New iridium(III) cyclometalates with extended absorption features for bulk heterojunction solar cells

Article abstract of DOI:10.1016/j.jorganchem.2016.03.003

Two new iridium(III) cyclometalates (A and B) based on 2-[5-(9,9′-diethyl-9H-fluoren-7-yl)thienyl]-pyridine were synthesized, characterized and applied in bulk-heterojunction solar cells (BHJSCs). Their absorption, electrochemical, thermal and photovoltaic properties have been investigated. The results reveal that the replacement of phenyl ring by thienyl one can extend the absorption wavelength up to 530 nm, thus narrowing the energy gap (E_g) to 2.93 eV and 2.81 eV for A and B, respectively. These complexes exhibit excellent thermal stability with the onset decomposition temperature at 5% weight-loss (T_d) of over 370 °C. The BHJSC device with A as donor blended with [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) gave the best power conversion efficiency (η) of 0.51%, with a short-circuit photocurrent density (J_sc) of 2.68 mA cm^-2, an open-circuit photovoltage (V_oc) of 0.66 V and a fill factor (ff) of 0.28 under illumination of an AM 1.5 solar cell simulator.

Full text of DOI:10.1016/j.jorganchem.2016.03.003

Accepted Manuscript
New iridium(III) cyclometalates with extended absorption features for bulk
heterojunction solar cells
Guiping Tan, Peng Liu, Hao Wu, Sze-Chun Yiu, Fengrong Dai, Ying-Hsuan Feng,
Xiaotian Liu, Yongfu Qiu, Yih Hsing Lo, Cheuk-Lam Ho, Wai-Yeung Wong
PII:
S0022-328X(16)30077-8
10.1016/j.jorganchem.2016.03.003
JOM 19428
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 27 January 2016
Revised Date: 4 March 2016
Accepted Date: 6 March 2016
Please cite this article as: G. Tan, P. Liu, H. Wu, S.-C. Yiu, F. Dai, Y.-H. Feng, X. Liu, Y. Qiu,
Y.H. Lo, C.-L. Ho, W.-Y. Wong, New iridium(III) cyclometalates with extended absorption features
for bulk heterojunction solar cells, Journal of Organometallic Chemistry (2016), doi: 10.1016/
j.jorganchem.2016.03.003.
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ACCEPTED MANUSCRIPT
Two new iridium(III) complexes were synthesized, characterized and applied in bulk
heterojunction solar cells. The complexes showed extended absorption and decent
photovoltaic effect.

ACCEPTED MANUSCRIPT
JOMC Special Issue on Lewis Memorial Issue
New iridium(III) cyclometalates with extended absorption
features for bulk heterojunction solar cells
Guiping Tana,b, Peng Liua, Hao Wub, Sze-Chun Yiub, Fengrong Daic, Ying-Hsuan
Fengd, Xiaotian Liua, Yongfu Qiua*, Yih Hsing Lod*, Cheuk-Lam Hob*,
Wai-Yeung Wongb*
a College of Chemistry and Environmental Engineering, Guangdong Engineering and
Technology Research Center for Advanced Nanomaterials, Dongguan University of
Technology, Dongguan 523808, P.R. China.
b
Institute of Molecular Functional Materials, Department of Chemistry and Institute
of Advanced Materials, Hong Kong Baptist University, Waterloo Road, Kowloon
Tong, Hong Kong, P.R. China.
c
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R.
China.
d
Department of Applied Physics and Chemistry, University of Taipei, Taipei 100,
Taiwan.
Electronic supplementary information (ESI) available.
_____________
* Corresponding author. Institute of Molecular Functional Materials, Department of
Chemistry and Institute of Advanced Materials, Hong Kong Baptist University,
Waterloo Road, Kowloon Tong, Hong Kong, P.R. China.
