New fluorenone-containing organic photosensitizers for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2013.03.007

Four new organic dyes (F1-F4) comprising the triarylamine or fluorene unit as an electron-donating group, a less commonly used fluorenone spacer and a cyanoacrylic acid as the anchoring group in the molecular framework were synthesized, characterized and utilized in dye-sensitized solar cells (DSSCs). Their absorption, photoluminescence, electrochemical and photovoltaic properties were fully investigated in detail. Electrochemical data indicate that the tuning of the HOMO and LUMO energy levels can be conveniently accomplished by alternating the donor moiety. The photovoltaic performance can be increased by adding an electron-donating triarylamine or fluorene unit next to the fluorenone ring or increasing the number of thienyl ring next to the cyanoacrylic acid group, which is consistent with the results from electrochemical impedance spectroscopy. The maximal monochromatic incident photon-to-current conversion efficiency (IPCE) can reach up to 80% for DSSC devices based on F4 with the overall light to electricity conversion efficiency up to 4.71% (V_oc = 565 mV, J_sc = 11.71 mA cm^-2 and FF = 0.71) under AM 1.5 irradiation (100 mW cm^-2).

Full text of DOI:10.1016/j.dyepig.2013.03.007

Accepted Manuscript
New fluorenone-containing organic photosensitizers for dye-sensitized solar cells
Lai-Fan Lai, Chuanjiang Qin, Chung-Hin Chui, Ashraful Islam, Liyuan Han, Cheuk-
Lam Ho, Wai-Yeung Wong
PII:
S0143-7208(13)00096-X
10.1016/j.dyepig.2013.03.007
DYPI 3876
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 27 January 2013
Revised Date: 9 March 2013
Accepted Date: 12 March 2013
Please cite this article as: Lai L-F, Qin C, Chui C-H, Islam A, Han L, Ho C-L, Wong W-Y, New
fluorenone-containing organic photosensitizers for dye-sensitized solar cells, Dyes and Pigments (2013),
doi: 10.1016/j.dyepig.2013.03.007.
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Entry for the Table of Contents
New fluorenone-containing organic photosensitizers for dye-sensitized solar cells
Lai-Fan Lai, Chuanjiang Qin, Chung-Hin Chui, Ashraful Islam, Liyuan Han, Cheuk-Lam
Ho and Wai-Yeung Wong
A new family of fluorenone-based organic photosensitizers were described. The
photovoltaic response can be enhanced by adding an electron-donating triarylamine or
fluorene unit next to fluorenone or increasing the number of thienyl ring next to
cyanoacrylic acid. These dyes showed the best power conversion efficiency of 4.71% in
nanostructured dye-sensitized solar cells.
e_
N
O
O
C
O
S
S
CN
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3I
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I2
η
= 4.71%

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Research highlights:
We report synthesis and characterization of four new fluorenone-containing organic
photosensitizers for dye-sensitized solar cells (DSSCs).
Effect of different electron donors on the photovoltaic performance was studied.
The maximal monochromatic incident photon-to-current conversion efficiency of
DSSCs can reach 80% with power conversion efficiency of 4.71%.

ACCEPTED MANUSCRIPT
New fluorenone-containing organic photosensitizers for
dye-sensitized solar cells
Lai-Fan Lai, Chuanjiang Qin*, Chung-Hin Chui, Ashraful Islam, Liyuan
Han, Cheuk-Lam Ho*, Wai-Yeung Wong*
[*] Prof. W.-Y. Wong, L.-F. Lai, Dr. C.-L. Ho
Institute of Molecular Functional Materials
Department of Chemistry and Institute of Advanced Materials
Hong Kong Baptist University
Waterloo Road, Kowloon Tong, Hong Kong (P.R. China)
Fax: 852-34117348
E-mail: rwywong@hkbu.edu.hk, clamho@hkbu.edu.hk
Dr. L. Han, Dr. C. Qin, Dr. A. Islam
Photovoltaic Materials Unit
National Institute for Materials Science
Sengen, 1-2-1, Tsukuba, Ibaraki 305-0047, Japan
Fax: +81 29-859-2304
Tel: +81 29-859-2305
E-mail: roverqin@gmail.com
Prof. C.-H. Chui
Clinical Division, School of Chinese Medicine,
Hong Kong Baptist University
Waterloo Road, Kowloon Tong, Hong Kong (P.R. China)
ꢀ Supporting information for this article is available on the WWW under xxxxxx.
