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Major Processes in Organic Solar Cells

2009-06-09T20:46:53Z

12.167.130.137: /* Semiconductors compared */

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Edmond Becquerel (1820-1891) was first to discover the photovoltaic effect. But it took quite a long time before there was a report on an efficient cell and that came from Chapin, Fuller and Pearson (1954) in Bell Labs. They reported a silicon cell with 6% efficiency.

== Semiconductor Fundamentals ==

===Semiconductors compared===
[[Image:led-opvcompared.JPG|thumb|300px|LEDs and photovolataics are opposite processes.]]
This slide gives a comparison of the processes that take place in organic light emitting dioes (OLEDs) and in organic solar cells (OPVs).

In an OLED the first step toward creating light requires injection of charge carriers from the contacting electrodes. Holes typically are injected into the hole-transport layer (HTL) at the bottom, transparent contact, electrons are typically injected at the top metallic contact into the electron transport layer (ETL). The next steps involve migration of these charge carriers toward the center of the device under the influence of the applied field. The rates of charge migration are field dependent, and dependent upon the mobilities of both holes in the HTL and electrons in the ETL. In the center of the device, at the HTL/ETL interface, these injected charges (which reside on molecular species as radical cations and radical anions, or polaronic states), must find each other and recombine to form an excited (excitonic) state, from which emission occurs to create the light in the OLED display. Exciton migration, energy transfer to dopants in the OLED, and energy losses occur afterward -- the entire process from injection to emission of a photon is over typically in well under one microsecond.

An organic photovoltaic cell works in a complementary fashion. Light is absorbed in either of two donor (D) or acceptor (A) layers, creating excitonic states which must diffuse to the D/A interface where differences in ionization potential and electron affinities of D and A cause these excited states to dissociate into free charge carriers (electrons and holes residing on molecular species). The combination of diffusion and migration of these charge carriers to the collection electrodes, and the harvesting of these charges by these electrodes, produces a cureent in the external circuit, as a specific voltage, the product of which is the power produced by the OPV.

===Field effect transistors===
[[Image:Field_effect_transistor.png|thumb|300px]]
When you look at the field-effect transistors there are three major processes. The first one is the injection of charges into your semi-conductor. In the case of light-emitting diode and photovoltaic cell there are only two electrodes. However, in a field-effect transistor, the charge injection is modulated through a third electrode called a gate. Now all is needed are the electrons and the holes. Let’s suppose you inject electrons. Those electrons must migrate and be collected. The first electrode will be referred to as a gate. Then you have a thin insulator called a dielectric. You have two other electrodes referred to as the source and the drain. Then you have your organic semi-conductor. This is one of the configurations possible. When you are given a voltage difference between the source and drain, the amount of charges that will be injected into your semi-conductor will be modulated by the voltage at the gate. The gate will modulate the injection and produce a switching effect. For a given voltage between the source and drain, the voltage of the gate can either be decreased such that there is a small injection or current or it can be increased to have a very large injection of charges into your semi-conductor and a large current. These are the components that make a transistor, which is also called a three terminal devices because you have 3 electrodes. So once you have an injection of charges into the organic semi-conductor, those charges will travel and be collected at the other electrodes. These are the main steps in an organic semi-conductor; charge injection, charge transport, and charge collection.

== Quantifying OPVs ==

===Exciton binding energies===
It is important for the material to absorb photons efficiently in the energy region that matches at best the solar spectrum. If you look at the solar spectrum, it goes very significantly into the Near IR. That means a pretty small bandgap system that can get down to 1 EV bandgap absorbing nearly down to 1 electron-volt. If you have a material that has a bandgap in the middle of the visible, then you will miss out on a lot of photons.

Also, the photon density shows that there are few photons that can be absorbed with energies like 4 electron volts. If the energy is very large, there are fewer photons. This requires a good match between the solar spectrum and the longer wavelengths with lower energies. In the case of π-conjugated systems the exciton binding energies are large about a few tenths of an eV. This is what makes the difference with the respect to the organic semi-conductor. The exciton binding energy in silicon or gallium arsenide is just a few milli -eV. Thus at room temperature, there is enough energy to allow the electron and the hole to separate.

