Solar Leaves

Introduction 

         It’s nothing if not frustrating. Everyday the Sun pours down on us ten thousand times more energy than we need to drive every human activity. Yet, we have still to figure out a cost-effective way of harnessing it. Nature doesn’t have this problem. Solar cells simply grow on trees where photosynthesis in leaves turns sunlight into the chemical energy that powers almost every natural ecosystem. Yes, plants can teach us a thing or two about making the most of sunlight.

          Almost all-commercial solar cells are made from amorphous silicon. However, the trouble is that silicon is expensive to process. Also, these cells aren’t very efficient either, typically converting only 13 to 16 % of the sunlight into electricity. So it’s still more practical to burn up our non-renewable fossil fuels.

          But low efficiency wouldn’t necessarily matter if huge arrays of cells could be produced cheaply. Wim Sinke, Head of the Solar Energy Group at the Netherlands Energy Research Foundation, at Petten says, “Today’s industry, is not about squeezing out another one or two percent efficiency. Turn out inefficient cells in large numbers and the unit price will drop, making them accessible to all.”

          That, after all, is nature’s strategy: chloroplasts, the organelles in a plant cell that trap the Sun’s energy, are less than 1 % efficient. However, it scarcely matters because they are scattered everywhere across the green forest canopies and grasslands of the world.

          Therefore, in search for low cost solar cells researchers have abandoned silicon in favour of more exotic substances that copy photosynthesis. Just months ago, for instance, chemists announced an advance in the development of solar cells built from dye and the pigment in the white paint. With the right dye, this bizarre mix is just as efficient at gathering sunlight as silicon.

          In a few months, the Swiss watch company will begin to sell solar powered watches that incorporate these cells in a glass cover. Since these cells are transparent, the watch face remains visible while the cells use the Sun’s energy to power the watch. Scale-up the technology and you might eventually be able to swap the windows of your house or office block for cheap, transparent solar cells.

          Even more remarkable are tiny solar powered molecular “machines”, the brainchild of a team of US researchers. These copy photosynthesis even more closely because they are assembled from artificial membranes stuffed with molecules that turn sunlight into chemical energy rather than electricity. These machines are still in an early stage of development, but by copying photosynthesis in this way, the researchers are well on the way to building an artificial leaf.

 Principle

        Most solar powered devices rely on the same principle: a photon of sunlight boosts an electron from some material into a mobile state. Once the electron is mobile, it can be used to generate energy. However, there is a problem with this simple mechanism. Since the electron is negatively charged, it leaves a positive charge behind. Opposite charges, attract one another so they tend to recombine, squandering the absorbed energy as heat or as re-emitted light. Every solar powered device must somehow overcome this difficulty. Here lies the crux of the difference between today’s commercial solar cells and green leaves.              

          Designers of silicon solar cells use sheer brutal force to get around this problem. They create an electric field in the silicon that pushes the negatively charged electrons and positive charges apart. However, chloroplasts adopt a more subtle approach. They separate charges by making a distinction between the units that generate the electron and those that transport it away. The photosynthetic apparatus inside the chloroplast is embedded in a membrane. The electron is shuttled rapidly from one molecule to another to take it progressively further from where it started out. It is carried across the membrane before it can recombine with the positive charge. At the far side of the membrane, its energy is stored as energy rich molecules of adenosine triphosphate (ATP) - the chemical fuel that powers many of the reactions in our body.

          So Michael Gratzel and Brian O’Regan at the Swiss Federal Institute of Technology, in Lausanne decided to steal a trick from the leaves. They have copied the chloroplast’s modular design and combined it with a semi-conductor to build solar cells that are both efficient and cheap to make.  

 Making of the Cells

         Instead of using silicon to absorb sunlight, their cell relies on dye molecules containing Ruthenium ions that absorb visible light. The dye is coated onto nanocrytsals of the semi-conductor titanium dioxide (Titania). This is where the white paint comes in - for the pigment that imparts whiteness there is nothing more than a fine suspension of Titania particles. The nanocrystals do more than just support the dye. They carry the photo excited electrons away rapidly. Titania has just the right electronic properties to pull electrons out of the Ruthenium dye and shuttles them off into an electrical circuit.

          To make a complete solar cell, Gratzel seals a 10-micrometer thick film of dye coated nanocrystals between two transparent electrodes. The nanocrystals are packed together to form a highly porous film that crams a large absorbing surface area into a small space ideal for harvesting light. A film just micrometers thick contains hundreds of layers of the dye. So, any photon passing through is almost certainly absorbed.

          The space between the nanocrystals is filled with a liquid electrolyte containing iodide ions. These ions provide a source of electrons to replace those knocked out of the dye by sunlight. The two electrodes are connected to a circuit and light is shone on the cell. And, hey presto, current begins to flow.

          Even the prototype cells the Swiss team created in 1991 were at least as efficient as amorphous silicon devices, converting between 10-15 % of light into electricity. These cells generate voltages of around 750 millivolts with a power output of about 100 watts per square meter. However, the great thing about using Titania is that it is dirt-cheap. About 10 grams is enough to make one square meter of film for about a penny.  No wonder, then, industries are already expressing an interest. Eight Swiss, Austrian and Japanese companies want to commercialize the technology and two are about to begin production of photovoltaic tiles 10 centimeters square.

 Cell Views and Limitations

         The Ruthenium dye in Gratzel’s cells absorbs light strongly in the red and green parts of the visible spectrum. This doesn’t matter in the glass cover of the Swatch watch, because the cells are so thin that they remain transparent. However, bigger, higher power cells need thicker films of dye coated nanocrytals. Because they absorb light more strongly, such cells are opaque.

