Band edge engineering of composite photoanodes for dye-sensitized solar cells

1. Band edge engineering of composite photoanodes for dye-sensitized
solar cells
Venkata Manthina a,b
, Alexander G. Agrios a,b,
*
a
Department of Civil & Environmental Engineering, University of Connecticut, 261 Glenbrook Rd Unit 3037, Storrs, CT 06269, USA
b
Center for Clean Energy Engineering, University of Connecticut, 44 Weaver Rd, Storrs, CT 06269, USA
A R T I C L E I N F O
Article history:
Received 13 February 2015
Received in revised form 14 April 2015
Accepted 15 April 2015
Available online 16 April 2015
Keywords:
zinc oxide
titanium dioxide
doping
charge transfer
nanocomposites
A B S T R A C T
As dye-sensitized solar cells (DSSCs) transition from iodide/triiodide-based electrolytes to organome-
tallic complex redox couples with higher rates of recombination with electrons in the semiconductor,
there is a need for semiconductor nanostructures that can rapidly transport electrons out of the device
while maintaining high surface areas for the semiconductor/dye/electrolyte interface. A previously
reported composite, with TiO2 nanoparticles coating ZnO nanorods, met these criteria but suffered from a
barrier to electron transfer from the TiO2 to the ZnO. Here, the band edge positions of the TiO2 and ZnO
have been shifted by doping with Zr4+
and Co2+
, respectively, to arrive at the desired energetic alignment.
The materials were characterized using diffuse-reflectance spectroscopy and a three-electrode
measurement of the open circuit photovoltage under bandgap excitation (OCV). The OCV measurement
indicated that the doping moved the conduction band minimum of ZnO to a more positive potential than
that of the TiO2, enabling electron transfer from dye-sensitized TiO2 nanoparticles to the underlying ZnO
nanorods for efficient charge collection. However, DSSC devices fabricated with the composite
nanostructures did not show improved performance. This paper details a methodology for producing and
measuring band-edge shifts along with the benefits and limitations thereof.
ã 2015 Elsevier Ltd. All rights reserved.
1. Introduction
One-dimensional nanostructured metal oxides are promising
materials for applications such as dye sensitized solar cells [1,2]
(DSSCs) and water splitting [3] where rapid electron transport and
high interfacial area are needed. This becomes especially impor-
tant as DSSCs move away from iodide/triiodide-based electrolytes
and toward redox couples, such as cobalt(II/III) [4] or ferrocene/
ferrocenium (Fc/Fc+
), [5] that have faster rates of recombination
with electrons in the photoanode, necessitating fast electron
transport out of the photoanode for efficient charge collection. ZnO
has been found particularly useful in DSSCs [1,6–9] since it can be
grown in 1-D morphologies using facile methods and has other
favorable material properties such as a high electron mobility and
appropriate band edge positions.
The major drawbacks of ZnO nanorods in DSSCs, as compared to
the standard TiO2 nanoparticle film [10], are a reduced surface area
and reactions with carboxylic dyes resulting in partial dissolution
of the surface. Some ZnO–TiO2 and ZnO–ZnO composite nano-
structures have been proposed to address one or both of these
problems [11–15], but in our own composite of ZnO nanorods
coated with TiO2 nanoaparticles, we reported evidence of an
energy barrier preventing electron injection from TiO2 nano-
particles to ZnO nanorods, hindering charge collection [16]. For an
efficient core-shell photoanode, the conduction band minimum
(CBM) potential of the shell material must be more negative than
that of the core material (but more positive than the dye LUMO
level). Achieving this condition requires raising the CBM energy of
the TiO2 and/or lowering the CBM energy of the ZnO, as depicted in
Fig. 1.
