+ (1 - The Hong Kong Polytechnic University

polyu.edu.hk

+ (1 - The Hong Kong Polytechnic University

Professor Chau Wai-yin Memorial Lecture on

Science and Science Education 2009

Presented by:

Prof. Donal Bradley

Deputy Principal , Faculty of Natural Sciences

Lee-Lucas Professor of Experimental Physics

Imperial College London


“Twenty Years of Plastic Electronics - The Science

and Application of Molecular Electronic Materials”

Professor Donal Bradley FRS

Lee-Lucas Professor of Experimental Physics,

Centre for Plastic Electronics & Department of Physics,

The Blackett Laboratory, Imperial College London

D.Bradley@Imperial.ac.uk

Chau Wai-yin Memorial Lecture,

Hong Kong Polytechnic University, Hong Kong

Friday 27th November 2009


Talk Outline

Introduction to Molecular Electronic Materials

Basic Properties & Application Potential

A Discovery and First Applications:

Conjugated Polymer LEDs for Displays

What’s Next:

Developing Further Applications

Some Recent Research:

High Mobility + High Gain

Conformation Control


“Just one word ….. Plastics”

“I just want to say one word to you. Just one word… Plastics.”

Mr McGuire to Benjamin Braddock in The Graduate (1967)

• Plastics aka Polymers aka Long chain molecules

• Structural materials, lightweight and easily manufactured

• Cheap / brightly coloured / insulating / good optical properties

• Function and processing - cheap and cheerful & jolly useful


Functional Plastics

• Imagine a combination of the desirable processing traits of

plastics with for example:

The conduction of metals,

The absorption / emission of dyes and phosphors that

give

colour to our displays and provide efficient lighting,

The controllable conduction and light emission /

absorption / detection of traditional semiconductors

that

power electronic logic, underpin the digital revolution in

entertainment and promise efficient solar energy.

• That is the exciting prospect for conjugated polymers in which

electronic charge can delocalize and move as for a traditional

conductor / semiconductor rather than be localized and static


Conjugated Polymers

Alternating sequences of single / double (sp 2 p z hybrid)

H

H

and / or single / triple (sp p z p y hybrid) C-C bonds

H

H

H

give delocalised π-electron systems.

Polyene

Benzene

1 st Generation Polyfluorenes:

Homopolymer:

Copolymer:

Fluor: highly fluorescent

Ene: conjugated molecule

PFO:

H 17 C 8

C 8 H 17

n

H 17 C 8

C 8 H 17

R

n

Copolymer R-moiety:

F8T2:

S

S

F8BT:

N

S

N

etc

Solution Photoluminescence:

• High chemical purity, good stability,

thermal processing, liquid crystallinity

• Generation one comprises homopolymers

and simple (alternating) copolymers


Conjugated Polymers as Molecular Solids:

Processing & Environment

Molecule-molecule interactions in a macroscopic sample typically involve

weak van der Waals forces (≈ 1/20 covalent bond strength): This enables

processing

• Weak interactions can generate complex structures: LC phases, “breath figures”, etc

• Interest in many different phases to enable / tune desired properties and

unravel complex structure / property relationships:

Tuning Physical

Structure

Solutions e.g. dye lasers

Increasing

Order/Interaction

Dispersions e.g. photocopiers, CD-R

Nature successfully engineers excellent

performance:

Solid films e.g. light-emitting- & photo-diodes,

transistors, lasers

• Chlorophyll molecules in Light Harvesting Complex

• Carotenoid molecules in Photoreaction Centre & Retina


Molecular Electronic Materials:

Attributes & Applications

µ -analysis

Display

Lighting

Laser

Large α

Datacomm

Large ΦPL

Large E B

Low µ

Anisotropic

Localised

Ψ

Low εR

Low ρ

Disordered

Amplifier

Large ∆n

Large ∆λ

Large J

Telecomm

Photodiode

Solar Cell

FET


Key Electronic Structure Features

• Localised wavefunctions (k not a good quantum number):

• Discrete electronic transitions

(oscillator strength)

• Absorption coefficient ≈ 10 5 cm -1 & radiative decay time ≈ 1 ns

• Coupling of electronic and local vibrational degrees of freedom

(i.e. not rigid band semiconductors with Bloch wave states)

• Vibronic structure in spectra with Stokes’ shift between

absorption and emission

(bandwidth)

• Relaxed charges occupy polaron-like states with optical transition

energies within the optical gap

(switching, loss)

