Synthesis and characterisation of a carbazole-based bipolar exciplex-forming compound for efficient and color-tunable OLEDs

Article abstract of DOI:10.1039/c6nj02865a

A new ambipolar fluorophore, 3,6-di(4,4′-dimethoxydiphenylaminyl)-9-(1-naphthyl)carbazole, was synthesized and its physical properties were studied by differential scanning calorimetry, thermogravimetric analysis, UV-vis absorption, luminescence and photo

Full text of DOI:10.1039/c6nj02865a

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Simokaitiene, J. Keruckas, D. Y. Volyniuk, O. Bezvikonnyi, V. V. Cherpak, P. Y. Stakhira, K. Ivaniuk, I.
Helzhynskyy, G. Baryshnikov, B. Minaev and J. V. Grazulevicius, New J. Chem., 2016, DOI:
1
0.1039/C6NJ02865A.
Volume 40 Number
1
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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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New Journal of Chemistry
ARTICLE
Synthesis and characterisation
мReceived 00th January 20xx,
of carbazole-based bipolar exciplex-
forming compound for efficient and color-
tunable OLEDs
a
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
a
a
a
a
Titas Deksnys, Jurate Simokaitiene, Jonas Keruckas , Dmytro Volyniuk , Oleksandr Bezvikonnyi ,
b
b
a,b
b
c,d
Vladyslav Cherpak , Pavlo Stakhira , Khrystyna Ivaniuk , Igor Helzhynskyy, Gleb Baryshnikov ,
c
a
Boris Minaev , Juozas Vidas Grazulevicius
New ambipolar fluorophore 3,6-di(4,4’-dimethoxydiphenylaminyl)-9-(1-naphthyl)carbazole was synthesized and its
physical properties were studied by differential scanning calorimetry, thermogravimetric analysis, UV-Vis absorption,
luminescence and photoelectron emission spectroscopies, cyclic voltammetry and time of flight method. The later
technique indicated that the compound demonstrated the bipolar semiconducting properties. Using the synthesized
compound as an emissive material, the single-layer OLED with electroluminescence spectrum containing the voltage-
dependent electroplex emission band in the region of 550-650 nm was fabricated. Another OLED was fabricated with the
additional electron-transporting bathophenanthroline layer that forms the direct interface with the layer of 3,6-di(4,4’-
dimethoxydiphenylaminyl)-9-(1-naphthyl)carbazole. The strong exciplex-type band in the electroluminescence spectrum of
this OLED with the emission maximum at ca. 540 nm was observed. The electroluminescence spectra of both the devices
were found to be clearly dependent on the applied bias. This effect can be useful for the development of the efficient and
colour-tunable OLEDs.
material charge carrier injection is the consistence of the HOMO
and LUMO energy levels with the work functions of the electrodes.
This condition can be successfully met using ambipolar
Introduction
The molecular structure of the organic semiconductors determine
to the great extent the structure and properties of their functional
films including the molecular conformation, absorption and
luminescence spectra, conduction type, charge carrier mobilities,
etc. The combining of the donor and acceptor moieties within a
single molecule provides the background for the good balance
between the electron and hole mobilities that constitutes one of
the mandatory conditions for the fabrication of the single-layer
semiconductors for the emissive layers.
The choice of the ambipolar semiconductors with the high electron
and hole mobilities and high luminescence quantum yields is rather
2
limited. The positive aspect of the high energetic barriers between
the electrode and functional material layer is that the additional
emission from electromers can be observed in the
3
electroluminescence (EL) spectra. The electromers are formed only
1
under an electric field and give light emission through
crossrecombination of electrons and holes, if the donor and the
high-efficiency organic light-emitting diodes (OLEDs). In this
context, the necessary condition for the efficient electrode-to-
+
– 3
acceptor are of the same kind of molecules (M M ). As the
electromer-type excited states can only be formed under an
electrical excitation, their emission is visible only in the EL spectra.
The existence of such electromer- or excimer-type excited states
leads to the appearance of the additional long-wavelength bands in
the luminescence spectra of the functional films relative to the
a.
Department of Polymer Chemistry and Technology, Kaunas University of
Technology, Radvilenu Plentas 19, LT-50254 Kaunas, Lithuania.
Lviv Polytechnic National University, S. Bandera 12, 79013 Lviv, Ukraine.
Bohdan Khmelnytsky National University, Shevchenko 81, 18031
Cherkassy, Ukraine.
Division of Theoretical Chemistry and Biology, School of Biotechnology,
b.
c.
*
3
d.
molecular excitons (M ).
KTH Royal Institute of Technology, 10691 Stockholm, Sweden.
Corresponding author: J.V. Grazulevicius; address: Department of Organic
Technology, Kaunas University of Technology, Radvilenu pl. 19, LT-50254 Kaunas,
Lithuania; tel.: +37037300193; e-mail: Juozas.Grazulevicius@ktu.lt.
