1. One pot synthesis of multi-functional tin oxide nanostructures
for high efficiency dye-sensitized solar cells
Qamar Wali, Azhar Fakharuddin, Amina Yasin, Mohd Hasbi Ab Rahim, Jamil Ismail, Rajan Jose ⇑
Nanostructured Renewable Energy Materials Laboratory, Faculty of Industrial Sciences & Technology, Universiti Malaysia Pahang, 26300, Malaysia
a r t i c l e i n f o
Article history:
Received 21 January 2015
Received in revised form 12 May 2015
Accepted 14 May 2015
Available online 8 June 2015
Keywords:
Photovoltaic
UV–VIS spectroscopy measurements
TiO2–SnO2 composite structure
Electron life time and recombination
a b s t r a c t
Photoanode plays a key role in dye sensitized solar cells (DSSCs) as a scaffold for dye molecules, transport
medium for photogenerated electrons, and scatters light for improved absorption. Herein, tin oxide
nanostructures unifying the above three characteristics were optimized by a hydrothermal process and
used as photoanode in DSSCs. The optimized morphology is a combination of hollow porous nanoparti-
cles of size $50 nm and micron sized spheres with BET surface area (up to 29 m2
/g) to allow large
dye-loading and light scattering as well as high crystallinity to support efficient charge transport. The
optimized morphology gave the highest photovoltaic conversion efficiency ($7.5%), so far achieved in
DSSCs with high open circuit voltage ($700 mV) and short circuit current density ($21 mA/cm2
) employ-
ing conventional N3 dye and iodide/triiodide electrolyte. The best performing device achieved an incident
photon to current conversion efficiency of $90%. The performance of the optimized tin oxide nanostruc-
tures was comparable to that of conventional titanium based DSSCs fabricated at similar conditions.
Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction
The photoanode or working electrode (WE) fabrication offers
significant challenges in achieving high efficiency dye-sensitized
solar cells (DSSCs). The WE is usually a mesoporous film of wide
band gap metal oxide semiconductor (MOS) and has typically three
functionalities: (i) to anchor large amount of dyes, (ii) to transport
of photogenerated electrons to the collecting electrode (FTO), and
(iii) to scatter the light additionally to improve the light absorption
by the solar cells [1]. Mesoporous particles ($20–30 nm) of large
surface area (P100 m2
/g) is required for loading large amount of
dyes, [2,3] highly crystalline particles or wires with less defects
are preferred for efficient transport, and larger particles ($200–
300 nm) are required for light scattering [4–6].
Tin oxide (SnO2) is a promising WE material in DSSCs owing to
its wider band gap ($3.6 eV vs. $3.2 eV of TiO2) and larger electron
mobility (ln $100–250 cm2
VÀ1
sÀ1
) than most frequently used
TiO2 (ln < 0.1 cm2
VÀ1
sÀ1
) [7]. The wider band gap of SnO2
improves device stability; whereas the UV absorption of TiO2
degrades the dye and considerably reduce the operating hours of
DSSCs [8]. The ln of SnO2 is one of the highest in MOS, even in
nanocrystalline form, in which form the ln sharply decreases by
several orders of magnitude. The SnO2 NPs have 50% higher ln
($3.63 Â 10À3
cm2
VÀ1
sÀ1
) than TiO2 ($2.47 Â 10À3
cm2
VÀ1
sÀ1
)
[9,10]. Order of magnitude higher ln is reported in SnO2 nanowires
and flowers than NPs [11]. Many wet-chemical methods are
reported for synthesis of various morphologies of SnO2 as WE in
DSSCs; [12–15] but produced inferior efficiency (g). For instance,
Wang et al. [16] synthesized hollow nanospheres (HNS) and
reported g < 1% in pristine SnO2. Liu et al. [17] fabricated
coral-like porous SnO2 HNS and developed DSSCs with g $ 1%.
