Pyrazine-incorporating panchromatic sensitizers for dye sensitized solar cells under one sun and dim light

Article abstract of DOI:10.1039/c8ta04774j

New D-A′-π-A type (D = electron donor, A′ = auxiliary acceptor, π = π-conjugated bridge, and A = electron acceptor/anchor) sensitizers with two bis(alkoxy)phenyl substituents incorporating thieno[3,4-b]pyrazines (TP) or a benzo[3,4-b]pyrazine (BP) entity

Full text of DOI:10.1039/c8ta04774j

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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DOI: 10.1039/C8TA04774J
Pyrazine Incorporated Panchromatic Sensitizers for Dye
Sensitized Solar Cells under One Sun and Dim Light
Mulu Berha Desta,a,b,c,d Nguyễn So'n Vinh,e CH Pavan Kumar,a Sumit Chaurasia,a Wei-Ti Wu,a
Jiann T. Lin,a* Tzu-Chien Wei,e* Eric Wei-Guang Diauc
aInstitute of Chemistry, Academia Sinica, Nankang, Taipei−115, Taiwan
bTaiwan International Graduate Program (TIGP), Sustainable Chemical Science and Technology
(SCST), Academia Sinica, Taipei, Taiwan
cDepartment of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan
dDepartment of Chemistry, Mekelle University, Mekelle, Ethiopia
eDepartment of Chemical Engineering, National Tsing Hua University, Hsinchu 300, Taiwan
jtlin@gate.sinica.edu.tw, tcwei@mx.nthu.edu.tw
ORCID ID: 0000-0002-9704-5322 (Lin); 0000-0002-9608-8275 (Wei)
Abstract: New D−A′−π−A type (D = electron donor, A′ = auxiliary acceptor, π = π-conjugated
bridge, A = electron acceptor/anchor) sensitizers with two bis(alkoxy)phenyl substituents
incorporated thieno[3,4-b]pyrazines (TP) or benzo[3,4-b]pyrazine (BP) entity as the auxiliary
acceptor have been synthesized for dye-sensitized solar cells (DSSCs) application. Under 1 sun
illumination, the DSSCs fabricated from the two BP dyes with co-adsorbent have efficiencies of
8.39% and 9.03%, respectively. The latter surpasses that of the standard N719 dye (8.87%). Under
dim light condition MD7 exhibited a power conversion efficiency of 18.95% and 27.17% under
300 lux and 6000 lux irradiance, respectively.
Keywords: Pyrazines; Electron deficient entities; Metal-free sensitizers; Dye-Sensitized Solar
Cells, Dim-light.
1

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1 Introduction
In recent years, the drastic improvement in efficiency1 of metal-free dye-sensitized solar cells
(DSSCs)2 has paved a path leading to their commercialization parallel to conventional Si-based
solar cells. Compared with Si-based solar cells, DSSCs have an immense potential owing to their
flexibility in molecular design, easy device fabrication, environmental friendliness and low cost of
production.3 A high power conversion efficiency (PCE) of 13% has achieved with metal-free dye1b
and Zn-porphyrin dyes.1c The record PCEs have also been achieved for a co-dyed single cell
(14.3%)1e and a tandem cell (14.64%),1j respectively, using cobalt-based redox mediator. These
values are very close to the threshold required for commercialization (15%). In order to further
boost performance of DSSCs under 1 sun, sensitizers with absorption extending to near-infrared
(NIR) region (~700−1400 nm) is very important as NIR light (700−2500 nm) constitutes ~50% of
solar spectrum. Among NIR sensitizers, the most representative dyes are porphyrins and
phthalocyanines.4 Though the Si solar cell (absorbing in the range of 350−1000 nm) has excellent
efficiency under 1 sun illumination, it is outperformed by DSSCs under dim light condition
because of two reasons: (1) indoor light sources such as fluorescence lamps and white LED lights
cover mainly the visible region from 400 to 650 nm; (2) being an indirect band gap material, Si
solar cells need intense light illumination for efficient light harvesting. Currently DSSCs are
deemed promising under indoor applications such as powering portable electronics (sensors,
wireless transmitters, etc.) used in the Internet of Things (IoTs).3a,5 However, there are only limited
reports on dye-sensitized photovoltaics under indoor lighting.6
In this research, we intended to develop metal-free sensitizers for both indoor and outdoor
DSSCs. We figured that dyes with panchromatic absorption in the visible region up to ~650 nm
could cover the spectrum of T5 or white LED light source. In order to achieve this goal, we set out
to develop D−A′−π−A (D = electron donor, A′ = auxiliary acceptor, π = π-conjugated bridge, A =
electron acceptor/anchor) type sensitizers.7 Incorporation of an electron deficient entity as
auxiliary acceptor in the conjugated spacer of D−π−A dyes is beneficial for red shifting the
absorption spectra due to deepened LUMO and improving the photostability of sensitizers. We
reckon that A′ is like a double-edged blade: on one hand, it helps with red shifting the absorption
spectra; on the other hand, it deteriorates electron injection of the excited dye molecule due to
charge trapping.8 Therefore, proper design of the conjugated spacers with A′ are needed for good
performance of DSSCs. For example, replacing benzothiadiazole (BT)9 with more electron
2

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deficient pyrido[2,1,3]thiadiazole (PyT)10 resulted in lower cell performance in spite of the longer
wavelength ICT band. In comparison, the cell performance was improved by one order when PyT
was replaced with less electron deficient 2H-[1,2,3]triazolo[4,5-c]pyridine (PT).11 Similarly,
replacing pyridoimidazole (PIm) by 1H-benzo[d]imidazole (BIm) led to better cell efficiency.12
We therefore set out to develop D−A′−π−A type sensitizers with benzo[3,4-b]pyrazine (BP), an
electron-withdrawing entity popularly used for small band gap D−A type polymeric materials,13
as the auxiliary acceptor (A′). The BP was selected as its moderate electron deficiency7b may avoid
spectral over red shift of the dye and charge trapping of the excited dye molecule. More electron
deficient entity, thieno[3,4-b]pyrazines (TP), was also chosen as the electron-withdrawing entity
for comparison. Incorporation of alkyl or aryl substituents at the pyrazine ring of BP or TP allows
tuning the solubility and optical property of the dyes.14 Herein we incorporate 3,4-
dihexyloxyphenyl moiety at the 2- and 3-site of BP and TP to alleviate serious dye aggregation.
