Controllably tuning excited-state energy in ternary hosts for ultralow-voltage-driven blue electrophosphorescence

Article abstract of DOI:10.1002/anie.201202702

A series of dibenzofuran-based ternary hosts were designed and prepared. The singlet energy levels and carrier injecting/transporting abilities were adjusted on the basis of the mixed meso and short-axis linkages (see picture). By harmonizing the optical and electrical properties, the blue-emitting diodes realized highly efficient blue electrophosphorescence with ultralow driving voltages. Copyright

Full text of DOI:10.1002/anie.201202702

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Chemie
DOI: 10.1002/anie.201202702
Excited-State Tuning
Controllably Tuning Excited-State Energy in Ternary Hosts for
Ultralow-Voltage-Driven Blue Electrophosphorescence**
Chunmiao Han, Zhensong Zhang, Hui Xu,* Jing Li, Guohua Xie, Runfeng Chen, Yi Zhao,* and
Wei Huang
Phosphorescent organic light-emitting diodes (PHOLEDs),
with 100% theoretical internal efficiency, are being rapidly
developed as a most promising approach to meet the urgent
and extensive demand of energy-efficient and portable digital
terminals and lighting sources.[1] Thanks to the recent break-
through of highly efficient blue PHOLEDs[2] and out-
coupling technologies, PHOLEDs in full color can already
realize extremely high efficiencies that approach those of
fluorescent tubes (about 70 LmWꢀ1).[3] Nevertheless, as the
hosts in the emitting layers (EMLs) should have higher triplet
excited energy levels (T1) to confine the excitons on
phosphorescent guests,[4] the highest occupied molecular
orbital (HOMO)–lowest unoccupied molecular orbital
(LUMO) energy gaps in PHOLEDs are often much larger
than their fluorescent counterparts, which consequently result
in poor energy-level alignment and thus higher driving
voltages. This drawback not only complicates the design of
driving circuit, but also directly reduces power efficiency
(PE).[5] Thus, the low-voltage-driving high-efficiency
PHOLED remains the biggest challenge. Su, Kido, et al.
have reported green PHOLEDs with extremely low operating
voltages of 2.18 V for onset and 2.41 V at 100 cdmꢀ2 through
good management of the interfacial contact between electron
transporting layers and anodes.[5] However, there are only
a few blue PHOLEDs that achieve low driving voltages; for
example, applicable luminance at a driving voltage of less
than 3 V.[6] The formidable challenge is the high barriers for
carrier injection and transportation deriving from the pre-
requisite of extremely high T1 of the hosts, for example,
2.85 eV (0.2 eV higher than that of blue phosphor iridium-
(III)bis(4,6-(difluorophenyl)pyridinato-N,C2)picolinate
(FIrpic; Scheme 1). This issue actually reflects the intrinsic
[*] C. Han, Dr. H. Xu, J. Li
Key Laboratory of Functional Inorganic Material Chemistry
Ministry of Education, Heilongjiang University
74 Xuefu Road, Harbin 150080 (P. R. China)
E-mail: hxu@hlju.edu.cn
Z. Zhang, Dr. G. Xie, Prof. Y. Zhao
State Key Laboratory on Integrated Optoelectronics
College of Electronics Science and Engineering, Jilin University
2699 Qianjin Street, Changchun 130012 (P. R. China)
E-mail: zhao_yi@jlu.edu.cn
Scheme 1. The energy-transfer process in phosphorescent doping
systems, and molecular structures of DBFxPOCzn.
