1. Organic Semiconductors for Flexible Electronics
Jessica Wade
Department of Physics & Centre for Plastic Electronics
Imperial College London, United Kingdom

2. Motivation and Outline
Global Power Consumption
Available Solar Power
2x1013 Watts
1017 Watts
Oil – 34 %
Coal – 27%
Gas – 21 %
Nuclear – 2.2 %
Solar Energy – < 1%
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 is high… 20 terawatts. We realistically have enough natural gas for around 50 years, and oil will run out by 2070.
Why aren’t we using the sun’s energy?
$$$
 OPV

3. Energy Bands
Energy
No of Atoms 1 2
1023
Bohr model: electrons occupy distinct energy levels
Multiple atoms side by side: spreading out of discrete levels
Si crystal with 1023 atoms per cm3  only distinguish 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 band
The energy bands:
Single atom: Bohr model, sharp, distinct energy levels are occupied by electrons.
Multiple adjacent atoms, discrete levels spread out.
In a crystal of Si there are 1023 atoms / cm3  individual energy levels form broad energy ranges or ‘bands’, where there width is determined by how strongly the electrons are bound.
The valence or outer electrons are in the highest energy level and strongly interact with the neighbouring atoms. These electrons form a ‘valence band’.
Conduction occurs when there are electrons in the conduction band.

4. Metals Metallic bonding free electrons
Energy
Valence Band
CONDUCTION BAND
Location in crystal
Metallic bonding free electrons
Valence and conduction band overlap.
Conductive material: electrons can be promoted from the valence to the conduction band
In a metal, metallic bonding leaves behind free electrons.
Electrostatic force between conduction electrons and positively charged metal ions
Valence electrons are free  conduction and valence band overlap so that some valence electrons can move through the metal.

5. Insulators Fully occupied valence bands in covalent bonds
CONDUCTION BAND
Energy
Valence Band EG
Location in crystal
Fully occupied valence bands in covalent bonds
Electrons can’t move (locked to atoms)
Large energy gap: can’t conduct
Valence electrons are all fully occupied in bonds.
Large band gap between valence and conduction band.

6. Semiconductor Intermediate conductivity Small band gap
CONDUCTION BAND
Energy
Valence Band EG
Location in crystal
Intermediate conductivity
Small band gap
Energy at room temperature can cause electrons to move from the to valence band
No conduction occurs at 0 K
Higher temperatures, some electrons can reach the conduction band and produce some current.
Can dope semiconductors; add impurities and introduce extra energy levels near the conduction or valence band.
N-type: new energy levels near top of band gap allows electrons to be easily excited into material.

7. Covalently bonded molecules with intermolecular van de Waals forces
Molecular Structures
Covalently bonded molecules with intermolecular van de Waals forces
PPV
PFO
P3AT
poly(p-phenylenevinylene)
polyfluorene
poly(3-alkylthiophene)
Reduced hardness
Lower melting point
Weaker delocalisation of electronic wavefunctions
Organic Semiconductors
Inorganic Semiconductors Si
GaAs
Inorganic- familiar with. Band structure means that at temperatures above 0 K there is a finite probability that an electron will move in the lattice.
Si is the most widely used semiconductor for electronics (high processing temperatures).
Abundant, good band gap for capturing sunlight.
Organic- novel semiconductors based on carbon polymers.
Can tune electronic properties by using all different atoms and changing electron distribution.
Covalent and Ionic bonds
Hard
High melting and boiling points
Electronic wavefunction spreads out over whole lattice

8. Inorganic semiconductors:
Why ?
Organic Semiconductors:
Inorganic semiconductors:
Expensive
Vacuum deposition
Brittle
Heavy
Adaptable
Solution processable
Cheap
Disposable
Printable
Flexible
Wearable
Lightweight

9. Saturated and Unsaturated Hyrdocarbons
Alkenes
Alkanes
Alkynes
Organic chemistry: hydrocarbon is an organic compound made from carbon and hydrogen- all familiar with.
Saturated hydrocarbons (alkanes)  single bonds, saturated with hydrogen.
Examples: petrol, chlorophyll.
Unsaturated hydrocarbons (alkenes etc)  double / triple bonds (alkenes, alkynes and aromatics).
Unsaturated hydrocarbons are more reactive and acidic.
Aromatics have strong smells (their name)- benzene.
Aromatics

