Low glass transition temperature hole transport material in enhanced-performance solid-state dye-sensitized solar cell Prof. Gregory L. Baker passed away on Oct. 18. 2012. We dedicate this work as a memorial to him.

Article abstract of DOI:10.1016/j.orgel.2014.09.017

A new triarylamine-based hole transport materials (HTM) with branched side chain giving low glass transition temperature (T_g) is synthesized and incorporated into a solid state dye sensitized solar cell. This designing of molecular structure of HTM for lowering the T_g along with viscosity and surface tension of the casting solution effectively increases the pore-filling fraction (PFF) as the cell is heated during the fabrication, leading to an 8 fold increasing in cell efficiency over cells without heat treatment. We relate the cell performance improvement not only due to the PFF of TiO₂ by the (HTM), but also because of morphological and thickness changes in the hole transport material (HTM) capping layer.

Full text of DOI:10.1016/j.orgel.2014.09.017

Organic Electronics xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Low glass transition temperature hole transport material in
enhanced-performance solid-state dye-sensitized solar cell
⇑
Wen Yuan , Hui Zhao, Gregory L. Baker
Department of Chemistry, Michigan State University, East Lansing 48824, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 July 2014
Received in revised form 8 September 2014
Accepted 11 September 2014
Available online xxxx
A new triarylamine-based hole transport materials (HTM) with branched side chain giving
low glass transition temperature (Tg) is synthesized and incorporated into a solid state dye
sensitized solar cell. This designing of molecular structure of HTM for lowering the Tg along
with viscosity and surface tension of the casting solution effectively increases the pore-
filling fraction (PFF) as the cell is heated during the fabrication, leading to an 8 fold
increasing in cell efficiency over cells without heat treatment. We relate the cell
performance improvement not only due to the PFF of TiO2 by the (HTM), but also because
of morphological and thickness changes in the hole transport material (HTM) capping layer.
Ó 2014 Elsevier B.V. All rights reserved.
Prof. Gregory L. Baker passed away on Oct.
18. 2012. We dedicate this work as a
memorial to him.
Keywords:
Solid-state dye sensitized solar cell
Low glass transition temperature
Heat-assisted infiltration
Morphology
Pore filling fraction
1. Introduction
hole-conducting polymer as measured against the ionic
conductivity of the liquid cells. Alternatives to spiro-MeO-
Solid-state dye sensitized solar cells (ss-DSSC) suffer
from incomplete pore-filling of the hole transport
materials into the porous TiO2, which leads to poor
dye-regeneration and inhibits cell performance. Using
spiro-MeOTAD (2,20,7,70-tetrakis-(N,N-di-p-methoxyphe-
nylamine) 9,90-spirobifluorene) as the hole transport
material (HTM) have achieved a cell efficiency of over
10% in ss-DSSC [1,2]. Compared to Iꢀ/I3ꢀ liquid electrolytes,
a solid state HTM may avoid the issues of solvent leakage
and evaporation. The solid-state cells have the potential
for greater stability and longer cell lifetime, making them
more suitable for buildings and other outdoor applications.
However, ss-DSSCs are currently limited by poorer
photocurrent due to the higher internal resistance of the
TAD, such as polymers [3–5] or oligomers [6–8] with
higher hole mobility, have been applied to ss-DSSC, but
their performance does not rival that of spiro-MeOTAD in
ss-DSSC because the larger size of these polymer or oligo-
mer structures inhibits their penetration into mesoporous
TiO2. Indeed not only charge mobility but also interfacial
contact resistance will limit performance of ss-DSSC. When
only limited amounts of HTM penetrate into the mesopor-
ous TiO2 there are gaps between the dye-coated TiO2 and
the conductive polymer that result in poor contact
between dye molecules and spiro-MeOTAD, inhibiting
dye regeneration [9,10].
Grätzel [6,9] and his coworkers were the first to study
the pore-filling of spiro-MeOTAD relating to the power
conversion efficiency of ss-DSSC. They suggested that both
the device performance and conductivity went down with
a thicker film device [9,10]. Balancing light absorption
against spiro-MeOTAD penetration, they obtained
⇑
Corresponding author. Tel.: +1 517 355 9715.
