Highly efficient benzodifuran based ruthenium sensitizers for thin-film dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2015.05.013

The synthesis of 4,4′-dibenzodifuran-2,2′-bipyridine derivatives as ligands of organic ruthenium dyes for DSSC applications is described. Two new heteroleptic ruthenium complexes have been prepared and compared with commercially available N719 and Z907 dyes, using dip dyeing and flow dyeing methods in large area testing cells prepared for industrial purposes. The newly synthesized dyes revealed higher solar-to-electric energy conversion efficiency (η), measured at the AM1.5G conditions, up to 21% compared to N719 and up to 46% compared to Z907, and external quantum efficiency of 53%. These results can be explained by enhanced light-harvesting of the benzodifuran moiety of the dyes that is related to photocurrent.

Full text of DOI:10.1016/j.dyepig.2015.05.013

Dyes and Pigments 121 (2015) 79e87
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Highly efficient benzodifuran based ruthenium sensitizers
for thin-film dye-sensitized solar cells
a
,
*
a
a
b
Mariusz J. Bosiak , Marcin Rakowiecki , Andrzej Wolan , Jolanta Szlachta ,
b
b
c
Edyta Stanek , Dawid Cyco nꢀ , Krzysztof Skupie nꢀ
Synthex Technologies Sp. z o.o., Gagarina 7, 87-100 Toru nꢀ , Poland
ML System Sp. z o. o., Warszawska 50 d, 35-230 Rzesz oꢀ w, Poland
D-nano, Powsta nꢀ c oꢀ w 64b, 31-670 Cracow, Poland
a
b
c
3
a r t i c l e i n f o
a b s t r a c t
0
0
Article history:
The synthesis of 4,4 -dibenzodifuran-2,2 -bipyridine derivatives as ligands of organic ruthenium dyes for
DSSC applications is described. Two new heteroleptic ruthenium complexes have been prepared and
compared with commercially available N719 and Z907 dyes, using dip dyeing and flow dyeing methods
in large area testing cells prepared for industrial purposes. The newly synthesized dyes revealed higher
Received 21 March 2015
Accepted 11 May 2015
Available online 19 May 2015
solar-to-electric energy conversion efficiency (h), measured at the AM1.5G conditions, up to 21%
Keywords:
DSSC
Benzodifuran
The Sonogashira reaction
Ruthenium dyes
Solar cells
compared to N719 and up to 46% compared to Z907, and external quantum efficiency of 53%. These
results can be explained by enhanced light-harvesting of the benzodifuran moiety of the dyes that is
related to photocurrent.
©
2015 Elsevier Ltd. All rights reserved.
Benzofuran
0
0
1
. Introduction
ancillary 4,4 -dinonyl-2,2 -bipyridine ligand [8e10]. Although sta-
bility of the device based on Z907 improved significantly, the effi-
ciency decreased, mainly due to lower light-harvesting, related to
the molar extinction coefficient for two UVeVis absorption bands e
at 350e450 nm (ligand-centered charge transfer (LCCT) transitions
Since 1991 when O'Regan and Gr a€ tzel [1] described the first
dye-sensitized solar cell (DSSC) containing ruthenium dye as a
photosensitizer, this technology have attracted tremendous inter-
est due to its low manufacturing cost, a remarkable stability, and
(p
-
p
*)), as well as at 500e600 nm (metal-to-ligand charge
respectable high solar-to-electric energy conversion efficiency (
h)
transfer (MLCT) transitions (4d - *)). To improve the light-
p
up to 12% [2]. Although the perovskite type sensitizers derived from
lead halides and applied by spin coating technique allowed to reach
the efficiency of ~19.3% [3], there is still place for organic ruthenium
dyes soluble in “green” organic solvents, such as ethanol, and easily
applicable by “dip coating” or “flow dyeing” techniques, well suited
for the preparation of large area solar panels.
harvesting efficiency via an enhanced molar extinction coeffi-
cient, many heteroleptic ruthenium complexes derived from 2,2 -
bipyridine substituted with aryl, heteroaryl and aryl-vinyl groups
were developed [11e25]. Herein, the synthesis and photovoltaic
characterization of benzodifuran (BDF) derived ruthenium dyes of
high energy conversion efficiency is described.
0
The first commonly used homoleptic ruthenium complexes for
DSSC applications (N3 and N719) developed in Gr €a tzel group [4,5]
2. Experimental
revealed
h~11% for devices based on these dyes under one-sun
illumination (Fig. 3) [6,7] The main drawback of the devices was
their low stability. To improve their durability an amphiphilic
2
.1. Materials and general methods
0
ruthenium sensitizer Z907 was developed by replacing one 4,4 -
Experiments with air and moisture sensitive materials were
0
dicarboxylic-2,2 -bipyridine (dcbpy) ligand in the N3 dye with an
carried under argon atmosphere. Glassware was oven dried for
several hours, assembled hot, and cooled in a stream of argon. Silica
gel 60, Merck 230e400 mesh, was used for preparative column
flash chromatography. Analytical TLC was performed using
SigmaeAldrich silica gel on TLC Al foils with fluorescent indicator
*
Corresponding author. Tel.: þ48 512038649; fax: þ48 56 6542477.
E-mail address: bosiak@synthex.com.pl (M.J. Bosiak).
http://dx.doi.org/10.1016/j.dyepig.2015.05.013
143-7208/© 2015 Elsevier Ltd. All rights reserved.
