SixLinks Wiki

Solar Thermal

Low Temperature Collectors:

This is where passive systems such as thermal mass systems, solar chimneys, evaporation ponds, and Trombe walls fall into place, but as stated above, we will be focusing on active systems here and passive systems in the energy usage link. Approximately 3/4 of all the solar thermal collector area produced in 2006 was of this type. These systems typically use solar thermal energy as heat and are used for heating pools and for space heating. They can greatly reduce the energy consumption of a building, however, as heating, ventillation, and air conditioning systems account for over 25% of the energy used in commercial buildings, and almost 50% of the energy used in residential buildings. One of the more popular low temperature solar thermal collectors is the flat plate collector. Flat plates are black plates enclosed in glass that have copper pipes running through them. The sun heats the plate, which heats the pipes, which heats the water running through the pipes. This hot water can be used to preheat the building's hot water, or more likely to heat a swimming pool. Another popular technology is that of Unglazed Transpired Collectors (UTCs). These are perforated sun-facing walls that preheat ventilation air. The can raise the incoming air temperature from 70 degrees Fahrenheit to 110 to 140 degrees. This super heated air can then be mixed with the cooler air to heat the building or for taking room temperature air and using it for drying purposes. These systems payback in about 3-12 years.

Medium Temperature Collectors:

• Often used for hot water and space heating, although can also be used for cooking, desalination, or water purification. Hot water and space heating systems are typically made up of either flat plate collectors or evacuated tubes and can be pressurized glycol, drainback or batch systems. All of these systems share a common concept with the flat plate system for heating water described above. These systems run at higher temperatures to be useful for domestic hot water as opposed to just pool water and, if the temperature is hot enough, can be used for space heating through either radiant floor systems or by running incoming air over a heating coil that contains fluid from this system. Need more detail on different systems (drainback, batch, etc) and some more general detail on hot water/space heating systems such as radiant floors
• Solar ovens can be used for cooking and primarily consist of either an insulated box, which can get up to 100 degrees C even with overcast skies or a parabolic, flat plate, or disk deign that get up to 350 degrees C that concentrate but need direct sunlight
• Solar water disinfection (SODIS), involves placing water in plastic PET bottles in the sun and is a viable method of decentralized water treatment and storage that is recommended by the World Health Organization. It is a very common practice in developing countries and has over two million users. (72)
• A solar still can be used to distill water to remove solids in suspension or solution. It works by heating the water into a gas and then having it collect as a liquid in a different vessel. It operates very similarly to a still used to produce alcohol, where you remove the ethanol from everything else, including water, but in this case, you are removing water from everything else in it. The three main types are cone shaped, boxlike, and pit. The box shaped types are the most sophisticated and the pit types are the least sophisticated.
• A definitive report by the German research instituted DLR in 2007 discusses the possibilities for solar thermal power for large scale desalination in sunny regions such as the Middle East and North Africa.

High Temperature Collectors:

High temperature solar thermal collectors are usually designed with concentrators to increase temperatures to eventually produce electricity, typically through steam or gas turbines. The high temperatures are needed because the theoretical efficiencies of heat engines used to produce electricity are limited by the temperature at which they operate, which higher temperatures allowing for an increase in efficiency. Check to make sure I got my understanding of Carnot efficiencies down Higher temperatures also make thermal storage more efficient, which as we will describe below, is an important part of any solar thermal power plant and they also reduce a plant's water use. These technologies are generically referred to as concentrating solar power (CSP). These plants can be reliable generators of electricity in the desert where there is more or less predictable solar insolation and if equipped with the appropriate heat storage mechanisms for 24 hour energy production and an appropriate back-up system. The important issue with CSP is finding a design simple enough that it is cost effective.

• Parabolic trough power plants use a curved trough to reflect the sun's rays to a thermal collector running along the trough above the collectors. The troughs run in a number of parallel straight lines from east to west so that changes in the sun's elevation can be tracked by rotating the trough, but that changes in the sun's location from east to west don't require tracking because the rays will always be hitting some part of the long trough Insert picture to demonstrate this The collector is typically a pipe containing a heat transfer fluid such as synthetic oil, molten salt, or pressurized steam, and may be encased in a vacuum chamber of glass to reduce heat losses.