To our knowledge, D−A′−π−A dyes incorporating dialkoxyphenyl unit at the auxiliary acceptor
have not been reported so far, though there were examples of dyes with 2,4-dialkoxyphenyl moiety
at the donor.15 In addition to ambient light, the DSSCs fabricated from the new dyes were also
explored under 1 sun. The result shows these novel dyes perform well in both 1 sun irradiation and
300 lux T5 fluorescent light, providing viable materials for realizing practical use of DSSC.
2 Experimental Section
2.1 Materials and Characterizations
All commercially purchased solvents were dried before use. Reactions were performed under inert
1
13
atmosphere using standard Schlenk techniques unless specified. H and C NMR spectra were
recorded on a Bruker AVA-300 and AVIII-400 spectrometer. UV-vis absorption spectra of dyes
in solution and on mesoporous TiO2 films were measured using Cary 300 Bio UV-vis
spectrophotometer (Varian) and photoluminescence measurements were carried out with a
HITACHI F-4500 fluorescence spectrophotometer. Cyclic voltammetry (CV) measurements were
performed on a CHI-621A potentiostat (CH-Instruments, Inc.), Pt-wire as the counter electrode
and Ag/AgCl electrode as the reference electrode at a scan rate of 50 mV s−1. Tetra n-
butylammonium hexafluorophosphate and THF were used as the supporting electrolyte and
solvent, respectively. Elemental analysis was performed on a PerkinElmer 2400 analyzer. Fast
atom bombardment mass spectrometry (FAB MS) analysis was performed on a JEOL Tokyo Japan
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JMS-700 mass spectrometer equipped with the standard FAB source. Photovoltaic parameters of
the DSSCs were measured by a potentiostat/galvanostat (CHI650B, CH Instruments, Inc.) at 100
mW cm-2 (1 sun) light illumination by using a class A quality solar simulator (XES–301S, AM1.5G,
San–Ei Electric Co., Ltd., Osaka, Japan). The incident light intensity of 100 mW cm–2 was
calibrated with a standard Si cell (PECSI01, Peccell Technologies, Inc., Kanagawa, Japan).
Incident photon–to–current conversion efficiencies (IPCE) of the DSSCs were recorded by the
same potentiostat/galvanostat under monochromatic light illumination in a wavelength range of
380-900 nm. To obtain the IPCE data, another class-A solar simulator (PEC–L11, AM1.5G,
Peccell Technologies, Inc., Kanagawa, Japan) equipped with a monochromator (model 74100,
Oriel Instrument, California, USA) was used. The incident radiation flux (φ) was obtained by using
an optical detector (model 818-SL, Newport, California, USA) and a power meter (model 1916-R,
Newport, California, USA). Electrochemical impedance spectra (EIS) of DSSCs were recorded by
the same potentiostat/galvanostat equipped with a FRA2 module at an applied voltage of -0.70 V.
The explored frequency range is set from 10 MHz to 100 kHz with an AC amplitude of ±10 mV
using an equipped FRA2 module. The TiO2 nanoparticles and the reference dye of N719 were
purchased from Solaronix, S.A., Switzerland.
2.2 Experimental Procedures
The synthesis of 1,2-bis(3,4-bis(hexyloxy)phenyl)ethane-1,2-dione, 3,6-dibromobenzene-1,2-
diamine, [2,2':5',2''-terthiophene]-3',4'-diamine are described in supporting information. The
syntheses of dyes and relevant precursors are also described in Supporting Information except for
MD4.
Synthesis of 2,3-bis(3,4-bis(hexyloxy)phenyl)-5,7-di(thiophen-2-yl)thieno[3,4-b]pyrazine (4a):
A mixture of 1,2-bis(3,4-bis(hexyloxy)phenyl)ethane-1,2-dione (3.0 mmol, 1.83 g), [2,2':5',2''-
terthiophene]-3',4'-diamine (3.0 mmol, 0.83 g), and para-toluenesulfonic acid (0.2 mmol) in dry
CHCl3 was refluxed for 12 h and monitored through TLC. On completion, the reaction mixture
was cooled to room temperature and then poured into the crushed ice, and finally extracted with
DCM (100 mL x 3). The organic extracts were combined and dried over anhydrous MgSO4. The
crude product was purified by flash column chromatography using hexane: DCM (1:4 by vol.) as
the eluent. Compound 4a was obtained as a red solid (82% yield). 1H NMR (400 MHz, CDCl3): δ
0.91-0.98 (m, 12H), 1.33-1.41 (m, 16H), 1.43-1.53 (m, 8H), 1.74-1.89 (m, 8H), 3.91 (t, J = 6.5 Hz,
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4H), 4.04 (d, J = 6.5 Hz, 4H), 6.85 (d, J = 8.4 Hz 1H), 7.12-7.16 (m, 2H), 7.20 (d, J = 8.4 Hz, 2H),
7.25-7.27 (m, 2H), 7.39 (d, J = 5.0 Hz, 2H), 7.66-7.68 (m, 2H). MS (HRMS): m/z calculated for
C50H64N2O4S3 [M] 852.4028; found [M + H]+ 853.4110.
Synthesis of 5-(2,3-bis(3,4-bis(hexyloxy)phenyl)-7-(thiophen-2-yl)thieno[3,4-b]pyrazin-5-
yl)thiophene-2-carbaldehyde
(4b):
2,3-Bis(3,4-bis(hexyloxy)phenyl)-5,7-di(thiophen-2-
yl)thieno[3,4-b]pyrazine (2.35 mmol, 2.0 g) was dissolved in dry DMF (10 ml) and the solution
o
was cooled to 0 C. Phosphorous oxychloride (2.81 mmol, 0.263 ml) was added to the solution
dropwise at 0 oC and then the solution was warmed slowly to room temperature and then stirred at
80 oC for overnight. After completion, the reaction mixture was poured into ice water (200 mL)
and stirred for 10 minutes, extracted with DCM (100 mL x 3). The organic layer was combined
and dried over anhydrous MgSO4. After filtration, the solvent of the filtrate was removed under
reduced pressure, and the product was purified by flash column chromatography using hexane:
DCM (1:4 by vol.) as the eluent. Compound 4b was obtained as a dark red solid (72% yield). 1H
NMR (400 MHz, CDCl3): δ 0.91-0.98 (m, 12H), 1.33-1.42 (m, 16H), 1.44-1.56 (m, 8H), 1.76-1.91
(m, 8H), 3.89-3.95 (m, 4H), 4.02-4.09 (m, 4H), 6.87 (t, J = 8.3 Hz, 2H), 7.15 (t, J = 4.00 Hz, 1H),
7.22 (d, J = 8.3 Hz, 1H), 7.25-7.29 (m, 3H), 7.45 (d, J = 5.00 Hz, 1H), 7.63 (d, J = 4.00 Hz, 1H),
7.71 (m, 2H), 9.93 (s, 1H). MS (FAB) m/z: 881.6 (M+).
