New sulfone-based electron-transport materials with high triplet energy for highly efficient blue phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1039/c4tc01474j

A series of new electron transport materials, which combines a diphenylsulfone core with different electron withdrawing end groups, has been synthesized, characterized, and found to exhibit high triplet energy (E_T > 2.8 eV) for use in phosphorescent organic light emitting diodes (PhOLEDs). The new materials, including 3,3′-(4,4′-sulfonylbis(4,1-phenylene))dipyridine (SPDP), 5,5′-(4,4′-sulfonylbis(4,1-phenylene)) bis(3-phenylpyridine) (SPPP), and 3,3′-(4,4′-sulfonylbis(4,1-phenylene))diquinoline (SPDQ) had wide band gaps (3.6-3.8 eV) and LUMO levels of -2.4 to -2.7 eV. The triplet energy measured from phosphorescence spectra at 77 K varied from 2.53 eV for SPDQ and 2.81 eV for SPPP to 2.90 eV for SPDP, which are in good agreement with density functional theory calculated values. High performance blue PhOLEDs using the sulfone-based materials are exemplified by devices containing a poly(N-vinylcarbazole) host and SPDP electron transport layer, which had a high quantum efficiency (19.6%) and a high current efficiency (33.6 cd A^-1) even at very high luminances (4500 cd m^-2). These results demonstrate that sulfone-based molecules are promising electron transport materials for application in developing highly efficient phosphorescent OLEDs.

Full text of DOI:10.1039/c4tc01474j

Journal of
Materials Chemistry C
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New sulfone-based electron-transport materials
with high triplet energy for highly efficient blue
phosphorescent organic light-emitting diodes†
Cite this: DOI: 10.1039/c4tc01474j
Soon Ok Jeon, Taeshik Earmme and Samson A. Jenekhe*
A series of new electron transport materials, which combines a diphenylsulfone core with different electron
withdrawing end groups, has been synthesized, characterized, and found to exhibit high triplet energy
T
(E > 2.8 eV) for use in phosphorescent organic light emitting diodes (PhOLEDs). The new materials,
0
0
0
0
including 3,3 -(4,4 -sulfonylbis(4,1-phenylene))dipyridine (SPDP), 5,5 -(4,4 -sulfonylbis(4,1-phenylene))
0
0
bis(3-phenylpyridine) (SPPP), and 3,3 -(4,4 -sulfonylbis(4,1-phenylene))diquinoline (SPDQ) had wide band
gaps (3.6–3.8 eV) and LUMO levels of ꢀ2.4 to ꢀ2.7 eV. The triplet energy measured from
phosphorescence spectra at 77 K varied from 2.53 eV for SPDQ and 2.81 eV for SPPP to 2.90 eV for
SPDP, which are in good agreement with density functional theory calculated values. High performance
blue PhOLEDs using the sulfone-based materials are exemplified by devices containing
a
Received 8th July 2014
Accepted 14th October 2014
poly(N-vinylcarbazole) host and SPDP electron transport layer, which had a high quantum efficiency
ꢀ1
ꢀ2
(
19.6%) and a high current efficiency (33.6 cd A ) even at very high luminances (4500 cd m ). These
DOI: 10.1039/c4tc01474j
results demonstrate that sulfone-based molecules are promising electron transport materials for
www.rsc.org/MaterialsC
application in developing highly efficient phosphorescent OLEDs.
oxide (PO) group. Sulfone-based materials can be expected to
have efficient electron-injection and transport due to the strong
Introduction
Organic light-emitting diodes (OLEDs) are nding applications electron withdrawing character of the disulfoxide group and the
in full-color at panel displays, exible displays, and solid-state high triplet energy and wide band gap of the materials. They are
1
lighting. Recent research on OLEDs has largely focused on also known to have good electrochemical and thermal
13,14
developing new materials and device strategies for achieving stability.
highly efficient phosphorescent organic light-emitting diodes
High triplet energy materials containing the sulfone group
PhOLEDs) because nearly 100% internal quantum efficiency were reported for applications in PhOLEDs. Hsu et al. reported
(
2
can be realized. One promising strategy towards highly effi- bis(phenylsulfonyl)uorine-based molecule as the host material
15
cient PhOLEDs is through the utilization of high triplet energy for red PhOLEDs. Recently, Marder and Kippelen et al.
materials to conne triplet excitons inside the emission layer synthesized and evaluated bis(phenylsulfonyl)biphenyl
molecule, and Kido et al. reported a sulfone derivative as a
have been high triplet energy (calcd. E
¼ 3.11 eV) host material for bis(3,5-
achieved by using various classes of electron-transport mate- diuoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium) (FIrpic)
a
3–5
16
(
EML) in multilayered device structures.
4–12
13,14
High-performance PhOLEDs
and OLEDs
T
7
8a
8b
9
17
rials (ETMs) such as pyridine, phenylpyrimidine, triazine,
triplet emitter in blue PhOLEDs. More recently, Adachi et al.
Phos- demonstrated diphenylsulfone-core structured molecules for
phine-oxide-based ETMs have gained much attention due to efficient thermally activated delayed uorescence (TADF) blue
10
11,12
quinoline and phosphine oxide (PO) derivatives.
