5... change in the spectral position of aborption /luminescence band due to change in the polarity of the medium solvation is a physical perturbation of lumophore’s molecular states isolated molecule (in a gas phase) and solvated moleculeare in the same chemical state(no solvent induced proton or electron transfer, ionization, complexation, isomerization)
6Solid State Solvation (SSS) polarlumophore“self polarization”for strongly dipolar lumophores(aggregation possible for highly polar molecules)dipolar hostwith moment μ
7on Chromatic Shift Direction Influence of μ0 and μ1on Chromatic Shift Directionsolute (chromophore)WITH DIPOLE MOMENT μsolventBathochromic (red) PL shiftHypsochromic (blue) PL shift
8PL of DCM2 Solutions and Thin Film Bulović et al., Chem. Phys. Lett. 287, 455 (1998).
10Thin Film Photoluminescence 1% DCM2 in Alq3polar hostμ ~ 5.5 D1% DCM2 in Zrq4non-polar host
11A “Cleaner” Experiment • Employ trace DCM2 so as to effectively eliminate aggregation• But still appreciably change local medium ⇒ use another dopant!• should be polar and optically inactive (i.e. wide band gap)
12CA Doping and Electronic Susceptibility Peak PL Energy of PS:CA:DCM2 Films42 nm red shift from 0 to 25% CAResults unchanged even for 10xhigher DCM2 concentration:DCM2 aggregation not the answerBulk Electronic Susceptibility of PS:CA:DCM2 FilmsLocal fields are responsible for thespectral shifts…… and dielectric measurementssuggest a “solvatochromic” effect.
27Parameters for Simulating Exciton Diffusion observed radiative lifetime (τ)Förster radius (RF)► Assign value for allowed transfers:Normalized IntegratedSpectral Intensityexcitonic density of states (gex(E))► Assume Gaussian shape of width, wDOS► Center at peak of initial bulk PL spectrum► Molecular PL spectrum implied…
28Fitting Simulation to Experiment – Doped Films • Good fits possible for all data sets• RF decreases with increasing doping,falling from 52 Å to 22 Å• wDOS also decreases with increasing doping,ranging from eV to eV
29Fitting Simulation – Neat Films • Spectral shift observed in each material system• Molecular dipole and wDOS are correllated: lowerdipoles correspond to less dispersion• Even with no dipole, some dispersion exists• Experimental technique general, and yields firstmeasurements of excitonic energy dispersionin amorphous organic solids
30Temporal Solid State Solvation upon excitation both magnitude and directionof lumophore dipole moment can changeFOR EXAMPLE for DCM: µ1 – µ0 > 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 µ • Eloc)
31Exciton Distribution in the Excited State (S1 or T1) ~ Time Evolved Molecular Reconfiguration ~log(Time)DIPOLE-DIPOLE INTERACTIONLEADS TO ENERGY SHIFT IN DENSITY OF EXCITED STATES
32Fusion of Two Material Sets Hybrid devicescould enableLEDs, Solar Cells,Photodetectors,Modulators, andLaserswhich utilize thebest properties ofeach individualmaterial.EfficientEmissiveOrganicSemiconductorsFabrication ofrational structureshas been the mainobstacle to date.Flexible
33Inorganic Nanocrystals – Quantum Dots Quantum Dot SIZESynthetic route of Murrayet al, J. Am. Chem. Soc.115, 8706 (1993).
34Fusion of Two Material Sets Quantum DotsOrganic Molecules
35Integration of Nanoscale Materials Quantum Dots and Organic 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)N,N'-Bis(3-methylphenyl)-N,N'-bis-(phenyl)-benzidine3-(4-Biphenylyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazoleN,N'-Bis(naphthalen-1-yl)-N,N'-bis(phenyl)benzidine
36Phase Segregation and Self-Assembly 1. A solution of an organicmaterial, QDs, and solvent…2. is spin-coated onto a cleansubstrate.3. During the solvent dryingtime, the QDs rise to thesurface…4. and self-assemble intograins of hexagonally closepacked 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.
37Monolayer Coverage – QD concentration As the concentration ofQDs in the spin-castingsolution is increased,the coverage of QDs onthe monolayer is alsoincreased.
39Full Size Series of PbSe Nanocrystals from 3 nm to 10 nm in Diameter
40Design of Device Structures QDs are poor chargetransport materials...But efficient emitters…Isolate layer functions of maximizedevice performanceUse organics for charge transport.1. Generate excitons on organic sites.2. Transfer excitons to QDs via Försteror Dexter energy transfer.3. QD electroluminescence.Need a new fabrication method inorder to be able to make such doubleheterostructures:Phase Segregation.
41A general method? • This process is robust, but further exploration Phase segregation occursfor 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,Glass, ITO.7) Spin parameters:speed, accelerationand time.• This process is robust, but further explorationis needed to broadly generalize these findings.• For the explored materials, consistentdescription is possible.• We have shown that the process is notdependent on any one material component.Phase segregation QD-LED structures
42EL Recombination Region Dependence on Current Coe et al., Org. Elect. (2003)
43Spectral Dependence on Current Density TOP DOWN VIEW of the QD MONOLAYERExciton recombinationwidth far exceeds the QDmonolayer thickness athigh current density.To achieve truemonochrome emission,new exciton confinementtechniques are needed.CROSS-SECTIONAL VIEW of QD-LED
44Benefits of Quantum Dots in Organic LEDs Demonstrated:•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!