New N-methyl pyrrole and thiophene based D-π-A systems for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2012.09.001

Dye-sensitized solar cells (DSSCs) containing novel DP-T and DP-P organic sensitizers having thiophene and N-methyl pyrrole moieties as π bridges in a D-π-A system were assembled and characterized. Incorporation of a thiophene bridge to give sensitizer DP-T enhanced solar energy capture, while impedance spectroscopy showed a much longer electron recombination lifetime (τ) for the DSSCs based on pyrrole containing sensitizer DP-P. The observed kinetics led to increased open circuit voltage (V_oc) for the device based on DP-P, regardless of types of additives (TBP or DCA) employed in the devices. Better photovoltaic performance was achieved using DP-T.

Full text of DOI:10.1016/j.dyepig.2012.09.001

Dyes and Pigments 96 (2013) 313e318
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
New N-methyl pyrrole and thiophene based DepeA systems for dye-sensitized
solar cells
*
Bo Hyung Kim, Harold S. Freeman
Fiber and Polymer Science Program, North Carolina State University, Raleigh, NC 27695, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Dye-sensitized solar cells (DSSCs) containing novel DP-T and DP-P organic sensitizers having thiophene
and N-methyl pyrrole moieties as bridges in a De eA system were assembled and characterized.
Incorporation of a thiophene bridge to give sensitizer DP-T enhanced solar energy capture, while
impedance spectroscopy showed a much longer electron recombination lifetime ( ) for the DSSCs based
on pyrrole containing sensitizer DP-P. The observed kinetics led to increased open circuit voltage (Voc) for
the device based on DP-P, regardless of types of additives (TBP or DCA) employed in the devices. Better
photovoltaic performance was achieved using DP-T.
Received 23 July 2012
Received in revised form
1 September 2012
Accepted 4 September 2012
Available online 12 September 2012
p
p
s
Keywords:
DSSCs
Ó 2012 Elsevier Ltd. All rights reserved.
Heterocyclic spacers
Recombination lifetime
Dep
eA system
IPCE
TiO2 film
1. Introduction
a donorespacereacceptor system (DepeA), to achieve effective
charge separation and transfer. Molecular engineering of donor
groups has evolved from the use of a simple triphenylamine moiety
to a starburst phenylene based moiety [12,13], to control electronic
levels or molecular geometry and to afford effective charge transfer.
Dye-sensitized solar cells (DSSCs) are of great interest because
their construction involves low cost components and they are
relatively simple, affording them the potential to compete with
fossil fuel-based electricity generation, a non-renewable process.
Ru (II) based polypyridyl complexes for DSSCs attracted early
attention in this arena, by giving a photon-to-current conversion
A
p bridging unit has been a common design feature, to influence
HOMO/LUMO energy levels and/or spectroscopic properties [14].
Generally, increased conjugation path length using oligoene [4] or
oligothiophene [5] was adopted as a basic way to achieve a red
shift. However, a flexible extended chain length had a negative
effect on charge separation or efficient generation of photons, thus
counteracting the positive effect of red shifted absorption spectra
contributing to large solar capture [15]. On the other hand, it was
reported that the presence of a rigid spacer [16,17] or introduction
of an alkyl group [18] as a side chain affected dye aggregate and/or
charge recombination and, consequently, photovoltaic perfor-
mance. Results from another recent study showed that the even
a small change in the bridging unit caused a significant change in
photovoltaic parameters [19]. In addition, aryl amine based donors
[20,21] have been used to give efficient charge separation and/or
control charge recombination. In view of these different results, the
rigid N-methyl pyrrole system, having both electron rich character
and low delocalization energy [22], attracted our attention and led
us to investigate its effect on photovoltaic parameters relative to the
thiophene ring system commonly incorporated into DSSC
efficiency (
tizers led to merocyanine [2], oligoene [3,4], coumarin [5,6], indo-
line [7,8], and cyanine dyes [9,10], with an efficiency ( ) up to 10%
h) of w12% [1]. Studies aimed at metal-free dye sensi-
h
achieved by using metal-free dye C219 containing ethyl-
enedioxothiophene and dithienosilole blocks [11]. Although the
performance of DSSCs based on metal-free organic dyes has not
exceeded that of solar cells based on Ru-complexed dyes, metal-
free types remain of interest for DSSCs due to advantages that
include high absorption coefficients resulting from intramolecular
pep* transitions, straightforward synthesis, color tuning, struc-
tural modification for desired physical and photochemical proper-
ties, and their economy. Typical metal-free sensitizers are based on
* Corresponding author. Tel.: þ1 919 515 6552; fax: þ1 919 515 3057.
