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Feeling blue? Blue phosphors for OLEDs
Feeling blue? Blue phosphors for OLEDs Research on organic light emitting diodes (OLEDs) has been revitalized, partly due to the debut of the OLED TV by SONY in 2008. While there is still plenty of room for improvement in efficiency, cost-effectiveness and longevity, it is timely to report on the advances of light emitting materials, the core of OLEDs, and their future perspectives. The focus of this account is primarily to chronicle the blue phosphors developed in our laboratory. Special attention is paid to the design strategy, synthetic novelty, and their OLED performance. The report also underscores the importance of the interplay between chemistry and photophysics en route to true-blue phosphors. Hungshin Fua, Yi-Ming Chengb, Pi-Tai Choua,*, and Yun Chib,* aDepartment of Chemistry, National Taiwan University, Taipei 10617, Taiwan bDepartment of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan *E-mail: [email protected]; [email protected] Organic light emitting diodes (OLEDs) have drawn great attention
Samsung introduced a 42” prototype in 2011. In addition to its
in the past two (plus) decades. Ever since the influential and
main application in display technology, OLEDs are also used as a
groundbreaking work by Tang and Van Slyke1 OLEDs have been
lighting source, namely, as white-emitting OLEDs, or WOLEDs2. For
viewed as the next generation flat panel display (FPD) technology
lighting purposes, WOLEDs have to meet stringent requirements.
as they offer several advantages for self-emitting displays, such
To compete with the fluorescent tube as a lighting source, a power
as a wide viewing angle (almost 180 º), a thin panel (< 2 mm),
efficiency of > 70 lm/W is preferred and the lifetime must be at
light weight, a fast response time (microseconds and less), bright
least 10 000 h @ 1000 cd/m2, with a color rendering index (CRI)
emission, and high contrast. Moreover, they can be made on flexible
greater than 80 %. For the OLED display, it is expected that the
substrates, and are thus highly versatile. In reality, OLEDs have
turning point will be around 2014 when the yield rate of production is
already been incorporated into some commercial products, like
expected to be greater than 70 %. The consequent cost-effectiveness,
MP3 players, mobile phones, digital cameras, PDAs, etc. The SONY
brightness, and portability of the OLED will represent a major threat
XEL-1 was the world’s first commercial OLED TV, featuring a 3 mm
to LCD products3.
thick panel, as well as breathtaking image contrast, brightness, and
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The basic OLED structure consists of an organic active layer
color. Later on, LG announced the debut of a 31” prototype and
sandwiched between two electrodes; usually a reducing-metal cathode
planned to have a 55” TV on the production line in 2012, while
and a transparent oxide anode. Upon applying a voltage, the electrons
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Feeling blue? Blue phosphors for OLEDs
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and holes are injected from their respective electrodes, generating
emissive state is in proximity to the metal-centered dπdσ* state, so
excitons, which then recombine at the designated organic layer and
that the phosphorescence is prone to quenching by the repulsive dd
release energy as visible light. Amid development, OLED technology
state via contact with the potential energy surface (PES) with respect
has been evolving from the initial single hetero-junction structure
to that of the ground state. As a result, there is a steep reduction of
into a double hetero-junction structure, taking advantage of the
the emission efficiency and likewise the photostability12. Attaining
separation of the charge transport and light emitting layers into two
blue phosphors with high quantum yields (QY) and better stability
territories. The technology has further evolved into a multi-hetero-
for OLEDs thus requires ingenious design and innovative synthetic
junction architecture, which not only separates the two functions,
pathways. It suffices to say that both the device encapsulation and
but also strictly confines the carriers, resulting in a vast improvement
adjustment of guest-host energy gaps play crucial roles; the latter is
in performance. Parallel to the structural evolution, key components
often sophisticated due to the difficulty of matching the large triplet
with different functions in the OLED have also been substantially
energy gap15.
developed. These include electrodes, electron/hole transports, hosts,
The poor stability of blue phosphors not only causes a shorter
and emitters, among which the emitters are the key issue, drawing
lifetime of the device, but perhaps more severely, it also induces poor
extensive academic and industrial interest. The driving force has been
color stability for white lighting devices. Ideally, there should not be
the development of suitable RGB emitters for both display and solid-
a noticeable color change during operation. In reality, however, the
state lighting sources.
distinctive stability of each emitter induces significant ratiometric color
To meet the above criteria, particularly the leap in efficiency
changes before a device reaches fatality. Although the color instability
and hence an intense light output, phosphorescent emitters (or
could be mitigated by device design, e.g., by using different pixel sizes
phosphors) turn out to be indispensible. Theoretically, an internal
according to their lifetime, practically, it is still of prime importance
quantum efficiency (ηint) as high as 100 % could be achieved for
for obtaining blue phosphorescence with high efficiency and long-term
phosphorescent OLEDs (PhOLED)4,5, so these emitting materials would
stability.
be superior compared to their fluorescent counterparts, for which only
Our aim in this account is to review progress on blue OLED phosphors;
singlet excitons can be harvested, giving an upper efficiency limit of
its past and current status, as well as the future perspectives. Particular
25 %. Likewise, phosphors with third-row transition-metal elements
attention will be paid to those new blue phosphors developed in our
as the core become crucial for the fabrication of PhOLEDs6-9. The
lab, including their design strategy, synthetic assessment, and OLED
strong spin-orbit coupling effectively promotes singlet-to-triplet
performance. A photograph of one such blue-emitting PhOLED is shown
intersystem crossing, and also enhances the subsequent radiative
in Fig. 1 to serve as an example. To be suitable for a general audience,
transition, i.e., phosphorescence, the results of which facilitate strong
both the underlining fundamental and photophysical properties will be
electroluminescence by harnessing both singlet and triplet excitons.
discussed as succinctly as possible. This report also unveils why we “felt
This superiority has led to the continuous trend of shifting research
blue” about research on blue phosphors, in particular, the early stage
endeavors towards these heavy transition-metal based phosphors.
true-blue phosphors, and how we partially circumvented the obstacles,
Among the three primary RGB colors, the synthetic protocols
underscoring the importance of the interplay between chemistry and
and fabrication methods of green and red phosphors to meet the necessary requirements have been well established10-14. Conversely, the design and fabrication of blue phosphors and ensuing devices is still an ongoing challenge. From our viewpoint, three challenges stand out regarding the development of blue phosphorescent emitters, particularly the phosphors: (i) the chromaticity of the most available blue phosphor is not a true-blue color, which gives a poorer color gamut for applications in full color OLED displays; (ii) the emission efficiency of blue phosphors, being inferior to green and red phosphors, has impeded the development of white lighting, because lighting demands a high power efficiency; (iii) the lifetime or long-term stability of phosphors in OLEDs, which is a property relevant to their phosphorescence efficiency and is influenced by their structural design and device architecture. The stability and efficiency of blue phosphors are, in many cases, significantly poorer than their green and red counterparts. One major deactivation mechanism should be ascribed to the fact that the
Fig. 1 Photograph of a blue-emitting PhOLED fabricated by our collaborators working at the Industrial Technology Research Institute of Taiwan.
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Feeling blue? Blue phosphors for OLEDs
photophysics. Nevertheless, complete success is yet to be achieved, but perhaps we are not too far away from some pragmatic solutions.
