APPLIED PHYSICS LETTERS 96, 193302 共2010兲
Charge generation layers comprising transition metal-oxide/organic
interfaces: Electronic structure and charge generation mechanism
J. Meyer,1,a兲 M. Kröger,2 S. Hamwi,3 F. Gnam,2 T. Riedl,4 W. Kowalsky,3 and A. Kahn1
1
Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544, USA
InnovationLab GmbH, Speyerer Straße 4, 69115 Heidelberg, Germany
3
Institute of High-Frequency Technology, Technical University of Braunschweig, 38106 Braunschweig,
Germany
4
Institute of Electronic Devices, University of Wuppertal, 42119 Wuppertal, Germany
2
共Received 15 February 2010; accepted 17 April 2010; published online 10 May 2010兲
The energetics of an archetype charge generation layer 共CGL兲 architecture comprising of
4 , 4⬘ , 4⬙-tris共N-carbazolyl兲triphenylamine 共TCTA兲, tungsten oxide 共WO3兲, and bathophenanthroline
共BPhen兲 n-doped with cesium carbonate 共Cs2CO3兲 are determined by ultraviolet and inverse
photoemission spectroscopy. We show that the charge generation process occurs at the interface
between the hole-transport material 共TCTA兲 and WO3 and not, as commonly assumed, at the
interface between WO3 and the n-doped electron-transport material 共BPhen: Cs2CO3兲. However, the
n-doped layer is also essential to the realization of an efficient CGL structure. The charge generation
mechanism occurs via electron transfer from the TCTA highest occupied molecular orbital level to
the transition metal-oxide conduction band. © 2010 American Institute of Physics.
关doi:10.1063/1.3427430兴
Charge generation layers 共CGL兲 are commonly used
as interconnecting unit in stacked organic light-emitting
devices 共OLED兲 or photovoltaic cells, and provide an attractive strategy for integrating multiple emission wavelengths,
tuning the emission color, and improving the device
performance.1–4 It was demonstrated that CGLs based on
transition metal-oxides 共TMO兲 such as molybdenum trioxide
共MoO3兲, vanadium oxide 共V2O5兲, and tungsten trioxide
共WO3兲 allow for the realization of very efficient stacked
devices.5–8 Qi et al.9 recently proposed a model to explain
the mechanism of CGLs containing TMOs, based on the assumption that MoO3 is a p-type material with a work function 共WF兲 of 5.7 eV. We recently investigated MoO3 films
grown in vacuum and reported WFs of 6.9 eV and 5.7 eV
before and after ambient exposure of the film, respectively.10
In all cases, however, the film was highly n-type doped due
to oxygen vacancies.
The present study, in parallel with another study based
on an investigation of angle-dependent emission, WO3 and
BPhen: Cs2CO3 thickness variations, and efficiency measurements in stacked OLEDs,11 demonstrates that the charge
generation process occurs at the hole-transport/TMO interface and not as previously proposed at the TMO/n-doped
electron-transport interface. This revised charge generation
model originates from the understanding that TMOs such
as MoO3 or WO3 offer very deep lying electronic states
and are highly n-doped by oxygen vacancies, so that the
Fermi level 共EF兲 is located very close to the conduction
band 共CB兲 of these materials.12–15 We focus here solely on
the energetics of the CGL, i.e., the interface alignment of
molecular levels at TMO/organic interfaces by ultraviolet
photoemission spectroscopy 共UPS兲, and inverse photoemission spectroscopy 共IPES兲, and provide a consistent model
of the CGL working mechanism. We use an archetype
CGL structure comprising a hole-transport layer 共HTL兲,
a兲
Electronic mail: [email protected].
