Phenothiazine derivatives-based D-π-A and D-A-π-A organic dyes for dye-sensitized solar cells

Article abstract of DOI:10.1039/c4ra01858c

D-π-A and D-A-π-A-structured organic dyes represent different developmental directions of photosensitizers in dye-sensitized solar cells (DSSCs). In this work, two phenothiazine derivatives-based D-π-A and D-A-π-A-structured organic dyes have been synthesized and applied in DSSCs. The physical and electrochemical properties of both dyes have been investigated systematically. The results show that the D-A-π-A-structured dye exhibits a broader spectrum response but lower molar coefficient of extinction when compared to the D-π-A-structured dye. Regarding photovoltaic performance, the D-π-A-structured dye yields a higher efficiency (η) of 7.5% with a higher short-circuit current density (J_sc) of 16.36 mA cm ^-2 and open-circuit voltage (V_oc) of 706 mV than that of the D-A-π-A-structured dye. Incident photon-to-electron conversion efficiency (IPCE) studies and impedance analysis also support these results. These results demonstrate that the phenothiazine derivatives-based D-π-A-structured organic dyes can compete with, and even exceed, D-A-π-A-structured organic dyes under the same test conditions. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c4ra01858c

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PAPER
Phenothiazine derivatives-based D–p–A and D–A–
p–A organic dyes for dye-sensitized solar cells†
Cite this: RSC Adv., 2014, 4, 24377
a
Xichuan Yang, Jianghua Zhao,a Lei Wang,a Jie Tiana and Licheng Sunab
*
D–p–A and D–A–p–A-structured organic dyes represent different developmental directions of
photosensitizers in dye-sensitized solar cells (DSSCs). In this work, two phenothiazine derivatives-based
D–p–A and D–A–p–A-structured organic dyes have been synthesized and applied in DSSCs. The
physical and electrochemical properties of both dyes have been investigated systematically. The results
show that the D–A–p–A-structured dye exhibits a broader spectrum response but lower molar
coefficient of extinction when compared to the D–p–A-structured dye. Regarding photovoltaic
performance, the D–p–A-structured dye yields a higher efficiency (h) of 7.5% with a higher short-circuit
current density (Jsc) of 16.36 mA cmꢀ2 and open-circuit voltage (Voc) of 706 mV than that of the D–A–
p–A-structured dye. Incident photon-to-electron conversion efficiency (IPCE) studies and impedance
analysis also support these results. These results demonstrate that the phenothiazine derivatives-based
D–p–A-structured organic dyes can compete with, and even exceed, D–A–p–A-structured organic
dyes under the same test conditions.
Received 3rd March 2014
Accepted 12th May 2014
DOI: 10.1039/c4ra01858c
www.rsc.org/advances
between the electron donor and the p-bridge, which could
effectively decrease DEg. Zhu and co-workers reported a highly
Introduction
efficient D–A–p–A-structured organic dye in 2012, which
produced an efficiency of 9.0%.6 D–p–A- and D–A–p–A-struc-
tured dyes represent two different developmental directions in
DSSCs. However, as we know, there are few reports on the
systematic study of both D–p–A- and D–A–p–A-structured dyes
with similar structures until now. By investigating the differ-
ence in the physical and electrochemical properties and
photovoltaic performance of both dye structures, the advan-
tages and disadvantages of both dye structures will pave the
way for further molecular design.
Triphenylamine and its derivatives7–12 have been successfully
used as electron donors in the construction of sensitizers.
However, the rotation of the triphenylamine ring decreases the
electron-donating ability of the electron donor. Indoline13,14 and
tetrahydroquinoline15 are stronger electron donors with more
planar spatial structures when compared to triphenylamine. In
addition, phenothiazine16–18 is also a strong electron donor,
which has been successfully employed as electron donor and p-
conjugated bridge in D–p–A-structured dyes. However, as we
know, there are no reports of introducing phenothiazine as
electron donor into a D–A–p–A structure until now. Thiophene
and its derivatives19–21 always serve as a p-conjugated bridge in
organic dyes. Recently, Wang et al. have reported a series of D–
A–p–A-structured organic dyes with thiophene and its deriva-
tives as p-conjugated bridge.22 The results indicate that the
introduction of a long alkyl chain in a p-conjugated bridge is
not benecial for the improvement of the efficiency of dyes.