E-mail addresses: bkim6@ncsu.edu (B.H. Kim), hfreeman@ncsu.edu, harold_
freeman@ncsu.edu (H.S. Freeman).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.09.001

314
B.H. Kim, H.S. Freeman / Dyes and Pigments 96 (2013) 313e318
sensitizers. The present paper reports the photovoltaic perfor-
mance of DSSCs sensitized with two new metal-free sensitizers in
which a diphenylamino-substituted indole moiety and cyanoa-
crylic acid were connected using either thiophene or N-methyl
2.2.3. Voltammetry and impedance measurements
The oxidation potential of dye adsorbed on TiO2 films was
measured using a three electrode electrochemical cell in acetoni-
trile containing 0.1
M TBAPF6 (tetrabutylammonium hexa-
pyrrole to form a De
p
eA system (cf. Fig. 1), as a part of a larger
fluorophosphate) at a scan rate of 100 mV sꢀ1. Dye coated TiO2 film
was used as a working electrode and Ag/Agþ electrode and Pt wire
were employed as a reference and counter electrode, respectively.
The potential of the working electrode was calibrated with ferro-
cene as an internal reference.
study in which heterocyclic systems in place of a benzene spacer
and a bulky bis-carbazoleaminor donor in place of diphenylamine
were examined for DSSC dye development [23].
The impedance spectroscopy of the cell was recorded using an
impedance analyzer connected to a potentiostat (reference 600TM,
Gamry Instruments, USA) in a frequency range of 0.1 Hze105 Hz at
room temperature under dark condition. The applied forward bias
was ꢀ 0.65 V and AC amplitude was set to 10 mV.
2. Methods
2.1. Materials
All reagents were purchased from SigmaeAldrich, TCI, or Fisher
Scientific. All chemicals used in this study were of reagent-grade
quality and solvents purchased from commercial suppliers were
used without further purification. Moisture-sensitive reactions
were performed under either nitrogen or argon gas. All reactions
were followed by thin-layer chromatography (TLC) using Analtech
silica gel GHLF plates. The N719 sample used for fabrication of the
reference solar cell was received as a gift from Dyesol and used
without further purification.
2.2.4. Photovoltaic measurements
Photocurrentevoltage characteristics of DSSCs were measured
using a Keithley 2400 source meter under illumination of AM 1.5 G
solar light coming from solar simulator (SOL3A, Oriel) equipped
with a 450 W xenon lamp (91160, Oriel). The incident light intensity
was calibrated using a reference Si solar cell (Newport Oriel,
91150V) to set 1 Sun (1 mW/cm2). The currentevoltage curve of the
cell was obtained by applying external voltage bias and the
measuring generated photocurrent. The measurement was fully
controlled under Oriel IV Test Station software. A mask (0.30 cm2)
was covered on the testing cell during photocurrent and voltage
measurement. The photo active area of cell was 0.5 ꢁ 0.4 cm.
IPCE (incident monochromatic photon to current conversion
efficiency) experiments were carried out using a system (QEX10, PV
Measurements, USA) equipped with a 75 W short arc xenon lamp
(UXL-75XE, USHIO, Japan) as a light source connected to a mono-
chrometer. Calibration of incident light was performed using
a silicone photodiode (IF035, PV Measurements). Monochromatic
quantum efficiency was recorded at short circuit conditions under
AC mode with white-light bias. Wavelength sampling interval was
10 nm. The beam size of monochromatic light illuminated on DSSCs
was 0.1 ꢁ 0.5 cm and chopping speed of AC was set to 10 Hz.
