Solution-processed white organic light-emitting diodes with bi-component emitting layer based on symmetry blue spiro-sulfone derivative

Article abstract of DOI:10.1016/j.orgel.2019.04.034

We report a white organic light-emitting diode (WOLED)of simple construction with a bi-component emitting layer, in which a novel, dual-role deep blue TADF emitter as both the non-doped blue emitter and efficient host of an orange TADF emitter of TXO-TPA.

Full text of DOI:10.1016/j.orgel.2019.04.034

Accepted Manuscript
Solution-processed white organic light-emitting diodes with bi-component emitting
layer based on symmetry blue spiro-sulfone derivative
Ruifang Wang, Yanwei Liu, Taiping Hu, Xiaofang Wei, Jianjun Liu, Zhiyi Li, Xiaoxiao
Hu, Yuanping Yi, Pengfei Wang, Ying Wang
PII:
S1566-1199(19)30205-8
DOI:
https://doi.org/10.1016/j.orgel.2019.04.034
ORGELE 5228
Reference:
To appear in: Organic Electronics
Received Date: 25 March 2019
Revised Date: 19 April 2019
Accepted Date: 24 April 2019
Please cite this article as: R. Wang, Y. Liu, T. Hu, X. Wei, J. Liu, Z. Li, X. Hu, Y. Yi, P. Wang, Y.
Wang, Solution-processed white organic light-emitting diodes with bi-component emitting layer based
on symmetry blue spiro-sulfone derivative, Organic Electronics (2019), doi: https://doi.org/10.1016/
j.orgel.2019.04.034.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Solution-Processed White Organic Light-Emitting Diodes
with bi-Component Emitting Layer Based on Symmetry
Blue Spiro-Sulfone Derivative
1
,2†
1,2†
3
1,2
1,2
1,2
1,2
3*
Ruifang Wang , Yanwei Liu , Taiping Hu , Xiaofang Wei , Jianjun Liu , Zhiyi Li , Xiaoxiao Hu , Yuanping Yi , Pengfei
1
,2
1,2*
Wang , and Ying Wang
1
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry,
Chinese Academy of Sciences, Beijing, 100190, China.
wangy@mail.ipc.ac.cn
2
University of Chinese Academy of Sciences, Beijing, 100049, China.
3
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese
Academy of Sciences, Beijing, 100190, China.
ypyi@iccas.ac.cn
†
These authors contributed equally to this work.
Abstract
We report a white organic light-emitting diode (WOLED) of simple construction with a
bi-component emitting layer, in which a novel, dual-role deep blue TADF emitter as both the
non-doped blue emitter and efficient host of an orange TADF emitter of TXO-TPA. The deep blue
molecule (SDPS-4PhCz, CIEy
∼ 0.04) was designed using spiro-diphenylsulfone (SDPS) as
electron-accepting group and four 9-phenyl-9H-carbazole (N-PhCz) as the electron-donating groups.
SDPS-4PhCz showed delayed fluorescence with short delayed lifetime of 1.4 µs via fast reverse
intersystem crossing (RISC) from “hot exciton” triplet excited state. The solution-processed,
non-doped deep blue inverted OLED device exhibited a maximum EQE of 4.92% with low efficiency
roll-off. The two-color WOLED with extremely simple device architecture showed an EQE above 17%,
CIE (0.39, 0.44) and high CRI of 64 by spin-coating method.
1
. Introduction
White organic light-emitting diodes (WOLEDs) have attracted significant attention in both

