1 Organic Semiconductor EE 4541.617A 2009. 1 st Semester 2009 5 12 K. C. Kao and W. Hwang, Electrical Transport in Solids, (Pergamon, New York, 1981), p.159 Changhee Lee, SNU, Korea 1/41 Changhee Lee School of Electrical Engineering and Computer Science Seoul National Univ. [email protected]2009. 5.12. Organic Semiconductor EE 4541.617A 2009. 1 st Semester Electron mobility of Si Carrier mobility of Si and Organic Materials hole mobility of Perylene hole mobility of MPMP Changhee Lee, SNU, Korea 2/41 370 nm thick perylene crystal 8.7 um thick unordered layer of MPMP ( sublimed).
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1
Organic SemiconductorEE 4541.617A
2009. 1st Semester
2009 5 12
K. C. Kao and W. Hwang, Electrical Transport in Solids, (Pergamon, New York, 1981), p.159
Changhee Lee, SNU, Korea1/41
Changhee LeeSchool of Electrical Engineering and Computer Science
370 nm thick perylene crystal 8.7 um thick unordered layer of
MPMP ( sublimed).
2
Organic SemiconductorEE 4541.617A
2009. 1st Semester
d 2
Mobility Measurement Techniques
10 2−
)/( 2 Vscmμ 10 4− 10 6− 10 8− 100 102
TOFMobility in the bulk
d = 0.5~5 mm
FETMobility in a thin layer
d ~ 50 nm
τμ
Vd 2
=∴
Vdμ
τ2
=
Changhee Lee, SNU, Korea3/41
d 0.5 5 mmOther mobility measurement techniques• Dark injection in the space-charge limited current regime• I-V characteristics of space charge limited current • Transient EL• SHG measurement [T. Manaka, E. Lim, R. Tamura, M. Iwamoto, Nature Photon.1, 581–584 (2007).]……
2009. 1st SemesterTransport of charge carriers in organic materials
60
40
20
0
Phot
ocur
rent
(arb
. uni
ts)
100806040200
0.20
0.15
0.10
0.05
0.00hoto
curre
nt (a
rb. u
nits)
a-NPD Alq3
Gaussian transport Dispersive transport Fast
Slow
P 1.00.80.60.40.20.0
Time (μs)
Ph 806040200
Time (μs)
Dispersive transportGaussian transport
er D
ensi
ty
Trappingand release
Band States
Changhee Lee, SNU, Korea6/41
Distance
Carr
ie
Localized states
Hopping
Deep trap
Ref.) Harvey Scher, Michael F. Shlesinger and John T. Bendler, Physics today, Jan. p.26 (1991)
4
Organic SemiconductorEE 4541.617A
2009. 1st SemesterMobility Measurement Techniques: Transient EL
30
25
20
rb.u
nit)
57V
Al
PPP15
10
5
0
Ligh
t (x
10-3
ar
500450400350300250200150100500
Time (x10-9 s)
57V
19V27V
39V
42V
46V52V
240
220
Mobility from the delay time
τ=d2/ μV
ITO glass
PMT
Changhee Lee, SNU, Korea7/41
220
200
180
160
140
120
100
Tim
e Del
ay (n
s)
605040302010
1/V (x10-3 V-1)
Hole mobility
in the vacuum-deposited PPP:
ITO/PPP/Al: μ=4.5x10-6 cm2/Vs
G. W. Kang, C. H. Lee, W. J. Song, and C. Seoul, SPIE 4105, 362 (2001).
Organic SemiconductorEE 4541.617A
2009. 1st SemesterTransient EL Mobility vs TOF-PC mobility in Alq3
Changhee Lee, SNU, Korea8/41
S. C. Tse, H. H. Fong, and S. K. So, J. Appl. Phys. 94, 2033 (2003)
For a thin layer of Alq3 (d<180 nm), it is found that td is affected by both the charging effect and carrier transit time through the Alq3 layer. For a thicker layer of Alq3 (d>200 nm), tdapproaches the intrinsic electron transit time through Alq3 .
