Triarylamine-based dual-function coadsorbents with extended π-conjugation aryl linkers for organic dye-sensitized solar cells

Article abstract of DOI:10.1016/j.orgel.2014.09.008

Triarylamine-based dual-function coadsorbents containing a carboxylic acid acceptor linked by extended π-conjugation aryl linkers (e.g., phenylene: HC-A3, naphthalene: HC-A4 and anthracene: HC-A5) were newly designed and synthesized. They were used as coadsorbents in organic dye-sensitized solar cells (DSSCs) based on a porphyrin dye (hexyloxy-biphenyl-ZnP-CN-COOH (HOP)). For comparison, the π-conjugated phenyl linker (HC-A3) previously developed by our group was also used as a coadsorbent. The structural effects on the photophysical and electrochemical properties and DSSC performance were systematically investigated. As a result, the DSSCs based on HC-A4 and HC-5 displayed power conversion efficiencies (PCEs) of 8.2% and 5.1%, respectively, while the HC-A3-based DSSC achieved a PCE of 7.7%. In the case of HC-A4, both the short-circuit photocurrent densities (J_sc) and open-circuit voltages (V_oc) of DSSCs were simultaneously improved to a large extent due to the more effective prevention of π-π stacking of organic dye molecules and the better light-harvesting effect at short wavelengths. The HC-A5-based DSSC exhibited a much lower short-circuit current (J_sc) and open-circuit voltages (V_oc) compared to the HC-A4-based DSSC, due to the fact that the dihedral angle of the π-conjugated linkers was too high for electron injection into the TiO₂ conduction band (CB) level. This had a reduced effect on preventing the π-π stacking of dye molecules, resulting in lower J_sc and V_oc values.

Full text of DOI:10.1016/j.orgel.2014.09.008

Organic Electronics 15 (2014) 3316–3326
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Triarylamine-based dual-function coadsorbents with extended
p-conjugation aryl linkers for organic dye-sensitized solar cells
In Taek Choi a,1, Ban Seok You a,b,1, Yu Kyung Eom a,1, Myung Jong Ju a, Won Seok Choi a,
Sung Ho Kang a, Min Soo Kang a, Kang Deuk Seo a, Ji Yeoun Hong a, Sang Hyun Song a,
a,
Ji-Woon Yang b, , Hwan Kyu Kim
⇑
⇑
a Global GET-Future Laboratory, Department of Advanced Materials Chemistry, Korea University, 2511 Sejong-ro, Sejong 339-700, Republic of Korea
b Department of Electronics and Information Engineering, Korea University, 2511 Sejong-ro, Sejong 339-700, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 June 2014
Accepted 6 September 2014
Available online 18 September 2014
Triarylamine-based dual-function coadsorbents containing a carboxylic acid acceptor
linked by extended
p-conjugation aryl linkers (e.g., phenylene: HC-A3, naphthalene: HC-
A4 and anthracene: HC-A5) were newly designed and synthesized. They were used as
coadsorbents in organic dye-sensitized solar cells (DSSCs) based on a porphyrin dye
(hexyloxy-biphenyl-ZnP-CN-COOH (HOP)). For comparison, the
p-conjugated phenyl
Keywords:
Dual-function coadsorbents
Extended p-conjugation aryl linkers
Prevention of dye aggregation
Light-harvesting
Dye-sensitized solar cells
linker (HC-A3) previously developed by our group was also used as a coadsorbent. The
structural effects on the photophysical and electrochemical properties and DSSC perfor-
mance were systematically investigated. As a result, the DSSCs based on HC-A4 and
HC-5 displayed power conversion efficiencies (PCEs) of 8.2% and 5.1%, respectively, while
the HC-A3-based DSSC achieved a PCE of 7.7%. In the case of HC-A4, both the short-circuit
photocurrent densities (Jsc) and open-circuit voltages (Voc) of DSSCs were simultaneously
improved to a large extent due to the more effective prevention of
p p stacking of organic
À
dye molecules and the better light-harvesting effect at short wavelengths. The HC-A5-
based DSSC exhibited a much lower short-circuit current (Jsc) and open-circuit voltages
(Voc) compared to the HC-A4-based DSSC, due to the fact that the dihedral angle of the
p
-conjugated linkers was too high for electron injection into the TiO2 conduction band
(CB) level. This had a reduced effect on preventing the
resulting in lower Jsc and Voc values.
pÀp stacking of dye molecules,
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
ful, and applicable solar cells [1–5]. Until now, high light to
power conversion efficiencies surpassing 12% have been
achieved co-sensitization with YD2-o-C8 and Y123 organic
sensitizer-based DSSCs using cobalt electrolyte [6]. The
PCE of DSSCs depends on the sensitizing ability of dyes,
and extensive efforts have been made to develop highly
efficient dyes for panchromatic sensitization [7,8]. On the
other hand, effective enhancement of the conversion
efficiency has been achieved through a co-sensitization
method using two organic dyes having complementary
spectral responses [9,10], an organic dye and multi-
functional coadsorbent [11–15], Zn-phthalocyanine
Dye-sensitized solar cells (DSSCs) have been exten-
sively studied as a promising candidate for low-cost, color-
⇑
Corresponding authors at: Global GET-Future Laboratory, Department
of Advanced Materials Chemistry, Korea University, 2511 Sejong-ro,
Jochiwon, Sejong 339-700, Republic of Korea. Tel.: +82 44 860 1493; fax:
+82 44 860 1331 (H.K. Kim), Tel./fax: +82 44 860 1426 (Ji-W. Yang).