• Power towers use an array of flat, moveable mirrors (heliostats) to focus the sun's rays unto a central collector tower. These systems are also called heliostat power plants or central power power plants. There are a number of advantages to this sort of system, the first of which is that they are capable of reaching higher temperatures. In addition, the plumbing and all power-producing components are concentrated in a single tower for a large amount of collecting area. There is also no need for this system to be built on flat ground, which makes it more flexible in terms of geography of placement. The primary disadvantage though is that each reflector must have its own dual axis tracking device, whereas parabolic collectors need to only track on one axis, and the tracking mechanism is shared for a greater area of reflectors. NREL did a cost/performance comparison between power towers and parabolic troughs and estimated that by 2020, power towers coupld produce electricity at 5.47 cents/kWh at a capacity factor of 72.9% whereas parabolic troughs could produce electricity for 6.21 cents/kWh with a capacity factor of 56.2%.

• Dish systems use large,reflective, parabolic dishes to concentrate the sun's rays onto a single point above the dish, similar to the way a satellite dish works. These have the similar advantage of power towers in that they can achieve high temperatures due to a high collection to receiving area ratio. They are often coupled with a Stirling engine or a steam engine to create electricity on site. This however has disadvantages because it increases the number of moving parts and also the weight that must be moved with the dual axis tracking mechanism that is association with each dish.

• Linear Fresnel reflector power plant consists of a a series of long, shallow-curvature or flat mirrors that focus light onto one or more linear receivers positioned above the mirrors that contains a small parabolic mirror. This is similar to a parabolic trough and so only needs to rotate about one axis, but it is lower cost because instead of each trough having one receiver, a number of parallel runs of collectors can all be pointed at one receiver above the ground. The one problem these systems run into is that of the receiver or other collectors shading the collectors, which can only be avoided by increasing the height of the receiver or the space between reflectors and the absorber, both of which increase cost.

• Compact Linear Fresnal reflector This is an adaptation on the linear Fresnal reflector power plant that places the number of absorbers (which is still much fewer than the number of reflectors) closer together, allowing the reflectors to change their orientation to point at one of at least a couple different absorbers, minimizing the risk of absorbers shading each other. These systems are by far the cheapest of the CSP systems, with lower parasitic pumping loses, lower maintenance, flatter glass, and a lack of flexible high pressure lines when compared to parabolic trough systems.

Possibly could include some research on Fresnal Lenses here

Efficiencies:

• Solar dish/sterling energy technologies tend to have the highest efficiencies, with one example of a single solar dish-Stirling engine installed at Sandia National Laboratories National Solar Thermal Test Facility producing up to 25 kW of electricity with a conversion efficiency of 30%
• Solar parabolic trough plants have been built with efficiencies of about 20%
• Fresnel reflectors have an efficiency just a bit below 20%, but this is compensated by the denser packing
• Another measure of efficiency that compensates for packing is the gross conversion efficiency, which is the amount of energy produced not over the collector area but over the entire area of the plant. These efficiencies are obviously very low because only a percentage of the land area used for a plant is actually collecting energy.

PVs:

Advantages of PVs

• There are 89 petawatts of sunlight reaching the earth's surface, more than 6,000 times more than the 15 terawatts of average power consumed by humans, a very small amount of which is actually recovered and utilized
• Solar has the highest power density of any renewable energy (global mean of 170 W/m^2)
• Solar power is pollution free during use
• Facilities require little maintenance once running
• Definitely economically superior energy source when grid connection or fuel transport is costly
• When grid-connected, solar electricity can displace the high cost, low efficiency electricity used during times of peak demand, which can reduce grid loading and increase the overall efficiency of the electrical grid. Time-of-use net metering can be highly favorable to small PV systems because of this.
• Grid-connected solar systems can use locally produced energy, reducing transmission costs and losses
• Operating costs for solar energy are incredibly low
• There is much room for improvement in solar cell production as there has been relatively little money spent on R&D of solar compared to nuclear and fossil fuels.