2.2. Characterizations
2.2.1. Structure confirmation
1H NMR spectra were recorded on a 300, 400, or 500 MHz
Bruker Advance spectrometer using DMSO-d6 or CDCl3 as a solvent.
13C NMR spectra were recorded on a 500 MHz Bruker Advance
spectrometer. 2-Dimensional NMR spectra including COSY, HSQC,
and HMBC were recorded on a 500 MHz or 700 MHz Bruker
Advance spectrometer. High resolution mass spectra (HRMS) were
obtained using electrospray ionization (ESI), in the positive mode,
on an Agilent Technologies (Santa Clara, California) 6210 LC-TOF
mass spectrometer.
2.2.2. Spectroscopic analysis
UVeVis spectra in solution and in dye-loaded TiO2 film were
recorded on a Varian Cary 300 UVeVis spectrophotometer. Pho-
toluminescence of dye solutions was recorded using a fluorometer
(FluorologÒ-3, HORIBA, USA) equipped with a 450 W xenon lamp
(FL-1039/40, HORIBA, USA). The concentration of dye solutions for
emission spectra was constrained to have absorbance of w0.2.
Fluorescence lifetime of each dye in THF was measured by a system
using a TCSPC (time-correlated single photon counting) controller.
LED (NanoLED-460, HORIBA) having a peak wavelength of 460 nm
was used as an excitation source and LudoxÒ in distilled water was
used as an internal reference. The instrument response was
measured below the 200 ns of time period. The decay curve ob-
tained was fitted to the multi-exponential function represented by
A þ B1exp(ꢀi/T1) þ B2exp(ꢀi/T2). In order to predict the amount of
dye adsorbed on the TiO2 surface, dyes on TiO2 film were desorbed
into a solution of 0.1 M NaOH in THF-H2O (v/v, 1:1) followed by
measurement of UVeVis spectra and calculation of the adsorbed
amount of dye using BeereLambert’s law. Thickness of dye loaded
2.3. Synthesis of target dye sensitizers
2.3.1. (2E)-2-cyano-3-(5-(5-(diphenylamino)-3,3-dimethyl-3H-
indol-2-yl)thiophen-2-yl)acrylic acid (DP-T)
To a solution of compound 1c (0.3 g, 0.70 mmol) and 2-
cyanoacetic acid (0.07 g, 0.85 mmol) in acetonitrile (70 ml) was
added a few drops of piperidine under N2 gas flow. The reaction
mixture was then stirred under reflux at 80 ꢂC. The precipitate
formed during reaction was collected by filtration and dried. Pure
DP-T was obtained by column chromatography (dichloromethane/
methanol, 12/1, v/v) on silica gel, to give a red solid. Yield: 87%, 1H
NMR (300 MHz, DMSO-d6)
d
¼ 8.13 (s, 1H), 7.87 (d, 1H, J ¼ 3.9 Hz),
7.76 (d, 1H, J ¼ 3.6 Hz), 7.54 (d, 1H, J ¼ 8.4 Hz), 7.33e7.28 (m, 4H),
7.21 (d, 1H, J ¼ 2.1 Hz), 7.06e7.01 (m, 6H), 6.93 (dd, 1H, J ¼ 8.2 Hz,
2.1 Hz), 1.47 (s, 6H). 13C NMR (500 MHz, DMSO-d6)
d
¼ 176.35,
162.84, 148.97, 148.39, 147.44, 145.84, 141.09, 140.07, 139.56, 135.83,
129.54, 129.52, 123.66, 123.56, 122.90, 121.25, 118.83, 117.23, 111.53.
HRMS-ESI: calculated for C30H23N3O2S (M þ H)/z 490.1584, Found:
490.1579.
TiO2 film for all characterizations was 6 mm.
2.3.2. (2E)-2-cyano-3-(5-(5-(diphenylamino)-3,3-dimethyl-3H-
indol-2-yl)-1-methyl-1H-pyrrol-2-yl)acrylic acid (DP-P)
CN
CN
CO2H
N
N
S
N
CO2H
To a stirred solution of 2d (0.13 g, 0.31 mmol) and 2-cyanoacetic
acid (0.05 g, 0.61 mmol) in ethanol (20 ml) was added NaOH (0.05 g,
1.22 mmol) under N2 gas flow. The reaction mixture was then
stirred under reflux at 80 ꢂC. The mixture formed during reaction
was filtered and product was dried. Pure DP-P was obtained by
N
N
DP-T
DP-P
Fig. 1. Chemical structures of DP-T and DP-P.

