1、1Organic LEDs part 8 Exciton Dynamics in Disordered Organic Thin Films Quantum Dot LEDsHandout on QD-LEDs:Coe et al.,Nature 420,800(2002).April 29,2003 Organic Optoelectronics-Lecture 20b2Exciton Dynamics in Time Dependant PL3Dynamic Spectral Shifts of DCM2 in Alq3 Measurement performed on doped DCM
2、2:Alq3 films Excitation at=490 nm(only DCM2 absorbs)DCM2 PL red shifts 20 nm over 6 ns Wavelength nm 4Time Evolution of 4%DCM2 in Alq3 PL Spectrum5Electronic Processes in Moleculesdensity of availableS1 or T1 states6Time Evolution of DCM2 Solution PL Spectra7Spectral Shift due to Exciton Diffusion I
3、ntermolecular Solid State Interactions 8Excitonic Energy Variations9Exciton Distribution in the Excited State(S1 or T1)Time Evolved Exciton Thermalization EXCITON DIFFUSION LEADS TO REDUCTION IN FWHM10111213Time Evolution of Peak PL in Neat Thin Films14Parameters for Simulating Exciton Diffusionobse
4、rved radiative lifetime()Normalized IntegratedSpectral IntensityFrster radius(RF)Assign value for allowed transfers:Assume Gaussian shape of width,wDOS Center at peak of initial bulk PL spectrum Molecular PL spectrum impliedexcitonic density of states(gex(E)15Fitting Simulation to Experiment Doped F
5、ilms Good fits possible for all data sets RF decreases with increasing doping,falling from 52 to 22 wDOS also decreases with increasing doping,ranging from 0.146 eV to 0.120 eV16Fitting Simulation Neat Films Spectral shift observed in each material system Molecular dipole and wDOS are correllated:lo
6、wer dipoles correspond to less dispersion Even with no dipole,some dispersion exists Experimental technique general,and yields firstmeasurements of excitonic energy dispersionin amorphous organic solids17Temporal Solid State Solvationupon excitation both magnitude and directionof lumophore dipole mo
7、ment can changeFOR EXAMPLE for DCM:1 0 20 Debye!from 5.6 D to 26.3 D following the excitation the environment surrounding theexcited molecule will reorganize to minimize the overallenergy of the system(maximize Eloc)18Exciton Distribution in the Excited State(S1 or T1)Time Evolved Molecular Reconfig
8、uration DIPOLE-DIPOLE INTERACTIONLEADS TO ENERGY SHIFT IN DENSITY OF EXCITED STATESlog(Time)19Fusion of Two Material SetsHybrid devicescould enableLEDs,Solar Cells,Photodetectors,Modulators,and Laserswhich utilize thebest properties of each individual material.EfficientOrganicSemiconductorsFlexibleE
9、missiveFabrication ofrational structureshas been the mainobstacle to date.20Inorganic Nanocrystals Quantum DotsQuantum Dot SIZESynthetic route of Murrayet al,J.Am.Chem.Soc.115,8706(1993).21Fusion of Two Material SetsQuantum DotsOrganic Molecules22Integration of Nanoscale MaterialsQuantum Dots and Or
10、ganic SemiconductorsZnS overcoating shell(0 to 5 monolayers)Oleic Acid orTOPO capsSynthetic routes of Murray etal,J.Am.Chem.Soc.115,8706(1993)and Chen,et al,MRS Symp.Proc.691,G10.2.Trioctylphosphine oxideTris(8-hydroxyquinoline)Aluminum(III)3-(4-Biphenylyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole
11、N,N-Bis(naphthalen-1-yl)-N,N-bis(phenyl)benzidineN,N-Bis(3-methylphenyl)-N,N-bis-(phenyl)-benzidine231.A solution of an organic material,QDs,and solvent2.is spin-coated onto a clean substrate.3.During the solvent drying time,the QDs rise to the surface4.and self-assemble into grains of hexagonally c
12、lose packed spheres.Organic hosts that deposit as flat filmsallow for imaging via AFM,despite theAFM tip being as large as the QDs.Phase segregation is driven by acombination of size and chemistry.Phase Segregation and Self-Assembly24As the concentration ofQDs in the spin-castingsolution is increase
13、d,the coverage of QDs onthe monolayer is alsoincreased.Monolayer Coverage QD concentration25CdSe(ZnS)/TOPOPbSe/oleic acidQD-LED Performance26Full Size Series of PbSe Nanocrystalsfrom 3 nm to 10 nm in Diameter27Design of Device StructuresQDs are poor chargetransport materials.Isolate layer functions
14、of maximizedevice performance1.Generate excitons on organic sites.2.Transfer excitons to QDs via Frster or Dexter energy transfer.3.QD electroluminescence.Phase Segregation.But efficient emittersUse organics for charge transport.Need a new fabrication method inorder to be able to make such doublehet
15、erostructures:28A general method?Phase segregation occurs for different1)organic hosts:TPD,NPD,and poly-TPD.2)solvents:chloroform,chlorobenzene,andmixtures with toluene.3)QD core materials:PbSe,CdSe,andCdSe(ZnS).4)QD capping molecules:oleic acid and TOPO.5)QD core size:4-8nm.6)substrates:Silicon,Gla
16、ss,ITO.7)Spin parameters:speed,accelerationand time.This process is robust,but further exploration is needed to broadly generalize these findings.For the explored materials,consistent description is possible.We have shown that the process is not dependent on any one material component.Phase segregat
17、ion QD-LED structures29EL RecombinationRegion Dependenceon CurrentCoe et al.,Org.Elect.(2003)30Spectral Dependence on Current DensityTOP DOWN VIEW of the QD MONOLAYERExciton recombinationwidth far exceeds the QDmonolayer thickness athigh current density.To achieve truemonochrome emission,new exciton
18、 confinementtechniques are needed.CROSS-SECTIONAL VIEW of QD-LED31Benefits of Quantum Dots in Organic LEDsDemonstrated:Spectrally Tunable single material set can access most of visible range.Saturated Color linewidths of 35nm Full Width at Half of Maximum.Can easily tailor“external”chemistry without affecting emitting core.Can generate large area infrared sources.Potential:High luminous efficiency LEDs possible even in red and blue.Inorganic potentially more stable,longer lifetimes.The ideal dye molecule!
