Characteristics of p-NiO Thin films prepared by spray pyrolysis and their application in CdS-sensitized photocathodes

Article abstract of DOI:10.1149/1.3590742

Compact nickel oxide (NiO) thin films were prepared on various substrates via a simple spray pyrolysis technique. Morphological and structural characterization indicates that these NiO films are very uniform in thickness (～100 nm) and possess the bunsenite crystal structure. Optical measurements show that the NiO films are highly transparent with a band gap of 3.70 0.05 eV. Mott-Schottky plots obtained from electrochemical impedance spectroscopy measurements reveal that the as-deposited NiO on fluorine-doped tin oxide (FTO) glass behaves as a p-type semiconductor. The flat band potential of NiO was estimated to be ～0.36 V (vs. NHE) in 0.10 M tetrabutylammonium perchlorate/acetonitrile electrolytes. Cyclic voltammetric measurements of the NiO films on FTO in various redox electrolytes show that electrochemical reactions proceed in the accumulation region but are completely inhibited in the depletion region, indicating the NiO films effectively block the FTO substrate. Using these NiO blocking layers, a CdS-sensitized mesoscopic NiO photocathode operating in a polysulfide electrolyte is unambiguously demonstrated for the first time. It is anticipated that NiO thin films synthesized by spray pyrolysis could find important applications as stable and transparent electron barrier layers for various optoelectronic devices.

Full text of DOI:10.1149/1.3590742

Journal of The Electrochemical Society, 158 (7) H733-H740 (2011)
0
H733
013-4651/2011/158(7)/H733/8/$28.00 VC The Electrochemical Society
Characteristics of p-NiO Thin Films Prepared by Spray
Pyrolysis and Their Application in CdS-sensitized Photocathodes
a,b
Xuan-Hao Chan, James Robert Jennings, Md. Anower Hossain, Karen Koh Zhen Yu,
and Qing Wang
a
a
a
a,z
a
Department of Materials Science and Engineering, Faculty of Engineering, NUSNNI-NanoCore, National University of
Singapore, Singapore 117576
b
Faculty of Engineering, Multimedia University, Jalan Multimedia, 63100 Cyberjaya, Selangor, Malaysia
Compact nickel oxide (NiO) thin films were prepared on various substrates via a simple spray pyrolysis technique. Morphological
and structural characterization indicates that these NiO films are very uniform in thickness (ꢀ100 nm) and possess the bunsenite
crystal structure. Optical measurements show that the NiO films are highly transparent with a band gap of 3.70 6 0.05 eV. Mott-
Schottky plots obtained from electrochemical impedance spectroscopy measurements reveal that the as-deposited NiO on fluorine-
doped tin oxide (FTO) glass behaves as a p-type semiconductor. The flat band potential of NiO was estimated to be ꢀ0.36 V
(vs. NHE) in 0.10 M tetrabutylammonium perchlorate/acetonitrile electrolytes. Cyclic voltammetric measurements of the NiO
films on FTO in various redox electrolytes show that electrochemical reactions proceed in the accumulation region but are com-
pletely inhibited in the depletion region, indicating the NiO films effectively block the FTO substrate. Using these NiO blocking
layers, a CdS-sensitized mesoscopic NiO photocathode operating in a polysulfide electrolyte is unambiguously demonstrated for
the first time. It is anticipated that NiO thin films synthesized by spray pyrolysis could find important applications as stable and
transparent electron barrier layers for various optoelectronic devices.
VC 2011 The Electrochemical Society. [DOI: 10.1149/1.3590742] All rights reserved.
Manuscript submitted January 25, 2011; revised manuscript received April 5, 2011. Published May 18, 2011.
Nickel oxide (NiO) is a wide band-gap (E > 3.60 eV) semicon-
g
evaluated electrochemically in different redox electrolytes. Lastly,
the critical role of a compact NiO layer in a CdS-sensitized NiO
photocathode is demonstrated, which represents the only unambigu-
ductor, which naturally possesses p-type conductivity and is one of
very few stable and transparent transition metal oxides known.
1–3
c,30
Owing to these intriguing optical and electrical properties, efforts
have been made to integrate NiO layers of different dimensions and
morphologies into various electronic and optoelectronic devices.
For instance, mesoscopic NiO thin films have been extensively stud-
ous report of a semiconductor-sensitized photocathode to date.
