Iridium-based emitters containing pendant triphenylene moieties for bluish-green OLEDs with improved efficiency upon thermal annealing

Article abstract of DOI:10.1039/c6nj03692a

Two new cyclometalated iridium complexes with non-conjugated triphenylene moieties attached to the ancillary ligand, namely YF3 and YF4, are reported. Both iridium complexes presented intense bluish-green emission both in solution (Φ_PL ～ 0.37-0

Full text of DOI:10.1039/c6nj03692a

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Iridiumꢀbased Emitters Containing Pendant Triphenylene
Moieties for Blue Green OLEDs with Improved Efficiency upon
Thermal Annealing
Yafei Wang†‡*, Mengbing Zhu‡, Jiyong Deng△, Guohua Xie§*, Etienne Baranoff†*
†School of Chemistry, University of Birmingham, Edgebaston, Birmingham, B15 2TT, UK
‡School of Materials Science & Engineering, Changzhou University, Changzhou 213164, P. R.
China
§Department of Chemistry, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials,
Wuhan University, Wuhan 430072, P. R. China
△
School of Chemistry and Chemical Engineering, Hunan Institute of Engineering, Xiangtan
411104, P.R. China
Corresponding author
Yafei Wang: qiji830404@hotmail.com
Guohua Xie: xghꢀ008@163.com
Etienne Baranoff: e.baranoff@bham.ac.uk
Abstract
Two new cyclometalated iridium complexes with nonꢀconjugated triphenylene
1

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moieties attached to the ancillary ligand, namely YF3 and YF4, are reported. Both
iridium complexes presented intense blue green emission both in solution (ФPL～
0.37ꢀ0.5) and in solid state. High hole mobility of 0.001 cm2/Vꢀs was obtained for
YF4 annealed film, evidenced by the spaceꢀchargeꢀlimitedꢀcurrent method. This result
is among the highest hole mobility for a cyclometalated iridium complex. Organic
lightꢀemitting diodes employing these iridium complexes as the dopant were
fabricated. The influence of the annealing temperature of both the emitting layer and
the host matrix on the devices performance was explored. Interestingly, YF4ꢀbased
o
device with the annealing temperature of 60 C shows the best performance with a
maximum current efficiency of 15.6 cd/A and external quantum efficiency of 6.8%
compared to 1.5 cd/A and 0.8% for the nonꢀannealed device.
Introduction
Phosphorescent organic lightꢀemitting diodes (OLEDs) have been the focus of
2

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intense research for the nextꢀgeneration display due to their high efficiency, low cost
and potential largeꢀarea.1ꢀ5 Over the past two decades, cyclometalated iridium
complexes have been widely used as the emitter in electroluminescent devices due to
their relatively short lifetime of triplet excited state, high emission efficiency and
tunable emission color.6ꢀ9 To date, various strategies for the design of highꢀefficiency
iridium complexes have been demonstrated. One particular approach is to
functionalize the complex with conjugated charge transfer unit such as fluorene,
carbazole or triphenylamine, which can achieve good charge transporting and charge
recombination properties in devices.10ꢀ14 For example, triarylborane15ꢀ19 and
triphenylamine11,20 have been widely incorporated into iridium complex owing to
their strong electronꢀaccepting and electronꢀdonating properties. Yet, the charge
mobility displayed by these phosphorescent materials is usually still low in the region
of 10–5 cm2/Vꢀs. In addition, the direct functionalization of the cyclometalated ligand
results in large changes in optoelectronic properties compared to the
nonꢀfunctionalized parent complexes.
