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)
trans-polyacetylene
soliton
antisoliton
positive soliton
negative soliton
cis-polyacetylene
hole polaron
hole polaron
N
N
N
N
H
N
N
N
H
N
polypyrrole
polyphenylene
electron polaron
R
R
hole polaron
R
R
polydiacetylene
R
R
R
R
R
R
positive bipolaron
S
S
S
S
S
S
S
S
polythiophene
6
Chemical term, charge and spin of soliton, polaron and bipolaron in
conducting polymers
• Filtration, membranes
• Rechargeable batteries
• Radar absorbers
Potential applications and corresponding physical properties of conducting
Polymers.
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)
BLUE light
emission
Poly(9,9-dialkyl fluorene)
CN-PPV: RED light emission
Nature 1993, 365, 628
Polythiophene derivatives
A blend of these polymers
produced variable colors,
depending on the composition
Nature 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?
Photons
Electrons
YES!
Photodiode
Production 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)
490 nm
Max. quantum efficiency: ~ 9 %
Open circuit voltage Voc: 0.8 V
Another example: Science 1995, 270, 1789.
ITO/MEH-PPV:C60/Ca
Active materials: MEH-PPV blended with a C60 derivative
light
MEH-PPV
ITO/MEH-PPV:C60/Ca
dark
e-
h+
light
C60
ITO/MEH-PPV/Ca
dark
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 V
Quantum 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)
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.
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.
http://www.phys.vt.edu/~rheflin/
Prof. Heflin's group is examining how
Prof. Heflin's group is using self-assembly of
nanoscale control of the composition of
nanoscale organic films to create organic
organic solar cells consisting of semiconducting electrochromic devices that change color when
polymers and fullerenes can improve their
a voltage is applied at rates up to 50 Hz.
power conversion efficiency.
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/