White light emission from an exciplex based on a phosphine oxide type electron transport compound in a bilayer device structure

Article abstract of DOI:10.1039/c3ra43017k

A white organic light emitting diode (OLED) with a simple structure using exciplex emission is fabricated. The white emission of exciplex is formed at the interface between new electron transport material anthracene-9,10- diylbis(diphenylphosphine oxide) (DPPA) and hole transporting layer N,N-bis (naphthalen-1-yl)-N,N-bis(phenyl)benzidine (NPB). Pure white emission with Commission International de L'Eclariage (CIE) coordinates (0.33, 0.33) at the equal-energy white point is obtained. The phosphine oxide type material DPPA based OLEDs work well when Al is directly deposited on them, without an electron injection layer. In order to investigate the mechanism of electron injection from Al to DPPA, X-ray photoelectron spectroscopy (XPS) measurements have been carried out and revealed that the improved electron injection is due to the existence of interaction between the phosphine oxide group of DPPA and Al at the DPPA/Al interface. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra43017k

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White light emission from an exciplex based on a
phosphine oxide type electron transport compound in a
Cite this: DOI: 10.1039/c3ra43017k
bilayer device structure†
Yan Zhao, Lian Duan,* Xiao Zhang, Deqiang Zhang, Juan Qiao, Guifang Dong,
Liduo Wang and Yong Qiu*
A white organic light emitting diode (OLED) with a simple structure using exciplex emission is fabricated.
The white emission of exciplex is formed at the interface between new electron transport material
0
anthracene-9,10-diylbis(diphenylphosphine oxide) (DPPA) and hole transporting layer N,N -bis
0
(
naphthalen-1-yl)-N,N -bis(phenyl)benzidine (NPB). Pure white emission with Commission International
de L'Eclariage (CIE) coordinates (0.33, 0.33) at the equal-energy white point is obtained. The phosphine
oxide type material DPPA based OLEDs work well when Al is directly deposited on them, without an
electron injection layer. In order to investigate the mechanism of electron injection from Al to DPPA, X-
ray photoelectron spectroscopy (XPS) measurements have been carried out and revealed that the
improved electron injection is due to the existence of interaction between the phosphine oxide group
of DPPA and Al at the DPPA/Al interface.
Received 17th June 2013
Accepted 9th September 2013
DOI: 10.1039/c3ra43017k
www.rsc.org/advances
12,13
not in PL spectrum.
Exciplex and electroplex can be used to
Introduction
simplify the structure of WOLEDs.
Since the great development of organic light emitting diodes
In this letter, a white OLED using exciplex emission is
(
OLEDs) due to the advantages of wide viewing angle, high- demonstrated. Although the photoluminescent properties of
1–3
efficiency, self-emission, and low driving voltage, white OLEDs the electron transporting material anthracene-9,10-diylbis(di-
14
(WOLEDs) have achieved more attention for applications in solid phenylphosphine oxide) (DPPA) have been reported, its elec-
state light sources, at-panel displays and even exible displays. tronic and optoelectronic properties have not been investigated.
For efficient WOLEDs, multi-emission layers or stacked emission
layers are necessary to optimize the performance. As to the
structure of multi-emission layers or stacked emission layers
In this work, DPPA is a multi-functional material in OLED. It is
used not only as an emission material to emit blue light, but
also as a electron transport material with high electron mobility
4–7
device, their complicated fabrication and high cost process may in OLED. The white emissions of exciplex is formed at the
0
be a great challenge for business application. In order to mini- interface DPPA and the hole transporting layer N,N -bis (naph-
0
mize the structure of the WOLEDs, many solutions can be thalen-1-yl)-N,N -bis(phenyl)benzidine (NPB), and a fairly pure
8
9
introduced, such as a single-emitting-compound, exciplexes or white emission at the equal-energy white point with Commis-
electroplexes. For the exciplex, it is happened between different sion International de L'Eclariage (CIE) coordinates (0.33, 0.33)
10
molecules in the excited state of the donor to the acceptor in
the ground state, and from the emission of exciplex, it can be
observed both in electroluminescence (EL) and photo-
is obtained. We also develop a white OLED with the structure of
using the phosphine oxide type material DPPA layer on which Al
is following directly deposited, without an electron injection
11
luminescence (PL) spectrum. For the electroplex, it is formed layer, exhibiting the efficient electron injection. In order to
between the lowest unoccupied molecular orbital (LUMO) of investigate the mechanism of electron injection from Al to
electron transport material and the highest occupied molecular DPPA layer, X-ray photoelectron spectroscopy (XPS) is carried
orbital (HOMO) of hole transport material, while from the out. It is found that the existence of interaction between the
emission of electroplex, it can be observed only in EL spectrum phosphine oxide group of DPPA and Al at the DPPA/Al interface
is the main reason for the efficient electron injection.
Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of
Results and discussion
Synthesis and characterization
Education, Department of Chemistry, Tsinghua University, Beijing 100084, China.
E-mail: duanl@mail.tsinghua.edu.cn; qiuy@mail.tsinghua.edu.cn; Fax: +86 10
6
2795137; Tel: +86 10 62788802
The synthesis route and chemical structure of DPPA are shown
†
CCDC 958068. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3ra43017k
in Scheme 1, which has some modications with previous
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The molecular structure of DPPA was further determined by
single-crystal X-ray crystallographic analysis, as shown in Fig. 1.
ꢁ
The dihedral angles of benzene–naphthalene were 85 and 55 ,
respectively. Single crystals of DPPA were grown from recrys-
tallization of methylbenzene solution during heating. The room
temperature single-crystal X-ray experiments were performed
on a RIGAKU SATURN 724 + CCD diffractometer equipped with
a graphite monochromatized Mo Ka radiation. The structure
was solved by direct methods and rened with a full-matrix
Scheme 1 Synthetic route toward DPPA.
2
least-squares technique based on F with the SHELXL-97 crys-
1
4
tallographic soware package. Selected crystal data of DPPA:
reports. 9,10-Dibromoanthracene (2.0 g, 6 mmol) was added to
a dried 150 mL ask followed by adding 60 mL distilled THF to
form a suspension under nitrogen. The reaction mixture was
˚
˚
˚
space group P2 /n with a ¼ 7.87 A, b ¼ 12.00 A, c ¼ 14.80 A, a ¼
1
ꢁ
ꢁ
ꢁ
9
0 , b ¼ 94.7 , g ¼ 90 .
ꢁ
cooled to ꢀ78 C and then n-BuLi (24 mmol) was added drop-
wise over 2 h. The suspension was stirred at room temperature Thermal properties
2
for another 12 h aer adding PPh Cl (5.3 g, 24 mmol). 5 mL
methanol was added to reaction mixture to quench the reaction
Thermal gravimetric analysis (TGA) and differential scanning
calorimetry (DSC) were carried out to measure the thermal
stability. As shown in Fig. 2, the decomposition temperature
and ltrated to collect the yellow solid. 15 mL 30% H O solu-
2
2
tion was added to a suspension of the yellow precipitate in
methylene chloride (50 mL). The reaction mixture was stirred
for 1 h and the organic layer was wash by 30 mL water for 2
times. The crude product was puried by silica column with
methylene chloride–acetone (5 : 1). Yellow-green 9,10-bis(di-
phenylphosphoryl)anthracene (DPPA) was obtained with a yield
of 61%. The compound was puried using the silica column
method with further purication by sublimation, producing
ꢁ
(T
d
) of compound DPPA was 339 C under nitrogen at the
ꢁ
ꢀ1
heating rate of 10 C min . In the rst DSC scan, an endo-
thermic peak appeared at 265 C, corresponding to the melting
ꢁ
m
point (T ) of DPPA. Aer quenched by liquid nitrogen, the
second DSC scan was measured and the glass transition
ꢁ
temperature (T ) of DPPA was 108 C.
g
1
Photophysical properties
very pure powders. H NMR (600 MHz, CDCl -d ): 8.53 (s, 4H),
3
1
7
.78 (d, J ¼ 7.2 Hz, 4H), 7.74 (d, J ¼ 7.5 Hz, 4H), 7.58 (d, J ¼ 7.2 The UV-vis absorption and photoluminescence (PL) spectrum
Hz, 4H), 7.51 (d, J ¼ 5.4 Hz, 4H), 7.31 (s, 4H), 7.17 (s, 4H); of DPPA in methylene chloride were measured, as shown
elemental analysis calcd for C38
H
28
P
2
O
2
: C 78.88, H 4.88, O 5.53; in Fig. 3. The absorption edge of UV-vis spectrum is about
448 nm, indicating that the energy gap of DPPA is 2.8 eV. The
found: C 78.92, H 4.65, O 5.79%.
