Effect of main ligands on organic photovoltaic performance of Ir(iii) complexes

Article abstract of DOI:10.1039/c1nj20446g

The photovoltaic performance of devices fabricated using three iridium complexes (1, 2, and 3) containing different main ligands (1-phenylisoquinoline, (4-isoquinolin-1-yl-phenyl)diphenylamine, and 1-pyren-1-yl-isoquinoline for 1, 2, and 3, respectively) was investigated. Two different devices, one fabricated by spin coating and one produced by vacuum deposition, were tested. Among the bulk heterojunction solar cells (BHJCs) fabricated by spin coating, the cell fabricated using 2 had the highest power conversion efficiency (PCE, 0.50%). The PCEs of 1 and 3 were 0.43% and 0.34%, respectively. These results suggested that the superior hole-transport ability of the triphenylamine moiety in 2 was responsible for the high photovoltaic performance of the device fabricated using this complex. This assumption was confirmed by fabricating electron-only devices using the three Ir complexes and comparing the turn-on voltage of each device. The photovoltaic performance of device C fabricated by the vacuum co-deposition of 2 and C₆₀ in a 50 nm-thick active layer was 50% higher than that of device A (bilayer heterojunction solar cell) and device B (fabricated by the co-deposition of 2 and C₆₀ with a 30 nm-thick active layer).

Full text of DOI:10.1039/c1nj20446g

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PAPER
Effect of main ligands on organic photovoltaic performance of Ir(III)
complexesw
a
ab
Woochul Lee,z Tae-Hyuk Kwon,z Jongchul Kwon,a Ji-young Kim,c
Changhee Leec and Jong-In Hong*a
Received (in Montpellier, France) 27th May 2011, Accepted 26th July 2011
DOI: 10.1039/c1nj20446g
The photovoltaic performance of devices fabricated using three iridium complexes (1, 2, and 3)
containing different main ligands (1-phenylisoquinoline, (4-isoquinolin-1-yl-phenyl)diphenylamine,
and 1-pyren-1-yl-isoquinoline for 1, 2, and 3, respectively) was investigated. Two different devices,
one fabricated by spin coating and one produced by vacuum deposition, were tested. Among the
bulk heterojunction solar cells (BHJCs) fabricated by spin coating, the cell fabricated using 2 had
the highest power conversion efficiency (PCE, 0.50%). The PCEs of 1 and 3 were 0.43% and
0.34%, respectively. These results suggested that the superior hole-transport ability of the
triphenylamine moiety in 2 was responsible for the high photovoltaic performance of the device
fabricated using this complex. This assumption was confirmed by fabricating electron-only devices
using the three Ir complexes and comparing the turn-on voltage of each device. The photovoltaic
performance of device C fabricated by the vacuum co-deposition of 2 and C60 in a 50 nm-thick
active layer was 50% higher than that of device A (bilayer heterojunction solar cell) and device B
(fabricated by the co-deposition of 2 and C60 with a 30 nm-thick active layer).
maximum thickness of the active layers.6 Bulk heterojunction
Introduction
solar cells (BHJCs)7–10 have been proposed as a possible solution
to this problem. In these devices, the acceptor molecules are
uniformly distributed in the donor matrix resulting in a three-
dimensional network of photoinduced charge-generating inter-
faces. An efficiency of more than 7% was recently achieved using
a BHJC composed of a benzodithiophene polymer derivative
(PTB7) and PCBM.10 However, efficient BHJCs are relatively
difficult to fabricate because of the difficulty involved in con-
trolling the morphology of their active layer. Further, it is
necessary to use high-purity materials to obtain BHJCs with
long-term stability and high efficiency. One of the primary
reasons for the low efficiency of OHSCs is the short (exciton)
diffusion length of the donor material. Therefore, it is essential
to develop organic dyes composed of donor molecules with long
exciton diffusion lengths to obtain high PCE.
