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Interfacial energetics of NaCl–organic composite layer at an OLED anode
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2012 J. Phys. D: Appl. Phys. 45 455304
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IOP PUBLISHING
JOURNAL OF PHYSICS D: APPLIED PHYSICS
J. Phys. D: Appl. Phys. 45 (2012) 455304 (7pp)
doi:10.1088/0022-3727/45/45/455304
Interfacial energetics of NaCl–organic
composite layer at an OLED anode
Jeongho Kim1,4,5 , Yeonjin Yi2 , Jeong Won Kim3,5 , Seok Hwan Noh1 and
Heon Kang4
1
2
3
4
LG Electronics, Seoul, 153-801, Republic of Korea
Department of Physics, Yonsei University, Seoul 120-749, Republic of Korea
Korea Research Institute of Standards and Science, Daejeon 305-340, Republic of Korea
Department of Chemistry, Seoul National University, Seoul 151-747, Republic of Korea
E-mail: [email protected] (J Kim) and [email protected] (J W Kim)
Received 7 August 2012, in final form 11 September 2012
Published 16 October 2012
Online at stacks.iop.org/JPhysD/45/455304
Abstract
Although low work function alkaline halides are widely used as a cathode interlayer for
organic light-emitting diodes (OLEDs), NaCl–organic composites are shown to be an efficient
anodic buffer. Here we suggest a mechanistic origin of the improved OLED performance upon
the use of a NaCl-containing organic buffer layer between an indium tin oxide (ITO) anode
and N ,N -bis(naphthalene-1-yl)-N ,N -bis(phenyl)benzidine (NPB), based on the studies with
ultraviolet photoelectron spectroscopy and atomic force microscopy. While a pure NaCl
interlayer has a high hole-injection barrier (1.40 eV), the NPB : NaCl composite layer exhibits
a substantially lower barrier (0.84 eV), which is comparable to the value at a bare ITO/NPB
interface. Furthermore, the wettability of the composite onto ITO is enhanced due to
significant adhesive interactions of NaCl with both ITO and NPB, leading to effective electrical
contacts. The two key factors, i.e. the plausible hole-injection barrier and better wettability of
the NPB : NaCl composite, contribute to the improved hole injection efficiency and lifetime.
(Some figures may appear in colour only in the online journal)
such as oxygen plasma and ultraviolet ozone (UVO) have been
widely used to enhance hole injection [3, 4]. These treatments
are effective in removing residual surface contaminants and
inducing oxygen-rich surface at ITO, resulting in an increase
in the work function via surface band bending of ITO or
interfacial dipole [5, 6]. However, the surface treatments are
not enough to obtain interfacial compatibility between the ITO
and organic hole transport layer (HTL) because the plasma
or UVO treatment makes the ITO surface more hydrophilic
[7, 8], possibly exhibiting adverse interfacial properties to
most organic HTL materials with high hydrophobicity [9].
Another or an additional important approach for modifying
the anodic interfacial region is to introduce metals or metal
oxides with high work function [10–15], organic materials such
as a star-burst amine [16] and fluorocarbon plasma polymers
[17–19], and even insulators [20–23] between the ITO and
organic HTL. Hole injection enhancement has variously been
ascribed to mechanisms such as energy level realignment
via interface dipoles or surface band bending [14, 24–26],
1. Introduction
Interface engineering between electrodes and organic layers
in organic light-emitting diodes (OLEDs) has been one of
the main technological approaches because charge injection
and device stability are strongly dependent on their interface
formation [1, 2]. This is why much effort has been made
to improve the interface properties and understand the
mechanism governing the charge injection and transport. In
general, to reduce charge injection barriers, the anodic side
consists of high work function materials and the cathodic
side of low work function materials. In addition, surface or
interface free energy is another important factor to be counted
in terms of interfacial compatibility between inorganic and
organic materials.
Various approaches have been tried to modify both the
work function and surface energy of indium tin oxide (ITO), the
most common transparent anode. Physical surface treatments
5
Authors to whom any correspondence should be addressed.