An alternative design to limit the impact of the charge transporting groups on the
optoelectronic properties of the complexes is to functionalize the ancillary ligand,21,22
especially when nonꢀconjugated linkages are used.23,24 For example, Wang and
coworkers synthesized a series of iridium complexes where the ancillary ligand
(acetylacetone derivative) bears carbazoleꢀ/oxadiazole units.23 The nonꢀdoped OLEDs
utilizing these iridium complexes as the emitter exhibited a maximum luminous
efficiency of 25.5 cd/A. Recently, we introduced a biphenyl mesogenic unit onto an
3

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acetylacetonate ancillary ligand through a flexible spacer.25 The resulting iridium
complexes possessed the same optical property as their parent complex while they
showed a smectic liquid crystalline phase induced by the peripheral biphenyl
derivative. Furthermore, these iridium complexes exhibited high holeꢀmobility up to
0.004 cm2/Vꢀs, which is the highest hole mobility reported for iridium complexes.25
These results indicate that modifying the ancillary ligand is an attractive strategy to
tune the properties of the complexes beyond the sole photophysical properties.
Triphenylene is a polycyclic aromatic hydrocarbon widely used in the field of
discotic liquid crystals because of its appealing charge transporting characteristics.26,27
It was therefore conceived that the introduction of triphenylene unit on
cyclometalated iridium complexes would result in interesting optical properties with
high charge mobility. Although some researches on OLEDs with triphenylene
derivatives have been reported,28ꢀ30 integrating a triphenylene moiety onto the
ancillary ligand of iridium complexes has not yet been explored. Herein, we report the
synthesis and characterization of two novel iridium complexes bearing an
acetylacetone (acac) ligand monosubstitutedꢀ and bisubstituted with mesogenic
triphenylene units, abbreviated YF3 and YF4. The photophysical properties and
electroluminescent properties were systematically investigated. As expected, YF4
annealed film showed a high hole mobility up to 0.001 cm2/Vs, which is among the
highest hole mobility reported for an iridium complex. Interestingly, when used as an
emitter in an electroluminescent device, YF4 exhibited much improved performance
with a maximum current efficiency of 15.6 cd/A and a maximum external quantum
4

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efficiency of 6.8% after thermal annealing of the emissive layer.
Reaction conditions: a) 1ꢀbromooctane, K2CO3, KI, acetone, reflux, overnight; b) FeCl3, H2SO4, RT, 3 h;
c)1,6ꢀdibromohexane, K2CO3, KI, acetone, reflux, overnight; d) acetylacetone, NaH, nBuLi, THF, 0oC to RT,
overnight; e) LG1 or LG2, TABOH, CH2Cl2, MeOH, reflux, overnight.
Scheme 1 Synthetic route for YF3 and YF4
Experimental
General information
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Pyrocatechol and all reagents were purchased from Aldrich and VWR and used as
received. All reactions and manipulations were carried out under Ar atmosphere with
1
the use of standard inert atmosphere techniques. H NMR spectra were measured in
CDCl3 solution on a Bruker DPX (300 MHz and 400 MHz) NMR spectrometer with
tetramethylsilane (TMS) as the internal standard. Mass spectra (MS) were recorded on
a Bruker Autoflex TOF/TOF (MALDIꢀTOF) instrument using dithranol as a matrix.
Thermogravimetric analysis (TGA) was carried out with a NETZSCH STA449 from
25 °C to 600°C at a 20 °C/min heating rate under N2 atmosphere. Differential
scanning calorimetry (DSC) was measured at the phase transition temperature with a
rate of 10 °C/min on the first heating circle and 10 °C/min on the first cooling process.
UVꢀvis absorption spectroscopy was measured by Shimadzu UVꢀ265 spectrometer at
room temperature. Emission spectra were recorded on RFꢀ5301 PC (Perkin Elmer) at
room temperature in dichloromethane (solutions 10–5 M) degassed by Ar.
Timeꢀresolved fluorescence lifetime measurements were carried out by using a
timeꢀcorrelated single photon counting lifetime spectroscopy system, Hamamatsu
QuantaurusꢀTau C11567ꢀ01. The quality of the fit has been judged by the fitting
parameters such as χ2 (<1.2) as well as the visual inspection of the residuals. Cyclic
voltammetry were measured by an electrochemical analyzer (CHI660) in degassed
CH2Cl2 solution with (Bu4N)PF6 as electrolyte. A platinum disk was used as the
working electrode. The platinum wire was served as the counter electrode and Ag/Ag+
was used as the reference electrode. Ferrocenium/ferrocene was considered as the
internal reference compound.