Synthesis
of
5-(2,3-bis(3,4-bis(hexyloxy)phenyl)-7-(5-bromothiophen-2-yl)thieno[3,4-
b]pyrazin-5-yl)thiophene-2-carbaldehyde (4c): 5-(2,3-Bis(3,4-bis(hexyloxy)phenyl)-7-(thiophen-
2-yl)thieno[3,4-b]pyrazin-5-yl)thiophene-2-carbaldehyde 4b (1.59 mmol, 1.4 g) was dissolved in
o
dry DMF (20 mL) and the mixture was cooled to 0 C. NBS (1.75 mmol, 283 mg) was added
slowly to the solution and the reaction mixture was warmed slowly and stirred for 12 h. Finally,
the reaction mixture was poured into ice water and extracted with DCM (100 mL x 2). The organic
extracts were combined and dried over anhydrous MgSO4. After filtration, the solvent of the
filtrate was removed under reduced pressure and the residue was purified by flash column
chromatography using hexane: CH2Cl2 (1:4 by vol.) as the eluent. Compound 4c was obtained as
1
a brown solid (68% yield). H NMR (300 MHz, CDCl3): δ 0.84-0.95 (m, 12H), 1.32-1.60 (m,
24H), 1.81-1.95 (m, 8H), 3.95-4.06 (m, 8H), 6.77-6.84 (m, 2H), 7.02 (s, 1H), 7.11-7.21 (m, 2H),
7.38-7.42 (m, 2H), 7.64 (s, 1H), 9.84 (s, 1H). MS (MALDI) m/z: 961.3 [M+ H]+.
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Synthesis of 5-(2,3-bis(3,4-bis(hexyloxy)phenyl)-7-(5-(4-(diphenylamino)phenyl)thiophen-2-
yl)thieno[3,4-b]pyrazin-5-yl)thiophene-2-carbaldehyde (4d): To a flame dried flask 5-(2,3-
bis(3,4-bis(hexyloxy)phenyl)-7-(5-bromothiophen-2-yl)thieno[3,4-b]pyrazin-5-yl)thiophene-2-
carbaldehyde 4c (0.94 mmol, 0.90 g), N,N-diphenyl-4-(tributylstannyl)aniline (1.41 mmol, 0.75 g)
and PdCl2(PPh3)2 (0.02 mmol, 13 mg) was added 20 mL of dry DMF via syringe and the mixture
o
was stirred under nitrogen atmosphere at 100 C for 15 h. Reaction was also monitored through
TLC. After completion of the reaction, the mixture was poured into ice water (200 mL) and
extracted with CH2Cl2 (100 mL x 3). The organic extracts were combined, washed with brine, and
dried over anhydrous MgSO4. Product 4d was obtained in 58% yield by flash column
chromatography using hexane: CH2Cl2 (1:4 by vol.) as the eluent. 1H NMR (400 MHz, CDCl3): δ
0.83-0.93 (m, 12H), 1.21-1.28 (m, 4H), 1.29-1.39 (m, 14H), 1.40-1.51 (m, 6H), 1.68-1.88 (m, 8H),
6.79 (d, J = 8.5 Hz, 1H), 6.85 (d, J = 8.5 Hz, 1H), 7.03-7.07 (m, 4H), 7.11-7.15 (m, 5H), 7.21-7.32
(m, 8H), 7.50 (d, J = 8.5 Hz, 2H), 7.60 (d, J = 4.0 Hz, 1H), 7.65 (d, J = 4.0 Hz, 1H), 7.69 (d, J =
4.0 Hz, 1H), 9.88 (m, 1H). MS (MALDI) m/z: 1124.5 [M + H]+.
Synthesis of 5-(7-(5-(4-(bis(4-(hexyloxy)phenyl)amino)phenyl)thiophen-2-yl)-2,3-bis(3,4-
bis(hexyloxy)phenyl)thieno[3,4-b]pyrazin-5-yl)thiophene-2-carbaldehyde (4e):
The synthetic procedure was similar to that of 4d except that 4-(hexyloxy)-N-(4-
(hexyloxy)phenyl)-N-(4-(tributylstannyl)phenyl)aniline was used instead of N,N-diphenyl-4-
(tributylstannyl)aniline. The crude product was purified by flash chromatography using hexane:
CH2Cl2 (1:2 by vol.) as the eluent to afford 4e as a brown solid (80% yield). 1H NMR (400 MHz,
CDCl3): δ 0.94-0.97 (m, 18H), 1.36-1.40 (m, 24H), 1.49-1.52 (m, 12H), 1.78-1.87 (m, 12H), 3.93-
3.98 (m, 8H), 4.04-4.08 (m, 4H), 6.80-6.93 (m, 14H),7.09-7.14 (m, 2H), 7.23 (s, 1H), 7.25-7.45
(m, 3H), 7.51 (s, 1H), 7.60-7.61 (m, 1H), 9.85 (s, 1H). MS (FAB): m/z calculated for
C81H101N3O7S3 [M] 1323.68; found [M + H]+ 1324.70.
Synthesis of 3-(5-(2,3-bis(3,4-bis(hexyloxy)phenyl)-7-(5-(4-(diphenylamino)phenyl)thiophen-
2-yl)thieno[3,4-b]pyrazin-5-yl)thiophen-2-yl)-2-cyanoacrylic acid (MD4): 5-(2,3-bis(3,4-
bis(hexyloxy)phenyl)-7-(5-(4-(diphenylamino)phenyl)thiophen-2-yl)thieno[3,4-b]pyrazin-5-
yl)thiophene-2-carbaldehyde 4d (0.27 mmol, 0.30 g), 2-cyanoacetic acid (0.40 mmol, 34 mg),
ammonium acetate (10 mg) and acetic acid (10 mL) were mixed in two neck round bottom flask
o
under nitrogen and stirred at 100 C for 12 h. The solution was cooled to room temperature and
poured into ice water (100 mL) and extracted with CH2Cl2 (100 mL x 2). The organic layer was
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combined and dried over anhydrous MgSO4. After filtration, the solvent of the filtrate was
removed under reduced pressure and crude product was purified by flash column chromatography
using 1% acetic acid in CH2Cl2 as the eluent. Compound MD4 was obtained as a dark green solid
(56% yield). 1H NMR (400 MHz, THF-d8): δ 0.86-0.98 (m, 12H), 1.25-1.70 (m, 28H), 1.71-1.92
(m, 4H), 3.74 (t, J = 6.4 Hz, 2H), 3.88 (t, J = 6.0 Hz, 2H), 4.02 (t, J = 6.6 Hz, 2H), 4.13 (t, J = 6.4
Hz, 2H), 6.78 (d, J = 8.3 Hz, 1H), 6.95-6.97 (m, 4H), 7.05-7.11 (m, 8H), 7.25-7.36 (m. 10H), 7.55
(m, 1H), 7.72 (d, J = 7.2 Hz, 1H), 8.26 (s, 1H). 13C NMR (75 MHz, THF-d8): δ 14.09, 22.67, 25.76,
29.30, 31.61, 69.04, 69.40, 122.75, 123.43, 124.85, 126.27, 127.59, 129.40, 147.24, 147.49,
148.82, 149.55, 162.39. MS (FAB): m/z calcd. for C72H78N4O6S3 [M]: 1190.5083 found: [M+H]+
1191.5173.
