Solar Technologies
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Although solar energy is a very small component of the overall sources of energy, the use of solar energy is growing significantly. But an exponential increase of the production capacity of the solar energy is still needed in order to satisfy the needs of energy.
Utilization of Solar Energy
A 42% annual growth rate of cumulative installed PV capacity has been fairly steady in recent years and been accompanied by a steady decrease of the price of solar modules (per watt), from nearly $100 in 1976 down to an average of $4. That trend is often referred to as the learning curve. In 2005, for the first time in history a total of over 1 GW (gigawatt) of power capacity was added, increasing the cumulative installed capacity to a value of 3.7 GW in established industrial countries. The greatest proportion was installed by Germany and Japan alone (85%).
"Generation I" (single crystal and poly-crystal silicon) solar cells may be found on roof tops, and in large power generating arrays around the world. The BOS (balance of system) cost is generally larger for totally off-grid applications because they require a storage device, today this is typically an array of lead-acid batteries. For houses or industries which are "on-grid" solar generated electricity can be distributed back into the electrical grid when there is an overproduction with respect to the needs of the user.
Organic solar cells (organic photovoltaics -- OPVs) are an emerging, "Generation III" technology, providing another light-weight option for "off-grid" applications (e.g. chargers for portable electronics). OPVs may eventually compete for power generation with Generation I on-grid devices, if their efficiencies and lifetimes are high enough, and their costs low enough.
Generations of Solar Cells
First Generation Solar Cells
Solar cells on the market now are almost all "Generation I" devices, made out of crystalline silicon, considered to be a "mature" technology. They are somewhat expensive to produce because of the high processing costs needed to create silicon cells with sufficient purity and long range order. Generation II materials are less mature as a PV technology, are generally deposited from vacuum or by chemical vapor deposition (CVD), and include CdTe or copper-indium-gallium-selenide (CIGS) technologies. Their costs are lower, as are their efficiencies, but they can be created in much thinner, light weigh formats versus silicon cells. Disposal at end-of-life is a consideration, because of the toxicity of Cd, Te, Ga, and Se, and because of concerns regarding their "earth abundance."
See DOE report on solar energy research[1]
Third Generation Organic Solar Cells
OPVs come in several different platforms, and when fully optimized, will be light-weight, low-cost, flexible and easily scalable energy conversion platforms. Their mode of energy conversion is based upon formation of a heterojunction between a donor (D) and an acceptor (A) material, which can be two different organic dyes, a polymer and a small molecule, a polymer and an ensemble of semiconductor or oxide nanoparticles, and a host of variations on this theme. Examples of companies attempting to bring OPVs to market include Konarka and Plextronics:
Konarka is one of the first commercial OPV products Plextronics
Solar Potential
A broad band light source is often used to illuminate OPVs under test, sometimes filtered to approximate the output of the sun at the earth's surface (air mass (AM) 1.5 = Air Mass, corresponding to an incident power of 100 mW/cm^2 or 1000 W/m).
Solar Spectrum
This green curve shows solar spectral distribution for one sun (the y-axis units on the right) for those standardized conditions. The dips in the bands are represent absorption by carbon dioxide and water in the atmosphere. These dips also account for reflectance back towards Earth, e.g. the greenhouse effect.
The black curve (y-axis on the left) represents the total current density you could produce in a photovoltaic device, performing the integration up to a certain wavelength. The maximum current density can be calculated by counting all the photons starting from 0 wavelength on the high energy side. This the maximum current density that can be obtained if the power efficiency were to be 100%, or in other words, if for every photon that comes, one electron enters the electrical circuit. This represents a photocurrent of 40 milliamps per square centimeter of solar cell area. No known solar cells have achieved such current densities, without "concentrating" the sun's power on the solar cell. Established efficiencies (NREL certified) for OPVs to date are, at best, just under 6%.
Advantages of Organic Photovoltaics
Cost Considerations
The major driver for the adoption of new technologies is often a combination of a lack of supply of the old technology (e.g. fossil fuels), and the introduction of a "disruptive" new technology. The increased cost of electricity generation from fossil fuels, coupled with climate change issues associated with their use, are driving the search for new forms of energy. The cost of electricity from coal fired thermal plants is about 4 cents a kilowatt hour, whereas silicon PVs create electricity at about 25- 35 cents a watt (that price continues to fall). The price for alternative solutions such has solar cells is still too high. For instance, Allen Heeger[2] has installed a solar cell roof on his house in Santa Barbara but it will take about 7-8 years before it becomes profitable. However, that is still better than losing money from electricity costs every year for the duration or lifetime of the house. Metrics: What is most important in terms of the overall production is power conversion efficiency of single cell. With respect to the given input power of the sun, what is the electrical output power that the solar cell can produce? In our case, we will refer to this power conversion efficiency
An economic example: with 5 hours of peak sun per day, 10% conversion efficiency and 10 m2 (1 kW capacity), 5kWh/day, 150 kWh per month, 1,800 kWh per year would product $600 of electricity per year if $0.3 per kWh. The cost of a 1kW capacity system, $7/W, $7,000.
Calculate the Carbon footprint to charge an iPod
Efficiency
The record in terms of power efficiency for a single crystal silicon cells is close to 25%. Multi-junction solar cells (created from multiple stacks of III-V semiconductor materials), with concentrators have power conversion efficiencies approaching 40%, but the electricity generated is still not competitive in cost with that generated from fossil fuels.
Both small molecule and polymer-based OPVs have recently shown dramatic improvements in efficiency, approaching 6%. Dye-sensitized solar cells (DSSCs) are hybrids of nano-porous metal oxides, like titanium oxide, and organic dyes, with solution electrolytes. DSSCs can demonstrate power conversion efficiencies of 11-12%, however they have proven difficult to manufacture in stable platforms, and current research is focussed on using condensed phase, polymeric charge transport agents, and it would not be surprising if OPVs and DSSCs converge on very similar materials combinations and device platforms.
5% is considered a reliable efficiency for OPVs at this time. This low efficiency is tolerable if the production expense is small compared to silicon devices. At 10-11% OPV will become economically viable. This should be achievable within a few years.
Weight and Flexibility
OPVs are also attractive because of their potential flexibility and light weight. Many "off-grid" applications in remote areas of the world would be greatly aided by "portable power" that you could pack in with you, use, and then pack out (think about taking your iPod with you on that next backpacking trip, or think about the dissemination of remote sensors throughout the food system, powering sensors that can detect pathogens in real time).
Manufacturing and Disposal
Organic solar cells may eventually be manufactured with easy to process plastic substrates, using standard printing and screen printing techniques rather than the elaborate methods required with silicon solar cells. At end of life they promise to be easily re-cycled or disposed of.
External Links
Explanation of spincoating process
Video on on OPV manufacturing process
References
http://en.wikipedia.org/wiki/Vacuum_deposition
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