The g in the above studies remarkably increased several folds upon
a TiCl4 treatment. This enhancement is related to the increase in
the Fermi energy of pristine SnO2 upon TiCl4 treatment, which
otherwise occurs at lower energies than that of TiO2 [18]. The
lower Fermi energy of SnO2 increase the energy loss at the
SnO2-dye interface and thereby impose a loss-in-potential at this
interface and subsequently reduces the open circuit voltage (VOC)
and recombination resistance in DSSCs. Furthermore, SnO2 has a
low iso-electric point (pH $ 4–5) than that of TiO2 (pH $ 6–7) so
resulting in poor dye-loading [19–21] and consequently lower
the short circuit current density (JSC).
Herein, we optimized a hydrothermal process, with slight mod-
ifications in that used by Wang et al. [16] that developed a series of
SnO2 nanostructures. A temperature dependent growth process
showed low crystallinity but high surface area for SnO2 NPs syn-
thesized at a low temperature ($150 °C) which gave inferior per-
formance in DSSCs as is conventionally observed. Increase in
temperature ($180 °C) lead growth of HNS ($700–800 nm) along
http://dx.doi.org/10.1016/j.jallcom.2015.05.120
0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
⇑ Corresponding author.
E-mail addresses: rjose@ump.edu.my, joserajan@gmail.com (R. Jose).
Journal of Alloys and Compounds 646 (2015) 32–39
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom

2. with small NPs. The particles synthesized at a higher temperature
$200 °C showed an optimum mixture of SnO2 NPs and HNS with
improved crystallinity, desirable light scattering and transport
properties. When these nanostructures were tested as a WE in
DSSCs, the SnO2 HNS synthesized at $200 °C showed best perfor-
mance among the others with a highest g ($4.0%) achieved using
pristine SnO2 till date. The g was increased to $7.5% upon TiCl4
treatment.
2. Experimental section
2.1. Synthesis of SnO2 nanostructures
The materials were synthesized following Wang et al. [16] but with modifica-
tions. In our experiment, we reduce the concentration of the growth solution
(16.5 mM) to nearly one half than that reported before (32.7 mM) [16]. In a typical
procedure, SnCl2Á2H2O (0.25 g) was added to a mixture of 1 N HCl (0.6 mL), ethanol
(6 mL) and DI water (60 mL) ultrasonicated for 1 h. The resultant transparent solu-
tion was then transferred to an autoclave and kept at a pre-heated furnace at
$150 °C for $24 h. After cooling to room temperature, the solution was centrifuged,
washed three times with DI water and dried it at $60 °C in the oven for overnight.
This sample was labelled as ‘‘sample A’’. The same procedure was applied for ‘‘sam-
ple B’’ and ‘‘sample C’’; however, the furnace temperature was increased to $180 °C
and $200 °C, respectively. The SnO2 HNS were also synthesized using the reported
procedure [16] to see its macroscopic particle distribution.
2.2. Characterizations
The annealed nanostructures were characterized for morphology, particulate
properties, and crystal structure. Morphology and microstructure of the materials
were studied by scanning electron microscopy (7800F, FESEM, JEOL, USA). The
BET surface area of the materials were measured using gas adsorption studies
employing Micromeritics (Tristar 3000, USA) instrument in the nitrogen atmo-
sphere. High resolution lattice images and selected area diffraction (SAED) patterns
were obtained using transmission electron microscope (TEM) operating at 300 kV
(FEI, Titan 80–300 kV). Crystal structure of the material was studied by X-ray
diffraction (XRD) technique using Rigaku Miniflex II X-ray diffractometer employ-
ing Cu Ka radiation (k = 1.5406 Å).
2.3. SnO2 paste preparation
Three hundred milligram of SnO2 was dispersed in ethanol and added
a-terpinol (18 wt.%) and ethyl cellulose (10 wt.%). The above solution was ultrason-
icated for 1 h and heated up to $70 °C to evaporate ethanol until viscous slurry was
formed.