===Photovoltaic Efficiency===
[[Image:currentvoltagecurve1.jpg|thumb|300px|]]
Photovoltaics efficiency can be described using a current voltage curve-- the voltage on the x axis and the current on the y axis. In the dark the device has very little current flowing (gray line). The forward bias region is to the right of the axis. When voltage is increased at past a critical onset voltage certain point the current begins to rise exponentially. This is the same with any diode build with build with two dissimilar electronic materials. When light is turned on the curve shifts down (black line) producing a photocurrent identified as Jph. The intersection with the Y axis is the short circuit photocurrent. The intersection with the x axis is the open circuit voltage which is the largest voltage the cell can generate. The behavior is governed by the Shockley equation. The dark current/voltage curve is simply displaced by an amount when light is applied.
<br clear='all'>

=== Photovoltaic maximum power ===

[[Image:pvpowergraph.jpg|thumb|300px|]]
Power is the product of voltage of current. If this were an ideal cell the maximum power would be the product of the open circuit volage and the short circuit current (P<sub>TH</sub>). We actually get a maximum power point on the curve. The fill factor is ratio of the maximum output measured vs the theoretical maximum. A fill factor of .7 or .8 is as good as you get with any solar cell. OPV have fill factors around .5 or at the most .6. The fill factor is determined by the shape of the curve, which is in turn governed by how fast the charges recombine before we can harvest the current.

[[OPV Fabrication and Testing]]
<br clear='all'>

== Steps of Organic Photoelectric Process ==

===Light absorption (exciton formation)===
[[Image:Opv15-photonabsorption.JPG|thumb|300px]]
Suppose you have a crystalline silicon that absorbs a photon from the solar spectrum, which leads to an excited state. The exciton binding energy is so small that the electron and hole can separate and thus, a current can be produced. However, in an organic material, the absorption of a photon creates a strongly bound exciton. Remember that an exciton is a neutral species. When you have the electron and the hole next to one another, the charge is 0 since 1+ + 1- = 0. A current can not be generated from an exciton. This is why crystalline silicon can perfectly lead to an absorption to the generation of free carriers also known as band to band process. Suppose you form an exciton on a polythiophene chain. To generate a current, you must disassociate the exciton, meaning, separate the electron and the hole in the exciton so that they can move away from one another and generate current.

===Exciton Migration===
[[Image:opv16_migration.JPG|thumb|300px]]
This is the key point that makes the processes in an organic solar cell different from the processes in a crystalline silicon or amorphous silicon solar cell. That is the fact that in the pi- conjugated materials, you have large binding energies. Excitons must be formed and disassociated to generate a current.
The structure of the organic cells includes a component that corresponds to an electron donor and a component that corresponds to an electron acceptor. There is an interface between those two components where the excitons must reach in order to disassociate. The critical condition for an efficient organic solar cell is the fact that the exciton can diffuse a pretty large distance within its lifetime. If the interface on average is 20 nanometers from where the exciton is generated, but during its lifetime the exciton only diffuses 5 nanometers, no current will be produced. You will have photons in and photons out or vibrations out or anything similar but a current will not be generated. In other words, during its lifetime, the exciton must be able to diffuse and reach the interface between the donor and acceptor components. If a system is designed with large diffusion lengths and the migration of the exciton takes too long, then the average distance between where the exciton is generated and the interface can be reduced. This results in thinner films. However, if the film too thin, only a few number of photons can be absorbed. This is a type of issue many organic cells engineers deal with. The film must be thick enough to absorb a reasonable amount of photons, but simultaneously, the film must also be thin enough so that the generated excitons can reach the interface efficiently.

=== Charge separation (exciton dissociation)===
[[Image:opv17_chargeseparation.JPG|thumb|300px]]
This state diagram shows what happens when the exciton reaches the interface. The D refers to the donor component and A refers to the acceptor component. At the interface there are donor molecules or donor polymer chains next to the acceptor molecules or acceptor polymer chains. The lowest energy state is the ground state of the two components. Suppose that the exciton is formed within the donor. This exciton will be labeled D*, an excited state in the donor. The exciton must reach the interface. There is a state and energy there that corresponds to the exciton energy above the ground state. What needs to happen is that the exciton at the interface must get into a charge transfer state in which then the electron finds itself on the electron acceptor and therefore, there is a hole on the donor component. Now we have what we call a charge transfer state D<sup>+</sup> A<sup>-</sup>. The + and – now need to move away from one another. This process is called a charge separated state. So you need not only to have a charge transfer but to have a charge separation. In the literature, the use of these terminologies can be confusing depending on the community you from a state like this could be in a molecule. For instance, people might say this is a charge separated state. It is possible to have a charge transfer state where the + and the – remain close to each other. However, in order to generate a electrical current a full separation of the + and the – is needed. So that is the distinction between a CT(Charge transfer) state and a CS(Charge separation) state shown on the diagram/slide. At the CT state, when the hole and electron, the D<sup>+</sup> and A<sup>-</sup>, are next to each other, there is a chance that they will recombine (aka '''recombination''') and lead to the ground state. If this happens a loss in terms of efficiency of the charge separation process will occur. Recombination is the main reason for the low efficiencies of OPVs.

