N,N′-Bis((6-methoxylpyridin-2-yl)methylene)-p-phenylenediimine based d10 transition metal complexes and their utilization in co-sensitized solar cells

Article abstract of DOI:10.1039/c4dt00437j

N,N′-Bis((6-methoxylpyridin-2-yl)methylene)-p-phenylenediimine based four-coordinated d¹⁰ transition metal complexes (named ML, M = Zn, Cd, Hg) were synthesized and employed as co-sensitizers and co-adsorbents in combination with a ruthenium complex N719 in dye sensitized solar cells. After co-sensitization, not only the incident-photon-to-current conversion efficiency is enhanced but also the dark current is reduced. A short circuit current density of 14.46 mA cm^-2, an open circuit voltage of 0.74 V and a fill factor of 0.62 corresponding to an overall conversion efficiency of 6.65% under AM 1.5 G solar irradiation were achieved when ZnL was used as a co-sensitizer, which are much higher than that for DSSCs only sensitized by N719 (5.22%) under the same conditions. The improvement in efficiency is attributed to the fact that N,N′-bis((6-methoxylpyridin-2-yl)methylene)-p- phenylenediimine coordinated complexes overcome the deficiency of N719 absorption in the low wavelength region of the visible spectrum, prevent its aggregation, offset competitive visible light absorption of I₃ ^- and reduce charge recombination due to formation of an effective cover layer of the dye molecules on the TiO₂ surface. As a result, the synthesized complexes are promising candidates as co-adsorbents and co-sensitizers for highly efficient DSSCs. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00437j

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N,N’-Bis((6-methoxylpyridin-2-yl)methylene)-
p-phenylenediimine based d10 transition metal
complexes and their utilization in co-sensitized
solar cells†
Cite this: Dalton Trans., 2014, 43,
11361
Liguo Wei,a,b Yulin Yang,*a Ruiqing Fan,*a Yong Na,a Ping Wang,a Yuwei Dong,a
Bin Yanga and Wenwu Caoa,c
N,N’-Bis((6-methoxylpyridin-2-yl)methylene)-p-phenylenediimine based four-coordinated d10 transition
metal complexes (named ML, M = Zn, Cd, Hg) were synthesized and employed as co-sensitizers and co-
adsorbents in combination with a ruthenium complex N719 in dye sensitized solar cells. After co-sensitiz-
ation, not only the incident-photon-to-current conversion efficiency is enhanced but also the dark
current is reduced. A short circuit current density of 14.46 mA cm−2, an open circuit voltage of 0.74 V and
a fill factor of 0.62 corresponding to an overall conversion efficiency of 6.65% under AM 1.5 G solar
irradiation were achieved when ZnL was used as a co-sensitizer, which are much higher than that for
DSSCs only sensitized by N719 (5.22%) under the same conditions. The improvement in efficiency is
attributed to the fact that N,N’-bis((6-methoxylpyridin-2-yl)methylene)-p-phenylenediimine coordinated
complexes overcome the deficiency of N719 absorption in the low wavelength region of the visible spec-
−
Received 11th February 2014,
Accepted 22nd May 2014
trum, prevent its aggregation, offset competitive visible light absorption of I3 and reduce charge
recombination due to formation of an effective cover layer of the dye molecules on the TiO2 surface. As a
result, the synthesized complexes are promising candidates as co-adsorbents and co-sensitizers for
highly efficient DSSCs.
DOI: 10.1039/c4dt00437j
www.rsc.org/dalton
harvesting spectrum so as to increase the photocurrents of the
devices. For example, the record high performance (12.3%) of
1. Introduction
Since O’Regan and Grätzel first reported dye sensitized solar co-sensitized DSSCs was reported by Grätzel et al. using a
cells (DSSCs) in 1991, DSSCs have drawn the attention of many metal-containing porphyrin dye (YD2-o-C8) and an organic dye
researchers due to their ability to convert sunlight to electricity (Y123) together with
a
Co(II/III)-based redox electrolyte.4
at moderate efficiencies with low cost fabrication techniques Nazeeruddin et al. employed different NIR dyes (TT1 and SQ1)
compared with conventional silicon solar cells.1–3 However, as the co-sensitizers of JK2,5,6 and an efficiency of over 7% was
much improvement in efficiency is desired for these devices achieved. Chen and Dehghani co-sensitized porphyrin dyes
before they could take the place of the Si based solar cells. with others respectively, resulting in an enhanced efficiency
Recently, several strategies have been utilized to improve the which was higher than individual dye sensitized solar cells.7,8
photovoltaic performance of DSSCs, among which co-sensitiz- Co-sensitization of TiO2 films with black dyes and organic dyes
ation is an effective approach to enhance the device perform- (D131 or Y1) significantly enhanced the photocurrent and the
ance through a combination of two or more dyes sensitized on device performance attained η = 11.0–11.1%.9–12 Ruthenium
the same semiconductor film together, extending the light- dyes such as N719 co-sensitized with organic dyes also showed
an enhanced device performance compared to single N719 sen-
sitized solar cells.13–15 And co-sensitization of two metal-free
aDepartment of Chemistry, Harbin Institute of Technology, Harbin 150001,
organic sensitizers has also resulted in a higher efficiency than
P. R. China. E-mail: ylyang@hit.edu.cn, fanruiqing@hit.edu.cn;
single DSSCs.16–18 Most of these studies mainly concentrated
Fax: +86-451-86418270
bModern Analysis and Research Center, Heilongjiang University of Science and
on the selection of sensitizer combinations aiming at maximiz-
ing spectral compensation, which is an effective approach to
Technology, Harbin 150027, P. R. China
cMaterials Research Institute, The Pennsylvania State University,
enhance the light-harvesting properties of DSSCs. However,
Pennsylvania 16802, USA
†CCDC 980463 and 980464. For crystallographic data in CIF or other electronic
the competitive light absorption of triiodide,19,20 aggregation
of dyes21–23 and recombination of charge24 are also the main
format see DOI: 10.1039/c4dt00437j
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factors affecting the performance of DSSCs.25 Therefore, syn- (0.1 mmol, 0.0358 g) was dissolved in 25 mL of CH2Cl2 and a
thesis of a co-sensitizer or co-adsorbent, which could not only solution of zinc chloride (0.2 mmol, 0.0272 g of ZnCl2 in
compensate the absorption spectra but also overcome the com- 10 mL of methanol) was added at room temperature and then
petitive light absorption by I−/I3−, avoid dye aggregation and heated and refluxed for 6 h. After cooling and removal of the
reduce charge recombination, is meaningful to enhance the solvent, the residue was dissolved in acetonitrile (30 mL),
performance of DSSCs by co-sensitization.