0003-6951/2010/96共19兲/193302/3/$30.00
4 , 4⬘ , 4⬙-tris共N-carbazolyl兲triphenylamine 共TCTA兲, WO3,
and an electron transport layer 共ETL兲, bathophenanthroline
共BPhen兲, doped with cesium carbonate 共Cs2CO3兲 共the molecular structures are given in Fig. 2兲. We determine the energetics of both TCTA/ WO3 and WO3 / BPhen: Cs2CO3 interfaces. In the case of TCTA/ WO3, a large dipole leads to a
downwards shift of the vacuum level from WO3 to the organic film, and the Fermi level is pinned close to the TCTA
highest occupied molecular orbital 共HOMO兲 level. Electrons
can easily overcome the energy barrier from the TCTA
HOMO to the WO3 CB, a process which is at the origin of
the CGL process. We show that the generated electrons can
tunnel from WO3 to BPhen through an energy barrier reduced by a strong dipole and doping-induced band bending
at that interface.
All organic and inorganic layers were thermally evaporated in a UHV growth chamber 共10−9 Torr兲 on Au-coated
n+-doped Si wafers cleaned by sonication in acetone and
methanol. For the evaporation of WO3 and Cs2CO3, a
temperature-controlled Knudsen cell was used. Films of
BPhen doped with Cs2CO3 共9 wt %兲 were grown by
co-evaporation of the two constituents. After deposition
the samples were transferred to an analysis chamber
共⬍10−10 Torr兲 without breaking vacuum to study the electronic structure by UPS and IPES. In UPS, both He I 共21.22
eV兲 and He II 共40.8 eV兲 radiation lines from a discharge
lamp were employed, with an experimental resolution of
0.15 eV. IPES was carried out in the isochromat mode, with
a resolution of 0.45 eV.16 To prevent electron beam-induced
damage to the organic and inorganic films, the e-beam current and recording times were kept to minimum, and the data
were averaged from nine different spots on the samples surface. The Fermi level reference was established by UPS and
IPES measurements on a freshly evaporated Au surface.
Electron affinity 共EA兲 and ionization energy 共IE兲 of all films
were determined by a linear extrapolation of the HOMO and
lowest unoccupied molecular orbital 共LUMO兲 edges, respec-
96, 193302-1
© 2010 American Institute of Physics
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193302-2
Meyer et al.
Appl. Phys. Lett. 96, 193302 共2010兲
FIG. 1. UPS spectra of WO3 layer with incrementally deposited TCTA layer
共1, 2, 4, 8, 16, and 32 Å兲. Left: magnified section of the photoemission
onset. Right: evolution of the density of states near the Fermi level.
tively, to the background intensity line. Typical resolution on
the gap using this procedure is ⫾0.2 eV.16
The UPS spectra of 10 nm WO3 with incrementally deposited TCTA are shown in Fig. 1. The photoemission onset
for the secondary electrons is displayed in the left panel,
while the right panel shows the magnified spectra of the density of states near the oxide valence band edge and the TCTA
HOMO. Vertical lines mark the photoemission onset and
HOMO edge, respectively. The photoemission onset of a
neat WO3 layer is at 14.54 eV, which corresponds to a WF of
6.68 eV. The valence band maximum is 3.15 eV below EF,
and corresponds to an IE of 9.83 eV. From the IPES measurements shown in Fig. 2共a兲, the CB minimum is 0.23 eV
above EF, leading to an EA of 6.45 eV. These numbers are in
excellent agreement with those reported by Kröger et al.14 As
mentioned above, oxygen vacancies in TMOs like MoO3 and
WO3 dope these materials n-type.
The fractional coverage of 1 Å of TCTA on the WO3
surface shifts the photoemission onset by 0.4 eV toward
higher binding energy, representing an abrupt lowering of
the vacuum level and a dipole field at the interface. The
shift increases until completion of one to two monolayers
共⬃16 Å兲 of TCTA, reaching a maximum of 1.5 eV. At the
same time, the occupied valence state features of TCTA shift
also slightly toward higher binding energy and become
clearly defined above 8–16 Å. The tail-like feature at the top
of the occupied states, also clearly visible on the He II spectrum 共not shown here兲, is intrinsic to, and corresponds to the
HOMO of, the material. The 32 Å TCTA layer has an IE of
5.82 eV, consistent with previous reports.17,18 The interface
dipole places the TCTA HOMO level very close 共0.6 eV兲
below EF. The TCTA EA measured via IPES is 2.14 eV 关Fig.
2共b兲兴, leading to a band gap of 3.68 eV.