Fig. 1 The single crystal structure of DPPA.
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Fig. 4 The cyclic voltammogram for DPPA.
The cyclic voltammogram measurement
The cyclic voltammograms (CV) were measured in dichloro-
methane to study the energy levels of DPPA. The highest occu-
pied and lowest unoccupied molecular orbital (HOMO/LUMO)
energy levels can be calculated from the CV measurement data
together with the UV-vis absorption spectrum. From the CV
measurement shown in Fig. 4, the HOMO energy level of DPPA
is determined to be 5.9 eV, while the LUMO energy level is
g
calculated from the HOMO level and the E (2.8 eV, calculated
from the UV-vis absorption spectrum) to be 3.1 eV.
Quantum chemical calculations
ꢁ
ꢀ1
Fig. 2 TGA and DSC curves of DPPA recorded at a heating rate of 10 C min
.
Density functional theory (DFT) calculations on the ground
state of DPPA were carried out at the B3LYP/6-31G(d) level. The
structure was rst optimized from the geometry in single
crystal. All calculations were performed with Gaussian 09 so-
ware package, as shown in Fig. 5.
Electron mobility
We also measured the electron mobility of DPPA. The structure
of the device is ITO/DPPA (2 mm)/Mg:Ag (150 nm). A nitrogen
pulsed laser was used as the excitation light source, which is
directed from the ITO side to generate a thin sheet of excess
carriers near the ITO/organic interface. The transient photo-
current signals were recorded by a digital storage oscilloscope.
The transient current is shown in Fig. 6. The transit time was
extracted from the double-logarithmic plot of the transient
15
photocurrent. The charge mobility m could be calculated as:
Fig. 3 UV-vis absorption and photoluminescence spectrum of DPPA in methy-
2
m ¼ L /t
s
V.
lene chloride solution.
Where L is the lm thickness, t is the transit time, and V is the
s
PL emission peak of DPPA in the methylene chloride is applied electric eld. Fig. 6 shows the electron mobility of DPPA
observed at 462 nm. The quantum efficiency of DPPA thin at different electric elds. The negative eld dependence may be
16

lm is 5.5%.
attributed to the large positional disorder.
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Fig. 7 The structures for the white OLED devices Device A and Device B.
Electroluminescent properties
As shown in Fig. 7, two series of devices were fabricated to
obtain white OLEDs. These are Device A, with a structure of ITO/
NPB (50 nm)/DPPA (50 nm)/Mg:Ag (150 nm)/Ag (50 nm), and
Device B, with a structure of ITO/NPB (50 nm)/DPPA (50 nm)/Al
(
120 nm). The thickness of these materials was measured by the
scanning probe microscope system-atomic force microscope
AFM). The luminance-current characteristics of devices were
(
measured using a PR705 spectroscan spectrometer and the
brightness–voltage (B–V) characteristics were measured by a
Keithley 4200 semiconductor characterization system and cali-
brated silicon. PL spectra were recorded with a uoro-spectro-
photometer (Jobin Yvon, FluoroMax-3).
The EL spectrum of the above mentioned devices were
shown in Fig. 8, and the PL spectrum for the thin lms of DPPA,
NPB and DPPA:NPB (1 : 1) on quartz glass substrate. For Device
A, the device exhibited a white EL spectrum with peaks at 486
and 662 nm, and as shown in Fig. 8(b), a fairly pure white
emission with CIE coordinates (0.33, 0.33) at the equal-energy
white point was exhibited. The emission with peak at 486 nm in
the blue region was corresponding well to the PL spectrum of
DPPA with peak at 482 nm, apparently generated from the
singlet excited state of the DPPA molecule in the DPPA layer.
Because NPB is a hole transport material and also provides an
electron block effect, most excitons will be accumulated in the
DPPA layer near the interface with NPB. The emission in the
long wavelength region with a peak at 662 nm was observed in
EL spectrum, red shied comparing with the emission of the
exciplex peak at 620 nm in the PL spectrum of the DPPA:NPB
Fig. 5 Isocontour plots of HOMO and LUMO wavefunctions of DPPA obtained at
the: B3LYP/6-31G(D) level. (a) HOMO orbital of DPPA. (b) LOMO orbital of DPPA.