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scientific and industrial communities due to their great applications in full color displays and solid-state
lighting [1-4]. To realize high efficiency and high color rendering index (CRI) of WOLEDs, much
progress on all-phosphorescence and fluorescence-phosphorescence hybrid WOLEDs has been widely
made [5-8]. However, the low efficiency of fluorescence and low stability of phosphorescence devices
spur the exploration of new alternatives. Since Adachi and co-workers firstly reported WOLEDs based
on thermally activated delayed fluorescence (TADF) emitters [9], TADF emitters have been widely
used for WOLEDs with high efficiency and high CRI due to their theoretical 100% internal quantum
efficiency (IQE) and broad full width at half maximum (FWHM) [10, 11]. For example, Ma and
co-workers reported a TADF-based hybrid WOLED with superior efficiency/CRI/color stability and
low efficiency roll-off [12]. Su’s group reported WOLEDs based on all-fluorescence to give high
’
external quantum efficiency (EQE), satisfied Commission International de I Eclairage (CIE)
coordinates, CRI and color stability [13]. However, high efficiency with low roll-off and high CRI can
only be achieved for complicated WOLED device structure. Very recently, our group reported a high
'
efficient bi-component WOLED by using a new blue TADF of o,o -NPh and a yellow TADF of
2
TXO-PhCz4, an EQE of 12.5% and a high CRI of 71.0 were obtained due to the broad emission spectra
'
of both blue and yellow TADF emitter [14]. o,o-NPh serves as not only blue energy source but also
2
the efficient host of yellow TADF emitter to produce properly balanced color chromaticity in the
simple WOLEDs. Thus, preparing WOLEDs with higher efficiency and simple structure was desirable
[
15-17]. Although several blue TADF emitters have been reported since 2012, blue-emitting materials
have been the short board that hampers the performance of WOLEDs due to the problem of efficiency
and stability [18-20]. Especially the lack of deep blue TADF materials may make it difficult to fully
cover a wide range of the visible-light spectral region (380-780 nm) in the simple-structure WOLEDs,
resulting in low CRI of the WOLEDs [21]. All these inspire us to develop novel deep blue emitting
materials with high efficiency for the WOLED with simple structure.
A limited π-conjugation length and high rigidity geometry is required to achieve a deep blue
emitter because of its wide band gap. In recent years, spiro molecular system with its unique
advantages has attracted much attention in the field of organic photoelectric materials [22, 23]. The
spiro structure can relieve the self-aggregation of molecules, thereby reducing exciton annihilation,
which is expected to solve the problem of efficiency roll-off in non-doped devices [24]. Our group
reported an effective strategy of constructing emitter MTXSFCz with a thioxanthone (TX) acceptor and

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3
PhCz donor linked by a saturated sp hybridized carbon of spirofluorene, which served as host of
highly efficient PHOLEDs with low EQE roll-off [25]. In addition, the spiro materials possess several
advantages, such as improving the thermal [26] and morphological stabilities [27] increasing the
solubility [28] and charge-transport properties [29]. Lee and his co-workers reported a series of
tert-butylated spirofluorene derivatives, all devices based on them showed efficient blue emission with
good device performances [30]. In 2012, Adachi and co-workers developed a spiro derivative using
cyano as acceptor and diphenylamine/acridine as donors showing TADF property, which yet exhibited
green emission color [31, 32]. Herein, we report a spiro-diphenylsulfone based TADF material that
emits in deep blue region with CIE coordinates of (0.15, 0.04). Notably, solution-processed non-doped
single blue inverted electroluminescent (EL) device and highly efficient single-EML WOLED device
doped with orange TADF emitter have been demonstrated.
2
. Experimental details
2
.1 Materials and instruments
Unless otherwise noted, all chemicals and reagents were commercially available and were used
1
without further purification. Proton nuclear magnetic resonance ( H NMR) spectra were recorded on a
Bruker Advance 400M spectrometer using TMS as internal standard at room temperature. Mass spectra
were recorded using a BIFLEXIII mass spectrometer. Ultraviolet-visible (UV-vis) absorption spectra
and photoluminescence (PL) spectra were measured by using Hitachi U-3900 spectrophotometer and
F-4600 fluorescence spectrometer. The transient PL decay characteristics were measured via Laser
Flash Photolysis Spectrometer (LP980-KS, Edinburgh Instruments). The photoluminescence quantum
yields (PLQYs) were measured by Hamamatsu absolute PLQY spectrometer C11347. Differential
scanning calorimetry (DSC) was carried out using a TA Instruments DSC 2910 thermal analyzer under
−1
a nitrogen atmosphere at a heating rate of 10 °C min . Thermogravimetric analyses (TGA) were
performed on a TA Instruments TGA 2050 thermal analyzer. Ultraviolet photoelectron spectroscopy
(
UPS) was performed in a Kratos Axis Ultra Dld system using a He-I gas discharge lamp source with
the excitation energy of 21.22 eV and the energy resolution of 150 meV.
2
.2 Theoretical Calculations
All the calculations were performed with the Gaussian 09 program package at the