Disadvantages of PVs

• Without incentives, the cost of solar panels may not cover lifetime savings
• Solar electricity has not reached grid parity on a large scale yet, and is still more expensive than most other renewable energies
• Solar power needs either a storage or complimentary power system because it is not available at all times
• Solar cells produce DC power, which must be converted to AC, incurring a small loss

Grid Parity

The economics of solar photovoltaic technologies are judged based on the levelized cost of energy (LCOE) as the measure of cost competitiveness. Grid parity is achieved when the LCOE of solar energy is equivalent to that of grid power that currently exists. Grid parity has already been reached in Hawaii and other islands that otherwise use diesel fuel to produce electricity. The next places to reach grid parity will be those with large amounts of sunshine and with high costs for electricity, such as California and Japan. President George W. Bush has set 2015 as a date for grid parity in the USA. General Electric's Chief Engineer predicts grid parity without subsidies in some of the sunnier parts of the US by around 2015, although other companies predict an earlier date. It is also estimated that solar power will be below grid parity for more than half of residential customers and 10% of commercial customers in the OECD, assuming at least constant electricity prices through 2010. While the LCOE is about $0.25/kWh in most OECD countries currently, within a few years it could be below $0.15/kWh in most, and below $0.10/kWh in a few of the sunnier OECD countries. This is due to three main trends:

• Vertical integration of the supply chain
• Origination of power purchase agreements by solar power companies
• Unexpected risk for traditional electrical generators, grid operators, and turbine manufacturers

There are three typical political incentives used to grow the PV industry even while it is above grid parity to help it achieve the economies of scale to reach grid parity, to promote national energy independence, high tech job creation, and to reduce CO2 emissions.

• Investment subsidies that refund part of the cost of the installation of a system
• Feed-in Tariffs/Net Metering that affect the rate at which an electricity provider must buy energy produced by solar energy systems
• Renewable Energy Certificates that give an additional value to any renewable energy produced

PV Cell Types

First Generation Solar Cells:

First generation solar cells consist of large-area, high quality, single junction devices. They involve high energy and labor inputs, which keep production costs high. They have a theoretical limiting efficiency of 33%. They were 89.6% of commercial production in 2007, but are unlikely to reach grid parity without incentives because of the high production costs.

Second Generation:

Second generation solar cells attempt to reduce the energy requirements and production costs of solar cells using alternative manufacturing techniques such as vapor deposition and electroplating. These technologies greatly reduce the temperatures needed for the production of solar cells, lowering the production cost and forcing the cost to vary mostly based on the materials used. These technologies are expected to gain market share in 2008. Examples of these second generation solar cells would be cadmium telluride (CdTe), copper indium gallium selenide, amorphous silicon, and micromorphous silicon. These materials are described below, but in general are applied in a thin film to a glass or ceramics substrate, reducing the amount of material used. In 2007, CdTe production represented 4.7% of total market share, thin film silicon 5.2% and CIGS 0.5%.

Third Generation:

Third generation solar cells attempt to maintain the low production costs of second generation technologies while maintaining and improving on the higher efficiencies of the first generation solar cells. The goal is to reach efficiencies of 30-60% while retaining low cost materials and manufacturing techniques. The techniques used to accomplish this are the following:

• Producing multijunction photovoltaic cells
• Modifying the incident spectrum (concentration)
• Using of excess thermal generation to enhance voltages or carrier collection

Graph of efficiencies over time in (74)

Concentrating Photovoltaic Systems

Similar to the way solar thermal systems are often concentrated, some photovoltaic systems attempt concentrating to reduce the collection area. They may have single or dual axis tracking and are called Heliostate Concentrator Photovoltaics (HCPV). This not only reduces the amount of expensive material that is currently in short supply, but also improves teh performance of the photovoltaic materials. The costs of focusing, tracking, and cooling has limited its usage, however.

Crystalline Silicon (c-Si)

This is by far the most common material used for solar cell production and is often referred to as \"solar grade\" silicon. Bulk silicon is separated into multiple categories according to crystallinity and crystal size in the resulting ingot, ribbon, or wafer, the difference of which is described below: Below is mostly verbatim, try to find a better way to say it, possibly with how it is used or simply move these descriptions to where they are applied

• Monocrystalline silicon (c-Si) is often made using the Czochralski process. Single-crystal wafer cells tend to be expensive, and because they are cut from cylindrical ingots, do not completely cover a square solar cell module without a substantial waste of refined silicon. Hence most c-Si panels have uncovered gaps at the four corners of the cells.
• Poly- or multicrystalline silicon (poly-Si or mc-Si): made from cast square ingots — large blocks of molten silicon carefully cooled and solidified. These cells are less expensive to produce than single crystal cells but are less efficient.
• Ribbon silicon: formed by drawing flat thin films from molten silicon and having a multicrystalline structure. These cells have lower efficiencies than poly-Si, but save on production costs due to a great reduction in silicon waste, as this approach does not require sawing from ingots.