All the MO surfaces correspond to an isocontour value of |J| ¼ 0.025 a.u.
(1 : 1) thin lm. This phenomenon indicated that the white EL
spectrum of the devices seem be originated from the exciplex
formed at the interface DPPA and NPB.
The red-shi is attributed to the possible electroplex
+
ꢀ
(
donor /acceptor )* formed at the DPPA/NPB interface, as the
columbic interaction is stronger in the electroplex than in the
17–19
exciplex.
The emission of exciplex or would obey the rela-
tionship as follows:
hnmax ¼ ID ꢀ AA ꢀ Ec
2
E
c
¼ e /4p3
0 c
3r
where ID is the ionization potential of the donor, AA is the
electron affinity of the acceptor and E is the columbic attrac-
c
20,21
tion energy between the ions situated at a distance rc.
The ionization potential of the donor (NPB) is 5.4 eV, and the
electron affinity of the acceptor (DPPA) is 3.1 eV. As to hnmax ¼ I
Fig. 6 The electron mobility of DPPA at different electric fields under the
structure of ITO/DPPA (2 mm)/Mg:Ag (150 nm).
D
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Fig. 8 (a) PL spectra of DPPA, NPB and DPPA:NPB (1 : 1) thin films on quartz glass substrate, (b) EL spectra of white OLEDs Device A and Device B. Device A: ITO/NPB/
DPPA/Mg:Ag/Ag, Device B: ITO/NPB/DPPA/Al.
ꢀ
A
A
ꢀ E
c
¼ 5.4 eV ꢀ 3.1 eV ꢀ E
c
. For the 662 emission, hnmax is electric pulse was applied, the traps for components of NPB and
about 1.9 eV that related to E ¼ 0.4 eV. Then an 3 value is close DPPA were lled gradually; when the electric pulse was off, the
c
21,22
to 2.6–3 for organic lms,
and we get a distance between charges in the traps returned to the components of NPB and
ions r around 1.2–1.4 nm. Stable exciplexes can be formed at DPPA, and then at the interface between NPB and DPPA existed
c
23,24
distances below 0.4 nm,
.4 nm, suggesting that the extra red-shi of the emission interface. For the decay curve measured at 662 nm, a prompt
spectra is due to the electroplex rather than exciplex. and a delayed component were also observed. The decay curves
c
and it is lower than r around 1.2– charge-trapping, allowing the formations of exciplex at the
1
To reveal the origin of the peaks of the spectra, we measured observed here may be due to the exciplex formed at the interface
the EL decay curves of the Device A, as shown in Fig. 9. A prompt between NPB and DPPA.
and a delayed component from the decay curve of Device A were
As for the electron transport material DPPA, we also fabri-
clearly observed measured at 486 nm, as shown in Fig. 9. This cated the Device B shown in Fig. 7, with the structure of on the
can be attributed to the triplet–triplet annihilation (TTA) DPPA layer which Al was directly deposited without an elec-
delayed uorescence which has been found for the devices tron injection layer. Learned from the EL spectrum, a white
25
based on anthracene deviations. From an indepth research emission was also obtained. Device B exhibited the CIE coor-
about the EL decay curve at 486 nm, we found that there exhibit dinate at (0.35, 0.36). The performance of Device A and Device
2
6,27
spike at 15–20 ms, due to charge-trapping
existed at the B exhibited a litter lower current efficiency at 0.08 and 0.05 cd
ꢀ
1
28
interface between NPB and DPPA. As shown in Fig. 9, the spike
A
without optimization. As reported by Chihaya Adachi,
of the curve consisted of two parts: the curve dropped quickly the structures consisting of proper donor and acceptor
and then the curve rose and dropped slowly to form the tail. For molecules will absolutely improve the device performance
the rst part, it was most probably due to the fast uorescence from exciplex, so higher-efficiency white Device A and Device B
of DPPA which was originated from direct exciton formation on would be obtained aer optimization. Apparently, Device B
DPPA at the beginning of the electric excitation pulse, and this showed the lower turn-on voltage than Device A by compar-
procedure was so fast that the curve drop quickly. Then the ison shown in Fig. 10.