Experimental
4
ied for use as an electrochromic material for smart windows, an
electroactive material for batteries and supercapacitors, a photoca-
Materials and chemicals.— All of the chemicals in this study are
commercially available and were used as received: nickel acetyla-
5
6,7
thode for dye-sensitized photoelectrochemical cells, etc. In addi-
tion, because of the p-type conductivity, compact nanocrystalline
NiO films may have important applications as robust electron barrier
layers in light emitting diodes and organic photovoltaic devices to
help facilitate charge separation.
cetonate (Ni(acac)
Sigma Aldrich, anhydrous 99.8%), ferrocene (Fc, Acros Organics,
98%), tetrabutylammonium perchlorate (TBAClO , Fluka, 99.9%),
lithium perchlorate (Fluka, 98.0%), tetrabutylammonium iodide
2
, Sigma Aldrich, 95%), acetonitrile (ACN,
4
2þ
(TBAI, Fluka, 99%), methyl viologen dichloride hydrate (MV
,
Various chemical and physical approaches have been developed to
synthesize NiO thin films, such as aerosol-chemical vapor deposition,
Sigma Aldrich, 98%), diisopropoxytitanium bis(acetylacetonate) so-
lution (TAA, Fluka, ꢀ75% in isopropanol), ethanol (Merck, 99.9%)
and denatured ethanol (Merck, 99.5%), NiO powder (Inframat,
8
9–13
chemical bath deposition,
14
electrodeposition, sputtering,
12,15–17
1
1
18
sol-gel, spin coating and spray pyrolysis,
13,19–27
etc., of which spray
99.9%, 20 nm), cadmium nitrate tetrahydrate (Cd(NO
3
)
Fluka, >99.0%), sodium sulfide nonahydrate (Na O, Sigma
2
ꢁ4H
2
O,
pyrolysis has several advantages over the others such as simple experi-
mental setup, cost-effectiveness, and ease of scale-up. In practice, spray
pyrolysis has become a standard technique used to prepare compact
2
Sꢁ9H
2
Aldrich). Commercially available FTO coated glass (TEC-15, Pil-
kington) and bare microscope glass slides were used as substrates
for sample synthesis. The substrates were cleaned by sonicating
them in 5% aqueous Decon 90 solution, deionized water and dena-
tured ethanol (10 min sonication each) then heated in a hot air
TiO thin films for use as a hole barrier layer in mesoscopic sensitized
2
solar cells, where a fast one-electron redox mediator or hole conductor
is used. Compared with n-type TiO , compact p-type NiO films could
2
ꢂ
potentially be used as an electron barrier layer in mesoscopic solar cells
with a sensitized photocathode (such as sensitized mesoporous NiO),
with which a tandem cell could be built by incorporating a sensitized
stream at 500 C for 15 min.
Compact NiO thin film preparation.— A nickel precursor solu-
tion of 0.02 or 0.08 M Ni(acac) in ACN was prepared. A reagent
2
sprayer (Kontes, TLC) and a vacuum pump (Vacuubrand ME 1C)
were used to create an aerosol from the solution. Substrates were
6
,28
photoanode.
could in theory be greatly improved, there has recently been much
With the impetus that the power conversion efficiency
endeavor to develop a viable sensitized photocathode, although p-NiO
is the only successful candidate so far.
29
ꢂ
ꢂ
kept heated at 450 C and all samples were sintered at 450 C for 30
min after deposition. Precursor solution was sprayed repeatedly in
very short pulses of 1 s followed by a 4 s pause throughout the depo-
sition period. Every 10 min the direction which precursor was
sprayed from was reversed. During depositions, a narrow area (of
width ꢀ0.50 cm) at the edge of the substrate (FTO/bare glass) was
covered by a glass mask to ensure that no material was deposited on
this area, which subsequently served as the baseline for
In this study, we report on the preparation of compact NiO thin
films on various substrates via spray pyrolysis, and their structural,
chemical, optical, electrical, and photoelectrochemical characteris-
tics; the final objective being to use NiO thin films as an electron
barrier layer in photoelectrochemical cells. The energetics of the
compact NiO thin films on fluorine-doped tin oxide (FTO) coated
glass, such as band gap energy and flat band potential, were deter-
mined optically and electrochemically, respectively. In addition, the
effectiveness of electron blocking by the compact NiO thin film was
c
2
We should note at this point that Rhee et al. have recently reported upon a Cu S/
NiO solar cell. However, these authors did not use a compact NiO barrier layer,
thus it is non-trivial to distinguish between true sensitization of the NiO and direct
hole injection from the Cu S into the substrate.