0

80
M.J. Bosiak et al. / Dyes and Pigments 121 (2015) 79e87
0
ꢀ
2
54 nm, 0.2 mm plates. 1-Octanol, iodine, morpholine, 4,4 -
to 150 C in opened flask and stirred at this temperature for 18 h. It
0
0
dibromo-2,2 -bipyridine, [1,1 -bis(diphenylphosphino)ferrocene]
dichloropalladium(II), copper iodide, diisopropylamine, ethynyl-
trimethylsilane, 1M tetrabutylammonium fluoride solution in
was poured into concentrated solution of sodium bicarbonate
(200 mL) and brine (100 mL), stirred for several minutes and
extracted with ethyl acetate (3 ꢁ 100 mL). Combined organic layers
were dried with anhydrous magnesium sulfate. The ethyl acetate
was removed using rotary evaporator and the remaining 1-octanol
was distilled off under reduced pressure. To a slurry n-heptane
(500 mL) was added, the mixture was heated to reflux and filtered
through a warm Büchner funnel. The filtrate was cooled overnight
and the flaky precipitate was filtered off and dried to obtain 32.87 g
0
0
THF, dichloro(p-cymene)ruthenium(II) dimer, 2,2 -bipyridine-4,4 -
dicarboxylic acid, ammonium thiocyanate, and tetrabutylammo-
nium hydroxide were commercially available from SigmaeAldrich,
Merck or Fluorochem, and were used without further purification.
0 0
2,2 -Bipyridine]-4,4 -dicarbaldehyde was prepared according to
[
the literature procedure [26]. Solvents were purchased from
Avantor and SigmaeAldrich. Toluene was distilled from sodium
benzophenone ketyl prior to use, DMF was stirred overnight with
calcium hydride, filtered, distilled at 20 mmHg, and stored over 4A
molecular sieves. Chloroform, diethyl ether, methanol, anhydrous
ethanol, THF, n-heptane, and ethyl acetate were used without
further purification. Deuterated solvents for NMR spectroscopy
were purchased from SigmaeAldrich. Sephadex LH-20 was pur-
chased from GE Healthcare. N719 and Z907 dyes were purchased
from Dyesol. Melting points were determined with a Büchi SMP 32
and Barnstead-Thermolyne Mel-Temp II apparatus in open capil-
laries and are uncorrected. Elemental analyses were performed at
Elementary Analysensysteme GmbH Vario MACRO CHN analyzer.
ꢀ
1
(80%) of pure solid, mp 93e94 C. H NMR (CDCl
ppm: 0.90 (t, J ¼ 6.8 Hz, 3H, CH ), 1.28e1.42 (m, 8H, 4CH
1.45e1.52 (m, 2H, CH ), 1.78e1.86 (m, 2H, CH ), 2.76 (s, 3H, CH
4.36 (t, J ¼ 6.8 Hz, 2H, CH
J ¼ 2.6 Hz,1H, CHAr), 7.30 (d, J ¼ 8.8 Hz,1H, CHAr), 7.46 (m, 1H, CHAr).
3
, 400 MHz);
d
3
2
),
),
2
2
3
2
), 5.23 (s, 1H, OH), 6.82 (dd, J ¼ 8.8 Hz,
13
C NMR (CDCl
3
, 100 MHz); ppm: 14.07 (CH ), 14.81 (CH ), 22.63
d
3
3
(CH
(CH
2
), 26.10 (CH ), 28.73 (CH ), 29.19 (CH ), 29.22 (CH ), 31.78
), 64.82 (CH ), 106.81 (CH), 108.90 (C), 111.25 (CH), 112.79 (CH),
127.31 (C), 148.50 (C), 152.85 (C), 164.38 (C), 165.07 (C). IR (ATR)
max cm : 3348, 2948, 2923, 2854, 1670, 1622, 1605, 1577, 1463,
1415,1394,1382,1359,1298,1268,1258,1212,1173,1111,1094,1020,
962, 944, 863, 837, 806, 789, 783, 735, 724, 648, 619, 611, 581. Anal.
2
2
2
2
2
2
ꢂ1
n
24 4
Calcd. for C18H O : C, 71.03; H, 7.95. Found: C, 71.21; H, 7.97.
2
.1.1. Spectroscopic measurements
1H and 13C NMR spectra were recorded on a Bruker Advance III
00 MHz or Bruker Advance III 700 MHz instrument at ambient
2.2.3. Ethyl 5-hydroxy-6-iodo-2-methylbenzofuran-3-
carboxylate (3)
4
temperature. Chemical shifts are reported in parts per million
scale), coupling constants (J values) are listed in Hertz. Anhydrous
ethanol (spectrometric grade from SigmaeAldrich) was employed
Ethyl
5-hydroxy-2-methylbenzofuran-3-carboxylate
(1)
(d
(26.43 g, 120 mmol) and iodine (48.72 g, 192 mmol, 1.6 equiv.) were
dispersed in anhydrous methanol (100 mL) and stirred for 5 min
under argon. The mixture was cooled to 0 C, and morpholine
ꢀ
as solvent for absorption measurements. UV/Vis absorption spectra
of dyes solutions and sensitized TiO
2
films were recorded by means
(30.32 g, 348 mmol, 2.9 equiv.) was added dropwise at a rate to
ꢀ
ꢀ
of a Jasco V670 UVeViseNIR spectrophotometer equipped with
liquid a cell holder (10 mm quartz cell, versus solvent blank) and
thin film holder. IR spectra were recorded on a Perkin Elmer FT-IR
Spectrum Two spectrometer equipped in diamant ATR.
