RIKEN KEIKI CO., LTD 1
Photo-electron spectrometer in airModel AC-2
The counting mechanism of the photoelectron
and
The application to studies of
The Organic Light Emitting Diode (OLED)
RIKEN KEIKI CO., LTD 2
5. Conclusion
4. The application to the OLED The measurements of the IPs and WFs of the Organic materials.
3. The data analysis What do the photoelectron spectrums mean?
2. The mechanism of counting photoelectrons How does the open counter detect the electrons in the air?
1. The outline of AC-2 applications, features
Contents
1. The outline of AC-2 applications, features
2. The mechanism of counting photoelectrons How does the open counter detect the electrons in the air?
3. The data analysis What do the photoelectron spectrums mean?
4. The application to the OLED The measurements of the IPs and WFs of the Organic materials.
5. Conclusion
RIKEN KEIKI CO., LTD 3
Contents
1. The outline of AC-2 applications, features
2. The mechanism of counting photoelectrons How does the open counter detect the electrons in the air?
3. The data analysis What do the photoelectron spectrums mean?
4. The application to the OLED The measurements of the IPs and WFs of the Organic materials.
5. Conclusion
RIKEN KEIKI CO., LTD 4
1. Outline ofPhoto-Electron Spectrometer in Air (PESA)
Light source part Measuring part Personal computer
More than 170sets are used in Japan and world market.
MODEL AC-2
120cm45cm
36cm
Mainly applications •Material research of the OLED .•The quality check of an ITO cleaning.•The surface research of an MgO film for the PDP.
Latest applications •Organic transistor•Organic solar battery•Catalyst of fuel cell
RIKEN KEIKI CO., LTD 5
The features of AC-2
•Measurements can be done in the air. Usually a photoelectron spectrometer needs a vacuum.
Because it is very difficult to detect and to count electrons in air.
•Easy operation & short measuring time.
•The work function and ionization potential can be
measured in the air in very high resolution.
•Measurement of contamination or film thickness on
a sample surface of 1mono layer-20nm.
RIKEN KEIKI CO., LTD 6
Contents
1. The outline of AC-2 applications, features
2. The mechanism of counting photoelectrons We employed the open counter for the detector of AC-2.
3. The data and the analysis What do the photoelectron spectrums mean?
4. The application to the OLED The measurements of the IPs and WFs of the Organic materials.
5. Conclusion
RIKEN KEIKI CO., LTD 7
UV Light
Sample
HV
Pulse counter
Vacuum chamber
A photoelectron is detected as an electric pulse.
Conventional methods for detecting electrons
1pA (10-12A )
6.24x106cps (62 million/s)
Photoelectrons are detected as quite small amount of
electric current.
Sample
UV light
Collector
e
e
Picoammeter Channeltron
e
A pulse of 10 million electrons
The electrons strike the channel walls and produce additional electrons.
This method is not sensitive enough for photoelectron spectroscopy.
This device works only in a vacuum.
RIKEN KEIKI CO., LTD 8
UV Light
Quenching GridSuppresser Grid
Anode Suppresser circuit
Quenching circuit
High Voltage Supply
Scaling circuit and Rate meter
Preamplifier
Sample holder
Sample
O2
e
The open counter
The open counter is employed the photoelectron spectrometer in air. The open counter is unique counter which can detect and count photoelectrons in the air.
e
Display
1 count
Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.
Open counter was invented in 1979 by Uda and Kirihata.
RIKEN KEIKI CO., LTD 9
Anode
Quenching GridSuppresser Grid
Structure
The open counter is composed of an anode, a quenching grid, suppresser grid and electric circuits.
Suppresser circuit
Quenching circuit
High Voltage Supply
Scaling circuit and Rate meter
Preamplifier
Display
0 count
Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.
14mm
80mm
RIKEN KEIKI CO., LTD 10
Quenching GridSuppresser Grid
Anode Suppresser circuit
Quenching circuit
High Voltage Supply
Scaling circuit and Rate meter
Preamplifier
Sample holder
Sample
The test sample is mounted on the sample stage which is kept at earth potential (0V).