10. Polymerisation of Unsaturated Hydrocarbons
Acetylene
Addition Polymerisation
+ H2
poly(acetylenes)
Ethyne/ acetylene is the simplest member of the family of hydrocarbons called alkynes
C-C triple bond places all four atoms in the same straight line; CCH bond angles of 180 °
Addition polymerisation forms poly(ethyne). Titanium and aluminium ‘ziegler-natta’ catalysts.
Alternating single and double bonds  conjugated system

11. Polymerisation of Unsaturated Hydrocarbons
1974
poly(acetylenes)
1977
All-cis-polyacetylene
-78° C
In 1977 Shirakawa and co-workers discovered oxidation with halogens made the films a 109 times more conductive than they previously were.
Conducting polymers had been around since the 1860s, where their discovery is credited to UCL.
Polyacetylene was known as a black powder since 1974, when prepared as a silvery film by Shirakawa et al.
In 1977, chemists carried out the polymerisation at the surface of a concentrated solution of catalyst in an inert solvent.
The catalyst solution was added to toluene, an inert solvent, and kept at room temperature for 20 minutes, then cooled it to -78 C.
Here the gas of acetylene was introduced and reacted with the catalyst on the walls of the reaction vessel where a film of polyacetylene was formed.
>> 95% cis-polyacetylene.
When they heated the mixture to 150 C mainly the trans-isomer formed; which has much higher conductivity and is more thermodynamically stable.
In 2000 the Nobel prize for chemistry was awarded to Shirakawa, MacDiarmic and Heeger for their work on conductive polymers.
All-trans-polyacetylene
150° C
More conductive!

12. Carbon Bonding Hybridisation Promotion Carbon 1s22s22p2
Three hybridised sp2
Un-hybridised pz
Hybridisation 2s 2p
Promotion 2s 2p
In carbon there are 6 electrons; 2 in 1s, 2 in 2s and 2 in 2p orbital.
There is only a small energy gamp between the 2s and 2p orbitals, and an electron is promoted from the 2s to the empty 2p orbital to give 4 unpaired electrons. The energy released when these electrons are used for bonding more than compensates for the energy used to promote the electron.
The carbon is now in an excited state.
Because carbon is only joining to three other atoms, when they hybridise their outer orbitals they only hybridise three rather than all four.
The 2s and 2px and 2py orbitals then hybridise to make three sp2 orbitals. The pz electron is left un-hybridised.

13. 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
The three sp2 orbitals arrange themselves as far apart as possible, which is 120 ° from one another in a plane. The remaining p-orbital is at right angles to them.
When neighbouring sp2 orbitals overlap end-to-end; -bonds are formed. Sigma bonds are the strongest type of covalent bonds due to the direct overlap of orbitals.
When pz orbitals overlap next to one another, -bonds form. Pi bonds are more diffuse than sigma bonds.

14. Delocalisation of Electrons
The extensive side-ways overlap of the pz electrons produces a system of -electrons which spreads down the length of the polymer.
Because the electrons are no longer held between just to carbon atoms, they are ‘delocalised’.
Conjugation refers to alternating single and double bonds, whereas delocalisation describes the spread of electrons.
Delocalised π electrons along the polymer chain (conjugation) produces semiconducting properties

15. What is organic electronics?
Organic PhotoVoltaics (solar cells)
Organic Field Effect Transistors
Absorption of light
Diffusion of charges
Charge separation
Charge transportation
Charge collection
Organic Material S D
Insulator
Gate Electrode
Organic Light Emitting Diode
All organic devices consist of many layers.
In an organic solar cell, two photoactive materials are sandwiched between two metal electrodes, where one is transparent (to collect photogenerated charge).
Light is absorbed and charges are generated in one material.
These then move to an ‘interface’ with another organic material, where the charges are separated into an electron and hole (positive electron).
These then move to the electrodes (one to the anode and one to the cathode) without recombining.
They then enter the external circuit.
In an organic field effect transistor, we have three terminals: a source, a drain and a gate.
The gate is separated from the source and drain contacts by an insulator. The organic material sits on top of the source and drain.
When we apply a voltage to the gate, charges are induced in the organic material. We then apply another voltage to the source and drain, and the charges move from one terminal to the other.
The speed they move at is an import property of a transistor. They are used as little electronic switches.
In an OLED structure, an organic film is sandwiched between two electrodes on a transparent substrate.
Electronic charges are injected and transported into the polymer from the electrodes (holes and electrons).
The electrons and holes capture each other though electrostatic interaction.
Radiative recombination of electron and hole  light
Wavelength of emitted light is dependent on the energy gap of the organic materials.
In an OLED

16. What do we do at Imperial?
Film preparation in the clean room
Thin film analysis
Polymer synthesis ✗ ✓ ✗ ✗ ✗ ✗ ✗ ✗ ✓ V ✗ ✗ ✗ ✗
Device Characterisation
Device Fabrication
Thin film optimisation

17. 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?