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Abstract
Four new organic dyes (F1–F4) comprising the triarylamine or fluorene unit as an
electron-donating group, a less commonly used fluorenone spacer and a cyanoacrylic acid as
the anchoring group in the molecular framework were synthesized, characterized and utilized
in dye-sensitized solar cells (DSSCs). Their absorption, photoluminescence, electrochemical
and photovoltaic properties were fully investigated in detail. Electrochemical data indicate
that the tuning of the HOMO and LUMO energy levels can be conveniently accomplished by
alternating the donor moiety. The photovoltaic performance can be increased by adding an
electron-donating triarylamine or fluorene unit next to the fluorenone ring or increasing the
number of thienyl ring next to the cyanoacrylic acid group, which is consistent with the
results from electrochemical impedance spectroscopy. The maximal monochromatic incident
photon-to-current conversion efficiency (IPCE) can reach up to 80% for DSSC devices based
on F4 with the overall light to electricity conversion efficiency up to 4.71% (Voc = 565 mV,
Jsc = 11.71 mA cm–2 and FF = 0.71) under AM 1.5 irradiation (100 mW cm–2).
Keywords: Arylamine / Fluorenone / Photosensitizer / Organic dye / Dye-sensitized solar
cells
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1. Introduction
In recent years, to overcome one of the topmost problems – energy that human faces,
many researchers have already started working on various issues by generating energy from
renewable alternatives. Solar energy conversion has become the major alternative to the
conventional fossil fuels, considering their drastic depletion over the last few decades and the
environmental issues caused by their heavy utilization. Dye-sensitized solar cells (DSSCs)
have attracted considerable scientific and industrial interest as a promising candidate for a
new renewable energy source in the past decades [1]. Especially, the extensively used
charge-transfer Ru(II)-polypyridyl photosensitizers in DSSCs such as N3 [2], N719 [3], N907
[4], and black dye [5] have achieved solar-to-electricity conversion efficiency higher than
10% under AM 1.5 simulator, which is a value close to an amorphous silicon-based
photovoltaic cell [6]. However, the rarity of the Ru(II) metal, lack of good absorption in the
red region of the visible spectrum and tricky purification steps all pose certain limitations in
the practical application of these complexes for DSSCs [7]. Despite the superiority of the
Ru(II)-based dyes, there is rapid progress in the development of the metal-free organic dyes
[8]. To date, the metal-free organic dyes are on a par with the benchmark Ru(II)-polypyridyl
complexes since they are a low-cost alternative of our renewable energy sources. Moreover,
they were shown to have great potential to get high power conversion efficiency (PCE) and
show good ease of device fabrication [9]. Also, their high molar extinction coefficients, ease
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of purification and flexible structural modifications render them to be attractive
light-harvesting materials [10].
Fluorenone and their derivatives are generally regarded as one of the most promising
candidates for photonic and optoelectronic devices due to their high thermal stability, ease of
preparation and good electron-transporting property [11]. They are also widely used in the
preparation and investigation of charge-transfer complexes [12], activation of
photoconductivity of organic semiconductors [13] and as electron transport materials. When
these 9-oxo-substituted fluorenes (i.e. fluorenones) have been extensively studied [14], many
research groups tempted to prepare copolymers of fluorenone with the goal of tuning the
resulting electronic and optical properties [15]. Therefore, fluorenone-based materials are
conceived to be attractive photovoltaic materials, especially because their weakly
electron-accepting property may result in a broader absorption profile and hence a much
better match between the solar radiation and absorption, leading in turn to a much better
device efficiency as compared to fluorene-derived counterparts. However, organic dyes
containing the fluorenone moiety are still not very well studied to date [16]. On the other
hand, the excellent stability, electron-donating ability and inhibition of aggregation due to a
non-planar molecular configuration of triphenylamine-based sensitizers have made them a
prospective chromophore for DSSC applications [17]. As a continuation of our efforts to
develop functional materials suitable for DSSCs [18−20], four new fluorenone-functionalized
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organic photosensitizers F1–F4 of the type D-A-π-A (D = donor, A = acceptor) were designed
and synthesized here with triarylamine or fluorene unit as the electron donor, an additional
fluorenone unit as a mild electron-withdrawing linkage and a cyanoacrylic acid moiety as the
electron-withdrawing and anchoring group (Chart 1).