The evolution of blue phosphors A great deal of credit must be given to Thompson and his co-workers for their seminal and elegant studies on the sky-blue phosphorescent Ir(III) complex (FIrpic, see Fig. 2) and its application in
PhOLEDs16.
Using
a carbazole based CBP as the host material, the power efficiency (ηp)
Compound
abs. λmax (nm)
PL λmax (nm)
Q.Y.
τ (μs)
1
311
430, 457, 480
0.14
18.5
2
247, 292, 320
460
0.26
143
based FCNIr25, bipyridine-based Ir(dfpypy)326, phenyltriazole-based Ir(taz)327 and carbene-based mer-Ir(cn-pmic)15, have also been reported. Evidently, tuning phosphorescence to the true-blue region with ideal
and external quantum efficiency (ηext) were reported to be 10.5 lm/W
CIEx,y coordinates of (0.14, 0.08) is highly challenging. The majority of
and 5.7 %, respectively. Subsequently, the efficiency was found to be
blue phosphors reported demonstrated inferior color chromaticity, with
closely related to the triplet state energy (ET) of the host material. When
the sum of CIEx + CIEy being much greater than 0.3 or with a single CIEy
doped in higher ET host materials like mCP and CDBP, the ηext improved
coordinate being higher than 0.2521, 28. To tackle this problem, we first
to 7.8 % and 10.4 %, respectively17,18. In 2005, Kido and co-workers
synthesized Os(II) complexes known as [Os(fppz)2(CO)2] (1)29,30 and
developed an FIrpic based OLED with a maximum ηp of 37 lm/W and
[Os(CO)3(tfa)(fppz)] (2)31, (see Fig. 3). The fppz chelate tends to exhibit
ηext of 19 % by using novel, high ET host materials such as 4CzPBP,
a much greater intra-ligand charge-transfer (ILCT) or ππ* energy gap
3DTAPBP, and mTPPP (see Fig. 2). The applications of these materials
versus that of the sky-blue emitting 4,6-difluorophenyl pyridinato chelate
are versatile, since they can provide highly balanced hole and electron
(dfppy). The electron-withdrawing CF3 group on fppz also stabilizes the
transport19. They later produced another FIrpic-based OLED with a
pyrazolate-centered highest occupied molecular orbital (HOMO) and
maximum ηp of over 37 lm/W and ηext of 24 % by using a wide energy
enlarges the respective ππ* energy level; thus, a true-blue phosphor could
gap electron transport layer20. Although the efficiencies of FIrpic-based
be realized. As a result, the photophysical data (see Table 1) show better
OLEDs have been improved over the years, a few critical problems need
chromaticity; unfortunately, the associated carbonyl ligands make them
to be addressed. For example, it exhibits 1931 Commission Internationale
electrochemically unstable. Moreover, the 2+ charged Os(II) cation is
de L’Eclairage coordinates, CIEx,y, of (0.16, 0.29), which are still far from
limited by its ligand field strength; therefore, its stability and tunability
the true-blue color (0.14, 0.08). Some endeavors were made to search for
toward shorter wavelengths are restricted compared to the isoelectronic
better blue phosphors by modifying the ancillary chelate of FIrpic. These
Ir(III) complexes and hence, it is unsuitable as a true-blue OLED dopant.
include the second generation FIr621-23, for which the picolinate ancillary
To circumvent the inherent disadvantage of Os(II) complexes, we
of FIrpic was replaced with a chelating pyrazolyl-borate, and FIrtaz
switched to Ir(III) complexes, which provide stronger ligand fields due
and FIrN424, which were chelated by pyridyl triazolate and tetrazolate
to the 3+ charged metal core. Nevertheless, en route to the required
ancillary, respectively. Other blue phosphors, such as phenylpyridine-
blue phosphor, some crucial factors must be optimized. Of prime
Fig. 2 The structures of FIrpic and related host and electron transporting materials.
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Table 1 Photophysical data for Os(II) complexes (1) and (2)
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Fig. 4 The MLCT, ILCT, and LLCT orbital transitions of complex (3). The orbital contour in purple corresponds to the HOMO, while the contour in yellow represents the second orbital above the LUMO (i.e., LUMO+2). Note that the iso-density of the orbital contours was set to 0.05.
density flow of the MLCT, ILCT, and LLCT, using complex (3) as an example (see Fig. 3). From the viewpoint of relaxation dynamics, LLCT may possess a well bounded PES due to the π-electron delocalization and hence the overall bond stabilization, which avoids intersection and/or thermal population to the dd excited state, resulting in an increased QY. Conversely, the associated vibration modes eligible for quenching increase in LLCT, which may facilitate radiationless deactivation and hence decrease QY. Therefore, the optimization of various ligand structures and compositions is necessary before the resulting transition metal complexes can maximize the blue emission efficiency. Fig. 3 Structural drawings of assorted complexes, as described in the text.
concern is increasing the contribution from the metal-to-ligand chargetransfer (MLCT) in the lowest-lying triplet
manifold32,33.
The increase
Based on the above, we have synthesized heteroleptic isomeric iridium complexes [Ir(dfppy)(fppz)2] (3) and (4)36. As demonstrated by the computational analysis (density functional theory, DFT), the
of the MLCT contribution enhances the coupling of the orbital angular
lowest lying state for both complexes (3) and (4) comprises MLCT and
momentum to the electron spin, such that the T1 → S0 transition
ILCT mixed with a substantial LLCT component. As a result, (3) and
would have a large first order spin-orbit coupling term, which would
(4) exhibit phosphorescence maxima at 450 and 480 nm with QYs as
result in a substantial decrease of the radiative lifetime and hence
high as 50 % and 15 %, respectively, supporting the positive effect of
the possibility of increasing the phosphorescence QY34. Moreover,
LLCT suppressing the radiationless pathways. Note that the higher QY
whilst enlarging the emission energy gap, care has to be taken to
in (3) also reflects its larger MLCT percentage (26.6 %) versus that of
avoid touching the metal-centered dd excited state, which may induce
(4) (16.9 %) and hence a radiative decay rate of 1.4 × 105 s-1 (3) versus
radiationless decay or, more seriously, bond dissociation toward the
3.2 × 104 s-1 (4), verifying the above first criterion. Accordingly, an
weakest metal-ligand site35. Third, upon increasing the energy gap
OLED device (see Fig. 5) prepared with complex (3) demonstrated CIE
towards blue, it becomes facile for the lowest-lying excited state,
coordinates of (0.16, 0.18), ηext of 8.5 %, and a peak ηp of 8.5 lmW-1.
which is mainly ILCT and MLCT in character, to mix with a thermally
While the delocalization concept mentioned above is beneficial
accessible ligand-to-ligand charge-transfer (LLCT) transition involving
for the emission QY, the π-delocalization among distinctive chelates
auxiliary chromophores. For clarity, Fig. 4 illustrates the electron
for LLCT, per se, may reduce the energy gap, which has adverse
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Feeling blue? Blue phosphors for OLEDs
Fig. 5 Architecture of the OLED using (3) as the dopant and molecular drawings of the electron or hole transporting material. Reproduced with permission from36. Copyright Wiley 2007.
