Acridan-Grafted Poly(biphenyl germanium) with High Triplet Energy, Low Polarizability, and an External Heavy-Atom Effect for Highly Efficient Sky-Blue TADF Electroluminescence

Article abstract of DOI:10.1002/anie.201904433

We propose the novel σ–π conjugated polymer poly(biphenyl germanium) grafted with two electron-donating acridan moieties on the Ge atom for use as the host material in a polymer light-emitting diode (PLED) with the sky-blue-emitting thermally activated de

Full text of DOI:10.1002/anie.201904433

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Accepted Article
Title: Acridan Grafted Poly(biphenyl germanium) with High Triplet
Energy, Low Polarizability and External Heavy-Atom Effect for
Highly Efficient Sky-Blue TADF Electroluminescence
Authors: Miao-Ken Hung, Kuen-Wei Tsai, Sunil Sharma, Jun-Yi Wu,
and Show-An Chen
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201904433
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10.1002/anie.201904433
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COMMUNICATION
Acridan Grafted Poly(biphenyl germanium) with High Triplet
Energy, Low Polarizability and External Heavy-Atom Effect for
Highly Efficient Sky-Blue TADF Electroluminescence
Miao-Ken Hung, Kuen-Wei Tsai, Sunil Sharma, Jun-Yi Wu, and Show-An Chen*
Abstract: We propose the novel σ-π conjugated polymer,
poly(biphenyl germanium) grafted with two electron-donating acridan
moieties on the germanium (Ge) atom for use as the host material in
polymer light-emitting diode (PLED) with sky-blue emitting thermally
activated delayed fluorescence (TADF) material, DMAC-TRZ as the
guest. It gives the high triplet energy (ET) 2.86 eV, which is
significantly higher than those of conventional π-π conjugated
polymers (ET= 2.65 eV as the limit) and this guest emitter (ET= 2.77
eV). The TADF emitter emits bluer emission than other host
materials due to the low orientation polarizability of this Ge-based
polymer host. Additionally, the Ge atom provides an external heavy-
atom effect leading to the promoted rate of reversed intersystem
crossing (RISC) in this TADF guest, and thus harvesting more triplet
excitons for light emission. The sky-blue TADF electroluminescence
using this host/guest pair gives the record-high external quantum
efficiency (EQE) 24.1% at maximum and 22.8% at 500 cd/m2. Thus,
the present study opens up a new promising design route of polymer
host for various TADF guests for highly efficient TADF PLED.
fabricate the devices. Since film quality of solution-processed
film of small molecular materials is poor in general, polymeric
TADF materials including intrinsic TADF-polymers and
conjugated polymer with small molecular TADF materials (host-
guest system) must be used for forming high quality film of the
EML. For the TADF-polymers (denotes as intrinsic TADF-
10]
polymers),[1d,
the reported studies can be classified into
conjugated and non-conjugated types based on their different
main-chain structures. From the viewpoint of charge balance,
conjugated TADF-polymers with good charge-transport
properties are much more desirable than non-conjugated TADF-
polymers whose devices usually give low brightness (below
2000 cd/m2) and high efficiency roll-off.[11] To date, the reported
EQEs of conjugated TADF-polymer electroluminescence
devices can achieve 18.1% and 19.4% for green and orange
emissions,[12] respectively. However, the blue conjugated TADF-
polymer device with CIE coordinate of y < 0.4 giving EQE only
6.1% as the reported record value which is far inferior in
performance to the other emission colors due to the limitation of
triplet energy (ET) level of its backbone structure (below 2.65
eV).[13] Therefore, a development of blue conjugated TADF-
polymer emitter is highly constrained. For the host-guest system
with conjugated polymer as host and small molecular TADF
materials as guest, up to the present, there is no report
employing this strategy to fabricate TADF PLEDs (denotes as
the EML using the blend of polymer host/ TADF guest).