refluxed for another 3 h at 85 °C, then filtered into a beaker
In the previous work of our group, metal–organic com- flask, volatilized for about 6 h, and a desired rhombus crystal
plexes including d10 transition metal complexes were intro- was obtained.
duced into the DSSC system as co-sensitizers.26–28 It has been
C15H19N3OZnCl2, Yield: 75%, UV-vis (nm, in ethanol,
1
found that d10 transition metal complexes with pyridine-type 25 °C): 213, 246, 287, 365, 500. H NMR (400 MHz, DMSO-d6):
ligands lead to a significant increase in overall conversion δ = 8.49 (s, 1H, CHvN), 7.81 (t, 1H, Py–H4), 7.70 (d, 1H, Py–
efficiency of DSSCs by co-sensitization with N719. In this work, H3). 7.23 (d, 2H, o-Ph), 6.89 (d, 1H, Py–H5), 6.63 (d, 2H, m-Ph),
−
in order to overcome the competitive light absorption by I−/I3
5.42 (s, 2H, NH2), 3.94 (s, 3H, OCH3). FT-IR (KBr, cm−1):
in the DSSC system sensitized by N719, another pyridine- 3429 (w, νN–H), 1630 (m, νCvN), 1599 (s), 1550 (s), 1479 (s),
containing ligand N,N′-bis((6-methoxylpyridin-2-yl)methylene)- 1374 (m), 1291 (s), 1171 (s), 1098 (m), 1016 (m), 960 (m),
p-phenylenediimine is selected to synthesize a series of new 830 (m), 797 (m), 512 (m). Elemental analysis for
d10 transition metal complexes (ML, M = Zn, Cd, Hg) as co- C15H19N3OZnCl2 (Mr: 365.56): calcd: C, 42.71%; H, 3.58%; N,
sensitizers of N719, respectively. All of these co-sensitized 11.49%. Found: C, 42.55%; H, 3.72%; N, 11.39%.
devices exhibited enhancements of the photovoltaic perform-
The synthesis procedure of CdL and HgL was the same as
ance both in Jsc and Voc. The prepared complexes can be used that for ZnL except that ZnCl2 was replaced by CdCl2
as co-sensitizers and co-adsorbents to absorb light in the low (0.2 mmol, 0.0456 g) and HgCl2 (0.2 mmol, 0.0544 g), respecti-
wavelength region of the visible spectrum, overcome the com- vely. The synthesis of complexes ZnL, CdL and HgL is sum-
petitive light absorption by I−/I3−, avoid dye aggregation and marized in Scheme 1.
reduce charge recombination. These synthesized Zn(II), Cd(II)
C15H19N3OCdCl2, yield: 78%, UV-vis (nm, in ethanol,
1
and Hg(II) d10 transition metal complexes do help in improving 25 °C): 217, 246, 304, 365, 497. H NMR (400 MHz, DMSO-d6):
the device performance.
δ = 8.48 (s, 1H, CHvN), 7.97 (t, 1H, Py–H4), 7.70 (d, 1H, Py–H3).
7.22 (d, 2H, o-Ph), 6.89 (d, 1H, Py–H5), 6.63 (d, 2H, m-Ph), 5.44
(s, 2H, NH2), 3.96 (s, 3H, OCH3). FT-IR (KBr, cm−1): 3437
(w, νN–H), 1629 (s, νCvN), 1594 (s), 1479 (s), 1434 (m), 1309 (m),
1094 (s), 1009 (m), 958 (m), 800 (m), 624 (m), 585 (w). Elemental
analysis for C15H16Cl2N3OCdCl2 (Mr: 412.59): calcd: C, 37.84%;
H, 3.18%; N, 10.18%. Found: C, 37.35%; H, 3.58%; N, 10.67%.
