UNIVERSITA’ DEGLI STUDI DI PADOVA
Laurea specialistica in Scienza e Ingegneria dei Materiali
Curriculum Scienza dei Materiali
Chimica Fisica dei Materiali Avanzati
Part 12 – Plastic electronics
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Basic questions
 Is it possible to do electronics with molecules?
 What sort of molecules to use? Carbon-based, similar to
those used by biology, e.g. for photosynthesis
 How will we manipulate and position molecules to
create the architectures we want?
 Transport molecules in solution (as biology does)
 Assemble molecules in correct juxtaposition through use of
‘weak’ intermolecular interactions (e.g., hydrophobic vs.
hydrophilic)
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Plastic electronics
 Plastics (or, more correctly, polymers), are traditionally used within
the electronics industry as ‘passive’ materials, for encapsulation or for
their electrically- insulating properties. However, there is now a class
of polymers which can behave as semiconductors or as metals.
 Our understanding of the semiconductor physics of these materials
has enabled us to use them as the active components in a range of
devices.
 Polymer light-emitting diodes, LEDs, providing full color range and
high efficiency as well as solar cells show particular promise.
 The electronic behavior of these polymers is very different from
inorganic semiconductors such as silicon or gallium arsenide.
 Polymer electronic devices require different strategies to make them
useful. In some respects, these strategies resemble those already
adopted by biology, for example in photosynthesis.
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Large electronic conductivities in organic
materials
 Charge transfer crystals
 E.g. TTF-TCNQ, first metallic
conductivity (1973)
 Organic superconductors
 E.g., (TMTSF)2PF6 (1980)
 (BEDT-TTF)2X
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Conducting Polymers
1977: First conducting polymer, Poly(acetylene)
Shirakawa, MacDiarmid, Heeger
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Structures of some conjugated polymers
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Electronic structure and charge carriers in
conducting polymers
polaron
bipolaron
In conducting polymers,
doping is the result of a
redox process.
Charges are bound and
deep in the gap
 A polaron (= radical ion) has both
charge (+e) and spin (±1/2)
 A bipolaron (dication) has charge
(+2e) but no spin
Polarons (A) and bipolarons (B) in PPP
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Electrochemical doping of polypyrrole
Doping effect on the optical properties:
electrochromism
Bipolaron absorptions (2)
polaron
Interband absorption (3 eV)
Polaron absorptions (3)
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bipolaro
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Current Uses of Conducting Polymers
Antistatic Coatings and Conducting Films
Electrochromic Displays?
Memory Devices? (HP Labs/Princeton)
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Light Emitting Diodes
1990: Burroughs, Friend (Cambridge)
light emission from undoped
semiconducting polymer
2003: full color range possible
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OLEDs Everywhere
2000: first commercial products with OLEDs
Advantage in color spectrum beats solid state materials
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Polymeric Photovoltaics
Solar cell efficiencies of ~ 2% (up to 6% in labs)
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Thin Film Transistors
2004: both p and n-type materials are known
Critical Advances: Crystallinity and purity
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Organic Semiconductors
Molecular Materials:
•polycrystalline
•vapor deposited
Polymeric Materials:
•semi-crystalline
•solution processed
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Mobility of organic semiconductors
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Motivations for organic electronics
 Organic TFTs show poor performance compared to
silicon CMOS
 But organic TFTs also show the potential for
extremely low cost production (printing)
 Organic TFTs are in a stage of development as
silicon MOSFETs were 30 years ago
 Organic TFT electronics certainly will not replace CMOS
 But organic TFT electronics may open new low cost / low
performance (but high volume!) markets
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Polymer electronics
 Low-end, high volume electronic applications, based on:
 Mechanical flexibility
 Low-cost
 Large area
 Potential applications:
 Electronic barcodes
 Memories
 Displays (e-paper)
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Rubber Stamped, Large-Area Plastic
Active Matrix Backplanes
10 µm Design Rules, Patterned by Single-Impression Microcontact Printing
PNAS 98(9), 4835-4840 (2001).
Science 291, 1502-1503 (2001).
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E-paper
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Key feature: solution processing
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Materials and technology
Flexible, all-plastic field effect transistor
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Technology
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Operation of the polymer transistor
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Light emitting diode
Organic light emitting diode consists of a thin film (30-500 nm) of
an emitting organic compound sandwiched between appropriate
anode and cathode layers. A relatively modest voltage (typically 2
- 10 Volts) applied across the material will cause it to emit light in
a process called electroluminescence.