• Hopping transport

(mobility)

• Strong interactions amongst electrons and due to small dielectric

constant (ε ≤ 4) there is weak screening:

• Strongly Bound Excitons (Coulomb) E B ≈ 0.2 to 0.5 eV (radiative)

• Distinct Triplet States (Exchange) J ≈ 0.5 to 1 eV (EL efficiency,

photochemistry)


Consequent Optical Properties

Configuration Coordinate Diagram:

S 1

S 1

S 0

S

3

0

2

1 2 3

1

0

0

Absorption & PL

3

2

1

0

Q

0 - 0

0 - 2 0 - 1

0 - 3

S 0 to S 1 absorption

0 - 0

0 - 1

0 - 2

0 - 3

S 1 to S 0 emission

Wavelength (nm)

1 2 3

0

• Typical vibrational quanta: hv = 0.15 - 0.20 eV

i.e. large bandwidth (∆λ)

• Stokes shift between absorption & emission:

i.e. reduced self-absorption

Q

g(n) = S n e -S /n!

Singlet and Triplet Manifolds:

Singlet Manifold

S 4

S 3

S 2

S 1

Abs.

S 0

1

√2

ISC

Fluor.

Triplet Manifold

S = 0 S = 1

-

Phosphorescence

1

√2

+

T 3

T 2

• Spin forbidden transitions between

manifolds c.f. Par (S = 0) and Ortho (S

= 1) Helium

• Spin orbit coupling promotes ISC and

phosphorescence: heavy metal complexes

T 1

T 0


Key Features: Anisotropy & Disorder

• Anisotropic electronic structure (strong intrachain vs weak interchain bonding)

manifest in optical and electrical response.

• Can be a large effect e.g. optical dichroism ≈ 100:1 in highly oriented polydiacetylenes.

• Anisotropy allows:

• polarised light emission

• large optical birefringence

• control over the optical gain

• control over charge transport

EL Intensity

6

ITO:r-PPV:PFO:Ca

4

25:1 @ 458 nm

2

0

400 450 500 550 600

Wavelength (nm)

Relative PL intensity

∆PL ≥ 300:1

420 440 460 480 500

Wavelength (nm)

• Processing and molecular environment play

an important role in controlling the optical

and electrical properties:

• Energetic & positional disorder.

• Static & dynamic effects.

Energy

Negative carrier transport states

Density of States

2

Mobility, ?[cm /Vs]

0.1

0.01

0.001

0.0001

Oriented LC Film

Positive carrier transport states

0.00001

Spin Coated Films

200 400 600 800

E 1/2 [(V/cm) 1/2 ]


There are also interesting structural confinement effects

e.g. glass transition temperature not the same as in bulk

Relevance of the Nano-scale?

• Not just a matter of working with nanometric sized objects.

• Nanometer dimensions provide the natural length scale on which many

properties can be tuned:

• Kuhn segment lengths for conjugated polymers: ~ 10 - 30 nm

• Exciton diffusion lengths: ~ 5 - 50 nm

• Förster transfer radii: ~ 1 - 10 nm

• 1/e absorption depths: ~ 20 - 200 nm

• Together with limitations on carrier transport (hopping between nm

domains) this leads to typical device layer thicknesses ~ 10 - 100

nm

• This in turn leads to important optical confinement and interference

effects in near UV through visible to near IR spectral range of interest

(nd ≈ λ) e.g. cavity resonances in diodes, etc


Plastic Electronics Applications

Electroluminescence for

Displays & lighting

Xuhua Wang

Peter Levermore

& Xuhua Wang

Jingsong Huang

FETs

Solar Cells

& Photodetectors

Optical Communications

Imaging & Analysis Systems

Monika Voigt

Monika Voigt

Jingsong Huang

Ruidong Xia

Tom Wellinger

Flexible / Integrable / Versatile


Talk Outline

Introduction to Molecular Electronic Materials

Basic Properties & Application Potential

A Discovery and First Applications:

Conjugated Polymer LEDs for Displays

What’s Next:

Developing Further Applications

Some Recent Research:

High Mobility + High Gain

Conformation Control


Conjugated Polymers

1970s - 1980s

R

R

trans-Polyacetylene

Conductivity: Synthetic Metals

(Chemistry Nobel Prize 2000:

Alan Heeger, Alan McDiarmid, Hideki Shirakawa)

n

R'

R'

n

Polydiacetylene [PDA]

Nonlinear Optics

1990s - 2000s

R

R

R'

n

Poly(p-phenylenevinylene)s [PPVs]

LEDs, Lasers, Photodiodes

Dow, Sumitomo, Covion, Merck….