E-mail addresses: qiuyf1979@foxmail.com (Y. Qiu), yhlo@utaipei.edu.tw (Y.H. Lo),
clamho@hkbu.edu.hk (C.-L. Ho), rwywong@hkbu.edu.hk (W.-Y. Wong)
1

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Abstract
Two new
iridium(III)
cyclometalates
(A
and
B)
based
on
2-[5-(9,9’-diethyl-9H-fluoren-7-yl)thienyl]-pyridine were synthesized, characterized
and applied in bulk-heterojunction solar cells (BHJSCs). Their absorption,
electrochemical, thermal and photovoltaic properties have been investigated. The
results reveal that the replacement of phenyl ring by thienyl one can extend the
absorption wavelength up to 530 nm, thus narrowing the energy gap (Eg) to 2.93 eV
and 2.81 eV for A and B, respectively. These complexes exhibit excellent thermal
stability with the onset decomposition temperature at 5% weight-loss (Td) of over 370
oC. The BHJSC device with A as donor blended with [6,6]-phenyl C61-butyric acid
methyl ester (PCBM) gave the best power conversion efficiency (η) of 0.51%, with a
short-circuit photocurrent density (Jsc) of 2.68 mA cm–2, an open-circuit photovoltage
(Voc) of 0.66 V and a fill factor (ff) of 0.28 under illumination of an AM 1.5 solar cell
simulator.
Keywords: Thiophene • Iridium(III) complex • Bulk-heterojunction solar cell •
Photovoltaics • Phosphorescence
1. Introduction
Research on organic light-emitting diodes (OLEDs) has shown tremendous progress
in recent years, and especially, cyclometallated iridium(III) complexes are attracting
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enormous attention because of their highly efficient phosphorescence even at room
temperature [1]. The iridium(III) ion (5d6) is commonly employed to be the metal
centre of triplet emitters for the following reasons: a) its quasi-octahedral geometry
can introduce specific ligands in a controlled manner; (b) the photophysical and
electrochemical properties of iridium complexes can be tuned in a predictable way; (c)
the iridium metal possesses stable and accessible oxidation states; and (d)
cyclometalated iridium(III) complexes are known to have the highest triplet quantum
yields [2]. The phosphorescence lifetime of iridium(III) complexes is usually less than
1 µs, and such a quick transfer within triplet excitation reduces the energy loss,
leading to the improvement of emission efficiency. These unique features render the
iridium(III) complexes excellent emitters for full color display and lighting
applications [3]. However, only very few papers report the photovoltaic properties of
iridium(III) complexes to date. Furthermore, most of these papers are related to the
application in the dye-sensitized solar cells (DSSCs) [4], while the reports on bulk
heterojunction solar cells (BHJSCs) are very rare indeed. 2008, Holdcroft and
co-workers
used
the
iridium(III)-containing
polymer
poly[9,9-dioctylfluorene-co-tris(2-phenylpyridine)iridium(III)] as the donor material
in a BHJSC. By utilizing the triplet energy state of iridium(III) core, the PCE of the
resulting BHJSC (0.07%) was improved largely from that for the pure organic ligand
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(0.002%) [5]. This is the first report about the photovoltaic research on iridium(III)
complex as the donor material. Yu et al. reported a BHJSC with (t-bt)2Ir(acac) as the
donor material in 2012, and the lower HOMO level of (t-bt)2Ir(acac) enhanced the
open-circuit voltage (Voc) of such doped BHJSC [6]. Therefore, as donor materials in
BHJSCs, iridium(III) complexes should own lower HOMO energy level and proper
LUMO energy level to achieve higher Voc and narrowed energy gap (Eg), resulting in
better separation and transfer of charges [7]. Introduction of thiophene to the ligand
system is a good solution, because the electron-rich thiophene could enhance the
delocalization of π electrons within the donor molecule and narrow its Eg without
increasing its HOMO energy level [8]. Hence, we introduced the thiophene ring into
the cyclometalating ligand of iridium(III) complexes and replaced the traditional
phenyl-pyridine unit with thienyl-pyridine unit, to develop the ligand
2-[5-(9,9’-diethyl-9H-fluoren-7-yl)thienyl]-pyridine (L), which afforded complexes A
and B (Fig. 1). The thiophene ring is electron-rich in nature that can improve the
π-electron delocalization in order to narrow the Eg, enhance the absorption spectral
coverage and molar extinction coefficient and also facilitate charge transfer. The
photophysical properties, electrochemistry, computational analyses, thermal
properties and photovoltaic performance of A and B will be discussed.
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B(OH)2
S
Pd(PPh3)4
n-EtBr, DMSO
S
Br
Br
NaOH (50% aq.)
r.t., overnight
2 M Na2CO3
THF, 70 oC, 48 h
Bu3Sn
Br
S
NBS, CHCl3
Overnight
N
S
N
Pd(PPh3)4, toluene
110 oC, 48 h
S
N
Ir(acac)3, glycerol
200 oC, overnight
S
L
N
3
Ir
IrCl3·nH2O
A
2-ethoxyethanol / H2O
90 oC, overnight
Na2CO3
Acetylacetone
N
O
Ir
N
N
Cl
Ir
Ir
2-ethoxyethanol
90 oC, overnight
O
S
S
S
Cl
2
2
2
B
Dimer
Fig. 1. The synthesis routes of A and B.