          So Gratzel and his colleagues are collaborating with the chemicals company Johnson Matthey to look for dyes that absorb light in the infrared part of the Sun’s spectrum instead. Replace the Ruthenium dye with these dyes that absorb in the infrared part, and even the hefty cells would be transparent. Best of all, they could double up as windows. If the project succeeds, your office could have windows that let visible light to pass through and yet absorb enough in the invisible part of the spectrum to provide electricity. Gratzel calculates that an energy conversion of 10 % should be possible in these cells.

          But there’s still one problem. Using a liquid electrolyte inside the cells is far from ideal. They need reliable, watertight seals. Spring a leak and all is lost. Gratzel has been searching for ways to make his cells as robust as silicon cells, but with little success. Then in October 1991, Gratzel announced a break through. Together with colleagues at Hoescht Research & Technology in Frankfurt and the Max Planck Institute for Polymer Research in Mainz, he has made versions of his cells that use an organic solid called OMeTAD to do a similar job to the liquid electrolyte in his earlier cells. So far, these solid-state cells have an efficiency of less than 1 %, which is nevertheless very high for an organic device. The low voltages that the cell generates  - about a third of a volt - could also be a big obstacle to most applications which require higher voltages. But these cells are promising and cheap to make.

 Recent Trend 

         Copying some of the tricks of photosynthesis is one thing, but Tom Moore and co-workers at the Arizona State University are mimicking nature more literally: capturing solar energy using "“biological" rather than electrochemical cells. In an ambitious project, they are using artificial cell like structures called liposomes as light harvesting machines.

          Chloroplasts do not convert sunlight into electrical current. They merely move electrons from one side of a membrane to another - a process that is eventually exploited to make chemicals. It would be a very useful ability to mimic in artificial systems: imagine chemicals factories where the reactions are not driven by, say, heat from a flame, but directly from the energy of sunlight. This is the kind of vision that Moore’s group is pursuing.

          The Gratzel cell produces elecromotive force (EMF), which is exactly what is needed to run the motors and electronic devices of the macroscopic world. But in a chloroplast, electron transfer has a different result. It leads to an electrochemical potential that drives microscopic motors and other devices of living cells. Electron transfer in chloroplasts occurs inside complex membrane structures. They are made from a collection of molecules called “phospholipids” - molecular “tadpoles” with a water soluble head group and a water insoluble tail. In water, phospholipids spontaneously form hollow spherical membranes called liposomes, driven by the tendency of the insoluble tail to shy away from the water.

 
Working Components

         The liposomes provide the fabric of Moore’s artificial chloroplasts. They are about the same size as real cells - a micrometer or so in diameter. Moore has embedded molecules into the liposome’s membrane that are designed to perform some of the functions of the chloroplast.

          The heart of the system is a molecular triad, three molecules linked by chemical bonds. One component is a porphyrin molecule (P) that is adept at soaking up photons of sunlight. When excited by light, it passes an electron to the second component of the triad - a quinone molecule (Q) - that hands it to a “shuttle” molecule (Qs) that is free to rove around inside the phospholipid membrane. To remain electrically neutral, the shuttle molecule grabs a positively charged hydrogen ion from outside the liposome and diffuses across the membrane. When it reaches the other side of the membrane, it passes the electron to the third molecule - cytochrome (C) of the triad and releases the hydrogen ion into the centre of the liposome.

          The result is that the electron efficiency ushered away before recombination can occur. At the same time, the shuttle carries hydrogen ions across the membrane. As light falls on the liposomes, the concentration of the hydrogen ions inside them goes up, creating an electrochemical potential across the membrane. The excess of hydrogen ions inside the liposome creates a “proton motive force” (PMF) - proton power.  PMF can in principle, drive any of the reactions and processes of the living cell.  

          Moore and his colleagues showed their liposomes could generate PMF in their initial experimentation. However, in March 1991, they added a crucial component that would let them harness PMF to drive a chemical reaction - and to use sunlight for chemical synthesis. They inserted an enzyme called ATP synthase into the liposome membranes. In nature, ATP synthase converts ADP to ATP by tacking on a phosphate group. Critically, ATP synthase is powered by a flow of protons, much as a water wheel runs on water. It sits in the membrane that has an excess of hydrogen ions on one side, and the enzyme acts as a kind of conduit for the hydrogen ions. As the hydrogen ions flow across the membrane, energy is released to make ATP.

          Moore found that the high hydrogen ion concentrations provided the driving force for the membrane bounded enzyme to do its job on ADP. The result: ATP from sunlight, just as in photosynthesis.

          However, making ATP is just the first step. Another important energy store is the enzyme co-factor NADPH. Like ATP, it provides the energy for a whole host of bio-chemical reactions. Moore’s team wants to demonstrate PMF-driven production of NADPH in the near future.

 
Conclusions 

         This would be a big step towards the first artificial leaf. However, Moore and his colleagues believe this system could also change the way chemists work. In the future, they say, an artificial leaf could provide a solar powered energy source for a vast range of chemical reactions.

           Just as our bodies use molecules of ATP to make hormones, enzymes and a host of other bio-molecules, by creating natural sources of chemical power from sunlight offers chemists a valuable source of energy. It might provide the power to manufacture drugs, for instance. Or how about generating clean fuel by splitting water into hydrogen and oxygen or degrading organic industrial wastes into harmless molecules? These are questions that are yet to be answered. With real, live leaves as its inspiration, this kind of chemical processing would be truly green.

References:  New Scientist

**This article was graciously submitted to www.cheresources.com for publication by
Shankara Narayanan K.R. from Bangalore, India. The author can be reached for questions/comments at k_r_shankar_nar@hotmail.com

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