Do TCOs contribute to electrical fatigue of organic · PDF fileOxygen vacancies Indium oxide and tin oxide are truly intrinsically n-type semiconductors ... O1s C1s UPS 0.8Å 0Å 536

Andreas Klein Surface Science (D3)

Karsten Albe Materials Modelling (C2)

Do TCOs contribute to electrical

fatigue of organic LEDs

5.5

5.0

4.5

wo

rk f

un

cti

on

[eV

]

4.0 3.8 3.6 3.4 3.2 3.0

ECB

SnO

2:F

EF-EVB [eV]

SnO2

SnO2:Sb

16.09.2014 | SFB Symposium Sellin | Andreas Klein | 2

What is a TCO

VB

CB

energ

y

Eg >~3eV

EF

k

Optical transparent material by large band gap

Highly conducting material by degenerate doping

Doped ZnO, SnO2, In2O3

undoped high doping higher doping

n~ 1021 cm-3


In2O3:Sn (ITO)

Highest conductivity and transparency of TCO materials

Good structurability by chemical etching

Surface properties modified by oxidation treatments (increase of work

function)

Bixbyite crystal structure with 80 atom unit cell and plenty interstitial

positions

Typically 10 mole% SnO2 doping

Sn dopants mainly compensated by interstitial oxygen


Self-compensation

Compensating defects (Oi in ITO) are formed spontaneously only

when the Fermi energy is deep in the conduction band

• The formation of defects requires a certain amount of energy DHdef

Example: Donor type defect (VO)

EVB

ECB

ED

eDD

EF ED

EF

occupied unoccupied

D0 D+

d

E

• The charge state of the defect depends on the Fermi level position

DH

def

EF

donor ED

EF

acceptor

EA

J Am Ceram Soc 96 (2013) 331

CB VB only n-type


Oxygen vacancies

Indium oxide and tin oxide are truly intrinsically n-type semiconductors

The behavior is more complex for ZnO

Phys Rev Lett 103 (2009) 245501


Compensating acceptors

Charged oxygen interstitials are very unfavorable in SnO2

Oxygen transport only via oxygen vacancies in SnO2

Stability of acceptor defects increases with increasing Fermi energy

J Appl Phys 108 (2010) 053511


The TCO electrode in OLEDs

grain orientation determined by

surface energy & deposition

Workfunction determined by

surface orientation and

termination

Hole injection determined

by workfunction

Oxygen exchange

determined by surface

properties

Defect concentration (e.g. Oi) and mobility

determined by material properties and doping

cathode


Possible contributions to fatigue

Change of injection barrier during operation

Change of work function

surface termination

Fermi level position (oxygen concentration)

Interfacial reaction

Release of oxygen

Chemical decomposition of organic

Change of TCO conductivity

Oxygen exchange surface vs. diffusion limitation


Performed work

Experimental

Systematic determination of TCO work functions

Interfaces between ITO and organic semiconductors

Conductivity relaxation experiments (oxygen exchange)

Building test OLEDs and study fatigue behaviour ( D4)

Theoretical

Thermodynamics of point defects in ZnO, In2O3 and SnO2

Anion and cation diffusion in ZnO and In2O3

Thermodynamics of surface structures of In2O3 and SnO2

Defect at (101) twin boundary in SnO2

Adsorption behaviour of organic compounds


DAISY-MAT (XPS/UPS + preparation)

electron analyser

UV-source Ion-source

monochromated X-ray source

sample handler

load lock

magnetron sputtering

MBE organics, metals

MOCVD ALD

Preparation

Analysis

magnetron sputtering

evaporation


TCO work functions

5.5

5.0

4.5

4.0 3.8 3.6 3.4 3.2 3.0

SnO

2:F

EF-EVB [eV]

SnO2:Sb

SnO2

3.6 3.2 2.8 2.4 2.0

EF-EVB [eV]

5.5

5.0

4.5

4.0

ITO

5.0

4.5

4.0

3.5

3.6 3.2 2.8 2.4

work

function [

eV]

EF-EVB [eV]

ZnO

ZnO:Al

Thin Solid Films 518 (2009) 1197

Large variation of work function due to changes in

Fermi level position, surface orientation and termination

ZnO SnO2 In2O3


Types of surfaces

In2O3 (110) In2O3 (111) In2O3(100)

non-polar polar - stable polar - unstable

stoichiometric stoichiometric non-stoichiometric

stable composition stable composition variable composition


In2O3 surfaces

oxygen chemical potential [eV]