E-mail addresses: jyang@korea.ac.kr (Ji.-W. Yang), hkk777@korea.ac.kr
(H.K. Kim).
1
The authors contributed equally to this work.
http://dx.doi.org/10.1016/j.orgel.2014.09.008
1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

I.T. Choi et al. / Organic Electronics 15 (2014) 3316–3326
3317
Scheme 1. Molecular structures of (a) HC-A3, (b) HC-A4, and (c) HC-A5.
Fig. 1. (a) Normalized UV–visible absorption (solid lines) and emission spectra (dashed lines) of HC-A3, HC-A4, and HC-A5 in THF solution. (b) Potential-
level diagram exhibiting the upper (LUMO) and lower (HOMO) potentials for the HOP dye and HC-A coadsorbents used in this study, the conduction band of
TiO2, and the potential of the IÀ/I3À redox couple.
Table 1
Optical and electrochemical properties of HOP, HC-A3, HC-A4, and HC-A5.
Dye & coadsorbent
Absorption kmax/nm (
e
/MÀ1cmÀ1
)
Eox/V(vs. NHE)
EoxÀEo-o/V(vs. NHE)
Eo-o/V (vs. eV)
HOP
419 (209,584), 632 (10,732)
358.5 (42,040)
359.5 (37,759), 408.5 (9786)
362 (51,353), 453.5(7104)
0.78
1.178
1.18
1.08
À1.05
À2.042
À1.58
À1.38
1.83
3.22
2.76
2.47
HC-A3
HC-A4
HC-A5
(or Zn-porphyrin) and an organic dye [6,16,17], and a
ruthenium complex dye and organic dye [18,19].
acid (CA) derivatives as coadsorbents in DSSCs, based on
a Ru-pyridyl complex [22,23], coumarin [24–26], porphy-
rin [6,27], phthalocyanine [28,29], and naphthalocyanine
dye [30] have been well-studied. Deoxycholic acid (DCA)
is commonly used as a coadsorbent to prevent the dye
aggregation, thus remarkably improving Voc and Jsc [24–
26,31]. Recently, we demonstrated that a newly discovered
The photovoltaic performance of DSSC is also influenced
on the low charge recombination at the interface of TiO2/
dye/electrolyte as well as on the dye regeneration effi-
ciency [20]. Moreover, charge recombination at the inter-
face of the TiO2/dye/electrolyte results in a decrease of
PCE due to losses in both the short-circuit current (Jsc)
and the open-circuit voltage (Voc). To reduce the possible
charge recombination at the TiO2/dye/electrolyte interface,
several kinds of additives have been adapted as coadsor-
bents onto the TiO2 surface. Decylphosphonic acid (DPA),
dineohexyl bis(3,3-dimethylbutyl)phosphinic acid (DIN-
HOP), and chenodeoxycholic acid (CDCA) have been
adsorbed onto the TiO2 surface [21]. Among them, cholic
coadsorbent based on the p-conjugated phenyl linker (HC-
A3) has dual functions, including both harvesting light
ability at shorter wavelength to improve Jsc and the pre-
vention of dye aggregation to enhance Voc by reducing
charge recombination [32].
In this study, in order to enhance the harvesting light at
shorter wavelengths with the increased UV–visible absor-
bance range by introducing extended
p-conjugation aryl

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I.T. Choi et al. / Organic Electronics 15 (2014) 3316–3326
Fig. 2. The optimized structures and the dihedral angles between the conjugated linker (anthracene) of (a) HC-A3, (b) HC-A4, and (c) HC-A5, and the
electron distribution of the HOMO and LUMO energy levels of three HC-A coadsorbents from DFT calculations.
Table 2
Photovoltaic parameters of the DSSCs with different coadsorbents under one sun illumination (AM 1.5 G (100 mW cmÀ2)).
Device
Adsorbed dye amount/(10À7 mol cmÀ2
)
Adsorbed coadsorbent amount/
Jsc/(mA cmÀ2
)
Voc/(mV)
FF (%)
g (%)
(10À7 mol cmÀ2
)
HC-A3: Device 1
HC-A4: Device 2
HC-A5: Device 3
HOP: Device 4
HOP/HC-A3: Device 5
HOP/HC-A4: Device 6
HOP/HC-A5: Device 7
Y123/CDCA: Device 8
Y123/HC-A4: Device 9
–
–
–
1.54
0.65
0.68
1.32
–
2.55
2.29
0.21
4.15
5.24
0.29
11.0
15.0
15.6
11.3
14.2
14.6
692
711
486
618
692
703
621
630
711
77.5
76.7
74.1
71.5
73.0
74.9
72.4
74.1
71.1
2.2
2.8
0.1
4.9
7.7
8.2
5.1
6.6
7.4
2.35
1.82
0.24
–
–
–
Performance of DSSCs was measured with a black metal mask (0.16 cm2). Electrolyte composition: 0.6 M DMPII, 0.5 M TBP, 0.05 M I2 and 0.1 M LiI in CH3CN.
