A new series of azobenzene-bridged metal-free organic dyes and application on DSSC

Article abstract of DOI:10.1016/j.dyepig.2017.07.049

A series of novel azobenzene-based dyes were designed and synthesized using azobenzene as the π-spacer unit to connect diarylamino and cyanoacetic acids for construction of the D–π–A conformation. The introduction of a thienyl into the anchor group (A) co

Full text of DOI:10.1016/j.dyepig.2017.07.049

Accepted Manuscript
A new series of azobenzene-bridged metal-free organic dyes and application on
DSSC
Kuo Yuan Chiu, Thai Thi Ha Tran, Sheng Hsiung Chang, Te-Fang Yang, Yuhlong
Oliver Su
PII:
S0143-7208(17)30633-2
10.1016/j.dyepig.2017.07.049
DYPI 6138
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 17 April 2017
Revised Date: 21 July 2017
Accepted Date: 21 July 2017
Please cite this article as: Chiu KY, Ha Tran TT, Chang SH, Yang T-F, Su YO, A new series of
azobenzene-bridged metal-free organic dyes and application on DSSC, Dyes and Pigments (2017), doi:
10.1016/j.dyepig.2017.07.049.
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ACCEPTED MANUSCRIPT
A New Series of Azobenzene-bridged Metal-free Organic
Dyes and Application on DSSC
Kuo Yuan Chiu,a Thai Thi Ha Tran,a Sheng Hsiung Chang,b Te-Fang Yang,a,**
Yuhlong Oliver Su a,*
a Department of Applied Chemistry, National Chi Nan University, 1 University Road,
Puli, Nantou 545, Taiwan
b Research Center for New Generation Photovoltaics, National Central University,
No.300, Zhongda Rd., Zhongli City, Taoyuan 320, Taiwan
Corresponding Author
** Email: tfyang@ncnu.edu.tw
* Email: yosu@nchu.edu.tw
Keywords: DSSC • Organic dye • Azobenzene • Electrochemistry • DFT
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Abstract
A series of novel azobenzene-based dyes were designed and synthesized using
azobenzene as the π-spacer unit to connect diarylamino and cyanoacetic acids for
construction of the D–π–A conformation. The introduction of a thienyl into the anchor
group (A) could lead to a red-shift in the absorption spectra and λonset of the dyes.
When the diaryamino substituent was changed from methyl to methoxyl, a slight
negative shift in the first oxidation peak could be observed. The spin density
distribution of the cation radicals of the four dyes was also investigated. The
qualitative order of the stability of these four cation radicals was found to be TA4 >
TA3 > TA2 > TA1 in air. They were then employed as the sensitizer in organic
dye-sensitized solar cells to achieve the highest power conversion efficiency (PCE) of
4.78 % under AM 1.5 conditions. By systematically changing the donor and acceptor
unit of these dyes, the new record on PCE with using azobenzene as the π-spacer unit
was achieved.
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Graphical abstract
12
10
8
TA1
TA2
TA3
TA4
6
R
4
N
N
N
Ar
2
COOH
NC
R
0
0.0
0.2
0.4
0.6
0.8
Voltage (V)
3

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Highlights
ꢀ
ꢀ
A series of novel azobenzene-based dyes were synthesized.
The stability order of these four dyes cation radicals was found to be TA4 > TA3
> TA2 > TA1 in air.
ꢀ
The new record on PCE with using azobenzene as the π-spacer unit was
achieved.
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1. Introduction
Recently, numerous metal-free organic dyes have been introduced for use in solar
cells which have the advantages of a wide variety of structural designs and high molar
absorption coefficients. The resultant solar cells often display respectable power
conversion efficiencies (PCE) [1-3]. The Donor-(π-spacer)-Acceptor (D-π-A) system
is the basic architecture used for the design of organic sensitizers due to the effective
photoinduced intramolecular charge transfer properties [4]. In general, TAA
(triarylamine) units are widely used as electron donors due to their strong
electron-donating ability. On the other hand, the cyanoacrylic acid moiety is usually
employed as the electron acceptor as well as the anchoring group that attaches to the
surface of the TiO2 [1,5].
