Organic Semiconductors for Flexible Electronics

Jessica Wade (jessica.wade08@imperial.ac.uk)Department of Physics & Centre for Plastic Electronics

Imperial College London, United Kingdom

Motivation and Outline

• Introduction

• What do we do in the Centre for Plastic Electronics at Imperial College?

• Research in the Nanoanalysis group

• Molecular Energy Levels and Spectroscopy

Global Power Consumption Available Solar Power

1017 Watts2x1013 Watts

34 % 27 % 21 % 2.2 %

< 1%

Energy BandsEn

ergy

No of Atoms

1 2 1023

• Electrons occupy distinct energy levels

• Lots of atoms side by side: spreading out of discrete levels

• Si crystal with 1023 atoms per cm3 only see bands

CONDUCTION BAND

Ener

gy

Location in crystal

VALENCE BAND

Interatomic Distance

• Valence (outer electrons) are in the highest energy levels and interact strongly with neighbouring atoms

• Valence electrons valence band

MetalsEn

ergy

Valence Band

CONDUCTION BAND

• Metallic bonding free electrons

• Valence and conduction band overlap.

• Conductive material: electrons can be promoted from the valence to the conduction band

Location in crystal

Insulators

CONDUCTION BAND

Ener

gy

Valence Band

• Fully occupied valence bands in covalent bonds

• Electrons can’t move (locked to atoms)

• Large energy gap: can’t conduct

Location in crystal

EG

Semiconductor

CONDUCTION BAND

Ener

gy

Valence Band

• Intermediate conductivity

• Small band gap

• Energy at room temperature can cause electrons to move from the to valence band

Location in crystal

EG

Molecular Structures

Inorganic Semiconductors

Covalently bonded molecules with intermolecular van de Waals forces

R RS

S

R

R

PPV

PFO

P3AT

poly(p-phenylenevinylene)

polyfluorene

poly(3-alkylthiophene)

• Reduced hardness • Lower melting point• Weaker delocalisation of electronic

wavefunctions

Organic Semiconductors

SiGaAs

Covalent and Ionic bonds

• Hard • High melting and boiling points• Electronic wavefunction spreads out

over whole lattice

Why ?

Organic Semiconductors:

Flexible

Lightweight

Adaptable

CheapSolution processable

Printable Wearable

Disposable

Inorganic semiconductors:

Expensive

Vacuum deposition

Brittle

Heavy

Saturated and Unsaturated Hydrocarbons

H C C HH C C H

H H

HH

C C

H H

HH

H H

H

HH

H

AlkanesAlkenes

Alkynes

Aromatics

Saturated Unsaturated

H C C H

Polymerisation of Unsaturated Hydrocarbons

H C C H H C C H

Acetylene

poly(acetylenes)

CH

CH

CH

CH

CH

CH

Alternating single and double bonds conjugated system

Polymerisation

+ H2

Titanium

Aluminium

Polymerisation of Unsaturated Hydrocarbons

poly(acetylenes)

CH

CH

CH

CH

CH

CH

All-cis-polyacetyleneAll-trans-polyacetylene

1974

1977

…More conductive!

Isomers: same molecular but different structural formula

TiAl

-78° C150° CAcetylene

Carbon Bonding

Carbon 1s22s22p2

2s

2p 2p 2p

2s

2p 2p 2p

Promotion

Three hybridised sp2

Un-hybridised pz

Hybridisation

Delocalisation of Electrons

sp2 orbitals are in a trigonal planar shape pz orbital perpendicular to the plane

End-to-end overlap of sp2 orbitals:-bonds

Side-to-side overlap of p orbitals:-bonds

Delocalised π electrons along the polymer chain (conjugation) produces semiconducting properties

Delocalisation of Electrons

What is organic electronics?

Organic PhotoVoltaics (solar cells)

+ -+

-

1. Light is absorbed

2. Charge Separation

3. Charge transport

4. Charge collection

Ener

gy

-

+

-

+

Organic Semiconductor 1 Organic

Semiconductor 2

What is organic electronics?

Organic PhotoVoltaics (solar cells) Organic Field Effect Transistors

Organic Light Emitting Diode

Organic Material

Gate ElectrodeInsulator

S D

V

What do we do at Imperial?

Polymer synthesis Film preparation in the clean room Thin film analysis

Thin film optimisation

✗

✗

✗ ✗

✗

✓

✗ ✗

✗ ✗ ✗ ✗Device Fabrication

✓

Device Characterisation

What do synthetic chemists think about?

