Benzoquinoline-based fluoranthene derivatives as electron transport materials for solution-processed red phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1039/c7ra03178e

A new electron transport material (ETM) with two fluoranthene and a benzoquinoline moiety was synthesized for the fabrication of solution-processed phosphorescent organic light-emitting diodes (PHOLEDs). In particular, we designed a bulky structure with a large twisted angle and improved thermal stability as compared to those of the common ETMs [e.g., 2,2′,2′′-(1,3,5-phenylene)tris(1-phenyl-1H-benzimidazole) (TPBI)] for device stability. As a result, we found that 4-(3-(fluoranthen-3-yl)-5-(fluoranthen-4-yl)phenyl)-2-phenyl-benzo[h]quinoline (FRT-PBQ) showed high glass transition temperature (T_g: ～184 °C) and moderately high electron transport behavior despite its bulky structural moieties. Moreover, we fabricated solution-processed red PHOLEDs with FRT-PBQ, and a relatively high current efficiency and external quantum efficiency of up to 20.7 cd A^-1 and 15.5%, respectively, were achieved. In addition, the device lifetime was extended by a factor of 1.7 as compared to that of the device fabricated with TPBI as a common ETM.

Full text of DOI:10.1039/c7ra03178e

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PAPER
Benzoquinoline-based fluoranthene derivatives as
electron transport materials for solution-processed
red phosphorescent organic light-emitting diodes†
a
Cite this: RSC Adv., 2017, 7, 28520
So-Ra Park,a Dong Heon Shin,a Sang-Mi Parkb and Min Chul Suh
*
A new electron transport material (ETM) with two fluoranthene and a benzoquinoline moiety was
synthesized for the fabrication of solution-processed phosphorescent organic light-emitting diodes
(PHOLEDs). In particular, we designed a bulky structure with a large twisted angle and improved thermal
stability as compared to those of the common ETMs [e.g., 2,20,200-(1,3,5-phenylene)tris(1-phenyl-1H-
benzimidazole) (TPBI)] for device stability. As
a result, we found that 4-(3-(fluoranthen-3-yl)-5-
(fluoranthen-4-yl)phenyl)-2-phenyl-benzo[h]quinoline (FRT-PBQ) showed high glass transition
temperature (Tg: ꢀ184 ꢁC) and moderately high electron transport behavior despite its bulky structural
moieties. Moreover, we fabricated solution-processed red PHOLEDs with FRT-PBQ, and a relatively high
current efficiency and external quantum efficiency of up to 20.7 cd Aꢂ1 and 15.5%, respectively, were
achieved. In addition, the device lifetime was extended by a factor of 1.7 as compared to that of the
device fabricated with TPBI as a common ETM.
Received 17th March 2017
Accepted 13th May 2017
DOI: 10.1039/c7ra03178e
rsc.li/rsc-advances
transport layer (HTL) and emitting layer (EML)).7,8 To achieve
orthogonal solubility, polymer-based cross-linkable mate-
Introduction
rials have been used as HTLs for solution-processed
OLEDs.1,2,9,10 However, cross-linkable HTLs mostly restrict
the hole ow because of the lower hole mobility [mhole (cross-
linkable HTL) z 10ꢂ6 cm2 Vꢂ1 sꢂ1] as compared to that of the
commonly used HTL materials.1,2,11 As a result, the recom-
bination zone of the solution-processed OLEDs should be
shied towards the HTL/EML interface although it could be
the main reason for the deterioration of device lifetime as
well as the induced exciton quenching.1,2 To solve this
problem, we need to control the charge balance of the devices
using various materials for EMLs and electron transport
layers (ETLs).12–17 In this study, we focused on new electron
transport materials (ETMs) for solution-processed OLEDs
because the common ETMs normally exhibited very low glass
transition temperature (TPBI ¼ 124 ꢁC), which resulted in
a reduced device lifetime because of molecular crystallization
from joule-heating of the devices.18 In other words, we
report a new ETM with pretty high glass transition tempera-
ture (Tg) (about 184 ꢁC) as well as moderately higher
electron transport behavior than those of the reported
previously ETMs [e.g. TPBI (2,20,200-(1,3,5-phenylene)tris
(1-phenyl-1H-benzimidazole)), BPhen (4,7-diphenyl-1,10-
phenanthro-line) Alq3 (tris-(8-hydroxyquinoline) aluminum),
etc.]. As a result, we obtained a moderately high device efficiency
up to 20.7 cd Aꢂ1 using 4-(3-(uoranthen-3-yl)-5-(uoranthen-4-
yl)phenyl)-2-phenyl-benzo[h]quinoline (FRT-PBQ).
