Frontiers in Energy Research: Winter 2015

Solar Energy Gets Flexible

New electrode and design creates cells that are more scalable and able to bend

Flexible electrodes with an embedded copper metal mesh for organic solar cells were developed by scientists at the Center for Interface Science: Solar Electric Materials.

Solar energy is abundant; however, carbon-based materials to capture that energy efficiently in a solar cell are not as readily available. A new design for a carbon-based, or organic, solar cell offers an exciting solution to this problem by making use of materials that are abundant on Earth while also requiring very little organic material to absorb sunlight. In fact, a 100-nanometer-thick organic layer, 1,000 times thinner than a sheet of paper, will efficiently absorb the sun’s rays. Additionally, these organic solar cells can be fabricated on flexible supports, meaning it might be possible to print solar cells onto different materials.

Currently, carbon-based solar cells, known as organic photovoltaics, are often made using indium-tin-oxide electrodes. While the technology of such cells is well established, the indium compound has significant drawbacks. It is brittle, limiting the overall cell flexibility and potential applications. It is also expensive, reducing the economic competitiveness of the cells. Furthermore, there is a very limited worldwide supply of indium.

So is it possible to work around these drawbacks to capitalize on the high efficiency of organic solar cells? Researchers at the Center for Interface Science: Solar Electric Materials think so. Bernard Kippelen and his team have developed a new type of organic solar cell that eliminates the need for brittle and expensive indium-tin-oxide electrodes without sacrificing efficiency. To conduct electricity, organic solar cells require a transparent electrode (such as indium-tin-oxide) that allows sunlight to pass into the organic layer. To circumnavigate this, Kippelen’s team embedded a thin, copper-based mesh into a plastic base layer and then deposited a conducting polymer layer and organic semiconductor layers on top of the plastic. The embedded grid with the conducting polymer overcoat replaces the indium-tin-oxide electrode, but because the grid is embedded in the substrate it does not get in the way during the subsequent deposition of the thin organic layers.

There are many benefits of this design. In addition to eliminating brittle indium-based electrodes and their economic challenges, this electrode design with an embedded metal grid allows for reduction of the resistive power losses. The high conductivity of the copper used for the metal mesh improves the overall efficiency of the cell.

Solar panels, like the ones we see on rooftops, are composed of individual solar cells linked together. Ideally, a solar panel would be large, so that it can generate more power. However, there is a limit to the number of cells that can be connected in a single panel because each connection increases the electrical resistance of the panel. These new cells are less resistive than traditional organic cells, so they can be made bigger without significant loss in efficiency. As Kippelen explains, “This device enables area scaling without paying a penalty in power conversion.”

In addition to being more scalable than indium-tin-oxide cells, the new design is flexible, allowing for numerous potential applications. Because they are so thin, flexible, and semitransparent, these devices could ultimately be applied to windows, absorbing only some of the sunlight and using it to power the building.

Kippelen notes this is just the beginning for these devices. “It’s not ready for prime time,” he cautions, “but this is sufficiently interesting and intriguing that it has a lot of potential.” He and his team plan to probe that potential in the coming years with funding in part from the Office of Naval Research, pushing these materials further, improving their efficiency, and ultimately providing a reliable source of solar energy to address our growing renewable energy needs.

Acknowledgments: 

This research was funded in part through the Center for Interface Science: Solar Electric Materials, an Energy Frontier Research Center funded from 2009 to 2014 by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

More Information: 

Choi S, Y Zhou, W Haske, JW Shim, C Fuentes-Hernandez, and B Kippelen. 2015. "ITO Free Large Area Flexible Organic Solar Cells with an Embedded Metal Grid." Organic Electronics 17:349-354. DOI: 10.1016/j.orgel.2014.12.029

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.

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