HOP dye concentration was 0.2 mM. Y123 dye concentration was 0.1 mM. HC-A3, HC-A4 and HC-A5 concentration was 2 mM. CDCA conenetraion was
5 mM. TiO2 nanocrystalline particle size was 20 nm.
linkers, two types of Y-shaped coadsorbents, 4-(bis(9,9-
dimethyl-9H-flouren-2-yl)amino)-1-naphthoic acid (HC-
A4) and 4-(10-(bis(9,9-dimethyl-9H-flouren-2-yl)amino)
anthracen-9-yl)benzoic acid (HC-A5), were newly
designed and synthesized for use as coadsorbents in
organic DSSCs based on a porphyrin dye (hexyloxy-biphe-
nyl-ZnP-CN-COOH (HOP)) previously developed by our
group [33]. For comparison, the -conjugated phenyl
linker (HC-A3) was also used as a coadsorbent [32]. The
structural effects on the photophysical and electrochemi-
cal properties and DSSC performance were systematically
investigated.
p

I.T. Choi et al. / Organic Electronics 15 (2014) 3316–3326
3319
coadsorbents, e.g., HC-A3, HC-A4 and HC-A5. Their optical
and electrochemical properties are summarized in Table 1.
HC-A4 and 5 exhibited one strong absorption band at the
absorption kmax of 359.5 (
362 nm
max = 51,353 MÀ1 cmÀ1
additional weak absorption band at the absorption kmax
of 408.5 and 453.5 nm
max = 9786 MÀ1 cmÀ1
max = 7104 MÀ1 cmÀ1), respectively, while HC-A3 exhib-
ited one strong absorption band at the absorption kmax of
358.5 (
max = 42,040 MÀ1 cmÀ1). The latter weak absorp-
e
max = 37,759 MÀ1 cmÀ1) and
as well as another
(
e
)
(
e
)
(e
e
tion band in HC-A4 and 5 corresponds to the naphthalene
and anthracene units. In addition, the tailed absorption
bands of HC-A3-5 in the edge region of absorbance were
at 405, 444 and 512 nm, respectively. As a consequence,
the absorbance edge of HC-A4 and HC-A5 was red-shifted
due to the extended
p-conjugation aryl unit between the
amino and carboxylic acid groups. This was attributed to
better delocalization of electrons over the whole molecule
when naphthalene or anthracene was used as the conju-
gated linker rather than a phenylene unit. In general, a
broader absorption wavelength range is favorable for coad-
sorbents used in DSSCs based on an organic dye, because of
the better light-harvesting effect to increase Jsc [9,10,11–
15,32,33].
The HOMO energy levels of the HC-A coadsorbents
were measured by cyclic voltammetry (CV) (see
Fig. S1).The cyclic voltammograms were obtained from a
three-electrode cell, which consisted of a dye-coated TiO2
electrode as the working electrode, a Pt wire counter elec-
trode, and an Ag/AgCl (saturated KCl) reference electrode
(+0.197 V vs. NHE) calibrated with ferrocene, in 0.1 M
TBAPF6 in CH3CN at a scan rate of 50 mV sÀ1. All of the
measured potentials were converted to the NHE scale.
The electrochemical data are presented in Fig. 2b and Table
1. The HOMO energy levels of three coadsorbents exhibited
higher potentials than that of the IÀ/I3À redox couple (ca.
0.35 V vs. NHE) [34] indicating effective regeneration of
the oxidized state. The zero–zero excitation energies (E0-0
)
was calculated from the intersection between absorption
and emission spectra, and the lowest unoccupied molecu-
lar orbital (LUMO) levels of the dye and coadsorbents were
determined using the formula: LUMO = HOMO – E0-0. As a
consequence, the band gaps of HC-A4 and HC-A5 were
decreased due to the extended
p-conjugation aryl unit
between the amino donor and carboxylic acid groups, com-
pared to HC-A3.
In order to evaluate the electron distribution of the
HOMO and LUMO energy levels of the HC-A coadsorbents,
density functional theory (DFT) calculations were achieved
by employing Becke’s three parameterized Lee–Yang–Parr
(B3LYP) exchange correlation functional and 6-31G* basis
sets using a suite of Gaussian 09 programs (see Table S2)
[35]. Fig. 2 shows the dominant electron transfer from
the donor unit to the acceptor one for the HOMO–LUMO
transition. The HOMO energy levels were localized on the
Fig. 3. (a) The UV–visible absorption spectra of dye only or dye with HC-A
coadsorbents adsorbed onto the TiO2 surface. (b) The absorption spectra
of the dye-desorbed solution, obtained from the treatment of the same
TiO2 films with 0.1 M aqueous NaOH solution, in
a 50:50:50 (vol%)
EtOH:H2O:THF mixture. (c) The emission spectra for the dye only and the
dye with HC-A coadsorbents adsorbed onto ZrO2 film.