18
their good exciton blocking ability enabled by their high triplet emitting materials. However, all previous reports on sulfone-
6
energy. The PO group is known to lower the LUMO level based materials to date have focused on their use as host or
15–18
predominantly by inductive electron-withdrawing effect, which emitting materials for PhOLEDs,
which involves vacuum co-
facilitates electron-injection while retaining high triplet energy. deposition of the materials.
The sulfonyl group (SO ) is another attractive group with good
In this paper, we report the synthesis, experimental and
2
electron withdrawing properties comparable to the phosphine theoretical electronic structure, photophysics, and thermal and
electron-transport properties of a series of new sulfone-based
high triplet energy materials and demonstrate their use as
Department of Chemical Engineering and Department of Chemistry, University of
efficient electron transport materials in high performance blue
Washington, Seattle, WA 98195-1750, USA. E-mail: jenekhe@u.washington.edu
PhOLEDs. The new electron transport materials (ETMs)
include 3,3 -(4,4 -sulfonylbis(4,1-phenylene))dipyridine (SPDP),
†
Electronic supplementary information (ESI) available: TGA thermograms and
0
0
cyclic voltammograms of sulfone-based ETMs. See DOI: 10.1039/c4tc01474j
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C

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0
,4 -Sulfonylbis(4,1-phenylene)bis(trimethylstannane) (2)
4
To a solution of bis(4-chlorophenyl) sulfone (3.0 g, 10.4 mmol)
in 50 mL anhydrous tetrahydrofuran, 2.5 M solution of n-BuLi
ꢁ
(
9.19 mL, 22.9 mmol) was added dropwise at ꢀ78 C. The
ꢁ
solution was stirred at ꢀ78 C for 2 h and 1.0 M solution of
trimethyltin chloride (22.9 mL, 22.9 mmol) in tetrahydrofuran
was added. The solution was warmed to room temperature and
50 mL of water and 50 mL of ethyl ether were added. The
organic layer was washed twice with 50 mL of water and dried
over anhydrous MgSO . The organic layer was evaporated and
4
1
Chart 1 Diagram of (a) singlet energy (S
1
) and triplet energy (T
1
) and (b) dried over vacuum to afford white solid (5.5 g, 97% yield). H
diphenylsulfone core structure with different substituents.
NMR (CDCl
3
, 300 MHz, ppm): d 7.72–7.66 (m, 4H), 7.34–7.28
(m, 4H), 0.42 (s, 18H).
0
0
5
(
(
,5 -(4,4 -sulfonylbis(4,1-phenylene))bis(3-phenylpyridine)
0 0
,3 -(4,4 -Sulfonylbis(4,1-phenylene))dipyridine (SPDP)
3
0
0
SPPP), and 3,3 -(4,4 -sulfonylbis(4,1-phenylene))diquinoline
SPDQ). The new sulfone-based ETMs, whose molecular struc-
tures are shown in Chart 1, have a diphenylsulfone core with
different electron withdrawing substituents (pyridine, phenyl-
pyridine, and quinoline) aimed at controlling the triplet energy
A mixture of bis(4-chlorophenyl) sulfone (2.0 g, 6.96 mmol),
-pyridinylboronic acid (1.02 g, 8.35 mmol) and tetrakis-
triphenylphosphine)palladium(0) (5 mol%) in 20 mL of anhy-
3
(
drous tetrahydrofuran was reuxed under argon for 12 h. To the
reaction mixture was slowly added a solution of potassium
carbonate (2 M, 20 mL). Aer cooling to ambient temperature,
the reaction mixture was extracted with dichloromethane and
water. The organic layer was evaporated with a rotary evapo-
rator. The product was puried by column chromatography
using ethyl acetate and n-hexane mixture (10 : 90) and a white
(E
T
), band gap (E
g
) and electron transport properties. The triplet
energy (E ) of the new electron transport materials was
T
measured from the low temperature (77 K) phosphorescence
spectra and compared to density functional theory (DFT) based
predicted values. The electron mobility of the ETMs was
measured in space-charge limited current (SCLC) experiments
on evaporated thin lms. Finally, a series of blue PhOLEDs was
fabricated and evaluated toward investigation of the electron
transport and exciton blocking properties of the new sulfone-
based ETMs. The results demonstrate that the new sulfone-
based molecules are promising electron transport materials
with high triplet energy for high-performance PhOLEDs.
ꢁ
1
solid product was obtained (1.65 g, 63% yield); Tm 239 C. H
NMR (300 MHz, CDCl , ppm): d 8.85 (s, 2H), 8.67 (s, 2H), 8.12–
3
8
2
1
.09 (d, 4H), 7.90–7.87 (d, 2H), 7.76–7.74 (d, 4H), 7.45–7.40 (m,
1
3
H). C NMR (500 MHz, CDCl
41.0, 134.6, 128.5, 128.1, 123.7. MALDI-TOF/MS: 373 [(M + H) ].
S: C, 70.95; H, 4.33; N, 7.52; S, 8.61.
Found: C, 70.99; H, 4.19; N, 7.50; S, 8.45.