This phenomenon was recently applied for the fabrication of high-
efficiency white organic light emitting diodes (WOLEDs) with the
†
4
simplified structure (using only the single emissive layer).
Electronic Supplementary Information (ESI) available: [Characterization of OLED
which contains TPBi electron-transport interlayer]. See DOI: 10.1039/x0xx00000x
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◦
Figure 1. TGA curve of DPNC (scan rate 20 C/min, N
2
◦
atmosphere) (a), DSC curves of DPNC (scan rate 10 C/min, N
2
atmosphere) (b).
1
Scheme 1. The synthetic route to DPNC. (a) Cu, K
crown-6, o-dichlorobenzene, 185°C under argon, 24 hrs; (b) KI /
KIO , AcOH, reflux, 2 hrs; (c) Cu, K CO , 18-crown-6, o-
dichlorobenzene, 185°C under argon, 24 hrs.
2
CO
3
, 18-
synthesized compound was characterized by mass spectrometry, H
1
3
and C NMR and IR spectroscopies. The obtained data were found
to be in good agreement with the desirable structure.
3
2
3
The behaviour of DPNC under heating was explored by TGA and
Another way to improve the efficiency of OLEDs and to extend the DSC.
EL spectra is an exciplex-type emission that can be induced at the The thermal properties of DPNC are illustrated by Figure 1 a, b.
5
a,b
interface between the functional organic layers.
Two different TGA revealed high thermal stability of DPNC with 5% weight loss
types of exciplex-forming interfaces were realized: 1) between two temperature (T ) of 413°C. After the synthesis and purification,
D
emissive layers; 2) between an emissive layer and hole- or electron- DPNC was obtained as amorphous powder. Several attempts to
transporting layer. The high charge carriers mobility of emissive obtain its crystals by simple means of crystallization from the
layer and adjacent transporting layer relative to the electrons or solutions were not successful. Only glass transition at 117 °C with
holes would provide the accumulation of charge carriers at the no signals of melting and crystallization was observed during DSC
interface. As a result the highly intensive exciplex emission can be experiments.
obtained together with the spectral broadening due to the intrinsic Electrochemical properties of DPNC were examined by cyclic
EL of emitter. In addition, the interface exciplexes can be utilized as voltammetry (CV). Cyclic voltammogram of DPNC is shown in Figure
active planar pn heterojunctions, which dramatically reduce turn-on 2 a. DPNC could be oxidized reversibly during multiple scans.
5
b
CV
p
voltage of OLEDs.
The ionization potential (I ) of DPNC evaluated by CV was found
8
In the present work we present the synthesis, spectroscopic, to be 5.0 eV.
thermal and electrochemical characterization of the ambipolar
fluorophore
3,6-di(4,4’-dimethoxydiphenylaminyl)-9-(1-
naphthyl)carbazole (DPNC) which demonstrates the voltage-
dependent green-blue electrofluorescence. We also fabricated the
bilayer OLED which demonstrated the exciplex emission from the
interface between the DPNC layer and the adjacent electron-
transporting layer of bathophenanthroline (Bphen) or 2,2′,2"-(1,3,5-
benzenetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi). We have
clearly observed that DPNC demonstrated the voltage-dependent
EL both in the short-wavelength region (0-0 and 0-1 bands of S -S
1
0
transition) as well as in the long-wavelength region due to the
electroplex emission. These observations open up the new
possibilities for the fabrication of efficient color-tunable OLEDs with
the minimal number of functional layers.
a)
Results and discussion
Synthesis and characterization.
DPNC was obtained by a three-step synthetic route as shown in
Scheme 1. The first step was the attachment of 1-naphthyl
fragment to a 9H-carbazole moiety through N-arylation reaction
6
under the modified Ullmann coupling conditions. The iodination of
7
b)
1
according to the procedure reported by Tucker yielded 3,6-
Figure 2. Cyclic voltammogram of the solution of DPNC in
dichloromethane (a); electron photoemission spectrum of the
vacuum deposited layer of DPNC (b).
diiodo-9-(1-naphthyl)carbazole (2). The last step was N-arylation
reaction of di(4-methoxyphenyl)amine and compound 2 which
resulted in the formation of DPNC in 74% yield. The newly
2
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Photoelectrical properties.
EP
The ionization potential (I
p
) of the vacuum deposited layer of
DPNC was estimated from electron photoemission spectrum (Figure
EP
p
2
b). To find I
of DPNC, photocurrent versus photon energy (hv)
9
was plotted. The extrapolation of the linear part of plot to the zero
EP
EP
photocurrent gave I
p
of 4.94 eV. The I
p
value is related to the
HOMO value (-4.94 eV) of DPNC which is by only 0.24 eV lower than
1
0
F
the Fermi level (E ) of the ITO electrode (-4.7 eV). Therefore,
effective hole injection from the ITO anode in OLED can be provided
using the studied material as the hole injecting or transporting
layers.