18. Plastic Electronics in Ji-Seon Kim’s Group
Controlling Thin Film Microstructure
Developing Nanoanalysis Techniques
Raman Spectroscopy
Raman-AFM towards Tip-Enhanced Raman Spectroscopy
Photoconductive AFM
There are two main themes of work in our group:
Microstructural control with solution processing
Development of spectroscopic and scanning probe techniques to characterise a material’s microstructure and properties.
We have worked extensively on zone-casting, a unique solution processing method to produce highly aligned films.
We are fabricating not only structures in films but also in real devices including LEDs, OPVs, and Transistors.
 Understanding of the thin film structure-property relationships in plastic electronic devices

19. Quantum Mechanics 𝐸= ℎ 
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 
c. Photoelectric effect

20. Molecular Energy Levels and Spectroscopy
E = Eelectronic + Evibrational + Erotational + Etranslational
Eelectronic : energy stored as potential energy in excited configurations
Energy
Internuclear Separation
Excited Electronic State S1
v'3
v'2
v'1
Evibrational : oscillation of atoms (kinetic  potential)
v'0 S0 v3
Ground State v2
Two atoms that can form a bond will do so when they approach each other closely.
The energy is described by a potential energy curve, called a potential well: ‘Morse potential’.
The vibrational energy levels are calculated using a harmonic oscillator model for the two atoms.
Erotational : kinetic energy associated with molecular rotations v1 rn r0 v0
Etranslation: ~ unquantized small amounts of energy stored as kinetic energy

21. Nanoanalysis Techniques
Energy
Internuclear Separation
Excited Electronic State S1
v'3
v'2
v'1
Ground State
v'0 S0 v3 v2 v1 v0
Electronic transitions are vertical or almost vertical lines on such a plot since the electronic transition occurs so rapidly that the internuclear distance can't change much in the process.
When light is shined on a sample, some of that light is absorbed to promote electrons to a higher energy level (ground to excited state). We measure this using absorption spectroscopy.
Vibrational transitions occur between different vibrational levels of the same electronic state. To measure this we use a technique called Raman spectroscopy using lasers.
AFM relies on van de Waals forces and maps the surface of an organic film.
Absorption Spectroscopy
Raman Spectroscopy
Atomic Force Microscopy

22. Absorption Spectroscopy
Xenon Lamp
Organic Thin Film
Energy
Internuclear Separation
v'3
Absorbance
Wavelength
S0  S1
v'2
Excited Electronic State
v'1
S0  S1 S1
v'0 v3 v2 v1
Ground State v0 S0

23. Absorption Spectroscopy
rraP3HT
rrP3HT
In twisted polymer chains, delocalisation is broken due to poor orbital overlap
Increase ‘delocalisation’:
Longer chain (electrons can spread around more easily)
Improve molecular order (better overlap of orbitals)
 Decrease energy gap
Red Shift absorption spectra
( longer , lower E)
Band gap

24. (R: Rayleigh, S: Stokes, A: Anti-Stokes)
Raman Spectroscopy
- Chemical structure
- Molecular conformation
- Molecular orientation
(R: Rayleigh, S: Stokes, A: Anti-Stokes)
v = 0
v = 2
v = 1
v = 3 S0 S1 um S A R
virtual state u0
Vibrational spectroscopy based on molecular polarizability
Excite molecule to a virtual excited state.
Most often we deal with Rayleigh- elastic scatter via virtual excited state.
Raman scattering: Inelastic scatter where a molecule ends up in a different vibrational state.
Raman is particularly appropriate for probing the microstructure of organic semiconductors in thin-film because:
Non-destructive technique
Minimal sample preparation required
Using Raman we can learn more about:
Chemical structure
Molecular conformation
Molecular orientation
Example Raman spectrum of RR-P3HT. Different peaks represent different chemical bonds.

