P-Type dye-sensitized solar cell based on nickel oxide photocathode with or without Li doping

Article abstract of DOI:10.1016/j.jallcom.2013.08.142

Nickel oxide (NiO) nanostructures are synthesized using a microwave-assisted hydrothermal method. The hydrothermal bath has a solution of nickel salt mixed with precipitating agent. During the synthesis the microwave temperature, the concentration of nickel salt and precipitating agent along with the pH of the reaction solutions are changed and different morphologies of nickel oxide are obtained. The resulting nickel oxide nanostructures are characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Brunauer-Emmett-Teller method and X-ray photoelectron spectroscopy. Thus formed NiO has been used as a photocathode in dye-sensitized solar cell. Lithium doped NiO showed better IPCE as well as solar to electrical conversion efficiency than the undoped NiO.

Full text of DOI:10.1016/j.jallcom.2013.08.142

Journal of Alloys and Compounds 584 (2014) 142–147
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
p-Type dye-sensitized solar cell based on nickel oxide photocathode
with or without Li doping
Hui-Tzu Wang a, D.K. Mishra a, Peter Chen b, Jyh-Ming Ting a,
⇑
a Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan
b Department of Photonics, National Cheng Kung University, Tainan, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 May 2013
Received in revised form 3 August 2013
Accepted 24 August 2013
Available online 2 September 2013
Nickel oxide (NiO) nanostructures are synthesized using a microwave-assisted hydrothermal method.
The hydrothermal bath has a solution of nickel salt mixed with precipitating agent. During the synthesis
the microwave temperature, the concentration of nickel salt and precipitating agent along with the pH of
the reaction solutions are changed and different morphologies of nickel oxide are obtained. The resulting
nickel oxide nanostructures are characterized using X-ray diffraction, scanning electron microscopy,
transmission electron microscopy, Brunauer–Emmett–Teller method and X-ray photoelectron spectros-
copy. Thus formed NiO has been used as a photocathode in dye-sensitized solar cell. Lithium doped
NiO showed better IPCE as well as solar to electrical conversion efficiency than the undoped NiO.
Ó 2013 Elsevier B.V. All rights reserved.
Keywords:
p-Nickel oxide
Li-doping
Hydrothermal
Microwave
Photocathode
Dye-sensitized solar cell
1. Introduction
(NiSO4ꢀ6H2O) and urea solution have been used by Lai et al. [12]
to synthesize coralloid nanostructure nickel hydroxide hydrate uti-
Nickel oxide is a very promising material owing to its p-type
conductivity with wide bandgap and has attracted numerous
attentions for its versatile applications including its use as catalysts
[1], electrode materials for lithium ion batteries and fuel cells [2,3],
electrochemical supercapacitors [4], magnetic materials [5], and
dye-sensitized photo-cathodes [6]. Many wet chemical methods
have been developed to fabricate micro/nanostructures of Nickel
oxide (NiO) such as sol–gel process [7], spray pyrolysis method
[8], microemulsion technique [9] and traditional hydrothermal
method [10]. Compared to these methods, the microwave-assisted
hydrothermal preparation [11,12] provides several advantages
such as rapid heating, high reaction rate, short reaction time, and
energy saving [13]. However, there are only limited reports in
which NiO has been synthesized using such a method.
lizing again a focus microwave assisted hydrothermal method.
Pure coralloid-like nanostructure nickel oxides have obtained by
the calcination of the nickel hydroxide hydrate at 400 °C. The pro-
posed mechanism involved a sequence of steps: nucleation of nick-
el precursor and urea to form tiny particles, growth of tiny particles
by aggregation to form sheets or plates, self-assembly and overlap
of sheets or plates to form coralloid nanostructure. Because of the
presence of NCO, CO23ꢁ, and OHꢁ groups, it has been believed that
the electrostatic and hydrogen bonds are the main driving forces
for self-assembly to form coralloid nickel hydroxide hydrate. Re-
cently a couple of reports have mentioned about DSCs using NiO
as photocathode and TiO2 as photoanode. Lithium doped NiO
(4%) photocathode has shown 0.3% efficiency [14] for TiO2 as pho-
toanode whereas 0.2 M KI with 0.02 M Iodine as the electrolyte.