Recently, the energy crisis and environmental problems have
led to intensification of the search for alternative energy
sources. Photovoltaic cells are considered a viable solution
to these problems. Specifically, organic photovoltaic devices
(OPV) are a promising alternative to silicon solar cells owing
to their flexibility, high absorption coefficient, ease of fabrica-
tion, and most importantly, low manufacturing cost. In addi-
tion, the chemical structure of the donor materials can easily be
modified to achieve the desired electronic properties. Organic
heterojunction solar cells (OHSCs) comprise separate donor
and acceptor layers analogous to a classical p–n junction.1–5 In
such bilayer cells, dissociation of photogenerated excitons
occurs at the interface between the donor (D) and the acceptor
(A). Hence, the power conversion efficiency is limited by the
average exciton diffusion length, which in turn determines the
In general, the conversion of solar energy to electrical
energy involves four major steps: absorption of light and
generation of excitons, diffusion of excitons, dissociation
of excitons, and charge accumulation at the electrodes.9 To
achieve high PCE, it is necessary to synthesize organic materials
that can absorb near-infrared (NIR) radiation, because approxi-
mately 50% of the total solar photon flux is in the NIR
region.4 Moreover, the organic active layer must be sufficiently
thick to ensure strong light absorption. However, the exciton
diffusion length (5–10 nm) of singlet materials limits the
absorption depth required for efficient light absorption.11b
Thus, synthesis of donor materials that have a long exciton
a Department of Chemistry, College of Natural Sciences,
Seoul National University, Seoul, 151-747, Korea.
E-mail: jihong@snu.ac.kr; Fax: +82-2-889-1568;
Tel: +82-2-880-6682
b School of Chemistry, Bio21 Institute, University of Melbourne,
Parkville, Victoria 3010, Australia
c School of Electrical Engineering and Computer Science,
Inter-University Semiconductor Research Center, Seoul National
University, Seoul 151-742, Korea. E-mail: chlee7@snu.ac.kr
w Electronic supplementary information (ESI) available: Cyclic
voltammograms and data of an electron-only OLED device. See
DOI: 10.1039/c1nj20446g
z These authors contributed equally to this study.
c
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Solar cell materials including CuPc, fullerene, PCBM, and
2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) were pur-
chased from Luminescence Technology Corporation (Taiwan).
Poly(styrene sulfonic acid)-doped poly(ethylenedioxythiophene)
(PEDOT:PSS, Baytron VP Al 4083) was purchased from
Bayer. Analytical thin-layer chromatography was conducted
using Kieselgel 60F-254 plates purchased from Merck. Column
chromatography was conducted using a Merck silica gel 60
(70–230 mesh) column. All solvents and reagents used in this
study were commercially available and used without further
purification unless otherwise specified. 1H and 13C NMR
spectra were recorded using an Advance 300 or 500 MHz
Bruker spectrometer. UV-vis spectra were recorded using a
Beckman DU 650 spectrophotometer. Mass spectra were
Scheme 1 Molecular structures of iridium complexes 1–3.
diffusion length is essential for the production of efficient
active layers. The exciton diffusion length depends on the
charge mobility and lifetime of the donor. Accordingly,
organic triplet materials, which have relatively long lifetimes,
are expected to be suitable alternatives to singlet materials.5,11
Iridium (Ir) complexes are increasingly being used in organic
light emitting Diodes (OLEDs) because the strong spin–orbit
coupling in Ir allows efficient intersystem crossing (ISC) between
the singlet and triplet excited states; thus, an internal quantum
efficiency of 100% can be achieved.12 Moreover, owing to the
relatively long-lived excited state formed in metal-to-ligand
charge-transfer (MLCT) processes, the excitons formed in Ir
complexes may have a long lifetime, which favors the subse-
quent exciton dissociation. However, in organic solar cell
research, very few studies have focused on the use of Ir com-
plexes for active layer formation, even though organic triplet
materials are suitable for the aforementioned purpose.5,11
In this study, the photovoltaic properties of Ir complexes
with isoquinoline-based ligands were investigated. As shown
in Scheme 1, to increase the power conversion efficiency of the
standard Ir complex, we made two simple changes to the
parent structure 1: (1) addition of a triphenylamine unit at
the 2 position of the isoquinoline moiety of 2, and (2) addition
of a pyrene unit to the isoquinoline unit to increase the
absorption range. Triphenylamine is known to be a good
hole-injecting/transporting material for OLEDs.13,14 Devices
were fabricated by spin coating using complexes 1–3 and their
efficiencies were then compared with that of a bilayer device
fabricated from 2 by vacuum deposition.
