organic electronics an introduction - pt. … electronics an introduction dr.sanjay tiwari professor...
TRANSCRIPT
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Organic Electronics
An introduction
Dr.Sanjay Tiwari
Professor & Head
SOS in Electronics & Photonics
Pt.Ravishankar Shukla University,Raipur
Disclaimer: Some of the material presented in this presentation is freely available on the internet
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A Little Background on Light
• Different colors of light have different
wavelengths and different energies
Source: http://www.mhhe.com/physsci/astronomy/arny/instructor/graphics/ch03/0305.html
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Absorption of Light by Atoms
Sources: http://members.aol.com/WSRNet/tut/absorbu.htm, http://csep10.phys.utk.edu/astr162/lect/light/absorption.html
Single electron transition in an isolated atom
• Absorption occurs only when the energy of the light equals the energy of transition of an electron
Light
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Review of Semiconductors
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CdTe Orange Yellow CdS Blue
SiC GaN ZnS
6.0 3.0
HgCdTe 2.0 1.5
1.0 0.9
GaAs1-yPy 0.8 0.7
0.6
λ (µm) 0.5
0.45 0.4 0.35
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Eg (eV) 2.2 2.4 2.6
2.8 3.0 3.2 3.4 3.6
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Light-Emitting Diodes (LEDs)
Light-emitting diode (LED) is a
semiconductor diode that emits
incoherent narrow-spectrum light when
electrically biased in the forward
direction of the p-n junction.
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Photon Emission in Semiconductor
EF
EC
EV
Conduction
band
Valence
band
Photon
Eg
When an electron meets a
hole, it falls into a lower
energy level, and releases
energy in the form of a
photon.
The wavelength of the light
depends on the band gap
of the semiconductor
material
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Operation Principle of LED
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March of LEDs
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Semiconductor Materials vs.
LED Color
General Brightness
GaP GaN GaAs GaAIAs --
Green, Red Blue Red, Infrared Red, Infrared --
Super Brightness
GaAIAs GaAsP GaN InGaN GaP
Red Red, Yellow Blue Green Green
Ultra Brightness
GaAIAs InGaAIP GaN InGaN --
Red Red, Yellow, Orange Blue Green --
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Application of LEDs
Display
Solid-state lighting
Communication
Remote control, etc
LED lights on an Audi S6
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Goal : Basket of Applications & Products
Source: w4.siemens.de/.../archiv/ pof/heft2_03/artikel18/
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Inorganic Vs. Organic
Material Properties
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Conducting polymers
10-18 10-14 10-10 10-6 10-2 102 106 S/cm
Quart
z
Dia
mond
Gla
ss
Sili
con
Germ
aniu
m
Copper
Insulators Semi- Conductors
Metals
Polymers: Insulators and Metals
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Limitations At Early Stage Organic materials have often proved to be
unstable. Making reliable electrical contacts to organic
thin films is difficult. When exposed to air, water, or ultraviolet light,
their electronic properties can degrade rapidly. The low carrier mobilities characteristic of
organic materials obviates their use in high-frequency (greater than 10 MHz) applications.
These shortcomings are compounded by the
difficulty of both purifying and doping the materials.
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Optical, Electronic and Structural Properties of Semiconductor
Nanostructures and Optoelectronic Devices
Inorganic Semi-
conductors & Devices
(Compounds of III-V, I-
III-VI2, II-IV-V2)
Organic Semi-
conductors & Devices
(Polymers & Functional
Polymers)
Solar Components &
Systems
(Photovoltaic
und Solar Thermal)
Department of Experimental Physics I
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The Nobel Prize
in Chemistry
2000
"for the discovery and development of conductive polymers"
Alan J. Heeger Alan G. MacDiarmid
Hideki Shirakawa
1/3 of the prize 1/3 of the prize 1/3 of the prize
USA USA and New Zealand
Japan
University of California Santa Barbara, CA, USA
University of Pennsylvania Philadelphia, PA, USA
University of Tsukuba Tokyo, Japan
b. 1936 b. 1927 (in Masterton, New Zealand)
b. 1936
“for the discovery and developement of conductive polymers“
Organic Optoelectronics
A new material class!