Spray pyrolysis has been carried out at 500 °C on FTO substrates
to prepare the Li-doped NiO film. When the electrolyte concentra-
tion of 0.1 M LiI and 0.05 M Iodine was used by Mizoguchi and Fuji-
hara [15], the cell efficiency reached a similar value as Joseph et al.
[14] at 0.31% for NiO–TiO2 tandem cell. They also used NiO photo-
cathode as p-DSC which has shown the efficiency of 0.057% for an
increased concentration of electrolyte to 0.6 M LiI and 0.3 M Iodine
in MPN. A high temperature profile has been used (500 °C in flow-
ing oxygen) to prepare the NiO here too. Nattestad et al. [16] have
used a commercial NiO nanopowder (ꢂ20 nm) for making p-DSC
and shown that Coumarin 343 dye has the best efficiency
Song and Gao [11] have used Nickel chloride (NiCl2ꢀ6H2O), so-
dium acetate, and ethylene glycol to synthesize NiO nanorods
using a focused microwave assisted hydrothermal method fol-
lowed by calcination. The diameter of resultant structures ranges
from 500 to 800 nm with a length up to 10 lm. The ethylene glycol
serves as a ligand that forms linear coordination complexes, which
then transformed to precursor nanorods. After calcination, the pre-
cursor became polycrystalline NiO nanorods. Later, Nickel sulfate
⇑
Corresponding author. Tel.: +886 6 2757575x62949; fax: +886 6 2385613.
E-mail address: jting@mail.ncku.edu.tw (J.-M. Ting).
0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2013.08.142

H.-T. Wang et al. / Journal of Alloys and Compounds 584 (2014) 142–147
143
composition was determined by X-ray photoelectron spectroscopy (XPS). The spe-
cific surface area was calculated using the Brunauer–Emmett–Teller (BET) method.
UV–Vis measurement was carried out to understand the absorption process. Cell
was evaluated using a solar simulator (100 mW/cm2, the equivalent of one sun at
AM1.5G) to obtain the J–V curve. A typical cell size of 4 ꢃ 4 mm was used. Incident
photon-to-electron conversion efficiency (IPCE) was also conducted under 10 mW/
cm2 light in the range of 400–800 nm.
(0.016%) among many other dyes for NiO film sintered at 500 °C.
The electrolyte has been 0.5 M LiI and 0.05 M Iodine in propylene
carbonate. When the Iodine concentration has increased to 0.5 M,
the efficiency also increased to 0.025%. Recently Nattestad et al.
[17] have modified the electrolyte and used three different colored
dyes for NiO cell and found the red dye has been most appropriate
for the NiO films. The highest efficiency of 0.41% reported for the
1.25 lm NiO film in a mixture of electrolytes such as guanadinium
3. Results and discussion
thiocyanate, TBP, and acetonitrile/valeronitrile. In the mean time
Zhang et al. [18] have reported the Voc as high as 350 mV for NiO
based DSC using thermolysis at 950 °C followed by screen printing.
When NiO microballs prepared by thermolysis and used in the DSC
by Power et al. [19], there has been an unprecedented high Jsc of
The schematics of the NiO growth process and various charac-
terizations are depicted in Fig. 1. XRD patterns of the precursor
and NiO powders are given in Fig. 2. All the peaks are consisted
of those of nickel oxalate precursor and after calcinations trans-
formed to NiO phase. Three major peaks belonging to (111),
(200) and (220) planes observed in the analyses range and the
structure of NiO is FCC similar to that of sodium chloride. The grain
sizes of these synthesized NiO, Dc, was calculated from the major
diffraction peak (200) using the Debye–Scherrer formula (Eq. (1)),
5.15 mA/cm2 using a novel sensitizer, PMI-6T-TPA for a 2.8
lm
film. Although there has been improvement in Jsc or Voc for NiO
DSCs, the requirement of a low temperature process for the growth
of highly crystalline NiO has been most sought after and here we
have synthesized Lithium doped NiO by microwave-assisted
hydrothermal method and utilized thus formed NiO as a photo-
cathode in dye-sensitized solar cell.