E-mail
addresses:
yuanwen@msu.edu,
yaji861112@gmail.com
(W. Yuan).
http://dx.doi.org/10.1016/j.orgel.2014.09.017
1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: W. Yuan et al., Low glass transition temperature hole transport material in enhanced-performance solid-
state dye-sensitized solar cell, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.09.017

2
W. Yuan et al. / Organic Electronics xxx (2014) xxx–xxx
maximum cell efficiency with a device having 2–3
lm
2.2. Instruments
thickness of TiO2 which had been treated with an amphi-
philic dye that made the spiro-MeOTAD ‘wet’ the TiO2 pore
[11–13] Subsequently, McGehee’s group [14–16] exten-
sively studied and quantitatively determined the pore-
filling fraction (PFF) in mesoporous TiO2 films with spiro-
MeOTAD. They showed that as the PFF of spiro-MeOTAD
increased from 26% to 65%, the cell efficiency increased
by nearly 300%. Very recently, Snaith’s group studied the
PFF in ss-DSSC via optical reflectometry, which did not
require any assumptions on material properties, and
re-estimated the PFF of ss-DSSC at about 80% [17].
One approach of increasing the PFF is to change the sur-
face properties of the TiO2 pores so that the spiro-MeOTAD
will easily pass through the ‘wet’ surface. Using an amphi-
philic dye such as Z907 dye [12] or C220 dye [18],
enhances the hydrophobility of the TiO2 surface, and
improves the infiltration of the spiro-MeOTAD because of
the strong hydrophobic–hydrophobic interaction between
the chlorobenzene solution of spiro-MeOTAD and longer
alky chain from the dye. Another approach to optimize
the pore filling is to tune the deposition parameters of
spiro-MeOTAD, such as the spin-coating rate, the concen-
tration of spiro-MeOTAD [14] during the spin-coating pro-
cess, or deposition techniques by doctor-blading instead of
spin-coating [15]. Later, Sellinger et al. designed new
organic HTMs (AS 37 and AS 44) with low melting point
(Tm), low glass transition temperature (Tg) and a 2 fold
higher solubility which allows facile interpenetration into
the TiO2 pore structure [19]. The AS 37 and AS 44 based
ss-DSSC reached efficiencies of 3%, rivaling that of conven-
tional spiro-MeOTAD based cells.
In contrast to these previous methods, here we present
a simple heat-assisting method to induce our low Tg HTM
(Tg = 20 °C) into the TiO2 pores. A remarkable increase in
cell performance is obtained especially the photocurrent
after the heat treatment. The improved performance is
attributed to more efficient dye regeneration. This simple
strategy with a unique HTM reduces the surface tension
of the HTM in the TiO2 pores by elevating the temperature
of both the HTM solution and deposition TiO2 substrate.
We calculate the PFF and conduct an SEM analysis that
reveals a uniform HTM thin layer coating the TiO2. This
thin layer serves as a blocking layer, preventing short-
circuits between the counter electrode and TiO2, which
can lead to charge recombination. This observation is also
consistent with the observed increase in open circuit
voltage in the ss-DSSC after heat treatment as well.
The capping layer on TiO2 was imaged by a Hitachi S-
4700 II field-emission scanning electron microscope
(FESEM). The cross-sectional structure of solar cell was
determined by a Carl Zeiss Auriga Dual Column FIB SEM.
The morphology of capping layer was obtained using a
Veeco Dimension 3100 scanning probe microscope.
Contact angles were measured using a VCA 2000 Video
Contact Angle Surface Analysis system. Absorption spectra
were obtained from Varian Carey Model 50 UV–Vis Spec-
trophotometer. Current–voltage measurements and open
circuit voltage decay measurements of ss-DSSC were per-
formed using a Xe Arc Lamp and CHI-650D Electrochemical
work station. An AM 1.5 solar filter and neutral density
filter were used to simulate sunlight at 100 mW cmꢀ2. A
mask with an area of 0.25 cm2 was used.
2.3. Solar cell fabrication and preparation
Devices of ss-DSSC were fabricated according to a pub-
lished procedure [9,20]. Briefly, a pre-cleaned FTO glass
was put on a hot plate with a temperature of 500 °C for
30 min before any deposition. A 200 nm thickness of TiO2
compact layer on FTO was formed by home-made spray-
pyrolysis set-up using air as the carrier gas and titanium
di-isopropoxidebis (acetylacetonate) as the precursor. A
2.5 lm thickness of mesoporous TiO2 film was deposited
by doctor-blading TiO2 pastes with scotch tape as a spacer.