25. 2. Polymer Molecular Order
rraP3HT
rrP3HT
This is an example of where we have used Raman spectroscopy to probe the conformation of a single material.
We know RRa-P3HT is more disordered, and we see a significantly blue-shifted absorption to ordered RR-P3HT.
We can see the difference more clearly when comparing the resonant Raman spectra, where we see three signatures of increased order:
Shift of C=C mode to lower frequency
Reduced FWHM of C=C peak
Increase of intensity ratio IC-C/IC=C
These changes are due to the backbone becoming more planar in the ordered structure, so there is a longer effective conjugation length.
An increase in -electron delocalisation in the ring causes the original benzoidal structure becomes more quinoidal like.
Bond-length change when resonantly excited: benzoidal  quinoidal. Bonds all become 1.5 long.
Emphasis on advantages of Raman over Abs. i.e. lateral resolution, non-invasive, in-situ. More direct morphological probe.
Tsoi et al., J. Am. Chem. Soc. (2011), 133, 9834
Razzell-Hollis et al., J. Mater. Chem. C (2013), 1, 6235
Tsoi et al., Macromolecules (2011), 44, 2944

26. 2. Molecular Order in OPV Blends
P3HT:PCBM (before annealing)
P3HT:PCBM (after annealing)
95%
40%
We said before to make a solar cell we need two organic materials: here we have used a polymer and a small molecule.
We can use this analysis to quantify the degree of molecular order in blend films to imagine what goes on in real devices.
We have separated the main vibrational modes of pristine and blend films into an ordered and disordered component.
We then ‘anneal’ or ‘cook’ the sample. When you anneal a thin film, we hold it at a temperature just below its melt temperature for a while to allow chains of the polymer to wriggle around and become more ordered.
We can see that after annealing the portion of order increases from 40 to 95 %. This explains the increase in device performance after annealing.
The amount of ordered P3HT increases from 40% to 95% upon thermal annealing,
which correlates with an increase in solar cell performance
Tsoi et al., J. Am. Chem. Soc. (2011), 133, 9834
Razzell-Hollis et al., J. Mater. Chem. C (2013), 1, 6235

27. Atomic Force Microscopy
Detector and feedback electronics
Scan a sharp ‘tip’ over surface
Vertical bending due to forces on tip detected by laser
Laser focussed on back of cantilever
Laser spot reflected to photodiode
Photodetector signal  feedback circuit  z-part of scanner
Z-movement  topography
Laser
Photodiode
z-scanner
Cantilever and Tip
In AFM we use a sharp tip, a laser, a photodiode and a detector.
The sharp tip (5-20 nm radius) is translated over the surface of the film.
The movement of the tip is controlled by a scanner and maintains a constant force between the tip and the sample.
Light from the laser is reflected off the back of the tip and into the photodiode, which sends it to a feedback circuit that controls z-movement of the scanner. This maintains tip-sample distance.
Because the cantilever acts as a spring, this fixed cantilever deflection means a fixed probe-sample force is maintained.
The amount by which the scanner has to move in the z axis to maintain the cantilever deflection is taken to be equivalent to the sample topography.
Sample Surface
Scanner
xy-scanner

28. Atomic Force Microscopy
Form polymer nanowires in solution
Well-ordered nanowires minimise disorder when blending to make solar cells
Here’s an example of AFM.
We have played with the polymer solution to allow ‘nanowires’ to form.
These nanowires are well-ordered domains which can be used to minimise disorder at the interface where charges are separated in solar cells.
We can see from this absorption spectrum that the different features are much more defined in the nanowire film  better order.
The nanowires are visible in an AFM image across the film surface.
Nanowire formation increases the solar cell efficiency by 10 x.
Polymer domains, 10s of nanometres across, are visible in AFM images
Solar cell device efficiency increases 10 x
J. Chem. Phys. 139, (2013)

29. Conclusion and Outlook
Energy
Internuclear Separation
Excited Electronic State S1
v'3
Tunable chemistry of carbon based polymers
v'2
v'1
Ground State
v'0 S0 v3 v2 v1 v0
Efficient flexible electronic devices
Control of structural and electronic properties
J. Chem. Phys. 139, (2013)

31. 1. Chemical Composition Mapping
Spin-coated at RT
After etching, RT Se
C6H13 n
P3HS
PCBM
(633nm ext)
Spin-coated at 80 deg
After etching, 80 deg
My first example is a use of resonant Raman scattering to look at the chemical composition of an OPV blend.
P3HS hole transporting polymer, PCBM electron transporting small molecule
We’ve excited the blend using a 633 nm excitation, which is resonant with P3HS. PCBM is not Raman active under 633nm excitation
First we identified the vibrational modes of P3HS.
Here we’ve mapped the intensity of main P3HS (C=C) mode.
Distinguish different chemical domains.
To check if this was only at the surface, we removed half the film by etching  still large domains of P3HS
By spin-coating at 80 degrees we achieve better mixing of the polymer and small order, which is the same throughout the film.
BUT we want more detail on molecular order.
Li et al., Nat. Commun. (2013), 4, 2227 & Ballantyne et al., Macromolecules (2010), 43, 1169