Re-aligning Molecules in Organic Solar Cells raises Efficiency

Molecules in face-on orientation inside organic solar cell.Energy is handled more efficiently in three-dimensional organic solar cells if the molecules are aligned face-on (shown in the illustration) instead of edge-on, according to a new study from North Carolina State University and UNC-Chapel Hill. The result may help design and manufacture more efficient and cost effective organic solar cells.

In bulk heterojunction organic solar cells, efficiency hinges on how easy with an exciton (the energy particle produced when light is absorbed by the cell) can find the interface between the donor and acceptor molecules within the cell.

At the interface, the exciton is converted into charges that travel to the electrodes, producing electrical power. But molecules in the donor and acceptor layers can get mixed up, cluster into domains, or both, leading to variances in domain purity and size which can affect the power conversion process.

Also, the donor and acceptor molecules have distinct shapes. How they are oriented relative to each other is also important. This intricacy makes it very hard to measure the key characteristics of their structure.

Efficient Molecular Structure

A team of scientists from both universities investigated the molecular composition of solar cells in order to determine what aspects of the structures have the most impact on efficiency.

“A face-on orientation is thought to allow favorable interactions for charge transfer and inhibit recombination, or charge loss, in organic solar cells,” NC State physicist Harald Ade says, “though precisely what happens on the molecular level is still unclear.

A heterojunction is the interface that occurs between two layers or regions of dissimilar crystalline semiconductors. These semiconducting materials have unequal band gaps as opposed to a homojunction. It is often advantageous to engineer the electronic energy bands in many solid state device applications including semiconductor lasers, solar cells and transistors.

The current project used advanced soft X-ray techniques to describe the orientation of molecules within the donor and acceptor materials. Through the manipulating of this orientation in different solar cell polymers, they were able to show that a face-on alignment between donor and acceptor was much more efficient in generating power than an edge-on alignment.

“Donor and acceptor layers don’t just lie flat against each other,” Ade said. “There’s a lot of mixing going on at the molecular level. Picture a bowl of flat pasta, like fettucine, as the donor polymer, and then add ‘ground meat,’ or a round acceptor molecule, and stir it all together. That’s your solar cell. What we want to measure, and what matters in terms of efficiency, is whether the flat part of the fettuccine hugs the round pieces of meat — a face-on orientation — or if the fettuccine is more randomly oriented, or worst case, only the narrow edges of stacked up pasta touch the meat in an edge-on orientation. It’s a complicated problem.This research gives us a method for measuring this molecular orientation, and will allow us to find out what the effects of orientation are and how orientation can be fine-tuned or controlled.”

Reference:

John R. Tumbleston, Brian A. Collins, Liqiang Yang, Andrew C. Stuart, Eliot Gann, Wei Ma, Wei You, Harald Ade.
The influence of molecular orientation on organic bulk heterojunction solar cells.
Nature Photonics, 2014; DOI:10.1038/nphoton.2014.55

Image courtesy of NC State University. Artist: Peter Allen