A Significantly Twisted Spirocyclic Phosphine Oxide as a Universal Host for High-Efficiency Full-Color Thermally Activated Delayed Fluorescence Diodes

Article abstract of DOI:10.1002/adma.201506286

A universal thermally activated delayed fluorescence (TADF) host, 4′-diphenylphosphinoylspiro[fluorene-9,9′-xanthene] (SFXSPO), is constructed with a highly distorted and asymmetric configuration and disordered molecular packing in its solid state. SFXSPO

Full text of DOI:10.1002/adma.201506286

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A Significantly Twisted Spirocyclic Phosphine Oxide as
a Universal Host for High-Efficiency Full-Color Thermally
Activated Delayed Fluorescence Diodes
Jing Li, Dongxue Ding, Youtian Tao, Ying Wei, Runfeng Chen, Linghai Xie, Wei Huang,*
and Hui Xu*
Thermally activated delayed fluorescence (TADF) materials and
diodes have emerged in recent years.[1] Most TADF emitters
are pure organic compounds, featuring donor–acceptor (D–A)
With an increasing requirement on device performance,
especially efficiency stability, in recent years, much attention
has been paid to TADF host material development with a focus
on molecular engineering pertinent to structural and optoelec-
[
2]
systems with near-zero singlet–triplet splitting (ΔEST). TADF
dyes can utilize triplet exciton through the efficient reverse
intersystem crossing from triplet (T ) to singlet (S ) excited
[
8]
tronic characteristics of specific TADF dyes. The state-of-the-
art external quantum efficiencies (EQE) (ηext) of monocolor and
white hybrid diodes were achieved as ≈15% for yellow and red,
≈20% for green and blue, and ≈15% for white, approaching
to the current levels of PHOLEDs. Nevertheless, the further
improvement of the device performance is limited by the small
number of high-efficacy TADF materials available currently.
Noticeably, recent studies about universal phosphorescent host
materials have already manifested the superiority of utilizing a
1
1
states for 100% theoretically internal quantum efficiency
[3]
(
η ). However, similar to phosphorescent organic light-emit-
int
ting diodes (PHOLEDs), the involvement of long-lifetime triplet
excitons amplifies the influence of collision-induced quenching
effects, including triplet–triplet annihilation (TTA), singlet–
triplet annihilation (STA), and triplet–polaron quenching, on
device performance.[ Consequently, TADF dyes are commonly
dispersed in host matrixes to mitigate intermolecular interac-
4]
[
9]
single host for multicolor devices with favorable efficiencies,
[
5]
tions, which substantially promotes the device efficiencies.
which is another main drawback of TADF host development
because none of the host materials reported so far are generally
suitable to full-color TADF devices. Universal host materials
have the superiority in fabrication simplification and produc-
tion cost saving, since display pixels with different colors can
be produced with similar and even the same configurations,
and multiple EMLs of lighting devices can be integrated. The
absence of universal TADF host materials should be attrib-
uted to the various molecular polarities and configurations of
TADF dyes with different emission colors, which give rise to
diverse intermolecular interactions and exciton quenching
However, different to PHOLEDs using heavy-metal complex
emitters, the dopants and hosts in the emissive layers (EMLs)
of TADF diodes are both pure organic compounds with sim-
ilar components and optical properties. In this case, not only
dopant–dopant but also host–dopant interactions would worsen
exciton quenching,[ making the hosts dominant in quenching
suppression, besides their functions in carrier injection/trans-
portation and energy transfer.[
6]
7]
channels. With respect to the essence of quenching effects as
J. Li, D. Ding, Dr. Y. Wei, Prof. H. Xu
Key Laboratory of Functional
Inorganic Material Chemistry
Ministry of Education and School of
Chemistry and Material Science
Heilongjiang University
[10]
collision induction,
the challenge for developing universal
TADF hosts is embodied in extremely rigorous demand on sup-
pressing dopant–dopant and host–dopant interactions.
