Printed organic transistors pave the way for intelligent windows

Researchers at DTU Energy have shown that transistors can be quickly and cheaply printed on a polymer foil. The ability to print them by the same processes and on the same foil as an array of polymer solar cells opens up the possibility of, e.g., intelligent windows operating without the need for batteries or external power.

Electrochromic materials can change their transparency when an external voltage is applied. Such materials have interesting applications in windows. Thus, one can imagine a window in an office or a greenhouse which automatically pulls the shade by making itself darker, when the temperature gets too high. Very handy, but so far also quite expensive due to the need for external circuits and power supplies.

Now a research team at DTU Energy has found a way to integrate printed electronics and printed solar cells with an electrochromic organic sheet, resulting in a device which can operate without batteries.

They have shown that one square meter of an electrochromic sheet can be powered by a 5 cm stripe made of printed solar cells and transistors along the bottom, and that all components can be manufactured by the same roll-to-roll printing processes.

One of the many difficulties was to achieve a small as possible interspace between the electrodes in order to support high current. Where printed state-of-the-art electronics has an interspace of 80 micrometer (µm), the DTU researchers are now down to 10-50 µm. To achieve these results they are building on years of experience in producing cheap and durable polymer solar cells using ambient air roll-to-roll techniques.

Smart windows is only one application of a self-powered sheet with printed electronics. Other possibilities are clothing with flexible electronics integrated without the need of a power source. Or the use of functional organic sheets as a cheap sensor for bio-testing

The researchers are very optimistic about the future of printed electronics. However, whatever the final product will be, it will still take years to perfect this techniques.

Francesco Pastorelli, Marie Curie Research Fellow, who is the main author of the study, said, "Normally electrochromic solutions are expensive because of external electrical components and installation cost, but we are able to print in ambient air on the same sheet the electrochromic surface, the solar cells and the transistors using roll-to-roll techniques… we print as if it was a newspaper! And we can directly glue the resulting foil on any surface with little effort. This will reduce costs, facilitate the installation and reduce the environmental impact."

Francesco Pastorelli, Marie Curie Research Fellow, added, "We have a vision of making post cards for Africa, where people can make a blood sample just by using the card with printed flexible electronics, without the need of a power source nearby. It is a cheap way to make a blood analysis, and we have already some groups within biomedicine showing interest."