A number of transition metal dopants have been incorporated
into metal oxide semiconductors to change their optical and
photoelectrochemical properties [17–20]. In particular, previous
reports have shown that Co2+
can substitute for Zn2+
atoms in ZnO,
causing the lattice cell to contract due to the slightly smaller
Shannon crystal radius of Co2+
(0.72 Å) compared to Zn2+
(0.74 Å) in
the wurtzite structure, resulting in a smaller bandgap [21–23]. In
TiO2, there are multiple reports that Zr4+
substitution of some Ti4+
atoms results in an increased bandgap [24–27]. If the O 2p orbitals
remain largely unchanged, keeping the valence band maximum
(VBM) potential constant, the altered bandgap corresponds mainly
to a CBM shift.
We have doped the ZnO with cobalt to lower its CBM energy and
doped TiO2 with zirconium to raise its CBM energy. In this report
* Corresponding author. Tel.: +1 860 486 1350.
E-mail address: agrios@engr.uconn.edu (A.G. Agrios).
http://dx.doi.org/10.1016/j.electacta.2015.04.080
0013-4686/ã 2015 Elsevier Ltd. All rights reserved.
Electrochimica Acta 169 (2015) 416–423
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2. we will discuss the effect of doping on those band potentials and on
performance of the DSSCs in two different electrolytes: one
containing the standard iodide/triiodide couple and one with the
fast recombining Fc/Fc+
couple.
2. Experimental
2.1. Reagents and Materials
All chemicals were purchased from Sigma-Aldrich and were
ACS grade or better, except where noted. N719 dye was purchased
from Dyesol (Australia). Fluorine-doped tin oxide glass (FTO, sheet
resistance 8 V /&) was purchased from Hartford Glass Co (USA).
2.2. Electrode Fabrication
ZnO nanorods were synthesized by a two-step chemical bath
deposition (CBD) technique, then coated with TiO2 nanoparticles
by electrostatic layer-by-layer deposition, as previously reported
[28]. The ethanolic ZnO seed layer precursor solution was modified
by the addition of 0.6 M H2O to increase the spacing between the
nanorods [29]. Doping ZnO with Co2+
and TiO2 with Zr4+
was
achieved by substituting 10 mol% of the zinc nitrate with cobalt(II)
nitrate and 5 or 10 mol% of titanium tetraisopropoxide with
zirconium tetraisopropoxide, respectively.
2.3. Surface/Structure Characterization
Scanning electron microscopy (SEM) of the nanomaterials on
FTO glass was performed using an FEI Quanta FEG250 SEM in high
vacuum mode. Elemental analysis of the doped and undoped ZnO
and TiO2 was made using the EDX attached to the SEM. The ZnO
nanorods and TiO2 nanoparticles were additionally characterized
by X-ray powder diffraction (XRD) using a Bruker D8 Advance X-
ray diffractometer using Cu Ka radiation (l= 0.154178 nm) at a
scanning rate of 0.04
sÀ1
in the 2 u range from 10
to 90
.
2.4. Optical Characterization
The UV–vis spectra of doped and undoped TiO2 and ZnO were
obtained by using Cary 50 UV–vis spectrophotometer. The diffuse
reflectance spectra were obtained using a Shimadzu 2450 UV–vis
spectrophotometer with an ISR-240A integrating sphere attach-
ment with barium sulfate as the standard.
2.5. Photoelectrochemical Characterization
Illuminated open circuit photovoltage (OCV) was measured to
find the flat band potential of each semiconductor in solution. A
ZnO nanorod array or TiO2 nanoparticle film on FTO glass was
immersed in a three-electrode electrochemical cell and served as
the working electrode, along with a Ag/AgCl reference electrode
and Pt wire counter electrode. The electrolyte was aqueous 0.1 M
LiClO4 basified to pH 11 with NaOH. The working electrode was
illuminated from the back (non-conducting) side with a 300-W Xe
arc lamp (Oriel). The cell open-circuit voltage was monitored as a
shutter kept the sample dark for 30 s, then illuminated for 60 s,
then dark for 60 s. To avoid reaction of photogenerated electrons
with dissolved oxygen in the electrolyte, the solution was purged
with nitrogen gas for 10 minutes before measurement. According
to data from the manufacturer, the lamp outputs about 100 mW in
the spectral region between the bandgap edge of ZnO (370 nm) and
the cutoff of FTO transmission (350 nm); this is the power available
for bandgap excitation of ZnO, with somewhat more available for
TiO2 (with a bandgap edge of 385 nm). Focused onto a 1 cm2
area,
for either semiconductor this was judged to be more than enough
UV intensity to achieve a saturation condition in which the bands
flatten and the Fermi level (measured as the working electrode
potential) is close to that of the conduction band edge potential
[30].
