Electronic structure of Vanadium pentoxide: An efficient hole injector for
organic electronic materials
J. Meyer, K. Zilberberg, T. Riedl, and A. Kahn
Citation: J. Appl. Phys. 110, 033710 (2011); doi: 10.1063/1.3611392
View online: http://dx.doi.org/10.1063/1.3611392
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JOURNAL OF APPLIED PHYSICS 110, 033710 (2011)
Electronic structure of Vanadium pentoxide: An efficient hole injector for
organic electronic materials
J. Meyer,1,a) K. Zilberberg,2 T. Riedl,2 and A. Kahn1
1
Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544, USA
Institute of Electronic Devices, University of Wuppertal, 42119 Wuppertal, Germany
2
(Received 13 April 2011; accepted 16 June 2011; published online 4 August 2011)
The electronic structure of Vanadium pentoxide (V2O5), a transition metal oxide with an exceedingly
large work function of 7.0 eV, is studied via ultraviolet, inverse and x-ray photoemission
spectroscopy. Very deep lying electronic states with electron affinity and ionization energy (IE) of
6.7 eV and 9.5 eV, respectively, are found. Contamination due to air exposure changes the electronic
structure due to the partial reduction of vanadium to Vþ4 state. It is shown that V2O5 is a n-type
material that can be used for efficient hole-injection into materials with an IE larger than 6 eV, such
as 4,40 -Bis(N-carbazolyl)-1,10 -bipheny (CBP). The formation of an interface dipole and band bending
is found to lead to a very small energy barrier between the transport levels at the V2O5/CBP
C 2011 American Institute of Physics. [doi:10.1063/1.3611392]
interface. V
I. INTRODUCTION
Organic electronic devices such as organic light-emitting diodes (OLED) and organic photovoltaic cells (OPV)
are comprised of organic multilayers sandwiched between
bottom and top electrodes. These electrode/organic interfaces are crucial for high device performances, as interface
energy barriers can hamper charge injection or lead to losses
due to poor charge extraction. In order to minimize interfacial power losses on the anode side, various hole-injection
layers (HIL) have been introduced. One of the most widely
used HIL is polyethylene dioxythiophene:polystyrene sulfonate (PEDOT:PSS).1,2 Yet, the material suffers from chemical stability issues with organic over-layers, and its relatively
limited work function of 5.0–5.1 eV prevents facile chargeinjection into organic materials with large ionization energy
(>5.5 eV). A powerful alternative are transition metal oxides
(TMO), such as molybdenum or tungsten trioxide (MoO3
WO3), nickel oxide (NiO), or vanadium pentoxide (V2O5),
which can effectively replace PEDOT:PSS.3–7 Recently, it
was demonstrated that TMOs can be deposited from solution
and, with mild post-deposition processing, achieve properties
comparable to those of vacuum-grown thin TMO films and
even superior to those of PEDOT:PSS, making these materials also applicable for low-cost, large area applications.7–9
Detailed studies of the electronic structure of MoO3, NiO,
WO3 and of corresponding TMO/organic hole-injection
mechanisms have shown that MoO3 and WO3 have very similar energetics and are highly n-type materials, whereas NiO
is a p-type material.5,10–13 However, as of yet, the electronic
structure of V2O5 and the resulting charge injection mechanism into organic hole transport materials remain unclear.
In this work, we study the energetics and charge injection properties of vacuum-grown V2O5 layers via ultraviolet,
inverse and x-ray photoemission spectroscopy (UPS, IPES,
XPS) and current density-voltage (J-V) measurements on
a)
Electronic mail: [email protected].
0021-8979/2011/110(3)/033710/5/$30.00
simple hole-only device structures. The electron affinity
(EA), ionization energy (IE) and work function (WF) of the
TMO films are found equal to 6.7 eV, 9.5 eV and 7.0 eV,
respectively. The Fermi level (EF) is nearly pinned against
the conduction band minimum, consistent with the fact that
V2O5, like MoO3 and WO3, is highly n-doped. This is in contrast to previous reports, which suggested that V2O5 is of ptype nature, as proposed based on density-functional theory14
and later assumed by several other groups.15–17 In addition,
the electronic structure of V2O5 is often schematically drawn
like a p-type material with IE and EA values ranging
between 4.7–5.6 eV and 2.4 eV, respectively.18–21 We show
here that air exposure affects the electronic structure of
V2O5, and might be the reason for the inconsistent reports in
the literature.