2. Experimental Section
2.1. Materials and Reagents
All the reactions were performed under a nitrogen atmosphere by using standard
Schlenk techniques. Solvents were dried by standard methods and distilled prior to use. The
matrix-assisted laser desorption ionization time of flight (MALDI-TOF) mass spectra were
recorded on a Autoflex Bruker MALDI-TOF system. NMR spectra were measured in CDCl3
or d6-DMSO on a Bruker AM 400 MHz FT-NMR spectrometer, and chemical shifts were
quoted relative to tetramethylsilane for 1H and 13C nuclei. UV-Vis spectra were obtained on a
HP-8453 diode array spectrophotometer.
2.2. General Procedure for the Synthesis of Compounds 4, 7, 13 and 14
Compounds 4, 7, 13 and 14 were synthesized by the same procedure as above based on
the Suzuki coupling reaction between 5-formylthiophenyl-2-boronic acid and different
brominated fluorenone derivatives. The synthetic procedure for 4 is described below as an
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example.
2.2.1. 5-(5-(9-Oxofluorenyl)thiophenyl)thiophene-2-carbaldehyde (4)
In a 50 mL round bottom flask, 2-(5-bromothiophenyl)fluoren-9-one (500 mg, 1.46
mmol) in toluene/ethanol (3:1, 40 mL) was added to
a
mixture of
5-formylthiophenyl-2-boronic acid (274 g, 1.76 mmol), Pd(PPh3)4 (168 mg, 0.14 mmol) and
o
2 M Na2CO3 (2.92 mL, 5.84 mmol). The mixture was heated at 80 C for two days.
Afterwards, water was added and the solution mixture was extracted with ethyl acetate. The
combined organic layer was dried by Na2SO4 and filtered. The mixture was then concentrated
under reduced pressure. The residue was purified by column chromatography using
CH2Cl2/n-hexane (1:1, v/v) as eluent to yield compound 4 as a yellow solid (250 mg, 0.67
1
mmol, 46%). H NMR (400 MHz, CDCl3): δ = 9.87 (s, 1H, CHO), 7.90 (m, 1H, Ar),
7.73−7.68 (m, 3H, Ar), 7.56−7.51 (m, 3H, Ar), 7.36−7.28 (m, 4H, Ar). 13C NMR (100 MHz,
CDCl3): δ 193.35 (C=O), 182.48 (CHO), 146.72, 144.68, 144.06, 143.71, 141.85, 137.36,
135.69, 135.05, 135.02, 134.48, 134.35, 131.51, 129.31, 127.21, 124.81, 124.58, 124.31,
121.37, 120.95, 120.53 (Ar). MALDI-TOF (m/z): 372.03 [M]+.
2.2.2. 5-(2-(9,9-Dibutylfluorenyl)-9-oxofluorenyl)thiophenecarbaldehyde (7)
1
Yield: 45%; orange solid. H NMR (400 MHz, CDCl3): δ = 9.91 (s, 1H, CHO),
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8.02−7.98 (m, 1H, Ar), 7.85−7.73 (m, 6H, Ar), 7.65−7.59 (m, 4H, Ar), 7.48−7.47 (m, 1H, Ar),
7.36−7.26 (m, 3H, Ar), 2.05−2.01 (m, 4H, butyl), 1.11−1.07 (m, 4H, butyl), 0.68−0.64 (m,
10H, butyl). 13C NMR (100 MHz, CDCl3): δ 193.13 (C=O), 182.63 (CHO), 152.65, 151.73,
151.06, 144.87, 143.40, 142.87, 142.30, 141.40, 140.49, 138.42, 138.32, 137.25, 135.44,
135.16, 133.93, 133.58, 132.57, 128.30, 127.40, 126.91, 125.70, 124.59, 123.24, 122.99,
122.02, 121.13, 121.09, 121.05, 120.15, 119.92 (Ar), 55.23, 40.24, 31.60, 26.04, 23.07, 22.66,
14.11, 13.82 (butyl). MALDI-TOF (m/z): 566.23 [M]+.
2.2.3.
5-(5-(2-(9,9-Dibutylfluorenyl)-9-oxofluorenyl)thiophenyl)thiophene-2-carbaldehyde
(13)
1
Yield: 48%; deep orange solid. H NMR (400 MHz, CDCl3): δ = 9.86 (s, 1H, CHO),
8.00−7.90 (m, 2H, Ar), 7.81−7.66 (m, 5H, Ar), 7.60−7.54 (m, 4H, Ar), 7.37−7.32 (m, 5H, Ar),
7.27−7.25 (m, 1H, Ar), 2.05−2.01 (m, 4H, butyl), 1.13−1.07 (m, 4H, butyl), 0.72−0.62 (m,
10H, butyl). 13C NMR (100 MHz, CDCl3): δ 193.45 (C=O), 182.42 (CHO), 151.67, 151.02,
146.70, 144.68, 143.64, 142.98, 142.56, 141.86, 141.29, 140.49, 138.371, 137.32, 135.69,
135.36, 135.09, 134.32, 133.48, 131.60, 127.35, 127.21, 126.88, 125.64, 124.77, 124.29,
123.13, 122.95, 121.42, 120.98, 120.89, 120.11, 119.88 (Ar), 55.20, 40.26, 31.60, 26.01,
23.07, 22.66, 14.11, 13.83 (butyl). MALDI-TOF (m/z): 649.29 [M]+.