The use of a suitable electron-rich group as the π-spacer between the electron
donor and electron acceptor was reported to be beneficial by inducing a red-shift in
absorption maximum during the charge-transfer transition [6-8]. Many studies have
focused on the conjugate chains such as vinyl [6], stilbene [7], anthrancene [8],
phenyl and thienyl. However, the use of azobenzene as the π-spacer in the D-π-A
structure for metal-free organic dyes has not been studied in detail thus far. The
azobenzene derivatives have revealed a number of important characteristics, such as
thermal stability over a wide temperature range [9], high chemical stability, stronger
electronic communication [10] and lower optical band gap (LUMO-HOMO), when
the azobenzene moiety is used for bridge conformation [11]. To the best of our
knowledge, there have been few reports of the construction of sensitizers comprised
of azobenzene units, even though they have shown very low PCE [12,13]. We herein
report a facile synthesis and the application of four novel organic dyes (TA1-TA4,
Scheme 1), in which diarylamine is used as the electron-donating moiety, azobenzene
moiety as the π-spacer, and aromatic-cyanoacrylic acid as the anchoring moiety
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(Figure 1). The best PCE is almost four times greater than the previous studies [12].
2. Experimental
2.1. Materials and physicochemical study
All chemicals used were obtained from the commercial resources and used as
received unless specified. HPLC grade CH2Cl2 was purchased from ACROS and was
distilled from CaH2 under an argon (Ar) atmosphere for electrochemical use.
Tetra-n-butylammonium hexafluorophosphate (TBAPF6) was recrystallized twice
1
from ethanol and then stored in a desiccator. H-NMR and 13C-NMR spectra were
recorded with a Bruker 300 MHz NMR spectrometer using CDCl3 as a solvent.
FAB-MS spectra were obtained using JMS-700 spectrometer. UV/Vis absorption and
emission spectra were measured using a Cary 300 Bio spectrophotometer and
FluoroMax-4 spectrofluorometer (HORIBA JOBIN YVON), respectively. Elemental
analyses were carried out with a Heraeus CHN-O-S Rapid-F002 analysis system.
Electrochemistry was conducted using a three-electrode cell with BAS glassy carbon
working electrode (GCE, area = 0.07 cm2). The glassy carbon electrode was polished
with 0.05 ꢀm alumina on Buehler felt pads and was ultrasonicated for 2 min to
remove the alumina residue. A platinum wire was used as the auxiliary electrode. A
home-made Ag/AgCl, KCl (saturated) was the reference electrode. The supporting
electrolyte was 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in CH2Cl2.
The square-wave voltammograms (frequency: 25 Hz; step potential: 0.0051 V;
amplitude: 0.01995 V) were recorded using a potentiostat/ galvanostat (PGSTAT 30,
Autolab, Eco-Chemie, the Netherlands) and ferrocene was used as a calibration
standard (+0.49 V versus Ag/AgCl).
2.2. Synthesis of TA1, TA2, TA3 and TA4
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Compounds 1, 2, 3, 4, and 1-bromo-4-nitrosobenzene were synthesized according
to the previous reports [14,15]. The synthesis pathway of TA series are shown in
Scheme 1.
Synthesis of compound 5. 1-Bromo-4-nitrosobenzene (3.45 g, 18.5 mmol, 1eq.)
was dissolved in a mixture of acetic acid / ethyl acetate 1/1 (200 mL). To this solution,
3 (4.28 g, 14.83 mmol, 0.8 eq.) was added directly. The reaction mixture was stirred at
40 ºC overnight. After cooling to room temperature, a precipitate appeared. The
precipitate was filtered and purified by silica-gel column chromatography using
CH2Cl2/hexane (1:1) as the eluent to afford compound 5 as a dark yellow solid (yield:
80.2%), m.p. 176-178 . IR (KBr), cm-1: 2916、1590、1505、1484、1316、1294、
1267、1135、1065、1002、836、809. 1H NMR (300 MHz, CDCl3) δ 7.77-7.71 (m, 4H),
13
7.65-7.58 (m, 2H), 7.14-7.00 (m, 10H), 2.34 (s, 6H). C NMR (75 MHz, CDCl3) δ
152.0, 151.5, 146.5, 144.4, 134.4, 132.4, 132.3, 130.4, 127.5, 126.1, 124.6, 124.2,
120.2, 21.1. MS (FAB+) m/z Calcd for C26H22BrN3, ([M + H]+) 455.0997, 457.0977;
found, 455.0999, 457.0981.
