Scientists at Siemens, Germany have developed a printed organic solar cell with an efficiency of 5%. In the present state of the art they expect this to reach 7%. If the cell is cheap enough in the market place, this will still be an attractive proposition especially where there is no constraint on the size of platform. The cost of electricity per kWh will still be substantially less than for conventional silicon PVs.
A further step has been taken by chemists at the University of California at Berkeley. They have developed a flexible solar cell that can be painted onto any surface. Their solar cell is a hybrid consisting of inorganic nanorods dispersed within an organic polymer or plastic. The nanorods are manufactured in a solution containing cadmium selenide to a diameter of around 7 nanometres (nm). These absorb sunlight. The nanorods are then mixed with a plastic semi-conductor to coat a transparent electrode to a thickness of 200 nm. The cell is completed with an aluminium coating to act as a back electrode.
The nanorods behave like wires able to absorb light of a particular wavelength. When subjected to light they generate an electron plus an electron hole. The electron travels the length of the rod to be collected by the aluminium electrode. The hole, which mimics an electron, is transferred to the plastic or hole carrier where it migrates to its electrode thereby creating a current.
At present hybrid solar cells based on nanorods and plastic semiconductors achieve an efficiency of 2%. However, there is confidence among the scientists that realizing an efficiency of 10% will only be a matter of time.
The researchers claim that using inorganic semiconductors coupled with organic polymers achieves the best of both worlds since polymers offer the advantage of processing at room temperature which is relatively low cost and permits the use of a flexible substrate. Inorganic semiconductors have well-established electronic properties well suited as solar cell material.
If efficiency can be raised to 10% this promises to revolutionize the PV industry, for example by incorporating light concentrators in the cell. A commercial solar cell might have three layers to absorb light across the spectrum of sunlight.
Researchers at the University of Toronto led by Edward Sargent have come up with a novel material which contains semi-conductor nanocrystals that respond to the infra-red end of the light spectrum. This enables them to produce more electricity pro rata than conventional solar cells. The cells are so small that they can be held in suspension in solution such as paint or dye. This gives the technology considerable potential, for example to be painted onto walls of buildings and incorporated into fabrics. These plastic solar cells are claimed to be cheap and amenable to mass production. The inventors are confident it will transform the PV industry within ten years.1
Scientists at Cambridge University are developing an organic cell which uses two different types of carbon molecule, which, when mixed together, separate into layers and start converting light to electricity. So far the device has achieved 34% efficiency but only in the blue-green part of the spectrum. The objective is to encompass the whole spectrum. This would seem to be a technology to watch.
Finally, one manufacturer, Nanosolar of Palo Alto, California, is developing 'next generation' solar cells based on ultra-thin light absorbers. The semi-conductor film will be —10,000 times thinner than crystalline silicon cells and will be based on three-dimensional nanocom-posite architecture. This will make it possible for the cells to absorb light from most wavelengths of the spectrum. In terms of material use and manufacture this technology promises to offer considerable savings compared with crystalline silicon, not least the capability of a bulk rolling manufacturing process. The company's chief executive claims that the cell can deliver a ten-fold improvement in the cost-to-performance ratio compared with crystalline silicon. Another important feature is that the cells last considerably longer than conventional cells - nearer to a lifetime than the 20 years of silicon-based PVs. They also do not degrade with exposure to the elements. These will be major considerations when it comes to selecting building-integrated systems. The company aims at volume production in 2007. By 2012 there are plans to introduce ultra-thin three-dimensional absorber cells. (www. nanosolar.com/thin-film.com.htm)
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The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.