Active matrix organic light-emitting diodes (AMOLEDs) have
received signicant attention because of their advantages such
as their ability to fabricate large-area displays and solid-state
lighting. It is very difficult to manufacture large-area and
high-resolution AMOLEDs as their production is based on the
thermal vacuum deposition method, which results in a high
manufacturing cost of mass production due to signicant
organic material consumption during deposition. Thus, to
reduce the manufacturing cost, many research groups have
focused on solution processes such as spin-coating, nozzle
printing, and ink-jet printing.1–6 Due to these efforts, the effi-
ciency of the devices has reached values similar to those ach-
ieved with vacuum deposition, but the lifetime of the devices is
still considerably shorter.7 Thus, solution-processed OLEDs
should be further developed to reach a level suitable for
commercialization.
The most important issue to obtain highly efficient solution-
processed OLEDs is to realize multi-layer deposition without
any mixing of each layer. In other words, it is important to
have perfect orthogonality for each functional layer (e.g., hole
aOrganic Electronic Materials Laboratory, Department of Information Display, Kyung
Hee University, Dongdaemoon-Gu, Seoul 02447, Republic of Korea. E-mail: mcsuh@
khu.ac.kr
bR&D Center OLED Team, AlphaChem, 145, Dongo 1-Gil, Hwaseong-Si, Gyeonggi-Do
18298, Republic of Korea
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra03178e
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temperature for 1 h. The resulting mixture was ltered and then
washed with ethyl ether. The product was dried under vacuum
and directly used for the next step. Then, a white powder was
obtained (13.0 g, yield: 77.5%). The obtained solid (12.8 g, 45.9
mmol) was added dropwise to a solution of compound FRT-PBQ
(A) (15.0 g, 38.3 mmol) and ammonium acetate (14.7 g, 191.3
mmol) in acetic acid (150 ml). The mixture was reuxed for 12 h
aer cooling down to room temperature. The resulting mixture
was ltered and then washed with methanol. White powder
(12.3 g, yield: 65.4%). 1H-NMR (CDCl3) d [ppm]: 8.30 [d, 2H],
8.14 [d, 1H], 7.96 [s, 2H], 7.54 [t, 2H], 7.45–7.35 [m, 6H], 2.99 [s,
4H].
FRT-PBQ (C): 4-(3,5-dibromophenyl)-2-phenylbenzo-[h]
quinoline. A mixture of compound FRT-PBQ (B) (12.0 g, 24.4
mmol) and 2,3-dichloro-5,6-dicyano-p-benzoquinone (12.2 g,
53.7 mmol) in 1,4-dioxane (180 ml) was reuxed for 12 h. Aer
being cooled down to room temperature, dichloromethane was
added to the reaction mixture. The organic phase was separated
and washed with brine before being dried over anhydrous
MgSO4. The solvent was evaporated and the residue was puri-
ed by column chromatography on silica gel using
dichloromethane/hexane. Ivory powder (8.10 g, yield: 67.8%).
1H-NMR (CDCl3) d [ppm]: 8.51 [d, 1H], 8.30 [m, 2H], 8.16 [d, 1H],
8.06 [d, 1H], 7.92 [s, 1H], 7.81 [d, 1H], 7.70–7.60 [m, 3H] 7.55–
7.40 [m 5H].
