Organic Semiconductors for Flexible Electronics Jessica Wade 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 Watts2x10 13 Watts 34 %27 %21 % 2.2 % < 1%

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 band

Metals Energy 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 Energy 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 EGEG

Semiconductor CONDUCTION BAND Energy Valence Band Intermediate conductivity Small band gap Energy at room temperature can cause electrons to move from the to valence band Location in crystal EGEG

Molecular Structures Inorganic Semiconductors 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 Si GaAs Covalent and Ionic bonds Hard High melting and boiling points Electronic wavefunction spreads out over whole lattice

Why ? Organic Semiconductors: Inorganic semiconductors:

Saturated and Unsaturated Hydrocarbons Alkanes Alkenes Alkynes Aromatics Saturated Unsaturated

Polymerisation of Unsaturated Hydrocarbons Acetylene poly(acetylenes) Alternating single and double bonds  conjugated system Polymerisation + H 2 Titanium Aluminium

Polymerisation of Unsaturated Hydrocarbons poly(acetylenes) All-cis-polyacetylene All-trans-polyacetylene …More conductive! Isomers: same molecular but different structural formula Ti Al -78° C150° C Acetylene

Carbon Bonding Carbon 1s 2 2s 2 2p 2 2s 2p 2s 2p Promotion Three hybridised sp 2 Un-hybridised p z Hybridisation

Delocalisation of Electrons sp 2 orbitals are in a trigonal planar shape p z orbital perpendicular to the plane End-to-end overlap of sp 2 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) Light is absorbed 2. Charge Separation 3. Charge transport 4. Charge collection Energy 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 Electrode Insulator SD

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?

 Understanding of the thin film structure-property relationships in plastic electronic devices 19 Controlling Thin Film Microstructure Developing Nanoanalysis Techniques 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

v0v0 v1v1 v2v2 v3v3 Energy Internuclear Separation v' 0 v' 1 v' 2 v' 3 S0S0 S1S1 Ground State Excited Electronic State Molecular Energy Levels and Spectroscopy E = E electronic + E vibrational + E rotational + E translational r0r0 rnrn E electronic : energy stored as potential energy in excited configurations E vibrational : oscillation of atoms (kinetic  potential) E rotational : kinetic energy associated with molecular rotations E translation : ~ unquantized small amounts of energy stored as kinetic energy

Nanoanalysis Techniques Raman Spectroscopy Absorption Spectroscopy v0v0 v1v1 v2v2 v3v3 Energy Internuclear Separation v' 0 v' 1 v' 2 v' 3 S0S0 S1S1 Ground State Excited Electronic State

Absorption Spectroscopy v0v0 v1v1 v2v2 v3v3 Energy Internuclear Separation v' 0 v' 1 v' 2 v' 3 S0S0 S1S1 Ground State Excited Electronic State Absorbance Wavelength S 0  S 1 Xenon Lamp Organic Thin Film

rraP3HT rrP3HT Absorption Spectroscopy Band gap In twisted polymer chains, delocalisation is broken due to poor orbital overlap  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 = 2 v = 1 v = 3 S0S0 v = 0 v = 2 v = 1 v = 3 S1S1 mm S A R virtual state  - Chemical structure - Molecular conformation - Molecular orientation Raman Spectroscopy

26 rraP3HT rrP3HT 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, Polymer Molecular Order

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 P3HT:PCBM (after annealing) 95% 40% Tsoi et al., J. Am. Chem. Soc. (2011), 133, 9834 Razzell-Hollis et al., J. Mater. Chem. C (2013), 1, Molecular Order in OPV Blends 27

Conclusion and Outlook v0v0 v1v1 v2v2 v3v3 Energy Internuclear Separation v' 0 v' 1 v' 2 v' 3 S0S0 S1S1 Ground State Excited Electronic State Tunable chemistry of carbon based polymers Control of structural and electronic properties Efficient flexible electronic devices