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A Cheaper Way to Touch the Sun
American chemists co uld be close to converting sunlight into a cheap source of energy. Dr Stuart Ucht and a team from Clark University in Worcester. Massachusetts, are perfecting a new solar cell that works like a conventional battery as It converts sunlight Into electrical power.
The more familiar solid-state solar cells or photovoltaic devices (which produce electric current at the junction of two substances exposed to light). are found in spacecraft and calculators. These are highly developed and efficient, but relatively expensive to produce. This makes them too costly to generate solar electricity on a large scale, even though solar power would not contribute to global warming. The new device, called a photoelectrochemical cell, though less efficient than solid-state devices. Is much cheaper to make. These cells could pose a serious challenge to the supremacy of solid-state devices for solar energy conversion.
Turning the sun's light Into chemical energy is not new Eor billions of years, photosynthesis has provided the basic chemical driving force for practically all life on earth
Plants convert sunlight carbon dioxide and water Into high-energy carbohydrates, liberating oxygen in the process.
From an industrial point of view, though, photosynthesis is highly inefficient On average, only 1 per cent of sunlight is converted Into energy-yielding compounds such as wood. For an industrial society, such a low efficiency would mean turning over vast tracts of land to solar energy conversion.
The most promising approach so far to solar energy seems to be to convert sunlight directly Into electricity. Up to now. this has meant moving away from chemistry and biochemistry and into the realms of solid-state physics.
When light shines on a semiconducting materia), it "knocks’ electrons out of the outer orbits of the atoms of the material and so generates an electric current. But there is a snag. The conductivity Is short-lived. Physicists tackle this by grafting on another piece of semi-conductor with a slightly different structure. Where the two semi-conductors meet is called a p-n junction and the effect Is to keep the current flowing for as long as light falls on the device.
These cells are reliable and highly efficient — the best lab-
oratory models work at 25 26 per cent efficiency. But they are expensive to make: fashioning the all-important p-n junction requires all the skills of modern electronics technology.
The photoelectrochemical cell has been developed to help solve the problem. Chemists have put the semiconductor in contact with an electrolyte — a water-based solution which has the same effect on the semi-conductor as the p-n junction.
The electrons knocked off the atoms of the semi-conductor by the light pass this time into the electrolyte, rather than into another piece of semi-conductor. The effect is rather like what happens at one terminal of a battery. If another electrode is placed In the electrolyte, current will flow through a completed circuit outside the cell, from semi-conductor to electrode. This is the photoelectrochemical ceil: to generate a current just shine light on the semiconductor.
Photoclectrochemlcal cells could also produce chemical fuels. The passage of an electric current through a waterbased electrolyte causes chemical reactions at one of the electrodes, producing hydro-
gen. This can be burnt or stored for later use.
The chemical cells are also cheaper than photo-voltaic devices. The semi-conductor electrode does not need the hi-tech treatment used in making silicon solar cells. The problem is that the chemical reactions started by the semiconductor can dissolve it
This is called photo-corrosion. a light-induced rusting which eats away at the semiconductor and covers it with an electrically insulating layer. This drastically reduces the life-span of, the cell, sometimes to the order of seconds. Moreover, the chemical cells are less efficient in turning light into electricity than the photo-voltaic ones.
In the early Eighties, chemists were so gloomy about the prospects for these cells that they were predicting efficiencies of about 10 per cent by the late Nineties. But in 1984. Nathan Lewis of Stanford University in California smashed the 10 per cent barrier by using electrolytes that did not contain any water. This beat the corrosion problem but at a price: no water must be let into the system. Later. Lewis was able to hoist the efficiency of his cells to 15 per cent in water-based electrolytes by
coating his semi-conductor, made from gallium arsenide, with an ultra-thin film of expensive osmium.
Now Stuart Licht has gone one better. He has used a special combination of semi-conductor (this time cadmium se-
lenide. which is cheaper than gallium arsenide and which reacts to light differently) and a water-based electrolyte. This enables the cell to produce its maximum operating efficiency — a new high at almost 17 per cent — over days instead of
seconds.
Apart from being much cheaper to make than a solid-state solar cell, Licht s new cell does something that no solid-state cell can do. It generates a comparatively high voltage — 1.2 volts (about the
same as a conventional ton
battery), compared to only 0.7 volts from a silicon solar cell.
"There is no reason why we shouldn t get even higher voltages tn the future, which will lead to efficiencies