All-organic thermally activated delayed fluorescence materials for organic light-emitting diodes

Abstract

Thermally activated delayed fluorescence (TADF) emitters, which produce light by harvesting both singlet and triplet excitons without noble metals, are emerging as next-generation organic electroluminescent materials. In the past few years, there have been rapid advances in molecular design criteria, our understanding of the photophysics underlying TADF and the applications of TADF materials as emitters in organic light-emitting diodes (OLEDs). This topic is set to remain at the forefront of research in optoelectronic organic materials for the foreseeable future. In this Review, we focus on state-of-the-art materials design and understanding of the photophysical processes, which are being leveraged to optimize the performance of OLED devices. Notably, we also appraise dendritic and polymeric TADF emitters — macromolecular materials that offer the potential advantages of low cost, solution processable and large-area OLED fabrication.

Full text

Organic light-emitting diodes (OLEDs) are important
for display and illumination technologies owing to their
high efficiency, flexible device structures and multicolour
emission1,2. However, conventional fluorescent OLEDs
suffer from an internal quantum efficiency (IQE) of 25%
as a consequence of spin statistics, and phosphorescent
OLEDs use rare noble metals (for example, Ir and Pt)3–5.
To overcome these drawbacks, noble metal-free, ther-
mally activated delayed fluorescence (TADF) OLEDs are
emerging as next-generation devices6–8. TADF OLEDs
are likely to be used in display and illumination tech-
nologies as well as fluorescence microscopy and sensing
applications.
In this Review, we describe the mechanism of TADF
and detail the progress that has been made in improv-
ing the efficiencies of all-organic TADF-based OLEDs.
We also highlight advances in the molecular design
and photo physical characteristics of the three classes
of organic TADF emitters — organic small molecules,
dendrimers and polymers — and OLEDs based on these
molecules.
TADF mechanism
During the electroluminescence of TADF emitters
(FIG.1), singlet and triplet states are generated by com-
bining holes and electrons through electrostatic binding.
The singlet excitons can decay radiatively with prompt
fluorescence (PF), decay nonradiatively or transform
into triplet excitons by intersystem crossing (ISC).
Simultaneously, low-energy triplet excitons decay non-
radiatively or up-convert to the emissive singlet level by
an endo thermic reverse ISC (RISC) process with delayed
fluorescence (DF). Thus, by harvesting both singlet and
triplet excitons, an IQE of nearly 100% can be achieved.
To utilize singlet and triplet excitons simultaneously
and maximize the IQE, the energy transition processes
for TADF emitters must be optimized. As shown in the
Arrhenius equation (equation 1), the RISC rate constant
kT
RISC from triplet to singlet is closely related to the singlet–
triplet energy splitting (EST), which is defined as the gap
between the lowest energy triplet (T1) and singlet (S1)
excited states:
kT
RISC ∝ exp (–EST /kBT)
in which kB is the Boltzmann constant and T is the
temperature.
Enhanced spin–orbit coupling (SOC) between sin-
glet and triplet states can simultaneously lead to rela-
tively high rates of the ISC process and the endothermic
reverse process9. The RISC rate constant can also be
computed in the framework of Fermi’s golden rule,
which is related to the SOC matrix elements10,11.
A small EST is the most important criterion for
achieving high TADF efficiency. A small EST will boost
up-conversion from triplet to singlet states and poten-
tially increase the IQE of a device. It is generally rec-
ognized that EST < 0.2 eV is favourable for an efficient
RISC process6. Higher temperatures provide sufficient
1Beijing Advanced Innovation
Center for Soft Matter
Science and Engineering,
State Key Laboratory of
Chemical Resource
Engineering, Beijing
University of Chemical
Technology, Beijing, China.
2Department of Chemistry,
Durham University, Durham,
UK.
3These authors contributed
equally: Yuchao Liu,
Chensen Li
*e‑mail: renzj@mail.buct.edu.
cn; skyan@mail.buct.edu.cn;
m.r.bryce@durham.ac.uk
doi:10.1038/natrevmats.2018.20
Published online 10 Apr 2018
All-organic thermally activated
delayed fluorescence materials for
organic light-emitting diodes
Yuchao Liu1,3, Chensen Li1,3, Zhongjie Ren1*, Shouke Yan1* and Martin R.Bryce2*
Abstract | Thermally activated delayed fluorescence (TADF) emitters, which produce light by
harvesting both singlet and triplet excitons without noble metals, are emerging as
next-generation organic electroluminescent materials. In the past few years, there have been
rapid advances in molecular design criteria, our understanding of the photophysics underlying
TADF and the applications of TADF materials as emitters in organic light-emitting diodes (OLEDs).
This topic is set to remain at the forefront of research in optoelectronic organic materials for the
foreseeable future. In this Review, we focus on state-of-the-art materials design and
understanding of the photophysical processes, which are being leveraged to optimize the
performance of OLED devices. Notably, we also appraise dendritic and polymeric TADF emitters
— macromolecular materials that offer the potential advantages of low cost, solution
processable and large-area OLED fabrication.
REVIEWS
NATURE REVIEWS
|
MATERIALS VOLUME 3
|
ARTICLE NUMBER 18020
|
1

energy to overcome the barrier to RISC. Moreover, a
small EST can be accomplished only by minimizing the
electron–electron repulsion of the triplet state12–14. From
a molecular viewpoint, a spatially twisted structure usu-
ally enables a small EST, because the effective separa-
tion of the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO)
signifies a small exchange integral (J). The relationship
between J and EST can be quantified using equation
2, in which EST is approximately twice the value of J
between the spatial wavefunctions of the HOMO and
LUMO15:
EST = ES – ET ≈ 2J
The effects of SOC are generally treated as a per-
turbation of the exciton transition process. The effects
can be enhanced by the nature of the excited states. In
general, the charge-transfer (CT) state is predominant in
the singlet excited state. Therefore, to achieve SOC and
efficient thermally activated RISC, the triplet excited
states on the donor or acceptor units should possess
predominant local triplet excited state (3LE) character.
If this occurs, one or both of the local triplets on the
donor or acceptor units can be in near-resonance with
the predominant singlet CT state (1CT). This situation
is consistent with the El-Sayed rule for ISC rates16,17.
CT emission is coupled with large transition dipoles but
low oscillator strength, whereas local excited (LE) emis-
sion is coupled with small transition dipoles and large
oscillator strength. Therefore, the proportion of CT or LE
can be controlled by the molecular structure. However,
if the excited states have hybrid LE and CT states18,19 or
if the singlet excited states are approximately equal to
the corresponding triplet states20, relatively weak SOC is
obtained. In some cases, the TADF efficiency is enhanced
by lone-pair orbitals on hetero atoms (for example, nitro-
gen and sulfur), which provide a 3nπ∗ triplet state that can
mediate the ISC process21.
Molecular rigidity is another important factor that
affects the TADF efficiency. Different conformations
of a molecule can yield different TADF efficiencies,
which means that flexible donors or acceptors should be
avoided22,23. For TADF molecules, molecular rigidity is
achieved by a large dihedral angle (θ) in a twisted donor–
acceptor bipolar structure, caused by bulky substituents
or a spiro-junction; all these features are favourable for
the RISC process6–8,24–26.
Increasing the radiative decay rate (kR) from S1 to S0
ensures high luminescence efficiency. A high kR requires
the oscillator strength and the transition dipole moment
to be balanced. However, kR tends to be suppressed by a
twisted structure with reduced conjugation. Therefore,
it is necessary to reduce the overlap of the HOMO and
LUMO orbitals while increasing the rate of decay24,27–30.
Separating donor and acceptor moieties by a phenyl
linker is an established approach to realize high effi-
ciency for TADF emitters. Moreover, a wide and delo-
calized HOMO distribution is favourable for increasing
the kR of TADF emitters31.
Compared with conventional fluorescent emitters,
which have small full width at half maximum, TADF
emitters usually display relatively poor mono chromaticity
Figure 1 | Mechanism of TADF OLEDs. a|The multilayer architecture of thermally activated delayed fluorescence
(TADF) organic light-emitting diodes (OLEDs) including a metallic cathode, electron-transport layer, emitting layer,
hole-transport layer and indium tin oxide (ITO) anode. Upon application of an electrical field, OLEDs convert electricity
into light. b|The TADF mechanism that occurs during electroluminescence: singlet and triplet excitons are formed after
electron–hole recombination in a singlet:triplet ratio of 1:3. The high exciton states are transferred to the lowest exciton
states (S1 or T1) via internal conversion (IC), and the accumulated triplet excitons at T1 are transferred back to S1 via a
reverse intersystem crossing (RISC) process with the aid of thermal activation. The singlet excitons at S1 formed either
initially after electronic excitation or back-transfer from T1 are radiatively deactivated to S0 following a prompt
fluorescence (PF) decay mechanism for fluorescent emissions with different luminescence lifetimes of PF and delayed
fluorescence (DF) decay or non-radiative (NR) decay. In addition, T1 states are radiatively deactivated to S0 following a
phosphorescence (Ph) decay mechanism for phosphorescent emissions or NR decay. ISC, intersystem crossing; kISC,
rate constant of ISC; kRISC, rate constant of endothermic RISC; NRS, singlet nonradiative decay; NRT, triplet nonradiative
decay; RIC, reverse internal conversion.
Nature Reviews | Materials
a b
IC
S2
S1
S0
T
2
T
1
PhPF DF NRSNRT
IC RIC
+ –
kRISC
kISC
Metallic cathode
Electron
transporting
layer
Emitting layer
Hole transporting
layer
ITO anode
Transparent
substrate
–
–
+
+
REVIEWS
2
|
ARTICLE NUMBER 18020
|
VOLUME 3 www.nature.com/natrevmats

because the broadened emission spectrum originates
from a singlet excited state with strong CT character.
A successful approach to improving the emission purity
is to suppress molecular rotation between the donor and
acceptor moieties32 by introducing large steric hindrance
to the core of the TADF emitter33 or to increase the
rigidity of the acceptor moiety34.
Good thermal and photochemical stabilities of TADF
emitters are important for their application. Stability can
be judged by bond dissociation energies under positive
or negative polarons. More specifically, the combination
of stable acceptor (for example, triazine) and donor units
(for example, carbazole), which have relatively high bond
dissociation energies, favours TADF devices with long
lifetimes6,31,35–37.
In a device, the external quantum efficiency (EQE or
ηext) is the key parameter. As shown in equation 3, the
contribution from TADF to the EQE is almost twice that
from PF, indicating highly efficient T1/S1 up-conversion
and that a high proportion of DF is responsible for a
high EQE value:
ηext
=
[0.25Φp + {0.75 + 0.25(1–Φp )} Φd/1–Φp ]γηout
in which γ is a charge recombinationfactor, ηout is the
light out-coupling efficiency and Φp and Φd are the photo-
luminescence quantum yield (PLQY) of the prompt and
delayed components, respectively.
The pre-coefficients of 0.25 and 0.75 are the branch-
ing ratios of singlet exciton and triplet exciton formation,
respectively. The value of ηout can be increased by prefer-
ential molecular orientation along the horizontal plane
of the dipoles of the emitting molecules in the OLED.
Higher horizontal dipole ratios improve the efficiency
of TADF OLEDs. The lifetime of DF (τDF) is another
key factor for the EQE: a short τDF is desirable to reduce
device efficiency roll-off, which is caused by triplet–
triplet annihilation, singlet–triplet annihilation and/or
triplet–polaron annihilation24.
Efficiency of TADF materials
TADF, initially named E-type delayed fluorescence,
was first reported in 1961 in a study on eosin38. In 2009,
Adachi’s group reported SnF2–porphyrin complexes
with TADF properties and OLEDs based on these TADF
complexes39. The DF of SnF2–porphyrin complexes is
dependent on the temperature, increasing from 0.6%
of the total emission at 300 K to 2.4% at 400 K (REF.39).
Among organometallic TADF-emitting materials, Cu()
complexes are widely used because of their low cost, facile
synthesis and potential for wet fabrication40,41. The highly
efficient luminescence of Cu() compounds with the for-
mula Cu2X2(NP) and with ΦPL = 92% has been reported42,
and for dinuclear Cu() complexes, NHetPHOS-Cu(),
a maximum EQE (EQEmax) of 23% has been obtained.
This EQE value is the highest reported for OLEDs with
a solution-processed emitting layer based on Cu() emit-
ters and is comparable to the efficiency of state-of-the-art
thermally evaporated devices based on Ir() emitters43.
The structures of these traditional TADF emitters are
shown in Supplementary Figure S1.
Recently, organic molecules have emerged as a class
of TADF emitter, providing flexible molecular design,
good morphological and electrochemical stabilities and
well-controlled photophysical characteristics7,44. In the
remainder of this Review, we focus on organic TADF
emitters, of which there are three distinct classes: small
molecules, dendrimers and polymers. The structures of
these compounds are shown in Supplementary Figures
S2–S6.
The EQEs of OLEDs based on TADF organic small
molecules, dendrimers and polymers are improving
steadily (FIG.2). In 2011, Adachi’s group25 reported the first
purely aromatic compound, PIC-TRZ, with highly effi-
cient TADF characteristics and a device EQEmax of 5.3% at
low current density. With the rational choice of molecules
with a twisted electron donor–acceptor structure and a
high radiative decay rate, highly efficient TADF emitters
with a wide range of emission colours have been reported.
For example, 4CzIPN (REF.6), the structure of which is
shown in FIG.3, exhibits green emission in a doped TADF
device with an EQEmax of 29.7% at 1,000 cd m−2 (REFS45–47).
Moreover, optimization of the molecular structure and
device architecture has resulted in the small-molecule
red TADF emitter HAP-3TPA achieving an EQEmax of
17.5 ± 1.3% and a peak luminance of 17,000 ± 1,600 cd m−2,
despite the inherently low PLQY for red emitters28.
Combining rigid electron donor and acceptor compo-
nents gives the orange–red TADF emitters NAI-DMAC
and NAI-DPAC, which have record EQEs of 21–29.2% for
orange to red TADF OLEDs48. The sky-blue TADF emitter
spiroAC-TRZ also exhibits excellent device performance,
with an EQEmax of 36.7% (REF.29).
Small molecules are usually dopants within host
materials, which can lead to phase separation and poor
morphological stability of the emitting layer. Therefore,
TADF dendrimers and polymers are attractive alterna-
tives to small-molecule emitters because these macro-
molecular TADF materials do not require a host material
and can be sufficiently soluble in organic solvents to pro-
duce thin films by solution processing techniques. For
Figure 2 | External quantum efficiencies of TADF emitters. Maximum external
quantum efficiencies (EQEs) of thermally activated delayed fluorescence (TADF)
emitters with red, green and blue emission colours. The data are taken from literature
published before the end of December 2017 (Web of Science search).
40
30
20
10
0
2017
2016
2015
2014
2013
2012
2011
Polymers
Dendrimers
Small molecules
(spin coating)
Small molecules
(vacuum evaporation)
EQE (%)
Year
Nature Reviews | Materials
REVIEWS
NATURE REVIEWS
|
MATERIALS VOLUME 3
|
ARTICLE NUMBER 18020
|
3