2
z
E-mail: qing.wang@nus.edu.sg
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H734
Journal of The Electrochemical Society, 158 (7) H733-H740 (2011)
Table I. Summary of experimental parameters and characterization results for NiO films.
a
b
c
Film thickness (nm)
d
Estimated band-gap energy (eV)
Sample
Substrate
Deposition time(min)
Ni(acac)
2
concentration
Crystal structure
NGL
NGH
NFL
NFH
Glass
Glass
FTO
FTO
40
40
40
40
L
H
L
135
105
112
91
3.72
3.75
3.72
3.62
bunsenite
bunsenite
bunsenite
bunsenite
H
a
b
c
d
N ¼ NiO sample
L ¼ 0.02 M, H ¼ 0.08 M.
Thickness was obtained from FESEM images.
2
direct band gap energy obtained from (ahv) vs. (hv) plots derived from UV-Vis measurements.
profilometric measurements and provided an area through which to
make ohmic contact to the electrode for samples deposited on bare
FTO. Table I summarizes important parameters, which were con-
trolled during sample deposition and includes selected results
obtained from several characterization tools.
Photoelectrochemical characterization.— NiO and TiO meso-
2
scopic films were prepared on various substrates following previously
32,33
published procedures.
2
CdS-sensitized NiO and TiO electrodes
34
were prepared by the SILAR method. The photoelectrochemical
cells were fabricated by sandwiching the sensitized electrodes with a
Pt counter electrode. The electrolyte was an aqueous solution of 1 M
Structural and optical characterization.— Thermogravimetric
analysis (TGA) of a nickel precursor source, Ni(acac) , was per-
2
formed using a Thermogravimetric Analyzer 2950 (DuPont) to
understand the decomposition of the material and give a firm confir-
mation of the appropriate synthesis temperature to be used. The
Na S, 1 M sulfur and 0.1 M NaOH. IPCE spectra were measured
2
with a spectral resolution of ca. 5 nm using a 300 W Xenon lamp and
a grating monochromator equipped with order sorting filters (New-
port/Oriel). The incident photon flux was determined using a cali-
brated silicon photodiode (Newport/Oriel). Photocurrents were
measured using an auto-ranging current amplifier (Newport/Oriel).
Control of the monochromator and recording of photocurrent spectra
were performed using a PC running the TRACQ Basic software
TGA was carried out in an atmosphere of air with a heating rate of
ꢂ
1
0 C/min. The geometric profile and thickness of the samples were
gauged by an Alpha-Step IQ surface profilometer (KLA-Tencor
instruments). The samples’ band-gap energies and transmission
spectra were determined by carrying out optical absorption studies
in the range 300–1000 nm utilizing a Varian Cary 5000 UV-VIS-
NIR spectrometer. Bare glass or FTO-coated glass was used as a
baseline for the absorption measurements. Determination of the
crystal structure of the samples was performed using a Bruker D8
(
Newport). Current-voltage characteristics under simulated AM 1.5
illumination were measured using a Keithley Source Meter and the
PVIV software package (Newport). Simulated AM 1.5 illumination
was provided by a Newport class A solar simulator and light intensity
was measured using a calibrated Si solar cell. The active area of the
2
cells was defined by a mask to be 0.1199 cm .
Advance X-ray diffractometer (XRD) with thin film attachment
ꢂ
(
monochromatic Cu Ka radiation source) by scanning from 20 /2h
ꢂ
ꢂ
to 80 /2h with a step size of 0.02 and dwell time of 24 s. A Zeiss
Supra 40 field-effect scanning electron microscope was used to
characterize the morphology and to determine film thickness. The
samples were sputtered and covered by a thin layer of gold before
the FESEM images were taken. X-ray photoelectron spectroscopy
Results and Discussion
Thermogravimetric analysis (TGA) was employed to determine
the characteristics of the precursor Ni(acac) upon thermal decom-
2
position at elevated temperature; The results are shown in Fig. 1.