2. Experimental
2.1. Materials and reagents
All the reactions were carried out under a nitrogen atmosphere. Solvents were
carefully dried and distilled from appropriate drying agents prior to use.
Commercially available reagents were used without further purification unless
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otherwise stated. All reagents for the chemical syntheses were purchased from
Aldrich or J&K Chemicals. All reactions were monitored by thin-layer
chromatography (TLC) with Merck pre-coated aluminum plates. Flash column
chromatography was carried out using silica gel from Merck (230 - 400 mesh). The
matrix-assisted laser desorption ionization time of flight (MALDI-TOF) mass spectra
were recorded on an Autoflex Bruker MALDI-TOF system. NMR spectra were
measured in CDCl3 on a Bruker Ultra-shield 400 MHz spectrometer and chemical
1
shifts are quoted relative to tetramethylsilane for H and 13C nuclei. UV-visible
absorption spectra were obtained with a Hewlett Packard 8453 spectrometer. The
photoluminescence (PL) spectra were recorded in CH2Cl2 by PTI Fluorescence
Master Series QM1 spectrophotometer. For lifetime measurements, 337 nm line of a
N2 laser was used as the excitation light source. Cyclic voltammograms were obtained
with the CH Instrument CV. The thermal gravimetric analysis (TGA) was conducted
on a Perkin Elmer TG-6 instrument under nitrogen at the heating rate of 10 oC min–1.
2.2. Procedures for the synthesis of ligand L
2-Bromo-5-(9,9-diethyl-9H-fluorenyl)thiophene (2.768 g, 7.22 mmol) was reacted
with 2-(tributylstannyl)pyridine (5.32 mg, 14.4 mmol) in dry toluene (50 mL) with the
catalyst Pd(PPh3)4 (200 mg) under an inert atmosphere of nitrogen. Such Stille
o
coupling reaction proceeded at 100 C for 48 h. After being cooled to room
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temperature, water was added and the solution mixture was extracted with ethyl
acetate. The combined organic layer was dried over Na2SO4, filtered and concentrated
under reduced pressure. The residue was purified on a silica column eluted by ethyl
acetate/hexane (1:6, v/v) solution mixture to afford 2-(5-(9,9’-diethyl-9H-fluorenyl)
thienyl)pyridine (L) as a yellow solid (1.588 g, 58%).
1
Spectral Data: MS (MALDI-TOF): m/z 381 [M]+. H NMR (400 MHz, CDCl3): δ
(ppm) 8.60-8.58 (m, 1H, Ar), 8.19-8.16 (m, 2H, Ar), 7.73-7.64 (m, 6H, Ar), 7.58-7.57
(d, J = 3.8 Hz, 1H, Ar), 7.40-7.39 (d, J = 3.9 Hz, 1H, Ar), 7.35-7.31 (m, 3H, Ar),
7.17-7.14 (m, 1H, Ar), 2.08-2.03 (m, 4H, CH2), 0.38-0.34 (t, J = 7.2 Hz, 6H, CH3);
13C NMR (CDCl3): δ (ppm) 152.53, 150.86, 150.21, 149.59, 146.93, 143.53, 141.54,
140.98, 136.59, 133.09, 132.76, 127.36, 127.06, 125.61, 125.22, 124.69, 123.82,
123.02, 121.81, 120.24, 120.20, 119.88, 118.44 (Ar), 56.23 (quat. C), 32.82 (CH2),
8.71 (CH3).
2.3. Procedures for the synthesis of iridium(III) complexes
2.3.1. Homoleptic iridium(III) complex A
Under a N2 atmosphere, cyclometalating ligand L (0.382 g, 1.0 mmol) and [Ir(acac)3]
(0.14 g, 0.29 mmol) were heated to 220 oC in glycerol (10 mL) for 16 h. The reaction
mixture was cooled to room temperature and water was added. After extraction with
CH2Cl2 (3 × 60 mL), the organic phase was dried over Na2SO4. The solvent was
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removed under reduced pressure. The residue was obtained as a crude product, which
was purified by chromatography on a silica column using ethyl acetate/hexane (1:6,
v/v) as eluent followed by pure Ch2Cl2 to produce A as a brown solid (67 mg, 18%).