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34 34
The Nobel Prize in Physics 2000
"for basic work on information and communication technology"
Zhores I.
Alferov b. 1930
Herbert
Kroemer b. 1928
Jack S.
Kilby 1923–2005
“for his part in the
invention of the
integrated circuit”
“for developing semiconductor
heterostructures used in high-speed- and
opto-electronics”
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Prof. Richard Friend, FRS
University of Cambridge
Department of Physics
Dr. C. Tang
Kodak
From Laboratory to Industry
From Nobel Prizes to Products : The New Gen Physicists
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Electronic and optoelectronic devices using organic
materials as active elements, for example, organic light-
emitting diodes (OLEDs), organic photovoltaic devices
(OPVs), organic field-effect transistors (OFETs), organic
photorefractive devices, and so forth, have recently
received a great deal of attention from the standpoint of
potential technological applications as well as
fundamental science. The devices using organic
materials are attractive because they can take
advantage of organic materials such as light weight,
potentially low cost, and capability of thin-film, large-
area, flexible device fabrication. OLEDs have also found
practical applications in small displays such as mobile
phones, digital camera finders, and car audios and are
expected to expand their markets to flat-panel
televisions and lighting in the future 36
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Organic LEDs: Organic FETs: Organic Solar Cells
→ Displays → Plastic Electronics
→ Photovoltaics
37
Why are we so excited about Organic Electronics ? We unfortunately missed microelectronics revolution (Silicon based) but cannot afford to miss Macroelectronics revolution. Compatible with our new found self confidence as a nation to excel in high tech Benefits: Low Cost, Light weight, High absorption coefficient, Tunable bandgap, capability of thin-film, large-area, flexible device fabrication, environment friendly
Polymer electronics research has developed rapidly over
the last decade
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Semiconductors:
Inorganic Semiconductors: Si, Ge, GaAs, GaN…
Organic Semiconductors: Molecules (oligomers) and Polymers
2pz
Chemistry Nobel Prize - 2000
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Characteristics:
Weak Intermolecular Interactions
Low Dielectric Constants
Presence of Disorder
Polymer Semiconductors
Mechanically flexible Excellent optoelectronic properties
Chemically tenable and tunable
Absorption & emission in the visible range
Low temperature, solution processed
Large area, inexpensive , “plastic” electronics
General Introduction
ORGANIC ELECTRONICS
Plastic Electronics
Flexible Electronics
Printed Electronics
Large Area Electronics
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Charge carrier mobilities
comparably small (FET) mobilities*:
*C. D. Dimitrakopoulos and D. J. Mascaro IBM J. Res. & Dev. 45 (1), 2001
low mobilities & large absorption coefficients thin absorber
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10/9/2014
conductivity Light Emission Photoelectric
response Bio-materials Current control
Conduct
electricity
Convert
electricity to
photons
Convert photons
to electricity
Manipulate
electricity Smart interface
Plastic
capacitor
High power
electrolyte
Battery organic
electrode
Organic EL
Organic
lighting
Organic
laser
Lumalive
fabrics
Super
capacitors
Organic optical
sensors
Organic
Solar-cells
Organic interface
device
Wearable
Information
device
Organic
transistors
Organic
IC circuits
Molecular
logic
circuits
Bio-sensors
Drug delivery
Organic Electronics
Roadmap
Properties (time)
Pote
nti
al a
ppli
cati
on (
tim
e)
41
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Organic Semiconductors –
Optical Properties
Organic Materials :
- Saturated Bonds : Insulators, glasses
- Conjugated Bonds organic compounds
(delocalized orbital)
Organic Semiconductors:
- Cyclic Conjugated Bonds (Benzene
rings)
- Linear Chains (Conjugated Polymers)
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Organic Semiconductors –
Optical Properties
Organic Semiconductors are Molecular
Solids
Monomers to Oligomers
Monomers to Polymers
