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S C HW E R P U N K T 16 Sustainable energy use is one of the greatest challenges of our time. Scien- tists across the globe are searching for ways to utilise renewable energies and to create self-sufficient systems. Hous- es that generate their own electricity by way of photovoltaics and electric cars powered by a battery charged pre- dominantly from wind, water and solar power are now commonplace. Materi- als scientists and chemists at Jena have been thinking on a smaller scale: they are focusing on small, mobile applica- tions. As far as Prof. Dr Ulrich S. Schubert and Dr Martin Hager are concerned, the key lies in flexible solar batteries, which combine the benefits of organic solar cells in terms of flexible solar fab- ric with organic batteries. The five-per- son Solar Batteries research group, which has been funded over the past two years as part of a research group from the Thüringer Aufbaubank with funds from the European Social Fund and the Free State of Thuringia, is searching for a suitable way to connect the two components and has already achieved some promising results. In addition to the CEEC Jena, the Leibniz Institute of Photonic Technology was also involved and provided production facilities for flexible solar fabric. Chemical liaison: marrying solar cells and batteries »We had the idea of creating an inte­ grated system that incorporates both energy conversion and storage«, ex- plains Martin Hager from CEEC Jena. »In order to achieve this objective, we married solar cells and batteries to de- velop a solar battery.« Benefits include the fact that cables are largely redun- dant and that the battery recharges in the sun even when it is currently in use. This guarantees a continuous power supply. »The battery stores the absorbed energy, which can then be used when there is no sun.« The researchers connected organic pol- ymer solar cells or a solar fabric com- prising silicon solar cells directly with an organic battery. The solar cells sup- ply the electricity and are thus able to charge the polymer batteries. This en- sures continuous charging when in the sun, as well as the discharge of the en- ergy when there is no sun. »This com- bination is highly complex and lots of adaptive processes are required«, says Dr Hager. »It was very important for us to be able to adjust the voltages.« A solar cell generates a maximum of 0.7 volts, but a mobile phone requires 3.8 volts to charge, for example. »Thus, we had to interconnect several solar cells to produce sufficient voltage.« The solar batteries are thin, lightweight and flexible—they are ideal for small, light applications that are regularly exposed to the sun and for which the low amounts of energy are sufficient. »The solar battery is ideal for safety clothing with reflectors, which light up independently. New sensory functions, such as connections to the smart watch, are also possible«, continues Prof. Schubert. »Furthermore, it is suitable Materials scientists and chemists at the Center for Energy and Environmental Chemistry Jena (CEEC Jena) have been working on the development of organic solar cells and organic batteries for many years. A recently concluded research project has now seen them combine both components to facilitate a more efficient energy generation for small applications, and thus to open up whole new processing and application options for the developed solar batteries as a result. Sunlight to go BY JULIANE DÖLITZSCH F E AT U R E