Surf

ace e

nerg

y [

eV/Å

2]

(110) stoichiometric

(111) stoichiometric

stoichiometric

(100)

UPS of epitaxial In2O3 films

Calculated ionization potentials

100-perox 100-met 100-st 111

7.7 eV 6.8 eV 8.6 eV 7.1 eV

J Phys CM 23 (2011) 334203


ITO/Al2O3 interface

strong increase in work function initial dipole formation

binding energy [eV]

18 14

SEC

12 8 4 0

VB

65Å

41Å

25Å

14Å

8.5Å

4.7Å

1.4Å

0.7Å

2 0

EF

6.4eV

4.5eV

5.9eV

thickness [Å]

9

8

7

6

5

IP

f

4

3

2

60 50 40 30 20 10 0

In 3d

Sn 3d

Al 2p

EF-E

VB [

eV

]

f /

IP [

eV

]

Change to anion-terminated surface ?

Phys Chem Chem Phys 11 (2009) 3049


ITO/organic interface

O O O O

O O O O O

O

ITO ITO + O2

EVB

ECB

EF

HOMO

ZnPC

energ

y

292 288 284

0.4Å

536 532 528

binding energy [eV]

-1 1

O1s C1s UPS

0.8Å

0Å

536 532 528 1 -1 292 288 284

C-O

binding energy [eV]

O1s C1s UPS

FB

J Phys Chem B 110 (2006) 4793


O-intersticialcy diffusion in ZnO

Phys Rev B 73 (2005) 115207


Oxygen diffusion in ZnO

Interstitialcy mechanism, neutral and positive charge states

Dependence of diffusivity on Fermi level and chemical potential

Phys Rev B 73 (2005) 115207


Zn diffusion in ZnO

Hierarchy of mobilities:

zinc interstitials

oxygen interstitials

zinc vacancies

oxygen vacancies

Appl Phys Lett 88 (2006) 201918


EB(A) =0.99 eV

EB(B) =1.28 eV

EB(G) =3.82 eV

VO••

Combined DFT+KMC Ansatz

Diffusion in In2O3

Phys Rev B 81 (2010) 195205


EM(V

O) = 1.01 eV

EM(O

i) = 1.40 eV

G. P. Wirtz and H. P. Takiar, J. Am. Ceram. Soc. 64, 748 (1981).

Diffusion in In2O3

Phys Rev B 81 (2010) 195205


Conductivity relaxation

glass

TCO

contacts

van der Pauw

conductivity measurement

gas flow gas flow

pO2

time

pO

2

conductivity s

O2,g

O2,ad

+𝑒 O2,ad

− +𝑒 O2,ad

2− 2Oad

− +2𝑒+2VO 2Os

2−

2O2−

Surface oxygen exchange reaction


Conductivity relaxation of ITO (1 bar)

6 ppm 16 ppm

1010 ppm

0.97 %

9.94 %

97.6 %

Conductivity depends on oxygen pressure

Slope related to dominant defect species

Log (pO2)

Log (

co

nductivity)

-1/8

T=590°C

d=400nm


Hall effect and conductivity relaxation


Hall effect measurement of ITO

Changes much slower than expected from oxygen diffusion

Changes not monotonic

Cation diffusion is also involved

Agrees with pO2 dependent Sn segregation from XPS

Solid State Ionics 262 (2014) 636

PRB 73 (2006) 245312


Enhancement of exchange by In2O3

Exchange at SnO2 possible with 1nm In2O3 on surface

1440 1200 960 720 480 240 0

time [min]

400

300

200

100

0

tem

peratu

re [

°C

]

0.01

0.1

1

10

con

du

cti

vit

y [

S/

cm

]

960 720 480 240 0

time [min]

s

T

0.6 Pa O 2 0.6 Pa O 2

SnO2 SnO2/In2O3

ITO conductivity during OLED operation

1st step: new contacting

method in order to assure

reproducibility

2nd step: remeasure, also

with ITOs with different

oxygen content:

ITO Conductivity

[S/cm]

concentration

[1/cm³]

Mobility µ

[cm²/Vs]

Commercial ITO 7400 1E+21 40

Commercial ITO

after lithography

5800 1E+21 35

most reduced

ITO 100% Ar

7700 1E+21 41

ITO 1% O2 1900 3E+20 36

ITO 10% O2 70 3E+19 17

30.09.2013 | Mareike Hohmann | SFB Meeting Bad Orb | 26

?


