptor in 80 ps the resulting
    charge-separated state has a lifetime of 15
    nanoseconds. Complexes such as the one shown
    provide the first steps in artificial
    photosynthesis. They have the potential to drive
    further chemical reactions, such as the oxidation
    of water to produce H2 or the reduction of CO2 to
    CH4, alcohol, or other fuel.
  • Cited (Crabtree Lewis, 2007)

Nonbiological ways of creating fuels
  • Solar fuels can be created in fully nonbiological
    way based on semiconductor solar cells rather
    than on photosynthesis. (Khaselev Turner, 1998)
  • In photoelectrochemical conversion
    (Photovoltaic-Photoelectrochemical Device), the
    charge-separated electrons and holes are used to
    split water or reduce CO2 at the interface with
    an electrolytic solution, rather than being sent
    through an external circuit to do electrical
    work. (Crabtree Lewis, 2007 Khaselev Turner,
  • Hydrogen was produced at the electrodewater
    interface with greater than 10 efficiency by
    Adam Heller in 1984 and by Oscar Khaselev and
    John Turner in 1998, but the fundamental
    phenomena involved remain mysterious, and the
    present devices are not practical. (Crabtree
    Lewis, 2007)

Conversion to Heat
  • The first step in traditional energy conversion
    is the combustion of fuel, usually fossil fuel,
    to produce heat. Heat produced by combustion may
    be used for heating space and water, cooking, or
    industrial processes, or it may be further
    converted into motion or electricity. (Crabtree
    Lewis, 2007)
  • The premise of solar thermal conversion is that
    heat from the Sun replaces heat from combustion
    fossil-fuel use and its threat to the environment
    and climate are thus reduced. (Crabtree Lewis,

Concentrating Sunlight for Heating
  • Unconcentrated sunlight can bring the temperature
    of a fluid to about 200 C, enough to heat space
    and water in residential and commercial
    applications. Many regions use solar water
    heating, though in only a few countries, such as
    Cyprus and Israel, does it meet a significant
    fraction of the demand. Concentration of sunlight
    in parabolic troughs produces temperatures of
    400 C, and parabolic dishes can produce
    temperatures of 650 C and higher. (Crabtree
    Lewis, 2007)
  • Image from (Wikipedia, 2007b)

Power towers
  • Power towers, in which a farm of mirrors on the
    ground reflects to a common receiver at the top
    of a tower, can yield temperatures of 1500 C or
    more. (Crabtree Lewis, 2007)
  • The high temperatures of solar power towers are
    attractive for thermochemical water splitting and
    solar-driven reforming of fossil fuels to produce
    H2 (Steinfeld, 2005)
  • The 11 megawatt PS10 solar power tower produces
    electricity from the sun using 624 large movable
    mirrors called heliostats.

Conversion to HeatConcentrated Sunlight
  • The temperatures produced by concentrated
    sunlight are high enough to power heat engines,
    whose Carnot efficiencies depend only on the
    ratio of the inlet and outlet temperatures.
  • Steam engines driven by solar heat and connected
    to conventional generators currently supply the
    cheapest solar electricity. Nine solar thermal
    electricity plants that use tracking
    parabolic-trough concentrators were installed in
    California's Mojave Desert between 1984 and 1991.
    Those plants still operate, supplying 354 MW of
    peak power to the grid. Their average annual
    efficiency is approximately 20, and the most
    recently installed can achieve 30.
  • Although those efficiencies are the highest for
    any widely implemented form of solar conversion,
    they are modest compared to the nearly 60
    efficiency of the best gas-fired electricity
  • Achieving greater efficiency for solar conversion
    requires large-scale plants with operating
    temperatures of 1500 C or more, as might be
    produced by power towers. Another alternative,
    still in the exploration stage, is a hybrid of
    two conversion schemes A concentrated solar beam
    is split into its visible portion for efficient
    photovoltaic conversion and its high-energy
    portion for conversion to heat that is converted
    to electricity through a heat engine.

Conversion to HeatThermoelectric materials
  • Thermoelectric materials, which require no moving
    parts to convert thermal gradients directly into
    electricity, are an attractive possibility for
    reliable and inexpensive electricity production.
  • Charge carriers in a thermal gradient diffuse
    from hot to cold, driven by the temperature
    difference but creating an electric current by
    virtue of the charge on each carrier. The
    strength of the effect is measured by the
    thermopower, the ratio of the voltage produced to
    the applied temperature difference.
  • Although the thermoelectric effect has been known
    for nearly 200 years, materials that can
    potentially convert heat to electricity
    efficiently enough for widespread use have
    emerged only since the 1990s.
  • Efficient conversion depends on minimizing the
    thermal conductivity of a material, so as not to
    short-circuit the thermal gradient, while
    maximizing the material's electrical conductivity
    and thermopower. Achieving such a combination of
    opposites requires the separate tuning of several
    material properties the bandgap, the electronic
    density of states, and the electron and phonon
  • The most promising materials are nanostructured
    composites. Quantum-dot or nanowire substructures
    introduce spikes in the density of states to tune
    the thermopower (which depends on the derivative
    of the density of states), and interfaces between
    the composite materials block thermal transport
    but allow electrical transport.
  • Proof of concept for interface control of thermal
    and electrical conductivity was achieved by 2001
    with thin-film superlattices of Bi2Te3/Sb2Te3 and
    PbTe/PbSe, which performed twice as well as
    bulk-alloy thermoelectrics of the same materials.
  • The next challenges are to achieve the same
    performance in nanostructured bulk materials that
    can handle large amounts of power and to use
    nanodot or nanowire inclusions to control the
    thermopower. Figure 5 shows encouraging progress
    structurally distinct nanodots in a bulk matrix
    of the thermoelectric material Ag0.86Pb18SbTe20.
    Controlling the size, density, and distribution
    of such nanodot inclusions during bulk synthesis
    could significantly enhance thermoelectric
  • (Crabtree Lewis, 2007)