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Organic Semiconductors –
Optical Properties
Organic Semiconductors are Molecular
Solids
- Covalent Bonding (Intra molecular)
- van der Waals interactions (inter-
molecular)
Low melting point
Soft structure
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Organic Semiconductors –
Optical Properties Electronic States are tightly bound to the
molecules
Solids – Crystals, Amorphous Thin Films or polycrystalline thin films
(Localized electronic states – molecular solids)
(Delocalized band states in Inorganic Crystalline Solids)
Optical Properties of Organic Semiconductors are governed by the optical properties of the
constituent molecules
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Optical Spectra of Molecules
• Far infrared region (>100 mm, ~10 meV,
Rotational Transitions)
• Mid infrared region (~1-100 mm, ~100
meV, Vibrational Transitions)
• Visible & Near infrared region (< mm, ~1
eV, Electronic Transitions)
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Definitions
•Disordered organic materials include: molecularly doped
polymers, -conjugated polymers, spin- or solution cast
molecular materials
•Mobility, m [cm2/Vs], is the velocity of the moving charge
divided with electric field (F) m=v/F
•Conductivity: =enm=epm
•Only discussing ”insulating” materials, i.e. < 10-6S/cm
•Current: j=F=epmF
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Ordered and disordered materials:
defects and impurities
Periodic potential distribution implies
the occurrence of extended (non-
localized) states for any electron (or
hole) that does not belong to an
atomic orbital
Coordinate
En
erg
y
Coordinate
En
erg
yA defect or an impurity atom,
embedded into a crystalline matrix,
creates a point-like localized state
but do not destroy the band of
extended states
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Disordered materials: positional
disorder and potential fluctuations
Coordinate
Ene
rgy
Potential landscape for electrons
Potential landscape for holes
Energ
y
Density of states
Positional disorder inevitably gives
rise to energy disorder that can be
described as random potential
fluctuations. Random distribution
of potential wells yields an energy
distribution of localized states for
charge carriers
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50
Disordered materials: deep traps
Coordinate
En
erg
y
Shallow (band-tail) states
Deep traps E
nerg
y
Density of states
Shallow localized states, that are
often referred to as band-tail states,
are caused by potential fluctuations.
Deep states or traps can occur due to
topological or chemical defects and
impurities. Because of potential
fluctuations the latter is also
distributed over energy.
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Overview of energy levels in inorganic semiconductors (left)
and molecular semiconductors (right).
Molecular materials that have a low ionisation potential and thus can easily donate an electron are denoted
as electron donors. Materials that have a high electron affinity and thus can easily take up an electron are
denoted as electron acceptors
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From a torch to Blue and White LEDs and to Solid State Lamps and to Microelectronics
ASIC Core
VCSEL
Receiver
Photodetector
interface
CMOS ASIC
GaAs
Virtual
Input Pad
Virtual
Output Pad
Laser
Driver
Electrical
Photonic Links
Opto-Electronics Applications
Solid State Lighting
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Organic Semiconductors: Processing
Solution processing Evaporation (polymers): (small molecules):
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Singlets and triplets (S = 0 or 1)
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Singlet and triplet excitons E
ne
rgy
S1
S0
T1
Abso
rpti
on
Inte
rnal
con
vers
ion
Flu
oresc
ence
Radiative process: energy released as photons Non-radiative process: energy released as vibrations, etc They all occur at different time scales: (fs) – ps – ns – ms - ms
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Deposition Techniques
Spin Coating
Dip Coating
t 100 Å
t 700 - 1000 Å
Drop Cast
t 5000 Å
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Inorganic Vs. Organic
LEDs