3
, ppm): d 149.7, 148.3, 142.9,
+
16 2 2
Anal. calcd. for C22H N O
Experimental
0
0
5
,5 -(4,4 -Sulfonylbis(4,1-phenylene))bis(3-phenylpyridine)
Materials
(SPPP)
0
Bis(4-chlorophenyl) sulfone, 3-pyridinylboronic acid, quinoline- 3-Bromo-5-phenylpyridine (1) (3.0 g, 12.8 mmol) and 4,4 -sul-
-boronic acid, phenylboronic acid, tetrakis(triphenylphosphine) fonylbis(4,1-phenylene)bis(trimethylstannane) (2) (4.18 g, 7.68
3
palladium(0), potassium carbonate, 3,5-dibromopyridine, mmol) and catalyst tetrakis(triphenylphosphine)palladium (0)
n-butyllithium, 1,4-dioxane, trimethyltin chloride, copper iodide, (5 mol%) and copper iodide (1 mol%) in anhydrous toluene (30
ꢁ
anhydrous toluene and tetrahydrofuran (Aldrich) were used mL) were heated at 120 C for 24 h. The solution was warmed to
without further purication.
room temperature and extracted with 50 mL of water and 50 mL
of dichloromethane. The organic layer was dried over anhy-
drous MgSO . The product was puried by column chroma-
Synthesis 3-bromo-5-phenylpyridine (1)
4
tography using ethyl acetate and n-hexane mixture (10 : 40) and
A mixture of 3,5-dibromopyridine (5.0 g, 21.1 mmol), phenyl-
boronic acid (3.08 g, 25.3 mmol) and tetrakis-
ꢁ
1
a white solid product was obtained. Yield 45%; T
NMR (CDCl , 300 MHz, ppm): d 8.93–8.90 (d, 4H), 8.12 (s, 2H),
m
180 C. H
3
(triphenylphosphine)palladium(0) (5 mol%) in 80 mL of 1,4-
1
3
7.69–7.47 (m, 18H). C NMR (500 MHz, CDCl
3
, ppm): d 148.0,
dioxane was reuxed under argon for 12 h. To the reaction
mixture was added slowly a solution of potassium carbonate
1
46.9, 137.3, 137.0, 133.4, 133.0, 129.3, 128.5, 127.3. MALDI-
+
TOF/MS: 525 [(M + H) ]. Anal. calcd. for C34
24 2 2
H N O S: C, 77.84;
(2 M, 80 mL). Aer cooling to room temperature, the reaction
H, 4.61; N, 5.34; S, 6.11. Found: C, 77.90; H, 4.58; N, 5.31; S, 5.98.
mixture was extracted with dichloromethane and water. The
organic layer was evaporated with a rotary evaporator. The
product was puried by column chromatography using ethyl
0 0
,3 -(4,4 -Sulfonylbis(4,1-phenylene))diquinoline (SPDQ)
3
acetate and hexane mixture (10 : 90) and a white solid product A mixture of bis(4-chlorophenyl)sulfone (1.77 g, 6.10 mmol),
1
was obtained (3.5 g, 72% yield). H NMR (300 MHz, CDCl
3
,
quinoline-3-boronic acid (2.56 g, 14.7 mmol) and tetrakis-
(triphenylphosphine)palladium(0) (5 mol%) in 20 mL of
ppm): d 8.74–8.64 (d, 2H), 7.99 (s, 1H), 7.54–7.38 (m, 5H).
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anhydrous tetrahydrofuran was reuxed under argon for 12 h. layer onto a precleaned ITO glass. Then the lm was annealed at
ꢁ
To the reaction mixture was slowly added a solution of potas- 150 C under vacuum to remove residual water. The 70 nm
sium carbonate (2 M, 20 mL). Aer cooling to ambient polymer EML was obtained by spin coating of the PVK : OXD-
temperature, the reaction mixture was extracted with dichloro- 7 : FIrpic blend in chlorobenzene onto the PEDOT:PSS layer and
ꢁ
methane and water. The organic layer was evaporated with a vacuum dried at 100 C overnight. Sulfone-based ETM was
rotary evaporator. The product was puried by column chro- vacuum-deposited onto the EML followed by deposition of 1 nm
matography using ethyl acetate and n-hexane mixture (10 : 90) LiF and 100 nm Al cathode without breaking the vacuum. The
and a white solid product was obtained (0.90 g, 32% yield); T
m
device structure with different thickness of SPDP ETL:ITO/
, ppm): d 9.17 (s, 2H), 8.35 (s, PEDOT:PSS/blue EML/SPDP/Al with 5, 15, and 30 nm thick
H), 8.20–8.10 (m, 4H), 7.98–7.78 (m, 8H), 7.67–7.62 (m, 2H), SPDP. A set of blue PhOLEDs were also fabricated; device I: ITO/
ꢁ
1
2
2
7
1
1
1
29 C. H NMR (300 MHz, CDCl
3
1
3
.55–7.53 (m, 2H). C NMR (500 MHz, CDCl , ppm): d 149.2, PEDOT:PSS/blue EML/LiF/Al; device II: ITO/PEDOT:PSS/blue
3
47.9, 143.1, 143.0, 140.9, 140.6, 140.1, 140.0, 134.1, 131.8, EML/SPDP (15 nm) LiF/Al; device III: ITO/PEDOT:PSS/blue
30.3, 129.7, 129.3, 129.2, 128.6, 128.5, 128.3, 128.2, 127.7, EML/SPPP (15 nm)/LiF/Al; and device IV: ITO/PEDOT:PSS/blue
27.5. MALDI-TOF/MS: 473 [(M + H) ]. Anal. calcd. for EML/SPDQ (15 nm)/LiF/Al.
+
C
30
H
20
N
2
O
2
S: C, 76.25; H, 4.27; N, 5.93; S, 6.79. Found: C, 76.14;
Film thickness was measured by using an Alpha-Step 500
H, 4.21; N, 4.89; S, 7.34.