0022-3727/12/455304+07$33.00
1
© 2012 IOP Publishing Ltd
Printed in the UK & the USA
J. Phys. D: Appl. Phys. 45 (2012) 455304
J Kim et al
interfacial stability induced by mechanical adhesion [10, 21–
23], or tunnelling probability due to a reduced effective
barrier [20].
On the other hand, metal halides which are usually
applied at cathode/organic interfaces, have also been tried
for better anode/organic interface properties [27, 28]. The
application of metal halides on the anodic side can allow
simpler fabrication of highly efficient OLEDs by introducing
the same metal halide for both electron and hole injection
enhancement. Furthermore, metal halides are usually cheap
and easy to handle, compared with other organic or metal
oxide interlayer materials such as V2 O5 [14], MoO3 [15] and
buckminsterfullerene [29, 30]. LiF, the most common electron
injection material on the cathode surface, was demonstrated
to be effective in hole injection enhancement on hydrogen
plasma treated ITO anodes but not on the ones with more
practical treatments such as oxygen plasma or UVO, based on
the tunnelling model [31]. A composite form of metal halides
such as LiF and MgF2 with hole transport materials improves
the thermal stability of OLEDs [28, 32]. Furthermore, we
have recently shown that a NaCl-incorporated interlayer at the
ITO and HTL interface improves not only the anodic interface
stability but also the hole injection efficiency with implication
of multiple mechanistic origins for improved performance
[33]. However, the reasons for hole injection and device
lifespan enhancement are not clear yet despite the practical
advantages of metal halides as an anodic buffer.
To tackle the underlying mechanism of OLED performance enhancement using NaCl composites, the interfacial
electronic and thermodynamic properties are studied. Here, we
prepared ITO/NaCl-containing buffer/N ,N -bis(naphthalene1-yl)-N ,N -bis(phenyl)benzidine (NPB) interfaces, and measured ultraviolet photoelectron spectroscopy (UPS) and the
wettability of the thin films by morphological study with
atomic force microscopy (AFM). First, the hole injection barrier at the interface of the ITO and NPB : NaCl composite layer
was comparable to that at the ITO/NPB interface, showing that
NPB at the interface is kept both chemically and electronically
intact even in the presence of NaCl. Second, the wettability of
NPB in the composite onto ITO was enhanced by the significant
adhesive interactions of NaCl with both ITO and NPB, leading
to effective electrical contacts across the anode side. This was
supported by the calculation of interface free energies, work
of adhesions and spreading coefficients. These studies reveal
the importance of concomitant consideration of both electronic
charge injection barrier and morphological adhesion property
for a rational design of organic/inorganic interfaces in organic
electronic devices.
Figure 1. UPS spectra collected near the Fermi level and secondary
cutoff region during the step-by-step layer deposition of NPB on
ITO.
NPB and NaCl films were deposited onto the ITO substrate
by resistive heating in the film growth chamber. The
deposition rate was 0.01 nm s−1 for both NPB and NaCl
layers. Mixed films of NPB : NaCl (1 : 3 wt%) were prepared
by coevaporation of NaCl and NPB. UPS spectra acquisition
was performed in the analysis chamber using the unfiltered
He I (21.2 eV) radiation lines of a gas discharge lamp and a
hemispherical electron energy analyser (VG ESCALAB 220i
system). At each step of interface formation, the valence
band and the onset of photoemission were recorded. The
onset or secondary electron cutoff, which represented the
vacuum level, was measured with a bias of −10 V on the
sample to clear the vacuum level of the detector. Vacuum
level shifts and HOMO level changes induced by the NPB
or NPB : NaCl composite overlayers were thus measured.
Surface morphology measurements were performed using
AFM (dimension 3100).