obtained using
(JEOL, JMS-AX505WA, HP 5890 Series II). Fluorescence
spectra were recorded with Jasco FP-7500 spectro-
a gas chromatograph-mass spectrometer
a
photometer. The CV measurements were made in a 0.1 M
Bu4NPF6/acetonitrile solution using a CH Instruments 660
electrochemical analyzer (CH Instruments, Inc., Texas). The
Ag reference electrode was calibrated using a ferrocene/
ferrocenium redox couple. The performance of the device
was measured under AM 1.5 illumination (100 mW cmꢀ2
)
using a solar simulator (Newport, 91160A) and Solar Cell
IPCE Measurement System (McScience, K3100). The light
intensity at each wavelength was calibrated using the standard
Si solar cell as a reference. OLED performances were obtained
using a PR 650 spectroradiometer and a source meter (Keithly
2400) connected to a computer was used to operate the device.
Synthesis
The main ligands in complexes 1–3 were synthesized according
to previously described methods, with some modifications
(Scheme 2).15 Cyclometalated Ir-m-chloro-bridged dimers
(general formula: C^N2Ir(m-Cl)2IrC^N2) were synthesized using
a modified version of the method proposed by Nonoyama16 and
then coupled with picolinic acid in 2-ethoxyethanol to produce
complexes 1, 2, and 3.
IrCl3ꢁnH2O was mixed with 2–2.5 equiv. of a cyclometalating
ligand in a 3 : 1 mixture of 2-methoxyethanol and water, after
which the mixture was refluxed for 6–7 h. The reaction mixture
was then cooled to room temperature, after which more water
was added to precipitate the product. The resulting mixture
Experimental section
Materials and instruments
All organic chemicals were purchased from Aldrich and TCI.
Iridium(III) chloride trihydrate was obtained from Strem.
was subsequently filtered through a Buchner funnel and then
¨
Scheme 2 Synthesis of Ir complexes.
c
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washed with hexane and ethyl ether several times to obtain
the crude product. Next, the crude chloro-bridged dimer
(0.08 mmol), 3-isoquinolinecarboxylic acid (0.25 mmol), and
sodium carbonate (1 mmol) were heated at reflux in 2-ethoxy-
ethanol in an inert atmosphere for 4–5 h. After cooling to
room temperature, the solvent was evaporated under high
vacuum and the residue was dissolved in methylene chloride.
The organic phase was then washed with water and dried
over Na2SO4. Next, the solvent was evaporated to afford
the crude product, which was subsequently purified by silica
gel column chromatography. Elution was conducted using
methylene chloride and methyl alcohol to obtain the desired
products. The yields of 1, 2, and 3 were 61%, 65%, and 59%,
respectively.
Fabrication of organic photovoltaic cells
Fabrication of an organic solar cell device by spin coating:
PEDOT:PSS (Baytron VP Al 4083) was spin coated for 30 s
at 4000 rpm and then baked for 30 min at 140 1C. This layer
was about 50 nm thick. Next, a 10 nm-thick copper phthalo-
cyanine (CuPc) layer was thermally deposited onto the
PEDOT:PSS layer. An Ir complex and PCBM were then
blended in dichlorobenzene and spin-coated onto the CuPc
layer in a nitrogen atmosphere. The active layer was heated on
a hot plate for 30 min at 110 1C. Measurement of the thickness
of the active layer using a KLA-TENCOR Alpha-step 500
profiler revealed that it was approximately 55 nm. In this
procedure, CuPc was not significantly influenced by dissolu-
tion in dichlorobenzene during spin coating, as confirmed by
fabrication of the devices using the thermal evaporation
method. Both C60 and BCP were systematically deposited
onto the active layer before aluminium evaporation.