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Hole mobility
The hole mobilities of both iridium complexes were estimated using the
spaceꢀcharge limited current (SCLC) method. Hole mobility was measured using the
space charge limited current method with a diode configuration of ITO/PEDOT:PSS
(40 nm)/iridium complex (50 nm)/MoO3 (10 nm)/Ag (100 nm) for hole mobility and
the results were fitted by space charge limited form. The rotate speed for PEDOT:PSS
and activity layer are 4000 rmp/min and 600 rmp/min, respectively. All devices were
o
measured at room temperature and an annealed temperature (150 C for 30 min). The
SCLC is described by:
film thickness of the active layer, µ0 is the hole mobility, εr is the relative dielectric
constant of the transport medium, ε0 is the permittivity of free space (8.85 × 10–12
m–1), is the internal voltage in the device.25
Devices fabrication and characterization
J = (9/8)ε0εrꢀ( / J is the current density, d is the
V2 d3), where
F
V
All devices employing YF3 or YF4 as the dopant were fabricated by
solutionꢀprocessed approach. In these OLEDs, ITO was treated with the same
procedures of our previous reports.31 PEDOT:PSS was spin coated onto ITO substrate
with a 30 nm thick layer, which is utilized as the smooth layer and hole inject layer.
The blend of mCP and OXDꢀ7 (or the neat mCP) was used as the host matrix.
TmPyPB is the electron transporting layer with 40 nm in thickness. Liq and Al are
used as the composite cathode. In order to explore the effect of temperature on the
device performance, the activity emitter was treated with annealing at 60 oC and 80 oC.
EL spectra were recorded using a spectroradiometer, PhotoꢀResearch PR735. The
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features of current and brightness versus applied voltage were obtained by combining
a Keithley 2400 and the spectroradiometer. The external quantum efficiency (EQE)
was calculated from the luminance, current density, and EL spectrum, assuming a
Lambertian distribution. The device configurations are as follows:
Device A: ITO/PEDOT:PSS (30 nm)/mCP:OXDꢀ7:YF3 or YF4 (70:20:10, RT, 40
nm)/TmPyPB (40 nm)/Liq(2 nm)/Al (100 nm)
Device B: ITO/PEDOT:PSS (30 nm)/mCP:OXDꢀ7: YF3 or YF4 (70:20:10, 80oC, 40
nm)/TmPyPB (40 nm)/Liq(2 nm)/Al (100 nm)
Device C: ITO/PEDOT:PSS (30 nm)/mCP:YF3 or YF4 (90:10, RT, 40 nm)/TmPyPB
(40 nm)/Liq(2 nm)/Al (100 nm)
Device D: ITO/PEDOT:PSS (30 nm)/mCP: YF3 or YF4 (90:10, 60oC, 40
nm)/TmPyPB (40 nm)/Liq(2 nm)/Al (100 nm)
Synthesis of 3,6,7,10,11ꢀpentakis(octyloxy)triphenylenꢀ2ꢀol (2)
Compound 1 (12.0 g, 36 mmol) and 3ꢀ4 drops H2SO4 was added to dry CH2Cl2.
Then, FeCl3 (17.4 g, 108 mmol) was added and the reaction mixture was stirred at
room temperature for 3.5 h. The reaction mixture was poured into MeOH and the
mixture left in freezer overnight. The solid was collected by filtrate and purified with
column chromatography using hexane/ethyl acetate = 35:1 (V/V) as the eluent. A grey
white solid (3.0 g, 28.6 %) was achieved. 1H NMR (CDCl3 300 MHz): δ 7.96 (s, 1H),
7.82 (d, J = 4.8 Hz, 4H), 7.76 (s, 1H), 5.92 (s, 1H), 4.28ꢀ4.18 (m, 10H), 1.97ꢀ1.92 (m,
10H), 1.58ꢀ1.32 (m, 52H), 0.93ꢀ0.89 (m, 15H). MS (MALDIꢀTOF) m/z (M•Na+):
calcd. for C58H92O6, 907.7; found, 907.7.
