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Chimica Fisica dei Materiali Avanzati Part 12 – Plastic electronics

Chimica Fisica dei Materiali Avanzati Part 12 – Plastic electronics. Laurea specialistica in Scienza e Ingegneria dei Materiali Curriculum Scienza dei Materiali. Basic questions. Is it possible to do electronics with molecules?

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Chimica Fisica dei Materiali Avanzati Part 12 – Plastic electronics

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  1. Chimica Fisica dei Materiali AvanzatiPart 12 – Plastic electronics Laurea specialistica in Scienza e Ingegneria dei Materiali Curriculum Scienza dei Materiali Corso CFMA. LS-SIMat

  2. 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) Corso CFMA. LS-SIMat

  3. 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. Corso CFMA. LS-SIMat

  4. Large electronic conductivities in organicmaterials • 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

  5. Conducting Polymers 1977: First conducting polymer, Poly(acetylene) Shirakawa, MacDiarmid, Heeger Corso CFMA. LS-SIMat

  6. Structures of some conjugated polymers Corso CFMA. LS-SIMat

  7. Electronic structure and chargecarriers in conductingpolymers In conductingpolymers, doping is the resultof a redoxprocess. Charges are bound and deep in the gap polaron bipolaron • 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 Corso CFMA. LS-SIMat

  8. Doping effect on the opticalproperties: electrochromism Electrochemical doping ofpolypyrrole Bipolaron absorptions (2) polaron bipolaron Interband absorption (3 eV) Polaron absorptions (3) Corso CFMA. LS-SIMat

  9. Current Uses of Conducting Polymers Antistatic Coatings and Conducting Films Electrochromic Displays? Memory Devices? (HP Labs/Princeton) Corso CFMA. LS-SIMat

  10. Light Emitting Diodes 1990: Burroughs, Friend (Cambridge) light emission from undoped semiconducting polymer 2003: full color range possible Corso CFMA. LS-SIMat

  11. OLEDs Everywhere 2000: first commercial products with OLEDs Advantage in color spectrum beats solid state materials Corso CFMA. LS-SIMat

  12. Polymeric Photovoltaics Solar cell efficiencies of ~ 2% (up to 6% in labs) Corso CFMA. LS-SIMat

  13. Thin Film Transistors 2004: both p and n-type materials are known Critical Advances: Crystallinity and purity Corso CFMA. LS-SIMat

  14. Organic Semiconductors • Molecular Materials: • polycrystalline • vapor deposited • Polymeric Materials: • semi-crystalline • solution processed Corso CFMA. LS-SIMat

  15. Mobility of organic semiconductors Corso CFMA. LS-SIMat

  16. 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 Corso CFMA. LS-SIMat

  17. Polymer electronics • Low-end, high volume electronic applications, based on: • Mechanical flexibility • Low-cost • Large area • Potential applications: • Electronic barcodes • Memories • Displays (e-paper) Corso CFMA. LS-SIMat

  18. 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). Corso CFMA. LS-SIMat

  19. E-paper Corso CFMA. LS-SIMat

  20. Key feature: solution processing Corso CFMA. LS-SIMat

  21. Materials and technology Flexible, all-plasticfieldeffect transistor Corso CFMA. LS-SIMat

  22. Technology Corso CFMA. LS-SIMat

  23. Operation of the polymer transistor Corso CFMA. LS-SIMat

  24. 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. Corso CFMA. LS-SIMat

  25. 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. Corso CFMA. LS-SIMat

  26. Mechanism of electroluminescence in organic semiconductors 1. Charge (electrons and holes) injection Positive polaron = radical cation Negative polaron = radical anion Corso CFMA. LS-SIMat

  27. Mechanism of electroluminescence in organic semiconductors (cont’d) Corso CFMA. LS-SIMat

  28. 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, Chemistry, Physics and Engineering” Hadziioannou G and van Hutten, P.F. Eds. Wiley-VCH 1999, pp1-32 Corso CFMA. LS-SIMat

  29. Light emitting metal chelates Mitschke, U.; Bauerle, P. J. Mater. Chem. 2000, 10, 1471 Corso CFMA. LS-SIMat

  30. Electroluminescence efficiency Adachi, C.; Baldo, M.A.; Thompson, M.E.; Forrest S.R. J. Appl. Phys. 2001, 90, 5048 Corso CFMA. LS-SIMat

  31. 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 Zhang, Q.; Zhou, Q.; Cheng, Y.; Wang, L.; Ma, D.; Jing, X.; Wang, F. Adv. Mater., 2004, 16, 432 Corso CFMA. LS-SIMat

  32. 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) Corso CFMA. LS-SIMat

  33. 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) Corso CFMA. LS-SIMat

  34. 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 • Fill factor FF Pel,max = VOC x ISC x FF Corso CFMA. LS-SIMat

  35. Correlation between morphology and transport 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 µh,bulk ~ µh polymer µe,bulk < µe polymer µh,bulk ~ µh polymer µe,bulk > µe polymer • Upon blending of materials, macroscopic transport properties of single components may change significantly Corso CFMA. LS-SIMat

  36. Integrated Circuits (IC) based on organics Corso CFMA. LS-SIMat

  37. Block diagram of an identification tag Corso CFMA. LS-SIMat

  38. Design of organic identification tags • The 48 bit identification IC Corso CFMA. LS-SIMat

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