Spirocyclic structures with two orthogonal conjugated rigid
planes are widely employed in organic semiconductors for var-
7
4 Xuefu Road, Harbin 150080, China
[11]
ious optoelectronic applications, by virtue of their steric hin-
E-mail: hxu@hlju.edu.cn, iamhxu@njtech.edu.cn
drance. Nevertheless, the orthogonal symmetry of spirocyclic
structures is still favorable for intermolecular ordered packing
and π–π interactions. In this sense, twisted and asymmetrical
spiro compounds with remarkably reduced intermolecular
interactions can be regarded as a feasible alternative as uni-
versal hosts for full-color TADF diodes. Our group developed
a series of spiro[fluorene-9,9′-xanthene] (SFX)-based organic
Prof. Y. Tao, Dr. Y. Wei, Prof. W. Huang, Prof. H. Xu
Key Laboratory of Flexible Electronics (KLOFE)
and Institute of Advanced Materials (IAM)
Jiangsu National Synergetic Innovation Center
for Advanced Materials (SICAM)
Nanjing Tech University (NanjingTech)
3
0 South Puzhu Road, Nanjing 211816, China
E-mail: wei-huang@njtech.edu.cn
[
12]
semiconductors for electroluminescence (EL). The xanthene
ring in SFX is relatively flexible for distortion, providing an
additional space for further configuration optimization. In this
contribution, on the basis of a short-axis linkage strategy, we
designed and prepared a highly twisted and asymmetrical SFX
phosphine oxide host substituted with a single diphenylphos-
phine oxide (DPPO) group at its 4′-position (Figure 1a). By
Prof. R. Chen, Prof. L. Xie, Prof. W. Huang
Key Laboratory for Organic Electronics
and Information Displays (KLOEID)
Institute of Advanced Materials (IAM)
Nanjing University of Posts and Telecommunications
9
Wenyuan Road, Nanjing 210023, China
DOI: 10.1002/adma.201506286
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relaxation suppression, a high T value of 2.97 eV for positive
1
energy transfer and favorable electrical performance, SFXSPO
successfully realized state-of-the-art ηext values up to 17.9%,
1
9.7%, 19.6%, 22.5%, 13.9%, and 19.0% and ηint of ≈100%
for blue, green, yellowish green, yellow, orange, and comple-
mentary nearly white TADF diodes, respectively, manifesting
SFXSPO as the best universal TADF host reported so far.
SFXSPO can be conveniently prepared from SFX through a
successive two-step lithiation and phosphorylation reaction with
a good yield of ≈60% (Figure 1a). Its chemical structure was
fully characterized by NMR spectroscopy, mass spectroscopy,
and elemental analysis. The configuration of SFXSPO was
further confirmed by single-crystal X-ray diffraction (Figure 1a).
As designed, the dihedral angle between the two phenyls of
its xanthene is decreased to 155.6°, indicating the nonplanar
structure with the spiro 9-C atom warped out. Consequently,
fluorene in SFX tilts toward one side, significantly reducing
the intersection angle between the fluorene and the xanthene
to 72.6°. In contrast, with DPPO at the para-position, SFX in
[
12c]
SFX2′PO
is exactly orthogonal with a regular dihedral angle
of xanthene of 173.6° and an intersection angle between
fluorene and xanthene of ≈91° (Figure S1a, Supporting Informa-
tion). Therefore, it is rational to attribute the highly twisted con-
figuration of SFXSPO to the amplified steric bulk of its DPPO
at the ortho-position. In consequence, even in single crystal, the
molecular packing of SFXSPO is still locally disordered such
that there are as many as six kinds of molecular orientations
without any SFX-involved interactions observed (Figure 1b).
On the contrary, the single-crystal packing diagram of SFX2′PO
shows the π–π interactions between SFX cores with a centroid–
centroid distance of 3.7 Å, ascribed to the regular molecular
alignment with V only two different molecular orientations
(
Figure S1a, Supporting Information).