B.H. Kim, H.S. Freeman / Dyes and Pigments 96 (2013) 313e318
315
column chromatography (dichloromethane/methanol, 10/1, v/v) on
silica gel, to afford an orange-yellow solid. Yield: 34%, 1H NMR
(300 MHz, DMSO-d6)
3.2. Synthesis
d
¼ 8.28 (s, 1H), 7.34 (d, 1H, J ¼ 8.4 Hz), 7.30e
Based on the results of the DFT calculations, target dye sensi-
tizers were synthesized (Fig. 3). Intermediates (i and ii), which
underwent Fisher indole cyclization [26], were prepared by the
reaction of each heterocyclic starting material with isobutyric
anhydride in the presence of Lewis acid [26,27]. Following Fisher
indole cyclization, amination under BuchwaldeHartwig reaction
conditions [28] was followed by formylation, which was done by
either metal-halogen or metal-hydrogen exchange using n-BuLi
[29] for DP-T. The two steps were reversed for DP-P. Reaction [30]
of the formylated intermediate (1c and 2d) with cyanoacetic acid
produced the target dyes. 2-Dimensional NMR including COSY
(correlation spectroscopy), HSQC (heteronuclear single quantum
coherence), and HMBC (heteronuclear multiple-bond correlation
spectroscopy) analysis (Supplementary Data S1eS4) allowed us to
distinguish the desired structural isomer (ii) from a possible
structural isomer of the 2-methyl-(1-(1-methyl-1H-pyrrol-3-yl)
propan-1-one).
7.25 (m, 4H), 7.18 (d, 1H, J ¼ 2.7 Hz), 7.15 (d, 1H, J ¼ 2.1 Hz), 7.02e
6.97 (m, 6H), 6.90 (dd, 1H, J ¼ 8.4 Hz, 2.4 Hz), 6.80 (d, 1H,
J ¼ 3.0 Hz), 1.40 (s, 6H). 13C NMR (500 MHz, DMSO-d6)
d
¼ 179.24,
164.08, 150.06, 148.09, 147.58, 144.32, 140.83, 129.64, 129.41, 129.01,
127.73, 124.11, 123.11, 122.37, 120.27, 119.36, 118.99, 117.90, 109.19,
53.58, 35.94, 24.19. HRMS-ESI: calculated for C31H26N4O2 (M þ H)/z
487.2129, Found: 487.2129.
2.4. Molecular modeling methods
DFT (density functional theory) calculations were conducted
using Gaussian 03 software. Geometries were optimized using the
B3LYP hybrid functional (Beck’s three-parameter functional and
LeeeYangeParr functional) as an exchange-correlation functional
and 3-21G(d) was used as the basis set. HOMO and LUMO orbitals
were determined using optimized geometries.
3.3. Optical and electrochemical properties
3. Results and discussion
The oxidation potential (Eox) of dye sensitizers adsorbed on TiO2
films was measured with a three electrode electrochemical cell at
a scan rate of 100 mV sꢀ1. The cyclovoltammogram (Supplementary
Data S5) of DP-T indicated a one-electron transfer process arising
from the oxidation of the diphenylamine moiety of this dye. The
corresponding oxidation and reduction potential for DP-T were
1.38 V and ꢀ0.92 V versus NHE, respectively. However, the oxida-
tion potential of DP-P obtained by cyclovoltammetric technique
could not be resolved due to the background charging current.