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Organic light emitting diode (OLED):
Organic Semiconductor Devices: OLED
1.) charge injection 2.) charge transport 3.) charge recombination exciton formation
4.) light emission
single layer device
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10/9/2014
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OLED display structure:
electrode bars one pixel = three devices
Organic Semiconductor Devices: OLED
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74
Outline • Introduction and Motivation
– Definitions
• Electronic structure in disordered solids
– Positional disorder
– Deep traps
• Trap controlled transport
– Multiple trapping
– Equilibrium transport
– TOF
– Field dependence
• Gaussian disorder formalism
– Predicitions
– Energy relaxation
– photo-CELIV
• Summary
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76
Definitions
•Disordered organic materials include: molecularly doped
polymers, -conjugated polymers, spin- or solution cast
molecular materials
•Mobility, m [cm2/Vs], is the velocity of the moving charge
divided with electric field (F) m=v/F
•Conductivity: =enm=epm
•Only discussing ”insulating” materials, i.e. < 10-6S/cm
•Current: j=F=epmF
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77
Ordered and disordered materials:
defects and impurities
Periodic potential distribution implies
the occurrence of extended (non-
localized) states for any electron (or
hole) that does not belong to an
atomic orbital
Coordinate
En
erg
y
Coordinate
En
erg
yA defect or an impurity atom,
embedded into a crystalline matrix,
creates a point-like localized state
but do not destroy the band of
extended states
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78
Disordered materials: positional disorder
and potential fluctuations
Coordinate
Ene
rgy
Potential landscape for electrons
Potential landscape for holes
Energ
y
Density of states
Positional disorder inevitably gives
rise to energy disorder that can be
described as random potential
fluctuations. Random distribution
of potential wells yields an energy
distribution of localized states for
charge carriers
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79
Disordered materials: deep traps
Coordinate
En
erg
y
Shallow (band-tail) states
Deep traps E
nerg
y
Density of states
Shallow localized states, that are
often referred to as band-tail states,
are caused by potential fluctuations.
Deep states or traps can occur due to
topological or chemical defects and
impurities. Because of potential
fluctuations the latter is also
distributed over energy.
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80
Trap-controlled transport
Mobility edge (E = 0)
Localized states
Important parameters:
mc - carrier mobility in extended states
c - lifetime of carriers in extended states
0 - attempt-to-escape frequency
Density-of-states
distribution
Extended states: jc = emc pcF
r(E)E = 0
En
erg
y
DOS, g(E)
pc - the total density of carriers in extended states (free
carriers) r (E) - the energy distribution of localized
(immobile) carriers
EdEpp c r
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81
Multiple trapping equations (1)
Since carrier trapping does not change the total
density of carriers, p, the continuity equation can
be written as
t
p
2
2
x
pD
x
pF c
cc
c
m 0
Change of the total
carrier density
Drift and diffusion of carriers in
extended states
Simplifications: (i) no carrier recombination;
(ii) constant electric field (no space charge)
A.I. Rudenko, J. Non-Cryst. Solids 22, 215 (1976); J. Noolandi PRB 16, 4466 (1977);
J. Marshall, Philos. Mag. B, 36, 959 (1977); V.I. Arkhipov and A.I. Rudenko, Sov.
Phys. Semicond. 13, 792 (1979)
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82
Multiple trapping equations (2)
r(E)E = 0
En
erg
yDOS, g(E)
Trapping rate:
0cp
Total trapping
rate
Share of carriers trapped by
localized states of energy E
Release rate:
EkT
Er
exp0
Attempt-to-escape
frequency
Boltzmann
factor
Density of
trapped carriers
E
kT
EpEg
Nt
Ec
t
r
r
exp
10
0
tN
EEg r
tN
Eg
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83
Equilibrium transport
E
kT
EpEg
Nt
Ec
t
r
r
exp
10
0
Since the equilibrium energy distribution of localized carriers is
established the function r(E) does not depend upon time.
