New D-π-A dyes incorporating dithieno[3,2-: B:2′,3′- d] pyrrole (DTP)-based π-spacers for efficient dye-sensitized solar cells

Article abstract of DOI:10.1039/c7ra08965a

Three D-π-A sensitizers incorporating dithieno[3,2-b:2′,3′-d]pyrrole (DTP)-based π-linkers were synthesized for fabricating efficient dye-sensitized solar cells (DSSCs). The DTP-based π-spacers were attached by 10-phenyl-10H-phenothiazine (JW1), 5,5,10,10

Full text of DOI:10.1039/c7ra08965a

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PAPER
New D–p–A dyes incorporating dithieno[3,2-
b:20,30-d]pyrrole (DTP)-based p-spacers for
efficient dye-sensitized solar cells†
Cite this: RSC Adv., 2017, 7, 45807
*
Jingwen Jia, Yu Chen,
Liangsheng Duan, Zhe Sun,
Mao Liang
*
and Song Xue
Three D–p–A sensitizers incorporating dithieno[3,2-b:20,30-d]pyrrole (DTP)-based p-linkers were
synthesized for fabricating efficient dye-sensitized solar cells (DSSCs). The DTP-based p-spacers were
attached by 10-phenyl-10H-phenothiazine (JW1), 5,5,10,10,15,15-hexapropyl-10,15-dihydro-5H-
diindeno[1,2-a:10,20-c]fluorene (JW2) and 4-hexyloxyphenyl (JW3). The bulky groups and alkyl chains are
introduced to suppress dye aggregation as well as reduce electron recombination. An additional double
bond spacer was introduced between the arylamine donor and DTP-based p-spacer in the studied dyes.
Based on the theoretical calculation results, all of the dye molecules display good electron transport.
The photovoltaic performances of device based on JW1, JW2 and JW3 were measured by J–V
characterizations in Co-phen based electrolyte, achieving power conversion efficiencies of 7.09%, 7.87%
and 6.50%, respectively. The incident photon-to-current conversion efficiency (IPCE) follows the order
JW2 > JW1 > JW3, which is accordance with their photovoltaic performances. IMVS results indicate that
the three dyes incorporating the modified DTP-based p-spacers have an influence on the CB shift of
TiO2 and the inhibition of charge recombination. Therefore, the DTP-based p-spacer is a promising
building block for developing new dyes to build highly efficient DSSCs.
Received 14th August 2017
Accepted 21st September 2017
DOI: 10.1039/c7ra08965a
rsc.li/rsc-advances
The design of metal-free organic dyes can focus on molecular
engineering of the electron donor, p-spacer, and acceptor for
1. Introduction
good light response, large anchoring amount and suitable
energy levels.19–21 Suitable molecular energy levels of organic
dyes to match electrolyte are benecial to achieve an efficient
regeneration of the oxidized dyes.22 Triarylamines are mostly
used as effective electron donor groups in the sensitizer mole-
cules, which can be feasibly synthesized and modied. Cyano-
acetic acid is widely used as end groups with good electron
accepting ability. The p-spacers of the organic dyes have a great
inuence on electron transfer between donors and acceptors in
a dye molecule. In recent years, p-conjugated spacers contain-
ing thiophene rings have been reported showing positive effects
on improving photovoltaic performances of organic sensitizers,
such as thieno[3,2-b]thiophene,23 cyclopenta[1,2-b:5,4-b0]
dithiophene24–26 and dithieno[3,2-b:20,30-d]pyrrole (DTP).27–29
DTP is accessible with a yield in one step up to 80%, and the
N-position provides great opportunity for the control of dye
aggregation, which makes it good for DSSC applications. As re-
ported in our previous study,30 DTP can be easily modied by the
introduction of long-chain alkyl groups on the bridging nitrogen,
and DSSCs fabricated by these DTP-based dyes achieved a PCE of
8.14%. Wang and co-workers reported a series of arylamine
organic dyes bearing N-heterocycle-substituted DTP spacers dis-
played a PCE of 8.20%.31 Polander and co-workers synthesized
two donor–p-spacer–acceptor (D–p–A) dyes based on the DTP p-
Dye-sensitized solar cells (DSSCs) have drawn much attention
1,2
¨
since the work of O'Regan and Gratzel in 1991. DSSCs can
reduce the production cost and exhibit good photovoltaic
performances so as to become a good substitute for expensive
silicon solar cells. To date, various sensitizers have been used to
fabricate highly efficient DSSCs, involving metal complex dyes3–8
and metal-free organic dyes.9–14 Metal-free organic dyes are
amenable to structure modication and easy to purify. The
power conversion efficiency (PCE) of DSSCs based on metal-free
organic dyes has been greatly improved by structure design.