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2.2.4. 5-(5-(2-(4-(Di-p-tolylamino)phenyl)-9-oxofluorenyl)thiophenyl)thiophene-2-carbalde
hyde (14)
Yield: 42%; red solid. 1H NMR (400 MHz, CDCl3): δ 9.84 (s, 1H, CHO), 7.84−7.82 (m,
2H, Ar), 7.65−7.64 (m, 2H, Ar), 7.50−7.41 (m, 4H, Ar), 7.30−7.22 (m, 3H, Ar), 7.10−7.02 (m,
13
11H, Ar), 2.33 (s, 6H, CH3). C NMR (100 MHz, CDCl3): δ 193.40 (C=O), 182.42 (CHO),
148.31, 146.72, 144.92, 144.72, 143.71, 142.02, 141.93, 141.75, 137.37, 135.54, 135.28,
135.04, 134.05, 133.03, 132.53, 131.99, 131.48, 131.24, 130.00, 129.82, 127.22, 124.99,
124.65, 124.23, 123.35, 122.38, 122.06, 121.24, 120.85, 120.78, 120.06 (Ar), 20.88 (CH3).
MALDI-TOF (m/z): 643.97 [M]+.
2.3. Synthesis of Organic Dyes F1–F4
Dyes F1–F4 were synthesized by the reaction between the aldehyde derivatives and
cyanoacetic acid in the presence of piperidine. A typical procedure is given for F1.
2.3.1. F1
5-(5-(9-Oxofluorenyl)thiophenyl)thiophene-2-carbaldehyde (100 mg, 0.26 mmol) in 4
mL of dry chloroform, cyanoacetic acid (45.7 mg, 0.53 mmol) and piperidine (0.1 mL) were
mixed. The mixture was heated at reflux temperature overnight. Afterwards, the mixture was
concentrated under reduced pressure. The residue was purified by chromatography on a silica
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gel column using CH2Cl2/MeOH (1:1, v/v) as eluent. The mixture was then washed with
1
methanol to give F1 as an orange solid (78 mg, 0.18 mmol, 69%). H NMR (400 MHz,
d6-DMSO): δ 7.73 (s, 1H, Ar), 7.58−7.45 (m, 5H, Ar), 7.37−7.30 (m, 4H, Ar), 7.18−7.15 (m,
1H, Ar), 7.08−7.05 (m, 1H, Ar). MALDI-TOF (m/z): 440.7 [M]+. C25H13NO3S2 (439.50):
calcd. C 68.32, H 2.98, N 3.19; found C 68.45, H 3.12, N 3.33.
2.3.2. F2
1
Yield: 65%; orange solid. H NMR (400 MHz, d6-DMSO): δ 8.10−7.75 (m, 13H, Ar),
7.46−7.45 (m, 1H, Ar), 7.36−7.34 (m, 2H, Ar), 2.11−2.03 (m, 4H, butyl), 1.07−1.02 (m, 4H,
butyl), 0.68−0.62 (m, 10H, butyl). MALDI-TOF (m/z): 634.1 [M]+. C42H35NO3S (633.80):
calcd. C 79.59, H 5.57, N 2.21; found C 79.49, H 5.68, N 2.43.
2.3.3. F3
1
Yield: 61%; deep orange solid. H NMR (400 MHz, d6-DMSO): δ 8.06 (s, 1H, Ar),
7.98−7.84 (m, 9H, Ar), 7.72−7.66 (m, 3H, Ar), 7.51−7.44 (m, 3H, Ar), 7.36−7.34 (m, 2H, Ar),
2.11−2.01 (m, 4H, butyl), 1.05−1.00 (m, 4H, butyl), 0.62−0.49 (m, 10H, butyl). MALDI-TOF
(m/z): 716.2 [M]+. C46H37NO3S2 (715.92): calcd. C 77.17, H 5.21, N 1.96; found C 77.30, H
5.23, N 2.13.
