Small-molecule solar cells get 50% increase in efficiency with optical spacer

August 14, 2013 by Lisa Zyga feature
(Left) A ZnO optical spacer viewed under an atomic force microscope. (Center) The device structure of the small-molecule solar cell. (Right) Solar cell performance with and without an optical spacer. Credit: Aung Ko Ko Kyaw, et al. ©2013 American Chemical Society

(Phys.org) —In the world of organic solar cells, polymer-based devices may currently be at the top, but other organic materials such as "small molecules" also prove to be promising. Although small-molecule organic solar cells currently have lower efficiencies than polymer solar cells, they are generally easier to fabricate and their efficiencies are improving.

In a new study, researchers have shown that they can increase the efficiency of one type of small-molecule organic solar cell from 6.02% to 8.94% simply by tuning the thickness of the active layer and inserting an optical spacer between the active layer and an electrode. The efficiency improvement demonstrates that small-molecule solar cells have the potential to compete with their polymer counterparts, which have efficiencies approaching 10%.

The researchers, led by Alan J. Heeger at the University of California at Santa Barbara, have published their paper on the efficiency improvement in small-molecule solar cells in a recent issue of Nano Letters.

As the scientists explain in their paper, small-molecule have several advantages over solar cells: relatively simple synthesis, high charge , similarly sized particles (monodispersity), and better reproducibility, among others. However, small-molecule solar cells have so far achieved top efficiencies of about 8%, lagging somewhat behind the best polymer devices.

By demonstrating how a few simple changes can increase the efficiency of one type of small-molecule organic solar cell by nearly 50%, the scientists here have shown that these devices still have the potential for vast improvements.

Tuning the thickness of the active layer and inserting a optical spacer between the active layer and enable the active layer to harvest more light, increasing . The insertion of the optical spacer places the active layer in a more favorable position within the optical electric field within the cell. As the scientists explained, the optical spacer contributes to increased light absorption in three ways: increasing the charge collection efficiency, serving as a blocking layer for holes, and reducing the recombination rate.

More information: Aung Ko Ko Kyaw, et al. "Improved Light Harvesting and Improved Efficiency by Insertion of an Optical Spacer (ZnO) in Solution-Processed Small-Molecule Solar Cells." Nano Letters. DOI: 10.1021/nl401758g

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