H 17 C 8

C 8 H 17

R

Polyfluorenes [PFs]

LEDs, Lasers, FETs, PDs

Dow, Sumitomo, Covion, …

n

S

Polythiophenes [PTs]

Conductivity, FETs, PDs & Solar Cells

Bayer (Starck), Agfa, Merck, ….

n


Discovery: Cavendish Laboratory 1989

• 1989 Jeremy Burroughes study on polymer FETs (RHF supervisor)

• Problem to find suitable polymer insulators (gate dielectrics):

Require:

(i) Dense & pin-hole free thin films

(ii) Low conductivity & impurity free

• DB PhD (1983-’87 Cavendish Laboratory, RHF supervisor) & Toshiba Research

Fellowship (Japan 1987-’88) studies on precursor route poly(p-phenylenevinylene)

(PPV):

4µm

(D.D.C. Bradley, J.Phys.D:Applied Physics 20 (1987) 1389; Wessling & Zimmerman (The Dow Chemical Company))

• PPV seemed to meet the requirements:

σ ≤ 10 -15 S/cm, N S ≤ 10 -6 spins/monomer

• Breakdown test on PPV film sandwiched between Al electrodes: Feb. ‘89


Making Light of the Breakdown Problem!

Al (130 nm)

PPV (90 nm)

Al/Al 2 O 3 (11 nm)

An OLED

Glass

30

20

Current (mA)

20

10

0

5 10 15 20 25 30

Bias Voltage (V)

Luminance (arb. units)

10

0

0 10 20 30

Current Density (mA/cm

2 )

K.E. Ziemelis 1989

• Light emission above diode turn-on @ 25 V

• Linear luminance vs current density

• Characteristic emission spectrum of PPV: Green light

• η ≤ 0.01% photons/electron

(i.e. E ≈ 2.8 x 10 6 V/cm)


Who In Their Right Mind Would Patent This?

• Only 0.01% efficiency but not optimised in any way:

Large injection barriers: ∆φ > 1eV

Poor transmission through metal electrodes

Low photoluminescence quantum efficiency: Φ ≈ 24%

• Target 1% efficiency (N.B. no GaN/GaInN, AlGaAs MQW diodes at that time)

• Red, Green, Blue possible via chemistry (N.B. No good blue diodes at that

time - main focus was on SiC)

• Desirability of solution fabrication process was recognised

• Potential for fast response as needed for video

• Decided to patent: Technology transfer office (Lynxvale) had no money

to cover patent costs!


Conjugated Polymer LED Impact

•First paper published in Nature 1990

(Has > 6800 ISI citations, the most cited paper in Plastic Electronics)

“Light-Emitting Diodes based on Conjugated Polymers” J.H.

Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K.D.

Mackay, R.H. Friend, P.L. Burn, and A.B. Holmes: Nature

347 (1990), 539 - 541

• Major Scientific Impact: Triggered an explosive growth in the

science and application of Plastic Electronic materials and

devices

c.f. Nature Physics Portal - Looking Back (Most prominent

Physics papers published in Nature since 1869):

www.nature.com/physics/looking-back/burroughes/index.html


Polymer Electroluminescence 1990-1991

An early Cambridge LED (1990):

(Area = 4mm x 2mm)

Luminance [cd/m 2 ]

An early Cambridge LED (1991):

10

1

10 -1

10 -2

10 -3

Adam Brown

L cd/m2

I [mA]

10 0

10 -1

10 -2

10 -3

10 -4

10 -5

Current [mA]

10 -4

-5 0 5 10 15 20 25 30

Voltage/V

10 -6

0.01 cd/m 2 0.1 cd/m 2 1 cd/m 2

V 8.25 15.75 25.50

J [A/m 2 ] 0.046 0.44 5.6

cd/A 0.22 0.22 0.18

lm/W 0.08 0.04 0.02


Device Physics & Ultimate Limits

Bipolar Injection EL Flow Diagram:

Internal Quantum Efficiency:

ANODE

"LEA K AG E" CURRENTS

CATHODE

η int = γ {β φ fl + (1 - β)φ ph }

h + COULO MB e -

CAPTURE

β

SINGLET

φ fl

ISC

Non-

Radiativ e

Deca y

GROUND STATE

(1 − β)

TRIPLET

φ ph

• γ = No of Coulomb capture events

per injected charge.

