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October 2012

Now Is the Time to Design a Novel Dye in Solar Cells

Design tools allow scientists to create a novel organic dye with an efficient light to electric energy conversion

Gyu Leem

Insight into the functioning of organic photosensitizing dyes for DSSCs was provided by combined computational and experimental design approach. The figure shows chemical structure of a novel organic dye, EB-01, and its design concept based on the dye-design principles. EB-01 has an extended donor/acceptor based pi conjugated systems (D-π-A structure) leading to binding excitons loosely. It resulted in highly efficient light harvesting.

Researchers at the Center for Solar and Thermal Energy Conversion (CSTEC) have explored molecular organic dye design tools for photocurrent generation efficiency in dye-sensitized solar cells (DSSCs) and developed a novel organic dye that exhibits excellent efficiencies and photocurrents. 

The need for environmentally sustainable and renewable energy technologies relies on natural resources such as sun and wind. Solar cells directly convert energy from the sun to electricity. In recent years, DSSCs have emerged as a low-cost and environmentally friendly alternative to conventional silicon-based solar cells. Organic dyes make DSSCs sensitive to sunlight and produce power by means of a porous semiconductor film. Upon light absorption, the organic dye molecule goes to its excited state and subsequently injects electrons into the film. The leftover holes are transported to another electrode through electrolytes. The circulation of electrons into DSSCs results in the production of electricity. Thus, solar energy is converted to electrical energy and stored.

To improve the performance of DSSCs, previous studies have examined adding an electron donor/acceptor system, technically called a D-π-A structure, into the organic dye framework. However, compared to similar structures without electron-withdrawing acceptor groups, they don't convert as much sunlight to energy. The CETEC researchers developed a reliable dye-design tool and synthesized a novel organic dye with higher efficiency for DSSCs.

The reliable dye-design tool clarifies how organic dyes are more efficient. Specifically, the researchers discovered the relationship between metal-free organic dyes' exciton binding energy and its photocurrent efficiency by using reliable computational tools and complex calculations. An exciton, an electron/hole pair, transports energy. Researchers inserted a designed molecular structure into the organic dyes; they hold excitons loosely, resulting in a more efficient light-to-electricity conversion for better performing DSSCs. The researchers revealed that the exciton binding energy is inversely related to the maximum efficiency of the dyes. With their well-established dye-design principles, the researchers developed a novel organic dye, termed EB-01. It showed an excellent light-to-electricity conversion efficiency and outstanding power conversion efficiency.  

 "Achieving power conversion efficiency over 9 percent from a metal-free organic dye through the computational/experimental hybrid type of approach was very exciting. Now, we have better understanding on rational dye design for excitonic solar cells," remarks Jinsang Kim, associate professor at University of Michigan, who led the team of researchers at CSTEC.

With this information, scientists can optimize dyes through molecular design to increase efficiency of the light-to-electricity conversion. This new tool and research provides a new path to develop novel organic dyes with better performance for DSSCs.

Acknowledgments

This research was funded by the Center for Solar and Thermal Energy Conversion in Complex Materials, an Energy Frontier Research Center funded by the Department of Energy, Office of Science, Office of Basic Energy Sciences.

More Information

Kim BG, CG Zhen, EJ Jeong, J Kieffer, and J. Kim. 2012. "Organic Dye Design Tools for Efficient Photocurrent Generation in Dye-Sensitized Solar Cells: Exciton Binding Energy and Electron Acceptors." Advanced Functional Materials 22:1606-1612. DOI: 10.1002/adfm.201101961

About the author(s):

  • Gyu Leem is a postdoctoral research associate in Kirk Schanze’s group at the University of Florida and a member of the Center for Solar Fuels, an Energy Frontier Research Center.  He received his Ph.D. in chemistry from the University of Houston in 2008.  Before joining Schanze’s research group in 2012, he worked in the division of petrochemical and polymer at LG Chem.  Currently, his research interests are in designing and synthesizing polymer assemblies for potential use as light-harvesting antenna.

Tying Dye’s Structure and Function Together Could Help Solar Cells

Letting energy move more freely could lead to more efficient devices

Insight into the functioning of dyes for dye-sensitized solar cells was provided by a combined computational and experimental design approach. 

Reducing the nation’s imported oil requirements could be done using solar cells that act like green leaves, capturing sunlight and creating energy. Instead of using chlorophyll, which gives leaves their greenish hue, certain low-cost solar cells use a synthetic dye, but these cells produce less electricity than desired. Combining computational and experimental tools, scientists discovered dyes are more efficient when they bind the excitons loosely. Excitons are mobile bits of energy, created by electrons and the holes they leave when they move. The team proved the value of loose binding by adding a molecular structure to a dye that holds excitons strongly and showing that the dye performed poorly. The team’s results provide insight into dyes’ behavior and could lead to more efficient dyes for solar cells. The Center for Solar and Thermal Energy Conversion of Complex Materials, led by the University of Michigan, conducted this work.

More Information

Kim BG, CG Zhen, EJ Jeong, J Kieffer, and J. Kim. 2012. "Organic Dye Design Tools for Efficient Photocurrent Generation in Dye-Sensitized Solar Cells: Exciton Binding Energy and Electron Acceptors." Advanced Functional Materials 22:1606-1612. DOI: 10.1002/adfm.201101961

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.