Electron transport and recombination in dye-sensitized solar cells made
from single-crystal rutile TiO2nanowires
Emil Enache-Pommer, Bin Liu and Eray S. Aydil*
Received 28th July 2009, Accepted 2nd September 2009
First published as an Advance Article on the web 10th September 2009
Contrary to expectations, the electron transport rate in
dye-sensitized solar cells made from single-crystal rutile
titanium dioxide nanowires is found to be similar to that
measured in dye-sensitized solar cells made from titanium
Electrodes made from one-dimensional nanostructures such as
nanowires, nanorods and nanotubes have been proposed for
improving the charge collection efficiencies of ordered-bulk-
heterojunction and dye-sensitized solar cells (DSSC).1–7DSSCs
based on polycrystalline2and single-crystal ZnO nanowires,1,3,4
amorphous and polycrystalline TiO2 nanotubes,8–10poly-
crystalline11and single-crystal TiO2nanowires,12–14have been
reported. In DSSCs a monolayer of photosensitive dye is
adsorbed on the internal surfaces of a nanostructured porous
film to form a photoanode. This film can be made of nano-
particles, nanotubes or nanowires. The pores are filled with a
liquid electrolyte containing the I?/I3?redox couple or a solid-
state electrolyte15–17to create a semiconductor–dye–electrolyte
interface with a high interfacial area. Upon illumination,
excited electrons from the dye are injected into the semi-
conductor and the positively charged dye is reduced through
an electrochemical reaction with I?. The electrons diffuse
through the semiconductor, flow through the load and arrive
at the cathode placed across from the photoanode to reduce
the I3?back to I?and to complete the circuit.
Competition between electron diffusion through the photo-
anode and recombination with I3?in the electrolyte determines
the electron collection efficiency in DSSCs. In nanoparticle-
based DSSCs, three factors influence the electron transport
rate across the nanoparticle films. These factors are the
residence time of electrons in traps, the morphology of
the nanoparticle network, characterized by a distribution of the
number of nearest neighbours, and the contact area between
the particles.18,19Single-crystal nanowires are advantageous
for DSSCs because they eliminate the latter two factors by
providing a direct path for electrons from the point of injection
to the collection electrode. Indeed, studies show that electron
transport is faster in both single-crystalline ZnO nanowires20,21
and in polycrystalline ZnO nanotubes22than in nanoparticle
films. While these results are encouraging, TiO2-based
photoelectrodes are preferred over ZnO because of their
superior stability, ability to adsorb dyes without forming
aggregates,23and higher density of states in the conduction
band. Record power conversion efficiencies obtained with
TiO2nanoparticle photoelectrodes are much higher (B11%)
than thoseachieved with
Surprisingly, recent studies showed that electron transport
rates in polycrystalline TiO2nanowires24and nanotubes9were
as slow as in TiO2nanoparticle electrodes. This was attributed
to a similar transport mechanism in polycrystalline TiO2
nanowires/nanotubes and TiO2 nanoparticle films. The
electrons in polycrystalline nanowires and nanotubes still have
to cross grain boundaries, which are similar to the interparticle
boundaries in nanoparticle films. Another possibility is that
surface traps control the diffusion rate and these exist on TiO2
nanoparticles, nanotubes and nanowires regardless of the
morphology. If the grain boundaries are responsible for slow
electron transport in polycrystalline nanowires then electron
transport in DSSCs made from single crystalline TiO2nano-
wires should be faster. However, growth of TiO2nanowires on
transparent conducting oxide films has been difficult and this
hypothesis could not be checked until now. Growth of single
crystalline TiO2nanowires on transparent conducting oxide
films was achieved only recently.14Herein, we report electron
transport and recombination rates in these single-crystal rutile
TiO2nanowires and compare them with TiO2nanoparticle
films of similar thickness. Surprisingly, we find that the
electron transport time constant is approximately the same
and even slightly slower in single-crystal rutile TiO2nanowires
than in TiO2 nanoparticle films, suggesting that electron
diffusion rate is still determined by the residence time in
surface traps even in single-crystal TiO2nanowires.