keep the temperature below 30 C, slowly heated to 35 C, and
stirred at this temperature for 120 h. Water (20 mL) was added and
ꢀ
the mixture was left at ꢂ20 C overnight. Precipitate was filtered
ꢀ
off, washed with cold methanol of (ꢂ25 C, 3 ꢁ 60 mL), and dried to
obtain 46.91 g of beige solid. It was extracted with diethyl ether
(450 mL) using a Soxhlet extractor for 48 h. The crude product was
finally crystallized from methanol to obtain 13.97 g (34%) of pure
2
2
.2. Synthesis of the compounds
ꢀ
ꢀ
1
.2.1. Ethyl 5-hydroxy-2-methylbenzofuran-3-carboxylate (1)
A solution of p-benzoquinone (54.045 g, 0.5 mol) and ethyl
white solid, mp ¼ 196e197 C. Lit [27]. 194e194.5 C. H NMR
(CDCl , 700 MHz); ), 2.73 (s, 3H,
ppm: 1.44 (t, J ¼ 7.2 Hz, 3H, CH
CH ), 5.28 (s, 1H, OH), 7.56 (s, 1H, CHAr),
), 4.40 (q, J ¼ 7.2 Hz, 2H, CH
7.73 (s, 1H, CHAr). C NMR (CDCl , 100 MHz); ppm: 14.42 (CH ),
14.56 (CH ), 60.47 (CH ), 80.68 (C), 106.25 (CH), 108.98 (C), 119.63
3
d
3
acetoacetate (200 mL, 1.583 mol) in dry acetone (200 mL) was
added dropwise during 9 h to a mechanically stirred solution of
anhydrous zinc chloride (70 g, 0.513 mol) in acetone (100 mL) and
3
2
13
3
d
3
3
2
ꢀ
acetic acid (5 mL) at 75 C. The mixture was stirred for additional
(CH),128.38 (C),148.73 (C), 151.25 (C), 164.11 (C),165.11 (C). IR (ATR)
ꢂ1
10 min, transferred into a large crystallizer and left opened in fume
n
max cm : 3255, 1672, 1614, 1582, 1447, 1405, 1380, 1345, 1279,
cupboard overnight. The crystallizer was transferred into freezer
and left for 24 h. Precipitate was filtered off, washed with cold
MeOH (2 ꢁ 50 mL) and dried on air. Crude product was crystallized
1268,1183,1166,1150,1115,1093,1033, 964, 909, 862, 841, 800, 786,
4
722, 657, 590. Anal. Calcd. for C12H11IO : C, 41.64; H, 3.20. Found: C,
41.48; H, 3.21.
ꢀ
from toluene to give 47.50 g (43%) of white solid, mp ¼ 143e144 C.
ꢀ
1
Lit [27]. 143.5e144 C. H NMR (700 MHz, CDCl
3
);
d
ppm: 1.41
2.2.4. Octyl 5-hydroxy-6-iodo-2-methylbenzofuran-3-
(
6
(
t, J ¼ 7 Hz, 3H, CH
.79e6.80 (dd, J ¼ 9.1 Hz, J ¼ 2.8 Hz, 2H, CH
d, J ¼ 4.2 Hz, 1H, CH), 7.26 (s, 1H, CH), 7.48 (d, J ¼ 2.8, 1H, CH).
NMR (CDCl , 175 MHz); ppm: 15.38 (CH ), 15.75 (CH ), 61.53
3
), 2.71 (s, 3H, CH
3
), 4.38 (q, J ¼ 7 Hz, 2H, CH),
carboxylate (4)
2
), 5.88 (s, 1H, CH), 7.24
Octyl
5-hydroxy-2-methylbenzofuran-3-carboxylate
(2)
13
C
(12.16 g, 40 mmol) and iodine (24.38 g, 96 mmol, 2.4 equiv.) were
dispersed in anhydrous methanol (60 mL) and stirred for 10 min
under argon. The mixture was cooled to 0 C and morpholine
(15.20 mL, 175 mmol, 4.35 equiv.) was added dropwise at a rate to
keep the temperature below 30 C, slowly heated to 35 C, and
stirred at this temperature for 48 h. Water (12 mL) was added and
3
d
3
3
ꢀ
(
CH
2
), 107.84 (CH), 109.86 (C), 112.25 (CH), 113.72 (CH), 128.26 (C),
ꢂ1
1
49.52 (C), 153.67 (C), 165.52 (C), 165.86 (C). IR (ATR)
n
max cm
:
ꢀ
ꢀ
3
1
6
323, 1668, 1623, 1607, 1580, 1475, 1464, 1417, 1382, 1348, 1300,
259, 1217, 1168, 1111, 1087, 1027, 954, 866, 853, 838, 809, 784, 736,
82, 652, 617, 573. Anal. Calcd. for C12 : C, 65.45; H, 5.49.
Found: C, 65.61; H, 5.44.