Suppresser circuit
Quenching circuit
High Voltage Supply
Scaling circuit and Rate meter
Preamplifier
Display
0 count
Structure Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.
15mm
RIKEN KEIKI CO., LTD 11
Quenching GridSuppresser Grid
Anode Suppresser circuit
Quenching circuit
High Voltage Supply
Scaling circuit and Rate meter
Preamplifier
Sample holder
Sample
+80V
+100V
+2900V
Suppresser Grid is kept at +80V, Quenching Grid is +100V, Anode is +2900V.
Display
0 count
Structure Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.
RIKEN KEIKI CO., LTD 12
UV Light
Quenching GridSuppresser Grid
Anode Suppresser circuit
Quenching circuit
High Voltage Supply
Scaling circuit and Rate meter
Preamplifier
Sample holder
Sample
+80V
+100V
+2900V
Measurement
Monochromatized UV photons are used to excite photoelectrons from the sample surface.
Display
0 count
Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.
RIKEN KEIKI CO., LTD 13
UV Light
Quenching GridSuppresser Grid
Anode Suppresser circuit
Quenching circuit
High Voltage Supply
Scaling circuit and Rate meter
Preamplifier
Sample holder
Sample
+80V
+100V
+2900V
Electrone
If the energy of an UV photon(=h) becomes bigger than a work function of a sample, a photoelectron is emitted from the sample surface.
Display
0 count
Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.
RIKEN KEIKI CO., LTD 14
UV Light
Quenching GridSuppresser Grid
Anode Suppresser circuit
Quenching circuit
High Voltage Supply
Scaling circuit and Rate meter
Preamplifier
Sample holder
Sample
+80V
+100V
+2900V
Electrone
The electron is accelerated by a weak electric field applied between the sample (0V) and the suppresser grid kept at +80V.
Display
0 count
Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.
RIKEN KEIKI CO., LTD 15
Sample surface
O2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
O2e
N2
N2N2
O2
N2
N2
O2-ion
The electron becomes attached to an oxygen molecule to form O2-
ion in the air during drift to the counter.
Form the O2- ion
Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.
RIKEN KEIKI CO., LTD 16
UV Light
Quenching GridSuppresser Grid
Anode Suppresser circuit
Quenching circuit
High Voltage Supply
Scaling circuit and Rate meter
Preamplifier
Sample holder
Sample
+80V
+100V
+2900V
O2
e
When an O2- ion reaches the inner cylinder of the open counter, the ion is accelerated again by a
strong electric field applied between the quenching grid ( +100V) and the anode kept at +2900V.
Display
0 count
Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.
RIKEN KEIKI CO., LTD 17
Anode surface
O2
O2
O2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2N2
N2
N2
N2
N2
O2
N2N2 N2
O2
Electric Field
N2
N2
N2N2 O2
e
O2-ion
O2
e
The electron avalanche
When the O2- ion arrives near the anode, the electron is detached from the O2
- ion and then the electron is accelerated again to the anode.
Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.
RIKEN KEIKI CO., LTD 18
Anode surface
O2
O2
O2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2N2
N2
N2
N2
N2
O2
N2N2 N2
O2
Electric Field
N2
N2
N2N2 O2
e
O2-ion
O2
e+
ee+
+
++
+
++
++
+
ee e e ee e e e ee e
The electron avalanche
This causes an electron avalanche, which produces many electrons and positive ions around the anode wire, where only the electrons are collected on the anode.
Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.
RIKEN KEIKI CO., LTD 19
UV Light
Quenching GridSuppresser Grid
Anode Suppresser circuit
Quenching circuit
High Voltage Supply
Scaling circuit and Rate meter
Preamplifier
Sample holder
Sample
+80V
+100V
+2900V
The electron avalanche makes a small electric pulse on anode. This pulse is detected and counted as one electron.