A great reduction of weight was observed in the temperature range
(
XPS) measurements were performed on samples synthesized on
ꢂ
of 350–390 C. The residue weight of the material was 27.3% of the
ꢂ
bare glass substrates using a Kratos Analytical – Axis Ultra DLD
with a monochromatic aluminum X-ray source.
original weight at temperatures above 400 C, being close to the
weight ratio of NiO to Ni(acac) — 29.0%. The material showed no
2
ꢂ
Electrochemical characterization.— All electrochemical meas-
urements were performed using an Autolab potentiostat/galvanostat
further weight loss as the temperature rose further to 450 C. This
(
PGSTAT 302N) and a one compartment, 3-electrode cell with the
2
NiO/FTO samples (exposed area of 0.25 cm ) as the working elec-
trode, a platinum (Pt) wire as counter electrode and a silver/silver
nitrate (Ag/AgNO ) reference electrode. The Ag/AgNO reference
3
3
electrode was calibrated against ferrocene internal standard (electro-
lyte consisting of 5 mM ferrocene and 0.1 M TBAClO in ACN; Pt
4
ꢃ1
disc working electrode and 10 mVs scan rate). The reduction
potential of ferrocene/ferrocenium (Fc/Fc ) reaction was measured
þ
to be 70 6 10 mV versus Ag/AgNO . All electrochemical potentials
3
are henceforth reported versus the normal hydrogen electrode
NHE) and were converted assuming Fc/Fc is 630 mV versus
þ
(
31
NHE. The flat band potentials of the samples in redox inactive
supporting electrolyte were determined by Mott-Schottky plots
derived from dark electrochemical impedance spectroscopy (EIS)
measurements. The impedance measurements were implemented in
two different electrolytes: (i) 0.1 M TBAP in ACN (E1) and (ii) 0.1
4
M LiClO in ACN (E2). The frequency-dependent impedance was
fitted using NLS software (Schribner Associates, (Z-view 3.1c) to
obtain the space charge capacitance of the samples. Cyclic voltam-
metry of the samples were measured in two different electrolytes:
2
þ
i) 1 mM MV and 0.1 M LiClO in ACN (E3) and (ii) 5 mM
4
(
Figure 1. (Color online) Thermogravimetric analysis (black curve) and dif-
ferential thermal analysis (red curve) of the precursor Ni(acac)2.
TBAI and 0.1 M TBAClO in ACN (E4).
4
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Journal of The Electrochemical Society, 158 (7) H733-H740 (2011)
H735
small difference of weight loss is presumably due to the existence of
nickel (III) oxide (Ni ) (Ref. 35) or equivalently excessive oxy-
gen which is normally observed on the surface of NiO nanocrys-
tallites. To ensure full decomposition of Ni(acac) and the formation
2 3
O
1
5
2
of NiO films, the temperature for spray pyrolysis was then set to
ꢂ
4
50 C.
To avoid complications arising from the crystalline FTO con-
ducting substrate, NiO films were firstly deposited onto bare micro-
ꢂ
scope glass slides by spray pyrolysis of (Ni(acac)
2
) at 450 C, with a
deposition time of 40 min. The thin films obtained were highly uni-
form and transparent to the naked eye. Figure 2 shows the XRD pat-
terns of NiO films on glass or FTO substrates, prepared using high
or low concentration (0.08 or 0.02 M, labeled H or L) Ni(acac) sol-
2
utions (labeled NGL and NGH or NFL and NFH, for bare glass and
FTO substrates, respectively, cf. Table I) according to the procedure
outlined in the Experimental Section. Well-defined diffraction peaks
ꢂ
can be observed at 37.20, 43.20, and 62.87 , being indicative of the
111), (200), (220) planes of the bunsenite crystal structure (JCPDS
(
No. 04-0835) of NiO. The optical characteristics of NGL and NGH
films were analyzed by UV-Vis spectroscopy with bare glass used
2
as the baseline for the measurements. As indicated by the (ahm) vs.
hm plots in Fig. 3a, the direct band gap energies of NGL and NGH
are estimated to be 3.72 and 3.75 eV, respectively, which are com-
parable to previously reported values that fall in the range 3.60–4.00
1
–3
eV.
To investigate the chemical and electronic states of the
elements in the NiO films, both NGL and NGH were further charac-
terized by XPS and the respective Ni 2p and O 1s spectra are shown
in Fig. 4. Consistent with the spectra of NiO thin films obtained
36,37
by others,
the primary Ni 2p and O 1s peaks were observed
at 854.0 and 529.1 eV, respectively. Apart from these peaks, addi-
tional features appear at ꢀ855.0 and ꢀ531.5 eV as small shoulders,
which are presumably due to the presence of a small quantity of
3
3
7,38
Ni
2
O
on the surface, as suggested by the TGA measurement.