prolometer (KLA-Tencor, San Jose, CA). Electroluminescence
(EL) spectra were obtained by using the same spectrouorim-
eter described above. Device performance of the PhOLEDs was
measured by using a HP4155A semiconductor parameter
analyzer (Yokogawa Hewlett-Packard, Tokyo). The luminance
was simultaneously measured by using a model 370 optometer
General analysis
1
H NMR spectra were recorded on a Bruker AV300 at 300 MHz,
1
3
whereas C NMR spectra were recorder on a Bruker AV500 at
00 MHz using CDCl as the solvent. The MALDI-TOF mass
5
3
(UDT Instruments, Baltimore, MD) equipped with a calibrated
spectra were recorded using a Bruker Autoex II spectrometer.
The ultraviolet-visible (UV-vis) absorption spectra were obtained
by means of a Perkin-Elmer model Lambda 900 UV/vis/near-IR
spectrophotometer and the photoluminescence (PL) spectra
were recorded on a Photon Technology International (PTI) Inc.
Model QM 2001-4 spectrouorimeter. The UV-vis absorption
and solution PL emission spectra of electron transport mate-
rials (ETMs) were obtained from dilute toluene solution, while
the solid PL spectra were obtained from thin lms prepared by
vacuum evaporation. Triplet energy values of the ETMs were
obtained from the photoluminescence spectra at 77 K using
liquid nitrogen. The differential scanning calorimeter (DSC)
measurements were performed on a TA Instruments Q100
luminance sensor head (Model 211) and a 5ꢂ objective lens.
The device external quantum efficiency (EQE) was calculated
from the luminance, current density and EL spectrum
assuming a Lambertian distribution using procedures reported
previously. All the device fabrication and device characteriza-
tion steps were carried out under ambient laboratory
conditions.
Devices for space-charge-limited current (SCLC) measure-
ment were fabricated with single carrier device structure: ITO/
sulfone-based ETM (100 nm)/Al. The 100 nm sulfone-based
ETM was deposited onto the plasma cleaned ITO substrate
followed by vacuum-deposition of Al electrode. Current density-
voltage (J–V) characteristics of SCLC devices were measured
using the same semiconductor parameter analyzer as for the
characterization of PhOLEDs. The SCLC measurements were
performed under dark and ambient conditions.
19
ꢁ
ꢀ1
under nitrogen at a heating rate of 10 C min to measure the
melting point (T ) and glass transition temperature (T ).
m
g
Thermogravimetric analysis (TGA) was done on a TA Instru-
ꢁ
ments Q50 under owing nitrogen at a heating rate of 20 C
ꢀ
1
min . The energy levels were measured by using cyclic vol-
tammetry (CV). Each ETM was dissolved in anhydrous aceto-
nitrile with 0.1 M tetrabutylammonium hexauorophosphate as
the electrolyte to measure the reduction CV from which the
LUMO energy level was estimated. We used a platinum wire
electrode both working and counter electrode and a saturated
Ag/AgCl reference electrode. Ferrocene was used as the standard
reference. All solutions were purged with nitrogen for 10
minutes before each experiment.
Results and discussion
Synthesis and characterization
The synthetic route to the new sulfone-based electron transport
0
0
0
0
materials is shown in Scheme 1. The compounds, 3,3 -(4,4 -
sulfonylbis(4,1-phenylene))dipyridine (SPDP) and 3,3 -(4,4 -
Device fabrication and characterization
The phosphorescent emission layer (EML) consisted of a blend
of poly(N-vinylcarbazole) (PVK) and 1,3-bis(2-(4-tert-butyl-
phenyl)-1,3,4-oxadiazo-5-yl)benzene (OXD-7) (PVK : OXD-7 ¼
60 : 40, wt% : wt%) as a host and 10.0 wt% FIrpic as the blue
dopant. A solution of Clevios P VP Al 4083 PEDOT:PSS (poly-
ethylenedioxythiophene)-polystyrenesulfonate) as received,
(
Scheme 1 Synthetic route of sulfone-based ETMs: SPDP, SPDQ, and
was ltered before spin-coating to make a 30 nm hole injection SPPP.
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ꢁ
sulfonylbis(4,1-phenylene))diquinoline (SPDQ), were synthe- The new ETMs are stable to temperatures above 350 C. The fact
sized by Suzuki coupling reaction of bis(4-chlorophenyl) sulfone that apparent complete thermal decomposition with zero%
with 3-pyridylboronic acid and quinoline-3-boronic acid, weight remaining is observed in the TGA curves suggests that
respectively using tetrakis(triphenylphosphine)palladium(0) in the materials can be readily evaporated to form thin lms.
anhydrous tetrahydrofuran. The reaction of 3,5-dibromopyri-
dine with phenylboronic acid in the presence of palladium
Photophysical properties
catalyst in 1,4-dioxane gave 3-bromo-5-phenylpyridine (1) in
0
72%
yield.
