1COURSE MISSIONexamine optical and electronic processes in organic molecules and polymersthat govern the behavior of practical organic optoelectronic devicesVladimir BulovićWelcome to 6.973Welcome to 6.973~ Organic ~ Organic OptoOpto--Electronics ~Electronics ~@ MITFebruary 4, 2003 - Lecture 12March Towards Molecular Electronics Adapted from L.L. Sohn, Nature 394, 131 (1998). The shrinkage of electronic components. The length scale reached by technology has dropped steadily from the millimeter scale of the early 1950s to the present-day atomic scale. In 1950, the first transistor measured 1mm. Quantum-dot turnstiles of the 1980s measured 10um. Quantum corrals, invented in the 1970s measured 100nm. The latest device is a one-atom point contact.3Organic Materials … TWO GENERAL CLASSESAlq3MOLECULAR MATERIALSAttractive due to:Attractive due to:•• Integrability with inorganic semiconductors•• Low cost (fabric dyes, biologically derived materials)•• Large area bulk processing possible•• Tailor molecules for specific electronic or optical properties•• Unusual properties not easily attainablewith conventional materials PPVPOLYMERSnBut problems exist:But problems exist:•• Stability•• Patterning•• Thickness control of polymers•• Low carrier mobility4Scientific Interest in Organic Materials• 1828 - Wöhler first synthesized ureawithout the assistance of a living organism• 1950’s - steady work on crystalline organics starts• 1970’s - organic photoconductors (xerography)• 1980’s - organic non-linear optical materials• 1987 - Kodak group published the first efficient organic light emitting device (OLED)• Since then, the field has dramatically expandedboth commercially and scientifically(OLEDs, transistors, solar cells, lasers, modulators, ... )to date, about two million organic compounds have been madeto date, about two million organic compounds have been made--this constitutes nearly 90% of all known materials this constitutes nearly 90% of all known materials --5The Royal Swedish Academy of Sciences awards the Nobel Prize in Chemistry for 2000 jointly to:•Alan J. Heeger, University of California at Santa Barbara, USA,•Alan G. MacDiarmid, University of Pennsylvania, Philadelphia, USA,•Hideki Shirakawa, University of Tsukuba, Japan "for the discovery and development of conductive polymers"Plastic that conducts electricityWe have been taught that plastics, unlike metals, do not conduct electricity. In fact plastic is used as insulation round the copper wires in ordinary electric cables. Yet this year's Nobel Laureates in Chemistry are being rewarded for their revolutionary discovery that plastic can, after certain modifications, be made electrically conductive. Plastics are polymers, molecules that repeat their structure regularly in long chains. For a polymer to be able to conduct electric current it must consist alternately of single and double bonds between the carbon atoms. It must also be "doped", which means that electrons are removed (through oxidation) or introduced (through reduction). These "holes" or extra electrons can move along the molecule - it becomes electrically conductive. Heeger, MacDiarmid and Shirakawa made their seminal findings at the end of the 1970s and have subsequently developed conductive polymers into a research field of great importance for chemists as well as physicists. The area has also yielded important practical applications. Conductive plastics are used in, or being developed industrially for, e.g. anti-static substances for photographic film, shields for computer screen against electromagnetic radiation and for "smart" windows (that can exclude sunlight). In addition, semi-conductive polymers have recently been developed in light-emitting diodes, solar cells and as displays in mobile telephones and mini-format television screens. Research on conductive polymers is also closely related to the rapid development in molecular electronics. In the future we will be able to produce transistors and other electronic components consisting of individual molecules -which will dramatically increase the speed and reduce the size of our computers. A computer corresponding to what we now carry around in our bags would suddenly fit inside a watch.http://www.nobel.se/chemistry/laureates/2000/press.htmlNobel Prize in Chemistry for 20006Electronic Processes inMolecules / Aggregates / Thin FilmsT1S1S0FLUORESCENCEPHOSPHORESCENCEENERGY TRANSFERFÖRSTER, DEXTERor RADIATIVEINTERNALCONVERSIONABSORPTION10 ps1-10 ns>100 nsS1 and T1 state densityEnergyBulovic et al., Chem. Phys. Lett. 287, 455 (1998); 308, 317 (1999).´AlqAlq33DCM2 in AlqDCM2 in Alq33low DCM2low DCM2high DCM2high DCM2Intensity [a.u.]Wavelength [nm]Time [ns]0.25 ns0.50 ns0.75 ns1.00 ns1.50 ns2.00 ns5.00 ns4812160123450600 650 700 75035 nm35 nmwavelength shift10% DCM2 in Alq310% DCM2 in Alq3Temporal ResponseTemporal ResponseSolid State SolvationSolid State Solvation7STM scan ofordered PTCDA monolayer on HOPGOrganic Thin Films … may be AMORPHOUS or CRYSTALLINEmolecular orbital calculation of the electron density in the highest occupied molecular orbital of a PTCDA moleculeAgreement between the calculation and the experiment exemplifies maturity of detailed understanding of electronic arrangement on molecules.However, …DYNAMIC ELECTRONIC PROCESSES in MOLECULES and MOLECULAR ASSEMBLIESare NOT WELL UNDERSTOODand present a topic of our research8Tetracene Thin Film Growth is affected by …Growth Rate20 Å/s0.2 Å/sSurface TreatmentSurface Nano-Structure(Guided Growth)4 x 4 µm4 x 4 µm16 x 16 µm2 x 2 µmMascaro, et al., unpublished.9-VGsource drainsubstrateinsulator gate-VDsemiconductorpentaceneTdeposition=27 °CDR=1.0 Å/secµ~0.6 cm2V-1s-1Adapted from Dimitrakopoulos, et. al., IBM J. Res. and Devel. 45, 11 (2001).IMPROVED MOLECULAR ORDERINGLarger grain sizesLower defect densitiesEnhanced mobilityCharge carrier mobility is dependent on molecular order within the semiconducting thin filmOrganic Field Effect TransistorsIBMPlastic Logic10+electrons and holesform excitonsexcitons(bound e--h+pairs)some excitons radiatesome excitons radiateHOMOLUMOrecombination regionETLHTLE_Kodak/Sanyo AM-OLED QVGA Display~2 mmOrganic Light Emitting DevicesV+-GlassITOAlq3TPDMg:AgAg~1000 Å~500 Å~500 ÅAlq3AlNO3Alq3AlNO3TPDNNTPDNN11Opportunities …• LEDs• Lasers (Optically and Electrically Pumped)• Solar Cells and Photodetectors• Transistors•