Figures 5a and 5b show the morphology and cross-sectional scan-
ning electron micrographs of NGL and NGH. A uniformly deposited
layer consisting of small granular particles of 20–30 nm can be
clearly observed from the face view of the NGL sample, while the
size of the granular particles is slightly larger for NGH films. The
cross-sectional views in Figs. 5a and 5b indicate very clearly that
continuous and compact layers with uniform thicknesses of ꢀ135
and ꢀ105 nm were successfully synthesized for NGL and NGH,
Figure 3. (Color online) UV-Vis spectra of NiO and TiO
(ahm) vs. hm plots of NiO samples: NGL (black), NGH (red), NFL (green),
and NFH (blue); (b) Transmission spectra of NiO (NFH; blue) on FTO, TiO
2
thin films. (a)
2
2
on FTO (red) and bare FTO (green) films. The inset of (b) shows photo-
graphs of the films on a piece of white paper printed with the NUS logo.
respectively. However, there has been difficulty in correlating the
amount of precursor sprayed with the thickness of synthesized films,
3
9
as similarly observed by Cameron et al. when synthesizing TiO
compact thin films.
2
After demonstrating that NiO thin films could be synthesized on
glass slides from Ni(acac) by spray pyrolysis (NGL and NGH sam-
2
ples), a similar synthetic procedure was applied using FTO-coated
glass substrates, from which samples referred to as NFL and NFH
were produced. XRD patterns of the two samples were shown in
Fig. 2 alongside those of NGH and NGL. Consistent with NGL and
NGH, the characteristic diffraction peaks of (111), (200) and (220)
planes are observed, being indicative of the same bunsenite crystal
structure. However, it seems the samples prepared on FTO sub-
strates have different crystal orientations from those deposited on
bare microscope glass slides, as indicated by the relative variation
of diffraction peak intensity in Fig. 2. The preferential growth plane
of the NiO film is altered from (200) to (111), presumably as a result
of the lattice matching between the (111) plane to the (110) plane of
Figure 2. (Color online) XRD diffraction patterns of the NiO samples syn-
thesized here, below is the bunsenite NiO reference pattern (JCPDS No. 04-
the underlying SnO substrate when NiO starts to grow. In addition,
2
0
835). The peaks marked with dots result from the FTO substrate.
it was observed that by increasing the deposition time, a more
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H736
Journal of The Electrochemical Society, 158 (7) H733-H740 (2011)
Figure 4. (Color online) XPS spectra of
the prepared NiO thin film on optical glass
slides: (a) Ni 2p3/2 and 2p1/2 of NGL; (b)
O 1s of NGL; (c) Ni 2p3/2 and 2p1/2 of
NGH; (d) O 1s of NGH. Black lines are
experimental data, red lines are fits, and
grey lines show the contribution of indi-
vidual peaks to the overall fit.
crystalline structure of NiO was obtained. Figure 3a also shows the
UV-Vis spectra of NFL and NFH, where bare FTO was used as a
baseline for the measurements, and both samples have an estimated
direct band-gap energy of 3.62–3.72 eV from (ahm) vs. hm plots,
which is similar to that of NGL and NGH on glass slides. Figure 3b
shows the transmission spectra of bare FTO, NFH and a commonly
dark electrochemical impedance spectroscopy (EIS) measurements
as a function of bias potential (E) in a Mott-Schottky analysis.
Although Efb must in fact lie at a more negative potential than the
VBM, for a moderately doped p-type semiconductor Efb should lie
fairly close to the VBM. For pure metal oxide semiconductors (i.e.
non-degenerate), the Mott-Schottky equation can be written as
2
used TiO
FTO substrate, there is 15–20% loss of transmission from 400 to
000 nm due to the absorption of the NiO layer. Encouragingly, the
2
compact layer (ꢀ100 nm) on FTO. Compared with the
ꢀ
ꢁ
1
2
kBT
e
¼
E ꢃ Efb ꢃ
[1]
2
sc
2
1
C
ee jN A
0
sc
transparency of the NiO thin film is only marginally lower than that
of the typical TiO compact layer, giving great promise for various
optoelectronic applications.
where Csc is the space charge region capacitance, e
permittivity, j is the dielectric constant, Nsc is the doping density,
and A is the surface area of the NiO electrode.