3. Results and discussion
The evolutions of secondary electron cutoff and valence region
spectra for NPB/ITO with increasing coverage are shown
in figure 1. The shape of the UPS spectrum agrees well
with the previous report along the measured whole energy
range, showing the characteristic features of the highest
occupied molecular orbital (HOMO) corresponding to the
central biphenyl unit of NPB [3, 34]. All spectra were
plotted with respect to the Fermi level (EF ). The normalized
secondary cutoff edges of NPB are shown on the left panel of
figure 1. The vacuum levels of the samples were determined
by linear extrapolation of secondary electron cutoffs on the
high-binding-energy side of the UPS spectra (15–19 eV). The
2. Experiment
Both the growth of thin solid layers and the UPS measurements
were performed in a ultrahigh vacuum system (base pressure
of 10−9 Torr) composed of three interconnected chambers
equipped for surface pretreatment, film growth and analysis.
An ITO-coated glass substrate with a sheet resistance of
50 cm−2 was used and UVO treatment was employed
to clean the substrate in the surface pretreatment chamber.
2
J. Phys. D: Appl. Phys. 45 (2012) 455304
J Kim et al
vacuum level shifts abruptly towards a higher binding energy
for the first sub-monolayer deposition (nominal thickness of
0.2 nm) by about 0.2 eV, thus indicating the presence of an
interface dipole as previously reported [35, 36]. As the NPB
layer becomes thicker after the second step of deposition, the
vacuum levels shift gradually towards higher binding energies.
The final secondary cutoff edge is assigned to the 4 nm
thickness NPB spectrum. The right panel of figure 1 shows
HOMO onsets for NPB. The HOMO onset was determined
via a linear extrapolation of the leading edge (closer to the
Fermi level) in the spectrum and is found at 0.84 eV for NPB
(0.2 nm)/ITO below EF , followed by a gradual higher binding
energy shift with a further increase in NPB thickness.
To investigate the electronic effect of the NaCl-containing
interlayer between ITO and NPB, NaCl was deposited on
the ITO surface in the forms of both pure NaCl layers and
NPB : NaCl mixed layers. Figure 2(a) shows the secondary
electron cutoffs and HOMO regions of UPS spectra for the
samples containing a pure NaCl interlayer between NPB and
ITO. The secondary cutoff shifts abruptly towards higher
binding energies at low coverages under 0.5 nm thickness
by 0.26 eV, and then saturates at a thickness of 1.0 nm. As
soon as the NPB is deposited on the pre-deposited NaCl
surface, the secondary cutoff shifts abruptly again in the same
direction by about 0.3 eV, and then moves gradually towards
higher binding energies with further deposition of NPB. Thus,
interface dipoles are expected to be present on both sides of
the NaCl interlayer between NPB and ITO. In the right panel
of figure 2(a), the NPB HOMO features are clearly observed
on the NaCl surface, showing that the HOMO onset at the
low coverage of 0.2 nm thickness is 1.40 eV and then shifts
slowly towards higher binding energies with the increase in
NPB coverage. Features above the HOMO level or in the
energy gap are replica of the valence state due to the weak He I
β-line from the unfiltered He I discharge lamp. Apart from
these, no new state appears at the NPB/NaCl interface.
Unlike the pure NaCl interlayer, the mixed film type of
NPB : NaCl shows a relatively pronounced gradual shift of
the secondary cutoff towards higher binding energies during
the deposition of the composite by 0.59 eV rather than sudden
shift(s), as shown in the left part of figure 2(b). The HOMO
features of NPB in the NPB : NaCl mixed phase are clearly
observed even though the intensities are relatively weak at
lower coverages of the composite, shown in the right panel
of figure 2(b). The shapes and widths of the NPB HOMO
features in the mixed phase are found to be similar to those
of the HOMO features of pure NPB overlayer in figure 2(a),
implying that NPB in the mixed phase with NaCl is kept
chemically intact. Even in the composite layer, NPB and NaCl
do not induce any new UPS feature and maintain their original
chemical properties. Although the HOMO cutoff position
of NPB : NaCl with 0.2 nm thickness is hard to be assigned
because of the weak signal from NPB, we could extract the
position to be 0.84 eV by considering the lower binding energy
shift of the observed HOMO maximum by 0.16 eV from that
of NPB : NaCl with 0.5 nm thickness, as marked in the right
panel of figure 2(b).