Complex 1. 1H NMR (300 MHz, CDCl3): d (ppm)
9.00–8.95 (m, 2H), 8.76 (d, 6 Hz, 1H), 8.36 (d, 6 Hz, 1H),
8.28 (d, 6 Hz, 1H), 8.19 (d, 6 Hz, 1H), 7.95 (m, 3H), 7.77–7.71
(m, 4H), 7.62 (d, 6 Hz, 1H), 7.50 (d, 6 Hz, 1H), 7.42 (d, 6 Hz,
1H), 7.33–7.26 (m, 2H), 7.03 (t, 6 Hz, 1H), 6.95 (t, 9 Hz, 1H),
6.80 (t, 6 Hz, 1H), 6.72 (t, 9 Hz, 1H), 6.54 (d, 6 Hz, 1H), 6.24
(d, 9 Hz, 1H). 13C NMR (125 MHz, CDCl3): d (ppm) 182.17,
179.50, 173.26, 153.42, 150.18, 150.10, 149.24, 148.91, 148.54,
141.45, 140.97, 138.13, 135.03, 134.24, 132.04, 131.44, 131.39,
131.02, 128.60, 128.13, 128.09, 127.26, 126.53, 126.17, 125.94,
125.16, 123.30, 122.44, 122.34, 121.70, 121.42, 118.06. HRMS
(FAB) m/z: calcd. for [C60H42IrN5O2+H]+ 723.1500. Found
[M+H]+: 723.1512.
Fabrication of the organic solar cell device by vacuum
deposition: heterojunction solar cell A: PEDOT:PSS (Baytron
VP Al 4083) was spin coated onto pre-cleaned ITO for 30 s at
4000 rpm by UV-ozone treatment and then baked for 30 min
at 140 1C. The thickness of this layer was approximately
50 nm. A 10 nm-thick CuPc layer was then thermally depos-
ited onto the PEDOT:PSS layer, after which a 20 nm-thick Ir
complex was deposited and a 40 nm-thick layer of C60 was
thermally deposited. Finally, C60 and BCP were systematically
deposited onto the active layer before aluminium evaporation.
BHJCs B and C: after thermal deposition of CuPc onto
the PEDOT:PSS layer, thermal co-deposition of 2 and C60
(1 : 4 ratio) was conducted. The deposition rate was controlled
using quartz crystal oscillators. To control the doping ratio
using one thickness monitor, the Ir complex was evaporated
until the evaporation ratio became constant. The C60 shutter
was then opened, after which the sample was heated until the
thickness monitor indicated the desired doping ratio. The Ir
complex shutter was simultaneously opened during the heating
process. When the desired doping ratio was reached, the main
shutter was opened to deposit the Ir complex and C60.
Complex 2. 1H NMR (300 MHz, DMSO-d6): d (ppm)
8.76 (d, 6 Hz, 2H), 8.14–8.03 (m, 5H), 8.36–8.15 (m, 2H),
7.87 (d, 6 Hz, 2H), 7.76–7.65 (m, 6H), 7.32 (d, 6 Hz, 1H),
7.13 (d, 6 Hz, 1H), 7.09–7.00 (m, 9H), 6.87 (dd, 18 Hz, 6 Hz,
13H), 6.55 (d, 9 Hz, 1H), 6.48 (d, 9 Hz, 1H), 5.81 (s, 1H), 5.57
(s, 1H). 13C NMR (125 MHz, DMSO-d6): d (ppm) 173.17,
169.19, 167.97, 167.41, 154.08, 152.98, 152.34, 150.27, 148.83,
148.53, 148.43, 147.77, 146.87, 146.82, 146.72, 144.01, 140.73,
140.08, 138.71, 138.52, 138.43, 137.52, 136.97, 136.83, 130.82,
130.61, 130.45, 130.34, 130.00, 129.93, 129.03, 128.85, 128.75,
128.30, 128.08, 127.72, 127.46, 127.42, 127.34, 127.15, 127.00,
126.89, 126.54, 126.17, 126.04, 125.93, 125.62, 125.58,
123.92, 123.85, 123.55, 123.42, 122.48, 122.33, 119.17,
118.95, 118.29, 113.46, 113.23, 112.89. HRMS (FAB) m/z:
calcd. for [C60H42IrN5O2+H]+ 1057.2973. Found [M+H]+:
1057.2985.
