Dye-sensitized solar cells

Overview
Dye-sensitized solar cells (DSC) are a relatively new class of low-cost solar cells. They are based on a semiconductor formed between a photo-sensitized anode and an electrolyte, a photoelectrochemical system. These cells were invented by Michael Grätzel and Brian O'Regan at the École Polytechnique Fédérale de Lausanne in 1991 and are also known as Grätzel cells.

These cells are extremely promising because they are made of low-cost materials and do not need elaborate apparatus to manufacture. In bulk they should be significantly less expensive than older solid-state cell designs. Although their conversion efficiency is less than the best thin-film cells, their price/performance ratio should be high enough to allow them to compete with fossil fuel electrical generation in Europe. 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.

Background
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. This transfer of electrons produces a potential barrier, typically 0.6V to 0.7V.

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, into the n-type material, 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.

This conventional approach has several disadvantages. In silicon the difference in energy between the valence and conduction bands, the bandgap, means that only photons with that amount of energy, or more, will contribute to producing a current. 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, and although some of this energy is transferred into the electrons, much 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 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 42.8% for the best laboratory modules.

But by far the biggest problem with this conventional approach is cost; it requires 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.

Principle


Dye-sensitized solar cells effectively 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 potential barrier to separate the charges and create a current. In the dye-sensitised solar cell, the semiconductor is used solely for charge separation, the photoelectrons are provided from a separate photosensitive dye. Additionally the charge separation is not provided solely by the semiconductor, but works in concert with a third element of the cell, an electrolyte in contact with both.

Since the dye molecules are quite small, 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 design, the cell has three primary parts. On the top is a transparent anode made of fluorine-doped tin oxide (SnO 2 :F) deposited on the back of a (typically glass) plate. On the back of the 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 plate is then immersed in a mixture of a photosensitive ruthenium-polypyridine dye 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 TiO 2. 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. 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. TiO 2, for instance, is already widely used as a paint base.

In operation, 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 will create an excited state of the dye, from which an electron can be "injected" directly into the conduction band of the TiO 2, and from there it moves by a chemical diffusion 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.

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 TiO 2 and the clear electrode, and the optical losses in the front electrode. The overall quantum efficiency is about 90%, with the "lost" 10% being largely accounted for by the optical losses in top electrode. 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 Fermi level of the TiO 2 and the redox potential of the electrolyte, about 0.7 V total (V oc ). That is, if a 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 in turning photons into electrons, it is only those electrons with enough energy to cross the TiO 2 bandgap that will result in current being produced. The bandgap is slightly larger than in silicon, which means that fewer of the photons in sunlight are usable for generation. In addition, the electrolyte limits the speed at which the dye molecules can regain their electrons and become available for photoexcitation again. These factors limit the current generated by a DSSc, for comparison, a traditional silicon-based solar cell offers about 35 mA/cm&sup2;, whereas current DSSc's offer about 20 mA/cm&sup2;.

Combined with a fill factor of about 70%, overall peak power production for current DSSc's represents a conversion efficiency of about 11%, whereas (as noted earlier) common low-cost commercial panels operate between 12% and 15%. Flexible thin-film cells are typically around 8%. This makes DSSc's extremely attractive as a replacement for existing technologies in "low density" applications like rooftop solar collectors. 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.

However, there is another practical issue to consider. The process of injecting an electron directly into the TiO 2 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. As mentioned earlier, this is a major limit on the efficiency of traditional designs.

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 the electrolyte is not possible, due to differences in energy levels. Simply put, the electron–hole recombination that affects the efficiency of traditional cells doesn't exist.

As a result of both of these features &mdash;low losses and lack of recombination&mdash; DSSc's work even in low-light conditions. DSSc's are therefore able to work under cloudy skies, 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.

The only major disadvantage to the 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.

So in spite of the slightly lower efficiency, DSSc's represent a major advance in solar cell technology. They offer efficiency close to that of the very best low-cost devices, but are far easier and less expensive to produce. Even small improvements in the efficiency of the electrolyte or dye would essentially eliminate silicon as a competitive approach to solar power generation. Their closest competitor in price-performance terms are the various thin-film approaches, which are currently somewhat more developed commercially.

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". 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.

A solar cell must be capable of producing electricity for at least twenty years, without a significant decrease in efficiency. 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 and appear to be ready for commercial applications.

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.

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. 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.

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 TiO 2 to better match it with the electrolyte being used.

Market introduction
Dyesol (Dyesol) in Queanbeyan, New South Wales has been producing "experimental quantities" of DSSc's for several years, and have also offered the chemicals and tools for others to build their own DSSc's by hand. They are currently in the process of building two manufacturing plants, one in Queanbeyan and another in Northern Wales, coming online in '09 and '10 respectively.

G24 innovations (G24i) in Cardiff, Wales, have recently opened their experimental roll-to-roll DSSc factory, and have a second line coming online to produce a total of 30 MW of solar cells per year.