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Steps of the electroluminescence process




Charge (electrons and holes) injection
Charge transport
Charge recombination and exciton formation
Exciton radiative relaxation
Friend, R.H.; Gymer, R.W.; Holmes, A.B.; Burroughes, J.H.; Marks, R.N.;
Taliani, C.; Bradley, D.D.C.; Dos Santos, D.A.; Brédas, J.L.; Logdlund, M.;
Salaneck, W.R. Nature, 1999, 397, 121.
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Mechanism of electroluminescence in organic
semiconductors
1. Charge (electrons and holes) injection
Negative polaron = radical anion
Positive polaron = radical cation
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Mechanism of electroluminescence in organic
semiconductors (cont’d)
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Some common electroluminescent polymers:
poly(phenylenevinylene)s (PPVs)
Murray, M.M.; Holmes, A.B. in “Semiconducting Polymers, Chemistry, Physics and Engineering” Hadziioannou G
and van Hutten, P.F. Eds. Wiley-VCH 1999, pp1-32Murray, M.M.; Holmes, A.B. in “Semiconducting Polymers,
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Chemistry, Physics and Engineering” Hadziioannou G and van Hutten, P.F. Eds. Wiley-VCH 1999, pp1-32
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Light emitting metal chelates
Mitschke, U.; Bauerle, P. J. Mater. Chem. 2000, 10, 1471
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Electroluminescence efficiency
Adachi, C.; Baldo, M.A.; Thompson, M.E.; Forrest S.R. J. Appl. Phys. 2001, 90, 5048
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PHOSPHORESCENT OLEDS (PHOLED)s
 The internal quantum efficiency of the phosphorescent OLEDs can be
in principle increased to 100%, because both singlet and triplet
excitons can emit radiatively. OLEDs prepared with these heavy metal
complexes are the most efficient OLEDs reported to date, with internal
quantum efficiencies > 75% and external efficiencies > 20%.
Baldo, M.A.; O’Brien, D.F.; You, Y.;
Shoutstikov, A.; Silbey, S.; Thompson,
M.E.; Forrest, S.R. Nature, 1998,
395, 151
Baldo, M.A.; Lamansky, S.; Burrows,
P.E.; Thompson, M.E.; Forrest, S.R.
Appl. Phys. Lett., 1999, 75, 4
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Zhang, Q.; Zhou, Q.; Cheng, Y.;
Wang, L.; Ma, D.; Jing, X.; Wang, F.
Adv. Mater., 2004, 16, 432
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Working principle of polymer photovoltaic
cells (OPV)
1. Absorption of incident light
by the active layer
2. Generation of charge carriers
3. Collection of separated
charge carriers at contacts
Separation of positive and
negative charge carriers by
an asymmetry (junction)
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Large area printed devices




Active area of a single stripe: 10 cm2
Isc: > 10 mA/cm2 (under 100 mW/cm² simulated AM1.5)
Voc: ~ 0.6 V
FF: < 0.5 (limited by serial resistivity of the substrate)
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Working principle of a bulk heterojunction
1. Incoming photons are absorbed
 Creation of excitons on the Donor
/Acceptor
2. Exciton is separated at the donor
/acceptor interface
 Creation of charge carriers
3. Charge carriers within drift distance
reach electrodes
 Creation of short circuit current ISC
1. The “photodoping” leads to splitting of
Fermi levels
 Creation of open circuit voltage VOC
2. Charge transport properties, module
geometry
Pel,max = VOC x ISC x FF
 Fill factor FF
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Correlation between morphology and transport
Fullerene traps e-
e- and h+ are able to go through
[Fullerene] < 17% (no Percolation !)
[Fullerene] > 17%
h+ are blocked
[Fullerene] >> 17%
µh,bulk ~ µh polymer
µh,bulk ~ µh polymer
µh,bulk < µh polymer
µe,bulk < µe polymer
µe,bulk > µe polymer
µe,bulk ~ µe polymer
• Upon blending of materials, macroscopic transport
properties of single components may change significantly
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Integrated Circuits (IC) based on organics
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Block diagram of an identification tag
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Design of organic identification tags
 The 48 bit identification IC
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