Insulated donor-π-acceptor systems based on fluorene-phosphine oxide hybrids for non-doped deep-blue electroluminescent devices

Article abstract of DOI:10.1039/c2cc31066j

An ambipolar ternary deep-blue emitter with CIE coordinates of (0.15, 0.07) and high electroluminescent performance was constructed on the basis of an insulated donor-π-acceptor system through an indirect linkage.

Full text of DOI:10.1039/c2cc31066j

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COMMUNICATION
Insulated donor–p–acceptor systems based on fluorene-phosphine oxide
hybrids for non-doped deep-blue electroluminescent devicesw
Donghui Yu,a Fangchao Zhao,b Zhen Zhang,a Chunmiao Han,a Hui Xu,*a Jing Li,a
Dongge Ma*b and Pengfei Yana
Received 14th February 2012, Accepted 26th April 2012
DOI: 10.1039/c2cc31066j
An ambipolar ternary deep-blue emitter with CIE coordinates of
(0.15, 0.07) and high electroluminescent performance was
constructed on the basis of an insulated donor–p–acceptor
system through an indirect linkage.
hole- and electron-transporting groups can be effectively
suppressed. In this sense, an indirect linkage provides a
promising alternative for designing ternary D–p–A type deep-
blue emitters. In this contribution, we present two ternary deep-
blue emitters, 9DPNAFSPO and 9TPAFSPO (Scheme 1),
composed of fluorene as chromophore, arylamines as hole-
transporting groups and diphenylphosphine oxide (DPPO)
as electron-transporting group. To investigate the advantages
of these ambipolar emitters, another unipolar analogue
9DEFFSPO was also prepared for comparison. The electronic
communication between the functional units in 9DPNAFSPO
and 9TPAFSPO is effectively suppressed by the saturated 9-C
of the p-spacer fluorene. Non-doped devices of 9DPNAFSPO
achieved ideal deep-blue EL with CIE coordinates of (0.15, 0.07)
and ideally balanced carrier injection/transportation.
Efficient deep-blue-emitting devices are always one of the most
significant challenges for organic light-emitting diodes
(OLEDs) and have become the main bottleneck hampering
commercial applications of electroluminescent (EL) technology.1
The huge energy gap (B2.8 eV) and high colour purity for
deep-blue emission is so demanding that only few luminophors
and devices can satisfy the requirements on CIE coordinates
(x = 0.15, y = 0.08), (x = 0.15, y = 0.06) defined by National
Television System Committee, USA (NTSC) and European
Broadcasting Union (EBU), respectively. Because the extended
conjugation and intramolecular charge-transfer (ICT) state
of donor–p–acceptor (D–p–A) type structures can induce
remarkable bathochromic shifts, it clear that limited conjugation
and functionalization are prerequisites for deep-blue emission
with y o 0.1.2 So far, there are only few reports of deep-blue
EL satisfying the NTSC standard on the basis of a single
molecule with both hole- and electron-transporting groups.3
Additionally, unbalanced carrier injection and transportation
in mono-4 or bi-functional1a,5 systems not only increases the
driving voltages and reduces EL efficiencies, but also broadens
the emissions due to the interface-localized recombination
zone. D–p–A type deep-blue emitters can however eliminate
these problems.3a,6 However, the difficulty is how to suppress
the interaction between donor and acceptor without increasing
p-conjugation of the spacer.
9DEFFSPO, 9DPNAFSPO and 9TPAFSPO, collectively
named 9ArFSPO, were conveniently prepared through a
similar procedure reported recently7b with good total yields
over 40% (Scheme SI1, ESIw). All of these materials reveal
high morphological stability with glass transition temperature
(Tg) of 4110 1C (Fig. SI1w). No melt point (Tm) of 9DEFFSPO
was observed, while Tm of 9DPNAFSPO was 286 1C, which
is about 30 1C higher than that of 9TPAFSPO.7b These
materials also exhibit outstanding thermal stability with the
decomposition temperatures (Td) at weight loss of 5% of
4400 1C. 9DPNAFSPO shows the highest thermal and
Recently, our group demonstrated an effective strategy of
indirect linkage for constructing high energy-gap materials.7
Based on this strategy, the communication between chromophors,
a Key Laboratory of Functional Inorganic Material Chemistry,
Heilongjiang University, Ministry of Education, Harbin 150080,
P. R. China. E-mail: hxu@hlju.edu.cn; Fax: +86 451 86608042
b State Key Laboratory of Polymer Physics and Chemistry,
Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, P.R. China.
E-mail: mdg1014@ciac.jl.cn
w Electronic supplementary information (ESI) available: Experimental
details, thermal properties, absorption and PL spectra, DFT calcula-
tion results, CV curves, I–V curves of single-carrier transporting
devices and Efficiency–J curves. See DOI: 10.1039/c2cc31066j
Scheme 1 Design strategy and molecular structures of 9ArFSPO
emitters.
c
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Chem. Commun., 2012, 48, 6157–6159 6157