• β = branching ratio for singlet

exciton formation during electron-hole

capture process.

• Spin independent formation process

leads to β = 0.25 i.e. dictated by spin

state degeneracy.

• Can therefore expect much higher

efficiency following optimisation.


OLED Device Physics & Engineering

(A) Exciton Formation

• Carrier Injection

• Carrier Transport

• Carrier Combination

(B) Exciton Decay

• Photophysics of Decay

Radiative vs Non-radiative

(C) Light Extraction

• Vertical Cavity Effects

• In-plane Waveguiding

• Daylight Contrast

(D) Stability

Relate to

internal

efficiency

Relates to

subsequent

external efficiency,

colour hue

& saturation

• Encapsulation against H 2 O & O 2

• Interface & Redox Chemistry, Morphology


Chemical

Tuning:

Tokmoldin

et al

CIE Chart:

(CDT Ltd)

Normalised Electroluminescence

OLED Spectral Engineering

Microcavity:

Lidzey et al

White Blend:

99% F8DP; 0.6% F8BT;

0.4% RedF

(0.3, 0.34)

Belton et al


Polymer Electroluminescence 1991 and 2000

An early Cambridge LED (1991):

A Sheffield Blend LED (2000):

Luminance [cd/m 2 ]

10

1

10 -1

10 -2

10 -3

Adam Brown

L cd/m2

I [mA]

10 0

10 -1

10 -2

10 -3

10 -4

10 -5

Current [mA]

Luminance [cd/m 2 ]

10 4

1000

100

10

10 5 0.01

Xuhua Wang

L cd/m2

I [mA]

1000

100

10

1

0.1

Current [mA]

10 -4

-5 0 5 10 15 20 25 30

Voltage/V

10 -6

1

0 2 4 6 8 10

Voltage [V]

0.01 cd/m 2 0.1 cd/m 2 1 cd/m 2

V 8.25 15.75 25.50

J [A/m 2 ] 0.046 0.44 5.6

cd/A 0.22 0.22 0.18

lm/W 0.08 0.04 0.02

100 cd/m 2 1,000 cd/m 2 10,000 cd/m 2

V 3.2 3.8 5.3

J [A/m 2 ] 25 126 1260

cd/A 4 8.2 7.7

lm/W 3.9 6.8 4.6


Polymer Electroluminescence after 15 years

An early Cambridge LED (1990):

(Area = 4mm x 2mm)

Epson Prototype 2004: 40” diagonal

(Area ≈ 555 mm x 830 mm)


Fast Forward: 1990 - 2007

• 1992 Efficiency reaches η = 1% photons/electron:

A.R. Brown et al Appl.Phys.Lett. 61 (1992), 2793

• 1992 Cambridge Display Technology Ltd founded:

Founders: DDCB, JHB, RHF, Andrew Holmes, Paul Burn, Arno Kraft

• PLED product development: Philips (‘02-’04), Osram (‘03-’07), Delta,

MicroEmissive Displays, Add-Vision, X’ian Smart Displays

World’s Smallest TV

• 2004 CDT lists on USA Nasdaq Exchange

• 2007 CDT acquired by the Sumitomo Chemical Company


OLED Device Engineering 2009

Gravure-printed

V

LED Structure:

Electrode

Injection Layer

Transport & Emission Layer(s)

Injection Layer

Electrode

Substrate

ITO-free/flexible

Peter Levermore

& Xuhua Wang

Micropatterned

Dae-Young Chung et al

Dong-Seok Leem &

Xuhua Wang

ITO-free/flexible

Inverted/flexible

Inverted

Nurlan Tokmoldin &

Xiangjun Wang

Xuhua Wang et al

Dae-Young Chung et al


Polymer Electroluminescence 2008

Imperial Lumation TM Green LED:

Imperial ITO-free LGreen LED:

Peter Levermore

Peter Levermore

10,000 cd/m 2 20,000 cd/m 2 40,000 cd/m 2

V 7.2 9.2 14

J [mA/cm 2 ] 119 250 512

cd/A 8.4 8.0 7.8

lm/W 3.7 2.7 1.75

5,000 cd/m 2 10,000 cd/m 2 20,000 cd/m 2

V 9 12 18

J [mA/cm 2 ] 45 100 222

cd/A 11 10 9

lm/W 3.8 2.6 1.6


Talk Outline

Introduction to Molecular Electronic Materials

Basic Properties & Application Potential

A Discovery and First Applications:

