Highly efficient, deep-red organic light-emitting devices using energy transfer from exciplexes

Article abstract of DOI:10.1039/c6tc04979f

We developed a highly efficient, deep-red organic light-emitting device (OLED) with an external quantum efficiency of nearly 18% with a very low turn-on voltage of 2.41 V and an electroluminescence emission wavelength (λ_EL) of 670 nm using energy transfer from an exciplex host to a deep-red phosphorescent emitter, bis(2,3-diphenylquinoxaline)iridium(dipivaloylmethane)[(DPQ)₂Ir(dpm)].

Full text of DOI:10.1039/c6tc04979f

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Highly efficient, deep-red organic light-emitting
devices using energy transfer from exciplexes†
Cite this:DOI: 10.1039/c6tc04979f
Yuji Nagai,a Hisahiro Sasabe,*ab Jun Takahashi,a Natsuki Onuma,a Takashi Ito,a
Satoru Ohisaab and Junji Kido*ab
Received 15th November 2016,
Accepted 20th December 2016
DOI: 10.1039/c6tc04979f
www.rsc.org/MaterialsC
We developed a highly efficient, deep-red organic light-emitting [bis(spirofluorenedibenzosuberene[d]quinoxaline-C2,N)-monoacetyl-
device (OLED) with an external quantum efficiency of nearly 18% acetone]iridium(III) with an ZPL of 51% in dichloromethane solution.
with a very low turn-on voltage of 2.41 V and an electrolumines- The corresponding deep-red OLED employing a co-host system of
cence emission wavelength (kEL) of 670 nm using energy transfer N,N0-di(naphthalene-1-yl)-N,N0-diphenylbenzidine (NPD) and bis(10-
from an exciplex host to a deep-red phosphorescent emitter, bis(2,3- hydroxybenzo[h]quinolinato)beryllium (BeBq2) showed a maximum
diphenylquinoxaline)iridium(dipivaloylmethane)[(DPQ)2Ir(dpm)].
Z
ext of 11.2%, a lEL of 688 nm, and CIE coordinates of (0.70, 0.27).5b
Recently, Wang and co-workers synthesized the TADF emitter, 7,10-
Organic light-emitting devices (OLEDs) have several outstanding bis(4-(diphenylamino)phenyl)dibenzo[f,h]quinoxaline-2,3-dicarbo-
characteristics compared to other light sources; for example, OLEDs nitrile (TPA-DCPP), which exhibited a ZPL of 50% in a co-doped
are thin, light-weight, flexible, and low-power-consuming.1,2 Among film. An OLED based on TPA-DCPP showed an Zext of 9.8%, a lEL
the emitters for highly efficient OLEDs, phosphorescent and/or of 668 nm, and CIE coordinates of (0.70, 0.29).5c As can be seen
thermally activated delayed fluorescent (TADF) emitters have from these examples, in order to improve the Zext values of deep-
received considerable attention because these emitters can red OLEDs, sophisticated device engineering along with the
realize 100% conversion from electrons to photons by harvesting development of a deep-red emitter with a high ZPL is necessary.
all the electrically generated molecular excitons.1,2
In this context, we developed deep-red OLEDs based on the
Although orange, green, and sky-blue OLEDs have already iridium complex, bis(2,3-diphenylquinoxaline)iridium(dipivaloyl-
achieved an external quantum efficiency (Zext) of over 30% by methane) [(DPQ)2Ir(dpm)]. By using energy transfer from exci-
using phosphorescent3 and/or TADF emitters,4 the Zext values of plexes, these OLEDs exhibit a record-breaking Zext of nearly 18%
deep-red OLEDs with an electroluminescence (EL) emission with a lEL of 670 nm. We designed and synthesized three new
wavelength (lEL) of approximately 680 nm have been reported types of electron-transporting host materials composed of dibenzo-
to be significantly low, around 10% (Table S1, ESI†).5
thiophene (DBT) and azine units: 2-(30-(dibenzo[b,d]thiophen-4-
For example, Fujii and co-workers synthesized a deep-red yl)-[1,10-biphenyl]-3-yl)-4,6-diphenylpyrimidine (4DBT46PM), 4-(30-
phosphorescent emitter, bis(2,3-diphenylquinoxaline)iridium- (dibenzo[b,d]thiophen-4-yl)-[1,10-biphenyl]-3-yl)-2,6-diphenylpyrim-
(acetylacetone) [(QH)2Ir(acac)], with
a
photoluminescence idine (4DBT26PM), and 2-(30-(dibenzo[b,d]thiophen-4-yl)-[1,10-
quantum efficiency (ZPL) of 79% in dichloromethane solution. biphenyl]-3-yl)-4,6-diphenyl-1,3,5-triazine (4DBT46TRZ). These
A deep-red OLED based on (QH)2Ir(acac) showed an Zext of host materials formed exciplexes in co-deposited films with
10.2%, a lEL of 675 nm, and Commission Internationale de di(naphthalene-1-yl)-N,N0-diphenylbenzidine (NPD). Combining
l’Eclairage (CIE) coordinates of (0.70, 0.27) using 3-(4-biphenylyl)- (DPQ)2Ir(dpm) with these exciplex hosts, the co-deposited