In order to diminish this effect, thiophene is considered to be a
¨
DSSCs have drawn increasing attention since Gratzel and
O'Regan reported them in 1991.1 Research on novel highly
efficient dyes has never been suspended. Compared to metal
complexes, metal-free organic dyes have many advantages such
as lower cost, environmental friendliness, higher molar
extinction coefficient and easier molecular design. The donor–
p-conjugated bridge–acceptor (D–p–A) structure is a classical
molecular conguration, which has helped in the designing of
many highly efficient organic dyes. However, the absorption
spectra of D–p–A-structured organic dyes are narrow, which is
not benecial for higher short-circuit current density. In order
to broaden the absorption band, one strategy is to decrease the
energy band (DEg) of the dyes by employing a stronger electron
donor or electron acceptor. Recently, a new kind of molecular
conguration named the donor–acceptor–p bridge–acceptor
(D–A–p–A) structure has been reported.2 For D–A–p–A-struc-
tured organic dyes, electron-withdrawing units, such as
benzothiadiazole,3 benzotriazole,4 and quinoxaline,5 are
generally embedded in the molecule of sensitizers located
aState Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research
Centre on Molecular Devices, Dalian University of Technology(DUT), 2 Linggong Rd,
116024 Dalian, China. E-mail: yangxc@dlut.edu.cn; Fax: +86 411 84986250; Tel:
+86 411 84986247
bKTH Royal Institute of Technology, School of Chemical Science and Engineering,
Department of Chemistry, Teknikringen 30, 10044 Stockholm, Sweden. E-mail:
lichengs@kth.se; Fax: +46 8 791 2333; Tel: +46 8 790 8127
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra01858c
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Fig. 1 Chemical structures of dyes JH305 and JH307.
p-conjugated bridge. As an electron acceptor, cyanoacrylic acid
is still a valid candidate for general organic dyes.
Here, we report two phenothiazine organic dyes based on the
D–p–A structure and D–A–p–A structure, coded JH305 and
JH307, respectively. As shown in Fig. 1, for both the dyes, a
phenothiazine derivative serves as an electron donor. Tradi-
tional cyanoacrylic acid is employed as an electron acceptor to
maintain the similar conjugation level, the only difference in
both the dyes is that the thiophene ring adjacent to the electron
donor in dye JH305 is replaced by benzothiadiazole in dye
JH307. The physical and electrochemical properties and the
photovoltaic performance of both dyes are investigated
systematically.
Scheme
1
Synthetic routes of JH305 and JH307. (a) K2CO3,
Pd(PPh3)4, THF–H2O, reflux, 12 h; (b) NBS, THF, 0 ꢁC, 2 h; (c) n-BuLi,
THF, ꢀ78 ꢁC, pinacolato boronate, 2 h; (d) Br2, HBr (40%), 90 ꢁC, 12 h;
(e) POCl3, DMF, CHCl3, 60 ꢁC; (f) cyanoacetic acid, pyperidine, THF,
reflux, 2 h.
(3.83 g, 14.4 mmol), K2CO3 (2.48 g, 18 mmol), and Pd(PPh3)4 (60
mg) were dissolved in THF–H2O (5 : 1, v/v) under N2. The
mixture was reuxed for 12 h. When the reaction was cooled to
room temperature, water (40 ml) was added and the mixture
was extracted by CH2Cl2. The crude product was puried by
petroleum–CH2Cl2 (8 : 1) to give compound 2 (2.33 g, 42%) as a
colourless oil. 1H NMR (400 MHz, CDCl3) d 7.43 (s, 1H), 7.28 (d, J
¼ 4.1 Hz, 1H), 7.09 (dt, J ¼ 13.3, 5.5 Hz, 3H), 6.82 (d, J ¼ 8.5 Hz,
3H), 6.50 (s, 1H), 4.01 (t, J ¼ 6.4 Hz, 3H), 3.88 (t, J ¼ 15.3, 8.9 Hz,
3H), 1.84–1.73 (m, 4H), 1.42 (tt, J ¼ 13.5, 6.9 Hz, 6H), 0.91 (dd, J
¼ 17.3, 10.1 Hz, 9H). [M]+ calcd for C30H37NO2S, 475.2545;
found, 475.2544.
Experimental section
Analytical measurements
The absorption spectra were recorded on HP8453 (USA). Elec-
trochemical redox potentials were obtained by cyclic voltam-
metry (CV) on an electrochemistry workstation (BAS100B, USA).