Electron-only OLEDs fabrication
PEDOT:PSS (Baytron VP Al 4083) was spin-coated onto a
patterned ITO glass substrate that had been pre-cleaned by
UV-ozone treatment. The active layer was fabricated by
spin-coating a solution of the Ir complex (10 wt%) and
4,40-bis(9-carbazolyl)-2,20-biphenyl (CBP) in dichlorobenzene
in a nitrogen atmosphere. The other organic layers were
fabricated onto the emitting layer by high-vacuum (10ꢀ7 Torr)
thermal evaporation. A 30 nm-thick layer of 4-biphenyloxolato
aluminium(III)bis(2-methyl-8-quinolinato)4-phenylphenolate
(BAlq), which acted as a hole-blocking layer (HBL) and
an electron-transporting layer, was deposited on the emitting
layer. Subsequently, LiF, an electron injection layer (1 nm),
was deposited by high-vacuum (10ꢀ7 Torr) thermal evapora-
tion. Finally, the metal mask was changed and a 100 nm-thick
aluminium layer was deposited onto the EIL. EL spectra were
obtained using a PR 650 spectroradiometer. A source meter
(Keithly 2400) connected to a computer was used to operate
the device.
Complex 3. 1H NMR (300 MHz, DMSO-d6): d (ppm)
8.82 (d, 6 Hz, 1H), 8.65 (d, 6 Hz, 1H), 8.38–7.69 (m, 21H),
7.38 (d, 6 Hz, 2H), 7.04–6.87 (m, 3H), 6.26 (d, 9 Hz, 1H), 6.03
(d, 6 Hz, 1H). 13C NMR (125 MHz, DMSO-d6): d (ppm)
172.04, 171.92, 170.33, 170.00, 169.82, 169.79, 151.60, 151.31,
151.14, 149.06, 148.35, 140.78, 140.06, 139.47, 139.08, 138.92,
138.71, 137.91, 136.87, 136.69, 136.65, 132.03, 131.97, 129.74,
129.63, 129.60, 129.32, 129.20, 129.12, 129.08, 129.04, 128.97,
128.87, 128.78, 127.68, 127.51, 127.24, 127.11, 126.37, 126.32,
126.28, 126.17, 126.06, 125.96, 125.81, 125.72, 125.21, 126.06,
124.91, 124.37, 124.22, 122.03, 121.68, 121.24, 120.95, 120.87.
HRMS (FAB) m/z: calcd. for [C56H32IrN3O2+H]+ 971.2128.
Found [M+H]+: 971.2123.
c
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The electrochemical properties of the Ir complexes were
studied by cyclic voltammetry (CV). The energy levels of
the highest occupied molecular orbital (HOMO) of the Ir
complexes were calculated from the oxidation potentials,
and the energy levels of the lowest unoccupied molecular
orbital (LUMO) were obtained from the HOMO and Eopt,g
values. As shown in Table 1 and Fig. S2 (ESIw), the HOMO
level was similar in complexes 1 and 2, and the LUMO energy
of 2 (ꢀ3.13 eV) was more stable than that of 1 (ꢀ3.02 eV). The
HOMO and LUMO energy levels (ꢀ5.37 eV and ꢀ3.30 eV,
respectively) in 3 were more stable than those in 1. Based on
these photophysical and electrochemical studies, the com-
plexes can be arranged in the increasing order of their energy
band gaps as follows: 1 4 2 4 3.