The thickness was measured by profilometer and con-
firmed by a cross-section SEM image. After baking the
TiO2 paste with a programmable heating procedure up to
450 °C, we let the substrate cool to 80 °C before absorbing
it into a 0.3 mM Z907 dye solution for a minimum of 16
hours, which was followed by rinsing with ethanol and
acetonitrile. A heated chlorobenzene solution of HTM was
used for spin-coating on heated dye-coated TiO2 sub-
strates. The concentration of hole conductors was 9 wt%
or 15 wt% in total with additives of lithium bis(trifluoro-
methylsulfonyl)imide (LiTFSI) and 4-tert butylpyridine
(4-tBP). A 36 lL electrolyte solution was first applied to
the substrate for one minute, and then was spin-coated
with 2000 rpm for 30 s in air. The cell was stored under
dark overnight before depositing the gold electrode. A
30 nm thickness of gold was applied by thermal evapora-
tion as back contact.
3. Results and discussions
2. Experimental
A new type of hole transport material is developed and
synthesized by tethering 2-ethyl hexyl chains onto fluo-
rine, leading to a low Tg of 20 °C (Scheme 1 and a NMR
spectrum and DSC curve shown in Figs. S5 and S6). A
longer chain allows a more flexible structure at tempera-
ture over Tg [19,21]. A further reason for introducing a
branched chain such as the 2-ethyl hexyl group is to min-
imize intermolecular hydrophobic–hydrophobic interac-
tion, which may cause voids generating a large bulk
resistance in solar cell [22]. A UV–vis spectrum of new
synthesized HTM is also provided in the supporting
2.1. Materials
Fluorine doped Tin Oxide (FTO) Glass purchased from
Hartford was pretreated by Alconox, distilled water, ace-
tone and UV–ozone before use. Low Tg HTM was synthe-
sized through a previous literature [19]. TiO2 paste (Ti-
Nanoxide T) was from Solaronix. Gold used as evaporation
source for counter electrode was from Kurt J. Lesker. All
other chemicals were purchased from Sigma Aldrich and
were used as receive unless mentioned otherwise.
Please cite this article in press as: W. Yuan et al., Low glass transition temperature hole transport material in enhanced-performance solid-
state dye-sensitized solar cell, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.09.017

W. Yuan et al. / Organic Electronics xxx (2014) xxx–xxx
3
Scheme 1. Synthesis of a low Tg hole transport material.
information (supporting information (Fig. S3)) with the
absorption below 420 nm, which proves no coverage on
visible region competing with Z907 dye. The ss-DSSC was
assembled using the following optimized procedure
[9,20]. TiO2 compact layer and mesoporous TiO2 film were
deposited onto the FTO substrate sequentially, followed by
absorption of Z907 dye. After drying the dye-coated TiO2
substrate from soaking solution, both the substrate and
prepared HTM solution are heated to 70 °C and the pre-
heated solution is quickly applied to the warmed substrate
by a spin-coating set (depicted in Scheme S1). We have
performed UV–Vis to test dye-coated TiO2 film before
and after heating. With pre-heating treatment the absorp-
tion spectra of dye/TiO2 film had no change at all, which
proved dye molecule was thermally stable at 70 °C for a
short period at least 60 s (Fig. S1). After the gold electrode
is thermal-deposited on top of the HTM covered TiO2, the
device is ready for performance measurement. Following
this heat treatment procedure on both the TiO2 substrate
and the HTM solution, we obtain ss-DSSCs with efficiencies
of 3.3%, a Jsc, of 8.5 mA/cm2, a Voc of 0.75 V, and a FF of 0.51.
This is even better than a spiro-based ss-DSSC with this
strategy (Fig. S4). However without the heat treatment,
we obtain cells with a poor efficiency of 0.44% and an
extremely low Jsc of 1.7 mA/cm2 (seen in Fig. 1). By taking
the advantage of low Tg of HTM and its flexible structure
under elevated temperature, we believe that the
heat-assisting method will reduce both the viscosity and
surface tension of the casting solution (Fig. S2), leading to
a more effective infiltration of the HTM into the TiO2 pores,
even with the smaller pore size. Without the heat treat-
ment, the cell has insufficient HTM for dye regeneration.
