Novel coatings boost solar cell efficiencies

[OKIsItJustMe]

(Feature Article)

http://www.engineerlive.com/features/20105/novel-coatings-boost-solar-cell-efficiencies.thtml

March 28, 2008

Novel coatings boost solar cell efficiencies

Low efficiency and high production costs have always dogged the development of solar cells. Now scientists in Switzerland and the US are tackling these problems with novel coating and sensitising solutions.

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Some years ago, Grätzel developed photoelectrochemical solar cells that are inexpensive, easy to produce, and able to withstand long exposure to light and heat. These ‘Grätzel cells’ contain a mesoscopic layer of titanium oxide (TiO2) particles coated with a sensitising dye.

Upon irradiation with light, electrons are injected from the dye adsorbed on the TiO2, which are then transferred to the conducting band of the TiO2 and collected at the back contact, and carried away by an external circuit. In order for the cell to work, the electrons that are injected into the TiO2 must not recombine with the oxidised dye.

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n contrast to earlier approaches for anode coating, the Northwestern nickel oxide coating is cheap, electrically homogeneous and non-corrosive. In the case of model bulk-heterojunction cells, the Northwestern team has increased the cell voltage by approximately 40 per cent and the power conversion efficiency from approximately 3 to 4 per cent to 5.2 to 5.6 per cent.

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http://www.northwestern.edu/newscenter/stories/2008/02/solarenergy.html

February 26 | Research

MEDIA CONTACT: Megan Fellman at 847-491-3115 or [e-mail]

Special Coating Greatly Improves Solar Cell Performance

EVANSTON, Ill. --- The energy from sunlight falling on only 9 percent of California’s Mojave Desert could power all of the United States’ electricity needs if the energy could be efficiently harvested, according to some estimates. Unfortunately, current-generation solar cell technologies are too expensive and inefficient for wide-scale commercial applications.

A team of Northwestern University researchers has developed a new anode coating strategy that significantly enhances the efficiency of solar energy power conversion. A paper about the work, which focuses on “engineering” organic material-electrode interfaces in bulk-heterojunction organic solar cells, is published online this week in the Proceedings of the National Academy of Sciences (PNAS).

This breakthrough in solar energy conversion promises to bring researchers and developers worldwide closer to the goal of producing cheaper, more manufacturable and more easily implemented solar cells. Such technology would greatly reduce our dependence on burning fossil fuels for electricity production as well as reduce the combustion product: carbon dioxide, a global warming greenhouse gas.

Tobin J. Marks, the Vladimir N. Ipatieff Research Professor in Chemistry in the Weinberg College of Arts and Sciences and professor of materials science and engineering, and Robert Chang, professor of materials science and engineering in the McCormick School of Engineering and Applied Science, led the research team. Other Northwestern team members were researcher Bruce Buchholz and graduate students Michael D. Irwin and Alexander W. Hains.

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http://www.pnas.org/cgi/content/full/105/8/2783

Published online on February 19, 2008, 10.1073/pnas.0711990105
PNAS | February 26, 2008 | vol. 105 | no. 8 | 2783-2787

PHYSICAL SCIENCES / CHEMISTRY

p-Type semiconducting nickel oxide as an efficiency-enhancing anode interfacial layer in polymer bulk-heterojunction solar cells

Michael D. Irwin, D. Bruce Buchholz, Alexander W. Hains, Robert P. H. Chang*, and Tobin J. Marks*

Department of Chemistry, Department of Materials Science and Engineering, and Materials Research Center, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208

Contributed by Tobin J. Marks, December 21, 2007 (received for review September 12, 2007)

Abstract

To minimize interfacial power losses, thin (5–80 nm) layers of NiO, a p-type oxide semiconductor, are inserted between the active organic layer, poly(3-hexylthiophene) (P3HT) + <6,6>-phenyl-C₆₁ butyric acid methyl ester (PCBM), and the ITO (tin-doped indium oxide) anode of bulk-heterojunction ITO/P3HT:PCBM/LiF/Al solar cells. The interfacial NiO layer is deposited by pulsed laser deposition directly onto cleaned ITO, and the active layer is subsequently deposited by spin-coating. Insertion of the NiO layer affords cell power conversion efficiencies as high as 5.2% and enhances the fill factor to 69% and the open-circuit voltage (V_oc) to 638 mV versus an ITO/P3HT:PCBM/LiF/Al control device. The value of such hole-transporting/electron-blocking interfacial layers is clearly demonstrated and should be applicable to other organic photovoltaics.

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Fig. 1. Schematic drawings of the bulk-heterojunction photovoltaic device structures used here without (A) and with (B) an interfacial electron-blocking layer/hole-transporting layer (EBL/HTL). The chemical structures of PEDOT:PSS and the active layer components P3HT and PCBM are also shown.


Fig. 2. Energy level diagrams of device components referenced to the vacuum level. (A) Typical P3HT:PCBM BHJ OPV with a PEDOT:PSS hole transport layer. (B) Device structure reported here. The published valence band, conduction band, and the Fermi level energies of NiO are shown.

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Fig. 3. Glancing-angle x-ray diffraction patterns of a NiO film grown on ITO/glass and the ITO/glass background. Features are labeled with the corresponding (hkl) reflections of cubic phase NiO.


Fig. 4. Optical transmission spectra of various-thickness NiO films grown on ITO/glass. ITO/glass is included as the blank.

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