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Department of Chemistry Jeanne McHale Group

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Dye-sensitized solar cells (DSSCs) are potentially economical and environmentally  friendly alternatives to silicon based solar photovoltaics. (See O’Regan, B.; Grätzel, M. “A low-cost, high-efficiency solar cell based on dye-sensitized colloidal titanium dioxide films,” Nature 1991, 353, 737-40.) However, reliance on ruthenium based metal-organic dyes to sensitize nanocrystalline TiO2 greatly mitigates their economic and environmental advantages. In our lab, we are replacing the costly ruthenium-containing sensitizers with plant pigments from the betalain family, which are found in plants of the order Caryophyllales:  such as beets, bougainvillea, amaranth and cactus pear.

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Betalains are found in nature in two forms: as purple compounds, called betacyanins, or yellow ones called betaxanthins. The former vary as to the sugar group R1 or R2, attached to the aromatic ring, and the latter are Schiff’s base adducts of betalamic acid with various amino acids.
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We have focused on the pigment betanin obtained from extracting red beet root. The first students to work on this project included Suzanne Lanier, Dongshe Zhang, Jon Downing, shown below with Prof. Jeanne McHale and Dr. Fritz Knorr.

JOnJeanneSuzanneDongsheFritzJonWe published the first report of using betalains in solar cells in 2008:

“Betalain Pigments for Dye-Sensitized Solar Cells,” Dongshe Zhang, Suzanne Lanier, Jonathan A. Downing, Jason Avent, June Lum, and Jeanne L. McHale,” J. Photochem. Photobio. A 2008, 195, 72-80.Since then we have made improvements to the performance of betanin-based DSSCs and have tested other sources of betalain pigments.
Betanin-based DSSCs show exceptional light harvesting owing to a broadened absorption spectrum when absorbed on TiO2, as shown below.
SpectraIPCE2 The figure at left shows the structure of betanin obtained from beet root, and its absorption spectrum in solution and adsorbed on nanocrystalline TiO2. The wavelength-dependent IPCE data (“incident photon-to-current conversion efficiency”) shows the quantum yield for converting incident photons into electrons, which can be quite high for betanin-based DSSCs. Note that IPCE has wavelength dependence similar to that of the absorption spectrum.  We have observed power conversion efficiencies in these solar cells to be as high as 2.7%, the highest efficiency that has been obtained to date for a DSSC sensitized by a plant pigment.
JustCody The data above is from: “Improved efficiency of betanin-based dye-sensitized solar cells,” Cody Sandquist and Jeanne L. McHale, J. Photochem.  Photobiol. A 2011, 221, 90-97.  Cody earned a masters degree in chemistry in 2010 and is now employed at Micron.

Using natural dyes as sensitizers has advantages and disadvantages. The potential advantage is that organic molecules can undergo two-electron, two-proton redox chemistry resulting in higher IPCE than a metal-centered dye which undergoes one-electron redox chemistry. However, the oxidation of organic compounds can be irreversible, which is detrimental to the stability of a DSSC. To test the stability of a betanin-based solar cells, Candy Mercado (PhD, 2012, now employed at the National Renewable Energy Lab), used to betanin-sensitized solar cells installed in the window of our lab to run a small clock, as shown below.

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Several current group members are working on this project.

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Riley Rex is working on surface modifications that will inhibit recombination of injected electrons with oxidized dye.

 

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Deborah Malamen is refining the chromatographic purification of  purple and yellow beet pigments.
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Nick Treat is using screen-printing techniques to prepare TiO2 films, and exploring the performance of yellow and purple betalains from various plants.

 

MarthaMPLC Martha Cuevas-Ramos, an undergraduate student, is working on the separation of pigments from red Hopi dye, a type of amaranth, using medium pressure liquid chromatography.  She has some very interesting preliminary results on the efficiency of solar energy conversion using the pigment amaranthin.

 

 

We are using our beet-based solar cells as science outreach projects. In this photo, Jeanne, Cody and Chris Rich visited the Skwant Summer Science Camp at the Paschal-Sherman Indian School in Omak, Washington.  Campers assembled small solar cells and then connected them to power a buzzer.We are presently partnering with the Palouse-Clearwater Environmental Institute to develop a solar energy demonstration that can be used in regional middle schools and high schools.