Influence of the anchoring number in a carbazole-based photosensitizer on the photovoltaic performance of p-type NiO dye sensitized solar cells

Article abstract of DOI:10.1039/c4ra08271k

The development of high performance p-type dye sensitized solar cells (DSSCs) is essential for producing tandem configurations that can overcome the theoretical limitations of single-junction DSSCs. This study evaluates a suitable organic dye for producing high performance p-type DSSCs. The photovoltaic properties of the carbazole monomer (MCBZ) and dimer (DCBZ) were examined to determine the effects of the molecular structure of the photosensitizer on the photovoltaic performance. MCBZ absorbs a larger amount of visible light photons with wavelengths between 400 and 600 nm, and provides better p-type DSSCs than conventional coumarin 343-sensitized solar cells. Furthermore, DCBZ exhibits better photovoltaic performance under standard global AM 1.5 solar conditions than the MCBZ-sensitized solar cells because of the synergistic effects of the enhanced electron injection efficiency and reduced charge recombination probability.

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Carbazole dimer enhances the charge injection and reduces the charge recombination to
exhibit superior p-type DSSCs performance.

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Influence of the anchoring number in carbazole-
based photosensitizer on the photovoltaic
Cite this: DOI: 10.1039/x0xx00000x
performance of p-type NiO dye sensitized solar cells†
Ji Young Parka, Bo Youn Janga, Chi Hwan Leea, Hyeong Jin Yuna*, Jae Hong
Kima*
Received 00th January 2014,
Accepted 00th January 2014
DOI: 10.1039/x0xx00000x
The development of high performance p-type dye sensitized solar cells (DSSCs) is essential for
producing tandem configurations that can overcome the theoretical limitations of single-junction
DSSCs. This study evaluates a suitable organic dye for producing high performance p-type
DSSCs. The photovoltaic properties of the carbazole monomer (MCBZ) and dimer (DCBZ) are
examined to determine the effects of the molecular structure of the photosensitizer on the
photovoltaic performance. MCBZ absorbs a larger amount of visible light photons with
wavelengths between 400 and 600 nm, and provides better p-type DSSCs than conventional
coumarin 343-sensitized solar cells. Furthermore, DCBZ exhibits better photovoltaic
performance under standard global AM 1.5 solar conditions than the MCBZ-sensitized solar
cells because of the synergistic effects of the enhanced electron injection efficiency and reduced
charge recombination probability.
www.rsc.org/
Introduction
Dye sensitized solar cells (DSSCs) have attracted considerable
interest as the next-generation solar cells1. DSSCs are efficient
photovoltaic devices, showing an overall peak power conversion of
approximately 11 %. In addition, they have been the focus of
economical solar electricity generation because the raw materials for
producing DSSCs are relatively inexpensive and the manufacturing
process is quite simple. Over the past two decades, a variety of dye-
sensitized photoanodes have been developed for making highly
efficient DSSCs. DSSCs with a wide-band gap p-type semiconductor
(p-type DSSCs), such as NiO, have attracted increasing attention2. As
shown in Scheme 1, p-type DSSCs work in an opposite manner to
typical n-type DSSCs, in which the dye is photo-excited and transfers
an electron to the electrolyte and a hole to the semiconductor (dye-
sensitized hole injection). The combination of an n-type
semiconductor-based photoanode with a p-type photocathode leads to
a tandem configuration that can break through the theoretical
limitations of single-junction DSSCs3. Moreover, these dye-sensitized
photocathodes can also be applied to produce hydrogen under visible
light irradiation4.
The photo-sensitizing dye plays an important role in capturing
photons, generating electron and hole pairs, and then transferring
them to the electrolyte and semiconductor, respectively. A proper
band structure is generally required to harvest solar light2c-e. Moreover,
the molecular structure of organic photosensitizers should facilitate
forward electron transfer from NiO to the photosensitizers with the
suppression of backward electron transfer, which causes charge
recombination2e. Thus far, coumarin 343 (C343, Fig. S1) has been
used as the p-type photosensitizer. On the other hand, C343 shows a
low extinction coefficient at the short wavelength visible light region
Scheme 1 Schematic diagram of charge transfer in a p-type DSSC.