The formation of a twisted configuration through the ortho-
linked DPPO gives rise to the enhanced molecular rigidity of
SFXSPO, resulting in a high temperature of the glass transi-
tion (T ) beyond 180 °C and melting point (T ) approaching
Figure 1. a) Synthetic procedure of SFXSPO: i) Phenol, CF SO H, 80 °C;
3
3
ii) n-BuLi, Ph PCl, THF, −78 °C to room temperature, 30% H O , and lat-
2
2
2
g
m
eral and bottom views of single crystal structure for SFXSPO; b) packing
diagram of single crystal for SFXSPO.
2
80 °C, which indicates the strong morphological stability of
SFXSPO (Figure S2, Supporting Information and Table 1). In
spite of the big intramolecular tension, SFXSPO still reveals a
good thermal stability with the temperature of decomposition
virtue of its nonplanar xanthene and oblique fluorene, 4′-diphe-
nylphosphinoylspiro[fluorene-9,9′-xanthene] (SFXSPO) realizes
highly disordered molecular packing and dramatically reduced
intermolecular interactions between the SFXSPO and TADF
dyes. With assistance from the rigid configuration for structural
(T ) more than 300 °C and can be facilely evaporated in vacuo
d
before 230 °C, making device fabrication feasible. The twisted
rigid structure and highly disordered molecular alignment of
SFXSPO make it easy to form amorphous thin films. The atom
Table 1. Physical properties of SFXSPO.
Compound
RMSh)
[nm]
Tg/Tm/Td
[°C]
HOMO
[eV]
LUMO
[eV]
µh/µe
k)
λAbs.
nm]
λEm.
[nm]
ηPLc)
[%]
FWHM
[nm]
S1
[eV]
T1
[eV]
[cm V s−1
8.98 × 10−7
7.34 × 10−6
2
−1
]
[
SFXSPO
308, 279, 228a) 311, 322a)
09, 273, 231b) 322b)
23
24a)
24b)
3.91d)
4.92e)
2.97g)
2.98e)
3.15f)
0.34
183/279/327
−2.47 (−2.31)i)
−2.61j)
−6.52 (1.74)i)
−6.09e)
3
−1.17e)
4
.34f)
a)
Absorption and emission in CH2Cl2 (10−6 mol L−1); b)Absorption and emission in film; c)PLQY calculated by using 9,10-diphenylanthracene as standard; d)Estimated
e)
f)
g)
according to the absorption edges; DFT calculated results; NTO calculated results; Calculated according to the 0–0 transitions of the phosphorescence spectra;
h)
Root-mean-square surface roughness of vacuum-evaporated film (100 nm); i)Calculated according to the equation HOMO/LUMO = 4.78 + onset voltage; j)Estimated
k)
according to the absorption bandgap and the HOMO energy level; Hole and electron mobility evaluated by I–V characteristics of single carrier transporting devices with
field-dependent space-charge-limited current model.
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force microscopy (AFM) image of a vacuum-deposited film of
SFXSPO (100 nm) shows a uniform and smooth surface with
a root-mean-square (RMS) roughness of 0.34 nm (Figure 2 and
Table 1). Significantly, after doping with bis[4-(9,9-dimethyl-
teristics of SFXSPO were investigated with natural transition
[
14]
orbitals (NTO). For the S →S excitation, the “hole” is mainly
0
1
localized on the xanthene; while the “particle” is dispersed on
the fluorene and the DPPO-linked phenyl of the xanthane, char-
acteristic of a hybrid transition with the major part of a charge
transfer (CT) excited state and the minor part of a locally excited
(LE) state. The CT-dominant transition renders the low oscil-
lator strength ( f ) of 0.0374, corresponding to the small transi-
tion probability of SFXSPO, which would facilitate host–dopant
energy transfer and exciton confinement on emitting dopants
[8]
9
2
2
,10-dihydroacridine)phenyl] sulfone (DMAC-DPS) (10 wt%),
,3,5,6-tetracarbazole-4-cyano-pyridine (4CzCNPy)[ (5 wt%),
13]
[
1b]
,3,5,6-tetrakis(carbazol-9-yl)-1,4-dicyanobenzene (4CzTPN)
wt%), 3,4,5,6-tetrakis(3,6-diphenylcarbazol-9-yl)-1,2-dicy-
(5
[1b]
anobenzene (4CzPNPh)
diphenylcarbazol-9-yl)-1,4-dicyanobenzene
(5 wt%), and 2,3,5,6-tetrakis(3,6-
[
1b]
(4CzTPNPh)
[
15]
(5 wt%) as blue, green, yellowish green, yellow, and orange
during the EL process.