4-phenyl-5-(4-tert-buthylphenyl)-1,2,4-triazole as an electron- films showed reasonably high ZPL values of over 50%. Finally,
transporting host material.5a Jou and co-workers synthesized we fabricated OLEDs using the exciplex host systems, and the
optimized OLED showed an Zext of 17.9% with a very low turn-on
a Department of Organic Materials Science, Graduate School of Organic Materials
Science, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata, 992-8510,
Japan. E-mail: hisahirosasabe@yz.yamagata-u.ac.jp, kid@yz.yamagata-u.ac.jp
b Research Center for Organic Electronics (ROEL), Frontier Center for Organic
Materials (FROM) Yamagata University, Yonezawa, Yamagata, 992-8510, Japan
† Electronic supplementary information (ESI) available: Synthetic route, results
of DFT calculation, UV-vis absorption spectra, phosphorescence lifetime of co-
deposited films, and energy diagrams of OLEDs. See DOI: 10.1039/c6tc04979f
voltage of 2.41 V, and CIE coordinates of (0.70, 0.29). This value
of Zext is 1.6 times higher than that of the current state-of-the-art
deep-red OLED.5
First, we designed three types of electron-transporting host
materials: 4DBT46PM, 4DBT26PM, and 4DBT46TRZ. Azine units,
such as pyrimidine and triazine, were introduced into these host
materials to improve their electron-transporting properties.
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led to the ability of 4DBT46TRZ to form an amorphous film with
good thermal stability. Actually, DBT-azine derivatives gave
smooth films with a small roughness of approximately 0.2 nm
(Fig. S2, ESI†). The ionization potential (Ip) was obtained by
photoelectron yield spectroscopy (PYS) in a vacuum of 10ꢀ3 Pa.
The DBT-azine derivatives showed Ip values of approximately
6.2 eV. The UV-vis absorption edges of the DBT-azine derivatives
were located at nearly the same wavelength of 350 nm, which is
equivalent to an energy gap (Eg) of approximately 3.54 eV. The
electron affinities (Ea) were estimated from the Ip and Eg values at
2.71 eV for 4DBT46PM, 2.71 eV for 4DBT26PM, and 2.79 eV for
4DBT46TRZ. The electron affinity (Ea) values were all estimated
Fig. 1 Chemical structures of materials.
The azine units are expected to interact with electron-donating to be approximately 2.7 eV by subtracting Eg from Ip.
moieties such as arylamines and carbazoles to form exciplexes.6,7
In the photoluminescence (PL) spectra (Fig. 2), the emission
Furthermore, the azine units are connected via meta-conjugation peaks (lem) were located at 405 nm for 4DBT46TRZ, 397 nm for
to enhance the solubility in organic solvents and film uniformity 4DBT46PM, and 392 nm for 4DBT26PM. We then investigated
to prevent crystallization of solid films. Prior to preparing the exciplex formation between the DBT-azine derivatives and NPD.
materials, we conducted a density functional theory (DFT) An exciplex is an excited complex formed between a donor material
calculation using the Gaussian 09 program package. The mole- and an acceptor material. The donor material contains electron-
cular structures of the employed materials are shown in Fig. 1, donating groups such as triphenylamine and carbazole derivatives,
and all the calculated data are summarized in Table 1. The DFT while the acceptor material contains electron-withdrawing groups
calculation indicated that the highest occupied molecular orbital such as pyrimidine and triazine. Therefore, the DBT-azine derivatives
(HOMO) and the lowest unoccupied molecular orbital (LUMO) can be expected to form exciplexes with NPD. To confirm exciplex
are completely separated; the HOMO is located on the DBT unit, formation, we recorded the UV-vis absorption and PL spectra of
whereas the LUMO is located on the azine unit (Fig. S1, ESI†). co-deposited films of DBT-azine derivatives and NPD (1 : 1 weight
The HOMO energy levels were calculated to be approximately ratio). The additional absorption peak derived from the charge-
6.00 eV, and the LUMO energy levels were approximately 2.10 eV. transfer (CT) complex was not observed in the UV-vis absorption
We also calculated the triplet energy (ET) by TD-DFT calculations. spectra (Fig. S3, ESI†). The DBT-azine derivatives showed strong
The studied derivatives had high ET values of over 2.90 eV, which absorption derived from p–p* transition around 260 nm and broad
are sufficiently high to confine the triplet excitons on the absorption derived from n–p* transition around 330 nm. In contrast,
phosphorescent emitter (DPQ)2Ir (dpm) (ET = 1.95 eV).
