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The two plates are then joined and sealed together to prevent the electrolyte from leaking. The construction is simple enough that there are hobby kits available to hand-construct them.
TiO 2 , for instance, is already widely used as a paint base. One of the efficient DSSCs devices uses ruthenium-based molecular dye, e. The excited dye rapidly injects an electron into the TiO 2 after light absorption.
Diffusion of the oxidized form of the shuttle to the counter electrode completes the circuit. The efficiency of a DSSC depends on four energy levels of the component: These nanoparticle DSSCs rely on trap-limited diffusion through the semiconductor nanoparticles for the electron transport.
This limits the device efficiency since it is a slow transport mechanism. Recombination is more likely to occur at longer wavelengths of radiation.
It has been proven that there is an increase in the efficiency of DSSC, if the sintered nanoparticle electrode is replaced by a specially designed electrode possessing an exotic 'nanoplant-like' morphology.
Sunlight enters the cell through the transparent SnO 2: F top contact, striking the dye on the surface of the TiO 2. Photons striking the dye with enough energy to be absorbed create an excited state of the dye, from which an electron can be "injected" directly into the conduction band of the TiO 2.
From there it moves by diffusion as a result of an electron concentration gradient to the clear anode on top.
Meanwhile, the dye molecule has lost an electron and the molecule will decompose if another electron is not provided.
The dye strips one from iodide in electrolyte below the TiO 2 , oxidizing it into triiodide. This reaction occurs quite quickly compared to the time that it takes for the injected electron to recombine with the oxidized dye molecule, preventing this recombination reaction that would effectively short-circuit the solar cell.
The triiodide then recovers its missing electron by mechanically diffusing to the bottom of the cell, where the counter electrode re-introduces the electrons after flowing through the external circuit.
Several important measures are used to characterize solar cells. The most obvious is the total amount of electrical power produced for a given amount of solar power shining on the cell.
Expressed as a percentage, this is known as the solar conversion efficiency. Electrical power is the product of current and voltage, so the maximum values for these measurements are important as well, J sc and V oc respectively.
Finally, in order to understand the underlying physics, the "quantum efficiency" is used to compare the chance that one photon of a particular energy will create one electron.
In quantum efficiency terms, DSSCs are extremely efficient. Due to their "depth" in the nanostructure there is a very high chance that a photon will be absorbed, and the dyes are very effective at converting them to electrons.
Most of the small losses that do exist in DSSC's are due to conduction losses in the TiO 2 and the clear electrode, or optical losses in the front electrode.
The quantum efficiency of traditional designs vary, depending on their thickness, but are about the same as the DSSC. In theory, the maximum voltage generated by such a cell is simply the difference between the quasi - Fermi level of the TiO 2 and the redox potential of the electrolyte, about 0.
That is, if an illuminated DSSC is connected to a voltmeter in an "open circuit", it would read about 0. This is a fairly small difference, so real-world differences are dominated by current production, J sc.
Although the dye is highly efficient at converting absorbed photons into free electrons in the TiO 2 , only photons absorbed by the dye ultimately produce current.
The rate of photon absorption depends upon the absorption spectrum of the sensitized TiO 2 layer and upon the solar flux spectrum.
The overlap between these two spectra determines the maximum possible photocurrent. Typically used dye molecules generally have poorer absorption in the red part of the spectrum compared to silicon, which means that fewer of the photons in sunlight are usable for current generation.
DSSCs degrade when exposed to ultraviolet radiation. In air infiltration of the commonly-used amorphous Spiro-MeOTAD layer was identified as the primary cause of the degradation, rather than oxidation.
The damage could be avoided by the addition of an appropriate barrier. This makes DSSCs attractive as a replacement for existing technologies in "low density" applications like rooftop solar collectors, where the mechanical robustness and light weight of the glass-less collector is a major advantage.
They may not be as attractive for large-scale deployments where higher-cost higher-efficiency cells are more viable, but even small increases in the DSSC conversion efficiency might make them suitable for some of these roles as well.
There is another area where DSSCs are particularly attractive. The process of injecting an electron directly into the TiO 2 is qualitatively different from that occurring in a traditional cell, where the electron is "promoted" within the original crystal.
In theory, given low rates of production, the high-energy electron in the silicon could re-combine with its own hole, giving off a photon or other form of energy and resulting in no current being generated.
Although this particular case may not be common, it is fairly easy for an electron generated in another molecule to hit a hole left behind in a previous photoexcitation.