Kakiage and co-workers reported that DSSCs co-sensitized by
ADKEA-1 and LEG-4 exhibited a highest PCE of over 14%.15–17
Recently, it has been demonstrated by Freitag and co-workers
that a DSSC achieved a very high PCE of 28.9% under
ambient light conditions, using two organic sensitizers coded
D35 and XY1.18 Therefore, metal-free organic dyes show prom-
ising advantages in fabricating highly effective DSSCs.
Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School
of Chemistry & Chemical Engineering, Tianjin University of Technology, Tianjin
300384, China. E-mail: cytjut@163.com; xuesong@ustc.edu.cn; Fax: +86-22-
60214252
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra08965a
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bridge and compared with their cyclopenta[1,2-b:5,4-b0]dithio-
phene (CPDT) analogues.32 Based on their results, DTP-based
device exhibited an increased open circuit photovoltage values
and improved charge-transfer kinetics relative to the CPDT
systems. The present researches suggest DTP has a great poten-
tial for the design of efficient metal-free organic sensitizers in the
eld of DSSCs. The effects of N-substituents on dyes properties
and device performances are worthy to explore.
In this work, three new D–p–A metal-free organic dyes were
developed for DSSC application (JW1, JW2 and JW3). JW1 and
JW2 have been synthesized by introducing bulky groups of 10-
phenyl-10H-phenothiazine and 5,5,10,10,15,15-hexapropyl-10,15-
dihydro-5H-diindeno[1,2-a:10,20-c]uorene (truxene) onto the N-
position of DTP spacers, respectively. JW3 was prepared by
incorporating 4-hexyloxyphenyl as a reference compound. In the
three dyes, the triphenylamine substituted by hexyloxy groups
were used as electron donating group, and 2-cyanoacrylic acid
was used as electron accepting group. A double bond was inser-
ted between triphenylamine donor and DTP spacer, aiming to
enhance p-conjugation length of dye molecules.33 The UV-vis
absorption, optimized geometrical and electronic structures by
density functional theory (DFT) calculations, and cyclic voltam-
metry behaviors of the three D–p–A dyes were characterized.
DSSCs based on JW1, JW2 and JW3 were fabricated by using Co-
phen based (phen ¼ 1,10-phenanthroline) electrolytes with 4-tert-
butylpyridine (TBP). The photovoltaic performances of DSSCs
based on the studied dyes are measured by J–V characteristics,
and the corresponding incident photon-to-current conversion
efficiency (IPCE) spectra of JW1, JW2 and JW3 were recorded. The
results show JW1 and JW2 bearing bulky groups exhibit a more
positive inuence on enhancing efficiency of DSSCs than JW3.
The dyes effect on device performance was further discussed by
conducting the measurements of the controlled intensity
modulated photovoltage spectroscopy (IMVS).
2. Experimental
2.1. Instrument and materials
The chemical structures of target products JW1, JW2 and JW3
are shown in Fig. 1. BINAP, P(t-Bu)3, t-BuONa, Pd2(dba)3 and
PPh3$HBr were purchased from J&K Chemical. Cyanoacetic acid
was purchased from Energy Chemical (China). The solvents and
chemicals used in this work were analytical grade and purchased
from Energy Chemical (China). The reagents for reactions and
spectral measurements were dried and distilled before use. The
treatment on solvents and reactants is shown in ESI.†
Fig. 1 Chemical structures of JW1, JW2 and JW3.
measurements were carried out with a HITACHI F-4500 uores-
cence Spectrophotometer.
1H NMR and 13C NMR spectra were recorded on a Bruker AM-
400 spectrometer. The reported chemical shis were against
TMS. High resolution mass spectra were obtained with a Micro-
mass GCT-TOF mass spectrometer. The melting point was taken
on a RY-1 thermometer and the temperatures were uncorrected.
Cyclic voltammetry (CV) measurements for sensitized lms
were measured by a Zennium electrochemical workstation
(ZAHNER, Germany), with sensitized electrodes as the working
electrode, Pt-wires as the counter electrode, and an Ag/AgCl
electrode as the reference electrode at a scan rate of 25 mV sꢀ1
.
Tetrabutylammonium perchlorate (TBAP, 0.1 M) and acetonitrile
were used as supporting electrolyte and solvent, respectively.