example, the dendritic TADF emitter D4 (Supplementary
Figure S5) exhibits an EQEmax of 13.8%, which remains
as high as 13.3% at the high luminance of 1,000 cd m−2 in
a green device49, while the polymeric TADF emitter P8
(Supplementary Figure S6) displays an EQEmax of 20.1%
in a yellow device50.
Narrow emission bands with high colour purity are
desirable for display applications24,33,51. Recently, poly-
cyclic aromatic backbone structures DABNA-1 and
DABNA-2 have shown pure blue emission with a full
width at half maximum of less than 30 nm, which is much
narrower than the typical values of 70–100 nm found for
the majority of emitters32. However, the full width at half
maximum of most macromolecules is still broader than
100 nm. It is noteworthy that a device based on a TADF
dendrimer with encapsulated hosts and guests exhibits
improved colour purity with a full width at half maxi-
mum of nearly 70 nm and Commission Internationale de
l’Éclairage (CIE) coordinates of (0.15, 0.30) owing to the
reduced host–host and host–guest intermolecular inter-
actions51. The design strategy of encapsulating the host
and guest separately provides an alternative method to
gain solution-processed macromolecular emitters with
narrow emission52.
The operating lifetime of TADF OLEDs is comparable
to traditional OLEDs and is continually improving. For
example, the lifetime of green OLEDs using the emitter
4CzIPN is 1,380hours at 1,000 cd m−2 after insertion of
ultrathin 8-hydroxyquinolatolithium interlayers, which
can reduce the number of charge traps and reduce exci-
ton–polaron annihilation; this is comparable to the
lifetimes of well-established phosphorescence-based
OLEDs53,54. Furthermore, the predicted half-life of 10,934
hours for an initial brightness of 1,000 cd m−2 can be
achieved using 4CzIPN as a green emitter and SF3-TRZ as
an n-type host. Compared with p-type hosts, n-type hosts
are more suitable for achieving stable electroluminescent
devices owing to their inherent ability to balance charge
fluxes and suppress high-energy exciton formation35.
Organic small-molecule TADF materials
Organic small-molecule TADF materials have the
benefits of precise molecular structures, high purity
(achieved by recrystallization and vacuum sublima-
tion), versatile chemical modification and high lumi-
nescence efficiency55. The small molecules typically
have intra molecular donor–acceptor-type structures
incorporating steric hindrance or a twisted conforma-
tion, which enhances the CT contribution to the S1 state
(FIG.3). Various acceptors and donors have been used
to tune the TADF characteristics31, namely, EST, emis-
sion colour, excited state lifetime and TADF quantum
yield. The structures of the materials covered in this
section are shown in Supplementary Figures S2–S4, and
TABLE1 provides some of their photo luminescence
and electroluminescence performances.
Structures and photophysical features
Emission colour. For the realization of full-colour
TADF OLEDs, it is vital to systematically modulate
the HOMO–LUMO gap and emission colours using
suitable donor–acceptor combinations. It is a challenge
to prepare blue TADF emitters owing to the required
high energies of the CT singlet and triplet states. In
this regard, studies on traditional fluorescent materials
have shown that limiting the conjugation length56 and
choosing the correct donor units are important to avoid
redshifts and to maintain a high triplet level57. For blue
TADF emitters, donors such as carbazole, dipheny-
lamine, 9,9-dimethyl- 9,10-dihydroacridine31 and their
derivatives are commonly used, owing to their moder-
ately strong electron-donating ability and high-energy
triplet states, in combination with acceptors with shallow
LUMOs that exhibit weak electron-accepting ability55.
This donor–acceptor pairing ensures that the CT emis-
sion is not shifted to lower energy. A typical series of blue
TADF emitters are based on a diphenylsulfone acceptor
with either diphenylamine, bis(4-tert-butylphenyl)amine
or 3,6-di-tert-butylcarbazole donors8, and their λmaxin
DPEPO films are 421, 430 and 423 nm, respectively.
Among efficient blue TADF molecules, Adachi’s group26
reported a device based on DMAC-DPS (FIG.3) with a
maximum electro luminescence at ~464 nm. A sky-blue
OLED based on a DMAC-TRZ emitter (FIG.3) gave an
EQEmax of 26.5% and a λmax of ~490 nm (REF.30).
Most green TADF emitters incorporate strong elec-
tron donors, such as phenoxazine, phenothiazine and
dihydrophenazine, as well as highly conjugated carbazole
and diphenylamine derivatives31, and acceptors such as
cyano-substituted aromatics, triazine and benzo phenone.
However, diphenylsulfone and pyrimidine acceptors
are less suitable because of their very weak electron-
accepting ability. The majority of green to yellow TADF
emitters contain cyano-based acceptors. For example,
the highly efficient TADF molecule 4CzIPN (FIG.3) has
intense green emission in toluene with a λmax of 507 nm
(REF.6). Numerous green TADF emitters contain triazine
as the acceptor. PXZ-TRZ (FIG.3) displays green emission
with a λmax of 529 nm (REF.58). A close analogue gives state-
of-the-art green OLEDs (λmax ≈ 520 nm) with an impres-
sive EQEmax of 29.6%59, originating from the optimized
oscillator strength andEST.
Compared with green emitters, relatively few red
TADF emitters have been reported. To increase conju-
gation length, an additional phenyl linker between the
donor and acceptor serves to reduce the twist angle
between the donor and acceptor24, thereby giving bet-
ter orbital overlap and lower-energy emission. However,
these features are likely to reduce the efficiency of the
RISC process. Orange–red emitters are generally based
on dicyanodiazatriphenylene, heptaazaphenalene and
anthracenedione acceptors owing to their strong elec-
tron-accepting ability with deeper LUMOs31. OLEDs
prepared with the first near-IR TADF emitter TPA-
DCPP (FIG.3) exhibited an EQEmax of 9.8% at a λmax of
645 nm (REF.19). Furthermore, an orange–red emitter
demonstrated an EQEmax of 17.5% owing to efficient
up-conversion from T1to S1 (REF.28). Two series of anth-
raquinone-based orange-to-red emitters with CIE
coordinates of (0.61, 0.39) and (0.63, 0.37) were also
developed, and the highest EQEmax among these emitters
was 12.5%24.
REVIEWS
4
|
ARTICLE NUMBER 18020
|
VOLUME 3 www.nature.com/natrevmats

Photoluminescence. DF arising from triplet states is
closely related to the environment because the emission
is easily quenched by oxygen25. Therefore, the photo-
luminescence intensity of TADF emitters is higher under
vacuum than in oxygen. The calculated emission ratio
(IVac /IO2 = (IPF + IDF)/IPF = IDF/IPF + 1) can effectively express
the contribution from the triplet state. Additionally, TADF
emitters show solvato chromic behaviour. For example, a
yellow TADF emitter, 2,8-bis [N,N-di(4-butylphenyl)
amino]dibenzo thiophene-S,S-dioxide, shows strong and
well-resolved emission in hexane, assigned to an excited
state with strong 1ππ∗ character21. By contrast, in ethanol,
the emission is broadened and redshifted, owing to a
redistribution of the electron density in the intra molecular
1CT state.
There have been many molecular design strategies to
enhance the PLQY of TADF emitters. A smaller EST can
contribute to a fast RISC process, which is advantageous
for a high PLQY. In one approach, a phenyl linker is
used to increase the CT character from the donor to the
acceptor in the S1 state to generate a smaller EST (REF.31).
For example, the sky-blue TADF emitter BCzT60 has a
phenyl group inserted between the donor and acceptor
of CzT61. The LUMO distribution of BCzT is extended
Figure 3 | Chemical structures of representative acceptors, donors and TADF small molecules. The colour bar
indicates the emission wavelength of the molecules. These molecules include the deep-blue emitter DMAC-DPS
(464 nm), the sky-blue emitter DMAC-TRZ (495 nm), the green emitter 4CzIPN (507 nm), the green-yellow emitter
PXZ-TRZ (545 nm) and the red emitter TPA-DCPP (708 nm). The dihedral angles between electron donor and acceptor
moieties are shown in the molecular structures. HOMO, highest occupied molecular orbital; LUMO, lowest
unoccupied molecular orbital; TADF, thermally activated delayed fluorescence.
Nature Reviews | Materials
O
–1.70
CN
–1.41 CN
NC
–2.36
CNNC
–2.23
NN
O
–0.55
–3.16
N N
N
–1.58
N
N
NC CN
–3.01
S
OO
–1.81
–1.37
S
OO
Acceptor (LUMO, eV)
Chemical structures of representative TADF molecules
4CzIPN
N
N
N
N
DMAC-TRZ
N
N
N
N
O
PXZ-TRZ
H
N
–5.44
H
N
–5.22
H
N
–5.08
H
N
–4.88
H
N
O
–4.68
H
N
S
–4.67
H
N
N
H
–4.14
Donor (HOMO, eV)
DMAC-DPS TPA-DCPP
88°
S
NN
O
O
89°
75°
CN
N
NC
N
N
N
71°
63°
63°
64°
NN
N
N
CN
NC
35°
N
N
N
N
N
NN
REVIEWS
NATURE REVIEWS
|
MATERIALS VOLUME 3
|
ARTICLE NUMBER 18020
|
5

to the electron-donating unit in contrast to CzT, which
resulted in a higher PLQY for BCzT (95.6%) than for
CzT (39.7%) measured in DPEPO films. Dispersing
the HOMO distribution is another way to improve the
PLQY because this increases the oscillator strength. For
example, a series of blue TADF emitters containing the
TRZ acceptor together with different carbazole donors
(Ph3Cz-TRZ, 3Cz-TRZ and 2Cz-TRZ) displayed PLQYs
of 100%, 95% and 93% in DPEPO films, respectively,
as Ph3Cz-TRZ has the widest HOMO distribution62.
A narrow emission spectrum with high colour purity
is advantageous in display applications. In general, CT
excited states can be generated with a range of energies,
resulting in very broad emission spectra. Molecular
design strategies to achieve a narrow emission spec-
trum include a rigid, planar core structure or a sterically
hindered structure32,33. DABNA-1-based OLEDs exhibit
emission at 459 nm with a full width at half maximum
of 28 nm and CIE coordinates of (0.13, 0.09)32. Because
of the rigid triphenylboron π-conjugated framework,
nitrogen and boron atoms exhibit opposite resonance
effects, and a separation of the HOMO and LUMO states
occurs at high oscillator strength. A deep-blue TADF
device based on CzBPCN shows electroluminescence at
460 nm with a full width at half maximum of 48 nm and
an EQEmax of 14.0%33. At the biphenyl core, two carba-
zole units of CzBPCN are interlocked because rotation
of the biphenyl unit is sterically prohibited.
Characteristics of singlet–triplet energy splitting. TADF
small-molecule OLEDs benefit from a small EST and a
short τDF. In general, EST is estimated experimentally
from the onsets of the fluorescence and phosphores-
cence spectra for the 1CT and 3CT states and from the
peak maxima of the phosphorescence spectra for tri-
plet states when 3LE transition character is dominant.
In addition, EST can be calculated by fitting an Arrhenius
plot of kRISC as a function of 1/T, according to equation1.
Many methods have been developed to optimize
these properties, as describedbelow.
Strong donors or acceptors can attain a small exchange
energy and small EST owing to the increased CT char-
acter and stabilization of the singlet energy. A green-
emitting Px-VPN with stronger donor units than Cz-VPN
achieves a smaller EST of 0.08 eV and a shorter τDF of
2.3 s, compared with 0.36 eV and 173 s for Cz-VPN, in
mCBP films63. The HOMO–LUMO energy gap decreases
from 3.2 eV (Cz-VPN) to 2.3 eV (Px-VPN) because the
electron-donating ability of their donor units affects
their HOMO energy levels. Analogously, Px-CNP, which
contains a strong electron-withdrawing dicyanopyra-
zine core, has a lower LUMO energy and larger electron
affinity, resulting in a shorter τDF (1.5 s) and smaller EST
(0.04 eV) than its phthalonitrile-based counterpart63.
Emitters with multiple donors or multiple acceptors
can also effectively decrease EST and τDF. Examples
are the blue TADF emitters DTC-pBPSB and DTC-
mBPSB, which have two sulfonyl groups as acceptors and
3,6-di-tert-butylcarbazole as the donor. These emitters
have comparatively small ESTvalues (0.19 and 0.26 eV,
respectively) and short τDF values (1.23 and 1.16 s,
respectively) in neat films (that is, undoped films)64. Their
single sulfonyl counterpart, tDCz-DPS, exhibits a larger
EST (0.32 eV) and a considerably longer τDF (270 s) in
neat films8. Similarly, donor–acceptor–donor-type mole-
cules generally have a smaller EST and a shorter τDF than
their corresponding donor–acceptor analogues65.
A distorted backbone structure between the donor
and acceptor units is also beneficial to decreasing the
EST. Therefore, an ortho-phenyl linkage is better than
a meta-linkage or para-linkage. This is illustrated with
a series of benzofurocarbazole (BFCz) donor and TRZ
acceptor molecules: the ortho-linkage, meta-linkage and
para-linkage isomers have EST values of 0.002, 0.30 and
0.19 eV, respectively, in DPEPO films36.
Another method to obtain a small EST is to use a
through-space (that is, non-covalent) interaction66. For
example, TPA-QNX(CN)2 (TABLE1), which has a homo-
conjugated structure, has a EST of 0.111 eV and τDF of
2.4 s in cyclohexane. In addition, the maximum emis-
sions of TPA-QNX(CN)2 are located at 554 nm in toluene
and 601 nm in the pure film, which shows that different
properties of the isolated molecules and large ensemble
of molecules can be attributed to the solid-state solvation
effect. Homoconjugation has also been used in spiro-
based TADF emitters. For example, spiro-CN shows
a small EST (0.057 eV) and short τDF (14 s) in a mCP
film, because the donor and acceptor are orthogonally
connected via a spirobifluorene moiety67.
Despite the examples provided above, it is possible to
achieve good OLED performance with a relatively large
EST and long τDF. For example, the blue–green emitter
HMAT-TRZ, which contains azatriangulene and diphe-
nyltriazine moieties, has a EST of 0.38 eV, τDF1 of 0.7 ms
and τDF2 of 7.18 ms. The doped mCBP film also has a
high PLQY of 84.7%68.
OLED fabrication and performance
Doped TADF-based OLEDs. In TADF-based OLEDs,
the emitter is usually dispersed in a solid host matrix
at a relatively low concentration to avoid exciton anni-
hilation. Host materials should be carefully chosen to
have the following features: a higher triplet energy than
the TADF emitter, suitably aligned HOMO and LUMO,
a large bandgap, bipolar charge-transport properties to
maximize exciton formation in the emitter layer and
minimize exciton quenching at the electrode interfaces,
high morphological stability and good film-forming
properties69.
In most cases, traditional host materials — ini-
tially developed for heavy metal phosphorescent com-
plexes70 — are used in the fabrication of OLEDs with
TADF emitters. Typical hosts with high triplet energies
include DPEPO (T1: 3.3 eV)26, PPF (T1: 3.1 eV)63, mCPCN
(T1: 3.03 eV)71, CzSi (T1: 3.0 eV)71, PPT (T1: 3.0 eV)72, mCP
(T1: 2.9 eV)26, mCBP (T1: 2.9 eV)62, TPBi (T1: 2.7 eV)73 and
CBP (T1: 2.64 eV)26 (Supplementary Figure S2). Several
host materials74–76 have been developed for the high-
efficiency green emitter 4CzIPN, for which an EQEmax
of 31.2% was reported (TABLE1). Many host materials77,78
for the blue emitter DMAC-DPS have been designed.
Most notably, an EQEmax of 23.0% was obtained with
REVIEWS
6
|
ARTICLE NUMBER 18020
|
VOLUME 3 www.nature.com/natrevmats