ꢀ
12
H O
4
the mixture was left at ꢂ20 C overnight. Precipitate was filtered
ꢀ
off, washed with cold methanol (ꢂ25 C, 5 ꢁ 25 mL) and dried to
obtain 17.10 g of beige solid. It was dispersed in chloroform
(300 mL) and stirred for 5 min. Insoluble precipitate of morpholine
hydroiodide was filtered off and the filtrate was concentrated to
obtain 10.02 g of crude brown product. Crystallization from
methanol/water (200 mL/25 mL) gave 8.89 g (52%) of light yellow
2
.2.2. Octyl 5-hydroxy-2-methylbenzofuran-3-carboxylate (2)
To a dispersion of ethyl 5-hydroxy-2-methylbenzofuran-3-
carboxylate (29.54 g, 134 mmol) in 1-octanol (300 mL), concen-
trated sulfuric acid (1 mL) was added and the mixture was heated

M.J. Bosiak et al. / Dyes and Pigments 121 (2015) 79e87
81
ꢀ
1
solid, mp ¼ 124e125 C. H NMR (CDCl
3
, 700 MHz);
), 1.26e1.42 (m, 8H, 4CH
), 1.78e1.86 (m, 2H, CH ), 2.75 (s, 3H, CH
t, J ¼ 6.8 Hz, 2H, CH ), 5.44 (s, 1H, OH), 7.59 (s, 1H, CHAr), 7.74 (s, 1H,
, 175 MHz); ppm: 14.01 (CH ), 14.36
), 25.59 (CH ), 28.23 (CH ), 28.67 (CH ), 28.69
), 64.17 (CH ), 80.95 (C), 105.15 (CH), 108.12 (C),
d
ppm: 0.90
), 1.44e1.51
), 4.36
1.51e1.55 (m, 4H, 2 ꢁ CH
2
), 1.87e1.91 (m, 4H, 2 ꢁ CH
2
), 2.82 (s, 6H,
(
(
(
t, J ¼ 6.8 Hz, 3H, CH
3
2
2 ꢁ CH
3
), 4.41 (t, J¼7.0 Hz, 4H, 2 ꢁ CH ), 7.46 (s, 2H, 2 ꢁ CHAr), 7.62
2
m, 2H, CH
2
2
3
(s, 2H, 2 ꢁ CHAr), 7.80 (dd, J ¼ 5.0 Hz, J ¼ 1.5 Hz, 2H, 2 ꢁ CHAr), 8.10
(s, 2H, 2 ꢁ CHAr), 8.82 (d, J ¼ 5.0 Hz, 2H, 2 ꢁ CHAr), 8.90 (s, 2H,
2
13
13
ꢂ1
CHAr). C NMR (DMSO-d
6
d
3
2 ꢁ CHAr). Solubility too low for C NMR. IR (ATR)
nmax cm : 2928,
(
(
CH
CH
3
), 22.14 (CH
), 31.23 (CH
2
2
2
2
2861, 1708, 1605, 1592, 1406, 1383, 1368, 1239, 1222, 1176, 1151, 1117,
1091, 1043, 988, 923, 863, 832, 808, 796, 701. Anal. Calcd. for
2
2
2
1
20.34 (CH), 127.04 (C), 147.63 (C), 153.25 (C), 163.38 (C), 164.22 (C).
50 52 2 8
C H N O : C, 74.24; H, 6.48; N, 3.46. Found: C, 74.43; H, 6.50; N,
3.49.
ꢂ1
IR (ATR)
n
max cm : 3266, 2959, 2918, 2854, 1669, 1610, 1580, 1467,
1
8
C
448, 1400, 1355, 1340, 1262, 1201, 1184, 1166, 1152, 1099, 1021, 962,
87, 865, 840, 789, 726, 676, 661, 593, 558, 474. Anal. Calcd. for
: C, 50.24; H, 5.39. Found: C, 50.53; H, 5.41.
0
0
2
.2.7. (TBA(Ru[(4-carboxylic acid-4 -carboxylate-2,2 -bipyridine)
18 4
H23IO
(Ligand-6a) (NCS) ])) (M44)
2
A mixture of dichloro(p-cymene)ruthenium(II) dimer (0.3062 g,
0
0
0
2
[
.2.5. Diethyl 6,6 -([2,2 -bipyridine]-4,4 -diyl)bis(2-methylbenzo
0
.5 mmol) and 6a (0,6402 g, 1 mmol) in dry DMF (40 mL) was
0
1,2-b:4,5-b ]difuran-3-carboxylate) (6a)
ꢀ
stirred in the dark, under argon at 150 C, monitoring the disap-
0
0
0
4
,4 -Dibromo-2,2 -bipyridine (1.5699 g, 5 mmol), [1,1 -bis(di-
phenylphosphino)ferrocene]dichloropalladium(II) (0.3660 g,
.5 mmol, 10% mol), and copper iodide (0.0952 g, 0.5 mmol, 10%
mol) were placed under argon in a Schlenk flask and dry toluene
20 mL) was added, followed by diisopropylamine (10 mL,
0 mmol). The mixture was stirred for 15 min and ethynyl-
pearance of substrates and appearance of product by UVeVis
0
0
spectrometry (~2 h). 2,2 -Bipyridine-4,4 -dicarboxylic acid
0
ꢀ
(
0.2442 g, 1 mmol) was added and the mixture was stirred at 150 C
for 4 h. Ammonium thiocyanate (2.74 g, 36.5 mmol) was added and
(
7
ꢀ
the mixture was stirred at 150 C for another 4 h. It was cooled to rt,
filtered, and DMF was removed with the rotary evaporator. To the
residual slurry water (100 mL) was added and filtered. Precipitate
was washed with water (100 mL) and dried under vacuum. The
crude product was dissolved in DMF and then passed through a
Sephadex LH-20 column with DMF as eluent. The main band was
collected and concentrated. This purification process was repeated
three times. Pure complex was dispersed in ethanol and titrated
with 0.5M tetrabutylammonium hydroxide solution to pH ¼ 9,
stirred for 5 min, filtered, and the filtrate was titrated with 0.01M
trimethylsilane (1.2766 g, 13 mmol) in dry toluene (5 mL) was
added in one portion. The mixture was heated to 80 C until all
ꢀ
ethynyltrimethylsilane was consumed, as judged by GC (1.5 h). It
was cooled to rt and 1M tetrabutylammonium fluoride solution in
THF (10 mL) was added. After 1 min ethyl 5-hydroxy-6-iodo-2-
methylbenzofuran-3-carboxylate (3.4630 g, 10 mmol) in dry
ꢀ
toluene (50 mL) was added and the mixture was heated to 80 C for
2
0 h. It was cooled to rt, water (40 mL) was added and filtered using
Büchner funnel. Precipitate was returned to the flask, water (50 mL)
and diethyl ether (50 mL) were added and stirred for 30 min. The
procedure was repeated using methanol (120 mL) and finally THF
HNO
rator, the product was washed with acidified water (pH ¼ 5.2), and
dried under vacuum and P to obtain 0.4966 g (36%) of pure
complex as black plates, mp > 300 C. IR-ATR, cm : 2962, 2935,
856, 2100, 1707, 1611, 1467, 1426, 1402, 1362, 1305, 1235, 1176,
083, 1019, 979, 921, 853, 807, 782, 705, 473. Anal. Calcd. for
3
to pH ¼ 5.2. Solvents were removed with the rotary evapo-
2 5
O
(
120 mL). The precipitate was filtered using Büchner funnel and
ꢀ
ꢂ1
dried under vacuum to give 2.5070 g (78%) of pure desired product,
2
1
ꢀ
mp > 300 C. 1H NMR (CDCl
6
7
3
, 700 MHz);
), 2.82 (s, 6H, 2 ꢁ CH ), 4.48 (q, J ¼ 7.1 Hz, 4H, 2 ꢁ CH
.47 (s, 2H, 2 ꢁ CHAr), 7.63 (s, 2H, 2 ꢁ CHAr), 7.81 (s, 2H, 2 ꢁ CHAr),
.11 (s, 2H, 2 ꢁ CHAr), 8.83 (s, 2H, 2 ꢁ CHAr), 8.91 (s, 2H, 2 ꢁ CHAr).
