Complete report - Donostia International Physics Center - Euskal ...

2011 DIPC Highlight
Dye sensitized ZnO solar cells:
attachment of a protoporphyrin
dye to different crystal surfaces
characterized by NEXAFS
R. González-Moreno, P.L. Cook, I. Zegkinoglou, X. Liu, P.S. Johnson, W. Yang,
R.E. Ruther, R.J. Hamers, R. Tena-Zaera, F.J. Himpsel, J.E. Ortega, and C. Rogero
The Journal of Physical Chemistry C. 115, 18195 (2011)
Figure 1. N1s absorption edge of
different Protoporphyrin molecules
attached to ZnO nanorods
(full lines) compared to the molecular
powders (dotted lines).
Protoporphyrin dimethyl ester
(esterH2PPIX) on ZnO is almost
not distinguished, demonstrating
the need of carboxylic groups
for attaching molecules to ZnO
efficiently.
In this work, element-specific spectroscopy, particularly Near Edge X-ray Absorption Fine Structure
(NEXAFS) spectroscopy, is used to investigate the chemistry and electronic structure at the
interface between dye molecules and the ZnO acceptor electrode for dye-sensitized solar cell
applications.
Dye sensitized solar cells are becoming a serious contender for low-cost solar cells. To improve their efficiency
it is necessary to find the optimal combination of organic dyes, electrolytes and nanostructured
anode materials. The best performance so far is achieved with a TiO2 nanocrystal anode sensitized by
ruthenium-based organic dyes. This design exhibits high quantum efficiency due to the fast electron injection
process into the oxide conduction band. An alternative is a ZnO nanorod array, which has the ideal
architecture to minimize the charge carrier path while keeping a large active surface. However, in ZnO
nanorod cells, the overall efficiency is poor. One possibe reason is a poor attachment of the dye to the
ZnO surface.
With the aim of understanding the physical and chemical properties of dye-sensitized ZnO surfaces, we
have analyzed the attachment of porphyrin molecules, such as free and metalated protoporphyrin, to the
three low-index ZnO single crystal faces and to ZnO nanorod arrays, using near edge X-ray absorption
fine structure (NEXAFS) spectroscopy. The selection of protoporphyrin was motivated by both the presence
of carboxylic acid anchoring groups, and the structural simplicity, i.e., a planar geometry that allows structural
characterization of the bonding through light polarization studies at the N 1s edge in NEXAFS.
Using this approach we have confirmed the importance of carboxylic groups in the anchoring of the molecules
to the ZnO surfaces, similar to the situation on single crystals as Cu(110). By comparing results obtained
for molecules with two propionic acid groups and molecules where the carboxylic acid groups have
been replaced by two ester groups, the last did not immobilize on the ZnO surface (illustrated in Figure 1).
Moreover, we have found that the crystallographic orientation of the exposed ZnO face has a strong influence
on the dye immobilization (illustrated in Figure 2). The O-terminated ZnO(0001) surface provides
the maximum total uptake, and molecules are oriented such that the central macrocycle ring is perpendicular
to the surface, as determined from the polarization dependence of NEXAFS. The non-polar Zn(10-
10), which makes up the lateral facets of the nanowires, exhibits a lower uptake and much less polarization
dependence. This can be interpreted as non-optimal anchoring, and could explain the poor efficiency of
ZnO cells fabricated with nanowires instead of nanoparticles. Finally we determine that different chemical
immobilization conditions produce etching on the ZnO surface. This could strongly affect to the efficiency
of the possible organic cell device.
ZnO nanorod arrays
are desirable for
organic photovoltaic
devices because
of the high crystallinity
and the optimum
architecture.
Figure 2 cross section SEM micrographs of the ZnO nanorods showing a
dense array of roughly hexagonal columns. Polarization-dependent N 1s
absorption edges of H2-Protoporphyrin IX on the three low-index ZnO single
crystal surfaces and for ZnO nanorods. Red lines are for a polarization
parallel to the surface and blue lines for mixed polarization. The (nominally)
oxygen-terminated ZnO(000-1) surface exhibits the strongest polarization
dependence and the highest coverage. Inserted is an illustration of the configuration
of the molecule standing upright on the ZnO (000-1) surface.
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