were investigated by use of a liquid junction, containing dioxygen
as a photoelectron scavenger. The main result was that the
transport ability depends on the type of coordinating ligand
present in the polymer.
4. Conclusions
Layer-by-layer structures constituted by CdSe quantum dots
linked by electron-transporting conjugated molecules, of various
lengths and conjugation patterns, have been produced and their
photoconductive properties investigated.
The linkers used in the present investigation bear carboxylic
functions and, as expected, following the same experimental
conditions of the previous study,12 provide the same photocur-
rent values. Values of 10–20 mA cmꢀ2 (measured at ꢀ0.5 V from
the onset potential) were found.
Photoconduction in these structures is high, in contrast with
that encountered in those formed by (p-type) oligothiophenes,
thus evidencing the n-type character of charge transport.
Moreover the photoconduction is dominated by interdot
distance and linker characteristics rather than by LUMO energy
levels. Photoconductivity and linker length are exponentially
related, like in the conduction of molecular wires, and the
tunnelling attenuation is comparable with other recently
produced phenylene-based molecular wires.
It may be surprising that photocurrents generated on the same
multilayer in the photoelectrochemical and in the solid-
state configuration are strikingly different, passing from
10–20 mA cmꢀ2 to 10–15 mA cmꢀ2 (i.e. with a 1000-fold gain)
under the same illumination. The explanation is in the different
contacts and is related to charge trapping.
Such structures, which display optical and electronic proper-
ties useful in photovoltaic devices, are still under our attention
with a particular focus on the linker core, which may be also light
absorbing, and head, the nature of which also may be crucial for
an efficient transfer between quantum dots.
Unpassivated states of the NC surface may trap electrons and
holes and when one charge carrier is trapped, the other carrier
cycles through the circuit until it recombines with the trapped
carrier.21 The enhancement of the photoconductance depends on
the nature of the contacts between the electrodes and the NC
film. If the contacts are blocking, then the maximum photo-
conductive gain is one electron per absorbed photon, and the
photocurrent is called primary. If the contact is ohmic or
injecting, which is the case of our solid-state device, then the gain
can be much greater than unity and the photocurrent is called
secondary.27
References
1 G. Zotti, B. Vercelli, A. Berlin, M. Pasini, T. L. Nelson,
R. D. McCullough and T. Virgili, Chem. Mater., 2010, 22, 1521.
2 T. Mokari, E. Rothenberg, I. Popov, R. Costi and U. Banin, Science,
2004, 304, 1787.
3 C. B. Murray, C. R. Kagan and M. G. Bawendi, Annu. Rev. Mater.
Sci., 2000, 30, 545.
In the case of secondary (trap-induced) photocurrents the
recombination lifetime is limited by the release rate of the trap.
This obviously results in slow response times, frequently higher
than some seconds,28 which is our actual case (Fig. 4).
4 D. Yu, C. Wang and P. Guyot-Sionnest, Science, 2003, 300, 1277.
5 C. J. Wang, M. Shim and P. Guyot-Sionnest, Science, 2001, 291, 2390.
6 (a) A. R. Murphy and J. M. J. Frechet, Chem. Rev., 2007, 107, 1066;
(b) V. Coropceanu, J. Cornil, D. A. da Silva Filho, Y. Olivier,
R. Silbey and J. L. Bredas, Chem. Rev., 2007, 107, 926.
7 J. G. Laquindanum, H. E. Katz, A. Dodabalapur and A. J. Lovinger,
J. Am. Chem. Soc., 1996, 118, 11331.
8 P. R. L. Malenfant, C. D. Dimitrakopoulos, J. D. Gelorme,
L. L. Kosbar and T. O. Graham, Appl. Phys. Lett., 2002, 80, 2517.
9 H. E. Katz, J. Johnson, J. Andrew and W. Li, J. Am. Chem. Soc.,
2000, 122, 7787.
