Renaissance of the Plastic Age Polymers for Electronics & Photonics T.P.Radhakrishnan School of Chemistry, University of Hyderabad Hyderabad 500 046, India tprsc@uohyd.ernet.in http://chemistry.uohyd.ernet.in/~tpr/ This file is available at http://chemistry.uohyd.ernet.in/~ch521/

Materials and civilisation (Before 5000 BC) Stone age (5000 - 3000 BC) Copper age (3000 - 800 BC) Bronze age (800 BC - 40 AD) Iron age Plastic age ?

Types of materials Molecular materials Metals / Alloys Ceramics * Ceramics Polymers Semiconductors Composites Biomaterials Molecular materials *Courtsey: W. D. Callister, Fundamentals of Materials Science and Engineering

Design of Molecular Materials Elements / Compounds Materials Chemical / Physical routes Molecules Crystals Thin films / LB films Polymers Nanostructures Chemical routes Chemical / Physical routes

Molecular materials / Polymers

Natural polymers

Synthetic polymers Polytetrafluoroethylene Polyethylene (Teflon) Phenol-formaldehyde (Bakelite) Polyethylene Polytetrafluoroethylene (Teflon) Polyhexamethylene adipamide (Nylon 6,6) Polyethyleneterephthalate (PET) Polycarbonate

Discovery of conducting polymers 1862 Lethby (College of London Hospital) Oxidation of aniline in sulfuric acid 1970’s Shirakawa (Japan) Acetylene gas Ti(OBu)4 & Et3Al Toluene –78oC Ti(OBu)4 & Et3Al Hexadecane 150oC silvery film trans-polyacetylene copper-coloured film cis-polyacetylene

Polyacetylene (PA) Electrical conductivity (s) cis PA 10-10 – 10-9 S cm-1 trans PA 10-5 – 10-4 S cm-1 For comparison : s (copper) ~ 106 S cm-1 : s (teflon) ~ 10-15 S cm-1

Doping leads to enhanced conductivity s ~ 10-5 S cm-1 Semiconductor + e- - e- s ~ 104 S cm-1 Metal

Discoverers - Nobel Prize 2000 A. Heeger, A. McDiarmid, H. Shirakawa (this photograph taken at the International Conference on Synthetic Metals, 2000, was kindly provided by Prof. Heeger)

Polyacetylene - electronic structure -electronic energy levels and electron occupation (a) ethylene (b) allyl radical (c) butadiene (d) regular trans-PA (e) dimerised trans-PA

How does a conducting polymer work ? Oxidative doping of polyacetylene by iodine Polaron and its delocalisation

Excitations Bipolaron Neutral Soliton Positive Soliton

Examples of conducting polymers

Electrical conductivities Copper Platinum Bismuth Graphite 10+6 10+4 10+2 100 10-2 10-4 10-6 10-8 10-10 10-12 10-14 10-16 10-18 S cm-1 Conducting Polymers Germanium Silicon Polyethylene Diamond Quartz

Synthesis of PANI Cathode Anode (ITO plate) Aniline + dil. HCl Instead of electrochemical oxidation, chemical oxidation may be carried out : Aniline + acid + oxidising agent ((NH4)2S2O8)

Voltage (~ 0.3 - 0.5 V) applied

Result of electropolymerisation The green coating on the ITO electrode is due to the formation of emeraldine salt form of PANI

Polyaniline (PANI) Leucoemeraldine Colorless Emeraldine base Blue (Insulator) Emeraldine base Blue (Insulator) Emeraldine salt Green (Conductor) Purple (Insulator) Pernigraniline Oxidation

Applications of conducting polymers Polyaniline (PANI) Transparent conducting electrodes Electromagnetic shield Corrosion inhibitor ‘Smart windows’ (electrochromism) Polypyrrole (Ppy) Radar-invisible screen coating (microwave absorption) Sensor (active layer) Polythiophene (PT) Field-effect transistor Anti-static coating Hole injecting electrode in OLED Polyphenylenevinylene (PPV) Active layer in OLED

Polypyrrole - conductivity switching

Enzyme Biosensor Using PPy Glucose oxidase -D-glucose + ½O2 + H2O  D-gluconic acid + H2O2 H2O2 + 2HCl + Ppy  2H2O + Ppy2+.2Cl-

PANI-PSS PSSn-(100 kDa) RT = 8.3x10-2 Scm-1 PSSn-(70 kDa)

Sensors Typical example : Ammonia sensing by PANI-PSSM film Resistance change with time Ammonia in Ammonia out

NH3 Constant Vacuum Two-way switch Digital Voltmeter Current Source Rotary Vacuum Pump Two-way switch Digital Voltmeter NH3 Vacuum FAN 10 V Battery

Resistance change at 150 sec. for different concentrations of ammonia

Electroluminescence - Electric field + Light Metal electrode Organic thin film Transparent electrode (ITO) Light Electric field

Principle of EL Cathode Anode e- HOMO LUMO HOMO LUMO Light h+

Polymers for Organic Light Emitting Diodes (OLED) PPV MEH-PPV Commercial materials like Mn2+ in ZnS require 100V DC PPV : requires 5 - 10V DC runs even with AC brightness ~40,000 cd/m2 ie. ~100 times brighter than a TV screen

Organic LED driven by organic transistor Ca/Ag MEH/PPV Silica Gold P3HT n+-Silicon Aluminium G D S

Electrochromic devices Polymer Undoped Doped Polythiophene Red Blue Polypyrrole Yellow-green Blue-black Polyaniline Yellow Green/Blue Viewing side Li anode Polymer electrolyte Conducting polymer ITO electrode V

On application of voltage Viewing side Li anode Polymer electrolyte Conducting polymer ITO electrode V

Conjugated polymers for nonlinear optics NLO materials interact with light Polydiacetylene ( ) n Light changes the material properties Changes the properties of the light

Photonic Application of Conducting Polymers - Kerr gate Polariser Laser 1 Crossed Polariser No light NLO (c(3)) polymer Laser 2 Laser 1 Polariser Crossed Polariser

All organic transistor Future Outlook All organic transistor

Plastic solar cell based on MDMO-PPV/PCBM (conducting polymer - fullerene composite) on flexible ITO coated PET

Thank you