Here, we report the host-guest TADF PLED for the first time
by developing the σ-π conjugated polymer host P(DMAC-Ge)
(Figure 1a) with Ge-biphenyl polymer as backbone and two
electron-donating acridan groups (DMAC) grafting on the Ge
central atom as the side arms for charge transport. It gives the
high ET (2.86 eV) and well-matched energy levels to the adopted
highly efficient sky-blue TADF emitter DMAC-TRZ (ET=2.77
eV)[14] here. Moreover, the polymer host P(DMAC-Ge) with
symmetric arrangement in polar transport moieties leading to
bluer emission from the guest, and the Ge atom on the host can
generate external heavy-atom effect. To the TADF guest for
enhancing spin-orbit coupling and promote the rate of RISC,
which can efficiently harvest triplet excitons for light emission. As
using P(DMAC-Ge) as host and DMAC-TRZ as guest, this TADF
PLED shows an EQE of 17.6% with sky-blue emission at the
CIE coordinates (0.16, 0.31). Upon further adopting the cohost
strategy in the EML, the sky-blue device gives new record
EQEmax over 24%.
Organic light-emitting diodes (OLEDs) using TADF
emitters
have
emerged
as
a
next-generation
electroluminescence for illumination and display applications.[1]
An effective TADF emitter can harvest both singlet and triplet
excitons to potentially achieve 100% internal quantum efficiency
by converting triplets to singlets via RISC due to the small
difference of singlet and triplet energy levels (∆EST) below 0.1
eV.[1a] Similar to phosphors, the triplet exciton lifetimes of TADF
emitters are usually several microseconds, which is prone to
result in triplet-triplet annihilation (TTA) for neat TADF emitter
(i.e. without host) in the emitting layer (EML).[2] Thus, the EML
with host-guest strategy is usually employed with TADF material
as guest dispersed uniformly in the host matrix for suppressing
TTA.[3] To date, the state-of-the-art TADF OLEDs with EQEs
over 30% were reported using the host-guest strategy in the
EML.[4] Like phosphorescence OLEDs, an efficient host for
TADF guest requires higher ET level than the guest to suppress
the non-radiative process,[5] reasonable charge balance to
reduce efficiency roll-off,[6] and well-matched HOMO/LUMO
levels to guest to reduce charge injection barrier.[3] Besides, the
host with low polarizability and heavy-atom is also crucial to
enhance emission color purity[7] and spin-orbit coupling
interaction in the TADF guest[8], respectively, and the latter can
promote the RISC rate and compensate the increased
intersystem crossing (ISC) rate that efficiently harvests triplet
excitons for light emission.[8b, 9]
To meet the requirements of high ET, reasonable charge
mobility and heavy-atom effect for the host design, the σ-π
conjugated polymer is a candidate structure,[15] in which the
repeat unit can be composed of a biphenyl unit linking with a
heavy Group IV atom such as Ge.[16] In our previous work,[15] the
silicon-based σ-π conjugated polymer has been realized having
semiconducting characteristic with charge mobility at the level of
10-5 and 10-7 cm2/Vs for hole and electron, respectively, its ET
(2.67 eV) though higher than π-π conjugated polymers, however,
is still insufficient to develop the polymer host for the common
TADF OLEDs are generally fabricated by the expensive
vacuum deposition process (dry process). For low-cost and
large area fabrication, it is desirable to use solution-process to
[*]
M.-K. Hung, K.-W. Tsai, S. Sharma, J.-Y. Wu, and Prof. S.-A. Chen
Chemical Engineering Department, National Tsing-Hua University,
Hsinchu 30013, Taiwan
Corresponding author E-mail: sachen@che.nthu.edu.tw
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10.1002/anie.201904433
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blue TADF emitters. To design a new σ-π conjugated polymer
with higher ET, we need to promote the ET of the backbone
structure due to the existence of multiple-triplet states.[15] Since
Ge atom has larger atomic radius (125 pm) than the original Si
central atom (110 pm),[17] a replacement of Si by Ge in the
backbone may reduce the extent of π-electron delocalization
along the backbone and therefore to promote its ET energy. For
clarifying if such strategy of varying the size of central atom can
lead to a promotion of the ET level of σ-π conjugated polymer
backbone, we prepare the analogues of σ-π conjugated polymer
backbone structures P(dBu-Si) and P(dBu-Ge) (Figure 1a). As
expected, P(dBu-Ge) obviously has higher ET of 2.85 eV than
P(dBu-Si) with 2.78 eV (Table S1), and both are much higher
than the π-π conjugated polymer with 2.65 eV as the limit.[18]
we choose the sky-blue TADF DMAC-TRZ because it has a
rather high PLQY of 90% that could facilitate high device
efficiency[14] and has the well-matched HOMO/LUMO of -5.38/-
3.10 eV (Table S1) to those of P(DMAC-Ge).