2.4. Solar cell fabrications and testing
The FTO substrates were immersed in 0.1 M aqueous TiCl4 solution at $80 °C for
$40 min followed by annealing for $30 min at $450 °C and subsequently cooled to
room temperature. The SnO2 pastes was then coated using Doctor–Blade technique
on the TiCl4 treated FTO substrates (1.5 cm  1 cm; sheet resistance $18 X sqÀ1
)
and then heated at $450 °C for $30 min. Thickness of the sintered electrodes was
studied by SEM. The thickness of the films was $8.5 lm and the active area of
the cells was $0.12 cm2
. The sintered electrodes were further treated with aqueous
TiCl4 solution (0.2 M) for improving the connectivity between the grains as well as
suppressing the electron recombination with the tri-iodide species in the elec-
trolyte [22]. This post TiCl4 treatments was done by dipping the electrodes in the
solution at $70 °C for $30 min, washed it with DI water to remove the residual
TiCl4 and then sintered at $450 °C for $30 min. The sintered electrodes was then
soaked in RuL2 (NCS)2Á2H2O (L = 2,2/
-bipyridyl-4,40
-dicarboxylic acid (N3 dye,
Solaronix) (0.3 mM) for $24 h at room temperature. The unanchored
dye-molecules were removed by washing with ethanol. The DSSCs were sealed
using a $25 lm spacer. A Pt-sputtered FTO glass was used as the counter electrode.
The electrolyte was acetonitrile containing 0.1 M lithium iodide, 0.03 M iodine,
0.5 M 4-tert-butylpyridine and 0.6 M 1-propyl-2,3-dimethyl imidazolium iodide,
which was injected through two small openings at the counter electrode.
Absorption and transmission spectra of the dye anchored electrodes as well as
dye desorption test were recorded using a UV–vis NIR spectrometer (UV-2600
SHIMADZU, Japan). The current–voltage (I–V) characteristics of the assembled
DSSCs were studied using a solar simulator (SOLAR LIGHT, Model 16-S 150)
employing single port simulator with power supply (XPS 400) at AM1.5 conditions.
The I–V curves were obtained using a potentiostat (Autolab PGSTAT30, Eco Chemie
B.V., The Netherlands) employing the NOVAÒ
software. The level of standard irradi-
ance (100 mW/cm2
) was set with a calibrated c-Si reference solar cell. To avoid
stray-light effects, devices were properly masked to expose only the WE area. In
DSSCs, two types of I–V characteristics measurement are performed, the normal
scan mode where the voltage is changing stepwise from the short circuit (V = 0 or
current = ISC) to the open circuit (V = VOC or current = 0), and the reverse scan mode
operate where the voltage sweep from open circuit state (V = VOC) to the short cir-
cuit current state (V = 0). Usually reverse scan mode delivered better results than
that of normal scan mode when the delay time is shorter than the time required
for a cell to acquire its equilibrium state. This discrepancy between normal and
reveres scan mode could be reduced by taking the delay time an order of magnitude
longer than that required for the silicon based solar cells. During the I–V measure-
ment the delay time is necessary after each step of sweep voltage in order to stabi-
lize the device. The most obvious characteristics of DSSCs I–V measurement is the
temporal response which is much low as compared to the silicon based solar cells.
Therefore, sufficient time is required for DSSCs transit time from short circuit to
open circuit [23,24]. In our experiment, the DSSCs were illuminated for 10 min
under solar cells simulator at room temperature $25 °C to stabilize the temperature
before measuring the I–V curve. The applied bias voltage source was swept from
short-circuit current (ISC, at V = 0) to open circuit (V = VOC, at ISC = 0). Following are
the I–V measurement parameters: current range $0–1 mA, maximum time for open
circuit potential determination $180 s, wait time $5 s, potential range $À0.200–
0.900 V, step potential $0.00244 s, scan rate $0.02 V/s and delay time $0.122 s.
Measurements were repeated for five times for each DSSCs.
The incident photon to current conversion efficiency (IPCE) measurements was
done using the Bukoh Keiki (CEP-2000) instrument, Japan. Five sets of devices were
fabricated using each SnO2 nanostructures and the measurements were repeated
for 10 times to assure the consistency in the values.
3. Results and discussions
3.1. Morphological properties
Fig. 1 shows the SEM images of the SnO2 nanostructures syn-
thesized at $150, $180, and $200 °C, respectively. They are
labelled as samples A, B, and C, respectively. The SEM image of
particles synthesized using the increased precursor concentration,
as reported in the reference [16] (32.7 mM), is also shown in Fig. 1.