The goal is to maximize kct(charge transfer) and kcs(charge separation) and minimize kcr(charge recombination). The rate of transfer and of charge separation need to be maximized while the rate of charge recombination must be minimized. With those rates, you can also evaluate the Marcus Theory expressions.

===Charge Migration===
[[Image:Opv19migration.JPG|thumb|300px]]
Inorganic solar cells can be very efficient because a single inorganic material can be used with an exciton binding energy is negligible at room temperature. The physics of organic solar cells very different because the exciton is strongly bound. You need to have an acceptor and donor component with as much interface as possible between them in order that the formed excitons can quickly reach the interface to disassociate.
Once the excitons have disassociated into two separated species, the electron and the hole, they must move efficiently toward the electrodes. Remember that the more you can order your material, in general, the better the mobility will be. The faster the electrons and the holes can move away from each other, the more efficient their separation will be.
<br clear='all'>

==Electronic energy Levels==
[[Image:opv18_energylevel.JPG|thumb|300px]]
In the literature, people will refer to the exciton states and the charge transfer state which is when the donor + and the acceptor – are right next to each other. But the electron and the hole still feel each other rather strongly. This charge transfer state CT<sub>1</sub> is the lowest in energy precisely because the electron and the hole are strongly bound together. Remember, the stronger the attraction between the + and – charges, the lower the energy state. So the lowest energy charge transfer state has the strongest coulomb attraction between the hole and electron. Therefore, to separate the charges, energy must be applied and a higher energy state must be acquired. In the literature there are a number of models that can show how this can be achieved. But it would be much simpler to just go from S<sub>1</sub> to higher lying charge states CT<sub>n</sub> and then to CS=E<sub>final</sub> rather then going all the way down to CT<sub>1</sub> and then to the final state. With atomic energy levels, the higher the energy level of a state, the farther away the electron is from the nucleus. By making the connection and comparing with the atomic energy levels, the higher the energy of a charge transfer state, the lesser the attraction is between the electron and the hole, and therefore, the further away the two species are from one another. So if a higher lying charge transfer state is achieved, that state will be more diffused; the wave functions will be more delocalized. If instead of having the exciton come at the interface and then having the plus and minus right next to each other instead the plus is far removed from the minus because the wave functions are much more delocalized, you can intuitively understand that it will be easier for the plus and minus to move away from one another. So this is the picture that is now discussed in the literature.

[[category:organic solar cell]]

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Solar Technologies

2009-06-09T20:30:40Z

12.167.130.137: /* Efficiency */

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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==
[[Image:PV worldwide production.jpg|thumb|300px|The U.S. lags other countries in PV shipments.]]
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==
[[Image:Cost-efficiency pv.jpg|thumb|300px|Graph of efficiency vs cost for generations of solar cells. Courtesy of Martin Green]]

=== 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<ref>http://www.sc.doe.gov/bes/reports/files/SEU_rpt.pdf</ref>

=== 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:

[http://konarka.com/ Konarka is one of the first commercial OPV products]
[http://www.plextronics.com/index.aspx Plextronics]

==Solar Potential==
[[Image:Direct_normal_solar_radiation_2004.jpg|thumb|300px|This map shows the average irradiance of the US.]]

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===
[[Image:Opv8-spectrum.JPG|thumb|300px|Solar spectral distribution for one sun. ]]

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%.

<br clear='all'>

== Advantages of Organic Photovoltaics ==

=== Cost Considerations ===
[[Image:pv_doe_price curve.jpg|thumb|300px|DOE graph shows price vs volume of pv production. The brown line shows what the curve might look like with a disruptive new technology such as OPVs.]]

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, Nobel Laureate Professor Alan Heeger<ref>http://www.esi-topics.com/conducting-polymers/interviews/Dr-Alan-Heeger.html</ref> has installed a solar cell roof on his house in Santa Barbara, California, but it will take about 7-8 years before it becomes he is able to recover his initial investment, where the technology becomes "cost-neutral." However, that is still better than losing money from electricity costs every year for the duration or lifetime of these PV devices.
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.