Display
0 countDisplay
1 count
Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.
RIKEN KEIKI CO., LTD 20
Quenching Grid
UV Light
Suppresser Grid
Anode Suppresser circuit
Quenching circuit
High Voltage Supply
Scaling circuit and Rate meter
Preamplifier
Sample holder
Sample
+2900V
Quenching Grid
+400V
When the quenching circuit detects the electric pulse, +400V is applied in place of +100V to the quenching grid.
Display
1 count
Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.
RIKEN KEIKI CO., LTD 21
Quenching Grid
UV Light
Suppresser Grid
Anode Suppresser circuit
Quenching circuit
High Voltage Supply
Scaling circuit and Rate meter
Preamplifier
Sample holder
Sample
+2900V
Quenching Grid
+400V
This reduction of the electric field around the anode causes the electron avalanche to be quenched.
Display
1 count
Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.
RIKEN KEIKI CO., LTD 22
Suppresser Grid UV Light
Quenching Grid
Anode Suppresser circuit
Quenching circuit
High Voltage Supply
Scaling circuit and Rate meter
Preamplifier
Sample holder
Sample
Suppresser Grid
-30V
+400V
+2900V
On the other hand, -30V is applied in place of +80V to the suppressor grid.
Display
1 count
Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.
RIKEN KEIKI CO., LTD 23
Suppresser Grid UV Light
Quenching Grid
Anode Suppresser circuit
Quenching circuit
High Voltage Supply
Scaling circuit and Rate meter
Preamplifier
Sample holder
Sample
Suppresser Grid
-30V
+400V
+2900V
Ion
This voltage switch prevents leaving of the positive ions to the sample surface, and entering of the next O2
- ions into the counter during quenching.
Display
1 count
Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.
RIKEN KEIKI CO., LTD 24
Suppresser Grid UV Light
Quenching Grid
Anode Suppresser circuit
Quenching circuit
High Voltage Supply
Scaling circuit and Rate meter
Preamplifier
Sample holder
Sample
Suppresser Grid
-30V
+400V
+2900V
3ms
Such voltages applied to the quenching grid and suppressor grid are kept for 3msec i.e. the quenching time.
Display
1 count
Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.
RIKEN KEIKI CO., LTD 25
Suppresser Grid UV Light
Quenching Grid
Anode Suppresser circuit
Quenching circuit
High Voltage Supply
Scaling circuit and Rate meter
Preamplifier
Sample holder
Sample
Suppresser Grid
-30V
+400V
+2900V
3ms
All positive ions produced around the anode wire arrive and neutralize at the quenching grid or suppressor grid during quenching time.
Display
1 count
Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.
RIKEN KEIKI CO., LTD 26
Suppresser Grid UV Light
Quenching Grid
Anode Suppresser circuit
Quenching circuit
High Voltage Supply
Scaling circuit and Rate meter
Preamplifier
Sample holder
Sample
Suppresser Grid
-30V
+400V
+2900V
3ms
If the next electron has emitted from the sample surface, the suppresser grid prevents entering of the electron during quenching time.
Display
1 count
Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.
RIKEN KEIKI CO., LTD 27
UV Light
Quenching GridSuppresser Grid
Anode Suppresser circuit
Quenching circuit
High Voltage Supply
Scaling circuit and Rate meter
Preamplifier
Sample holder
Sample
+80V
+100V
+2900V
After the quenching time (3ms), the initial voltages (+100V and +80V, respectively) are restored and the quenching procedure is over.
Display
1 count
Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.
RIKEN KEIKI CO., LTD 28
UV Light
Quenching GridSuppresser Grid
Anode Suppresser circuit
Quenching circuit
High Voltage Supply
Scaling circuit and Rate meter
Preamplifier
Sample holder
Sample
O2
e
Now the counter is ready to count the next electron.
e
Display
1 countDisplay
2 count
Measurement Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.