2.6. Solar Cell Assembly
After sintering, films were allowed to cool to 100
C then
immediately immersed in 0.3 mM N719 in ethanol. After 12 h they
were removed and rinsed in acetonitrile and dried in air. Each
sensitized electrode was sealed against a counter electrode on a
hot plate at 120
C using a hot-melt plastic frame (Solaronix,
Meltonix 1170, 25 mm thick) applying light pressure with a glass
rod. The assembled cell was filled with electrolyte through two
holes in the counter electrode. The holes were then sealed using
hot-melt plastic and a thin glass cover slide. The exposed
conducting glass leads of each electrode were coated with copper
tape (3M) for improved electrical conductivity.
2.7. Electrolyte Composition
Iodide/triiodide (IÀ
/I3
À
) electrolyte was prepared with 0.5 M
tetrabutylammonium iodide and 0.05 M iodine (I2) in 3-methox-
ypropionitrile. The ferrocene/ferrocenium (Fc/Fc+
) electrolyte
contained 0.1 M ferrocene and 0.05 M ferrocenium hexafluoro-
phosphate in 3-methoxypropionitrile and was deoxygenated by
bubbling nitrogen for 10 minutes prior to cell fabrication to
minimize reaction of ferreocene with oxygen [5,31]. Additives such
as 4-tert-butylpyridine were avoided to minimize complications
arising from band-edge energy shifts.
2.8. Solar Cell Characterization
Current-voltage (J–V) measurements were made using a
Keithley 2400 source/meter controlled by a PC, while irradiating
at 1000 W/m2
with AM 1.5G simulated sunlight produced by a solar
simulator (Newport 91160). The DSSC photocurrent and photo-
voltage were measured with an active area of 1 cm2
.
2.9. Transient Measurements
Measurements of electron transport time were made using
transient decay of photocurrent under square wave modulation of
a white light LED as described previously except with a 40-V
resistor [16]. Electron lifetime was measured in a similar way using
the photovoltage transient at open circuit. At least 50 transients
were averaged for noise reduction.
e-
ConductingGlass(FTO)
ZnO
VALENCE
BAND
COND.
BAND
TiO2
LUMO
HOMO
Dye
VALENCE
BAND
COND.
BAND
Fig. 1. Schematic of band edge engineering to overcome barrier to electron transfer
from TiO2 to ZnO.
V. Manthina, A.G. Agrios / Electrochimica Acta 169 (2015) 416–423 417

3. 3. Results and Discussion
3.1. Synthesis and morphology of cobalt-doped ZnO nanorods and
zirconium-doped TiO2 nanoparticles
Cobalt-doped ZnO nanorods were fabricated on FTO by
chemical bath deposition. Scanning electron microscope (SEM)
images of the doped nanorods are shown in Fig. 2. No phase
separation of the Co2+
dopant is observed in these images, which is
consistent with the growth of a solid solution of (Zn0.9Co0.1)O.
Dopant elements can alter the thermodynamics of crystal growth
and affect growth kinetics, [32–34] and this was reflected in
altered nanorod dimensions in this study. Compared to undoped
nanorods, the presence of Co2+
in solution increased the average
diameter from 600 nm to 1100 nm, increased the average length
increased from 5 mm to 6 mm, and decreased the number density
from 1.2 Â108
rods per cm2
to 0.9 Â108
rods per cm2
. EDX analysis
of the broken rod cross-section (shown in Fig. 2c) shows the
presence of the Co2+
in the ZnO matrix with a ratio of Co/(Co + Zn)
equal to 0.08. The voids seen in the cross section have been
observed in other annealed ZnO nanorods [35].
