all about Dye-sensitized solar cells and how they work

Dye-sensitized solar cell

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A selection of dye-sensitized solar cells

A dye-sensitized solar cell (DSSc, DSC or DYSC^[1]) is a relatively new class of low-cost solar cell, that belong to the group of thin-film solar cells.^[2] It is based on a semiconductor formed between a photo-sensitized anode and an electrolyte, a photoelectrochemical system. This cell was invented by Michael Grätzel and Brian O'Regan at the École Polytechnique Fédérale de Lausanne in 1991^[3] and are also known as Grätzel cells.

This cell is extremely promising because it is made of low-cost materials and does not need elaborate apparatus to manufacture. In bulk it should be significantly less expensive than older solid-state cell designs. It can be engineered into flexible sheets and is mechanically robust, requiring no protection from minor events like hail or tree strikes. Although its conversion efficiency is less than the best thin-film cells, its price/performance ratio (kWh/M2/annum) should be high enough to allow them to compete with fossil fuel electrical generation (grid parity). Commercial applications, which were held up due to chemical stability problems, are now forecast in the European Union Photovoltaic Roadmap to be a potentially significant contributor to renewable electricity generation by 2020.

Previous technology: semiconductor solar cells

In a traditional solid-state semiconductor, a solar cell is made from two doped crystals, one with a slight negative bias (n-type semiconductor), which has extra free electrons, and the other with a slight positive bias (p-type semiconductor), which is lacking free electrons. When placed in contact, some of the electrons in the n-type portion will flow into the p-type to "fill in" the missing electrons, also known as an electron hole. Eventually enough will flow across the boundary to equalize the Fermi levels of the two materials. The result is a region at the interface, the p-n junction, where charge carriers are depleted and/or accumulated on each side of the interface. In silicon, this transfer of electrons produces a potential barrier of about 0.6V to 0.7V^[4].

When placed in the sun, photons in the sunlight can strike the bound electrons in the p-type side of the semiconductor, giving them more energy, a process known technically 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 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 electrical current.^[4]

In any semiconductor, the bandgap 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 enough energy to make this happen. Unfortunately this also means that the higher energy photons, at the blue and violet end of the spectrum, have more than enough energy to cross the bandgap; although some of this extra energy is transferred into the electrons, the vast majority of it is wasted as heat. Another issue is that in order to have a reasonable chance of capturing a photon in the p-type layer it 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. These effects produce an upper limit on the efficiency of silicon solar cells, currently around 12% to 15% for common examples and up to 25% for the best laboratory modules.

By far the biggest problem with the conventional approach is cost; solar cells require a relatively thick layer of silicon in order to have reasonable photon capture rates, and silicon is an expensive commodity. 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.

[edit] DSC

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.

Construction

In the case of the original Grätzel design, the cell has three primary parts. On the top is a transparent anode made of fluorine-doped tin oxide (SnO₂:F) deposited on the back of a (typically glass) plate. On the back of the conductive plate is a thin layer of titanium dioxide (TiO₂), which forms into a highly porous structure with an extremely high surface area. TiO₂ only absorbs a small fraction of the solar photons (those in the UV).^[5]

The plate is then immersed in a mixture of a photosensitive ruthenium-polypyridine dye (also called molecular sensitizers^[5]) and a solvent. After soaking the film in the dye solution, a thin layer of the dye is left covalently bonded to the surface of the TiO₂. A separate backing is made with a thin layer of the iodide electrolyte spread over a conductive sheet, typically platinum metal. The front and back parts are then joined and sealed together to prevent the electrolyte from leaking. The construction is simple enough that there are hobby kits available for hand-constructing them.^[6] Although they use a number of "advanced" materials, these are inexpensive compared to the silicon needed for normal cells because they require no expensive manufacturing steps. TiO₂, for instance, is already widely used as a paint base.

Operation

Sunlight enters the cell through the transparent SnO₂:F top contact, striking the dye on the surface of the TiO₂. Photons striking the dye with enough energy to be absorbed will create an excited state of the dye, from which an electron can be "injected" directly into the conduction band of the TiO₂, and 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 TiO₂, 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.

Efficiency

Main article: Solar conversion efficiency

There are several important measures that 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, DSSc's 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 TiO₂ and the clear electrode, or optical losses in the front electrode. The overall quantum efficiency for green light is about 90%, with the "lost" 10% being largely accounted for by the optical losses in top electrode.^[7] The quantum efficiency of traditional designs vary, depending on their thickness, but are about the same as the DSSc.

The maximum voltage generated by such a cell, in theory, is simply the difference between the (quasi-)Fermi level of the TiO₂ and the redox potential of the electrolyte, about 0.7 V under solar illumination conditions (V_oc). That is, if an illuminated DSSc is connected to a voltmeter in an "open circuit", it would read about 0.7 V. In terms of voltage, DSSc's offer slightly higher V_oc than silicon, about 0.7 V compared to 0.6 V. This is a fairly small difference, so real-world differences are dominated by current production, J_sc.