Synthesis of compound 6. 1-Bromo-4-nitrosobenzene (3.45 g, 18.5 mmol, 1eq.)
was dissolved in a mixture of acetic acid / ethyl acetate 1/1 (200 mL). To this solution,
4 (4.75 g, 14.83 mmol, 0.8 eq.) was added directly. The reaction mixture was stirred at
40 ºC overnight. After cooling to room temperature, a precipitate appeared. The
precipitate was filtered and purified by silica-gel column chromatography using
CH2Cl2/hexane (1:1) as the eluent to afford compound 6 as a orange-red solid (yield:
85.1%), m.p. 169-171 . IR (KBr), cm-1: 2997、1593、1508、1316、1288、1234、
1132、1023、824. 1H NMR (300 MHz, CDCl3) δ 7.73 (t, J = 7.5 Hz, 6.9 Hz, 4H), 7.58
13
(d, J = 6.9 Hz, 2H), 7.13 (d, J = 6.6 Hz, 4H), 6.94-6.85 (m, 6H), 3.81 (s, 6H). C
NMR (75 MHz, CDCl3) δ 157.0, 152.0, 151.6, 146.0, 139.8, 132.4, 127.8, 124.7,
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124.1, 118.6, 115.1, 55.7. MS (FAB+) m/z Calcd for C26H22BrN3O2, ([M + H]+)
487.0895, 489.0875; found, 487.0901, 489.0877.
Synthesis of compound 5a. Compound 5 (27.3 mg, 6×10-2 mmol) and
4-formylphenylboronic acid (13 mg, 9×10-2 mmol) were dissolved in anhydrous
toluene (10 mL), then Pd(PPh3)4 (17 mg, 1.5×10-2 mmol) and Na2CO3 (63.6
mg ,6×10-1 mmol) in water (5 mL) was added as a catalyst. The mixture was refluxed
under argon for 24 hours. After cooling to the room temperature, the crude compound
was extracted by ethyl acetate/water, then washed with brine and finally dried with an
anhydrous sodium sulfate. The solution was concentrated under reduced pressure and
the residue was purified by column chromatography (CH2Cl2/hexane = 2:1, v/v) on a
silica gel to yield a red powder 5a (80% yield), m.p. 182-183 . IR (KBr), cm-1:
3018、1686、1593、1499、1406、1312、1288、1207、1138、824. 1H NMR (300
MHz, CDCl3) δ 10.07 (s, 1H), 7.99-7.95 (m, 4H), 7.84-7.75 (m, 6H), 7.15-7.04 (m,
13
10H), 2.35 (s, 6H). C NMR (75 MHz, CDCl3) δ 192.1, 146.8, 1.46.6, 144.4, 134.4,
130.6, 130.4, 128.3, 127.9.3, 126.1, 124.6, 123.3, 120.3, 21.15. MS (FAB+) m/z Calcd
for C33H27N3O, ([M + H]+) 481.2154; found, 481.2155.
Synthesis of compound 6a. The synthesis method resembles that of compound 5a
and the compound was purified by silica-gel column chromatography using
CH2Cl2/hexane (1:1) as the eluent to afford compound 6a as a yellow solid (yield:
40.2%), m.p. 177-178 . IR (KBr), cm-1: 3020、1698、1587、1499、1328、1240、
1
1135、1020、824. H NMR (300 MHz, CDCl3) δ10.05 (s, 1H), 7.97-7.93 (m, 4H),
7.81-7.73 (m, 6H), 7.14-7.11 (d, J = 8.4 Hz, 4H), 6.95 (d, J = 9 Hz, 2H), 6.86 (d, J =
13
8.7 Hz, 4H), 3.81 (s, 6H). C NMR (75 MHz, CDCl3) δ 192.1, 157.0, 153.1, 151.9,
146.6, 146.2, 140.9, 139.8, 135.6, 130.5, 128.2, 127.8, 124.7, 123.3, 118.6, 115.1,