Table 2 Comparison of photophysical properties between complexes (5) and (6) abs. λmax (nm)
PL λmax (nm)
Q.Y. (Φ)
τobs (ns)
kr (×105 s-1)
[Ir(dfb-pz)2(fptz)] (5)
261, 368
437
0.1
100
10
[Ir(dfpz)2(fptz)] (6)
300, 349
457
0.0046
8.6
5.4
Compound
consequences for reaching true-blue emission. We then proceeded to
substantial LLCT contribution38. The low emission QY for complex (6)
design a class of Ir(III) complexes, for which the (lowest lying) emitting
(c.f., 5) is consistent with the studies by Thompson and co-workers
state was restrained to a single chromophore. Experimentally, we were
on the temperature dependence of Ir(III) complexes based on the
keen to exploit fppz or an analogous chelating triazolate (c.f., fptz) as
N-phenylpyrazole chelate (ppz)39. It was found that Ir(ppz)3 is non-
the key chromophore, thanks to their high energy gaps and versatility
emissive at room temperature due to the small activation energy to a
in chemical modification. Using the aforementioned Ir(III) complexes
non-emissive dd excited state.
(3) and (4) as the prototypes, a way to reduce the involvement of
476
Note that the much lower QY for (6), to a certain extent, reflects
the auxiliary dfppy ligand is to cut off the π-conjugation between the
the negative effect of LLCT compared to complexes (3) and (4) (see
2,4-difluorophenyl and pyridyl moieties. Thus, we proceeded to study
above). Thus, the counteracting effect of LLCT on the QY seems to vary
the so-called nonconjugated chelate, in which the two designated
case by case, the trend of which is unfortunately still unpredictable at
moieties are strategically linked by a methylene group to reduce the
this stage11,12. Nevertheless, via an ingenious chemical design, if the
cross-talk. Accordingly, a blue phosphorescent Ir(III) complex (5) with
LLCT is intentionally placed in the higher excited state near to the dd
nonconjugated N-benzylpyrazole (dfb-pz) ligands was synthesized37.
state, LLCT with a steeper PES should be able to by-pass/cross the dd
Table 2 lists the comparison between compound (5) with dual dfb-pz
repulsive PES. In this case, the bond dissociation occurring in the higher
chelates and its counterpart (6) that possesses N-phenylpyrazole
excited states can be mitigated, increasing the emission efficiency of
chelates (dfpz)38. Clearly, the higher emission QY for (5), versus
the blue phosphor. Together with a single emitting chromophore, i.e.,
that of (6), proves the concept of non-conjugating auxiliary ligands,
ILCT (mixed with MLCT) that may also be easily chemically modified, it
which is also verified by DFT calculation, confirming that only the
is thus possible to produce a blue phosphor possessing a true-blue hue,
ILCT of 2-pyridyl triazolate fragment (fptz) in (5) is involved in
high emission yield, and great stability. A conceptual design strategy
the emitting state, while the lowest lying state for (6) comprises a
for such a system is depicted in Fig. 6.
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Fig. 6 Concept for obtaining blue phosphors with a high quantum yield and photostability.
Bearing this concept in mind, we reported modified blue phosphors
[Ir(fppz)2(P^N)] (9), were used in the fabrication of OLED devices. Of
employing benzyldiphenylphosphine and its difluorinated analogue
particular interest was the (9)-doped OLEDs, which exhibited remarkable
as the ancillary chelate40. As for the strategy, cyclometalation of
peak ηext, ηl, and ηp values of 6.9 %, 8.1 cdA-1, and 4.9 lmW-1, and with
benzyldiphenylphosphine can produce the required anionic chelate.
a true-blue chromaticity CIEx,y = (0.16, 0.15).
Moreover, both the PPh2 unit and the non-conjugated benzyl group
An additional strategy to achieve efficient blue emission was to use
exert the greatest ligand field stabilization energy as well as the
high-field-strength ligands such as NHC carbenes15,32. In this contribution,
largest ππ* energy gap among all established chelates41. A deep-blue
we have reported the use of benzyl carbene chelate and allowed it
phosphor, possessing a superior emission QY, was thus anticipated.
to react with [IrCl3(tht)3] in synthesizing a series of Ir(III) complexes,
In this approach, a heteroleptic complex [Ir(dfppy)2(dfbdp)] (7) was
[(fbmb)2Ir(bptz)] (10) and [(dfbmb)2Ir(fptz)] (11)43. Complexes (10)
successfully isolated, which showed an improved efficiency with
and (11) exhibited blue emission λmax at 460 and 458 nm, and QYs of
respect to its chloride precursor, [Ir(dfppy)2(dfbdpH)Cl], as well as two
0.22 and 0.73, respectively. Complex (11) was used as the dopant for
blue-shifted emission maxima at 457 and 480 nm versus those of
the fabrication of blue PhOLEDs due to its superior emission quantum
standard FIrpic (470 and 490 nm). An OLED device fabricated with (7)
efficiency. The respective device configuration and materials used were:
demonstrated a low turn-on voltage (defined as the voltage obtained at
ITO/α-NPD (30 nm)/TCTA (20 nm)/CzSi (3 nm)/CzSi: 6 % (11) (3 nm)/
1 cd m-2) of 4.6 V. The peak ηext, ηl, and ηp are approximately 10.24 %,
UGH2 (2 nm)/BCP (50 nm)/CsCO3 (2 nm)/Al (150 nm). Figs. 7a and
15.95
cdA-1,
and 10.07
lmW-1,
respectively. The CIE coordinates
7b show the electroluminescence (EL) spectrum and the corresponding
calculated from the resulting spectrum are (0.16, 0.20), which is more
CIE color coordinates, while Figs. 7c and 7d depict the current-voltage-
blue-shifted compared to the majority of dfppy based phosphors.
luminance (I-V-L) characteristics and the EL efficiencies of the device. The
A parallel study resulted in another class of Ir(III) complexes
EL spectrum exhibits two distinct emission peaks at 434 and 460 nm. The
with either dfppy or chelating azolate chromophores, plus one non-
CIEx,y coordinates were (0.16, 0.13). The respective peak ηext and ηp were
conjugated phosphine chelate42 The non-conjugated phosphine
6.0 % and 4.0 lmW-1, but dropped to about 2.7 % and 0.9 lmW-1 at a
chelate not only greatly restricted its participation in the lowest-lying
higher current density. Despite the common efficiency roll-offs, their color
electronic transition but also enhanced the coordination strength.
chromaticities were far better than other PhOLEDs fabricated from well-
These two factors led to authentic blue phosphorescence as well as
known blue phosphors such as FIrpic17, FIr621, FIrtaz24, and FIrN424.
suppressed nonradiative deactivation, thus improving the emission efficiency. Thus, the blue-emitting complexes [Ir(dfpbpy)2(P^N)] (8) and
Finally, we move on to the recent discovery of heteroleptic Ir(III) complexes with a tripodal, facially coordinated phosphite (or phosphonite)
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Feeling blue? Blue phosphors for OLEDs
Fig. 7 (a) EL spectrum, (b) CIE chromaticity coordinates, (c) I-V-L characteristics (J = current density, L = brightness), and (d) external quantum efficiency (ηext) versus L for the device containing dopant (11). Reproduced with permission from43. © Wiley 2008.