Dr. R. Chen, Prof. W. Huang
Key Laboratory for Organic Electronics and Information Displays
(KOLED), Institute of Advanced Materials (IAM)
Nanjing University of Posts and Telecommunications
Nanjing 210046 (China)
contradiction between optical and electrical properties of the
materials. As the singlet excited energy levels (S1) are related
to HOMO–LUMO energy gaps, the challenge has actually
evolved into an original scientific problem: how to control-
lably adjust S1 without changing T1. Unfortunately, as most of
the modification approaches have the same effects on both
singlet and triplet states,[7] only few hosts possess both of the
energy difference between S1 and T1 (DEST) of less than
0.4 eV and T1 of more than 2.85 eV.[6b]
We recently reported several ambipolar ternary aryl
phosphine oxide (APO) hosts based on indirect linkage.[6b]
Their DEST was tuned to 0.45 eV, and low driving voltages and
high efficiencies were realized. However, the indirect linkage
can hardly afford multiple modifications. We believe that the
[**] C.H. and Z.Z. contributed equally to this work. This work was
financially supported by the National Key Basic Research and
Development Program of China (2010CB327701), NSFC (50903028,
61176020, and 60977024), Key Project of Ministry of Education
(212039), New Century Talents Developing Program of Heilongjiang
Universities (1252-NCET-005), Education Bureau of Heilongjiang
province (2010td03), and Heilongjiang University (2010hdtd-08).
C.H. thanks the support of the Postgraduate Innovation Funds of
Heilongjiang Province and Heilongjiang University. H.X. thanks the
kind help of Dr. Chao Zheng for DFT calculations.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201202702.
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development of ternary systems composed of chromophores
and hole and electron transporting moieties on the basis of
reasonable linkages should be one of the most promising
pathways to achieve high energy-gap hosts with improved
carrier injecting/transporting ability,[8] because 1) high T1 hole
and electron injecting can be contributed by the relative
groups; 2) more complicated molecular structures can be
established to finely tune the optoelectronic properties; and
3) the carrier injecting/transporting can be accurately modu-
lated through varying the number and ratio of the carrier
transporting moieties so as to find the ideal molecular
structures.
the less controllable deposition for bigger molecules under
same evaporation condition.
The UV/Vis spectra of DBFxPOCzn include three main
absorption bands in the ranges of 200–250, 250–300 and 300–
375 nm, corresponding to the p!p* transitions of phenyl in
DPPOs, DBF, and the mixed transitions of limited charge-
transfer (CT) states and p!p* transitions from carbazolyl to
DBF moieties, respectively (Figure 1a; Supporting Informa-
Herein, we present an effective strategy for constructing
ambipolar hosts with high T1 through the mixed meso- and
short-axis linkages, which is proved by four ternary hosts
based on dibenzofuran (DBF) as chromophore, which mainly
determines the optical characteristics, hole-transporting car-
bazolyl, and electron-transporting diphenylphosphine oxide
(DPPO), namely 2-carbazolyl-6-(diphenylphosphinoyl)di-
benzofuran (DBFSPOCz), 2,8-dicarbazolyl-4-(diphenylphos-
phinoyl)dibenzofuran (DBFSPOCz2), 2-carbazolyl-4,6-bis-
(diphenylphosphinoyl)dibenzofuran (DBFDPOCz), and 2,8-
dicarbazolyl-4,6-bis-(diphenylphosphinoyl)dibenzofuran
(DBFDPOCz2; Scheme 1 and Supporting Information,
Scheme S1). They are collectively named DBFxPOCzn.
Upon their gradually increased p-conjugation by changing
the number and ratio of carbazolyl and DPPO from 1:1, 2:1,
and 1:2 to 2:2, we successfully reduced S1 values whilst
simultaneously maintaining T1 values of these hosts at around
2.90 eV. DBFDPOCz supported FIrpic-based PHOLEDs
with the extremely low driving voltages, such as 2.4 V for
onset, which is even 0.25 V lower than that corresponding to
the photon energy (2.65 V, hn/e). The luminance of 100 cdmꢀ2
for display and 1000 cdmꢀ2 for indoor lighting were also
achieved at < 2.8 Vand < 3.3 V. To the best of our knowledge,
these data are the lowest among the prototype blue PHO-
LEDs without additional enhancing technology. This work
suggests a brand new concept to construct high energy-gap
hosts with mixed linkages and shows the huge potential of
ternary hosts in highly efficient blue PHOLEDs for portable
applications.