Vertically aligned single-crystal rutile TiO2nanowires were
grown on transparent conductive fluoride-tin oxide (FTO)
substrates using the hydrothermal method developed by Liu
et al.14Briefly, two pieces of ultrasonically cleaned FTO
substrates were placed at an angle against the wall a Teflon-
lined stainless steel autoclave and immersed in 30 mL of
deionized water, 30 mL of concentrated hydrochloric acid
(36.5–38% by weight), and 1 mL of titanium isopropoxide.
The autoclave was placed and kept in an oven for 3 h at
200 1C. Then the autoclave was cooled rapidly to room
temperature under flowing water. The substrates were removed
from the autoclave, rinsed and dried. Scanning electron micro-
graphs (SEMs) of the nanowires are shown in Fig. 1a and c.
The nanowire filmthicknessis1.2
Department of Chemical Engineering and Materials Science,
University of Minnesota, 151 Amundson Hall,
421 Washington Avenue SE, Minneapolis, Minnesota, USA.
E-mail: email@example.com; Fax: +1 612 626 7246;
Tel: +1 612 625 8593
9648 | Phys. Chem. Chem. Phys., 2009, 11, 9648–9652This journal is ? c the Owner Societies 2009
COMMUNICATION www.rsc.org/pccp | Physical Chemistry Chemical Physics
approximately the average nanowire length. Nanoparticle
films were deposited on FTO substrates by drop-casting from
a dispersion of P25 nanoparticles (Degussa) in ethanol. The
solution contained 100 mg of P25 particles in 20 mL ethanol
and was sonicated for 1 h prior to deposition to disperse the
nanoparticles. FTO substrates were cleaned by sonication in a
1 : 1 : 1 mixture of deionized water, acetone and isopropanol
before 170 mL of the nanoparticle dispersion was drop-cast
onto the FTO substrate to deposit a nanoparticle film with
comparable thickness to the nanowire film. The thickness of
the nanoparticle film used in the comparisons shown herein
was 1.2 ? 0.02 mm (Fig. 1b and d); this value is an average of
six different measurements across the substrate indicating that
the film was very uniform. After drying, the nanoparticle
covered substrates were annealed at 450 1C for 10 min. Both
the nanowire and nanoparticle substrates were then immersed
in an aqueous TiCl4solution at 50 1C for 2 h. The TiCl4
solution was prepared by mixing 0.09 mL of TiCl4with 0.4 mL
of concentrated hydrochloric acid followed by the addition of
deionized water to reach a final volume of 100 mL. After TiCl4
treatment, the substrates were rinsed with deionized water,
allowed to dry in ambient air and annealed at 450 1C for
an additional 30 min. Both the nanowire and nanoparticle
covered substrates were then immersed in a 0.2 mM solution
of N719 dye (Solaronix) in ethanol for 24 h. After dyeing, the
substrates were rinsed with pure ethanol and dried in ambient
air for 30 min. Solar cells were assembled by pressing TCO
substrates coated with 10 nm Pt against the nanowire or
nanoparticle substrates. The electrodes were separated by
25 mm Teflon spacers (Pike Technologies) and liquid electro-
lyte Iodolyte MPN-100 (Solaronix) was infiltrated between the
electrodes using capillary action. Current–voltage (I–V)
characteristics of the solar cells were recorded using a Keithley
2400 sourcemeter while the solar cells were illuminated with
AM 1.5 light from a solar simulator described previously.25
Electron transport and recombination time constants were
measured using intensity modulated photocurrent spectro-
scopy (IMPS)26,27and open-circuit photovoltage decay,28,29
(OCVD), respectively. In IMPS, the DSSCs were illuminated,
at short-circuit, with a small sinusoidally modulated light
intensity (6% of the mean value), superimposed on a constant
monochromatic light intensity (658 nm from a Newport LQA
series laser). The photocurrent modulation amplitude and the
phase lag between illumination and photocurrent were
measured using a low noise current-to-voltage amplifier
(Stanford Research Systems SR570) and a lock-in amplifier
(Stanford Research Systems SR810). The amplitude and phase
lag were converted to real and imaginary components of the
photocurrent and plotted on the complex plane for a range of
modulation frequencies. The electron transport time constant
(tc) was evaluated using the frequency at the minimum of the
imaginary part of the photocurrent (fc) using tc= (2pfc)?1. In
OCVD, the DSSC is illuminated with AM 1.5 light and
allowed to reach a steady-state VOC. The illumination is then
turned off and the photovoltage decay is measured using an
oscilloscope (Tektronix TDS 1000 series). Recombination
times are obtained from the photovoltage decay using
tr = ?(kT/e)(dVOC/dt)?1, where kT/e is the potential of
thermal energy and (dVOC/dt)?1is the instantaneous open
circuit voltage decay rate.