RIKEN KEIKI CO., LTD 29
The calculation of counts
Electron counts may be lost during the quenching time. The counting rates of electrons are estimated based on the counting rates of counter pulses by calculation.
NO
1-tNO
N =
NO: Counting rate of counter pulses
N : Counting rate of photo electronst :Quenching time
Reference: H.Kirihata, and M.Uda; Rev. Sci. Instrum. 52 (1981) 68.
RIKEN KEIKI CO., LTD 30
Measuring part
open counter
Light source part
grating monochromator
deuterium lamp
e
: UV light : photoelectrone
sample
sample stage
Structure and functions of AC-2
open counter controller
personal computer
optical fiber
deuterium lamp
grating monochromator
open counter
3.4eV 6.2eV photodiode
RIKEN KEIKI CO., LTD 31
Measuring part
open counter
Light source part
grating monochromator
deuterium lamp
: UV light : photoelectrone
sample stage
Structure and functions of AC-2
open counter controller
personal computer
optical fiber
deuterium lamp
grating monochromator
open counter
3.4eV 6.2eV photodiode
RIKEN KEIKI CO., LTD 32
Contents
1. The outline of AC-2 applications, features
2. The mechanism of counting photoelectrons How does the open counter detect the electrons in the air?
3. The data and the analysis What do the photoelectron spectrums mean?
4. The application to the OLED The measurements of the IPs and WFs of the Organic materials.
5. Conclusion
RIKEN KEIKI CO., LTD 33
Yield : counting rate after calibtation
counting rate of photoelectrons (E) / intensity of UV-ray (E) x intensity of UV-ray (5.9eV)
(Yie
ld[c
ps]
)1/2
Incident Photon Energy [eV]
Photoelectron Spectrum
Photoemission Threshold Energy[eV]
Metal : the relationship between the photon energy and yield1/2 looks like a linear line. Semiconductor : yield1/3 gives a linear line.
RIKEN KEIKI CO., LTD 34
The relationship between the threshold energy and the energy diagrams
Conduction Band Valence Band Fermi Level
The energy level diagrams of metals, semiconductors and general materials
Energy
Vacuum level
General materialMetal Semiconductor
Lowest unoccupied molecular orbital (LUMO) Highest occupied molecular orbital (HOMO)
RIKEN KEIKI CO., LTD 35
The relationship between the threshold energy and the energy diagrams
Conduction Band Valence Band Fermi Level
Energy
Vacuum level
General materialMetal Semiconductor
Lowest unoccupied molecular orbital (LUMO) Highest occupied molecular orbital (HOMO)
A work function is an energy difference between a vacuum level and a Fermi level. On the other hand, the ionization potential is an energy difference between a vacuum level and highest occupied molecular orbital.
Ionization potential
Ionization potential
Ionization potential
Work function Work function Work function
RIKEN KEIKI CO., LTD 36
The relationship between the threshold energy and the energy diagrams
Conduction Band Valence Band Fermi Level
Energy
Vacuum level
General materialMetal Semiconductor
Lowest unoccupied molecular orbital (LUMO) Highest occupied molecular orbital (HOMO)
A UV photon excites an electron from the occupied states to the higher energy states than the vacuum level. And this electron can be emitted from the sample surface. The electron is called the photoelectron.
Photoelectrone eeUV photon
RIKEN KEIKI CO., LTD 37
The relationship between the threshold energy and the energy diagrams
Conduction Band Valence Band Fermi Level
Energy
Vacuum level
General materialMetal Semiconductor
Lowest unoccupied molecular orbital (LUMO) Highest occupied molecular orbital (HOMO)
Therefore, the photoemission threshold energy is the ionization potential.
Ionization potential
Ionization potential
Ionization potential
PhotoelectronUV photon e ee
RIKEN KEIKI CO., LTD 38
The relationship between the threshold energy and the energy diagrams
Conduction Band Valence Band Fermi Level
Energy
Vacuum level
Ionization potential
General materialMetal Semiconductor
Ionization potential
Lowest unoccupied molecular orbital (LUMO) Highest occupied molecular orbital (HOMO)
The metal is special material. Because, the energy of highest occupied molecular orbital and the Fermi level are same. Therefore the photoemission threshold energy of a metal is the work function.