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2.3.4. F4
Yield: 67%; red solid. 1H NMR (400 MHz, d6-DMSO): δ 7.99 (s, 1H, Ar), 7.80−7.69 (m,
7H, Ar), 7.61−7.58 (m, 2H, Ar), 7.54−7.50 (m, 2H, Ar), 7.42−7.39 (m, 2H, Ar), 7.14−7.08 (m,
4H, Ar), 7.90−7.85 (m, 5H, Ar), 2.23 (s, 6H, CH3). MALDI-TOF (m/z): 711.0 [M]+.
C45H30N2O3S2 (710.86): calcd. C 76.03, H 4.25, N 3.94; found C 76.24, H 4.35, N 4.09.
2.4. Computational Studies
The geometry and electronic properties of the dyes were performed with the Gaussan09
program package. All calculations were performed by the Gaussian software using density
functional theory (DFT) with B3LYP/6-31G* basis set [21].
2.5. Assembly and Characterization of DSSCs
A double-layer TiO2 photoelectrode (thickness 15 ꢀm; area 0.25 cm2) was used as a
working electrode. A 10 ꢀm main transparent layer with titania particles (~20 nm) and a 5 ꢀm
scattering layer with titania particles (~400 nm) were screen-printed on the fluorine-doped tin
oxide conducting glass substrate. A solution of F1−F4 (3 × 10–4 M) in acetonitrile/tert-butyl
alcohol (1/1, v/v) was used to coat the TiO2 film with the dye. Deoxycholic acid (20 mM)
was added to the dye solution as a co-adsorbent to prevent aggregation of the dye molecules.
o
The electrodes were immersed in the dye solutions and then kept at 25 C for 24 h to adsorb
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the dye onto the TiO2 surface. The dye-coated TiO2 film was used as the working electrode,
and platinum-coated conducting glass was used as the counter-electrode. The two electrodes
were separated by a Surlyn spacer (50 ꢀm thick) and sealed up by heating the polymer frame.
The electrolyte was composed of 0.6 M dimethylpropylimidazolium iodide, 0.05 M I2, 0.1 M
LiI and 0.5 M tert-butyl pyridine in in acetonitrile. The current–voltage characteristics were
measured using a black metal mask and edge with an aperture area of 0.25 cm2 under
standard AM 1.5 sunlight (100 mW cm–2, WXS−155S−10: Wacom Denso Co. Japan) [22].
Monochromatic incident photon-to-current conversion efficiency (IPCE) spectra were
measured with monochromatic incident light of 1 × 1016 photons cm–2 under 100 mW cm–2 in
direct current mode (CEP-2000BX, Bunko- Keiki). The electrochemical impedance spectra
(EIS) were measured with an impedance analyzer (Solartron Analytical, 1255B) connected
with a potentiostat (Solartron Analytical, 1287) under light illumination using a solar
simulator (WXS-155S-10: Wacom Denso Co. Japan). The frequency range was 0.1 Hz–100
kHz at 298 K. The electrical impedance spectra were characterized using Z-View software
(Solartron Analytical).
3. Results and Discussion
3.1. Materials Synthesis
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The synthetic routes of the target dyes are outlined in Schemes 1–3. The key materials
2-bromofluoren-9-one 1 and 2,7-dibromofluoren-9-one 5 were firstly obtained by oxidation
of 2-bromofluorene and 2,7-dibromofluorene with potassium dichromate in acetic acid,
respectively [23]. These two compounds then reacted with the corresponding arylboronic acid
derivatives to give 2, 6 and 8–10 by the Suzuki coupling reaction. Monobromination of these
compounds by N-bromosuccinimide (NBS) gave 3 and 11–12. The intermediates 4, 7, 13 and
14 were subsequently made by the Suzuki coupling reaction of 3, 11 and 12 with
5-formylthiophenyl-2-boronic acid. The final step involved a Knoevenagel condensation with
cyanoacetic acid in the presence of piperidine to convert the carbaldehydes to cyanoacrylic
acids [24]. Purification of the reaction mixtures by silica gel chromatography furnished
F1–F4 as air-stable powders with high purity. The structures of all the dyes were verified by
NMR spectroscopy and MALDI-TOF mass spectrometry. Cyclic voltammetry was performed
on a CH Instruments 600D electrochemistry station. The dyes are orange or red in color and
are readily soluble in common organic solvents such as THF, CH2Cl2, CHCl3 and DMSO.
