Nanowires and colloidal quantum dots for improved light harvesting and energy conversion
efficiency in excitonic solar cells
Authors: G. Sberveglieri, A. Vomiero, I. Concina, A. Braga, C. Baratto, V. Galstyan.
Institution: SENSOR Lab. CNR-IDASC and University of Brescia - Italy
Photoelectrochemical solar cells, which use quantum dots (QDs) as active media for light
harvesting, are a specific type of the general class of the excitonic solar cells.1 They are based on
QD excitons for light trapping and charge transfer to the photoanode. QD-based solar cells 2 have
the potential to induce a revolution in the field of photoconversion and electric current generation
thanks to their peculiar characteristics: low cost, low environmental impact, improved
photoconversion efficiency, while maintaining good long term stability.3 Recent theoretical
investigations indicate that it should be possible to obtain a photoconversion efficiency up to
45%,4 thanks to two main principal photogeneration processes: (a) multiple exciton generation
(MEG) thanks to the absorption of a photon with sufficient energy; (b) presence of intra-gap
energy bands, which allow the absorption of sub-bandgap photons and creation of electron-hole
pairs.
A further possibility for increasing the photoconversion efficiency is based on the improvement
of the charge transport mechanism inside the photoanode, which can be gained by applying
nano-engineered photoanodes based on either single crystalline nanowires 5 or cylindrical
nanotubes.6 These structures have been demonstrated to enhance charge collection and inhibiting
the recombination processes at the photoanode. However, optimization of exciton
photogeneration, separation and charge collection is a major bottleneck for improving efficiency
in QDSC, which is still low compared to commercial devices (5%). 7
We have investigated several structures of photoanodes composed of quasi-one dimensional
nanostructure arrays as scaffold for charge collection: nanowires, nanotubes, engineered
networks of mixed nanowires and polycrystal powders. The functional properties of the solar
cells related to the different anode morphologies will be discussed in detail.
The photoanodes have been sensitized using QDs prepared via successive ionic layer adsorption
and reaction (SILAR) technique, which consists of in situ generation of QDs directly on the
photoanode, and has recently demonstrated the possibility of radically improve photoconversion
efficiency in QD solar cells.
The effect on the overall photoconversion efficiency of different types of QDs composed of one
single metal (CdS, CdSe, CdTe, PbS, PbSe, PbTe, ZnS, ZnSe, ZnTe) as well as composite
systems (e.g. CdS-PbS) will be considered and discussed.
1
B.A. Gregg J. Phys. Chem. B 107, 4688 (2003).
P.V. Kamat J. Phys. Chem. C 111, 2834 (2007); P.V. Kamat J. Phys. Chem. C 112, 18737
(2008).
3
A. Luque et al. MRS Bull. 32, 236 (2007).
4
A.J. Nozik, Physica E 14, 115 (2002).
5
J. Baxter et al. Appl. Phys. Lett. 86, 053114 (2005); M. Law et al. Nature Mater. 4, 455 (2005).
6
O.K. Varghese, et al. Nature Nanotechnol. 4, 592 (2009).
7
H.J. Lee, et al. Nano Lett., 9, 4221 (2009).
2