Plastic solar cells: two ultrafast processes producing electricity in harmony

Research by a team headed by Prof. Paul van Loosdrecht and Prof. Kees Hummelen of the Zernike Institute for Advanced Materials of the University of Groningen has shown how two processes in plastic solar cells cooperate harmoniously to produce electricity. Last week, the research was the cover story for Advanced Functional Materials, an American scientific journal.

Plastic solar cells consist of a combination of two components – buckyballs (football-shaped C60 or rugby ball-shaped C70 molecules) and a polymer that can conduct electricity. It was already known that the polymer after absorption of light could release an electron to the buckyballs in the extremely short period of time of 30 femtoseconds (a millionth of a billionth of a second), which eventually leads to the generation of electricity.

 
The Groningen research team has succeeded in establishing that after absorption of light by the buckyballs, a mirror-image of this process also takes place – in other words, a positively charged hole jumps from the buckyball to the polymer. In addition, this mirrored process is just as ultrafast.
The measurements of the speed of charge transfer between the two types of molecules are done with the help of special techniques based on lasers, explains Van Loosdrecht, professor of experimental physics. Two extremely short light pulses that fall onto the solar cell material quickly after each other are used.

The first pulse is absorbed either by the polymer or the buckyball, depending on the colour of the light. The second pulse is used to see how long it takes before the material is charged. Van Loosdrecht: ‘To our surprise we found that that speed was virtually identical in both cases, which means that the two processes are equally efficient in generating electricity.’ 

The researchers combined the measurements of the speed of the charge build-up with images of various mixtures created with an atomic force microscope. This research emphasizes the importance of how the two types of molecule are interwoven. In order to achieve a high yield, there must be a large contact surface between the two types of molecule and both the (negative) electrons and the positive holes must be able to find their way to the electrodes quickly and easily.

The greatest challenge in the development of plastic solar cells is improving the yield. Currently this is still much lower than that of silicon solar cells, which are much more expensive to produce. Plastic solar cells have recently reached the point where practical applicability is an option, says Kees Hummelen, professor of organic chemistry, proudly. Last year, the first bus shelter with a roof of plastic solar cells was installed in San Francisco. It collects sustainable energy to power an interactive map of the city. Hummelen: ‘Solar power is taking over the world.’

More information:
Prof. Kees Hummelen, tel. 050-363 5553, e-mail: j.c.hummelen rug.nl
Prof. Paul van Loosdrecht, tel. 050-363 8149, e-mail: p.h.m.van.loosdrecht rug.nl