More SEM images are shown in Supporting Information (SI)
(Fig. S1, SI). Sample A consists of uniform particles of size
$100 nm; a closer examination revels that the particles are aggre-
gates of <10 nm sized grains. At $180 °C (sample B), a wider dis-
tribution of aggregates were observed with size up to $800 nm;
however, basic building blocks of each particle were remained
same ($10 nm). Bigger particles further grow >1 lm at 200 °C
with relatively large size distribution (Sample C). On the other
hand, the particles synthesized following the increased precursor
concentration showed coarsening (Fig. 1d). Size distribution and
properties of the aggregates were closely analyzed using TEM.
Fig. 2a shows typical TEM images of the sample C. Spherical aggre-
gates of large size distribution in the range of $50 nm–1 lm was
observed; particles in each aggregate remain practically constant
at $10 nm. All aggregates including the larger ones showed partial
transparency to the electron beam; from which we infer that the
aggregates are hollow. Hollow nature of the aggregates could also
be observed from SEM (Fig. 1c). Crystallinity of the sample was
judged from the high resolution transmission electron microscopic
(HRTEM) lattice images and selected area electron diffraction
(SAED) patterns, which are shown in Fig. 2b. More TEM and
HRTEM images are presented in SI (Fig. S2, SI). The HRTEM images
showed aggregates of defect free nanograins and SAED pattern
showed diffraction spots oriented along a circle. These observa-
tions show that the particles are of high crystallinity. In the
X-ray diffraction (XRD) pattern, sharp and intense peaks of the
sample C reveal that they are highly crystalline as observed from
the HRTEM images and SAED patterns. Smaller particles with
superior crystallinity are recommended for efficient charge trans-
port in DSSCs while anchoring large amount of dyes [25]. The
specific surface area, pore size and volume distribution were stud-
ied by Brunauer–Emmett–Teller (BET) method in nitrogen adsorp-
tion and desorption environment. The particulate properties of the
samples from the above study, such as surface area, pore size and
volume distributions are listed in Table 1. The BET surface area were
$50, $45, and $29 m2
gÀ1
, respectively for samples A, B, and C. The
Q. Wali et al. / Journal of Alloys and Compounds 646 (2015) 32–39 33

3. lowering of surface area could be due to the formation of aggregates
at high temperature for samples B & C. The high surface area and
varying pore size are beneficial for the DSSCs as they help in large
dye loading and help improving permeations of the electrolyte.
Adsorption–desorption isotherm of the three samples corresponds
to type IV isotherm (Fig. S3, SI). The area under hysteresis loops
increased with processing temperature thereby indicating increas-
ing the pore size distribution. The XRD patterns (Fig. 3) show that
all the samples have the similar crystal structure (cassiterite phase,
tetragonal crystal system, space group P42/mnm, JCPDS file card #
41–1445). The lattice parameters calculated from the XRD patterns
were a = 4.7380 Å and c = 3.1865 Å. The sample C has remarkablydif-
ferent crystallinity than the others, which was judged through a
lower value of the full width at half maximum of the diffraction
peaks. High crystallinity is suggested to improve the conductivity
of metal oxide semiconductors and improve the photovoltaic prop-
erties of DSSCs fabricated using them [25].
The samples were sintered onto transparent conducting glass
substrates (FTO) duly spin coated with a thin ($500 nm) TiO2 layer
for fabrication of solar cells (Fig. S4, SI). The DSSCs fabricated using
samples A, B, and C were termed as ADSSCs, BDSSCs, and CDSSCs,
respectively. One set of electrodes using the commercial TiO2 paste
was also fabricated and used as a reference cell (PDSSCs).
Fig. 4 shows a typical SEM image (Sample C) showing the
cross-section of the electrode of thickness $8.5 lm. The particles
were well sintered onto FTO; a closer examination shows sporadic
distribution of larger particles as well as their shell structure
(Fig. 4d). The hollow structures retained their initial morphology
even after extensive mechanical agitation during the paste making
procedure and subsequent thermal annealing. No agglomeration
was found in the WE film; ensuring high porosity for electrolyte
permeation [26].