Although the dye is highly efficient at turning absorbed photons into free electrons in the TiO₂, it is only those photons which are absorbed by the dye that ultimately result in current being produced. The rate of photon absorption depends upon the absorption spectrum of the sensitized TiO₂ 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. These factors limit the current generated by a DSSc, for comparison, a traditional silicon-based solar cell offers about 35 mA/cm², whereas current DSSc's offer about 20 mA/cm².

Combined with a fill factor of about 70%, overall peak power production for current DSSc's is about 11%.^[8][9]

Degradation

DSSC degrades from UV light. The barrier may include UV stabilizers and/or UV absorbing luminescent chromophores (which emit at longer wavelengths) and antioxidants to protect and improve the efficiency of the cell ^[10].

Advantages and drawbacks

DSSc's are currently the most efficient third-generation solar technology available. Other thin-film technologies are typically around 8%, and traditional low-cost commercial silicon panels operate between 12% and 15%. This makes DSSc's 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 TiO₂ is qualitatively different to 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 TiO₂, 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 TiO₂ to species in the electrolyte is also possible although, again, for optimized devices this reaction is rather slow.^[11] On the contrary, electron transfer from the platinum coated electrode to species in the electrolyte is necessarily very fast.

As a result of these favorable "differential kinetics", DSSc's work even in low-light conditions. DSSc's 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.^[12]

A practical advantage, one DSSc's 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. DSSc's 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 major drawback is the electrolyte solution, which contains volatile organic solvents and must be carefully sealed. This, along with the fact that the solvents permeate plastics, has precluded large-scale outdoor application and integration into flexible structure.^[13]

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.^[14]

Development

The dyes used in early experimental cells (circa 1995) were sensitive only in the high-frequency end of the solar spectrum, in the UV and blue. Newer versions were quickly introduced (circa 1999) that had much wider frequency response, notably "triscarboxy-terpyridine Ru-complex" [Ru(2,2',2"-(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".^[15] The dyes have an excellent chance of converting a photon into an electron, originally around 80% but improving to almost perfect conversion in more recent dyes, the overall efficiency is about 90%, with the "lost" 10% being largely accounted for by the optical losses in top electrode.^[7]

A solar cell must be capable of producing electricity for at least twenty years, without a significant decrease in efficiency (lifespan). The "black dye" system was subjected to 50 million cycles, the equivalent of ten years' exposure to the sun in Switzerland. No discernible decrease of the performance 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, which were completed in 2007.^[7]

The team has also worked on a series of newer dye formulations while the work on the Ru-complex continued. These have included 1-ethyl-3 methylimidazolium tetrocyanoborate [EMIB(CN)4] which is extremely light- and temperature-stable, copper-diselenium [Cu(In,GA)Se2] which offers higher conversion efficiencies, and others with varying special-purpose properties.

DSSc's 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 TiO₂ to better match it with the electrolyte being used.

New developments

2006

The first successful solid-hybrid dye-sensitized solar cells were reported.^[14]

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.^[16]

On August 2006, to prove the chemical and thermal robustness of the 1-ethyl-3 methylimidazolium tetracyanoborate solar cell, the researchers subjected the devices to heating at 80°C in the dark for 1000 hours, followed by light soaking at 60°C for 1000 hours. After dark heating and light soaking, 90% of the initial photovoltaic efficiency was maintained – the first time such excellent thermal stability has been observed for a liquid electrolyte that exhibits such a high conversion efficiency. Contrary to silicon solar cells, whose performance declines with increasing temperature, the dye-sensitized solar-cell devices were only negligibly influenced when increasing the operating temperature from ambient to 60°C.

April 2007

Wayne Campbell at Massey University, New Zealand, has experimented with a wide variety of organic dyes based on porphyrin.^[17] In nature, porphyrin is the basic building block of the hemoproteins, which include chlorophyll in plants and hemoglobin in animals. He reports efficiency on the order of 7.1% using these low-cost dyes.^[18]

June 2008

In a joint article published in Nature Materials, Michael Grätzel and colleagues at the Chinese Academy of Sciences demonstrated cell efficiencies of 8.2% using a new solvent-free liquid redox electrolyte consisting of a melt of three salts, as an alternative to using organic solvents as an electrolyte solution. Although the efficiency with this electrolyte is less than the 11% being delivered using the existing iodine-based solutions, the team is confident the efficiency can be improved.^[19]

[edit] Market introduction

DSC is the only third generation technology ready for mass production^[20]. DSC's are currently available from several commercial providers:

• G24innovations, founded in 2006, based in Cardiff, South Wales, UK. On October 17, 2007, claimed the 'production of the world’s first commercial grade Dye Sensitised Thin Film'.
• Dyesol officially opened its new manufacturing facilities in Queanbeyan on the 7th of October 2008
• Konarka, announced in 2002 that they were granted licensee rights to dye-sensitized solar cell technology from the Swiss Federal Institute of Technology (EPFL).
• Aisin Seiki has worked with Toyota Central R&D Labs to develop dye-sensitized solar cells (DSC) for applications in cars and homes.^[21]
• Sony Corporation has developed dye-sensitized solar cells with an energy conversion efficiency of 10 percent, a level seen necessary for commercial use.^[22]

How Dye-Sensitized Solar Cells Work

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