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Corso CFMA. LS-SIMat 1
Chimica Fisica dei Materiali AvanzatiChimica Fisica dei Materiali Avanzati
Part 12 – Plastic electronicsPart 12 – Plastic electronics
Laurea specialistica in Scienza e Ingegneria dei MaterialiCurriculum Scienza dei Materiali
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Corso CFMA. LS-SIMat 2
Basic questionsBasic 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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Corso CFMA. LS-SIMat 3
Plastic electronicsPlastic 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 Large electronic conductivities in organic materialsorganic materials
Charge transfer crystals E.g. TTF-TCNQ, first metallic
conductivity (1973)
Organic superconductors E.g., (TMTSF)2PF6 (1980)
(BEDT-TTF)2X
Corso CFMA. LS-SIMat 4
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Corso CFMA. LS-SIMat 5
Conducting PolymersConducting Polymers1977: First conducting polymer, Poly(acetylene)
Shirakawa, MacDiarmid, Heeger
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Corso CFMA. LS-SIMat 6
Structures of some conjugated Structures of some conjugated polymerspolymers
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Electronic structure and charge carriers in Electronic structure and charge carriers in conducting polymersconducting polymers
Corso CFMA. LS-SIMat 7
Polarons (A) and bipolarons (B) in PPP
polaron bipolaron
A polaron (= radical ion) has both charge (+e) and spin (±1/2)
A bipolaron (dication) has charge (+2e) but no spin
In conducting polymers, doping is the result of a redox process.
Charges are bound and deep in the gap
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Doping effect on the optical Doping effect on the optical properties: electrochromismproperties: electrochromism
Corso CFMA. LS-SIMat
Interband absorption (3 eV)
Bipolaron absorptions (2)
Polaron absorptions (3)
polaron bipolaron
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Corso CFMA. LS-SIMat 9
Current Uses of Conducting Current Uses of Conducting PolymersPolymers
Antistatic Coatings and Conducting Films
Electrochromic Displays?
Memory Devices? (HP Labs/Princeton)
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Corso CFMA. LS-SIMat 10
Light Emitting DiodesLight Emitting Diodes1990: Burroughs, Friend (Cambridge)
light emission from undoped semiconducting polymer
2003: full color range possible
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Corso CFMA. LS-SIMat 11
OLEDs EverywhereOLEDs Everywhere
2000: first commercial products with OLEDs
Advantage in color spectrum beats solid state materials
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Corso CFMA. LS-SIMat 12
Polymeric PhotovoltaicsPolymeric Photovoltaics
Solar cell efficiencies of ~ 2% (up to 6% in labs)
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Corso CFMA. LS-SIMat 13
Thin Film TransistorsThin Film Transistors
2004: both p and n-type materials are known
Critical Advances: Crystallinity and purity
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Corso CFMA. LS-SIMat 14
Organic SemiconductorsOrganic SemiconductorsMolecular Materials:•polycrystalline•vapor deposited
Polymeric Materials:•semi-crystalline•solution processed
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Corso CFMA. LS-SIMat 15
Mobility of organic semiconductorsMobility of organic semiconductors
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Corso CFMA. LS-SIMat 16
Motivations for organic electronicsMotivations 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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Corso CFMA. LS-SIMat 17
Polymer electronicsPolymer 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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Corso CFMA. LS-SIMat 18
Rubber Stamped, Large-Area Plastic Rubber Stamped, Large-Area Plastic Active Matrix BackplanesActive 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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Corso CFMA. LS-SIMat 19
E-paperE-paper
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Corso CFMA. LS-SIMat 20
Key feature: solution processingKey feature: solution processing
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Corso CFMA. LS-SIMat 21
Materials and technologyMaterials and technology
Flexible, all-plastic field effect transistor
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Corso CFMA. LS-SIMat 22
TechnologyTechnology
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Corso CFMA. LS-SIMat 23
Operation of the polymer transistorOperation of the polymer transistor
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Corso CFMA. LS-SIMat 24
Light emitting diodeLight emitting diodeOrganic 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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Corso CFMA. LS-SIMat 25
Steps of the electroluminescence Steps of the electroluminescence processprocess
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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Corso CFMA. LS-SIMat 26
Mechanism of electroluminescence in Mechanism of electroluminescence in organic semiconductorsorganic semiconductors
1. Charge (electrons and holes) injection
Negative polaron = radical anion Positive polaron = radical cation
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Corso CFMA. LS-SIMat 27
Mechanism of electroluminescence in Mechanism of electroluminescence in organic semiconductors (cont’d)organic semiconductors (cont’d)
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Corso CFMA. LS-SIMat 28
Some common electroluminescent Some common electroluminescent polymers:polymers:
poly(phenylenevinylene)s (PPVs)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, Chemistry, Physics and Engineering” Hadziioannou G and van Hutten, P.F. Eds. Wiley-VCH 1999, pp1-32
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Corso CFMA. LS-SIMat 29
Light emitting metal chelatesLight emitting metal chelates
Mitschke, U.; Bauerle, P. J. Mater. Chem. 2000, 10, 1471
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Corso CFMA. LS-SIMat 30
Electroluminescence efficiencyElectroluminescence efficiency
Adachi, C.; Baldo, M.A.; Thompson, M.E.; Forrest S.R. J. Appl. Phys. 2001, 90, 5048
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Corso CFMA. LS-SIMat 31
PHOSPHORESCENT OLEDS (PHOLED)sPHOSPHORESCENT 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
Zhang, Q.; Zhou, Q.; Cheng, Y.; Wang, L.; Ma, D.; Jing, X.; Wang, F. Adv. Mater., 2004, 16, 432
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Corso CFMA. LS-SIMat 32
Working principle of polymer Working principle of polymer photovoltaic cells (OPV)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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Corso CFMA. LS-SIMat 33
Large area printed devicesLarge 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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Corso CFMA. LS-SIMat 34
Working principle of a bulk heterojunctionWorking 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 Fill factor FF Pel,max = VOC x ISC x FF
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Corso CFMA. LS-SIMat 35
Correlation between morphology and Correlation between morphology and transporttransport
Fullerene traps e- e- and h+ are able to go through h+ are blocked
[Fullerene] < 17% (no Percolation !) [Fullerene] > 17% [Fullerene] >> 17%
µh,bulk ~ µh polymer
µe,bulk < µe polymer
• Upon blending of materials, macroscopic transport properties of single components may change significantly
µh,bulk ~ µh polymer
µe,bulk > µe polymer
µh,bulk < µh polymer
µe,bulk ~ µe polymer
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Corso CFMA. LS-SIMat 36
Integrated Circuits (IC) based on organicsIntegrated Circuits (IC) based on organics
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Corso CFMA. LS-SIMat 37
Block diagram of an identification tagBlock diagram of an identification tag
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Corso CFMA. LS-SIMat 38
Design of organic identification tagsDesign of organic identification tags
The 48 bit identification IC