The deposition of 4,40 -Bis(N-carbazolyl)-1,10 -bipheny
(CBP) on the TMO yields a very small injection barrier
between the V2O5 conduction band minimum and the highest
occupied molecular orbital (HOMO) of the organics. As in
the case of previously investigated TMO/organic interfaces,
we propose that hole injection occurs via electron extraction
from the CBP HOMO to the conduction band minimum of
V2O5. The characterization of hole-only devices confirms
V2O5 as an efficient hole-injector for organic materials with
large IE, such as CBP.
II. EXPERIMENTAL
Indium-tin oxide (ITO) was chosen as substrate for this
series of experiments. Indium-tin oxide was cleaned by sonication in acetone and methanol followed by exposure to UV
ozone for 30 min. V2O5 (Sigma-Aldrich) and CBP (SigmaAldrich) films were thermally evaporated in an ultrahigh
vacuum (UHV) growth chamber (109 Torr) and transferred
to an analysis chamber (<1010 Torr) without breaking vacuum. A temperature-controlled Knudsen cell was used to
evaporate V2O5 at a temperature of around 700 C at a rate
of 0.2 Å/s. CBP was evaporated from a simple quartz
110, 033710-1
C 2011 American Institute of Physics
V
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033710-2
Meyer et al.
crucible at a rate of about 0.3 Å/s. UPS and IPES were used
to study the electronic structure of V2O5 and the alignment
of the TMO energy levels with those of ITO and CBP. He I
(21.22 eV) and He II (40.8 eV) radiation lines from a discharge lamp were used in UPS, with an experimental resolution of 0.15 eV. IPES was done in the isochromat mode,
with a resolution of 0.45 eV.22 For composition and chemical
analysis, the O 1s and V 2p core levels of V2O5 films were
measured by XPS using the Al Ka (1486.6 eV) photon line
with a spectral resolution of 0.8 eV. PEDOT:PSS (Baytron P
VP CH 8000) was used as an alternative hole-injection layer
to be compared with V2O5. PEDOT:PSS was first filtered
(0.45 lm PEET), then spun at 3000 rpm for 80 sec and
finally baked at 180 C for 45 min. The J-V measurements
were done with a mercury-probe station in nitrogen, using a
semiconductor parameter analyzer (HP 4155 A).
III. RESULTS AND DISCUSSION
Figure 1 shows the UPS spectra of ITO with incrementally
deposited V2O5 and CBP (2 Å, 5 Å, 10 Å, 25 Å and 50 Å).
The photoemission onset of the initial ITO substrate is at 16.7
eV, giving a WF of 4.5 eV. The deposition of small amounts
(2 – 25 Å) of V2O5 leads to shifts of the photoemission onset
toward lower binding energy, indicating the formation of an
interface dipole consistent with an upward shift of the vacuum
level, and which totals about 2.5 eV. This corresponds to the
formation of a film with a WF equal to 7.0 eV. The V2O5
valence bandedge shifts only slightly toward lower binding
energy with increasing film thickness, and reaches a final position of 2.5 eV below EF. A careful look at the TMO spectrum
during the evolution of these states also reveals the formation
of gap states at about 1.5 eV below EF (position marked with
arrows). Interestingly, these states are observable only for very
thin layers, i.e., maximum thicknesses of about 10 Å, and disappear at higher coverage, indicating that they correspond to
an interaction limited to the interface with the ITO substrate.
Kanai et al.5 observed a similar effect at the ITO/MoO3 interface, and attributed it to a change in stoichiometry in the TMO
FIG. 1. (Color online) UPS spectra of ITO with incrementally deposited
V2O5 and CBP layer (2, 5, 10, 25, and 50 Å). (a) magnified section of the
photoemission onset. (b) evolution of the density of states near the Fermi
level including a blow up on the 2 Å and 5 Å spectra. Position of gap states
marked with arrows.