1
Spectral data: MS (MALDI-TOF): m/z 1333 [M]+. H NMR (400 MHz, CDCl3): δ
(ppm) 7.65-7.59 (m, 9H, Ar), 7.54-7.50 (m, 12H, Ar), 7.31-7.28 (m, 9H, Ar), 6.98 (s,
3H, Ar), 6.75-6.71 (m, 3H, Ar), 2.07-1.94 (m, 12H, CH2), 0.35-0.31 (t, J = 7.3 Hz, 9H,
CH3), 0.29-0.26 (t, J = 7.3 Hz, 9H, CH3); 13C NMR (CDCl3): δ (ppm) 162.36, 161.61,
150.44, 150.12, 148.21, 147.86, 141.21, 140.84, 136.59, 133.87, 132.06, 126.84,
126.75, 125.13, 122.81, 120.35, 119.72, 119.55, 118.89, 117.42 (Ar), 56.13 (quat. C),
32.70 (CH2), 32.62 (CH2), 8.57 (CH3), 8.49 (CH3).
2.3.2. Heteroleptic iridium(III) complex B
Under a N2 atmosphere, ligand L (0.31 g, 0.81 mmol) and IrCl3·nH2O (97 mg, 0.33
o
mmol) were heated to 100 C in a mixture of 2-ethoxyethanol and water (9 mL, 3:1,
v/v) for 16 h. The reaction mixture was cooled to room temperature and water was
added. The cyclometalated IrIII ꢀ-chloro-bridged dimer was formed as a precipitate
which was collected and dried under vacuum. The dimeric Ir complex, 3 equiv. of
acetylacetone and 10 equiv. of Na2CO3 were added to 2-ethoxyethanol and the
o
mixture was heated to 90 C for 18 h. After cooling to room temperature and the
addition of water, the brown precipitate was collected by filtration and washed with
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water and dried. The crude product was purified by preparative TLC plates eluting
with ethyl acetate/hexane (1:3, v/v) to produce B (83 mg, 42%) as a brown solid.
1
Spectral Data: MS (MALDI-TOF): m/z, 1052 [M]+. H NMR (CDCl3): δ (ppm)
8.39-8.38 (d, J = 5.6 Hz, 2H, Ar), 7.68-7.62 (m, 6H, Ar), 7.57-7.55 (d, J = 8.0 Hz, 2H,
Ar), 7.53-7.51 (d, J = 8.1 Hz, 2H, Ar), 7.47-7.45 (m, 2H, Ar), 7.40 (s, 2H, Ar),
7.28-7.27 (m, 4H, Ar), 6.98-6.94 (m, 2H, Ar), 6.50 (s, 2H, Ar), 5.26 (s, 1H, acac),
2.00-1.95 (m, 8H, Et), 1.84 (s, 6H, acac), 0.29-0.23 (m, 12H, Et); 13C NMR (CDCl3):
δ (ppm) 184.58, 165.01, 150.39, 150.28, 150.05, 148.55, 148.30, 141.20, 140.98,
137.35, 135.93, 133.66, 128.36, 126.92, 126.79, 125.01, 122.80, 120.31, 119.63,
119.54, 118.10, 117.08 (Ar), 100.63 (acac), 56.10 (quat. C), 32.73, 30.91 (Et), 28.64
(acac), 8.46, 8.42 (Et).
2.5. Bulk heterojunction solar cell fabrication
ITO glass substrates (10 ꢁ/square) were cleaned by sonication in toluene,
acetone, ethanol and deionized water and dried in an oven. The samples were then
cleaned with UV ozone for 300 s. As-received PEDOT:PSS solution was passed
through the 0.45 m filter and spin-coated on patterned ITO substrates at 5000 rpm for
3 min. The substrates were then baked in N2 at 150 ºC for 15 min. The active layer
was prepared by spin-coating the chlorobenzene solution (Ir(III) : PCBM = 1 : 4, v/v)
at 1000 rpm or 600 rpm for 2 min. The substrates were dried in a vacuum oven at
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room temperature for 1 h, and then stored in high vacuum (10-5 to 10-6 Torr) overnight.