A traditional three-electrode system was used. The working
electrode, auxiliary electrode and reference electrode are a
glassy carbon disk electrode, Pt wire and Ag/Ag+ electrode,
respectively. The photocurrent–voltage (J–V) properties were
measured under AM 1.5G illumination (16S-002, Solar Light Co.
Ltd., USA). The incident light intensity was 100 mW cmꢀ2
,
3-Bromo-10-butyl-7-(2,4-dibutoxyphenyl)-10H-phenothiazine
(3). Compound 2 (2.2 g, 4.63 mmol) was dissolved in THF
solution at 0 ꢁC. NBS (824 mg, 4.63 mmol) was added slowly for
2 h. Water (30 ml) was added when the reaction was nished,
and the mixture was extracted by CH2Cl2. The organic layer was
collected and concentrated by rotary evaporation. The crude
product was puried by petroleum–CH2Cl2 (8 : 1) to give
compound 3 (2.28 g, 89%) as a colourless oil. 1H NMR (400
MHz, CDCl3) d 7.42 (s, 1H), 7.26 (dd, J ¼ 6.3, 3.0 Hz, 2H), 7.24 (d,
J ¼ 2.8 Hz, 1H), 7.20 (d, J ¼ 2.3 Hz, 1H), 6.84 (d, J ¼ 8.7 Hz, 1H),
6.69 (d, J ¼ 8.2 Hz, 1H), 6.51 (d, J ¼ 8.1 Hz, 1H), 4.01 (t, J ¼ 24.8,
6.4 Hz, 3H), 3.91 (t, J ¼ 15.7, 9.1 Hz, 3H), 1.90–1.73 (m, 6H), 1.44
(m, J ¼ 14.6, 7.3, 4.4 Hz, 6H), 0.97–0.91 (m, 9H). [M]+ calcd for
C30H36NO2SBr, 553.1650; found, 553.2.
which was calibrated with a standard Si solar cell. The working
areas of the cells were masked to be 0.1256 cm2. The J–V data
were collected by an electrochemical workstation (LK9805,
Lanlike Co. Ltd., China). The measurement of the IPCE was
obtained by a Hypermono-light (SM-25, Jasco Co. Ltd., Japan).
Synthesis of JH305 and JH307
Scheme 1 shows the synthetic routes of JH305 and JH307. The
organic solvents used in these reactions were freshly distilled
according to standard procedures. The commercial reagents
were directly used without further purication. Compound 1
and (2,4-dibutoxyphenyl)boronic acid were synthesized
according to a previously reported method by our group.23,24
Almost all the structures were identied by 1H NMR and MS
except for pinacolato boronate, which was identied by MS.
10-Butyl-3-(2,4-dibutoxyphenyl)-7-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-10H-phenothiazine (4). Compound 3 (2.1 g,
3.8 mmol) was dissolved in dry THF under N2. Aer the addition
10-Butyl-3-(2,4-dibutoxyphenyl)-10H-phenothiazine
(2).
ꢁ
of n-BuLi (1.9 ml, 4.6 mmol) at ꢀ78 C, the resulting mixture
Compound 1 (4 g, 12 mmol), (2,4-dibutoxyphenyl)boronic acid
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was stirred for 2 h at ꢀ78 C. Aer isopropyl pinacolato boro- reaction nished, 1 M NaOH solution was added slowly to deal
nate (0.85 ml, 4.2 mmol) was added, the mixture was heated to with excess POCl3. Then, water was added to the mixture and
room temperature and maintained for 12 h. The mixture was the organic phase was concentrated by rotary evaporation. The
then poured into water and extracted by CH2Cl2. The crude crude product was puried by CH2Cl2 to give compound 9 (262
product was puried by petroleum–CH2Cl2 (4 : 1) to give mg, 12%) as an orange solid. 1H NMR (400 MHz, CDCl3) d 8.04
compound 4 (1.32 g, 58%). [M]+ calcd for C36H48BNO4S, (dt, J ¼ 3.7, 1.7 Hz, 1H), 7.82–7.74 (m, 1H), 7.67–7.61 (m, 1H),
601.3405; found, 601.3397.