3.2. Photophysical Properties
The UV-Vis absorption spectra of the photosensitizers F1–F4 in THF at room
temperature are displayed in Figure 1 and the data are collected in Table 1. The more intense
absorption band located in the range of 365 to 398 nm corresponds to the π–π* transition of
the conjugated aromatic rings. The other weaker low-energy bands appeared as a shoulder
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between 450 and 550 nm which are attributed to the intramolecular charge transfer (ICT) [25].
The lowest energy transition in fluorenone possesses n–π* or a π–π* character [26] with a
contribution from ICT which gives rise to a shoulder band in the absorption spectrum [27].
The spectra of all the dyes are quite similar, but those with two thiophene groups F1, F3 and
F4 are red-shifted in comparison to that of F2, which is mainly due to the effective
conjugation caused by the introduction of one more thiophene unit [28]. In addition, F3 and
F4 are more red-shifted than that of F1, which could be ascribed to the presence of one more
electron-rich moieties to produce more extended π-conjugation system. The molar extinction
coefficients of the π–π* bands for F1−F4 were also investigated in THF and increase in the
order of F1 < F2 < F3 < F4, which indicates that F4 should possess the best light-harvesting
ability. The high molar extinction coefficients of the organic dyes allow a correspondingly
thinner nanocrystalline film so as to avoid the decrease of the film mechanical strength. This
also benefits the electrolyte diffusion in the film and reduces the recombination possibility of
the light-induced charges during transportation [29].
As shown in Figure 2, the absorption bands of sensitizers F1–F4 attached to the
transparent TiO2 electrode (4 ꢀm thickness) are broadened significantly as compared to those
in THF solution, indicating that aggregation of the sensitizers occurs commonly on the TiO2
surface [30, 31]. In addition, the entire dyes exhibit a slight red shift of 29 nm (F1), 13 nm
(F2), 23 nm (F3), and 11 nm (F4) in wavelength as compared to that measured in THF
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solution. The red shifts of the absorption spectra on TiO2 of F1–F4 are probably due to the
J-aggregation of the dyes on the TiO2 surface in the presence of the carboxyl moiety in the
molecules [32]. This may also be due to the increased delocalization of the π* orbital of the
conjugated framework caused by the interaction between the carboxylate group and the Ti4+
ions that directly decreases the energy of the π* level [33].
All of the dyes display photoluminescence at room temperature (Figure 3) when they
are excited at their respective absorption bands and the fluorescence peak maximum ranges
from 547 to 620 nm. Similar to the absorption spectra, the presence of electron-donating
groups and extension of the conjugation by thiophene group would shift the emission
maximum to the red. The most electron-donating triarylamine unit gave the most red-shifted
peak at 620 nm for F4.
3.3. Electrochemical Properties
To investigate the electron transfer from the excited dye molecule to the conductive
band (Ecb) of TiO2 and understand the molecular orbital energy levels, cyclic voltammetry
(CV) was performed for the dyes in thin films on glassy carbon electrode measured in 0.1 M
tetrabutylammonium hexafluorophosphate solution in acetonitrile at a scan rate of 50 mV s–1.
The reference electrode was Ag/AgCl calibrated with ferrocene-ferrocenium (Fc+/Fc) as an
internal reference. The potentials versus NHE were calibrated by the addition of 0.63 V to the
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potentials versus Fc/Fc+ [34]. The results are summarized in Table 2.
As shown in Table 2, the HOMO levels of F1–F4 are calculated to be 1.11 to 1.30 V.
−
These values are more positive than that of the I−/I3 electrolyte (0.4 V vs. NHE), thereby
ensuring an effective sensitizer regeneration process to give an efficient charge separation
[35]. The LUMO levels of these sensitizers are –0.87 to –0.99 V. These values are more
negative than the bottom level of the conduction band of TiO2 (0.5 V vs. NHE), and hence
indicate that the dyes provide sufficient driving forces for electron injection and the charge
injection from the excited sensitizer molecule to the TiO2 conduction band is viable [35]. In
addition, it is clear that the HOMO level of F4 is the highest among the dyes, which is in
agreement with the order of the electron-donating ability of the triarylamine donor [36].
3.4. Computational Studies
The density functional theory (DFT) was used to optimize the ground state geometries
as well as gain better insights into the electronic and spectroscopic properties of F1–F4
(Figure 4). For F1, the electron density in HOMO is equally distributed all along the
molecule, which is also observable for F2 and F3. Porzio et al. also reported a similar
fluorenone-based molecule TFT that can be compared to F1 by removing the cyanoacrylic
acid moiety [37]. Both F1 and TFT have almost the same HOMO characters. For F4, the
electron density is mainly delocalized on the triphenylamine moiety in the HOMO which
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indicates that delocalization of the electrons on the triphenylamine moiety may facilitate
redox reaction with liquid electrolyte, making the dye suitable for highly efficient solar cells.
