Understanding how electrons travel in crystal

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Organic solar cells

Organic solar cells (OPV) are an architect’s dream, because they are pliable, light, and colorful. But researchers still have a way to go to make them robust and efficient. There are many flavors of OPV cells. Typically, they consist of a donor- and acceptor material that is organized in layers (planar), or in random blobs (bulk). Today’s most used architecture is a bulk heterojunction cell using fullerene as acceptor material. The matching donor material may be one of many.

Fundamental research at the heterojunction

Many universities and research centers are doing fundamental research into the nature and functioning of organic solar cells. An ongoing discussion is what exactly happens at the heterojunction. Recently, researchers from Imec and U.S. universities lifted a tip of the veil. Their work was done using heterojunction cells with fullerene as acceptor material and NPD as donor material.

The experiments, conclusion and lessons learned

The researchers made solar cells with varying rates od donor- and acceptor material. From morphological measurements (X-ray diffraction), they learned that this influences the size of the crystals in the fullerene material. In addition, optical measurements revealed the recombination (when electron and holes pair up again). All these parameters were coupled to the cells’ efficiency in turning light into electricity.

From all these measurements, the researchers deduced that a 4nm minimal size of the crystals of the fullerene material led to the best results (best conversion efficiency). They also revealed why this is so. At these sizes, the electrons in the crystal are sufficiently delocalized from the holes to reduce the chances of recombination.

This is best pictured in the following way: when light strikes the interface, a molecule is excited. As a result, an electron loosens from the molecule and starts moving in the crystal. When this crystal is larger than 4nm, the electron is delocalized enough from the molecule so that electron and hole can be pulled away from each other easier, and can move to the respective electrodes. To make this happen as best as possible is what counts in an organic solar cell.

Although this insight is fundamental, the conclusion is important for making organic solar cells. We now know that the fullerene material works best when organized as crystals with a minimal size. In this research, that size was varied by the change of rate of the donor/acceptor materials. But this can also be done by changing the deposition parameters. A second practical lesson is that when researchers look for alternatives for the fullerene material (that have e.g. a better absorption spectrum), they will have to look for materials that readily form crystals.

This is yet another result to boost the research into organic solar cells. As a result, architects and product developers will soon shape the world with a new exciting type of flexible solar cell.

David Cheyns, one of the authors of the paper, is a senior researcher at Imec, a leading nanoelectronics research facility based in Leuven, Belgium.