The situation for NTOs of S →T
0 1
TADF dyes, respectively, the coevaporated SFXSPO films pre-
served the smooth surface with almost unchanged RMS rough-
ness around ≈0.3–0.4 nm, evidencing uniform dispersion of
these TADF dyes in SFXSPO matrixes and their perfect com-
patibility (Figure 2 and Table S1, Supporting Information). In
this sense, the aggregation and phase separation of the dopants
were effectively suppressed by the rigid molecular structure
and disordered molecular alignment of SFXSPO.
excitation is different that both the “hole” and the “particle” are
thoroughly localized on the fluorene with transition characteris-
tics of the LE state, in accord with the spin-density distribution
of the optimized T state on fluorene (Figure S4, Supporting
1
Information). The nonplanar xanthene with reduced conjuga-
tion induces the T state to concentrate on fluorene, giving rise
1
to a high calculated T energy of 2.98 eV for positive energy
1
transfer to blue TADF dyes, such as DMAC-DPS (T = 2.91 eV).
1
Density function theory (DFT) and time-dependent DFT cal-
culations on the ground (S ) state and the first singlet (S ) and
The electronic absorption spectrum of SFXSPO in dilute
−6
solution (10 in CH Cl ) consists of three bands around
0
1
2
2
triplet (T ) excited states of SFXSPO show that the SFX con-
figurations of these three states approximately keep stable with
similar intersection angles between fluorene and xanthene of
308, 279, and 228 nm, corresponding to the n→π* transition
from xanthene to fluorene, mixed π→π* transitions of xan-
thene and fluorene, and π→π* transition of DPPO, respec-
1
−6
7
2.6°, 75.6°, and 78.1°, which is beneficial to suppress struc-
tively (Figure 3a). In dilute solution (10 in CH Cl ), SFXSPO
2 2
tural-relaxation-induced exciton quenching (Figure S3, Sup-
porting Information). The singlet and triplet transition charac-
reveals two fluorescence (FL) peaks at 310 and 322 nm, corre-
sponding to 0→0 and 0→1 radiative transitions, respectively
Figure 2. AFM images of vacuum-evaporated neat and TADF dyes-doped SFXSPO thin films with thickness of 100 nm (testing area: 3 µm × 3 µm).
Doping concentrations were 10 wt% for DMAC-DPS and 5 wt% for 4CzCNPy, 4CzTPN, 4CzPNPh, and 4CzTPNPh, respectively.
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from FL (Figure 3a). According to the first peak at 418 nm from
0
→0 transition, the T energy of SFXSPO is evaluated as 2.97 eV,
1
identical to the DFT-calculated value, which can support the
positive triplet energy transfer to full-color TADF dyes.
SFXSPO films doped with DMAC-DPS (10 wt%), 4CzCNPy
(
(
5 wt%), 4CzTPN (5 wt%), 4CzPNPh (5 wt%), and 4CzTPNPh
5 wt%), respectively, were prepared through vacuum evapora-
tion (Figure 3b, and Figure S5 and Table S1, Supporting Infor-
mation). The emissions from all of these films are thoroughly
originated from their dopants, manifesting efficient host–
dopant energy transfer. The absolute ηPL values of these films
are as high as 83%, 95%, 99%, 100%, and 76% for DMAC-DPS,
4
CzCNPy, 4CzTPN, 4CzPNPh, and 4CzPNTPh, accompanied
with delayed fluorescence lifetimes (τDF) of 6.4, 14.2, 5.3, 5.1,
and 5.1 µs, respectively. Except for 4CzCNPy, τ less than 10 µs
of these TADF dyes in SFXSPO matrix is beneficial to suppress
the triplet-involved quenching effects.