3.2. Optical properties of the dye-anchored electrodes
The light harvesting properties of the dye-anchored WE were
studied by UV–Vis absorption spectroscopy. Dye-loading was
Fig. 1. (a) Illustrates SEM images of the synthesized sample A, (b) depicts sample B, while (c and d) illustrate sample C and the reference, respectively.
Fig. 2. (a) TEM images of the nanostructure (sample C) at different magnification
level while, (b) show the HRTEM and SAED pattern, respectively.
Table 1
BET surface area, pore sizes and volume distribution for the respective synthesized
Sample A, B and C.
Sample BET surface area
(m2
gÀ1
)
Pore size
(nm)
Pore volume
(cm3
gÀ1
)
Sample A 50 15.1 0.19
Sample B 45 13.1 0.15
Sample C 29 11.7 0.09
34 Q. Wali et al. / Journal of Alloys and Compounds 646 (2015) 32–39

4. measured using desorption test (Fig. S5, SI), which were $278,
$203, and $131 nmol/cm2
for electrodes based on sample A, B
and C, respectively. Difference in dye-loading is attributed to the
difference in their surface area. The light scattering properties of
the electrodes were studied by recording their absorbance
(Fig. S5a, SI), transmittance (Fig. S5b, SI) and reflectance spectra
(Fig. S5c, SI); Fig. 5 compares normalized absorbance, transmit-
tance and reflectance of the electrodes A, B and C, respectively.
Electrodes of samples A and B showed similar absorbance although
slightly improved absorbance was observed for the later despite of
its lower dye-loading. Light scattering by the larger particles is
responsible for this increment in absorbance. The electrode of sam-
ple C showed larger absorption cross-section, i.e., the area under
the absorbance curve, due to the presence of larger particles
despite its inferior dye-loading, than that of the other two.
Superior light harvesting property of the electrode C is more
obvious in the transmittance spectra (Fig. S5b in SI and Fig. 5). As
the size of the particles in sample C corresponds to the wavelength
of the visible ($360–700 nm) and near-infrared ($700 nm–
$2.5 lm) regions a strong light scattering could be expected.
Moreover, the presence of micron and mesoporous sized NPs
would increase the reflection of light and eventually enhance the
optical path length for incident photons [27]. The transmittance
of the electrodes were $34, $30, and <10% at the dye’s absorption
wavelength range for electrodes A, B, and C, respectively. Thus,
$90% of the incident light is absorbed by the electrode C, whereas
considerable portion of the incoming light is transmitted in the
other electrodes. In a diffuse reflection (Figs. 5 and S5c), incident
photons are reflected in all directions by the photoanode thereby
increasing the light harvesting efficiency. If the internal diffuse
reflection is taken into consideration, electrode C has a higher
reflectance >700 nm owing to its comparable size together with
macro and mesopores, which is expected to increase the incident
light reflected and scattered inside the photoanode. However, if
we take external reflectance into account, the electrode C pos-
sesses lower reflectance in the 350–700 nm range than electrodes
A and B. This low reflectance of the electrode C reveals high absor-
bance; and therefore, leads to high photovoltaic parameters.
10 20 30 40 50 60 70 80
(321)
(202)
(301)
(112)
(310)
(002)
(220)
(211)
(200)
(101)
(110)
Sample C
Sample B
Sample A
Intensity(a.u)
2θ (deg)
Fig. 3. XRD patterns of the respective three synthesized samples A, B and C.
Fig. 4. FESEM cross section of the fabricated DSSCs based on sample C at various magnifications.
300 400 500 600 700 800 900
0.0
0.2
0.4
0.6
0.8
1.0
Abs.(a.u)
λ (nm)
A T R (Electrode A)
A T R (Electrode B)
A T R (Electrode C)
0
20
40
60
80
100
Tra.Ref(%)
Fig. 5. Normalized absorbance, transmittance and reflectance curves of the
electrodes A, B and C.