Silicon processing

Almost all of currently produced solar panels are made using silicon, so one means of reducing the cost of solar panels would be to develop cheaper methods of obtaining sufficiently pure silicon. Silicon is a very common element, but is usually bound in silica, or silica sand. Current processing techniques are very high energy processes. The current process is called carbothermic reduction and involves reacting carbon (charcoal) with silica (SiO2) at 1700 degrees Celsius. Each ton of silicon produced releases about 1.5 tons of carbon dioxide, this in addition to the carbon dioxide released from the heating of the reaction. Silica can be reduced to pure silicon by electrolysis in a molten salt bath at only 800 to 900 degrees Celsius. While this is still very high temperature, it is a great improvement over the current process. This process was essentially discovered in 1996 and is referred to as the FFC Cambridge Process, and the output is silicon as a fine powder. Another way this process is improved is simply by doing it less often, by reducing the amount of silicon used. If the wafers are very thin and cut into slivers I know I have notes on this somewhere, a guy from Solar Lab talked about it, but for now I'll leave it verbatim until I can rewrite it and recategorize it under its appropriate heading

\"The technique involves taking a silicon wafer, typically 1 to 2 mm thick, and making a multitude of parallel, transverse slices across the wafer, creating a large number of slivers that have a thickness of 50 micrometres and a width equal to the thickness of the original wafer. These slices are rotated 90 degrees, so that the surfaces corresponding to the faces of the original wafer become the edges of the slivers. The result is to convert, for example, a 150 mm diameter, 2 mm-thick wafer having an exposed silicon surface area of about 175 cm² per side into about 1000 slivers having dimensions of 100 mm x 2 mm x 0.1 mm, yielding a total exposed silicon surface area of about 2000 cm² per side. As a result of this rotation, the electrical doping and contacts that were on the face of the wafer are located the edges of the sliver, rather than the front and rear as is the case with conventional wafer cells. This has the interesting effect of making the cell sensitive from both the front and rear of the cell (a property known as bifaciality).44 Using this technique, one silicon wafer is enough to build a 140 watt panel, compared to about 60 wafers needed for conventional modules of same power output.\"

Thin-film processing

Thin-film solar cells use less than 1% of the raw material wafer-based solar cells do, leading to a great decrease in the cost. There are a number of thin-film technologies under research because at the moment, while costing much less than wafer-based solar cells, they are also much less efficient over an equivalent area. Another promising advantage of thin-film is the fact that the solar cells have the potential to be deposited on a number of different materials, moving solar panels away from the bulky rectangular modules of the past. Below are a number of the thin-film technologies currently being researched.

• Cadmium Telluride (CdTe)
  • Cadmium telluride is an efficient light-absorbing material for thin-film solar cells. Compared to other thin-film materials, CdTe is easier to deposit and, along with amorphous silicon, has shown the most promise for large-scale production. There is fear of this technology, however, because cadmium is a heavy metal that is a cumulative poison. The reality is that more cadmium is released into the atmosphere with the production of first generation silicon panels and some other thin-film technologies. Double check this claim
• Copper-Indium Selenide (CuInSe2) a.k.a CIS
  • The semiconductors used in these panels are especially attractive for thin film solar cell application because of their high optical absorption coefficients and versatile optical and electrical characteristics which can in principle be manipulated and tuned for a specific need in a given device. CIS films achieved greater than 14% efficiency, however manufacturing costs of CIS solar cells are currently high when compared with amorphous silicon solar cells but continuing work is attempting to lower the cost of production processes.
• CIGS
  • When gallium is substituted for some of the indium in CIS, the material is sometimes called CIGS , or copper indium/gallium diselenide, a solid mixture of the semiconductors CuInSe2 and CuGaSe2, often abbreviated by the chemical formula CuInxGa(1-x)Se2. Unlike the conventional silicon based solar cell, which can be modelled as a simple p-n junction, these cells are best described by a more complex heterojunction model. These panels have, as of March 2008, recorded the highest efficiency of any of the thin-film technologies at 19.9%. The use of gallium increases the optical bandgap of the CIGS film, increasing the open-circuit voltage over that of CIS films. Gallium is also much more readily available than indium, 70% of which is used by the flat-screen monitor industry. While this is a concern for the future of CIGS, to match the capacity of silicon-based solar cells produced in 2006 would take up 10% of the indium produced in 2004, whereas silicon solar cells used up 33% of the world's electronic grade silicon produced in 2006. Indium can also be easily recycled from PV modules as shown by a recycling program introduced in Germany.
  • One of the R&D Magazine's prestigious R&D 100 Awards — also called the “Oscars of Invention”— for 2008, has gone to National Renewable Energy Laboratory Hybrid CGIS (or Thin-Film Photovoltaic Manufacturing Process). This process involves manufacturing hybrid CIGS cells in layers by using ink-jet and ultrasonic technology to precisely apply metal-organic inks in separate layers directly into common building materials such as metal and glass.
• Silicon Thin Films Needs to be further reviewed by someone who understands it more, this is still verbatim
  • \"Silicon thin-films are mainly deposited by chemical vapor deposition (typically plasma-enhanced (PE-CVD)) from silane gas and hydrogen gas. Depending on the deposition's parameters, this can yield:

    1. Amorphous silicon (a-Si or a-Si:H) 2. Protocrystalline silicon or 3. Nanocrystalline silicon (nc-Si or nc-Si:H).

    These types of silicon present dangling and twisted bonds, which results in deep defects (energy levels in the bandgap) as well as deformation of the valence and conduction bands (band tails). The solar cells made from these materials tend to have lower energy conversion efficiency than bulk silicon, but are also less expensive to produce. The quantum efficiency of thin film solar cells is also lower due to reduced number of collected charge carriers per incident photon.

    Amorphous silicon has a higher bandgap (1.7 eV) than crystalline silicon (c-Si) (1.1 eV), which means it absorbs the visible part of the solar spectrum more strongly than the infrared portion of the spectrum. As nc-Si has about the same bandgap as c-Si, the two material can be combined in thin layers, creating a layered cell called a tandem cell. The top cell in a-Si absorbs the visible light and leaves the infrared part of the spectrum for the bottom cell in nanocrystalline Si.

    Recently, solutions to overcome the limitations of thin-film crystalline silicon have been developed. Light trapping schemes where the incoming light is obliquely coupled into the silicon and the light traverses the film several times enhance the absorption of sunlight in the films. Thermal processing techniques enhance the crystallinity of the silicon and pacify electronic defects.

    A silicon thin film technology is being developed for building integrated photovoltaics (BIPV) in the form of semi-transparent solar cells which can be applied as window glazing. These cells function as window tinting while generating electricity.

• Nanocrystalline solar cells These structures make use of some of the same thin-film light absorbing materials as amorphous silicon, but are overlain as an extremely thin absorber on a supporting matrix of conductive polymer or mesoporous metal oxide having a very high surface area to increase internal reflections (and hence increase the probability of light absorption). Using nanocrystals allows one to design architectures on the length scale of nanometers, the typical exciton diffusion length. In particular, single-nanocrystal ('channel') devices, an array of single p-n junctions between the electrodes and separated by a period of about a diffusion length, represent a new architecture for solar cells and potentially high efficiency.\"

Metamorphic Multijunction Solar Cell

NREL won another R&D Magazine's R&D 100 Award for its Metamorphic Multijunction Solar Cell, an ultra-light and flexible cell that has recorded record efficiencies for thin-films. This new class of solar cells are grown upside down using high-energy materials with extremely high quality crystals, especially in the upper layers of the cells where most of the power are produced. The atoms do not have the even lattice spacing of most solar cells, but have varying spacings. The germanium layer found in many other cells on the bottom layer is removed, reducing the cell's cost and 94% of its weight. These high efficiency cells were originally developed for special applications such as satellites and other space programs, but despite the higher cost, may have a cheaper cost per watt. They are produced by laying multiple thin films with different semiconductors using Metalorganic vapor phase epitaxy. The different semiconductors will each have a different characteristic band gap energy, which allows the cell to absorb most efficiently at a wider range of incident light wavelengths, often covering the entire solar light spectrum.

• Gallium arsenide (GaAs) multijunction
  • These are the most efficient solar cells to date, reaching a record high of 40.7% efficiency under concentrated solar irradiation and laboratory conditions. These cells are usually made with GaAs, Ge, and GaInP2

PV Processing Techniques

Polymer processing

Conductive polymers, invented by Alan Heeger, Alan G. MacDiarmid, and Hideki Shirakawa, may lead to the development of much cheaper cells available on inexpensive plastics. This may allow for the production of organic solar cells, but so far they have all suffered from degradation upon exposure to UV light, so do not have lifespans long enough to be viable. These conductive polymers move charge through conjugated double bond systems, but are susceptible to breaking up when radiated with shorter wavelengths. They are also highly sensitive to atmospheric moisture and oxidation because they are highly unsaturated.

