www.phi.com

Depth Profiling of Organic Photovoltaic and

OLED Materials by Cluster Ion Beams

J.S. Hammond1, S. N. Raman1, S. Alnabulsi1, N. C. Erickson2 and R. J.

Holmes2

1. Physical Electronics, 18725 Lake Drive East, Chanhassen, MN.

*2. University of Minnesota, Minneapolis, MN.

www.phi.com

Challenges for Organic Electronics Research

Efficiencies are based on optimum band matching and the

physical dispersions of components

– Work functions at discrete interfaces e. g. metal electrodes

– Diffusion lengths of charge carriers

Device lifetimes influence customer acceptance and profit

margins

– Atmospheric contamination (H20, O2) degrade chemistry

– Flexible designs challenge encapsulation technologies

– Breakdown mechanisms are not well understood

Chemical and molecular specific depth profiling is desired

– Depth resolution of a few nm with high sensitivity

– Cluster ion depth profiling interleaved with XPS and TOF-SIMS may

be solution

www.phi.com

Why use Depth Profiling for Organic Electronics?

New “nanotechnology” products use extremely thin

organic and polymer structures

– OLEDS

– Energy conversion materials and fuel cell membranes

Fabrication process producing molecular gradients can

result in significant differences in efficiency

Product degradation can result from molecular oxidation

and molecular diffusion

Spectroscopy with a nano-scale depth of analysis (XPS

and TOF-SIMS) needed for surface and depth profiling

characterization of molecular composition and diffusion

www.phi.com

Comparison of XPS and TOF-SIMS

XPS TOF-SIMS

Probe Beam Photons Ions

Analysis Beam Electrons Ions

Spatial Resolution 10 µm 0.10 µm

Sampling Depth(Å) 5-75 1-10

Detection Limits 0.01atom % 1ppm

Information Content Elemental Elemental

Chemical Chemical

Molecular

Quantification Excellent Std. needed

www.phi.com

XPS System Schematic

Electron Source Quartz Crystal

Monochromator

Aluminum Anode Sample

Rowland

Circle

Multi-channel

Detector

Al ka x-rays

Photoelectrons

X-ray Source

Hemispherical

mirror analyzer

Energy Analyzer

15 kv electrons

www.phi.com

Scanning Micro Focused X-ray Source

Al X-rays

Raster Scanned

Micro-Focused

Electron Beam

Al Anode

Monochromatic

Raster Scanned

Micro-Focused

X-ray Beam

Sample

Analyzer

Input Lens

Al X-rays

Electron Gun

Analyzer

Input Lens

Quartz Crystal

Monochromator

www.phi.com

0 200 400 600 800 1000 Binding Energy (eV)

c/s

-O

KL

L

-O

1s

-C

1s

-O

2s

280 285 290 295 300 Binding Energy (eV)

c/s

XPS survey spectra provide

quantitative elemental

information

High resolution XPS spectra

provide quantitative chemical state

information

C 1s

CH C-O C-O=O

C C O O

O O

CH2 CH2

PET Atom %

C 70.9

O 29.1

% of C 1s

CH 62.7

C-O 20.2

O=C-O 17.1

Typical XPS Spectra Poly(ethylene terephthalate)

www.phi.com

PHI TRIFT V nanoTOF

8

www.phi.com

Unique Polymer "Fingerprint” Identification

Using TOF-SIMS Spectra XPS shows identical spectra for both polymers

0 20 40 60 80 100 0

2.0E5

4.0E5

6.0E5

8.0E5

1.0E6

To

tal C

ou

nts

101 85 29 45 117

87 109

99

57 41

59

0 20 40 60 80 100 0

2.0

4.0E4

6.0E4

8.0E4

1.0E5

To

tal C

ou

nts

29 115 41 97 87

45

71 59 85

101 75

(CH2CHO)n

CH3

Polypropylene glycol

(CH2CH)n

O-CH3

Polyvinylmethyl ether

www.phi.com

Damage is observed.

275 280 285 290 295 300

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

Binding Energy (eV)

c/s

PET C 1s

surface

20 nm sputtered

522 524 526 528 530 532 534 536 538 540 542 544

0

500

1000

1500

2000

2500

3000

3500

Binding Energy (eV)

c/s

PET O 1s

surface

20 nm sputtered

O=C-O C-O

C-C

C-H

O=C O-C

500 eV Ar+ Sputter

Damage Accumulation Indicated by XPS

www.phi.com

275 280 285 290 295 300

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

Binding Energy (eV)

c/s

PET C 1s

surface

20 nm sputtered

522 524 526 528 530 532 534 536 538 540 542 544

0

500

1000

1500

2000

2500

3000

3500

Binding Energy (eV)

c/s

PET O 1s

surface

20 nm sputtered

O=C-O

O=C O-C

C-O

C-C

C-H

No damage observed.