Experimental
Instruments
1H NMR spectra were obtained using a Bruker AVANCE II 400
MHz NMR spectrometer. The absorption spectrum was ob-
tained using a ultraviolet-visible (UV-vis) spectrophotometer
(UV-1650PC). Photoluminescence (PL) spectra were acquired
using a Perkin Elmer photoluminescence spectrophotometer
(LS 55 model). Thermogravimetric analysis (TGA) was per-
formed using TA INSTRUMENT TA4100. Differential scanning
calorimetry (DSC) was performed using a Perkin Elmer model
DSC-N650, operated under a nitrogen atmosphere. Matrix-
assisted laser desorption/ionization time-of-ight mass spec-
trometry (MALDI-TOF MS) experiments were performed via
a Bruker Daltonics model Ultraex III MALDI TOF/TOF. High
performance liquid chromatography (HPLC) was performed
using a Waters model 2695 HPLC system. The positions of the
highest occupied molecular orbital (HOMO) was measured
using a photoelectron spectrometer (AC2, Riken Keiki). Carbon,
hydrogen, and nitrogen analyses were conducted using an
elemental analyzer (Flash1112/Flash2000, CE Instrument,
Italy).
Materials
The following materials were either purchased or synthesized.
Poly(3,4-ethylene dioxythiophene) (PEDOT) doped with poly
(styrenesulfonate) anions (PSS) (PEDOT:PSS) was purchased
from Heraeus (Heraeus Clevios™ P VP CH 8000) as a hole
injection layer (HIL) and HL-X026, a cross-linkable material
(XM, T1: 2.25 eV), was purchased from Merck as an HTL.1,2,19,20
11-[20-Diphenyl-[1,3,5]triazin-2-yl]-biphenyl-4-yl]-11H-benzo[a]
carbazole as a red host material (RH, TRZ-PBC) was purchased
from KISCO and iridium(III) bis(2-(3,5-dimethylphenyl)quinoli-
FRT-PBQ: 4-(3-(uoranthen-3-yl)-5-(uoranthen-4-yl)phenyl)-
2-phenylbenzo[h]-quinoline. A mixture of compound FRT-PBQ
(C) (15.0 g, 30.7 mmol), uoranthen-3-yl-3-boronic acid
(12.2 g, 53.7 mmol), Pd(pph3)4 (1.06 g, 0.92 mmol), and 1 M
K2CO3 (150 ml) in THF (300 ml) was reuxed for 12 h. Aer
being cooled down to room temperature, the reaction mixture
was extracted with dichloromethane and deionized water. The
solvent was evaporated, and the residue was puried via column
chromatography on silica gel using dichloromethane/hexane.
White powder (13.4 g, yield: 60%). 1H-NMR (CDCl3) d [ppm]:
9.59 [d, 1H], 8.40 [d, 2H], 8.16 [m, 3H], 8.12 [d, 1H], 8.04 [d, 3H],
7.94 [m, 9H], 7.85 [d, 3H], 7.78 [m, 4H], 7.68 [t, 2H], 7.54 [t, 1H],
7.40 [m, 4H]. MALDI-TOF MS: m/z 731.88, cal. 731.914. Anal.
calc. for C57H33N: C, 93.54; H, 4.54; N, 1.91; found: C, 93.62; H,
4.55; N, 1.99.
nato-N,C20)tetra-methylheptadionate,
Ir(mphq)2tmd
(Red
Dopant, RD) was purchased from Lumtec and used as a phos-
phorescent red dopant for EML.1,2,21 In addition, FRT-PBQ was
synthesized and used as a material for ETL. 2,20,200-(1,3,5-Phe-
nylene)tris(1-phenyl-1H-benzimidazole) (TPBI) as an ETL,
lithium uoride (LiF) as a material for an electron injection
layer (EIL), and aluminum (Al) as a cathode were also purchased
from commercial suppliers and used without purication.