55.9. MS (FAB+) m/z Calcd for C33H27N3O3 ([M + H]+) 513.2052; found, 513.2050.
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Synthesis of compound 5b. Compound 5 (27.3 mg, 6×10-2 mmol) and
5-Formyl-2-thienylboronic acid (14 mg, 9×10-2 mmol) were dissolved in anhydrous
toluene (10 mL), then Pd(PPh3)4 (17 mg, 1.5×10-2 mmol) and Na2CO3 (63.6
mg ,6×10-1 mmol) in water (5 mL) was added as a catalyst. The mixture was refluxed
under argon for 24 hours. After cooling to the room temperature, the crude compound
was extracted by ethyl acetate/water, then washed with brine and finally dried with an
anhydrous sodium sulfate. The solution was concentrated under reduced pressure and
the residue was purified by column chromatography (CH2Cl2/hexane = 2:1, v/v) on a
silica gel to yield a dark red powder 5b (yield: 88%), m.p. 186-187 . IR (KBr), cm-1:
2910、1665、1590、1508、1319、1270、1229、1147、1056、839、809. 1H NMR
(300 MHz, CDCl3) δ 9.90 (s, 1H), 7.91-7.77 (m, 6H), 7.46-7.40 (m, 2H), 7.11-7.02 (m,
13
10H), 2.34 (s, 6H). C NMR (75 MHz, CDCl3) δ 183.0, 153.6, 153.5, 151.6, 146.7,
144.3, 143.0, 137.6, 134.5, 134.4, 130.4, 129.4, 127.2, 126.6, 126.1, 124.7, 124.3,
123.5, 120.2, 21.1. MS (FAB+) m/z Calcd for C31H25N3OS, ([M + H]+) 487.1718;
found, 487.1720.
Synthesis of compound 6b. The synthesis method resembles that of compound 5b
and the compound was purified by silica-gel column chromatography using
CH2Cl2/hexane (1:1) as the eluent to afford compound 6b as a yellow solid (yield:
40.2%), m.p. 153-155 . IR (KBr), cm-1: 2922、1737、1656、1508、1243、1035、
1
855、822. H NMR (300 MHz, CDCl3) δ 9.89 (s, 1H), 7.89 (d, J = 8.1 Hz, 2H),
7.78-7.76 (m, 6H), 7.13 (d, J = 8.7 Hz, 4H), 6.93 (d, J = 9 Hz, 2H), 6.87 (d, J = 9 Hz,
4H), 3.81 (s, 6H). 13C NMR (75 MHz, CDCl3) δ183.0, 157.1, 153.7, 153.6, 152.0,
146.2, 143.0, 139.8, 137.6, 137.1, 134.2, 127.8, 127.2, 124.8, 124.7, 123.4, 118.5,
115.1, 55.7. MS (FAB+) m/z Calcd for C31H25N3O3S, 519.1617; found, 519.1621.
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Synthesis of TA1. An oven-dried 100 mL one-necked flask was charged with
compound 5 (3.5 × 10-1 mmol), cyanoacrylic acid (7 mmol), dry THF (10 mL) and
piperidine (0.4 mL). The solution was heated at reflux for 15 h and the color turned
deep red. After cooling to room temperature, dichloromethane was added and the
organic phase was washed with water (3×20 mL) and brine (3×20 mL). The combined
organic phase was dried on MgSO4, filtered and evaporated under reduced pressure
and the residue was purified by silica-gel column chromatography to give a purple
solid (86.5%), m.p. 197-198 . IR (KBr), cm-1: 3033、1590、1505、1319、1138、
818. 1H NMR (300 MHz, CDCl3) δ 8.14 (br, 1H), 7.72-7.55 (m, 2H), 7.58-7.56 (d, J
13
= 8.1 Hz, 6H), 6.96 (d, J = 8.4 Hz 4H), 6.86 (d, J = 8.1 Hz, 6H), 2.24 (s, 6H). C
NMR (75 MHz, CDCl3) δ 210.2, 171.0, 144.6, 137.3, 134.6, 133.8, 130.3, 127.7,
125.8, 124.6, 123.2, 120.6, 21.1. MS (FAB+) m/z Calcd for C36H28N4O2, ([M + H]+)
548.2212; found, 548.2219.