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Synthesis of 2ꢀ(6ꢀbromohexyloxy)ꢀ3,6,7,10,11ꢀpentakis(octyloxy)triphenylene (3)
A mixture of compound 2 (3.0 g, 3.39 mol), 1,6ꢀdibromohexane (3.0 g, 12.3 mol)
and K2CO3 (2.4 g, 17.0 mol) was dissolved in acetone (300 mL). The mixture was
refluxed for 48 h under Ar. After cooling to room temperature, the reaction mixture
was poured into water and extracted with CH2Cl2. The organic layer was collected,
washed with water and dried with MgSO4. The solvent was removed under vacuum
and the residual will be purified with column chromatography using hexane/ethyl
1
acetate = 35:1 (V/V) as the eluent. A light yellow solid was obtained (2.6 g, 73%). H
NMR (CDCl3 300 MHz): δ 7.83 (s, 6H), 4.23 (d,
J
= 6.3 Hz, 12H), 3.45 (t,
J
= 6.6 Hz,
= 6.6 Hz,
2H), 2.0ꢀ1.89 (m, 12H), 1.59ꢀ1.56 (m, 12H), 1.39ꢀ1.28 (m, 44H), 0.90 (t,
J
15H). 13C NMR (100 MHz, CDCl3): δ 148.98, 123.56, 107.37, 69.72, 33.74, 32.73,
31.85, 29.48, 27.99, 26.20, 25.40, 22.69, 14.11.
General procedures for synthesis of ancillary ligands LG1 and LG2
Acetylacetone (1 eq) was added carefully to the stirred suspension of the sodium
hydride (1.2 eq) in THF (5 ml) at 0°C. The resultant mixture was stirred for 30 min. at
0°C. Butyllithium (2.2 eq) was added dropwise over 30 min. to the stirred mixture at
0°C. After 30min., a solution of compound 3 (2.2 eq) in THF (5 ml) was added
dropwise to the solution at 0°C. The reaction mixture was stirred for 2 hours at 0°C
and for overnight at RT, after which it was quenched with aqueous ammonium
chloride. The mixture was acidified with concentrated hydrochloric acid to pH = 1
and the aqueous phase was separated and extracted with CH2Cl2. The combined
organic layers were washed with brine and dried with anhydrous Na2SO4, and the
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solvent was evaporated in vacuo. The crude product was simply purified by column
chromatography on silica (CH2Cl2ꢀMethanol) to give the mixture of LG1 and LG2 as
a yellow solid (0.1 g, 12.2 %). It is failed to separate them and the mixture was used
to next step.
General procedures for iridium complexes YF3 and YF4
A mixture of cyclometalated ligand (dfppy) (2.5 eq) and IrCl3•3H2O (1 eq) in the
o
2ꢀethoxylethanol (15 mL) and water (5 mL) was heated to 100 C for 16 h. After
cooling; the precipitate was collected and washed with water and hexane. The solid
was used to the next step without any further purification. A mixture of LGꢀ1 (or
LGꢀ2) (2.2 eq), TBAOH (2.2 eq) and CH2Cl2/MeOH (V/V = 7:3) was added into the
solution of the dimer (1 eq) and CH2Cl2. The reaction mixture was stirred at 40 oC for
overnight. After cooling to room temperature, the solution was removed by evaporator
and the residue was purified by silica gel chromatography (CH2Cl2) to get the target
iridium complexes.
YF3. Yellow glassy material, yield: 35.1 %. 1H NMR (300 MHz, CDCl3): δ 8.44ꢀ8.41
(m, 2H), 8.25ꢀ8.21 (m, 2H), 7.85 (s, 6H), 7.80ꢀ7.72 (m, 2H), 7.20ꢀ7.12 (m, 2H),
6.37ꢀ6.29 (m, 2H), 5.72 (dd,
1H), 5.65 (dd, = 8.7 Hz, 2.4 Hz, 1H), 5.24 (s, 1H), 4.26ꢀ4.19 (m, 12H), 2.03ꢀ1.89 (m,
12H), 1.82 (s, 3H), 1.61ꢀ1.54 (m, 12H), 1.42ꢀ1.25 (m, 48H), 0.89 (t, = 3 Hz, 15H).