To investigate the influence of the heat treatment on
the HTM penetration, we first study the ‘‘wetting’’ behavior
of the room temperature TiO2 substrate. We drop HTM
solution on the substrate, and measure the dynamic con-
tact angle as a function of temperature. With or without
heating, the initial contact angle was around 18° when
the droplet first touched the TiO2 surface (see Fig. 2b and
c). As the solution began to infiltrate into the pores, the
contact angle decreased and quickly saturated. After only
30 s, the contact angle decreased to 4° with heat treatment,
while without the heat treatment the contact angle satu-
rated at 6° (see Fig. 2a). In particular, the initial variation
of the contact angle was quite rapid, requiring just 2–3 s,
and was very sensitive to temperature. The heated solution
saturated more quickly, taking a shorter time to absorb
into the TiO2 pores as compared to the non-heat treated
solution. We conclude that the reduction in surface tension
of the solution and viscosity is sufficient to increase the
permeation of the HTM into the porous TiO2 substrate.
Another side, the feather of low Tg of our HTM aligns with
the conclusion of a smaller contact angle as well, allowing
more materials soaking into the pores under annealing
temperature. Different annealing temperature of spin-
coating for hole conductor infiltration has also been
explored from 25 °C up to 110 °C. The power conversion
efficiency arises from without heating to 70 °C and then
reduces from 70 °C to 110 °C (shown in Table S1). Since
all the heating-assisting infiltration took place in the air
not in a glove box. The efficiency reducing at elevated tem-
perature might be from an oxygen-induced over-doping of
triarylamine to its radical cation form. This triarylamine
radical cation generally improves the conductivity, but
absorbs only at visible region which competes with
absorption of Z907 dye and further gives less light harvest-
ing. 70C so far is the optimized condition to build the best
performance of this type of cell without killing either dye
(seen in Fig. S1) or hole conductor.
The morphology of the HTM capping layer on the outer
surface of the TiO2 also has a significant impact on the ss-
DSSC performance [14,23]. A uniform capping layer mini-
mizes the chance of short-circuit by direct contact between
the TiO2 and the counter electrode. To examine the
Fig. 1. J–V curves of ss-DSSC fabricated with (red curve) and without
(green curve) the heat-assisted method. (For interpretation of the
references to color in this figure legend, the reader is referred to the
web version of this article.)
Please cite this article in press as: W. Yuan et al., Low glass transition temperature hole transport material in enhanced-performance solid-
state dye-sensitized solar cell, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.09.017

4
W. Yuan et al. / Organic Electronics xxx (2014) xxx–xxx
Fig. 2. Dynamic contact angle measurements of HTM solution on TiO2 substrate with heat treatment (black) and without heat treatment (red). (a) The
variation of the contact angle is monitored for the first 30 s after the droplet touches the TiO2 substrate. (Inset for the first 2 s). (b) The initial stage t = 0 s and
(c) the final stage t = 30 s. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
uniformity of the HTM capping layer, we compared three
different heat treatments: (1) both substrate and HTM
solution heated, (2) only HTM solution heated, (3) without
any heating. The concentration of HTM solution was 9 wt%
and 15 wt%, respectively. The corresponding ss-DSSC per-
formance is summarized in Fig. 3 and Table 1.
exhibited a few pinholes (Fig. 4b). Without pre-heat of
either solution or substrate, there was almost no HTM
covering on the TiO2 surface (Fig. 4c). Fig. 5, presents the
results of the atomic force microscopy (AFM) imaging of
the HTM capping layer, confirming the results from the
SEM images. Both the SEM and AFM image reveals a
smooth HTM capping film when applying heating treat-
ment. There is no observation of large crystal, which is con-
sistent with our hypothesis by introducing branched 2-
ethyl hexyl chain to minimize the possible crystal forma-
tion via intermolecular interaction. In the AFM experiment,
the pre-heated or non-pre-heated HTM solution was
placed on the TiO2 substrate for 60 s before the spin-coat-
ing, affording enough time for the HTM solution to pene-
trate into the pores. Because of the pre-heating of the
substrate and HTM solution, the chlorobenzene solvent
evaporated during the soaking period, which concentrated
the HTM solution. After 60 s, the heated solution cooled
down to room temperature but was more concentrated
and viscous. The more concentrated, less viscous solution
formed with the pre-heated sample form a more uniform
capping layer even with only 9% HTM solution. In contrast,
the un-heated solution with low concentration of HTM
With low concentration of HTM solution (9 wt%), the cell
efficiency reached the highest value of 3.3% (Table 1) when
both substrate and HTM solution were heat-treated. If only
the HTM solution is pre-heat but the TiO2 substrate is left a
room temperature, the resulting cell has 2.0% efficiency
with a lower Jsc of 4.4 mA/cm2. When neither the solution
nor the substrate is pre-heated we obtain a cell with an
extremely low current and efficiency. This impact of heat-
treatment upon cell performance is not found in the litera-
ture; moreover we have replicated this phenomenon in 12
different cells, thereby establishing its reproducibility.