Device fabrication
Materials synthesis of a new electron transport material
To fabricate OLED devices with solution-processed red PHO-
FRT-PBQ
(A):
(E)-2-(3,5-dibromobenzylidene)-3,4-dihy- LEDs, a 150 nm-thick patterned indium–tin oxide (ITO) glass
dronaphthalen-1(2H)-one. A mixture of 3,5-dibromobenzalde- with an open emission area of 4 mm2 was used. The ITO glass
hyde (20.0 g, 75.8 mmol), a-tetralone (15.5 g, 106.1 mmol), and was cleaned in acetone and isopropyl alcohol via ultra-
ethanol (400 ml) was added dropwise to a 8.5 M NaOH solution sonication and rinsed in deionized water. Then, the ITO glasses
(in water) at 0 ꢁC. The resulting mixture was stirred for 1 h and were treated in UV-ozone to eliminate all the organic impurities
then ltered. The obtained solid was washed with water and that remained during the previous fabrication processes.
ethanol to obtain the product. Yellowish powder (22.0 g, yield: PEDOT:PSS was spin-coated on an ITO glasses under ambient
74%). 1H-NMR (CDCl3) d [ppm]: 7.88 [s, 1H], 7.73 [d, 1H], 7.68 [t, conditions and annealed at 150 ꢁC for 15 min in a nitrogen
1H], 7.61 [s, 1H], 7.51 [s, 2H], 7.46–7.48 [m, 2H], 3.10–2.90 [m, atmosphere. Subsequently, the HTL material was dissolved in
4H].
chlorobenzene at 0.5 wt% and spin-coated and cross-linked via
FRT-PBQ (B): 4-(3,5-dibromophenyl)-4a,5,6,10b-tetrahydro- the standard process. For the EML materials, the RH and red
2-phenylbenzo-[h]quinoline. Pyridine (120.0 ml) was added phosphorescent dopant (RD) were dissolved in toluene to
dropwise to phenacyl bromide (12.0 g, 60.3 mmol) at room obtain a 1 and 0.5 wt% solution. The red EML solution obtained
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by mixing the host and dopant solutions was spin-coated and
ꢁ
dried at 100 C for 10 min in a glove box. Then, the ETL was
thermally deposited at a rate of 0.5 A sꢂ1 under typical vacuum
˚
conditions (<10ꢂ7 Torr). Aer the completion of all the solution
and evaporation processes of the organic species, LiF andꢂA1l
ꢂ1
˚
˚
were successively deposited at 0.3 A s
and 3 A s ,
respectively.
Measurements
The current density–voltage (J–V) and luminance–voltage (L–V)
data of OLEDs were obtained using a Keithley SMU 2635A and
Minolta CS-100A, respectively. The OLED area was 4 mm2 for all
the samples studied in this work. Electroluminescence (EL)
spectra and the Commission Internationale De'Eclairage (CIE)
coordinates were obtained using
spectroradiometer.
a
Minolta CS-2000A
Results and discussion
Scheme 1 Synthesis of the new electron transport material.
Basic properties of FRT-PBQ
As previously reported, the carrier mobility m of the hole and/or
electron transport materials can be inuenced by the molecular
geometry of these materials (e.g. twisted angle between shell
moieties, size of the pendant groups, planarity of the molecules
near the core moiety, etc.). Especially, the mobility signicantly
decreases if the carrier hopping is limited due to the molecular
geometry of these materials.22 In other words, m is seriously
affected by the molecular structure, which can cause molecular
stacking through p–p interaction. Thus, m increases if the
hopping charge interacts with randomly oriented and located
dipoles in the bulk lm. However, Tg increases with molecular
size and molecular weight. In other words, it normally increases
if the sizes of the pendant moieties are bigger because they limit
the segmental motion of the molecule. Based on these aspects,
we synthesized a new ETM with two large uoranthene and
a benzoquinoline moiety for solution-processed OLEDs. Espe-
cially, we adopted the phenyl linker unit as a core moiety for
these two uoranthene and benzoquinoline moieties. As
a result, we could increase the thermal stability as well as
electron transporting behavior as compared to those of TPBI.23
The detailed synthetic route for the preparation of these mate-
rials is shown in Scheme 1.
and the central phenyl linker) is about 53.8ꢁ. It was larger than
that (about 34.0ꢁ) of TPBI (between the benzimidazole moiety
and the central phenly linker), which may cause interruption of
the p–p stacking. However, we could not expect that those
twisted angles can interrupt to reduce the electron mobility
because the planar fused aromatic systems such as uo-
ranthene and benzoquinoline moieties are large enough to be
separated from the central core unit (e.g., phenyl linker).