Fig. 9 Room temperature transient EL intensity for Device A (ITO/NPB/DPPA/
Mg:Ag/Ag) at different observed wavelengths (486 nm and 662 nm).
Fig. 10 The curve of brightness–voltage for Device A (ITO/NPB/DPPA/Mg:Ag/
Ag) and Device B (ITO/NPB/DPPA/Al).
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Fig. 11 (a) XPS Al2p core level spectra for the sample Si/NPB (50 nm)/Al (120 nm). (b) XPS Al2p core level spectra for the sample Si/DPPA (50 nm)/Al (120 nm).
Although the using of the electron injection layer of Mg:Ag,
For the Si/DPPA/Al sample shown in Fig. 11(b), with the
the OLED with high work-function Al cathode exhibited a lower sputter time increased, the spectrum of Al XPS was different
turn-on voltage. This phenomenon demonstrated that the from the sample Si/NPB/Al. The peak of Al2p was shied from
combination of DPPA and Al metal cathode indeed receives 73.0 eV to 75 eV during the sputter time 46.3 min to 66.3 min,
more efficient electron injection. In order to investigate the indicated that there exist the strong interaction of P–O–Al at
29,30
mechanism of efficient electron injection at the DPPA/Al inter- DPPA/Al interface.
The XPS data clearly indicated that the
face, X-ray photoelectron spectroscopy (XPS) measurement was existence of the interaction at DPPA/Al interface is mainly the
carried out to research the mechanism. 50 nm DPPA layer and reason for efficient electron injection of DPPA/Al device.
120 nm Al layer were prepared on Si substrate. For comparison,
the sample Si/NPB (50 nm)/Al (120 nm) was also fabricated. The
samples of Si/DPPA (50 nm)/Al (120 nm) and Si/NPB (50 nm)/Al
Conclusions
+
(120 nm) were investigated via Ar ion to etch the Al layer
gradually.
In summary, we successfully fabricated a white-OLED that the
The peaks of Al2p for the two samples with increased white light emission is from exciplex based on a phosphine
sputtering time are shown in Fig. 11. As shown in Fig. 11, it oxide type electron transport compound in a bilayer device
could be learned that the peaks of Al2p are shown at 73.0 eV structure with Mg:Ag cathode and a fairly pure white emission
and 75.8 eV from both samples' surface, respectively. This at the equal-energy white point with CIE coordinates (0.33, 0.33)
exhibition indicated that there exists metallic oxide of AlO
x
is obtained. Another white OLED with the structure without an
at initial sputter time from both samples, formed by ex- electron injection layer, using the phosphine oxide type mate-
posed in atmosphere. As to the sample Si/NPB/Al shown in rial DPPA layer on which Al was directly deposited, is also
Fig. 11(a), with the increased sputtering time, the Al2p demonstrated. Because of the existence of interaction between
peaks of AlO disappear gradually and the Al2p peaks of Al the phosphine oxide group of DPPA and Al at the DPPA/Al
x
only existed in the sample Si/NPB/Al subsequently. This interface, the DPPA/Al device with efficient electron injection
result indicated that there is no interaction at the NPB/Al shows the lowest turn-on voltage. These contributions can be
interface.
used to simply the structure of white OLEDs.
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component organic electroluminescent device, Adv. Mater.,
Acknowledgements
2004, 16(17), 1538–1541.
This research was supported by the National Natural Science 14 Z. F. Fei, N. Kocher, C. J. Mohrschladt, H. Ihmels and
Foundation of China (Grant no. 51173096, 21161160447 and
D. Stalke, Single crystals of the disubstituted anthracene
9,10-(Ph2P]S)(2)C14H8 selectively and reversibly detect
toluene by solid-state uorescence emission, Angew. Chem.,
Int. Ed., 2003, 42(7), 783–787.
50990060) and the National Key Basic Research and Develop-
ment Program of China (Grant no. 2009CB623604).
1
5 V. Coropceanu, J. Cornil, D. A. da Silva, Y. Olivier, R. Silbey
and J. L. Bredas, Charge transport in organic
semiconductors, Chem. Rev., 2007, 107(4), 926–952.
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