Thermoelectric materials
  • A nanodot inclusion in the bulk thermoelectric
    material Ag0.86Pb18SbTe20, imaged with
    high-resolution transmission electron microscopy.
    Despite a lattice mismatch of 25, the nanodot
    (indicated by the dotted line) is almost
    perfectly coherently embedded in the matrix. The
    arrows show two dislocations near the interface,
    and the white box indicates the unit cell. The
    nanodot is rich in silver and antimony relative
    to the matrix. (Crabtree Lewis, 2007)

Storage and distribution
  • IN addition to efficient conversion, another
    challenge related to solar eergy usage is energy
    (electrical or heat) storage.
  • Access to solar energy is interrupted by natural
    cycles of daynight, cloudysunny, and
    wintersummer variation that are often out of
    phase with energy demand.
  • Solar fuel production automatically stores energy
    in chemical bonds. Electricity and heat, however,
    are much more difficult to store. Cost
    effectively storing even a fraction of our peak
    demand for electricity or heat for 24 hours is a
    task well beyond present technology.
  • Storage is such an imposing technical challenge
    that innovative schemes have been proposed to
    minimize its need.
  • Sci-Fi solutions
  • Baseload solar electricity might be generated on
    constellations of satellites in geosynchronous
    orbit and beamed to Earth via microwaves focused
    onto ground-based receiving antennas.
  • A global superconducting grid might direct
    electricity generated in sunny locations to
    cloudy or dark locations where demand exceeds
  • But those schemes, too, are far from being
    implemented. Without cost-effective storage and
    distribution, solar electricity can only be a
    peak-shaving technology for producing power in
    bright daylight, acting as a fill for some other
    energy source that can provide reliable power to
    users on demand.

  • (Krauter, 2006)

  • The Sun has the enormous untapped potential to
    supply our growing energy needs.
  • The barrier to greater use of the solar resource
    is its high cost relative to the cost of fossil
    fuels, although the disparity will decrease with
    the rising prices of fossil fuels and the rising
    costs of mitigating their impact on the
    environment and climate.
  • The cost of solar energy is directly related to
    the low conversion efficiency, the modest energy
    density of solar radiation, and the costly
    materials currently required.
  • The development of materials and methods to
    improve solar energy conversion is primarily a
    scientific challenge Breakthroughs in
    fundamental understanding ought to enable marked
  • There is plenty of room for improvement, since
    photovoltaic conversion efficiencies for
    inexpensive organic and dye-sensitized solar
    cells are currently about 10 or less, the
    conversion efficiency of photosynthesis is less
    than 1, and the best solar thermal efficiency is
    30. The theoretical limits suggest that we can
    do much better.

  • If solar energy is to become a practical
    alternative to fossil fuels, we must have
    efficient ways to convert photons into
    electricity, fuel, and heat. (Crabtree Lewis,
  • The need for better conversion technologies is a
    driving force behind many recent developments in
    biology, materials, and especially nanoscience.
    (Crabtree Lewis, 2007)

  • Crabtree, G. W., Lewis, N. S. (2007). Solar
    Energy Conversion. Physics Today, 60(3), 37-42.
  • Gust, D., Moore, T. A., Moore, A. L. (2001).
    Mimicking Photosynthetic Solar Energy
    Transduction. Acc. Chem. Res., 1, 40 -48.
  • Intergovernmental Panel on Climate Change (IPCC).
    (2007). Climate Change 2007 The Physical Science
    Basis. Cambridge UK and New York, NY USA.
    Cambridge Univ. Press.
  • Kazmerski, L., Gwinner, D., Hicks, A. (2007).
    Reported Timeline of Solar Cell Energy Conversion
    Efficiencies by National Renewable Energy
    Laboratory (USA). Retrieved Nov 12, 2007, from
  • Khaselev, O., Turner, J. A. (1998). A
    Monolithic Photovoltaic-Photoelectrochemical
    Device for Hydrogen Production Via Water
    Splitting. Science, 280(5362), 425 - 427.
  • Krauter, S. C. W. (2006). Solar Electric Power
    Generation - Photovoltaic Energy Systems
  • Lewis, N. S., Crabtree, G. W. (Eds.). (2005).
    Basic Research Needs for Solar Energy
    Utilization Report of the Basic Energy Sciences
    Workshop on Solar Energy Utilization, April
    1821, 2005 US Department of Energy Office of
    Basic Energy Sciences, Avaliable at
  • NOAA Earth System Research Laboratory. (2007).
    The Noaa Annual Greenhouse Gas Index (Aggi).
    Retrieved Nov 12, 2007, from http//www.esrl.noaa.
  • Nozik, A. J. (2005). Exciton Multiplication and
    Relaxation Dynamics in Quantum Dots Applications
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