Ionization potential
Metal
Work function
PhotoelectronUV photon e ee
We can estimate the work functions or ionization potentials of the materials from the photoemission threshold energy.
RIKEN KEIKI CO., LTD 39
The shape of a photoelectron spectrum
Potential energy
DOS
Reference: M.Uda, Y.Nakagawa, T.Yamamoto, M.Kawasaki, A.Nakamura, T.Saito, and K.Hirose ”Successive change in work function of Al exposed to air”, J. Electron. Spectrosc. and Related Phenom. 88 (1998) 767.
Energy level diagramE
nergy
Vacuum level
unoccupied molecular orbital occupied molecular orbital
RIKEN KEIKI CO., LTD 40
Potential energy
Energy level diagram
DOS
the relationship between an energy level diagram and the photoelectron spectrum
Reference: M.Uda, Y.Nakagawa, T.Yamamoto, M.Kawasaki, A.Nakamura, T.Saito, and K.Hirose ”Successive change in work function of Al exposed to air”, J. Electron. Spectrosc. and Related Phenom. 88 (1998) 767.
Photoemission yield (Y)
Photoelectron spectrum
Photon energy
0
dYdE=
The DOS is estimated by the differential of the photoelectron spectrum with respect to the photon energy E.
RIKEN KEIKI CO., LTD 41
WFf> WFs
The difference of a photoelectron spectrum caused by the thickness of a surface film
Contaminationor
Oxide film(0-20nm)
Substrate
Incident photon (E)
WFf
WFs
-
--
-
--
--
The cross section of the sample surface covered with a thin film
WFf> E > WFs
RIKEN KEIKI CO., LTD 42
WFf> WFs
The difference of a photoelectron spectrum caused by the thickness of a surface film
Contaminationor
Oxide film(0-20nm)
Substrate
Incident photon (E)
WFf
WFs
-
--
-
--
--
When photoelectrons pass through a surface film, some electrons collide with a molecule forming the surface film, and the photoelectrons are scattered.So some of photoelectrons can not escape from the sample surface.
WFf> E > WFs
RIKEN KEIKI CO., LTD 43
WFf> WFs
The difference of a photoelectron spectrum caused by the thickness of a surface film
Contaminationor
Oxide film(0-20nm)
Substrate
Incident photon (E)
WFf
WFs
-
--
-
--
--
So, when the surface film is thick, many photoelectrons can not be emitted from the sample surface.
WFf> E > WFs
RIKEN KEIKI CO., LTD 44
The relationship between the slope and film thickness
Large slope
Thin contamination film-
--
-
--
- -
(Yie
ld[c
ps]
)n
Incident Photon Energy [eV]
-
--
-
--
- -Thick contamination film
(Yie
ld[c
ps]