2. Results and discussion
The molecular structures of HC-A3, HC-A4 and HC-A5
have low molecular weight Y-shaped structures with an
anchorable carboxylic acid acceptor linked by various
bis(dimethylfluorenyl)amino donor and
ker. The LUMO energy levels of HC-A3 and HC-A4 were
delocalized across the -conjugated linker and acceptor
p-conjugated lin-
p
p
Àconjugated aryl moieties, such as phenylene, naphtha-
system, whereas the LUMO energy levels of HC-A5 were
not concentrated on the acceptor unit but rather distrib-
lene, and anthracene units (Scheme 1). Fig. 1a exhibits
the UV–vis absorption and emission spectra of the three
uted over a range of the
p-conjugated anthracene linker.

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I.T. Choi et al. / Organic Electronics 15 (2014) 3316–3326
Fig. 4. (a) IPCE action spectra and (b) J–V curves of the DSSCs based on various HC-A coadsorbents. HOP only (black), HOP/HC-A3 (red), HOP/HC-A4 (green),
and HOP/HC-A5 (blue). The IPCE and J–V curves of HC-A3, HC-A4, and HC-A5 only are shown in the insets. (C) J–V curves of the DSSCs based on Y123/CDCA
and Y123/HC-A4. The TiO2 film thickness is 16 (12 + 4)
interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
l
m. All cells were tested with a metal shadow mask having an aperture area of 0.16 cm2. (For
The theoretical calculation of HC-A5 suggests that the
dihedral angle between anthracene and phenyl is too large,
which is unfavorable for intramolecular charge transfer
from the HOMO to the LUMO orbitals [36,37]. This means
that the electron distribution from the donor unit to the
anchoring moiety is expected to be less susceptible to exci-
tation from the HOMO to the LUMO orbitals, such that
charge transfer in HC-A5 is expected to be less effective
than any other HC-As, leading to less electron injection
from the excited coadsorbents into the CB of TiO2 in the
DSSC.
We obtained the dye desorption solution to investigate
the dye adsorption amount on the TiO2 surface by UV–vis
absorption spectra (Table 2 and Table S1). Fig. 3a exhibits
the UV–visible spectra of the HOP dye only or the HOP
dye with coadsorbents adsorbed onto a TiO2 surface. The
UV–visible spectrum of the TiO2 film adsorbed with dye
only showed a much broader absorption than the TiO2
films adsorbed with both dye and coadsorbents. This phe-
nomenon comes from the strong intermolecular interac-
tions among the dye molecules aggregated on the TiO2
surface. When the dye with coadsorbents was adsorbed
onto the TiO2 film, the deprotonation of the carboxylic acid
group in the dye induced the electrostatic repulsion of neg-
atively charged carboxylic groups onto the TiO2 surface,
leading to the reduction of the adsorption of the HOP dye
onto the TiO2 surface [31,32]. In this study, we confirmed
that coadsorbents prevent dye aggregation in another
way as well. Fig. 3c shows the PL emission spectra at the
excitation wavelength (kex) of 500 nm on a nanocrystalline
ZrO2 thin film. ZrO2 film was employed to prevent electron
injection from the excited dye state [38]. We noted that the
emission intensity for the ZrO2/HOP film was much lower
than those for ZrO2/HOP/HC-A3 and ZrO2/HOP/HC-A4
films, consistent with the much higher aggregation of
HOP only. This aggregation has been suggested to promote
intermolecular interactions which compete with charge
separation [39,40] and/or form a layer of non-injecting
dyes [41].
The amount of adsorption of the coadsorbents on the
TiO2 film was determined to be 2.35 Â 10À7, 1.82 Â 10À7
,
and 0.24 Â 10À7 mol cmÀ2 for HC-A3, HC-A4, and HC-A5,
respectively (see Table 2). The adsorption amount of dye
on the TiO2 film decreased from 1.54 Â 10À7 to
0.65 Â 10À7, 0.68 Â 10À7, and 1.32 Â 10À7 mol cmÀ2, lead-
ing to a 58%, 57%, and 14.28% drop with HCA3, HC-A4,

I.T. Choi et al. / Organic Electronics 15 (2014) 3316–3326
3321
Fig. 5. (a) Nyquist and (b) Bode plots of the DSSCs measured at À0.62 V from 106 Hz to 0.1 Hz under dark condition. (c) The gcc and Rrec, and (d) dark
currents measured by EIS for the same DSSCs.
and HC-A5, respectively. Judging by the differences in the
amount of dye adsorption, coadsorbents with Y-shaped
structures may be necessary for effective spacing of adja-
spectra for the TiO2 film loaded with the HC-A coadsorbent
series alone. As shown in Fig. 4, the maximum IPCE was
only ꢀ59% for HOP-sensitized solar cells. It was much
lower than 85%, taking into account the light loss (ꢀ15%)
by the reflection and absorption of the TCO glass. Upon
coadsorption of the HC-A coadsorbent in the dye solution,
except for HC-A5, which had the lowest light-harvesting
effect in UV–visible regions, due to the less effective charge
transfer caused by the large dihedral angle between
anthracene and phenyl, the IPCE was increased
remarkably, even with relatively lower dye adsorption.