(
a)
The experimental charge mobilities (μ) were calculated by the
2
11a,b
formula μ = d /Vttr
,
where d is the thickness of the layer, V is the
Holes
70V
60V
50V
applied voltage to the electrodes, and ttr is the TOF transit time
1
0
1
which was determined from the TOF photocurrent transients
10
4
3
0V
0V
(
Figure 3b,c). The hole and electron mobilities of DPNC show linear
1
/2
dependencies on the square root of the electric field (E ) with the
20V
10V
10
1
/2
good agreement to Poole–Frenkel relationship µ=µ
0
×exp(α×E
Figure 3a). This result was not unexpected since bipolar charge
transport was previously observed for naphthylamine-based
)
11b
(
-
10
1
2a
compounds and for carbazole derivatives having only carbazolyl
1
2b
d= 1.5 ꢀm
electroactive groups. The values of the zero electric field charge
mobility (µ ) and the field dependence parameter (α) are given in
-7
0
-6
-5
-4
1
10
10
10
0
Time, s
the inserts of Figure 3a. The field dependence of hole mobility for
(
b)
DPNC was found to be higher than that of the electron mobility.
1
/2
The dependency of µ versus E
for organic materials were
60V
0V
40V
0V
20V
0V
Electrons
1
10
5
described by the disorder formalism in which α is proportional to
2
2
C(
σ
ˆ − Σ ) where
C
is an empirical constant,
σ
ˆ
is an energetic
3
13
disorder parameter, and
Σ
is the degree of positional disorder.
0
10
1
The charge transport occurs by hopping through a manifold of
localized states which are distributed in energy due to this
formalism. Apparently, the energetic disorder parameters (σˆ and
-
1
10
Σ
) could be different for holes and electrons in DPNC which leads
to the differences in hole and electron mobility values as well as in
the hole and electron field dependences. In general, low values of
the field dependence parameter are related to non-dispersive
-2
d= 1.5 ꢀm
10
-
6
-5
10
1
0
Time, s
c)
14
charge transport of organic semiconductors. Distinctive plateau
and relative short tails in the TOF transient show that hole and
electron transports of DPNC are nondispersive.
(
Figure 3. Electric field dependences of hole and electron
mobilities for the layer of DPNC (a) and TOF current transients
for holes (b) and electrons (c) in DPNC.
Well balanced hole and electron transport was found for the
vacuum deposited layer of DPNC. The values of hole and electron
-4
2 -1 -1
mobilities well exceeded 10 cm V s in the range of electric fields
5
5
Table 1. Calculated wavelengths, oscillator strength and orbital
assignment of the singlet-singlet vertical electronic transitions
for the molecule of DPNC.
from 0.5×10 to 5×10 V/cm. Such values are acceptable for organic
15
semiconductors used in efficient OLEDs.
The experimentally observed ambipolar charge-transporting
Sta
te
λ,
λ
max
,
exp
λ ,
properties of DPNC were found to be in good agreement with the
f
Assignment
nm
459
417
414
387
329
316
312
nm
nm
electron (λ
B3LYP/6-31G(d). The values of λ
.25 eV respectively, i.e. they are comparable indicating the
ambipolar nature of conductivity of DPNC. Somewhat higher value
of λ corresponds to to the lower parameter of hole mobilities
relative to the electron mobilities (in the framework of Marcus
-
) and hole (λ
+
) reorganization energies calculated by
S
S
1
0.019
0.012
0.024
0.067
0.391
0.101
0.253
HOMO → LUMO (98%)
HOMO-1 → LUMO (98%)
HOMO → LUMO+1 (98%)
HOMO-1 → LUMO+1 (96%)
HOMO → LUMO+5 (82%)
HOMO-2 → LUMO (90%)
HOMO → LUMO+7 (52%)
HOMO-1 → LUMO+3 (29%)
HOMO → LUMO+7 (23%)
-
and λ were found to be 0.2 eV and
+
2
0
S3
393
377
S
S
4
8
+
S
S
10
1
6
12
312
296
theory). This is in a good congruence with the experimental data
which show that the hole mobility of DPNC is five times lower than
the electron mobility at zero electric field (Figure 3a).