Q. Wali et al. / Journal of Alloys and Compounds 646 (2015) 32–39 35

5. 3.3. Photovoltaic characteristics of SnO2 DSSCs
Fig. 6 shows the photocurrent density (J)–photovoltage (V)
characteristics of the best performing devices; a statistics of the
PV parameters of all tested devices are listed in Table 2. The g of
the DSSCs increased in the order gADSSCs < gBDSSCs < gCDSSCs <
gPDSSCs. The superior g of the PDSSCs arises from its $80% higher
VOC and $50% higher fill factor (FF) (Table 2) which are due to
lower loss-in-potential at the dye-TiO2 and dye-electrolyte inter-
faces of PDSSCs [28] compared to the other devices. However, by
using perylene dyes [29] with lower unoccupied molecular orbital
energies and cobalt based electrolyte [30] the loss-in-potential at
the dye-SnO2 and dye-electrolyte interfaces could be reduced con-
siderably thereby achieving high VOC and FF in SnO2 DSSCs. The
CDSSCs showed the highest JSC compared to A & BDCSSs which
could be attributed to the increased light scattering of the elec-
trode C. The CDSSCs showed the highest g $ 4.0% (JSC $ 16
mA cmÀ2
, VOC = 491 mV and FF $ 0.50) so far achieved using pris-
tine SnO2 nanostructures. The performance of the CDSSCs was fur-
ther improved by TiCl4 treatment (TCDSSCs); which showed $80%
higher g ($7.5%) than the parent device. Although the TCDSSCs
gave a higher JSC ($17%) than the PDSSCs lower FF ($20%) of the
former restricts its performance beyond that of PDSSCs. However,
compared with A & BDSSCs, the CDSSCs offered improved FF, which
could be attributed to the widening of its pore-size distribution as
reported by Chen et al. [31].
A systematic increase in VOC (VOC(CDSSCs) > VOC(BDSSCs) >
VOC(ADSSCs)) was observed in the three set of devices despite the
chemical similarity of the WE material. This increase in VOC could
be due to increased light scattering achieved using varying particle
size and crystallinity. The high VOC and the suppression of electron
recombination by TiCl4 blocking layer were evaluated from the
charge transport parameters by electrochemical impedance spec-
troscopy (EIS). The EIS is a powerful tool used to analyze the charge
transport through the WE material, electron transfer and recombi-
nation at the dye anchored WE/electrolyte interface, charge trans-
fer at the counter electrode, and the ions diffusion of the
electrolyte in DSSCs [32]. The EIS curves of the three devices, viz.
CDSSCs, TCDSSCs and PDSSCs, and the descriptive model to elabo-
rate the frequency response in the mentioned processes of DSSCs
are presented in the SI (Fig. S6, SI). The transmission line shows
parallel channels representing the transport of electrons through
the MOS and redox species in the electrolyte. The charge recombi-
nation occurs at the electrode–electrolyte interface. If L is the
thickness of MOS film, the electron transport resistance, RT = rtL,
interfacial charge recombination resistance, RCT = rct/L where all
these parameters are defined in the reference [32]. The Nyquist
plots of DSSCs display three regions. The first small circle of
Nyquist plot represents the charge transport resistance at the
higher frequency regime (>1000 Hz), whilst the second
semi-circle at the middle frequency (1000 Hz < f < 1 Hz) symbolize
the electrons recombination at the dye anchored WE/electrolyte
interface. The low frequency (<1 Hz) reveals the diffusion of ions
in the electrolyte which reflects in the third semi-circle. The
Nyquist plot for TCDSSCs shows larger diameter than the other
two as shown in Fig. 7, thereby, assures the suppression of elec-
trons recombination resistance and consequently, long electron life
time (sn). To further identify the high performance of TCDSSCs, we
extracted charge transport parameters using Z-view software of
Bisquert transmission line model [32]. TCDSSCs exhibit outstand-
ing performance which obeys the ideal condition RCT ) RT of any
high performing DSSCs. The calculated RCT were $19, $145 and
$70 O, while RT ($5, $10 and $43 O), observed for CDSSCs,
TCDSSCs and PDSSCs at 0.7 V, respectively. The large semi-circle
for TCDSSCs showed increase in RCT and consequently increased
sn according to the relation, sn = RCT Â Cl, where Cl is the chemical
capacitance [32]. The sn was calculated for the respective DSSCs
using mid frequency of the Bode-Phase plots using sn = 1/2pfo
where fo is the maximum frequency at the mid peak. The calcu-
lated values of fo were $21.98, $4.14 and $16.26 Hz, with corre-
sponding sn $7, $38 and $10 ms for CDSSCs, TCDSSCs and
PDSSCs, respectively. Long sn implies that [33] electrons could sur-
vive for long time before recombination, therefore, leads to high JSC
and VOC, respectively.