(fppz)H = 3-(trifluoromethyl)-5-(2-pyridyl) pyrazole tfa = trifluoroacetate dfbdpH = 4,6-difluorobenzyl diphenylphosphine (dfpbpy)H = 2-(4,6-difluorophenyl-4-tert-butylpyridine) (P^N)H = 5-(diphenylphosphinomethyl)-3-trifluoromethylpyrazole) tht = tetrahydrothiophene fbmbH = 1-(4-fluorobenzyl)-3-methylbenzimidazolium dfbmbH = 1-(2,4,-difluorobenzyl)-3-methylbenzimidazolium) ITO = indium tin oxide TAPC = di[4-N,N-ditolylamino]phenyl)cyclohexane TCTA = 4,4’,4”-tris(carbazol-9-yl)-triphenylamine CzSi = 9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole UGH2 = p-bis(triphenylsilyl)benzene TmPyPB = 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene)
Fig. 8 Reaction protocol that provided the blue-emitting Ir(III) complexes with the tripodal ancillary ligand.
(denoted as P^C2) to serve as the ancillary ligand44. In addition to
by using double emitting layers and double buffer layers to balance the
fulfilling the criteria discussed above, of both non-conjugation and a
charge transport and to move the exciton-formation zone away from
strong ligand field, the tridentate ligand offers great binding stability.
the adjacent carrier-transport layers. The device demonstrated a turn-
As a result, highly efficient blue phosphorescence is attained with
on voltage of 4.1 V, and peak ηext, ηl, and ηp values of 11.0 %, 22.3
good OLED performance. The synthetic protocol of this class of Ir(III)
cdA-1, and 16.7 lmW-1, respectively.
complexes, (12) and (13), is depicted in Fig. 8. Complex (13) was chosen
478
for the fabrication of an OLED device due to its high solid state emission
Concluding remarks
QY of 0.97. The device configuration and materials being used were:
Despite the intensive progress on blue phosphors that has been made, the
ITO/TAPC (30 nm)/TCTA (10 nm)/CzSi (3 nm)/CzSi: 4 % (13) (25nm)/
design of a highly efficient blue phosphor based on a third-row transition
UGH2: 4 % (13) (2nm)/TmPyPB (50nm)/LiF (0.8nm)/Al (150 nm). The
metal is still a challenge today. We have demonstrated that pushing the
wide-gap host CzSi and UGH2, which have a triplet energy gap of 3.02
emission gap towards the authentic blue region requires not only ingenious
eV and 3.08 eV, respectively, were employed for optimal efficiency. In
molecular design but also consideration of the subtle interference between
addition, good confinement of the excitons and carriers was realized
various low-lying electronic states. The chelates chosen for pursuing the
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blue phosphor include: (i) C^N chelates such as 2,4-difluorophenylpyridine,
the case of fluorescence emitters, it has been proposed that a single
(ii) N^N chelates like 2-pyridyl azolate, (iii) C^C chelates comprising both
system utilizing both normal Franck-Condon and exciplex emissions may
C-H site and neutral NHC carbene fragment, (iv) C^P chelates with both
accomplish this goal45-48. A similar strategy, however, is not attainable
benzylic C-H site and neutral phosphine donor, and (v) N^P chelates with
for the Ir(III) complexes because of the octahedral configuration. Since
an azolic N-H group11. These cyclometalating chelates, in theory, may
the heavy atom effect and hence the spin-orbit coupling is empirically
afford a series of Ir(III) based phosphors with photophysical properties
inversely proportional to r6, where r is the distance between the emissive
tuned specifically for the fabrication of PhOLEDs.
chromophore and core Ir(III) element, one approach is to de-emphasize
We have systematically pushed the chromaticity of phosphors closer
the spin-orbit coupling by elongating their spatial separation49,50. If the
to the true-blue region. However, even if they reach the authentic
S1-Tm (m ≥ 1) intersystem crossing has a low efficiency for a designated
blue color, the close proximity between the metal-centered dd and
Ir(III) complex and the fluorescence yield is virtually 100 % upon optical
the emissive states may inevitably lead to a lower emission efficiency
excitation, the electrical pumping (in OLED applications) may then give
and photostability. These inferiorities make us feel somewhat “blue”
25 % and 75 % population in the S1 and T1 states, respectively. As
about attaining a true-blue hue, though some proposed mechanisms
opposed to the null phosphorescence upon optical excitation, this 75 %
such as increasing the ligand-field strength or placing LLCT near the dd
T1 population may exhibit phosphorescence due to the heavy atom effect,
state may partly improve the performance. An alternative solution for
producing dual emission (fluorescence + phosphorescence). On the basis
achieving more stable OLEDs may rely on the sky-blue phosphor, for
of proper chemical derivation, white light generation is not impossible,
which the excited state is lower in energy which would thus strengthen
per se. Of course, such a system may be limited by the maximal 25 %
the photostability. Moreover, by selecting better host/charge transport
fluorescence yield, considering white light generation. Another possibility
materials and optimizing the OLED device structure, we believe that these
may count on the design of transition-metal complexes that show
sky-blue phosphors, together with orange ones, have latent potential for
fast isomerization at the triplet manifolds. What if the isomerization is
making all phosphorescent WOLEDs.
adiabatic, meaning the reaction is along the triplet PES, with a certain
Nevertheless, white light generation using multiple chromophores,
barrier, so that the system exhibits dual phosphorescence covering a
namely RGB triads or dual colors, such as sky-blue plus orange (or blue
gamut of the visible spectrum? We believe that such a system should be
plus yellow), would still suffer from relative-stability issues due to the
both theoretically and synthetically feasible. Certainly, more ingenious
intrinsically different lifespans of different materials. In theory, a single
and original concepts should be available, and we hope this review will
system emitting white light could ultimately overcome this hurdle. In
stimulate and encourage the OLED community.
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Tang, C. W., and VanSlyke, S. A., Appl Phys Lett (1987) 51, 913. Zhou, G., et al., J Photochem Photobio C (2010) 11, 133. Tsai, J. -H., and Chen, L. -R., Wealth Magazine (2011) 373, 66. Kawamura, Y., et al., Appl Phys Lett (2005) 86, 071104. Adachi, C., et al., J Appl Phys Lett (2001) 90, 5048. Baldo, M. A., et al., Nature (1998) 395, 151. Holder, E., et al, Adv Mater (2005) 17, 1109. Gong, X., et al., Appl Phys Lett (2002) 81, 3711. Wong, W. -Y., and Ho, C. -L., J Mater Chem (2009) 19, 4457. Chou, P. -T., and Chi, Y., Chem-Eur J (2007) 13, 380. Chi, Y., and Chou, P. -T., Chem Soc Rev (2010) 39, 638. Chou, P. -T. et al., Coord Chem Rev (2011), doi:10.1016/j.ccr.2010.12.013 Wong, W. -Y., and Ho, C. -L., Coord Chem Rev (2009) 253, 1709. Zhou, G., et al., Chem Asian J (2011) 6, 1706. Haneder, S., et al., Adv Mater (2008) 20, 3325. Adachi, C., et al., Appl Phys Lett (2001) 79, 2082. Holmes, R. J., et al., Appl Phys Lett (2003) 82, 2422. Tokito, S., et al., Appl Phys Lett (2003) 83, 569. Sasabe, H., and Kido, J., Chem Mater (2011) 23, 621. Sasabe, H., et al., J Chem Mater (2008) 20 5951. Holmes, R. J., et al., Appl Phys Lett (2003) 82, 3818. Ren, X., et al., Chem Mater (2004) 16, 4773. Li, J., et al., Polyhedron (2004) 23, 419. Yeh, S. J., et al., Adv Mater (2005) 17, 285.