S
13
310
0.255
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Photophysical properties
UV-Vis absorption and fluorescence spectra of the dilute solutions
of DPNC in THF are shown in Figure 4a. UV-Vis absorption spectrum
covers only the short wavelengths region up to 450 nm. The THF
solution of the compound showed blue fluorescence with the
emission maximum at ca. 490 nm. The photoluminescence
quantum yield (PLQY) of 26.4% was observed for THF solution and
of 4.6 % for the solid layer. The TD DFT calculations (Figure 4b,
Table 1) indicate that fluorescence of DPNC should not be very
strong since the first singlet-singlet electronic transition is weakly
allowed by the symmetry selection rules. In accordance with TD DFT
(a)
0 1
calculations, the first S -S electronic transition corresponds to the
one-electron excitation of HOMO-LUMO type (Table 1). The plots of
these frontier molecular orbitals are presented in Figure 5 together
with some other close-lying molecular orbitals. The HOMO orbital
corresponds to the π-type wave-function that is mainly localized on
the diphenylamine and carbazole fragments. In contrast, the LUMO
orbital represents the pure π-type wave-function of the
naphthalene moiety. Thus, the HOMO-LUMO electronic transition
corresponds to the charge-transfer nature that determine its weak
intensity (the transition dipole moment is very low).
The single visible absorption band at 377 nm in the experimental
spectrum of DPNC correspond to the superposition of three
(b)
Figure 4. UV-Vis absorption and fluorescence (λ = 310 nm)
spectra of the dilute THF solutions (10 M) of DPNC (a);
theoretical absorption spectrum of DPNC (b).
ex
-5
electronic singlet-singlet transitions
S
2
–S
4
with the resulting
absorption maximum at 393 nm. All of them demonstrate the π–π*
charge-transfer nature (Figure 6, Table 1) similarly to S
state.
1
electronic
it possible to estimate approximately the optical HOMO-LUMO
A
The short-wavelength absorption of DPNC THF solution can be
apparently assigned to the S8, S10, S12 and S13 excited states with
the small contribution of other close-lying transitions with the small
intensities.
λ
band gap (E
g
) accordingly to the relationship 1239.84/ onset, where
A
λ
onset corresponds to absorption onset (absorption edge). The
experimentally estimated E value was found to be of 2.81 eV which
is in a good agreement with the result of DFT calculations (E
g
calc
g
=
The analysis of absorption and fluorescence spectra of DPNC makes
3
.14 eV) accounting the solvent effect in the framework of PCM
HOMO-2
HOMO-1
HOMO
LUMO
LUMO+1
LUMO+2
Figure 5. Selected molecular orbitals of the DPNC molecule calculated by the B3LYP/6-31G(d) method (controlling value of isosurface is
.02 a.u).
0
4
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a)
(
a)
(
b)
b
Figure 6. Normalized electroluminescence spectra of the single-
layer OLED of the structure ITO/CuI/DPNC/Ca:Al at the
different applied voltage (a) and PL spectrum of the thin film of
DPNC (b).
Figure 7. Energy-band diagrams of the fabricated
devices: ITO/CuI/DPNC/Ca:Al and ITO/CuI/DPNC/Bphen(TPBi)
/
Ca:Al (a). CIE1976 chromaticity diagram of devices
ITO/CuI/DPNC/Ca:Al and Bilayer I at 11, 13, 15 V (b).
model.
energy barrier at the DPNC/Ca interface (Figure 7), that causes the
The photoluminescence intensity maximum of DPNC was observed
at ca. 490 nm. This peak can be assigned to the 0-1 vibronic satellite
20
misbalance between the injected charge carriers.
The characteristics of OLED are presented in Figure 8 and the data
are collected in Table 2. The obtained max. external quantum
efficiency of 0.18 % for the single-layer device is in good agreement
with maximum theoretical external quantum efficiency (ESI). The
efficiency can be increased using additional electron-transport layer
1 0
of the S -S electronic transition, while the 0-0 band is clearly visible
as the left-side shoulder at 445 nm. The same band-shape remains
in the EL spectra of the fabricated devices (0-0 band is less intensive
than the 0-1 vibronic band).
Characterization of the single-layer OLED.
2
,2′,2"-(1,3,5-benzenetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi)
As one can see from Figure 6 the EL spectra of the single-layer OLED (ESI). The relatively low brightness and current efficiency of the
of the structure ITO/CuI/ DPNC/Ca:Al are considerably different single-layer OLED can partially be explained by the electromer
from the PL spectrum of the dilute solution of DPNC in THF (Figure emission domination that naturally results in the decrease of OLED
2
1
4a) and of the thin film (Figure 6b).
efficiency. On the other hand, the presence of the electroplex-
In the EL spectrum of the single-layer OLED the emission bands of type excited states makes it possible to expand the EL spectrum
the neat DPNC remain at the same energies as those of the PL towards the red region and to obtain white OLED with the
3,20
spectrum (0-0 band at 450 nm, 0-1 band at 490 nm), but the new simplified structure.