Then, Al electrode (100 nm) was evaporated through a shadow mask to define the
active area of the devices (2 mm and 4 mm diameter circles). All the fabrication
procedures were performed in air, while the cells were measured in N2 inside the
sealed sample holder with a quartz window. The current density-voltage
characteristics were measured using a Keithley 2400 sourcemeter. For white light
efficiency measurements (at 100 mW/cm2), Oriel 66002 solar light simulator with
AM1.5 filter was used. The light intensity was measured by a Molectron Power Max
500D laser power meter. For the measurement of the external quantum efficiency,
different wavelength of light was selected with a Thermo Oriel 257 monochromator.
The photocurrent generated was measured with a Keithley 2400 sourcemeter. The
light intensity was measured with a Newport 1830-C optical power meter equipped
with an 818-UV detector probe.
2.6. Computational studies
The ground-state geometries of the molecules were optimized in vacuo at the
DFT level using the B3LYP [9] hybrid functional with the 6-31G* basis set for C, H,
N and S atoms, while LanL2DZ basis set for Ir atom was implemented in Gaussian 09
package [10]. Calculations of the electronic transition energies were performed with
time-dependent DFT (TD-DFT) using B3LYP method at the optimized ground-state
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geometry in vacuo. The calculated absorption spectra and related MO contributions
were obtained from Multiwfn [11].
3. Results and discussion
3.1 Synthesis
As shown in Fig. 1, 2-bromofluorene underwent alkylation with 2-bromoethane
in the presence of a strong base, sodium hydroxide, as the deprotonating reagent, and
then the corresponding bromide reacted with thiophen-2-yl-2-boronic acid via the
palladium-catalyzed
Suzuki
cross
coupling
to
give
2-(9,9-diethyl-9H-fluorenyl)thiophene. Subsequently, the resulting compound was
transformed into the corresponding bromide derivative via bromination with NBS,
followed by its coupling with 2-(tributylstannyl)pyridine via Stille coupling reaction
to give 2-[5-(9,9’-diethyl-9H-fluoren-7-yl)thienyl]-pyridine (L).
To obtain the homoleptic iridium(III) complex, the cyclometalating ligand L
underwent cyclometalation with Ir(acac)3 (Hacac = acetylacetone) in refluxing
glycerol at a high temperature (≥ 200 oC) to produce A after purification by silica gel
column chromatography. For the heteroleptic species, IrCl3·nH2O was reacted with an
excess of L to give a chloride-bridged iridium(III) dimer and it can be readily cleaved
to the monomeric complex B by replacing the bridging chlorides with acetylacetonate
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anion (acac) derived from the deprotonation of acetylacetone (Hacac) by sodium
carbonate [12]. Purification of B was achieved by preparative TLC plate to furnish B
as an air-stable brown powder in high purity.
3.2. Photophysical properties and thermal stability
The photophysical properties of the iridium(III) complexes have been recorded
in CH2Cl2 at 298 K. In Fig. 2, their UV-Vis spectra show extended absorption profiles
as caused by mixing ligand-based 3π - π* states, spin forbidden triplet metal-to-ligand
charge-transfer (3MLCT) states and the heavy-atom effect of metal center [13]. For A
and B, the UV-Vis absorption band can be extended up to 530 nm with relatively high
extinction coefficient values, suggesting that they are potential photovoltaic materials
with good absorptivity to sunlight.
1.0
0.8
0.6
0.4
0.2
0.0
A
B
250 300 350 400 450 500 550 600
Wavelength (nm)
Fig. 2. UV-Vis spectra of A and B in CH2Cl2 at 298 K.
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Table 1. UV-Vis absorption, emission and thermal stability data of A and B.
Tdec (onset)
λabs
(nm)[a]
λem
[c]
Complex
Φp
(nm, 298 K)[b]
(oC)[d]
390 (6.3),
421 (7.1),
480 (2.3)
365 (5.0),
439 (0.8),
485 (1.2)
670 (1.94)
0.02
0.05
408
A
736 (sh, 1.86)
676 (2.63)
379
B
741 (sh, 1.53)
[a] In CH2Cl2 at 298 K, extinction coefficients (104 M-1 cm-1) are shown in
parentheses.
[b] Excited by 400 nm in CH2Cl2. Lifetime (µs) data obtained by 337 nm of a nitrogen
laser in CH2Cl2 are shown in parentheses, sh means shoulder peak.