3.4. Incident photon to current conversion efficiency
The difference in collection efficiencies of the DSSCs were stud-
ied from their incident photon to current conversion efficiency
(IPCE) spectra also called external quantum efficiency (EQE). The
IPCE could be defined as the number of electrons generated in
the external circuit by light divided by the number of photons
hit the cell. Fig. 8a compares the IPCE spectra of the CDSSCs,
TCDSSCs and PDSSCs, whereas Fig. 8b compares the action spectra
with absorbed photons to converted electrons (APCE) or internal
quantum efficiency (IQE) and reflectance of electrode C, respec-
tively. The improved PV performance and long sn in TCDSSC is also
reflected from its IQE (APCE) spectrum, which is defined as the
ratio between number of photoelectrons collected in a solar cell
and the number of photons absorbed. The IQE (APCE) calculated
using the relation EQE = IQE À R À T, and the spectrum shows
nearly unity quantum yield in TCDSSCs for generated and collected
charge carriers at $520 nm wavelength. The spectra displayed sig-
nificant enhancement in the IPCE of the TCDSSCs devices compared
to the other two. The highest IPCE for the devices were $90%,
$78%, and $72% at k $ 520 nm near the peak absorbance of the
N3 sensitizer for all types of DSSCs. The improvement in TCDSSCs
can be attributed to the enhanced light scattering efficiency of
the particles composing variable particle sizes despite its relatively
low surface area and subsequent inferior dye-loading.
Furthermore, the J–V data was validated from the IPCE measure-
ments of the DSSCs by calculating the JSC by the relation
IPCE ð%Þ ¼
JSC ðmA=cm2
Þ
PðmW=cm2Þ
Â
1240
kðnmÞ
 100%
The integrated IPCE over the entire wavelength (k $ 300–
800 nm) was used to calculate the JSC as depicted in Fig. 8a. The cal-
culated JSC of the CDSSCs, TCDSSCs and PDSSCs were $14, $16 and
$15 mA/cm2
, respectively, which agree with their measured JSC
(Table 2). Integrated IPCE curves of C, TC and PDSSCs, from which
the JSC is evaluated, are depicted (Fig. S7, SI).
0
5
10
15
20
J(mA/cm2
)
TCDSSCs
PDSSCs
CDSSCs
BDSSCs
ADSSCs
0.0 0.2 0.4 0.6 0.8
Potential (V)
Fig. 6. J–V characteristics curve of five devices ADSSCs, BDSSCs, CDSSCs, TCDSSCs
and PDSSCs under 1 sun condition, respectively.
36 Q. Wali et al. / Journal of Alloys and Compounds 646 (2015) 32–39

6. Table 2
The photovoltaic characteristics (JSC, VOC, FF and g) along with dye loading of the five respective A & BDSSCs, CDSSCs, TCDSSCs and PDSSCs.
Electrode JSC (mA/cm2
) VOC (V) FF (%) g (%) Dye loading n-mole/cm2
Mean Best Mean Best Mean Best Mean Best
ADSSCs 6.7 7.2 0.35 0.37 0.36 0.38 0.85 1.02 278
BDSSCs 10.1 11.1 0.39 0.40 0.34 0.37 1.34 1.62 203
CDSSCs 14.7 16.3 0.46 0.49 0.48 0.50 3.34 4.0 131
TCDSSCs 19.7 21.3 0.69 0.71 0.48 0.50 6.56 7.5 –
PDSSCs 16.4 17.5 0.72 0.731 0.61 0.63 7.21 8.04 259
Fig. 7. Nyquist plot illustration shown in (a) the normalized Bode Phase diagram depicts in (b) while the Bode Phase illustrate in (c) for three respective devices CDSSCs,
TCDSSCs and PDSSCs.
