Removing Structural Disorder from Oriented TiO2 Nanotube Arrays: Reducing the Dimensionality of Transport and Recombination in Dye-Sensitized Solar Cells
We report on the influence of morphological disorder, arising from bundling of nanotubes (NTs) and microcracks in films of oriented TiO{sub 2} NT arrays, on charge transport and recombination in dye-sensitized solar cells (DSSCs). Capillary stress created during evaporation of liquids from the mesopores of dense TiO{sub 2} NT arrays was of sufficient magnitude to induce bundling and microcrack formation. The average lateral deflection of the NTs in the bundles increased with the surface tension of the liquids and with the film thicknesses. The supercritical CO{sub 2} drying technique was used to produce bundle-free and crack-free NT films. Charge transport and recombination properties of sensitized films were studied by frequency-resolved modulated photocurrent/photovoltage spectroscopies. Transport became significantly faster with decreased clustering of the NTs, indicating that bundling creates additional pathways via intertube contacts. Removing such contacts alters the transport mechanism from a combination of one and three dimensions to the expected one dimension and shortens the electron-transport pathway. Reducing intertube contacts also resulted in a lower density of surface recombination centers by minimizing distortion-induced surface defects in bundled NTs. A causal connection between transport and recombination is observed. The dye coverage was greater in the more aligned NT arrays, suggesting that reducing intertube contacts increases the internal surface area of the films accessible to dye molecules. The solar conversion efficiency and photocurrent density were highest for DSSCs incorporating films with more aligned NT arrays owing to an enhanced light-harvesting efficiency. Removing structural disorder from other materials and devices consisting of nominally one-dimensional architectures (e.g., nanowire arrays) should produce similar effects.
- Research Organization:
- National Renewable Energy Lab. (NREL), Golden, CO (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC36-08GO28308
- OSTI ID:
- 982263
- Journal Information:
- Nano Letters, Vol. 7, Issue 12, 2007
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
77 NANOSCIENCE AND NANOTECHNOLOGY
CHARGE TRANSPORT
CONVERSION
DEFECTS
DENSITY
DIMENSIONS
DRYING
DYES
EFFICIENCY
EQUIPMENT
EVAPORATION
FILMS
JOINTS
LIQUIDS
MATERIALS
MOLECULES
NANOTUBES
PHOTOCURRENTS
RECOMBINATION
SOLAR CELLS
SURFACE AREA
SURFACE TENSION
SURFACES
TRANSPORT
Materials Science and Semiconductors