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Jeon, S. O., et al., Adv Funct Mater (2009) 19, 3644. Lee, S. J., et al., Inorg Chem (2009) 48, 1030. Lo, S. -C., et al., Chem Mater (2006) 18, 5119. Zheng, Y., et al., Appl Phys Lett (2008) 92, 223301. Wu, P. -C., et al., Organometallics (2003) 22, 4938. Yu, J. -K., et al., Chem-Eur J (2004) 10, 6255. Cheng, Y. -M., et al., Inorg Chem (2007) 46, 10276. Sajoto, T., et al., Inorg Chem (2005) 44, 7992. Li, J., et al., Inorg Chem (2005) 44, 1713. Hsu, C. -W., et al., J Am Chem Soc (2011) 133, 12085. Yeh, Y. -S., et al., ChemPhysChem (2006) 7, 2294. Yang, C. -H., et al., Angew Chem Int Ed (2007) 46, 2418. Song, Y. -H., et al., Chem-Eur J (2008) 14, 5423. Yang, C. -H., et al., Inorg Chem (2005) 44, 7770. Sajoto, T., et al., J Am Chem Soc (2009) 131, 9813. Hung, J. -Y., et al., Dalton Trans (2009) 38, 6472. Chiu, C. -L., et al., Adv Mater (2009) 21, 2221. Chiu, Y. -C., et al., ACS Appl Mater Inter (2009) 2, 433. Chang, C. -F., et al., Angew Chem Int Ed (2008) 47, 4524. Lin, C. -H., et al., Angew Chem Int Ed (2011) 50, 3182. Zhou, G., et al., Chem Commun (2009) 45, 3574. Zhou, G., et al., J Mater Chem (2010) 20, 7472. Williams, E. L., et al., Adv Mater (2007) 19, 197. Kalinowski, J., et al., Adv Mater (2007) 19, 4000. Chen, Y. -L., et al., ChemPhysChem (2005) 6, 2012. Kozhevnikov, D. N., et al., Inorg Chem (2011) 50, 3804.
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Samsung introduced a 42” prototype in 2011. In addition to its
in the past two (plus) decades. Ever since the influential and
main application in display technology, OLEDs are also used as a
groundbreaking work by Tang and Van Slyke1 OLEDs have been
lighting source, namely, as white-emitting OLEDs, or WOLEDs2. For
viewed as the next generation flat panel display (FPD) technology
lighting purposes, WOLEDs have to meet stringent requirements.
as they offer several advantages for self-emitting displays, such
To compete with the fluorescent tube as a lighting source, a power
as a wide viewing angle (almost 180 º), a thin panel (< 2 mm),
efficiency of > 70 lm/W is preferred and the lifetime must be at
light weight, a fast response time (microseconds and less), bright
least 10 000 h @ 1000 cd/m2, with a color rendering index (CRI)
emission, and high contrast. Moreover, they can be made on flexible
greater than 80 %. For the OLED display, it is expected that the
substrates, and are thus highly versatile. In reality, OLEDs have
turning point will be around 2014 when the yield rate of production is
already been incorporated into some commercial products, like
expected to be greater than 70 %. The consequent cost-effectiveness,
MP3 players, mobile phones, digital cameras, PDAs, etc. The SONY
brightness, and portability of the OLED will represent a major threat
XEL-1 was the world’s first commercial OLED TV, featuring a 3 mm
to LCD products3.
thick panel, as well as breathtaking image contrast, brightness, and
472
The basic OLED structure consists of an organic active layer
color. Later on, LG announced the debut of a 31” prototype and
sandwiched between two electrodes; usually a reducing-metal cathode
planned to have a 55” TV on the production line in 2012, while
and a transparent oxide anode. Upon applying a voltage, the electrons
OCTOBER 2011 | VOLUME 14 | NUMBER 10
ISSN:1369 7021 © Elsevier Ltd 2011
Feeling blue? Blue phosphors for OLEDs
REVIEW
and holes are injected from their respective electrodes, generating
emissive state is in proximity to the metal-centered dπdσ* state, so
excitons, which then recombine at the designated organic layer and
that the phosphorescence is prone to quenching by the repulsive dd
release energy as visible light. Amid development, OLED technology
state via contact with the potential energy surface (PES) with respect
has been evolving from the initial single hetero-junction structure
to that of the ground state. As a result, there is a steep reduction of
into a double hetero-junction structure, taking advantage of the
the emission efficiency and likewise the photostability12. Attaining
separation of the charge transport and light emitting layers into two
blue phosphors with high quantum yields (QY) and better stability
territories. The technology has further evolved into a multi-hetero-
for OLEDs thus requires ingenious design and innovative synthetic
junction architecture, which not only separates the two functions,
pathways. It suffices to say that both the device encapsulation and
but also strictly confines the carriers, resulting in a vast improvement
adjustment of guest-host energy gaps play crucial roles; the latter is
in performance. Parallel to the structural evolution, key components
often sophisticated due to the difficulty of matching the large triplet
with different functions in the OLED have also been substantially
energy gap15.
developed. These include electrodes, electron/hole transports, hosts,
The poor stability of blue phosphors not only causes a shorter
and emitters, among which the emitters are the key issue, drawing
lifetime of the device, but perhaps more severely, it also induces poor
extensive academic and industrial interest. The driving force has been
color stability for white lighting devices. Ideally, there should not be
the development of suitable RGB emitters for both display and solid-
a noticeable color change during operation. In reality, however, the
state lighting sources.
distinctive stability of each emitter induces significant ratiometric color
To meet the above criteria, particularly the leap in efficiency
changes before a device reaches fatality. Although the color instability
and hence an intense light output, phosphorescent emitters (or
could be mitigated by device design, e.g., by using different pixel sizes
phosphors) turn out to be indispensible. Theoretically, an internal
according to their lifetime, practically, it is still of prime importance
quantum efficiency (ηint) as high as 100 % could be achieved for
for obtaining blue phosphorescence with high efficiency and long-term
phosphorescent OLEDs (PhOLED)4,5, so these emitting materials would
stability.
be superior compared to their fluorescent counterparts, for which only
Our aim in this account is to review progress on blue OLED phosphors;
singlet excitons can be harvested, giving an upper efficiency limit of
its past and current status, as well as the future perspectives. Particular
25 %. Likewise, phosphors with third-row transition-metal elements
attention will be paid to those new blue phosphors developed in our
as the core become crucial for the fabrication of PhOLEDs6-9. The
lab, including their design strategy, synthetic assessment, and OLED
strong spin-orbit coupling effectively promotes singlet-to-triplet
performance. A photograph of one such blue-emitting PhOLED is shown
intersystem crossing, and also enhances the subsequent radiative
in Fig. 1 to serve as an example. To be suitable for a general audience,
transition, i.e., phosphorescence, the results of which facilitate strong
both the underlining fundamental and photophysical properties will be
electroluminescence by harnessing both singlet and triplet excitons.
discussed as succinctly as possible. This report also unveils why we “felt
This superiority has led to the continuous trend of shifting research
blue” about research on blue phosphors, in particular, the early stage
endeavors towards these heavy transition-metal based phosphors.