5
Organic SemiconductorEE 4541.617A
2009. 1st SemesterSpace-charge-limited current
Hole only device
0.3 μm
d=0.13 μm
0.7 μm
OC1C10-PPV
Electron only device
3
2
89
dVJ roSCLC μεε=
d=0 22 m
ITO Au+
OC1C10-PPV-
Changhee Lee, SNU, Korea9/41
d=0.22 μm 0.37 μm
0.31 μm12
1
+
+
∝ m
m
dVJ
(a)Ca Ca
Poly(dialkoxy-p-phenylene vinylene) (OC1C10-PPV)P. W. M. Blom M. J. M. de Jong, and J. J. M. Vleggaar, Appl. Phys. Lett. 68, 3308 (1996).
Organic SemiconductorEE 4541.617A
2009. 1st Semesterd
T it tiV R
++++++
Dark injection SCL transient current
Transient SCLC
Transit time Ohmic contact τ787.0=pt
PTPBpoly (tetraphenylbenzidine) PTPB
1.0
0.8
0.6
0.4i(t) (
arb.
uni
t)
Dark injection SCL transient currentτ787.0=pt
t
Changhee Lee, SNU, Korea10/41
M. Abkowitz, J. S. Facci, and M. Stolka, Appl. Phys. Lett. 63, 1892(1993).
0.2
0.01.00.80.60.40.20.0
Time (ms)
787.0pt
=τ
TOF-PC
S.C. Tse, S.W. Tsang, and S.K. So, J. Appl. Phys. 100, 063708 (2006).
6
Organic SemiconductorEE 4541.617A
2009. 1st SemesterMobility Measurement Techniques: dark charge injection
ITO/300nm m-MTDATA/Ag with ITO biased positively.
Edttr 786.0=
In a monopolar and single-layer configuration, the carrier transit time is shorter than in the absence of
space-charge effects due to the enhancement of the electric field at the leading edge of the carrier packet.
Etr μ
The transient current overshoots its steady-state value by a factor of 1.21 and starts at 0.44 times the
steady-state value.
Changhee Lee, SNU, Korea11/41
M. Stossel et al, Phys. Chem. Chem. Phys.1, 1791 (1999)A. Many and G. Rakavy, Phys. Rev. 126, 1980 (1962)
Organic SemiconductorEE 4541.617A
2009. 1st Semester
nj.
Carrier injection efficiency
contact limiting-currentfor 1contact ohmicfor 1
current injected :efficiencyInjection
<=
=
ηη
ηSCLC
3
2
89
dVJ ro
SCLCμεε
=
Inje
ctio
n E
ffici
ency
ηin
Changhee Lee, SNU, Korea12/41
Hol
e
Hole Injection Barrier (eV)M. Abkowitz, J. S. Facci, J. Rehm, J. Appl. Phys. 1998, 83, 2670.
T. Manaka, E. Lim, R. Tamura, M. Iwamoto, Nature Photon.1, 581 (2007).
T. Manaka, M. Nakao, D. Yamada, E. Lim and M. Iwamoto, Optics Express 15, 15964 (2008).
Organic SemiconductorEE 4541.617A
2009. 1st Semester
(1) Poole-Frenkel Model
Conduction band mxx =x
)(xV)(xV
Charge Transport in Disordered Organic Solids
EEePF
2/1
0
3
PF βπεε
φ =⎟⎟⎠
⎞⎜⎜⎝
⎛=Δ
Conduction band
edge at ETunneling
PFφΔedge at E=0
Zero field (E=0) E≠0
x
eEx−x
e
oεπε4
2
−
Changhee Lee, SNU, Korea14/41
Zero field (E 0) E≠0
⎟⎟⎠
⎞⎜⎜⎝
⎛⎟⎟⎠
⎞⎜⎜⎝
⎛ Δ−=
TkE
TkEμF,T
B
PF
BPF expexp)( βμ EΔ : Activation energy at
E=0
: PF Mobility
: PF constant
PFμ
PFβ
0
3
PF πεεβ e
=
8
Organic SemiconductorEE 4541.617A
2009. 1st Semester
0
111TTTeff
−= : Empirical parameter
0T
(2) Gill’s modified Poole-Frenkel model
⎟⎟⎠
⎞⎜⎜⎝
⎛⎟⎟⎠
⎞⎜⎜⎝
⎛ Δ−=
effeff TkE
TkEμF,T
B
PF
BPF expexp)( βμ
Charge Transport in Disordered Organic Solids
PVKpoly-n-vinylcarbazole
(hole conductor)
Nn
O2N C
O
NO2
Changhee Lee, SNU, Korea15/41
TNF2,4,6-trinitro-9-fluorenone
(electron acceptor)