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B3LYP/6-31G** level. The molecular geometries of ground states for SDPS-4PhCz were optimized by
density functional theory (DFT). Based on the optimized geometries, time-dependent DFT (TDDFT)
was then used to calculate the lowest singlet (S ) and triplet (T ) states.
1
1
2
.3 Devices Fabrication and Characterization
Before device fabrication, the ZnO precursor was synthesized according to the method that was
described in previous report [33]. Accordingly, zinc acetate dehydrate (Zn(CH COO) ⋅2H O) and
3
2
2
−1
ethanolamine (NH CH CH OH, 99.0%) (0.45 mol L ) were stirring vigorously for 12 h in air.
2
2
2
Afterwards, we filtered the solution with a polytetrafluoroethylene (PTFE) filter (0.2 µm) to remove
precursor aggregates. The ITO glass substrates were sequentially cleaned by standard procedure and
dried at 75 °C (15 min) in an oven. 10 min of oxygen plasma treat was conducted before spin-coating
in order to remove any organic residues and to polarize the ITO surface for better ZnO precursor
adhesion. The ZnO precursor was spin-coated onto the pre-patterned ITO substrates at a spin speed of
7
000 rpm and annealed at 150 °C for 1 h in ambient condition. After that, PEIE: Rb CO (m/m = 1:1)
2 3
blend solution was spin-coated on the ZnO layers at a spin speed of 7000 rpm and the substrates were
rinsed with DMF, leaving ultrathin layers of PEIE: Rb CO on top of the ZnO films. After annealing at
2
3
1
00 °C for 10 min in a glove box, the solution of SDPS-4PhCz in chlorobenzene was spin-coated on
the PEIE: RbCO layer at a spin speed of 3000 rpm. For the WOLED, different concentrations of
3
TXO-TPA were added into SDPS-4PhCz and dispersed in toluene. The binary constituent solution was
spin-coated onto the PEDOT:PSS-PFI (m/m = 3/1) as same as above steps and annealed at 90 ℃ for 15
min. Then the sample was transferred into a vacuum deposition system, organics and the corresponding
−4
electrodes were deposited under a pressure below 1 × 10 Pa. The current-voltage characteristics were
measured with a computer-controlled Keithley 2400 Source Meter and CS-200 under ambient
atmosphere. The transient EL decay characteristics were measured using a pulse generator (Tektronix
AFG3011C) and a spectrometer (Princeton Instruments, Acton Spectrapro SP-2300).
3
. Results and discussion
3
.1 Synthesis and thermal Properties
Based on the design principles and the excellent characteristics of spiro structure, we selected
spiro-diphenylsulfone (SDPS) as electron-accepting group and four 9-phenyl-9H-carbazole as the

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donors which are connected to the para-position of phenylsulfone. SDPS-4PhCz is synthesized in
six-steps with overall yield of 18% (Scheme 1 and Supporting Information). Addition of thioxanthone
to (2-Iodophenyl) (phenyl) sulfane with n-BuLi in THF at −78 °C resulted in the formation of
spiro-diphenylsulfide. Subsequently, spiro-diphenylsulfide underwent selective bromine and oxidation
to produce bromo-spiro-diphenylsulfide. Finally, bromo-spiro-diphenylsulfide was allowed to react
with 9-phenyl-9H-carbazol-3-yl boronic acid under palladium-catalysed Suzuki-Miyaura cross
1
coupling conditions to obtain the target compound SDPS-4PhCz, which was characterized by H,
1
3
C-NMR, high-resolution mass spectrometry and elemental analysis. In addition, due to the
introduction of N-PhCz, SDPS-4PhCz showed good solubility in solvents as toluene and chlorobenzene
which is favorable for device fabrication. The thermal properties of SDPS-4PhCz were investigated by
TGA and DSC. Owing to the spiro and rigid skeleton of the emitter, SDPS-4PhCz has a high
decomposition temperature (T , 5% weight loss) of 574 °C and a glass transition temperature (T ) of
d
g
2
52 °C (Fig. S1). The HOMO energy level of SDPS-4PhCz was estimated to be 5.74 eV by UPS
measurements, and the LUMO energy level was calculated to be 2.49 eV according to the band gap of
.25 eV (Fig. S2). The thermal stability and frontier molecular orbital energy levels of SDPS-4PhCz
are summarized in Table 1.
3
Scheme 1 Synthetic route of SDPS-4PhCz
3
.2 Photophysical properties