Synthesis of TA2. The synthesis method resembles that of TA1 and the compound
was purified by silica-gel column chromatography using CH2Cl2/MeOH (10:1) as the
eluent to afford com TA2 as a yellow solid (yield: 90%), m.p. 165-167 . IR (KBr),
cm-1: 3031、1593、1508、1243、1141、825. 1H NMR (300 MHz, CDCl3) δ 8.17 (br,
1H), 7.94-7.60 (m, 6H), 7.28-7.26 (m, 4H), 6.97 (d, J = 8.7 Hz, 4H), 6.77-6.74 (m, J =
13
8.7 Hz, 6H), 3.74 (s, 6H). C NMR (75 MHz, CDCl3) δ 195.8, 156.7, 152.6, 151.3,
146.4, 140.4, 140.1 131.6, 127.7, 127.5, 124.7, 123.2, 119.0, 115.0, 55.6. MS (FAB+)
m/z Calcd for C36H28N4O4, ([M + H]+) 580.2111; found, 580.2115.
Synthesis of TA3. The synthesis method resembles that of TA1 and the compound
was purified by silica-gel column chromatography using CH2Cl2/MeOH (10:1) as the
eluent to afford TA3 as a yellow solid (yield: 88.6%), m.p. 173-174 . IR (KBr), cm-1:
3019、2361、1593、1511、1322、1270、1129、842、824. 1H NMR (300 MHz, CDCl3)
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δ 8.23 (br, 1H), 7.53 (m, 6H), 7.30 (m, 4H), 7.00-6.85 (m, 8H), 2.26 (s, 6H). 13C
NMR (150 MHz, CD2Cl2) δ 206.9, 153.1, 151.4, 146.7, 144.5, 136.4, 134.6, 134.29,
130.4, 127.0, 126.2, 124.7, 123.4, 120.0, 116.3, 110.4, 21.0. MS (FAB+) m/z Calcd for
C34H26N4O2S, ([M + H]+) 554.1776; found, 554.1775.
Synthesis of TA4. The synthesis method resembles that of TA1 and the compound
was purified by silica-gel column chromatography using CH2Cl2/MeOH (10:1) as the
eluent to afford TA4 as a yellow solid (yield: 85.9%), m.p. 181-183 . IR (KBr), cm-1:
3016、2361、1593、1508、1237、1129、1032、775. 1H NMR (300 MHz, CDCl3)
δ 8.28 (br, 1H), 7.45 (m, 4H), 7.15 (m, 2H), 6.91-6.72(m, 12H), 3.70 (s, 6H). 13C
NMR (150 MHz, CD2Cl2) δ 192.1, 160.9, 157.2, 146.3, 140.0, 138.9, 128.0, 124.8,
123.4, 118.5, 115.2, 55.8. MS (FAB+) m/z Calcd for C34H26N4O4S, ([M + H]+)
586.1675; found, 586.1669.
2.3. Device fabrication and photovoltaic performance test
Anatase TiO2 nanoparticles (NP) (ca. 20 nm diameter) were hydrothermally
prepared in a Ti based autoclave as reported previously [16]. The screen-printable 20
nm TiO2 paste was prepared by thoroughly mixing of 4.23 g TiO2 NP, 55 mL ethyl
cellulose, and 45 mL terpineol. The TiO2 paste was screen-printed to form an active
area of 0.16 cm2 TiO2 photoanode (11 µm thick mesoporous anatase-TiO2 film and 4
ꢀm thick TiO2 scattering layer with particle size of ca. 400 nm) on the fluorine-doped
tin oxide glass (FTO glass, 7 ꢁ/square, 2.2 mm thick) with pretreatment with 40 mM
TiCl4(aq) at 70 oC for 30 min and control the thickness. Then the TiO2 electrodes were
heated under an air flow at 500 °C for 60 minutes. After cooling to room temperature,
the TiO2 electrode was immersed in the TEDBn or TEDTh organic sensitizer solution
(0.2 mM) with 20 mM chenodeoxycholic acid (CDCA, Sigma-Aldrich) in THF at
40°C for 3 hours, and then rinsed with acetonitrile. The cell was covered with
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platinized FTO glass incorporating a drilled hole, to form the counter electrode and
complete the sandwich configuration. The cell interior was separated by a 30 ꢀm
surlyn and the electrolyte was injected into the cell via the hole in the counter
electrode. The electrolyte was composed of BMII (0.6 M), LiI (0.1 M),
tert-butylpyridine (TBP, Sigma-Aldrich) (0.5 M), 0.1 M GuNCS and I2 (0.05 M) in
acetonitrile. Finally, drill holes were sealed using Surlyn hot-melt polymer and cover
glass.