Dc ¼ Kk=ðb cos hÞ
ð1Þ
where K is a constant (ca. 0.9) [20], k is the X-ray wavelength used
in XRD (1.5418 Å), h is the Bragg angle and b is the diffraction
broadening of a peak at half-height, that is, broadening due to the
crystallite dimensions. The grain size in all samples calculated by
the Scherrer formula is 9–11 nm.
2. Experimental procedure
In this study, we have reported the use of microwave heating to nickel oxalate
(NiC2O4) precursor prepared via chemical reaction between Ni salts (nickel sulfate)
and precipitating agent (oxalic acid). The precursor was then subjected to calcina-
tion to obtain NiO.
The BET specific surface areas are measured for the prepared
samples and found to be around 75 m2/g as shown in the Supple-
mentary material Table S1(a). The pore size and the total pore vol-
ume of all samples are around 9.0 nm and 0.15 cm3/g respectively.
The surface morphological study of the nickel oxide nanoparti-
cles was carried out using SEM image. Fig. 3 is the SEM images of
the prepared precursor of sample A and the obtained NiO of all
samples. The SEM images of sample A in Fig. 3(a) shows that the
2.1. Materials
Nickel sulfate (NiSO4ꢀ6H2O), oxalic acid (C2H2O4), Lithium hydroxide (LiOHꢀH2-
O), Poly-vinyl pyrrolidone (PVP, MW = 4000), ethyl cellulose, terpineol, ethanol, in-
dium-tin-oxide (ITO), coumarin 343 (C343), 1-Propyl-2,3-dimethylimidazolium
iodide (DMPII), 4-tert-Butylpyridine (TBP), 3-Methoxypropionitrile (MPN).
2.2. Synthesis of NiO powder with and without Li doping
average particles size of the obtained NiO is about 2.9 0.15 lm
Nickel sulfate solution was mixed with oxalic acid solution having a protecting
agent (PVP). The concentrations of both nickel salt and precipitating agent solution
were varied (shown in Table 1). The mixed solution was then transferred to a Teflon
container of the microwave system then heated at 100 °C for 15 min. The green pre-
cipitate was separated by centrifugation, washed by deionized water several times,
and then dried in an oven at 60 °C. Finally, the green powder was calcined at 400 °C
for 2 h to obtain the black product. For Li-doping, lithium hydroxide was added to
the mixed solution of Sample A at various concentrations of of 1%, 2.5%, and 5%.
2.3. DSC fabrication
NiO powders, ethyl cellulose, and terpineol were mixed homogeneously in eth-
anol to form a paste as described elsewhere. [17] The paste was then applied to ITO
coated glass substrate using a doctor-blading technique. The coating was heat trea-
ted at 450 °C for 30 min to form NiO photocathode. The photocathode was then
soaked in C343 dye for 20 h. The electrolyte consisted of I2, LiI, DMPII, and TBP,
mixed in MPN. The counter electrode was Pt coated ITO glass.
Fig. 1. Schematics of the NiO growth process and the characterisations.
2.4. Characterization
The phase compositions of the powders were identified using X-ray diffractom-
etry (XRD). The morphology of NiO nanoparticles was studied using scanning elec-
tron microscope (SEM) and transmission electron microscope (TEM). The purity and
Precursor
NiO
Table 1
The reaction solutions with different nickel sulfate and oxalic acid concentration and
different adding sequence of PVP.
After calcination
Sample Nickel salt
(concentration)
Precipitating agent (concentration) (pH
value)
Precursor
A
B
C
D
E
NiSO4 (0.1 M)
H2C2O4 (0.1 M)
H2C2O4 (0.1 M)
H2C2O4 (0.1 M)
H2C2O4 (0.02 M)
H2C2O4 (0.02 M) pH = 4.5
NiSO4 (0.02 M)
NiSO4 (0.01 M)
NiSO4 (0.02 M)
NiSO4 (0.02 M)
0
20
30
40
50
60
2θ
Fig. 2. XRD of precursor and after the calcination.