around 400 nm, which limits the use of the solar spectrum2c, 2d, 5. C343
have shown a solar energy conversion efficiency of approximately
2d,
0.02 %2c,
5. Therefore, the development of novel p-type
photosensitizers is essential for achieving high performance tandem-
configured DSSCs. In this study, an efficient photosensitizing system
capable of operating in the long wavelength range of the solar
spectrum was fabricated by applying two types of carbazole-based
chromophores dyes to p-type DSSCs. Carbazole-based chromophores
dyes have been studied extensively in n-type DSSCs because of their
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high molar extinction coefficient with visible light irradiation6. In
addition, their synthetic process is convenient and their molecular
design can be customized easily. Interestingly the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular
orbital (LUMO) of carbazole-based chromophores dyes have a
suitable band configuration with the band structure of NiO for
applications to p-type DSSCs6a. Organic photosensitizers, such as
mono (MCBZ) or double carbazole-based chromophores (DCBZ),
were prepared and their photovoltaic properties for p-type DSSCs
were compared. Indolium acceptor are used in these dyes, which has
been recently utilized for enabling the dye to absorb the long
wavelength visible light in p-type DSSCs7. The dyes have different
numbers of anchoring moieties in their chromophores. Organic dyes
containing double-electron acceptors/anchors were recently reported
to be more efficient than the single-electron acceptor/anchor type
because the former provides more electron extraction paths between
the photosensitizing dye and TiO2, forming stronger electronic
8
coupling with TiO2 . The effects of the molecular structure of the
carbazole-based photosensitizer on the p-type DSSCs photovoltaic
performance were investigated systemically. Their photovoltaic
performance was examined by measuring the photocurrent density-
voltage (J-V) characteristics under simulated solar light irradiation.
The interfacial electron transfer processes were studied by measuring
the incident photon to current efficiency (IPCE) and by
electrochemical impedance spectroscopy (EIS).
Scheme 2 Synthesis of MCBZ and DCBZ.
Experimental
phenothiazine (10 g, 35.3mmol) was then dissolved in 80 ml of 1,2-
dichloroethane. Phosphorus oxychloride (27.06 g, 176.5 mmol) was
added slowly to this mixture under reflux for 2 hours. Subsequently,
the mixture was neutralized with Na2CO3. A yellow solid was
obtained by recrystallization by removing the solvent. Yield: 94%. ¹H
NMR (300MHz, DMSO-d6): δ 9.83 (s, 1H), 7.77 (d, J=8.1Hz, 1H),
7.64 (s, 1H), 7.28-7.02 (m, 5H), 4.02-3.97 (m, 2H), 1.76-1.71 (m, 2H),
1.43 (s, 2H), 1.30 (s, 4H), 0.87 (s, 3H).
Synthesis
Scheme 2 presents the scheme for the synthesis of MCBZ and DCBZ
photosensitizer and their structures.
Trimethyl-3H-indole-5-carboxylic acid (1). 4-(2-(3-Methylbutan-
2-ylidene)hydrazinyl)benzoic acid (11.5g, 52.2mmol) was dissolved
in acetic acid (115 ml). The mixture was stirred for 24 hours under a
nitrogen atmosphere. The solvent was then removed and the product
was purified by column chromatography on silica gel with methanol :
chloroform of 1:9 (v/v). The product was obtained as a yellow solid.
Yield: 73%. ¹H NMR (300MHz, DMSO-d6): δ 7.98 (s, 1H), 7.92 (d,
J=8.1Hz, 1H), 7.51 (d, J=8.1Hz,1H), 2.25 (s, 3H), 1.27 (s, 6H).
(E)-5-carboxy-2-(2-(9-hexyl-9H-carbazol-3-yl)vinyl)-3,3-
dimethyl-1-octyl-3H-indolium (5)
- (MCBZ). 10-hexyl-10H-
phenothiazine-3-carbaldehyde (Product 4, 1g, 3.58 mmol) and 5-
Carboxy-2,3,3-trimethyl-1-octyl-3H-indolium iodide (Product 2,
2.06g, 4.65 mmol) were dissolved in 35 ml of methanol. 5-carboxy-
2,3,3-trimethyl-1-octyl-3H-indolium iodide (Product 2) (0.23 g,
0.514mmol) was added slowly to the mixture under reflux for 12
hours. A dark red product was obtained by recrystallization with ethyl
acetate. Yield: 50 %. ¹H NMR (300 MHz, DMSO-d6) : δ 9.11 (s,
1H) ,8.64 (d, J=16.2Hz, 1H), 8.41 (s, 2H), 8.19 (q, 2H), 7.66 (d, J=9Hz,
1H), 7.84 (d, J=8.4Hz, 1H), 7.20 (t, 2H), 7.61 (t, J=6.9Hz, 1H), 7.37
(t, J=6.9Hz, 1H), 4.70 (s, 2H), 4.50 (s, 2H), 1.88 (s, 10H), 1.26-1.19
(m, 16H), 0.79 (q, 6H). 13C NMR (600MHz, DMSO-d6): 182.859,
166.474, 157.492, 144.134, 143.942, 143.513, 140.816, 1303615,
130.469, 129.167, 127.044, 125.940, 125.710, 123.856, 123.097,
122.231, 120.713, 120.621, 114.567, 110.551, 108.551, 51.762,
45.999, 42.665, 31.039, 30.802, 28.449, 28.428, 28.395, 27.882,
26.035, 25.897, 25.690, 21.919, 21.904, 21.866, 13.819, 13.734,
13.696. Its mass spectrum is presented Fig. S2(A), and this has a good
agreement of the molecular structure of MCBZ.