true-blue phosphors, and how we partially circumvented the obstacles,
Among the three primary RGB colors, the synthetic protocols
underscoring the importance of the interplay between chemistry and
and fabrication methods of green and red phosphors to meet the necessary requirements have been well established10-14. Conversely, the design and fabrication of blue phosphors and ensuing devices is still an ongoing challenge. From our viewpoint, three challenges stand out regarding the development of blue phosphorescent emitters, particularly the phosphors: (i) the chromaticity of the most available blue phosphor is not a true-blue color, which gives a poorer color gamut for applications in full color OLED displays; (ii) the emission efficiency of blue phosphors, being inferior to green and red phosphors, has impeded the development of white lighting, because lighting demands a high power efficiency; (iii) the lifetime or long-term stability of phosphors in OLEDs, which is a property relevant to their phosphorescence efficiency and is influenced by their structural design and device architecture. The stability and efficiency of blue phosphors are, in many cases, significantly poorer than their green and red counterparts. One major deactivation mechanism should be ascribed to the fact that the
Fig. 1 Photograph of a blue-emitting PhOLED fabricated by our collaborators working at the Industrial Technology Research Institute of Taiwan.
OCTOBER 2011 | VOLUME 14 | NUMBER 10
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Feeling blue? Blue phosphors for OLEDs
photophysics. Nevertheless, complete success is yet to be achieved, but perhaps we are not too far away from some pragmatic solutions.
The evolution of blue phosphors A great deal of credit must be given to Thompson and his co-workers for their seminal and elegant studies on the sky-blue phosphorescent Ir(III) complex (FIrpic, see Fig. 2) and its application in
PhOLEDs16.
Using
a carbazole based CBP as the host material, the power efficiency (ηp)
Compound
abs. λmax (nm)
PL λmax (nm)
Q.Y.
τ (μs)
1
311
430, 457, 480
0.14
18.5
2
247, 292, 320
460
0.26
143
based FCNIr25, bipyridine-based Ir(dfpypy)326, phenyltriazole-based Ir(taz)327 and carbene-based mer-Ir(cn-pmic)15, have also been reported. Evidently, tuning phosphorescence to the true-blue region with ideal
and external quantum efficiency (ηext) were reported to be 10.5 lm/W
CIEx,y coordinates of (0.14, 0.08) is highly challenging. The majority of
and 5.7 %, respectively. Subsequently, the efficiency was found to be
blue phosphors reported demonstrated inferior color chromaticity, with
closely related to the triplet state energy (ET) of the host material. When
the sum of CIEx + CIEy being much greater than 0.3 or with a single CIEy
doped in higher ET host materials like mCP and CDBP, the ηext improved
coordinate being higher than 0.2521, 28. To tackle this problem, we first
to 7.8 % and 10.4 %, respectively17,18. In 2005, Kido and co-workers
synthesized Os(II) complexes known as [Os(fppz)2(CO)2] (1)29,30 and
developed an FIrpic based OLED with a maximum ηp of 37 lm/W and
[Os(CO)3(tfa)(fppz)] (2)31, (see Fig. 3). The fppz chelate tends to exhibit
ηext of 19 % by using novel, high ET host materials such as 4CzPBP,
a much greater intra-ligand charge-transfer (ILCT) or ππ* energy gap
3DTAPBP, and mTPPP (see Fig. 2). The applications of these materials
versus that of the sky-blue emitting 4,6-difluorophenyl pyridinato chelate
are versatile, since they can provide highly balanced hole and electron
(dfppy). The electron-withdrawing CF3 group on fppz also stabilizes the
transport19. They later produced another FIrpic-based OLED with a
pyrazolate-centered highest occupied molecular orbital (HOMO) and
maximum ηp of over 37 lm/W and ηext of 24 % by using a wide energy
enlarges the respective ππ* energy level; thus, a true-blue phosphor could
gap electron transport layer20. Although the efficiencies of FIrpic-based
be realized. As a result, the photophysical data (see Table 1) show better
OLEDs have been improved over the years, a few critical problems need
chromaticity; unfortunately, the associated carbonyl ligands make them
to be addressed. For example, it exhibits 1931 Commission Internationale
electrochemically unstable. Moreover, the 2+ charged Os(II) cation is
de L’Eclairage coordinates, CIEx,y, of (0.16, 0.29), which are still far from
limited by its ligand field strength; therefore, its stability and tunability
the true-blue color (0.14, 0.08). Some endeavors were made to search for
toward shorter wavelengths are restricted compared to the isoelectronic
better blue phosphors by modifying the ancillary chelate of FIrpic. These
Ir(III) complexes and hence, it is unsuitable as a true-blue OLED dopant.
include the second generation FIr621-23, for which the picolinate ancillary
To circumvent the inherent disadvantage of Os(II) complexes, we
of FIrpic was replaced with a chelating pyrazolyl-borate, and FIrtaz
switched to Ir(III) complexes, which provide stronger ligand fields due
and FIrN424, which were chelated by pyridyl triazolate and tetrazolate
to the 3+ charged metal core. Nevertheless, en route to the required
ancillary, respectively. Other blue phosphors, such as phenylpyridine-
blue phosphor, some crucial factors must be optimized. Of prime
Fig. 2 The structures of FIrpic and related host and electron transporting materials.
474
Table 1 Photophysical data for Os(II) complexes (1) and (2)
OCTOBER 2011 | VOLUME 14 | NUMBER 10
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Fig. 4 The MLCT, ILCT, and LLCT orbital transitions of complex (3). The orbital contour in purple corresponds to the HOMO, while the contour in yellow represents the second orbital above the LUMO (i.e., LUMO+2). Note that the iso-density of the orbital contours was set to 0.05.
density flow of the MLCT, ILCT, and LLCT, using complex (3) as an example (see Fig. 3). From the viewpoint of relaxation dynamics, LLCT may possess a well bounded PES due to the π-electron delocalization and hence the overall bond stabilization, which avoids intersection and/or thermal population to the dd excited state, resulting in an increased QY. Conversely, the associated vibration modes eligible for quenching increase in LLCT, which may facilitate radiationless deactivation and hence decrease QY. Therefore, the optimization of various ligand structures and compositions is necessary before the resulting transition metal complexes can maximize the blue emission efficiency. Fig. 3 Structural drawings of assorted complexes, as described in the text.
concern is increasing the contribution from the metal-to-ligand chargetransfer (MLCT) in the lowest-lying triplet
manifold32,33.
