organic electronics an introduction - pt. … electronics an introduction dr.sanjay tiwari professor...

Organic Electronics

An introduction

Dr.Sanjay Tiwari

Professor & Head

SOS in Electronics & Photonics

Pt.Ravishankar Shukla University,Raipur

stiwari@fulbrightmail.org

Disclaimer: Some of the material presented in this presentation is freely available on the internet

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

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

Review of Semiconductors

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

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

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.

Photon Emission in Semiconductor

Conduction

Valence

Photon

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

Operation Principle of LED

March of LEDs

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 --

Application of LEDs

Display

Solid-state lighting

Communication

Remote control, etc

LED lights on an Audi S6

Goal : Basket of Applications & Products

Source: w4.siemens.de/.../archiv/ pof/heft2_03/artikel18/

Inorganic Vs. Organic

Material Properties

Conducting polymers

10-18 10-14 10-10 10-6 10-2 102 106 S/cm

Copper

Insulators Semi- Conductors

Metals

Polymers: Insulators and Metals

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.

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

The Nobel Prize

in Chemistry

"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

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!

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”

Prof. Richard Friend, FRS

University of Cambridge

Department of Physics

Dr. C. Tang

From Laboratory to Industry

From Nobel Prizes to Products : The New Gen Physicists

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

Organic LEDs: Organic FETs: Organic Solar Cells

→ Displays → Plastic Electronics

→ Photovoltaics

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

Semiconductors:

Inorganic Semiconductors: Si, Ge, GaAs, GaN…

Organic Semiconductors: Molecules (oligomers) and Polymers

Chemistry Nobel Prize - 2000

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

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

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

Lumalive

fabrics

capacitors

Organic optical

sensors

Organic

Solar-cells

Organic interface

device

Wearable

Information

device

Organic

transistors

Organic

IC circuits

Molecular

circuits

Bio-sensors

Drug delivery

Organic Electronics

Roadmap

Properties (time)

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)

Optical Properties

Organic Semiconductors are Molecular

Solids

Monomers to Oligomers

Monomers to Polymers

Optical Properties

Organic Semiconductors are Molecular

Solids

- Covalent Bonding (Intra molecular)

- van der Waals interactions (inter-

molecular)

Low melting point

Soft structure

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

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)

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

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

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

Disordered materials: positional

disorder and potential fluctuations

Coordinate

Potential landscape for electrons

Potential landscape for holes

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

Disordered materials: deep traps

Coordinate

Shallow (band-tail) states

Deep traps E

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.

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

From a torch to Blue and White LEDs and to Solid State Lamps and to Microelectronics

ASIC Core

Receiver

Photodetector

interface

CMOS ASIC

Virtual

Input Pad

Virtual

Output Pad

Driver

Electrical

Photonic Links

Opto-Electronics Applications

Solid State Lighting

Organic Semiconductors: Processing

Solution processing Evaporation (polymers): (small molecules):

Singlets and triplets (S = 0 or 1)

Singlet and triplet excitons E

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

Deposition Techniques

Spin Coating

Dip Coating

t 100 Å

t 700 - 1000 Å

Drop Cast

t 5000 Å

Inorganic Vs. Organic

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

10/9/2014

OLED display structure:

electrode bars one pixel = three devices

Organic Semiconductor Devices: OLED

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

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

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

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

Disordered materials: positional disorder

and potential fluctuations

Coordinate

Potential landscape for electrons

Potential landscape for holes

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

Disordered materials: deep traps

Coordinate

Shallow (band-tail) states

Deep traps E

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.

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

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

Multiple trapping equations (1)

Since carrier trapping does not change the total

density of carriers, p, the continuity equation can

be written as

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)

Multiple trapping equations (2)

r(E)E = 0

yDOS, g(E)

Trapping rate:

Total trapping

Share of carriers trapped by

localized states of energy E

Release rate:

Attempt-to-escape

frequency

Boltzmann

factor

Density of

trapped carriers

Equilibrium transport

Since the equilibrium energy distribution of localized carriers is

established the function r(E) does not depend upon time.

Solving (*) yields the equilibrium energy distribution of carriers

c exp00

Integrating (**)

relates p and pc as

cc exp

c exp00

EdEpp c rand bearing in mind that

Equilibrium carrier mobility and diffusivity

The relation between p and pc can be written as

where 1

00 exp

EEgdENT t

Substituting this relation into the continuity equation yields

With the equilibrium trap-controlled mobility, m, and diffusivity, D,

defined as

cTT mm cDTTD

Equilibrium carrier mobility: examples

1) Monoenergetic localized states E = 0 DOS

E = Et

tt EENEg

c exp00 mm

2) Rectangular (box) DOS distribution E = 0 DOS

E = Et

t EEEgEEE

NEg ,0;,

Time-of-flight (TOF) measurements

Field Light

c txpdxL

Fetxjdx

Transient current

Equilibrium transport:

ceqc TtxpTtxp mm ,,,

eqtxpdx

Time ttr

Equilibrium transit time

Trap controlled transport:

field dependent mobility

J. Frenkel, Phys. Rev. 54, 647-648 (1938)

•E-field lowers the barrier

•Poole-Frenkel coefficient

100 150 200 250 300 35010

[V/cm]1/2

Problem: does not fit!

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

Equilibrium carrier distribution: Gaussian DOS

DOS r(E) E

ENEg t

2 The width of the r(E)

distribution is the same as

that of the Gaussian DOS !

Equilibrium mobility: Gaussian DOS

2 kTT c

Eac exp

DOS r(E) E

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

bility,

1000/T, K-1

20 40 60 8010

bility,

(1000/T)2, K

> > > >

Bässler model in RRa-PHT

200 300 4001x10

1x10-7

1x10-6

1x10-5

[(V/cm)0.5

2/1222/1

0 F Cexp3

2exp),,(

m0=2.5 10-3 cm2/Vs =100 meV

=3.71 C=8.110-4 (cm/V)1/2

RRa-PHT

d=2.5mm

ttr(50V)

t [ms]

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!

Carrier equilibration: a broad DOS

distribution DOS

req(E)

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.

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

j(0)=0A/d

t [ms]

0 1000 2000 3000 4000

t [ms]

Photo-CELIV

0 1000 2000 3000 4000

t [ms]

0 1000 2000 3000 4000

t [ms]

Photo-CELIV

)0(36.013

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)

Mobility Relaxation measurements

200 ms

500 ms

1000 ms

2000 ms

t [ms]

The tmax shifts to longer times

as a function of tdel

t [ms]

4.5 mJ

2.8 mJ

0.94 mJ

1.59 mJ

0.33 mJ

The tmax is constant as a

function of intensity

Photo-CELIV is the only possible method to measure the

equilibration process of photogenerated carriers.

Mobility relaxation

t-0.58

+ tmax

R. Österbacka et al., Current Appl. Phys. 4, 534-538 (2004)

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

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