C15H19N3OHgCl2, yield: 78%, UV-vis (nm, in ethanol,
2. Experimental section
2.1. The synthesis of ZnL, CdL and HgL
All reagents were commercially available and used with further
purification. The N,N′-bis((6-methoxylpyridin-2-yl)methylene)-
p-phenylenediimine based d10 transition metal complex of
ZnL was synthesized according to the following method:
6-methoxypyridine-2-carbaldehyde (40 mmol, 3.80 mL) was
added to a mixture of 1,4-phenylenediimine (20 mmol, 2.16 g)
and ethanol (70 mL). The mixture was heated and refluxed for
6 h, then cooled down and the yellow powder of N,N′-bis-
((6-methoxylpyridin-2-yl)methylene)-p-phenylenediimine was
filtered out, washed with ether and recrystallized with hexane.
Then the obtained yellow needle like crystal sample
1
25 °C): 207, 244, 307, 367. H NMR (400 MHz, DMSO-d6): δ =
8.79 (s, 1H, CHvN), 8.05 (t, 1H, Py–H4), 7.73 (d, 1H, Py–H3).
7.33 (d, 2H, o-Ph), 7.16 (d, 1H, Py–H5), 6.63 (d, 2H, m-Ph), 5.64
(s, 2H, NH2), 4.01 (s, 3H, OCH3). FT-IR (KBr, cm−1): 3445 (b,
νN–H), 1627 (s, νCvN), 1596 (s), 1554 (s), 1515 (m), 1475 (s),
1293 (s), 1172 (s), 1098 (m), 957 (w), 838 (m), 793 (m), 584 (m).
Elemental analysis for C15H16Cl2N3OHgCl2 (Mr: 500.76): calcd:
C, 31.18%; H, 2.62%; N, 8.39%. Found: C, 31.35%; H, 2.78%;
N, 8.29%.
Scheme 1 Synthesis routes of ZnL, CdL and HgL.
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2.2. Assembly of DSSCs
Vmax)/(Jsc × Voc), where Jmax and Vmax are the photocurrent
density and photovoltage for the maximum power output; Jsc
and Voc are the short-circuit photocurrent density and open-
circuit photovoltage, respectively. The overall energy conver-
sion efficiency η is defined as: η = (FF × Jsc × Voc)/Pin where Pin
is the power of the incident light. The monochromatic incident
photon-to-electron conversion efficiency (IPCE) was measured
on an EQE/IPCE spectral response system (Newport). The
surface photovoltage spectrum (SPS) instrument was
assembled by Jilin University, in which monochromatic light
was obtained by passing light from a 500 W xenon lamp
through a double-prism monochromator (SBP300, China), and
the signals were collected by a lock-in amplifier (SR830, Stan-
ford). Electrochemical impedance spectra (EIS) were recorded
using the CHI660D Electrochemical Analyzer (Chenhua,
China), and the measurements were taken over a frequency
range of 0.1–100 kHz under standard global AM 1.5 solar
irradiation or in the dark by applying a forward bias of
−0.75 V. 1H NMR (400 MHz) spectra were recorded on a Bruker
Avance-400 spectrometer using Si(CH3)4 as an internal stan-
dard at room temperature. Infrared spectra (IR) were obtained
using KBr pellets on a Nicolet Avatar-360 Infrared spectrometer
in the 4000–100 cm−1 region. Elemental analyses were per-
formed on a Perkin-Elmer 2400 element analyzer. The single-
crystal X-ray diffraction data for ML were collected on a Rigaku
R-AXIS RAPID IP diffractometer using graphite-monochro-
mated Mo KR radiation (λ = 0.71073 Å), operating at 293 2 K.
The structures were solved by direct methods and refined by
full-matrix least-squares based on F2 using the SHELXTL 5.1
software package (Sheldrick G M. SHELXTL NT Crystal Struc-
ture Analysis Package [CP] version 5.10; Bruker AXS, Analytical
X-ray System: Madison, WI, 1999). The hydrogen atoms resid-
ing on the carbon atoms were located geometrically. All non-
The FTO conducting glass (fluorine-doped SnO2, sheet resist-
ance 15 Ω per square, transmission 90% in the visible) was
purchased from NSG, Japan, and cleaned by a standard
procedure. N719 [cis-bis(isothiocyanato)bis(2,2-bipyridyl-4,4-
dicarboxylato)-ruthenium(II)bis-tetrabutylammonium] was pur-
chased from Solaronix Company, Switzerland. Dye-sensitized
solar cells were fabricated using the following procedure. The
TiO2 paste was cast onto the FTO substrate by the screen-print-
ing method, followed by drying at 100 °C for 5 min and this
process was repeated six times, followed by sintering at 500 °C
for 15 min in air to obtain a transparent TiO2 photoelectrode
with the thickness of ca. 10 µm. The co-sensitized electrodes
were prepared by immersing the obtained mesoporous TiO2
photoelectrode in 0.3 mM ZnL, CdL or HgL solution in absol-
ute ethanol for 2 h and washing with ethanol and drying with
a blower, then further immersing the electrodes in a 0.3 mM
N719 solution in absolute ethanol for 12 h, and then washing
with ethanol and drying with a blower again. The single N719
sensitized electrodes were prepared by only immersing the
TiO2 photoelectrode in a 0.3 mM N719 solution in absolute
ethanol for 14 h. The electrolyte used in this work was 0.5 M
LiI + 0.05 M I2 + 0.1 M tert-butyl pyridine in a 1 : 1 (volume
ratio) of acetonitrile–propylene carbonate. The platinum
counter electrode was prepared by depositing H2PtCl6 paste
onto the FTO glass substrates and then sintering at 450 °C for
30 min. The cells were assembled by sandwiching the electro-
lyte between the dye sensitized photoanode and the counter
electrode and the assembly was held together using mini-
binder clips.