The crystal structure and morphology of the TiO2nano-
wires and nanoparticle films were studied using X-ray
diffraction (XRD, Bruker-AXS Microdiffractometer D5005,
l = 1.5406 A˚), field-emission scanning electron microscopy
(FESEM, JSM-6700F), transmission electron microscopy,
selected area electron diffraction (TEM/SAED, FEI Tecnai T12),
(HRTEM, FEI Tecnai G2 30).
Results and discussion
Fig. 2 shows the XRD from nanowires and from a P25
nanoparticle film. The XRD from nanowires matches the
tetragonal rutile phase with a = b = 0.4517 nm and
c = 0.2940 nm. When compared to the powder diffraction
pattern, the (002) diffraction peak appears significantly
enhanced, indicating that the nanowires are oriented with
respect to the substrate. The P25 nanoparticle film shows
diffraction peaks of both anatase and rutile phases consistent
with a mixture. Transmission electron micrographs (TEM)
and electron diffraction reveals that nanowires are single
crystalline along their entire length (Fig. 3). Lattice spacing
obtained from HRTEM images are consistent with atomic
plane spacings in rutile. Fig. 3 is representative of all nano-
wires and additional information regarding the growth
mechanism and structural characteristic of the nanowires
can be found in ref. 14.
Fig. 4 shows the I–V characteristics of typical TiO2nano-
wire and nanoparticle DSSCs. The nanoparticle-based DSSC
exhibits a short-circuit current (ISC) of 3.81 mA cm?2,
open-circuit voltage (VOC) of 0.65 V and a fill factor (FF) of
0.70, resulting in an overall efficiency of 1.73% while the
nanowire-based DSSC exhibit ISCof 3.54 mA cm?2, VOCof
0.68 V and FF of 0.68 resulting in an overall efficiency of
1.58%. Dye desorption experiments reveal that the roughness
factor for nanoparticle films is B130, while the roughness
films. (a) and (b) show top views of nanowires and nanoparticles,
respectively; (c) and (d) show cross-sectional views of nanowires and
SEM images of typical TiO2 nanowire and nanoparticle
This journal is ? c the Owner Societies 2009Phys. Chem. Chem. Phys., 2009, 11, 9648–9652 | 9649
factor of the nanowire film is B85 despite the fact that the
nanowire and nanoparticle films have similar thicknesses.