The carrier-injection ability of SFXSPO was investigated
with cyclic voltammetry (Figure S6a, Supporting Information).
SFXSPO shows one quasi-reversible anodic peak and one quasi-
reversible cathodic peak at 2.53 and −2.93 V, corresponding
to fluorene-attributed oxidation and reduction, respectively.
According to the redox onsets, the experimental values of the
highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) are −6.52 and −2.47 eV,
which are almost equivalent to those of the spirofluorene-based
[
16]
analogue.
Therefore, in accordance to DFT calculation
results (Figure S4, Supporting Information), the twisted and
nonplanar xanthene in SFXSPO is hardly involved in the car-
rier-injection processes. In view of the LUMO and HOMO
levels for electron- and hole-transporting layers around −2.7
and −5.2 eV, respectively, electron injection with a smaller bar-
rier would be dominant in SFXSPO-based EMLs. The intrinsic
carrier-transport performance of SFXSPO was evaluated by the
current–voltage (I–V) characteristics of its nominal single-car-
rier transporting devices (Figure S6b, Supporting Information).
SFXSPO displayed electron-dominant transportation with an
electron-only current density (J) about one order of magnitude
larger than the hole-only J. Estimated with the field-dependent
space-charge-limited current model, the electron and hole
Figure 3. a) Electronic absorption, fluorescence (FL) in dilute solution
−
6
−1
(10 mol L ), FL in film and phosphorescence (PH) spectra of SFXSPO;
b) time decay curves and emission spectra (inset) of vacuum-evaporated
TADF dyes doped SFXSPO films (100 nm). DMAC-DPS for blue, 4CzCNPy
for green, 4CzTPN for yellowish green, 4CzPNPh for yellow, and 4CzT-
PNPh for orange.
(
(
Figure 3a). The relative photoluminescence quantum yield
PLQY) (η ) of SFXSPO is as low as 23%, due to its reduced
PL
conjugation and CT-featured excited state transition (Table 1).
In accord with almost same optimized SFX configurations in
the S and S states, as shown by quantum simulation, the first
−6
mobility (µ and µ ) of SFXSPO are appropriate as 7.34 × 10
0
1
e
h
−7
2
−1 −1
peaks in the absorption and FL spectra of SFXSPO are basi-
and 8.98 × 10 cm V s , respectively. Therefore, in spite of
the highly disordered molecular packing, the intermolecular
charge hoping in SFXSPO film can still be effective by virtue
of the charge-transport network established by the pendent
fluorene and DPPO.
−1
cally overlapped with a negligible Stokes shift of 21 cm , mani-
festing an extremely suppressed excited-state structural relaxa-
tion. A single fluorescence peak at 322 nm from 0→1 transition
is recognized in the emission spectrum of SFXSPO film. The
absence of the peak originating from the 0→0 transition should
be attributed to the restrained molecular vibration in solid state.
From solution to film, the ignorable emission bathochromic
shift further verifies the suppressed intermolecular aggrega-
tion and π–π interactions in the solid state by the distorted
and asymmetrical configuration and disordered molecular
packing of SFXSPO. Furthermore, with the full width at half
maximum (FWHM) as small as 24 nm (Table 1), SFXSPO
possesses fluorescent emissions even narrower than those of
condensed aromatics, e.g., perylene with FWHM of ≈35 nm,
ascribed to the limited transition processes due to its highly
rigid structure. The phosphorescence spectrum was measured
with time-resolved technology to get rid of the interferences
In consequence, despite the distorted configuration and
disordered molecular packing, SFXSPO still preserves the
favorable electrical performance with major contributions from
its fluorene and DPPO groups on the basis of its spirocyclic
structure and short-axis linkage. The electron-predominant
characteristics of SFXSPO can improve the charge flux bal-
ance in hole-redundant organic light-emitting diodes (OLEDs)
employing conventional carrier transporting materials.