Results and discussion
Photophysical and electrochemical properties
As shown in Fig. 1, all Ir complexes yielded similar absorption
spectra at low wavelengths owing to the p - p* transition
(250–350 nm) in the main C^N ligand. However, at the
wavelength at which singlet and triplet MLCT transitions
occur (4400 nm),17 the characteristics of the absorption
spectrum differed from those of the main ligand present in
the complex. Specifically, the presence of a triphenylamine
unit in 2 produced an increase in the molar extinction coefficient
(430 000 Mꢀ1 cmꢀ1) in the MLCT transition range of
400–450 nm, and this molar extinction coefficient was higher
than that observed for 1 (7000 Mꢀ1 cmꢀ1). Extension of
conjugation by the addition of a pyrene unit in 3 caused the
onset of the MLCT to be red-shifted, although the p - p*
transitions were unaffected. Complex 3 had the highest molar
extinction coefficient (440 000 Mꢀ1 cmꢀ1) in the 400–450 nm
region. The photoluminescence (PL) spectra of the complexes
Photovoltaic performances—spin coating method
For systematic investigation of the effect of the main ligand of
the Ir complexes on the photovoltaic performance, a series of
BHJCs with a multilayer structure were fabricated using a
wide range of D/A blending ratios. The cell representation is
as follows: ITO/PEDOT:PSS (50 nm)/CuPc (10 nm)/active
layer (Ir complex was blended with PCBM before spin coating,
55 nm)/C60 (10 nm)/BCP (10 nm)/Al (100 nm). The cell
structure was very similar to the conventional bilayer struc-
ture, except for the active layer.5a,b The devices were tested
under AM 1.5 illumination (100 mW cmꢀ2). The current–
density (I–V) characteristics are shown in Fig. 2, and the
overall photovoltaic performances are summarized in Table 1.
The PCE of the devices fabricated using Ir complexes 1–3
was best when the D : A ratio was 1 : 4. A higher PCE was
achieved when using 2 (0.50%) than when using 1 (0.45%).
are also shown in Fig. 1. The emission spectrum of 2 (lmax
621 nm) was red-shifted with respect to that of 1 (lmax
=
=
600 nm). The emission peak of 3 was not observed because
of its very low intensity; however, the lmax value for the
electroluminescence spectrum of OLEDs fabricated using
3 appeared to be far red-shifted (lmax = 736 nm). For
investigating the photo-induced charge transfer process from
the donor to the acceptor, we fabricated a blending film of
Ir complex : PCBM = 1 : 1. Ir complexes were used as the
electron donor material, and PCBM as the electron acceptor
material. As shown in Fig. S1 (ESIw), the PL of the donor was
significantly quenched by PCBM. This suggested that an
intermolecular photoinduced charge transfer process from
the Ir complex to PCBM occurred.
This is because the current density (Jsc) of 2 (2.65 mA cmꢀ2
)
was greater than that of 1 (2.32 mA cmꢀ2), which was due to
the increased hole mobility in 2. The presence of a triphenyl-
amine group in 2 resulted in a slight decrease in the open-
circuit voltage (Voc) from 0.46 V to 0.44 V because of a slight
increase in the HOMO energy of 2. In contrast, the photo-
voltaic performance of 3 unexpectedly decreased to 0.34%,
even though it had the highest light harvesting ability among
all three complexes. The fill factor (FF) of 3 was 0.34, which
was considerably lower than that of 1 (FF = 0.42) and
2 (FF = 0.43). We tentatively assumed that these results
were related to the poor hole-transport capability of 3.
We attempted to confirm this assumption by fabricating
electron-only OLED devices without a hole-transport layer.
Study of electron-only OLED devices
Fig. 1 Optical absorption spectra and PL spectra of Ir complexes
To analyze the hole-transporting capability of complexes 1–3,
we designed electron-only OLEDs (Fig. 3; Fig. S3 and S4, ESIw).