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Photophysical properties of SDPS-4PhCz were analyzed using UV-vis and PL spectrometer. The
absorption and PL peaks of SDPS-4PhCz in 2-methyltetrahydrofuran (2-Me THF) and in neat thin film
and its PLQY are summarized in Table 1. SDPS-4PhCz exhibits absorption maximum at 337 nm and
unstructured deep blue emission band at 413 nm in 2-Me THF. The absorption spectra were slightly
influenced by the polarities of solvents, while remarkable positive solvatochromism property for
emission spectra of SDPS-4PhCz was observed (Fig. S3), which was typical for those of reported
intramolecular charge transfer (ICT) compounds [34-36]. The SDPS-4PhCz exhibited slightly red-shift
absorption and PL spectra at 353 nm and 429 nm, respectively, as well as PLQY of 50% under nitrogen
atmosphere in pure film (Fig. 1a).
The fluorescence and phosphorescence emission of SDPS-4PhCz were examined at 77 K in 2-Me
THF. As shown in Fig. 1b, SDPS-4PhCz exhibited unstructured fluorescence band at 390 nm, which is
assigned to the fluorescence from S state. The band at 466 nm in the phosphorescence spectrum is
1
assigned to the phosphorescence from T state. Accordingly, the energy levels of S and T were
1
1
1
achieved to be 3.18 eV and 2.66 eV, and a large ∆EST of 0.52 eV was qualitatively achieved. Noticeably,
after magnification, it is visible for the phosphorescence spectrum at 388 nm, which was consistent
with the fluorescence emission at 77 K, suggesting the delayed fluorescence properties.
Figure 1 (a) UV-vis absorption and PL spectra of SDPS-4PhCz in 2-Me THF and thin film. (b) Fluorescence and
phosphorescence spectra of SDPS-4PhCz in 2-Me THF at 77 K. ( λex = 350 nm)
Table 1 Optical, electrochemical and thermal data of SDPS-4PhCz.
b)
c)
d)
d)
e)
e)
T
d
/T
g
HOMO
[eV]
LUMO
[eV]
τ
p
τ
d
a)
a)
a)
Compound
λabs[nm]
λPL[nm]
φPL[%]
(℃ )
(ns)
(µs)
SDPA-4PhCz
337/353
413/429
83/50
574/252
−5.74
−2.49
100
1.45
a)
−
5
Measured in 2-Me THF (1×10 M) solutions and neat thin film at room temperature;
b)
c)
T
d
: decomposition temperature (5% weight loss);
T
g
: glass transition temperature;

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d)
HOMO is calculated from the UPS spectra of SDPS-4PhCz thin film, LUMO is calculated from the onset of the absorption
edge of the thin film;
e)
PL lifetime of prompt and delayed decay component in deoxygenated 2-Me THF (10 µM) solution at room temperature.
To confirm the delayed fluorescence of SDPS-4PhCz, the transient photophysical properties were
investigated. The transient PL decay curves of SDPS-4PhCz in oxygen-free 2-Me THF revealed
obvious two-order exponential decay, with a fast 100 ns relaxation and a second delayed 1.45 µs
relaxation (Fig. 2a). The microsecond-scale delayed component of SDPS-4PhCz was much shorter than
those of traditional blue TADF materials, which helped to solve the problem of efficiency roll-off of the
device [37]. As shown in Fig. 2b, the good overlap between the PL spectra of prompt component and
delayed component indicated that both the prompt and delayed emissions stem from the radiative decay
of the singlet excitons. Similar to other conventional TADF emitters, the emission intensity and delayed
lifetime of SDPS-4PhCz were sensitive significantly toward oxygen in 2-Me THF [38]. The oxygen
quenching results are summarized in Table S1 and Table S2. The delayed lifetime of SDPS-4PhCz was
suppressed by the presence of oxygen and the PLQYs significantly enhanced with nitrogen bubbling
(
Fig. 2c and 2d). Consequently, it can be expected that the triplet states were involved in the PL process
and the delayed fluorescence of SDPS-4PhCz arise from the emission harvesting from the triplet via
reverse intersystem crossing (RISC). Additionally, similar temperature-dependence of the delayed
lifetime was observed for the classical TADF emitters. At lower temperature (77 K), the delayed
lifetime of SDPS-4PhCz dramatically decreased to 0.61 µs when compared with that at room
temperature, implying RISC process of SDPS-4PhCz from lower triplet to singlet state. Transient
experiment results showed that photophysical properties of SDPS-4PhCz were significantly different
with CzS2 reported by Zhang and co-workers [39]. The RISC rate constant was calculated to be
5
−1
5
.6×10 s , which was smaller than those of the conventional blue TADF emitters (Table S3). The
small kTADF indicated a large energy gap between the singlet and triplet (∆E ) for the RISC.
ST

