1、1,Organic LEDs part 8,Exciton Dynamics in Disordered Organic Thin FilmsQuantum Dot LEDs,Handout on QD-LEDs: Coe et al., Nature 420, 800 (2002).,April 29, 2003 Organic Optoelectronics - Lecture 20b,2,Exciton Dynamics in Time Dependant PL,3,Dynamic Spectral Shifts of DCM2 in Alq3, Measurement performe
2、d on doped DCM2:Alq3 films Excitation at =490 nm (only DCM2 absorbs), DCM2 PL red shifts 20 nm over 6 ns ,Wavelength nm ,4,Time Evolution of 4% DCM2 in Alq3 PL Spectrum,5,Electronic Processes in Molecules,density of available S1 or T1 states,6,Time Evolution of DCM2 Solution PL Spectra,7,Spectral Sh
3、ift due to, Exciton Diffusion Intermolecular Solid State Interactions ,8,Excitonic Energy Variations,9,Exciton Distribution in the Excited State (S1 or T1), Time Evolved Exciton Thermalization ,EXCITON DIFFUSION LEADS TO REDUCTION IN FWHM,10,11,12,13,Time Evolution of Peak PL in Neat Thin Films,14,P
4、arameters for Simulating Exciton Diffusion,observed radiative lifetime (),Normalized Integrated Spectral Intensity,Frster radius (RF), Assign value for allowed transfers:, Assume Gaussian shape of width, wDOS Center at peak of initial bulk PL spectrum Molecular PL spectrum implied,excitonic density
5、of states (gex(E),15,Fitting Simulation to Experiment Doped Films, 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 eV,16,Fitting Simulation Neat Films, Spectral shift observed in
6、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 first measurements of excitonic energy dispersion in amorphous organic solids,17,Temporal Solid State Solv
7、ation,upon excitation both magnitude and direction of lumophore dipole moment can changeFOR EXAMPLE for DCM: 1 0 20 Debye ! from 5.6 D to 26.3 D ,following the excitation the environment surrounding the excited molecule will reorganize to minimize the overall energy of the system (maximize Eloc),18,
8、Exciton Distribution in the Excited State (S1 or T1), Time Evolved Molecular Reconfiguration ,DIPOLE-DIPOLE INTERACTION LEADS TO ENERGY SHIFT IN DENSITY OF EXCITED STATES,log(Time),19,Fusion of Two Material Sets,Hybrid devices could enableLEDs, Solar Cells, Photodetectors, Modulators, andLaserswhich
9、 utilize the best properties ofeach individualmaterial.,Efficient,Organic Semiconductors,Flexible,Emissive,Fabrication of rational structures has been the main obstacle to date.,20,Inorganic Nanocrystals Quantum Dots,Quantum Dot SIZE,Synthetic route of Murray et al, J. Am. Chem. Soc. 115, 8706 (1993
10、).,21,Fusion of Two Material Sets,Quantum Dots,Organic Molecules,22,Integration of Nanoscale Materials Quantum Dots and Organic Semiconductors,ZnS overcoating shell (0 to 5 monolayers),Oleic Acid or TOPO caps,Synthetic routes of Murray et al, J. Am. Chem. Soc. 115, 8706 (1993) and Chen, et al, MRS S
11、ymp. Proc. 691,G10.2.,Trioctylphosphine oxide,Tris(8-hydroxyquinoline) Aluminum (III),3-(4-Biphenylyl)-4-phenyl-5- tert-butylphenyl-1,2,4-triazole,N,N-Bis(naphthalen-1-yl)- N,N-bis(phenyl)benzidine,N,N-Bis(3-methylphenyl)- N,N-bis-(phenyl)-benzidine,23,1. A solution of an organicmaterial, QDs, and s
12、olvent 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 films allow for imaging via AFM, despite the AFM tip being as large as the QDs.,Phase se
13、gregation is driven by a combination of size and chemistry.,Phase Segregation and Self-Assembly,24,As the concentration of QDs in the spin-casting solution is increased, the coverage of QDs on the monolayer is also increased.,Monolayer Coverage QD concentration,25,CdSe(ZnS)/TOPO,PbSe/oleic acid,QD-L
14、ED Performance,26,Full Size Series of PbSe Nanocrystals from 3 nm to 10 nm in Diameter,27,Design of Device Structures,QDs are poor charge transport materials.,Isolate layer functions of maximize device performance,1. Generate excitons on organic sites. 2. Transfer excitons to QDs via Frsteror Dexter
15、 energy transfer. 3. QD electroluminescence.,Phase Segregation.,But efficient emitters,Use organics for charge transport.,Need a new fabrication method in order to be able to make such double heterostructures:,28,A general method?,Phase segregation occursfor different1) organic hosts: TPD,NPD, and p
16、oly-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 furt
17、her 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,29,EL Recombination Region Dependence on Current,Coe et al.,
18、 Org. Elect. (2003),30,Spectral Dependence on Current Density,TOP DOWN VIEW of the QD MONOLAYER,Exciton recombination width far exceeds the QD monolayer thickness at high current density. To achieve true monochrome emission, new exciton confinement techniques are needed.,CROSS-SECTIONAL VIEW of QD-L
19、ED,31,Benefits 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!,