(0.02 mM CH2Cl2 solution at 298 K).
Table 1 Summary of the electrochemical and photophysical data and device performance obtained for Ir complexes 1–3
E
ox/V vs. Ered/V vs.
Complex Absa (e ꢂ 104 cmꢀ1 molꢀ1
)
(Fc/Fc+) (Fc/Fc+) Eopt.gb/V HOMOc/eV LUMOd/eV Jsc/mA cmꢀ2 Voc/V FF PCEe (%)
1
2
3
296 (0.4), 341 (2.4), 400 (1.0), 460 (0.7) 0.47
297 (0.4), 430 (3.5), 493 (1.3) 0.44
311 (7.3), 426 (4.1), 448 (4.1), 545 (0.7) 0.57
ꢀ2.17
ꢀ2.20
ꢀ1.85
2.25
2.11
2.07
ꢀ5.27
ꢀ5.24
ꢀ5.37
ꢀ3.02
ꢀ3.13
ꢀ3.30
2.32
2.65
2.17
0.46 0.42 0.45
0.44 0.43 0.50
0.45 0.34 0.34
a
b
c
d
0.02 mM CH2Cl2 solution at 298 K. Obtained at the onset of UV absorption. HOMO = ꢀ(Eox + 4.8 eV). LUMO = HOMO + Eopt.g
.
e
Device structure: ITO/PEDOT:PSS (50 nm)/CuPc (10 nm)/Ir complex + PCBM (1 : 4) (55 nm)/C60 (10 nm)/BCP (10 nm)/Al (100 nm).
c
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Table 2 Electron-only device performance
Complexa Vturn-on/V PE/lm Wꢀ1 CE/cd Aꢀ1 Emission peaks/nm
1
2
3
8.2
6.8
13
0.10
0.34
0.002
0.45
1.20
0.008
624
636
736
a
Device structure: ITO/PEDOT:PSS (50 nm)/Ir complex with CBP
(10 wt%)/BAlq (30 nm)/LiF (1 nm)/Al (100 nm).
energies of the Ir complexes (complex 1, ꢀ5.27 eV; complex 2,
ꢀ5.24 eV; complex 3, ꢀ5.37 eV) was less than 0.1 eV.
Therefore, we assumed that the turn-on voltage (Vturn-on
)
and device efficiency were primarily influenced by the hole
mobility of the Ir complex, with a lower Vturn-on occurring in
response to relatively better hole mobility. According to the
data shown in Table 2, the complexes could be arranged in the
increasing order of the Vturn-on values as follows: 2 (Vturn-on
=
6.8 V) o 1 (Vturn-on = 8.2 V) o 3 (Vturn-on = 13 V). This order
is in good agreement with that proposed on the basis of the
photovoltaic performances of the complexes (2 (PCE =
0.50%) 4 1 (PCE = 0.43%) 4 3 (PCE = 0.34%)). These
results demonstrate that 2 had the relatively fastest hole
mobility among the synthesized complexes. Further, the electron-
only OLED fabricated using 2 was also found to have better
current efficiency (CE) (1.20 cd Aꢀ1), power efficiency (PE)
(0.34 lm Wꢀ1), and luminance (494 cd mꢀ2) than the OLED
fabricated using 1. In contrast, the lowest OLED performance
of 3 was caused by its poor hole mobility. These results
demonstrate that the triphenylamine unit in 2 plays a crucial
role in enhancing the performance of the organic solar cell by
increasing the hole mobility. Therefore, it is important to focus
on the hole-transport capability when designing dyes for
organic solar cells.
Fig. 2 I–V curves of Ir complex/PCBM blending systems (top) and
complex 2/C60 vacuum-deposition systems (bottom).