[c] Phosphorescence quantum yields ΦP relative to fac-[Ir(ppy)3] in CH2Cl2 (ΦP
=
0.40) at 298 K, λex = 400 nm.
o
[d] Tdec is the decomposition temperature, measured at a heating rate of 10 C/min
under N2.
A
B
1.0
0.8
0.6
0.4
0.2
0.0
600
650
700
750
800
Wavelength (nm)
Fig. 3. PL spectra of A and B in CH2Cl2 (at 298 K, λex = 400 nm).
A and B show very weak phosphorescent emissions at 670 nm (ΦP = 0.02) and
676 nm (ΦP = 0.05), respectively (Fig. 3, Table 1). The microsecond scale for τP (1.94
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ꢀs for A at 670 nm and 2.63 ꢀs for B at 676 nm) reveals the phosphorescent nature of
such emissions.
The thermal properties of these iridium(III) complexes were examined by
thermogravimetric analysis (TGA) under a nitrogen stream. Homoleptic iridium(III)
complex showed much better thermal ability than the heteroleptic analogue,
presumably because of the stronger binding of Ir(III) ion with C^N ligands than the
coordination of iridium(III) ion with diketone ancillary ligands. Benefited from the
more robust structure, A started to decompose at 408 oC, showing higher resistance to
heat than its bis(cyclometallated) species B, which decomposed at 379 oC.
3.3. Computational studies and electrochemical properties
The nature of the frontier MOs for A and B has been addressed by means of
density functional theory (DFT) and time-dependent density functional theory
(TD-DFT) calculations. The contour plots of the frontier MOs for A and B and the
computed electronic density of the frontier orbitals are presented in Fig. 4 and Table 2,
respectively. The HOMO of A is mainly composed of the π-system of the
thienyl-pyridine and the Ir(III) core. The LUMO also possesses a major contribution
from the cyclometallating C-N ligands with much reduced effect from the iridium
atom. For B, the same description may be given except that there is a little
contribution from the π-orbital of acetylacetone ligand. Consequently, there are more
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atomic contributions from the π-systems of the cyclometallating C-N ligands of A, but
the acetylacetone ligand did not contribute very much to the HOMO and LUMO
orbitals in B. Based on the comparison between the computed HOMO and LUMO
electronic density data (Table 2), the large decrease in iridium(III) atomic
contrubutions on going from the HOMO to the LUMO confirms the strong
metal-to-ligand charge transfer character for this transition. TD-DFT computations
were performed to investigate the lowest energy excited states of A and B. Table S1
and Table S2 in the electronic supplementary information (ESI) provides the
calculated transition energies, position of the 0-0 peaks, oscillator strengths (f) and
major contributions for the 20 first transitions for A and B. The first calculated
transitions are placed at 486.4 and 512.2 nm for A and B, respectively. These values
reveal that the lowest energy transition is composed mainly of HOMO→LUMO,
which is in accordance with the results of the DFT computations [3c].
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Fig. 4. Representations of the frontier MOs for optimized geometries of the Ir(III)
complexes A (left) and B (right).
By assigning a full width at half maximum (FWHM) of 1000 cm–1, spectra as
shown in Fig. 5 are generated. These calculated spectra do not include the presence of
vibronic progressions but the shape of the spectra compares favorably well to the
experimental ones (see Fig. 2). According to the similarity between the HSOMO and
LUMO and the LSOMO and HOMO (shown as Fig. S1 in ESI), the contribution to
the first spin-forbidden singlet-triplet transition is supposed to be composed of the
same transition (i.e. HOMO→LUMO). Hence, these computations support the
assignment of the nature of the lowest energy excited states as described above. Based
on the DFT calculations, the calculated Eg of A and B is 3.07 eV and 2.99 eV,
respectively, which are narrower than the common cyclometalated Ir(III) complexes.
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Table 2. Computed electronic density of the frontier orbitals of A and B.
% of electronic density
A
B
Atom
C
HOMO
68.72
0.14
LUMO
84.5
HOMO
73.34
0.18
LUMO
81.30
0.19
H
0.37
S
4.14
6.25
4.86
5.60
N
4.70
7.57
2.82
7.66
Ir
22.30
1.31
17.50
4.76
200000
150000
100000
50000
0
200000
0.9
0.9
0.6
0.3
0.0
150000
100000
50000
0
0.6
0.3
0.0
250 300 350 400 450 500 550 600
250 300 350 400 450 500 550 600
Wavelength (nm)
Wavelength (nm)
Fig. 5. Calculated spectra of A (left) and B (fight).