Although DPV (differential pulse voltammetry) might be employed
to enhance resolution of the voltammogram [31], given the more
electron rich nature of the N-methyl pyrrole ring than the thio-
phene ring, it is anticipated that the redox potentials of DP-P are
negatively shifted relative to those of DP-T. This is because oxida-
tion of the diarylamine moiety would be enhanced by the intro-
duction of an electron rich moiety. These results are supported by
our molecular modeling study, which showed an EHOMO increase of
33 eV (6 eV for ELUMO) for DP-P compared to the ELUMO value of DP-
T. Judging from the electrochemical analysis combined with
modeling study, enough driving force was obtained for the electron
injection process for both dyes.
3.1. Modeling studies
Prior to the synthesis of the target dyes, molecular modeling
studies involving Gaussian 03 [24] were used to calculate: (1)
thermodynamically favorable electron injection into the conduc-
tion band edge of TiO2 and (2) electronic energies and densities at
HOMO and LUMO levels. Fig. 2 shows the calculated electronic
energies of frontier orbitals and corresponding surface isodensity
plots. For both dyes, the Elumo values were located above the
conduction band edge (Ecb) of TiO2 (ꢀ4.0 eV versus vacuum). The
Ehomo values were below the energy of the redox species of iodide/
triiodide (ꢀ4.8 eV versus vacuum) [25]. It was concluded that these
relative matchings of electronic levels of sensitizers would lead to
energetically favorable electron injection as well as regeneration of
oxidized dye during DSSC operation. Furthermore, HOMO-LUMO
excitation moved the electron distribution from the donor side
(diarylamine substituted indole moiety) to the acceptor side (the
cyanoacrylic acid group), ensuring efficient charge separation by
photo-excitation of dye.
O
Ph2N
S
Br
S
S
(b)
N
N
i
(a)
1b
1a
(c)
(c)
Br
CHO
Ph2N
S
1c
(d)
NHNH2HCl
DP-T
N
(a)
O
N
(e)
Br
N
I
Br
N
N
ii
N
2b
2a
CHO
Ph2N
N
CHO
Br
N
(f)
(d)
N
DP-P
N
2d
2c
Fig. 3. Synthetic protocol for DP-T and DP-P. (a) TsOH$H2O at 85 ꢂC. (b) Pd2(dba)3, X-
Phos, sodium tert-butoxide, NHPh2, Toluene, 1 day, 80 ꢂC. (c) (i) n-BuLi, THF, ꢀ78 ꢂC;
(ii) DMF, ꢀ78 ꢂC, 1 h; (iii) ꢀ78 ꢂC to rt for 2 h; (iv) 10% HCl (d) cyanoacetic acid,
piperidine, CH3CN, reflux, 3 h (e) NIS, THF, rt, 4 h (f) Pd2(dba)3, X-Phos, Cs2CO3, NHPh2,
Toluene, 1 day, 80 ꢂC.
Fig. 2. Schematic representation of electronic densities, energies, and geometries for
DP-T and DP-P. All parameters were obtained using DFT-B3LYP at 3-21G(d) level. Inset
figure (a) and (b) indicate dihedral angles for DP-T and DP-P, respectively.

316
B.H. Kim, H.S. Freeman / Dyes and Pigments 96 (2013) 313e318
Fig. 4 provides a schematic representation of possible forward
1.2
1.0
0.8
0.6
0.4
0.2
DP-T
DP-T on TiO
reactions (solid arrows) by photo-excitation of dyes and back
electron transfers (dashed arrows) impeding high efficiency of
DSSCs.
2
DP-T w/DCA
DP-P
Related research on the metal-free sensitizers afforded an elec-
tron injection time of <100e200 fs (K2) and an emission lifetime
(B3) on a ns scale [4,6,32]. In this study, we examined the relaxa-
tion time constants (K2) of excited dye sensitizers by time-resolved
single photon counting methods (TCSPC) [33]. The decay curves
obtained (Supplementary Data Fig. S6) after photo-excitation of
dyes was fit to an exponential function: F(t) ¼ A þ B1$exp
(ꢀt/T1) þ B2$exp (ꢀt/T2), where B is light intensity and T is lifetime
DP-P on TiO
2
DP-P w/DCA
300
350
400
450
500
550
600
650
Wavelength (nm)
Fig. 5. Absorption spectra of dye sensitizers DP-T and DP-P and dye-loaded TiO2 film.