Ferrocene was used as standard in the measurements.
2.2. Photophysical properties and electrochemical
properties
The absorption spectra of dyes and sensitized lms were measured
Charge densities at open circuit and intensity modulated
by SHIMADZU UV-2600 spectrophotometer. Fluorescence photovoltage spectroscopy (IMVS) were performed on a Zennium
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electrochemical workstation (ZAHNER, Germany), which sensitized electrode and Pt-counter electrode were assembled
includes a green light emitting diode (LED, 525 nm) and the into a sandwich type cell by a 25 mm-thick Surlyn (DuPont) hot-
corresponding control system. Charge densities and IMVS were melt gasket and sealed up by heating. The Co-phen electrolyte is
measured on a two electrode system with the TiO2-photoanode composed of 0.25 M [Co(II) (phen)3](PF6)2, 0.05 M [Co(III)
as a working electrode and the photocathode as the counter/ (phen)3](PF6)3, 0.5 M TBP and 0.1 M LiTFSI in acetonitrile.
reference electrodes. The intensity-modulated spectra were
The photocurrentevoltage (J–V) characteristics of the solar
measured at room temperature with light intensity ranging from cells based on the studied dyes were carried out using a Keithley
5 to 75 W mꢀ2, in modulation frequency ranging from 0.1 Hz to 2400 digital source meter controlled by a computer and a stan-
10 kHz, and with modulation amplitude less than 5% of the light dard AM1.5 solar simulator-Oriel 91160-1000 (300 W) SOLAR
intensity.
SIMULATOR 2 ꢂ 2 BEAM. The light intensity was calibrated by
an Oriel reference solar cell. The action spectra of mono-
chromatic incident photon-to-current conversion efficiency
(IPCE) for solar cell were performed by using a commercial
setup (QTest Station 2000 IPCE Measurement System,
CROWNTECH, USA).
2.3. The DSSCs fabrication and characterization
A TiO2 electrode (0.156 cm2) was developed on a precleaned
uorine-doped tin oxide (FTO) conducting glass (Nippon Sheet
Glass, Hyogo, Japan, sheet resistance of 14 U sqꢀ1), and the
detailed fabrication procedure was included in ESI.† The dye
was loaded by immersing it into a 300 mM dye solution (ethanol/
CH2Cl2 ¼ 2/3, v/v) for 12 h at room temperature. Then washed
2.4. Synthesis
the dye coated titania electrode with dry ethanol. The pre- The synthesis route for JW1, JW2 and JW3 are shown in
washed FTO glass was deposited with Pt catalyst by coating with Scheme 1. The intermediates of compounds 1–5 have been re-
H2PtCl6 solution (40 mM in ethanol), and used the heat treat- ported in literatures.34–36 The 1H NMR and 13C NMR data of 7a–
ment sinter at 395 ꢁC for 15 min to get photocathode. The c, 8a–c, JW1, JW2 and JW3 are listed in ESI.†
Scheme 1 The synthesis route of JW1, JW2 and JW3. Conditions: (i) t-BuONa, Pd2(dba)3, P(t-Bu)3, toluene, N2, reflux, 14 h; (ii) NaBH4, EtOH, N2,
RT, 4 h; (iii) PPh3$HBr, CHCl3, reflux, 4 h; (iv) t-BuOK, THF, RT, 24 h; (v) OCl3, DMF, DEC, 0 ꢁC then reflux, 8 h; (vi) piperidine, cyano-acetic acid,
CHCl3/CH3CN (v/v, 1/2), reflux, 8 h.
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anhydrous Na2SO4. The solvent was evaporated, and the
remaining crude product was puried by column chromatog-
raphy to give desired compounds 7a–c.
2.4.1 General procedure for the synthesis of compounds
7a, 7b, 7c. To a 100 mL two-neck round-bottom ask, dissolved
compound 5 (0.38 mmol, 317 mg) in anhydrous THF (10 mL).
