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Innovative solar cell mimics photosynthesis: a 'leaf-inspired' PV cell design offers the possibility of inexpensively converting solar to chemical and electrical energy.

The U.S. alone uses about 30 trillion kilowatt-hours of energy each year, and with energy usage in other regions of the world rapidly growing, it is clear that developing alternative sources is imperative. Each square meter at mid-latitude locations in the U.S. receives 4 to 5 kilowatt-hours of solar energy per day; so tapping into that energy source seems an obvious path to alleviate energy shortages. But traditional methods of capturing solar energy are expensive to generate and deploy. For 15 years now, researchers around the world have been improving the efficiency and manufacturability of a non-traditional approach: the dye-sensitized solar cell. Their efforts are beginning to pay off.

Capture & harvest

There's plenty of solar energy around, the problem is converting it to a usable form. Because of the established infrastructure developed for distributing and using electrical energy, one of the most desirable approaches is to convert solar energy to electrical energy. Photovohaic (PV) cells do just that, by transferring energy from an incident photon to an electron. That absorption occurs when the energy of the incident photon matches the energy needed to push an electron from one energy level to a higher available energy level. The trick is harvesting the energetic electrons before other naturally occurring processes return them to their lower energy state.

Traditional PV cells, for their part, are constructed from semiconductor crystals. The band gap of the semiconductor is tuned to match the energy available in sunlight so that a decent percentage of the incident light is converted into electrical energy. But then the real battle begins. If left to itself, the energetic photon will drop back down into the vacancy it left in the lower energy band. So PV ceils are engineered with an electric field across the absorption region. When electrons are promoted into the higher energy band of the semiconductor, the applied electric field rapidly draws them away, eventually to an electrode that harvests them. But any disturbance in the process as the electron travels to the electrode will tend to bring the electron back to its lower energy state, so great care must be taken to ensure the purity and order of the semiconductor.

Industrial processes for producing semiconductors with high purity and order are already well-advanced because of the maturity of the semiconductor industry; so there's not too much of a technological problem. There is, however, an economic problem. Semiconductor manufacturing facilities are extremely expensive to build, operate, and maintain. For electronic circuits, the cost is offset as devices get smaller and smaller, offsetting the cost of production through high volume. But solar cells need to be large to collect incident sunlight. And large pieces of semiconductor are expensive.


A "new" approach

In 1991, Prof. Michael Gratzel and his colleague Brian O'Regan, at the Ecole Polytechnique Federale de Lausanne in Switzerland developed a device architecture that introduced a new conceptual approach to PV energy generation. It would be more accurate to state they refashioned an old idea--a three-billion-year-old idea. They reasoned that the light-harvesting approach used by plants for billions of years must have features that could be adapted for use in PV cells. One of the primary features is a separation between the light absorption and electron transfer mechanisms. An energetic electron generated in chlorophyll is rapidly transferred from molecule to molecule until it reaches a chlorophyll reaction center--a chlorophyll molecule modified with a metallic atom to modify its electron energy level structure. The reaction center then transfers the electron to an energy storage molecule. But this leaves the chlorophyll "short" an electron. It grabs the electron it needs from a surrounding water molecule. Gratzel followed that same model.

A dye molecule, usually aruthenium complex, is surface bound to a Ti[O.sub.2] crystal. An electron within the dye molecule is excited by a photon into an energy level just above that of the conduction band within the titania crystal. In just femtoseconds, the electron will transfer into the titania. When the attachment mechanism between the dye and the Ti[O.sub.2] is engineered properly, the back reaction--the return of the electron to the dye--is discouraged. The immediate problem, the relaxation of the excited electron into a lower state, is avoided. Still, the vacancy in the dye molecule created by the transfer of the electron is still there, and eventually would draw the electron back, but, again analogous to photosynthesis, an electron is pulled from a molecule in the surrounding environment. In this case, the environment is provided by an electrolyte solution. An electrolyte molecule is oxidized by the dye complex, leaving it now shy an electron. Meanwhile, the initial photoelectron is travelling through the conduction band in the titania, where it is collected at an electrode. The circuit is completed when the electron is sent through the device to be powered or charged and then returned to a counter electrode in contact with the electrolyte. The oxidized electrolyte is then reduced by the addition of the electron, regenerating the PV cell.