The LUMO electron densities of F1–F4 are all located predominantly on the cyanoacetic acid
units and extended to the fluorenone moieties. These kinds of electron distributions facilitate
electron injection from the photoexcited dye into the conduction band of the TiO2 oxide. As a
consequence, it was revealed that the HOMO→LUMO excitation transferring electrons from
the electron-donating group through the π-framework to the anchoring ligand via the
acceptor-fluorenone moiety can facilitate electron injection from the excited state of the
sensitizer to TiO2.
3.5. Photovoltaic Performance of DSSCs
DSSCs photosensitized by F1–F4 were measured under standard AM 1.5 irradiation
(100 mW cm-2) using a black metal mask with an aperture area of 0.25 cm2 and an electrolyte
composed of 0.6 M dimethylpropyl-imidazolium iodide, 0.05 M I2 and 0.1 M LiI in
acetonitrile.
Figure 5 shows the photocurrent action spectra of incident photon-to-current conversion
efficiency (IPCE) for DSSCs using F1–F4. The losses of light reflection and absorption by
the conducting glass were not corrected for. All of the dyes can efficiently convert the visible
light to photocurrent in the region from 300 nm to 600 nm. Generally, the IPCE values of
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DSSCs depend on a number of factors, including the molar absorptivity of the molecule, the
amount of materials being absorbed by the TiO2, the efficiency of charge injection,
transportation and collection [38]. Charge transfer kinetics calculations are needed in each
step in order to establish the relationship between the structure of dye and IPCE. It is known
from the equation IPCE = (1240 × Jsc)100 / (λ × Pin) (in %) that the IPCE value is closely
related to Jsc at a specific wavelength and fixed input power (Pin), and the IPCE values of our
dyes are in line with the order of Jsc values observed (F1 < F2 < F3 < F4). Their IPCE
reaches a maximum of 80% in the range of 450 to 600 nm. F4 exhibits a broader
photocurrent action spectrum than that of F3, which is attributed to the lower bandgap of F4
in the presence of the electron-donating triphenylamine groups. Also, as the IPCE value is a
function of the wavelength, it is difficult to make an accurate comparison here for the
performance among the dyes, and we can only compare them at a fixed wavelength. Hence,
the extinction coefficient becomes a major factor in governing the IPCE at a particular
wavelength, which is in agreement with the trend for the molar extinction coefficient of our
dyes (F1 < F2 < F3 < F4). Moreover, for the issue on dye regeneration, it depends mainly on
–
the composition of the electrolyte. As all of the DSSCs are working in the I–/I3 redox couple
as the active component in the electrolyte, the dye regeneration rate (dye regeneration charge
kinetics) should be similar for each case [1]. This factor would not contribute significantly to
the variation of the IPCE of the dyes. More detailed studies are currently underway to
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establish the correlation between the dye structure and IPCE.
Figure 6 shows the photocurrent–voltage (J–V) characteristics of DSSCs fabricated with
these fluorenone-based photosensitizing dyes (F1–F4) under simulated sunlight illumination.
Compared with the control molecule F1, the photovoltaic performance can be significantly
improved by introducing an electron-donating group such as triarylamine and fluorene in the
molecule for F2–F4. A high molar extinction coefficient of the dye molecule as well as a
broad absorption spectrum can help enhance the light-harvesting ability, thus yielding a
higher Jsc. F4 has the highest light-harvesting efficiency and consequently an improved Jsc
(11.71 mA cm–2) due to its largest molar extinction coefficient, broader absorption spectrum
and highest IPCE value among all the dyes. The light-harvesting efficiency of F3 is slightly
lower than that of F4 and this may be attributed to the smaller molar extinction coefficient.
The device photovoltaic statistics such as short-circuit current density (Jsc), open-circuit
voltage (Voc), fill factor (FF) and photovoltaic conversion efficiency (η) or PCE are
summarized in Table 3. For the overall light to electricity conversion efficiency of F1–F4,
dye F4 exhibited the best η of 4.71%, with Voc of 565 mV and a FF of 0.71. Under the same
device operating conditions, F3 shows a slightly poorer device performance with fluorene in
place of triarylamine as the donor with η of 4.06%, Jsc 10.90 mA cm–2, Voc of 549 mV and a
FF of 0.68. The incorporation of additional thiophene ring as the conjugated bridge between
F2 and F3 shows distinct characteristics which broaden the light-harvesting capacity and
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hence higher efficiency was obtained for F3. In addition, the Voc value of F3 is higher than
that of F2. This may be due to the increasing length of the π-bridge of F3. It would be more
difficult for the injected electron in the TiO2 conduction band to recombine with the redox
electrolyte due to the formation of a thicker dye block layer. Additionally, electron
recombination with the photooxidized adsorbed sensitizers could also be reduced with
increasing the length of the π-bridge because the more extended charge delocalized HOMO is
more far away from the TiO2 substrate [39].