Conjugated Polymer LEDs for Displays

What’s Next:

Developing Further Applications

Some Recent Research:

High Mobility + High Gain

Conformation Control


Plastic Electronics Applications

Electroluminescence for

Displays & lighting

Xuhua Wang

Peter Levermore

& Xuhua Wang

Jingsong Huang

FETs

Solar Cells

& Photodetectors

Optical Communications

Imaging & Analysis Systems

Monika Voigt

Monika Voigt

Jingsong Huang

Ruidong Xia

Tom Wellinger

Flexible / Integrable / Versatile


Some Visions of Future OLED Lighting

Makoto Tojiki

Jonas Samon


Polymer Diode Structures: LEDs and PDs

Basic diode characteristics:

J

V

A: Dark response

B: Light response

J (mA/cm 2 )

(i)

B

A

V (V)

(i): Quadrant of interest for OLEDs (Dark)

(ii): Quadrant of interest for Solar Cells (Light)

(iii): Quadrant of interest for photodetectors (Light)

(iii)

(ii)

• Polymer light emitting diodes under study for displays and lighting

• Polymer photodiodes under study for:

Solar energy conversion

Photodetection: UV detectors; Fast detectors for data links; Optical scanners/imaging

Direct competition with Si devices: Similar QE (~ 50%), responsivity (~ 0.3 A/W) and

limit of detection (~ 10 -13 - 10 -14 W) but slower rise time (µs vs ns). Spectral selectivity

without requirement for colour filters.


MV Fluidic Technology Platform

Organic devices integrated on-chip:

• Organic LEDs & Photodetectors:

Discrete, array, variable size, selectable spectral characteristics

• Simple, low-cost deposition procedures compatible with planar

microfluidic chip structures: Coating and printing

• Matched molecular light sources & photodetectors

• Quantitative analysis at a throw-away cost

Channel wall

photodetector

light

Channel wall

light

pLED

“Polymer Detection System” J.C. De Mello, A.J. De Mello, D.D.C. Bradley; UK Patent Application 0028482.8

(22/11/00); Granted as GB 2369428B; US 6,995,348; EP 1336089; JP 2002-544636; HK 1045190


First ‘Manufacturable’ Prototype: 2008

Discrete component demonstrator

OLEDs and OPDs fabricated by OTB


Polymer Optical Gain Media

• Expect high gain (~ 50 cm -1 ) & large bandwidth (~ 100 nm)

• Coverage of the full visible spectrum, compatible with POF

1-D DFB:

η ≈ 10%

pJ-nJ

few Hz

few ns

Pulses

Quasi 4-level:

S 1

S 0

0

1

2

Absorption

1

0

ASE

2

2-D DFB:

George Heliotis, Ruidong Xia, Paul

Stavrinou, Tom Wellinger, …..


Polymer Gain Media Research Themes

• Fundamental Laser- and Photo-physics and Sensing/Metrology:

Relative ease of fabrication enables extensive studies to test/develop theory

Stimulated emission characteristics help to understand excitation

transfer & decay dynamics

Resonant structures are very sensitive to changes (EPSRC S&I

Nanometrology)

• Functional components to complement passive polymer waveguide

structures:

Opportunities to use conjugated polymer gain media to provide integrable

functionality for:

Polymer Lightwave Circuits (PLC) (telecomm splitters for FTTX)

Polymer Optical Fibre (POF) (visible automotive data systems)

Short haul polymer photonic data systems & chip-to-chip comms

Organics for broad band amplifiers & optical switches (UPC; POLYCOM)

• Hybrid structures:

CMOS/Nitride/Organic hybrids (HYTEC; 1000 microemitters/mm 2 ; HYPIX)

Organic/Plasmonic hybrids (EPSRC Programme Grant)

• Electrically pumped organic lasers & amplifiers:


Talk Outline

Introduction to Molecular Electronic Materials

Basic Properties & Application Potential

A Discovery and First Applications:

Conjugated Polymer LEDs for Displays

What’s Next:

Developing Further Applications

Some Recent Research:

High Mobility + High Gain

Conformation Control


Polymer Thin Films: Mobility vs PLQE

• High mobility and high PLQE are important for:

PM-OLEDs

Light sources for data communications

Light emitting TFTs

Laser diodes

• A long standing issue for conjugated polymers:

How does one simultaneously achieve high PLQE

and charge carrier mobility?