Fig. 3a shows the XRD pattern of cobalt-doped ZnO nanorods
recorded in the 2 u range of 20–70
. All diffraction peaks can be
attributed to crystalline ZnO with the hexagonal wurtzite structure
[space group: P63mc(186); a) 0.3249 nm, c) 0.5206 nm] and are in
agreement with a reference standard for zinc cobalt oxide (JCPDS
072-8025). No additional peaks were detected that would indicate
formation of cobalt oxide or presence of starting precursor
materials. Sharp XRD peaks imply a high degree of crystallization,
which is expected from the hexagonal shape of the nanorods in
Fig. 2. The intense peak assigned to the (0 0 2) plane of ZnO
indicates the formation of nanorods through elongation along the
c-axis perpendicularly to the substrate. Fig. 3b shows representa-
tive X-ray powder diffraction patterns of doped and undoped TiO2
nanoparticle films deposited on FTO glass. The patterns showed no
detectable peak shifts in 0%, 5% or 10% Co-doped samples, in
agreement with previous reports [36]. This is likely because the
similarity of the ionic radii of Co2+
(0.58 Å) and Zn2+
(0.60 Å) results
in a lattice deformation too small to be observed with our
diffractometer [37]. An increase in the crystallite size from 16 nm
to 20 nm after 10 at% zirconium doping was found by using Scherer
equation from the XRD pattern. EDX analysis (not shown) indicates
9.9 at% Zr is present for 10 at% substitution of the zirconium
precursor for titanium during the TiO2 synthesis.
3.2. Flat Band Potential (Efb) Measurements
In altering the ZnO and TiO2 materials by doping, the important
target parameter is not the bandgap of either material but the
potential of their conduction band edges in the electrolyte in a
DSSC. In particular, the goal is to shift the ZnO CBM to a more
negative potential than the TiO2 CBM. The CBM in ZnO nanorods
(and many other semiconductors) is often measured using the
Mott–Schottky method [38]. But Mott–Schottky theory is based on
band bending, which TiO2 nanoparticles have too few carriers to
sustain, and is therefore inappropriate for nano TiO2 [39]. To use a
single technique valid for all samples studied, we instead measured
the CBM potential in aqueous solution (0.1 M LiClO4 at pH 11) for
Fig. 2. SEM images of 10% Co-doped ZnO nanorods: (a) top view, (b) cross section, (c) area of a broken rod used for EDX analysis.
418 V. Manthina, A.G. Agrios / Electrochimica Acta 169 (2015) 416–423

4. doped and undoped ZnO and TiO2 films on conducting glass
samples using the open circuit photovoltage (OCV) technique [30]
under bandgap irradiation and assumed that the CBM shifts will be
similar in the DSSC electrolyte (Fig. 4). Before illumination, the
potential is dominated by surface properties and shows variability
between samples, since it is sensitive to subtleties in the
preparation and history of each sample. Under illumination, the
electrode saturates and the potential shifts negatively to values
close to ECBM, and the potentials converge for replicate samples to
quite similar values.
Plotted on the same potential scale, the OCV traces show that in
this electrolyte the CBM of ZnO lies at a more negative potential
than that of TiO2 by about 0.1 V. This would tend to block the
desired electron transfer from TiO2 to ZnO in a DSSC device based
on a hybrid of these two materials. However, doping ZnO with 10 at
% Co results in a positive shift of the CBM by about 0.15 V, while
doping TiO2 with Zr results in a negative shift of about 0.1 V at the
5 at% doping level and 0.2 V at the 10% doping level. Together, the
dopants of the two materials combine to give a driving force of
more than 0.2 V for electron transfer from Zr-doped TiO2 to Co-
doped ZnO.