J = 8.7 Hz, 2.4 Hz, 1H), 5.72 (dd, J = 8.7 Hz, 2.4 Hz,
J
J
13C NMR (100 MHz, CDCl3): δ 188.53, 185.09, 151.70, 149.05, 148.15, 137.90,
128.61, 123.60, 122.51, 121.52, 115.21, 107.41, 100.37, 97.24, 69.76, 41.46, 31.87,
29.51, 28.84, 27.01, 26.22, 22.71, 14.13. MS (MALDIꢀTOF) m/z (M−):
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C91H121F4IrN2O8, 1638.87; found, 1662.0 (Na+).
YF4. Yellow glassy material, yield: 22.3 %. 1H NMR (300 MHz, CDCl3): δ 8.41 (dd,
= 5.7 Hz, 0.9 Hz, 2H), 8.22 (d, = 8.4 Hz, 2H), 7.83 (s, 12H), 7.77ꢀ7.71 (m, 2H),
7.16ꢀ7.11 (m, 2H), 6.36ꢀ6.28 (m, 2H), 5.71 (dd, = 8.7 Hz, 2.4Hz, 2H), 5.22 (s, 1H),
4.25ꢀ4.19 (m, 24H), 2.06ꢀ1.81 (m, 20H), 1.61ꢀ1.52 (m, 24H), 1.39ꢀ1.26 (m, 100H),
0.88 (t,
= 4.2 Hz, 30H). 13C NMR (100 MHz, CDCl3): δ 188.87, 152.40, 149.32,
J
J
J
J
148.40, 138.14, 123.89, 122.77, 121.72, 115.71, 107.70, 100.24, 96.14, 70.051, 41.84,
32.16, 29.80, 27.38, 26.51, 23.0, 14.42. MS (MALDIꢀTOF) m/z (M−):
C155H223F4IrN2O14, 2605.64; found, 2606.17.
Results and discussion
Synthesis and structure
The synthetic routes for the ancillary ligands and the iridium complexes are shown
in Scheme 1. Compound 1 was prepared from pyrocatechol according to a previous
report.32 For intermediate 2, a multiꢀstep synthetic route is usually carried out,33 which
is costly and timeꢀconsuming. Herein, an improved one step reaction of intermediate 1
using H2SO4 and FeCl3 in CH2Cl2 afforded 2 in 28 % yield.34 The precursor 3 was
obtained via etherification between intermediate 2 and 1,6ꢀdibromohexane in good
yield.
The ancillary ligands were prepared via oneꢀpot alkylation between acetylacetone
and the precursor 3 using NaH and n
ꢀBuLi in dry THF with low yield (10ꢀ23%).23,25
Both monoꢀ and dialkylated compounds were observed with the monoalkylated ligand
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(LG1) being the major species compared to the bialkylated ligand (LG2), which is
attributed to increased steric effect of the pendent unit limiting the dialkylation.
However, they could not be separated and were used directly as a mixture in the
complexation reaction. Then, the cyclometalated iridium complexes were prepared
from diꢀꢁꢀchlorodiiridiumꢀ(III) dimer [(C^N)4Cl2Ir] and ancillary ligands (LG1 and
LG2), in which the (C^N)4Cl2Ir was synthesized according to previous reports.22
Finally, the target iridium complexes were purified and characterized by 1H NMR, 13C
NMR and TOFꢀMS (ESI†). Both iridium complexes have good solubility in organic
solvent, such as CH2Cl2 and hexane.
