An electron transporting unit linked multifunctional Ir(iii) complex: A promising strategy to improve the performance of solution-processed phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1039/c3cc49796h

An oxadiazole based electron transporting (ET) unit was glued to the heteroleptic Ir(iii) complex (TPQIr-ET) and used as a dopant for phosphorescent organic light-emitting diodes (PhOLEDs). It shows superior device performance than the dopant without the

Full text of DOI:10.1039/c3cc49796h

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An electron transporting unit linked multifunctional
Ir(III) complex: a promising strategy to improve the
performance of solution-processed phosphorescent
organic light-emitting diodes†
Cite this: Chem. Commun., 2014,
50, 4000
Received 27th December 2013,
Accepted 25th February 2014
Thota Giridhar,a Chinnusamy Saravanan,a Woosum Cho,a Young Geun Park,b
DOI: 10.1039/c3cc49796h
Jin Yong Lee*b and Sung-Ho Jin*a
www.rsc.org/chemcomm
An oxadiazole based electron transporting (ET) unit was glued to or electron mobility.11 Alternatively, the mixed and/or bipolar host
the heteroleptic Ir(III) complex (TPQIr-ET) and used as a dopant for materials can provide a more balanced injection and transport of both
phosphorescent organic light-emitting diodes (PhOLEDs). It shows the charge carriers due to the balance between the charges.8,9 Thus, we
superior device performance than the dopant without the ET unit glued the oxadiazole based ET unit to the ancillary ligand of hetero-
(TPQIr) due to the balanced charge carrier injection by the ET unit. leptic Ir(III) complex via an ether linkage to analyze its effect on the
device performance without changing the luminance maximum. Here,
The development of high performance solution-processed phos- the ET unit may further improve the balance between the charges.
phorescent organic light-emitting diodes (PhOLEDs) has attracted
For this purpose, a thiophene–phenylquinoline (TPQ) based main
significant research interest owing to their foreseeable impact in the ligand and picolinic acid with the oxadiazole based ET unit as an
field of inexpensive large area and flexible display devices.1 PhOLEDs ancillary ligand for the heteroleptic Ir(III) complex, TPQIr-ET, were
using Ir(III) complexes as emitters have drawn more attention due to successfully designed and synthesized; for comparison, a similar Ir(III)
their potential of 100% internal quantum efficiency via harvesting complex without the ET group, TPQIr, was also synthesized. The
of both singlet and triplet excitons.2 Among the phosphorescent structures of TPQIr-ET and TPQIr are shown in Scheme 1. The
emitters, homoleptic and heteroleptic cyclometalated Ir(III) complexes solution-processed deep-red PhOLEDs were fabricated using
are still the most promising phosphors due to their relatively short these Ir(III) complexes as dopants and tris(4-carbazoyl-9-ylphenyl)-
triplet lifetimes, high quantum yields, and emission wavelength amine (TCTA)/1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl
tunability from blue to deep-red.3–5 Particularly, the color of the (TPBi) or TCTA/1,3-bis(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl)-
emitters can be tuned by the introduction of electron-donating or benzene (OXD-7) as mixed-host materials with the configuration
electron-accepting units and increasing or decreasing the extended of ITO/PEDOT:PSS/TCTA:TPBi (1 : 1) or TCTA : OXD-7 (1 : 1):dopant/
p-conjugation in the ligands. In contrast, the performance of the TmPyPB/LiF/Al. The poly(3,4-ethylenedioxythiophene):poly(styrene-
particular emitter depends mainly on its interaction with the host sulfonate) (PEDOT:PSS) and 3,30-[50-[3-(3-pyridinyl)phenyl]-
materials, which are holes6 or electrons7 or a mixture of hole and [1,10:30,100-terphenyl]-3,300-diyl]bispyridine (TmPyPB) were used
electron transporting8 or bipolar materials.9 Generally, the similar as the hole transporting layer and the electron transporting
structural features of the ligands of emitter and host materials facili- layer, respectively. The devices based on TPQIr-ET show an EQE
tate charge injection and transporting ability of the resultant of 20.59%, which is the highest value reported to date for red
PhOLEDs. For example, Ir(III) complexes with carbazole based ligands PhOLEDs prepared using a solution-process (Table S2, ESI†).
as the dopant and CBP [4,40-di(carbazole-9-yl)biphenyl] or PVK [poly- Additionally, we found that the TPQIr-ET shows 25% higher
(N-vinylcarbazole)] as host materials enhance the hole injection and external quantum efficiency (EQE) than the TPQIr due to the
transport ability of the PhOLEDs.10 Particularly, upon using either balanced charge carrier injection by the ET group.
hole transporting or electron transporting (ET) single host materials,
there has been a charge imbalance due to the existence of higher hole
a Department of Chemistry Education, Graduate Department of Chemical Materials,
and Institute for Plastic Information and Energy Materials, Pusan National University,
Busan, 609-735, Republic of Korea. E-mail: shjin@pusan.ac.kr
b Department of Chemistry, Sungkyunkwan University, Suwon, 440-746,
Republic of Korea. E-mail: jinylee@skku.edu
† Electronic supplementary information (ESI) available: Experimental details,
thermal stabilities and additional figures. See DOI: 10.1039/c3cc49796h
Scheme 1 Structure of TPQIr-ET and TPQIr.