The blocking action of oligo(poly)thiophenes. Organic semi-
conductors are commonly classified as either p-type (hole-con-
ducting) or n-type (electron-conducting) depending on which
type of charge carrier is more efficiently transported through the
material.
The photoconductance in CdSe-NCs is likely due to mobile
electrons rather than holes4,29 so that CdSe-NCs should behave
as n-type semiconductors. Thus it is conceivable that n-type
linkers are mostly efficient in favoring charge transport in CdSe-
NCs. As a consequence the results are negative with (p-type)
oligothiophenes but positive with (n-type) oligophenylenes.
One possibility is that in the case of oligothiophenes photo-
current is mainly dominated by the light induced enhancement
effect of electron tunnelling,30 while in the case of oligopheny-
lenes, both the aforementioned effect and the n-type linkers play
roles in the photoconductivity.
10 S. A. P. Guarin, S. Dufresne, D. Tsang, A. Sylla and W. G. Skene, J.
Mater. Chem., 2007, 17, 2801.
11 D. I. Schuster, K. Li, D. M. Guldi, A. Palkar, L. Echegoyen,
C. Stanisky, R. J. Cross, M. Niemi, N. V. Tkachenko and
H. Lemmetyinen, J. Am. Chem. Soc., 2007, 129, 15973.
12 G. Zotti, B. Vercelli, A. Berlin, P. T. K. Chin and U. Giovanella,
Chem. Mater., 2009, 21, 2258.
13 W. W. Yu, L. Qu, W. Guo and X. Peng, Chem. Mater., 2003, 15, 2854.
14 V. K. Ol’klovik, A. A. Pap, V. A. Vasilevskii, N. A. Galinovskii and
S. N. Tereshko, Russ. J. Org. Chem., 2008, 1172.
15 M. L. Tomlinson, J. Chem. Soc., 1946, 756.
16 T. Yamamoto, I. Nishiyama and H. Yokoyama, Chem. Lett., 2007,
1108.
17 G. Zotti, B. Vercelli, A. Berlin, unpublished.
About the origin of the p-type character of oligothiophenes,
which does not appear to be accounted for in the literature, we
present here a possible explanation. The conductivity of poly-
conjugated polymers in general is expected to be comparable for
p- and n-doping, as e.g. in polyacetylene.31 Yet in polythiophenes
the n-conductivity is ca. 100–300 times lower.32 Localization of
charge in oligothiophenes may in principle be traced to the
different localization of the LUMO (on the sulfur atoms) and of
the HOMO (on the polyacetylene CH backbone) orbitals.33 Thus
the blocking action of oligothiophenes in the photoconduction in
CdSe-NCs may be traced to a strong localization of photo-
generated electrons.
18 (a) G. H. Aylward, J. L. Garnett and J. H. Sharp, Anal. Chem., 1967,
39, 457; (b) J. L. Sadler and A. J. Bard, J. Am. Chem. Soc., 1968, 90,
1979.
19 H. Lund, in Organic Electrochemistry, ed. H. Lund and M. M. Baizer,
M. Dekker, New York, 1991, p. 401.
20 (a) C. A. Leatherdale, C. R. Kagan, N. Y. Morgan, S. A. Empedocles,
M. A. Kastner and M. G. Bawendi, Phys. Rev. B: Condens. Matter,
2000, 62, 2669; (b) D. S. Ginger and N. C. Greenham, J. Appl.
Phys., 2000, 87, 1361; (c) M. V. Jarosz, V. J. Porter, B. R. Fisher,
M. A. Kastner and M. G. Bawendi, Phys. Rev. B: Condens.Matter
Mater. Phys., 2004, 70, 195327; (d) V. J. Porter, T. Mentzel,
S. Charpentier, M. A. Kastner and M. G. Bawendi, Phys. Rev. B:
Condens. Matter Mater. Phys., 2006, 73, 155303.
21 V. J. Porter, S. Geyer, J. E. Halpert, M. A. Kastner and
M. G. Bawendi, J. Phys. Chem. C, 2008, 112, 2308.
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