Moreover, these bulky and planar moieties could increase the
thermal stability, as abovementioned. We also calculated the
excited state of the FRT-PBQ molecule using the same theory
(TD-DFT, B3LYP/6-311++G(d)) and obtained a triplet energy
level (T1) of 2.30 eV, which is also summarized in Table 1.
Fig. 2(a) displays the UV-vis absorption and PL spectra of
FRT-PBQ. The band gap Eg that was determined from the band
edge of the UV-vis spectrum was 3.09 eV for FRT-PBQ. The
HOMO energy level measured by the AC2 analyzer and the
LUMO energy level determined via the subtraction of the band
Fig. 1(a) and (b) show the molecular geometry of the ground
state (S0) of TPBI and FRT-PBQ, which were obtained from the
molecular simulation using the Gaussian 09 soware. The
HOMO and lowest unoccupied molecular orbital (LUMO) were
optimized using Becke's three-parameter method and the Lee–
Yang–Parr correlation (B3LYP) functional with the 6-31G (d)
basis set. From this result, we found that the HOMO was mostly
distributed over the entire molecule, whereas the LUMO was
mostly localized on the uoranthene and benzene core moie-
ties, as shown in Fig. 1(b). This means that the electron trans-
port process through the FRT-PBQ molecule may occur at the
uoranthene moiety although the benzoquinoline moiety has
an electron-withdrawing heteroaromatic component. Moreover,
Fig. 1 Optimized molecular geometry of (a) TPBI and (b) FRT-PBQ
molecule showing HOMO (left), LUMO (middle), and side-view of the
the twisted angle of FRT-PBQ (between the uoranthene moiety molecule (right).
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Table 1 A summary of the physical properties of the electron transport material
lplb [nm]
T1 [eV]
297 K 77 K (exp)
HOMO [eV]
T1 [eV] (cal) (AC2)
a
c
Compound labs [nm]
LUMO [eV] Eg [eV] Tg [ꢁC] Tm [ꢁC] Td [ꢁC]
FRT-PBQ
246, 269, 290, 365 455
569
2.18
2.30
ꢂ5.99
ꢂ2.90
3.09
184
—
567
a
labs: peak wavelength observed via UV-visible absorption spectroscopy (at 297 K). b lpl: peak wavelength measured from the photoluminescence
c
(PL) spectra (at 297 K and 77 K). T1 was calculated using the Gaussian 09W program [B3LYP, 6-311++G].
EOD A: ITO/LiQ (1.5 nm)/TPBI (100 nm)/LiF (1.0 nm)/Al (100
nm).
EOD B: ITO/LiQ (1.5 nm)/FRT-PBQ (100 nm)/LiF (1.0 nm)/Al
(100 nm).
Fig. 3(a) shows the energy band diagram of the EODs of the
ETMs. These materials exhibit LUMO energy levels of ꢂ2.70 and
ꢂ2.90 eV for TPBI and FRT-PBQ, respectively. For this reason,
we expected that FRT-PBQ might show better electron injection
behavior than TPBI. However, we observed lower electron
density from the EOD B (prepared with FRT-PBQ) as compared
to that form the EOD A (with TPBI) at the voltage range less than
1.5 V. Hence, we cannot conclusively say that FRT-PBQ has
better electron injection property than TPBI. In other words, the
electron mobility of TPBI seems to be greater than that of FRT-
PBQ at low electric eld. Actually, we calculated the electron
mobility of FRT-PBQ and TPBI from the space-charge-limited
current (SCLC) region as follows in the next equation (eqn (1)):25
9
V2
L3
J ¼ 3r30m
(1)
8
and
Fig. 2 (a) UV-visible absorption and photoluminescence spectra, (b)
TGA, and (c) DSC results of FRT-PBQ.
