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September 2011

A Simpler Way to an Efficient Solar Cell

New method yields better solar cells and eliminates cumbersome fabrication steps

Ralph L. House

A new technique allows even distribution of the dye-coated particles in a solar cell. This technique eliminates several processing steps and improves the cell’s efficiency.

Photographs of hydrophilic titanium dioxide particles in water (A) and ethanol (B), which is partially hydrophobic. The hydrophilic particles agglomerate in the ethanol and precipitate, making film preparation cumbersome. Particles coated with the tetraalkylammonium hydroxide remain dispersed in both water (C) and ethanol (D), greatly simplifying the fabrication procedure. (Reprinted with permission from Jeong NC, et al. (2011) Langmuir, 27 (5), 1996-1999. Copyright 2011 American Chemical Society).

A new method for preparing titanium dioxide thin-film electrodes yields efficient solar cells and greatly reduces the number of fabrication steps. The films are highly transparent, making them ideal for use in dye-sensitized solar cells where maximum light penetration to the photosensitive dye is critical. When coupled with the popular ruthenium-based molecular dye, N719, the electrodes produce high photocurrent densities (17.7 mA cm-2) and have a light-to-electrical energy conversion efficiency of 9.6%, about 1.4 times greater than photoelectrodes prepared using commercial titanium dioxide particles. This work, conducted by the Argonne-Northwestern Solar Energy Research Center, greatly simplifies thin-film fabrication for use in dye-sensitized solar cell electrodes without compromising overall efficiency.

Challenges in thin film fabrication: Dye-sensitized solar cells represent one of the most promising advances in solar energy conversion science. A critical component of the dye-sensitized solar cell is a thin semiconducting film that bridges the photosensitive dye molecules to the metallic electrode. The quality of this film is determined in part by particle size, transparency and degree of electrical connectivity. Titanium dioxide nanoparticles are commonly used; however, these films can often yield poor results and are cumbersome to prepare, largely due to particle agglomeration.

Solving the problem: Titanium dioxide particles are generally hydrophilic, which ensures adequate dispersion in aqueous solutions, but agglomeration as the film dries diminishes overall transparency, thereby reducing the solar cell’s efficiency. Pastes prepared in hydrophobic environments can yield better results, but also cause the particles to agglomerate, requiring a number of intermediate steps to achieve adequate dispersion. Lead author, Nak Cheon Jeong, eliminated these steps by coating the particles with hydrophobic tetraalkylammonium ions, giving them both hydrophilic and hydrophobic characteristics; i.e., the particles are amphiphilic. In addition, the particles used were ~9 nanometers in size, yielding superior transparency and high energy-conversion efficiency.

While there are dye-sensitized solar cells that have yielded higher efficiencies, around 11%, using the N719 dye, the work described in this article provides a simple method to produce highly transparent films that also yield highly efficient dye-sensitized solar cells. The simplicity of this method will prove useful to scientists and greatly facilitate progress in solar energy-conversion science.

More Information

Jeong NC, OK Farha, and JT Hupp. 2011. "A Convenient Route to High Area, Nanoparticulate TiO2 Photoelectrodes Suitable for High-Efficiency Energy Conversion in Dye-Sensitized Solar Cells." 27(5), 1996-1999. DOI: 10.1021/la104297s

Acknowledgments

The synthesis and characterization work done by N. C. Jeong was supported by the Argonne-Northwestern Solar Energy Research Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences. Additionally, Jeong was supported by the Korean Research Foundation in the form of a postdoctoral fellowship in place for a fraction of the time the project was underway. Supplemental materials characterization was done by Omar K. Farha who was funded by Northwestern Nanoscale Science and Engineering Center.

About the author(s):

  • A member of the Solar Fuels Energy Frontier Research Center, Ralph is a Research Associate specializing in the use of multiple spectroscopic techniques to analyze the steps leading to the generation of solar fuels. Ralph is also involved with coordinating projects leading to the fabrication of a prototype solar fuels device and is the UNC-EFRC Liaison for External Outreach and Collaboration.

Solar Cells Made Easy

New technique cuts fabrication time, improves performance for turning sunlight into electricity

A new technique allows even distribution of the dye-coated particles in a solar cell. This technique eliminates several processing steps and improves the cell’s efficiency.

To make solar power mainstream, the devices that collect sunlight and pump out electricity need to be a bit more effective. Take, for example, the hardy, dye-sensitized solar cell. Slightly improving the efficiency of this already cheap solar cell could expand its use in different markets, pushing out fossil fuel power supplies. To improve the efficiency of these solar cells, scientists are tackling the thin film of dye-coated particles that turn sunlight into hard-working electrons. The film suffers because, instead of spreading out evenly to capture more light, the particles clump together. In a recent study, scientists coated the particles with a chemical that allows even mixing and drying on a surface. This new approach also increases the efficiency of the cell, allowing it to capture more light and produce more electricity. This research is a major step in creating efficient, affordable films for solar cells. The work was done by the Argonne-Northwestern Solar Energy Research Center, led by Northwestern University.

More Information

Jeong NC, OK Farha, and JT Hupp. 2011. "A Convenient Route to High Area, Nanoparticulate TiO2 Photoelectrodes Suitable for High-Efficiency Energy Conversion in Dye-Sensitized Solar Cells." 27(5), 1996-1999. DOI: 10.1021/la104297s

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.