DPETPO as the host79. Polar hosts can stabilize the CT
excited state of the guest emitter owing to local dipole
interactions and thereby redshift the emission. For exam-
ple, host–guest interactions have been studied in detail
for the blue TADF emitter DDMA-TXO2. By combining
the emitter with a host of the correct polarity, the energy
of the 1CT state is lowered, and the EST is minimized,
resulting in an EQEmax of 22.4% for a device with CIE
coordinates of (0.16, 0.24)80.
White TADF OLEDs are a growing area of research81–83.
For example, mixing green (4CzPN), red (4CzTPN-Ph)
and blue (3CzTRZ) emitters with hosts (mCBP and PPT)
in the emitting layer gives white OLEDs81. With an opti-
mized light-emitting layer composed of 4CzPN:mCBP
(3 nm)/4CzTPN-Ph:mCBP (2 nm)/3CzTRZ:PPT (10 nm),
the device achieved an EQEmax of 17.6% andCIEx,y of
(0.26, 0.38). In addition, phosphorescent and fluorescent
emitters have also been doped to prepare TADF white
OLEDs. Hybrid ‘warm-white’ OLEDs using the blue
TADF emitter 2CzPN and yellow phosphor PO-01 in
separate emitting layers (FIG.4a; Supplementary Figure
S1) achieved an EQEmax of 22.6% (CIEx,y = (0.45, 0.48))82.
By combining the yellow TADF emitter PXZDSO2
with the deep-blue fluorescence emitter NI-1-PhTPA,
Table 1 | Photoluminescence and electroluminescence characteristics of representative small-molecule TADF materials
Molecule Medium λPL
(nm)
ΔEST
(eV)
ΦPL Host/
toluene (%)
τPF (ns)/
τDF (μs)
CE
(cd A−1)
PE
(lm W−1)
EQEmax
(%)
Refs
DMAC-DPS PL: mCP film (10 wt%)
EL: DPEPO film (10 wt%)
464 0.09 90/80 21/3.1 −–19.5 26
mCP film (10 wt%) −19 19.5 87
PL: mCP film (10 wt%)
EL: DPEPO film (10 wt%)
39.7 44.4 23.0 79
DMAC-TRZ mCPCN film (8 wt%) 495 0.05 90/83 20.3/1.9 66.8 65.6 26.5 30
PL: mCPCN film (10 wt%)
EL: neat film
61.1 45.7 20 30
4CzIPN CBP film (6 wt%) 507 0.08 93.8/− 17.8/5.1 − − 19.3 6
3CzPFP (1%) − − 31.2 46
PXZ-TRZ CBP film (6 wt%) 545 0.08 66/43 20/1.1 − − 12.5 58
DACT-II CBP film (9 wt%) 529 0.009 63.7/100 −/− − − 29.6 59
TPA-DCPP Neat film 708 0.13 14/84 20.8/0.76 4.0 −9.8 19
HAP‑3TPA 26mCPy film (6 ± 1 wt%) 610 0.17 91/− −/100 25.9 22.1 17.5 28
BCzT DPEPO film (6 wt%) 483 0.31 95.6/− 5.5/33 − − 21.7 60
Ph3Cz‑TRZ DPEPO film (6 wt%) 480 0.09 100 6.3/− − − 20.6 62
Px-VPN mCBP film (6 wt%) 540 0.08 77/42 27/2.3 45.4 26.7 14.9 63
TPA-QNX(CN)2 PL: toluene or cyclohexane for τDF 554 0.111 44/− −/2.4 − − 9.4 66
EL: mCP film (10 wt%) 601
DABNA-1 PL: neat film 460 0.20 88/− 8.8/93.7 10.6 8.3 13.5 32
SpiroAC-TRZ mCPCN film (12 wt%) 480 0.072 100/− 17/2.1 94 98.4 36.7 29
DMAC-BP mCP film (10 wt%) 498 0.07 90/85 −/3.0 −59 18.9 87
DBT-BZ-DMAC PL: neat film
EL: CBP film (6 wt%)
505 0.08 80/− 40.4/2.9 51.7 50.7 17.9 88
Neat film 43.3 35.7 14.2 88
ACRDSO2 PL: neat film or CBP film (6 wt%) for τPF/τDF
EL: CBP film (6 wt%) and evaporation process
578 0.058 71/34 36/8.3 61.8 54.0 19.2 98
PL: neat film or CBP film (6 wt%) for τPF/τDF
EL: CBP film (6 wt%) and solution process
53.3 −17.5 98
PXZDSO2 PL: neat film or CBP film (6 wt%) for τPF/τDF
EL: CBP film (6 wt%) and evaporation process
608 0.048 62/37 36/5.0 49.3 38.5 16.7 98
PL: neat film or CBP film (6 wt%) for τPF/τDF
EL: CBP film (6 wt%) and solution process
45.1 −15.2 98
CE, maximum current efficiency; EL, electroluminescence; EQEmax, maximum external quantum efficiency; EST, singlet−triplet energy splitting; PE, maximum
power efficiency; PL, photoluminescence; TADF, thermally activated delayed fluorescence; ΦPL, photoluminescence quantum yield; λPL, wavelength of maximum
photoluminescence; τDF
, delayed fluorescence lifetime; τPF
, prompt fluorescence decay lifetime. ‘−’ indicates that the reference did not provide the data.
REVIEWS
NATURE REVIEWS
|
MATERIALS VOLUME 3
|
ARTICLE NUMBER 18020
|
7

two- colour warm-white OLEDs were prepared, achiev-
ing an EQEmax of 15.8% (CIEx,y = (0.401, 0.477)) with a
colour rendering index (CRI) of 68 (REF.83). Furthermore,
when a deep-red fluorescent component was introduced,
three-colour white OLEDs were obtained with an EQEmax
of 19.2% (CIEx,y = (0.348, 0.577)), a very high CRI of 95
and an EQE of 15.6%83.
Oriented molecules can enhance the efficiency of
doped devices84. An emitter with a horizontal transition
dipole moment results in a much higher out-coupling
efficiency than the vertically aligned dipole, and hence
higher EQE85. Theoretically, it is possible to increase the
EQE up to 46% for perfectly horizontally oriented emit-
ters without the use of external out-coupling structures
with ΦPL = 1 and Θ = 1, where ΦPL is the PLQY and Θ is
the percentage of horizontal dipoles among all emitting
dipoles (REF.84). For example, combining the host DPEPO
and TADF emitter CC2TA as the emitting layer resulted
in an EQEmax of 11 ± 1% owing to the high percentage of
all emitting dipoles (Θ = 92%)86 (FIG.4b). The out- coupling
efficiency of the emitters arranged horizontally (31.3%)
was higher than that of the isotropic emitters (20.6%).
In another example, the TADF emitter spiroAC-TRZ,
which has strongly horizontally oriented emitting
dipoles (Θ = 83%), displayed extremely efficient electro-
luminescence with an IQE of nearly 100% and an EQE
of 37%29.
Non-doped TADF-based OLEDs. Non-doped emit-
ter layers are more attractive than doped (host–guest)
systems for two reasons. First, the fabrication processes
are easier. Second, heterogeneities (such as those that
arise from phase separation of the components), which
are detrimental to colour stability and device efficiency,
cannot occur. However, non-doped OLEDs are rela-
tively rare. Most non-doped devices exhibit a lower
voltage, higher luminance and smaller efficiency roll-
off than the corresponding doped devices. Holes and
electrons recombine efficiently into excitons within the
broader recombination zone of a neat film owing to the
superior bipolar charge-transport properties of TADF
molecules30. In addition, increasing the doping concen-
tration can lower the probability of an emitter being in
its excited state and thus reduce excitation-dependent
dissociation87. The shorter decay lifetime of DF in a neat
film can also help to decrease efficiency roll-off88.
Non-doped green and blue OLEDs employing
DMAC-BP and DMAC-DPS displayed EQEs and a high-
est luminance of 18.9% and ~50,000 cd m−2, and 19.5% and
100 cd m−2, respectively87. The blue emitter DMAC-TRZ
attained an EQE of 18.9% at a brightness of 100 cd m−2 in
non-doped OLEDs30. OLEDs based on DBT-BZ-DMAC,
showing combined aggregation-induced emission and
TADF, displayed an EQEmax of 14.2% and a negligible cur-
rent efficiency roll-off of 0.46% from the peak values to
those at 1,000 cd m−2 (REF.88). In a recent example, the green
luminogen CP-BP-PXZ, which exhibits aggregation-in-
duced DF properties, was used in a non-doped OLED.
This device displayed an EQE of 18.4%, with a negligible
current efficiency roll-off of 1.2% at 1,000 cd m−2; these
data are comparable to those of doped devices89.
TADF exciplex OLEDs are another kind of non-doped
device (FIG.4c; Supplementary Figure S3). An exciplex
is a CT state, and its emission occurs as a result of an
electron transition from the LUMO of the accepter to
the HOMO of the donor under photoexcitation and
electrical excitation7. The intermolecular excited states
(that is, exciplex state) should provide a smaller exchange
energy than the intramolecular excited states, resulting
in the triplet levels being very close to the singlet levels7.
The first reported TADF exciplex emission was from
m-MTDATA:3TPYMB, which demonstrated an EQE
of 5.4%. Furthermore, the triplet exciplex state must be
confined using donor and acceptor molecules with high
triplet energy levels to decrease triplet state quenching.
OLEDs based on exciplex emission from m-MTDATA-
:PPT achieved an EQEmax of 10.0%90. These results suggest
that high-PLQY exciplex emitters are required for further
efficiency improvement. The TAPC:DPTPCz exciplex
system with high T1 triazine/carbazole exhibited a high
PLQY of 68%, a small EST of 0.047 eV and an EQE of
15.4% in a green OLED91. The best-performing device
based on an exciplex is that of MAC:PO-T2T, which
achieved an EQEmax of 17.8%. In this design, two RISC
channels on both the pristine TADF MAC molecules and
the exciplex system can be utilized92.
Solution-processed TADF-based OLEDs. OLEDs with a
solution-processed emitter layer have attracted attention
for large-area displays owing to their simple fabrication
processes and relatively low cost. The performance of
solution-processed OLEDs is generally inferior to that
of vacuum-deposited OLEDs93 (FIG.4d). Recently, a few
TADF OLEDs with good performance have been fab-
ricated by the solution processing of small molecules.
The solubility of small molecules in aromatic solvents
can be improved to apply solution fabrication techniques
by introducing methyl groups71, tert-butyl groups71, flu-
orine atoms94, trifluoromethyl groups95 or sec-butoxy
groups96. Other TADF molecules that are applicable for
solution processing without specific solubilizing groups
include 3ACR-TRZ, ACRDSO2 and PXZDSO2 with
an EQEmax of 18.6%97, 17.5%98 and 15.2%98, respectively
(FIG.4d). The HOMO levels are estimated to be −5.26 eV
for ACRDSO2 and −5.06 eV for PXZDSO2. Compared
with PXZDSO2, the weaker electron-donating group
lowers the HOMO level of ACRDSO2, which matches
well with the HOMO level of the CBP host, leading to
improved device performance.
Device stability. In addition to high-performance
light-emitting characteristics, a long operational life-
time under electrical excitation is necessary for com-
mercial applications. Suppressing the degradation of
hole-transport and electron-transport materials has a
positive effect on the lifetime of a device, because this
ensures that charges are effectively confined in the
emitting layer owing to large energy barriers for hole
and electron leakage53. In addition, reduction of charge
accumulation at the interface between the emitting
layers and the charge-transport layers can reduce exci-
ton–polaron quenching, thereby enhancing the device
REVIEWS
8
|
ARTICLE NUMBER 18020
|
VOLUME 3 www.nature.com/natrevmats

stability 54. The chemical stability of the molecules99
and the matching of energy levels between hosts and
emitters53 can also contribute to long device lifetimes.
For example, a DCzDCN:4CzIPN (15 wt%) device had
a lifetime of 200hours with up to 90% of the initial
luminance of 1,000 cd m−2 (LT90/1,000). By contrast, a
CBP:4CzIPN (15 wt%) device exhibited a lifetime of only
10.2hours under the same conditions. This is because
the low singlet energy of 2.88 eV for DCzDCN increases
the energy-transfer efficiency during the light-emission
process54. Devices composed of SF34:4CzIPN (8 wt%)
exhibited considerably longer lifetimes (LT80/2,000) of
252.4hours, compared with the control device using
CBP as the host (61.5hours). This indicates that pure
Figure 4 | The performance of small-molecule TADF
OLEDs. a|Energy levels, exciton energies and the
mechanism of light emission for materials used in hybrid
white organic light-emitting diodes (OLEDs). A hybrid
white OLED has two emitting layers (EMLs). The first EML,
which is close to the hole-transporting layer (NPB),
contains 2CzPN in a mCP host. In the second EML, which is
close to the electron-transporting layer, PO-01 is doped in
the host. The emitting layers are sandwiched between the
electron-transporting layer of TAZ and the electron/
exciton-blocking layer of TCTA. HATCN is used as the
hole-injection layer. The large energy offset of the highest
occupied molecular orbital (HOMO) of TAZ and the HOMO
of mCP prevents hole transport between the adjacent
hole-transporting layer. In the exciton-generation zone
(grey), a hole and an electron can combine via Coulomb
forces to form excitons. b|Individual contributions of
isotropic and horizontal singlets to the external quantum
efficiency (EQE or ηEQE) in the oriented EML. The left
column lists typical values for an isotropic singlet emitter,
and the right column lists typical values for an oriented
thermally activated delayed fluorescence (TADF) emitter
enabling EQE beyond the classical limit. TADF increases
the percentage of radiative exciton, ηr, as a consequence of
the up-conversion of triplet excitons to the singlet state,
which in turn boosts the internal quantum efficiency (ηint).
The horizontal orientation of the transition dipole
moments in the EML increases the out-coupling efficiency
(ηout). It is assumed that the radiative quantum efficiency is
the same in both scenarios. However, because Purcell
factors depend on orientation, slightly different radiative
quantum efficiency values are used in the calculation of
ηEQE. c|Electronic energy diagram showing the exciplex
formation process and energy-level relationships. First,
donors and acceptors form excited donors and ground
state acceptors or form excited acceptors and ground
state donors under high-energy excitation. Then, donor
excitons and acceptor excitons combine into an exciplex.
Finally, the exciplex decays into light and ground state
donors and acceptors. d|Device structures and EQEs of
vacuum-deposited and solution-processed OLEDs based
on ACRDSO2 and PXZDSO2. The structures of HATCN,
NPB, TCTA, 2CzPN, mCP, PO-01, TAPC and TAZ are given in
Supplementary Figures S1–S4. A, acceptor; D, donor; DF,
delayed fluorescence; EA*, exciton energy of the acceptor;
ED* exciton energy of the donor; Eexciplex, exciplex photon
energy; ET, triplet energy; −Gcs, driving force; ISC,
intersystem crossing; kcs, rate constant of exciplex
formation; LUMO, lowest unoccupied molecular orbital;
PF, prompt fluorescence; Ph, phosphorescence decay;
qeff, effective radiative quantum efficiency; RISC,
reverse intersystem crossing; γ, charge balance factor; Γr,
radiative decay rates of the emitting molecule; Γnr,
non-radiative decay rates of the emitting molecule; hυ,
optical radiation. Panel a is adapted with permission from
REF.82, RSC. Panels b and d are reproduced with permission
from REFS86,98, Wiley-VCH. Panel c is adpated with
permission from REF.91, Wiley-VCH.
TADF molecule Solution process Evaporation process
Nature Reviews | Materials
a
c
b
d
–2
–3 3
2
1
0
–4
–5
–6
Energy levels (HOMO and LUMO) [eV]
Exciton energy (singlet and triplet) [eV]
Energy level (eV)
ITO
NPB
TCTA
TAZ
PO-01
LiF/AI
mCP
HATCN
Exciton-generation
zone
A + D*
A + D
(Aδ– Dδ+)*
kcs
kcs
A + D*
ED*
Eexciplex E
exciplex
hvexciplex
hv hv EA*
–∆Gcs
–∆Gcs
S1
S1T1
S0
PF + DF
ET
ISC
S0S0
T1
RISC
ISC
11%3.4%
Horizontal
31.3%
Isotropic
20.6%
TADF
ηr = 56%
Classical
ηr = 25%
γ ≤ 100%
qeff = 63.1%qeff
= 65.4%
ΓrΓnr Ph
Ph
ηint
ηout
η
EQE
S1
T1
RISC
2CzPN
DFPF
19.2%17.5%
ACRDSO2
PXZDSO2
15.2%16.7%
EML
TAPC
EML
TmPyBP
TmPyBP
LiF/AI
LiF/AI
ITO/
PEDOT:PSS
ITO/
HATCN
NS
S
O
O
O
O
N
OS
S
O
O
O
O
REVIEWS
NATURE REVIEWS
|
MATERIALS VOLUME 3
|
ARTICLE NUMBER 18020
|
9

