Substantially Conductive Polymers

Substantially Conductive Polymers

Part 02

Usually, soliton is served as the charge carrier for a degenerated conducting polymer (e.g. PA) whereas polaron or bipolaron is used as charge

carrier in a non-degenerated conducting polymer (e.g. PPy and PANI)

Schematic structure of (a) a positive polaron, (b) a positive bipolaron,and (c) two positive bipolarons in polythiophenes

Typical Charge Carriers (via doping)

6

soliton

antisoliton

positive soliton

negative soliton

trans-polyacetylene

hole polaron cis-polyacetylene

NN

NH

NN

NNH

Nhole polaron polypyrrole

electron polaron polyphenylene

R

RR

R

R

R

R

RR

R

polydiacetylenehole polaron

SS

SS

SS

SS polythiophenepositive bipolaron

Chemical term, charge and spin of soliton, polaron and bipolaron inconducting polymers

• Filtration, membranes• Rechargeable batteries• Radar absorbers

Potential applications and corresponding physical properties of conductingPolymers.

Organic Light Emitting Polymer• First reported in 1990 (Nature 1990, 347, 539)

• Based on poly(p-phenylenevinylene) (PPV), with a bandgap of 2.2 eV

ITO: Indium-tin-oxide-A transparent electrical conductor

• Threshold for charge injection (turn-on voltage): 14 V (E-field = 2 x 106 V/cm

• Quantum efficiency = 0.05 %• Emission color: Green• Processible ? No!!• Polymer is obtained by precursor

approach. It cannot be redissolved once the polymer is synthesized

Other PPV Derivatives • MEH-PPV• More processible, can be dissolved in

common organic solvents (due to the presence of alkoxy side chains)

• Fabrication of Flexible light-emitting diodes(Nature 1992, 357, 477)

Substrate: poly(ethylene terephthlate) (PET)Anode: polyaniline doped with acid-a flexible and transparent conducting polymer

EL Quantum efficiency: 1 %Turn-on voltage: 2-3 V

Other Examples of Light Emitting Polymers

Poly(p-phenylene) (PPP)

Poly(9,9-dialkyl fluorene)

CN-PPV: RED light emissionNature 1993, 365, 628

BLUE lightemission

Polythiophene derivativesA blend of these polymers produced variable colors, depending on the compositionNature 1994, 372, 443

Applications

• Flat Panel Displays: thinner than liquid crystals displays or plasma displays (the display can be less than 2 mm thick)

• Flexible Display Devices for mobile phones, PDA, watches, etc.

• Multicolor displays can also be made by combining materials with different emitting colors.

For an Electroluminescence process:

Electrons Photons

Can we reverse the process?

Electrons Photons YES!

PhotodiodeProduction of electrons and holes in a semiconductor device under illumination of light, and their subsequent collection at opposite electrodes.Light absorption creates electron-hole pairs (excitons). The electron is accepted by the materials with larger electron affinity, and the hole by the materials with lower ionization potential.

A Two-Layer Photovoltaic Devices• Conversion of photos into electrons• Solar cells (Science 1995, 270, 1789; Appl. Phys. Lett. 1996, 68, 3120)

(Appl. Phys. Lett. 1996, 68, 3120)

Max. quantum efficiency: ~ 9 %Open circuit voltage Voc: 0.8 V

490 nm

Another example: Science 1995, 270, 1789.

ITO/MEH-PPV:C60/CaActive materials: MEH-PPV blended with a C60 derivative

dark

dark

light

light

ITO/MEH-PPV:C60/Ca

ITO/MEH-PPV/Ca

MEH-PPV

C60

e-h+

A Photodiode fabricated from polymer blend(Nature 1995, 376, 498)

Device illuminated at 550 nm (0.15 mW/cm2)

Open circuit voltage (Voc): 0.6 VQuantum yield: 0.04 %

• Field Effect Transistors (FET)– Using poly(3-hexylthiophene) as the active layer– “All Plastics” integrated circuits

(Appl. Phys. Lett. 1996, 69, 4108; recent review: Adv. Mater. 1998, 10, 365)

More Recent Development• Use of self-assembled monolayer organic

field-effect transistors• Possibility of using “single molecule” for

electronic devices(Nature 2001, 413, 713)

Polymer light-emitting diodes, such as the one produced by Martin Drees (Ph.D. 2003) in Prof. Heflin's laboratory, may potentially yield flexible, inexpensive flat-panel displays.

Prof. Heflin's group is developing organic solar cells that have the potential to be flexible, lightweight, efficient renewable energy sources. Photograph by John McCormick.

http://www.phys.vt.edu/~rheflin/

Prof. Heflin's group is examining how nanoscale control of the composition of organic solar cells consisting of semiconducting polymers and fullerenes can improve their power conversion efficiency.

Prof. Heflin's group is using self-assembly of nanoscale organic films to create organic electrochromic devices that change color when a voltage is applied at rates up to 50 Hz.

http://www.phys.vt.edu/~rheflin/

Prof. Heflin's group is using self-assembly of nanoscale organic films to create organic electrochromic devices that change color when a voltage is applied at rates up to 50 Hz.

http://www.phys.vt.edu/~rheflin/