The increase
Based on the above, we have synthesized heteroleptic isomeric iridium complexes [Ir(dfppy)(fppz)2] (3) and (4)36. As demonstrated by the computational analysis (density functional theory, DFT), the
of the MLCT contribution enhances the coupling of the orbital angular
lowest lying state for both complexes (3) and (4) comprises MLCT and
momentum to the electron spin, such that the T1 → S0 transition
ILCT mixed with a substantial LLCT component. As a result, (3) and
would have a large first order spin-orbit coupling term, which would
(4) exhibit phosphorescence maxima at 450 and 480 nm with QYs as
result in a substantial decrease of the radiative lifetime and hence
high as 50 % and 15 %, respectively, supporting the positive effect of
the possibility of increasing the phosphorescence QY34. Moreover,
LLCT suppressing the radiationless pathways. Note that the higher QY
whilst enlarging the emission energy gap, care has to be taken to
in (3) also reflects its larger MLCT percentage (26.6 %) versus that of
avoid touching the metal-centered dd excited state, which may induce
(4) (16.9 %) and hence a radiative decay rate of 1.4 × 105 s-1 (3) versus
radiationless decay or, more seriously, bond dissociation toward the
3.2 × 104 s-1 (4), verifying the above first criterion. Accordingly, an
weakest metal-ligand site35. Third, upon increasing the energy gap
OLED device (see Fig. 5) prepared with complex (3) demonstrated CIE
towards blue, it becomes facile for the lowest-lying excited state,
coordinates of (0.16, 0.18), ηext of 8.5 %, and a peak ηp of 8.5 lmW-1.
which is mainly ILCT and MLCT in character, to mix with a thermally
While the delocalization concept mentioned above is beneficial
accessible ligand-to-ligand charge-transfer (LLCT) transition involving
for the emission QY, the π-delocalization among distinctive chelates
auxiliary chromophores. For clarity, Fig. 4 illustrates the electron
for LLCT, per se, may reduce the energy gap, which has adverse
OCTOBER 2011 | VOLUME 14 | NUMBER 10
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Feeling blue? Blue phosphors for OLEDs
Fig. 5 Architecture of the OLED using (3) as the dopant and molecular drawings of the electron or hole transporting material. Reproduced with permission from36. Copyright Wiley 2007.
Table 2 Comparison of photophysical properties between complexes (5) and (6) abs. λmax (nm)
PL λmax (nm)
Q.Y. (Φ)
τobs (ns)
kr (×105 s-1)
[Ir(dfb-pz)2(fptz)] (5)
261, 368
437
0.1
100
10
[Ir(dfpz)2(fptz)] (6)
300, 349
457
0.0046
8.6
5.4
Compound
consequences for reaching true-blue emission. We then proceeded to
substantial LLCT contribution38. The low emission QY for complex (6)
design a class of Ir(III) complexes, for which the (lowest lying) emitting
(c.f., 5) is consistent with the studies by Thompson and co-workers
state was restrained to a single chromophore. Experimentally, we were
on the temperature dependence of Ir(III) complexes based on the
keen to exploit fppz or an analogous chelating triazolate (c.f., fptz) as
N-phenylpyrazole chelate (ppz)39. It was found that Ir(ppz)3 is non-
the key chromophore, thanks to their high energy gaps and versatility
emissive at room temperature due to the small activation energy to a
in chemical modification. Using the aforementioned Ir(III) complexes
non-emissive dd excited state.
(3) and (4) as the prototypes, a way to reduce the involvement of
476
Note that the much lower QY for (6), to a certain extent, reflects
the auxiliary dfppy ligand is to cut off the π-conjugation between the
the negative effect of LLCT compared to complexes (3) and (4) (see
2,4-difluorophenyl and pyridyl moieties. Thus, we proceeded to study
above). Thus, the counteracting effect of LLCT on the QY seems to vary
the so-called nonconjugated chelate, in which the two designated
case by case, the trend of which is unfortunately still unpredictable at
moieties are strategically linked by a methylene group to reduce the
this stage11,12. Nevertheless, via an ingenious chemical design, if the
cross-talk. Accordingly, a blue phosphorescent Ir(III) complex (5) with
LLCT is intentionally placed in the higher excited state near to the dd
nonconjugated N-benzylpyrazole (dfb-pz) ligands was synthesized37.
state, LLCT with a steeper PES should be able to by-pass/cross the dd
Table 2 lists the comparison between compound (5) with dual dfb-pz
repulsive PES. In this case, the bond dissociation occurring in the higher
chelates and its counterpart (6) that possesses N-phenylpyrazole
excited states can be mitigated, increasing the emission efficiency of
chelates (dfpz)38. Clearly, the higher emission QY for (5), versus
the blue phosphor. Together with a single emitting chromophore, i.e.,
that of (6), proves the concept of non-conjugating auxiliary ligands,
ILCT (mixed with MLCT) that may also be easily chemically modified, it
which is also verified by DFT calculation, confirming that only the
is thus possible to produce a blue phosphor possessing a true-blue hue,
ILCT of 2-pyridyl triazolate fragment (fptz) in (5) is involved in
high emission yield, and great stability. A conceptual design strategy
the emitting state, while the lowest lying state for (6) comprises a
for such a system is depicted in Fig. 6.
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Feeling blue? Blue phosphors for OLEDs
REVIEW
Fig. 6 Concept for obtaining blue phosphors with a high quantum yield and photostability.
Bearing this concept in mind, we reported modified blue phosphors
[Ir(fppz)2(P^N)] (9), were used in the fabrication of OLED devices. Of
employing benzyldiphenylphosphine and its difluorinated analogue
particular interest was the (9)-doped OLEDs, which exhibited remarkable
as the ancillary chelate40. As for the strategy, cyclometalation of
peak ηext, ηl, and ηp values of 6.9 %, 8.1 cdA-1, and 4.9 lmW-1, and with
benzyldiphenylphosphine can produce the required anionic chelate.
a true-blue chromaticity CIEx,y = (0.16, 0.15).
Moreover, both the PPh2 unit and the non-conjugated benzyl group
An additional strategy to achieve efficient blue emission was to use
exert the greatest ligand field stabilization energy as well as the
high-field-strength ligands such as NHC carbenes15,32. In this contribution,
largest ππ* energy gap among all established chelates41. A deep-blue
we have reported the use of benzyl carbene chelate and allowed it
phosphor, possessing a superior emission QY, was thus anticipated.
to react with [IrCl3(tht)3] in synthesizing a series of Ir(III) complexes,
In this approach, a heteroleptic complex [Ir(dfppy)2(dfbdp)] (7) was
[(fbmb)2Ir(bptz)] (10) and [(dfbmb)2Ir(fptz)] (11)43. Complexes (10)
successfully isolated, which showed an improved efficiency with
and (11) exhibited blue emission λmax at 460 and 458 nm, and QYs of
respect to its chloride precursor, [Ir(dfppy)2(dfbdpH)Cl], as well as two
0.22 and 0.73, respectively. Complex (11) was used as the dopant for
blue-shifted emission maxima at 457 and 480 nm versus those of
the fabrication of blue PhOLEDs due to its superior emission quantum
standard FIrpic (470 and 490 nm). An OLED device fabricated with (7)
efficiency. The respective device configuration and materials used were:
demonstrated a low turn-on voltage (defined as the voltage obtained at
ITO/α-NPD (30 nm)/TCTA (20 nm)/CzSi (3 nm)/CzSi: 6 % (11) (3 nm)/
1 cd m-2) of 4.6 V. The peak ηext, ηl, and ηp are approximately 10.24 %,
UGH2 (2 nm)/BCP (50 nm)/CsCO3 (2 nm)/Al (150 nm). Figs. 7a and
15.95
cdA-1,
and 10.07
lmW-1,
respectively. The CIE coordinates
7b show the electroluminescence (EL) spectrum and the corresponding
calculated from the resulting spectrum are (0.16, 0.20), which is more
CIE color coordinates, while Figs. 7c and 7d depict the current-voltage-
blue-shifted compared to the majority of dfppy based phosphors.