d
ppm: 1.53 (t, J ¼ 7.1 Hz,
H, 2 ꢁ CH
3
3
2
),
C
52
35
H N
6
2 2
O12RuS ·TBA·2H O: C, 59.20; H, 5.48; N, 7.11. Found: C,
59.41; H, 5.52; N, 7.16.
8
13
ꢂ1
nmax cm : 2979, 1709,
Solubility too low for C NMR. IR (ATR)
601, 1368, 1174, 1157, 1081, 853, 832, 809, 701, 533. Anal. Calcd. for
0
0
1
2.2.8. (TBA(Ru[(4-carboxylic acid-4 -carboxylate-2,2 -
bipyridine)(Ligand-6b)(NCS) ])) (M43)
C
4
38
H
.39.
28
N
2
O
8
: C, 71.24; H, 4.41; N, 4.37. Found: C, 71.40; H, 4.38; N,
2
A mixture of dichloro(p-cymene)ruthenium(II) dimer (0.3062 g,
0
.5 mmol) and 6a (0.809 g, 1 mmol) in dry DMF (40 mL) was stirred
0
0
0
ꢀ
2
[
.2.6. Dioctyl 6,6 -([2,2 -bipyridine]-4,4 -diyl)bis(2-methylbenzo
in the dark, under argon at 150 C, monitoring the disappearance of
substrates and appearance of product by UVeVis spectrometry
(~2 h). 2,2 -Bipyridine-4,4 -dicarboxylic acid (0.2442 g, 1 mmol)
was added and the mixture was stirred at 150 C for 4 h. Ammo-
nium thiocyanate (2.74 g, 36.5 mmol) was added and the mixture
was stirred at 150 C for another 4 h. It was cooled to rt, filtered, and
0
1,2-b:4,5-b ]difuran-3-carboxylate) (6b)
0
0
0
0
0
4
,4 -Dibromo-2,2 -bipyridine (1.5699 g, 5 mmol), [1,1 -bis(di-
phenylphosphino)ferrocene]dichloropalladium(II) (0.3660 g,
.5 mmol, 10% mol), and copper iodide (0.0952 g, 0.5 mmol, 10%
mol) were placed under argon in a Schlenk flask and dry toluene
20 mL) was added followed by diisopropylamine (10 mL,
0 mmol). The mixture was stirred for 15 min and ethynyl-
trimethylsilane (1.2766 g, 13 mmol) in dry toluene (5 mL) was
added in one portion. The mixture was heated to 80 C until all
ethynyltrimethylsilane was consumed as judged by GC (2 h). It was
cooled to rt and 1M tetrabutylammonium fluoride solution in THF
ꢀ
0
ꢀ
(
7
DMF was removed with the rotary evaporator. To the residual slurry
water (100 mL) was added and filtered. Precipitate was washed
with water (100 mL) and dried under vacuum. The crude product
was dissolved in DMF and then passed through a Sephadex LH-20
column with DMF as eluent. The main band was collected and
concentrated. This purification was repeated three times. Pure
complex was dispersed in ethanol and titrated with 0.5M tetra-
butylammonium hydroxide solution to pH ¼ 9. It was stirred for
ꢀ
(
13 mL) was added. After 1 min octyl 5-hydroxy-6-iodo-2-
methylbenzofuran-3-carboxylate (4.3040 g, 10 mmol) in dry
toluene (50 mL) was added and the mixture was heated to 80 C for
ꢀ
3
5 min, filtered and the filtrate was titrated with 0.01M HNO to
1
4 h. It was cooled to rt, water (40 mL) was added and filtered using
pH ¼ 5.2. Solvents were removed with the rotary evaporator, the
Büchner funnel. Precipitate was returned to the flask, water (50 mL)
and diethyl ether (50 mL) were added and stirred for 10 min. The
precipitate was filtered off and dried under vacuum, 3.8870 g (96%)
product was washed with acidified water (pH ¼ 5.2), and dried
under vacuum and P
mp > 300 C. IR-ATR, cm-1: 2962, 2935, 2856, 2100,1707,1611,1467,
2
O
5
to obtain 0.5418 g (35%) of black plates,
ꢀ
ꢀ
of pure desired product, mp ¼ 212e214 C. 1H NMR (CDCl
3
,
1426, 1402, 1362, 1305, 1235, 1176, 1083, 1019, 979, 921, 853, 807,
7
4
00 MHz);
d
ppm: 0.89 (t, J ¼ 7.0 Hz, 6H, 2 ꢁ CH
3
), 1.28e1.35 (m, 8H,
782, 705, 473. Anal. Calcd. for C64
59 6 2 2
H N O12RuS ·TBA·2H O: C,
ꢁ CH ), 1.36e1.39 (m, 4H, 2 ꢁ CH
2
2
), 1.41e1.45 (m, 4H, 2 ꢁ CH
2
),
62.08; H, 6.45; N, 6.33. Found: C, 61.98; H, 6.42; N, 6.37.