Light illumination for the photovoltaic measurements employed a 100 mW/cm2
simulated Sun light source (Yamashita Denso, YSS-50A), and the light-source power
output was calibrated using a reference Si photodiode (BS-520, Bunko Keiki). The
DSC photovoltaic characteristics were obtained by applying an external potential bias
to the cell, and measuring the generated photocurrent using a Keithley model 2400
digital source meter (Keithley, USA). Photocurrent action spectra for the incident
monochromatic photon to current conversion efficiency (IPCE) were taken with an
IPCE measurement system (EQE-R-3011, ENLI Technology Co. Ltd., Taiwan)
calibrated with a single-crystal silicon reference cell for each measurement.
3. Results and discussion
The absorption spectra of TA1-TA4 in CH3CN are shown in Figure 2. It can be
seen that the main absorption wavelength in CH3CN is between 400 and 650 nm, as
contributed by the azobenzene π-spacer. This allows the efficient absorption of
photons with lower energy [17]. The onset value of the visible absorption maxima
(λonset) of the dyes corresponds to the energy gap (E0-0) between the HOMO to LUMO
transitions (Table 2) [18]. With the change in the diarylamino group from methyl
(TA1) to methoxyl (TA2), there is a red shift in the value of λonset. This phenomena
can also be observed in TA3 and TA4. A comparison of TA1 with TA3 shows that the
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introduction of a 2,5-thienyl moiety into the anchor group can lead to a red-shift in the
absorption spectra and λonset (from 610 nm to 660 nm) of the dye, which is desirable
for a lower energy gap and for harvesting more solar energy [17].
The first oxidation potential (Eox) of these dyes was measured by square wave
voltammetry (SWV) in CH3CN (Table 1), and was shown to correspond to the
HOMO level (Table 2). The Eox of the dyes TA1, TA2, TA3 and TA4 were +1.05,
+1.04, +1.08 and +1.06 V vs NHE, respectively. When the diaryamino substituent was
changed from methyl to methoxyl (compare TA1 with TA2), only a slight negative
shift in the first oxidation peak was observed. The values of Eox for all the dyes were
more positive than the potential of the I-/I3 - redox mediator (~0.4 V vs NHE) when
employed as an electrolyte in DSCs [19-21].
The LUMO levels of these dyes were calculated by Eox+E0-0, where E0-0 is the
optical transition energy of the dyes estimated by the onset value of the absorption
spectrum (Table 2). The E0-0 values of TA1, TA2, TA3 and TA4 were 2.03, 1.91, 1.88
and 1.82 eV, yielding LUMO levels for these dyes of -3.52, -3.63, -3.70 and -3.74 eV,
respectively. The frontier orbital energy levels of the dyes (Figure 3) not only
confirmed the facile electron injection from photo-excited sensitizers to the TiO2
conduction band (Ecb, ca. -0.3 V vs NHE, − 4.2 eV vs vacuum) [21], but also revealed
that azobenzene, used as the π-spacer, caused lower energy level than the conduction
band edge of TiO2. For all of the dyes, the process of electron injection into Ecb of
TiO2 could be completed for oxidization of the dyes.
Density functional theory (DFT) using the hybrid B3LYP function along with the
6-31G(d,p) basis set was employed to investigate the electronic structures of the four
dyes, TA1, TA2, TA3, and TA4. At the HOMO level, electrons are delocalized from
the donor unit to the π-bridge while at the LUMO level, the electron delocalization is
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shown to range from the π-bridge to the acceptor moiety (Figure 3). The trend in the
calculated HOMO-LUMO gap matches the experimental data obtained from the CV
results. The HOMO-LUMO gap decreased with the addition of one more
electron-donating group at the donor unit and/or electron-withdrawing group at the
acceptor unit. For example, in the experiments, the HOMO-LUMO gaps of TA1 and
TA2, (replacing methyl by methoxyl at the donor moiety), were 2.03 and 1.91 eV,
respectively. Those gaps obtained with DFT calculations were 2.41 and 2.32 eV.