For the PL measurements of the host-guest system (Figure
2a), we find that the P(DMAC-Ge) film doped with 8wt% DMAC-
TRZ exhibits the maximum PL emission (λPL, max) at 480 nm and
the relatively narrow full-width at half-maximum (FWHM) of PL
with 77.8 nm, which show the obvious blue-shift of λPL, max and
the narrower FWHM relative to the same emitter in the other
hosts: in P(V-DMAC) film (λPL, max= 488 nm, FWHM= 84.0 nm)
and in the common host materials, PVK film (λPL, max= 493 nm,
FWHM= 91.3 nm) and TAPC film (λPL, max= 502 nm, FWHM=
91.4 nm). While the doped P(dBu-Ge) films (λPL, max= 473 nm,
FWHM= 76.2 nm) shows more blue-shift of λPL, max and narrower
FWHM as compared to the doped P(DMAC-Ge) film. All the PL
emission colors of the doped films in the CIE 1931 color space
are shown in Figure 2b. As can be seen, the doped P(DMAC-Ge)
and P(V-DMAC) films are located in the sky-blue region (CIEy <
0.4),[19] while the doped PVK and TAPC films shift to the green
region (CIEy≧0.4).[20] Evidently, these differences in PL spectra
arise from the difference in the emitter’s environments. For
instance, P(DMAC-Ge) gives the blue-shift of λPL, max by 8 nm
compared to P(V-DMAC) with non-conjugated vinyl backbone,
indicating the molecular configuration of host is a crucial factor
to affect PL emission; while P(V-DMAC) with acridan group
gives the blue-shift of λPL, max by 5 and 13 nm as compared to
PVK and TAPC that contain carbazole and triphenylamine
groups, and the P(dBu-Ge) gives 7 nm blue-shift of λPL, max than
P(DMAC-Ge), both are suggesting the influences of various
functional groups. These results reveal that the PL emissions of
DMAC-TRZ are largely affected by the natural properties of
hosts, which inspire us to consider the possibility if the PL
emission of DMAC-TRZ affected by the polarizability of host.
Figure 1. a) Molecular structures of the σ-π conjugated polymers P(DMAC-
Ge), P(dBu-Si) and P(dBu-Ge), and their ET values are show in the brackets.
b) DFT calculation of HOMO/LUMO distributions for the repeat unit of
P(DMAC-Ge) using the Gaussian 09 package at the B3LYP/6-13G(d) level.
Based on the above results, P(dBu-Ge) is further modified
by replacing two butyl side arms with the two electron-donating
moieties 9,9-dimethyl acridan (DMAC) to give P(DMAC-Ge). For
demonstrating the significance of the σ-π conjugated backbone
in charge transport, the control polymer P(V-DMAC) containing
non-conjugated backbone was also synthesized (Figure 2d).