In DBFxPOCzn, carbazolyl moieties are introduced at
2,8-positions of DBF, while DPPOs bond to DBF along
molecular short axis. Carbazolyl moieties and DPPOs are at
meta position to each other so as to suppress the interplay
between them. The decomposition temperature at a weight
loss of 5% (Td) from DBFSPOCz to DBFDPOCz2 increased
remarkably owing to the reduced molecular volatility and
increasing p-conjugation (Supporting Information, Figure S1
and Table S1). The rigid molecular configurations of
DBFxPOCzn endow them with high glass-transition temper-
atures (Tg) of over 1008C. The good morphological properties
were further indicated through AFM (Supporting Informa-
tion, Figure S2) and SEM images (Supporting Information,
Figure S3) of the vacuum-evaporated thin films. No aggrega-
tion and crystallization were observed. The root-mean-square
roughness (RMS) of 0.24–0.94 nm testifies the excellent film-
forming ability of these compounds. More carbazolyl moieties
or DPPOs slightly increase the roughness, which is ascribed to
Figure 1. a) UV/Vis absorption spectra and fluorescence spectra of
DBFxPOCzn in dilute dichloromethane (10ꢀ6 molLꢀ1); b) Phosphores-
cence spectra of DBFxPOCzn in dichloromethane at low temperature
(77 K) after a delay of 300 ms to eliminate fluorescence.
tion, Figure S4). The molar extinction coefficients of
DBFDPO derivatives are much smaller than DBFSPOCz2.
Furthermore, DBFDPOCz2, with the biggest hindrance,
showed a slightly lower extinction coefficient than that of
DBFDPOCz. The excitation spectra also exhibit the same
situation (Supporting Information, Figure S5). The main
reason might be that the much enhanced steric hindrance
by doubled DPPOs in DBFDPOCz and DBFDPOCz2
restrains their configuration adjustment in excited states.
Nevertheless, it is not surprising that the optical energy gaps
are remarkably reduced from 3.52 to 3.26 eV, along with the
increase of p-conjugation (Table 1). The variation tendency of
the fluorescent emissions of DBFxPOCzn is the same. A more
careful comparison reveals that the second carbazolyl group
results in the reduction of energy gap for 0.1 eV, while the
second DPPO seems more effective with the reduction of
0.15 eV. This is much different in binary systems (such as
carbazole-DPPO hybrids[9]), in which DPPO moieties nearly
do not change the energy gaps. In films, the absorption spectra
of DBFxPOCzn still retain the corresponding fine structures
(Supporting Information, Figure S6 and Table S1). The shift
of their solid-state emissions can also be negligible. It further
implies limited intermolecular interaction and aggregation.
More significantly, it is inspiring that the phosphorescence
spectra of DBFxPOCzn are perfectly fixed in shape and range
(Figure 1b; Supporting Information, Figure S7). All of these
compounds have high T1 of about 2.90 eV for efficient
exothermic energy transfer to FIrpic. T1 of 2,8-di(carbazol-
9-yl)dibenzofuran (m-CzOF)[10] was reported as 2.97 eV;
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Table 1: Physical properties and device performance of DBFxPOCzn.
increase the frontier molecular orbital (FMO) density by
inserting new MOs into the interspaces without remarkably
influencing the original MO distributions. Thus, in ternary
systems, the influence of electron-donating groups and
electron-withdrawing groups on energy gaps can be combined
rather than neutralized by suitable linkages. HOMOs of
DBFxPOCzn are similar about ꢀ6.1 eV, as determined by
cyclic voltammetry (Supporting Information, Figure S10 and
Table S1). However, DBFDPOCz and DBFDPOCz2 have
lower LUMOs. This implies that the number of DPPO groups
and electron-injecting ability are proportional. The nominal
single-carrier transporting devices based on DBFxPOCzn and
DBFCz2 were also fabricated to investigate their carrier
transporting abilities (Supporting Information, Figure S11).