Summary

Carrier concentration in TCOs determined by doping and

intrinsic defects (self-compensation)

Work function determined by doping, surface orientation

and surface termination

Inhomogeneous work function (charge injection)

Oxygen exchange at ITO limited by bulk diffusion and not

by surface exchange coefficient in contrast to SnO2

Oxygen exchange in principle also at T < 200°C

No dominant influence of ITO electrode on OLED fatigue

identified


Contributors

Ph.D. students

Paul Erhart, Péter Ágoston, Arno Fey, Yvonne Gassenbauer,

André Wachau, Mareike Frischbier (Hohmann)

Bachelor, Master, Diploma students

Péter Ágoston, Arno Fey, Thorsten Bayer, Kai Kühne, André

Wachau, Mareike Hohmann, Karsten Rachut, Hans Wardenga,

Robert Schafranek, Mirko Weidner, Timo Noll, Jonas

Deuermeier

International cooperations

T.O. Mason (Northwestern University), R.G. Egdell (Oxford),

Y. Shigesato (Yokohama), R. Nieminen, K. Nordlund (Helsinki)



Publication Highlights

7 joint publications with 432 citations

34 publication of project D3 with 1253 citations


Relaxation setup


Discussing the carrier concentration

Gassenbauer, Y., et al. (2006).

Physical Review B 73: 245312. 30.09.2013 | Mareike Hohmann | SFB Meeting Bad Orb | 32

Effects affecting carrier concentration n:

1. oxygen incorporation n ↓

2. Sn segregation n ↓

3. T↑ µo ↓ n ↑

0

0 ln+=p

pRTµµ

O

O


Discussing the carrier concentration

30.09.2013 | Mareike Hohmann | SFB Meeting Bad Orb | 33

Effects affecting carrier concentration n:

1. oxygen incorporation n ↓

2. Sn segregation n ↓

3. T↑ µo ↓ n ↑

0

0 ln+=p

pRTµµ

O

O

seg

i

SnO

eSnO

SnO

In

In

2+2

1

2+2+2

1

2+

2

'•

2

•"

First ideas on

complex interplay

of several effects


In2O3 – work function

Almost no change of surface termination with oxygen

Work function depends on surface orientation

Differences between In2O3 and ITO explained by

texture of films

Surface oxidation (e.g. via ozone) only possible for

(100) orientation

(100)

(111)

DFT

J. Phys. CM 23 (2011) 334203


Doping limit of ITO (In2O3:Sn)

Self-compensation provides a natural explanation for the

transition from electronic to ionic compensation of SnIn

1 eV

In O2 3

EF

DHd (Oi)

EF (max)

ECB

EVB log [Sn]

log c

Mostly electronic compens.

Mostly ionic

compens.

n

[Sn]

[Oi] O-p

oor

O-r

ich

J Am Ceram Soc 96 (2013) 331


Fermi level of TCO films

J. Am. Ceram. Soc. 96 (2013) 331

energ

y


Work function and ionisation potential

doping

wo

rk f

un

cti

on

EF-EVB [eV]

Evac-E

F [

eV

] Evac

ECB

EVB

EF

IP

IP= constant

Work function affected by Fermi level and ionisation

potential


Band gap of In2O3 and ITO

In2O3

Eg,opt Eg

0.8 eV

Forbidden

transitions

DEF (XPS) ~ DEF (optic)

Fundamental gap Eg ~ 2.8 eV

3.4

3.2

3.0

2.8

4.4 4.2 4.0 3.8

0% Sn

2% Sn

10% Sn

Eg,opt –

1.04 eV

EF-E

VB [

eV]

Eg,opt [eV]

XPS vs. Optik Theorie ITO = In2O3:Sn

PRL 100 (2008) 167402


organic

TCO (ITO)

structurestoichiometry

surfacedefects

exchangekinetics

concentration, type and charge of point defects

mobility of defects

dopant distribution

and mobility

orientationtermination

charge injectionwork function

grain orientation


Conductivity and carrier concentration

CBE

dEEfEDn )()(

Non-degenerate semiconductors

ECB-EF > 3 kBT

kT

EENn FCB

C exp

Electron concentration

Electrical conductivity determined by carrier concentration

Carrier concentration determined by Fermi level position

n [

cm

-3]