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Both TPQIr-ET and TPQIr were synthesized in quantitative Finally, from both optical and electrochemical properties, it was
1
yields (Scheme S1, ESI†) and confirmed their structures by H, found that the introduction of the oxadiazole based ET group
13C NMR, elemental analysis, and high resolution mass spectro- into TPQIr-ET does not have any significant influence on the
scopy (details are in the ESI†). They possess excellent solubility in energy levels of Ir(III) complexes. Density functional theory
common organic solvents, making them a suitable candidate for calculations were performed to gain an insight into the photo-
solution-processed PhOLEDs. Thermal gravimetric analysis under physical properties of both Ir(III) complexes. Becke’s three para-
an N2 atmosphere (Fig. S1, ESI†) reveals the onset decomposition meter Lee–Yang–Parr exchange functional (B3LYP) and a suite of
temperatures to be 349 and 343 1C for TPQIr-ET and TPQIr, Gaussian 09 programs were employed.12 Except for treating the
respectively, indicating their high thermal stability. Differential Hay–Wadt effective core potential of a double zeta basis set
scanning calorimetry analysis (Fig. S2, ESI†) reveals the glass (LANL2DZ) on Ir(III) metal, split valence 6-311+G** basis sets
transition temperatures (Tg) to be 202 and 213 1C for TPQIr-ET were used. The HOMOs are largely distributed over the Ir(III)
and TPQIr, respectively, indicating their amorphous nature.
metal and over one of the TPQ based main ligands while LUMOs
The UV-visible absorption and photoluminescence (PL) spectra are predominant on the ancillary ligand (Fig. S4, ESI†). These
of TPQIr-ET and TPQIr in methylene chloride (MC) solution at results show that the MLCT is likely to contribute to the transi-
room temperature are shown in Fig. 1a. Both TPQIr-ET and TPQIr tion properties. The calculated HOMO–LUMO energy levels of
show two absorption bands: the broad bands between 550 and TPQIr-ET and TPQIr were 3.15 and 3.16 eV, which are almost
450 nm corresponding to the admixture of singlet and triplet similar to the experimentally determined energy levels of 3.09
3
metal to ligand charge transfer (1MLCT, MLCT) transitions and and 3.12 eV, respectively. Particularly, the ET group in TPQIr-ET
the bands between 470 and 330 nm with a peak at 350 nm are cannot involve in any of the energy levels.
attributed to the spin allowed p–p* transitions of the ligands. This
To demonstrate the potential of the ET group in TPQIr-ET
indicates that the ET group in the TPQIr-ET does not show any on the device performance, the solution-processed deep-red
significant effects on the ground state energy levels of Ir(III) PhOLEDs (Fig. S5, ESI†) with various combinations of host and
complexes because the ET group is linked to the ancillary ligand dopant materials (I, II, III, and IV), as shown in Table 1, were
by ether linkage, which prevents the extended p-conjugation fabricated with the configuration of ITO/PEDOT:PSS/TCTA:TPBi
between them. Additionally, the PL patterns of TPQIr-ET and or TCTA:OXD-7:dopant/TmPyPB/LiF/Al. Here, the emitting layer
TPQIr are also exactly similar with a maximum at 612 nm as (EML) consists of a 1 : 1 ratio of TCTA and TPBi or OXD-7 with
shown in Fig. 1a and the quantum yield of 0.25 and 0.23, 8wt% of the TPQIr-ET or TPQIr. Fig. 2 shows the current density–
respectively. It also reveals that the ET group cannot induce voltage–luminance (I–V–L) and luminance efficiency–current
any significant changes in the excited state of the Ir(III) complex, density–power efficiency (LE–I–PE) curves of the all the devices
which leads to similar emission patterns and maxima.
and the data are summarized in Table 1.
In order to find highest occupied molecular orbital (HOMO)
Device I shows a current efficiency of 13.69 cd Aꢀ1 and an
and lowest unoccupied molecular orbital (LUMO) energy EQE of 15.28%, which are around 70% higher than that of
levels, the cyclic voltammetry experiments were carried out device II, with a current efficiency of 5.09 cd Aꢀ1 and an EQE of
for TPQIr-ET and TPQIr in MC solution (Fig. S3, ESI†). Both 5.92%, respectively. It reveals that TPBi has a more balanced ET
the Ir(III) complexes exhibited a well-defined reversible redox property with the hole transporting host TCTA than the OXD-7.
process. The HOMO energy levels were calculated from the onset
of oxidation potentials and found to be ꢀ5.26 and ꢀ5.24 eV for
Table 1 Device characteristics of TPQIr-ET and TPQIr with various ratios
of mixed host systems
TPQIr-ET and TPQIr, respectively. The LUMO levels were calcu-
lated from their HOMO levels and optical band gaps were
Von EQE Zc
Zp
Lmax
determined from their absorption edges and found to be
ꢀ3.07 and ꢀ3.08 eV for TPQIr-ET and TPQIr, respectively. As
shown in Fig. 1b, the HOMO and LUMO energy levels of Ir(III)
complexes are well matched with the adjacent layers of devices.