This indicates that the better light harvesting effect in
shorter-wavelength regions induces an increased photo-
current of the DSSCs. The maximum IPCE increased from
59% to 73% when HC-A4 was added to the dye solution,
and increased from 59% to 66% with HC-A3 in the dye
solution. Upon HC-A coadsorption, except for HC-A5, the
cent HOP dye molecules. The prevention of
with an additive in the dye solution is a typical method
p p stacking
À
to enhance the PCE efficiency of organic DSSCs. The advan-
tage of
due to broad absorption in the UV–vis absorption spec-
trum, but stacked aggregates usually lead to low
pÀp stacking is to have a better light harvesting
pÀp
short-circuit (Jsc) current and low PCE, due to inefficient
electron injection [33].
Fig. 4a exhibits the incident photon to current conver-
sion efficiency (IPCE) action spectra as a function of the
light wavelength and the corresponding current–voltage
(JÀV) characteristics for the DSSCs using various coadsor-
bents under one sun illumination (AM 1.5G, 100 mW cmÀ2
irradiation at 25 °C). The inset in Fig. 4a shows the IPCE
Table 3
EIS values and charge collection efficiency of the DSSCs.
Device
Rt/X
Rrec
/
X
Cu/mF
s
t/ms
sn/ms
gcc
HOP: Device 4
4.2
9.3
10.6
7.1
34.2
148.3
160.1
40.6
0.69
0.97
0.90
0.67
2.93
9.06
9.65
4.88
23.6
0.89
0.94
0.93
0.85
HOP/HC-A3: Device 5
HOP/HC-A4: Device 6
HOP/HC-A5: Device 7
144.2
145.2
27.6
Calculated values from EIS data measured at a forward bias of À0.62 V under dark conditions.

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I.T. Choi et al. / Organic Electronics 15 (2014) 3316–3326
maximum IPCE improved significantly, up to 55% at
360 nm than that of the HOP-sensitized solar cell, which
could have contributed to the increased photocurrent of
the DSSCs, due to the better light harvesting ability in the
UV wavelength regions of 350–400 nm. Moreover, a pla-
teau from 550 to 650 nm was observed in the IPCE action
spectrum with an IPCE of ꢀ55% on the dye/HC-A TiO2 film.
This demonstrated that the HC-A coadsorbent is effective
spectra of the HC-A3-5 only desorbed on TiO2 film. The
adsorption amounts of the HCA3-5 on the TiO2 film were
determined to be 2.55 Â 10À7
,
2.29 Â 10À7 and
0.21 Â 10À7 mol cmÀ2 for HC-A3, HC-A4, and HC-A5,
respectively (see Table 2). Among the HC-A series, since
the amount of HC-A5 adsorption is the lowest, its preven-
tion effect of
A5 shows the lowest cell performance.
p p stacking to dyes is the lowest so that HC-
À
in dissociating
p
À
p
stacking by strong intermolecular
The DSSC with HOP only displayed
a
Jsc of
interactions. This higher IPCE value from 550 to 650 nm
implies that a bulky coadsorbent can act as a more effec-
tive spacer, suppressing the dye aggregation and thereby
retarding the charge recombination. Thus, it significantly
improves device performance. As a result, they have dual
11.0 mA cmÀ2, a Voc of 618 mV, an FF of 71.50%, and PCE
of 4.9%. The JSC of the DSSCs fabricated with the dye and
different coadsorbents gradually enhanced in the this
order: HOP/HC-A5 < HOP/HC-A3 < HOP/HC-A4. The Jsc
strongly relied on the light harvesting efficiency, charge
separation (such as charge injection, dye regeneration
and recombination) and charge collection. In particular,
in the DSSC with HC-A4, the Jsc and Voc were significantly
enhanced compared to the others. Moreover, it is notewor-
thy that the DSSC with HC-A4 improved approximately
41.8% and 13.8% for both Jsc and Voc, respectively, despite
a 26% lower HOP dye coverage, and enhanced around
67% for a PCE compared to the DSSC with HOP only. Also,
typical organic dye (Y123)-based DSSCs with CDCA and
HC-A4 were also fabricated to explore the cell performance
enhancement by HC-A4, according to a previously pub-
lished report [42]. The Y123-based DSSCs with CDCA and
HC-A4 gave a Jsc of 14.25 and 14.61 mA cmÀ2; a Voc of
630 and 711 mV; a fill factor (FF) of 74.1% and 71.1%;
functions, i.e. both the prevention of the
pÀp stacking of
organic dye molecules such as DCA and the light harvest-
ing in shorter-wavelength regions. Thus, the IPCE of the
HOP-sensitized solar cell was lower than that of the others.
Table 2. summarizes the photovoltaic properties of the
DSSCs with HC-A3, HC-A4, or HC-A5 only. The results were
as follows: a Jsc of 4.15, 5.24 and 0.29 mA cmÀ2; a Voc of
692, 711, and 486 mV; a fill factor (FF) of 77.5, 76.7 and
74.1%; and a PCE (g) of 2.20%, 2.80% and 0.11%, respec-
tively. Among them, HC-A4 showed the best cell perfor-
mance because of its highest light harvesting effect at
shorter wavelengths. The IPCE and J–V spectra for the
DSSCs with HC-A3, HC-A4, and HC-A5 alone are presented
in the insets in Fig. 4. The HC-A4-based solar cell exhibited
slightly higher maximum IPCE from 300 nm to 550 nm
than that of HC-A3. These results indicate that it had a bet-
ter light harvesting effect in shorter wavelength regions for
co-sensitization. UV–visible spectra and molar extinction
coefficients were broadened and increased with the
and a PCE (g) of 6.6 and 7.4%, respectively (see Fig. 4 and
Table 2). It indicates that The HC-A4 coadsorbent enhanced
the cell performance of other organic dye-sensitized solar
cells.