The Commission Internationale de
broad emission band appears in the region of 550-650 nm. This l’Eclairage (CIE) chromaticity coordinates (x, y) of the single-layer
band is probably originated from the electric-field induced device correspond to the near white color (0.31; 0.36) at 13 V
1
7
electromer emission. The electromer emission takes place mainly (Figure 7b).
from the direct irradiative recombination of holes and electrons The electroplex emission band demonstrates the clear dependence
residing at two neighboring molecules, or from two molecules on the applied voltage that allows to fabricate the color-tunable
1
8
within some appropriate close distance. We have predicted that devices. Indeed, the simple device ITO/CuI/DPNC/Ca:Al was found
the intermolecular π–π interactions (between the diphenylamine to be capable of yielding daylight chromaticity with color-
and naphthalene moieties) would be expected to enhance the temperature ranging from 6000 K (at 15 V) to 7400 K (at 11 V),
electronic interaction within the molecules which can facilitate the compared with that of sunlight, i.e., 5500 K at noon or 8000 K at
1
9
22
formation of trapped level of hole-electron pair. The formation of noon in high-latitude country.
the electric-field-induced electromer emission can be conditioned
by the complicated electron injection into DPNC because of the Characterization of the bilayer OLEDs.
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(a)
Figure 9. Normalized EL spectra of the device
ITO/CuI/DPNC/Bphen/Ca:Al.
(b)
Figure 8. Current density-voltage, luminance-voltage (a) and
current efficiency-current density (b) characteristics for the
device ITO/CuI/DPNC/Ca:Al.
Figure 10. PL spectrum of spin coated films of the blends:
DPNC(50%):Bphen (50%) and DPNC(50%):TPBi (50%).
In order to improve the efficiency of OLED we added the additional
electron-transporting layer between the Ca:Al cathode and the
layer of DPNC. First of all, we expected improvement of charge
carriers balance in the DPNC layer. In addition, we hoped to
observe the exciplex emission from the DPNC/Bphen interface. It is
well known that one of the necessary conditions for the formation
of exciplexes is availability of high energy barriers for the charge
5
carriers in the organic-organic interface. From the number of
2
3,24
available acceptors
we have chosen Bphen because of the low
LUMO level (3.0 eV, Figure 7a). As a result, we have observed the
strong exciplex emission at 530 nm as the main emission band
(
Figure 9) generated by the electron cross-transition from the
Figure 11. PL decay curves of the films of the blends:
DPNC(50%):Bphen (50%) at 580 nm and DPNC(50%):TPBi (50%)
at 540 nm.
23
LUMO of Bphen to the HOMO of DPNC.
Appearance of exciplex-type emission can be explained by the
presence of the energy barriers for both electrons and holes at the
interface DPNC/Bphen. One energy barrier of 1.00 eV is for electron
injection from the LUMO level of Bphen into the LUMO level of are accumulated at the interface DPNC/Bphen due to the high
DPNC (Figure 7a). The second one of 1.4 eV is for injection of holes energy barriers. The formation of exciplex at the DPNC/BPhen
from HOMO level of DPNC into the HOMO level of Bphen (Figure interface is confirmed by appearance of the long-wavelength
7a). When the direct bias (+ to ITO) is applied, electrons and holes emission band at 580 nm in the photoluminescence spectrum of
the spin coated film of the blend: DPNC (50%):Bphen (50%) (Figure
Table 2. EL characteristics of the fabricated devices.
1
0). One can stress that because of the bulk interface character
Max
external
quantum
efficiency,
%
Max
brightness
at 15 V,
Max
current
efficiency,
cd/A
between the molecules of DPNC and Bphen, the exciplex band
observed in PL spectrum is predominant and red-shifted relative to
the EL spectrum of the device Bilayer I ITO/CuI/DPNC/Bphen/Ca:Al.
PLQY of 9.31 % was found for the solid layer of the DPNC:Bphen
blend. For the origin of the corroboration of the emission from the
DPNC/Bphen interface the fluorescence decay curve was recorded
for the mixture DPNC:Bphen at the emission maximum of 580 nm.
Max power
efficiency,
Lm/W
V
V
on
,
Device
2
cd/m
Single-
layer
2
.6
3600
0.83
0.54
0.18
Bilayer I
Bilayer II
4.0
2.3
27000
10.5
8.5
3.6
2.2
3.3
1.2
a
983
a
–
the value was taken at applied voltage of 6V.
6
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However, the bilayer device Bilayer I exhibited high current
2
efficiency of 10.5 cd/A and maximum brightness of 27000 cd/m (at
1
5V), and external quantum efficiency of ca 3.3 %.