0
Solving (*) yields the equilibrium energy distribution of carriers
kT
EEg
N
pE
t
c exp00
r
Integrating (**)
(*)
(**)
relates p and pc as
p
kT
EEgdE
N
pp
t
cc exp
00
kT
EEgdE
N
p
t
c exp00
EdEpp c rand bearing in mind that
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84
Equilibrium carrier mobility and diffusivity
pTpc
The relation between p and pc can be written as
where 1
00 exp
kT
EEgdENT t
t
p
2
2
x
pD
x
pF c
cc
c
m 0
Substituting this relation into the continuity equation yields
02
2
x
pDT
x
pFT
t
pcc m
02
2
x
pTD
x
pFT
t
pm
With the equilibrium trap-controlled mobility, m, and diffusivity, D,
defined as
cTT mm cDTTD
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85
Equilibrium carrier mobility: examples
1) Monoenergetic localized states E = 0 DOS
E = Et
tt EENEg
kT
ET t
c exp00 mm
2) Rectangular (box) DOS distribution E = 0 DOS
E = Et
tt
t
t EEEgEEE
NEg ,0;,
kT
r(E)
1exp
00
kT
EkT
ET
t
tcmm
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86
Time-of-flight (TOF) measurements
Field Light
L
cc
L
c txpdxL
Fetxjdx
Lj
00
,,1 m
Transient current
Equilibrium transport:
ceqc TtxpTtxp mm ,,,
L
eqtxpdx
L
Fej
0
,m
Time ttr
Equilibrium transit time
F
Lt
eq
trm
tr
eqFt
Lm
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87
Trap controlled transport:
field dependent mobility
J. Frenkel, Phys. Rev. 54, 647-648 (1938)
•E-field lowers the barrier
•Poole-Frenkel coefficient
2/1
0
3 ePF
100 150 200 250 300 35010
-6
10-5
10-4
mp [
cm
2/V
s]
F1/2
[V/cm]1/2
Problem: does not fit!
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88
Gaussian Disorder formalism • The Gaussian Disorder formalism is based on fluctuations
of both site energies and intersite distances (see review in: H. Bässler, Phys. Status Solidi (b) 175, 15 (1993) )
• Long range order is neglected
– > Transport manifold is split into a Gaussian DOS!
• Distribution arises from dipole-dipole and charge-dipole interactions
• Field dependent mobility arises from that carriers can reach more states in the presence of the field.
• It has been argued that long range order do exist, due to the charge-dipole interactions.
(see Dunlap, Parris, Kenkre, Phys. Rev. Letters 77, 542 (1996) )
-> Correlated disorder model
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89
Equilibrium carrier distribution: Gaussian DOS
DOS r(E) E
ner
gy
E = 0
2
2
2exp
2
ENEg t
2
2
2exp
2
1
r mEE
E
kTEm
2 The width of the r(E)
distribution is the same as
that of the Gaussian DOS !
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90
Equilibrium mobility: Gaussian DOS
2
2
00
2exp
2 kTT c
mm
kT
Eac exp
2
00m
22
2
ma
E
kTE
DOS r(E) E
ner
gy
E = 0
kTEm
2
Ea
Activation energy of the equilibrium mobility
Ea is two times smaller than the energy Em
around which most carriers are localized !
2 3 4 5 6 7 8 9 1010
-15
10-13
10-11
10-9
10-7
10-5
10-3
10-1
Mo
bility,
a.
u.
1000/T, K-1
20 40 60 8010
-15
10-13
10-11
10-9
10-7
10-5
10-3
10-1
Mo
bility,
a.
u.