2.3 Fabrication of DSSC
Dye-sensitized solar cells were fabricated using conductive glass, fluorine-doped tin oxide (FTO,
TEC-7, 7 Ω/sq, NSG America Inc., New Jersey, USA), as the substrate for photoanode and
counter-electrode (CE) after proper cleaning. The FTO glasses were firstly cleaned via the ultra-
sonication in the bathes of neutral cleaner, de-ionized water, acetone, and isopropanol,
sequentially. A 12 μm-thick TiO2 film, containing a 7 μm-thick transparent layer (Solarnix
transparent paste) and a 5 μm-thick scattering layer, was casted on the cleaned FTO with a
controlled area of 0.16 cm2 and was sintered at 450 oC for 30 min in an air atmosphere.16 The film
thickness is measured by a profilometer (Dektak3, Veeco/Sloan Instruments Inc., USA). The
sintered TiO2 film was immersed in 3×10−4 M dye solution at room temperature for 12 h in THF.
For the cases of the dyes co-adsorbed with chenodeoxycholic acid (CDCA), 1 mM of CDCA is
added into each dye solution. After rinsing with acetonitrile, the dye-adsorbed TiO2 photoanode
was obtained. Finally, a photoanode was assembled with a sputtered-platinum/FTO counter
electrode using a 25 μm-thick Surlyn® (SX1170-25, Solaronix S. A., Switzerland) as the cell
spacer. The iodide-based electrolyte, containing 0.1 M lithium iodide (LiI), 1.0 M 1,2-dimethyl-3-
propylimidazolium iodide (DMPII), 0.04 M iodine (I2), and 0.5 M 4-tert-butylpyridine (TBP) in
acetonitrile/3-methoxypropionitrile (ACN/MPN = 8:2 by vol.), was injected into the cell gap
between these two electrodes by capillarity. The cell parameters for the dim light irradiation have
been adjusted as follows: a mesoporous TiO2 (12 μm active layer ~ 4 μm-thick scattering layer)
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with an effective area of 0.16 cm2 and a redox electrolyte composition of LiI (0.3 M), I2 (0.5 M),
DPMII (1M) and TBP (0.5 M) in ACN/MPN (80:20 v/v).
2.4 Computational Details
All the calculations have been performed using the Gaussian 09 program package.17 Geometrical
optimization was performed in vacuo using B3LYP exchange-correlation functional and a 6-311G
(d,p)18 basis set. The optimized geometries were then used to obtain frontier molecular orbitals
(FMOs). To simulate the optical spectra, the lowest spin allowed singlet-singlet transitions were
computed on the ground state geometry. TDDFT calculations of the lowest singlet-singlet
excitations were performed in THF solution, on the optimized geometries using the CAM-
B3LYP/6-311G (d,p) level of theory. The integral equation formalism polarizable continuum
model (PCM)19 within self-consistent reaction field (SCRF) theory, has been used to describe the
solvation of the dyes. The TDDFT calculations were also performed using CAM-B3LYP with a
6-311G (d, p) basis set. The software GaussSum 2.2.5 was utilized to simulate the major portion
of absorption spectrum and to analyze the nature of transitions.20
3
Results and Discussion
3.1 Synthesis and Characterization
The structures of the new sensitizers are shown in Fig. 1, and Schemes 1 and 2 describe the
synthetic pathway for MD4/MD6 and MD5/MD7 respectively. Pyrazine cores (4a, 5a) were
synthesized by refluxing a mixture of the corresponding diamine21 and diketone22 in suitable
solvent as mentioned in the schemes. It is worth noting that we failed to isolate dihexyloxyphenyl
moiety-free analogues of 4a and 5a, possibly due to low solubility of intermediates stemmed from
serious intermolecular aggregation. Vilsmeier-Haack reaction was performed on 4a to provide
aldehyde intermediate, 4b, which underwent subsequent bromination with NBS to afford 4c. Next,
Stille cross coupling reaction between 4c and N,N-diphenyl-4-(tributylstannyl)aniline23 was
carried out to incorporate the arylamine donor, and subsequent Knoevenagel condensation reaction
furnished the desired MD4 dye. Stille cross coupling of tributyl(thiophen-2-yl)stannane23 with 5a
afforded 5b, which then underwent formylation to give 5c via Vilsmeier-Haack reaction.
Subsequent bromination of 5c with NBS provided 5d. Stille cross coupling reaction of 5d with
N,N-diphenyl-4-(tributylstannyl)aniline afforded 5e incorporating the donor group, which
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underwent subsequent Knoevenagel condensation with cyanoacetic acid to provide MD5. The
synthetic procedure of MD6 and MD7 was similar to that of MD4 and MD5 except that 4-
(hexyloxy)-N-(4-(hexyloxy)phenyl)-N-(4-(tributylstannyl)phenyl)aniline was used instead of
N,N-diphenyl-4-(tributylstannyl)aniline.
