Nano-patterning gives polymer solar cells a big boost

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Polymer solar cells are still in their infancy compared to their silicon-based counterparts, but thanks to their low cost and rapidly advancing efficiency, the outlook is encouraging for cheap end-user power generation. Researchers have developed a new nano-patterned array production technique that showed a roughly seven-fold increase in efficiency when compared to the traditional sandwich-style construction.

The important bits of the physics that go into generating energy in a solar cell only happen at the interface of the electron donor and receptor layers, which is a few nanometers thick, so optimizing this interface is an important area of study. In a new study, researchers used a piece of aluminum oxide to pattern the polymer comprising the electron donor part of the solar cell, P3HT. The P3HT was drawn into a honeycomb-like array of nanometer pores using vacuum and capillary forces—the resultant structure was a 30nm thick film of P3HT with a nano-forest of pillars roughly 150nm tall and 75nm thick.

Backfilling the P3HT array with the electron receptor, C60, created a complete heterojunction that could then be used in a solar cell, with an interface area 2.6 times greater than if it were flat. Thanks to the use of aluminum oxide as a patterning material, the P3HT polymer chains would align themselves in a stacked array, which greatly increased the conductivity of the pillar. The conductivity increase, coupled with the higher interfacial area yielded a an efficiency 6.6 times greater than a planar configuration of the same donor and receptor polymers.

While the absolute efficiency of the new array—just 1.12 percent—is not cutting edge, the patterning technique is cheap and can be done on a large scale, and is unlikely to be limited to just this material system. Other recent polymer cells have claimed efficiencies of 5.5 percent, for example, and the micro- and nano-pillar approach works with traditional photovoltaic materials, too. There is still much work to be done in the optimization of the processing conditions, but this is yet another piece of the puzzle that may make polymer solar cells a viable option for power generation.

Advanced Functional Materials, 2010. DOI:10.1002/adfm.200901760

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