82
M.J. Bosiak et al. / Dyes and Pigments 121 (2015) 79e87
The NMR spectra of the dyes are complex because they contain
24 h at room temperature. After dyeing the plates were rinsed with
pure ethanol and dried under nitrogen. The dye-coated TiO elec-
trodes and Pt counter electrodes were assembled into a sealed
mixture of two diastereoisomers. As a result, the NMR spectra
exhibit groups of broad peaks in the 6ꢂ9 ppm region that cannot be
separated and unambiguously assigned, and have not been
described.
2
sandwich-type cell by heating with a hot-melt ionomer film (Surlyn
1702, 25
m
m thickness, DuPont) as a spacer between the electrodes.
ꢀ
An encapsulation process was performed at 110 C. The cell internal
space was filled with a liquid electrolyte (EL-HPE, Dyesol) using the
holes on the counter electrode, which were sealed by heating with a
Surlyn sheet and a thin glass cover. The flow dyeing procedure was
conducted after electrodes encapsulation. The same dyes solutions
were flown through the cell for 8 h using peristaltic pomp and
electrolyte-injecting holes. The obtained DSSC devices consists of
2
.3. Device fabrication
2
Fluorine-doped SnO (FTO) glass plates (AGC ANS10ME, thick-
ness 3.2 mm, 10
U
/sq) with dimensions 200 ꢁ 70 mm were struc-
tured by means of ytterbium laser in order to obtain eight separated
areas on photoanode and counter electrode. Then two holes
2
(
Ø1 mm) were drilled in counter electrode. After that glass plates
were washed in ultrasonic bath containing de-ionized water,
acetone and ethanol. A transparent single layer of 20-nm sized TiO
eight 8 individual cells (Fig. 2), active area of single cell was 5 cm . In
order to assure reliable results, six DSSC devices for each dye were
prepared, where three of them were used in dip dyeing and next
three in flow dyeing method.
2
particles (18NR-T, Dyesol) and silver stripes (AG 7500-80, Johnson
Matthey) were screen-printed on photoanode. A nanocolloidal Pt
(
3D-nano) layer and silver stripes were applied on counter elec-
2.4. Photovoltaic measurements
trode in the same way. All layers thickness was precisely deter-
mined using optical profiler (Sensofar PLU Neox), and the fabricated
2
The DSSC devices were evaluated under 100 mW/cm standard
TiO
2
film thickness was around 5
m
m (Fig.1). Finally, the plates were
air mass 1.5 global (AM 1.5G) sunlight simulation with a class AAA
solar cell analyzer (Model No. #SS150AAA, PV Test Solutions)
conjugated with Keithley model 2401 digital source meter. A silicon
solar cell fitted with a KG3 filter tested and certified by Fraunhofer
ISE was used for calibration. The KG3 filter accounts for the
ꢀ
ꢀ
dried at 150 C for 30 min and annealed at 480 C for 45 min. Dyeing
process was conducted using dip dyeing and flow dyeing methods.
In dip dyeing process photoanodes were immersed in solutions
containing 1 ꢁ 10 M dyes (M43, M44, N719, Z907) in ethanol for
ꢂ4
2
Fig. 1. Topography and profile of nanocrystalline TiO layer deposited on photoelectrode.

M.J. Bosiak et al. / Dyes and Pigments 121 (2015) 79e87
83
Fig. 2. Test plate with eight cells (M43 dye) prepared in ML System Laboratory.
different light absorption of the dye-sensitized solar cell and sili-
con, and ensues that the spectral mismatch correction factor ap-
proaches unity. The voltage step and scan rate were 10 mV and
Addition of 3 or 4 to the reaction flask resulted in the second
Sonogashira reaction followed by palladium catalyzed cyclization
of hydroxyl group and triple bond to form a new furan ring in 78%
and 96% yield, respectively. Unfortunately all attempts to hydrolyze
the ester groups failed, mainly due to low solubility of both esters
and acid in polar solvents.
1
00 ms, respectively. A special black plastic mask with an aperture
2
area of 5 cm was covered on each testing cell during measurement
to eliminate light reflection leading to unreliable results. Incident
photo-current efficiency (IPCE) studies were carried out by means
of Bentham PVE 300 analyzer, with xenon/quartz halogen light
source and measured in DC mode with low chopping frequency
The desired heteroleptic ruthenium complexes were prepared
in one-pot process (Scheme 3). The BDF ligands were reacted with
dichloro(p-cymene)ruthenium(II) dimer until all substrates were
consumed, as judged by UVeVis spectra (~2h). The resulting in-
(
<1 Hz). The device was calibrated with reference photodiode DH
ꢀ
0
0
Si. A cell spectral response was obtained at 25 C for wavelengths
termediate complexes were reacted with 2,2 -bipirydylo-4,4 -
dicarboxylic acid and then with ammonium thiocyanate to replace
chlorine atoms with isothiocyanate groups.
range 300e800 nm with 5 nm step.
The reaction proceeds similarly when ruthenium(III) chloride is
used as a source of ruthenium. Crude products were purified by
column chromatography on Sephadex LH-20 with DMF as eluent.
The main band was collected, and the process was repeated three
times. Pure complexes were titrated in ethanol with tetrabuty-
lammonium hydroxide solution to pH ¼ 9 and then with 0.01M
3
. Results and discussion
3.1. Synthesis
The aim of this work was the development and photovoltaic
characterization of new heteroleptic ruthenium complexes for
DSSC application, containing BDF moiety combined with 2,2 -
HNO
3
to pH ¼ 5.2. The complexes (dyes) were obtained as black,
0
shiny microcrystalline solids.
bipyridine as ligands and their comparison with commercially
available N719 and Z907 dyes (Fig. 3). The heteroleptic complexes
3.2. Spectroscopic properties of the dyes
0
were prepared using ester form of BDF ligands and 2,2 -bipyridine-
0
4
,4 -dicarboxylic acid. Two alkyl esters, ethyl and 1-octyl, have been
The dyes M43 and M44 showed good solubility in ethanol and
ꢂ4
chosen to investigate the effect of the ester chain length on the
solubility of dyes and efficiency of fabricated dye-sensitized solar
cells based on these dyes.