luminance (I-V-L) characteristics and the EL efficiencies of the device. The
A parallel study resulted in another class of Ir(III) complexes
EL spectrum exhibits two distinct emission peaks at 434 and 460 nm. The
with either dfppy or chelating azolate chromophores, plus one non-
CIEx,y coordinates were (0.16, 0.13). The respective peak ηext and ηp were
conjugated phosphine chelate42 The non-conjugated phosphine
6.0 % and 4.0 lmW-1, but dropped to about 2.7 % and 0.9 lmW-1 at a
chelate not only greatly restricted its participation in the lowest-lying
higher current density. Despite the common efficiency roll-offs, their color
electronic transition but also enhanced the coordination strength.
chromaticities were far better than other PhOLEDs fabricated from well-
These two factors led to authentic blue phosphorescence as well as
known blue phosphors such as FIrpic17, FIr621, FIrtaz24, and FIrN424.
suppressed nonradiative deactivation, thus improving the emission efficiency. Thus, the blue-emitting complexes [Ir(dfpbpy)2(P^N)] (8) and
Finally, we move on to the recent discovery of heteroleptic Ir(III) complexes with a tripodal, facially coordinated phosphite (or phosphonite)
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Feeling blue? Blue phosphors for OLEDs
Fig. 7 (a) EL spectrum, (b) CIE chromaticity coordinates, (c) I-V-L characteristics (J = current density, L = brightness), and (d) external quantum efficiency (ηext) versus L for the device containing dopant (11). Reproduced with permission from43. © Wiley 2008.
(fppz)H = 3-(trifluoromethyl)-5-(2-pyridyl) pyrazole tfa = trifluoroacetate dfbdpH = 4,6-difluorobenzyl diphenylphosphine (dfpbpy)H = 2-(4,6-difluorophenyl-4-tert-butylpyridine) (P^N)H = 5-(diphenylphosphinomethyl)-3-trifluoromethylpyrazole) tht = tetrahydrothiophene fbmbH = 1-(4-fluorobenzyl)-3-methylbenzimidazolium dfbmbH = 1-(2,4,-difluorobenzyl)-3-methylbenzimidazolium) ITO = indium tin oxide TAPC = di[4-N,N-ditolylamino]phenyl)cyclohexane TCTA = 4,4’,4”-tris(carbazol-9-yl)-triphenylamine CzSi = 9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole UGH2 = p-bis(triphenylsilyl)benzene TmPyPB = 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene)
Fig. 8 Reaction protocol that provided the blue-emitting Ir(III) complexes with the tripodal ancillary ligand.
(denoted as P^C2) to serve as the ancillary ligand44. In addition to
by using double emitting layers and double buffer layers to balance the
fulfilling the criteria discussed above, of both non-conjugation and a
charge transport and to move the exciton-formation zone away from
strong ligand field, the tridentate ligand offers great binding stability.
the adjacent carrier-transport layers. The device demonstrated a turn-
As a result, highly efficient blue phosphorescence is attained with
on voltage of 4.1 V, and peak ηext, ηl, and ηp values of 11.0 %, 22.3
good OLED performance. The synthetic protocol of this class of Ir(III)
cdA-1, and 16.7 lmW-1, respectively.
complexes, (12) and (13), is depicted in Fig. 8. Complex (13) was chosen
478
for the fabrication of an OLED device due to its high solid state emission
Concluding remarks
QY of 0.97. The device configuration and materials being used were:
Despite the intensive progress on blue phosphors that has been made, the
ITO/TAPC (30 nm)/TCTA (10 nm)/CzSi (3 nm)/CzSi: 4 % (13) (25nm)/
design of a highly efficient blue phosphor based on a third-row transition
UGH2: 4 % (13) (2nm)/TmPyPB (50nm)/LiF (0.8nm)/Al (150 nm). The
metal is still a challenge today. We have demonstrated that pushing the
wide-gap host CzSi and UGH2, which have a triplet energy gap of 3.02
emission gap towards the authentic blue region requires not only ingenious
eV and 3.08 eV, respectively, were employed for optimal efficiency. In
molecular design but also consideration of the subtle interference between
addition, good confinement of the excitons and carriers was realized
various low-lying electronic states. The chelates chosen for pursuing the
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REVIEW
blue phosphor include: (i) C^N chelates such as 2,4-difluorophenylpyridine,
the case of fluorescence emitters, it has been proposed that a single
(ii) N^N chelates like 2-pyridyl azolate, (iii) C^C chelates comprising both
system utilizing both normal Franck-Condon and exciplex emissions may
C-H site and neutral NHC carbene fragment, (iv) C^P chelates with both
accomplish this goal45-48. A similar strategy, however, is not attainable
benzylic C-H site and neutral phosphine donor, and (v) N^P chelates with
for the Ir(III) complexes because of the octahedral configuration. Since
an azolic N-H group11. These cyclometalating chelates, in theory, may
the heavy atom effect and hence the spin-orbit coupling is empirically
afford a series of Ir(III) based phosphors with photophysical properties
inversely proportional to r6, where r is the distance between the emissive
tuned specifically for the fabrication of PhOLEDs.
chromophore and core Ir(III) element, one approach is to de-emphasize
We have systematically pushed the chromaticity of phosphors closer
the spin-orbit coupling by elongating their spatial separation49,50. If the
to the true-blue region. However, even if they reach the authentic
S1-Tm (m ≥ 1) intersystem crossing has a low efficiency for a designated
blue color, the close proximity between the metal-centered dd and
Ir(III) complex and the fluorescence yield is virtually 100 % upon optical
the emissive states may inevitably lead to a lower emission efficiency
excitation, the electrical pumping (in OLED applications) may then give
and photostability. These inferiorities make us feel somewhat “blue”
25 % and 75 % population in the S1 and T1 states, respectively. As
about attaining a true-blue hue, though some proposed mechanisms
opposed to the null phosphorescence upon optical excitation, this 75 %
such as increasing the ligand-field strength or placing LLCT near the dd
T1 population may exhibit phosphorescence due to the heavy atom effect,
state may partly improve the performance. An alternative solution for
producing dual emission (fluorescence + phosphorescence). On the basis
achieving more stable OLEDs may rely on the sky-blue phosphor, for
of proper chemical derivation, white light generation is not impossible,
which the excited state is lower in energy which would thus strengthen
per se. Of course, such a system may be limited by the maximal 25 %
the photostability. Moreover, by selecting better host/charge transport
fluorescence yield, considering white light generation. Another possibility
materials and optimizing the OLED device structure, we believe that these
may count on the design of transition-metal complexes that show
sky-blue phosphors, together with orange ones, have latent potential for
fast isomerization at the triplet manifolds. What if the isomerization is
making all phosphorescent WOLEDs.
adiabatic, meaning the reaction is along the triplet PES, with a certain
Nevertheless, white light generation using multiple chromophores,
barrier, so that the system exhibits dual phosphorescence covering a
namely RGB triads or dual colors, such as sky-blue plus orange (or blue
gamut of the visible spectrum? We believe that such a system should be
plus yellow), would still suffer from relative-stability issues due to the
both theoretically and synthetically feasible. Certainly, more ingenious
intrinsically different lifespans of different materials. In theory, a single
and original concepts should be available, and we hope this review will
system emitting white light could ultimately overcome this hurdle. In
stimulate and encourage the OLED community.
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