Fig. 1 Molecular structures of the pyrazine-based MD4–MD7 dyes
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3.2 Photophysical and Electrochemical Characteristics
The UV−vis absorption spectra of MD dyes were measured in THF and on TiO2 film (Fig. 2a and
2b) and the optical HOMO/LUMO energy gap of the dyes were estimated from the intersection of
normalized absorption and fluorescence spectra. The dyes show panchromatic absorption: an
intense band at the higher energy region is attributed to π–π* transition and a broad intramolecular
charge transfer (ICT) band appears at the longer wavelength region. The thieno[3,4-b]pyrazine-
based dyes, MD4 and MD6, show absorption maxima (λmax) at 633 nm and 642 nm with the λonset
reaching >750 nm (Fig. 2, Table 1) and the molar absorption coefficient exceeding 10000 M-1cm-
1 at 700 nm. Compared with MD4 and MD6, the absorption maxima of MD5 (513 nm) and MD7
(532 nm) are blue-shifted by 120 nm and 110 nm respectively, which is consistent with the more
electron deficient nature of thieno[3,4-b]pyrazine (in MD4 and MD6) than benzo[3,4-b]pyrazine
(in MD5 and MD7).24 Incorporation of the alkoxy substituents at the arylamine donor increases
its strength and results in red shift of the absorption: MD6 > MD4 and MD7 > MD5. It is
noteworthy that the molar extinction coefficient of MD4 (ε = 32,231 M-1cm-1), MD6 (ε = 33,530
M-1cm-1), MD5 (ε = 32,231 M-1cm-1) and MD7 (ε = 39,360 M-1cm-1) are greater than BP and TP
congeners with similar skeletons, CR25 and DJ26 (BT dyes), and NL14d and FNE (TP dyes).27 This
11

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can be attributed to better planarity of the conjugated skeleton and incorporation of the alkoxy
substituents for the MD dyes. Though there is negligible dye aggregation in the solution up to
concentration of 3 x 10‒4 M in view of linearity of the absorbance vs. concentration (in THF) plot
(Fig. 2b), slight dye aggregation still exists in the film state. Fig. 2c displays the absorption spectra
for the dyes adsorbed on the TiO2 film (~4 µm in thickness). Broadened absorption spectra in the
longer wavelength region with bathochromic shift of λonset indicates the presence of J-aggregation
of the dyes in the film state.28 MD4 (or MD6) appears to have higher tendency to aggregate than
its BP congener (Supporting Information: Fig. S1), possibly due to more planar skeleton in the
former. The blue shift of λmax of the dyes on TiO2 is attributed to the deprotonation of the carboxylic
acid.29
6.0x104
1.0
MD4
b)
1.0
0.8
0.6
0.4
0.2
0.0
MD4
MD5
MD6
MD7
a)
MD5
MD6
MD7
MD4
MD5
MD6
MD7
0.8
0.6
0.4
0.2
0.0
4.0x104
2.0x104
0.0
1.0x10-4
2.0x10-4
3.0x10-4
400
500
600
700
800
400
500
600
700
800
900
0.0
Wavelength (nm)
Wavelength (nm)
Concentration (M)
Fig. 2 (a) Absorption spectra in THF solution (10 µM), (b) absorbance vs. concentration plot in
THF and (c) absorption spectra on TiO2 thin film for MD4‒MD7 dyes.
To gain a better understanding of the structure-property relationship, we have carried out
TDDFT studies on these dyes using CAM-B3LYP level of theory, with inclusion of the solvent
(THF). A detailed list of the calculated transition energies for all the compounds are provided in
the Supporting Information (Tables S1−S4), and the simulated spectra are shown in Fig. S2. The
TDDFT results are in good agreement with the experimental UV-Vis spectra. The spectra have
been produced by convoluting Gaussian functions with FWHM = 0.35 eV centered at the
excitation wavenumbers. Relevant ground-state dihedral angles between aromatic entities of the
conjugated skeleton are shown in Fig. S3 (SI). There is significant twist (dihedral angles: 46.9°‒
43.2° ) between the BP (or TP) core with its two alkoxyphenyl substituents. As expected, TP core
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has better planarity with the neighboring thiophene rings compared with the BP core, and the
dihedral angles are smaller in MD4 and MD6 (6.6°‒7.7°) than in MD5 and MD7 (24.2°‒28.3°).
Selected frontier orbitals are shown in Fig. 3 and Fig. S4 (SI). The HOMO orbital is mainly
distributed from the arylamine donor extending to the conjugated spacer, and the LUMO orbital is
distributed from the anchor extending to the spacer. Charge transfer character is evident for the
lowest-energy transition (S0 → S1): >75% of HOMO → LUMO for MD4 and MD6, and >50% of
HOMO → LUMO for MD5 and MD7. Computation on compounds MD4'−MD7', the congeners
of MD4−MD7 without alkoxy chains at the BP or TP core, were also carried out in order to
evaluate the influence of the alkoxy chains on the electronic property of the dyes, and the results
are compiled in Table S5. Compared with MD4'−MD7', MD4−MD7 appear to have LUMO
lowered by 0.13−0.18 eV, and therefore HOMO/LUMO gap lowered by 0.17−0.24 eV. The
Mulliken charges difference between the excited-state and ground states for different segments of
the MD dyes were also calculated from the CAM-B3LYP/6-311G for the S1 and S2 states in order
to estimate the extent of charge separation upon excitation (Fig. S5). The dye molecules were
grouped into several segments: the arylamine donor (D), the thiophene unit between D and BD or
TP (Th-L), BD or TP (A′), the thiophene unit between A′ and the anchor (Th-R), and the anchor
(A). The Mulliken charges are compiled in Table S6. The Mulliken charges changes at A′/A for
S0 → S1 are: MD4, -0.11/-0.06 |e|; MD5, -0.11/-0.11 |e|; MD6, -0.11/-0.06 |e|; and MD7, -0.04/-
0.12 |e| respectively. The products of the oscillator strength and the Mulliken charge changes in A
are as below: MD4, 0.06 |e|; MD5, 0.15 |e|; MD6, 0.07 |e|; and MD7, 0.17 |e| respectively.
Therefore, the TP-based dyes (MD4 and MD6) have more charges trapped at A′ and less charges
residing at A compared with the BP-based dyes (MD5 and MD7). This implies that electronic
injection from the excited dye molecule to TiO2 is more facile for the BP-based dyes.
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Fig. 3 HOMO and LUMO orbitals and the corresponding energies.
