to be coated with 5TPY. The organic layer is approximately
1–2 nm thick, suggesting monolayer or near-monolayer coverage.
The theoretical molecular length calculated using a MM2
force field (HyperChem) is 2.3 nm. It is possible that the angle
of the molecular backbone is tilted towards the tube, nearly
prone to the surface of the tube.
Notes and references
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Photovoltaic devices were fabricated by depositing a thin
(25–50 nm) 5TPY–CNT film onto an ITO coated glass
substrate via the filter dissolution process described above.
This film was then covered with 150–200 nm of an n-type
acceptor layer of PCBM that was spin cast at 1000 rpm from a
25 mg mLꢁ1 solution in an equal mixture of chloroform and
chlorobenzene. The top contacts were formed by thermal
evaporation of 250 nm of aluminium using a shadow mask
and the devices (4 mm2) were tested in air immediately after
fabrication under standard AM1.5 illumination. Fig. 3d shows
a representative I–V curve of a device with a power conversion
efficiency of 0.02%. Six devices were fabricated with short-
circuit currents varying from ꢁ3.1 to ꢁ2.3 mA cmꢁ2 and open
circuit voltages from 12.5 to 27.5 mV. Dark curves for these
devices all pass through zero and controls without 5TPY
showed no photovoltaic effect. The very low open circuit
voltages and linear I–V characteristics of these devices are
likely the result of significant shorting between the CNTs and
the Al top contact. This shorting is difficult to entirely
eliminate when using a solution processed acceptor layer. To
avoid shorting it is also helpful to use very thin 5TPY–CNT
films, but this results in limited light absorption and subsequent
low short circuit currents. Future device optimization may
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In conclusion, we have developed a strategy to assemble
electronically active coatings on carbon nanotubes that creates
a hybrid material for photovoltaic devices. This method
uses rational molecular design to assemble quinquethiophene
segments around the nanotube without changing the covalent
structure of CNTs. Molecular design for self-assembly of
p-type overcoats on carbon nanotubes may be a useful
chemistry to create flexible organic solar cells.
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This work was supported by the Department of Energy
(DE-FG02-00ER45810) and the National Science Foundation
(DMR 0605427). Experiments made use of the IMSERC, EPIC
and Keck-II Facilities of the NUANCE Center, the Keck
Biophysics Facility and the Institute for BioNanotechnology
in Medicine. The NUANCE Center is supported by the
NSF-NSEC, NSF-MRSEC, Keck Foundation, the State of
Illinois and Northwestern University. I.P.M. thanks the ASEE
for the National Defense Science and Engineering graduate
fellowship. J.G. thanks the NIH for a Ruth L. Kirschstein
postdoctoral fellowship. We thank Dr Liam Palmer for helpful
discussions.
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ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 3705–3707 | 3707