Work functions or vacuum levels, and HOMO onsets
with different NPB coverages were extracted and summarized
Figure 2. UPS spectra collected near the Fermi level and secondary
cutoff region during the step-by-step layer deposition of (a) NaCl
and (b) NPB : NaCl, followed by NPB, on ITO.
by the schematic energy level diagrams for NPB/ITO,
NPB/NaCl/ITO, and NPB/NPB : NaCl/ITO interfaces, as
shown in figure 3. With respect to the Fermi level, the
change in the secondary cutoff corresponds directly to the work
function or vacuum level change. For all the samples, both
surface work functions and HOMO onsets shift downwards
with the increase in NPB or NPB : NaCl thickness. Thus,
the polarization effect on the inorganic substrate (ITO) could
be considered to be negligible. Otherwise, a difference in
the HOMO and vacuum level shifts from the interface to the
film surface would be observed by the substrate polarization
3
J. Phys. D: Appl. Phys. 45 (2012) 455304
J Kim et al
Figure 3. Energy level diagram of (a) NPB/ITO, (b) NPB/NaCl/ITO and (c) NPB/NPB : NaCl/ITO. Evac , EF , eD and Φh correspond to the
vacuum level, Fermi level, interface dipole and hole injection barrier, respectively.
effect upon photoionization [37]. Thus, the overall downward
shifts are attributed mainly to the space charge redistribution
[36, 38], compensating for the interface dipole formation. The
pure NaCl interlayer raises the hole injection barrier from
ITO to NPB by 0.56 eV compared with the direct contact
of ITO/NPB. The specific mechanisms related to the charge
injection efficiency might be extremely complicated, but it is
clear that the higher hole injection barrier contributes to the
higher turn-on voltage of OLEDs with a pure NaCl thin film
buffer in the previous report [27]. Meanwhile, the injection
barrier of NPB : NaCl on ITO is comparable to that of pure
NPB on ITO, implying that the NPB maintains its intrinsic
injection property even in the composite phase on the ITO
surface. Furthermore, the pure NaCl interlayer between ITO
and NPB shows two stepwise decreases in the vacuum level
by the interface dipoles formed on both sides of the interlayer,
whereas the NPB : NaCl composite layer shows a gradual
decrease throughout the mixed phase. The total vacuum level
change from ITO to the NPB : NaCl/NPB interface is 0.59 eV
in figure 3(c), which is similar to that of 0.60 eV from ITO
to the NaCl/NPB interface in figure 3(b). Thus, the amount
of vacuum-level lowering by NaCl observed in the pure NaCl
interlayer between NPB and ITO via interface dipoles is likely
expressed as a large band bending in the mixed phase of the
NPB : NaCl composite interlayer, even though the origin of
the large band bending is not yet clearly revealed. Due to
the large band bending in the mixed phase, the HOMO level
is relatively far from the Fermi level at a thickness of 4 nm
(1.28 eV) compared with that of pure NPB at the same nominal
thickness (1.09 eV).
Based on the above energy level alignment diagrams, the
schematic energy level changes for pure NPB and NPB : NaCl
composite layers on ITO under a bias condition are compared
in figures 4(a) and (b), respectively. When a forward bias
is applied, the NPB : NaCl layer creates a larger electric field
due to its large resistivity induced by the insulating property
of NaCl compared with pure NPB. In fact, the thickness and
composition of the composite interlayer could be optimized
for the best device performance. As the layer thickness goes
above several nm, the current density of the device rapidly
becomes worse [33]. Thus, a larger energy level gradient in
Figure 4. Schematic band diagram: (a) without and (b) with
NPB : NaCl buffer layer. Energy level changes with applied bias are
indicated. The shaded area denotes the approximate tunnelling
barrier under the bias voltage.
the mixed buffer layer reduces the tunnelling barrier, which
is approximately displayed by the shaded area in figure 4(b).
Compared with the pure NPB layer in figure 4(a), this reduced
effective tunnelling barrier may lead to an easier hole injection
into the HTL through the NPB : NaCl buffer layer. Therefore,
the hole injection enhancement could be partially attributed to
the tunnelling mechanism [1, 39, 40].