NO2
W. D. Gill, J. Appl. Phys. 43, 5033 (1972).
Organic SemiconductorEE 4541.617A
2009. 1st Semester(3) Bassler’s Gaussian Disorder Model• The energy of each site is distributed in accordance with the Gaussian distribution• Energies of adjacent sites are uncorrelated and motion between sites is Markovian (no phase memory)• The transition rates for phonon-assisted tunneling (Miller and Abrahams):
⎪⎬⎫
⎪⎨⎧
>−
−−= jiji
kTRvW εεεε
α ),exp()2exp( εεj
Charge Transport in Disordered Organic Solids
α = inverse localization length, Rij = distance between the localized states, εi = energy at the state i.• Since the hopping rates are strongly dependent on both the positions and the energies of the localized states, hopping transport is extremely sensitive to structural as well as energetic disorder.
σLUMO
⎪⎭⎬
⎪⎩⎨
>=
ji
jijphij kTRvW
εεα
,1)2exp( εi
A. Miller, E. Abrahams, Phys. Rev. 120 (1960) 745. Hopping
radiuson localizaticarrier anddistance site-inter average where
),/2exp()( 2
==
−=
o
ooo aμ
ρρ
ρρρρ
H. Bässler, Phys. Status Solidi B 175, 15 (1993).M. Stolka, J. F. Janus and D. M. Pai, J. Phys. Chem. 88, 4707 (1984).
1.5)( ,32exp
32exp)(
1.5)( ,32exp
32exp)(
22
B
2
B
22
B
2
B
≥Σ⎪⎭
⎪⎬⎫
⎪⎩
⎪⎨⎧
⎥⎥⎦
⎤
⎢⎢⎣
⎡Σ−⎟⎟
⎠
⎞⎜⎜⎝
⎛
⎥⎥⎦
⎤
⎢⎢⎣
⎡⎟⎟⎠
⎞⎜⎜⎝
⎛−=
<Σ⎪⎭
⎪⎬⎫
⎪⎩
⎪⎨⎧
⎥⎥⎦
⎤
⎢⎢⎣
⎡Σ−⎟⎟
⎠
⎞⎜⎜⎝
⎛
⎥⎥⎦
⎤
⎢⎢⎣
⎡⎟⎟⎠
⎞⎜⎜⎝
⎛−=
ETk
CTk
μE,T
ETk
CTk
μE,T
o
o
σσμ
σσμ
: Constant ~2.9x10-4 (cm/V)1/2
: High temperature limit of the mobilityC
oμ
P. M. Borsenberger, L. Pautmeier and H. Bassler, J. Chem. Phys. 94, 5447 (1991).
Organic SemiconductorEE 4541.617A
2009. 1st Semester
MPMPbis(4-N,N-diethylamino-2-methylphenyl)-4-
methylphenylmethane (thickness~8.7 μm)
Hole mobility of MPMP
Charge Transport in Disordered Organic Solids
Changhee Lee, SNU, Korea18/41
P. M. Borsenberger, L. Pautmeier and H. Bässler, J. Chem. Phys. 95, 1258 (1991)
10
Organic SemiconductorEE 4541.617A
2009. 1st SemesterDisorder parameters• σ: The width of the DOS. Random distribution of both permanent and van der Waals dipoles lead to local fluctuations in electric potential increase σ by an amount proportional to the square root of the dipole concentration and to the strength of the dipole moment. reduce the carrier mobility. The smaller dipolar interaction is better for the carrier transport. • Σ: The degree of positional disorder. Amorphous morphology of molecular solids or doped polymers lead to the variation in the intermolecular distances.
Changhee Lee, SNU, Korea40/41P. E. Burrows, Z. Shen, V. Bulovic, D. M. McCarty, S. R. Forrest, J. A. Cronin and M. E. Thompson, J. Appl. Phys. 79, 7991
(1996).
21
Organic SemiconductorEE 4541.617A
2009. 1st SemesterTrap-limited SCL current with field-dependent μ
Changhee Lee, SNU, Korea41/41
W. Brütting, S. Berleb and A. G. Mückl, Org. Electronics 2, 1 (2001)