Nanoparticle processing

Non-silicon solar panels made of quantum heterostructures such as carbon nanotubes or quantum dots embedded in conductive polymers or mesoporous metal oxides are currently being experimented with. In addition to cells made of these structures, the addition of these structures to silicon solar cells can boost their overall efficiency. Varying the size of quantum dots can tune cells to absorb different wavelengths. Quantum dot-modified photovoltaics may have a future that rivals GaAs cells for the highest conversion efficiency.

Transparent conductors

Many new solar cells that are being experimented with use transparent thin films that are also conductors of electricity. The majority of these are transparent conductive oxides (TCOs) and include fluorine-doped tin oxide (SnO2:F, or \"FTO\"), doped zinc oxide (such as ZnO:Al), and indium tin oxide (\"ITO\"). These conductive films are currently being used for LCD flat panel displays. The purpose of these films in solar cells would be to help transport photogenerated charge carriers away from the light absorbing materials below. Currently, however, these films require very special production circumstances including a high vacuum. They also are weak materials and have poor transmittance in the infrared portion of the spectrum. There has been new research into using carbon nanotube networks as a transparent conductor for organic solar cells. These networks are flexible and are not as particular in how they are deposited onto the material. They not only can be made to be highly transparent in the infrared spectrum, but may even enable low bandgap solar cells. One other major advantage is that nanotube networks are p-type conductors, whereas the inorganic ones are only n-type. A p-type transparent conductor to lead to new cell designs that are much easier to manufacture, as well as more efficient.

Silicon wafer based solar cells

The dominant technology on the market by a huge degree and so are what almost all solar energy product manufacturers are setup to be producing at this point. For this reason, it may make more sense to focus research on improving the cost and efficiency of these cells rather than needing to retool the entire manufacturing process. Then again, there may be a limit to how far silicon solar cells can go, and it may make sense to pursue and alternative path.

Infrared solar cells

There has been discovered a way to produce plastic sheets that contain billions of nanoantennas that collect heat energy from the sun and other sources. This research is being done at the US DOE's Idaho National Laboratory and is one of the technologies out there that may create a solar energy collector that would be mass-produced on flexible materials. The hope is to use these as lightweight skins that could power everything from small electronics to electric vehicles. There is no means of turning this energy into electricity yet, however, which is a large holdup. They have been proposed alternatively as an energy-free means of removing heat away from electronics or even buildings. If used for electricity, they could be used in conjunction with current solar panels to collect over a wider spectrum of energies and also to collect at night and in other low-light situations.

Light-absorbing dyes (DSSC)

English please? We need a less techy description of this, but this is what I found on first look through Typically a ruthenium metalorganic dye (Ru-centered) is used as a monolayer of light-absorbing material. The dye-sensitized solar cell depends on a mesoporous layer of nanoparticulate titanium dioxide to greatly amplify the surface area (200-300 m²/g TiO2, as compared to approximately 10 m²/g of flat single crystal). The photogenerated electrons from the light absorbing dye are passed on to the n-type TiO2, and the holes are passed to an electrolyte on the other side of the dye. The circuit is completed by a redox couple in the electrolyte, which can be liquid or solid. This type of cell allows a more flexible use of materials, and is typically manufactured by screen printing, with the potential for lower processing costs than those used for bulk solar cells. However, the dyes in these cells also suffer from degradation under heat and UV light, and the cell casing is difficult to seal due to the solvents used in assembly. In spite of the above, this is a popular emerging technology with some commercial impact forecast within this decade.

Organic/polymer solar cells

These are solar cells built from thin films of organic semiconductors. Examples of materials used are polyphenylene vinylene, copper phthalocyanine, and carbon fullerenes. These cells typically have low conversion efficiencies, with the record so far being 6.5%, but may be more useful for areas where flexibility and disposability are more important than size. The low efficiency is due to the fact that organic solar cells do not have a p-n junction to separate electrons and holes when photons are absorbed. Instead, these cells simply have two different materials, one of which acts as an electron donor, and one as an acceptor. This decreases the length of travel of the exciton, limiting performance possibilities. The creation of nanostructured interfaces, possibility in the form of bulk heterojunctions, can improve performance. Lets make sure I understand that whole exciton diffusion length thing