Similarly, after replacing the 1,4-phenylene by a 2,5-thienyl unit at the acceptor
moiety, the HOMO-LUMO gaps of TA1 and TA3 were 2.03 and 1.88 eV, respectively,
while, they were 2.41 and 2.32 eV based on the DFT calculations. The trend of shown
by the calculated ionization potentials was also consistent with that of the oxidation
potential obtained from the CV results (Table 2 and Table 3).
The spin density distribution of the cation radicals of the four dyes was also
investigated. As shown in Figure 4, delocalization mainly occurred at the donor
moiety and the calculated spin density values were at the ND and CA positions. The
trend of the total spin density values at these two positions followed that of the
HOMO-LUMO gap, namely that it decreases with the addition of one more
electron-donating group at the donor unit or/and electron-withdrawing group at the
acceptor unit. The spin density values can be calculated to show the qualitative order
of the stability of these four cation radicals was found to be TA4 > TA3 > TA2 > TA1
in air [22].
The J-V curves and IPCE spectra of these DSSCs are shown in Figure 5. The
detailed photovoltaic parameters are summarized in Table 4. By systematically
changing the donor and acceptor unit of these dyes, we can increase the PCE. TA2
had a higher Voc than the other dyes because it possessed a strong electron-donating
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group (-OMe) in the donor unit with phenyl as the anchoring moiety. The IPCE
spectra of the DSCs sensitized using dyes with the phenyl anchoring group were
comparatively shorter and narrower than those of the DSCs sensitized using dyes with
other anchoring groups. The J-V curves show a trend that is consistent with the
stability of the cation radicals of the dyes as predicted by the DFT calculation. The
IPCE for the TA4-sensitized cell reached its maximum of 53% at 470 nm,
significantly higher than that of the TA1 based device (36% at 545 nm). In the
wavelength range from 300 to 600 nm, the TA4-sensitized cell also showed a stronger
IPCE than others, resulting in the highest PCE of 4.78%.
4. Conclusion
In conclusion, a new series of unsymmetrical dyes containing electron-abundant
azobenzene units (π-spacers) between the diarylamine and the terminal cyanoacetic
acid have been studied. With this design we could get a lower LUMO, bathochromic
shifts of absorption spectra (small energy gap) and increase the value of the Jsc and
FF because of the change in the electron-donating group on the diarylamine and the
difference in the aromatic group on the anchoring moiety. The improvement of Jsc
can be attributed to the much broader and red-shift absorption spectra of the dyes with
the thiophene moiety on the anchoring group. The highest Voc value was observed
when using bis(4-methoxyphenyl)amine as the donor unit and phenyl as the anchoring
group. The high PCEs of these dyes constructed with the azobenzene spacer were
investigated. The results present a new option for developing colorful azobenzene
sensitizers for DSC applications.
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Acknowledgement
The authors would like to extend their thanks to the Ministry of Science and
Technology of Taiwan, R.O.C. for financial support of this work (103-2113-M-260
-003 -MY3).
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[22] Definition of stability: the lifetime of cation radical in air.
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NO2
NH2
NO2
NH2
R
Pd/C, N2H4
Cu / K2CO3
+
N
N
18-crown-6 ether
dichlorobenzene
reflux, 18 hr
Ethanol
reflux, 10 hr
I
R
R
R
R
R = CH3
R = CH3
R = CH3
1
2
3
4
OCH3
OCH3
OCH3
NH2
Br
NO
Oxone
Dichloromathane / H2O
3 hr
Br
1-bromo-4-nitrosobenzene
R
R
NH2
NO
Acetic Acid
Ethanol
N
N
N
+
Br
N
Br
reflux , 48 hr
R = CH3
OCH3
R
R
5
6
R
R
Pd(PPh3)4 / Na2CO3
HO
HO
Cyanocarboxylic acid
N
N
N
N
N
N
TA1-TA4
+
B
Br
Toluene / H2O
reflux, 24 hr
piperidine / THF
reflux, 24 hr
O
O
R
R
=
5a,
5b,
6a
6b
S
Scheme 1. Synthetic route of the dyes.