2.3. Instrumentation and measurements
UV-visible absorption spectra in ethanol solution were hydrogen atoms were refined anisotropically.
recorded on a model SPECORD S600 UV-visible spectrophoto-
meter (Jena, Germany), and absorption spectra of TiO2 films
were recorded on
(Shimadzu, Japan). Fluorescence properties of the synthesized
complexes were obtained using an FLS920 spectrometer (Edin-
a model UV-2250 spectrophotometer
3. Results and discussion
3.1. Synthesis and structures of ZnL, CdL and HgL
burgh) equipped with a peltier-cooled R928 photomultiplier The synthetic routes and molecular structures of ZnL, CdL and
tube (Hamamatsu). A Xe900 450 W xenon arc lamp was used HgL are depicted in Scheme 1. All of the three complexes were
as the exciting light source. Cyclic voltammetry (CV) were obtained in good yields (ca. 75–78%) as air stable yellowish
1
performed with an electrochemical workstation (CHI660d, solids. And all the complexes were characterized by H NMR,
Chenhua, Shanghai) using a three-electrode cell with a Pt UV-vis, and IR spectroscopy as well as elemental analysis.
working electrode, a Pt wire auxiliary electrode, and a saturated Molecular structures of ZnL and HgL were further determined
calomel reference electrode in saturated KCl solution. The sup- by single-crystal X-ray diffraction analysis, but a suitable single
porting electrolyte was 0.1 M tetrabutylammonium hexafluoro- crystal of CdL was not obtained. Crystal structure data and
phosphate (TBAPF6, Fluka, electrochemical grade) in ethanol details of data collection together with the structure refine-
as the solvent. Photocurrent–photovoltage (I–V) curves were ment are given in Table 1. Selected bond lengths and angles of
recorded on the Keithley model 2400 digital source meter complexes ZnL and HgL are listed in Table 2. Hydrogen
using a mask with an aperture area of 0.16 cm−2. The irradi- bonding data of complexes ZnL and HgL are listed in Table 3.
ance of AM 1.5 global sunlight from a filtered 500 W xenon
The X-ray structural analysis of the single-crystals reveals
lamp light source was set at 100 mW cm−2 and was calibrated that ZnL and HgL are isostructural. Single crystal X-ray diffrac-
using a standard silicon solar cell (No. NIMMS1123, calibrated tion analysis also reveals that the complexes of ZnL and HgL
ˉ
at the National Institute of Metrology, P. R. China). Based on crystallize in the triclinic system, P1 space group. The complex
the I–V curve, the fill factor (FF) is defined as: FF = (Jmax
×
of ML (M = Zn, Hg) has 20 non-hydrogen atoms in the asym-
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arrangement (Fig. 1). For ZnL, the bond angles of N–Zn–N and
Cl–Zn–Cl are 81.39(15)° and 113.43(6)°, respectively; the di-
hedral angle of the benzene ring and the pyridine ring is
5.77°, which is almost coplanar due to coordination with Zn2+;
the Zn–N bond has an average bond length of 2.074 Å and the
imino CvN bond has typical double-bond characteristics,
with a bond length of 1.287(6) Å. For HgL, the bond angles of
N–Hg–N and Cl–Hg–Cl are 73.00(30)° and 111.06(12)°, respecti-
vely; the dihedral angle is 4.11°; the average Hg–N bond length
is 2.323 Å and the imino CvN bond with a bond length of
1.265(12) Å also has typical double-bond characteristics.
A 3-D supramolecular structure of HgL is obtained by π–π
stacking interactions (π–π stacking interactions of 3.5 Å) and
hydrogen bonding interactions (see Fig. 2). As for ZnL, it is
similar to HgL.
Table 1 Crystal data and crystal refinement for ZnL and HgL
Data
ZnL
HgL
Formula
C15H19N3OZnCl2 C15H19N3OHgCl2
Formula weight
Crystal system
Space group
365.56
Triclinic
ˉ
500.76
Triclinic
ˉ
P1
P1
Unit cell dimensions
a/Å
b/Å
c/Å
α/°
β/°
γ/°
7.5850(15)
9.3580(19)
13.408(3)
99.77(3)
94.79(3)
108.62(3)
879.1(3)
2
7.5770(15)
9.3950(19)
13.630(3)
99.83(3)
95.31(3)
108.01(3)
898.2(3)
2
Volume (Å3)
Z
Dcalcd (g m−3
)
1.528
1.708
1.996
8.873
μ (mm−1
θrange (°)
)
3.03° to 27.48°
−8 ≤ h ≤ 9
−12 ≤ k ≤ 12
−17 ≤ l ≤ 17
3967/0/209
0.941
0.0515
0.1399
0.0970
0.1725
3.03° to 27.48°
−9 ≤ h ≤ 9
−12 ≤ k ≤ 12
−17 ≤ l ≤ 17
4072/0/209
0.963
0.0505
0.1367
0.0866
0.1666
Limiting indices
3.2. Optical properties of ZnL, CdL and HgL
Data/restraints/parameters
The absorption spectra of the prepared complexes and N719 in
ethanol solutions are shown in Fig. 3, and the corresponding
data are listed in Table 4. In the UV-vis spectra, the complexes
of ZnL, CdL and HgL exhibit two major prominent bands. The
first one at ca. 280–325 nm is attributed to localized aromatic
π–π* transitions, and the other one at ca. 325–700 nm,
325–675 nm and 325–525 nm for ZnL, CdL and HgL, respecti-
vely, is attributed to intramolecular charge transfer (ICT)
absorption. Compared with the absorption spectra of N719,
apparently, the absorption spectra of ZnL, CdL and HgL could
compensate for that of N719 in the low wavelength region of
the visible spectrum.