144
H.-T. Wang et al. / Journal of Alloys and Compounds 584 (2014) 142–147
Fig. 3. SEM images of NiO samples prepared at different concentration of Ni salt and oxalate. (a) NiSO4 [0.1 M]; H2C2O4 [0.1 M]. (b) NiSO4 [0.02 M]; H2C2O4 [0.1 M]. (c) NiSO4
[0.01 M]; H2C2O4 [0.1 M]. (d) NiSO4 [0.02 M]; H2C2O4 [0.02 M]. (e) NiSO4 [0.02 M]; H2C2O4[0.02 M]; pH = 4.5.
and the morphology is clustered and flower-like. Additionally, the
morphology of the NiO obtained by calcination in sample A is sim-
ilar to its nickel oxalate precursor (Supplementary material
Fig. S1(a)). In high resolution images (Supplementary material
Fig. S1(b)), the precursor shows smooth surface which is quite dif-
ferent from the surface of the obtained NiO (Fig. S1(c)). The NiO
surface is granular and rough which could be due to the result of
degradation of organic groups and phase transformation. When
the nickel sulfate concentration reduced in sample B (0.02 M)
and sample C (0.01 M) keeping the same oxalic acid concentration
(0.1 M) as sample A, morphology of the NiO have changed, one is
rods with flower-like and the other is mostly rod-like as shown
in Fig. 3(b) and (c). In sample B, the particle size of the flower is
fore, it seems like the morphology of the obtained NiO has been
controlled by the reactant with lower concentration. For the sam-
ple E, the concentration of the nickel salt and oxalic acid are same
as sample D, but the pH value of the reaction solution has a higher
value and the Fig. 3(e) shows that the length of the particles are
shorter then sample D. The change in morphology could be due
to the higher interaction of sodium ion and hydroxyl ions with
nickel and oxalate ions respectively as pH increases. Such an inter-
action probably has disrupted the bonding between nickel ions and
oxalate ions. Therefore, at higher pH value the NiO particles be-
come shorter. On the other hand, when the microwave heated tem-
perature changed in sample D, some different morphology has
obtained for NiO. When the temperature of microwave increased,
the length of the NiO particles has increased so does the aspect ra-
tio. The relationship between aspect ratio and temperature is
shown in Supplementary material Fig. S2.
about 2.6 0.1
length is 2.2 0.1
diameter of the rod is 0.5 0.05
l
m. The diameter of a typical rod is 1.0 0.05
m and the aspect ratio is 2.2. In sample C, the
m, the length is 1.7 0.1 lm
lm,
l
l
and the aspect ratio is 3.4. When the concentration of nickel sulfate
is lowest the aspect ratio found to be larger. When the nickel salt
concentrations low in the solution, the interactions of nickel ion
with oxalate ion become less. That means there are less nucleation
site and the precipitated particles are rod-like. If the nucleation
sites are less, the size of rod is smaller. For the sample D in
Fig. 3(d), the morphology shows similar to sample B as both have
similar nickel sulfate concentration but lower oxalic acid. There-
The TEM images of sample A and E in Fig. 4 have shown that the
obtained NiO are polycrystalline. The selected area electron diffrac-
tion pattern shows several Debye–Scherrer rings, corresponding to
the reflections of the NiO polycrystals. The HRTEM image has
shown lattice spacing for (111) and (200) planes obtained from
X-ray diffraction consolidating the polycrystalline nature of NiO.
Although the morphologies of the obtained NiO in the above
samples have varied with different concentration and temperature,

H.-T. Wang et al. / Journal of Alloys and Compounds 584 (2014) 142–147
145
the particles are mostly rod shaped. Fig. 5 outlines a plausible
mechanism of the formation of nickel oxalate (NiC2O4) obtained
from the microwave heating process. The precipitation of NiC2O4
is due to the interaction of nickel ions (Ni2+) ions and oxalate ions
(C2O2ꢁ) which is shown in following equation:
4
Ni2þ þ C2O2ꢁ ! NiC2O4
ð2Þ
4
When nickel sulfate and oxalic acid are mixed, there are interac-
tions between the lone pair electron on the oxygen and the nickel
cation, so the oxalate ions chelated with the nickel ions side by side,
resulted in the formation of nickel oxalate chains. These chain-like
complexes aggregate due to van der Waals force, then the rod-like
morphology has formed. When the concentration of the reactants
(nickel sulfate and/or oxalic acid) is high, there is much nucleation
site, so the chain-like complexes cross to others eventually forming
flower-like morphology. After calcined at 400 °C for 2 h, the green-
ish precursor has transformed into polycrystalline NiO structures.