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arylamine groups. The lowest unoccupied molecular orbitals
(LUMOs) of 9DEFFSPO and 9DPNAFSPO are located on
fluorenyl or naphthyl, while the LUMO of 9TPAFSPO is
mainly located on its chromophore fluorene moiety. The
strong inductive effect of PQO enhances the contributions
of 9-phenyls linked with DPPO to LUMOÀ1 of 9DEFFSPO
and 9DPNAFSPO and LUMO of 9TPAFSPO. It is also
noticeable that each functional group hardly influences the
contributions of other groups to molecular orbitals. The
remarkable improvement of carrier injecting ability was further
shown by cyclic voltammetric (CV) analysis (Fig. SI6w).
HOMOs of 9DEFFSPO, 9DPNAFSPO and 9TPAFSPO are
estimated by the onset voltages of their first oxidation peaks as
À5.75, À5.32 and À5.25 eV, while their LUMOs are À2.58,
À2.58 and À2.52 eV, respectively. This trend is in accord with
the results of DFT calculation. The current density (J)–voltage
characteristics of the single-carrier transporting devices based
on 9ArFSPO revealed a similar tendency of the carrier trans-
porting abilities (Fig. SI7w). All of these three materials have
similar electron-only voltammetric characteristics, but J of the
electron-only devices based on 9DPNAFSPO was the largest.
There was no hole-only J at voltage less than 15 V for
9DEFFSPO. Contrarily, 9DPNAFSPO and 9TPAFSPO have
similar and excellent hole-transporting ability exhibited by the
high J of their hole-only devices. Therefore, both of them are
ambipolar, although the hole transportation in 9TPAFSPO is
still predominant. It can be concluded that each functional
moiety in 9ArFSPO provides its contribution to FMOs and
carrier transporting ability independently rather than influen-
cing others significantly. 9DPNAFSPO with the strongest
electron transporting ability possesses the most balanced
carrier injecting/transporting ability among these molecules.
We further demonstrated the application of 9ArFSPO as
ternary emitters with the device configuration of ITO|MoO3
(5 nm)|NPB (50 nm)|TCTA (5 nm)|9ArFSPO (20 nm)|TPBi
(30 nm)|LiF (1 nm)|Al (Fig. 1 and ESIw). Devices A, B and C
were based on 9DEFFSPO, 9DPNAFSPO and 9TPAFSPO,
respectively, and B and C successfully realized deep-blue
emissions with CIE coordinates of (0.15, 0.07) and (0.16, 0.07),
respectively. These data not only meet the NTSC standard,
but also approach to the criterion of EBU. Device A showed
inferior color saturation with CIE coordinates of (0.15, 0.12).
Its broad and remarkably red-shifted emission corresponded
to the formation of exciplexes at the interface between the
EML and TCTA layer. This should originate from the
unipolar electron-transporting characteristic of 9DEFFSPO.
As predicted, the ambipolar characteristic of the emitters is
significant to suppress colour change during the EL process.
The turn-on voltages of B and C were extremely low at 3.1
and 3.3 V, respectively. Brightness (B) of B100 cd mÀ2 for
displays was achieved at 6.5 and 7.1 V for B and C, respectively
(Fig. 2a). These driving voltages were much less than those of
A (4.1 V for onset and 8.3 V for 100 cd mÀ2). Obviously, these
results originate from the superiority of the ambipolar char-
acteristics of 9DPNAFSPO and 9TPAFSPO. Device C had
the highest maximum efficiency: 1.50 cd AÀ1 for current
efficiency (C.E.), 2.19 lm WÀ1 for power efficiency (P.E.) and
2.71% for external quantum efficiency (E.Q.E.) (Fig. 2b). This
is attributed to the highest radiative transition efficiency
Fig. 1 EL spectra, energy level diagram and CIE coordinates of the
devices.
morphological performance owing to its more bulky and rigid
structure.
In dilute solutions (1 Â 10À6 mol LÀ1 in CH2Cl2) of
9ArFSPO, the absorption bands originated from the
9-[4-(diphenylphosphinoyl)phenyl]-9-phenyl-9H-fluorene (FSPO)
cores at 308 and 275 nm are fully preserved (Fig. SI2w).7a
The additional bands at longer wavelengths are attributed to
the p-p* transitions of fluorenyl or arylamine groups in
9DEFFSPO (319 nm), 9DPNAFSPO (336 nm) and 9TPAFSPO