As the measurement was carried out in an inert electrolyte, no
Faradaic process was involved. A simplified equivalent circuit
s
consisting of the series resistance (R ) in series with the total capaci-
tance (C) was used to fit the impedance spectra. The total capaci-
tance contains the space charge capacitance (Csc) of the NiO film in
series connection with the double layer (Helmholtz layer) capaci-
tance (Cdl) at the NiO/electrolyte interface. In the potential range
where the total capacitance is dominated by the space charge layer
is the vacuum
2
0
Apart from the preferential growth of (111) planes, the morphol-
ogy of NFL and NFH seems to follow the mosaic-like surface struc-
ture of the supporting FTO substrate (Fig. 5e), as clearly shown by
the face views in Figs. 5c and 5d. However, the cross-sectional
views distinctly indicate that the deposited NiO layer is filling the
gaps between FTO grains and is continuous on top of the FTO layer.
Similar to those on glass slides, NiO films deposited from a less con-
2
centrated Ni(acac) solution (NFL) on FTO are slightly thicker than
those produced from a more concentrated solution (NFH) as shown
in Fig. 5 and Table I. A systematic study is required to elucidate the
effects of synthesis parameters on the characteristics of the NiO
films, in order to obtain a better understanding of the film growth
mechanism.
As a p-type, hole-selective layer with various optoelectronic
applications, the energetics of the synthesized NiO films, such as the
valance band maximum (VBM), and the resulting electrical proper-
ties are imperative. The VBM of NiO was estimated by determining
the flat band potential (Efb) of the NiO films prepared on an FTO
substrate, through deriving the space charge capacitance value from
capacitance (Csc ꢄ Cdl), the R
ꢃ Csc circuit was used directly to fit
s
the high frequency range of the impedance spectra. A constant phase
ꢃ1
ꢃp
element (CPE), with impedance defined by ZCPE ¼ T (xi) , was
used in the place of Csc to improve the fitting. In all cases the CPE
exponent, p, was in the range 0.95–1 and henceforth T values are
treated as capacitances (with units of Farads) without correction.
Figure 6 shows Mott-Schottky plots for NiO thin films prepared
2
on FTO substrates. The negative slopes of 1/Csc ꢃE plots in Fig. 6
indicate the NiO is p-type, which is in contrast to the positive slope
2
of n-TiO reported elsewhere. The Efb of the NiO electrode is ca.
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Journal of The Electrochemical Society, 158 (7) H733-H740 (2011)
H737
Figure 5. (Color online) Scanning electron micrographs showing the top and cross-sectional views of the prepared NiO thin films on different substrates: (a)
NGL; (b) NGH; (c) NFL; (d) NFH; (e) bare FTO substrate.
0.36 V (vs. NHE) in electrolyte E1, as determined from the intercept
of the fitted straight line on the E axis corrected by the thermal volt-
age (k T/e; cf. Eq. 1). The same NiO electrode measured in an Li -
slightly outside the range of previously reported values for NiO that
19
ꢃ3
we are aware of, which can be as low as 9.5 ꢅ 10 cm or as high
þ
21
as 1.8 ꢅ 10 cm , depending on the precise preparation and mea-
ꢃ3
B
40,42
containing electrolyte (E2) gave an Efb of ca. 0.38 V (vs. NHE),
which is slightly more positive than that in the Li -free electrolyte,
surement conditions. The precise doping density is not crucial
þ
for application of the NiO films as electron blocking layers. All that
is required is that the conductivity of the layer is sufficiently high so
that it does not impart too great a series resistance to the electrode,
and that the doping density is not so high that the electrode becomes
degenerate.
The electrical properties of these NiO thin films were examined
electrochemically by using samples deposited on FTO substrates as
working electrodes in voltammetric experiments. Two types of
þ
E1. A positive shift in E is expected in a Li containing elector-
fb
4
0
lyte, but the very small shift found here is probably within experi-
mental uncertainty.