Since the wettability of the charge injection or transport
layer onto the electrode is another factor determining the
charge injection efficiency, the surface morphologies of the
pure NPB and NPB : NaCl composite (1 : 7 wt%) films were
compared by AFM images. In figures 5(a) and (b), the pure
NPB film forms large aggregates showing numerous dewetting
spots on the revealed ITO grain pattern. The root mean square
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J. Phys. D: Appl. Phys. 45 (2012) 455304
J Kim et al
Figure 5. AFM images (2 × 2 µm2 ) of 4 nm thickness of (a) NPB and (b) NPB : NaCl, and 8 nm thickness of (c) NPB and (d) NPB : NaCl.
The thin films were deposited onto the oxygen plasma pretreated ITO surface. The root mean square roughness values are 5.13 nm, 4.17 nm,
6.06 nm and 6.86 nm for (a), (b), (c) and (d), respectively.
(rms) roughness value is 5.13 nm at the nominal thickness
of 4 nm. However, the NPB : NaCl composite film does not
show NPB aggregates with a lower rms roughness of 4.17 nm,
implying that the mixed film is more wettable on the ITO
surface. With the increase in the film thickness, the surface
roughness of the composite film increases more rapidly and
surpasses that of pure NPB. The rms roughness values at 8 nm
thickness for pure NPB and NPB : NaCl composite films are
6.06 nm and 6.86 nm, respectively, as shown in figures 5(c)
and (d). Therefore, NaCl significantly affects the film growth
mode in the NaCl : NPB mixed film. The film wetting and
growth behaviour can be described by work of adhesion and
surface free energy at each interface. The interfacial free
energy (γ ), work of adhesion (W A ) and spreading coefficient
(S) were calculated, according to equations (1), (2) and (3),
respectively [41, 42]:
1/2
1/2
γA : B = (γA − γB )2
(1)
WAA: B = γA + γB − γA : B
(2)
SA : B = γB − γA − γA : B
(3)
Table 1. Calculated values of interfacial free energy (γA : B ), work of
adhesion (WAA: B ) and spreading coefficient (SA : B ) in mJ m−2 . Each
surface free energy of NPB, NaCl and ITO is referenced from the
literature, 31.1 mJ m−2 , 227 mJ m−2 and 70.1 mJ m−2 , respectively
[43–45].
A:B
γA : B
WAA: B
SA : B
NPB : ITO
NaCl : ITO
NPB : NaCl
7.82
44.9
90.1
93.4
252
168
31.2
−202
106
where γA : B is the interfacial free energy of the A/B interface,
γA is the surface free energy of A, WAA: B is the work
of adhesion between A and B, and SA : B is the spreading
coefficient of A on surface B. The surface free energies of
NPB, NaCl and ITO were referenced from the reported values,
31.1 mJ m−2 , 227 mJ m−2 and 70.1 mJ m−2 , respectively [43–
45]. Table 1 exhibits the summarized values of γA : B ,
WAA: B and SA : B where A : B is NPB : ITO, NaCl : ITO or
NPB : NaCl. Although the interface free energy of NPB : ITO
is less than that of NaCl : ITO, NaCl shows a higher work
5
J. Phys. D: Appl. Phys. 45 (2012) 455304
J Kim et al
of adhesion (252 mJ m−2 ) onto the ITO surface than that of
NPB (93.4 mJ m−2 ) due to the high surface free energy of
NaCl, but its spreading coefficient is less and even negative
(−202 mJ m−2 ). This means that NaCl can strongly adhere
to the ITO surface but is not so wettable on it—reminiscent
of Volmer–Weber (V–W) film growth mode in which threedimensional crystallites nucleate immediately upon contact
[46]. As indeed shown in the above AFM results, NaCl
in the NPB : NaCl mixed film makes the composite film
growth mode close to the V–W type, preventing the NPB
aggregation observed in pure NPB films. Furthermore, NPB
is expected to be a good adhesive to NaCl from the high
W A of NPB : NaCl (168 mJ m/2 ), as additionally confirmed by
AFM images showing well-intermixed uniform morphologies
in figures 5(b) and (d). NaCl, thus anchoring to the ITO surface
with its relatively high W A , seems to facilitate the contact
between ITO electrode and NPB by suppressing the cohesive
interactions between NPB molecules and simultaneously
inducing moderate adhesive interactions between NPB and
NaCl. Therefore, such a morphological effect caused by
NaCl incorporation into the NPB film contributes to the
previous report of hole injection enhancement by better
mechanical adhesion, leading to ‘effective’ electrical contacts
and prolonged lifetime.