Encouraged by the well-controlled intermolecular interac-
tions and favorable optoelectronic properties, SFXSPO was
utilized to fabricate full-color TADF devices with DMAC-DPS,
4CzCNPy, 4CzTPN, 4CzPNPh, and 4CzPNTPh as blue, green,
yellowish green, yellow, and orange TADF dopants, respectively,
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Figure 4. EL performance of blue, green, yellowish green, yellow, and orange TADF devices using SFXSPO as host. a) Device structures, energy level
diagrams (left for blue; right for green, yellowish green, yellow, and orange), photos and chemical structures of employed materials; b) EL spectrum;
c) luminance–current-density (J)–voltage curves; d) efficiency–luminance curves.
to verify its potential as universal TADF host (Figure 4a). To
remedy the inferiority of DMAC-DPS in charge capture and
stability, the doping concentration of the blue devices was opti-
mized as 10 wt% (Figure S7, Supporting Information), accom-
panied by the employment of 4,7-diphenyl-1,10-phenanthroline
respectively, manifesting complete energy transfer from
SFXSPO to these TADF dyes (Figure 4b). It is interesting that
the EL spectra were identical to the PL spectra of these TADF
dyes in toluene solution, which should be attributed to the uni-
form and neutral environment provided by the SFXSPO matrix.
The blue devices achieved low driving voltages of 3.0,
≈4.5 and ≈6.5 V for the onset and at 100 and 1000 cd m ,
respectively (Figure 4c and Table S2, Supporting Informa-
tion), ascribed to the barrier-free carrier injection through
the HOMO and LUMO energy levels of DMAC-DPS equiva-
lent to the corresponding molecular orbital energy levels of
(Bphen) with higher electron mobility and the deeper LUMO
−2
for charge flux balance (Figure S8, Supporting Information).
Pure TADF dye-originated EL emissions were achieved with
peaks at 464, 516, 540, 560, and 580 nm and favorable Com-
mission Internationale de I′Éclairage (CIE) coordinates of (0.17,
0
.20), (0.31, 0.59), (0.41, 0.56), (0.48, 0.51), and (0.52, 0.46),
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N,N′-dicarbazole-3,5-benzene (mCP) and Bphen (Figure 4a).
In this case, the direct carrier capture and exciton recombina-
tion on DMAC-DPS would be the main EL mechanism, with
assistance from the host–dopant energy transfer at high driving
voltages. In contrast, although the carrier-injection abilities of
The state-of-the-art performance of the SFXSPO-based
monocolor TADF devices motivated us to demonstrate the first
example of the efficient single-host-based full-TADF comple-
mentary nearly white OLEDs (WOLED) using DMAC-DPS and
4CzPNPh as blue and yellow emitters (Scheme S1, Supporting
Information). Dual-peak emission was achieved with CIE coor-
dinates of (0.32, 0.43) at 500 cd m (inset in Figure 5a). Due to
the thicker device structure, the driving voltages of these devices
were slightly higher than those of blue and yellow analogues
(Figure 5a and Table S2, Supporting Information). Signifi-
cantly, state-of-the-art efficiencies were realized with maxima of
4
CzCNPy, 4CzTPN, 4CzPNPh, and 4CzPNTPh are stronger
−2
with the shallower HOMOs and the deeper LUMOs, SFXSPO
would be simultaneously involved in electron injection, owing
to its LUMO energy level comparable to that of 1,3,5-tri(1-
phenyl-benzimidazole-2-yl)phenyl (TPBi), which makes the EL
mechanism of host–dopant energy transfer considerable. In
consequence, the green, yellowish green, yellow, and orange
devices showed the higher onset voltages of 3.5–3.8 V, in coinci-
dence with the triplet energy gaps between SFXSPO and these
TADF dopants.