In comparison, the injection process used in the DSSC does not introduce a hole in the TiO 2 , only an extra electron. Although it is energetically possible for the electron to recombine back into the dye, the rate at which this occurs is quite slow compared to the rate that the dye regains an electron from the surrounding electrolyte.
Recombination directly from the TiO 2 to species in the electrolyte is also possible although, again, for optimized devices this reaction is rather slow.
As a result of these favorable "differential kinetics", DSSCs work even in low-light conditions. DSSCs are therefore able to work under cloudy skies and non-direct sunlight, whereas traditional designs would suffer a "cutout" at some lower limit of illumination, when charge carrier mobility is low and recombination becomes a major issue.
The cutoff is so low they are even being proposed for indoor use, collecting energy for small devices from the lights in the house. A practical advantage, one DSSCs share with most thin-film technologies, is that the cell's mechanical robustness indirectly leads to higher efficiencies in higher temperatures.
In any semiconductor, increasing temperature will promote some electrons into the conduction band "mechanically". The fragility of traditional silicon cells requires them to be protected from the elements, typically by encasing them in a glass box similar to a greenhouse , with a metal backing for strength.
Such systems suffer noticeable decreases in efficiency as the cells heat up internally. DSSCs are normally built with only a thin layer of conductive plastic on the front layer, allowing them to radiate away heat much easier, and therefore operate at lower internal temperatures.
The major disadvantage to the DSSC design is the use of the liquid electrolyte, which has temperature stability problems.
At low temperatures the electrolyte can freeze, ending power production and potentially leading to physical damage.
Higher temperatures cause the liquid to expand, making sealing the panels a serious problem. Another disadvantage is that costly ruthenium dye , platinum catalyst and conducting glass or plastic contact are needed to produce a DSSC.
A third major drawback is that the electrolyte solution contains volatile organic compounds or VOC's , solvents which must be carefully sealed as they are hazardous to human health and the environment.
This, along with the fact that the solvents permeate plastics, has precluded large-scale outdoor application and integration into flexible structure.
Replacing the liquid electrolyte with a solid has been a major ongoing field of research. Recent experiments using solidified melted salts have shown some promise, but currently suffer from higher degradation during continued operation, and are not flexible.
Dye sensitised solar cells operate as a photoanode n-DSC , where photocurrent result from electron injection by the sensitized dye.
Photocathodes p-DSCs operate in an inverse mode compared to the conventional n-DSC, where dye-excitation is followed by rapid electron transfer from a p-type semiconductor to the dye dye-sensitized hole injection, instead of electron injection.
A standard tandem cell consists of one n-DSC and one p-DSC in a simple sandwich configuration with an intermediate electrolyte layer.
Thus, photocurrent matching is very important for the construction of highly efficient tandem pn-DSCs. However, unlike n-DSCs, fast charge recombination following dye-sensitized hole injection usually resulted in low photocurrents in p-DSC and thus hampered the efficiency of the overall device.
Researchers have found that using dyes comprising a perylenemonoimid PMI as the acceptor and an oligothiophene coupled to triphenylamine as the donor greatly improve the performance of p-DSC by reducing charge recombination rate following dye-sensitized hole injection.
Photocurrent matching was achieved through adjustment of NiO and TiO 2 film thicknesses to control the optical absorptions and therefore match the photocurrents of both electrodes.
The energy conversion efficiency of the device is 1. The results are still promising since the tandem DSC was in itself rudimentary.
The dramatic improvement in performance in p-DSC can eventually lead to tandem devices with much greater efficiency than lone n-DSCs.
The dyes used in early experimental cells circa were sensitive only in the high-frequency end of the solar spectrum, in the UV and blue.
Newer versions were quickly introduced circa that had much wider frequency response, notably "triscarboxy-ruthenium terpyridine" [Ru 4,4',4"- COOH 3 -terpy NCS 3 ], which is efficient right into the low-frequency range of red and IR light.
The wide spectral response results in the dye having a deep brown-black color, and is referred to simply as "black dye".
A solar cell must be capable of producing electricity for at least twenty years, without a significant decrease in efficiency life span.
The "black dye" system was subjected to 50 million cycles, the equivalent of ten years' exposure to the sun in Switzerland.
No discernible performance decrease was observed. However the dye is subject to breakdown in high-light situations. Over the last decade an extensive research program has been carried out to address these concerns.
The newer dyes included 1-ethyl-3 methylimidazolium tetrocyanoborate [EMIB CN 4 ] which is extremely light- and temperature-stable, copper-diselenium [Cu In,GA Se 2 ] which offers higher conversion efficiencies, and others with varying special-purpose properties.