hydrocarbon hosts have good electroluminescence
stability and a suitable glass transition temperature for
OLEDs100. Electron-transporting n-type hosts can effi-
ciently increase the lifetime of OLEDs, as mentioned
earlier. Following this strategy, a sky-blue TADF OLED
with a high EQE of 8.8% and a lifetime of 454hours
was produced35. This lifetime is three orders of magni-
tude higher than that of the device hosted by the p-type
material CzSi (REF.101).
Dendritic TADF materials
Structure and photophysical features
Dendrimers have been widely used in organic optoelec-
tronics because of their solution processability, homo-
geneous film morphology, relatively high molecular
weight but precise molecular structure, and tuneable
energy gap and emission colour (by controlling the num-
bers of dendrons)102,103. In this section, we outline guide-
lines for controlling the photophysical properties and
device performance of TADF dendrimers, followed by
specific examples. The structures of the materials covered
are shown in Supplementary Figure S5, and TABLE2 pro-
vides their photoluminescence and electro luminescence
performances.
Star-shaped or dendrimer TADF emitters are typ-
ically composed of a TADF core with branched, den-
dritic molecular structures (namely, dendrons) attached.
These dendrons isolate the core and prevent concentra-
tion quenching and efficiency roll-off caused by inter-
molecular interactions and exciplex formation. TADF
dendrimers are classified into either conjugated104–108
or non-conjugated structures109–114, depending on the
functional group that links the core to the dendrons.
A drawback of π-conjugated linkages between the core
and dendrons is that this can lead to reduced solubility
and a change in the emission colour compared with that
of the isolated core. Therefore, non-conjugated structures
are preferable. An example of this class of structure is a
TADF core and carbazole dendrons connected through
non-conjugated aliphatic chains109 (D7; TABLE2), which
has good solubility and charge-transport ability but
maintains the emission of the TADF core115,116.
In general, EST depends on the TADF core and is
not significantly affected by the generation (that is, the
number of repeated branching cycles) of the dendrimers,
especially for non-conjugated structures109,110. Carbazole
dendrons are typically used to minimize concentra-
tion quenching or triplet–triplet annihilation because
of the good hole-transport properties117, high triplet
energy103, thermal stability118,119 and excellent crosslink-
ing ability120,121 of carbazole. Yamamoto and co-workers104
reported the first conjugated nonpolar TADF dendrimer,
which was a second-generation dendrimer containing
a triphenyl-s-triazine acceptor core and multiple donor
carbazoles (D1;FIG. 5; TABLE2). In addition, DMAC-
DPS emitters encapsulated by multiple carbazoles (D2;
TABLE2) exhibit blue emission with TADF characteristics,
high PLQY and excellent photophysical and electrolumi-
nescent properties105. In some cases, TADF dendrimers
with carbazole dendrons show a significantly reduced
EST, favouring efficient RISC104,105, which is ascribed
to an extended potential gradient. In addition, substitu-
ents or end groups also affect EST. For example, methyl,
tert- butyl and phenyl favour efficient CT processes from
donor to acceptor in the S1 state and give a small EST
for TADF dendrimers. Moreover, a series of terminally
substituted dendrimers exhibit a slightly smaller EST than
their unsubstituted analogues106 (D5, D6; TABLE2). In addi-
tion to peripheral carbazole dendrons, bipolar groups bal-
ance carrier transport and thus improve the efficiency. For
example, a blue TADF dendrimer 110 (D9;FIG.5;TABLE2) in
which DMOC-DPS serves as the TADF core and POCz as
the bipolar dendrons exhibited excellent charge-transport
properties. In another example, alkyl chains were intro-
duced as bridges between the core and dendrons to retain
the frontier orbital distribution of the components and the
colour purity of the TADF core110.
TADF dendrimers exhibit more complex photo-
luminescence spectral characteristics than individual
TADF components. This is particularly the case for
conjugated dendrimers: with increasing dendrimer gen-
eration, the HOMO migrates from the internal donor
moieties to the peripheral dendrons, while the LUMO
remains located on the acceptors. Consequently, the dis-
tance between the HOMO and LUMO levels increases
and the absorption coefficient of the CT band decreases.
However, the photoluminescence spectra are almost
identical for dendrimers with different generations.
As previously demonstrated104, emission from a CT state
is related only to the neighbouring carbazole moieties
of the TADF core because of a highly twisted spatial
structure. Nevertheless, in some cases, the HOMO will
delocalize over both the donor moieties and the periph-
eral dendrons owing to a relatively planar conjugated
structure. For example, the photoluminescence emis-
sion of DCzDMAC-DPS105 (D3, TABLE2) shows a slight
blueshift compared with the DMAC-DPS core, whereas
a slight redshift for CzDMAC-DPS (D2, TABLE2) is
observed. This can be explained in terms of the different
distributions of the HOMO, in contrast to the LUMO,
which remains located on DPS. For non-conjugated
TADF dendrimers, the absorption bands induced by
intramolecular CT and the emission peaks from the
lowest excited singlet state are essentially unchanged
(that is, the TADF core behaves independently of the
peripheral dendrons)109,110. Dendrimers also exhibit
distinct solvatochromic effects with a weaker solvent-
dependent shift than their small-molecule counterparts,
which can be attributed to the reduced interactions
between solvent molecules and the encapsulated emis-
sive core110. The temperature dependence of transient
photoluminescence decay in dendrimers is generally
the same as that of their small-molecule counterparts
(that is, DF gradually increases with temperature)104.
Nevertheless, the contribution of the DF slightly
decreases at high temperature, which might be caused
by increased non-radiative processes104.
The dendrimer generation has a marked effect on
the DF and PLQY. The DF proportion of nonconjugated
TADF dendrimers is higher than that of the correspond-
ing TADF monomers. This is because reduced quench-
ing by molecular encapsulation facilitates the RISC
REVIEWS
10
|
ARTICLE NUMBER 18020
|
VOLUME 3 www.nature.com/natrevmats

and radiative deactivation processes of triplet excitons.
The proportion of DF in solution generally increases
with increasing generation of dendrimer; for example,
the contributions of DF at 300 K were calculated to be
43% for a second-generation dendrimer (D1; TABLE2)
and 81% for a third-generation dendrimer104. However,
for higher generations, the proportion of DF decreases,
possibly owing to the long lifetime and quenching of
excitons. Moreover, TADF dendrimers with higher
generations usually result in lower PLQY. This can be
ascribed to the long photoluminescence lifetime and
increased exciton quenching caused by intermolecular
interactions112 (D10, D11; TABLE2).
OLED fabrication and performance
The EQEs of solution-processed dendrimer devices are
still rather low, and further work is required to realize
high-efficiency devices104,122. However, in some cases,
dendrimer emitters can exhibit higher luminescence and
a lower turn-on voltage than their small-molecule coun-
terparts49,109. The TADF core is the main factor that deter-
mines the device efficiency. For example, a DMAC-DPS
core encapsulated by multiple carbazoles exhibited an
EQEmax of 12.2% and brightness of ~3,000 cd m−2 (REF.105).
By replacing the core with DMAC-BP (D4; TABLE2),
the non-doped OLEDs exhibit a slightly higher EQE of
13.8% and brightness over 10,000 cd m−2 (REF.49), which
can be partly ascribed to the full harvesting of exci-
tons generated from TADF and exciplex formation123.
In addition to the TADF core, dendrimer generation is
an important factor. A higher dendrimer generation
usually improves the charge injection and transport
and suppresses exciton quenching owing to the more
effective encapsulation of the emissive core. As a result,
exciton recombination is more efficient, resulting in
reduced efficiency roll-off. TADF dendrimers with
selective interfacial exciplex-forming dendrons can
lead to relatively efficient solution-processed OLEDs113
(D12; TABLE2). An exciplex, which can be formed at the
interface between hole-transporting dendrons and the
adjacent electron-transport layer, will effectively boost
charge injection and transport to the TADF core.
The structures of the linkage between the core and
dendrons can affect the device efficiency. In general, non-
conjugated flexible connections not only ensure the rela-
tively independent optical and electrical properties of the
subunits but also suppress aggregation and crystallization,
thereby favouring amorphous films and improving the
Table 2 | Summary of the photoluminescence and electroluminescence characteristics of representative dendrimer TADF materials.
Dendrimer λmax
(nm)
ΔEST
(eV)
ΦPL/ΦDF (%) τPF (ns)/
τDF (μs)
Von (V) Lmax (cd m−2) CE (cd A−1)PE (lm W−1) EQEmax (%) CIE (x, y) Refs
D1 475a,
500d
0.06 59a, 100b,
31c/24.8
−/− 3.5 ~1,000 8.5 −3.4 (0.27, 0.49) 104
D2 498a,
492d
0.09 67.5c/− 28.5/1.54 3.6 ~3,000 30.6 24.0 12.2 (0.22, 0.44) 105
D3 464d0.20 0.48c/− 25.8/1.91 5.2 ~500 3.8 2.0 2.2 (0.18, 0.27) 105
D4 520d0.11 77c/− 15/0.523 4.4 >10,000 38.9 17.3 13.8 (0.40, 0.54) 49
D5 422 0.07 62a, 92b
40c/2/11c
15.7/3.3 3.0 2,235 26.5 21.5 9.4 −106
D6 422 0.07 63a, 96b, 45c/14 13.3/5.3 3.5 2,423 25.4 16.1 9.5 −106
D7 535a,
487d
0.20 76c/− 16/− 3.6 22,950 30.5 −10.1 (0.24, 0.51) 109
D8 591a,
541d
0.20 71c/− 16/0.8 2.4 23,145 39.0 40.8 11.8 (0.39, 0.56) 111
D9 490a,
460d
0.23 61/− −/− 5.4 2,700 12.6 −7.3 (0.18, 0.30) 110
D10 395a,
517a,
520 d
0.079 69.2c/33.8 21.7/2.9 4.0 2,336 30.8 24.2 9.5 (0.32, 0.57) 112
D11 392a,
412a,
495a,
494d
0.134 59.4c/27.3 16.1/25.2 4.5 2,770 20.7 14.5 8.1 (0.22, 0.43) 112
D12 527a,
548d
0.08 90/− 16/1.0 2.7 18,800 44.5 46.6 16.5 (0.42, 0.55) 113
D13 452a,
475d
0.22 78b, 25a/− 2.8/4.5 3.5 7,800 48.6 35.2 23.5 (0.15, 0.30) 52
aMeasured in solution in air. bMeasured in solution under N2. cMeasured in film under N2. dMeasured in film in air. CE, maximum current efficiency; CIE, Commission
Internationale de l’Éclairage; EQEmax, maximum external quantum efficiency; EST, singlet−triplet energy splitting; Lmax, maximum luminance; PE, maximum power
efficiency; TADF, thermally activated delayed fluorescence; Von, turn-on voltage at 1 cd m−2; ΦDF
, delayed fluorescence quantum yield; ΦPL, photoluminescence
quantum yield; λmax, wavelength of maximum luminescence; τDF
, delayed fluorescence lifetime; τPF
, prompt fluorescence decay lifetime. ‘−’ indicates that the
reference did not provide the data.
REVIEWS
NATURE REVIEWS
|
MATERIALS VOLUME 3
|
ARTICLE NUMBER 18020
|
11

device efficiency. Using this approach, with a TADF core
and carbazole dendrons connected by flexible aliphatic
chains, an EQEmax of 10.1% and a maximum luminance of
22,950 cd m−2 was achieved in non-doped OLEDs109. This
approach can also result in fewer peripheral dendrons
contributing to concentration quenching of the TADF
core. However, careful selection of peripheral dendrons
is important for self-host dendrimers. Tricarbazole den-
drons attached to a triazine core lead to better perfor-
mance than the monocarbazole analogue (D8; TABLE2).
OLEDs based on the tricarbazole dendron display a
low turn-on voltage of 2.4 V and an EQEmax of 11.8%111.
The low turn-on voltage can be ascribed to the appro-
priate energy-level matching of the HOMO and the
LUMO. In addition, good smooth film forming ability105
and the favourable molecular orientation of dendrimers
significantly enhance device efficiency106.
For doped dendrimer-based OLEDs, the choice
of host is crucial for achieving relatively high device
efficiency. For example, instead of using a common,
previously used host, a TADF material with an emis-
sive core encapsulated by multiple carbazole dendrons
(D12; Supplementary Figure S5) was used as the host
for D13 (Supplementary Figure S5). This strategy led to
suppression of the energy loss induced by aggregation of
the host molecules and the creation of an efficient energy
and CT channel with suppressed exciton quenching. The
resulting solution- processed blue TADF OLEDs incorpo-
rating D12:D13 as the emitting layer exhibited an EQEmax
of 23.5%114.
The examples outlined above demonstrate that precise
molecular design of the core, linkers and dendron struc-
ture, as well as the dendrimer generation, is indispensable
for attaining high-efficiency, solution-processed TADF
OLEDs.
Polymeric TADF materials
Structure and photophysical features
Thin films of small TADF molecules are usually fabricated
by thermal evaporation, which compromises film quality
because of the tendency of the molecules to crystallize
and aggregate, in turn reducing the efficiency and long-
term stability of the OLEDs. By contrast, polymeric and
dendritic TADF materials are suitable for film formation
by low-cost solution processes, such as spin-coating, die-
casting or ink-jet printing124,125. However, it is still challeng-
ing to prepare polymeric and dendritic TADF materials,
and this is an expanding research area. The reason for this
difficulty is twofold. First, it is difficult to simultaneously
achieve a small EST and to suppress the nonradiative
internal conversion in macromolecules containing a large
number of atoms. Second, the triplet excitons generated
from TADF are more readily quenched by intermolecular
and intramolecular triplet–triplet annihilation in poly-
mers than in small molecules. In addition, a common
feature of TADF polymers reported to date is their low
molecular weights and high polydispersities, meaning
that they are not well-defined materials. To address these
issues, many strategies have been applied to the design of
TADF polymers126. The structures of the materials covered
in this section are shown in Supplementary Figure S6,
Figure 5 | Molecular structures of TADF dendrimers. The conjugated non-polar
thermally activated delayed fluorescence (TADF) dendrimer D1 contains a triphenyl-
s-triazine moiety as an acceptor and multiple carbazole groups as donors. D9 is a
dendrimer with bipolar dendrons, in which DMOC-DPS serves as the TADF core and
POCz imparts excellent charge-transport properties. Lower panel is adapted with
permission from REF.110, American Chemical Society.
Nature Reviews | Materials
D1
D9
N
N
N
N
N N
TADF core
Second-generation dendrimer
TADF core (DMOC-DPS)
Second-generation dendrimer
Bipolar dendron (POCz)
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
P
P
O
O
N
P
P
O
O
NP
PO
O
NP
PO
O
66
66
S
OO
NN
O
OO
O
HN
P
P
O
O
REVIEWS
12
|
ARTICLE NUMBER 18020
|
VOLUME 3 www.nature.com/natrevmats