−
1
−1
50.5 cd A , 40.6 lm W , and 19.1%, corresponding to a η
int
as high as 95.5%, which are the record values of full-TADF
[
19]
[9a,11r,20]
WOLEDs
and comparable to full-phosphorescence
[
21]
and TADF-phosphorescence hybrid WOLEDs (Figure 5b and
Table S2, Supporting Information). Nevertheless, these devices
still suffered the EQE roll-offs of 4% and 18% at 100 and
−1
The maximum efficiencies of 30.3 cd A for current effi-
−
1
ciency, 30.7 lm W for power efficiency, and 17.9% for ηext
−
2
were respectively achieved by the blue devices, accompanied
1000 cd m , respectively, which were about two folds of those
of the yellow devices but approximate to those of the blue ana-
logues. Furthermore, the intensity of blue component in their
spectra was gradually decreased along with voltage increasing,
making the emission shift to yellow zone (Figure 5c and Figure
S9, Supporting Information). Through multipeak fitting anal-
−
2
with EQE roll-offs as 3% and 17% at 100 and 1000 cd m com-
parable to the lowest values reported so far (Figure 4d and Table
S2, Supporting Information).[ The green devices displayed
17]
−
1
−1
state-of-the-art efficiencies up to 60.9 cd A , 54.6 lm W , and
1
9.7%, but their EQE roll-off as 19% at 1000 cd m− was the
2
most serious among these monocolor devices, which revealed
ysis, the contributions of blue and yellow components to η of
ext
[
22]
the worst triplet exciton quenching due to the longest τDF of
white emission can be roughly evaluated by:
4
CzCNPy among these TADF dyes. It is rational that with the
C
shorter τDF, 4CzTPN rendered the reduced EQE roll-off as 14%
A
C
ext
η
= ηext
×
(2)
−
2
B
Y
at 1000 cd m for its yellowish green devices with the max-
imum efficiencies equivalent to those of the green analogues.
Inspiringly, SFXSPO endowed its yellow devices with the record
A + A
in which superscript C can be B or Y referring to blue and
yellow components, respectively, and A stands for peak area
(Figure 5d). It is showed that ηext of yellow component was only
slight decreased from ≈10.5% to 9.5%, in contrast to the remark-
able EQE reduction of blue component. Therefore, the consid-
erable EQE roll-offs and unstable EL spectra of these devices
should be assigned to rapid efficiency decrease of their blue-
emitting units at high luminance. In this sense, the employ-
ment of highly efficient and stable blue TADF dyes can further
improve the efficiency and spectral stability of SFXSPO-based
full-TADF WOLEDs. To figure out the emission mechanism of
the nearly white OLEDs, control devices with reversed blue and
yellow EMLs were fabricated, which revealed stable pure yellow
light from 4CzPNPh along with increasing voltage (Figure S10,
Supporting Information). Accordingly, excitons should be first
formed in blue EMLs of the nearly white OLEDs, and then par-
tially transferred to yellow EMLs to generate the nearly white
emissions. Considering the Dexter energy transfer distance and
their yellow-dominant emissions, the recombination zones of
these nearly white OLEDs should be close to the interface of
two EMLs and keep stable during voltage variation.
−
1
−1
efficiencies as high as 63.3 cd A , 53.7 lm W , and 22.5%,
accompanied by excellent efficiency stability with negligible
−2
EQE roll-offs of 1% and 7% at 100 and 1000 cd m , respec-
tively, verifying SFXSPO as the most efficient yellow TADF host
to the best of our knowledge.[ Its orange devices also realized
1c]
−
1
−1
the high efficiencies of 32.4 cd A , 26.8 lm W , and 13.9%,
−2
and reduced EQE roll-offs of 4% and 12% at 100 and 1000 cd m ,
respectively, which were remarkably improved in comparison to
,4′-bis(N-carbazolyl)-1,1′-biphenyl-based analogues.