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Figure 2 (a) PL transient spectra of SDPS-4PhCz in 2-Me THF with saturated nitrogen at 300 K and 77 K. (b) Prompt and
delayed fluorescence spectra of SDPS-4PhCz in 2-Me THF at 300 K. (c) PL transient spectra of SDPS-4PhCz in 2-Me THF
under oxygen and nitrogen atmospheres, respectively. Concentration of SDPS-4PhCz is 10 µM. (d) Fluorescence spectra of
SDPS-4PhCz in 2-Me THF with saturated nitrogen and oxygen at 300 K. (excitation and detection wavelength were 355 nm and
4
04 nm)
3
.2 Theoretical calculations
To gain further insight into the excited state properties of SDPS-4PhCz, the landscape for singlet
and triplet excited states and the highest occupied and lowest unoccupied natural transition orbitals
NTO) of the S and T (n = 1∼ 3) states were performed. The NTO particle wave function of the S
(
1
n
1
state is mainly localized on the SDPS unit and the adjacent linked phenyl of PhCz, while the NTO hole
wave function is localized on the PhCz unit and the adjacent linked phenyl of the SDPS (Fig. 3a). The
substantial overlap between the NTO hole and particle leads to the weak CT characteristics in S state
1
in nature, corresponding to the large exchange energy, K [40]. However, for the T state, NTO hole and
1
particle wave functions are mainly confined the PhCz moiety and adjacent phenyl ring on SDPS moiety,
suggesting the locally excited (LE) characteristics in T . Large ∆E (S -T ) of 0.57 eV was achieved
1
ST
1
1
(
Fig. 3b), which coincides with the results obtained from steady PL at 77 K and exceeds the upper limit

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value for ∆E of TADF emitters (0.3 eV). For the NTO of higher excited states, ∆EST (S -T ) of 0.44
ST
1
2
eV and ∆EST (S -T ) of 0.16 eV were obtained due to the significant LE characteristics of the T and T
1
3
2
3
1
−
state. The spin orbital coupling (SOC) matrix element values were calculated to be 0.05751 cm
−1
between S and T , 0.17920 cm between S and T for SDPS-4PhCz, combined with the larger ∆E
1
1
1
2
ST
(
S -T ) and ∆E (S -T ), the small SOC will inhibit the germination of RISC process between S and
1 1 ST 1 2 1
T or S and T . Therefore, we speculated that the RISC process occurs from higher energy T to S
1
1
1
2
3
−
1
because of the small ∆EST of 0.16 eV and stronger SOC matrix element of 0.12759 cm between S1
and T . Thus, SDPS-4PhCz exhibited fast RISC via high-lying triplet excited state, and “hot exciton”
3
TADF mechanism is expected in our systems [41-43]. Therefore, the calculations further demonstrate
that the energy levels of the singlet and triplet excited states in SDPS-4PhCz changes significantly as
compared with CzS2, ultimately resulting in different photophysical properties.
1 1 2 3
Figure 3 (a) Natural transition orbitals (hole: upper, particle: lower) for the S , T , T and T excitations for SDPS-4PhCz (the
weight for the transition is given on the right of the arrow). (b) The energy diagram of the singlet and triplet excited states of
SDPS-4PhCz.
3
.3 Electroluminescent Performance
To study the electroluminescence properties of the blue emitter, solution-processed devices were
fabricated using inverted device structure of ITO/ZnO (30 nm)/PEIE: Rb CO (5 nm)/EML (80
2
3
nm)/TCTA (30 nm)/MoO (10 nm)/Al (100 nm) (Fig. S4). In the devices, ZnO and MoO served as the
3
3
electron-injection and hole-injection layers, respectively, and PEIE: Rb CO and TCTA served as
2
3
electron-transporting and hole-transporting layers, respectively. The EL spectrum of device was
blue-shifted to 417 nm, compared to the PL spectrum of the solid thin film. The device exhibited
excellent color saturation with CIE coordinates (0.15, 0.04) and showed a turn on voltage (V ) of 3.0 V,
T
a current efficiency (CE) of 1.15 cd/A, a power efficiency (PE) of 1.12 lm/W, and the external quantum