We turn next to the WO3 / BPhen: Cs2CO3 interface. UPS
measurements for the incrementally formed interface are
shown in Fig. 3, including the photoemission onset 共left兲 and
the full valence spectra 共right兲. The photoemission onset
shifts by 4.19 eV toward higher binding energy with up to 64
Å BPhen: Cs2CO3, under the combined effect of a large dipole and molecular level bending. The BPhen: Cs2CO3 density of states becomes clearly distinguishable from the superposed WO3 features only beyond a layer thickness of 32 Å,
due to an inhomogeneous surface coverage 共polycrystalline
FIG. 2. UPS and IPES spectra near the band gap of 共a兲 WO3, 共b兲 TCTA, 共c兲
BPhen: Cs2CO3, and 共d兲 BPhen. Positions of HOMO edge, LUMO edge,
and vacuum level are marked.
film growth兲 and a strong WO3 photoemission cross section.
The HOMO edge of the 264 Å BPhen: Cs2CO3 layer is 4.33
eV below EF, corresponding to an IE value of 6.82 eV. The
IE of undoped BPhen is measured at 6.53 eV 关Fig. 2共d兲兴, in
very good agreement with previous reports.19 Wu et al.20 also
observe a slight increase in IE in Cs2CO3-doped organic
films compared to undoped films. The EA values of doped
and undoped BPhen are 2.35 eV and 2.37 eV, respectively.
Finally, Figs. 2共c兲 and 2共d兲 show that the Fermi level shifts to
0.14 eV below the LUMO edge in BPhen: Cs2CO3, demon-
FIG. 3. UPS spectra of WO3 layer with incrementally deposited
BPhen: Cs2CO3 共2, 4, 8, 16, 32, 64, 128, and 264 Å兲. Left: magnified section
of the photoemission onset. Right: full valence spectrum.
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193302-3
Appl. Phys. Lett. 96, 193302 共2010兲
Meyer et al.
and the WO3 CB minimum, which allows for charge transfer
共known as generation兲 at the interface.
FIG. 4.
共Color online兲 Energy level alignment in the
TCTA/ WO3 / BPhen: Cs2CO3 CGL structure. Position of the CGL origin is
marked.
Work in Princeton was supported by the National Science Foundation 共Grant No. DMR-0705920兲, the Princeton
MRSEC of the NSF 共Grant No. DMR-0819860兲, and the
Office of Science DOE Energy Frontier Research Center for
Interface Science: Solar Electric Materials 共DE-S0001084兲.
Work in Braunschweig was financially supported by the German Federal Ministry for Education and Research 共FKZ:
13N8995, 13N9152兲. J. M. acknowledges the Deutsche
Forschungsgemeinschaft 共DFG兲 for generous support within
the postdoctoral fellowship program.
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strating that the film is indeed highly n-doped.
A summary of the energetics of the CGL structure is
given in Fig. 4. This schematic, which is also used to explain
the behavior of the CGL and the performance of a stacked
double OLED,11 indicates the position at which the charge
generation takes place. Note the small energy barrier between TCTA and WO3, which allows for the electron transfer
from the HOMO level of TCTA to the CB of WO3. However,
this charge generation process is not restricted to the type of
CGL structure described here, and takes place in all devices
where TMOs such as MoO3 and WO3 are used as holeinjection layer.14 Kröger et al.21 demonstrated that the
mechanism of doped organic/organic CGL heterostructures
can be explained by a field-induced tunneling process from
the HOMO state of a p-doped layer to the LUMO state of an
n-doped layer, in very much the same way as the charge
generation at the organic/TMO interface investigated here.
From the energy level alignment scheme illustrated in Fig. 4,
it is also apparent that an n-doped ETL is essential to build
an efficient CGL structure for device application. An undoped layer would present a large energy barrier for electrons between the oxide CB and the LUMO of the ETL, and
would require a high electric field for efficient electron injection into the subsequent device.11
In conclusion, we have shown that the charge generation
process in CGLs based on TMO occurs at the interface between the HTL and the TMO. Due to the formation of a large
interfacial dipole between TCTA and WO3, the energy level
alignment leads to a small barrier between the TCTA HOMO
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