5-carboxy-2,3,3-trimethyl-1-octyl-3H-indolium iodide (2). 2,3,3-
Trimethyl-3H-indole-5-carboxylic acid (Product 1, 6.15g, 30.0mmol)
and 1-iodooctane (21.85 ml, 120 mmol) were dissolved in 100 ml of
acetonitrile, and stirred at 90 °C for 30 hours under a nitrogen
atmosphere. The solvent was removed and the product was
recrystallized with ethyl ether. The product was obtained as a yellow
solid. Yield: 61 %. ¹H NMR (300MHz, DMSO-d6) : δ 8.38 (s, 1H),
8.15 (d, J=8.3Hz, 1H), 8.07 (d, J=8.3Hz, 1H), 4.45 (t, J=7.5Hz, 2H),
2.87 (s, 3H), 1.57 (s, 6H), 1.45-1.10 (m, 12H), 0.83 (t, J=5.5Hz,3H).
9-hexyl-9H-carbazole (3). Carbazole (12g, 71.76 mmol), 1,6-
dibromohexane (14.21 g, 86.08 mmol) and NaH (3.44 g, 143.35 mmol)
were dissolved in 100 ml of dimethylformamide (DMF), and stirred
at 60 oC for 4 hours. After the reaction, the product was separated by
extraction using water and chloroform. The supernatant was then
dried with MgSO4. A white solid was obtained by removing the
organic solvent followed by recrystallization with methanol. Yield:
89.44 %. 1H NMR (300 MHz, DMSO-d6): δ 7.20 (d, J=5.7 Hz, 2H),
7.14 (d, J=6.9 Hz, 2H), 6.99 (d, J=8.1 Hz, 2H), 6.91 (t, J=7.2 Hz, 2H),
3.84 (t, J=6.9 Hz, 2H), 1.67-1.60 (m, 2H), 1.41-1.27 (m, 2H), 1.23-
1.19 (m, 4H), 0.86-0.80 (m, 3H).
1,6-di(9H-carbazol-9-yl)hexane (6). Carbazole (11.99 g,
71.70mmol), 1,6-dibromohexane (7 g, 28.69 mmol) and NaH (3.44 g,
143.35 mmol) were dissolved in 100 ml of DMF, and stirred at 60 °C
for 4 hours. After the reaction was complete, the product was
separated by extraction using water and chloroform. The supernatant
was dried with MgSO4. A white solid was obtained by removing the
organic solvent followed by recrystallization with methanol. Yield:
10-hexyl-10H-phenothiazine-3-carbaldehyde (4). 10-Hexyl-10H-
2 | RSC Adv., 2014, 00, 1-3
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89.44 %. ¹H NMR (300MHz, CDCl3): 8.11 (d, J=7.5Hz, 4H), 7.43 (d, measured potential is calibrated with respect to the redox potential of
J=7.4Hz, 4H), 7.34 (t, J=8.1Hz, 4H), 7.23 (t, J=7.8 Hz, 4H), 4.25 (t, ferrocene.
J=6.9 Hz, 4H), 1.83–1.81 (m, 4H), 1.41–1.39 (m, 4H).
2.3. Assembly and Characterization of the DSSCs.
9,9'-(hexane-1,6-diyl)bis(9H-carbazole-3-carbaldehyde) (7). 1,6-
di(9H-carbazol-9-yl)hexane (Product 6, 5g, 12.00mmol) and DMF
(17.54 g, 240.06 mmol) were dissolved in 50 ml of dichloroethane.