The purity and composition was determined by XPS studies and
the spectra are shown in Fig. 6. The Ni2p signal has been deconvo-
luted into six peaks. The binding energy at ꢂ853.6, ꢂ855.1, and
ꢂ860.6 eV are attributed to the Ni2p3/2, and the ꢂ873.0, ꢂ874.5
and ꢂ880.0 eV are attributed to Ni2p1/2. The positions of the peaks
are almost the same for all samples as seen from the Table 2. But
there have been slight peak shift to higher binding energy when
nickel concentration decreased from 0.1 M to 0.01 M. The peak at
ꢂ855.1 eV is ambiguous and could be due to 1.5 eV satellite peak
(1.5 eV sat.). This satellite peak attributed to Ni3+ species on the
NiO surface, and the peak intensity increased with the concentra-
tion of the defects [21]. Soriano et al. [22] has shown that the peak
at 853.6 eV could be due to NiO6 in octahedral symmetry (bulk),
and the peak at 855.1 eV attributed to NiO5 (with a lower coordi-
nate number) in pyramidal symmetry (surface). Therefore, higher
1.5 eV sat. area ratio implies the surface area is larger and/or the
concentration of Ni3+ species on the NiO surface is higher.
Fig. 5. Schematic illustrations of linear complexes that were formed by nickel ions
and oxalate ions. These chain-like complexes could further aggregate into micro/
nanostructures due to a weaker interaction (van der Waals force) between chains
relative to the interaction between atoms with each chain.
Ni2p3/2
1.5 eV sat.
7 eV sat.
Ni2p1/2
1.5 eV sat.
7 eV sat.
850
860
870
880
890
Binding energy (eV)
Fig. 6. XPS of NiO structure.
anomaly could be due to the decreased dye uptake of the film.
Again by further study of absorbance for different thickness (Sup-
plementary material Fig. S3(b)), it has been observed that thickest
film has shown highest absorbance and a clear hump near 700 nm.
This hump also has been observed in NiO with dyes. Therefore the
enhanced absorbance of NiO could be due to scattering rather than
absorption. To understand optical properties of doped films, more
studies on reflection and scattering have to carry out, so that the
enhanced absorption can be explained well. When the ratio of nor-
malized absorbance of NiO with dye and without dye was analysed
(Fig. 7b), the result reflected exactly similar trend of dye loading.
The preliminary studies of the Li-doped NiO films with dye have
shown enhanced relative absorbance with increasing Li as shown
in Fig. 7a. The absorbance of undoped NiO has been similar to
the NiO microball reported by Powar et al. [19] The normalized
absorbance with respect to thickness also reflected the similar
trend. The absorbance has increased for all the Li doped NiO sam-
ples in the range 500 to 800 nm similar to the films without dye
(Supplementary material Fig. S3(a))). But in 400–500 nm range
the absorbance of 5% doped Li has lower than without Li. The
d200 = 2.10 Å
d111 = 2.41 Å
Fig. 4. Selective area diffraction, TEM and HRTEM images of typical NiO structure obtained after microwave treatment.

146
H.-T. Wang et al. / Journal of Alloys and Compounds 584 (2014) 142–147
The ratio of dye loading per micron for without Li and with 5% Li
has been 2.28 and the absorbance ratio also around 2.0 at
405 nm and 436 nm. The dye loading has been decreasing with
the increase of doping. Further study of lithium doped NiO has
been carried out by XRD (Fig. 7c), and the peak position of (111)
and (200) plane for NiO phase remained the same after lithium
doping, but the FWHM decreased from 1.09 to 0.55 (for 200 plane).
The narrower FWHM indicating a better crystal structure for doped
NiO. The intensity ratio for (200) and (111) plane also increased
from 1.44(undoped) to 1.71(5% doped) indicating a better phase
crystallinity after doping (see Fig. 8).