Although the process sounds ideal and the materials are inexpensive, there is a problem: the optical cross-section of the dye molecules is small enough that they only absorb a small fraction of the incident light. The obvious solution--add more layers of dye molecules--will not work because the electron transfer depends upon immediate contact between the dye and the Ti[O.sub.2]. Rather than making an increasingly large flat titania surface with a monolayer of partially-absorbing dye, Gratzel created a mesoporous nanocrystalline layer of Ti[O.sub.2]. Using a sol-gel process, 10 to 20 nm nanocrystalline particles of titania are generated in a loose, randomly arranged 5-10 [micro]m layer with 10-20 nm pores interspersed throughout. The nanocrystalline titania layer is deposited on a transparent conductive electrode so that light enters through the electrode, and then passes across hundreds of surfaces within the mesoporous nanocrystalline dyesensitized layer, maximizing the opportunity for absorption.

Tweaking away

The dye-sensitized solar cell (DSSC), commonly called the Gratzel cell, is a photoelectrochemical cell, using oxidation and reduction reactions to mediate energy transfer, rather than the electronic band structure of a semiconductor. Because the different functions of a PV cell are carried out by different elements within the DSSC, each aspect may be separately optimized. For example, at the recent American Chemical Society meeting in San Francisco, about a dozen presentations discussed various dyes that offer different advantages. Gratzel introduced an infrared-absorbing phthalocyanine dye, which can efficiently capture the half of the solar radiation incident at wavelengths longer than the visible. This has two potential advantages. First, a solar window could be used in architectural design: a normal window that allows visible light to transmit while it converts the infrared region of the spectrum to electrical energy. Second, an infrared dye can be used in conjunction with other visible dyes to provide broad spectrum absorption. Gratzel believes that a "dye cocktail," a combination of several dyes linked to the nanocrystalline surface, may eventually be the solution that provides optimum efficiency. "We have already done that, and we've found that they behave in a complementary way," says Gratzel.

Other groups are also working on enhancing DSSC efficiency. Prof. Hernan Miguez of the Institute of Materials Science of Seville, Spain, has developed a DSSC architecture that uses photonic crystals to enhance the optical absorption. Light enters the cell through the transparent electrode then reaches the dye sensitized nc-Ti[O.sub.2] layer and then a photonic crystal.The photonic crystal consists of Ti[O.sub.2] spheres about 200 nm in diameter, created through an inexpensive spin-coating process. Miguez's team extended that concept by designing in three different photonic crystals, one on top of the other. "If you build up three photonic crystals, each one with a different band gap, on top of the DS layer you will increase the number of localized modes and the spectral range at which they appear, so you can enhance the absorption of the dye at all frequencies," adds Miguez.

Making it real

There are other technological advances in process for the Gratzel cell: attempts to increase the open circuit voltage, additional dye enhancements, and replacing the initial liquid electrolyte with more manufacturable and ecologically-friendly liquids or even solids. But the efficiencies are already around 11% and the costs are low; so much of the effort is now focussed on improving the manufacturability. "We have to learn how to produce these cells in an automated and high yield way," says Gratzel. "This is not a problem a scientist can solve."

The Ecole Polytechnique Federale de Lausanne has licensing agreements with several manufacturing partners. The most recent indication of the progress towards commercialization came when UK-based, G24 Innovations, in partnership with Konarka, a Massachusetts company, announced plans for a 17,373 [m.sup.2] DSSC manufacturing facility in Cardiff.

--Richard Gaughan

Founder and Chief Engineer

Mountain Optical Systems Technology,
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Title Annotation:PHOTONICS
Author:Gaughan, Richard
Publication:R & D
Date:Nov 1, 2006
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