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efficiency (EQE) is determined to be 4.92% (Fig. 4), which represent the state-of-the-art device
performance for “hot exciton” deep blue TADF OLEDs (Table S4). The exciton utilization of the
OLED is estimated according to the following equation:
EQE = η ×η ×η ×η
out
(1)
rec
PL
s
Where the PL efficiency of thin film ηPL was determined to be 50%, the charge balance factor ηrec
and optical out-coupling ηout are assumed to be 1.0 and 0.2~0.3, respectively. The fraction of singlet
exciton η is estimated to be 49.4~32.8%, overcoming the 25% theoretical limit imposed by spin
s
statistics. These results confirmed that the triplet excitons of SDPS-4PhCz were effectively utilized.
According to the energy-gap law [44], the small energy gap between T and T may significantly
2
3
increase the internal conversion rate. As a result, serious internal conversion from T can be expected,
3
leading to the low triplet excitons utilization. More fascinatingly, the deep blue device exhibited low
efficiency roll-off compared with most of the traditional blue TADF OLEDs, giving EQE value up to
2
2
.08% at the luminescence of 1000 cd/m (Table S5). The low efficiency roll-off was ascribed to the
relatively short lifetime of the delayed component of SDPS-4PhCz [45-47]. Furthermore, the transient
EL was measured to confirm the TADF property of the emitter in the device. The delayed EL
component lasted for several microseconds and the EL spectra with different delay times well matched
(
Fig. 5). Thus, the efficient RISC facilitated the delayed fluorescence under electric excitation and
afforded a high EQE.

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Figure 4 The performances of blue OLED: (a) current efficiency and power efficiency versus luminance curves; (b) current
density and luminance versus voltage (J-V-L) curves; (c) EQE versus luminance curves; (d) EL spectra of blue device.
Figure 5 Transient EL decay under electrical excitation at 300 K (inset: EL spectra were resolved into a prompt component
(black) and delayed component (red)).
We further applied the deep blue emitters for the fabrication of simple structure WOLEDs.
TXO-TPA was chosen as the orange TADF emitter due to its appropriate CIE coordination [36]. As
shown in Fig.6a, the emission band of SDPS-4PhCz and the absorption band of TXO-TPA showed
significant overlap which is favorable for the Förster resonance energy transfer from SDPS-4PhCz host
to TXO-TPA emitter. Increasing the dopant concentration from 1 wt% to 10 wt% decreased the
emission intensity of SDPS-4PhCz, while retained the intensity of TXO-TPA in blend films (Fig.6b).

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The red-shifted emission of TXO-TPA is not only attributed to the interaction between the host and
dopant but also between the guests. As a result, a pure white spectrum was achieved by tuning the
concentration of TXO-TPA.
Figure 6 (a) The absorption spectra of TXO-TPA and PL spectra of SDPS-4PhCz in pure film; (b) The PL spectra of
SDPS-4PhCz: TXO-TPA film with different doping concentrations; (c) The CIE coordinates alignment of SDPS-4PhCz and
TXO-TPA used in the WOLED.
Based on the above results, we fabricated a simple white EL device via solution method with a
structure of ITO/PEDOT:PSS-PFI (30 nm)/x wt% TXO-TPA:SDPS-4PhCz (60 nm)/TMPYPB (65
nm)/LiF (1 nm)/Al (100 nm), where PEDOT:PSS-PFI served as the hole-injection layer, TMPYPB
served as electron-transporting layer, and TXO-TPA worked as orange emitter (Fig. S5). All devices
2
start to emit at 3.1∼ 3.3 V, and display intense white light (Lmax = 3000∼ 8000 cd/m ). The device with
0
.2 wt% TXO-TPA afforded an EQE of 10.4% and a CIE coordinates of (0.35, 0.40), very close to the
pure white emission. More significantly, high CRI value of 71 of WOLEDs was achieved. When
TXO-TPA concentration was 1.0 wt%, the optimized performance of WOLEDs was achieved with
EQE, CE and PE of 17.6%, 45.6 cd/A, 40.9 lm/W, respectively (Fig. 7 and Table 2). The CIE
coordinates was calculated to be (0.39, 0.44). These results are significantly higher than previously
reported single-EML WOLED device by solution process (Table 3).