EF-ECB [eV]

1020

0.8 0.4 0.0 -0.4

D(E)

E

VB

CB

log n

log p EF

1018

1016

1014

1012

Conductivity he epen s

RT


Optical properties

data: K. Orgassa, IPE Stuttgart

transm

issio

n [

%]

wave length [nm]

400 600 800 1000 1200 1400 1600 1800 2000 0

20

40

60

80

100

I II

I: Burstein-Moss shift

II: Free-carrier induced infrared absorption

increase of carrier

concentration

Plasmon energy (Drude theory): eVm

en5.0~

0

22


TCO applications

Energy band alignment at interfaces important for function


Topics

Transparent conducting oxides

Basic electrical and optical properties

Applications and importance of surfaces and interfaces

Experimental Approach

Surface Properties

Work function and ionization potential

Oxygen exchange

Interface properties

Energy band alignment

Redox processes at interface


Photoemission (XPS, UPS) – Basics

Electron analyser

Light source:

•X-ray (Mg, Al)

•Gas discharge (He)

•Synchrotron

Vacuum chamber UHV (p=10-10 mbar)

e-

sample

Electron detector

schematics

Count

rate

590 585 580 575 570

Te 3d5/2 Te 3d3/2

CdTe

TeO2

Binding energy [eV]

CdTe

TeO2

Chemical shift

Chemical analysis Elemental analysis

Count

rate

Binding energy [eV]

1200 800 400 0

Cd Te

O

C

Mean f

ree p

ath

[Å]

5 10 100 1000 2

kinetic energy of electrons [eV]

Surface sensitivity

100

50

10

5


Surface vs. Bulk Fermi level

Fermi level in bulk corresponds with Fermi level at surface

J. Am. Ceram. Soc. 96 (2013) 331

EF

EVB

ECB

EF

EVB

ECB

or ?

In2O3(:Sn)


Photoemission – Semiconductors

Evac

f

ECB

EVB

EF hn – workfunction

counts

binding energy [eV] 0=EF

EF-EVB

valence band

core level secondary electrons

DEB

DEB

Df

EF-EVB

f

DEB

l


In-situ vs. ex-situ

Spectral shape and

binding energies (Fermi

level position in band

gap) are affected by

contact with air

J. Am. Ceram. Soc. 96 (2013) 331

EF

EVB

ECB

in situ ex situ


Topics





Surface Properties


Oxygen exchange





Preferred orientation

J. Phys. D 43 (2010) 055301

• Change of stable surface

orientation with oxygen

pressure

norm

iert

e I

nte

nsitä

t [w

illk.

Ein

h.]

908070605040302010

2

11

0

10

1

11

1

21

1

11

2

22

2

22

00

02

32

1

100% Ar

1% O2

2% O2

5% O2

10% O2

25% O2

(110)

(101) xO

2 [

%]

XRD

norm

alized inte

nsity


ZnO – work function

Change of ionisation potential due

to different surface orientation

Single crystal ZnO

(wurtzite)

IP = 7.0eV (0001)

IP = 7.8eV (000-1)

IP = 7.8eV (10-10)

5.0

4.5

4.0

3.5

3.6 3.2 2.8 2.4

work

function [

eV]

EF-EVB [eV]

ECB

ZnO

ZnO:Al

(1

10

)

(100)

-q

-q

+q

+q

cation anion

GaAs

TSF 518 (2009) 1197

Ranke (1983)

Swank (1967)

Jacobi (1984)

Materials 3 (2010) 4892


SnO2 – work function

5.5

5.0

4.5

wo

rk f

un

cti

on

[eV

]

4.0 3.8 3.6 3.4 3.2 3.0

ECB

SnO

2:F

EF-EVB [eV]

SnO2

SnO2:Sb

pO2

Change of ionization potential with surface termination

Change of surface termination with oxidation/reduction

Evac

ECB

EVB

EF

IP

TSF 518 (2009) 1197 Materials 3 (2010) 4892


Post deposition treatment

After O2 annealing

SnO2 (100% Ar)

In

ten

sit

y [

a.u

.]