(Solid, rectangle shaped, and dashed lines indicate the spectra obtained from THF
solutions, dye-loaded TiO2 film, and dye/DCA loaded TiO2 film, respectively.)
obtained. Fitting was repeated until CHISQ (c
2) value [34] was close
to 1 to ensure good fitting of decay curves.
Among the two lifetimes obtained (Supplementary Data
Table S1), the lifetime of higher relative amplitude was taken as the
lifetime for a given dye. The resultant lifetime constants were
4.11 ns for DP-T and 3.55 ns for DP-P. Considering the pico or femto
second scale of the electron injection process for most metal-free
organic dyes [4,6,32,35,36], the relaxation process (Fig. 4, B1) of
the present dye sensitizers would not hinder electron injection
process (Fig. 4, K2).
Light absorption phenomena, one of the important features of
high efficiency DSSCs were examined using UVeVis spectroscopy.
Absorption (Fig. 5) and emission spectra (Supplementary Data
Fig. S7) of the present dye sensitizers were recorded on THF solu-
tions. Among the three absorption peaks for dye DP-T, the one in
the low energy region was assigned to the ICT (internal charge
transfer) band, while peaks in the UV region arose from the local-
TiO2 film, which was used as a reference, showed an absorption
starting around 380 nm (Supplementary Data S8). DP-T loaded TiO2
film showed a red-shifted absorption spectrum (lmax ¼ 476 nm)
with broadened features. This suggests that strong dyeedye
interactions or/and dyeeTiO2 interactions are present, leading to
dye aggregates on the TiO2 electrode [37,38]. In the case of DP-P, its
lmax on TiO2 film could not be resolved due to the relatively large
absorption arising from the bare TiO2 film. However, it appears that
the absorption spectrum of DP-P loaded TiO2 film also gives shifted
or broadened feature compared to that in solution, judging from
the part of absorption edge above 450 nm. Absorption in this region
was not caused by the reference TiO2 film. In order to gain further
insight into the spectral changes of dyes on TiO2 film, DCA (deox-
ycholic acid) [39], a suppressing agent of dye aggregates on TiO2
films, was co-adsorbed with dyes on the TiO2 electrode. This caused
the absorption maximum of DP-T/DCA loaded TiO2 film to shift
toward the one in solution, providing evidence for dye aggregate
formation on TiO2 surface in the absence of DCA (Fig. 5). At the
same time, addition of DCA lowered the adsorbed amount of dye on
the TiO2 surface as it competes with dye molecules for TiO2 sites
(Supplementary Data S9). According to the normalization of
absorbance of dye/DCA loaded TiO2 film at a given Emax, the
adsorption of DP-T and DP-P was reduced by 5 and 20%, respec-
tively, in the presence of DCA.
ized
pep* transition of the conjugated system [19]. Notably, dye
DP-T (lmax ¼ 464 nm) having thiophene unit was characterized by
a 69 nm bathochromic shift of absorption spectrum compared to
that of dye DP-P (lmax ¼ 395 nm). DP-P hypsochromism might be
due to a lower electron push from donor side toward acceptor side
in the presence of an electron rich pyrrole ring in this De
system, resulting in an increased energy gap between HOMO and
LUMO. Also, DP-P hypochromism (
peA
3
¼ 17,000) arises from the
max
twisted molecular structure induced by steric interactions between
N-methyl groups and neighboring units. From the investigation of
optimized geometries, DP-P exhibited dihedral angles of 17ꢂ and
24ꢂ with the neighboring indole moiety and anchoring unit,
Bearing in mind that fast recombination components have been
identified as an efficiency-limiting factor, electrochemical imped-
ance spectroscopy (EIS) [31] measured under dark condition would
be useful in obtaining the time constant related to a back electron
transfer (route B3 in Fig. 4). To investigate the effects of additives on
recombination kinetics, device B having DCA and device C with 4-
tert-butylpyridine (TBP) were fabricated as well as a reference
device A, which had no additives.
respectively, whereas DP-T (
geometry (Fig. 2(b)).