What kind of device am I making?

Do I want to capture the sun’s energy or emit light?

What units should my polymer be made of?

Can I add any elements to change where the polymer absorbs or emits light?

Can I control the way the polymer units align in thin films?

S

S

R

R

Understanding of the thin film structure-property relationships in plastic electronic devices

19

Controlling Thin Film Microstructure Developing Nanoanalysis Techniques

030

60

90

120

150180

210

240

270

300

330

50μm

Optical Image Polarized Raman Plot

(top view)

Molecular Orientation

Raman Spectroscopy

Raman-AFM towards Tip-Enhanced Raman Spectroscopy

Photoconductive AFM

Plastic Electronics in Ji-Seon Kim’s Group

Quantum Mechanics

• Quantum mechanics describes the wave-particle nature of light

• Light travels in waves of electromagnetic radiation

• Photons carry a discrete amount of energy

• Some physical quantities can only be described in discrete amounts and not in a continuous way

𝐸=h

v0

v1

v2

v3

Ener

gy

Internuclear Separation

v'0

v'1

v'2

v'3

S0

S1

Ground State

Excited Electronic State

Molecular Energy Levels and Spectroscopy

E = Eelectronic + Evibrational + Erotational + Etranslational

r0

rn

Eelectronic : energy stored as potential energy in excited configurations

Evibrational : oscillation of atoms (kinetic potential)

Erotational : kinetic energy associated with molecular rotations

Etranslation: ~ unquantized small amounts of energy stored as kinetic energy

Nanoanalysis Techniques

Raman Spectroscopy

Absorption Spectroscopy

v0

v1

v2

v3

Ener

gy

Internuclear Separation

v'0

v'1

v'2

v'3

S0

S1

Ground State

Excited Electronic State

Absorption Spectroscopy

v0

v1

v2

v3

Ener

gy

Internuclear Separation

v'0

v'1

v'2

v'3

S0

S1

Ground State

Excited Electronic State

Abso

rban

ce

Wavelength

S0 S1

S0 S1

Xenon Lamp

Organic Thin Film

rraP3HT rrP3HT

Absorption Spectroscopy

Band gap

In twisted polymer chains, delocalisation is broken due to poor orbital overlap

S S

R

R

S

R

S

R

S

R

S S

R

R

S

R

S

R

S

R

Decrease energy gap

Red Shift absorption spectra ( longer , lower E)

Increase ‘delocalisation’:

• Longer chain (electrons can spread around more easily)

• Improve molecular order (better overlap of orbitals)

(R: Rayleigh, S: Stokes, A: Anti-Stokes)

v = 0

v = 2v = 1

v = 3S0

v = 0

v = 2v = 1

v = 3

S1

um

S AR

virtual state

u0

S

R

S

R

S

R

- Chemical structure- Molecular conformation- Molecular orientation

Raman Spectroscopy

26

rraP3HT rrP3HT

Tsoi et al., J. Am. Chem. Soc. (2011), 133, 9834Razzell-Hollis et al., J. Mater. Chem. C (2013), 1, 6235Tsoi et al., Macromolecules (2011), 44, 2944

2. Polymer Molecular Order

S

R

S

R

P3HT:PCBM (before annealing)

The amount of ordered P3HT increases from 40% to 95% upon thermal annealing,which correlates with an increase in solar cell performance

0

1x103

2x103

3x103

4x103

5x103

6x103

7x103

8x103

1400 1420 1440 1460 1480 1500R

aman

Inte

nsity

(cou

nts)

Wavenumber (cm-1)

P3HT:PCBM (after annealing)

95%

0

1x103

2x103

3x103

4x103

5x103

6x103

7x103

8x103

Expt dataOrderedDisorderedFitted data

1400 1420 1440 1460 1480 1500

Ram

an In

tens

ity (c

ount

s)

Wavenumber (cm-1)

40%

Tsoi et al., J. Am. Chem. Soc. (2011), 133, 9834Razzell-Hollis et al., J. Mater. Chem. C (2013), 1, 6235

2. Molecular Order in OPV Blends

27

Conclusion and Outlook

R R

v0

v1

v2

v3

Ener

gy

Internuclear Separation

v'0

v'1

v'2

v'3

S0

S1

Ground State

Excited Electronic State

Tunable chemistry of carbon based polymers

Control of structural and electronic properties

Efficient flexible electronic devices