15 10 5 0

(001)

(110)

binding energy [eV]

Large change of work

function and ionization

potential independent on

orientation

ionis

ation p

ote

ntial [e

V]


Conductivity relaxation of SnO2 (1 bar)

Almost no change of s with pO2 at 400°C

Equilibrium carrier concentration not achieved

time [h]

co

nd

ucti

vit

y [

S/

cm

]

oxyg

en

pressu

re [

Pa]

400°C 8

6

4

2

0

30 24 18 12 6 0

10 -1

10 0

10 1

10 2

10 3

10 4

10 5

time [h] co

nd

ucti

vit

y [

S/

cm

]

oxyg

en

pressu

re [

Pa] 600°C

3.0

2.5

2.0

1.5

1.0

0.5

0.0

36 30 24 18 12 6 0

10 -1

10 0

10 1

10 2

10 3

10 4

10 5


Relaxation at low pressure

Relaxation observed when starting from reduced surface

Saturated conductivity does not correlate with pO2

PCCP 13 (2011) 3223


Oxygen exchange of SnO2

Surface properties are crucial for oxygen exchange

reduced-SnO2 SnO2/In2O3 oxidized-SnO2


Relaxation measurements

Kinetics of oxygen exchange not accessible

contact failure

10% O2

100% Ar


Mobility and carrier concentration

Carrier concentration

(defect concentration) in

equilibrium

Carrier mobility different

for differently prepared

samples

Possible influence of

microstructure (texture,

grain size, segregation)

Necessary to understand

the evolution and the

control of microstructure


Topics





Surface Properties


Oxygen exchange





Band alignment

Type I alignment before contact Type II alignment

ECB

EVB

Energy band alignment described by band discontinuities

Each material combination has characteristic alignment

Semiconductor I Semiconductor II

Eg,1 Eg,2

DECB DECB

DEVB

DEVB

DECB: conduction band discontinuity (offset)

DEVB: valence band discontinuity (offset)


Interface ITO/Al2O3

Pinning in ALD-Al2O3 leads to modified band alignment

PCCP 11 (2009) 3049, Chem. Mater. 24 (2012) 4503


SnO2/Pt – interface chemistry

Reversible oxidation/reduction of Sn

Sn 3d

490 488 486 484

binding energy [eV]

co

un

t ra

te [a

rb. u

nits]

oxygen p

ressure

T=150°C

oxidation @ 150°C reduction @ 150°C

490 488 486 484

binding energy [eV]

Sn4+ Sn0

• 150°C: Sn0 <-> Sn4+

with intermediate

Sn2+ state

• 100°C: Sn0 <-> Sn2+

•Oxidation/reduction

not observable for

- large Pt islands

- bare SnO2 surface


Chemistry at buried interface

SnO2

EF

Pt

EVB

ECB

SnO2

EF

Pt

EVB

ECB

Reducing environment Oxidizing environment

H

O H

O

O O

SnO2 Pt SnO2 Pt

Sn0 Sn4+

Oxygen is reversibly transported to/from the interface

Barrier changes with oxidation/reduction


Summary

Transparent Conducting Oxides are important

technological materials

Surface and Interface properties can be systematically

adressed using photoelectron spectroscopy with in-situ

sample preparation

Work function and oxygen exchange determined by

doping, surface orientation and surface termination

Energy band alignment governed by orbital contribution

to the valence band density of states

Defects limit dopability and can modify the energy band

alignment


Acknowledgement

Frank Säuberlich, Yvonne Gassenbauer, Christoph Körber, André

Wachau, Jürgen Gassmann, Mareike Hohmann, Thorsten Bayer, Jonas

Deuermeier, Mirko Weidner, Anne Fuchs, Sebastian Siol

Paul Erhart, Péter Ágoston, Karsten Albe (TUD – Modelling)

Steven P. Harvey, Diana E. Proffit, E. Mitch Hopper, Thomas O. Mason

(Northwestern University)

German Science Foundation (SFB 595) MWN program)

BMBF (ZnO network project)

State of Hessen (LOEWE center AdRIA)

Work summarized in: J. Am. Ceram. Soc. 96, 331-345 (2013) Transparent Conducting Oxides: Electronic Structure – Property Relationship from

Photoelectron Spectroscopy with in-situ Sample Preparation

Documents

Do TCOs contribute to electrical fatigue of organic · PDF fileOxygen vacancies Indium oxide and tin oxide are truly intrinsically n-type semiconductors ... O1s C1s UPS 0.8Å 0Å 536