GOF on F2
a
Final R indices [I > 2σ(I)] R1
b
b
wR2
a
R indices (all data)
R1
wR2
)
Largest diff. peak/hole (e Å−3
0.365/−0.484
0.910/−1.548
a R1 = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = [∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]]1/2
.
Table 2 Selected bond lengths (Å) and angles (°) for ZnL and HgL
ZnL(M = Zn)
HgL(M = Hg)
M(1)–N(1)
2.063(4)
2.288(8)
As shown in Table 4, the ICT absorption (λmax) appears at
365 nm for ZnL and CdL, and at 367 nm for HgL. The molar
extinction coefficients in the visible region are 32 571 M−1
cm−1 for ZnL, 29 860 M−1 cm−1 for CdL and 28 353 M−1 cm−1
for HgL. All of these are much higher than that of the ruthe-
nium complex N719.29 A higher molar extinction coefficient
around 365–367 nm indicates that the ZnL, CdL, and HgL
possess a higher light harvesting ability in this wavelength
M(1)–N(2)
M(1)–Cl(1)
M(1)–Cl(2)
N(2)–C(6)
N(1)–M(1)–N(2)
N(1)–M(1)–Cl(1)
N(2)–M(1)–Cl(1)
N(1)–M(1)–Cl(2)
N(2)–M(1)–Cl(2)
Cl(1)–M(1)–Cl(2)
2.086(4)
2.206(2)
2.244(2)
1.287(6)
81.39(15)
119.62(11)
116.74(11)
113.73(11)
107.58(10)
113.43(6)
2.358(7)
2.377(3)
2.467(3)
1.265(12)
73.00(30)
126.80(20)
123.50(20)
112.65(19)
103.30(20)
111.06(12)
region compared with N719 and I3 (25 000 M−1 cm−1).30
−
Hence it can be predicted that the photon lost due to the light
absorption by I3− will be suppressed by the use of ZnL, CdL or
HgL as a co-sensitizer and co-adsorbent due to the compe-
tition between ZnL, CdL or HgL and I3− to absorb light.
Table 3 Hydrogen-bonds (Å, °) for ZnL and HgL
D–H⋯A
d(H⋯A)
<DHA
d(D⋯A)
ZnL
The absorption spectra of ZnL/N719, CdL/N719 and HgL/
N719 co-sensitized TiO2 films are shown in Fig. 4. As shown in
Fig. 4, the absorption of N719 on TiO2 films in the visible light
region is remarkably broadened and its peak at 475 nm in
ethanol solutions is red-shifted to 555 nm. This could be
ascribed to the aggregation31 or electronic coupling of the dyes
on the TiO2 surface.32 The interaction between the carboxyl
group and the surface Ti4+ ions may lead to increased delocali-
zation of the π* orbital of the conjugated framework. The
energy of the π* level is decreased by this delocalization, result-
ing in the red shift of the absorption spectra.
N3–H3A⋯N4 [−x, −y + 1, −z]
N3–H3B⋯Cl2 [−x, −y, −z + 1]
HgL
2.331
2.549
165.56
166.01
3.172
3.389
N3–H3A⋯Cl2 [−x, −y − 1, −z + 1]
N3–H3B⋯N4 [−x, −y, −z + 1]
2.601
2.294
160.79
170.51
3.425
3.146
metric unit, which contain one M2+ cation, one ligand
(N-((6-methoxylpyridin-2-yl)methylene)-p-phenylenediimine, half-
hydrolyzed from N,N′-bis((6-methoxylpyridin-2-yl)methylene)-
p-phenylenediimine) and one acetonitrile molecule with all in
general positions (Fig. 1). The center metal M2+ is four-co-
ordinated: one pyridine N atom, one imino N atom and two
chlorine atoms, and displays a slightly distorted tetrahedron
In comparison with the absorption of N719 on the TiO2
film, the absorption peaks for ZnL/N719, CdL/N719 and HgL/
N719 on the TiO2 film are blue-shifted by 23, 43 and 62 nm,
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Fig. 1 Molecular structural unit of ZnL and HgL (hydrogen atoms and free acetonitrile molecules are omitted for clarity).