3
¼ 25,500) showed almost plane
max
Dye adsorption on a TiO2 surface showed somewhat different
absorption spectra than those recorded in solution (Fig. 5). Bare
Fig. 6 shows Nyquist plots for these three devices. According to
the charge recombination lifetimes (
the Nyquist plot, DP-P (
¼ 40 ms) showed a 5-fold higher lifetime
than DP-T (
s) of device A extracted from
s
s
¼ 8 ms). The superior kinetic properties of devices
based on DP-P relative to those based on DP-T remain consistent
regardless of the presence of DCA or TBP. Use of TBP resulted in
a much higher
recombination via
s
, verifying its effect on controlling Ecb and/or
surface passivation effect [5,40]. These
a
recombination kinetics were also supported by dark-current
measurements (Supplementary Data S10eS11).
Concerning factors affecting
s values of DSSCs, factors such as
blocking effects, surface coverage [41], or complexation of dyes
with oxidizing species in electrolyte [42] have been pointed out. In
this regard, device A sensitized with DP-T (8.79E-0.8 mol/cm2) gave
higher dye loads on TiO2 than the device based on DP-P (1.16E-
07 mol/cm2). Assuming that the higher amount of adsorbed DP-T
could be a result of easier tendency of dye aggregate formation on
the TiO2 surface [6], a device based on DP-T might impede efficient
Fig. 4. Schematic representation of kinetic processes during the operation of DSSCs.
(Forward reactions (solid arrows) by photo-excitation of dye include light absorption
of dye (K1), electron injection (K2), regeneration of oxidized dye by redox shuttle (K3),
and transport of generated electrons (K4). These reactions compete with back reactions
(dashed arrows) which limit the efficiency of DSSCs. The major back reactions are
relaxation of excited dye (B1), back electron transfers of generated electrons with
oxidized dye (B2) and oxidized electrolyte (B3).)

B.H. Kim, H.S. Freeman / Dyes and Pigments 96 (2013) 313e318
317
device A
a DP-T
b DP-P
device B
150
25 ms
63 ms
device C
100
50
100
50
50 ms
8 ms
10 ms
40 ms
0
0
0
100
200
300
400
100 200 300 400 500
-Z'( )
Ω
-Z'( )
Ω
Fig. 6. Electrochemical impedance spectra of DSSCs fabricated with different additives: device A contained 0.6 M DMPII, 0.1 M LiI, 0.05 M I2, in dry CH3CN; device B was adsorbed
with 0.3 mM dye and 1 mM DCA and contained same electrolytes as device A; device C contained 0.6 M DMPII, 0.1 M LiI, 0.05 M I2, 0.5 M TBP in dry CH3CN.
surface blocking of dyes on TiO2 electrode. However, this assumes
that dye aggregates are adsorbed on TiO2 surfaces in the form of
“highly ordered layers”. Furthermore, dye loss by the DCA addition
was small in the case of DSSCs based on DP-T (5%), which can be
indicative of a good alignment of dye molecules on TiO2 surfaces.
Therefore, the origin of a significantly longer DP-P lifetime at even
lower dye loads could be ascribed to less tendency for complexa-
tion of DP-P with oxidizing species in electrolyte. According to
a previous finding that polarizability increases the local concen-
tration of oxidizing species in the electrolyte [42], polarizability
was calculated for isolated dye molecules. As a result, DP-P
(65.7 Å3) showed less polarity relative to that of DP-T (68.0 Å3),
electron injection followed by dye regeneration. This suggests that
either the spectral properties of dye sensitizers were insufficient for
high light harvesting efficiency [44] or that alignment of dyes
anchored toward the TiO2 surface negatively affected electron
transfer yield defined as Finj(l)$Fc(l) [45]. From the comparison of
current densities of device B (containing co-adsorbate DCA) and
device A, it was evident that DP-P showed a higher current density
(6.08 mA cmꢀ2
)
while DP-T reflected
a
slight increase
(8.21 mA cmꢀ2). Considering the ability of DCA to suppress dye
aggregation, a lower tendency for DP-Taggregate formation on TiO2
could be responsible for the slight Jsc increase compared to DP-P in
device B. On the other hand, device C having TBP as an additive
afforded a loss of current density at the expense of driving force for
electron injection. Current losses were more significant for DP-T
because of its lower LUMO, giving rise to the reduced energy gap
between Ecb and Elumo of the dye. Regarding the open-circuit
voltage (Voc) for reference device A, the device based on DP-P
showed 664 mV which is 46 mV higher than that of the device
based on DP-T. Assuming the Ecb of TiO2 as ꢀ0.5 V versus NHE for all
cases, it was found that a significantly increased electron lifetime of
the DSSC with sensitizer employing the N-methyl pyrrole spacer
seems to contribute to enhancement of Voc for DP-P. Also,
compared to device A, enhanced Voc for device C was ascribed to
which might impart DP-P an increased s.