DSSCs are still at the start of their development cycle. Efficiency gains are possible and have recently started more widespread study.
These include the use of quantum dots for conversion of higher-energy higher frequency light into multiple electrons, using solid-state electrolytes for better temperature response, and changing the doping of the TiO 2 to better match it with the electrolyte being used.
A group of researchers at the Swiss Federal Institute of Technology has reportedly increased the thermostability of DSC by using amphiphilic ruthenium sensitizer in conjunction with quasi-solid-state gel electrolyte.
The stability of the device matches that of a conventional inorganic silicon-based solar cell. In addition, the group also prepared a quasi-solid-state gel electrolyte with a 3-methoxypropionitrile MPN -based liquid electrolyte that was solidified by a photochemically stable fluorine polymer, polyvinylidenefluoride-co-hexafluoropropylene PVDF-HFP.
The use of the amphiphilic Z dye in conjunction with the polymer gel electrolyte in DSC achieved an energy conversion efficiency of 6.
More importantly, the device was stable under thermal stress and soaking with light. These results are well within the limit for that of traditional inorganic silicon solar cells.
The enhanced performance may arise from a decrease in solvent permeation across the sealant due to the application of the polymer gel electrolyte.
The polymer gel electrolyte is quasi-solid at room temperature, and becomes a viscous liquid viscosity: The much improved stabilities of the device under both thermal stress and soaking with light has never before been seen in DSCs, and they match the durability criteria applied to solar cells for outdoor use, which makes these devices viable for practical application.
The first successful solid-hybrid dye-sensitized solar cells were reported. To improve electron transport in these solar cells, while maintaining the high surface area needed for dye adsorption, two researchers have designed alternate semiconductor morphologies, such as arrays of nanowires and a combination of nanowires and nanoparticles , to provide a direct path to the electrode via the semiconductor conduction band.
Such structures may provide a means to improve the quantum efficiency of DSSCs in the red region of the spectrum, where their performance is currently limited.
Wayne Campbell at Massey University , New Zealand, has experimented with a wide variety of organic dyes based on porphyrin. He reports efficiency on the order of 5.
An article published in Nature Materials demonstrated cell efficiencies of 8. A group of researchers at Georgia Tech made dye-sensitized solar cells with a higher effective surface area by wrapping the cells around a quartz optical fiber.
The cells are six times more efficient than a zinc oxide cell with the same surface area. These devices only collect light at the tips, but future fiber cells could be made to absorb light along the entire length of the fiber, which would require a coating that is conductive as well as transparent.
Dyesol Director Gordon Thompson said, "The materials developed during this joint collaboration have the potential to significantly advance the commercialisation of DSC in a range of applications where performance and stability are essential requirements.
Dyesol is extremely encouraged by the breakthroughs in the chemistry allowing the production of the target molecules.
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When placed in contact, some of the electrons in the n-type portion flow into the p-type to "fill in" the missing electrons, also known as electron holes.
Eventually enough electrons will flow across the boundary to equalize the Fermi levels of the two materials. In silicon, this transfer of electrons produces a potential barrier of about 0.
When placed in the sun, photons of the sunlight can excite electrons on the p-type side of the semiconductor, a process known as photoexcitation.
In silicon, sunlight can provide enough energy to push an electron out of the lower-energy valence band into the higher-energy conduction band.
As the name implies, electrons in the conduction band are free to move about the silicon. When a load is placed across the cell as a whole, these electrons will flow out of the p-type side into the n-type side, lose energy while moving through the external circuit, and then flow back into the p-type material where they can once again re-combine with the valence-band hole they left behind.
In this way, sunlight creates an electric current. In any semiconductor, the band gap means that only photons with that amount of energy, or more, will contribute to producing a current.
In the case of silicon, the majority of visible light from red to violet has sufficient energy to make this happen. Unfortunately higher energy photons, those at the blue and violet end of the spectrum, have more than enough energy to cross the band gap; although some of this extra energy is transferred into the electrons, the majority of it is wasted as heat.
Another issue is that in order to have a reasonable chance of capturing a photon, the n-type layer has to be fairly thick.
This also increases the chance that a freshly ejected electron will meet up with a previously created hole in the material before reaching the p-n junction.
By far the biggest problem with the conventional approach is cost; solar cells require a relatively thick layer of doped silicon in order to have reasonable photon capture rates, and silicon processing is expensive.
There have been a number of different approaches to reduce this cost over the last decade, notably the thin-film approaches, but to date they have seen limited application due to a variety of practical problems.
Another line of research has been to dramatically improve efficiency through the multi-junction approach, although these cells are very high cost and suitable only for large commercial deployments.