in the PL spectra of the co-deposited films, a novel emission,
The synthetic route to the DBT-azine derivatives is shown in completely different from those of the DBT-azine derivatives and
Schemes S1–S5 (ESI†). Three derivatives were easily synthesized NPD, was observed at 522 nm for 4DBT46TRZ, 495 nm for
on a multigram scale via Suzuki–Miyaura cross-coupling reac- 4DBT46PM, and 500 nm for 4DBT26PM. These novel emissions can
tions with dibenzothiophene-containing boron derivatives and be attributed to the exciplexes.
azine-containing bromides in 62–81% yields. The products
We then evaluated the ZPL values of exciplex host films
were purified by column chromatography and train sublima- doped with 1 wt% (DPQ)2Ir(dpm). The ZPL value was measured
1
tion, and confirmed using H and 13C NMR and mass spectro- under N2 flow using an integrating sphere excited at 330 nm
metry. The purities were confirmed by high-performance liquid using a multichannel spectrometer as the optical detector.
chromatography to be over 99.0%.
The ZPL values were measured to be 52% to 54%. The transient
We investigated the thermal properties, including the glass phosphorescence spectra of the (DPQ)2Ir(dpm)-doped exciplex
transition temperature (Tg), melting point (Tm), and decomposi- host films were recorded at room temperature. The phosphor-
tion temperature (Td5), using thermal gravimetric analysis (TGA) escence decay curves showed a single exponential decay curve
and differential scanning calorimetry (DSC) under nitrogen flow. with a phosphorescence lifetime (tp) of approximately 1.0 ms
The thermal properties are listed in Table 1. Among these (Fig. S4, ESI†). Thus, these exciplex hosts can confine the triplet
materials, 4DBT46TRZ showed a high Tg of over 100 1C, which excitons of (DPQ)2Ir(dpm).
Table 1 Physical properties of DBT-azine derivatives
b
c
d
f
g
h
Tga/Tma/Td5 [1C]
HOMO/LUMO/DEH–L [eV]
ES/ET/DEST [eV]
Ipe/Eg /Ea [eV]
FPL [%]
4DBT46PM
4DBT26PM
4DBT46TRZ
92/n.d./445
88/261/444
104/248/438
5.96/2.10/3.86
5.97/2.09/3.88
6.04/2.22/3.82
3.59/2.90/0.69
3.63/2.93/0.70
3.55/2.94/0.61
6.25/3.54/2.71
6.25/3.54/2.71
6.23/3.44/2.79
53 ꢁ 1
52 ꢁ 1
54 ꢁ 1
a
Determined by DSC measurements. b Obtained from TGA. c Calculated at the RB3LYP 6-311(d,p)//RB3LYP 6-31G(d) level; DEH–L = HOMO–LUMO. d ES and
ET are energies from TD-DFT at the RB3LYP 6-31G(d)//RB3LYP 6-31G(d) level; calculated using ES and ET. e Obtained from PYS. f Taken as the point of
intersection of the normalized absorption spectra. g Calculated using Ip and Eg. h FPL of the DBT-host:NPD film doped with 1 wt% Ir(DPQ)2(dpm).
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tetrakis(pentafluorophenyl)borate (PPBI)9 (20 nm)/NPD (20 nm)/
NPD:DBT-azine derivatives: (DPQ)2Ir (dpm) (50 : 50 : 1) (wt/ratio)
(40 nm)/DBT-azine derivatives (20 nm)/DPB (30 nm)/20 wt% Liq
doped DPB (20 nm)/Liq (1 nm)/Al (80 nm)]. The energy diagram
of this device is shown in Fig. S5 (ESI†). The EL spectra showed
deep-red emission with an EL emission peak at 670 nm and CIE
coordinates of (0.63, 0.30) for 4DBT46PM, (0.68, 0.29) for
4DBT26PM, and (0.70, 0.29) for 4DBT46TRZ (Fig. S6, ESI†).