Fig. 1 1H NMR (400 MHz, CDCl3, room temperature) spectra of YF3 (A) and YF4 (B)
YF3 and YF4 were successfully separated via thinꢀlayer chromatography, and can
be clearly distinguished by 1H NMR spectroscopy. In YF4, the protons on
triphenylene are 12H (aʹ), while the protons on triphenylene unit are 6H (a) in YF3
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(Fig. 1). Additionally, a singlet resonance with 3H at 1.82 ppm is observed in YF3,
assigning to the methyl on acac moiety. Furthermore, a series of multiplets in the
range of 8.22ꢀ8.44 ppm and two different doubletꢀdoublet signals in 5.64ꢀ5.74 ppm
are presented in YF3, indicating that the asymmetric structure leads to different
chemical environments of protons on the cyclometalating ligand. In additional, 13C
NMR and TOFꢀMS also proved YF3 and YF4 were separated well (ESI†). Both
iridium complexes present good thermal properties measured by TGA (ESI†). The
o
o
decomposed temperatures of 5wt% are 332 C and 343 C for YFꢀ3 and YFꢀ4,
respectively. However, YFꢀ3 and YFꢀ4 did not display visible phase transition
o
between 50 C to 250 oC, evidenced by DSC (ESI†), pointing to glassy materials with
low transition temperatures.
Photophysical property
Fig. 2. UVꢀvis absorption spectra of YF3 and YF4 in CH2Cl2 solution at room temperature
Table 1 photophysical data of iridium complexes
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aAbsorption /nm
(ε/105 Mꢀ1cmꢀ1)
dΦPL eτ /ꢁs fkr /105 s–1 gknr/105 s–1
λ
em /nm
YF3 260 (0.97), 268 (1.19), 278
(1.56), 307 (0.46) 344 (0.15),
389 (0.05), 435 (0.03)
b 482
0.50
0.37
1.07
1.06
4.67
3.49
4.68
5.94
c 483,509
YF4 260 (1.40), 268 (1.75), 278
(2.38), 307 (0.66), 345 (0.16),
388 (0.05),437 (0.03)
b 482
c 483, 509
a,b,d,eThe data was collected in CH2Cl2 at room temperature under Ar. Emission was measured in neat film at
c
room temperature. fRadiative decay rate Kr PL/τ. gNonꢀradiative decay rate Knr =τꢀ1– Kr
=
Φ
.
The UVꢀvis absorption spectra measured in CH2Cl2 solution (10–5 M) at room
temperature are depicted in Fig. 2, and the corresponding data are summarized in
Table 1. Intense absorption peaks centered at 279 nm with high molar extinction
coefficient (ε~105 M–1 cm–1) are observed for YF3 and YF4, generated from the S0ꢀS4
transition of the triphenylene.35 The weaker absorption bands (~103 M–1 cm–1) from
320 nm to ~470 nm are attributed to the mixture of singlet and triplet metalꢀtoꢀligand
3
3
charge transfer and ligand centered transitions (1MLCT, MLCT, LC) involving the
cyclometalating ligand and the iridium center.36 Due to the two triphenylene units,
YF4 shows clearly increased absorption intensity in the range of 250ꢀ320 nm
compared to YF3.
Under the excitation wavelength of 400 nm, the iridium complexes present similar
emission spectra both in solution and neat film (Fig. 2). All emission profiles show
intense bluishꢀgreen emission with the maximum emission peak at about 482 nm and
a shoulder at 509 nm. Employing FIrpic as the reference compound,37 YF3 and YF4
present the high PL quantum yields (Фem) of 0.5 and 0.37 in degassed CH2Cl2,
respectively. The excited state lifetimes of 1.07 ꢁs and 1.06 ꢁs were measured for YF3
and YF4 at room temperature (Fig. 2 insert, Table 1). Consequently both the radiative
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and nonꢀradiative rate constants are calculated to be in the 105 s–1 range, similar to the
corresponding acetylacetonate complexes. Additionally, both iridium complexes are
luminescent in the solid state (ESI†). It is particularly interesting for YF3 and YF4 as
blue emitters generally suffer significantly from redꢀshifted emission when in the bulk.
Additionally, the PL property of pristine film and annealing film were preliminary
explored. Taking YF3 as an example (ESI
†
), the emission intensity of annealing film
2 nm red shift
is obviously weak than that of the pristine film, accompanied with
～
for the annealing film. Correspondingly, the emission efficiency of the pristine film is
better than that of the annealing film.