10keV C60+ Sputter

No Damage Observed by XPS

www.phi.com

Altered Volume & Sub-surface Damage

15 keV Ga 15 keV C60

1 nm

Graphic courtesy B. Garrison & Z. Postawa.

Non-C60 sputter sources result in the

accumulation of sub-surface damage.

Residual C60 sputter damage is mostly

removed with the next C60 impact event.

www.phi.com

Molecular Dynamics: Ar9000+ GCIB on PS/Ag

The permission of the pictures is by courtesy of Professor Zbigniew Postawa, Jagiellonian University (Poland); L. Rzeznik, B. Czerwinski, B.J. Garrison, N.

Winograd and Z. Postawa, "Microscopic Insights into the Sputtering of Thin Polystyrene Films on Ag{111} Induced by Large and Slow Ar Clusters", J. Phys.

Chem C 112 (2008) 521.

0.5eV/Ar atom

2.0eV/Ar atom

10eV/Ar atom

Lateral Jet Momentum Transfer Sputtering

www.phi.com

Single Layer, Graded Composition OLED

structure

www.phi.com

Previous Graded Composition OLED Research

15

www.phi.com

Graded Composition Structures Overview

Graded Electron and Hole Transport Materials (ETM

and HTM) with a green phosphorescent emitter

100% HTM at the Anode and 100% ETM at the Cathode

Result

– At 600 cd/m2

• Peak external quantum efficiency ƞEQE = (19.3 + 0.4)%

– Corresponding to internal quantum efficiencies approaching 100%

• Peak power efficiency ƞp = (66.5 + 1.3) lm/W

These structures are simple to grow

But need to confirm the chemistry of these structures

is essential

16

www.phi.com

Analytical Technique

Thin film samples were characterized in a PHI

VersaProbe II Scanning XPS system equipped with an

Ar+ Gas Cluster Ion Beam source for depth profiling

– The GCIB source can be operated

• With Beam energies from 2.5 kV to 20 kV

• Cluster size from <1000 to 5000

The XPS system has excellent charge neutralization

capability to compensate for differential charging at

various interfaces

17

www.phi.com

Structure of the two molecules for OLED

Tris(4-carbazoyl-9-ylphenyl)amine

(TCTA) (Hole Transfer Material)

Bathophenanthroline

(BPhen) (Electron Transfer Material)

18

www.phi.com

C 1s and N 1s Spectra for TCTA and BPhen

19

395 400 405 410 0

0.2

0.4

0.6

0.8

1

Binding Energy (eV)

Norm

aliz

ed

In

ten

sity

N 1s TCTA

BPhen

395 400 405 410 Binding Energy (eV)

Norm

aliz

ed

In

ten

sity

N 1s

TCTA

BPhen

280 285 290 295 0

0.2

0.4

0.6

0.8

1

Binding Energy (eV)

Norm

aliz

ed

In

ten

sity

C 1s

TCTA

BPhen

280 285 290 295 300 Binding Energy (eV)

No

rma

lize

d In

ten

sity

C 1s

TCTA

BPhen

1.8 eV Separation

www.phi.com

Sample Structure for OLED

20

Silicon

100 nm BPhen/TCTA gradient

With and without dopant

Silicon

100 nm TCTA

100 nm BPhen

Glass

150 nm ITO

100 nm BPhen/TCTA gradient

With Ir (ppy3) dopant

1 nm LiF

75 nm Al

DEVICE

XPS TEST STRUCTURES

www.phi.com

GCIB Depth Profile of TCTA:Bphen/SiO2/Si

21

0 50 100 150 200 250 0

20

40

60

80

100

Sputter Depth (nm)

Ato

mic

Concentr

ation (

%)

N1s (BPhen) N1s (TCTA)

Silicon

100 nm TCTA

100 nm BPhen

Ar2500,10 kV, 2 nA, 3 mm x 3 mm

395 400 405 410

0

100

200

Binding Energy (eV)

N1s (BPhen)

N1s (TCTA)