A conducting glass substrate (FTO; TEC8, Pilkington, 8 Ω/cm2,
thickness of 2.3 mm) was cleaned in ethanol by ultrasonication. A
NiO paste (NiO particles size: approximately 8.03nm) was prepared
Phosphorus oxychloride (36.80 g, 240.06 mmol) was then added
using ethyl cellulose (Aldrich) and terpineol (Aldrich) 9. The prepared
NiO paste was coated on a pre-cleaned glass substrate using the
doctor-blade method, and sintered at 450 °C for 30 min. For dye
adsorption, the annealed NiO electrodes were immersed in a solution
of each dye (0.3 mM in ethanol). The Pt counter electrodes were
prepared by the thermal reduction of a thin film formed from 7 mM
H2PtCl6 in 2-propanol at 400 °C for 20 min. The dye-adsorbed NiO
electrode and Pt counter electrode were assembled using a 60 μm-
thick Surlyn (Dupont 1702) layer. A liquid electrolyte was introduced
through a pre-punctured hole on the counter electrode. The electrolyte
was composed of lithium iodide (LiI, 0.5 M) and iodine (I2, 0.05 M)
in propylene carbonate. The J-V characteristics of the prepared
DSSCs were measured under 1 sunlight intensity (100 mW/cm2, AM
1.5), which was verified using an AIST-calibrated Si-solar cell (PEC-
L11, Peccell Technologies, Inc.). Three solar cells were fabricated
under each condition. The J-V curves were obtained from the solar
cell showing medium efficiency among the three tested-samples. The
monochromatic IPCEs were plotted as a function of the wavelength
using an IPCE measurement instrument (PEC-S20, Peccell
Technologies Inc.). Electrochemical impedance spectroscopy (EIS)
slowly to this mixture under reflux at 110 oC for 2 hours. After
reaction, the product was separated by extraction using water and
chloroform. The supernatant was purified by column chromatography
on silica gel with chloroform : hexane at a V/V ratio of 1:3. A white
solid was obtained. Yield: 67.5 %. ¹H NMR (300MHz, CDCl3) : 10.04
(s, 2H), 8.73 (s, 2H), 8.27 (d, J=7.8Hz, 2H), 7.94 (d, J=8.4Hz, 2H),
7.70 (d, J=8.7Hz, 2H), 7.62 (d, J=8.1 Hz, 2H), 7.50 (t, J=7.2Hz, 2H),
7.29 (t, J=7.5Hz, 2H), 4.39 (t, J=6.9Hz, 4H), 1.71 (m, 4H), 1.30 (m,
4H).
2,2'-((1E,1'E)-(9,9'-(hexane-1,6-diyl)bis(9H-carbazole-9,3-
diyl))bis(ethene-2,1-diyl))bis(5-carboxy-3,3-dimethyl-1-octyl-3H-
indol-1-ium) iodide
- (DCBZ). 9,9'-(hexane-1,6-diyl)bis(9H-
carbazole-3-carbaldehyde) (Product 7, 0.7g, 1.48mmol) and 5-
carboxy-2,3,3-trimethyl-1-octyl-3H-indolium iodide (Product 2,
2.06g, 3.70mmol) were dissolved in 35 ml of methanol. 5-Carboxy-
2,3,3-trimethyl-1-octyl-3H-indolium iodide (Product 2) (0.23 g,
0.514mmol) was added slowly to this mixture under reflux for 12
hours. A black product was obtained by recrystallization with ethyl
acetate. Yield: 80.24 %. ¹H NMR (300MHz, DMSO-d6) : 9.11 (s, 2H),
8.80 (d, J=15.9, 2H), 8.42 (s, 2H), 8.39 (d, J=8.4, 2H), 8.23 (d, J=7.5,
2H), 8.18 (d, J=8.7, 2H), 7.98 (d, J=8.7, 2H), 7.82 (d, J=8.7, 2H), 7.72
(d, J=16.2, 2H), 7.68 (d J=4.8, 2H), 7.57 (t, J=7.5, 2H), 7.38 (t, J=7.2,
2H), 4.71 (t, 4H), 4.46 (t, 4H), 1.89 (s, 10H), 1.5-1.16 (m, 28H), 0.73
(t J=6, 6H). 13C NMR (600MHz, DMSO-d6): 183.152, 166.782,
157.746, 144.419, 144.181, 143.806, 141.070, 130.930, 130.777,
129.681, 127.313, 126.026, 124.156, 123.435, 122.546, 121.036,
120.983, 114.905, 110.790, 110.721, 108.882, 52.062, 46.345, 42.858,
31.316, 28.741, 28.710, 28.596, 28.404, 28.189, 26.350, 26.281,
25.967, 22.181, 14.004. Its mass spectrum is presented Fig. S2(B),
and this has a good agreement of the molecular structure of DCBZ.
was performed using
a
computer-controlled potentiostat
(IVIUMSTAT, IVIUM) over the frequency range, 10 kHz - 100 mHz,
with an AC voltage amplitude of 10 mV at 0 V vs. the open circuit
voltage (VOC) in a two electrode measurement. The impedance spectra
were interpreted using a nonlinear least-square fitting procedure on
commercial software (ZView 2, Scribner Associates Inc.).