Table 4 Experimental data for spectral and electrochemical properties
of the synthesized complexes
εa (M−1
cm−1
λem
E0–0
(eV)
EHOMO
(eV)
ELUMO
(eV)
a
a,b
c
d
d
λabs
Dyes
(nm)
)
(nm)
ZnL
CdL
HgL
365
365
367
32 571
29 860
28 353
481
481
477
2.90
2.89
2.92
−5.09
−5.01
−5.06
−2.19
−2.12
−2.14
a Absorption and emission spectra were recorded in ethanol solution
(3 × 10−4 M) at room temperature. b Complexes were excited at their
absorption maximum value c Optical band gap calculated from
intersection between the absorption and emission spectra. d The
values of EHOMO and ELUMO were calculated with the following
ox
formula: HOMO (eV) = −e(E
V + 4.4 V); LUMO (eV) = EHOMO + E0–0.
onset
where E0–0 is the intersection of absorption and emission of the
Fig. 2 Packing diagram of the complex HgL. Hydrogen bonds are indi-
cated by dashed lines.
complexes.
Fig. 3 UV-visible absorption spectra of ZnL, CdL, HgL and N719 in
ethanol.
Fig. 4 UV-visible absorption spectra of ZnL, CdL, HgL and N719 on
TiO2 films.
respectively, displaying the bands at 532, 512 and 493 nm. wavelength region of the visible spectrum based on the spectra
This indicated that when ZnL, CdL or HgL is used as a co- shown in Fig. 3.
sensitizer and co-adsorbent, the aggregation of N719 is alle-
The emission spectra of ZnL, CdL and HgL are illustrated
viated and the new interactions between dye–dye and dye–TiO2 in Fig. 5. All of them exhibit strong luminescence in the region
are introduced, leading to a blue-shift in the absorption of of wavelengths 350–750 nm, and it is worth noting that all of
N719 on the TiO2 film. Furthermore, the absorption on the co- the emission spectra of ZnL, CdL and HgL overlap with the
sensitized film is enhanced remarkably when ZnL, CdL or HgL excitation spectra of N719 inordinately. This indicates that
was introduced, especially in the region of 320–550 nm. This is N719 could synchronously accept the energy from the incident
consistent with the result that the absorption spectra of ZnL, light and the excited ML, which will broaden the spectral
CdL and HgL could compensate for that of N719 in the low response of N719 in the region of 350–750 nm.
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further improved negatively by about 0.3 V by adding additives
such as 4-tert-butyl pyridine (TBP) to the I−/I3 redox electro-
−
lyte,34 providing sufficient driving force for dye regeneration.35
3.4. Photovoltage properties of DSSCs
ZnL, CdL and HgL were employed as co-sensitizers and co-
adsorbents to fabricate ML/N719 photoanodes. The current–
voltage (J–V) characteristics of the DSSC devices based on
N719, ZnL/N719, CdL/N719 and HgL/N719 photoanodes are
shown in Fig. 6, and the corresponding cells performances are
summarized in Table 5. Upon co-sensitization, the perform-
ances of cells are all improved. The values of Jsc, Voc and η are
improved in the order of ZnL/N719 > CdL/N719 > HgL/N719,
respectively. The ZnL/N719 device yields a Jsc of 14.46 mA
cm−2, a Voc of 0.74 V, a fill factor (FF) of 0.62 and a η of 6.65%.
All the parameters are significantly higher than that of the
device only sensitized by N719.
Fig. 5 The emission spectra of dyes in ethanol solution.
The IPCE spectra were collected in order to interpret the
enhancement of Jsc (Fig. 7). The DSSCs containing only the
N719 dye show a broad IPCE spectrum from 300–750 nm but a
decrease in the wavelength range of 340–450 nm, which is due
3.3. Electrochemical properties of ZnL, CdL and HgL
Energy-level matching is crucial in selecting the sensitizer. In
order to estimate the HOMO and LUMO energy levels of ZnL,
CdL and HgL, cyclic voltammetry (CV) was carried out in a
three-electrode cell on an electrochemical workstation. The
experimental data for electrochemical properties of ML are
summarized in Table 4. As estimated from the intersection of
the absorption and emission spectra, the excitation transition
energy values (E0–0) of ZnL, CdL and HgL are 2.90, 2.89 and
2.92 eV, respectively. Consequently, the HOMO values of ZnL,
CdL and HgL are calculated based on their first redox poten-
tials as −5.09, −5.01 and −5.06 eV, respectively, and the LUMO
levels of ZnL, CdL and HgL calculated from EHOMO + E0–0, are
−2.19, −2.12 and −2.14 eV, respectively.33 The HOMO and
LUMO energy levels of ZnL, CdL and HgL are shown in
Scheme 2. It shows that the energy levels of ZnL, CdL and HgL
are appropriate for the DSSC system containing TiO2. The
LUMO levels lay above the conduction band (CB) of the TiO2
semiconductor (−4.40 eV vs. vacuum), indicating efficient elec-
tron injection, and the HOMO energy levels lay below that of
−
to the competitive light absorption between I3 and N719.
When ML is used as a co-sensitizer and co-adsorbent, this
decrease is restored in the order of ZnL/N719 > CdL/N719 >
HgL/N719, which is attributed to the fact that the ML com-
plexes are attached to the TiO2 surface effectively and contribu-
ted to the electron injection into the conduction band of the
the I−/I3 redox electrolyte (−4.60 eV vs. vacuum) which is
−
Fig. 6 J–V curves for DSSCs based on co-sensitized photoelectrodes
and N719 sensitized photoelectrodes under irradiation.