With the scanning electron microscope (SEM), we
obtained images that show the morphology of the capping
layer varies with the different heat treatments. The most
uniform film formed after pre-heating both the substrate
and HTM solution prior to spin-coating (Fig. 4a). When
only the HTM solution is pre-heated, the capping layer
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W. Yuan et al. / Organic Electronics xxx (2014) xxx–xxx
5
Fig. 3. J–V curves of ss-DSSC measured under different heat treatments and with solution concentrations. (a) 9 wt% and (b) 15 wt% HTM in chlorobenzene
solution.
Table 1
Summary of ss-DSSC performance with different deposited condition.
Condition
9 wt%
Jsc (mA/cm2)
Voc (V)
FF
g (%)
Substrate & HTM heated
Only HTM heated
No heating
8.5
4.4
1.7
0.75
0.65
0.55
0.51
0.70
0.47
3.3
2.0
0.44
15 wt%
Substrate & HTM heated
Only HTM heated
No heating
7.5
7.0
3.2
0.70
0.68
0.66
0.44
0.44
0.57
2.3
2.1
1.2
polymer and high viscosity could was unable to generate a
capping layer at all.
layer. The 340 nm thick HTM capping layer gave a trade-
off of the internal resistance and short-circuits insulation,
providing the optimal performance.
To maximize the infiltration of HTM into the TiO2 sub-
strate, we also tried spin-coating a high concentrated solu-
tion with 15 wt% HTM in chlorobenzene. We observed the
same morphology these of 9 wt% HTM (Fig. 4). The high
concentration of HTM yielded a more viscose solution,
and under equivalent pre-heating conditions the capping
layer formed from the high concentration solution dis-
played even fewer defects than that formed from the low
concentration solution. However, the improved capping
layer did not improve cell performance, but generated a
noticeable reduction in the FF. The FF is strongly influenced
by the cell’s bulk resistance and we conclude that a thick,
uniform capping layer leads to a larger internal resistance.
To confirm this, we monitored the thickness of capping
layer for different deposited condition via both SEM imag-
ing and profilometer. The results are summarized in the
Table 2. Without any heating, both the 9% and 15% HTM
solution could not form any capping layer covering on
the TiO2. This observation is consistent with previous mor-
phology study presented in Fig. 4c and Fig. 4f. When pre-
heating only the HTM solution, the 9% HTM solution pro-
duced a 130 nm thick capping layer while the 15% HTM
solution produced a 680 nm thick capping layer. With
pre-heating of both the substrate and the HTM solution
the 9% HTM solution produced a 340 nm thick capping
layer while the 15% HTM solution yielded a 950 nm thick
Several studies [9,14] have related the PFF to the dye
regeneration and cell performance in ss-DSSC. They
approximate the PFF by extracting HTM from fabricated
cell. PFF has also been determined for the cases of a uni-
form capping layer or no capping layer. The results are
summarized in Table 3. For cells fabricated with pre-heat-
ing of only the HTM solution, the volume of the capping
area was hard to estimate because of the pin-holes. We
observe a strong, positive correlation between the PFF
and the cell efficiency. The cell with the 86% PFF, has the
highest current and efficiency, despite the use of a lower
concentration of HTM in the solution. The high concentra-
tion HTM solution achieved only a 77% PFF and a lower cell
efficiency. However, cells assembled without heat treat-
ment, whether at low or high concentration of HTM solu-
tion, had low PFF. This analysis supports our assumption
that heat treatment leads to superior infiltration of HTM
into the TiO2 substrate, yielding a higher PFF and higher
overall cell performance, even at lower HTM loads.