Table 1 Electro-optical parameters of MD4−MD7 dyes
Eox
(∆Ep)c
b
b
d
λabs (ε)a
λabs
λonset
Eox
Eox*e
E0-0 (λ)f
dye
nm (M-1 cm-1)
nm
nm
V
V
eV (nm)
mV
261
427 (33832),
633 (32231)
(122),
613
800
0.96
-0.80
1.76 (705)
MD4
511
(109)
356 (41667),
513 (37998)
498
629
630
815
360 (95) 1.06
142 (86) 0.84
-1.04
-0.90
2.10 (590)
1.74 (713)
MD5
MD6
420(40188)
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642(33530)
427
(104)
365(47545)
532(39360)
524
660
177 (90) 0.88
-1.13
2.01 (615)
MD7
a Recorded in THF (solution). b Absorption maxima and onset wavelength on TiO2 film. c Recorded
in THF. scan rate = 100 mVs-1; electrolyte = [(n-C4H9)4][NPF6]; Eox = 1/2(Epa + Epc), ∆Ep = (Epa −
Epc) where Epa and Epc are peak anodic and cathodic potentials, respectively. Oxidation potential
reported is adjusted to that of ferrocene (Fc) used as an internal reference. d Calculated by using
e
the formula Eox (vs. NHE) = 0.77 + Eox (vs. Fc). Eox* = Eox ‒ E0–0. f The optical HOMO/LUMO
energy gap, E0-0, was derived from the intersection of absorption and fluorescence spectra using
the formula 1240/λ.
3.3 Electrochemical Properties
Cyclic voltammetry (CV) measurements generally provide information on the feasibility of the
electron injection and dye regeneration. The CV measurements of MD dyes were carried out in
THF using tetra-n-butylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte.
The cyclic voltammograms and energy levels of the sensitizers are shown in Fig. 4a and 4b,
respectively, and the relevant data are compiled in Table 1. We can observe two features: (1)
incorporation of hexyloxy chains at the arylamine significantly increases the electron density and
lower the oxidation potential of arylamine, i.e., MD4 vs. MD6 and MD5 vs. MD7; (2) both MD4
and MD6 have two quasi-reversible oxidation waves and MD5 and MD7 have only one.
Apparently the three electron excessive thiophene rings in MD4 and MD6 render the molecule
more electron rich, and have higher capability to accommodate two positive charges. The HOMO
levels of MD4 (0.96 V), MD5 (1.06 V), MD6 (0.84 V) and MD7 (0.88 V vs. NHE), calculated
30
−
from the first oxidation potential, are more positive than the I−/I3 redox couple (∼0.4 V vs. NHE)
and ascertain the efficient dye regeneration. The LUMO energy levels of the dyes MD4 (-0.80 V),
MD5 (-1.04 V), MD6 (-0.90 V) and MD7 (-1.13 V vs. NHE), calculated from Eox − E0-0, are
remarkably more negative than the TiO2 conduction band-edge energy (~-0.5 V vs. NHE),31
ensuring favorable electron injection (see Table 1 and Fig. 4b).
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6.0x10-6
4.0x10-6
MD4
MD5
MD6
MD7
a)
2.0x10-6
0.0
-2.0x10-6
-4.0x10-6
-6.0x10-6
-8.0x10-6
-1.0x10-5
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
Potential (V)
Fig. 4 (a) Cyclic Voltammograms and (b) energy-level diagram of MD4–MD7 dyes.
3.4 Photovoltaic Properties
−
DSSCs fabricated with MD dyes and I−/I3 redox mediator were measured under AM 1.5 G
illumination. The current density−voltage (J−V) curves and the incident photon-to-current
conversion efficiency (IPCE) plots of DSSCs are shown in Fig. 5a and 5b, respectively. The
relevant photovoltaic parameters of the cells are summarized in Table 2 The photocurrent densities
estimated by integrating the product of the IPCE value at each wavelength and the photon flux
density data listed in the AM 1.5 solar spectrum are also listed in the parentheses of the JSC value
for reference. They deviate from the experimental values by <15%. The dyes with BP core (MD5:
7.88%; MD7: 8.22%) have significantly higher performance than those with TP core (MD4:
3.92%; MD6: 5.08%). Compared with the solution spectra, the IPCE spectra are broadened in the
longer wavelength region, which is consistent with the film spectra of the λonset (vide supra). The
best performed dye, MD7, has IPCE values above 80% in the range of 400‒620 nm. Though MD4
and MD6 have absorption extending to longer wavelength spectral region, their IPCE values are
lower than 60%. They may stem from several reasons: (1) higher electron density is trapped at the
more electron deficient TP entity of MD4 and MD6, leading to lower electron injection efficiency
(see Mulliken charge analysis, vide supra); (2) there is lower dye loading for MD4 and MD6; (3)
higher degree of dye aggregation in MD4 and MD6 due to more planar skeleton (vide supra)
jeopardizes the electron injection. Moreover, MD4 and MD6 are less effective in dark current
suppression compared to MD5 and MD7 (vide infra, see EIS), leading to smaller photovoltages.
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Consequently, MD5 and MD7 exhibit better conversion efficiencies than MD4 and MD6. The
largest dye loading amount and most efficient dark current suppression (vide infra) certainly
contribute significantly to the best performance of MD7 among all. In comparison, the lowest dye
loading, most serious dye aggregation and largest dark current (vide infra) render the cell
performance of MD4 the lowest among all. Better performance of MD6 than MD4 (or MD7 than
MD5) can be attributed mainly to two reasons: (1) better light harvesting because of higher dye
loading as well as broader and more intense absorption spectra (vide supra); (2) better dark current
suppression, leading to marginal increment of the photovoltage.
-
The higher VOC values of MD4 and MD6 than other TP dyes14c,14d,27 using I-/I3 redox mediator
suggest more effective dark current suppression in the former owing to the bulky
dihexyloxyphenyl moieties at the TP entity. Dihexyloxyphenyl moiety at the BP entity also
benefits dark current suppression in BP dyes, as MD5 and MD7 have relatively higher
photovoltages than their congeners without substituents at 2- and 3-site of BP, FNE45 and
FNE46.32a Among BP dyes with triphenylamine donor, the VOC values of MD5 and MD7 also
−
appear to be higher for DSSCs using I−/I3 redox couple.14a,25,32 Though the cell efficiencies of
MD5 (7.88%) and MD7 (8.22%) are somewhat lower than the best efficiency (9.83%) reported
for single BP dye,33 they compete favorably with other triphenylamine-based BP dyes.14a,25,32
Adding co-adsorbent during dye soaking has been proven to help with anti-aggregation of the
dyes in addition to passivation of TiO2 surface for dark current suppression.34 As there exists slight
dye aggregation, addition of co-adsorbent was also tested. After adding 1 mM CDCA co-adsorbent,
both VOC and JSC were improved and the power conversion efficiencies increased accordingly:
MD4, 4.58%; MD5, 8.39%; MD6, 5.26%; MD7, 9.03%. The J−V curves and IPCE plots with
CDCA are shown in Fig. 5c and Fig. 5d, respectively. Both VOC and JSC increase in spite of
decreasing dye loading, providing good evidence for alleviation of dye aggregation upon addition
of CDCA. Accordingly, all dyes exhibit higher IPCE values after addition of CDCA.