(336 nm),7b respectively. These bands are enhanced in films
because of p–p interaction between adjacent molecules
(Fig. SI3w). However, the fine structures of the solid-state
absorption spectra are still retained, and implies suppressed
intermolecular aggregation due to their unsymmetrical config-
urations. Moreover, no ICT band is observed in the spectra of
9DPNAFSPO and 9TPAFSPO. The suppressed ICT is
further testified by the reduced bathochromic shift of their
emissions in solvents with different polarities (Fig. SI4w).
9DPNAFSPO and 9TPAFSPO emit bright violet light in
solution with peaks at 435 and 407 nm, respectively, while
their solid-state emissions are nearly the same with maxima at
442 and 407 nm. This further demonstrates the suppressed
intermolecular interaction. Due to their too violet emissions
and limited proportion of chromophore in the molecules, PL
quantum yields (PLQY) of 9DEFFSPO, 9DPNAFSPO and
9TPAFSPO are only 0.19, 0.19 and 0.46 measured by using
9,10-diphenylanthracene as the standard. These values are
much smaller than PLQYs of other deep-blue emitters
reported (commonly 40.70).1,3 Nevertheless, since EL is a
‘‘dual-injection/transportation’’ procedure, the balanced elec-
tron and hole in emitting layers (EMLs) is crucial to high
efficiencies. Therefore, we believe the optimized bipolar struc-
tures of 9DPNAFSPO and 9TPAFSPO can still provide good
EL performance.
The electrical characteristics of 9ArFSPO were evaluated by
density function theory (DFT) calculation through employing
the Gaussian 03 package at the level of B3LYP/6-31G*
(Fig. SI5w). The highest occupied molecular orbitals (HOMOs)
of 9ArFSPO are contributed by their 9-subsituted fluorenyl or
c
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establishes a further example revealing the significance of
charge balance in EMLs of deep-blue-emitting devices.
Further modification through utilizing chromophores with
higher luminescent properties is ongoing in our lab.
D. Y. and F. Z. contributed equally to this work. This
project was financially supported by NSFC (50903028,
61176020 and 21102039), Key Project of Ministry of Education
(212039), New Century Talents Developing Program of
Heilongjiang Province (1252-NCET-005), Education Bureau
of Heilongjiang Province (10td03), the Supporting Program of
Highlevel Talents of HLJU (2010hdtd08) and HLJU Fund for
Distinguished Young Teachers.
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Fig. 2 B/J–V curves (a) and efficiency curves (b) of blue-emitting
devices based on 9TPAFSPO.
of excitons produced by the highest PLQY of 9TPAFSPO.
B achieved comparable maximum efficiencies of 1.18 cd AÀ1
,
1.13 lm WÀ1 and 1.96%, although the PLQY of 9DPNAFSPO
is less than a half of that of 9TPAFSPO. EL E.Q.E. can be
expressed as8
E.Q.E. = gwZPLZoc
(1)
where ZPL is PLQY, Zoc is the outcoupling factor (0.25 for
organic solid), g is the recombination efficiency of holes and
electrons and w is the fraction of excitons (0.4 for conjugated
fluophors). For totally balanced carrier injection and recombi-
nation g = 1 while g of C is calculated as 0.6 according to its
maximum E.Q.E., which should be ascribed to the predomi-
nant hole transportation in its EML. The electron-only trans-
porting characteristic of 9DEFFSPO was also further proved
by g of A as low as 0.4. Significantly, g of B is 1, indicating
ideal balance of carrier injection/transportation. The much
more stable efficiencies of B at high brightness further reveal
the balanced carrier injecting/transporting.
In summary, we have demonstrated the ternary deep-blue
emitters with balanced carrier injection and transportation on
the basis of indirect linkage. 9DPNAFSPO realized the most
favorable EL performance among the deep-blue emitters
reported so far (Table SI1w), even though its PLQY is only
0.19. This result not only shows the great potential of indirect
linkage in constructing ambipolar deep-blue emitters, but also
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c
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Chem. Commun., 2012, 48, 6157–6159 6159