The doping density, N , was calculated from the slopes of the
sc
4
1
MS plots taking j ¼ 11.9. Unsurprisingly, both electrolytes E1
19
ꢃ3
and E2 yield similar results for N , with values of 4.9 ꢅ 10 cm
sc
1
9
ꢃ3
and 4.8 ꢅ 10 cm being found, respectively. These values are
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H738
Journal of The Electrochemical Society, 158 (7) H733-H740 (2011)
Figure 6. (Color online) Mott-Schottky plots of a NiO thin film (NFL; 0.02
M precursor solution, FTO substrate) measured in a 0.1 M TBAP acetonitrile
4
electrolyte (E1; black) and in a 0.1 M LiClO acetonitrile electrolyte (E2;
red).
electrolytes containing redox active molecules were employed, with
redox potentials falling in the accumulation and depletion regions of
NiO, so that the extent of interfacial charge transfer occuring in dif-
ferent operating regiemes (i.e. accumulation or depletion) could be
2
þ
evaluated. With methyl viologen (MV ) as the redox mediator and
LiClO in actonitrile as supporting electrolyte (E3), cyclic voltam-
4
metric measurements of the NiO thin films were conducted, and
results were compared with those obtained for Pt disk and bare FTO
electrodes. As shown by the cyclic voltammograms in Fig. 7a, reac-
2
þ
tion of MV on a Pt electrode gives two consecutive reduction
peaks at ꢃ0.31 and ꢃ0.64 V (vs. NHE), corresponding to the forma-
þ
43
tion of MV and MV, respectively. In comparison, the reduction
on bare FTO is less reversible, with a small shoulder at around
ꢃ0.18 V and a broad, but larger peak at ꢃ0.69 V (vs. NHE). The
2þ
above redox behavior of MV is similar to that observed by Gem-
43
2þ
borys et al. Evidently, the reduction of MV takes place in a
potential range much more negative than the flat band potential of
Figure 7. (Color online) (a) Cyclic voltammograms of NiO (NFL: blue;
NFH: green), Pt disk (black) and bare FTO (red) electrodes measured in an
2
þ
NiO, namely in the depletion region. So MV could be a suitable
indicator molecule for gauging the electron-blocking capabilities of
the compact NiO films on FTO.
2þ
MV containing electrolyte (E3). (b) Cyclic voltammograms of NiO (NFH:
green), Pt disk (black) and bare FTO (red) electrodes measured in an iodide
containing electrolyte, E4. The scan rate was 0.1 V/s.
Cyclic voltammograms of NFL and NFH in electrolyte E3 are
2þ
also shown in Fig. 7a. In contrast to the clear reduction of MV on
Pt and FTO electrodes, no appreciable reduction peak is observed in
the same potential range on NiO electrodes, as a result of an absence
of carriers in the depletion region. In addition, stemming from the
same reason, the non-Faradaic current of the charging of the electrical
double layers at electrode-electrolyte interface, and the space charge
region within the electrode, is also greatly suppressed, being similar
to an insulating material. The marked cathodic current-suppressing
capability explicitly indicates that the NiO thin films effectively block
the transport of electrons through the layer and act as an electron
scanned positively, two oxidation peaks appear at ꢀ0.87 and 1.0 V
(vs. NHE), corresponding to the formation of triiodide and iodine,
45
respectively. The oxidation of iodide on bare FTO is much less re-
versible, requiring a large overpotential, so that oxidation peaks
now occur at ꢀ1.63 and 1.88 V (vs. NHE) for formation of triiodide
2þ
and iodine, respectively. In contrast to the reduction of MV
shown previously, the oxidation of iodide is located well inside the
accumulation region of NiO in terms of the position of the flat band
potential, where holes are accumulated in the space charge region
close to the electrode-electrolyte interface.
“
barrier” layer, which is desirable in many optoelectronic applica-
tions. The lack of cathodic current also indicates that the NiO film
itself is not reduced to any significant extent at the potentials we study
here, which might have resulted in accompanying Li intercalation.
To investigate the charge transport properties of the NiO films in
the accumulation region, similar electrochemical tests were per-
formed in electrolyte E4, where the redox species iodide was dis-
solved in acetonitrile with TBAClO as the supporting electrolyte.