Using these devices, we investigated the relative hole mobility of
the Ir complexes. The HOMO energy levels of each Ir complex
affected the hole injection; however, the difference in the HOMO
Photovoltaic performances—evaporation method
Because the solar cell fabricated from 2 by spin coating
delivered the best performance, this dye was used to optimize
the devices fabricated by the evaporation method. The I–V
characteristics of different photovoltaic devices under AM
1.5 illumination (100 mW cmꢀ2) are shown in Fig. 2, and
the performance data are summarized in Table 3. Two types of
multilayer photovoltaic devices were fabricated. Device A is a
multilayer heterojunction device (bilayer structure). Devices B
and C are co-deposition structures that are similar to BHJCs
(see Fig. 3). Because of the reduction in the effective charge
separation between 2 and the C60 layer, the photovoltaic
performance of device A fabricated by the evaporation
method (PCE = 0.38%) was poorer than that of the cells
fabricated by spin coating (PCE = 0.50%).
To prevent a decrease in charge separation, devices B and C
were fabricated as multilayer BHJCs in which the Ir complex
Table 3 Heterojunction photovoltaic device performance
Device structure D/A ratio Jsc/mA cmꢀ2 Voc/V FF PCEa (%)
A
B
C
1 : 0
1 : 4
1 : 4
1.62
1.55
2.32
0.58
0.49
0.49
0.40 0.38
0.53 0.40
0.53 0.60
Fig. 3 Energy level diagram of an electron-only device (top) and
a
Device structure: see Fig. 3.
structure of photovoltaic devices A, B, and C (bottom).
c
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performance. The photovoltaic performance of 3, which had a
pyrene unit, was relatively poor, although the energy band gap
in 3 was narrower than that in 1. The photovoltaic perfor-
mance of the complexes (2 4 1 4 3) was dependent on their
hole-transporting capabilities. The relative hole mobility of the
complexes was determined by comparing the turn-on voltages
of the electron-only OLED devices. The turn-on voltage of the
device fabricated using 2 was lowest, demonstrating that 2 had
the relatively highest hole mobility. Therefore, there should be
adequate focus on the hole mobility of organic dyes when
designing organic solar cells.
Moreover, the photovoltaic performance of device C fabricated
by the vacuum co-deposition of 2 and C60 in a 50 nm-thick active
layer was 50% higher than that of device A (bilayer heterojunction
solar cell) and device B (fabricated by the co-deposition of 2 and
C60 with a 30 nm-thick active layer). In spite of the low EQE,
OPVs of Ir complexes generated photocurrent over the wavelength
range from 350 to 600 nm. These results indicate that the
co-deposition method of fabricating BHJCs effectively enhances
the photovoltaic performance. This study provides the first
example of the use of Ir complexes as potential donor molecules
in the fabrication of organic photovoltaic cells.
Fig. 4 The IPCE spectra with an Ir/PCBM blending layer and with-
out a blending layer (reference cell structure: ITO/PEDOT:PSS
(50 nm)/CuPc (10 nm)/C60 (10 nm)/BCP (10 nm)/Al (100 nm)).
and C60 were co-deposited in the same layer (1 : 4 ratio). The
thickness of the deposited active layer was 30 nm and 50 nm
in devices B and C, respectively. All other parameters
corresponding to devices B and C were identical to those
corresponding to device A. The lower current density of device B
than device A appeared to be because of the inefficient
optical depth. Conversely, when the co-deposition layer thick-
ness was increased to 50 nm (device C), the PCE increased to
0.60% because Jsc of this device (2.32 mA cmꢀ2) increased
considerably than those of devices A and B. The Voc and FF
values (0.49 V and 0.53, respectively) of device C were identical
to those of device B. These results showed that by using the
co-deposition method (device C), the charge separation ratio
and absorption efficiency could be increased.
Acknowledgements
This work was supported by the Basic Science Research
Program through a National Research Foundation of Korea
(NRF) grant funded by the Ministry of Education, Science
and Technology (MEST) of Korea for the Center for Next
Generation Dye-sensitized Solar Cells (No. 2011-0001055).
We also thank the BK21 for fellowship grants to W. L.
The EQE spectra—spin coating method
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