Table 5 J–V
photoelectrodes
performance
of
DSSCs
based
on
different
η/%
Photoelectrode
Jsc/mA cm−2
Voc/V
FF
N719/TiO2
12.36
14.46
14.35
13.92
0.68
0.74
0.73
0.70
0.62
0.62
0.60
0.61
5.22
6.65
6.30
5.96
ZnL/N719/TiO2
CdL/N719/TiO2
HgL/N719/TiO2
Scheme 2 Schematic energy diagram of HOMO and LUMO for dyes
compared to the energy levels calculated for TiO2.
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−
tive light absorption of I3 and compensation of N719 absorp-
tion; besides, the enhancement of the photovoltage signals in
the region of 300–700 nm may also be attributed to the
reduction of N719 aggregation and the interaction of ML and
N719, which further support the results from IPCE.
The dark current–voltage characteristics of the DSSCs based
on the ML/N719 and N719 dyes are shown in Fig. 9, which
indicates the recombination of injected electrons with I3−. It
shows that the dark current is lower for the co-sensitized
system compared with that of single N719 sensitized DSSCs,
which is in the order of ZnL/N719 < CdL/N719 < HgL/N719.
This could be explained as follows: as shown in absorption
spectra, co-sensitization of N719 with ML on the TiO2 film pro-
duces a blue shift, indicating that the N719 molecules orien-
tate themselves in a more parallel conformation, leaving space
for the ML co-sensitizer. The interaction with each other forms
a dense packing of dyes on the TiO2 electrode or an insulating
layer composed of ML and N719, which could effectively sup-
press charge recombination and thereby enhance Voc. It is
noted that the Voc value is in the opposite order with the dark
current, that is ZnL/N719 > CdL/N719 > HgL/N719 > N719.
When ML was used as a co-sensitizer and co-adsorbent, the Jsc
and Voc were all enhanced, both of which lead to the improve-
ment of η. The improved Jsc and Voc with the unpronounced
difference in FF make it obtain higher η, which is on the same
order as Jsc and Voc.
When the crystal structures of the complexes are taken into
account, it is found that the enhancement of the co-sensitized
DSSC performance compared with that of single N719 sensi-
tized DSSCs may be due to the key structural feature of the
complex. In the complex, the center metal chelates with two N
electron donors of the ligand and form a five membered ring
which is adjacent to the pyridine ring. The five membered
ring, pyridine ring and benzene ring are almost coplanar;
thereby the rigidity to the molecular structure is increased.
The good co-planarity and rigidity mean easier electron transfer
Fig. 7 IPCE spectra of DSSCs based on single N719 sensitized and co-
sensitized photoelectrodes.
TiO2. This means that the co-sensitization of N719 and ML has
a significant synergy and compensatory effect on light harvest-
ing and electron collection in TiO2. Based on the IPCE and the
absorption spectra, the cell’s higher Jsc in the case of co-sensit-
ization is ascribed to better light harvesting in the low wave-
length region, where the absorption of N719 is compensated
and the competitive light absorption of I3− is overcome.
To further investigate the enhanced performance of DSSCs,
the surface photovoltages of DSSCs based on different photo-
electrodes were investigated, which is a well-established con-
tactless technique for surface state distribution.36,37 The
curves in Fig. 8 indicate that co-sensitization could result in
stronger photovoltage signals between 300 and 750 nm,
especially in the region of 300–450 nm. The photovoltage
signals increased in the order of ZnL/N719 > CdL/N719 > HgL/
N719, which indicates that the separation efficiency of elec-
trons and holes correlates strongly with ZnL, CdL and HgL,
and the enhancement of the photovoltage signals in the region
of 300–450 nm is attributed to the overcoming of the competi-
Fig. 9 J–V curves for DSSCs based on co-sensitized photoelectrodes
Fig. 8 SPS of DSSCs based on different photoelectrodes.
and N719 sensitized photoelectrodes in the dark.
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and are favorable for electrochemical properties. Furthermore, reported in the literature that three semicircles should be
the complexes ML extend as supermolecules through hydrogen observed in the frequency range of 0.1 to 10 kHz in the
bonding and π–π interactions, which make the electron-donat- Nyquist plots. This can be explained as the conventional
ing ability of ML better when they are used in DSSCs as a co- diffusion resistance of the redox couple is greatly overlapped
sensitizer. All of these structural features of ML result in by the charge transfer resistance due to the short diffusion
obtaining higher performance of co-sensitized cells than that length of I− ions available with the thin spacer and the low
of cells sensitized by single N719. Meanwhile, it is noted that viscosity of the solvents used in the electrolyte.