Additionally, we have carried out the experiments with the purpose
of selection of more efficient electron-transport layers for the
improvement of technical characteristics of DPNC-based OLEDs. We
have fabricated OLED which contains electron-transporting
interlayer of TPBi. The structure of the device was as follows:
ITO/CuI/DPNC/TPBi/Ca/Al (device Bilayer II) (Figure 7a). Figure S1
shows that the EL spectrum of this device is similar to the PL
spectrum of the solid film of DPNC (Figure 6b). However, the red
shift of EL maximum by ca.10 nm and the slight broadening of the
EL spectrum in the red region is probably caused by some additional
intermolecular interactions that can occur at the DPNC/TPBi
interface. Indeed, the formation of exciplex between DPNC and
TPBi was also confirmed similarly to DPNC:Bphen exciplex by
appearance of the long-wavelength emission band at 540 nm in the
photoluminescence spectrum of the spin coated film of the blend:
DPNC (50%):TPBi (50%) (Figure 10). Fluorescence decay curve of the
mixture DPNC:TPBi at the emission maximum of 540 nm was well
fitted by double exponential function similarly to that of
DPNC:Bphen exciplex (Figure 11). Despite the enhancement of
electron injection process into the DPNC emissive layer by the
reduction of energy barrier for electron hopping between the DPNC
and TPBi adjacent layer in device Bilayer II comparing to device
Bilayer I, the maximum external quantum efficiency of device
Bilayer II were found to be worse than those observed for device
Bilayer I (Table 2, Figure S2). This observation can be explained by
the shorter life-time of DPNC:TPBi exciplex compared to that of
DPNC:Bphen exciplex which can lead to lower RISC efficiency of
DPNC:TPBi exciplex (Figure 11). We note that the smaller external
quantum efficiency of 1.2 % observed for device Bilayer II
corroborates the advantage of exciplex emission based
DPNC/BPhen device Bilayer I.
(
a)
(
b)
Figure 12. Current density-voltage, luminance-voltage (a) and
current efficiency-current density (b) characteristics for the
device ITO/CuI/DPNC/Bphen/Ca:Al.
(
Figure 11). Two components with the lifetimes of 6.4 ns and 42 ns
could be identified. The first component was apparently due to the
singlet exciplex relaxation and the second one apparently
originated from the reverse intersystem crossing (RISC) from T
1
to
S
1
state of exciplex, resulting in the effect of thermally activated
2
5,26
delayed fluorescence (TADF).
The chromaticity coordinates (x, y) of the device Bilayer I
ITO/CuI/DPNC/Bphen/Ca:Al depend on the applied bias (0.33, 0.48
at 11V; 0.32, 0.46 at 13V; 0.31, 0.45 at 15V; Figure 7b) due to the
voltage dependence of DPNC exitonic-type emission. This
phenomenon can be useful for the tuning of the OLED emission
colour only by the applied bias variation. We think that this is one of
the simplest ways to fabricate the color-tunable OLEDs as
compared with the multilayer devices which require much more
complex fabrication technologies.
At the same time, the exciton-type emission bands (0-0 at
4
55 nm and 0-1 at 490 nm) are observed as the left-side shoulders
on the main exciplex band in the EL spectrum (Figure 9).
Noteworthy, that upon the gradually increasing the bias voltages,
the intensity of 0-0 and 0-1 bands of the EL spectra clearly
increased. However, the emission profile remained essentially the
2
7
same as suggested by the Kasha's rule. The enhanced intensity is
caused by the increased population of the excited state at high bias
voltage. Due to the decay of population, only the photon emission
from the lowest vibrational level in the excited state can be
observed. Thus the observed spectra can be understood by the
Conclusions
27
Kasha's rule.
In the present work we report on the new approach for the
fabrication of colour-tunable organic light-emitting diodes being
characterized by a broad electroluminescence which consists of the
combination of pure exciton-type fluorescence from the ambipolar
emitter, its electroplex emission and exciplex emission (in the case
of bilayer device containing electron-transporting layer). For the
preparation of emissive layer, we used the newly synthesized
dimethoxydiphenylamino-substitted 9-naphthylcarbazole which
showed bipolar charge-transporting properties. The synthetic
protocol of the compound and extensive characterization of its
thermal, spectral and conductive properties are presented. The
Thus, we can conclude that the EL spectrum of the bilayer
device Bilayer I ITO/CuI/DPNC/Bphen/Ca:Al can be represented as
the superposition of the exciton-type emission bands of pure DPNC,
exciplex-type emission band of DPNC:BPhen interface and
electroplex long-wavelength emission band at ca 650 nm (Figure 9).
Figure 12 shows the current density-voltage characteristics and
luminance-voltage characteristics of the device Bilayer
I
ITO/CuI/DPNC/Bphen/Ca:Al. In contrast to the single-layer OLED,
the bilayer device Bilayer I demonstrated the relatively higher turn-
2
on voltage (Von) of 4 V for electroluminescence (at 1 cd/m ).