[[Carbon footprint to charge an iPod|Calculate the Carbon footprint to charge an iPod]]

===Efficiency===

[[Image:Opvefficiency.png|thumb|300px|]]
The record in terms of power conversion efficiency for a single crystal silicon cells is close to 25%. Multi-junction solar cells (created from 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 focused 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% OPVs are projected to become economically viable.
[[Image:Efficiency_OPV.jpg|thumb|500px| NREL plot of efficiency of various photovoltaic devices. The red line in the lower right is OPVs.]]
<br clear='all'>

===Weight and Flexibility===
[[Image:Opv-flexible.JPG|thumb|440px|Organic cells use manufacturing techniques similar to OLEDs and printed electronics
]]
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.

<br clear="all">

==External Links==
[http://en.wikipedia.org/wiki/Spin_coating Explanation of spincoating process]

[http://www.ipc.uni-linz.ac.at/index.html Video on on OPV manufacturing process]

== References ==

http://en.wikipedia.org/wiki/Vacuum_deposition
<references />
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Solar Technologies

2009-06-09T20:27:59Z

12.167.130.137: /* Cost Considerations */

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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==
[[Image:PV worldwide production.jpg|thumb|300px|The U.S. lags other countries in PV shipments.]]
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==
[[Image:Cost-efficiency pv.jpg|thumb|300px|Graph of efficiency vs cost for generations of solar cells. Courtesy of Martin Green]]

=== 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<ref>http://www.sc.doe.gov/bes/reports/files/SEU_rpt.pdf</ref>

=== 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:

[http://konarka.com/ Konarka is one of the first commercial OPV products]
[http://www.plextronics.com/index.aspx Plextronics]

==Solar Potential==
[[Image:Direct_normal_solar_radiation_2004.jpg|thumb|300px|This map shows the average irradiance of the US.]]

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===
[[Image:Opv8-spectrum.JPG|thumb|300px|Solar spectral distribution for one sun. ]]

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%.

<br clear='all'>

== Advantages of Organic Photovoltaics ==

=== Cost Considerations ===
[[Image:pv_doe_price curve.jpg|thumb|300px|DOE graph shows price vs volume of pv production. The brown line shows what the curve might look like with a disruptive new technology such as OPVs.]]

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, Nobel Laureate Professor Alan Heeger<ref>http://www.esi-topics.com/conducting-polymers/interviews/Dr-Alan-Heeger.html</ref> has installed a solar cell roof on his house in Santa Barbara, California, but it will take about 7-8 years before it becomes he is able to recover his initial investment, where the technology becomes "cost-neutral." However, that is still better than losing money from electricity costs every year for the duration or lifetime of these PV devices.
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.

[[Carbon footprint to charge an iPod|Calculate the Carbon footprint to charge an iPod]]

===Efficiency===

[[Image:Opvefficiency.png|thumb|300px|]]
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.
[[Image:Efficiency_OPV.jpg|thumb|500px| NREL plot of efficiency of various photovoltaic devices. The red line in the lower right is OPVs.]]
<br clear='all'>

===Weight and Flexibility===
[[Image:Opv-flexible.JPG|thumb|440px|Organic cells use manufacturing techniques similar to OLEDs and printed electronics
]]
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.

<br clear="all">

==External Links==
[http://en.wikipedia.org/wiki/Spin_coating Explanation of spincoating process]

[http://www.ipc.uni-linz.ac.at/index.html Video on on OPV manufacturing process]

== References ==

http://en.wikipedia.org/wiki/Vacuum_deposition
<references />
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Solar Technologies

2009-06-09T20:24:17Z

12.167.130.137: /* First Generation Solar Cells */

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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==
[[Image:PV worldwide production.jpg|thumb|300px|The U.S. lags other countries in PV shipments.]]
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==
[[Image:Cost-efficiency pv.jpg|thumb|300px|Graph of efficiency vs cost for generations of solar cells. Courtesy of Martin Green]]

=== 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<ref>http://www.sc.doe.gov/bes/reports/files/SEU_rpt.pdf</ref>

=== 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:

[http://konarka.com/ Konarka is one of the first commercial OPV products]
[http://www.plextronics.com/index.aspx Plextronics]

==Solar Potential==
[[Image:Direct_normal_solar_radiation_2004.jpg|thumb|300px|This map shows the average irradiance of the US.]]

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===
[[Image:Opv8-spectrum.JPG|thumb|300px|Solar spectral distribution for one sun. ]]

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%.