Results and discussion
Band structures
Fig. 1 shows the absorbance spectra of C343, MCBZ and DCBZ. The
concentration of each dye in ethanol is 0.03 mM for all the tested
samples. The absorbance peaks correspond to a π–π* charge transfer
transition in the organic dyes. The absorption maximum (λmax) of
Characterization
All 1H nuclear magnetic resonance (NMR) spectra were recorded on
a Varian Mercury NMR 300 Hz spectrometer using CDCl3 and
DMSO-d6 purchased from Cambridge Isotope Laboratories, Inc. 13
C
NMR spectra were recorded on a Varian Mercury NMR 600MHz
spectrometer using CDCl3 and DMSO-d6. Elemental analysis was
performed at the Center for Organic Reactions using an elemental
analyzer (EA1112, Thermo Electron Corporation). The optical
extinction spectra were recorded using
a
UV/vis/NIR
spectrophotometer (845X, Agilent). Cross-section images of the
NiO/FTO electrode were taken by scanning electron microscopy
(SEM, S-4800, Hitachi, LTD). The microstructure of NiO was
examined by X-ray diffraction (XRD, MPD for bulk (powder),
PANalytical) using CuKα radiation (wavelength = 0.154 nm). The
Fourier transform infrared (FT-IR, Perkin Elmer) spectra were
obtained using a Miracle single bounce diamond ATR cell from
PIKET Technologies. The redox properties of the dyes are examined
by cyclic voltammetry (IVIUMSTAT, IVIUM).The electrolyte
solution is 0.10
M tetrabutylammonium hexafluo-rophosphate
(TBAPF6) in freshly dried dimethylformamide (DMF). Ag/AgCl and
Pt wire (0.5 mm in diameter) electrodes are used as the reference and
counter electrodes, respectively. The scan rate is 30 mV/s. The
Fig. 1 Absorption spectra of C343, MCBZ, and DCBZ in ethanol. The
concentration of each dye in ethanol is 0.03 mM for all the samples.
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1.0
0.5
MCBZ
DCBZ
0.0
-0.5
-1.0
-1.5
-0.8
-0.4
0.0
0.4
0.8
AppliedPotential vs. Ag/AgCl (V)
Fig. 3 Cross section SEM image of a NiO/FTO electrode prepared
using the doctor blade method. The in-set shows a high resolution
SEM image of NiO nanoparticles.
(A)
Adsorption on NiO films.
To fabricate the dye sensitized photocathodes, the NiO-coated FTO
glass substrates are dipped in ethanol solutions of each dye. Before
dyeing, the mesoporous NiO film is deposited on FTO glass substrates
using the doctor blade method. S. Ito et al. introduced a novel method
for preparing a transparent TiO2 paste with α-terpineol and ethyl
cellulose9. This method is used to prepare a NiO paste to fabricate a
nanocrystalline layer for the photoactive electrodes of p-type DSSCs.
The NiO photocathodes are well fabricated using the prepared NiO
paste without cracking and peeling-off. Fig. 3 shows a SEM image of
a cross-section of the NiO/FTO electrode. The NiO film does not
contain any aggregates, and the nanoparticles are dispersed
homogeneously throughout the entire layer (inset of Fig. 3). The
thickness of the mesoporous NiO layer on FTO is approximately 7 μm.
No cracking or peeling-off are observed on the fabricated NiO
nanocrystalline layers after dipping the photocathode into the dyeing
solution. Fig. 4 shows the XRD patterns of the NiO-coated FTO
substrate. The characteristic peaks correspond to the rock salt-
structured NiO with crystallographic preferred orientations. The XRD
patterns shows only the presence of rock salt-structured NiO without
other crystalline structures.
(B)
Fig. 2 (A) Cyclic voltammograms and (B) estimated band-structures
of MCBZ and DCBZ.
C343 is observed at 419 nm with a molar extinction coefficient (εmax
)
of 1.91 × 104 M-1cm-1. MCBZ absorbs a large number of photons with
a longer wavelength between 400 and 600 nm (λmax of MCBZ is 516
nm) and shows a high εmax value of 4.23 × 104 M-1cm-1. In contrast,
DCBZ exhibits an εmax value of 6.77 × 104 M-1cm-1 at 516 nm without
showing a peak shift compared to MCBZ.