7.48–7.41 (m, 1H), 7.17 (ddd, J ¼ 6.2, 5.1, 3.7 Hz, 1H). [M]+ calcd
[2,20-Bithiophene]-5-carbaldehyde (5). 5-Bromothiophene-2- for C11H5BrN2OS2, 323.9027; found, 323.9027.
carbaldehyde (1.5 g, 7.9 mmol), thiophen-2-ylboronic acid (1.23
50-(10-Butyl-7-(2,4-dibutoxyphenyl)-10H-phenothiazin-3-yl)-
g, 9.48 mmol), K2CO3 (1.64 g, 11.85 mmol), and Pd(PPh3)4 (30 [2,20-bithiophene]-5-carbaldehyde (10). Compound 6 (150 mg,
mg) were dissolved in THF–H2O (5 : 1, v/v) under N2. The 0.55 mmol), compound 4 (400 mg, 0.67 mmol), K2CO3 (114 mg,
mixture was reuxed for 12 h. When the reaction was nished, 0.86 mmol), and Pd(PPh3)4 (10 mg) were dissolved in THF–H2O
water (30 ml) was added and the mixture was extracted by (5 : 1, v/v) under N2 and the mixture was reuxed for 12 h. When
CH2Cl2. The crude product was puried by petroleum–CH2Cl2 the reaction nished, water (20 ml) was added and the mixture
(1 : 2) to give compound 5 (1 g, 67%) as a yellow solid. 1H NMR was extracted by CH2Cl2. The crude product was puried by
(400 MHz, CDCl3) d 9.85 (s, 1H), 7.66 (d, J ¼ 3.9 Hz, 1H), 7.39– petroleum–CH2Cl2 (1 : 3, v/v) to give compound 10 (286 mg,
1
7.30 (m, 2H), 7.24 (d, J ¼ 3.9 Hz, 1H), 7.06 (dd, J ¼ 4.9, 3.8 Hz, 64%) as an orange solid. H NMR (400 MHz, CDCl3) d 9.81 (s,
1H). [M]+ calcd for C9H6OS2, 193.9860; found, 193.9865.
1H), 7.64–7.57 (d, 1H), 7.37–7.28 (m, 3H), 7.27–7.22 (m, 2H),
50-Bromo-[2,20-bithiophene]-5-carbaldehyde (6). Compound 7.21–7.14 (m, 2H), 7.11 (d, J ¼ 6.6 Hz, 1H), 6.85 (d, J ¼ 7.8 Hz,
ꢁ
5 (950 mg, 4.92 mmol) was dissolved in THF solution at 0 C. 2H), 6.54–6.47 (s, 2H), 4.09–3.71 (m, 6H), 1.83–1.72 (m, 4H),
NBS (875 mg, 4.92 mmol) was added slowly several times. When 1.54–1.40 (m, 6H), 1.02–0.88 (m, 9H). [M]+ calcd for
the reaction nished, water (30 ml) was added and the mixture
C
39H41NO3S3, 667.2249; found, 667.2263.
was extracted by CH2Cl2. The organic layer was then collected
10-Butyl-3-(2,4-dibutoxyphenyl)-7-(7-(thiophen-2-yl)benzo[c]-
and concentrated by rotary evaporation. The crude product was [1,2,5]thiadiazol-4-yl)-10H-phenothiazine (11). Compound 10
puried by petroleum–CH2Cl2 (1 : 2, v/v) to give compound 6 (150 mg, 0.46 mmol), compound 4 (331 mg, 0.67 mmol), K2CO3
(986 mg, 74%) as yellow solid. 1H NMR (400 MHz, CDCl3) d 9.85 (95 mg, 0.69 mmol), and Pd(PPh3)4 (10 mg) were dissolved in
(d, J ¼ 16.9 Hz, 1H), 7.66 (d, J ¼ 4.0 Hz, 1H), 7.18 (d, J ¼ 3.9 Hz, THF–H2O (5 : 1, v/v) under N2 and the mixture was reuxed for
1H), 7.11 (d, J ¼ 3.9 Hz, 1H), 7.04 (d, J ¼ 3.9 Hz, 1H). [M]+ calcd 12 h. When the reaction nished, water (20 ml) was added and
for C9H5BrOS2, 271.8965; found, 271.8974.
the mixture was extracted by CH2Cl2. The crude product was
4,7-Dibromobenzo[c][1,2,5]thiadiazole
(7).