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3
3
fabricated organic light-emitting diodes demonstrate the voltage- CuI was used for the hole-transporting layer and Bphen was
25
dependent electroluminescence in a short range of applied bias applied as acceptor for exciplex forming layer. Calcium (Ca) layer
that allows to tune the device emission by the minor voltage topped with aluminum (Al) layer was used as the cathode. The
2
variation. The single-layer device showed color-temperature active area of the obtained device was 6 mm .
ranging from 6000 K (at 15 V) to 7400 K (at 11 V), compared with The current density-voltage and luminance-voltage dependences
that of sunlight, i.e., 5500 K at noon or 8000 K at noon in high- were recorded with a semiconductor parameter analyzer (HP
latitude country. The best fabricated bilayer device exhibited high 4145A) using it in the air without passivation immediately after
current efficiency values of 10.5 cd/A, maximum brightness of fabrication of the device. The measurement of brightness was
2
2
7000 cd/m (at 15V), and external quantum efficiency of ca. 3.3 %.
performed using a calibrated photodiode. The electroluminescence
spectra were recorded with an Ocean Optics USB2000
spectrometer.
Experimental
Photoluminescence (PL) spectra and PL decay curves were recorded
with the Edinburgh Instruments FLS980 spectrometer at room
temperature using a low repetition rate μF920H Xenon Flash lamp
as the excitation source.
Instrumentation
A TA Instruments Q2000 thermosystem was used for differential
scanning calorimetry (DSC) measurements under nitrogen
The PL measurements were performed for the layer prepared by
thermo vacuum deposition and for the layer of the molecular
mixtures DPNC:Bphen and DPNC:TPBi prepared by casting the THF
solution of the mixtures consisting of 50% of DPNC and 50% of
Bphen (or TPBi) onto clean quartz substrate.
ο
atmosphere. A heating/cooling rate of 10 C/min was choosen for
recording DSC tomograms of the sample. A TA Instruments Q50
devise was used for thermogravimetric analysis (TGA) under
ο
nitrogen atmosphere with the heating rate of 20 C/min.
A glassy carbon electrode in a three-electrode cell of an Autolab
type potentiostat-galvanostat was used for cyclic voltammetry (CV)
Materials
1
-Iodonaphthalene, di(4-methoxyphenyl)amine, 9H-carbazole, 18-
measurements was used for the estimation of the ionization
CV
crown-6 were used as received from Sigma-Aldrich. The solvents
were used as supplied or purified by the standard techniques.
Synthesis
potential (I
p
). The measurements were done for the solutions in
tetrabutylammonium
hexafluorophosphate at room temperature. The potentials were
found against Ag/AgNO
dry dichloromethane containing 0.1
M
3
as a reference electrode. A platinium 9-(1-Naphthyl)carbazole (1). 9H-carbazole (4 g, 24 mmol), 18-
counter electrode was used in the CV measurements. The crown-6 (0.6 g, 10 mol%) and 1-iodonaphthalene (12.2 g, 48 mmol)
potentials were compared to those of the ferrocene/ferrocenium were dissolved in o-dichlorobenzene (30 ml). The solution was
+
28
(Fc/Fc ) redox system.
heated under stirring. When 9H-carbazole completely dissolved,
2
9
The electron photoemission method was used for the estimation potassium carbonate (K CO ) (12 g, 87 mmol) and copper powder
of the ionization potential (I
2
3
EP
p
) of a vacuum deposited layer of (3.8 g, 60 mmol) were added. The mixture was stirred for 24 hours
DPNC. We recorded the electron photoemission spectra in the air under argon at 185 °C. After 24 hours, the complete consumption
using a deep UV deuterium light source ASBN-D130-CM, CM110 of 9H-carbazole and the formation of a single product spot was
1/8m monochromator and a 6517B Keithley electrometer. The observed by thin-layer chromatography (TLC). The mixture was then
details of electron photoemission experiment were described cooled down and filtered off washing with chloroform which was
3
0
before.
further removed by rotary evaporator under reduced pressure. The
Time of flight (TOF) technique was used for the estimation of product 1 was purified by column chromatography using the
charge-transporting properties of the layers of DPNC. The samples mixture of ethyl acetate and hexane (volume ratio 1 to 5
for TOF measurements were prepared by vacuum deposition of the respectively) as an eluent. The target compound was recrystallized
layer of DPNC on a pre-cleaned glass/indium tin oxide (ITO) from hexane. The procedure afforded 5.3 g (76% yield) of white
3
4
substrate and the aluminum top electrode. The thickness (d) of the needles, T = 124–125°C (lit. 123–124 ). M (C H N) = 293.36. MS
m
r
22 15
+
1
layer of DPNC was ca. 1.5 μm. In principle, the TOF experiment was (ESI ) m/z (%): 293.3 (100%), 294.3 (50%). H NMR (400 MHz, CDCl
such as described earlier. The setup included a pulsed Nd:YAG δ, ppm) 7.02 (d, 2H, J = 7.6 Hz), 7.28 – 7.40 (m, 6H), 7.55 (t, 1H, J =
laser (EKSPLA NL300, a wavelength of 355 nm, pulse duration 3-6 7.3 Hz), 7.60 – 7.74 (m, 2H), 8.03 (d, 2H, J = 8.3 Hz), 8.06 (d, 2H, J =
3
,
31
1
3
ns), a Keithley 6517B electrometer, and a Tektronix TDS 3052C 8.8 Hz), 8.23 (d, 2H, J = 7.0 Hz). C NMR (100 MHz, CDCl δ, ppm)
3
,
oscilloscope.