A more detailed understanding of the energy level of the highest
occupied molecular orbital (HOMO) and lowest occupied molecular
orbital (LUMO) can provide essential information regarding the
charge transfer kinetics. The energy level of HOMO (EHOMO) can be
approximated by measuring the oxidation potential (EOX) of the dye
by cyclic voltammetry (Fig. 2(A)). The EOX of MCBZ and DCBZ is
0.39 and 0.42 V vs. NHE, respectively. These values are converted to
the EHOMO of MCBZ and DCBZ on an absolute scale of -4.69 and -
4.72 eV, respectively, with comparing to the redox potential of
ferrocene as a reference reaction. The energy level of LUMO (ELUMO
)
is given by ELUMO = EHOMO - Eg, where Eg is the band-gap energy of
each photosensitizer estimated from absorbance data. Table S1 lists
the electronic absorption properties and electrochemical parameters
of MCBZ and DCBZ. Their apparent band structures are presented in
Fig. 2(B). As shown in their band diagrams, the band structures of
MCBZ and DCBZ are not remarkably different from each other. Thus,
the Eg and ELUMO are not regarded as the factors to determine the
differences in the photovoltaic performance of MCBZ and DCBZ.
Fig. 4 XRD patterns of the FTO and NiO/FTO electrodes.
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(A)
(B)
Fig. 5 FT-IR spectra of (A) MCBZ & MCBZ on NiO and (B) DCBZ
& DCBZ on NiO. ◆ indicates the C=O stretching peak at 1710 cm-1.
Fig. 7 Current density-voltage (J-V) characteristics of C343, MCBZ,
and DCBZ sensitized solar cells under simulated solar light
illumination (AM 1.5, 100 mW/cm2).
(A)
(B)
Fig. 6 Schematic diagrams of the adsorption of (A) MCBZ and (B)
DCBZ on the surface of NiO.
The annealed NiO electrodes are immersed in each dye solution as
the loading photosensitizer. The C343 photosensitizer adsorbs well on
metal oxide with a carboxylic anchoring group in their structure (Fig.
S1). The adsorption mechanism of MCBZ and DCBZ on NiO is
investigated by performing FT-IR spectroscopy. Fig. 5 shows the FT-
IR spectra of the photosensitizer itself and photosensitizer on NiO.
FT-IR spectroscopy reveals that MCBZ and DCBZ has a C=O
stretching peak at 1710 cm-1, which is in the carboxylic anchoring
group. As shown in Fig. 5(A), this peak is reduced significantly when
MCBZ is adsorbed on NiO. This suggests that most of the MCBZ is
adsorbed on NiO with a bidendate bridging structure, as shown in the
left sketch in Fig. 6(A). A small amount of MCBZ is adsorbed with
the monodendate mode, which yields a small peak in the FT-IR
spectra at 1710 cm-1. Interestingly, the peak for the C=O stretching
band have decreased significantly when DCBZ is adsorbed on NiO,
which means that DCBZ is adsorbed parallel to the NiO surface with
a bidendate bridging structure (Fig. 6(B)). In particular, double
anchoring of DCBZ leads to strong electronic coupling with NiO. This
enhanced electronic coupling can cause significant enhancement of
the injection of the photo-generated hole from the photosensitizer to
NiO. The amounts of dye loaded on the NiO electrode films are
determined by UV-Vis spectroscopy, and the results are summarized
in Table 1. Each dye is detached from mesoporous NiO film using a
0.1 mM KOH solution. 1.44 × 10-4 mol/cm3 of C343 is loaded on NiO,
which is the largest value among the samples tested. The amount of
MCBZ and DCBZ is 7.69 × 10-5 and 2.21 × 10-5 mol/cm3, respectively.
MCBZ with mono-anchoring moiety might be adsorbed vertically on
Fig. 8 IPCE curves for C343, MCBZ, and DCBZ sensitized solar cells.
the NiO surface. On the other hand, because DCBZ has two anchoring
moieties at the opposite position of the chromophore, it might be
adsorbed along the NiO surface. Therefore, larger amounts of MCBZ
are adsorbed on the NiO electrode than DCBZ.