Compound
Eg [eV] T1 [eV]
Voltage [V][c]
Efficiency roll-off [%][d]
DBFSPOCz 3.52[a] 2.90[a] 2.5, <3.0, <3.6
3.91[b] 2.98[b]
27, 45, 27
DBFSPOCz2 3.42[a] 2.91[a] 2.4, <2.8, <3.5
3.78[b] 2.85[b]
10, 26, 14
7, 15, 7
DBFDPOCz 3.37[a] 2.88[a] 2.4, <2.8, <3.3
3.67[b] 2.82[b]
DBFDPOCz2 3.26[a] 2.88[a] 2.4, <2.8, <3.5
3.51[b] 2.76[b]
15, 31, 16
[a] Calculated by the absorption edges.ꢀ[2b] DFT calculation results. [c] In
the order of onset, 100, and 1000 cdm . [d] In the order of CE [cdAꢀ1],
PE [lmWꢀ1], and EQE [%] at 1000 cdmꢀ2
.
Compared
with
the
monocarbazolyl
derivatives,
therefore, the incorporation of DPPO has the slight influence
of about 0.07 eV on T1. Furthermore, the introduction of
another DPPO along short axis can only induce the reduction
of T1 for 0.02 eV, and T1 cannot be changed by the second
carbazolyl group. DFT calculations of their T1 states showed
a similar tendency (Table 1). Actually, the spin-density
distributions of T1 states indicate that although T1 state of
DBFSPOCz is determined by the carbazolyl group, DBF in
the other three molecules has the dominant influence on their
T1 states (Supporting Information, Figure S8). As the inter-
actions between DPPO and carbazolyl are blocked by DBF
and meso linkage, the only intramolecular interplay is the
weak charge transfer between carbazolyl and DBF, which is
shown by the stable PL emissions of DBFxPOCzn in solvents
with various polarities and the similar bathochromic interval
less than 20 nm with that of 2,8-di(carbazol-9-yl)dibenzofuran
(DBFCz2; Supporting Information, Figure S9). Therefore, in
these ternary systems, all of the building blocks have been
well-managed to maintain T1 to the greatest extent. The DEST
value is consequently re-
DBFSPOCz2 and DBFDPOCz2 have the more balanced
carrier transporting ability owing to their enhanced hole-only
current densities (J) and reduced electron-only J. This is
attributed to the embedding and blocking of DBF by more
peripheral carbazolyl moieties. Actually, DBF can also make
considerable contributions to electron transportation. There-
fore, DBFSPOCz with the least substituents and
DBFDPOCz2 with the symmetrical configuration have the
stronger electron transporting ability than DBFDPOCz and
DBFSPOCz2, respectively. Simultaneously, the hole trans-
porting abilities of DBFSPOCz2 and DBFDPOCz2 are
comparable to that of DBFCz2, while their electron trans-
porting abilities are much stronger. This further indicates the
suppressed interference between carbazolyl moieties and
DPPOs owing to the rational linkages. In general, we can
readily control the carrier injecting/transporting ability of the
molecules through changing the number, ratio, and type of the
functional groups and the molecular configurations. The
dramatic progress is that it has almost no influence of T1.
duced from 0.62 to 0.38 eV.
Thus, it is feasible to fix T1
and simultaneously tune S1
on the basis of the reason-
able combination of molec-
ular composition and link-
age modes.
DFT calculation further
presents the same tendency
for the energy gaps to
decrease along with the
increase of p-conjugation
(Figure 2). The HOMO and
LUMO + 1 of these mole-
cules are located on their
carbazolyl
and
DPPO
groups, respectively, while
LUMOs are mainly contrib-
uted by DBF. Therefore,
through introducing carba-
zolyl and DPPO groups, the
carrier injection ability is
effectively improved. Nota-
bly, more functional groups Figure 2. DFT-calculated energy levels of DBFSPO, DBFDPO, and DBFxPOCzn.