3.3. Optical Measurements
The photoluminescence (PL) spectra of the films at room
temperature are presented in Fig. 5. The spectrum of the nanorods
is characterized by an intense UV emission at 386 nm due to
electron–hole recombination. No emission in the visible range
(which would indicate radiative recombination at defect sites) is
observed, indicating that the film is of high structural quality. A
sharp decrease in the PL intensity is observed after doping and is
likely due to luminescent quenching via dopant-dopant inter-
actions [40].
UV-vis spectroscopy was employed for optical characterization
of the bandgaps of the doped and undoped materials. Because of
the high scattering from the nanorods, a diffuse-reflectance
measurement was used. The reflectance data R were transformed
using the Kubelka-Munk function F(R), which is proportional to the
extinction coefficient a [41]:
FðRÞ ¼ ð1 À RÞ2
=2R
The optical bandgap can then be found by plotting, as a function of
photon energy hn, [F(R) Â hn]n
, where the exponent n is set to 2 for
materials with a direct transition, 3/2 for a forbidden transition,
and 1/2 for an indirect transition. We have therefore used n = 2 for
ZnO and n = 1/2 for TiO2. The plot leads to a linear rise near the
bandgap; the intersection of the extrapolation of this line with a
line extrapolated from the preceding baseline occurs where hn= Eg
(Fig. 6).
The analysis finds a bandgap for the undoped TiO2 of 3.13 eV,
which is slightly lower than the accepted value of 3.23 eV but
within the range of variability seen in TiO2 samples [42,43].
Replacing 5% of the Ti with Zr (in the precursor solution) raises the
Fig. 3. Normalized XRD spectra for (a) doped and undoped ZnO nanorods and (b)
doped and undoped TiO2 nanoparticles, on FTO substrates. Peaks are indexed to the
ZnO wurtzite or TiO2 anatase structure; peaks assigned to FTO are marked “F”.
(a) (b)
Fig. 4. Illuminated open-circuit photovoltage of (a) ZnO and Co0.1Zn0.9O nanorods, (b) TiO2 undoped or doped with Zr, on FTO substrates (each line represents one sample).
V. Manthina, A.G. Agrios / Electrochimica Acta 169 (2015) 416–423 419

5. bandgap to 3.27 eV. With 10% Zr substitution, the bandgap is
3.34 eV. The increases of Eg of 0.14 and 0.21 eV at the 5% and 10% Zr
doping levels, respectively, are in quite good agreement with the
OCV results above that indicated respective increases in the
conduction band energy of 0.1 and 0.2 eV for the same doping
levels.
In the case of ZnO, however, the optical analysis indicates a
minimal bandgap shift of 0.03 eV due to 10% substitution of the Zn
precursor with Co. Eg increased upon doping from 3.23 eV to
3.26 eV. The virtually unchanged bandgap contrasts with the OCV
observation of a negative CBM energy shift of 0.15 eV upon Co
doping. Taken together, the results of these two measurements can
only mean that the Co doping caused a downward shift of both the
valence and conduction bands. This suggests a modification of the
nanorod surface by Co altering the surface potential. For the
purposes of a DSSC, the position of the valence band is not
important, since bandgap excitation under sunlight will be
minimal. The CBM shift should therefore be favorable for the
hybrid DSSC device regardless of whether the bandgap contracted
or both bands shifted downwards in energy.