cantly, according to the relational expression:
[1b]
4
Signifi-
(1)
[18]
η
ext = ηint ⋅ηOC = γ ⋅ χ ⋅ηPL ⋅ηOC
in which ηoc refers to light out-coupling efficiency, regarded as
a constant of 0.20 for indium tin oxide (ITO) glass substrate
without any additional out-coupling enhancement, γ stands for
hole–electron recombination efficiency given 1 when carrier flux
balance and χ is radiative exciton fraction related to spin selec-
tion rule of 25% for singlet and 75% for triplet, ηint of SFXSPO-
based devices were evaluated as high as 89.5%, 98.5%, 98.0%,
1
12.5%, and 69.5% for blue, green, yellowish green, yellow, and
As D–A systems featured with CT excited states and ambi-
polar characteristics, most TADF dyes are superior in charge
flux balance and exciton recombination. In the case of heavy
doping devices, actually, carrier injection and transportation
can be basically supported by TADF dopants rather than host
matrixes. The primary function of the TADF hosts is more
embodied in uniformly dispersing dopants and suppressing
quenching effects induced by dopant–dopant and host–dopant
interactions. On account of the much shorter lifetime for
orange, respectively. Meanwhile, their χ values reached to 108%,
1
04%, 99%, 113%, and 91%, respectively, manifesting ≈100%
utilization of both singlet and triplet exciton.
Table S2 (Supporting Information) lists the representative
results of monocolor TADF diodes, in comparison to which
SFXSPO displays comparable or dramatically improved device
performance, testifying SFXSPO as the best universal TADF
host to date.
Adv. Mater. 2016, 28, 3122–3130
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Figure 5. EL performance of full-TADF complementary nearly white diodes employing SFXSPO as a single host. a) Luminance–J–voltage curves and EL
−2
spectrum and device photo at 500 cd m (inset); b) efficiencies versus luminance relationship; c) EL spectra at different driving voltages; d) variation
tendency of EQE contributions from blue and yellow emission parts along with luminance increasing.
singlet excitons, STA requires more effective intermolecular
collisions in contrast to TTA. It is noteworthy that the EQE
decrease tendency of SFXSPO-based devices basically fitted
with the TTA model with small deviations, manifesting the
significantly reduced STA (Figure S11, Supporting Informa-
diodes and single-host full-TADF complementary nearly white
devices, respectively, manifesting SFXSPO as the best universal
TADF host reported so far. Nevertheless, the efficiency stability
of its blue and nearly white TADF diodes, as well as spectral
stability of its WOLEDs, can be further improved when more
efficient and stable blue TADF dyes are available, which leaves
the space for further device optimization.
[
23]
tion), which was remarkably different from other host mate-
rials based TADF devices.[
4,24]
In this sense, the great success of
SFXSPO in full-color TADF diodes were doubtlessly attributed
to its highly distorted configuration and disordered inter-
molecular packing.
Supporting Information
In summary, a novel spirocyclic phosphine oxide host
SFXSPO was constructed on the basis of a short-axis linkage
strategy. The amplified steric bulk of its DPPO at the ortho-
position effectively distorts the SFX core, giving rise to the non-
planar xanthene and oblique fluorene. The extremely twisted,
rigid, and asymmetric conformation of SFXSPO renders the
highly disordered molecular packing in its solid states, effec-
tively mitigating intermolecular interactions and facilitating
uniform dispersion of the TADF dyes in its amorphous films.
With the superiority in effectively suppressing the quenching
effects induced by structural relaxation and dopant–dopant and
host–dopant interactions, SFXSPO successfully provided state-
of-the-art performance to its full-color devices, e.g., the record
ηext of 22.5% and 19.1% and ηint of ≈100% for its yellow TADF
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
J.L. and D.D. contributed equally to this work. This project was financially
supported by the NSFC (Grant Nos. 61176020 and 51373050), New
Century Excellent Talents Supporting Program of MOE (NCET-12-0706),
Program for Innovative Research Team in University (MOE) (IRT-1237),
Science and Technology Bureau of Heilongjiang Province (Grant Nos.
ZD201402 and JC2015002), Education Bureau of Heilongjiang Province
(Grant No. 2014CJHB005), the Fok Ying-Tong Education Foundation
for Young Teachers in the Higher Education Institutions of China
3
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www.advmat.de
www.MaterialsViews.com
(
2
141012) and the Harbin Science and Technology Bureau (Grant No.
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