Electrochemical impedance spectroscopy (EIS) taken in the dark was used to analyze the
interfacial electron transfer properties in DSSCs. Fig. 6a shows the Nyquist plots for DSSCs based
on MD4–MD7 dyes. The semicircle with larger impedance is related to charge recombination
resistance (Rrec) of the injected electron with the oxidized electrolyte occurring at the
TiO2/dye/electrolyte interface, and the larger Rrec means lesser tendency for recombination, i.e.
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lower dark current. The Rrec value for sensitizers decreases in the order of N719 (206 Ω) > MD7
(171 Ω) > MD5 (165 Ω) > MD6 (139 Ω) > MD4 (114 Ω), which is in agreement with the order of
the dark current (Fig. 6a). The trend is also in line with that of VOC value, N719 (751 mV) > MD7
(725 mV) > MD5 (718 mV) > MD6 (705 mV) > MD4 (700 mV). The electron recombination
lifetime extracted from the Bode phase plots (Fig. 7b) using τrec = 1/2πf, MD7 (40.83 ms) > MD5
(18.73 ms) > MD6 (12.94 ms) > MD4 (7.96 ms), provides additional supporting evidence. The
larger Rrec value of BP than TP dyes, i.e., MD7 > MD6 and MD5 > MD4, may be attributed to
two reasons: (1) more compact packing (higher dye loading) of the dyes on TiO2, resulting in less
free volume for penetration of the electrolytes to the TiO2 surface; (2) the larger twist angles
between BP and two neighboring thiophene rings than those between TP and the neighbouring
thiophene rings, resulting in more effective blockade of the electrolytes from approaching the TiO2
surface. The two alkoxy chains at the arylamine donor also help with dark current suppression, i.e.,
MD7 > MD5 and MD6 > MD4. MD7 stands out to be the best for dark current suppression due
to its higher dye loading, twist structure and the presence of the alkoxy chains at the donor.
Table 2 Photovoltaic parameters of MD dyes with and without CDCA under AM 1.5Ga
dye
η
VOC
V
JSC
FF
Rrec
τrec
dye loading
10-7mol cm-2
%
mA cm-2
Ω
ms
3.92 ± 0.02
4.58 ± 0.02
7.88 ± 0.03
8.39 ± 0.02
5.08 ± 0.03
5.26 ± 0.02
8.22 ± 0.04
9.03 ± 0.02
8.87 ± 0.01
0.631 ± 0.003
0.687 ± 0.004
9.56 ± 0.02 (8.13)
0.65 ± 0.03 112 7.92
1.29
1.01
3.10
2.12
2.81
2.02
5.03
4.51
MD4
MD4*
MD5
MD5*
MD6
MD6*
MD7
MD7*
N719
10.11 ± 0.01 (8.65) 0.66 ± 0.01
0.715 ± 0.002 16.50 ± 0.01 (15.05) 0.67 ± 0.03 163 19.41
0.727 ± 0.003 17.16 ± 0.03 (15.67) 0.67 ± 0.04
0.695 ± 0.002
11.25 ± 0.01 (9.85) 0.65 ± 0.01 136 12.75
0.700 ± 0.004 11.58 ± 0.01 (10.16) 0.65 ± 0.02
0.719 ± 0.002 17.12 ± 0.04 (15.78) 0.67 ± 0.02 172 37.91
0.753 ± 0.003 17.65 ± 0.01 (16.28) 0.68 ± 0.02
0.745 ± 0.001 17.50 ± 0.03
0.68 ± 0.01 204
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aVOC: open-circuit voltage; JSC: short-circuit current and the values in the parentheses are the
products of the IPCE value at each wavelength and the photon flux density data listed in the AM
1.5 solar spectrum; FF: fill factor; PCE: power-conversion efficiency. The data are based on five
measurements. Experiments were conducted by using TiO2 photo-electrodes with approximately
12 μm thickness and 0.16 cm2 working area on fluorine-doped tin oxide substrates. MDn: without
CDCA; MDn*: with CDCA where n = 4, 5, 6 and 7.
100
MD4
a)
MD4
MD5
MD6
MD7
20
15
10
5
b)
MD5
MD6
MD7
N719
80
60
40
20
0
0
-5
0.0
0.2
0.4
0.6
0.8
1.0
400
500
600
700
800
900
Photovoltage (V)
Wavelength (nm)
MD4-CDCA
MD5-CDCA
MD6-CDCA
MD7-CDCA
d)
MD4
MD5
MD6
MD7
N719
c)
100
80
60
40
20
0
20
15
10
5
0
-5
0.0
0.2
0.4
0.6
0.8
1.0
400
500
600
700
800
900
Photovoltage (V)
Wavelength (nm)
Fig. 5 (a) Current density-voltage (J−V) curves and (b) IPCE spectra without CDCA, and (c) J−V
curves and (d) IPCE spectra with CDCA for the DSSCs measured under simulated AM 1.5G
illumination.
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70
MD4
MD5
MD6
MD7
MD4
b)
a)
MD5
MD6
MD7
N719
60
50
40
30
20
10
200
150
100
50
0
10-1
100
101
102
103
100
200
300
400
Z'()
Frequency (Hz)
Fig. 6 Electrochemical impedance spectra, (a) Nyquist plots and (b) Bode plots of DSSCs based
on the dyes MD4–MD7 measured in the dark at applied bias of -0.7 V.