!
rffiffiffiffi
V
m ¼ m0 exp b
;
(2)
L
gap from the HOMO energy level were ꢂ5.99 and ꢂ2.90 eV,
respectively. Moreover, FRT-PBQ exhibited featureless emission
spectral patterns, whereas some vibronic ne emission peaks
were observed in the emission spectra obtained under low-
temperature condition (77 K). From this spectrum, we
conrmed the T1 of 2.18 eV for FRT-PBQ. In addition, we
investigated the thermal stability behavior of FRT-PBQ, as
shown in Fig. 2(b) and (c). As a result, the decomposition
temperature Td and Tg were determined to be 567 and 184 ꢁC for
FRT-PBQ, respectively. As was expected, FRT-PBQ possesses
much higher Tg of 184 ꢁC as compared to those of common
where 3r is the relative permittivity (ꢀ3), 30 is the vacuum
permittivity (ꢀ8.85 ꢃ 10ꢂ14 F cmꢂ1), m is the carrier mobility
determined via SCLC measurements, L is the cathode–anode
ꢁ
ETMs [e.g., Tg (TPBI) ¼ 124 ꢁC; Tg (TmPyPB) ¼ 79 C; and Tg
(BPhen) ¼ 66 ꢁC], which might be due to its rigid structure.24 All
the abovementioned data are also summarized in Table 1.
Device engineering for highly efficiency OLEDs
Design of electron only device. To investigate the electron
transport behavior, we prepared electron only devices (EODs)
with FRT-PBQ that have the following device congurations (see
also Fig. 3(a)). We also investigated a reference device with the
common ETL material (TPBI) as a bulk state.
Fig. 3 Energy band diagram of the fabricated (a) electron only devices
and (b) red PHOLEDs.
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distance, m0 is the zero-eld mobility, and b is a coefficient that luminance (J–V–L) characteristics. At a given constant voltage of
is proportional to the Poole–Frenkel factor. m0 and b were 5.0 V (>1.5 V, as aforementioned), the current density values
determined via linear tting. From eqn (1) and (2), we obtained were found to be 0.16 and 1.57 mA cmꢂ2 for device A and B,
the electron mobility, as shown in Fig. 4(b).
respectively. This is well consistent with the results obtained
As a result, the calculated electron mobilities are 1.70 ꢃ 10ꢂ5 from the EODs. In other words, the higher current transporting
and 1.66 ꢃ 10ꢂ6 cm2 Vꢂ1 sꢂ1 for TPBI and FRT-PBQ, respec- behavior of FRT-PBQ makes device B much more electron
tively, revealing the much lower electron mobility of FRT-PBQ conductive. For this reason, the turn-on voltages (Von) at a given
than that of TPBI under low electric eld. The electron mobility constant luminance of 1 cd mꢂ2 were 3.8 and 3.0 V for device A
of FRT-PBQ is a reliable value because the TPBI mobility and B, respectively.
matches quite well with the previously reported value.26 From
In addition, the operation voltages (Vop) required to achieve
the results, it was observed that FRT-PBQ provided lower elec- 1000 cd mꢂ2 were 6.7 and 6.0 V for device A and B, respectively
tron mobility than TPBI due to the increased twisted angle. (see Table 2). At a given constant luminance of 1000 cd mꢂ2, the
However, the electron current density of FRT-PBQ rapidly current and power efficiencies were 15.4 cd Aꢂ1 and 7.1 lm Wꢂ1
increased in the higher voltage regions as aforementioned (>1.5 and 18.6 cd Aꢂ1 and 9.3 lm Wꢂ1 for device A and B, respectively,
V) [see also Fig. 4(a)]. This type of reversal phenomenon in as shown in Fig. 5(b) and Table 2. In addition, this efficiency
electron current density might be originated from the intrinsic data correspond to the external quantum efficiencies (EQEs) of
molecular structure of FRT-PBQ. In other words, the weak 11.1 and 13.0% for device A and B, respectively, as shown in the
electron accepting ability of FRT-PBQ presumably due to its inset of Fig. 5(b). The maximum current and power efficiencies
lower polarity characteristic (1.5814 D) as compared to that of were 15.5 cd Aꢂ1 and 11.4 lm Wꢂ1 for device A and 20.7 cd Aꢂ1
TPBI (2.4206 D) may cause relatively poorer electron injection and 21.7 lm Wꢂ1 for device B, respectively, as summarized in
behavior, whereas the at and planar uoranthene moieties Table 2. In particular, the maximum EQE value of device A
help to efficiently transport the electron carriers at higher increased more than that of device B (i.e., device A: 11.4%;
electric eld region.27
device B: 15.5%). From the abovementioned results, we found
Solution-processed OLEDs with a new ETM. To verify the that the predominant electron transport characteristics were
FRT-PBQ characteristics, we prepared the solution-processed necessary because the devices fabricated with cross-linkable
red PHOLEDs with the following device congurations HTL materials via the solution process normally exhibited
(Fig. 3(b)). We also fabricated reference devices with TPBI to pretty low hole conductivity behavior. As a result, device B with
compare the performance levels.