Electrochemical property
Table 2 Electrochemical data of iridium complexes
aEred/V
–1.31
bEoptgap/eV
2.77
cELUMO/eV
dEHOMO/eV
–6.26
YF3
YF4
–3.49
–1.34
2.79
–3.46
b
–6.25
athe data were recorded by CV in CH2Cl2 solution; the data were recorded at the crossing point of
UVꢀvis spectra and emission spectra; cELUMO = – (4.8 +
E
red) eV; dEHOMO = (Eopt
– ELUMO) eV.
gap
The electrochemical property of both YF3 and YF4 were evaluated by cyclic
voltammetry in CH2Cl2 solution (ESI†), and the relevant data are listed in Table 2.
The irreversible reduction waves rather than oxidation waves were detected for YF3
(V
onset: –1.31 V; vs. Fc/Fc+; the same below) and YF4 (–1.34 V). According to the
reduction potential, the lowest unoccupied molecular orbitals levels (ELUMO) of YF3
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and YF4 are calculated to be –3.49 eV and –3.46 eV, respectively, via the empirical
formula of ELUMO = – (4.8 + Ered) eV.38 Based on the optical bandꢀgap (Table 2) and
the value of ELUMO, the highest occupied molecular orbital levels (EHOMO) were
evaluated to be –6.26 eV and –6.25 eV. Notably, negligible effect on the
electrochemical property of both iridium complexes was found due to the
nonꢀconjugation of triphenylene unit in ancillary ligand.
Hole mobilities
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Fig. 3 The curves of current densitiesꢀvoltage of holeꢀonly devices based on the annealed films of
YF3 and YF4. The solid lines represent the fit using a model of single carrier SCLC with
fieldꢀindependent mobility.
As high charge mobility is expected from triphenylene derivatives, we evaluated
the hole mobilities of these iridium complexes via SCLC method. Both iridium
complexes show the hole mobilities of 2.5×10–6 (YF3) and 7.7×10–4 (YF4) cm2/Vꢀs in
the pristine film. After thermal annealing, greatly enhanced hole mobilities were
obtained for both iridium complexes. Interestingly, YF4 annealed film displays the
hole mobility up to 0.001 cm2/Vꢀs (Fig. 3), which is among the highest report for the
hole mobilities of iridium complex.
Electroluminescent properties
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Fig. 4 EL spectra of YF3 and YF4 collected at 10 V
To evaluate the electroluminescent properties of both iridium complexes, the
OLEDs with different architectures (see experimental part) have been fabricated, in
which YF3/or YF4 were used as the dopant. Both the blend of mCP and OXDꢀ7 and
neat mCP are chosen as the host matrix. Due to the enhanced hole mobility in the
annealed film, the effect of annealed emitter layer on device performance was also
explored.
All devices based on YF3 dopant show almost identical EL spectra with the
maximum emission at 483 nm (Fig. 4), matching perfectly with the PL emission in
solution. This phenomenon implies that the EL emission is dominant from the iridium
complex. Additionally, no emission from the host matrix was detected, even for the
annealed emissive layer, implying efficient energy transfer from the host to the dopant.
YF4ꢀbased devices display similar EL spectra to YF3ꢀbased devices, except for some
highꢀenergy emission observed in devices B and D (Fig. 4, annealed emissive layer).
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Importantly, the thermal annealing process has a distinct favorable effect on the EL
spectra of YF4, probably due to the presence of two triphenylenes per complex.