and TABLE3 provides their respective photoluminescence
and electroluminescence performances.
A simple method for producing TADF polymers is
to functionalize and then polymerize a monomer unit
that already exhibits TADF characteristics and excellent
(small-molecule) device performance. Using this strategy,
a series of main chain (that is, TADF units located in the
backbone)127–132, side chain (that is, TADF units as pen-
dant groups)50,130,133,134, crosslinking135 and through-space
CT (that is, between isolated pendant electron donor and
acceptor units rather than intermolecular CT)136 TAD F
polymers have been synthesized.
There are several examples of main chain TADF poly-
mers with alternating donor and acceptor units in the
backbone or with acceptor units grafted onto a donor
backbone. However, TADF is not observed for conju-
gated polymers in which the donors and acceptors are
directly connected to each other in the backbone because
of the large EST of around 0.7 eV (REF.137). Therefore, con-
jugated or nonconjugated spacers are usually inserted to
isolate the TADF units and thus reduce EST and pre-
vent concentration quenching. Stronger electron donor
or acceptor units can also reduce EST. For example,
pAcBP (P3; TABLE3), which is based on acridan, has a
smaller EST than pCzBP (P2; TABLE3), which is based
on carbazole, because of the stronger electron-donating
ability of acridan127. Modulating the location of donors
and acceptors can also decrease EST. For example, accep-
tors were grafted onto a conjugated backbone consisting
of alternating electron donors and host moieties128. As a
consequence, a EST of 0.37 eV was achieved for PAPCC
(P5; TABLE3), and a value of 0.13 eV was achieved for
PAPTC (P6; TABLE3).
In the first report of a main chain TADF material
(albeit without molecular weight data) (P1;FIG.6;TABLE3),
nonconjugated monomers with high triplet energy were
introduced to separate TADF units and prevent quench-
ing of triplet excitons129. However, conjugated TADF poly-
mers are much more desirable than main chain TADF
polymers with nonconjugated backbones, which suffer
from poor charge-transport properties. For example, ben-
zophenone-based conjugated polymers were obtained127
(P2, P3; TABLE3) by adding a third conjugated monomer
unit (carbazole and acridan, respectively) as spacers
into the donor–acceptor backbones. By grafting accep-
tor units onto the donor backbone, the twisted donor–
acceptor structure also ensures effective TADF. Using
this strategy, two conjugated polymers were synthesized
containing carbazole and 9,10-dihydroacridine donors in
the backbone and cyanobenzene or triazine acceptors as
pendants128 (P5, P6; TABLE3). As expected, they exhibited
excellent TADF characteristics.
For side chain polymers (TABLE 3; Supplementary
Figure S6), the TADF features of monomers can be eas-
ily inherited owing to the independent (non-interacting)
TADF units. The first nonconjugated side chain poly-
mer with an aliphatic main chain and phenothiazine-
dibenzothiophene-S,S-dioxide TADF pendants (P8;
TABLE3) was synthesized by free-radical copolymeri-
zation50,130. Thus, the near-perpendicular orientation
of donors and acceptors within the corresponding
small-molecule precursor is retained in P8 (REFS50,130).
Typically, efficient TADF units are grafted onto a conju-
gated carbazole backbone133,134 (P9, P1; TABLE3), and the
photophysical features of the polymers are dramatically
regulated by controlling the feed ratios of monomers.
Alternative strategies to obtain polymeric TADF
materials use thermally crosslinkable TADF materials135
or conjugation-induced TADF polymers138 (TABLE3;
Supplementary Figure S6). Specifically, a crosslinked
TADF polymer was synthesized by combining crosslink-
able vinyl benzyl ether groups with TADF units135 (P11;
TABLE3). In addition, conjugated TADF cyclic oligomers
and polymers were synthesized from non-TADF mono-
mers (benzophenone acceptor and carbazole donor138)
(P7; TABLE3). The TADF characteristics in these poly-
mers are attributed to an extension of the π-conjugated
donor, which reduces EST. Before choosing this method
for a TADF polymer, a calculation of the energy levels
is desirable for guiding the synthesis. In contrast to tra-
ditional TADF polymers, which feature through-bond
intramolecular CT, through-space CT can be exploited
to separate the HOMO and LUMO and give a rela-
tively narrow EST (REF.136). The first such polymer was
based on a nonconjugated polyethylene backbone with
through-space CT between the pendant electron donor
9,9- dimethyl-10-phenyl-acridan and acceptor TRZ
units136. The resulting polymer with 5 mol% acceptor unit
displays blue electroluminescence with CIE coordinates
of (0.176, 0.269) and an EQE of 12.1%.
TADF polymers show similar photophysical sig-
natures to their small-molecule counterparts, such as
solvatochromism, oxygen quenching and CT emission.
For example, the IVac:IO2 ratio of P8 is 1.35, indicating a
strong TADF effect50. The polymer P7 shows fluores-
cence spanning from blue to yellow in solvents such as
toluene, tetrahydrofuran, o-dichlorobenzene, chloroform
and dichloromethane138. Usually, the photoluminescence
emission of the donors and acceptors is masked owing
to CT processes in the polymers. Thus, polymers typi-
cally exhibit much broader spectra than small molecules.
In addition, there is often a small redshift of the photo-
luminescence for polymers because of the increased
conjugation. This is exemplified in the absorption
and emission spectra of TADF copolymers130 (P4, P8;
TABLE3). The photoluminescence intensity increases
gradually and redshifts with an increasing ratio of TADF
units in the side chain conjugated copolymers P9 and P10
(REFS133,134). There are also differences between the prop-
erties of isolated molecules and large assemblies of mol-
ecules, especially for side chain polymers. The molecules
in dilute solution are isolated; thus, the emission of the
backbone (spacer units) can be observed through energy
transfer, which mainly occurs from the backbone to side
chain TADF moieties. By contrast, for aggregated mol-
ecules in a film, the emission of the backbone is absent,
and this can be ascribed to increased inter molecular and
intramolecular interactions within the film133,134.
A common belief about TADF is that triplet excitons
are up-converted into singlet states and that delayed sin-
glet excitons then transform radiatively to the ground
state. However, in some cases, the calculated thermal
REVIEWS
NATURE REVIEWS
|
MATERIALS VOLUME 3
|
ARTICLE NUMBER 18020
|
13

Table 3 | Summary of photoluminescence and electroluminescence characteristics of representative polymer TADF materials.
Polymer λmax
(nm)
ΔEST (eV) ΦPL/ΦDF (%) τPF (ns)/τDF (μs) Von (V) Lmax
(cd m−2)
CE (cd A−1)PE (lm W−1) EQEmax
(%)
CIE (x, y) Refs
P1 535 0.22 41.0 ± 0.8a,
43.6 ± 0.9b/− −/− 3.2 >1,000 − − 10 ± 0.5 (0.32,
0.58)
129
P2 472a,
508d
0.18 28b, 23c/16 13/74 6.0 5,100 24.9 9.0 8.1 (0.28,
0.43)
127
P3 550a,
540d
0.10 26b, 46c/27 24/10 4.3 30,800 31.8 20.3 9.3 (0.38,
0.57)
127
P4 535 −−/− −/− >1,000 10 3.9 11.05 −130
P5 472a,
487d
0.37 9a, 9b, 8d/12b, 6c5.8/0.50 3.0 554 3.6 3.67 1.34 (0.25,
0.47)
128
P6 510a,
507d
0.13 22a, 40b,
28d/42b, 14c
13.8/0.68 2.6 10,251 41.8 37.1 12.63 (0.30,
0.59)
128
− − − − 2.5 26,321 48.2 50.1 14.9 (0.34,
0.56)
145
P7 475 0.023 71/50.79 9.44 ± 0.23/296 ± 16 − − − − − − 138
P8 535 0.35 −/44.4 −/− −>1,000 6.3 2.5 2.3 −130
− − − − 5.8 >1,000 61.3 40.1 20.1 (0.36,
0.55)
50
P9 494 −33.7/1.5b, 4.5c9.59b, 8.82c/4.06b,
2.36c
3.1 >4,000 10.7 11.2 4.3 (0.24,
0.43)
133
P10 511b,
500c
−67b, 74c/72 19.5/2.0 7.5 >1,000 38.6 14.3 16.1 (0.22,
0.40)
134
P11 430 0.31 69b, 71c/− 11/50 5.3 899 1.6 0.9 2.0 (0.12,
0.13)
135
P12 402a,
581a/
416d,
567d
0.26 99c/10.3 25.4/6.2 7.0 15,410 42.4 17.9 19.4 (0.51,
0.47)
131
P13 420a,
550a/
531d
0.09 89c/51 27.9/1.1 2.57 15,456 48.7 50.5 15.5 (0.41,
0.55)
132
− − − − 2.80 25,864 49.8 49.6 16.0 (0.41,
0.55)
132
P14 489a0.021 60c, 39d/13 24.3/1.2 3.2 6,150 24.8 −12.1 (0.18,
0.27)
136
P15 369a,
444a
−9c, 8d/− 15.5/− 5.8 140 0.51 −0.33 (0.21,
0.20)
136
aMeasured in solution in air. bMeasured in solution under N2. cMeasured in film under N2. dMeasured in film in air. CE, maximum current efficiency; CIE, Commission
Internationale de l’Éclairage; EQEmax, maximum external quantum efficiency; EST, singlet−triplet energy splitting; Lmax, maximum luminance; PE, maximum power
efficiency; TADF, thermally activated delayed fluorescence; Von, turn-on voltage at 1 cd m−2; ΦDF
, delayed fluorescence quantum yield; ΦPL, photoluminescence
quantum yield; λmax, wavelength of maximum luminescence; τDF
, delayed fluorescence lifetime; τPF
, prompt fluorescence decay lifetime.
activation energy is much smaller than EST (REF.129).
T1 states are locally excited (3LE) states (or regarded as
the presence of 3nπ∗), based on the phosphorescence
spectra21. Accordingly, TADF processes in polymers
occur in two stages: an internal conversion from the
backbone-centred 3LE into the 3CT state and a RISC
from the 3CT to 1CT state.
Unlike small-molecule and dendritic TADF emit-
ters, the PF and DF of polymers usually show com-
plex decay dynamics at room temperature that cannot
be fitted by sums of exponentials. This is because DF
components originate from TADF, not triplet–triplet
annihilation. Moreover, the DF of TADF polymers, as
with small molecules, exhibits perfect linear depend-
ence with excitation dose. This is because DF originates
from a monomolecular process, whereas triplet–triplet
annihilation is a bimolecular process3,25. In terms of the
temperature dependence of TADF polymer emission,
the PF components show nearly no variation with tem-
perature because of negligible internal conversion. By
contrast, the DF components increase with increasing
temperature, indicating definite TADF from RISC,
rather than triplet–triplet annihilation. Moreover, the
fluorescence decay in the DF region becomes more
complex with temperature, accounting for the increas-
ing contribution from long lifetime 3LE phosphores-
cence and increasing triplet–triplet annihilation. The
large proportion of DF is favourable for producing
TADF poly meric emitters with relatively high efficiency.
However, the DF lifetime of polymers must be rationally
REVIEWS
14
|
ARTICLE NUMBER 18020
|
VOLUME 3 www.nature.com/natrevmats

balanced to improve the efficiency. A long lifetime of
excitons would increase the nonradiative transitions,
resulting in low PLQY and strong efficiency roll-off.
For example, pCzBP127 (P2; TABLE3) displays relatively
low PLQY and increased efficiency roll-off owing to the
longer TADF lifetime.
PLED fabrication and performance
To date, most TADF polymer LEDs (PLEDs) suffer from
unsatisfactory EQEs and appreciable efficiency roll-off
compared with small-molecule OLEDs. However, if
improvements in these properties can be achieved, the
inherent advantages of PLEDs (for example, large-area
fabrication with flexible substrates using low-cost solu-
tion processing) could be exploited.
Both doped and non-doped emitting layers have been
used in solution-processed TADF PLEDs. For a non-
doped emitting layer, the presence of a single-component
system (as opposed to a host–dopant system) is ben-
eficial for eliminating phase separation and reducing
charge trapping and scattering defects139–143. For exam-
ple, PLEDs fabricated using only the TADF polymer (P1;
TABLE3) displayed an EQEmax of 10% at low current den-
sities129. In general, high efficiency roll-off is observed,
which is ascribed to triplet quenching144. However,
extremely low-efficiency roll-off has been achieved by
tuning the ratios of donor and acceptor units in a con-
jugated backbone, thus balancing charge injection and
transport132 (P13; TABLE3). In addition, PLEDs with
an insitu thermally crosslinked emitting layer135 (P11;
TABLE3) exhibit an EQEmax of 2.0% and small roll-off
owing to the short triplet exciton decay lifetime.
In contrast to non-doped emitting layers, a doped
emitting layer can overcome the problem of exciton
quenching caused by aggregation and unbalanced
charge transport. For example, the electroluminescence
performance can be improved with TAPC as a host for
PAPTC compared with a pure PAPTC emitting layer.
The improved efficiency roll-off can be partly ascribed
to the formation of an interfacial exciplex host of TAPC/
TmPyPB and a further increase in RISC rates of tri-
plets145 (P6; TABLE3). In another example, a mixed host
(TCTA and TAPC) with high T1 energies was used to
fabricate the emitting layer by blending TADF polymer
P2 to improve charge injection and balance, to broaden
the recombination zone and to reduce efficiency roll-off.
The device structure is shown in FIG.7a (REFS144,146,147).
In addition, the blended films have very smooth sur-
face topographies. On the basis of a blended host–guest
emitting layer, the assistant dopant DMAC-DP-Cz and
mCP host, together with the TADF polymer PCzDP-10
(P10; FIG.6; TABLE3), were mixed as the light-emitting
layer. The resulting PLEDs show remarkably improved
performance with an EQE of 16.1% with a 110 nm-thick
emitting layer134. In these PLEDs, the injected charges
are first trapped on the assistant dopant owing to the
high HOMO and low LUMO energies of DMAC-DP-Cz;
subsequently, the excitons transfer to the singlet and
triplet states of the TADF polymer (FIG.7b).
Future perspectives
The electroluminescent performance of TADF mole-
cules has recently exceeded that of phosphorescent com-
plexes29,46–48. In addition, TADF molecules have other
advantages for use in OLEDs and solid-state lighting
technologies — for example, the use of heavy metals
is circumvented, starting materials are of low cost, the
chemical structures are versatile and controllable, and
the materials are easily prepared. However, the intrica-
cies of the TADF mechanism and the development of
higher-efficiency devices remain to be fully explored
and unravelled.
In terms of designing TADF molecules, new donors
and acceptors, as well as their bridginglinkers, are
needed to refine our understanding of the fundamental
photophysics of small molecules, including the roles of
conformational effects. Optimization of the molecular
geometry will lead to smaller EST values, shorter DF
lifetimes, higher PLQY and a narrower full width at half
Figure 6 | Molecular structures of TADF polymers. P1 is a thermally activated delayed
fluorescence (TADF) polymer in which the charge transfer emitter is a triazine–amine–
triazine unit and the non-conjugated moiety serves as the backbone to isolate the TADF
units. P10 is a typical side-chain polymeric TADF molecule (PCzDP-10) with an
asymmetric donor1–acceptor–donor2‑type TADF moiety and poly(3,6‑carbazole) as the
conjugated backbone.
Nature Reviews | Materials
P1
N N
N
BrBr
C12H25
Acceptor (50%)
Donor (5%)
NN
C8H17
C8H17
BO
O
B
O
O
Backbone (45%)
A
B
DA
AB
B
BO
O
O
O
P10
NN
N
N
S OO
N
O
0.5 0.4
0.1
REVIEWS
NATURE REVIEWS
|
MATERIALS VOLUME 3
|
ARTICLE NUMBER 18020
|
15