3.5 DSSCs Performance under Dim-Light Condition
Due to panchromatic absorption, the absorption spectra of MD dyes have good spectral coverage
with the emission spectrum of the T5 fluorescence lamp (see Fig. S7 in SI). The high photovoltage
of the MD dyes should also be beneficial for dim light environments. The photovoltaic
performance of MD dyes was therefore evaluated using T5 fluorescent lamp (0.1960 mW cm-2)
illumination with irradiance in the range of 300−6000 lux. CDCA (1 mM) was added as the co-
-
adsorbent during dye soaking and I-/I3 was used as the redox mediator for all the cells. The current
density−voltage (J−V) curve of DSSCs under 300 lux T5 fluorescent light illuminationis shown in
Fig. 7 and Table 3 displays the summarized dim-light cell efficiency data of the dyes at 300, 600,
900 and 6000 lux. The performance data for all other irradiance are compiled in Table S8. Both
BP-based dyes (MD5 and MD7) have cell efficiency of >15% even at only 300 lux illumination,
and MD7 has the best performance among all: 18.95% (300 lux), 20.16% (600 lux), 21.10% (900
lux) and 27.17% (6000 lux). The efficiencies are comparable with those of N719-based cell
fabricated and measured under similar condition: 19.02% (300 lux), 20.23% (600 lux), 20.94%
(900 lux) and 27.64% (6000 lux). The performance of MD7, though lower than the record
efficiency reported by Freitag et. al. on a co-dyed system using Cu(II/I)(tmby) (tmby = 4,4’,6,6’-
tetramethyl-2,2’-bipyridine) as the redox mediator (25.5% at 200 lux; 28.9% at 1000 lux),6a is very
−
competitive with the best cell based on single sensitizer and I−/I3 redox couple reported by Tingare
et. al.: 21.40% (300 lux), 21.88% (600 lux), 23.64% (900 lux) and 28.56% (6000 lux).6c Similar
to the case of 1 sun illumination, the efficiencies of TP-based dyes (MD4 and MD6) are
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significantly lower: for MD4, 6.39% to 8.62% as the irradiance varies from 300 to 6000 lux; for
MD6, 12.08% to 16.86% as the irradiance varies from 300 to 6000 lux. Based on the results at 1
sun, the lower VOC values of MD4 and MD6 can be attributed to less effective dark current
suppression, and lower JSC values due to their less efficient electron injection.
MD4
MD5
MD6
MD7
0.04
N719
0.03
0.02
0.01
0.00
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Voltage (V)
Fig. 7 Current density-voltage (J−V) curves for MD dyes under T5 illuminance (300 lux).
Table 3 Photovoltaic parameters of MD dyes measured using T5 fluorescent lighta
dye
illuminance irradiance
Voc
mV
Jsc
FF
%
η
%
Lux
300
mW cm−2
0.09
µA cm−2
499.16 ± 2.61
15.49 ± 0.51
75.18 ± 0.91
6.39 ± 0.13
MD4
600
900
6000
300
600
0.18
0.27
1.7
516.03 ± 2.57
530.11 ± 0.69
582.26 ± 5.09
543.26 ± 18.25
568.20 ± 15.53
29.95 ± 0.13
44.82 ± 0.79
75.58 ± 0.31
76.35 ± 0.13
6.67 ± 0.11
6.81 ± 0.19
8.62 ± 0.16
15.10 ± 0.67
16.49 ± 0.40
315.06 ± 0.00 76.63 ± 0.06
0.09
0.18
33.47 ± 0.66
67.95 ± 1.07
77.24 ± 2.01
77.73 ± 1.73
MD5
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900
6000
300
0.27
1.7
586.23 ± 16.40 104.61 ± 1.96 78.04 ± 1.67
651.16 ± 10.24 762.13 ± 26.12 79.01 ± 1.27
17.38 ± 0.38
23.17 ± 0.22
12.08 ± 0.02
12.79 ± 0.05
13.24 ± 0.11
16.86 ± 0.23
18.95 ± 0.69
20.16 ± 1.10
21.10 ± 1.20
27.17 ± 1.44
19.02 ± 0.70
20.23 ± 0.89
20.94 ± 1.06
27.64 ± 0.54
0.09
0.18
0.27
1.7
542.85 ± 7.68
561.95 ± 8.00
575.60 ± 9.52
634.48 ± 8.30
572.67 ± 3.79
596.77 ± 4.40
611.89 ± 4.22
26.87 ± 0.81
53.46 ± 1.19
80.72 ± 1.14
76.21 ± 0.94
77.05 ± 0.68
77.76 ± 1.01
MD6
MD7
N719
600
900
6000
300
562.20 ± 1.51 77.40 ± 0.82
0.09
0.18
0.27
1.7
40.92 ± 1.76
81.80 ± 2.15
75.85 ± 0.68
76.59 ± 0.77
600
900
124.33 ± 3.47 76.83 ± 0.83
6000
300
676.32 ± 6.32 913.69 ± 37.34 76.42 ± 0.38
0.09
0.18
0.27
1.7
550.53 ± 12.34
582.94 ± 13.91
41.48 ± 0.51
82.17 ± 2.50
75.34 ± 1.17
75.72 ± 1.50
600
900
597.55 ± 14.28 126.06 ± 4.43 75.72 ± 2.27
676.37 ± 24.38 923.38 ± 27.29 75.85 ± 1.98
6000
aVOC: open-circuit voltage; JSC: short-circuit current; FF: fill factor; PCE: power-conversion
efficiency. The data are based on three measurements. Experiments were conducted by using TiO2
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photoelectrodes, soaked with dyes and CDCA (1 mM), with approximately 12 μm thickness and
0.16 cm2 working area on fluorine-doped tin oxide substrates.
4 Conclusions
We have developed four new D−A′−π−A type sensitizers (MD4−MD7) with thieno[3,4-
b]pyrazines (TP) and benzo[3,4-b]pyrazine (BP) as the auxiliary acceptor. Incorporation of 3,4-
dihexyloxyphenyl moiety at the 2- and 3-site of TP and BP improves the solubility of the dyes in
organic solvents and helps with suppression of dark current in DSSCs. The best efficiency of
8.21% for MD7 reaches 92% of N719-based standard cell. Addition of 1 mM CDCA as co-
adsorbent helps with alleviation of dye aggregation and improvement of cell performance.
Consequently, MD7 has an efficiency of 9.00 %, which surpassed that of the N719-based cell.
DSSCs based on MD5 and MD7 have cell efficiencies of >15% at 300 lux irradiance (T5
fluorescent light source). The cell efficiency of MD7 reaches 18.95% at 300 lux and 27.17% at
6000 irradiance.
Associated Content
Supporting Information
It includes the synthesis procedures, 1H NMR spectra, 13C NMR spectra, Mass spectra , details of
theoretical calculations and all necessary figures and tables of the new compounds.
Author information
*Corresponding Authors
E-mail: jtlin@gate.sinica.edu.tw, tcwei@mx.nthu.edu.tw.
Conflict of interest
There are no conflicts to declare
Acknowledgment
We thank the Academia Sinica (AS), National Tsing Hua University and Ministry of Science and
Technology for financial support, and the Instrumental Center of Institute of Chemistry, AS.
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DSSCs fabricated from benzo[3,4-b]pyrazine-based metal-free sensitizer have efficiencies of 9.03%
at 1 sun and 18.95% at 300 lux irradiance.