J. Appl. Phys. 110, 033710 (2011)
(MoO3 ! MoO2). The He I* parasitic photon line (h ¼ 23.09
eV), which is always present when running the He discharge
lamp in He I, can lead to an artifact reproduction of the TMO
VB edge at around 1.5 eV below EF. Therefore, we measured
the samples with the He II excitation energy, and demonstrate
that (i) a similar density of gap states is detected near the interface, and (ii) no gap states are observable away from the interface on a 50 Å thick V2O5 layer. Figure 2 shows the combined
UPS (He II)/IPES spectra of the V2O5 density of states near the
bandgap. The conduction bandedge is located 0.3 eV above
EF. V2O5 is therefore an n-type material, most likely doped by
oxygen vacancies. Yet the density of these vacancies is too
small for detection via X-ray photoemission spectroscopy, and
the V2p3/2 core level peak position is found at 516.8 eV (shown
in Fig. 5), which corresponds to stoichiometric V2O5.23 The
resulting bandgap of V2O5 is 2.8 eV, which is slightly larger
than the gap measured via optical absorption.24
Also shown in Fig. 1 are the UPS spectra of incrementally deposited CBP on V2O5. The photoemission onset shifts
abruptly toward higher binding energy upon deposition of
the first 2–5 Å of CBP, corresponding to the formation of an
interface dipole (1.0 eV) with downwards shift of the vacuum level from the high work function TMO film into the organic film. In addition, the CBP HOMO edge shifts with
increasing coverage by 0.5–0.6 eV toward higher binding
energy, indicating some “band bending” away from the interface. Ultimately, the CBP photoemission onset and the
HOMO edge saturate at 15.8 eV and 0.8 eV below EF,
respectively, corresponding to a WF and IE of the organic
films of 5.4 eV and 6.2 eV, respectively. The latter is in
excellent agreement with previously reported IE.12
Interface dipole and “band bending” stem from the
extremely high V2O5 work function. In order to establish
thermodynamic equilibrium across the interface, electrons
from CBP are initially transferred from the organics to the
TMO and establish the interface dipole. The charge transfer
results in an effective “p-doping” of the CBP close to the
interface. The carrier concentration in that region is considerably increased, with pinning of the Fermi level 0.2–0.3 eV
above the CBP HOMO edge. As the film thickness increases,
the molecular levels, and the HOMO, move away from EF to
FIG. 2. (Color online) UPS and IPES spectra of the density of states close
to the bandgap of V2O5. Positions of the VB edge, CB edge and vacuum
level are marked.
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Meyer et al.
J. Appl. Phys. 110, 033710 (2011)
FIG. 4. (Color online) UPS and IPES spectra of freshly evaporated V2O5
and air-exposed V2O5 (cV2O5) for 1, 10 and 60 min. (a) magnified section
of the photoemission onset. Middle: evolution of the occupied density of
states near the Fermi level including a blow up (x6) for air-exposed samples.
(b) evolution of the unoccupied density of states.
FIG. 3. Energy level diagram of the ITO/V2O5/CBP structure.
decrease the charge density in the tail of gap states,25 as has
already been shown for other organic/electrode interfaces.26
The energy diagram of the ITO/V2O5/CBP structure
deduced from the measurements reported above is shown in
Fig. 3. As seen with previously studied organic/TMO interfaces,5,10,27,28 the HOMO edge of the CBP lies very close
below the TMO CB edge. In an ITO/V2O5/CBP structure
under bias, holes are therefore injected into the organics by
electron transfer from the CBP HOMO level to the TMO
conduction band, and then to the ITO electrode. Because of
the small interface barrier at the V2O5/CBP interface, this
process is very efficient. Note that the electron transfer from
the CBP HOMO to the V2O5 CB is also known as charge
“generation” process, frequently used to interconnect stacked
OLED. Thus, the V2O5/CBP interface acts as injection and
charge generation layer at the same time.
We now turn to the difference between the energetics of
V2O5 reported here and those reported in the literature,
e.g., WF values of 4.7–5.3 eV.17,18,20,29 We observe that airexposure of freshly evaporated V2O5 films drastically reduces
the WF to similar values. To further address this issue, we performed a series of experiments whereby V2O5 films are contaminated (cV2O5) by air-exposure for 1, 10 and 60 min. The
UPS and IPES spectra in Fig. 4 show a significant evolution
upon air-exposure with respect to those of fleshly evaporated
V2O5. Short exposure times (1 min) already shift the photoemission onset by around 1.5 eV toward lower binding energies, corresponding to a decrease of the WF to 5.5 eV. For
longer exposure times, the WF is further decreased and
reaches a value of 5.2 eV after 60 min. In the valence band
spectrum, the O 2p peak at about 3 eV below EF decreases
due to air-exposure and an additional broadening of the neighboring peaks can be observed. Interestingly, the position of
the valence band with respect to EF does not change upon air-
exposure, however, a close inspection of the energy gap window reveal additional gap states around 0.4 eV below EF in
the cV2O5 film. These gap states become more distinctive
with increasing exposure time. Furthermore, the conduction
bandedge measured by IPES (Fig. 4) shifts away from the
Fermi level with increasing exposure time. An initial large
shift of 0.7 eV occurs as soon as the V2O5 sample is taken out
of the vacuum, and this shift saturates after 10 min exposure
at a total of 1 eV. Since the VB edge energy of V2O5 remains
unchanged upon contamination, air-exposure appears to lead
to a widening of the bandgap from 2.8 eV to 3.6 eV. A similar
effect was recently observed for air-exposed MoO3 clusters by
Gwinner et al.30 They speculated that absorbed water disrupts
the cluster size of MoO3, which consequently exhibits a larger
bandgap. This widening of the gap was not observed in our
previous investigations of MoO3 exposed to ambient for short
periods of time (1–3 min.),31 however, a recent re-
FIG. 5. (Color online) XPS spectra of freshly evaporated V2O5 and air
exposed V2O5 (cV2O5) for 1, 10 and 60 min including Shirley background
fits (dotted lines) and Lorentzian-Gaussian fits.
