Key parameters for high-efficiency plastic solar sells revealed

By Angela HardinAugust 1, 2010

Since their discovery about fifteen years ago, plastic solar cells made from semiconducting polymers and fullerene derivatives ("buckyballs") have shown promise for future commercialization due to their low cost, ease of fabrication, and small environmental impact (relative to silicon and heavy metal-based semiconductor counter parts, the majority of current solar cells).

However, a few obstacles remain, such as low device efficiency below 5 percent—compared to more than 12 percent for commercial silicon-based devices—and short shelf lifetime. It has been recognized in general that a high power conversion efficiency (PCE) in such plastic solar cells (also referred to as bulk heterojunction solar cells) requires more efficient light harvesting, exciton splitting, charge carrier generation and transport and collection.

Recently, a polymer poly(thienothiophene-benzodithiophene) (PTB) synthesized by Luping Yu of the University of Chicago yielded unprecedented ~8% PCE when blended in a bulk heterojunction solar cell—but the reasons for such a significant improvement was initially unclear.

Using the Advanced Photon Source at Argonne National Laboratory, researchers in Argonne's Chemical Sciences and Engineering Division revealed the structural origins of the solar cells by probing polymer packing and interfacial structures at the electrode surface through the grazing angle x-ray scattering method that examines the thin film or interfacial layer selectively.

Seven PTB polymers having the same zigzag alternating thienothiophene (TT) and benzodithiophene (BDT) backbone sequence but various side chains were investigated; their corresponding solar cell PCEs range from 2 - 8 percent.

The strikingly different packing structures of these polymers in solar cells compared to the most commonly studied polymers have been revealed:

  1. the PTB polymer backbones assembling zigzag ribbons are all "face-down", or laying flat on the electrode surface, instead of "edge down", maximizing the contact with the electrode in favor of the charge transport to the electrode.
  2. These zigzag ribbon-like backbones are stacked on top of each other, forming channels in favor of charge carrier transport.

Meanwhile, these polymer blends in solar cells conduct very efficient exciton splitting to generate holes and electrons with very little trapping (one of the mechanisms for poor solar cell performance).

These discoveries inspire a new set of structural parameters to be optimized for high efficiency solar cell performance leading to future commercialization of plastic solar cells.

The researchers on this project at Argonne were Jianchang Guo, Jodi Szarko, Byeongdu Lee, Brain S. Rolczynski and Lin X. Chen from the Chemical Sciences and Engineering Division and X-ray Science Division in collaboration with Yongye Liang, Hea Jun Son and Luping Yu from the University of Chicago. The Argonne part of this work was supported by the U.S. Department of Energy.

Citation: Jianchang Guo, Yongye Liang, Jodi Szarko,  Byeongdu Lee, Hae Jun Son, Brian S. Rolczynski, Luping Yu, and Lin X. Chen, Journal of Physical Chemistry B 2010, 114, 742-748.