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U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat1 Chimica Fisica dei Materiali Avanzati Part 12 Plastic electronics Laurea specialistica in Scienza e Ingegneria dei Materiali Curriculum Scienza dei Materiali Slide 2 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat2 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) Slide 3 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat3 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. Slide 4 U NIVERSITA DEGLI S TUDI DI P ADOVA Large electronic conductivities in organic materials Charge transfer crystals E.g. TTF-TCNQ, first metallic conductivity (1973) Organic superconductors E.g., (TMTSF) 2 PF 6 (1980) (BEDT-TTF) 2 X Corso CFMA. LS-SIMat4 Slide 5 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat5 Conducting Polymers 1977: First conducting polymer, Poly(acetylene) Shirakawa, MacDiarmid, Heeger Slide 6 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat6 Structures of some conjugated polymers Slide 7 U NIVERSITA DEGLI S TUDI DI P ADOVA Electronic structure and charge carriers in conducting polymers Corso CFMA. LS-SIMat7 Polarons (A) and bipolarons (B) in PPP polaronbipolaron 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 Slide 8 U NIVERSITA DEGLI S TUDI DI P ADOVA Doping effect on the optical properties: electrochromism Corso CFMA. LS-SIMat Electrochemical doping of polypyrrole Interband absorption (3 eV) Bipolaron absorptions (2) Polaron absorptions (3) polaronbipolaro n Slide 9 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat9 Current Uses of Conducting Polymers Antistatic Coatings and Conducting Films Electrochromic Displays? Memory Devices? (HP Labs/Princeton) Slide 10 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat10 Light Emitting Diodes 1990: Burroughs, Friend (Cambridge) light emission from undoped semiconducting polymer 2003: full color range possible Slide 11 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat11 OLEDs Everywhere 2000: first commercial products with OLEDs Advantage in color spectrum beats solid state materials Slide 12 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat12 Polymeric Photovoltaics Solar cell efficiencies of ~ 2% (up to 6% in labs) Slide 13 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat13 Thin Film Transistors 2004: both p and n-type materials are known Critical Advances: Crystallinity and purity Slide 14 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat14 Organic Semiconductors Molecular Materials: polycrystalline vapor deposited Polymeric Materials: semi-crystalline solution processed Slide 15 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat15 Mobility of organic semiconductors Slide 16 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat16 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 Slide 17 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat17 Polymer electronics Low-end, high volume electronic applications, based on: Mechanical flexibility Low-cost Large area Potential applications: Electronic barcodes Memories Displays (e-paper) Slide 18 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat18 Rubber Stamped, Large-Area Plastic Active Matrix Backplanes 10 m Design Rules, Patterned by Single-Impression Microcontact Printing PNAS 98(9), 4835-4840 (2001). Science 291, 1502-1503 (2001). Slide 19 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat19 E-paper Slide 20 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat20 Key feature: solution processing Slide 21 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat21 Materials and technology Flexible, all-plastic field effect transistor Slide 22 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat22 Technology Slide 23 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat23 Operation of the polymer transistor Slide 24 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat24 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. Slide 25 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat25 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.; Brdas, J.L.; Logdlund, M.; Salaneck, W.R. Nature, 1999, 397, 121. Slide 26 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat26 Mechanism of electroluminescence in organic semiconductors 1. Charge (electrons and holes) injection Negative polaron = radical anion Positive polaron = radical cation Slide 27 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat27 Mechanism of electroluminescence in organic semiconductors (contd) Slide 28 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat28 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 Slide 29 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat29 Light emitting metal chelates Mitschke, U.; Bauerle, P. J. Mater. Chem. 2000, 10, 1471 Slide 30 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat30 Electroluminescence efficiency Adachi, C.; Baldo, M.A.; Thompson, M.E.; Forrest S.R. J. Appl. Phys. 2001, 90, 5048 Slide 31 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat31 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.; OBrien, 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 Slide 32 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat32 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) Slide 33 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat33 Large area printed devices Active area of a single stripe: 10 cm 2 Isc: > 10 mA/cm 2 (under 100 mW/cm simulated AM1.5) Voc: ~ 0.6 V FF: < 0.5 (limited by serial resistivity of the substrate) Slide 34 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat34 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 P el,max = V OC x I SC x FF Slide 35 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat35 Correlation between morphology and transport Fullerene traps e-e- and h+ are able to go through h+ are blocked [Fullerene] 17% [Fullerene] >> 17% h,bulk ~ h polymer e,bulk < e polymer Upon blending of materials, macroscopic transport properties of single components may change significantly h,bulk ~ h polymer e,bulk > e polymer h,bulk < h polymer e,bulk ~ e polymer Slide 36 U NIVERS

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