The system was purged with nitrogen several times. At 0 ꢁC, the
THF solution of t-BuOK (0.69 mmol, 71 mg) was added drop-
wise, and the solution became deep red. The system was kept at
0 ꢁC. Aer 1 h, the THF solution of 6a–c (0.345 mmol) was added
slowly. The mixture was cooled to room temperature and stirred
overnight. Water was added to pull the plug on the reaction and
the product was extracted with dichloromethane. The
combined organic layer was washed with brine and dried over
2.4.2 General procedure for the synthesis of compounds
8a, 8b, 8c. In a 100 mL round bottom ask, compound 7a–c
(0.215 mmol) was taken in dichloroethane (15 mL) followed by
the addition of anhydrous DMF (0.237 mmol) at room temper-
ature. Then the reaction solution was cooled to 0 ꢁC, and POCl3
(0.86 mmol) was added dropwise. The reaction mixture was
allowed to warm to room temperature and reuxed overnight,
and waked up with saturated sodium acetate solution. The
Fig. 2 The absorption and emission spectra of JW1, JW2 and JW3 (a) in dichloromethane and (b) on TiO2 films.
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Table 1 Photophysical and electrochemical data of JW1, JW2 and JW3
HOMOe/vs.
NHE (eV)
LUMOf/vs.
NHE (eV)
E0–0
c
Adsorbed
3 (104 Mꢀ1 cmꢀ1
)
lmax,lm (nm)
lint (nm)
dyed 10ꢀ6 mol cmꢀ2
(eV)
a
b
c
Dye
lmax,sol (nm)
JW1
JW2
JW3
510
515
516
3.23
4.63
3.48
482
490
472
614
622
620
4.6
1.6
3.1
1.014
0.996
0.914
ꢀ1.186
ꢀ1.174
ꢀ1.246
2.20
2.17
2.16
a
The maximum absorption wavelength of UV-vis spectra measured in dichloromethane. b The maximum absorption wavelength of UV-vis spectra
measured on TiO2. c lint is the intersections of normalized absorption and emission spectra in dichloromethane; E0–0 values were estimated from
d
E0–0 ¼ 1240/lint
.
The adsorbed amount of dye on the TiO2 surface. e HOMO (vs. NHE) were measured in acetonitrile and calibrated with ferrocene
(0.63 V vs. NHE). f LUMO (vs. NHE) were estimated from calculated from LUMO ¼ HOMO–E0–0
.
Fig. 3 (a) Cyclic voltammograms of JW1, JW2 and JW3 measured in acetonitrile solutions; (b) energy levels of dyes and redox couples.
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product was extracted with dichloromethane, washed with from the intersections of normalized absorption and emission
water and brine, and then dried over anhydrous Na2SO4. spectra. Fig. 2(a) shows the normalized absorption and emis-
2.4.3 General procedure for the synthesis of compounds sion spectra of JW1, JW2 and JW3 in dichloromethane. The
JW1, JW2 and JW3. To a stirred solution of compound 8a–c maximum absorption wavelength in dichloromethane of JW1,
(0.11 mmol) and cyanoacetic acid (0.164 mmol, 14 mg) in JW2 and JW3 show little difference, which are 510 nm, 515 nm
acetonitrile (6 mL) was added chloroform (3 mL) and piperidine and 516 nm, respectively. This suggests the bulky groups are not
(0.33 mmol, 28 mg). The reaction mixture was reuxed for 8 h. key point for increasing the light response, while the basic D–p–
Then additional cyanoacetic acid (0.164 mmol) and piperidine A skeleton is a more important factor for their absorption
(0.33 mmol, 28 mg) were added. The mixture was reuxed ability. Subsequently, the molar extinction coefficients of the
continuously for 8 hours and then acidied with 1 M hydro- dye solution with the concentration of 1.5 ꢂ 10ꢀ5 M were
chloric acid aqueous solution (15 mL). The crude product was measured. As listed in Table 1, the corresponding molar
extracted into CH2Cl2, washed with water and brine, and then extinction coefficients of JW1, JW2 and JW3 in dichloromethane
dried over anhydrous Na2SO4. Aer that the solvent was evap- are 3.23 ꢂ 104 Mꢀ1 cmꢀ1, 4.63 ꢂ 104 Mꢀ1 cmꢀ1 and 3.48 ꢂ
orated, and the remaining crude product was puried by 104 Mꢀ1 cmꢀ1, respectively. JW2 shows the highest molar
column chromatography to give desired compounds JW1, JW2 extinction among the three dyes in solution. The intersections
and JW3.