To further elucidate the photovoltaic results and gain
more information on electron transfer and charge recombi-
nation in the TiO2 photoanode in the DSSCs, electrochemi-
cal impedance spectroscopy (EIS) was studied in the dark
under a forward bias of À0.62 V. The Nyquist and Bode
plots for the DSSCs with various coadsorbents are shown
in Fig. 5a and b, respectively. A greater recombination
resistance and longer electron lifetime on the TiO2
increased
p-conjugated extension of coadsorbents. How-
ever, HC-A5 exhibited the lowest cell performance com-
pared with the HC-A3 and HC-A4, probably due to lower
electron injection efficiency into the TiO2 CB due to its
higher dihedral angle, while its UV–visible spectra and
molar extinction coefficients were broadened and
increased. The dihedral angle between anthracene and
phenyl was too large in HC-A5, which is unfavorable for
intramolecular charge transfer from the HOMO to the
LUMO orbitals [36,37]. Fig. S2 shows the UV–visible
film(
s
n = C ÁRrec) [43–51] were obtained for the DSSC with
l
HOP/HC-A4 (160.1
X
, 145.2 ms) compared to those for
HOP/HC-A3 (148.3
X
, 144.2 ms) and HOP/HC-A5 (40.6 X,
27.6 ms). HOP/HC-A3 and HOP/HC-A4 had almost the
same electron lifetime. The photovoltaic performance
depended on the charge collection efficiency derived from
À1
g
cc = (1 + Rt/Rrec
)
[52]. The charge collection efficiencies
evaluated from the EIS parameters with the transmission
line model [50,51] are summarized in Table 3, and the fit-
ted Nyquist plots are shown in Fig. S3. The gcc of the DSSCs
with HC-A3 and HC-A4 was almost same, but the charge
recombination resistance of the DSSC with HC-A4 was
greater than that of HC-A3. It resulted in an increased Voc
of the DSSC (Fig. 5c). Coadsorbents may induce both the
adsorption state of the dye and the electronic state of the
dye-sensitized TiO2 to be changed, thereby influencing
the electron injection rate and efficiency [53–55]. Wenger
and co-workers [56] reported that the charge injection rate
of non-aggregated dyes, from the excited dye to the CB of
the TiO2 electrode, was faster than that of aggregated dyes
by three orders of magnitude.
Fig. 6. The density of states (DOS) for the HOP-coated TiO2 films with
different coadsorbents as a function of applied potential.

I.T. Choi et al. / Organic Electronics 15 (2014) 3316–3326
3323
Scheme 2. Schematic diagram of the possible charge or energy transfer pathways taking place at the dyed TiO2/HC-A series/electrolyte interface. ET1
indicates the direct injection of an electron from the HC-A series to the TiO2 surface. ET2 and ET3 correspond to the energy transfer from the HC-A series to
the dye and to IÀ3 ions, respectively.
The difference in Voc for the DSSCs with various coad-
sorbents was plotted as a function of the JÀV curve
obtained from the EIS measurements (Fig. 5d). The dark
current, measured by EIS, decreased in the following order:
HOP, HOP/HC-A5, HOP/HC-A3, and HOP/HC-A4. A higher
dark current could reduce Voc because it is governed by
back electron transfer from the TiO2 surface into the redox
couple. In particular, among the coadsorbents used in this
study, it was obvious that HC-A4 had the highest Voc value
with the lowest dark current. The Y-shaped bulky and
robust insulating layer of HC-A4 induced fewer recombina-
tion reactions at the interface between the HOP-sensitized
TiO2 and the electrolytes. Therefore, HC-A4 might be more
effective than HC-A3 and HC-A5 in screening I2 or IÀ3 , which
resulted in the simultaneous improvements in Jsc and Voc in
the DSSC with HC-A4.
To investigate the change in the trap state caused by the
HC-A coadsorbents and to reveal the change in Voc, cyclic
voltammetry was achieved on each TiO2 electrode HOP-
sensitized with HC-A4 and HC-A5 together with LiClO4
(see Fig. S3). The capacitive currents of different electrodes
at the TiO2/LiClO4 interface, which exhibited gradual
onsets under a forward potential, were shown in Fig. 6.
The distribution of the trap states can be estimated from
the density of states (DOS) calculated from DOS = dQ/
dV Â (NA/F) [55], where Q is the total number of surface
trapping sites, NA is Avogadro’s number, F is Faraday’s con-
stant, and V is the potential applied to the electrode. As
shown in Fig. 6, the onsets of the capacitive currents were
around À0.194 V for the TiO2 electrode covered with HOP
Fig. 7. PL emission spectra for (a) 3.0 Â 10À6
M HC-A3 (black line),
and HC-A5, whereas the TiO2 electrode covered with HOP
and HC-A4 together demonstrated onsets of around
À0.335 V, which indicates that the edge of the TiO2 CB
shifted to lower values (toward the vacuum level). In par-
ticular, the TiO2 electrode with HC-A4 gave a more nega-
tive shift in the onset than that evaluated with HC-A5.