4
Fig. 7b shows the cyclic voltammograms for a Pt disk electrode in
this electrolyte over a wide potential range. When the potential is
Cyclic voltammograms of NFH in electrolyte E4 are shown in
Fig. 7b. Similar to the cyclic voltammograms in Fig. 7a, NiO elec-
trodes exhibit a similar “blocking” effect for electron transfer when
scanned in the negative potential region in electrolyte E4, where the
current is greatly suppressed. Interestingly, the oxidation of iodide
on the NiO electrode is kinetically more favorable than on bare
FTO, giving two well-defined oxidation peaks at lower potentials of
ꢀ1.13 and 1.63 V (vs. NHE). The accelerated oxidation of iodide on
NiO is presumably due to the catalytic effect of the surface of NiO,
þ
44
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Journal of The Electrochemical Society, 158 (7) H733-H740 (2011)
H739
Figure 8. (Color online) (a) Incident photon-to-current efficiency spectra for a CdS-sensitized mesoporous NiO electrode with a compact NiO underlayer (green
line), a CdS-sensitized mesoporous TiO electrode (dashed red line) and an unsensitized (bare) mesoporous NiO electrode (blue dash-dot line). (b) Absorbance
spectra of SILAR-deposited CdS on NiO (thin film) and TiO (mesoporous) electrodes, for ease of comparison both spectra have been normalized to the absorb-
ance at 350 nm. (c) Current-voltage (j-V) characteristics measured under simulated 1 Sun AM 1.5 illumination for a CdS-sensitizized NiO electrode with a com-
pact NiO underlayer (green line), a CdS-sensitized NiO electrode without a compact NiO underlayer (blue dash-dot line) and a CdS-sensitized TiO electrode
dashed red line). For all IPCE and j-V tests the electrodes were in contact with a polysulfide electrolyte and a Pt-coated counter electrode was used. (d) Sche-
2
2
2
(
2
matic diagrams illustrating the charge separation process in both CdS-sensitized TiO and NiO upon illumination.
although the slightly larger surface area of NiO as compared to bare
FTO may also contribute to the decrease of overpotential, and more
studies are required to get a mechanistic interpretation of the intrigu-
ing phenomenon.
The electrochemical tests shown in Fig. 7 clearly demonstrate
that these NiO thin films are effective at blocking the transport and
transfer of electrons in the depletion region, while facilitating the
transport and transfer of holes in the accumulation region. Thus, the
films are suited for application as electron barrier layers in various
optoelectronic devices. To demonstrate the effectiveness of the NiO
thin films prepared here, a CdS-sensitized NiO electrode with a
NFH under-layer was tested in a two-electrode thin-layer cell con-
figuration using a polysulfide based electrolyte and a Pt-coated
counter electrode. For comparison, cells were also fabricated with-
out compact NiO under-layers, with unsensitized (bare) mesoporous
NiO, and with CdS-sensitized mesoporous TiO . As described in the
2
experimental section, CdS was deposited by the SILAR technique.
Figure 8a shows the incident photon-to-current efficiency (IPCE)
spectra for a CdS/NiO cell with a NiO under-layer, a bare NiO cell
with an under-layer, and a CdS/TiO cell. The bare NiO cell pro-
2
duces negligible photocurrent over the entire wavelength range
tested, and thus IPCEs of practically zero, while the CdS/NiO cell
produces a small, but noticeable photocurrent at wavelengths where
CdS absorbs light, as shown in Fig. 8b which shows normalized ab-
sorbance spectra for SILAR-deposited CdS layers on two different
substrates (mesoporous TiO2 and a compact NiO thin fim). The
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H740
Journal of The Electrochemical Society, 158 (7) H733-H740 (2011)
CdS/TiO
2
cell exhibits far greater IPCEs than the CdS/NiO cell, but
various optoelectronic applications. As demonstrated in CdS-sensi-
tized NiO photocathode cells, the NiO thin films have potentially im-
portant applications as stable and transparent electron barrier layers
for various optoelectronic devices, such as semiconductor-sensitized
nanocrystalline NiO solar cells.
crucially the wavelength region where both cells produce photocur-
rent is very similar, and coincides with the known light absorption
range of CdS.
A very important difference between the CdS-sensitized NiO
and TiO cells is that the sign of the photocurrents is opposite, as
2
shown by the current-voltage characteristics in Fig. 8c. The sensi-
tized NiO electrode works in a photocathodic mode when a compact
NiO under-layer is used, with a positive open-circuit photovoltage
Acknowledgment
This work was financially supported by NUS startup grant No.
R-284-000-064-133, URC grant No. R-284-000-068-112, and NRF
CRP grant No. R-284-000-079-592.
(
measured with respect to the counter electrode) being developed
upon illumination, in contrast to the negative photovoltage of the
CdS-sensitized TiO photoanode. As illustrated in Fig. 8d, it is well
2
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