The electrochemical behavior of these iridium(III) complexes was investigated
by cyclic voltammetry (CV) using ferrocene as the internal standard in THF solutions
of the complexes containing 0.1 M tetrabutylammonium hexafluorophosphate
([Bu4N]PF6) as supporting electrolyte. All the measurements were performed with a
conventional three-electrode configuration consisting of a glassy carbon working
electrode, one Ag/Ag+ reference electrode and one Pt-wire counter electrode. The scan
rate was set at 100 mV/s for all the measurements. On the basis of the potentials of the
oxidation and reduction couples, the HOMO and LUMO energy levels of all the
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complexes were estimated with regard to the energy level of the ferrocene reference
(4.8 eV below the vacuum level) [14]. The results are listed in Table 3.
60
50
30
20
A
B
40
30
10
20
0
10
0
-10
-20
-30
-40
-10
-20
-30
-40
-50
-3.0
-2.5
-2.0
0.0
0.5
1.0
1.5
-3.5 -3.0 -2.5 -2.0
0.0
0.5
1.0
1.5
Potential (V)
Potential (V)
Fig. 6. Cyclic voltammograms for A and B.
All the iridium(III) complexes show a reversible oxidative couple in the range of
0.30 V to 0.40 V, which is attributed to the IrIII/IrIV redox process, in accordance with
the reported cyclometallated iridium(III) systems [15]. Moreover, A possesses a
second quasi-reversible anodic wave at 0.78 V (Fig. 6), which is possibly ascribed to
the oxidation of thiophene-containing ligands. The reduction potential of A is –2.63 V,
affording the Eg of 2.93 eV. A quasi-reversible cathodic wave is shown at –2.51 V for
B, leading to the Eg of 2.81 eV.
Table 3. Electrochemical properties of the iridium(III) complexes.
ox
red
E1
E1
HOMO
[eV][b]
–5.10
LUMO
[eV][c]
–2.17
Eg
[eV][d]
／
／
2
2
Compound
[V][a]
0.30, 0.78
0.30
[V][a]
–2.63
–2.51
2.93
A
B
–5.10
–2.29
2.81
[a] 0.1 M [Bu4N]PF6 in THF, versus Fc/Fc+ couple.
[b] HOMO = –(4.8 eV + E1/2ox) eV
[c] LUMO = –(4.8 eV + E1/2red) eV
[d] Eg = LUMO – HOMO.
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3.4. Photovoltaic performance
Bulk heterojunction (BHJ) solar cells were fabricated by using A and B as
electron donors and PCBM as an electron acceptor. The blend ratio of iridium(III)
complex to PCBM was fixed at 1:4 (w/w). The hole-collection electrode was ITO
glass coated with a PEDOT:PSS layer, whereas Al served as the electron-collecting
electrode. The solar cells were manufactured via spin-coating method using a
chlorobenzene solution (30 mg/ml). The influence of spinning speed was investigated
by fabricating devices at 1000 rpm and 600 rpm. All the data were collected under
simulated solar light AM 1.5.
Table 4. Photovoltaic performance of A and B.
Spinning speed
(rpm)
Voc
Jsc
(mA/cm2)
1.55
PCE
(%)
Devices Dopant
ff
(V)
600
0.63
0.66
0.45
0.45
0.27
0.28
0.27
0.26
0.26
0.51
0.14
0.14
SC1
SC2
SC3
SC4
A
A
B
B
1000
2.68
600
1.13
1000
1.22
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1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
SC1
SC2
SC3
SC4
0.0
0.2
0.4
0.6
0.8
Voltage (V)
Fig. 7. J-V curves of solar cell devices with
A
/PCBM (1:4) and
B
/PCBM (1:4) active
layers under simulated solar light AM 1.5.
0.4
0.3
0.2
0.1
0.0
SC2
SC4
400
500
600
Wavelength (nm)
700
800
Fig. 8. Absorption spectra for the blend films of devices SC2 and SC4
.
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45
40
35
30
25
20
15
10
5
SC2
SC4
0
400
500
600
700
800
Wavelength (nm)
Fig. 9. EQE versus wavelength curves for devices SC2 and SC4
.