(1000/T)2, K
-2
> > > >
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91
Bässler model in RRa-PHT
200 300 4001x10
-8
1x10-7
1x10-6
1x10-5
305 K
290 K
285 K
260 K
240 K
215 K
m [cm
2/V
s]
F0.5
[(V/cm)0.5
]
2/1222/1
2
0 F Cexp3
2exp),,(
mm
F
m0=2.5 10-3 cm2/Vs =100 meV
=3.71 C=8.110-4 (cm/V)1/2
10-1
100
101
102
103
104
10-10
10-9
10-8
10-7
10-6
10-5
10-4
RC
RRa-PHT
d=2.5mm
ttr(50V)
50V
46V
42V
38V
34V
30V
26V
22V
18V
14V
10V
6V
2V
j [A
]
t [ms]
/kT
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92
Disorder formalism, predictions
A.J. Mozer et. al., Chem. Phys. Lett. 389, 438 (2004).
A.J. Mozer et. al, PRB in press
A cross-over
from a
dispersive to
non-dispersive
transport regime
is observed.
Borsenberger et. al, PRB 46, 12145 (1992)
The Bässler model predicts a
negative field dependent
mobility!
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93
Carrier equilibration: a broad DOS
distribution DOS
req(E)
En
ergy
E = 0 After first trapping events the energy
distribution of localized carriers will
resemble the DOS distribution. The
latter is very different from the
equilibrium distribution.
Those carriers, that were initially trapped by shallow
localized states, will be sooner released and trapped
again. For every trapping event, the probability to be
trapped by a state of energy E is proportional to the
density of such states. Therefore, (i) carrier
thermalization requires release of trapped carriers
and (ii) carriers will be gradually accumulated in
deeper states.
Concomitantly, (i) equilibration is a long process and
(ii) during equilibration, energy distribution of carriers
is far from the equilibrium one.
r1(E)
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94
G. Juška, et al., Phys. Rev. Lett. 84, 4946 (2000)
G. Juška, et al., Phys. Rev. B62, R16 235 (2000)
G. Juška, et al., J. of Non-Cryst. Sol., 299, 375 (2002)
R. Österbacka et. al., Current Appl. Phys., 4, 534-538 (2004)
0 1000 2000 3000 4000
0
5
10
15
j(0)=0A/d
j [m
A/c
m2]
t [ms]
0 1000 2000 3000 4000
AU=At
t [ms]
Photo-CELIV
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95
0 1000 2000 3000 4000
0
5
10
15
j0
tmax
j
j [m
A/c
m2]
t [ms]
0 1000 2000 3000 4000
tdel
AU=At
t [ms]
Photo-CELIV
)0(36.013
2
2
max
2
j
jAt
dm
G. Juška, et al., Phys. Rev. Lett. 84, 4946 (2000)
G. Juška, et al., Phys. Rev. B62, R16 235 (2000)
G. Juška, et al., J. of Non-Cryst. Sol., 299, 375 (2002)
R. Österbacka et. al., Current Appl. Phys., 4, 534-538 (2004)
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Mobility Relaxation measurements
0 1 2
0
5
10
15
20
50 ms
200 ms
500 ms
1000 ms
2000 ms
10 ms
dark
j [m
A/c
m2]
t [ms]
The tmax shifts to longer times
as a function of tdel
0 1 2
0
5
10
15
20
t [ms]
8 mJ
4.5 mJ
2.8 mJ
0.94 mJ
1.59 mJ
0.33 mJ
dark
j [m
A/c
m2]
The tmax is constant as a
function of intensity
Photo-CELIV is the only possible method to measure the
equilibration process of photogenerated carriers.
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97
Mobility relaxation
10-4
10-3
10-2
10-1
10-7
10-6
10-5
t-0.58
m
[cm
2/V
s]
tdel
+ tmax
[s]
R. Österbacka et al., Current Appl. Phys. 4, 534-538 (2004)
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98
Summary
• An introduction to carrier transport in disordered organic materials is given
• Disorder gives rise to potential fluctuations
– > Energy distribution of localized states
• By knowing the DOS: equilibrium transport can be calculated
• In the disorder formalism (Bässler) carrier equilibration is a long process
– > Decrease of mobility as a function of time!
• We have shown a possible method (CELIV) to measure the equilibration process