The detailed synthesis procedures for all polymers in Figure 1a
and P(V-DMAC) are given in the Supporting Information. The
ultraviolet-visible (UV-vis) absorption, photoluminescence (PL)
and phosphorescence spectra of these polymers are shown in
Figures S1 and S2, and their corresponding physical property
data are listed in Table S1. The phosphorescence spectra
measured at 77K show that P(DMAC-Ge) has ET of 2.86 eV
(Figure S2c), which is significantly higher than the TADF emitter
9,9-Dimethyl-9,10-dihydroacridine (DMAC-TRZ, 2.77eV) used
as guest here. The HOMO/ LUMO levels of the polymer host
P(DMAC-Ge) measured by cyclic voltammetry (Figure S3) are -
5.43/-2.27 eV, respectively. Besides, the density functional
theory (DFT) calculation shows the major LUMO distribution on
the Ge-biphenyl backbone (Figure 1b), indicating that the σ-π
conjugated characteristics on Ge-biphenyl units have stronger
electron-deficient nature than DMAC group, which may provide
an additional channel for electron transport. For the TADF guest,
Figure 2. a) PL spectra of 8wt% DMAC-TRZ doped in the various host films,
where the λPL, max and FWHM are maximum PL emission and narrow full-width
at half-maximum of PL, respectively and b) the CIE coordinates for their PL
emissions. c) Plot for the maximum PL of 8wt% DMAC-TRZ doped in various
host films versus the orientation polarizability of host (Inset: the photographs of
the doped P(dBu-Ge) film (left) and the doped TAPC film (right) under
photoexcitation by UV-lamp), and d) the structures of the molecular materials
used in this measurement.
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It has been reported that the PL emission of TADF can be
also applicable to the emission color adjustment for other TADF
emitters.
affected by the environmental polarity, so-called as solid-state
21]
solvation.[7a,
However, no investigation on the relationship
Concerning the external heavy-atom effect on TADF guest,
we believe that the Ge atom with high atomic number in
P(DMAC-Ge) should exist heavy-atom effect, which can
enhance the spin-orbit coupling (HSO) and then promote the
RISC rate in the TADF guest that converts more triplet excitons
to singlet excitons for generating more delayed fluorescence.[8b,
between TADF emission and host configuration is given. To
explore the host-guest interaction in solid state on the
photophysical properties of the guest, the measurements for the
λPL, max of 8wt% DMAC-TRZ doped host materials with various
orientation polarizabilities (f) are carried out for the first time
(Figure 2c). The orientation polarizability of the host material is
calculated from the equation f(εr,n)=[(εr-1)/(2εr+1)]-[(n2-
1)/(2n2+1)], where εr and n are the relative permittivity and
23]
To confirm if the heavy-atom Ge on P(DMAC-Ge) has an
opportunity to provide the HSO to the guest DMAC-TRZ molecule,
the accommodation of the guest into the vicinity of the host is
presented in a 3D plot as shown in Figure 3a. The open space in
the repeat unit analogue of P(DMAC-Ge) shows sufficiently
large to allow the guest DMAC-TRZ molecule to approach the
Ge central atom, and the closest distance between the center of
the guest molecule and the central atom Ge is 5.4 Å, which is
close to the reported effective distance of external heavy-atom
effect 5.6~7.4 Å.[24] Based on this simulation result, we infer that
the external heavy atom effect on DMAC-TRZ should occur by
using P(DMAC-Ge) as the host.
refractive index, respectively.[22] The measured λPL,
and f
max
values are given in Table S2. The magnitude of f value decides
the ability to form instantaneous dipoles of the host in the
presence of applied electric field,[7b] and the structures of the
host molecules we used in this measurement are shown in
Figure 2d. The λPL, max shift of DMAC-TRZ is then plotted versus f
as shown in Figure 2c and showing that a linear relationship is
found, the emission λPL, max increases from 473 to 503 nm with
increasing the f values of host in the range from -9.12×10-3 to
33.4×10-3, and reflecting that the PL emission shift of DMAC-
TRZ is highly correlating to the polarizability of the host. The f
value of P(DMAC-Ge) (3.9×10-3) giving about one-third smaller
than that of P(V-DMAC) (1.2×10-2) can be attributed to more
symmetric arrangement for the former than the latter, which are
supported by the lower dipole moment (μ) of the repeat unit of
P(DMAC-Ge) with μ= 2.21 D than the repeat unit of P(V-DMAC)
with μ= 2.58 D (Figure S5). For the influence of functional group
on host, P(V-DMAC), mCP, PVK and TAPC containing electron-
donating groups show increased f values with the sequence of
acridan (1.2×10-2) < carbazole (1.5-1.6×10-2) < triphenylamine
(2.6×10-2). In contrast, the molecules DCzPPy, DMAC-TRZ,
TP3PO and POPH containing electron-withdrawing groups such
as pyridine, triazine and diphenylphosphine oxide generally give
high f values (2.4×10-2~3.3×10-2). Consequently, P(DMAC-Ge) is
an ideal host for DMAC-TRZ because the Ge-based polymer
backbone give it much low molecular polarizability to make its
PL emission color purer and bluer.