Photovoltaic performances of p-type DSSCs
The photocurrent density-voltage (J-V) characteristics of the C343,
MCBZ and DCBZ sensitized photovoltaic cells under the illumination
of AM 1.5G solar simulated light are examined (Fig. 7). Table 1 lists
the short circuit current density (Jsc), open circuit voltage (Voc), fill
factor (FF), and overall efficiency of power conversion (η). C343,
which is generally used as a photosensitizer for p-type DSSCs2c, 2d
,
shows the lowest photovoltaic performance with JSC, VOC and FF
values of 0.558 mA/cm2, 0.1122 V and 0.31, respectively, to give a η
value of 0.0194. These results show good agreement with the results
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Table 1 Photoelectrochemical parameters for each DSSC under
simulated solar light illumination (AM 1.5, 100 mW/cm2). M is the
amount of adsorbed dye on NiO.
Dye
Jsc
Voc
(V)
FF
(%)
η
(%)
M
(mA/cm2)
(mol/cm3)
0.56
(±0.010)
0.11
31.02
0.0194
C343
MCBZ
DCBZ
1.44 × 104
7.69 × 105
2.21 × 105
(±0.001) (±0.209) (±0.0003)
0.110 30.83 0.0387
(±0.006) (±0.249) (±0.0007)
0.13 30.96 0.0538
1.16
(±0.062)
1.35
(±0.009)
(±0.002) (±0.082) (±0.0007)
2d,
reported previously2c,
5. MCBZ provides better performance,
showing double the JSC value than C343, despite twice as much C343
being loaded on NiO than MCBZ. The absorbance experiments
showed that MCBZ absorbs larger amounts of visible light photons
than C343. These photophysical properties of MCBZ result in better
photovoltaic performance than the C343-sensitized solar cells under
simulated solar light irradiation. DCBZ shows better solar energy
conversion efficiency than MCBZ, showing higher JSC and VOC values,
despite the smaller amount of DCBZ loaded on NiO than MCBZ. The
VOC of the DCBZ-sensitized solar cell is higher than that of the
MCBZ-sensitized cell. The increase in electron injection efficiency
and the decrease in charge recombination probability between the
redox couple and metal oxide might increase the VOC10. Because
DCBZ has two anchoring moieties at the opposite position of the
chromophore, it might occupy more adsorption sites on the NiO
surface over a wider region than the MCBZ dye. Bi-anchoring leads
to stronger electronic coupling between DCBZ and NiO compared to
that between MCBZ and NiO. This enhancement of electron coupling
between DCBZ and NiO leads to the injection of photo-generated
holes in DCBZ into the valence band of NiO with rapid charge transfer
kinetics, leading to the generation of a high photo-current in p-type
DSSCs. Moreover, the bi-anchoring structure of DCBZ helps decrease
the probability of charge recombination at the interfacial region
between NiO and the electrolyte. As shown in Scheme 2, there is a
long - C6H12 - linkage connecting the two monomers in DCBZ. This
long chain enables DCBZ to cover more of the NiO surface than
MCBZ, blocking charge transfer between NiO and the electrolyte.
The synergistic effects of the enhanced electron injection efficiency
and reduced charge recombination probability leads a DCBZ
Fig. 9 Nyquist plots for C343, MCBZ, and DCBZ sensitized solar
cells under the illumination of simulated solar light (AM 1.5, 100
mW/cm2). The inset shows the equivalent circuit of the DSSCs.
Table 2 Resistance extracted from the fitted results of the EIS spectra
for each p-type DSSCa
Dye
RS (Ω)
R1 (Ω)
R2 (Ω)
17.49
(±0.723)
36.65
(±0.856)
410.3
(±5.236)
C343
16.31
(±0.612)
38.98
(±0.731)
145.9
(±1.963)
MCBZ
DCBZ
13.93
(±0.854)
35.28
(±0.845)
139.7
(±1.478)
aRs: ohmic serial resistance, R1: carge transfer resistance at the Pt
anode, R2: charge transfer resistance at the NiO/dye/electrolyte
interface
which is useful for quantifying the interfacial electrochemical
behaviour of DSSCs11, QDSSCs12, and photoelectrochemical
cells13. Fig. 9 presents Nyquist plots of the C343-, MCBZ- and
DCBZ-sensitized solar cells. The ohmic serial resistance (RS)
corresponds to the electrolyte and FTO resistance, and the
resistances, R1 and R2, denote the charge transfer process
occurring at the Pt counter electrode and NiO/dye/electrolyte
interface, respectively. Table 2 lists the extracted resistances for
the circuit elements. Two semicircles are observed for each
Nyquist plot, which are assigned to interfacial electrochemical
charge transfer on the Pt anode (at higher frequency) and
photocathode (at lower frequency). Their radius is strongly
related to the faradaic Red/Ox charge transfer resistance of each
electrode. No significant change in the radius of the semicircle at
higher frequencies is observed within the Nyquist plot for each
photovoltaic cell because the Pt-coated counter electrode is used
for all experiments. On the other hand, R2 changes remarkably
according to the type of photosensitizer. C343 exhibits the
highest charge transfer resistance on the photocathode under
sensitized solar cell with a higher VOC
.