3.4. Kinetic Measurements
Characteristic time constants for electron transport and
recombination processes were determined from exponential fits
to the decay of photocurrent or photovoltage at short-circuit or
open-circuit, respectively, in response to square-wave modulation
of illumination intensity from an LED. The measurements were
performed on complete DSSC devices with iodide/triiodide
electrolytes. Representative electron lifetimes, i.e., the time
constants for recombination of the electron with dissovled
triiodide, are shown in Fig. 7(a). Electron lifetimes are similar
for the three TiO2 nanoparticle samples, with the 10% Zr-doped
TiO2 having slightly slower recombination. As has been observed in
other reports, ZnO nanorods exhibit significantly slower recombi-
nation than TiO2 nanoparticles. Our hybrid (ZnO–TiO2) samples
exhibit intermediate electron lifetimes between ZnO and TiO2
samples, with no clear trend between them except that the doped
materials had slightly shorter lifetimes than a combination of
undoped ZnO and TiO2. The intermediate lifetimes may reflect the
fact that a typical electron spends time in both materials, being
first injected in TiO2 but later transferring to ZnO.
Time constants for electron transport are shown in Fig. 7(b). The
samples divide into clear groupings, with nanoparticle films giving
the slowest transport, nanorod films giving the fastest transport,
and hybrid films giving intermediate transport times. Most
samples give the familiar inverse power-law dependence of ttr
on JSC. The exceptions are the nanorod films, which are flat with
time constants of about 0.3 ms. This is probably a measurement of
the RC time constant of the devices, which gives the upper limit of
the transport time [16]. The true transport times of the ZnO and Co-
doped ZnO nanorods cannot be distinguished, but both are
significantly faster than any of the other films. Zirconium doping
of TiO2 at the level of 5 at% reduces ttr relative to undoped TiO2; at
10 at% doping, ttr rebounds toward the value of undoped TiO2 but
remains significantly faster. The improvement of electron trans-
port rates upon Zr-doping has been previously observed [44] and
may be due to a mitigation of grain boundary scattering at the
interfaces between nanoparticles. The decrease in transport speed
with increasing doping levels likely reflects increased ionized
impurity scattering.
Among the hybrid films, at or near one-sun illumination, doping
has little effect on transport rates. At lower illumination intensities,
the traces diverge based on the doping of the ZnO nanorods, with
Co-doping reducing the transport time of the composite film. Zr-
doping of the TiO2 does not have a significant effect. Others have
reported increased transport resistance in ZnO nanorods upon Co-
doping [45,46]. We cannot make a direct comparison in the
nanorods since both have transport times below the RC time. The
interpretation of ttr in the hybrid films is not straightforward due
to complex interactions between the two materials.
Fig. 5. PL measurement of doped and undoped ZnO nanorods.
Fig. 6. Optical bandgap determination of the doped and undoped TiO2 (a) and ZnO
(b).
420 V. Manthina, A.G. Agrios / Electrochimica Acta 169 (2015) 416–423

6. 3.5. Device Performance
DSSC devices were assembled and tested using photoanodes
based on either ZnO nanorods (doped or undoped), TiO2 nano-
particles (doped or undoped), or combinations thereof with the
TiO2 nanoparticles coated over the ZnO nanorods. As a reference,
we measured the current–voltage characteristics for devices made
using an electrolyte made using only iodine and tetrabutylammo-
nium iodide in 3-methoxypropionitrile (Fig. 8). The films based
only on TiO2 nanoparticles yield the highest efficiencies. This is not
surprising, since recombination is slow in an IÀ
/I3
À
electrolyte, and
the fast transport provided by the ZnO nanorods is of little benefit
[16]. Among the TiO2-only films, 5% Zr doping results in a
considerable improvement in photocurrent, but the gains are lost if
the Zr doping level is increased to 10%. Efficiencies of devices based
on the ZnO nanorod films were low, although they were notably
improved by Co doping. Hybrid devices had intermediate
performance. Among these, undoped films gave the highest
short-circuit current density (JSC) and open-circuit voltage (VOC)
but with a low fill factor. Co doping gave a different J–V
characteristic but a similar solar photoconversion efficiency.
Doping the TiO2 harmed performance on either doped or undoped
ZnO.