0
400 500 600 700 800
0
20
40
60
80
100
IPCE(%)
λ (nm)λ
CDSSCs
TCDSSCs
PDSSCs
Fig. 8a. Incident photon to current conversion efficiency of the three devices
CDSSCs, TCDSSCs and PDSSCs, respectively.
400 500 600 700 800
0
20
40
60
80
100
IQE--EQE--Reflectance(%)
λ (nm)
Electrode C(IPCE)
Electrode C(APCE)
Electrode C(R)
Fig. 8b. IPCE, absorbed photon to current converted electrons (APCE) and
reflectance of the electrode C.
Q. Wali et al. / Journal of Alloys and Compounds 646 (2015) 32–39 37

7. 3.5. Open circuit voltage decay measurement
In order to investigate and validate the long sn of the TCDSSCs,
open circuit voltage decay (OCVD) measurements were performed
[34]. The OCVD measures the temporal decay of VOC upon remov-
ing the illumination source in DSSC operating at steady state
thereby providing a real time measurement of the charge transport
parameters through the device [35,36]. As OCVD measurement is
performed in the dark; no electrons could recombine with oxidized
dye molecules, therefore, electrons solely recombine with the
tri-iodide species of the electrolyte. The recombination kinetics
of electrons can also verified by plotting sn against the VOC as
shown in Fig. 9 by the following equation [34].
sn ¼ À
kBT
e
dVOC
dt
À1
where kBT is the thermal energy e is the elementary positive charge
and dVOC/dt represent the decaying of VOC with respect to time.
Measured OCVD curves of the three types of devices are in SI
(Fig. S8). Fig. 9 compares the sn of the devices measured from the
OCVD curves. Interestingly, TCDSSCs exhibit remarkably increased
sn over the entire voltage range as compared to the other two
devices, thereby providing a real time measurement evidence for
increased carrier lifetime in the TCDSSCs. At a typical voltage of
$300 mV, the calculated sn for the CDSSCs, TCDSSCs and PDSSCs
were $0.7 s, $15 s and $1.5 s, respectively, which clearly validated
the high performance of the TCDSSCs device.
4. Conclusions
In conclusion, three SnO2 morphologies were synthesized at dif-
ferent temperatures ($150, $180, and $200 °C) by hydrothermal
method and employed as photoanodes in DSSCs. The material syn-
thesized at $200 °C unify three functionalities, viz. moderate sur-
face area for dye-loading, presence of larger particles for light
scattering, and high crystallinity for efficient charge transport to
enable fabrication of high performance DSSCs. The smaller NPs
($50 nm) in the sample synthesized at $200 °C enabled loading
of large amount of dyes and presence of scattering particles (up
to 1 lm) in it resulted high absorbance ($90%) by the photoan-
odes. These properties enabled collection of $90% of photogener-
ated electrons from $8.5 lm thick photoanode film. High
photovoltaic conversion efficiency of $7.5% is obtained using the
SnO2 nanostructures with a VOC $ 700 mV and JSC $ 21 mA/cm2
.
The DSSCs fabricated using the optimized SnO2 morphology
excelled the performance of TiO2 based devices despite the latter’s
superior dye-loading. These achievements are particularly inter-
esting using the N3 dye and iodide/triiodide electrolyte as they
are not good choice for SnO2 DSSCs because conduction band and
valence band energies of SnO2 are much lower than that of TiO2
which increases the over potential and charge recombination.
Appendix A. Supplementary material
‘‘More transmission electron microscopic images, electrode
diffraction pattern, FESEM images showing the cross-section of
the working electrode films, adsorption-desorption plots, absorp-
tion spectra of the desorbed dye-solution, EIS data and OCVD
curve’’ these material are available free of charge via the Internet
at http://www.elsevier.com.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jallcom.2015.05.
120.
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