Compound puried by petroleum–CH2Cl2 (1 : 2) to give compound 11 (259
1
benzo[c][1,2,5]thiadiazole (10 g, 73.6 mmol) was dissolved in mg, 56%) as an orange-red solid. H NMR (400 MHz, CDCl3) d
250 ml HBr (40%) solution at room temperature. Br2 (11 ml, 220 9.98 (d, J ¼ 1.7 Hz, 1H), 8.22 (dd, J ¼ 6.8, 4.0 Hz, 1H), 8.10–7.99
mmol) was added slowly, and then the mixture was heated to 90 (m, 1H), 7.91–7.81 (m, 2H), 7.77 (dd, J ¼ 9.0, 4.3 Hz, 1H), 7.71
ꢁC for 12 h. When the reaction nished, Na2S2O3 was added to (dt, J ¼ 14.4, 4.4 Hz, 1H), 7.55 (s, 1H), 7.35 (d, J ¼ 2.0 Hz, 1H),
deal with the excess Br2. CH2Cl2 was then added to the mixture 7.08–6.98 (m, 1H), 6.89 (dd, J ¼ 11.4, 6.6 Hz, 2H), 6.68 (d, J ¼ 7.4
and the organic phase was concentrated by rotary evaporation. Hz, 1H), 6.54–6.43 (m, 1H), 4.02–3.93 (m, 6H), 1.58–1.49 (m,
The crude product was puried by petroleum–CH2Cl2 (4 : 1, v/v) 6H), 1.33–1.24 (m, 6H), 1.05–0.94 (m, 9H). [M]+ calcd for
to give compound 7 (10.28 g, 48%) as a white solid. 1H NMR (400
MHz, CDCl3) d 9.82 (d, J ¼ 1.9 Hz, 1H), 7.57 (d, J ¼ 3.8 Hz, 1H),
C
41H41N3O3S3, 719.2310; found, 719.2318.
(E)-3-(50-(10-Butyl-7-(2,4-dibutoxyphenyl)-10H-phenothiazin-
6.97–6.88 (m, 1H), 2.61–2.51 (m, 4H), 1.74–1.51 (m, 4H), 1.34– 3-yl)-[2,20-bithiophen]-5-yl)-2-cyanoacrylic
acid
(JH305).
1.28 (m, 12H), 0.93–0.85 (m, 9H). [M]+ calcd for C6H2N2SBr2, Compound 10 (200 mg, 0.3 mmol) and cyanoacetic acid (31 mg,
291.8305; found, 291.8314.
0.36 mmol) were dissolved in THF solution and reuxed for 12
(8). h. Water was added to the mixture, and the mixture was
4-Bromo-7-(thiophen-2-yl)benzo[c][1,2,5]thiadiazole
Compound 7 (4 g, 13.8 mmol), thiophen-2-ylboronic acid (2.49 extracted by CH2Cl2. The solvent was dried by evaporation and
g, 20.6 mmol), K2CO3 (2.85 g, 20.6 mmol), and Pd(PPh3)4 (70 the crude product was puried by column purication with
mg) were dissolved in THF–H2O (5 : 1, v/v) under N2. The CH2Cl2–CH3OH (1 : 8, v/v) to give red solid JH305 (158 mg,
mixture was reuxed for 12 h. When the reaction nished, water 72%).1H NMR (400 MHz, DMSO) d 8.23–8.18 (m, 1H), 7.98–7.86
(30 ml) was added and the mixture was extracted by CH2Cl2. The (m, 1H), 7.61–7.53 (m, 2H), 7.32 (d, J ¼ 14.2 Hz, 2H), 7.20 (d, J ¼
crude product was puried by petroleum–CH2Cl2 (2 : 1, v/v) to 8.3 Hz, 2H), 7.06 (d, J ¼ 12.2 Hz, 1H), 6.78 (d, J ¼ 8.7 Hz, 2H),
1
give compound 8 (2.11 g, 52%) as a yellow solid. H NMR (400 6.66 (s, 2H), 4.11–3.83 (m, 6H), 1.81–1.68 (m, 4H), 1.46 (m, 7.4
MHz, CDCl3) d 9.99 (s, 1H), 8.18 (d, 1H), 7.92 (d, 1H), 7.86–7.84 Hz, 6H), 1.00–0.83 (m, 9H). [M]+ calcd for C42H42N2O4S3,
(m, 2H). [M]+ calcd for C10H5N2S2Br, 295.9078; found, 295.9081. 734.2307; found, 734.2319.