Device Host
Dopant [V] (%) [cd Aꢀ1] [lm Wꢀ1] [cd mꢀ2
]
I
TCTA/TPBi TPQIr
TCTA/OXD-7 TPQIr
6.6 15.28 13.69
6.3 5.92 5.09
TCTA/TPBi TPQIr-ET 6.0 20.59 17.20
TCTC/OXD-7 TPQIr-ET 5.7 7.40 6.54
2.87
1.66
6.72
1.31
2013
3013
1334
2661
II
III
IV
Fig. 2 (a) Current density–voltage–luminance (I–V–L) and (b) luminance
efficiency–current density–power efficiency (LE–I–PE) curve of the
devices based on TPQIr-ET and TPQIr dopants.
Fig. 1 (a) UV-visible and PL spectra of TPQIr and TPQIr-ET in MC and
(b) energy level diagram of the materials.
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Similarly, device III shows a current efficiency of 17.20 cd Aꢀ1
and an EQE of 20.59%, which are around 60% higher than that
of device IV, and a current efficiency of 7.40 cd Aꢀ1 and an EQE
of 6.54%, respectively. Although both TPBi and OXD-7 have a
good ET nature, the TPBi based devices show better device
performance than that of OXD-7. This can be explained by the
nature of the morphology engineering between TPBi and OXD-7
with TCTA (vide infra). Interestingly, the TPQIr-ET based
devices show improved device performance compared to TPQIr
due to the introduction of the ET group into the dopant
materials, which is only 8 wt% of the dopant in EML and has Fig. 4 (a) EL spectra and (b) CIE coordinate of devices I, II, III, and IV.
a strong influence on balanced charge carrier injection. The
hole transporting and ET properties of TPQIr-ET and TPQIr were
further studied by fabricating hole and electron only devices
(Fig. S6, ESI†). The mobility data are extracted in Table S1 (ESI†).
It is obvious that the balance between hole and electron trans-
port properties of TPQIr-ET is higher than that of TPQIr due to
the presence of the ET group.
regardless of the nature of the host materials. As shown in
Fig. 4b, devices I, II, III, and IV emit a deep-red light with the
CIE coordinates of (0.671, 0.326), (0.671, 0.326), (0.673, 0.323),
and (0.672, 0.325), respectively.
In summary, two heteroleptic Ir(III) complexes TPQIr-ET and
TPQIr were designed, synthesized, and applied as a dopant for
solution-processed deep-red PhOLEDs. Here, we introduced the
The morphology engineering between the host and the dopant
materials can be analyzed using atomic force microscopy (AFM).
ET group into the ancillary ligand of the dopant materials to
AFM topographic images of 40 nm thick mixed-host TCTA : TPBi
improve the balanced charge carrier injection in the devices. As
a result, TPQIr-ET shows an EQE value of 20.59%, which is 25%
higher than that of TPQIr. This work provides the first successful
(1: 1) and TCTA:OXD-7 (1 :1) with 8 wt% of the TPQIr doped
EML film are shown in Fig. 3a and b, respectively. Unexpectedly,
the TCTA:TPBi based mixed-host system shows a higher root
example of the use of a dopant with the ET group in PhOLEDs to
mean square surface roughness value (0.76 nm) than that of
realize efficient device performance. This also opens up new
TCTA:OXD-7 (0.55 nm). This is due to the fact the TCTA and TPBi
perspectives in the designing of new phosphorescent emitters
have a disk-like structure, which may be arranged in a columnar
for efficient solution-processed PhOLEDs.
manner, but in the case of TCTA and OXD-7 based systems they
This work was supported by grant funding from the National
may have an interdigitated future due to the rod-like nature of
Research Foundation of Korea (NRF) of the Ministry of Science,
OXD-7. Thus, a balanced charge carrier injection is more feasible
ICT & Future Planning (MSIP) of Korea (NRF-2011-0028320) and the
in the TCTA:TPBi system than in the TCTA:OXD-7 system.
Pioneer Research Center Program through the National Research
A similar trend has also been observed for TPQIr-ET (Fig. 3c
Foundation of Korea funded by the Ministry of Science, ICT &
and d). Interestingly, the introduction of the ET group into
Future Planning (MSIP) of Korea (NRF-2013M3C1A3065522).
dopant materials improves the efficiency without engineering
JYL acknowledges financial support from the NRF Grant
the morphology of the EML of the devices.
(No. 2007-0054343) funded by the Korean Government.
As shown in Fig. 4a, all the devices exhibited narrow deep-
red electroluminescence (EL) spectra, which are similar to the
PL spectra of the dopants. It suggests that the emission mainly
originates from the triplet states of the Ir(III) complexes
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