Applied Physics

Carrier Trapping

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Science  29 Oct 2010:
Vol. 330, Issue 6004, pp. 563
DOI: 10.1126/science.330.6004.563-c
CREDIT: RICHTER AND SCHMUTTENMAER, NAT. NANOTECHNOL. 5, 10.1038/NNANO.2010.196 (2010)

Ideally, solar cells should have high surface areas at the molecular level, in order to maximize the proportion of sites where light absorption can produce a current carrier. Although networks of nanoparticles can optimize carrier production, they also restrict current flow efficiency on account of scattering at all the interfaces. Nanotube architectures with well-defined flow paths would thus seem to offer the best of both worlds, yet their performance has not met expectations, instead proving little better than that of the particulate assemblies. Richter and Schmuttenmaer used ultrafast THz spectroscopy to probe the origins of this inefficiency, and they found that in an array of titanium dioxide nanotubes (shown above), a sharp resonance at 7.5 meV identified the culprit: excitonic trapping. They propose that the traps result from a comparatively high concentration of trivalent titanium ions associated with the fabrication technique, raising hopes that improved synthetic methods could clear the path.

Nat. Nanotechnol. 5, 10.1038/nnano.2010.196 (2010).

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