5-(7-Bromobenzo[c][1,2,5]thiadiazol-4-yl)thiophene-2-carbal-
(E)-3-(5-(7-(10-Butyl-7-(2,4-dibutoxyphenyl)-10H-phenothia-
dehyde (9). Compound 8 (2 g, 6.78 mmol) and DMF (0.99 g, 1.36 zin-3-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)-2-cyanoa-
mmol) were dissolved in CHCl3 (40 ml) solution, and then crylic acid (JH307). Compound 11 (200 mg, 0.29 mmol) and
POCl3 (0.93 ml, 10.17 mmol) was added at room temperature. cyanoacetic acid (30 mg, 0.35 mmol) were dissolved in THF
The mixture was then heated to 60 ꢁC for 12 h. When the solution and reuxed for 12 h. Water was added to the mixture,
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and the mixture was extracted by CH2Cl2. The solvent was dried
by evaporation and the crude product was puried by column
purication with silica gel CH2Cl2–CH3OH (1 : 30) to give red
solid JH307 (175 mg, 77%).1H NMR (400 MHz, DMSO) d 8.14 (s,
1H), 7.71 (d, J ¼ 5.8 Hz, 1H), 7.55–7.44 (m, 4H), 7.42 (s, 1H),
7.32–7.27 (m, 2H), 7.20 (d, J ¼ 8.3 Hz, 1H), 7.04 (dd, J ¼ 8.0, 5.5
Hz, 2H), 6.63–6.52 (s, 1H), 3.98 (dd, J ¼ 14.9, 6.5 Hz, 3H), 3.90 (d,
J ¼ 7.1 Hz, 2H), 1.75–1.65 (m, 4H), 1.43 (td, J ¼ 14.5, 7.3 Hz, 6H),
0.93 (m, 9H). [M]+ calcd for C44H42N4O4S3, 786.2368; found,
786.2359.
Preparation of the DSSCs
The DSSCs sensitized by JH305 and JH307 were fabricated by
modifying those previously reported.25 A layer of 2 mm TiO2
(13 nm paste, Heptachroma, China) was coated onto the
F-doped tin oxide conducting glass (TEC15, 15 U per square,
Pilkington, USA) by screen printing, and then dried for 5 min at
120 ꢁC. This procedure was repeated 5 times (10 mm) and a nal
coating of 4 mm of TiO2 paste (DHS-SLP1, Heptachroma, China)
as the scattering layer was applied. The double-layer TiO2 elec-
trodes (area: 6 ꢂ 6 mm) were sintered under an air ow at 500
ꢁC for 60 min, and then cooled to room temperature. The sin-
tered lm was further treated with 40 mM TiCl4 aqueous solu-
tion at 70 ꢁC for 30 min, washed with water, and then sintered at
ꢁ
500 C for 60 min. Aer the lm was cooled to room tempera-
ture, it was immersed into a 2 ꢂ 10ꢀ4 M dye bath for 12 h. The
electrode was then rinsed with ethanol and dried. Hermetically
sealed cells were fabricated by assembling the dye-loaded lm
as the working electrode and Pt-coated conducting glass as the
counter electrode separated by a Surlyn 1702 lm (60 mm,
Dupont).
Fig. 2 Absorption spectra of JH305 and JH307 in CH2Cl2 solution (a)
and on TiO2 film (b).
carboxylate-titania nanocrystal structure, which decreases the
intensity of the electron withdrawing ability of an electron
Results and Discussion
The UV-visible absorption spectra of JH305 and JH307 are acceptor. It can be noted that D–A–p–A-structured organic dye
shown in Fig. 2, and the corresponding data are listed in Table JH307 shows a widened spectrum response when compared to
1. As displayed in Fig. 2(a), D–p–A-structured organic dye JH305 D–p–A-structured organic dye JH305.
and D–A–p–A-structured organic dye JH307 show two main
The electrochemical properties of JH305 and JH307 on TiO2
absorption bands. In the higher energy region, the absorption lm have been studied by cyclic voltammetry, and the corre-
band for JH305 at 362 nm and JH307 at 410 nm is assigned to sponding data are given in Table 1. The highest occupied
the p / p* transition. The distinct absorption band at 457 nm molecular orbitals (HꢀOMO) of JH305 and JH307 are more
for JH305 and 498 nm for JH307 relates to the S0 / S1 transi- positive than the Iꢀ/I3 shuttle,27 indicating that the oxidized
tion. It can be noted that JH307 exhibits a red shi of 41 nm dyes can regenerate effectively. Meanwhile, the lowest unoccu-
compared with that of JH305. This phenomenon is probably pied molecular orbitals (LUMO) of both the structured dyes are
due to the introduction of benzothiadiazole, which exhibits a more negative than the conduction band (Ecb) of TiO2,28 which
strong electron-withdrawing ability. Furthermore, it can be ensures electron injection thermodynamically. Because of the
found that JH305 presents a higher molar extinction coefficient introduction of benzothiadiazole, D–A–p–A-structured organic
(33 542 Mꢀ1 cmꢀ1) compared with JH307 (16 256 Mꢀ1 cmꢀ1), dye JH307 exhibits positive LUMO levels in comparison with D–
indicating that D–p–A-structured organic dye JH305 shows a p–A-structured organic dye JH305. It can be noted that the
stronger ability to capture photons than that of D–A–p–A- introduction of benzothiadiazole has a negligible effect on the
structured organic dye JH307.