the DSSC performance is gradually enhanced in the order of
Under light illumination, EIS was utilized to analyze the
HgL < CdL < ZnL. To some extent, this difference observed in charge transport resistance at the TiO2/dye/electrolyte interface
the DSSC performance likely has its origin including the metal for its significance in the efficiency of DSSCs.40–42 As shown in
ion in the unit cell and the steric proximity of ligand L. For Fig. 10a, the two semicircles located in high and middle
HgL, the larger atom radius of Hg likely increases the Hg–N frequency regions (left to right) are attributed to the electro-
average bond length, which is 2.323 Å (the average Zn–N bond chemical reaction at the Pt/electrolyte interface and the charge
length is 2.074 Å), and the narrow angle formed by the chelat- transfer at the TiO2/dye/electrolyte interface.43–45 The radius of
ing L in HgL, N1–Hg–N2, is 73.00°, which is significantly the large semicircle located in middle frequency regions in the
smaller than that in ZnL (the bond angle of N1–Zn–N2 is Nyquist plots decreases after co-sensitization with ML, and the
81.39°). In this case, the distance between M to electron donat- values are in the order of ZnL/N719 < CdL/N719 < HgL/N719 <
ing group N is increased. Also, it is worthwhile to note that the N719, which indicates a decrease of the electron transfer impe-
distance between the benzene ring and a five membered ring dance (Rct) and an increase of charge transfer rate at this inter-
including M is increased, from ZnL to HgL, namely, the bond face after co-sensitization.
length C8–N2 is 1.41 Å in HgL, which is longer than the corres-
Under dark conditions, as shown in Fig. 10b, the two semi-
ponding value in ZnL (the bond length C8–N2 is 1.40 Å in circles located in high, middle and low frequency regions (left
ZnL). This is probably caused by the steric hindrance. Appar- to right) of Nyquist plots are attributed to the redox reaction at
ently, the increased distance is not good for electron transfer. the Pt counter electrode and the electron transfer at the TiO2/
Therefore, the performance of the co-sensitized cell is in the dye/electrolyte interface.43–46 Therefore the larger semicircle
order of ZnL/N719 > CdL/N719 > HgL/N719.
observed in the middle frequency region represents the resist-
ance of the charge transfer from the TiO2 to the electrolyte
(back recombination resistance Rrec). The radius of this semi-
circle increases after co-sensitization with ML, indicating an
increase of Rrec. A large Rrec means a small charge recombina-
tion rate and vice versa. The radius of the semicircle observed
in the middle frequency range lies in the order of ZnL/N719 >
CdL/N719 > HgL/N719 > N719, indicating the sequence of Rrec
at the TiO2/dye/electrolyte interface. The increased value of Rrec
for DSSCs implies the retardation of the charge recombination
3.5. Electrochemical impedance spectroscopy studies of
DSSCs
EIS is a powerful technique for the analysis of interfacial elec-
tronic and ionic transport processes in an electrochemical
device.38,39 It is a steady state method to measure the current
response based on application of an ac voltage at different fre-
quencies. Herein, we utilized EIS to analyze charge carrier
dynamics in the interfacial regions of solid–liquid layers.
The Nyquist plots of EIS for DSSCs based on different photo-
electrodes measured under standard global AM 1.5 solar
irradiation and in the dark by applying a forward bias of −0.75
V are illustrated in Fig. 10. Only two semicircles were observed
for DSSCs in our experiments, which is different from what is
between injected electrons and I− ions in the electrolyte, with
3
a consequent increase of Voc. This appears to be consistent
with the larger Voc value sequence.
Also, a suggested impedance model by Bisquert is used to
fit curves in the middle frequency region of Nyquist plots
Fig. 10 Nyquist plots of EIS for DSSCs based on different photoelectrodes measured under standard AM 1.5 G solar irradiation (a) or in the dark (b)
at forward bias −0.75 V.
11368 | Dalton Trans., 2014, 43, 11361–11370
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formance is competitive with solar cells made from the single
ruthenium complex N719, reaching up to 6.65% under similar
conditions. The high efficiency and simple fabrication
suggests very bright application prospects for this method in
large-scale production of DSSCs.
Table 6 Parameters obtained by fitting the impedance spectra of solar
cells
DSSCs
Rct/Ω
Rrec/Ω
N719
13.60
13.02
13.07
13.11
68.30
83.50
76.64
73.07
ZnL/N719
CdL/N719
HgL/N719
Acknowledgements
using the Z-view software.38 Rct and Rrec at the TiO2 surface
were deduced by this method and are listed in Table 6. The
values of Rct are in the order of ZnL/N719 < CdL/N719 < HgL/
N719 < N719 and those of Rrec are in the order of ZnL/N719 >
CdL/N719 > HgL/N719 > N719. These again confirm the ana-
lysis above that after co-sensitization with ML, the charge
transfer rate increased and the charge recombination rate
decreased, both of which are favorable for enhancing the per-
formance of DSSCs.
This work was supported by National Natural Science Foun-
dation of China (Grant 21171044 and 21371040), the National
key Basic Research Program of China (973 Program, No.
2013CB632900), The Fundamental Research Funds for the
Central Universities (Grant No. HIT. IBRSEM. A. 201409), and
Program for Innovation Research of Science in Harbin Insti-
tute of Technology (PIRS of HIT No. A201416 and B201414).
According to the EIS analysis, the charge recombination
resistance is larger and the charge transport resistance is
smaller for the DSSCs based on ML/N719 than that for the
DSSCs based on N719, indicating that co-sensitization has
effectively modified the TiO2/dye/electrolyte interface and
increased the Jsc and the Voc. In the co-sensitized system, a
better dye coverage helps to passivate the TiO2 surface or form
an insulating molecular layer composed of ML and
N719 molecules and thus reduces the recombination due to
electron back-transfer between TiO2 and I3−, which ultimately
increases the Voc compared with those of individual sensitized
devices.
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