Poly(3-hexylthiophene)

Poly(9,9-dialkylfluorene)

S

S

S

vs

Strong lateral π-overlap

Chains well separated

(ordered lamella structure) (even in crystalline films)

µ ≈ 10 -1 cm 2 /Vs µ ≈ 10 -3 cm 2 /Vs

PLQE ≤ 5% PLQE ≥ 50%


Intermolecular Packing: Transport

Processing:

Oriented LC ordering

Crystallization

Spin-coating

µ PI

0.1

E

2 /Vs]

Mobility, ?[cm

0.01

0.001

0.0001

0.00001

Oriented LC Film

Crystalline Film

Spin Coated Films

200 400 600 800

E 1/2 [(V/cm) 1/2 ]


Structured Heterogeneity

• Best of Both Worlds:

High intra-chain µ plus preferential inter-chain

hopping sites among otherwise isolated chains

• Want hopping sites to substantially enhance

transport but not quench PL:

Larger effect than statistical fluctuations

in geometry/distance found in LC glass

• Want to avoid backbone re-design:

Control peripheral chemistry to

heterogeneously mediate packing

• Don’t want elaborate processing:

Ideally just spin-coat

B.K. Yap et al Nature Materials 7 (2008), 376-380


B.K. Yap et al Nature Materials 7 (2008), 376-380

Poly(dialkylfluorene) Copolymers

Chemical structures: Yamamoto and Suzuki route synthesis of

polyfluorenes with 9,9-dioctyl & 9,9-di(2-methyl)butyl substituents

PFO

Y80F8:20F5

S50F8:50F5

Synthesis route, backbone architecture, conformation, solubility

and film morphology/uniformity are intimately entangled:

An interesting problem in materials design


Absorbance, PL & Refractive index

Absorbance [10 5 cm -1 ]

0

0

S50F8:50F5

Y80F8:20F5

PFO

PFO

α (10 5 cm -1 ) n 0-1 PLQE n (850 nm) ρ/ρ PFO

PFO 2.1 1.770 50±5% 1.590 1

Y80F8:20F5 2.7 1.960 70±5% 1.705 1.24

S50F8:50F5 2.7 1.945 60±5% 1.715 1.27

T g (°C)

51

82

100

B.K. Yap et al Nature Materials 7 (2008), 376-380


Time of Flight Hole Mobility

S50F8:50F5

0.1

Y80F8:20F5

2 /Vs]

0.01

Oriented LC Film

PFO

Mobility, ?[cm

0.001

0.0001

Crystalline Film

Spin Coated Films

0.00001

200 400 600 800

High mobilities:

µ ≤ 6 x 10 -2 cm 2 /Vs (E = few x 10 4 V/cm)

(also achieved in top gate TFT structures)

E 1/2 [(V/cm) 1/2 ]

B.K. Yap et al Nature Materials 7 (2008), 376-380


Y80F8:20F5 2 nd Order DFB Lasers

• 1-D, 75% grating, Λ = 290 nm

(185 x 165 µm 2 )

• 10Hz, 10ns, 390 nm excitation

20 x

2 x

I th

η = 10 ± 2 %

(c.f. 2-2.5% for PFO)

• I th = 0.1 nJ/pulse (0.3 µJ/cm 2

i.e. 30 W/cm 2 )

(c.f. 100 W/cm 2 for PFO

including β-phase)

• ASE measured gain 87 cm -1

and mode loss 1.8 cm -1

B.K. Yap et al Nature Materials 7 (2008), 376-380


Metamaterials-Inspired Conformation Approach to

Polymer Optical Structures

• Polymer optical structures of increasing interest for communications:

POF datacomms in automotive sector; PLC for FTTX splitters;

Chip-to-chip interconnects, etc

• Metamaterials:

Sub-wavelength physical structures used in place of changes in

chemical composition to modify light-matter interactions.

• Molecular conformation approach to visible spectral range metamaterials?