When 4DBT46PM or 4DBT26PM was used as the host, weak
exciplex emission was observed, likely because of the inefficient
energy transfer from the exciplex host to the emitter. The
4DBT46TRZ-based device showed the highest Zext of 17.9% at
100 cd mꢀ2; this record-breaking efficiency is approximately
1.6 times higher than the Zext of the current state-of-the-art
deep-red OLED.5 It can be considered that an EQE of nearly
18% is realized by a contribution of horizontal molecular
orientation because Ir(DPQ)2(dpm) is
a
heteroleptic
complex.10 The current density–voltage–luminance ( J–V–L)
characteristics are shown in Fig. 3(a). The 4DBT46TRZ-based
device showed a very low turn-on voltage (Von) of 2.41 V at
1 cd mꢀ2, while the 4DBT26PM-based device showed a higher
Von of 3.66 V. To investigate the differences in the device
performances, we fabricated and evaluated electron only
devices with a structure of [ITO (130 nm)/DPB (40 nm)/DBT-
azine derivatives (40 nm)/DPB (30 nm)/20 wt% Liq doped DPB
(20 nm)/Liq (1 nm)/Al (80 nm)]. The energy diagram of the
electron only device is shown in Fig. S7 (ESI†). As a result,
4DBT46PM showed superior electron transporting ability than
4DBT26PM (Fig. S8, ESI†). At this stage, it can be deduced
that 4DBT46PM has less steric bulkiness around the electron
cloud of LUMO compared to 4DBT26PM leading to better
overlapping of LUMOs for higher electron mobility. The
Z
ext–luminance characteristics are shown in Fig. 3(b). Com-
pared with the 4DBT26PM-based device, the 4DBT46TRZ-
based device showed a much higher current density due to
the deeper Ea, which is attributed to the superior electron
injection from DPB. Moreover, the 4DBT46TRZ-based device
showed a long device lifetime (LT70) of approximately 890 h at
7.75 mA cmꢀ2 (Fig. 3(c)), which is equivalent to approximately
5,000 cd mꢀ2 for a yellow OLED (lEL = 585 nm).7 Assuming
that the acceleration factor is 1.5, the device lifetime is
equivalent to an LT70 of approximately 1970 h at 100 cd mꢀ2
(4.04 mA cmꢀ2). All the device performances are summarized
in Table 2.
In conclusion, we designed and synthesized a series of
DBT-azine host materials. By combining DBT-azine and NPD as
an exciplex-forming host system, we developed highly efficient
and long-lifetime deep-red OLEDs with an Zext of nearly 18% and
an LT70 of 890 h at 7.75 mA cmꢀ2 (1970 h at 4.04 mA cmꢀ2).
Fig. 2 PL spectra of (a) 4DBT46PM, (b) 4DBT26PM and (c) 4DBT46TRZ.
To investigate the functionality of the exciplex hosts, we These results clearly show the strong advantages of an exciplex
fabricated OLEDs using the (DPQ)2Ir(dpm)-doped exciplex host system that can create superior carrier balance by changing
hosts as emission layers. We used 1,4-di(1,10-phenanthrolin- the ratio of hole-transporting and electron-transporting hosts
2-yl)benzene (DPB)8 as an electron transporter, and 8-quino- without preparing the materials for realizing high Zext, low-
linolatolithium (Liq) as an electron-injection layer. The structures driving voltage, and long-lifetime deep-red OLEDs. Finally,
of the resultant devices were [ITO (130 nm)/triphenylamine- the achieved Zext is 1.6 times higher than that of the current
containing polymer: 4-isopropyl-4-methyldiphenyl-iodonium state-of-the-art deep-red OLED.5
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Notes and references
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a
b
c
Von V100/Zp,100/Zc,100/Zext,100 V1000/Zp,1000/Zc,1000/Zext,1000
[V] [V/lm Wꢀ1/cd Aꢀ1/%]
[V/lm Wꢀ1/cd Aꢀ1/%]
4DBT46PM 2.61 3.65/2.03/2.36/15.0
4DBT26PM 3.66 4.93/1.49/2.34/16.6
4DBT46TRZ 2.41 3.50/2.21/2.47/17.9
5.89/0.99/1.87/11.6
7.00/0.86/1.93/12.7
5.80/1.06/1.96/13.5
10 (a) D. Yokoyama, J. Mater. Chem., 2011, 21, 19187;
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a
b
Turn-on voltage (Von) at 1 cd mꢀ2
.
Driving voltage (V), power
B. C. Krummacher, T. D. Schmidt, W. Bru¨tting and N. Danz,
Org. Electron., 2011, 12, 1663.
efficiency (Zp), current efficiency (Zc), and external quantum efficiency
(Zext) at 100 cd mꢀ2
. .
V, Zp, Zc, and Zext at 1000 cd mꢀ2
c
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