1 ꢁ 10 M solutions, required for dyeing process, were prepared.
Their UVeVis spectra in comparison with commercial N719 and
ꢂ6
Z907 (Fig. 4) were recorded at lower concentration (5 ꢁ 10 M) to
The synthesis of BDF ligands started from ethyl and n-octyl
achieve absorbance below 0.1 for low-energy CT band at ~530 nm
and to avoid deviations from the BeereLambert law. The visible
region band is typical for heteroleptic polypyridyl ruthenium(II)
complexes [13,29].
5
-hydroxy-6-iodo-2-methylbenzofuran-3-carboxylates
(3,
4)
obtained as shown in Scheme 1. Ethyl 5-hydroxy-2-
methylbenzofuran-3-carboxylate (1) was prepared according to
Bernatek and Ledaal [28] procedure from p-benzoquinone and
ethyl acetylacetate in the presence of anhydrous zinc chloride in
The spectra revealed higher molar extinction coefficient of M43
and M44 for both LCCT (350e450 nm) and MLCT (500e600 nm)
absorption bands in comparison to commercial dyes (Fig. 4).
Although the low energy MLCT band for M43 has almost the same
4
3% yield. Acid catalyzed transesterification with 1-octanol led to
octyl 5-hydroxy-2-methylbenzofuran-3-carboxylate (2) in 80%
yield. Both 1 and 2 were iodinated by modified Giza and Hinman
ꢂ1
ꢂ1
molar absorption coefficient as N719 (ε ¼ 13600 M cm vs.
ꢂ1
ꢂ1
[27] protocol to give the desired products 3 and 4 in 34% and 52%
ε ¼ 13760 M cm ) the molar absorption coefficient for LCCT
ꢂ1
ꢂ1
ꢂ1
ꢂ1
yield respectively.
band is much higher (ε ¼ 23200 M cm vs. ε ¼16000 M cm
)
The BDF ligands 6a and 6b were prepared in the one-pot pro-
cedure involving the Sonogashira coupling reaction and palladium
(Table 1).
The M44 dye revealed even higher molar absorption coefficient
for both LCCT and MLCT bands than M43. The MLCT transition ab-
sorption for M43 and M44 peaks at 538 nm and 533 nm, respectively,
are red-shifted compared to Z907 (521 nm) and N719 (526 nm).
The spectra of M43 and M44 solutions in ethanol, and anchored
0
catalyzed cyclization leading to a new furan ring (Scheme 2). 4,4 -
0
Dibromo-2,2 -bipyridine reaction with ethynyltrimethylsilane in
0
the presence of 10% mol [1,1 -bis(diphenylphosphino)ferrocene]
dichloropalladium(II) and copper iodide in toluene followed by
cleavage of trimethylsilyl groups with tetrabutylammonium fluo-
ride led to 4,4 -diethynyl-2,2 -bipyridine (5).
on a 5
2
mm thick transparent nanocrystalline TiO film, have almost
0
0
identical profiles (Fig. 5).

84
M.J. Bosiak et al. / Dyes and Pigments 121 (2015) 79e87
Fig. 3. Molecular structures of N719, Z907, M43 and M44.
Scheme 1. Synthesis of 5-hydroxy-6-iodo-2-methylbenzofuran-3-carboxylates for the Sonogashira coupling.
3.3. Photovoltaic characterization
cells were measured: 24 for dip dyeing and 24 for flow dyeing
method. The obtained average results were summarized in Table 2.
Moreover, Fig. 6 and Fig. 7 present currentevoltage characteristics
and spectral response as a function of wavelength for chosen cells,
respectively.
As shown in Table 2, the short-circuit current (Isc) of cells with
newly synthetized M44 and M43 dyes is higher than with
commercially available N719 and Z907 sensitizers, what corre-
sponds to their higher efficiency and power. This tendency is
visible for both dyeing methods. However, a significant difference
Two methods of dyeing test cells coated with a thin layer (5 mm)
of titanium dioxide were examined e dip dyeing and flow dyeing.
DSSC photovoltaic performances were used in order to compare
these methods and applied dyes. For each cell, photovoltaic pa-
rameters such as: open-circuit voltage (Voc), short-circuit current
(
(
I
V
sc), fill factor (FF), max power output (P
), max power current (I ), and solar-to-electric energy con-
) were determined. For each applied dye 48
m
), max power voltage
m
m
version efficiency (
h

M.J. Bosiak et al. / Dyes and Pigments 121 (2015) 79e87
85
Scheme 2. The “one pot” synthesis of BDF-bipyridine ligands.
Scheme 3. Synthesis of heteroleptic BDF-bipyridineedcbpy ruthenium(II) complexes.
Table 1
Molar absorption coefficients and wavelengths maxima for M43, M44, N719,
and Z907 in ethanol.
ꢂ1
ꢂ1
ꢂ1
ꢂ1
l
Dye
LCCT ε [M cm ]/
l
max [nm] MLCT ε [M cm ]/ max [nm]
M43
M44
23,200/391
31,940/377
13,600/538
15,800/533
13,760/526
6440/521
N719 16,000/382
Z907 12,534/411
between these methods was noticed. Thus, considering the same
photovoltaic parameters, the obtained values for the dip dyeing are
lower than for the flow dyeing process, e.g. the cell with M44 dye
exhibits an increase in Isc and in
h from 29.7 mA to 32.7 mA and
2.56%e2.81%, respectively. This trend is also observed in incident
photon-to-current efficiency (IPCE) plots. IPCE indicates the ratio of
Fig. 4. Absorption spectra of M43, M44, N719, and Z907 in ethanol.
the number of photons incident on a solar cell to the number of

86
M.J. Bosiak et al. / Dyes and Pigments 121 (2015) 79e87
Fig. 7. Incident-photon-to-current conversion efficiency spectra of DSSC based on
nanocrystalline TiO
methods.
