1@ MITApril 29, 2003 – Organic Optoelectronics - Lecture 20Organic LEDs – part 7• Solvation Effect - Review• Solid State Solvation• Exciton Dynamics in Disordered Organic Thin Films• Quantum Dot LEDsHandout on QD-LEDs: Coe et al., Nature 420, 800 (2002).Electroluminescence in Doped Organic Films2.Excitons transfer toluminescent dye1.Excitons formedfrom combinationof electrons andholes6.0 eVa-NPD2.6 eV5.7eVAlq32.7 eVelectronsexcitontrap stateslow work functioncathodetransparent anodeholesdopant molecule(luminescent dye)host molecules(charge transportmaterial)2400 500 600 700 8000.00.20.40.60.81.0Intensity [a.u.]Wavelength [nm]0.5 %1.5 %2.5 %4.5 %6 %Electroluminescence of x% DCM2 in Alq3OLEDsAlq335 nm35 nmtuning rangeAlqAlq33DCM2 in AlqDCM2 in Alq33low DCM2low DCM2high DCM2high DCM23... change in the spectral position of... change in the spectral position ofaborptionaborption/luminescence band/luminescence banddue to change in the polarity of the mediumdue to change in the polarity of the mediumÖsolvationsolvation is a physical perturbationphysical perturbation of lumophore’s molecular statesÖ isolated molecule (in a gas phase) and solvated moleculeare in the same chemical state(no solvent induced proton or electron transfer, ionization, complexation, isomerization)dipolarlumophoredipolarmoleculedipole - dipole interactionmodifies the energy structuremodifies the energy structureof the molecules∆E = ∆ 〈µ•Eloc〉∆E = ∆ 〈µ•Eloc〉Solid State Solvation (SSS)ElocRdipolar hostwith moment µpolarlumophore〈µ〉 > 0〈µ〉→ 0as R → large“self polarization”for strongly dipolar lumophores(aggregation possible for highly polar molecules)Eloc~ 107V/cm4Influence of µ0and µ1on Chromatic Shift Directionsolventsolute (chromophore)WITH DIPOLE MOMENT µµ0< µ1µ0< µ1SOLVENT POLARITYS0S1groundstateexcitedstateµ0> µ1µ0> µ1SOLVENT POLARITYS0S1∆E = ∆ 〈µ•E〉∆E = ∆ 〈µ•E〉Æ Bathochromic (red) PL shift Æ Hypsochromic (blue) PL shiftC6H6CHCl3C2H5OH (CH3)2S:O600 700 8000.00.51.0Intensity [a.u.]Wavelength [nm]C6H6CHCl3C2H5OH (CH3)2S:Othin filmthin film0 1.15 1.69 3.9OCNNCNSOLVENTDIPOLE MOMENTBulović et al., Chem. Phys. Lett. 287, 455 (1998).PL of DCM2 Solutions and Thin Film~ SOLVATOCHROMIC ~SHIFT[bathochromic (red) shift]5Ground State(equilibrium)µg, Eg,loc1µe, E*g,locExcited State(non-equilibrium)2Excited State(equilibrium)µe, Ee,loc3µg, E*e,locGround State(non-equilibrium)4Continuum, Dipole in Spherical Cavity Model:312)1(2aµεεElocrr+−=≡ΓεDynamic Relaxation Picture (a.k.a. solvation)500 600 700 800 9000.00.20.40.60.81.0Intensity [a.u.]Wavelength [nm]0.00.20.40.60.81.0Intensity [a.u.]1% DCM2 in Alq3Alq31% DCM2 in Zrq4Zrq4polar hostµ ~ 5.5 Dnon-polar host635 nm605 nmThin Film Photoluminescence6A “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)DCM2Camphoric Anhydride (CA)Polystyrene (PS)1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.50.00.20.40.60.81.025%0%0.005% DCM2 DopingPL Spectra of Different PS:CA:DCM2 Films Normalized IntensityEnergy (eV)CA %:CA Doping and Electronic Susceptibility 0 5 10 15 20 2523456ε = 2.44 + 0.13 x (CA %)0.005% DCM2 DopingBulk Electronic Susceptibility of PS:CA:DCM2 Films Epsilon (at 100 KHz)Camphoric Anhydride Concentration (%)42 nm red shift from 0 to 25% CAResults unchanged even for 10x higher DCM2 concentration:DCM2 aggregation not the answerLocal fields are responsible for the spectral shifts…… and dielectric measurements suggest a “solvatochromic” effect.0 5 10 15 20 252.042.062.082.102.122.142.162.182.202.22605 nm563 nm0.005% DCM2 DopingPeak PL Energy of PS:CA:DCM2 Films Energy (eV)Camphoric Anhydride Concentration (%)7Dynamic Relaxation Picture (a.k.a. solvation)Solvation TheoryGround State(equilibrium)µg, Eg,loc1µe, E*g,locExcited State(non-equilibrium)2Excited State(equilibrium)µe, Ee,loc3µg, E*e,locGround State(non-equilibrium)4Continuum, Dipole in Spherical Cavity Model:312)1(2aµεεElocrr+−=≡Γεconstant with CA%n nearly constant with CA% (ranging from ~1.55 to ~1.65)gasE↓AEεΓ∆ −≈↓where3∆aµµAerr⋅=()32ΓΓ∆ aµµµEEgneεgasrrr+⋅−=↓↓Connecting Theory to Experiment0.4 0.5 0.6 0.7 0.8 0.9 1.01.901.952.002.052.102.152.20 Benzene Toluene Chloroform Dichlormethane Acetone Acetonitrile MethanolDCM2 Peak PL in Various SolventsSlope givesA ~ 0.55 eV Energy (eV)Γ (no units)()+−≡1212Γεεε0246810120.00.10.20.30.40.50.60.70.80.91.0ε (no units)Γ (no units)Gamma Functional Shape-5 0 5 10 15 20 25 302.022.042.062.082.102.122.142.162.182.202.220.52 eV0.62 eV0.57 eVA =Evolution of Peak PL Energy for PS:CA:DCM2 Films0.005% DCM2 Doping Experiment Energy (eV)Camphoric Anhydride Concentration (%)-5 0 5 10 15 20 25 302.022.042.062.082.102.122.142.162.182.202.22Evolution of Peak PL Energy for PS:CA:DCM2 Films0.005% DCM2 Doping Experiment Energy (eV)Camphoric Anhydride Concentration (%)8Exciton Dynamics in Time Dependant PLwavelengthtime2 ns25 nmtimewavelengthDynamic Spectral Shifts of DCM2 in Alq31.7 1.8 1.9 2.0 2.10.00.20.40.60.81.0720 680 640 6002% DCM2:Alq36.2 ns3.4 ns2.2 ns1.6 ns1.0 ns0.6 ns0.3 ns0.1 ns0 nstimeNormalized IntensityEnergy [ eV ]Wavelength [ nm ]DCM2Alq3~ ~ DCM2 PL red shifts > 20 nm over 6 ns ~DCM2 PL red shifts > 20 nm over 6 ns ~• Measurement performed on doped DCM2:Alq3films• Excitation at λ=490 nm (only DCM2 absorbs)1.71.81.92.02.1024681012Energy [ eV ]Intensity [ arb. units ]91.6 1.8 2.0 2.20.00.20.40.60.81.0Integrated Spectrum(0-10 ns)Spectrumat 1 nsNormalized IntensityEnergy [eV]750 700 650 600 550Wavelength [nm]Spectrumat 4 nsTime Evolution of 4% DCM2 in Alq3PL SpectrumT1S1S0FLUORESCENCEPHOSPHORESCENCEENERGY TRANSFERFÖRSTER, DEXTERor RADIATIVEINTERNALCONVERSIONABSORPTION10 ps1-10 ns>100 nsEnergydensity of availableS1 or T1 states Electronic Processes in Molecules100123451.801.851.901.952.002.052.102.152.20CH3CNAcetoneDMSOCHCl3CHCl2BenzeneEnergy [eV]Time [ns]680660640620600580Wavelength [nm]Time Evolution of DCM2 Solution PL SpectraWavelength [nm]Time [ns]012345600 650 700 75010% DCM2 in Alq310% DCM2 in Alq3Spectral Shift due to~ Exciton Diffusion ~~ Intermolecular Solid State Interactions ~35 nm35 nmwavelength shift11Each dye molecule experiences a different local medium⇒ variations in excitonic