• To succeed we require:

(i) That molecular conformation engenders a significant

change in electromagnetic properties

(ii) That conformation is spatially controllable to allow patterning of

practical structures

Spatial patterning of poly(9,9-dioctylfluorene) β-phase conformation

via solvent-vapour exposure

G. Ryu, P.N. Stavrinou, D.D.C. Bradley, Adv.Funct.Mater. (2009), 3237-3242.


The PFO β-Phase Conformation

• Under certain processing conditions a new red-shifted peak contributes to the

absorption

R

R

R

R

R

R

R

R

R R R R

1.0

R R R R

Absorbance (a.u.)

0.8

0.6

0.4

0.2

R

0.0

320 360 400 440 480

Wavelength (nm)

R

R

R

R

R

R

R

R

R

R R

R

R

R

R

A fraction of chains adopt a well-defined, extended, rigid structure

Bradley et al Proc. SPIE 3145 (1997) 254; Grell et al Acta Polym. 49 (1998) 439; Grell et al Macromol. 32 (1999), 5810; Ariu et al Phys.Rev.B67 (2003) 195333


Change in Optical Properties for PFO β-Phase

• Refractive index change:

∆n max ≈ 0.175 for 23% β-phase

∆n ≈ 0.06 ± 0.03 in spectral range

(450-480 nm) that overlaps S 0 to S 1

0-1 PL vibronic peak (maximum

optical gain for four level system)

• Planar structures c.f. ion-implanted

waveguides

G. Ryu, P.N. Stavrinou, D.D.C. Bradley, Adv.Funct.Mater. (2009), 3237-3242.


Spatial Patterning of the β-Phase Conformation

• Masked solvent vapour exposure:

Spectrosil

PDMS

Spectrosil

Toluene vapour

Glassy PFO film (d ≈ 200 nm)

β-phase PFO

• Simple structures: Image patterns using PL contrast (red shifted β-phase)

Passive:

Active:

Buried

Grating

β-phase

PFO

Spectrosil

PFO

Spectrosil

G. Ryu, P.N. Stavrinou, D.D.C. Bradley, Adv.Funct.Mater. (2009), 3237-3242.


Spatial Patterning of the β-Phase Conformation

• Optical Cross-section for 1.8 mm stripe:

x x’

x x’

• ∆n max (450 nm) ≈ 0.025 for 8.5% β-phase fraction

i.e. ∆n increment ≈ 1.4% c.f. 1.3% typical for silica fibre

• Graded index profile good for guiding

i.e. reduced scattering losses c.f. step index

G. Ryu, P.N. Stavrinou, D.D.C. Bradley, Adv.Funct.Mater. (2009), 3237-3242.


Spatial Patterning of the β-Phase Conformation

• Pattern PFO film atop a 1-D DFB grating

10 x 10 mm 2 area; 270 nm period; 100 nm depth; 50% fill factor

PL image:

outside

1

1

2

3

4

5

Glassy

2

3

4

Glassy

β-Phase

5

• Track spatial variation in λ laser

8ns; 10Hz; 390 nm; 200µm excitation spot

• No change in threshold/slope efficiency - patterning does no damage

• Lasing wavelength tuned by index, gain and loss properties

Here ∆n ≈ 0.01-0.03; ∆λ laser ≈ 2.3 nm (from 455.4 nm to 457.7 nm)

G. Ryu, P.N. Stavrinou, D.D.C. Bradley, Adv.Funct.Mater. (2009), 3237-3242.


Molecular Electronic Materials:

Attributes & Applications

µ -analysis

Display

Lighting

Laser

Large α

Datacomm

Large ΦPL

Large E B

Low µ

Anisotropic

Localised

Ψ

Low εR

Low ρ

Disordered

Amplifier

Large ∆n

Large ∆λ

Large J

Telecomm

Photodiode

Solar Cell

FET


Acknowledgements

Imperial College London Centre for Plastic Electronics

Physics: Professor Jenny Nelson, Drs Thomas Anthopoulos, Alasdair Campbell,

Amanda Chatten, Nicholas Eakins-Daukes, Ji-Seon Kim & Paul Stavrinou

Chemistry: Professors James Durrant, Iain McCulloch & Andrew de Mello, Drs John

de Mello, Saif Haque, Martin Heeney, Joachim Steinke & Charlotte Williams

Materials: Drs Sandrine Heutz, Natalie Stingelin-Stutzmann and Professor Paul

Smith (ETH)

Institute for Bioengineering: Dr Patrick Degenaar

The Sumitomo Chemical Company Ltd (Toshihiro Ohnishi, Takeshi Yamada)

UK EPSRC, RCUK, Royal Society:

More magazines by this user
Similar magazines