4,4 -Sulfonylbis(4,1-phenylene)bis(trimethyl- Optical absorption and photoluminescence (PL) spectra of
stannane) (2) was prepared by stannylation of bis(4-chlor- the SPDP, SPPP and SPDQ molecules in dilute toluene solution
0
0
ꢀ5
ophenyl) sulfone with 97% yield. The compound 5,5 -(4,4 - (5 ꢂ 10 M) and thin lms are shown in Fig. 2. The solid state
sulfonylbis(4,1-phenylene))bis(3-phenylpyridine) (SPPP) was absorption and PL emission spectra of SPDP, SPPP and SPDQ
synthesized by Stille coupling reaction between 1 and 2 with were obtained from thermally evaporated thin lms. The key
tetrakis(triphenylphosphine)palladium(0) and copper iodide in numerical values of the photophysical properties of the sulfone-
abs
anhydrous toluene. The three nal products (SPDP, SPPP, based ETMs, including absorption maximum (l ), molar
max
em
max
SPDQ) were puried by column chromatography and molecular absorption coefficient (log 3), PL emission maximum (l ) and
1
13
structures were conrmed by H NMR, C NMR and elemental optical band gap (E
g
) are summarized in Table 1. A strong
analysis. solution absorption peak was observed between 281 nm and
Thermal properties of the sulfone-based ETMs were char- 298 nm which is assigned to the absorption of the diphe-
acterized by thermogravimetric analysis (TGA) and differential nylsulfone unit in the molecules. Similar absorption spectra
scanning calorimetry (DSC). TGA and DSC curves of the new were observed in the three ETMs due to the common diphe-
sulfone-based ETMs are shown in Fig. S1 (ESI†) and 1, respec- nylsulfone core in the molecules. In the case of SPDP, it showed
abs
max
tively. Numerical values extracted from the TGA and DSC scans an absorption maximum (l ) at 285 nm (log 3 ¼ 4.33). The
are given in Table 1. Three distinct transitions were seen in the thin lm absorption spectrum has a similar line shape except
abs
second-heating/cooling DSC scans of SPDP, SPPP and SPDQ. that lmax is at 296 nm. SPPP has an absorption with a maximum
These materials showed a high melting point (T
and 229 C, respectively, whereas they did not exhibit a glass 289 nm in thin lm. The absorption spectrum of the quinoline
m
) of 239, 180 centered at 281 nm (log 3 ¼ 4.36) in solution but red shis to
ꢁ
abs
transition temperature (T ) up to the T . The sulfone-based functionalized ETM, SPDQ, has a l
max
at 298 nm (log 3 ¼ 3.47)
g
m
materials crystallize upon cooling from the melt. The SPDP, in solution. However, the absorption spectrum of SPDQ thin
SPPP and SPDQ showed onset crystallization temperature (T
c
) at lms showed two absorption peaks at 279 nm and 306 nm; the
ꢁ
abs
10
223, 146 and 197 C, respectively. The onset decomposition latter l
value originates from quinoline ring. The red-shi
max
d
temperature (T ) of the new sulfone-based materials deter- of the p–p* band between solution and thin lm was 11 nm for
mined from TGA thermograms showed high values in the range SPDP, indicating an increase in electron delocalization in the
ꢁ
of 386 to 453 C, which demonstrate their thermal robustness. bulk solid state. In addition, the p–p* band in SPPP and SPDQ
in both dilute solution and thin lm is red-shied by 8 nm
compared to that of SPDP. This means that the extent of elec-
tron delocalization is greater with phenylpyridine and quinoline
em
than with the pyridine. The PL emission maximum (lmax) of
SPDP, SPPP and SPDQ was observed at 291, 294 and 375 nm,
respectively. The emission maximum is red-shied with
increasing p-conjugation. Optical band gaps of SPDP, SPPP and
SPDQ were estimated from the absorption edge of the UV-vis
spectra, revealing E
g
of 3.6, 3.79 and 3.64 eV, respectively.
) of the new ETMs was estimated from
The triplet energy (E
T
the shortest wavelength emission peak of the phosphorescence
spectrum obtained at low temperature (77 K) in dilute 2-methyl
tetrahydrofuran solution. The phosphorescent spectra of the
sulfone-based ETMs are shown in Fig. 3. The measured triplet
energies of the three ETMs are given in Table 1. In the case of
SPDP and SPPP, their E
T
values of 2.90 eV and 2.81 eV, respec-
tively, are high enough to conne the triplet excitons of blue
10
emitting FIrpic which has a triplet energy of 2.7 eV. These
triplet energy values are much higher than those of commercial
ETMs, such as 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline
2
0
(
BCP) (E ¼ 2.5 eV). The E value of SPDQ (2.53 eV), which is
T
T
comparable to that of BCP, is too low to conne the triplet
excitons of blue emitters but could be more useful for green and
red triplet emitters.
Fig. 1 DSC thermograms of sulfone-based ETMs: (a) SPDP; (b) SPPP;
and (c) SPDQ.
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Table 1 Thermal and photophysical properties of sulfone-based ETMs
abs
em
l
max (nm)
lmax (nm)
ꢁ
ꢁ
ꢀ1
ꢀ1
ETM
T
m
( C)
T
d
( C)
Solution
log 3 (M cm
)
Film
Solution
Film
HOMO/LUMO (eV)
E
g
(eV)
T
E (eV)
SPDP
SPPP
SPDQ
239
180
229
386
342
453
285
281
298
4.33
4.36
3.47
296
289
279, 306
291
294
375
399
393
409
ꢀ6.17/ꢀ2.57
ꢀ6.16/ꢀ2.37
ꢀ6.32/ꢀ2.7
3.60
3.79
3.64
2.90
2.81
2.53
Fig. 2 Normalized absorption and PL emission spectra of sulfone-
ꢀ5
based ETMs in dilute toluene solution (5 ꢂ 10 M) and thin solid films:
a) SPDP; (b) SPPP; and (c) SPDQ.
