Changhee Lee, SNU, Kore
1/14
EE 430.859 2014. 1st Semester
2014. 4. 15.
Changhee Lee
School of Electrical and Computer Engineering Seoul National Univ.
Metal/Organic Semiconductor Contacts
Changhee Lee, SNU, Kore
2/14
EE 430.859 2014. 1stSemester
Workfunction & IP Measurement: Photoelectric effect
Energy level alignment at interface: Molecular orientation, reaction with metal, distortion of electronic distribution, existence of electric dipoles, etc.
Changhee Lee, SNU, Kore
3/14
EE 430.859 2014. 1stSemester
Energy levels of Organic Materials
Weiying Gao and Antoine Kahn (Princeton Univ.), NSF workshop, "Technological Challenges for Flexible, Light-weight, Low-cost and Scalable Organic Electronics and Photonics," January 16-17, 2003
Ionization Potential (eV)
H. Ishii, K. Sugiyama, E. Ito, and K. Seki, Adv. Mater. 11, 605 (1999).
Changhee Lee, SNU, Kore
4/14
EE 430.859 2014. 1st Semester
Neutral contact, Schottky contact
neutral contacts : (1) we assumed that the semiconductor is not doped and thus contains no
charge carriers of its own, and (2) we assumed that the contact interfaces consist only of the pure materials, i.e. that they are ideally clean and the states at the surface are the same as those in the bulk.
Schottky contacts: the depletion zones are formed near the contacts, for example, in doped
semiconductors, and for one of the two polarities of the applied voltage they are inhibiting
contacts (Schottky barriers) that prevent the flow of current.
Changhee Lee, SNU, Kore
5/14
EE 430.859 2014. 1st Semester
Contacts
tor semiconduc the
of potential Ionization
tor semiconduc the
of affinity electron
metal, of
on workfuncti
c c
m h
c m
c m
e
, I A
Φ Φ
A , Φ
A Φ
Φ
Changhee Lee, SNU, Kore
6/14
EE 430.859 2014. 1st Semester
field electric
in built
potential Contact
) 1 (
1 2
d , F V
, Φ
e Φ e
V Φ
BI BI
m m
BI BI
Contact potential and built-in electric field
Changhee Lee, SNU, Kore
7/14
EE 430.859 2014. 1st Semester
Schottky barrier
Schottky barrier
Fermi level is equalized throughout the M/S/M structure by the diffusion of the carriers: Diffusion of holes from the molecular material into the metals leaves the negative
ionized acceptor dopants at the interface (band bending):
This diffusion will continue until the internal energy
barrier (eVD: diffusion potential) is large enough to stop it.
Depletion region
2 /
]
1) / (
[ 2
a D r
o
d
qN
q kT
w V
2 /
)
11 ( 2
V V
N e
A C
d a
r o
2 /
]
1) (
[ 2
a d r o
eN
V
w V
S. Karg, W. Riess, V. Dyakonov, M. Schwoerer, Synth. Met., 54, 427 (1993).
Under reverse bias the measured capacitance corresponds to the junction capacitance
m . w
V cm
N
d a
1 0
V, 1 voltage
diffusion or
in voltage -
Built
, 1017 3
Al/PPV/ITO Schottky
diode
Changhee Lee, SNU, Kore
8/14
EE 430.859 2014. 1st Semester
Incident Light
Reflected Light
Lock-in Amp.
a.c. Field
Electroabsorption Measurement
2 )
3
(
( )
Im )
( )
(
field E
an to response orption
Electroabs
E h T h
h T
) cos( t E
E
E
dc
ac
Measurement of an internal electric field
Metal
Metal
d
Metal
Metal
d
DC bias No DC Bias
Internal Field = (1-2)/d
DC bias = (1-2)/d Internal Field = 0
Phase (deg)
Bias (V)
0 2
-2 -1 1
180
90
0
T
T
Bias (V)
0 2
-2 -1 1
Same work function
Changhee Lee, SNU, Kore
9/14
EE 430.859 2014. 1st Semester
Measurement of an internal electric field
Ian H. Campbell, John P. Ferraris, Thomas W. Hagler, Michael D. Joswick, Ian D. Parker, Darryl L. Smith Polymers for Advanced Technologies, 8 (7), pp. 417 – 423
Al/MEH-PPV/Al
Changhee Lee, SNU, Kore
10/14
EE 430.859 2014. 1st Semester
MULTILAYER ORGANIC LED
Internal electric field and accumulated charges
At the largest forward bias voltage measured,
electron density at the PQ/PVK interface: 2x1012 electrons/cm2 hole density at the PBD/PQ interface: 3x1011 holes/cm2.
) 4 (
1
E
dc
I.H. Campbell, M.D. Joswick, I.D. Parker, Appl. Phys. Lett. 67 (1995) 3171.
Changhee Lee, SNU, Kore
11/14
EE 430.859 2014. 1st Semester
Fermi level alignment at the metal/semiconductor contact
m:
금속의
work functions :
반도체의
work function:electron affinity
Barrier height for e injection
Bn= m –
built-in potential eVbi= | m – s |
Formation of interface dipole inducing vacuum level shift ()
Barrier height is modified:
Bn= m –
Vbi is also modified:
eVbi= | m – s |
. H. Ishii, N. Hayashi, E. Ito, Y. Washizu, K. Sugi, Y.
Kimura, M. Niwano, Y. Ouchi, and K. Seki, phys. stat.
sol. (a) 201, 1075 (2004)
Changhee Lee, SNU, Kore
12/14
EE 430.859 2014. 1st Semester
I. H. Campbell, S. Rubin, T. A. Zawodzinski, J. D. Kress, R. L. Martin, D. L. Smith, N. N. Barashkov, and J. P. Ferraris, Phys. Rev. B 54, R14321 (1996).
i
N p
Vacuum level
Metal
No interface dipole
Metal
- + - + - + - + - + - + - +
mE
Fm h
IP
m
EA
e
E
FΔ<0
e mEA
p
Interface dipole
Improved e
-injection
Metal
+ -
E
FΔ>0
hIP
mInterface dipole
IP EA
+ - + - + - + - + - + -
Organic
Organic
Organic
Improved h
+injection
p
(a) (b) (c)
Metal/Organic interface
Schottky-Mott model
Common vacuum level assumption
Interface Dipole Model
Changhee Lee, SNU, Kore
13/14
EE 430.859 2014. 1stSemester
Origin of interface dipole
H. Ishii, K. Sugiyama, E. Ito, and K. Seki, Adv. Mater.11, 605 (1999).
Possible factors forming and affecting the interfacial dipole layer.
a1) and a2): Charge transfer across the interface,
b) Concentration of electrons in the
adsorbate leading to positive charging of the vacuum side,
c) Rearrangement of electron cloud at the metal surface, with the reduction of tailing into vacuum,
d) Strong chemical interaction between the surface and the adsorbate leading to the rearrangement of the electronic cloud and also the molecular and surface
geometries (both directions of dipoles possible),
e) Existence of interface state serving as a buffer of charge carriers,
f) Orientation of polar molecules or functional groups.
Changhee Lee, SNU, Kore
14/14
EE 430.859 2014. 1stSemester
EF
PEDOT:PSS
n
S O3-S O3HS O3H S O3HS O3-S O3H
S S S S S S
O O
O O
O O
O
O O O
n
N N
Au
N. Koch, A. Kahn, J. Ghijsen and J.-J. Pireaux, J. Schwartz, R. L. Johnson, A. Elschner, Appl. Phys. Lett. 82, 70 (2003)
Effect of interface dipole on the hole injection barrier