maximum in the emission spectra. In terms of dendrim-
ers, the effect of different dendrons on the same TADF
core should be studied. Furthermore, hyperbranched
polymers incorporating donor–acceptor structural units
should be developed owing to their good solubility and
low viscosity, which could improve processability in
comparison with straight chain TADF polymers148.
For TADF polymers, an important research focus is
the design of copolymers and homopolymers for white-
light emission or emission of a specific colour. In TADF
copolymers, monomers without TADF characteristics
are generally copolymerized with TADF monomers
to separate TADF units and thus prevent fluorescence
quenching. Therefore, in addition to the choice of TADF
monomers, the type of spacers, such as electron trans-
porting or hole transporting, should be carefully selected
to optimize the TADF characteristics of the copoly-
mers149. Developing high-purity polymers (that is, poly-
mers with narrow polydispersity and negligible defects)
is still demanding. In this situation, the exact relationship
between the TADF characteristics, device performance
and polymer structures can potentially be established.
Additionally, comparing the optoelectronic properties
(EST, PLQY and τDF) of dendrimers or polymers with
those of the corresponding monomers may help in the
design of high-efficiency TADF polymers.
Low-cost solution processing is suitable for large-
area production and makes the practical application of
OLEDs feasible. For this processing method, good sol-
ubility of small-molecule and macromolecular TADF
materials in common solvents is required. However, at
present, most TADF materials for the active layers of
OLEDs are prepared by thermal evaporation in vacuum
owing to their limited solubility. However, in some cases,
alkyl or alkoxyl substitutes, which serve to enhance sol-
ubility, can reduce or even prevent TADF150; thus, the
emission characteristics and solubility must be balanced
with caution.
Recently, aggregation-induced emission or aggre-
gation-enhanced emission combined with TADF has
improved the PLQY in solid films88,151 and enabled
variable emission colours in water–solvent solutions152.
Designing TADF molecules with aggregation-induced
emission or aggregation-enhanced emission properties
is promising for applications such as hole-transport or
electron-transport materials, exciton-blocking materials
and structurally integrated OLED-based luminescent
chemical and biological sensors153.
For macromolecular TADF materials, photophysical
mechanisms are very complex and require further study.
For example, the effect of systematic changes in the con-
formation of molecular chains and end-group structure
on the properties of TADF polymers is an unexplored
topic. In addition to the molecular compositions of TADF
polymers, the effect of the condensed state of polymer
films on the TADF characteristics and performance also
needs to be clarified. The stacking of the polymer chains,
the crystallization of the polymers and the orientation of
the polymer chains in the thin films may influence the
TADF characteristics and device performance and may
improve the efficiency of OLEDs129,154,155.
Figure 7 | Device structure of TADF polymer OLEDs. a|Energy‑level diagram and
device structures of pCzBP-based and pAcBP-based organic light-emitting diodes
(OLEDs): ITO/PEDOT:PSS (40 nm)/10 wt% polymer:TCTA:TAPC blend (40 nm)/TmPyPB
(50 nm)/LiF (0.8 nm)/Al (80 nm), with pCzBP as the emitter in Device A and pAcBP as the
emitter in Device B127. b|The proposed energy‑transfer process in the emitting layers of
assistant dopant-based TADF polymer OLEDs. In the emitting layer, the injected carriers
are transported on the polymer backbone and the mCP host. The carriers are eventually
trapped on the assistant dopant as a consequence of the favourable alignment of the
highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) levels of the dopant compared with those of the host materials. The singlet and
triplet excitons in the thermally activated delayed fluorescence (TADF) assistant dopant
of DMAC-DP-Cz are transferred into the singlet and triplet states of the side-chain TADF
units in the polymer. The triplet excitons of DMAC-DP-Cz convert into singlet states by a
reverse intersystem crossing (RISC) process. The same process occurs for side-chain
TADF units in the polymer134. DF, delayed fluorescence; EMLs, emitting layers; ETL,
electron-transport layer; HTL, hole-transport layer; ISC, intersystem crossing; PF, prompt
fluorescence. Panel a is adapted with permission from REF.127, Wiley-VCH. Panel b is
adapted with permission from REF.134, Wiley-VCH.
N
S OO
N
O
Nature Reviews | Materials
a
b
Device A
EML ETLHTL
PEDOT:PSS
ITO
TAPC
pCzBP
40 nm
pAcBP
TmPyPB
50 nm
TAPC
TCTA
TCTA
40 nm
5.1
Device B
LiF/AI
4.3
T
1
S1
S0
S0
S0
S1T1
S1T1
5.0
5.3 5.3
1.8
5.7 5.7
2.9 2.8
5.4
2.3
1.8
2.7
6.7
2.3
5.7
75%25%
SS
S S
T T T
h+e–
T T
T
SS S S
TADF emitterAssistant dopantHost
PF + DF
RISC
ISC RISC
ISC
Backbone
DMAC-DP-Cz PXZ-DP-Cz
mCP
N
R
n
NN
N
S OO
N
REVIEWS
16
|
ARTICLE NUMBER 18020
|
VOLUME 3 www.nature.com/natrevmats

In-depth computational predictions and optimi-
zation of TADF materials are expected to contribute
significantly to the future development of the field.
For example, quantum chemical calculations could
screen out low-efficiency molecules and allow experi-
mental effort to focus on the more promising molecular
candidates. For quantum chemical calculations, time-
dependent density functional theory is commonly used
to theoretically predict the singlet and triplet transition
energies of TADF molecules20,154. Specifically, appli-
cation of the descriptor-based optimal Hartree–Fock
percentage method and the optimally tuned range-
separated functional is a promising approach for the
more precise prediction of transition energies of TADF
systems156–158.
Finally, from the viewpoint of electroluminescent
devices, high performance needs to be combined with
low efficiency roll-off, good colour stability and long
device lifetimes159,160. These factors will determine the
commercial future of TADF materials. Realizing high-
performance solution-processable non-doped devices
remains a challenge. In this regard, TADF dendrimers
and polymers with blue, red and white emission colours
are particularly desirable targets. Energy transfer using
TADF assistant dopants combined with a conventional
fluorescent emitter dopant (in small-molecule and poly-
mer devices) is an emergent strategy to improve OLED
performance161. To date, traditional device structures
for fluorescent and phosphorescent emitters have gen-
erally been used in TADF OLEDs and to increase the
performance of TADF OLEDs, but alternative device
structures should be considered. These device archi-
tectures should be tailored to the molecular features of
TADF materials to improve the overall performances of
OLEDs. Specifically, there is a need to reduce the con-
centration quenching effect caused by triplet excitons.
As an alternative to host–guest emitting layers, quantum
well structures, which can effectively confine the charge
carriers and excitons, afford high-efficiency devices
with low efficiency roll-off162. Alternatively, p–n hetero-
junction device configurations canimpart extremely low
operating voltages and high efficiencies to OLEDs123.
1. Tang,C.W. & VanSlyke,S.A. Organic
electroluminescent diodes. Appl. Phys. Lett. 51,
913–915 (1987).
This is a seminal paper describing the first OLEDs
with a double-layer thin-film structure operating at a
voltage below 10 V.
2. Friend,R.H. etal. Electroluminescence in conjugated
polymers. Nature 397, 121–128 (1999).
3. Adachi,C., Baldo,M.A., Thompson,M.E. &
Forrest,S.R. Nearly 100% internal phosphorescence
efficiency in an organic light-emitting device. J.Appl.
Phys. 90, 5048–5051 (2001).
4. Lee,C.W. & Lee,J.Y. Above 30% external quantum
efficiency in blue phosphorescent organic light-
emitting diodes using pyrido[2,3-b]indole derivatives
as host materials. Adv. Mater. 25, 5450–5454
(2013).
5. Baldo,M.A. etal. Highly efficient phosphorescent
emission from organic electroluminescent devices.
Nature 395, 151–154 (1998).
6. Uoyama,H., Goushi,K., Shizu,K., Nomura,H. &
Adachi,C. Highly efficient organic light-emitting diodes
from delayed fluorescence. Nature 492, 234–238
(2012).
This is a representative paper on TADF small
molecules achieving nearly 100% IQE and nearly
20% electroluminescence efficiency.
7. Goushi,K., Yoshida,K., Sato,K. & Adachi,C. Organic
light-emitting diodes employing efficient reverse
intersystem crossing for triplet-to-singlet state
conversion. Nat. Photon. 6, 253–258 (2012).
8. Zhang,Q. etal. Design of efficient thermally activated
delayed fluorescence materials for pure blue organic
light emitting diodes. J.Am. Chem. Soc. 134,
14706–14709 (2012).
9. Hedley,G.J., Ruseckas,A. & Samuel,I.D.W. Ultrafast
luminescence in Ir(ppy)3. Chem. Phys. Lett. 450,
292–296 (2008).
10. Lawetz,V., Siebrand,W. & Orlandi,G.J. Theory of
intersystem crossing in aromatic hydrocarbons. Chem.
Phys. 56, 4058–4072 (1972).
11. Robinson,G.W. & Frosch,R.P. Electronic excitation
transfer and relaxation. J.Chem. Phys. 38,
1187–1203 (1963).
12. Kono,H., Lin,S.H. & Schlag,E.W. On the role of low-
frequency modes in the energy or temperature
dependence of intersystem crossing. Chem. Phys. Lett.
145, 280–285 (1988).
13. Fukumura,H., Kikuchi,K., Koike,K. & Kokubun,H.
Temperature effect on inverse intersystem crossing of
anthracenes. J.Photochem. Photobiol. A Chem. 42,
283–291 (1988).
14. Tanaka,F., Okamoto,M. & Hirayama,S. Pressure
and temperature dependences of the rate constant
for S1-T2 intersystem crossing of anthracene
compounds in solution. J.Phys. Chem. 99, 525–530
(1995).
15. Ziegler,T., Rauk,A. & Baerends,E.J. On the calculation
of multiplet energies by the Hartree-Fock-Slater
method. Theor. Chim. Acta 43, 261–271 (1977).
16. El-Sayed,M.A. Spin-orbit coupling and the
radiationless processes in nitrogen heterocyclics.
J.Chem. Phys. 38, 2834–2838 (1963).
17. Czerwieniec,R., Leitl,M.J., Homeier,H.H.H. &
Yersin,H. Cu(I) complexes — thermally activated
delayed fluorescence. Photophysical approach and
material design. Coord. Chem. Rev. 325, 2–28
(2016).
18. Pan,Y. etal. High yields of singlet excitons in organic
electroluminescence through two paths of cold and hot
excitons. Adv. Opt. Mater. 2, 510–515 (2014).
19. Wang,S. etal. Highly efficient near-infrared delayed
fluorescence organic light emitting diodes using a
phenanthrene-based charge-transfer compound.
Angew. Chem. Int. Ed. 54, 13068–13072 (2015).
20. Samanta,P.K., Kim,D., Coropceanu,V. & Brédas,J.-L.
Up-conversion intersystem crossing rates in organic
emitters for thermally activated delayed fluorescence:
impact of the nature of singlet versus triplet excited
states. J.Am. Chem. Soc. 139,
4042–4051 (2017).
21. Dias,F.B. etal. Triplet harvesting with 100% efficiency
by way of thermally activated delayed fluorescence in
charge transfer OLED emitters. Adv. Mater. 25,
3707–3714 (2013).
This study provides a systematic discussion of the
factors affecting the efficiency of TADF
small-molecule OLEDs, in which detailed molecular
structure–photophysics relationships are revealed.
22. Malrieu,J.-P. & Pullman,B. Configuration spatiale et
propriétés électroniques du noyau d’isoalloxazine.
Theor. Chim. Acta 2, 302–314 (1964).
23. Aizenshtat,Z., Klein,E., Weiler-Feilchenfeld,H. &
Bergmann,E. D. Conformational studies on xanthene,
thioxanthene and acridan. Isr. J.Chem. 10, 753–763
(1972).
24. Zhang,Q. etal. Anthraquinone-based intramolecular
charge-transfer compounds: computational molecular
design, thermally activated delayed fluorescence, and
highly efficient red electroluminescence. J.Am. Chem.
Soc. 136, 18070–18081 (2014).
25. Endo,A. etal. Efficient up-conversion of triplet
excitons into a singlet state and its application for
organic light emitting diodes. Appl. Phys. Lett. 98,
083302 (2011).
This study was the starting point for purely organic
TADF emitters with a very small singlet–triplet gap.
26. Zhang,Q. etal. Efficient blue organic light-emitting
diodes employing thermally activated delayed
fluorescence. Nat. Photon. 8, 326–332 (2014).
27. Lee,D.R. etal. Design strategy for 25% external
quantum efficiency in green and blue thermally
activated delayed fluorescent devices. Adv. Mater. 27,
5861–5867 (2015).
28. Li,J. etal. Highly efficient organic light-emitting diode
based on a hidden thermally activated delayed
fluorescence channel in a heptazine derivative. Ad v.
Mater. 25, 3319–3323 (2013).
29. Lin,T.A. etal. Sky-blue organic light emitting diode
with 37% external quantum efficiency using
thermally activated delayed fluorescence from
spiroacridine-triazine hybrid. Adv. Mater. 28,
6976–6983 (2016).
This study demonstrates that the maximum EQE of
TADF small-molecule OLEDs fabricated by vacuum
deposition reaches 36.7%.
30. Tsai,W.L. etal. A versatile thermally activated
delayed fluorescence emitter for both highly efficient
doped and non-doped organic light emitting devices.
Chem. Commun. 51, 13662–13665 (2015).
31. Im,Y. etal. Molecular design strategy of organic
thermally activated delayed fluorescence emitters.
Chem. Mater. 29, 1946–1963 (2017).
32. Hatakeyama,T. etal. Ultrapure blue thermally
activated delayed fluorescence molecules: efficient
HOMO-LUMO separation by the multiple resonance
effect. Adv. Mater. 28, 2777–2781 (2016).
33. Cho,Y.J., Jeon,S.K., Lee,S.-S., Yu,E. & Lee,J.Y.
Donor interlocked molecular design for fluorescence-
like narrow emission in deep blue thermally activated
delayed fluorescent emitters. Chem. Mater. 28,
5400–5405 (2016).
34. Lee,I. & Lee,J.Y. Molecular design of deep blue
fluorescent emitters with 20% external quantum
efficiency and narrow emission spectrum. Org.
Electron. 29, 160–164 (2016).
In this study, deep-blue TADF OLEDs are reported
based on the rational design of the emitter
molecule and host material.
35. Cui,L.-S. etal. Long-lived efficient delayed
fluorescence organic light-emitting diodes using n-type
hosts. Nat. Commun. 8, 2250 (2017).
36. Kim,H.M., Choi,J.M. & Lee,J.Y. Blue thermally
activated delayed fluorescent emitters having a
bicarbazole donor moiety. RSC Adv. 6, 64133–64139
(2016).
37. Lee,D.R., Choi,J.M., Lee,C.W. & Lee,J.Y. Ideal
molecular design of blue thermally activated delayed
fluorescent emitter for high efficiency, small singlet-
triplet energy splitting, low efficiency roll-off, and long
lifetime. ACS Appl. Mater. Interfaces 8,
23190–23196 (2016).
38. Parker,C.A. & Hatchard,C.G. Triplet-singlet emission
in fluid solutions: phosphorescence of eosin. Trans.
Faraday Soc. 57, 1894–1904 (1961).
REVIEWS
NATURE REVIEWS
|
MATERIALS VOLUME 3
|
ARTICLE NUMBER 18020
|
17