of normalized absorption and emission spectra of JW1, JW2 and
JW3 are at 614 nm, 622 nm and 620 nm, respectively. Thus, the
E0–0 values of JW1, JW2 and JW3 are 2.20 eV, 2.17 eV, and
2.16 eV, separately.37 The multi-alkyl groups of JW2 and JW3
3. Results and discussion
The synthetic routes for JW1, JW2 and JW3 were shown in relatively reduce the E0–0 value as compared to JW1. The
Scheme 1. The aldehyde intermediates (6a–c) play a key role in absorption spectra of the studied dyes anchored on mesoporous
obtaining the target dye molecules. All intermediates and nal TiO2 were also investigated, as shown in Fig. 2(b). Three dyes
products are puried and characterized by NMR spectra. The absorbing on TiO2 display obvious absorption extended to
spectroscopic and electrochemical properties of JW1, JW2 and 700 nm. While the weaker absorbance of JW2 on TiO2 due to its
JW3 were tested and analyzed, respectively.
larger molecular size than JW1 and JW3 causing a decrease of
absorbed amount on TiO2 per square centimeter. The absorbed
amounts of JW1, JW2 and JW3 on 3 mm-thick TiO2 lms
are 4.6 ꢂ 10ꢀ6 mol cmꢀ2, 1.6 ꢂ 10ꢀ6 mol cmꢀ2 and 3.1 ꢂ
10ꢀ6 mol cmꢀ2, respectively. The spectroscopic data are
summarized in Table 1.
3.1. Absorption spectra
Absorption and emission spectra of JW1, JW2 and JW3 were
studied by UV-vis spectroscopy and uorescence spectroscopy,
and the optical band gaps (E0–0) of the three dyes are calculated
Fig. 4 The Frontier molecular orbital distributions of JW1, JW2 and JW3 optimized by DFT calculations at the B3LYP/6-31G level.
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in Fig. 3(a). The electrochemical data and related calculation
methods are given in Table 1. The HOMO levels of JW1, JW2 and
JW3 corresponding to the rst oxidation potentials was
1.104 eV, 0.996 eV and 0.914 eV (vs. NHE), respectively. The
LUMO levels of JW1, JW2 and JW3 are ꢀ1.186 eV, ꢀ1.174 eV and
ꢀ1.246 eV (vs. NHE), respectively. The LUMO levels of the three
dyes are more negative than the conduction band (CB) of the
3.2. Electrochemical properties and theoretical calculations
The structure effect on energy levels has a great impact on
getting a suitable drive force for dye regeneration in DCCSs.
Cyclic voltammograms (CV) was conducted to determine the
accurate highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) levels of JW1, JW2
and JW3. The CV curves of three dyes in acetonitrile are shown
Fig. 5 (a) J–V curves and (b) IPCE spectra of DSSCs based on JW1, JW2 and JW3.
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Table 2 Photovoltaic parameters of the DSSCs based on JW1, JW2
and JW3
TiO2 electrode (ꢀ0.5 V vs. NHE), making it easily for electron
injection into the CB of TiO2. Based on the spectroscopic and
electrochemical data, the energy-level diagram of JW1, JW2 and
JW3 are depicted in Fig. 3(b). The energy levels of the studied
dyes are well matched with the CB of TiO2 and the redox energy
of the selected Co-phen electrolyte.
Dye
Jsc/MA cm2
Voc/mV
FF
PCE%
JW1
JW2
JW3
15.79
16.45
15.57
788
784
773
0.57
0.61
0.54
7.09
7.87
6.50
To gain insight into the electronic properties and geometries
of JW1, JW2 and JW3, quantum chemical calculations were
Fig. 6 (a) Charge density as the open circuit and (b) electron lifetime as the charge density for DSSCs based on the dyes JW1, JW2 and JW3.
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performed. Their geometric structures were optimized on the the introduction of bulky groups on DTP has an obvious impact
basis of density functional theory (DFT) at the B3LYP/6-31G on the CB shi of TiO2.27,41 TiO2 graed with JW2 caused
level. All structural optimization and energy calculations were a downward CB shi of TiO2 compared to JW1 and JW3.
performed using the GAUSSIAN 09 program. The optimized However, the LUMO level of JW2 is low enough to make sure
structures and the Frontier orbitals of molecules of JW1, JW2 efficient electron injection from JW2 to TiO2. Therefore, the
and JW3 are shown in Fig. 4. The HOMOs of JW1, JW2 and JW3 different bulky groups feasibly regulate HOMO–LUMO energy
are extended over the whole backbone of dye molecules, and the gap of the studied sensitizers, the suitable energy levels have
LUMOs are effectively transferred from the arylamine donor to a positive effect on fabricating highly-efficient DSSCs. As shown
the acceptor part unit via the DTP spacer. The optimized in Fig. 6(b), the s value of JW2 exhibits a certain increase than
structures of JW1, JW2 and JW3 are shown in Fig. S1 and S2,† those of devices based on JW1 and JW3, which may be more
and the corresponding data on the representative dihedral favorable to reduce charge recombination in the device. Trux-
angles and bond lengths are listed in Table S1 (ESI).† In ene is a good building block for the modication on p-spacers
Fig. S2,† the dihedral angles between DTP and its chains for of sensitizers in DSSCs.