In general terms the types of cells suitable for rooftop deployment have not changed significantly in efficiency, although costs have dropped somewhat due to increased supply.
In the late s it was discovered that illuminated organic dyes can generate electricity at oxide electrodes in electrochemical cells. Its efficiency could, during the following two decades, be improved by optimizing the porosity of the electrode prepared from fine oxide powder, but the instability remained a problem.
A modern DSSC is composed of a porous layer of titanium dioxide nanoparticles , covered with a molecular dye that absorbs sunlight, like the chlorophyll in green leaves.
The titanium dioxide is immersed under an electrolyte solution, above which is a platinum -based catalyst. As in a conventional alkaline battery , an anode the titanium dioxide and a cathode the platinum are placed on either side of a liquid conductor the electrolyte.
Sunlight passes through the transparent electrode into the dye layer where it can excite electrons that then flow into the titanium dioxide.
The electrons flow toward the transparent electrode where they are collected for powering a load. After flowing through the external circuit, they are re-introduced into the cell on a metal electrode on the back, flowing into the electrolyte.
The electrolyte then transports the electrons back to the dye molecules. Dye-sensitized solar cells 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 electric field to separate the charges and create a current.
In the dye-sensitized solar cell, the bulk of the semiconductor is used solely for charge transport, the photoelectrons are provided from a separate photosensitive dye.
Charge separation occurs at the surfaces between the dye, semiconductor and electrolyte. The dye molecules are quite small nanometer sized , so in order to capture a reasonable amount of the incoming light the layer of dye molecules needs to be made fairly thick, much thicker than the molecules themselves.
To address this problem, a nanomaterial is used as a scaffold to hold large numbers of the dye molecules in a 3-D matrix, increasing the number of molecules for any given surface area of cell.
In existing designs, this scaffolding is provided by the semiconductor material, which serves double-duty. In the case of the original Grätzel and O'Regan design, the cell has 3 primary parts.
On top is a transparent anode made of fluoride-doped tin dioxide SnO 2: F deposited on the back of a typically glass plate. On the back of this conductive plate is a thin layer of titanium dioxide TiO 2 , which forms into a highly porous structure with an extremely high surface area.
The TiO 2 is chemically bound by a process called sintering. TiO 2 only absorbs a small fraction of the solar photons those in the UV. After soaking the film in the dye solution, a thin layer of the dye is left covalently bonded to the surface of the TiO 2.
The bond is either an ester, chelating, or bidentate bridging linkage. A separate plate is then made with a thin layer of the iodide electrolyte spread over a conductive sheet, typically platinum metal.
The two plates are then joined and sealed together to prevent the electrolyte from leaking. The construction is simple enough that there are hobby kits available to hand-construct them.
TiO 2 , for instance, is already widely used as a paint base. One of the efficient DSSCs devices uses ruthenium-based molecular dye, e.
The excited dye rapidly injects an electron into the TiO 2 after light absorption. Diffusion of the oxidized form of the shuttle to the counter electrode completes the circuit.
The efficiency of a DSSC depends on four energy levels of the component: These nanoparticle DSSCs rely on trap-limited diffusion through the semiconductor nanoparticles for the electron transport.
This limits the device efficiency since it is a slow transport mechanism. Recombination is more likely to occur at longer wavelengths of radiation.
It has been proven that there is an increase in the efficiency of DSSC, if the sintered nanoparticle electrode is replaced by a specially designed electrode possessing an exotic 'nanoplant-like' morphology.
Sunlight enters the cell through the transparent SnO 2: F top contact, striking the dye on the surface of the TiO 2. Photons striking the dye with enough energy to be absorbed create an excited state of the dye, from which an electron can be "injected" directly into the conduction band of the TiO 2.
From there it moves by diffusion as a result of an electron concentration gradient to the clear anode on top. Meanwhile, the dye molecule has lost an electron and the molecule will decompose if another electron is not provided.
The dye strips one from iodide in electrolyte below the TiO 2 , oxidizing it into triiodide. This reaction occurs quite quickly compared to the time that it takes for the injected electron to recombine with the oxidized dye molecule, preventing this recombination reaction that would effectively short-circuit the solar cell.
The triiodide then recovers its missing electron by mechanically diffusing to the bottom of the cell, where the counter electrode re-introduces the electrons after flowing through the external circuit.
Several important measures are used to characterize solar cells. The most obvious is the total amount of electrical power produced for a given amount of solar power shining on the cell.