Given that the surface area of the nanoparticle film is more
than 50% greater than that of the nanowire film and, at low
film thicknesses, the collection efficiency is high in nanoparticle
films, one would expect nanoparticle-based DSSCs to show
substantially higher short-circuit current than the nanowire-based
DSSCs. Instead, the nanoparticle-based DSSC short circuit
current is only marginally higher than that of the nanowire-based
DSSC. In fact, the close performance of the two devices,
despite the significantly different roughness factors, is due to
strong light scattering within the nanowire film compared to
the nanoparticle film. This is illustrated in Fig. 5, which shows
digital photographs of an uncoated FTO substrate (left), an
FTO substrate coated with a 1 mm thick TiO2nanoparticle
film (center) and an FTO substrate coated with 1 mm thick
TiO2 nanowires (right). While the nanoparticle coated
substrate is nearly transparent, the nanowire substrate is
opaque due to stronger light scattering by the nanowires as
compared to nanoparticles. This strong light scattering by the
nanowires enhances the light-harvesting efficiency of the nano-
wire cells. The stronger light scattering of the nanowires
counteracts the lower surface area, resulting in similar
short circuit currents and similar overall efficiencies for the
nanowire- and nanoparticle-based DSSCs.
Fig. 6a and b show typical IMPS responses and OCVD
curves for nanoparticle- and nanowire-based DSSCs. The
IMPS response crosses the imaginary axis at high frequencies
and spirals to the origin. This is characteristic of cells
illuminated from the electrolyte side.26While our cells are
illuminated from the photoanode direction, the TiO2nano-
particle and nanowire layers are so thin that substantial
fraction of the light goes through these layers (e.g., see center
sample in Fig. 5) and reflect back to impinge on the photo-
anode from the electrolyte side. For nanowires, this crossing
occurs at a somewhat lower value of the imaginary axis
because nanowires scatter light and fewer photons reflect from
the platinized cathode. Crossing also occurs at lower values for
thicker films. Recombination in both nanowires and nano-
particles is very slow and the cells can sustain significant
voltages many seconds after the light has been turned off.
(b) P25 TiO2nanoparticles.
X-Ray diffraction patterns from (a) TiO2 nanowires and
the diffraction pattern from this nanowire while the lower left inset is
the HRTEM image showing the lattice spacings.
TEM image of a TiO2nanowire. The upper right inset shows
nanowires (NW) and TiO2nanoparticle (NP) recorded while they
were illuminated with 100 mW cm?2AM 1.5 radiation.
Current–voltage characteristics of DSSCs based on TiO2
FTO substrate coated with a 1 mm thick TiO2 nanoparticle film
(center) and an FTO substrate coated with 1 mm thick TiO2nanowires
Digital photographs of an uncoated FTO substrate (left), an
nanoparticle (NP) based DSSCs.
(a) IMPS response and (b) OCVD of the nanowire (NW) and
9650 | Phys. Chem. Chem. Phys., 2009, 11, 9648–9652 This journal is ? c the Owner Societies 2009
Initially, the nanoparticles decay faster than the nanowires but
the rates appear to asymptote to similar values after the initial
How to compare electron transport times fairly in two
photoanodes with dramatically different morphologies is a
complicated issue. One can compare cells with equal
roughness factor and porosity, equal photoanode thickness,
equal photocurrents or equal efficiencies. We chose nano-
particle and nanowire films with similar thicknesses and
similar photocurrents and efficiencies. We expected, as in
studies by Martinson,21the transport time constants in nano-
wires to be faster than those in nanoparticles by at least two
orders of magnitude so that small differences in geometric
characteristics, photocurrents and efficiencies between these
two cells were less important. Fig. 7 shows transport and
recombination time constants for both nanowire- and
nanoparticle-based DSSCs as a function of light intensity.
We find that transport time constant in single-crystalline
rutile TiO2 nanowires exhibit a power law dependence on
light intensity similar to that observed with nanoparticle films.