References
[1] Parker I D 1994 J. Appl. Phys. 75 1656–66
[2] Malliaras G G and Scott J C 1998 J. Appl. Phys. 83 5399–403
[3] Ding X M, Hung L M, Cheng L F, Deng Z B, Hou X Y,
Lee C S and Lee S T 2000 Appl. Phys. Lett. 76 2704–6
[4] Sugiyama K, Ishii H, Ouchi Y and Seki K 2000 J. Appl. Phys.
87 295–8
[5] Milliron D J, Hill I G, Shen C, Kahn A and Schwartz J 2000
J. Appl. Phys. 87 572–6
[6] Kim S Y, Lee J-L, Kim K-B and Tak Y-H 2004 J. Appl. Phys.
95 2560–3
[7] Huang Z H, Zeng X T, Sun X Y, Kang E T, Fuh J Y H and
Lu L 2008 Org. Electron. 9 51–62
[8] Kim J S, Friend R H and Cacialli F 1999 J. Appl. Phys.
86 2774–8
[9] Cui J, Huang Q, Veinot J C G, Yan H, Wang Q,
Hutchison G R, Richter A G, Evmenenko G, Dutta P and
Marks T J 2002 Langmuir 18 9958–70
[10] Shen Y, Jacobs D B, Malliaras G G, Koley G, Spencer M G
and Ioannidis A 2001 Adv. Mater. 13 1234–8
[11] Chan I M and Hong F C 2004 Thin Solid Films
450 304–11
[12] Moon J-M, Bae J-H, Jeong J-A, Jeong S-W, Park N-J,
Kim H-K, Kang J-W, Kim J-J and Yi M-S 2007 Appl. Phys.
Lett. 90 163516
[13] Hu W, Manabe K, Furukawa T and Matsumura M 2002 Appl.
Phys. Lett. 80 2640–1
[14] Zhang H M and Choy W C H 2008 IEEE Trans. Electron
Devices 55 2517–20
[15] You H, Dai Y, Zhang Z and Ma D 2007 J. Appl. Phys.
101 026105
[16] Chen S-F and Wang C-W 2004 Appl. Phys. Lett.
85 765–7
[17] Tong S W, Lee C S, Lifshitz Y, Gao D Q and Lee S T 2004
Appl. Phys. Lett. 84 4032–4
[18] Hung L S, Zheng L R and Mason M G 2001 Appl. Phys. Lett.
78 673–5
[19] Hsiao C-C, Chang C-H, Lu H-H and Chen S-A 2007 Org.
Electron. 8 343–8
[20] Qiu C, Xie Z, Chen H, Wong M and Kwok H S 2003 J. Appl.
Phys. 93 3253–8
[21] Poon C O, Wong F L, Tong S W, Zhang R Q, Lee C S and
Lee S T 2003 Appl. Phys. Lett. 83 1038–40
[22] Deng Z B, Ding X M, Lee S T and Gambling W A 1999 Appl.
Phys. Lett. 74 2227–9
[23] Gyoutoku A, Hara S, Komatsu T, Shirinashihama M,
Iwanaga H and Sakanoue K 1997 Synth. Met. 91 73–5
[24] Kim S Y, Hong K and Lee J-L 2007 Appl. Phys. Lett.
