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

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 Corso CFMA. LS-SIMat

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 Corso CFMA. LS-SIMat

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 Corso CFMA. LS-SIMat

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

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

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 Corso CFMA. LS-SIMat

Doping effect on the optical properties: electrochromism Electrochemical doping of polypyrrole Bipolaron absorptions (2) polaron bipolaron Interband absorption (3 eV) Polaron absorptions (3) Corso CFMA. LS-SIMat

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

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

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

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

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

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

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

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 Corso CFMA. LS-SIMat

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 Corso CFMA. LS-SIMat

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 Corso CFMA. LS-SIMat

E-paper Corso CFMA. LS-SIMat Corso CFMA. LS-SIMat

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

Materials and technology Flexible, all-plastic field effect transistor Corso CFMA. LS-SIMat Corso CFMA. LS-SIMat

Technology Corso CFMA. LS-SIMat Corso CFMA. LS-SIMat

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

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 Corso CFMA. LS-SIMat

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 Corso CFMA. LS-SIMat

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

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

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 Corso CFMA. LS-SIMat

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

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

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 Corso CFMA. LS-SIMat

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 Corso CFMA. LS-SIMat

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 Corso CFMA. LS-SIMat

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 Corso CFMA. LS-SIMat

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 Corso CFMA. LS-SIMat

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

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

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