2
film sensitized by M43, M44, N719 and Z907 dyes for two dyeing
Fig. 5. Absorption spectra of M43 and M44 in ethanol and anchored on a 5 mm thick
2
transparent nanocrystalline TiO film.
dyes absorption properties. Having compared Fig. 4 and Fig. 7 a
good relationship was observed between behavior of the dyes in
solution and in working DSSC device. The investigated dyes indicate
the highest absorption in the spectral region from 350 to 560 nm
which is correlated with the achieved IPCE values. To sum up, the
Table 2
Average photovoltaic parameters of the dye-sensitized solar cells with different
sensitizers for two dyeing methods.
2
evaluated DSSCs, based on nanocrystalline TiO film sensitized by
Complex
Voc [mV]
Isc [mA]
V
m
[mV]
I
m
[mA]
P
m
[mW] FF
h [%]
M43, M44, N719 and Z907 dyes for two dyeing methods, exhibit the
same trend with the order M44 > M43 > N719 > Z907 for
currentevoltage characteristics and IPCE curves. However, the
flowing process gives better photovoltaic parameters than the
dipping process.
Moreover, the flow dyeing method has further advantages in
comparison to dip dyeing process from practical point of view.
Firstly, it does not need as much dye solution as the second method.
Secondly, flow dyeing time is shorter than dip dyeing and takes
only 8 h whereas dipping needs 24 h. Additionally, flow dyeing
process is carried out in a closed system, without contact with
moisturizing air, which may cause dyes degradation.
Dip dyeing
M43
M44
N719
Z907
664
666
679
742
29.2
29.7
24.2
19.1
479
479
511
529
26.5
26.4
22.2
17.0
12.7
12.8
11.3
0.66 2.54
0.65 2.56
0.69 2.26
0.64 1.75
8.91
Flow dyeing
M43
M44
N719
Z907
662
28.8
32.7
25.5
21.2
479
473
495
550
26.2
29.7
23.5
19.0
12.5
14.0
11.6
10.5
0.66 2.51
0.64 2.81
0.68 2.32
0.65 2.06
670
671
764
Its noteworthy that values of efficiency for cells sensitized by
commercially available dyes are ~4 times smaller than reported in
literature. This can be explained by the fact that we applied testing
conditions more similar to the industrial ones. There was used
2
2
much larger testing cell area (5 cm ) than 0.158 cm standard
testing cell reported in the literature, and only single 5 m trans-
m
parent layer without scattering layer was used to maintain
maximum transparency. The “green” solvent, ethanol, preferred in
industrial applications was used. In these conditions our hetero-
leptic dyes revealed much higher efficiency than the most
commonly used heteroleptic dye Z907 (145% and 146% for M43 and
M44, respectively using dip dyeing, and 121% and 136%, respec-
tively using flow dyeing process). Moreover, both newly synthe-
sized dyes revealed also higher efficiency than the most commonly
used homoleptic dye N719 (112% and 113% for M43 and M44,
respectively using dip dyeing, and 108% and 121%, respectively
Fig. 6. Current e voltage characteristics of DSSC with M43, M44, N719 and Z907
sensitizers for two dyeing methods measured under illumination of AM1.5G sunlight
2
2
(
100 mW/cm ). Aperture area of the testing mask: 5 cm .
using flow dyeing process). Assuming that the efficiency of
2
0
.158 cm standard testing cell sensitized with N719 in laboratory
conditions is 11.18% [6] using dip dyeing in 1:1 mixture of aceto-
ꢂ4
generated charge carriers. The obtained results for devices with
newly synthesized dyes show higher IPCE values in the wavelength
range of 350e490 nm and 540e700 nm in comparison to N719 and
Z907 dyes (Fig. 7). However, N719 has slightly better performance
than M43 above 490 nm up to 540 nm, but worse than M44. The
M44 sensitizer has an advantage over the other studied dyes and
achieves the highest IPCE values in the entire wavelength range. It
was noticed that photovoltaic parameters strongly depend on the
nitrile:tert-butanol, c ¼ 5 ꢁ 10 M, we predict that M43 and M44
could achieve in these conditions efficiency of ~12.6%.
4. Conclusion
We report herein the design, synthesis, and characterization
of novel heteroleptic ruthenium dyes M43 and M44 containing

M.J. Bosiak et al. / Dyes and Pigments 121 (2015) 79e87
87
0
benzodifuran moieties conjugated with the anchoring 2,2 -
polypyridyl photosensitizers in solar cells based on nanocrystalline TiO
Langmuir 2002;18:952e4.
2
Films.
bipyridine group as a ligand. Both newly synthesized dyes revealed
increased molar extinction coefficients compared with Z907 and
N719 dyes in the visible light and the near-IR regions. Two dyeing
methods, dip and flow dyeing, were used for the preparation of
large area testing cells. The evaluated DSSCs based on nano-
[
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2
5
m
m transparent film, sensitized by M43, M44,
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2
N719 and Z907 dyes for two dyeing methods, exhibit the
same trend with the order M44 > M43 > N719 > Z907 for
currentevoltage characteristics and IPCE curves, however, cells
prepared in the flowing process revealed better photovoltaic pa-
rameters than prepared in dipping. The newly synthesized dyes
revealed higher energy conversion efficiency (h), measured at the
AM1.5G conditions, up to 21% compared to N719 and up to 46%
compared to Z907.
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Acknowledgments
We acknowledge financial support of this work by The National
Centre for Research and Development, Warsaw, POIG.01.04.00-04-
0
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