The current density-voltage (J-V) curve of solar cells is shown in Fig. 7. The
performance data of these devices are summarized in Table 4. The PCE value of SC2
is 0.51%, about seven-fold higher than that in the literature report using an
iridium(III)-containing polymer as donor with a similar configuration [5]. The Voc of
SC2 is 0.66 V, with Jsc of 2.68 mA/cm2 and ff of 0.28. Better results were obtained by
using the thinner active layer fabricated at a spinning speed of 1000 rmp, since SC1
only gave PCE of 0.26%. However, for SC3 and SC4, the thickness of active layer
did not seem to make a large difference, because both of them lead to the same PCE at
0.14%. The ff values of the solar cells are relatively low, from 0.26 to 0.28, suggesting
the charge transfer within the cells was hindered once the charges in the active layer
are formed [8a]. It is well known that fluorene unit possesses good charge carrier
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mobility [16], which may benefit sunlight harvesting in SC1 and SC2 over other
devices, because the former devices were blended with the tris(cyclometalated)
iridium(III) complex
A. Another possible reason to explain the better performance of
SC2 is the better transformation from excitons to charges within the active layer.
Since the absorption abilities of SC2 and SC4 are similar in the range of 400 nm to
800 nm in the blend films (with PCBM) as depicted in Fig. 8, SC2 exhibited much
higher EQE values than the other devices, especially between 400 nm and 600 nm, as
shown in Fig. 9. The maximum ηext of SC2 is located at 438 nm with a value of 41%,
which is more than two-fold to that of SC4 (18.1%) at the same absorption
wavelength.
4. Conclusions
We had explored the possibility of exploiting thiophene-containing iridium(III)
cyclometalated complexes in photovoltaic applications in this work. Two novel
iridium(III)
2-[5-(9,9’-diethyl-9H-fluoren-7-yl)thienyl]-pyridine were successfully synthesized
and fully characterized. It is impressive to see that SC2 blended with achieved a
cyclometallated
complexes
based
on
A
PCE of 0.51%, that is much higher than that for other previously reported neutral
iridium(III) complexes. In the future study, it would be more meaningful for us to
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enhance the charge transfer ability within BHJSCs, as well as the absorption ability
between 600 nm and 800 nm, since most of the solar light energy is located in this
region, according to the AM 1.5 spectrum [17]. We believe that these are effective
ways to enhance the PCE of iridium(III) complexes and further research along this
line is currently underway.
Acknowledgments
The first two authors contributed equally to this work. We thank Hong Kong
Research Grants Council (HKBU12304715), Areas of Excellence Scheme, University
Grants Committee of HKSAR (project No. [AoE/P-03/08]) and Hong Kong Baptist
University (FRG1/14-15/084 and FRG1/14-15/066) for financial support. Dr. Guiping
Tan acknowledges the financial support from the Department of Education of
Guangdong Province (No.2014KQNCX219) and Guangdong Natural Science
Foundation (No.2015A030310242). Dr. Yongfu Qiu acknowledges the financial
support from the National Natural Science Foundation of China (No.51206029). Dr.
Xiaotian Liu acknowledges the financial support from the Guangdong Natural
Science Foundation of China (No.S2013010015690). Y. H. Lo gratefully
acknowledges the financial support from the National Science Council of Taiwan
(NSC 102-2113-M-845-001) and the project of the specific research fields in the
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University of Taipei, Taiwan.
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Captions for Figures
Fig. 1. The synthesis routes of
Fig. 2. UV-Vis spectra of and
Fig. 3. PL spectra of and in CH2Cl2 (at 298 K, λex = 400 nm).
Fig. 4. Representations of the frontier MOs for optimized geometries of the Ir(III)
complexes (left) and (right).
Fig. 5. Calculated spectra of (left) and
Fig. 6. Cyclic voltammograms for and
A
and
B.
A
B in CH2Cl2 (298 K).
A
B
A
B
A
B
B
(fight).
A
.
Fig. 7. J-V curves of solar cell devices with A/PCBM (1:4) and B/PCBM (1:4) active
layers under simulated solar light AM1.5.
Fig. 8. Absorption spectra for the blend films of SC2 and SC4
.
Fig. 9. EQE versus wavelength curves for devices SC2 and SC4
.
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ꢀ Two
new
iridium(III)
cyclometalates
based
on
2-[5-(9,9’-diethyl-9H-fluoren-7-yl)thienyl]-pyridine were synthesized and
characterized
ꢀ They showed extended absorption features
ꢀ They have been applied in bulk heterojunction solar cells as donor materials