The Voc is estimated by the difference between the quasi-
Fermi level and the redox potential of the IÀ3 /IÀ couple in
2.0 Â 10À6 M HOP (red line), 4.0 Â 10À6 M I2 (green line), 3.0 Â 10À6
M
HC-
HC-A3 + 2.0 Â 10À6
M
HOP
(blue
line),
3.0 Â 10À6
M
A3 + 2.0 Â 10À6
M
HOP + 4.0 Â 10À6
M I2 (cyan line) at the kex of
358 nm in THF and (b) 3.0 Â 10À6 M HC-A4 (black line), 2.0 Â 10À6
M
M HC-
HOP (red line), 4.0 Â 10À6
M
I2 (green line), 3.0 Â 10À6
A4 + 2.0 Â 10À6 M HOP (blue line), 3.0 Â 10À6 M HC-A4 + 2.0 Â 10À6
M
HOP + 4.0 Â 10À6 M I2 (cyan line) at the kex of 359 nm in THF solution.
(For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)

3324
I.T. Choi et al. / Organic Electronics 15 (2014) 3316–3326
the electrolyte; therefore, the bigger negative shift of the
quasi-Fermi level of TiO2 photoanode with HOP and HC-
both HOP and I2 were much lower than those of HC-A3
and HC-A4. As shown in Fig. 7a and b, the emission inten-
sity of HC-A was significantly decreased for the HC-A/HOP
and HC-A/HOP/I2 systems. The reduction in the HC-A/HOP
system was similar to the value for HC-A3 (48%) and HC-
A4 (49%), while the reduction for the HC-A/HOP/I2 system
was different from that of HC-A3 (70%) and HC-A4 (67%).
From these results, it seems that energy transfer occurred
from HC-A3 and HC-A4 to the HOP dye [57]. Once excited,
HC-A3 and HC-A4 can transfer their excited energy to a
certain acceptor [6,9,10,16,17,58,59], and the energy trans-
fer rate strongly depends on the separation distance (r)
between the coadsorbent and the nearby acceptor mole-
cule [60]. Both HC-A3 and HC-A4 were assumed to be
within the Förster radius (Ro) of the HOP dye (see
Fig. S6), and the separation distance between the donor
of HC-A3/4 and the acceptor of HOP dye may be suffi-
ciently close in the solution state. In this investigation,
with a similar concentration of each solution estimated
from the real DSSCs fabricated here, an increase in the
emission spectrum of HOP over 600 nm was not observed,
probably due to the fact that the molar excitation
coefficient of HOP has an extremely low value of
11,650 MÀ1 cmÀ1 and 11,700 MÀ1 cmÀ1 at 358 and
359 nm, respectively, and its PL quantum efficiency is also
A4 induced higher Voc
.
Furthermore, in order to understand the role of these
coadsorbents on cell performance enhancement, the
photophysical properties of HC-A3, HC-A4 and HOP dye
were investigated in the solution state (see the ESI for
extended discussion). Scheme 2 shows the schematic dia-
gram of the possible charge or energy transfer pathways
taking place at a dyed TiO2/HC-A/electrolyte interface.
ET1 means a direct injection of electron from the HC-A
coadsorbent to the TiO2 surface. ET2 and ET3 correspond
to the energy transfer from the HC-A to the dye, and
quenching of the HC-A by triiodide, respectively. Fig. 7a
and b show the photoluminescence (PL) spectra with dif-
ferent conditions at the excitation wavelengths (kex) of
358 nm and 359 mm in THF, respectively. Each solution
was made to similar concentrations estimated from real
DSSCs fabricated here (the molar ratio of HC-A3,4/HOP/
I2 = 1.5/1/2). Upon photoexcitation at 358 and 359 nm for
HC-A3/HOP/I2 and HC-A4/HOP/I2, the emission maximum
of HC-A3, HOP and I2 in the HC-A3/HOP/I2 system
appeared at 462, 689 and 456 nm, and those of HC-A4,
HOP and I2 in the HC-A4/HOP/I2 system appeared at 516,
691, and 461 nm, respectively. The emission intensities of
Fig. 8. (a) and (b) Emission spectra for the HC-A3, dye, the dye with CDCA and the dye with HC-A3 adsorbed onto ZrO2 film at a kex of 358 nm. (c) and (d)
Emission spectra for the HC-A4 dye, the dye with CDCA and the dye with HC-A4 adsorbed onto ZrO2 film at a kex of 359 nm.

I.T. Choi et al. / Organic Electronics 15 (2014) 3316–3326
3325
very low at 358 and 359 nm (see Fig. S7). From these
3. Conclusions
results, it is presumed that the energy transfer from HC-
A3 and HC-A4 to HOP happens to be low as well. In other
words, the molar extinction coefficient of HOP is very low
from 12,825 MÀ1 cmÀ1 to 11,000 MÀ1 cmÀ1 in the UV
wavelength region of 350–375 nm (see Fig. S7), such that
the energy transfer efficiency from HC-A3 and HC-A4 to
HOP could be low.