N 1s

C1s

Si2p

O1s

4 eV/atom GCIB

www.phi.com 22

0 50 100 150 200 250 0

20

40

60

80

100

Sputter Depth (nm)

Ato

mic

Concentr

ation (

%)

C1s

Si2p

O1s

N1s (BPhen)

5x N1s (TCTA)

5x

Silicon

100 nm TCTA

100 nm BPhen

Ar2500,10 kV, 2 nA, 3 mm x 3 mm

395 400 405 410

0

100

200

Binding Energy (eV)

N1s (BPhen)

N1s (TCTA)

N 1s

4 eV/atom GCIB

GCIB Depth Profile of TCTA:Bphen/SiO2/Si

www.phi.com 23

0 50 100 150 0

20

40

60

80

100

Sputter Depth (nm)

Ato

mic

Con

ce

ntr

atio

n (

%)

C1s

Si2p

O1s

N1s (BPhen) N1s (TCTA)

390 395 400 405 410 Binding Energy (eV)

N1s (BPhen)

N1s (TCTA)

N 1s

Ar2500,10 kV, 2 nA, 3 mm x 3 mm Si

100%

0%

100 nm Native Oxide

4 eV/atom GCIB

GCIB Depth Profile of Bphen:TCTA/SiO2/Si

www.phi.com 24

0 50 100 150 0

20

40

60

80

100

Sputter Depth (nm)

Ato

mic

Con

ce

ntr

atio

n (

%)

C1s

Si2p

O1s

390 395 400 405 410 Binding Energy (eV)

N1s (BPhen)

N1s (TCTA)

N 1s

Ar2500,10 kV, 2 nA, 3 mm x 3 mm

N1s (BPhen)

5x

N1s (TCTA)

5x

Si

100%

0%

100 nm Native Oxide

4 eV/atom GCIB

GCIB Depth Profile of Bphen:TCTA/SiO2/Si

www.phi.com 25

395 400 405 410 0

20

40

60

80

Binding Energy (eV)

N1s (BPhen) N1s (TCTA)

Binding Energy (eV) 395 400 405 410

0

20

40

60

80

N1s (BPhen)

N1s (TCTA)

Si

100%

0%

100 nm Native Oxide

N 1s Spectra from GCIB Depth Profile of

Bphen:TCTA/SiO2/Si

www.phi.com 26

Si

100%

0%

100 nm Native Oxide

Ir(ppy3) Dopant

100

0 50 100 150 0

20

40

60

80

Sputter Depth (nm)

Ato

mic

Concentr

ation (

%)

C1s

Si2p

O1s

N1s (BPhen) N1s (TCTA)

Ir 4f (500x)

Ir 4f concentration 0.04 at %

Ar2500,10 kV, 2 nA, 3 mm x 3 mm

4 eV/atom

Ir Emissive Compound from GCIB Depth Profile of

Bphen:TCTA/SiO2/Si

www.phi.com

Composite Multi-layer

Inverted Organic Photovoltaic Device

27

Ag Cap

PEDOT:PSS

P3HT:TiO2 Nanorods

TiO2

ITO

Glass

Bulk Heterojunction

Electron Transport Layer

Hole Transport Layer

Sample is comprised of various material types: Metal / Polymer / Oxide

www.phi.com

Composite – Organic Photovoltaic Multi-layer

28

20 kV C60 Cluster

0 20 40 60 80 100 0

10

20

30

40

50

60

70

80

90

100

Sputter Time (min)

Ato

mic

Concentr

atio

n (

%)

Ag 3d

In 3d5

Sn 3d5

C 1s

C 1s

Ti 2p

O 1s

TiO2

O 1s

ITO

O 1s

Glass

S 2p (5×)

S 2p (5×)

Si 2p

0 20 40 60 80 100 0

10

20

30

40

50

60

70

80

90

100

Sputter Time (min)

Ato

mic

Concentr

atio

n (

%)

C 1s

S 2p

O 1s

Si 2p

Ag 3d

In 3d5

Sn 3d5

Ti 2p

Ag

Cap

PE

DO

T:P

SS

P3H

T:T

iO2

TiO

2

ITO

Gla

ss

• With Compucentric Zalar Rotation™

• Well defined layers

• Consistent sputter speed throughout the multilayer stack

www.phi.com

Ag

Cap

PE

DO

T:P

SS

P3H

T:T

iO2

TiO

2

ITO

Composite – Organic Photo Voltaic Multi-layer

0 20 40 60 80 100 120 140 160 0

10

20

30

40

50

60

70

80

90

100

Sputter Time (min)