Device A: ITO/PEDOT:PSS (40 nm)/XM (17 nm)/RH:RD (5%, values although it displayed more serious roll-off behavior as
30 nm)/TPBI (50 nm)/LiF (1 nm)/Al (100 nm). compared to device A, plausibly due to its narrower recombi-
Device B: ITO/PEDOT:PSS (40 nm)/XM (17 nm)/RH:RD (5%, nation zone originating from its higher electron transporting
higher electron transporting materials showed higher efficiency
30 nm)/FRT-PBQ (50 nm)/LiF (1 nm)/Al (100 nm).
property.
Moreover, we investigated the EL spectral response, as
The red PHOLEDs with TBPI and FRT-PBQ exhibited
different device characteristics depending on the ETM, as shown in Fig. 5(c). From this measurement, we found that all
shown in Fig. 5. Fig. 5(a) shows the current-density–voltage–
Fig. 4 (a) J–V characteristics of the electron only devices with EOD A Fig. 5 (a) J–V–L, (b) current-efficiency–L–power efficiency, and (c)
and EOD B and (b) calculated electron mobility for TPBI and FRT-PBQ normalized EL spectra of the fabricated red PHOLEDs (at brightness of
from the SCLC region.
1000 cd mꢂ2).
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Table 2 Summary of the device characteristics of the solution-processed red PHOLEDs
CE/PE/EQE [cd Aꢂ1/lm Wꢂ1/%]
Device
Vona/Vopb [V]
Maximum
At 1000 cd mꢂ2
CIE (x, y)bc
Lifetimeb (hours)
Device A
Device B
3.8/6.7
3.0/6.0
15.5/11.4/11.4
20.7/21.7/15.5
15.4/7.1/11.1
18.6/9.3/13.0
(0.65, 0.35)
(0.65, 0.35)
180.0
301.4
a
b
c
Measured at 1 cd mꢂ2
.
Measured at 1000 cd mꢂ2
.
CIE: Commission International de L'Eclairage.
Acknowledgements
This research was supported by the Ministry of Trade, Industry
& Energy (MOTIE, Korea) under the Industrial Technology
Innovation Program. No. 10067715, ‘Development of cross-
linkable organic materials for highly efficient multi-stacked
OLEDs fabricated by continuous printing process’. This
research was also supported by MOTIE (Ministry of Trade,
Industry & Energy (project number #10051655)) and KDRC
(Korea Display Research Consortium) support program for the
development of future devices technology for display industry.
Moreover, the authors are grateful to Mr Sang Kyu Kwak for his
helpful assistance in verifying the purity of FRT-PBQ.
Fig. 6 Operational lifetime of device A and device B (with an initial
brightness of 1000 cd mꢂ2).
the normalized EL spectra obtained from device A and B at
a brightness of 1000 cd mꢂ2 were almost the same, showing
a maximum EL peak wavelength (lmax) of 611 nm.
Notes and references
Finally, we measured the operational lifetimes of device A
and B, as shown in Fig. 6. The lifetimes were measured under
constant current density condition, giving rise to an initial
brightness of 1000 cd mꢂ2. The lifetimes at 50% drop of the
initial brightness (T50) of device A and B were 180.0 and 301.4 h,
respectively. Very interestingly, device B exhibited a much better
lifetime behavior, which was 1.7 times higher than that of
device A. This could be due to improved device efficiency as well
as a reduction in the probability of crystallization and/or
aggregation of the ETM during device operation, as
abovementioned.
Based on these results, we believe that FRT-PBQ could
provide fairly good device performances, including increased
operational lifetime, because of its suitable electron trans-
porting behavior as well as thermal stability when it is applied
to solution-processed PHOLEDs.
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