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Figure 7 The performances of WOLED: (a) current density and luminance versus voltage (J-V-L) curves; (b) current efficiency
and power efficiency versus luminance curves; (c) EQE versus luminance curves; (d) EL spectra of WOLED.
Table 2 The EL parameters of the WOLEDs with different concentrations of TXO-TPA.
V
on
CE
PE
EQE
(%)
CIE
CRI
Concentration
Luminescence
2
2/
(V)
(cd/A)
(lm/W)
@1000cd/m
@1000cd/m /max
0.2 wt%
0.5 wt%
1.0 wt%
3.1
3.3
3.2
24.06
33.73
45.56
22.90
30.28
40.90
3000
4000
8000
10.40
13.89
17.57
(0.35,0.40)
(0.39,0.42)
(0.39,0.44)
63/72
61/65
58/64

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Table 3 The representative solution-processed two-color WOLEDs.
EML
CE
PE
EQE (%)
CIE
CRI
Ref
-1
-1
(
blue/orange)
(cd A )
(lm W )
DMAC-TRA/Ir(dpm)P
CzOXD/4CzPNPh
B-G2:Ir(Fipy-CF
G2/ Ir(bt) (acac)
G0/CN-DPASDB
tBuCN-FIrpic-Mcp/
m-CF DPQ) Ir(pic)
2
28.80
21.60
59.40
17.69
17.00
18.10
4.71
7.20
(0.38, 0.44)
(0.35, 0.44)
(0.44,0.45)
(0.32, 0.33)
(0.32, 0.37)
67
62
43
-
48
15
49
50
3
5
26.10
58.8
3
)
3
19.60
10.10
6.45
2
7.88
15.60
64
4
3.50
15.70
7.20
20.60
6.10
(0.35, 0.35)
(0.40,0.34)
(0.32,0.33)
78
-
51
52
16
3
2
PTPATPPO/Ir(2-phq)
2
acac
10.50
interface exciplex/
TXO-TPA
2
4
2
1.10
5.60
4.10
20.71
10.02
85
SDPS-4PhCz/
40.90
22.90
17.60
10.40
(0.39, 0.44)
(0.35,0.40)
64
71
This work
This work
1
.0 wt% TXO-TPA
SDPS-4PhCz/
0
.2 wt% TXO-TPA
4
. Conclusions
In summary, the broad and continuous emission covering the full visible lighting spectral region
was required in high efficiency and high CRI WOLEDs. A new deep blue spiro-diphenylsulfone was
reported, non-doped single inverted EL device fabricated with SDPS-4PhCz showed EQE of nearly 5%
with a low efficiency roll-off. Steady and transient photophysical characterization combined with
quantum-chemical calculations confirm that fast RISC process can occur via a higher excited triplet
state T , the “hot exciton” mechanism is responsible for the short delayed lifetimes. The best WOLED
3
device was fabricated by using SDPS-4PhCz as deep blue emitter and simultaneously as the host of an
orange TADF emitter TXO-TPA by solution process, which realized intense white light (Lmax = 8000
2
cd/m ), a maximum EQE of 17.6%, a maximum current efficiency of 45.56 cd/A and a maximum
power efficiency of 40.90 lm/W.

ACCEPTED MANUSCRIPT
Keywords：White organic light-emitting diodes, Thermally activated delayed fluorescence, Deep blue,
Hot exciton, Solution-processed
Acknowledgments
This work was financially supported by the National Key Research and Development Project (No.
2
016YFB0401004), the National Science Foundation of China (Grant No. 61420106002, No.5137318
9
, No. 91833304 and No. 21772209), and the National Program for Support of Top-notch Young Profes
sionals.
Conflict of interest
The authors declare that they have no conflict of interest.
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Highlights
ò
ò
ò
ò
A new deep blue emitter via a spiro concept was designed.
The non-doped deep blue OLED showed a low efficiency roll-off.
The fast RISC process in SDPS-4PhCz coincided with “hot exciton” mechanism.
The two-color WOLED showed an EQE above 17% by spin-coating method.

ACCEPTED MANUSCRIPT
Author contributions
Ying Wang supervised the project.
Ruifang Wang carried out the experiments, analyzed the data and wrote the manuscript.
Yanwei Liu helped for the devices fabrication.
Taiping Hu and Yuanping Yi helped for the theoretical calculations.
Pengfei Wang, Xiaofang Wei, Jianjun Liu, Zhiyi Li and Xiaoxiao Hu helped for the theoretical
analysis of photophysical properties.