Ato

mic

Concentr

atio

n (

%)

C 1s Ti 2p

S 2p (5×)

O 1s

Ag 3d

In 3d5

Sn 3d5

0 20 40 60 80 100 120 140 160 0

10

20

30

40

50

60

70

80

90

100

Sputter Time (min)

Ato

mic

Concentr

atio

n (

%)

Ti 2p

O 1s

Ag 3d

In3 d5

Sn 3d5

C 1s

C 1s

S 2p (5×)

S 2p (5×)

• Poor layer definition

• Large variation in sputter rates

(13 ev/atom)

20 kV Ar1500+ Gas Cluster, 13.3 eV/atom

www.phi.com

Fabrication Process versus OLED Efficiency

Two fabrication processes of small molecular OLED’s

– Spin coating (wet process): typically easier fabrication, lower efficiency

– Evaporation has higher efficiency

Emissive layers (ELs) studied for high efficiency green OLED’s

– Guest

– Bis[5-methyl-7-trifluoromethyl-5H-benzo©(1,5)naphthyridin-6-one]iridium (picolinate) (CF3BNO)2IrPLA

– Host

– 4-4’-bis(carbazol-9-y)biphenyl (CBP)

– Wet efficiency 70 lm W-1

– Dry efficiency 21 lm W-1 WHY?

www.phi.com

Wet versus Dry Process: XPS Depth Profiles

• Same Ir composition similar when normalized by film thickness

• Wet process Ir guest has higher concentration at interface relative to dry process

www.phi.com

• TOF-SIMS with C60

sputtering shows no change

in molecular structure

between outer surface

spectra and 25 seconds

sputtering

• XPS profile with C60 is not

a result of molecular

decomposition

www.phi.com

• Same Ir composition similar when normalized by film thickness

• Wet process Ir guest has higher concentration at interface relative to dry process

Wet versus Dry Process: TOF-SIMS Depth Profiles

www.phi.com

Wet versus Dry Process Model of Efficiencies

• Higher relative guest concentration at HT interface give lower turn-on voltage

• More hole trapping in dry process

• Wet process efficiency is ~ 3.5x higher than dry process

www.phi.com

Cluster Ion Source Summary

Sample Type

Ion Gun Type

Ar Monoatomic C60 Cluster Ar Gas Cluster

Metal

• Preferred approach • Carbide formation 3 - 20 % on reactive

metals

• Slow etch rates

• Small differential sputtering for some alloys

Ceramics

• Differential

sputtering and

chemical change

• Ideal Approach on Soda-lime glass

• Works well on metalloid oxides and

conducting oxides: ITO / InGaO

• Slow etch rates

• Small oxygen loss for some oxides at high

acceleration voltages (ITO)

• Not suitable for transition metal oxides

TiO2 / HfO2 / WO3

• Not suitable for transition metal oxides

TiO2 / HfO2 / WO3

Organics

Polymers

• Significant

Chemical Damage

• Suitable for Type-II degrading polymers

(5× to 10× between Polymers and

Metal oxides)

• Suitable for Type-II degrading polymers

(50× to 100× between Polymers and Metal

oxides)

• Some Type-I crosslinking polymer to a

depth of a few hundred nanometers

• Ideal Approach for Type-I crosslinking polymers,

unless an inorganic filler is present

Complex

Materials

• Sample Dependent • Able to maintain sputter speed across

layers, with low level damage on Carbon

• Large variation in sputter rate causes depth

resolution problems

Semiconductors • Preferred approach • Carbide formation on reactive metals • Slow etch rates, variable sputter rates, surface

roughening

35

The Choice of Ion Source for XPS Compositional Depth Profiling Is Application Dependent

www.phi.com

Summary and Conclusions

Using Ar GCIB depth profiling and XPS we have been able to

confirm the graded composition emissive layer structures

The chemistry of the materials remains unaltered by the Ar

cluster ion beam

Sharp interfaces measured for layered structures

The depth profile of the Ir from Ir(ppy3) dopant is clearly

observable with a concentration of < 0.04 at. %

GCIB depth profiling is reliable reproducible and fast for these

materials

C60 provides excellent depth profiling of metal/organics/metal

oxides/glass multi-layer structures

Similar use of C60 and GCIB combined with TOF-SIMS can

provide more molecular information

36