The electrochemical impedance spectroscopy (EIS) technique was applied to study the
electron recombination process in DSSCs based on these four dyes under open circuit and
illuminated conditions. As shown in the Nyquist plots in Figure 7 under light illumination,
the Nyquist plots exhibited three semicircles, the largest semicircle (intermediate frequencies,
1–100 Hz) represents the electron transfer resistance (RCT) at the TiO2/dye/electrolyte
interface. The radius of the largest semicircle increased in the order F1 < F2 < F3 < F4,
indicating that the electron recombination resistance increases from F1 to F4. This result is
also in good agreement with the Voc results above and suggests that the electron
recombination rate between the TiO2 film and the electrolyte has strong relationships with
their structures in this series of fluorenone-based dyes.
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4. Conclusions
In this paper, four new metal-free organic dyes (F1–F4) comprising triarylamine or
fluorene moiety as the electron donor, cyanoacrylic acid as the anchoring group, and
fluorenone group as the linkage were synthesized and used in the fabrication of DSSCs. The
power conversion efficiency of the DSSCs depend significantly on the nature and strength of
the electron-donating end group along the conjugated main as well as the length of the thienyl
rings. Among all of the dyes examined, DSSCs based on F4 exhibited the best overall light to
electricity conversion efficiency of 4.71% (Voc = 565 mV, Jsc = 11.71 mA cm–2, FF = 0.71)
under AM 1.5 irradiation (100 mW cm–2). All of these results reveal that these metal-free
fluorenone-based organic dyes are promising molecular materials in the development of
high-performance DSSCs. Future work will be focused on developing other dyes bearing
double acceptor anchors in the DSSC applications [40].
Acknowledgements
This work was supported by the University Grants Committee Areas of Excellence
Scheme (AoE/P-03/08), Hong Kong Research Grants Council (HKBU203011) and Hong
Kong Baptist University (FRG2/10-11/101). We also thank Prof. He Tian at the East China
University of Science and Technology for collaboration and the National Natural Science
Foundation of China (NSFC) (project number: 21029001) for financial support.
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Figure and Scheme Captions
Fig. 1. Normalized absorption spectra of F1–F4 measured in THF.
Fig. 2. Normalized absorption spectra of F1–F4 measured on TiO2 film.
Fig. 3. Normalized PL spectra of F1–F4 in THF at 298 K.
Fig. 4. Contour plots of selected frontier orbitals of F1–F4.
Fig. 5. Photocurrent action spectra of the TiO2 electrode sensitized with F1–F4.
Fig. 6. J–V curves of the DSSC devices under AM 1.5G simulated sunlight illumination.
Fig. 7. The EIS Nyquist plots for DSSCs sensitized with F1–F4 under light illumination.
Chart 1. The chemical structures of new organic dyes F1–F4 in this study.
Scheme 1. Synthesis of F1: i) K2Cr2O7, acetic acid; ii) Pd(PPh3)4, Na2CO3 (2 M, aq), THF; iii)
NBS, CHCl3/acetic acid; iv) Pd(PPh3)4, Na2CO3 (2 M, aq), toluene/EtOH; v) Cyanoacetic
acid, piperidine, dry CHCl3.
Scheme 2. Synthesis of F2: i) K2Cr2O7, acetic acid; ii) Pd(PPh3)4, Na2CO3 (2 M, aq), THF; iii)
Pd(PPh3)4, Na2CO3 (2 M, aq), toluene/EtOH; iv) Cyanoacetic acid, piperidine, dry CHCl3.
Scheme 3. Synthesis of F3 and F4 : i) Pd(PPh3)4, Na2CO3 (2 M, aq), THF; ii) Pd(PPh3)4,
Na2CO3 (2 M, aq), THF; iii) NBS, CHCl3/acetic acid; iv) Pd(PPh3)4, Na2CO3 (2 M, aq),
toluene/EtOH; v) Cyanoacetic acid, piperidine, dry CHCl3.
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