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The harmonized optoelectronic characteristics of
DBFxPOCzn encouraged us to fabricate blue-emitting PHO-
LEDs based on FIrpic (Supporting Information, Scheme S2).
Devices A–D used DBFSPOCz, DBFSPOCz2, DBFDPOCz,
and DBFDPOCz2 as hosts, respectively. For comparison,
Devices E and F were also fabricated with the conventional
hosts 1,3-dicarbazolylbenzene (mCP) and tris(4-(9H-carba-
zol-9-yl)phenyl)amine (TCTA). It is inspiring that the turn-on
voltages of B–D were extremely low as 2.4 V, which is even
0.25 V lower than the theoretical threshold voltage corre-
sponding to the emission photon energy of FIrpic (2.65 eV;
Figure 3a and Table 1).[5] This value also approaches the limit
and F; furthermore, its efficiency roll-off was much reduced.
At 1000 cdmꢀ2, its external quantum efficiency decreased for
only 7% of the maximum. It is noteworthy that the EL
performance of B and D was much worse than that of A and
C, although DBFSPOCz2 for B and DBFDPOCz2 for C have
more balanced carrier transporting abilities than those of
DBFSPOCz and DBFDPOCz. As in the common devices the
current is hole-dominant and Ir3+ complexes often have
strong trapping ability, the moderately dominant electron
transportation in the host may facilitate the charge balance in
EMLs. In this case, it is crucial to modulate the carrier
transporting ability of the host accurately to fit to the entire
conditions of the devices, which is one of the unique
advantages of ternary host systems.
In summary, on the basis of the mixed meso and short-axis
linkages, we have fixed T1 and continuously tuned S1 of DBF-
based ternary hosts. The S1 variation actually corresponds to
the accurate modulation of carrier injection and transporta-
tion in the host materials. In this sense, we have successfully
overcome the intrinsic contradiction between optical and
electrical properties, and have consequently constructed the
promising hosts with both high T1 of about 2.90 eV and
specific ambipolar characteristics. The efficient blue electro-
phosphorescence was achieved under the extremely low
driving voltages, such as 2.8 V at 100 cdmꢀ2 for displays.
This proof-of-concept study not only develops an effective
method to control molecular excited energy levels, but also
presents a novel strategy for construct ambipolar high-
energy-gap hosts and the great potential of these ultralow-
voltage-driven PHOLEDs for the future applications in
pocket digital terminals directly driven by low-voltage
sources, such as solar cells.
Received: April 7, 2012
Revised: June 29, 2012
Published online: && &&, &&&&
Keywords: dibenzofuran · electrophosphorescence ·
.
excited-state tuning · light-emitting diodes · low-voltage driving
Figure 3. a) Current density J versus voltage (hollow symbols) and
luminance (solid symbols) versus voltage curves of devices A–F;
b) efficiency–luminance curves of A–F.
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Table S2). Although A achieved the highest efficiencies
among these devices, its sharp decline of efficiencies at high
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transportation in its EML. This should be attributed to the
much stronger electron transporting ability of DBFSPOCz. C
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Communications
Excited-State Tuning
C. Han, Z. Zhang, H. Xu,* J. Li, G. Xie,
R. Chen, Y. Zhao,*
W. Huang
&&&& — &&&&
Controllably Tuning Excited-State Energy
in Ternary Hosts for Ultralow-Voltage-
Driven Blue Electrophosphorescence
A series of dibenzofuran-based ternary
hosts were designed and prepared. The
singlet energy levels and carrier injecting/
transporting abilities were adjusted on
the basis of the mixed meso and short-
axis linkages (see picture). By harmoniz-
ing the optical and electrical properties,
the blue-emitting diodes realized highly
efficient blue electrophosphorescence
with ultralow driving voltages.
6
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!