<br clear='all'>

== Advantages of Organic Photovoltaics ==

=== Cost Considerations ===
[[Image:pv_doe_price curve.jpg|thumb|300px|DOE graph shows price vs volume of pv production. The brown line shows what the curve might look like with a disruptive new technology such as OPVs.]]

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<ref>http://www.esi-topics.com/conducting-polymers/interviews/Dr-Alan-Heeger.html</ref> 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.

[[Carbon footprint to charge an iPod|Calculate the Carbon footprint to charge an iPod]]

===Efficiency===

[[Image:Opvefficiency.png|thumb|300px|]]
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.
[[Image:Efficiency_OPV.jpg|thumb|500px| NREL plot of efficiency of various photovoltaic devices. The red line in the lower right is OPVs.]]
<br clear='all'>

===Weight and Flexibility===
[[Image:Opv-flexible.JPG|thumb|440px|Organic cells use manufacturing techniques similar to OLEDs and printed electronics
]]
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.

<br clear="all">

==External Links==
[http://en.wikipedia.org/wiki/Spin_coating Explanation of spincoating process]

[http://www.ipc.uni-linz.ac.at/index.html Video on on OPV manufacturing process]

== References ==

http://en.wikipedia.org/wiki/Vacuum_deposition
<references />
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Solar Cell Introduction

2009-06-09T20:21:54Z

12.167.130.137: /* Organic Photovoltaics */

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=== Organic Photovoltaics ===

[[Image:Organicphotovoltaic_assort_800.jpg|thumb|400px|A large OPV test cell built on glass substrate.]]
An organic solar cell is an electricity generating device comprised of thin layers of conductors, and photoelectrically active organic compounds. This section is based on lectures presented by Professor Jean Luc Bredas at Georgia Tech in 2007, and by materials contributed from several other technical sources, including Department of Energy workshops on Solar Electric energy conversion.

We will look at the major processes in solar cells and overview of the organic solar solar cell.

<swf width="500" height="400">http://depts.washington.edu/cmditr/media/opvanim.swf</swf>

===Energy Consumption===

There are serious issues regarding the energy needs of this country. Even during slow economic growth we continue to use an ever increasing amount of energy, and our energy consumption world-wide is projected to more than double by the middle of this century. Organic photovoltaic devices are an important emerging technology, part of a broad spectrum of "Generation III" photovoltaics, which promise to provide solar-to-electric energy conversion in portable, light-weight packages, at extremely low cost. We will look here at the major processes in solar cells and overview of the organic solar solar cell.

[[category:organic solar cell]]

=== External Links ===

http://en.wikipedia.org/wiki/Energy_in_the_United_States

An informative lecture by Dr. Nate Lewis, CalTech who provides some perspective on energy consumption and alternative energy conversion technologies on the horizon:
http://www.jpl.nasa.gov/events/lectures/feb08.cfm
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Solar Cell Introduction

2009-06-09T20:20:43Z

12.167.130.137: /* Energy Consumption */

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=== Organic Photovoltaics ===

[[Image:Organicphotovoltaic_assort_800.jpg|thumb|400px|A large OPV test cell built on glass substrate.]]
An organic solar cell is an electricity generating device comprised of thin layers of conductors, and photoelectrically active organic compounds. This section was based on lectures presented by Jean Luc Bredas at Georgia Tech in 2007.

We will look at the major processes in solar cells and overview of the organic solar solar cell.

<swf width="500" height="400">http://depts.washington.edu/cmditr/media/opvanim.swf</swf>

===Energy Consumption===

There are serious issues regarding the energy needs of this country. Even during slow economic growth we continue to use an ever increasing amount of energy, and our energy consumption world-wide is projected to more than double by the middle of this century. Organic photovoltaic devices are an important emerging technology, part of a broad spectrum of "Generation III" photovoltaics, which promise to provide solar-to-electric energy conversion in portable, light-weight packages, at extremely low cost. We will look here at the major processes in solar cells and overview of the organic solar solar cell.

[[category:organic solar cell]]

=== External Links ===

http://en.wikipedia.org/wiki/Energy_in_the_United_States

An informative lecture by Dr. Nate Lewis, CalTech who provides some perspective on energy consumption and alternative energy conversion technologies on the horizon:
http://www.jpl.nasa.gov/events/lectures/feb08.cfm
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<td style="text-align: center; width: 33%">[[Main_Page#Organic Solar Cells|Return to OPV Menu]]</td>
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