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O
N
N
N
N
N
N
COOH
COOH
NC
NC
O
TA2
TA1
O
N
N
N
N
S
S
N
N
COOH
COOH
NC
NC
O
TA3
TA4
Figure 1. Chemical structures of the azobenzene-bridged materials used in this study.
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0.75
0.50
0.25
0.00
TA1
TA2
TA3
TA4
300 350 400 450 500 550 600 650 700
Wavelength (nm)
Figure 2. Absorption spectra of the dyes (TA1-TA4) in CH3CN.
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Figure 3. Calculated frontier molecular orbitals and experimental energy level
diagram of TA1-TA4 measured in CH3CN as well as the conduction band of TiO2 and
the redox potential of I-/I3-.
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Figure 4. DFT-calculated spin density distribution of (a) TA1+., (b) TA2+., (c) TA3+.,
and (d) TA4+.. Isovalue: 0.004
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(A)
12
10
8
TA1
TA2
TA3
TA4
6
4
2
0
0.0
0.2
0.4
0.6
0.8
Voltage (V)
(B)
60
50
40
30
20
10
0
TA1
TA2
TA3
TA4
300
400
500
600
700
800
Wavelength (nm)
Figure 5. (A) J-V curves and (B) IPCE spectra of DSCs sensitized by the dyes
TA1-TA4 with electrolytes containing 0.6M BMII, 0.1M LiI, 0.05 M I2, 0.1 M
GuNCS, 0.5 M TBP in dried THF.
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Table 1. Peak potentials (V vs. Ag/AgCl) of TA1-TA4 (determined by square wave
voltammetry) in CH2Cl2/TBAPF6.
Oxidation
Reduction
1st
2nd
1st
+1.02
+1.40
+1.04
+1.09
+0.85
+0.84
+0.88
+0.86
-1.66
-1.28
TA1
TA2
TA3
TA4
-1.14
could not observed
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Table 2. Optical and electrochemical properties of the dyes
1st E1/2
1st E1/2
HOMO
(eV)b
-5.55
-5.54
-5.58
-5.56
λonset
(nm)
610
E0-0
(eV)c
2.03
1.91
1.88
1.82
LUMO
(eV)d
-3.52
-3.63
-3.70
-3.74
Dye
(V vs. Ag/AgCl) (V vs. NHE)
+0.85
+0.84
+0.88
+0.86
+1.05
+1.04
+1.08
+1.06
TA1
TA2
TA3
TA4
650
660
680
a. FeCp2 +/0 = +0.49 V in CH2Cl2/TBAPF6.
b. The HOMO energy level was calculated from the half-wave potential during
oxidation according to the empirical formula: EHOMO = -(E + 4.5) V by assuming
the energy level of ferrocene to be - 4.5 eV below vacuum level. (E (V) vs. NHE)
c. The gap energy level was calculated from the onset wavelength of the UV-Vis
spectrum according to the empirical formula: Egap = 1240/λonset
d. LUMO = HOMO + gap
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Table 3. Calculated ionization potentials (IP, eV) and HOMO-LUMO gaps (eV).
Index
IP
HOM LUMO HOMO-LUMO gap
(eV) O (eV)
(eV)
-2.68
-2.63
-2.83
-2.77
(eV)
2.41
2.32
2.32
2.23
TA1
TA2
TA3
TA4
6.13
5.93
6.13
5.95
-5.09
-4.94
-5.15
-5.00
Calculated in the gas phase. B3LYP/6-31G(d,p)
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Table 4. Photovoltaic Performance Data of TA Dyes.
Jsc
(mA/cm2)
5.40
Voc
(V)
Dye
FF
PCE (%)
0.662
0.704
0.654
0.662
0.715
0.718
0.723
0.728
2.56
3.45
3.69
4.78
TA1
TA2
TA3
TA4
6.83
7.81
9.92
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Highlights
ꢀ
ꢀ
A series of novel azobenzene-based dyes were synthesized.
The stability order of these four dyes cation radicals was found to be TA4 > TA3
> TA2 > TA1 in air.
ꢀ
The new record on PCE with using azobenzene as the π-spacer unit was
achieved.