(
Cyclic voltammetry and electronic structure
Fig. 3 Normalized phosphorescence spectra of sulfone-based ETMs
Electronic structure (LUMO/HOMO energy levels) of the at 77 K: (a) SPDP; (b) SPPP; and (c) SPDQ.
sulfone-based ETMs was studied by cyclic voltammetry (CV).
The cyclic voltammograms of the ETMs in solution are shown in
Fig. S2 (ESI†). SPDP show two reversible reduction waves respectively. Oxidation CV was not observed for any of the
whereas the reduction CVs of SPPP and SPDQ were not revers- sulfone-based materials. The HOMO levels of SPDP, SPPP and
onset
ible. The onset reduction potentials (Ered ) of the sulfone-based SPDQ were found to be ꢀ6.17, ꢀ6.16 and ꢀ6.32 eV, respectively,
ETMs SPDP, SPPP and SPDQ are ꢀ1.83, ꢀ2.03 and ꢀ1.7 V (vs. as estimated from the difference between the LUMO level and
SCE), respectively. The LUMO energy levels of the ETMs were the optical band gap (E ). The HOMO levels of SPDP and SPPP
g
onset
red
21
obtained according to the equation ELUMO ¼ eE
+ 4.4 eV,
are similar to those of other ETMs with deep HOMO level such
using ferrocene as the internal standard. The LUMO levels of as BCP (ꢀ6.1 eV) or 4,7-diphenyl-1,10-phenanthroline (BPhen)
4a
SPDP, SPPP and SPDQ were found to be ꢀ2.57, ꢀ2.37 and ꢀ2.7 eV, (ꢀ6.2 eV). Similar with BCP and Bphen, we believe that the
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22
low-lying HOMO energy levels of the new materials suggest performed using a suite of Gaussian programs. Molecular
good exciton and hole blocking properties. The LUMO/HOMO structures of the SPDP, SPPP and SPDQ were optimized by
energy levels of the sulfone-based ETMs are shown together in density functional theory (DFT) using Becke's three parame-
Fig. 4.
terized Lee–Yang–Parr exchange functional (B3LYP) with
23
6-31G* basis sets. The results show that the HOMO orbitals
are distributed mostly on the diphenylsulfone core of SPDP,
SPDQ and SPPP (Fig. 5a). This indicates that HOMO levels of the
Molecular simulation of the sulfone-based ETMs
The results of molecular simulations of the new sulfone-based
ETMs are shown in Fig. 5. The ab initio calculations were
new sulfone-based ETMs are mainly determined by the SO
2
group. However, the LUMO orbitals of SPDP and SPDQ are
dispersed throughout the whole molecule from SO2 to the
functional side groups whereas the LUMO orbital of SPPP is not
distributed to the attached phenyl group. We can conclude that
the LUMO orbital largely depends on the attached strong elec-
tron withdrawing group. Based on this, we expect that SPDP and
SPDQ may have better electron transport properties than SPPP.
The calculated triplet energy values of these molecules are
shown in Fig. 5b in comparison with the measured triplet
energy values of the sulfone-based ETMs. The results indicate
that the calculated triplet energy values of SPDP, SPPP and
SPDQ, which are 3.17, 3.09 and 2.67 eV, respectively, are very
close to the measured values. The calculated and measured E
T
values agree to within 5–10%, which is very good. SPDP and
SPPP with phenyl and phenylpyridine substituents combine a
high triplet energy with a wide band gap because those
substituents do not affect the overall conjugation length, while
the triplet energy value of SPDQ decreased signicantly due to
the extended conjugation length through attached quinoline
rings. These results suggest that DFT calculation of electronic
Fig. 4 Energy level diagram of the blue PhOLEDs with sulfone-based structure including the triplet energy can be a promising
ETMs.
strategy to designing new high triplet energy electron transport
materials for highly efficient PhOLEDs.
Space-charge-limited current (SCLC) carrier mobility
We investigated the electron transport properties of the sulfone-
based materials by space-charge-limited current (SCLC)
measurement of the electron mobility. The current density–
voltage (J–V) characteristics of the SCLC devices with the
structure of ITO/sulfone-based ETM (100 nm)/Al are shown in
Fig. 6. The sulfone-based ETMs were thermally vacuum-evapo-
rated to form 100 nm thick layers followed by Al deposition. The
electron mobility was extracted by tting the current density–
voltage (J–V) curves in the near quadratic region according to
24
the modied Mott–Gurney equation,
!
pffiffiffiffi
2
9
J ¼ 33
8
V
V
exp 0:89b pffiffiffi
0
m
L3
L
0
where J is the current density, 3 is the permittivity of free space,
3
is the relative permittivity, m is the zero-eld mobility, V is the
applied voltage, L is the thickness of active layer, and b is the
eld-activation factor. We xed the maximum electric eld at

5
ꢀ1
3
.0 ꢂ 10 V cm . The zero-eld electron mobility of SPDP was
ꢀ
6
2
ꢀ1 ꢀ1
ꢀ5
1/2 ꢀ1/2
found to be 5.9 ꢂ 10 cm V
s
(b ¼ 3.9 ꢂ 10 cm
V
)
ꢀ
6
2
ꢀ1 ꢀ1
whereas the value for SPDQ was 1.6 ꢂ 10 cm V
s
(b ¼ 4.7
ꢀ
5
1/2 ꢀ1/2
ꢂ 10 cm
V
). The electron mobility of SPDP is higher
Fig. 5 (a) Calculated HOMO/LUMO energy levels and orbitals of
SPDP, SPPP and SPDQ; (b) calculated (black bold lines) and measured
than that of SPDQ although they are of the same order of
magnitude. However, SPPP showed a relatively low electron
(
gray bold lines) triplet energy values of the sulfone-based ETMs.