90 183508
[25] Lee H, Cho S W, Han K, Jeon P E, Whang C-N, Jeong K,
Cho K and Yi Y 2008 Appl. Phys. Lett. 93 043308
[26] Tong S W, Lau K M, Sun H Y, Fung M K, Lee C S, Lifshitz Y
and Lee S T 2006 Appl. Surf. Sci. 252 3806–11
[27] Shi S W, Ma D G and Peng J B 2007 Eur. Phys. J. Appl. Phys.
40 141–4
[28] Tokito S and Taga Y 1995 Appl. Phys. Lett. 66 673–5
[29] Yuan Y Y, Han S, Grozea D and Lu Z H 2006 Appl. Phys. Lett.
88 093503
[30] Yuan Y, Grozea D and Lu Z H 2005 Appl. Phys. Lett.
86 143509
[31] Zhao J M, Zhang S T, Wang X J, Zhan Y Q, Wang X Z,
Zhong G Y, Wang Z J, Ding X M, Huang W and Hou X Y
2004 Appl. Phys. Lett. 84 2913–5
[32] Grozea D, Turak A, Yuan Y, Han S, Lu Z H and Kim W Y
2007 J. Appl. Phys. 101 033522
[33] Kim J, Kim M, Kim J W, Yi Y and Kang H 2010 J. Appl. Phys.
108 103703
4. Conclusions
We investigated the role of a NaCl thin film and a NPB : NaCl
composite layer as anodic buffer between an ITO anode and
a NPB transport layer using UPS and AFM measurements.
The hole injection barrier is increased with the insertion of
the pure NaCl layer compared with the interface without a
buffer layer. The NPB : NaCl composite, however, shows
a nearly identical injection barrier to that of the NPB/ITO
interface. Furthermore, NaCl in the NPB : NaCl mixed film
assists the mechanical adhesion and the resultant electrical
contact between ITO and NPB via better wettability of
the mixed film on the ITO surface. As a whole the
tunnelling through the highly resistive composite layer with
a comparable injection barrier height and enhanced electrical
contact are associated with the improved performance of the
OLEDs using a NPB : NaCl composite anodic buffer. To
achieve high-end OLEDs, interface control in terms of both
electronic charge injection barrier and mechanical adhesion is
necessary.
Acknowledgments
This work was supported by Brain Korea 21 (BK21) project
of the Ministry of Education, Science and Technology, by
research projects of the National Research Foundation of Korea
(Grants 2011-0026100 and 2012-0002303), and by the Korea
Research Council of Fundamental Science and Technology
(KRCF) through Basic Research Project managed by the Korea
Research Institute of Standards and Science (KRISS).
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J Kim et al
[41] Murakami T, Kuroda S-i and Osawa Z 1998 J. Colloid
Interface Sci. 202 37–44
[42] Adamson A W 1990 Physical Chemistry of Surfaces
(New York: Wiley-Interscience)
[43] Kim J K, Park J W, Yang H, Choi M, Choi J H and Suh K Y
2006 Nanotechnology 17 940
[44] Somorjai G A 1994 Introduction to Surface Chemistry and
Catalysis (New York: Wiley-Interscience)
[45] You Z Z and Dong J Y 2006 J. Colloid Interface Sci.
300 697–703
[46] Zangwill A 1988 Physics at Surfaces (Cambridge: Cambridge
University Press)
[34] Zhang R Q, Lee C S and Lee S T 2000 J. Chem. Phys.
112 8614–20
[35] He P, Wang S D, Wong W K, Lee C S and Lee S T 2001 Appl.
Phys. Lett. 79 1561–3
[36] Ishii H, Sugiyama K, Ito E and Seki K 1999 Adv. Mater.
11 605–25
[37] Gao W and Kahn A 2003 J. Appl. Phys. 94 359–66
[38] Yan L, Mason M G, Tang C W and Gao Y 2001 Appl. Surf.
Sci. 175–176 412–8
[39] Kim Y-E, Park H and Kim J-J 1996 Appl. Phys. Lett.
69 599–601
[40] Zhang S T et al 2004 Appl. Phys. Lett. 84 425-7
7