organic semiconductors for flexible electronics jessica wade department of physics & centre for...
DESCRIPTION
Energy Bands Energy N o of Atoms Electrons occupy distinct energy levels Lots of atoms side by side: spreading out of discrete levels Si crystal with atoms per cm 3 only see bands CONDUCTION BAND Energy 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 bandTRANSCRIPT
Organic Semiconductors for Flexible Electronics
Jessica Wade ([email protected])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