3.4. Photovoltaic performance of DSSCs based on DP-T and DP-P
Photovoltaic parameters measured under simulated AM 1.5 G
irradiation are listed in Table 1. Four devices were employed which
differed in the additives used during cell fabrication. For reference
device A without additives, DP-P showed lower current densities
(5.85 mA cmꢀ2) with respect to DP-T (8.19 mA cmꢀ2). IPCE (incident
photon to current conversion efficiency) reflected this relative
trend in current densities where absorption band edge of the
device based on DP-T extended to 700 nm. On the other hand, the
IPCE spectrum of the device sensitized with DP-P reached
w650 nm for optimized device A and device B (Fig. 7). These action
spectra correlated well with absorption spectra recorded in solu-
tion in which DP-T showed both bathochromism and hypochrom-
ism relative to DP-P. Bearing in mind that IPCE is defined by
the increased
s by the addition of TBP. These results clearly show
that the passivation effect of TBP [40], which contributes to a Voc
increase. Nonetheless, increased Voc did not compensate for large
decrease in current densities, resulting in the lower
h compared to
reference device A. Device D fabricated with both DCA and TBP
gave efficiencies essentially between those from devices B and C.
Interestingly, the Voc of DSSC based on DP-P was increased to
736 mV in the presence of TBP and DCA. From the photovoltaic
performances of the four types of devices containing the present
a product of LHE(
l), Finj(l), and Fc(l) where LHE(l), Finj(l), and
Fc ) means light harvesting efficiency, quantum yield of electron
(
l
injection, and collection efficiency of injected electrons, respec-
tively [43]. For the present DSSCs based on DP-Tand DP-P, quantum
efficiencies were not high despite the presence of driving forces for
a
b
device A
device B
device C
device D
80
60
40
20
0
80
60
40
20
0
Table 1
DSSCs performance based on dyes DP-T and DP-Pa.
Dye
Jsc [mA cmꢀ2
]
Voc [V]
ff
h [%]
DP-T
Device A
Device B
Device C
Device D
Device A
Device B
Device C
Device D
8.19
8.21
6.09
6.77
5.85
6.08
4.55
4.53
0.618
0.618
0.700
0.691
0.664
0.655
0.718
0.736
0.697
0.696
0.668
0.749
0.738
0.761
0.724
0.776
3.53
3.53
2.85
3.50
2.86
3.03
2.37
2.59
DP-P
400 500 600 700
Wavelength (nm)
400
500
600
700
Wavelength (nm)
a
Photovoltaic parameters were obtained under AM 1.5
G
irradiation
(100 mW cmꢀ2). All devices contained electrolyte composition of 0.6 M DMPII/
0.1 M LiI/0.05 M I2 þ 0.3 mM dye; Device B contained 1 mM DCA as an additive;
Device C contained 0.5 M TBP as an additive; Device D contained 0.5 M TBP and
1 mM DCA as additives.
Fig. 7. IPCE plots for DSSCs sensitized with DP-P (a) and DP-T (b)*. * Denotes: All
devices contained electrolyte composition of 0.6 M DMPII/0.1 M LiI/0.05 M I2 þ 0.3 mM
dye; Device B contained 1 mM DCA as an additive; Device C contained 0.5 M TBP as an
additive; Device D contained 0.5 M TBP and 1 mM DCA as additives.

318
B.H. Kim, H.S. Freeman / Dyes and Pigments 96 (2013) 313e318
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