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Fig. 7 (a) Current density–voltage–luminance (J–V–L) characteristics
(b) current efficiency–luminance (CE–L) of blue PhOLEDs with
different thicknesses of SPDP ETL.
The maximum luminance (brightness) decreased with
increasing thickness of the SPDP ETL. However, the perfor-
mance of the PhOLEDs signicantly increased with 15 and 30
nm SPDP ETL compared to the 5 nm ETL. The PhOLEDs with 5
nm thick SPDP showed a high turn-on voltage of 6.6 V and a
ꢀ1
current efficiency (CE) of 13.4 cd A (power efficiency (PE) ¼ 3.9
ꢀ1
lm W ). The performance of the PhOLEDs dramatically
changes with thicker SPDP ETL (15 and 30 nm). PhOLEDs with
15 nm SPDP ETL showed a reduced turn-on voltage (5.2 V) and
also a signicantly increased efficiency (Fig. 7b). This represents
a 2.5-fold higher efficiency compared to the device with 5 nm
ETL thickness. As the thickness of SPDP increases to 30 nm,
device showed a slightly higher turn-on voltage (5.5 V)
compared to the device with 15 nm ETL thickness. The CE and
PE of the PhOLEDs with 30 nm thick SPDP ETL was comparable
to the device with 15 nm thick SPDP. Nevertheless, the current
efficiency of the 30 nm SPDP device showed the highest value of
ꢀ
1
3
3.9 cd A . The device with 15 nm SPDP ETL had a high current
ꢀ1
efficiency of 33.6 cd A , which is similar to the 30 nm SPDP ETL
Fig. 6 Current–voltage characteristics of the SCLC devices with the device performance.
structure of ITO/sulfone-based ETM (100 nm)/Al; (a) SPDP; (b) SPPP;
We could observe severe roll-off in efficiency in the PhOLED
and (c) SPDQ.
with 30 nm thick SPDP whereas the device with 15 nm thick
SPDP showed a reduced roll-off in efficiency at high luminance.
Electroluminescence (EL) spectra of all the blue PhOLEDs
with different thicknesses of SPDP are shown in Fig. 8. The EL
mobility of 7.3 ꢂ 10ꢀ cm V
8
2
ꢀ1 ꢀ1
s
(b ¼ 1.3 ꢂ 10 cm
ꢀ5
1/2 ꢀ1/2
V
),
which is two orders of magnitude lower than the electron
mobility of SPDP and SPDQ. The low carrier mobility in SPPP
could be due to the poor intermolecular interactions as a result
of the phenyl groups on each side. Attached phenyl ring
disturbs planar molecular structure and thus hinders efficient
molecular packing.
10
spectra showed that the blue emission originated from FIrpic
triplet emitter with the maximum peak at 472 nm. We can
Blue PhOLEDs with sulfone-based electron transport layer
To evaluate electron-transport and exciton blocking ability of
the new sulfone-based ETMs, we initially fabricated PhOLEDs
with SPDP ETL. In order to optimize device performance, the
thickness of SPDP ETL was varied as 5, 15, and 30 nm in blue
PhOLEDs. Fig. 7 shows the electrical characteristics and
performance of PhOLEDs with different thicknesses of SPDP.
The PhOLEDs with 5 and 15 nm thick SPDP showed similar
current densities whereas the device with 30 nm thick SPDP
Fig. 8 Normalized EL spectra of the blue PhOLEDs with different
showed a lower current density due to its much larger thickness. thicknesses of SPDP ETL.
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ꢀ
1
clearly see the EL intensity of shoulder peak around 500 nm and had an increased current efficiency (CE) of 33.6 cd A with
increases as the thickness of SPDP increases. The increased EL an external quantum efficiency (EQE) of 19.6%, while device I
25
ꢀ1
intensity of the shoulder peak is due to a microcavity effect,
showed a high turn-on voltage (6.3 V) and a CE of 16.2 cd A
which is also consistent with the increased device performance. with an EQE of 10.2%. The device with SPDP ETL showed much
Commission Internationale de L'Eclairage (CIE) 1931 coordi- higher performance than using commercial BPhen ETL (CE ¼
ꢀ
1
4b
nates of the devices changed from (0.13, 0.28) to (0.14, 0.34) 16.5 cd A , EQE ¼ 8.4%). Devices III and IV also had enhanced
ꢀ
1
when the thickness of SPDP changed from 5 to 30 nm. This device performance, showing CE of 27.1 and 30.6 cd A (EQE ¼
result imply that although increased EL intensity of the 13.9 and 15.7%), respectively. However, the maximum lumi-
shoulder peak around 500 nm is effective for the enhancement nance of device IV was much lower than that of device II and
of the device performance, there is an undesirable change in the device III, which is presumably due to the low current density.
purity of blue color due to the green emission. Therefore, we Comparing the current densities, device II showed the highest
consider the 15 nm thickness as the optimum for high effi- current density and the device IV showed the lowest current
ciency and low roll-off in efficiency for blue PhOLEDs using density, which can be correlated with the energy levels of the
sulfone-based ETMs.
sulfone-based ETMs. SPDP has an energy barrier of only 0.27 eV
We fabricated blue PhOLEDs with all the sulfone-based with PVK-based polymeric host material (Fig. 4), whereas SPDQ
ETMs (SPDP, SPPP, and SPDQ) with the ETL thickness xed at has a large energy barrier of 0.4 eV with the host material.