Fig. 5 JꢁVꢁB curves of iridium complexesꢀbased devices AꢀD (■□: device A; ●○: device B; ▲△:
device C; ▼▽: device D)
The current densityꢀvoltageꢀbrightness (JꢁVꢁB) characteristics of the devices are
depicted in Fig. 5. YF4ꢀbased devices possess a lower turn on voltage (Von, ca. 5 V)
than that of YF3ꢀbased devices (ca. 6 V), which is attributed to the higher hole
mobility of YF4. It appears that mCP:OXDꢀ7 is a better host matrix than mCP in these
devices (Fig. 5 and Fig. 6). For the EL devices of YF3, both the annealed emitters
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exhibit much higher luminance than that of pristine film. The maximum brightness of
11380 cd/m2 (12 V, 169 mA/cm2) was achieved in device B. In contrast, YF4 based
devices show the highest luminance of 7237 cd/m2 (9 V, 149 mA/cm2) in the device D,
o
of which the emissive layer was annealed at lower temperature (60 C). This result
implies that the temperature plays a key role on the performance of YF4ꢀbased
devices.
Table 3. The performances of devices AꢀD of YF3 and YF4
Type
Maximum efficiency
L (cd/m2) CE (cd/A) PE (lm/W)
Von (V)
EQE (%)
3.5
YF3
YF4
device A
device B
device C
device D
device A
device B
device C
device D
6
6
5333
11380
3694
10900
4114
1280
2052
7237
8.6
9.2
2.9
3.0
5.9
2.5
3.1
0.6
3.2
7.5
3.8
6.5
6
16.1
8.1
6.6
3.4
5.5
5.5
6
8.3
3.5
1.5
0.8
8.1
3.4
5
15.6
6.8
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Fig. 6 Current efficiencyꢀcurrent density characteristics of YF3 and YF4 based devices
For the pristine activated emitter, devices of YF3 presented a best performance of
ηCE,
= 16.1 cd/A, ηPE,
= 5.9 lm/W and ηexe,
= 6.6 % (device C), whereas
max
max
max
devices of YF4 obtained the best EL performance of ηCE, max = 8.3 cd/A, ηPE, max = 3.1
lm/W, ηexe, = 3.5 % (device A, Table 3). Notably, YF3ꢀbased devices with a
max
pristine film have better performance than YF4ꢀbased devices, which is in agreement
with their ΦPL in solution. Inspired by the enhanced mobility in annealed film, we
o
o
treated the emitter layer with thermal annealing (60 C and 80 C). It is worth noting
that the devices showed an increased performance after annealing except device D of
YF3. Especially, device D with YFꢀ4 dopant achieved a sharply increased
performance of ηCE, max = 15.6 cd/A, ηPE, max = 7.5 lm/W, ηexe, max = 6.8 % (device D).
This exciting result could be ascribed to the enhanced charge transport in the annealed
film, leading to better device performance. Therefore, this result would be in favor of
designing the novel iridium complex for enhancing the device performance with
annealed film.
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Conclusions
Two novel iridium complexes bearing nonꢀconjugated triphenylene units on the
ancillary ligand have been successfully synthesized and characterized. Both iridium
complexes showed intense blue green emission in solution and solid state. Their
optical and electrochemical properties were similar to the parent complex
(dfppy)2Iracac due to the nonꢀconjugated linkage between the iridium emission core
and the triphenylene units. Interestingly, YF4 annealed film exhibited high hole
mobility up to 0.001 cm2/Vs, which is among the highest hole mobility reported for a
cyclometalated iridium complex. After thermal annealing of the emissive layer, the
YF4ꢀbased device displayed largely enhanced performance with the maximum current
efficiency of 15.6 cd/A and external quantum efficiency of 6.8%. This research
demonstrated that introducing a proper functionalized unit on the ancillary ligand of
iridium complex is an attractive strategy to improve the performance of
electroluminescent devices.
Acknowledgments
We thank the European Union (MCꢀIIFꢀ329199), the financial support from the
Natural Science Foundation of China (21202139, 51473140, 61575146 and
51273168), the Program for Innovative Research Cultivation Team in University of
Ministry of Education of China (1337304), The Petrochemical Joint Fund Between
National Natural Science Foundation & China Petroleum and Chemical Corporation
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(U1663229), and Natural Science Foundation of Hunan (2015JJ2042)
..
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Table of Content
High hole mobility was obtained for iridium complex bearing triphenylene unit, and
the annealing emitter shows the best device performance.