JW1, JW2 and JW3 are 126.9ꢁ, 125.9ꢁ and 126.9ꢁ, respectively.
The DTP spacer on JW2 exhibits a little more twisted structure
due to the larger size of truxene unit. Based on the photovoltaic
4. Conclusions
New D–p–A organic dyes with dithieno[3,2-b:20,30-d]pyrrole
(DTP) p-spacers (JW1, JW2 and JW3) were synthesized for
fabricating efficient DSSCs in Co-phen based electrolyte.
Different bulky groups were introduced onto the DTP unit, and
the structure–efficiency relationship was investigated by theo-
retical calculations, spectroscopic, electrochemical and photo-
voltaic measurements. The three dyes display strong absorption
at the range of 400–700 nm both in solution and on TiO2 lm.
CV measurements show that JW2 incorporating DTP with
truxene shows modest energy levels for reducing drive force for
electron injection and dye regeneration. Based on J–V results,
DSSCs fabricated by JW2 exhibit the highest PCE of 7.87%, and
that of cells based on JW1 and JW3 are 7.09% and 6.50%.
Devices based on JW1 and JW2 with lager bulky units yield
higher Jsc and Voc values than that based on JW3. IMVS results
indicate the three dyes have an obvious inuence on the CB
shi of TiO2 via the introduction of different substitutes on DTP
p-spacer, and differs the inhibition ability of charge recombi-
nation in device. Therefore, suitable design of dye molecules
has a great effect on improve the photovoltaic performances of
DSSCs.
performances discussed on Section 3.4, the little twisted change
on JW2 has a positive effect on reducing charge recombination
on interfaces in DSSCs.
3.3. Photovoltaic performances of DSSCs
DSSCs based on JW1, JW2 and JW3 were fabricated with Co-
phen electrolyte. The photovoltaic performances of devices
based on JW1, JW2 and JW3 were investigated by photocurrent–
voltage (J–V) measurements. The J–V curves are shown in
Fig. 5(a), and the corresponding photovoltaic parameters are
listed in Table 2. In Fig. 5(a), devices based on JW1, JW2 and
JW3 show a PCE of 7.09%, 7.87% and 6.50%, respectively. JW2-
based DSSCs exhibit the highest PCE, yielding a Jsc of
16.45 mA cm2, a Voc of 784 mV and a FF of 0.61. As shown in
Fig. 3(b), the suitable HOMO–LUMO energy gap of JW2 can both
strengthens electron injection from dye to TiO2 and reduces
driving force for the regeneration of oxidized dye among the
three dyes, making the corresponding device based on JW2
display the highest Jsc value. In Fig. 5(b), the corresponding
incident photon-to-current conversion efficiency (IPCE) spectra
of DSSCs fabricated by JW1, JW2 and JW3 were recorded.
Correspondingly, IPCE values of JW1 and JW2-based DSSCs are
higher than that of JW3-based DSSCs. As listed in Table 1, the
Conflicts of interest
absorbed JW1, JW2 and JW3 on TiO2 are 4.6 ꢂ 10ꢀ6 mol cmꢀ2
,
There are no conicts to declare.
1.6 ꢂ 10ꢀ6 mol cmꢀ2 and 3.1 ꢂ 10ꢀ6 mol cmꢀ2, respectively.
Although the amount of JW2 absorbed on TiO2 is the least
among the three dyes due to the large size of truxene unit, JW2-
based cells achieved the best performance.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21376179, 21506164).
In order to further study the relationship between dye
structures and device performances, the charge density (dn) at
open circuit and the electron lifetime (s) as a function of charge
density were measured by controlled intensity modulated pho-
tovoltage spectroscopy (IMVS).38–40 The results indicate that the
introduction of 10-phenyl-10H-phenothiazine and truxene onto
DTP spacers of JW1 and JW2 is benet to enhance the photo-
voltaic performances of DSSCs by more efficiently reduce elec-
tron recombination, as compared to the introduction of 4-
hexyloxyphenyl onto DTP spacers of JW3. As shown in Fig. 6(a),
TiO2 graed with JW1, JW2 and JW3 displays an increased Voc
value in the order of JW2 < JW3 < JW1 at a xed dn, indicating
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