On the other hand, Figure 2c also suggests that the extent
of intramolecular charge transfer (ICT) between DMAC (electron
donor) and TRZ (electron acceptor) on DMAC-TRZ that varying
its emission color not only occurs in solution state but also exists
in the solid state, in which the slight emission color modulation
can be achieved by controlling the polarizability of host material.
Besides, the same linear trends for the other TADF emitters
consisting of fixed 8wt% doping to the hosts with different
orientation polarizabilities were also observed (Figure S6a),
indicating that the effect of host orientation polarizability could be
Figure 3. a) Molecular modeling study for the repeat unit analogue of
P(DMAC-Ge) doped with DMAC-TRZ (expressed in yellow color). The
schematic 3D chemical structures were under minimized energy by use of
Chem3D (version 15.1). PL decays of b) P(DMAC-Ge) and P(V-DMAC) films,
and c) P(dBu-E), E= C, Si and Ge films doped with 8wt% DMAC-TRZ under
excitation by a fs pulse laser (350nm) at room temperature, and the emissions
were monitored at their maximum PL values. d) Schematic single-triplet
interaction between Ge-based polymer host and TADF guest.
Table 1. Photophysical data and rate constants of the host doped with 8wt% DMAC-TRZ films.
a
b
c
d
e
PLQY
[%]
Φp
Φd
τp
[ns]
τd
[μs]
% of τd
[%]
kISC
kRISC
EML
χ2 g
[%]
16.0
21.9
23.4
[%]
75.0
67.1
57.6
[107s-1]
[106s-1]
P(DMAC-Ge): DMAC-TRZ
P(V-DMAC) : DMAC-TRZ
P(dBu-C): DMAC-TRZ
91
89
81
36.4
42.9
36.2
2.16
1.81
2.03
82.5
75.4
71.1
2.27
2.64
1.42
1.52
1.26
1.76
2.25
1.96
1.70
P(dBu-Si): DMAC-TRZ
P(dBu-Ge): DMAC-TRZ
83
92
18.6
14.2
64.4
77.8
31.3
31.1
2.16
2.38
77.6
84.6
2.48
2.72
2.07
2.73
1.30
1.23
aPLQY for the prompt fluorescent (Φp) component calculated from ϕp=PLQY×(% of τp). bPLQY for the delayed fluorescent (Φd) component calculated from
Φd=PLQY×(% of τd). c % of τd= A2/(A1+A2)×100. d&e The rate constants of intersystem crossing (kISC) and reverse intersystem crossing (kRISC), and their detailed
calculation methods are demonstrated in Supporting Information.fChi-spaure value for exponentially fitting of the decay curve.