In addition, the electron transport rate is approximately a
factor of two slower in rutile nanowires than in P25 nano-
particles (B70% anatase, 30% rutile). Previous studies on
nanoparticle-based DSSCs showed that the electron transport
rate in rutile TiO2nanoparticle films is one order of magnitude
slower than anatase TiO2nanoparticle films.30This order of
magnitude difference in transport times was attributed to a
smaller number of interparticle connections in the rutile films30
as compared to the anatase films. Fig. 7 shows that the
electron transport rate is only slightly slower in single
crystal rutile nanowires than in nanoparticles, indicating
that, while the effect of interparticle connections is eliminated,
traps still dominate electron transport. Moreover, the fact
that the TiO2nanowires are single crystalline along their entire
length suggests that the traps reside on the surface of the
nanowires though we can not completely discount the
possibility of bulk traps. The scaling exponents for the trans-
port and recombination time constants were ?0.24 and ?0.45
for nanoparticles and ?0.35 and ?0.55 for nanowires,
respectively. Recombination rates were slightly slower in the
nanowires than in the nanoparticles, possibly due to different
surface trap distributions in nanowires vs. nanoparticles.
The ratios of the recombination time constant to the electron
collection time constant at the highest light intensity (tr/tc),
were B40 for nanowire films and B55 for nanoparticle
films. Although these ratios are high enough to give nearly
100% electron collection efficiency for 1 mm thick films, the
similarity between ratios of transport and recombination
rates in rutile TiO2nanowires and P25 nanoparticles indicates
that rutile nanowires, without passivation of traps, may have a
limited potentialfor improving
of DSSCs. While the potential of single-crystalline rutile
nanowires appears limited for DSSCs, the situation may be
different for single crystal anatase nanowires. Assuming
that the results obtained by Park et al.30for anatase and
rutile nanoparticle films also hold for nanowire films, an
order of magnitude improvement in electron transport with
respect to nanoparticle cells would be possible by fabricating
single-crystalline anatase TiO2nanowires on FTO substrates.
Such nanowires have already been fabricated on Ti foil but, to
our knowledge,not on transparent
substrates.13Also, it may be possible to improve the transport
rate in rutile nanowires through surface treatments or growth
of semiconductor shell layers at the nanowire surface,
provided that these modifications reduce the density of
Finally, the difference in electron transport rates in ZnO and
TiO2nanowires is major and striking. Even 4.5 mm long ZnO
nanowires exhibit much faster (tc B 0.1 ms)21electron
transport than 1.2 mm long rutile nanowires (tc 4 1 ms).
Moreover, the electron transport in ZnO has been shown to be
independent of light intensity20,21whereas rutile nanowires
exhibit light-intensity-dependent electron transport rates, a
signature of trap involvement in the electron transport
mechanism; in rutile nanowire-based DSSCs electrons diffuse
faster when the cell is illuminated and the number of filled
traps increases. The differences between the ZnO and rutile
TiO2nanowires may be due to the intrinsic differences in the
surfaces of the two materials, trap concentrations, carrier
densities or a combination of these differences. Whatever the
reason, having single crystalline high quality nanowires of the
acceptor material does not guarantee fast electron transport in
dye sensitized solar cells and one still may have to worry about
trapping and develop methods for trap passivation.
Electron transport and recombination rates have been
measured in single crystal rutile nanowire-based DSSCs.
Surprisingly, the electron transport rate is on the order of
the electron transport rate in nanoparticle-based DSSCs and
not as fast as would be expected from single crystal nanowires.
The slow and light intensity dependent electron transport rate
indicates that trapping and detrapping, most likely in surface
traps, still play an important role in electron transport even
when DSSC photoanodes are made from single crystalline one
constants for TiO2 nanowire (circles) and nanoparticle (squares)
DSSCs as a function of light intensity. Error bars represent standard
deviations and illustrate reproducibility for DSSCs from three differ-
ent batches (nanowires) and four different batches (nanoparticles).
(a) Recombination (K, ’) and transport (J, &) time
This journal is ? c the Owner Societies 2009Phys. Chem. Chem. Phys., 2009, 11, 9648–9652 | 9651
Acknowledgements Download full-text
This work was supported primarily by the NSF under the
NIRT program (CBET-0506672) and partially by the MRSEC
program of the NSF under the award number DMR-0819885.
We utilized the University of Minnesota Characterization
Facility and the University of Minnesota Nanofabrication
Center, which receive partial support from the NSF under
the NNIN program.
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