1
5 nm. Standard device conguration for the blue PhOLEDs Although the energy barrier between SPPP and PVK host
were ITO/PEDOT:PSS/PVK:OXD-7:FIrpic/ETM/LiF/Al. Four material is very small (0.07 eV), the electron mobility of SPPP is
different series of PhOLEDs were fabricated to investigate the very low (ꢃ10 cm V
ꢀ
8
2
ꢀ1 ꢀ1
s
) compared to the SPDP and SPDQ
), which is consistent with the performance
on device performance. Device I had no ETL and devices II, III, of the blue PhOLEDs.
and IV had SPDP, SPPP, and SPDQ ETL, respectively. Device IV with SPDQ also showed signicantly enhanced
The current density–voltage–luminance (J–V–L), and current device performance despite the relatively low triplet energy
efficiency–luminance (CE–L) characteristics of PhOLEDs with value of SPDQ (E
¼ 2.53 eV) compared to the other high
sulfone-based ETMs as the electron transport layer are shown in triplet energy sulfone-based ETMs (SPDP, E
ꢀ
6
2
ꢀ1 ꢀ1
effect of high triplet energy, electron mobility and energy levels (ꢃ10 cm V
s
T
T
¼ 2.90 eV and
Fig. 9. PhOLEDs with sulfone-based ETLs all showed enhanced SPPP, E ¼ 2.81 eV). We note that in order to conne triplet
T
device performance (devices II, III, and IV) compared to the excitons effectively inside the EML, the triplet energy of ETL
8,26
device without ETL (device I) as shown in Fig. 9 and Table 2. should be higher than that of the FIrpic emitter (2.7 eV).
Device II showed a signicantly reduced turn-on voltage (5.2 V) Nevertheless, SPDQ ETL with a low triplet energy signicantly
improved the PhOLED efficiency, which implies that facilitating
electron transport to achieve better charge carrier balance in the
emission layer by an ETM with high electron mobility is also an
important factor that can lead to high performance PhOLEDs
even without a high triplet energy. Furthermore, EL spectrum of
device IV with SPDQ ETL showed blue emission with very weak
shoulder peak around 500 nm with CIE coordinates of (0.13,
0
.28) while the EL spectrum of device III with SPPP ETL had a
more prominent shoulder peak with CIE coordinates (0.14,
.32) with unwanted greenish emission (Fig. 10). These results
0
suggest that an appropriate LUMO level of the ETM is essential
Fig. 9 (a) Current density–voltage–luminance (J–V–L) of blue PhO- while high electron mobility is also important to facilitate
LEDs with sulfone-based ETLs. (b) Current efficiency–luminance
electron-transport to achieve high-performance PhOLEDs.
(
CE–L) of blue PhOLEDs with sulfone-based ETMs.
a
Table 2 Device characteristics of blue PhOLEDs based on sulfone-based ETMs
Drive voltage
[V]
Current density
Luminance
Current efficiency
Power efficiency
External quantum
efficiency [%]
b
ꢀ2
ꢀ2
ꢀ1
ꢀ1
Device
ETL
V
on [V]
[mA cm
]
[cd m
]
[cd A
]
[lm W
]
Device I
Device II
Device III
Device IV
None
SPDP
SPPP
SPDQ
6.3
5.2
6.7
6.3
15.4
53.4
2.2
39.6
33.4
49.7
4.1
3490
360
6.5
16.2
19.2
33.6
12.1
27.1
16.1
30.6
1.3
4.1
4.3
10.6
2.1
8.9
3.2
8.2
4.1
10.2
11.2
19.6
6.2
13.9
7.3
15.7
9
.8
13.8
0.4
15.4
0.0
14.8
0.2
7810
2730
6010
1100
5990
1390
1
1
37.2
4.5
1
a
Values in italic correspond to those at maximum device efficiencies. Turn-on voltage (at luminance of ꢃ1 cd mꢀ2).
b
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5
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1
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2
Conclusion
A series of new sulfone-based electron transport materials with 11 P. E. Burrows, A. B. Padmaperuma, L. S. Sapochak,
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ꢀ
6
2
ꢀ1 ꢀ1
was (2–6) ꢂ 10 cm V
was signicantly lower in the case of SPPP (7 ꢂ 10 cm V
Density functional theory (DFT) calculated triplet energy values
s
for SPDP and SPDQ thin lms but
ꢀ
8
2
ꢀ1 ꢀ1
s
).
of the sulfone-based materials were found to be in good 15 F.-M. Hsu, C.-H. Chien, Y.-J. Hsieh, C.-H. Wu, C.-F. Shu,
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