To study the effect of the different nanostructures and materials
in systems with high recombination rates, we made devices using
ferrocene/ferrocenium electrolytes, which typically exhibit very
rapid scavenging of conduction-band electrons in DSSCs. The rapid
recombination in these samples makes the results highly sensitive
to transport and recombination rates. Nanoparticle-only films
(Fig. 9a) have very low currents (below 20 mA/cm2
). Despite the
extreme conditions in this sample of very low charge collection,
the trends are the same as in an iodide/triiodide electrolite: JSC
increases with 5% Zr doping, then decreases at the 10% Zr doping
level. Nanorod-only films (Fig. 9b) have currents more than an
order of magnitude higher, despite having much less surface area
compared to the nanoparticle films. The Co-doped ZnO nanorod
sample achieves a higher VOC than the undoped sample despite
having a more positive conduction band; this can only be the result
of suppressed recombination by the Co-doping as was observed in
the lifetime measurements. Among hybrid films (Fig. 9c), the
combination of undoped nanorods and undoped nanoparticles
performs best, and outperforms all nanoparticle-only and nano-
rod-only films. This is because the nanoparticles result in higher
surface area than the nanorod-only films, while the nanorod core
gives faster electron transport (and therefore better charge
collection efficiency) than the nanoparticle-only films. However,
the films with doping of either the TiO2 nanoparticles, or the ZnO
nanorods, or both, have performance similar to each other but
worse than the undoped sample.
It appears, then, that doping of the ZnO with Co and of the TiO2
with Zr achieved the immediate objective of shifting the
conduction band in the desired direction, but did not achieve
the broader objective of improving DSSC performance in the
context of electrolytes with fast recombination. According to the
results shown in Fig. 4, the correct band alignment exists that
should allow electrons to transfer from doped TiO2 nanoparticles
to doped ZnO nanorods. Indeed, doping just one of the materials
should enable this transfer. Yet the DSSC performance of doped
materials in Fc/Fc+
is below that of undoped materials. The doped
materials are not found to have inferior transport or recombination
properties. In fact, the doped materials surprisingly surpassed the
undoped materials on these measures. It is most likely that a
barrier to electron transfer remains between the two materials.
While the OCV measurements in water indicate the relative band
edge shifts due to doping, the absolute potentials of the band edges
in a DSSC device will differ from those in the OCV measurement
due to the adsorbed dye, the non-aqueous solvent, and the
adsorption of ions from the electrolyte. Therefore, a barrier may
still exist for electron transfer from TiO2 to ZnO in a DSSC
electrolyte despite the correct alignment indicated by the aqueous
OCV measurements.
Fig. 7. Characteristic times extracted from exponential fits for (a) electron lifetime
versus open circuit photovoltage, or (b) electron transport time versus short-circuit
current, for DSSC devices with IÀ
/I3
À
electrolytes.
Fig. 8. Current-voltage (J–V) characteristics for solar cells using electrolyte based
on IÀ
/I3
À
, under 1 sun.
V. Manthina, A.G. Agrios / Electrochimica Acta 169 (2015) 416–423 421

7. 4. Conclusions
The conduction band edge of the ZnO and TiO2 were engineered
to remove the energy barrier and provide effective electron
transport. The 5% Zr doping increased the photocurrent of the DSSC
due to faster electron transport, whereas 10% Zr doping decreased
the photocurrent probably due to an unfavorable conduction band
edge for electron injection. Among the doped nanocomposites,10%
Co-doped ZnO with 5% Zr-doped TiO2 performed best in Fc/Fc+
due
to fast electron transport and favorable electron injection of the 5%
Zr doped TiO2. The recombination is much lower for nano-
composite nanostructures when compared to nanoparticulate TiO2
films. However, the undoped nanocomposites outperformed
doped materials. These band edge engineered nanostructures
are promising for application in DSSCs, particularly those using
cobalt complex and ionic liquid electrolytes where mass transport
becomes limiting.
Acknowledgements
This material is based on work supported by the National
Science Foundation under Grant No. CBET-1332022. The authors
thank the University of Connecticut Center for Clean Energy
Engineering for the usage of the XRD and SEM.
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