Expressed as a percentage, this is known as the solar conversion efficiency. Electrical power is the product of current and voltage, so the maximum values for these measurements are important as well, J sc and V oc respectively.
Finally, in order to understand the underlying physics, the "quantum efficiency" is used to compare the chance that one photon of a particular energy will create one electron.
In quantum efficiency terms, DSSCs are extremely efficient. Due to their "depth" in the nanostructure there is a very high chance that a photon will be absorbed, and the dyes are very effective at converting them to electrons.
Most of the small losses that do exist in DSSC's are due to conduction losses in the TiO 2 and the clear electrode, or optical losses in the front electrode.
The quantum efficiency of traditional designs vary, depending on their thickness, but are about the same as the DSSC. In theory, the maximum voltage generated by such a cell is simply the difference between the quasi - Fermi level of the TiO 2 and the redox potential of the electrolyte, about 0.
That is, if an illuminated DSSC is connected to a voltmeter in an "open circuit", it would read about 0. This is a fairly small difference, so real-world differences are dominated by current production, J sc.
Although the dye is highly efficient at converting absorbed photons into free electrons in the TiO 2 , only photons absorbed by the dye ultimately produce current.
The rate of photon absorption depends upon the absorption spectrum of the sensitized TiO 2 layer and upon the solar flux spectrum. The overlap between these two spectra determines the maximum possible photocurrent.
Typically used dye molecules generally have poorer absorption in the red part of the spectrum compared to silicon, which means that fewer of the photons in sunlight are usable for current generation.
DSSCs degrade when exposed to ultraviolet radiation. In air infiltration of the commonly-used amorphous Spiro-MeOTAD layer was identified as the primary cause of the degradation, rather than oxidation.
The damage could be avoided by the addition of an appropriate barrier. This makes DSSCs attractive as a replacement for existing technologies in "low density" applications like rooftop solar collectors, where the mechanical robustness and light weight of the glass-less collector is a major advantage.
They may not be as attractive for large-scale deployments where higher-cost higher-efficiency cells are more viable, but even small increases in the DSSC conversion efficiency might make them suitable for some of these roles as well.
There is another area where DSSCs are particularly attractive. The process of injecting an electron directly into the TiO 2 is qualitatively different from that occurring in a traditional cell, where the electron is "promoted" within the original crystal.
In theory, given low rates of production, the high-energy electron in the silicon could re-combine with its own hole, giving off a photon or other form of energy and resulting in no current being generated.
Although this particular case may not be common, it is fairly easy for an electron generated in another molecule to hit a hole left behind in a previous photoexcitation.
In comparison, the injection process used in the DSSC does not introduce a hole in the TiO 2 , only an extra electron.
Although it is energetically possible for the electron to recombine back into the dye, the rate at which this occurs is quite slow compared to the rate that the dye regains an electron from the surrounding electrolyte.
Recombination directly from the TiO 2 to species in the electrolyte is also possible although, again, for optimized devices this reaction is rather slow.
As a result of these favorable "differential kinetics", DSSCs work even in low-light conditions. DSSCs are therefore able to work under cloudy skies and non-direct sunlight, whereas traditional designs would suffer a "cutout" at some lower limit of illumination, when charge carrier mobility is low and recombination becomes a major issue.
The cutoff is so low they are even being proposed for indoor use, collecting energy for small devices from the lights in the house.
A practical advantage, one DSSCs share with most thin-film technologies, is that the cell's mechanical robustness indirectly leads to higher efficiencies in higher temperatures.
In any semiconductor, increasing temperature will promote some electrons into the conduction band "mechanically".
The fragility of traditional silicon cells requires them to be protected from the elements, typically by encasing them in a glass box similar to a greenhouse , with a metal backing for strength.
Such systems suffer noticeable decreases in efficiency as the cells heat up internally. DSSCs are normally built with only a thin layer of conductive plastic on the front layer, allowing them to radiate away heat much easier, and therefore operate at lower internal temperatures.
The major disadvantage to the DSSC design is the use of the liquid electrolyte, which has temperature stability problems. At low temperatures the electrolyte can freeze, ending power production and potentially leading to physical damage.
Higher temperatures cause the liquid to expand, making sealing the panels a serious problem. Another disadvantage is that costly ruthenium dye , platinum catalyst and conducting glass or plastic contact are needed to produce a DSSC.
A third major drawback is that the electrolyte solution contains volatile organic compounds or VOC's , solvents which must be carefully sealed as they are hazardous to human health and the environment.
This, along with the fact that the solvents permeate plastics, has precluded large-scale outdoor application and integration into flexible structure.
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