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TiO2 Nanotubes Improve Solar Cells

4/23/2008

Quantum dots, which are nanoscale semiconductors, have been identified as important light harvesting material for building highly efficient solar cells. When quantum dots are exposed to light at certain wavelengths, they can generate free electrons and create an electrical current. Now, by combining spectroscopic and photoelectrochemical techniques, researchers at Notre Dame University in Indiana have demonstrated size-dependent charge injection from different-sized cadmium selenide (CdSe) quantum dots into titanium dioxide (TiO₂) nanoparticles and nanotubes, showing a way to maximize the light absorption of quantum dot-based solar cells.

Termed 'rainbow solar cells', these next-generation solar cells consist of different size quantum dots assembled in an orderly fashion. Just as a rainbow displays multiple colors of the visible light spectrum, the 'rainbow solar cell' has the potential to simultaneously absorb multiple wavelengths of light and convert it to electricity in a very efficient manner. The ability to tune the photoelectrochemical response and photoconversion efficiency is possible via size control of CdSe quantum dots. The smallest quantum dots (absorbing the shortest wavelengths of the solar spectrum) perform the best because they move electrons through the cell (i.e. create current) at the fastest rate. Size selection in the 2-8 nm region allowed selectively tuning of the photocurrent response in the visible region.

The photoconversion efficiency is improved by facilitating the charge transport through TiO₂ nanotube architecture. The maximum IPCE (photon-to-charge carrier generation efficiency) obtained with 3-nm diameter CdSe nanoparticles was 35% for particulate TiO₂ compared to 45% for tubular TiO₂ morphology. By utilizing a tubular titanium dioxide base instead of a spherical titanium dioxide base, electrons generated by the quantum dots travel more freely within the cell because a tube provides an elongated direct pathway for electron travel, whereas multiple spheres permit electron travel only if the electron "hops" between them, potentially slowing electron transport and hindering overall cell efficiency.

The maximum IPCE observed at the excitonic band increases with decreasing particle size, whereas the shift in the conduction band to more negative potentials increases the driving force and favors fast electron injection. The maximum power-conversion efficiency 1% obtained with CdSe-TiO₂ nanotube film highlights the usefulness of tubular morphology in facilitating charge transport in nanostructure-based solar cells. This research is supported by Basic Energy Sciences of the US Department of Energy.