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Thus, the followed up transient PL measurement on the
DMAC-TRZ doped P(DMAC-Ge), P(V-DMAC) and P(dBu-E), E=
C, Si, Ge films were carried out (Figure 3), from which various
quantum yields, exciton lifetimes, rate constants and ΔEST are
listed in Table 1. First, we compare the transient PL curves of
the doped P(DMAC-Ge) and P(V-DMAC) films (Figure 3b), and
find that the prompt lifetime (τp) for former (τp =36.4 ns) is
obviously smaller than that of the latter (τp= 42.9 ns), and the
decrease of τp indicates that the radiative transition is quenched
by an incresed ISC process[25] arising from the heavy-atom Ge
on P(DMAC-Ge). The delayed lifetime (τd) and delayed
component (% of τd) for the doped P(DMAC-Ge) film are 2.16
μs and 82.5%, which are larger than the doped P(V-DMAC) film
(τd= 1.81 μs, 75.4%) and can be attributed to the increased
RISC process leading to more delayed fluorescence
production.[23] Second, we calculate their rate constants of ISC
(kISC) and RISC (kRISC) via the results of PLQY and exiton
lifetime (Table 1). The doped P(DMAC-Ge) film shows kISC and
kRISC of 2.27×107 s-1 and 2.64×106 s-1, respectively, which are
larger than the doped P(V-DMAC) film (kISC= 1.76×107 s-1 and
kRISC= 2.25×106 s-1), and the increased kISC and kRISC for the
doped P(DMAC-Ge) film suggest that the spin-orbit coupling
interaction in TADF could be intensified by the heavy-atom Ge.
For further exploring the heavy-atom effect of the host on
the influence of kRISC in DMAC-TRZ, we also prepare the
polymers comprising of various IV A group central atoms with
the same non-polar butyl side arm P(dBu-E), E= C, Si, Ge
(Figure 3c) for comparison. Among them, the doped P(dBu-Ge)
film shows more pronounced TADF character (τd= 2.38 μs, % of
τd= 84.6%) than the doped P(dBu-C) film (τd=2.03 μs, 71.1%)
and the doped P(dBu-Si) film (τd=2.16 μs, 77.6%). The kISC and
kRISC values for the doped P(dBu-Ge) film are 2.72×107 and
2.73×106, which are higher than the doped P(dBu-Si) film
(kISC=2.48×107 and kRISC=2.07×106) and the doped P(dBu-C) film
(kISC=1.96×107 and kRISC=1.70×106). These enhanced TADF
characters and rate constants for the Ge-based host confrim the
existance of extenal heavy-atom effect. In addition, the higher
PLQY for the doped P(dBu-Ge) film (92%) than the doped
P(dBu-C) film (81%) and P(dBu-Si) film (83%) also indicats that
presence of Ge atom has a positive effect on TADF emitter. In
Figure 3d, we show the advantages of introducing the Ge atom
to the σ-π conjugated polymer host. First, it can give high ET to
avoid the back energy transfer from TADF guest. Second, it can
generate external heavy-atom effect to enhance spin-orbit
coupling and promote the rate of RISC in the TADF guest, Both
of the aboves can efficiently harvest triplet excitons for
promoting light emission.
based device gives better charge injection capability than P(V-
DMAC) based device under the same electrical field, which also
reflects on their brightness profiles and specifically on maximum
brightness (Bmax) values, the former Bmax ~11000 cd/m2 being
~2.8 times higher than that of the latter (Bmax~4000 cd/m2). For
the device performance, Figure 4b shows the maximum EQEs
with 17.6% and 10.0% for P(DMAC-Ge) and P(V-DMAC) based
devices, respectively, and the charge balance factor (γ) values
are 0.77 for the former and 0.45 for the latter (Table S3). The
EQE and the corresponding efficiency roll-off extent relative to
EQEmax at 500 cd/m2 are 15.9% (-9.7%) for P(DMAC-Ge) based
device, but decrease to 7.0% (-30%) for P(V-DMAC) based
device (Table S3). The P(DMAC-Ge) giving higher EQE and γ
and lower efficiency roll-off than P(V-DMAC) is due to that the
Ge-based σ-π conjugated backbone could act as an additional
electron transport channel. In addition, we believe that this high
device performance partly contributed from the increased HSO of
TADF guest by external heavy-atom effect of the Ge based host,
which is able to harvest more triplet excitons and thus give
higher EL efficiency.