Incident photon to current efficiency (IPCE) measurements are
conducted to further understand the photovoltaic cell performance of
the DSSCs. Fig. 8 shows the IPCE results of the C343, MCBZ and
DCBZ sensitized solar cells. The IPCE is calculated by measuring the
photocurrent of each cell at different levels of monochromatic
excitation. The photocurrent action spectra match the absorbance
spectra of the corresponding photocathode, as shown in Fig. 1.
Although C343 produces a large photo-current under the irradiation
of visible light with a short wavelength (~ 400 nm), it does not
adequately cover longer wavelengths (> 500 nm). This explains the
poor photovoltaic performance of C343-sensitized solar cells under
irradiation with simulated solar light. MCBZ and DCBZ, however,
generate a photocurrent over a wide wavelength range between 300
and 700 nm, which yield better solar cells than the C343-sensitized
cells. In particular, DCBZ produces a larger number of charges than
MCBZ due to the synergic effect of the enhanced electron injection
efficiency and reduced charge recombination probability.
solar light irradiation (R2 of C343 is 410.3 Ω). The use of MCBZ
reduces the R2 value of the DSSCs (145.9 Ω) remarkably because
the optical properties of the MCBZ dye are more suitable for
solar energy conversion than those of C343. Furthermore, DCBZ
shows the lowest R2 value in the impedance results because of
The overall charge transfer resistance in the photoanode devices
can be estimated by electrochemical impedance spectroscopy (EIS),
6 | RSC Adv., 2014, 00, 1-3
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Hammarström, D. Jacquemin and F. Odobel, Dyes Pigment., 2014, 105,
174; (c) D. Xiong, Z. Xu, X. Zeng, W. Zhang, W. Chen, X. Xu, M. Wang
and Y.-B. Cheng, J. Mater. Chem., 2012, 22, 24760.
the synergic effect of enhanced electron injection efficiency and
the reduced charge recombination probability. These findings are
in agreement with the photocurrent measurement presented
above.
6. (a) M. S. Kim, H. S. Yang, D. Y. Jung, Y. S. Han and J. H. Kim, Colloids
Surf. A. Physicochem. Eng. Aspects, 2013, 420, 22; (b) A. Michaleviciute,
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Conclusions
This study evaluates a suitable organic dye for producing high
performance p-type DSSCs. The photovoltaic properties of
MCBZ and DCBZ with different numbers of anchoring groups
in their chromophores are examined to determine the effects of
the molecular structure of the organic dye on the solar energy
conversion efficiency. MCBZ absorbs a larger number of
photons with a longer wavelength between 400 and 600 nm. The
absorption properties of MCBZ allow the production of much
better p-type DSSCs than conventional C343-sensitized solar
cells. Furthermore, DCBZ exhibits better photovoltaic
performance, showing higher JSC, VOC and η values, and lower
charge transfer resistance over the photoanode than the mono-
anchored MCBZ dye sensitized solar cells.
9. S. Ito, P. Chen, P. Comte, M. K. Nazeeruddin, P. Liska, P. Péchy and M.
Grätzel, Prog. Photovoltaics, 2007, 15, 603.
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Acknowledgements
13. H. J. Yun, H. Lee, J. B. Joo, W. Kim and J. Yi, J. Phys. Chem. C, 2009,
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This study was supported by the “New & Renewable Energy
Core Technology Program” of the Korea Institute of Energy
Technology Evaluation and Planning (KETEP) granted financial
resource from the Ministry of Trade, Industry & Energy (No.
20133010011750) and Basic Science Research Program through
the National Research Foundation of Korea (NRF) funded by the
Ministry of Education, Science and Technology, Republic of
Korea (grant number: NRF-2012R1A2A2A01014395).
Notes and references
aDepartment of Chemical Engineering, Yeungnam University,
Gyeongsangbuk-do, South Korea 712-749
Jae Hong Kim Tel: +82-53-810-2521 E-mail: jaehkim@ynu.ac.kr
Hyeong Jin Yun Tel: +82-53-810-3716 E-mail: hjyun@ynu.ac.kr
† Electronic Supplementary Information (ESI) available: Molecular
structure of C343 (Fig. S1), mass spectra of MCBZ and DCBZ, and
electrochemical
parameters
of
MCBZ
and
DCBZ.
See
DOI: 10.1039/b000000x/
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