The mobile world reaches its limits every time the power supply runs out. There must surely be huge market potential for devices that take their energy from the daylight just as effortlessly as we breathe oxygen from the air. Integrated solar modules can provide the solution. Integrated solar cells are already being used to power wrist watches and pocket calculators. But a significantly higher output is needed in order to feed power-hungry devices, such as cellphones or palmtops.
A few years ago, German researchers from the Fraunhofer Institute for Solar Energy Systems, in Freiburg, demonstrated a mobile telephone and a personal digital assistant which drew their entire power supply from light. Unlimited standby times were assured by high-energy solar cells integrated directly in the housing. All the prototypes presented were highly sophisticated. The researchers had fitted the cellphone, a standard Siemens model, with high-energy solar cells that they had integrated directly in the device. Thanks to the tilelike, overlapping arrangement of the cells and the special way in which they were connected, it was possible to charge a small solar battery inside the cellphone. A second accumulator, which acted as a standby, was also charged without connecting to the power-distribution network. The values were impres- sive: the solar module delivered an output of 250 mW and a module efficiency of 20%.
Although such high-energy solar cells deliver the best efficiency factors, they also have a drawback ¡V it is not always easy to integrate them in the devices. For ease of integration, the ideal solar cells would be thin, flexible ones capable of adapting to fit the given geometry. That is the reason why the researchers are now exploring an alternative approach ¡V they are working on the development of modules with organic solar cells that are cheap, flexible and can be mass-produced.
Organic solar cells have an efficiency of only about 3%, much lower than today's high-energy cells, but they have the great advantage that they can be applied as thin layers to flexible films in wet chemical processes. This allows them to fit in effortlessly with polytronic applica- tions, where the objective is to print electronic circuit elements directly onto thin polymer films. For some time, researchers ¡V including scientists from the German Institute for Reliability and Microintegration, in Munich ¡V have been trying to find ways to circumvent the handling of fiddly little chips by printing the circuits that they carry directly onto thin film.
It makes sense to use this technique wherever the main consideration is less the high-performance electronics, and more the need for low-cost, high-volume production of products such as tiny radio frequency identification (RFID) chips. Polytronic structures are a little larger and not quite as precise as silicon chips, but the technology has undeniable advantages in terms of production engineering: there are fewer steps in the manufacturing process, and the difficult component asssembly procedure is unnecessary. The starting material is cheap, and the structures can be manufactured in batches of millions or even billions by using the familiar roll-to-roll production technique from the printing industry.
In some RFID applications, the radio chips need their own power supply. All active systems that autonomously transmit messages are, therefore, potential candidates for a solar power supply. Scientists at the Fraunhofer Institute for Solar Energy Systems are working to improve the low power efficiency of organic solar cells by improving their light-absorption properties. This can be done by embossing them with nanostructures. Fur-ther, smart circuits will make sure that the required operation voltage is achieved.
RFID smart labels will not be the only application to use microsolar modules in future. Another application will be in sensor and display technology, where it is important to be independent of power sources ¡V in security and monitoring applications, for example. The film-based solar cells are elastic and flexible enough to be incorporated in garments, too. They will be able, in future, to supply the energy for 'wearable electronics' ¡V energy 'off the cuff'.
"We don't want any more miniature components that have to be painstakingly connected using tiny wires ¡V what we want are thin, flexible, fully integrated systems," says Andreas Gombert, head of the Department of Materials Research and Applied Optics at the Institute for Solar Energy System. In their own Smart Plastics project, the researchers are developing systems of this kind based entirely on organic materials, ranging from solar power supplies and electronic charge regulators to film-based batteries, sensors, and organic light-emitting diode displays.