To investigate the electroluminescence (EL) properties of
these polymer hosts, the devices were fabricated with the
following configuration of ITO/PEDOT:PSS /host: 8wt% DMAC-
TRZ (30nm)/TP3PO (3nm)/ TmPyPB (52nm) /CsF (1nm)/Al, in
which the hosts including P(DMAC-Ge) and P(V-DMAC). All of
the structures of molecules used in the devices and the energy
diagram are shown in Scheme S4 and the device performances
are shown in Figure 4 and their parameters are listed in Table
S3. For the EL emissions, the inset of Figure 4b shows that the
EL spectra of 8wt% DMACT-TRZ doped in P(DMAC-Ge) gives
ELmax at 482 nm with the CIE coordinates of (0.16, 0.31) as sky-
blue emission, which is more blue-shifting compared to the P(V-
DMAC) based device at 487 nm with CIE (0.17, 0.34), the PVK
based device at 497 nm with CIE (0.19, 0.45) and the TAPC
based device at 495 nm with CIE (0.21, 0.41) (Figure S7). For
charge transport behaviors, Figure 4a shows that P(DMAC-Ge)
Figure 4. Performances of P(DMAC-Ge), P(V-DMAC) and P(DMAC-
Ge)/POPH based sky-blue TADF PLEDs: a) current density-voltage-
brightness (I-V-B) characteristics and b) EQE versus brightness (inset: the EL
spectra at 7V and the photograph of P(DMAC-Ge) based device).
We further improve the performance of P(DMAC-Ge) based
sky-blue device by blending the electron transport material 5-
terphenyl-1,3-phenylene-bis(diphenylphosphine oxide) (POPH)
[26]
into the EML, under the same device configuration as the
above sky-blue TADF PLEDs (Figure S9 and Table S4).
Impressively, the device using P(DMAC-Ge)/ POPH (70:30 by
weight ratio) as the host gives maximumλEL at 490 nm with CIE
(0.17, 0.37) and the extremely high device performance
EQEmax=24.1%, LEmax= 55.0 cd/A and PEmax= 48.0 lm/W, as well
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Chem. Mater. 2017, 29, 1946-1963; d)Y. Liu, C. Li, Z. Ren, S. Yan, M.
R. Bryce, Nat. Rev. Mater. 2018, 3, 18020.
as very low efficiency roll-off showing high EQE= 22.8% at 500
cd/m2. The EL spectrum has a slighly red-shift because its f
value of P(DMAC-Ge)/ POPH increses to 1.6×10-2 (Figure 2b),
but it still locates in the sky-blue emission region. Moreover, the
high efficiency for this cohost device is attributed to chage
balance, which is supported by more balanced hole and electron
current (Figure S10) and higher γ value (0.99) (Table S3) as
compared to P(DMAC-Ge) based device. This successful cohost
strategy on device performance indicates that if we want to
construct a bipolar polymer host with high device efficiency but
without cohost, an electron transport moiety with
diphenylphosphine oxide could be a good candidate. Note that
although the ET of HIL (PEDOT:PSS) is low, a triplet energy
transfer from the EML to it does not occur since the emission
zones are located away from the HIL (Scheme S5 and Figure
S10). To the best of our knowledge, this device performance is
much higher than the reported state-of-the-art performance of
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Conflict of interest
The authors declare no conflict of interest.
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Keywords: polymer light-emitting diode (PLED), thermally
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This article is protected by copyright. All rights reserved.

10.1002/anie.201904433
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Miao-Ken Hung, Kuen-Wei Tsai, Sunil
Sharma, Jun-Yi Wu, and Show-An
Chen*
Page No. – Page No.
Acridan Grafted Poly(biphenyl
germanium) with High Triplet Energy,
Low Polarizability and External
Text for Table of Contents
Heavy-Atom Effect for Highly Efficient
Sky-Blue TADF Electroluminescence
Heavy-atom germanium in σ-π conjugated polymer host provides high ET level
to give efficient energy transfer from host to guest, low orientation polarizability
leading to bluer emission from the guest, and the enhancement of spin-orbit
coupling to promote the RISC rate of guest to host and thus harvesting more triplet
excitons for light emission. The sky-blue TADF electroluminescence using this
host/guest pair gives the record-high EQE 24.1%.
This article is protected by copyright. All rights reserved.