)n
Small slope
WFf WFs
WFf
WFs
WFf
WFs
- -
-
--
-
--
- -
RIKEN KEIKI CO., LTD 45
Contents
1. The outline of AC-2 applications, features
2. The mechanism of counting photoelectrons How does the open counter detect the electrons in the air?
3. The data and the analysis What do the photoelectron spectrums mean?
4. The application to the OLED The measurements of the IPs and WFs of the Organic materials.
5. Conclusion
RIKEN KEIKI CO., LTD 46
Glass SubstrateITO transparence anode
Organic layers
Metal cathode
The applications to the OLEDs
+
-
OLED device
Photon
WF
Ionization Potential
HOMO
ITO
Vacuum Level
Organic
LUMO
Fermi Level
Energy diagram of ITO and Organic Layer
HOMO
BarrierFermi Level
Hole
+
RIKEN KEIKI CO., LTD 47
Work functions of several metals
In Air 1)[eV] In UHV2) [eV]Fe 4.35 4.50Ni 4.25 5.15Cu 4.45 4.65Al 3.60 4.20Zn 3.80 -Au 4.78 5.101) M. Uda ; Jpn. J.Appl.Phys. 24,284 (1985)
2) D.E. Eastman ; Phys.Rev. B2, 1 (1970)
RIKEN KEIKI CO., LTD 48
Work functions or Ionization potentials of EL materials
AC-2 [eV] UPS 1) [eV]
Alq3 5.84 5.8
-NPD 5.50 5.4
CuPc 4.99 5.2
1) I.G.Hill and A.Kahn, J.Appl.Phys. 86,4515 (1999)
RIKEN KEIKI CO., LTD 49
4.7
4.9
5.1
5.3
5.5
0 10 20 30 40 50 60duration time after treatment (min)
Wor
k f
un
ctio
n (
eV)
before cleaningUV-ozone 10min
UV-ozone 20min
UV-ozone 60min
UV-ozone 0min (boiling in IPA )
The temporal change in the work function of ITO treated with UV-ozone
Reference: Y. Nakajima, T. Wakimoto, T. Tuji, T. Watanabe, M.Uda, The 10th International Workshop on Inorganic and Organic Electroluminescence (2000.12.4), Hamamatu, Japan.
RIKEN KEIKI CO., LTD 50
Change in WF of Al by Cl2 Contamination
Al in air : 4.05eV
Al exposed mixed gas 20sec
(Cl21.34ppm + air) : 4.33eV
(Yie
ld[c
ps]
)0.5
Incident Photon Energy [eV]
RIKEN KEIKI CO., LTD 51
5. Conclusion
1. We can detect and count photoelectrons in the air by the open counter.
2. We can measure the work function or the ionization potential of the OLED materials easily.
3. We can estimate the amount of the contamination on the ITO surface from 1mono-layer to 20nm in the thick.
4. AC-2 is the de facto standard equipment of the work function measurement on the OLED development.
RIKEN KEIKI CO., LTD 52
References
[1] H.Kirihata, and M.Uda “Externally quenched air counter for low-energy electron emission measurements”, Rev. Sci. Instrum. 52 (1981) 68.
[2] M.Uda “Open counter for low energy electron detection”, Jpn. J.Appl. Phys. 24 (1985) 284.
[3] T. Noguch, S. Nagashima and M. Uda “An electron counting mechanism for the open counter operated in air” , Nucl. Instr. Meth. A342 (1994) 521.
[4] S. Nagashima, T. Tsunekawa, N, Shiroguchi, H. Zenba, M. Uda “Double cylindrical open counter of pocket size”, Nucl. Instr. Meth. A373 (1996) 148.
[5] A. Koyama, M. Kawai, H. Zenba, Y. Nakajima, A. Yoneda and M. Uda “Electron counting by a double cylindrical open counter in mixtures of N2 and inert gases of various concentrations” , Nucl. Instr. and Meth. in Phys. Res. A422 (1999) 309.
RIKEN KEIKI CO., LTD 53
References
[6] M.Uda, Y.Nakagawa, T.Yamamoto, M.Kawasaki, A.Nakamura, T.Saito, and K.Hirose ”Successive change in work function of Al exposed to air”, J. Electron. Spectrosc. and Related Phenom. 88 (1998) 767.
[7] Y. Nakajima, M. Hoshino, D. Yamashita and M. Uda “ Near Edge Structures of Tetraphenylporphyrins Measured by PESA and Calculated with DV-Xα” , Adv. Quantum Chem. 42 (2003) 399.
[8] Y. Nakajima, T. Wakimoto, T. Tuji, T. Watanabe, M.Uda “Measurements of the work function of ITO Using an electron spectroscopy in air and a contact potential difference method” ,The 10th International Workshop on Inorganic and Organic Electroluminescence (2000.12.4), Hamamatu, Japan.