39. Endo,A. etal. Thermally activated delayed
fluorescence from Sn4+-porphyrin complexes and their
application to organic light emitting diodes — a novel
mechanism for electroluminescence. Adv. Mater. 21,
4802–4806 (2009).
40. Chen,X.-L. etal. Rational design of strongly blue-
emitting cuprous complexes with thermally activated
delayed fluorescence and application in solution-
processed OLEDs. Chem. Mater. 25, 3910–3920
(2013).
41. Liang,D. etal. Highly efficient cuprous complexes with
thermally activated delayed fluorescence for solution-
processed organic light-emitting devices. Inorg. Chem.
55, 7467–7475 (2016).
42. Hofbeck,T. Monkowius,U. & Yersin,H. Highly efficient
luminescence of Cu(I) compounds: thermally activated
delayed fluorescence combined with short-lived
phosphorescence. J.Am. Chem. Soc. 137, 399–404
(2015).
43. Volz,D. etal. Bridging the efficiency gap: fully bridged
dinuclear Cu(I)-complexes for singlet harvesting in
high-efficiency OLEDs. Adv. Mater. 27, 2538–2543
(2015).
44. Mehes,G., Nomura,H., Zhang,Q., Nakagawa,T. &
Adachi,C. Enhanced electroluminescence efficiency in
a spiro-acridine derivative through thermally activated
delayed fluorescence. Angew. Chem. Int. Ed. 51,
11311–11315 (2012).
45. Kim,B.S. & Lee,J.Y. Engineering of mixed host for
high external quantum efficiency above 25% in green
thermally activated delayed fluorescence device. Ad v.
Funct. Mater. 24, 3970–3977 (2014).
46. Lee,D.R. etal. Above 30% external quantum
efficiency in green delayed fluorescent organic light-
emitting diodes. ACS Appl. Mater. Interfaces 7,
9625–9629 (2015).
47. Sun,J.W. etal. A fluorescent organic light-emitting
diode with 30% external quantum efficiency. Ad v.
Mater. 26, 5684–5688 (2014).
48. Zeng,W. etal. Achieving nearly 30% external
quantum efficiency for orange-red organic light
emitting diodes by employing thermally activated
delayed fluorescence emitters composed of 1,
8-naphthalimide-acridine hybrids. Adv. Mater. 30,
1704961 (2017).
49. Li,Y., Xie,G., Gong,S., Wu,K. & Yang,C. Dendronized
delayed fluorescence emitters for non-doped, solution-
processed organic light-emitting diodes with high
efficiency and low efficiency roll-off simultaneously: two
parallel emissive channels. Chem. Sci. 7, 5441–5447
(2016).
50. Ren,Z. etal. Pendant homopolymer and copolymers
as solution-processable thermally activated delayed
fluorescence materials for organic light-emitting
diodes. Macromolecules 49, 5452–5460
(2016).
In this study, the maximum EQE of a polymer OLED
reaches 20% using the strategy of pendant TADF
moieties in the side chain of a polymer.
51. Lee,S.Y., Yasuda,T., Yang,Y.S., Zhang,Q. &
Adachi,C. Luminous butterflies: efficient exciton
harvesting by benzophenone derivatives for full-color
delayed fluorescence OLEDs. Angew. Chem. Inter. Ed.
53, 6402–6406 (2014).
52. Ban,X. etal. Design of encapsulated hosts and guests
for highly efficient blue and green thermally activated
delayed fluorescence OLEDs based on a solution-
process. Chem. Commun. 53,
11834–11837 (2017).
53. Tsang,D.P.K., Matsushima,T. & Adachi,C.
Operational stability enhancement in organic light-
emitting diodes with ultrathin Liq interlayers. Sci. Rep.
6, 22463 (2016).
54. Cho,Y.J., Yook,K.S. & Lee,J.Y. A universal host
material for high external quantum efficiency close to
25% and long lifetime in green fluorescent and
phosphorescent OLEDs. Adv. Mater. 26, 4050–4055
(2014).
55. Wong,M.Y. & Zysman-Colman,E. Purely organic
thermally activated delayed fluorescence materials for
organic light-emitting diodes. Adv. Mater. 29,
1605444 (2017).
56. Ritchie,J., Crayston,J.A., Markham,J.P. &
Samuel,I.D. Effect of meta-linkages on the
photoluminescence and electroluminescence
properties of light-emitting polyfluorene alternating
copolymers. J.Mater. Chem. 16, 1651–1656
(2006).
57. Ahn,T., Jang,M.S., Shim,H.K., Hwang,D.H. &
Zyung,T. Blue electroluminescent polymers: control of
conjugation length by kink linkages and substituents
in the poly (p-phenylenevinylene)-related copolymers.
Macromolecules 32, 3279–3285 (1999).
58. Tanaka,H., Shizu,K., Miyazaki,H. & Adachi,C.
Efficient green thermally activated delayed
fluorescence (TADF) from a phenoxazine-
triphenyltriazine (PXZ-TRZ) derivative. Chem. Commun.
48, 11392–11394 (2012).
59. Kaji,H. etal. Purely organic electroluminescent
material realizing 100% conversion from electricity to
light. Nat. Commun. 6, 8476 (2015).
60. Shizu,K. etal. Highly efficient blue
electroluminescence using delayed-fluorescence
emitters with large overlap density between
luminescent and ground states. J.Phys. Chem. C 119 ,
26283–26289 (2015).
61. Serevicius,T. etal. Enhanced electroluminescence
based on thermally activated delayed fluorescence
from a carbazole-triazine derivative. Phys. Chem.
Chem. Phys. 15, 15850–15855 (2013).
62. Hirata,S. etal. Highly efficient blue
electroluminescence based on thermally activated
delayed fluorescence. Nat. Mater. 14, 330–336
(2015).
This report reveals the compatibility between low
ΔEst and high PLQY and produces a blue emitter
with near 100% internal electroluminescence
quantum yield.
63. Park,I.S., Lee,S.Y., Adachi,C. & Yasuda,T. Full-color
delayed fluorescence materials based on wedge-
shaped phthalonitriles and dicyanopyrazines:
systematic design, tunable photophysical properties,
and OLED performance. Adv. Funct. Mater. 26,
1813–1821 (2016).
64. Liu,M. etal. Blue thermally activated delayed
fluorescence materials based on bis(phenylsulfonyl)
benzene derivatives. Chem. Commun. 51,
16353–16356 (2015).
65. Wang,Z. etal. Structure-performance investigation of
thioxanthone derivatives for developing color tunable
highly efficient thermally activated delayed
fluorescence emitters. ACS Appl. Mater. Interfaces 8,
8627–8636 (2016).
66. Kawasumi,K. etal. Thermally activated delayed
fluorescence materials based on homoconjugation
effect of donor-acceptor triptycenes. J.Am. Chem. Soc.
137, 11908–11911 (2015).
67. Nakagawa,T., Ku,S.Y., Wong,K.T. & Adachi,C.
Electroluminescence based on thermally activated
delayed fluorescence generated by a spirobifluorene
donor-acceptor structure. Chem. Commun. 48, 9580–
9582 (2012).
68. Chen,X.-K. etal. A new design strategy for efficient
thermally activated delayed fluorescence organic
emitters: from twisted to planar structures. Adv.
Mater. 29, 1702767 (2017).
69. Nishimoto,T., Yasuda,T., Lee,S.Y., Kondo,R. &
Adachi,C. A six-carbazole-decorated
cyclophosphazene as a host with high triplet energy to
realize efficient delayed-fluorescence OLEDs. Mater.
Horiz. 1, 264–269 (2014).
70. Lin,M.-S. etal. Incorporation of a CN group into mCP:
a new bipolar host material for highly efficient blue
and white electrophosphorescent devices. J.Mater.
Chem. 22, 16114–16120 (2012).
71. Cho,Y.J., Yook,K.S. & Lee,J.Y. High efficiency in a
solution-processed thermally activated delayed-
fluorescence device using a delayed-fluorescence
emitting material with improved solubility. Adv. Mater.
26, 6642–6646 (2014).
This paper pioneers solution-processed TADF
small-molecule emitting materials: the
corresponding OLED device exhibits a high electro-
luminescence efficiency of 18.3%.
72. Tsai,Y.S., Hong,L.A., Juang,F.S. & Chen,C.Y. Blue
and white phosphorescent organic light emitting diode
performance improvement by confining electrons and
holes inside double emitting layers. J.Lumin. 153,
312–316 (2014).
73. Senes,A. etal. Increasing the horizontal orientation of
transition dipole moments in solution processed small
molecular emitters. J.Mater. Chem. C 5, 6555–6562
(2017).
74. Kim,B.S. & Lee,J.Y. Phosphine oxide type bipolar
host material for high quantum efficiency in thermally
activated delayed fluorescent device. ACS Appl. Mater.
Interfaces 6, 8396–8400 (2014).
75. Seino,Y., Inomata,S., Sasabe,H., Pu,Y.J. & Kido,J.
High-performance green OLEDs using thermally
activated delayed fluorescence with a power efficiency
of over 100 lm W−1. Adv. Mater. 28, 2638–2643
(2016).
76. Gaj,M.P., Fuentes-Hernandez,C., Zhang,Y.,
Marder,S.R. & Kippelen,B. Highly efficient organic
light-emitting diodes from thermally activated
delayed fluorescence using a sulfone–carbazole host
material. Org. Electron. 16, 109–112 (2015).
77. Fan,C. etal. Dibenzothiophene-based phosphine
oxide host and electron-transporting materials for
efficient blue thermally activated delayed fluorescence
diodes through compatibility optimization. Chem.
Mater. 27, 5131–5140 (2015).
78. Ding,D., Zhang,Z., Wei,Y., Yan,P. & Xu,H. Spatially
optimized quaternary phosphine oxide host materials
for high-efficiency blue phosphorescence and
thermally activated delayed fluorescence organic
light-emitting diodes. J.Mater. Chem. C 3,
11385–11396 (2015).
79. Zhang,J. etal. Multiphosphine-oxide hosts for
ultralow-voltage-driven true-blue thermally activated
delayed fluorescence diodes with external quantum
efficiency beyond 20%. Adv. Mater. 28, 479–485
(2016).
80. Dos Santos,P. etal. Using guest-host interactions to
optimize the efficiency of TADF OLEDs. J.Phys. Chem.
Lett. 7, 3341–3346 (2016).
81. Nishide,J., Nakanotani,H., Hiraga,Y. & Adachi,C.
High-efficiency white organic light-emitting diodes
using thermally activated delayed fluorescence. Appl.
Phys. Lett. 104, 233304 (2014).
82. Zhang,D., Duan,L., Li,Y., Zhang,D. & Qiu,Y. Highly
efficient and color-stable hybrid warm white organic
light-emitting diodes using a blue material with
thermally activated delayed fluorescence. J.Mater.
Chem. C 2, 8191–8197 (2014).
83. Li,X.L. etal. High-efficiency WOLEDs with high color-
rendering index based on a chromaticity-adjustable
yellow thermally activated delayed fluorescence
emitter. Adv. Mater. 28, 4614–4619 (2016).
84. Kim,S.-Y. etal. Organic light-emitting diodes with
30% external quantum efficiency based on a
horizontally oriented emitter. Adv. Funct. Mater. 23,
3896–3900 (2013).
85. Nowy,S., Krummacher,B.C., Frischeisen,J.,
Reinke,N.A. & Brütting,W. Light extraction and
optical loss mechanisms in organic light-emitting
diodes: Influence of the emitter quantum efficiency.
J.Appl. Phys. 104, 123109 (2008).
86. Mayr,C. etal. Efficiency enhancement of organic
light-emitting diodes incorporating a highly oriented
thermally activated delayed fluorescence emitter.
Adv. Funct. Mater. 24, 5232–5239
(2014).
87. Zhang,Q. etal. Nearly 100% internal quantum
efficiency in undoped electroluminescent devices
employing pure organic emitters. Adv. Mater. 27,
2096–2100 (2015).
88. Guo,J. etal. Achieving high-performance nondoped
OLEDs with extremely small efficiency roll-off by
combining aggregation-induced emission and
thermally activated delayed fluorescence. Adv. Funct.
Mater. 27, 1606458 (2017).
89. Huang,J. etal. Highly efficient nondoped OLEDs with
negligible efficiency roll-off fabricated from
aggregation-induced delayed fluorescence
luminogens. Angew. Chem. Int. Ed. 129,
13151–13156 (2017).
90. Goushi,K. & Adachi,C. Efficient organic light-emitting
diodes through up-conversion from triplet to singlet
excited states of exciplexes. Appl. Phys. Lett. 101,
023306 (2012).
91. Liu,X.K. etal. Prediction and design of efficient
exciplex emitters for high-efficiency, thermally
activated delayed-fluorescence organic light-emitting
diodes. Adv. Mater. 27, 2378–2383
(2015).
92. Liu,W. etal. Novel strategy to develop exciplex
emitters for high-performance OLEDs by employing
thermally activated delayed fluorescence materials.
Adv. Funct. Mater. 26, 2002–2008 (2016).
93. Duan,L. etal. Solution processable small molecules
for organic light-emitting diodes. J.Mater. Chem. 20,
6392–6407 (2010).
94. Cho,Y.J., Chin,B.D., Jeon,S.K. & Lee,J.Y. 20%
external quantum efficiency in solution-processed
blue thermally activated delayed fluorescent devices.
Adv. Funct. Mater. 25, 6786–6792 (2015).
95. Mei,L. etal. The inductive-effect of electron
withdrawing trifluoromethyl for thermally activated
delayed fluorescence: tunable emission from tetra-
to penta-carbazole in solution processed blue OLEDs.
Chem. Commun. 51, 13024–13027
(2015).
REVIEWS
18
|
ARTICLE NUMBER 18020
|
VOLUME 3 www.nature.com/natrevmats