HOMO levels of the dye.
Fig. 2(b) shows the normalized absorption spectra of both
The optimized ground state geometries of JH307 and JH305
dyes on TiO2 lm. When anchored on TiO2 lm, a blue shi of were obtained in the gas phase using density functional theory
19 nm and 33 nm has been observed for dye JH305 and dye (DFT) calculation at B3LYP/6-31G level. The results are shown in
JH307, respectively. This kind of blue shi results from H-type Table 2. Excited energies of both the dyes were calculated by TD-
aggregation.26 This is mainly caused by the formation of the DFT at the CAMB3LYP/6-31G levels of theory with CPCM mode
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Table 1 Absorption and electrochemical data of JH305 and JH307
3 at lmax (Mꢀ1 cmꢀ1
)
lmaxb on lm (nm)
E0–0 (V)
EHOMOd (V) (vs. NHE)
ELUMO (V) (vs. NHE)
c
Dye
lmaxa (nm)
JH305
JH307
457
498
33500
16200
438
465
2.28
2.01
0.61
0.62
ꢀ1.67
ꢀ1.39
a
Absorption spectra in solution were measured in CH2Cl2 solution (2 ꢂ 10ꢀ5 M). b Absorption spectra on TiO2 lm were measured with dye-loaded
TiO2 lm immersed in CH2Cl2 solution (2 ꢂ 10ꢀ5 M). c E0–0 was determined from the intersection of the tangent of absorption on TiO2 lm and the
X axis by 1240/l. d The oxidation potentials of the dyes were measured on TiO2 lm with TBAPF6 (0.1 M) as an electrolyte, ferrocene/ferrocenium (Fc/
Fc+) as an internal reference and converted to NHE by the addition of 440 mV.
Table 2 Optimized Structures of Electron Distribution in HOMO and LUMO Levels of JH305 and JH307
Dye
Optimized structure
HOMO
LUMO
JH305
JH307
Table 3 Photovoltaic performancea of DSSCs based on JH305 and
JH307
Dyeb
Jsc (mA cmꢀ2
)
Voc (mV)
FF (%)
h (%)
JH305
JH307
16.36
14.71
706
645
64.5
67.3
7.5
6.4
a
Irradiation light: AM 1.5 simulated solar light (100 mW cmꢀ2) at room
temperature; working area: 0.1256 cm2, electrolyte: 0.6 M DMPII, 0.06 M
b
LiI, 0.4 M TBP, 0.02 M I2 in dry acetonitrile. Dye bath 2 ꢂ 10ꢀ4 M in
CH2Cl2.
in CH2Cl2 solution, and the corresponding data are listed in
Table SI (see ESI†). As shown in Table 3, for JH305 and JH307,
effective electron separation can be observed on HOMO and
LUMO levels, respectively. Meanwhile, the electron distribution
is similar for both the dyes on HOMO levels. However, an
Fig. 3 J–V curves of the devices sensitized by JH305 and JH307.
obvious difference can be observed on LUMO levels. Compared of 16.36 mA cmꢀ2, a Voc of 706 mV and an FF of 64.5%. In
to dye JH305, dye JH307 has more electron distribution on the contrast, the D–A–p–A-structured organic dye JH307-based
benzothiadiazole unit rather than the cyanoacrylic group due to device produces a lower h of 6.4% with a lower Jsc of 14.71 mA
the strong withdrawing ability of the benzothiadiazole unit, cmꢀ2, a lower Voc of 645 mV but a slightly higher FF of 67.3%.
which does not facilitate electron injection.
The reason for JH307 exhibiting a lower Jsc in comparison with
The current density–voltage behaviour of DSSCs based on JH305 is partially due to the poor light harvesting efficiency.