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033710-4
Meyer et al.
J. Appl. Phys. 110, 033710 (2011)
plified device structures, as its unusually large WF allows efficient charge injection into materials with IE significantly
larger than 6 eV.
IV. SUMMARY
FIG. 6. (Color online) Current density-voltage characteristics of hole-only
devices. Holes are injected from an ITO electrode covered with either
PEDOT:PSS or V2O5.
In summary, this work shows that V2O5 films grown
under vacuum exhibit very deep-lying electronic states and
are n-type, in a way very similar to MoO3 and WO3. In comparison, V2O5 exhibits the largest WF of the three TMOs,
which is highly favorable for efficient hole-injection into organic materials with large ionization energy. Air-exposure of
V2O5 changes the electronic structure due to the formation
of a reduced oxidation state. However, when, as demonstrated, processed under vacuum or nitrogen V2O5 can be a
very powerful hole-injection and extraction layer for organic
electronic applications.
ACKNOWLEDGMENTS
investigation of this issue done in our laboratory for longer exposure times confirm the observation made on the clusters.
Based on XPS analysis, we attribute the bandgap widening to
a change in chemical composition. As shown in Fig. 5, an
additional peak in the V 2p3/2 core level arises at lower binding energy with increasing air-exposure time. A LorentzianGaussian fit yields a peak position of 515.5 eV, which is close
to that reported for single crystalline VO2.32 The oxidation
states of Vanadium V2O5, VO2 and V2O3 are V5þ, V4þ and
V3þ, respectively. It is likely that air-exposure leads to adsorbates that partially reduce V5þ to V4þ. This would be consistent with the change observed in the electronic structure of
cV2O5 films and the formation of additional gap states, since
V4þ states are expected to exhibit states close to the Fermi
level. A widening of the bandgap has been also observed by
other authors when the V2O5 is intercalated with Na or Li.
The charge transfer from the alkali metal leads to a filling of
the split-off CB, which is present in the band structure of
V2O5 below the conduction bandedge. In these reports, the
observed bandgap changed from 2.3 eV to 3.1 eV for sputter
deposited V2O5 (Refs. 33 and 34). A similar finding has been
reported by Wu et al. for Na intercalation in thermally evaporated V2O5 (Ref. 35). It is therefore proposed that adsorbates
due to air exposure of V2O5 samples lead to a partial filling of
these gap states.
Finally, to demonstrate the efficiency of the charge
injection via V2O5 into a high ionization energy organic material like CBP, hole-only devices were built and tested.
PEDOT:PSS is used as hole-injection layer in a control device. The J-V characteristics in the log-log plot (Fig. 6) show
vastly superior hole-injection from V2O5. The large slope
clearly indicates a deviation from Child’s law most likely
due to a trap-charge limited current in CBP. To operate at a
current density of 0.1 mA/cm2, the PEDOT:PSS device
requires a driving voltage 15 V higher than the V2O5 device,
showing that the current is strongly injection-limited in the
former case. The work function of the conducting polymer is
indeed too small for efficient hole-injection in an organic
material like CBP. On the other hand, V2O5 allows for sim-
Work in Princeton was supported by the National Science
Foundation Grant No. DMR-0705920 (A.K), the Princeton
MRSEC of the NSF Grant No. DMR-0819860 (A.K), and as
part of the Center for Interface Science: Solar Electric Materials, an Energy Frontier Research Center funded the U. S.
Department of Energy, Office of Science, Office of Basic
Energy Sciences, under Award Number DE-SC0001084
(J.M). J.M. acknowledges the Deutsche Forschungsgemeinschaft (DFG) for generous support within the postdoctoral fellowship program.
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