We have designed and synthesized multi-functional
coadsorbents with naphthalene and anthracene as an
extended
p-conjugated aryl unit. Co-adsorption of HC-A
coadsorbents with HOP broke up dye aggregates and
improved the light harvesting effect in UV–vis wavelength
regions, leading to a significant enhancement in the photo-
current density (Jsc) and the open circuit voltage (Voc) by
suppressing charge recombination and increasing electron
lifetime. The HOP/HC-A4-based DSSC exhibited higher Voc
and Jsc values than the HC-A3- and HC-A5-based DSSCs.
According to the EIS and IPCE analyses, HC-A4-based DSSC
showed greater charge resistance, longer electron lifetime
and a higher IPCE value compared to the HC-A3 and HC-
A5-based DSSCs. As a consequence, we can conclude that
coadsorbents have a better light-harvesting effect with a
We also examined no direct energy transfer from HC-A
to the HOP dye. Fig. 8a and c show the photoluminescence
spectra of HC-A3, HC-A4, HOP, HOP/CDCA, HOP/HC-A3,
and HOP/HC-A4 at the excitation wavelengths (kex) of
358 nm and 359 nm on nanocrystalline ZrO2 thin films.
ZrO2 films were employed to prevent electron injection
from the excited dye state [38]. It is interesting that the
emission intensity for the ZrO2/HOP film is much less than
those for ZrO2/HC-A3 and ZrO2/HC-A4 films, indicating
that it is not effective to excite the HOP at the kex of
358 nm and 359 nm. On the other hand, ZrO2/HC-A3 and
ZrO2/HC-A4 films have effective emission intensity at the
kex of 358 nm and 359 nm. This result is similar to the
result observed in the real solution state with the DSSCs
fabricated here. Therefore, if energy transfer happens from
HC-A to the dye, the photoluminescence spectra of HOP
with HC-A3, HCA4 on the ZrO2 film should exhibit stronger
intensity than the emission of HOP itself. As expected, the
emission intensities of HOP with both HC-A and CDCA
were increased (Fig. 8b and d).
more extended
angle for efficient electron injection into the CB of TiO2, a
molecular structure preventing the stacking of dye
p-conjugated aryl unit, a smaller dihedral
p–p
molecules, and a compact blocking layer on the TiO2 sur-
face for reducing charge recombination, simultaneously.
Particularly, HC-A4-based DSSC showed a higher PCE of
up to 8.2% than those of the HC-A3-based DSSC (7.7%)
and HC-A5-based DSSC (5.1%).
Acknowledgments
CDCA is often adapted as a coadsorbent to prevent the
dye aggregation [24–26,31], thereby remarkably increas-
ing Voc and Jsc, as the aggregation of the dye has been sug-
gested to promote intermolecular interactions which
compete with charge separation [39,40] and/or form a
layer of non-injecting dyes[41]. An increasing tendency in
PL intensity was observed, even with a lower adsorption
amount of dye from the dye load amount determination
from the absorption spectra of the dye-desorbed solution
with the ZrO2 films, obtained from the treatment of in
This work was supported by the Converging Research
Program through the Korean Government Ministry of Edu-
cation, Science and Technology Science, ICT and Future
Planning (2013K000203) and by the New and Renewable
Energy R&D Program (20123010010070) of the Korean
Institute of Energy Technology Evaluation and Planning
(KETEP) grant through the Korean Government Ministry
of Trade, Industry and Energy. It would be also acknowl-
edged that Mr. BS You is a visiting graduate student in
the GET-future Lab at KU.
0.1 M aqueous NaOH solution, in 0.1 M NaOH in
a
50:50:50 (vol%) EtOH:H2O:THF mixture: 1.92 Â 10À8 for
HOP; 1.09 Â 10À8 for HOP/CDCA; 0.91 Â 10À8 for HOP/
HC-A3; 0.96 Â 10À8 mol cmÀ2 for HOP/HC-A4 (Fig. S8).
This can be attributed to the prevention of dye aggregation,
because CDCA has no absorption band. Consequentially,
the increase in emission intensity of HOP by coadsorption
with HC-A and CDCA might be ascribed to the prevention
of dye aggregates (Fig. 8 and Fig. S8) rather than the ET2
pathway (Scheme 2).
Appendix A. Supplementary material
Electronic Supplementary Information (ESI) available:
[details of any supplementary information available should
be included here]. See doi:http://dx.doi.org/10.1039/
b000000x/http://dx.doi.org/10.1016/j.orgel.2014.09.008.
On the basis of the results described above, it can be
concluded that the energy transfer efficiency from HC-A
to HOP is expected to be low or to not occur (ET2 pathway
in Scheme 2). Furthermore, triiodide is a highly mobile ion
that is known as a ‘‘perfect quencher’’ and it means that its
collisions with HC-A have a near unity probability of
quenching the excited state of HC-A [57,61]. Thus, HC-A
cannot influence photovoltaic performance enhancement
through the energy transfer pathways of HC-A into HOP
in real DSSCs with IÀ/I3À systems. In other words, the ET1
electron injection pathway is dominant compared to
the other energy transfer pathways, i.e. ET2 and ET3
(Scheme 2).
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