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May 2011

Improving Solar Cell Performance: One More Solvent

Researchers use solvent additives to induce dramatic improvements in film formation and organic photovoltaic performance

Thomas van der Poll

Images from conducting atomic force microscopy of a two-component organic film a) without and b) with additive. Each image is of a 4-µm x 4-µm area, and the contrast represents differences in current, represented in pico-amps. In this example, the film with additive shows a six-fold increase in current.

The performance of organic or carbon-based solar cells can be greatly improved by using a second solvent, or additive, during the processing of the organic materials, according to scientists at the University of California, Santa Barbara working in the Center for Energy Efficient Materials. The team gained insight into what that additive does using highly specialized imaging techniques. Their research was published in Advanced Energy Materials.

Organic solar cells can be prepared by dissolving organic light-absorbing materials that actually convert light into energy in an organic solvent and depositing that solution onto an electrode (later to be sandwiched by another). The resulting films are thinner than paper, flexible, lightweight and inexpensive compared to their inorganic counterparts. In the future, one can imagine having printed OSCs in any shape for any use; however, the full potential of OSCs has yet to be unlocked, and research is ongoing.

One factor that dictates how well an OSC device will perform is how the different organic components organize when they become a solid film. This is especially challenging because most OSCs have more than one organic material, and ideally the materials should be in contact but not mixed. Scientists spend months optimizing the preparation of a device, tuning variables like the concentration of organic materials, different types of filters for solutions or heating films to allow materials to crystallize. What scientists at Center for Energy Efficient Materials have done is add a tool to the metaphorical toolbox for fabricating OSCs.

In a film deposited without an additive, one often sees large globs of one type of material, or conversely, that everything has mixed completely. But mix two solvents with different properties and two organic materials and something interesting happens – in the deposited film, organic materials tend to form their own discrete phases with good contact. Basically, with good contact comes better conduction, and using additives increased the current produced by devices prepared at the Center for Energy Efficient Materials, and in turn the efficiency, more than three-fold.

More Information

Hoven CV, X Dang, RC Coffin, J Peet, TQ Nguyen, and GC Bazan. 2010. “Improved performance of polymer bulk heterojunction solar cells through the reduction of phase separation via solvent additives.” Advanced Energy Materials22, E63-E66. DOI: 10.1002/adma.200903677.

Acknowledgments

Materials synthesis and characterization was funded by the Institute for Collaborative Biotechnology at University of California, Santa Barbara. Device fabrication and characterization was funded by the Center for Energy Efficient Materials, a DOE Energy Frontier Research Center.

About the author(s):

  • A member of the Center for Energy Efficient Materials, an Energy Frontier Research Center, Thomas van der Poll is studying the surface modification of metal oxides used for organic electronics.

Tiny Solar Cells

Second additive creates more efficient collectors

Images from conducting atomic force microscopy of a two-component organic film a) without and b) with additive. Each image is of a 4-µm x 4-µm area, and the contrast represents differences in current, represented in pico-amps. In this example, the film with additive shows a six-fold increase in current.

Imagine a power supply that is cheap, lightweight and thinner than a sheet of paper. That’s the goal of creating organic solar cells. These cells use a carbon-based compound to convert sunlight into electricity. The challenge is efficiency. Organic solar cells only convert about 5% of the sunlight they capture, compared to 10 to 20% achieved with more expensive, bulky inorganic cells. Scientists found that using additives in the organic cell manufacturing process can increase efficiency by 3 fold. With the additive, the molecules in the carbon-based films become more ordered, which is essential for extracting current from the cell. The research was done by the Center for Energy Efficient Materials, a DOE Energy Frontier Research Center led by the University of California at Santa Barbara.

More Information

Hoven CV, X Dang, RC Coffin, J Peet, TQ Nguyen, and GC Bazan. 2010. “Improved performance of polymer bulk heterojunction solar cells through the reduction of phase separation via solvent additives.” Advanced Energy Materials22, E63-E66. DOI: 10.1002/adma.200903677.

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.