Li-doping of NiO has been done to improve the conductivity as
reported by Joseph et al. [14] and conducted here to understand
the doping process. The Hall measurement study of Li-doped films
indicated an increased order of conductivity with doping so also
the electron density as shown in Table 3. The solar cell parameters
have been measured for the Li-doped films (Supplementary mate-
rial Fig. S4) as shown in Table 4 and we observed that 1% Li-doped
has shown the best results of Jsc, Voc, and efficiency of 0.91 mA/cm2,
120.2 mV and 0.039 respectively. The Voc and efficiency are very
much closer to the report of Mizoguchi et al. [15], but lesser Jsc va-
lue, probably due to the difference in the calcinations temperature.
When the devices were measured for the IPCE, ꢂ14% photon to
electron conversion efficiency was observed at 430 nm for 1% Li-
doped film consolidating the enhanced photo activity. But for all
the films the only feature appears at 430 nm of the visible spec-
trum, suggesting the enhancement of photo activity could be due
to electronic effect rather than optical one. The crystallinity and
better charge collection efficiency has improved the IPCE for 1%
Li doped even though with lesser dye loading than undoped NiO.
The relative absorbances of Li-doped films have two features, one
at 380–430 nm range and the other one at around 700 nm. The
absorbance band at 700 nm seems to have no influence on photon
Fig. 7b. Normalised UV–Vis absorbance ratio of NiO with dye and without dye.
NiO
Li10
Li25
Li50
5.0% Li
500
2.5% Li
1.0% Li
NiO
0
40
2θ
Table 2
XPS results of NiO samples.
Fig. 7c. XRD of NiO doped with lithium.
Sample Binding energy Ni2p3/2
(eV)
Area ratio(1.5 eV sat./total Ni2p3/2
(%)
)
A
B
C
D
E
853.4, 854.9, 860.4
853.6, 855.1, 860.6
853.7, 855.2, 860.7
853.4, 854.9, 860.4
853.5, 855.0, 860.5
37.9
38.5
34.9
38.9
35.5
16
14
12
10
8
w/o Li+
1% Li+
2.5% Li+
5% Li+
6
1.2
1.0
0.8
0.6
0.4
0.2
0.0
4
NiO-w/o Li+/C343
NiO-1% Li+/C343
NiO-2.5% Li+/C343
NiO-5% Li+/C343
2
0
300
400
500
600
700
800
Wavelength (nm)
Fig. 8. IPCE of Li-doped NiO.
Table 3
Hall measurement of doped NiO films.
Sample
Resistivity (ohm cm)
Mobility (cm2/V s)
Bulk n (cmꢁ3
)
w/o Li+
1% Li+
1.20E+04
4.17E+03
2.19E+03
2.45E+03
1.33E+03
3.43E+02
1.76E+02
3.82E+01
3.91E+11
4.37E+12
1.62E+13
6.66E+13
300
400
500
600
700
800
Wavelength (nm)
2.5% Li+
5% Li+
Fig. 7a. UV–Vis absorbance of dye absorbed NiO doped with lithium.

H.-T. Wang et al. / Journal of Alloys and Compounds 584 (2014) 142–147
147
Table 4
Solar cell parameters of Li-doped NiO films at AM1.5.
Sample
JSC (mA/cm2)
VOC (mV)
F.F.
g
(%)
Thickness (
lm)
Dye loading (10ꢁ7mol/cm2)
w/o Li+
0.74
0.91
0.88
0.59
114.7
120.2
115.3
104.9
0.35
0.36
0.32
0.28
0.030
0.039
0.033
0.017
1.8
2.0
2.1
2.1
0.89
0.77
0.52
0.45
1 at.% Li+
2.5 at.% Li+
5 at.% Li+
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4. Conclusions
In conclusion, the structure of the NiO obtained after calcination
is FCC and oriented mostly in (200) and (111). The absorbance
result indicated an increasing trend with doping, but could be asso-
ciated with the scattering rather than doping as an effect of micron
size/flower shaped structures. The overall solar to electrical effi-
ciency also maximum for 1% doped films. The dye loading has been
maximum for undoped films but IPCE has shown 1% Lithium doped
NiO have maximum efficiency probably due to higher charge
collection efficiency and crystallinity.
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Acknowledgement
We would like to thank NCKU Top University funding for
providing the financial support to carry out the research.
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