JH305 and JH307 was monitored under the standard AM 1.5G Meanwhile, DFT calculation indicates that JH307 has more
illumination and the curves are given in Fig. 3. The detailed electron distribution on the benzothiadiazole unit rather than
data are tabulated in Table 3. It can be noted that the D–p–A- the cyanoacrylic acid group in LUMO levels, which hinders
structured organic dye JH305-based device yields a photon-to- effective electron injection. This can be considered as another
electron conversion efficiency (h) of 7.5%, corresponding to a Jsc support for the lower Jsc of JH307.
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The incident photon-to-electron conversion efficiency (IPCE) transfer resistance on the counter electrode/electrolyte (Rce)
spectra of the JH305 and JH307-based devices are displayed in interface, corresponding to the rst small semicircle. For the
Fig. 4. The result indicates that the JH305-based device exhibits similar Pt counter electrode and electrolyte, a similar Rce for
a higher IPCE spectrum response than the JH307-based device both JH305 and JH307-based devices can be obtained. The
in the range of 400–600 nm. This trend can be explained by the larger semicircle in the mid-frequency region (104 to 105 Hz)
higher molar extinction coefficient and greater electron injec- reects the recombination resistance (Rct) on the TiO2/dye/
tion dynamics for dye JH305 than that of JH307 in this spectral electrolyte surface. It can be observed that the JH305-based
region. However, dye JH307-based device gives a higher IPCE device exhibits a larger Rct (91.8 U cmꢀ2) compared with the
response at 640 nm and it can even reach up to 900 nm, which is JH307-based device with a Rct of 19.6 U cmꢀ2, indicating that
probably due to the broadened absorption spectrum of dye the JH305-based device can suppress recombination on the
JH307 in distant wavelength regions. These results indicate that TiO2/dye/electrolyte interface more effectively than the JH307-
although D–A–p–A-structured organic dye JH307 shows a based device. This phenomenon can be explained by the
widened absorption and IPCE response, unfortunately, these introduction of the benzothiadiazole unit, which aggravates
advantages do not facilitate an improvement in the Jsc and Voc recombination with electrons in TiO2 due to strong electron-
when compared to D–p–A-structured organic dye JH305.
withdrawing ability. Regarding J–V parameters, the JH305-based
Electrochemical impedance spectroscopy (EIS) analysis29 was device produces a higher Voc than that of the JH307-based
performed to investigate the interface charge transfer process of device.
the DSSCs based on JH305 and JH307. EIS spectra of solar cells
sensitized by both the dyes were measured with an impedance/
gain-phase analyzer (PARSTAT 2273, USA) under dark condi-
Conclusions
tions with a bias of ꢀ0.7 V. The alternating current (AC) D–p–A-structured organic dye JH305 and D–A–p–A-structured
amplitude was set at 10 mV. As shown in Fig. 5, Nyquist plots in organic dye JH307 were synthesized and investigated system-
the high-frequency (105 to 106 Hz) region indicate an electron atically. The results indicate that the phenothiazine-based D–A–
p–A-structured organic dye shows a wider spectrum response
but a lower molar extinction coefficient in comparison with the
D–p–A-structured organic dye with a similar structure. When
applied in DSSCs, the D–p–A-structured organic dye yields a
higher Jsc, Voc, and h than the D–A–p–A-structured organic dye.
IPCE study indicates that the D–p–A-structured organic dye
exhibits a higher IPCE response in certain regions, which
provides a support for the higher Jsc. The recombination
behaviour on a TiO2/dye/electrolyte interface explains the
higher Voc of D–p–A-structured dyes than that of D–A–p–A-
structured organic dyes. In summary, this work reveals that not
all D–A–p–A-structured organic dyes exhibit a better photovol-
taic performance than that of D–p–A-structured dyes. We can
conclude that phenothiazine is more suitable for D–p–A dyes,
while indoline and triphenylamine, which have been reported
with similar structures, are more suitable for D–A–p–A dyes,
which will pave the way for further molecular design of highly
efficient organic dyes.
Fig. 4 IPCE spectra of the DSSCs based on JH305 and JH307.
Acknowledgements
We gratefully acknowledge the nancial support for this work
from China Natural Science Foundation (Grant 21276044,
21120102036, 91233201), the National Basic Research Program
of China (Grant no. 2014CB239402).
Notes and references
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