An ‘over-shooting’ of photocurrent was also observed in
Fig. 6. With only HTM heated or without any heating, the
steady-state current density was significantly lower than
the initial-state current density. We attributed it to incom-
plete poor filling by the HTM, and some dye molecules
could only ‘fire’ once as they were not getting regenerate
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W. Yuan et al. / Organic Electronics xxx (2014) xxx–xxx
Fig. 4. Morphology of HTM capping layer deposited on TiO2 film after spin-coating under various conditions. 9 wt% HTM used for spin-coating with (a) both
substrate and HTM solution pre-heated; (b) only HTM solution pre-heated; and (c) no pre-heating. 15 wt% HTM used for spin-coating with (d) both
substrate and HTM solution pre-heated; (e) only HTM solution pre-heated; and (f) no pre-heating.
Fig. 5. AFM study of HTM capping layer on TiO2. (Left: both substrate and HTM solution pre-heated; Right: without any pre-heating).
Table 2
Summary of capping layer thickness under various deposited condition.
9% HTM
Capping layer thickness (nm)
15% HTM
Capping layer thickness (nm)
Both substrate and HTM heated
Only HTM heated
Without any heating
320 nm
100 nm
NA
Both substrate and HTM heated
Only HTM heated
Without any heating
750 nm
620 nm
NA
effectively. This similar mass transport problem has been
reported previously [24,25]. We’ve also thought this
‘over-shooting’ resulting from ‘short-circuit’ because of
non-uniform capping layer. However, if both HTM solution
and substrate had been heated, this phenomenon turned
attenuated (black curve in Fig. 6). We concluded that our
heating treatment could effectively eliminate this sort of
instantaneous current density. Pore filling of hole
Please cite this article in press as: W. Yuan et al., Low glass transition temperature hole transport material in enhanced-performance solid-
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W. Yuan et al. / Organic Electronics xxx (2014) xxx–xxx
7
Table 3
Capping layer thickness, PFF and cell performance from under the various deposited conditions. The PFF is estimated from Eqs. (1)–(4) (seen in supporting
information).
Condition
9% HTM
15% HTM
Substrate & HTM heated
No heating
Substrate & HTM heated
No heating
Thickness (nm)
PFF (%)
320
82%
8.5
NA
32%
1.7
750
77%
7.5
NA
65%
3.2
1.2
Jsc (mA/cm2)
g
(%)
3.3
0.44
2.3
that heating treatment enable to eliminate ‘over-shooting’
of photocurrent from mass transport problem. The heat
treatment strategy is a simple, cost effective means to
obtain an optimized distribution of the HTM within the
TiO2 substrate. For future work we will investigate
whether melting infiltration can replace spin-coating as a
means to infiltrate a low Tm and Tg solid electrolyte in
the absence of liquid solvent.
Acknowledgements
This work is financially supported by the. National Sci-
ence (DMR-0934568) and MSU Center for Alternative
Energy Storage Research and Technology (CAESRT). W.Y.
thanks Dr. Keith Promislow, Dr. James McCusker, and Dr.
Lawrence Drzal for valuable discussion.
Fig. 6. Plots of current transients of ss-DSSCs employing different heat
treatments with 9 wt% HTM solution under 1 sun illumination.
Appendix A. Supplementary material
conductor in TiO2 is one of the most dominant factor to cell
performance.
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.orgel.2014.09.017.
4. Conclusion
References
In this paper, we presented a new, simple method to
effectively infiltrate low Tg HTM into the TiO2 porous sub-
strate in DSSC. Before spin-coating the HTM solution, both
the TiO2 substrate and the HTM solution are pre-heated,
reducing the surface tension and viscosity of casting solu-
tion, while concentrating the HTM polymer by evaporating
a portion of the solvent. Following the drop casting of the
hot solution on the warm substrate, we observed the for-
mation of a more uniform capping layer on the outer sur-
face of the TiO2 substrate as compared to the capping
layer formed from the process with either a partial pre-
heating or no heat treatment. An analysis of performance
data indicates that a uniform capping layer minimizes
the current loss due to direct contact between counter
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confirmed that the highest PFF was achieved from the
heat-treated solution with a moderate amount of HTM.
We conclude that the heat treatment procedure yields
the highest PFF with a thin and uniform capping layer, at
a relatively low level of HTM concentration. Also by
conducting current transient measurements, we confirm
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