The OLEDs
60nm)/Ca(50nm)/Al(200nm)
ITO)/CuI(8nm)/DPNC(30nm)/Bphen(30nm)/Ca(50nm)/Al(200nm)
double-layer device (Bilayer I)) and ITO/ CuI(4nm)/ DPNC(60nm)/ 20 ml of glacial acetic acid (AcOH) at 120 °C. Potassium iodate (KIO3)
110.35, 119.89, 120.44, 123.34, 123.66, 126.06, 126.84, 126.89,
with
the
structures
ITO/CuI(8nm)/DPNC 127.09, 128.58, 129.14, 131.17, 134.17, 134.99, 142.31.
(
(
(
(single-layer
device), 3,6-Diiodo-9-(1-naphthyl)carbazole (2). Compound 1 (4 g, 13.65
mmol) and potassium iodide (KI) (3 g, 18.2 mmol) were dissolved in
TPBi(60nm)/ Ca(50nm)/ Al(240nm) (double-layer device (Bilayer II)) (3.9 g, 18.2 mmol) was divided into 4 equal parts (4.55 mmol each)
32
were fabricated as it was reported earlier by means of vacuum and added to the mixture within 30 min. The reaction mixture was
deposition of organic semiconductor layers and metal electrodes stirred for 2 hours at 120 °C. Then the hot mixture was filtered. The
−5
onto pre-cleaned ITO coated glass substrate under vacuum of 10
Torr.
target compound was crystalized from diethyl ether as white
crystals. Yield: 69% (5.13 g). T = 139–141 °C. M (C H NI ) =
m
r
22 13
2
+
1
5
45.15. MS (ESI ) m/z (%): 544.9 (30%). H NMR (400 MHz, CDCl
3
, δ,
8
| J. Name., 2012, 00, 1-3
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ARTICLE
ppm) 6.77 (d, 2H, J = 8.6 Hz), 7.14 (d, 1H, J = 8.5 Hz), 7.34 (t, 1H, J = Modeling of Molecular Materials”, SNIC 020/11-23. This
7.7 Hz), 7.53 – 7.58 (m, 1H), 7.58 (d, 2H, J = 1.7 Hz), 7.60 (d, 1H, J = research was also supported by the Ministry of Education and
1
=
1
1
3
.7 Hz), 7.66 (t, 1H, J = 7.7 Hz), 8.02 (d, 1H, J = 8.3 Hz), 8.07 (d, 1H, J Science of Ukraine (project number 0115U000637). B. F.
13
8.3 Hz), 8.47 (s, 2H). C NMR (100 MHz, CDCl3, δ, ppm) 112.54, Minaev acknowledges the fellowship of Chinese Academy of
23.05, 124.40, 126.02, 126.75, 127.10, 127.46, 128.76, 129.74, Sciences under the CAS President’s International Initiative for
30.65, 132.97, 134.97, 135.08, 141.41.
Visiting Scientists.
,6-Bis[N,N-di(4-methoxyphenyl)]amino-9-(1-naphthyl)carbazole
(
DPNC). DPNC was synthesized in the similar way as compound 1
using compound 2 (0.5 g, 0.9 mmol) and di(4-methoxyphenyl)amine
0.63 g, 2.75 mmol). The product (DPNC) was purified by column
chromatography using the mixture of chloroform and hexane
volume ratio 1 to 2 respectively) as an eluent and then precipitated
into methanol. After filtration and washing DPNC was obtained as a
greenish amorphous solid. Yield: 74% (0.5 g). M ) =
47.3. MS (ESI ) m/z (%): 747.28 (100%), 748.28 (55%). H NMR (400
MHz, DMSO, δ, ppm) 3.66 (s, 12H), 6.52 – 7.08 (m, 20H), 7.16 (d,
Notes and references
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Financial support of the People Programme (Marie Curie
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Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J.
L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R.
Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E.
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K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand,
3
1
0 | J. Name., 2012, 00, 1-3
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New Journal of Chemistry
Page 11 of 11
Journal Name
View Article Online
DOI: 10.1039/C6NJ02865A
ARTICLE
Graphical abstract
The single-layer device showed electroluminescence
containing blue monomer and electromer emission,
while the bilayer device exhibited electroluminescence
containing interface exciplex emission.
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J. Name., 2013, 00, 1-3 | 11
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