96. Chen,P. etal. Delayed fluorescence in a solution-
processable pure red molecular organic emitter based
on dithienylbenzothiadiazole: a joint optical,
electroluminescence, and magneto
electroluminescence study. ACS Appl. Mater.
Interfaces 7, 2972–2978 (2015).
97. Wada,Y. etal. Highly efficient electroluminescence
from a solution-processable thermally activated
delayed fluorescence emitter. Appl. Phys. Lett. 107,
183303 (2015).
98. Xie,G. etal. Evaporation- and solution-process-
feasible highly efficient thianthrene-9,9,10,10’-
tetraoxide-based thermally activated delayed
fluorescence emitters with reduced efficiency roll-off.
Adv. Mater. 28, 181–187 (2016).
99. Schmidbauer,S., Hohenleutner,A. & König,B.
Chemical degradation in organic light-emitting
devices: mechanisms and implications for the design
of new materials. Adv. Mater.25, 2114–2129
(2013).
100. Cui,L.S. etal. Pure hydrocarbon hosts for ≈ 100%
exciton harvesting in both phosphorescent and
fluorescent light-emitting devices. Adv. Mater. 27,
4213–4217 (2015).
101. Cui,L.S. etal. Controlling synergistic oxidation
processes for efficient and stable blue thermally
activated delayed fluorescence devices. Adv. Mater.
28, 7620–7625 (2016).
102. Kanibolotsky,A.L., Perepichka,I.F. & Skabara,P.J.
Star-shaped π-conjugated oligomers and their
applications in organic electronics and photonics.
Chem. Soc. Rev. 39, 2695–2728 (2010).
103. Burn,P.L., Lo,S.C. & Samuel,D.W. The development
of light-emitting dendrimers for displays. Adv. Mater.
19, 1675–1688 (2007).
104. Albrecht,K., Matsuoka,K., Fujita,K. & Yamamoto,K.
Carbazole dendrimers as solution-processable
thermally activated delayed-fluorescence materials.
Angew. Chem. Int. Ed. 54, 5677–5682
(2015).
In this study, a solution-processed dendrimer TADF
emitter is reported with a maximum EQE of 3.4%.
105. Luo,J. etal. Multi-carbazole encapsulation as a simple
strategy for the construction of solution-processed,
non-doped thermally activated delayed fluorescence
emitters. J.Mater. Chem. C 4,
2442–2446 (2016).
106. Albrecht,K. etal. Thermally activated delayed
fluorescence OLEDs with fully solution processed
organic layers exhibiting nearly 10% external
quantum efficiency. Chem. Commun. 53, 2439–2442
(2017).
107. Li,J. etal. Deep-blue thermally activated delayed
fluorescence dendrimers with reduced singlet-trip let
energy gap for low roll-off non-doped solution-
processed organic light-emitting diodes. Dyes
Pigments 140, 79–86 (2017).
108. Matsuoka,K., Albrecht,K., Yamamoto,K. & Fujita,K.
Mulifunctional dendritic emitter: aggregation-induced
emission enhanced, thermally activated delayed
fluorescent material for solution-processed
multilayered organic light-emitting diodes. Sci. Rep. 7,
41780 (2017).
109. Ban,X. etal. Self-host thermally activated delayed
fluorescent dendrimers with flexible chains: an
effective strategy for non-doped electroluminescent
devices based on solution processing. J.Mater. Chem.
C 4, 8810–8816 (2016).
110 . Ban,X., Jiang,W., Sun,K., Lin,B. & Sun,Y. Self-host
blue dendrimer comprised of thermally activated
delayed fluorescence core and bipolar dendrons for
efficient solution-processable nondoped
electroluminescence. ACS Appl. Mater. Interfaces 9,
7339–7346 (2017).
111. Sun,K. etal. Design strategy of yellow thermally
activated delayed fluorescent dendrimers and their
highly efficient non-doped solution-processed OLEDs
with low driving voltage. Org. Electron. 42, 123–130
(2017).
112 . Godumala,M. etal. Novel dendritic large molecules as
solution-processable thermally activated delayed
fluorescent emitters for simple structured non-doped
organic light emitting diodes. J.Mater. Chem. C 6,
1160–1170 (2018).
113 . Sun,K. etal. Thermally activated delayed
fluorescence dendrimers with exciplex-forming
dendrons for low-voltage-driving and power-efficient
solution-processed OLEDs. J.Mater. Chem. C 6,
43–49 (2018).
114 . Wang,Y. etal. Engineering the interconnecting
position of star-shaped donor-pi-acceptor molecules
based on triazine, spirofluorene, and triphenylamine
moieties for color tuning from deep blue to green.
Chem. Asian J. 11, 2555–2563 (2016).
115 . Xia,D. etal. Self-host blue-emitting Iridium dendrimer
with carbazole dendrons: nondoped phosphorescent
organic light-emitting diodes. Angew. Chem. Int. Ed.
53, 1048–1052 (2014).
116 . Chen,L. etal. Self-host heteroleptic green iridium
dendrimers: achieving efficient non-doped device
performance based on a simple molecular structure.
Chem. Commun. 47, 9519–9521 (2011).
117 . Albrecht,K., Kasai,Y., Kimoto,A. & Yamamoto,K. The
synthesis and properties of carbazole-
phenylazomethine double layer-type dendrimers.
Macromolecules 41, 3793–3800 (2008).
118 . Zhao,Z.H. etal. Synthesis and properties of dendritic
emitters with a fluorinated starburst oxadiazole core
and twisted carbazole dendrons. Macromolecules 44,
1405–1413 (2011).
119 . Ding,J. etal. Bifunctional green Iridium dendrimers
with a “self-host” feature for highly efficient nondoped
electrophosphorescent devices. Angew. Chem. Int. Ed.
48, 6664–6666 (2009).
120. Albrecht,K., Pernites,R., Felipe,M.J., Advincula,R.C.
& Yamamoto,K. Patterning carbazole-
phenylazomethine dendrimer films. Macromolecules
45, 1288–1295 (2012).
121. Kimoto,A. etal. Novel hole-transport material for
efficient polymer light-emitting diodes by
photoreaction. Macromol. Rapid Commun. 26,
597–601 (2005).
122. Yook,K.S. & Lee,J.Y. Small molecule host materials
for solution processed phosphorescent organic light-
emitting diodes. Adv. Mater. 26, 4218–4233 (2014).
123. Chen,D. etal. Fluorescent organic planar p-n
heterojunction light-emitting diodes with simplified
structure, extremely low driving voltage, and high
efficiency. Adv. Mater. 28, 239–244 (2016).
124. Sekine,C., Tsubata,Y., Yamada,T., Kitano,M. &
Doi,S. Recent progress of high performance polymer
OLED and OPV materials for organic printed
electronics. Sci. Technol. Adv. Mater. 15, 34203
(2014).
125. Grimsdale,A.C., Chan,K.L., Martin,R.E.,
Jokisz,P.G. & Holmes,A.B. Synthesis of light-emitting
conjugated polymers for applications in
electroluminescent devices. Chem. Rev. 109,
897–1091 (2009).
126. Rothe,C. & Monkman,A. Regarding the origin of the
delayed fluorescence of conjugated polymers. J.Chem.
Phys. 123, 244904 (2005).
127. Lee,S.Y., Yasuda,T., Komiyama,H., Lee,J. &
Adachi,C. Thermally activated delayed fluorescence
polymers for efficient solution-processed organic light-
emitting diodes. Adv. Mater. 28, 4019–4024 (2016).
128. Zhu,Y. etal. Synthesis and electroluminescence of a
conjugated polymer with thermally activated delayed
fluorescence. Macromolecules 49, 4373–4377
(2016).
129. Nikolaenko,A.E., Cass,M., Bourcet,F., Mohamad,D.
& Roberts,M. Thermally activated delayed
fluorescence in polymers: a new route toward highly
efficient solution processable OLEDs. Adv. Mater. 27,
7236–7240 (2015).
130. Nobuyasu,R.S. etal. Rational design of TADF
polymers using a donor-acceptor monomer with
enhanced TADF efficiency induced by the energy
alignment of charge transfer and local triplet excited
states. Adv. Opt. Mater. 4, 597–607 (2016).
131. Wang,Y. etal. Bright white electroluminescence from
a single polymer containing a thermally activated
delayed fluorescence unit and a solution-processed
orange OLED approaching 20% external quantum
efficiency. J.Mater. Chem. C 5, 10715–10720
(2017).
132. Wang,Y. etal. Efficient non-doped yellow OLEDs
based on thermally activated delayed fluorescence
conjugated polymers with an acridine/carbazole donor
backbone and triphenyltriazine acceptor pendant.
J.Mater. Chem. C 6, 568–574(2018).
133. Luo,J., Xie,G., Gong,S., Chen,T. & Yang,C. Creating
a thermally activated delayed fluorescence channel in
a single polymer system to enhance exciton utilization
efficiency for bluish-green electroluminescence. Chem.
Commun. 52, 2292–2295 (2016).
134. Xie,G. etal. Inheriting the characteristics of TADF
small molecule by side-chain engineering strategy to
enable bluish-green polymers with high PLQYs up to
74% and external quantum efficiency over 16% in
light-emitting diodes. Adv. Mater. 29, 1604223
(2017).
135. Sun,K. etal. Thermally cross-linkable thermally
activated delayed fluorescent materials for efficient
blue solution-processed organic light-emitting diodes.
J.Mater. Chem. C 4, 8973–8979
(2016).
136. Shao,S. etal. Blue thermally activated delayed
fluorescence polymers with nonconjugated backbone
and through-space charge transfer effect. J.Am.
Chem. Soc. 139, 17739–17742 (2017).
137. Köhler,A. & Beljonne,D. The singlet-triplet exchange
energy in conjugated polymers. Adv. Funct. Mater. 14,
11–18 (2004).
138. Wei,Q. etal. Conjugation-induced thermally activated
delayed fluorescence (TADF): from conventional non-
TADF units to TADF-active polymers. Adv. Funct.
Mater. 27, 1605051 (2017).
139. Reineke,S. Organic light-emitting diodes:
Phosphorescence meets its match. Nat. Photon. 8,
269–270 (2014).
140. Dias,F.B. Kinetics of thermal-assisted delayed
fluorescence in blue organic emitters with large
singlet-triplet energy gap. Phil. Trans. R.Soc. A 373,
20140447 (2015).
141. Uchida,M., Adachi,M., Koyama,T. & Taniguchi,Y.
Charge carrier trapping effect by luminescent dopant
molecules in single-layer organic light emitting
diodes. J.Appl. Phys. 86, 1680–1687
(1999).
142. Tsung,K.K. & So,S.K. Carrier trapping and scattering
in amorphous organic hole transporter. Appl. Phys.
Lett. 92, 103315 (2008).
143. Wang,S. etal. Ultrahigh color-stable, solution-
processed, white OLEDs using a dendritic binary host
and long-wavelength dopants with different charge
trapping depths. Adv. Opt. Mater. 3, 1349–1354
(2015).
144. Zhang,Y. & Forrest,S.R. Triplets contribute to both
an increase and loss in fluorescent yield in organic
light emitting diodes. Phys. Rev. Lett. 108, 267404
(2012).
145. Lin,X. etal. Highly efficient TADF polymer
electroluminescence with reduced efficiency roll-off via
interfacial exciplex host strategy. ACS Appl. Mater.
Interfaces 10, 47–52 (2018).
146. Murawski,C., Leo,K. & Gather,M.C. Efficiency roll-off
in organic light-emitting diodes. Adv. Mater. 25,
6801–6827 (2013).
147. Fu,Q., Chen,J., Shi,C. & Ma,D. Solution-processed
small molecules as mixed host for highly efficient blue
and white phosphorescent organic light-emitting
diodes. ACS Appl. Mater. Interfaces 4, 6579–6586
(2012).
148. Wu,W., Tang,R., Li,Q. & Li,Z. Functional
hyperbranched polymers with advanced optical,
electrical and magnetic properties. Chem. Soc. Rev.
44, 3997–4022 (2015).
149. Li,C. etal. Thermally activated delayed fluorescence
pendant copolymers with electron and hole-
transporting spacers. ACS Appl. Mater. Interfaces 10,
5731–5739 (2018).
150. Ward,J.S. etal. The interplay of thermally activated
delayed fluorescence (TADF) and room temperature
organic phosphorescence in sterically-constrained
donor-acceptor charge-transfer molecules. Chem.
Commun. 52, 2612–2615 (2016).
151. Furue,R., Nishimoto,T., Park,I.S., Lee,J. &
Yasuda,T. Aggregation-induced delayed fluorescence
based on donor/acceptor-tethered janus carborane
triads: unique photophysical properties of nondoped
OLEDs. Angew. Chem. Int. Ed. 128, 7287–7291
(2016).
152. Li,C. etal. Solution-processable thermally activated
delayed fluorescence white OLEDs based on dual-
emission polymers with tunable emission colors and
aggregation-enhanced emission properties. Adv. Opt.
Mater. 5, 1700435 (2017).
This report provides a novel route of combining
TADF and aggregation-induced emission in a
polymeric emitter to enhance the efficiency.
153. Shinar,J. & Shinar,R. Organic light-emitting devices
(OLEDs) and OLED-based chemical and biological
sensors: an overview. J.Phys. D Appl. Phys. 41,
133001 (2008).
154. Freeman,D.M.E. etal. Synthesis and exciton
dynamics of donor-orthogonal acceptor conjugated
polymers: reducing the singlet-triplet energy gap.
J.Am. Chem. Soc. 139, 11073–11080
(2017).
155. Xie,Y. & Li,Z. Thermally activated delayed fluorescent
polymers. J.Polym. Sci. Part A Polym. Chem. 55,
575–584 (2017).
REVIEWS
NATURE REVIEWS
|
MATERIALS VOLUME 3
|
ARTICLE NUMBER 18020
|
19

156. Huang,S. etal. Computational prediction for singlet-
and triplet-transition energies of charge-transfer
compounds. J.Chem. Theory Comput. 9, 3872–3877
(2013).
157. Peng,Q. etal. Theoretical study of conversion and
decay processes of excited triplet and singlet states
in a thermally activated delayed fluorescence
molecule. J.Phys. Chem. C 121, 13448–13456
(2017).
158. Tian,X., Sun,H., Zhang,Q. & Adachi,C. Theoretical
predication for transition energies of thermally
activated delayed fluorescence molecules. Chinese
Chem. Lett. 27, 1445–1452 (2016).
159. Chen,X.K., Zhang,S.F., Fan,J.X. & Ren,A.M.
Nature of highly efficient thermally activated delayed
fluorescence in organic light-emitting diode emitters:
nonadiabatic effect between excited states. J.Phys.
Chem. C 119 , 9728–9733 (2015).
160. Marian,C.M. Mechanism of the triplet-to-singlet up
conversion in the assistant dopant Acrxtn. J.Phys.
Chem. C 120, 3715–3721 (2016).
161. Nakanotani,H. etal. High-efficiency organic light-
emitting diodes with fluorescent emitters. Nat.
Commun. 5, 4016 (2014).
162. Meng,L. etal. Highly efficient nondoped organic
light emitting diodes based on thermally activated
delayed fluorescence emitter with quantum well
structure. ACS Appl. Mater. Interfaces 8,
20955–20961 (2016).
Author contributions
All authors researched data for the article and contributed to
the discussion of content, as well as the writing and editing of
the article, before submission.
Acknowledgements
The financial support of the National Natural Science
Foundations of China under Grant Nos. 51521062 and
21274009 (Z.R. and S.Y.) and the Engineering and Physical
Sciences Research Council (EPSRC) Grant No. EL/L02621X/1
(M.R.B.) is gratefully acknowledged.
Competing interests
The authors declare no competing interests.
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional
claims in published maps and institutional affiliations.
REVIEWS
20
|
ARTICLE NUMBER 18020
|
VOLUME 3 www.nature.com/natrevmats