Solar Cell Researcher Explores Nanotech Possibilities

National Science Foundation Grant Aids Quest for Low-Cost, Flexible Solution

Oct. 5, 2009

A UT Dallas researcher envisions a time soon when plastic sheets of solar cells are inexpensively stamped out in factories and then affixed to cell phones, laptops and other power-hungry mobile devices.

And a new $330,000 grant from the National Science Foundation should help him come closer to realizing that vision.

Many researchers are investigating the development of flexible solar cells in hopes of improving efficiency and lowering manufacturing costs, however Walter Hu’s novel approach would use nanoimprint lithography to produce precisely nanostructured devices rather than using chemical methods of manufacturing.

Nanoimprinting is an emerging technology that’s been used to produce various electronic and optical devices by imprinting the pattern from a mold onto a surface. But Dr. Hu’s team is exploring how the thermal imprinting can not only impart a pattern to the solar cell material but also change properties of the material in ways that maximize light absorption, increasing the efficiency of the resulting cells well beyond what anyone has done to date with these so-called organic solar cells.

That requires exploring all of the interrelated properties of the solar cell material’s plastic polymer surface (such as crystallinity, molecular orientation, stability and thermal dynamics) and their correlation with geometry, imprint conditions, surface effects and the quantum efficiency of the resulting solar cells.

“Scientifically, we would like to understand – materials-wise, engineering-wise – how we can solve the problem of engineering materials at the nanoscale to improve solar cell performance,” said Dr. Hu, an assistant professor of electrical engineering in the Erik Jonsson School of Engineering and Computer Science at UT Dallas.

Understanding these basic questions is important to developing new methods to improve the power conversion efficiency as well as the stability of next-generation solar cells, he added.

His team is collaborating with J.C. Hummelen at the University of Groningen, Netherlands, who is internationally recognized for the creation of PCBM, a popular nanomaterial for organic solar cells. Dr. Hummelen will customize those materialls to fit the nanoimprint process. Dr. Hu’s group also works closely with Anvar Zakhidov, a renowned expert in solar cells at the UT Dallas Nanotech Institute, to study various material systems and device architectures.

Dr. Hu’s team is also addressing energy payback, which is the time needed once a solar-cell system is operable to both recover the amount of energy used to manufacture it and offset carbon emissions from that manufacturing.

Conventional silicon-based solar cells are manufacturing-intensive. The energy payback for these systems is two to four years. The organic solar cells his team is developing would have an energy payback that can be measured in a few months.

“The fundamental science of this project is very interesting to us,” he said. “Plus it’s a great project for training graduate students to do advanced engineering research.”


Media Contact: The Office of Media Relations, UT Dallas, (972) 883-2155, newscenter@utdallas.edu

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A researcher holds a sample of prototyped solar cells. The inset is an electron micrograph of the nanostructures.

Walter Hu (right) has been aided in his research by graduate students Mukti Aryal and Mindy Zhou.

Photovoltaic Cell
Production Issues

Power-Conversion Efficiency
vs. Manufacturing Cost

The trick to creating the ideal, practical solar cell is to reduce costs while maintaining relatively-high power-conversion-efficiency (PCE) rates. These seem to be, however, mutually exclusive goals. The cheaper the manufacturing costs of a solar cell, the less efficient the product. The more efficient the solar cell, the higher the cost.

Conventional crystalline silicon-based solar cells have become highly efficient, with a PCE of more than 20 percent. Even so, they remain expensive to manufacture. Today’s solar cell manufacturers have focused on cost-cutting by using fewer materials, but with a sacrifice on efficiency. For example, organic solar cells use only 100 nm thick material and offer much lower cost, but with a PCE of only 5 percent. The proposed nanostructured solar cells are one attempt to increase efficiency while maintaining low costs.

Energy Payback
Energy payback is the time needed once a solar cell system is operable to recover the amount of energy used to manufacture it and to offset the carbon dioxide and pollution generated while doing so.

Conventional silicon-based wafers remain manufacturing-intensive. The energy payback for these systems is two to four years. Thin film and organic solar cells have an energy payback that can be measured in a few months.

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