Organic LEDs part 8. Exciton Dynamics in Disordered Organic Thin Films. Handout on QD-LEDs: Coe et al., Nature 420, 800 (2002).

Transcription

1 Organic LEDs part 8 Exciton Dynamics in Disordered Organic Thin Films Quantum Dot LEDs Handout on QD-LEDs: Coe et al., ature 420, 800 MIT April 29, 2003 Organic Optoelectronics - Lecture 20b 1

2 Exciton Dynamics in Time Dependant PL 2 time 25 nm 2 ns time wavelength wavelength

3 Intensity [ arb. units ] Dynamic Spectral Shifts of DCM2 in Alq 3 Measurement performed on doped DCM2:Alq 3 films Excitation at λ=490 nm (only DCM2 absorbs) Wavelength [ nm ] Alq 3 DCM2 ormalized Intensity % DCM2:Alq 3 time 0 ns 0.1 ns 0.3 ns 0.6 ns 1.0 ns 1.6 ns 2.2 ns 3.4 ns 6.2 ns Energy [ ev ] Energy [ ev ] ~ DCM2 PL red shifts > 20 nm over 6 ns ~ 3

4 Time Evolution of 4% DCM2 in Alq 3 PL Spectrum Wavelength [nm] ormalized Intensity Spectrum at 4 ns Integrated Spectrum (0-10 ns) Spectrum at 1 ns Energy [ev] 4

5 Electronic Processes in Molecules 10 ps Energy density of available S1 or T1 states FÖRSTER, DEXTER or RADIATIVE EERGY TRASFER S 1 ABSORPTIO ITERAL COVERSIO FLUORESCECE 1-10 ns PHOSPHORESCECE T 1 >100 ns S 0 5

6 Time Evolution of DCM2 Solution PL Spectra Benzene 580 Energy [ev] CHCl 3 CHCl 2 Acetone Wavelength [nm] CH 3 C DMSO Time [ns]

7 Spectral Shift due to ~ Exciton Diffusion ~ ~ Intermolecular Solid State Interactions ~ 5 4 wavelength shift 35 nm 10% DCM2 in Alq 3 Time [ns] Wavelength [nm] 7

8 Excitonic Energy Variations Each dye molecule experiences a different local medium variations in excitonic energies on-zero width to excitonic density of states Intensity Molecular PL Spectra Frequency Excitonic Density of States Energy Energy 8

9 Exciton Distribution in the Excited State (S 1 or T 1 ) ~ Time Evolved Exciton Thermalization ~ S 1 or T 1 Energy fwhm hν log(time) EXCITO DIFFUSIO LEADS TO REDUCTIO I FWHM 9

10 2.10 Low E midpoint 4% DCM2 in Alq 3 FWHM High E midpoint 2.05 High E midpoint T = 15~125 K 600 Energy Energy [ev] Low E midpoint Wavelength [nm] Time [ns] 10

11 x% DCM in Alq Energy [ev] Alq3 0.2 % 0.4 % 0.6 % Wavelength [nm] % 2 % Time [ns] 11

12 X% DCM in Alq Initial Peak PL 570 Energy [ev] Final Peak PL Wavelength [nm] % of DCM in Alq 3 12

13 Time Evolution of Peak PL in eat Thin Films 2.45 Energy [ev] Alq Ir(ppy) Time [ns] 13

14 Parameters for Simulating Exciton Diffusion ormalized Integrated Spectral Intensity observed radiative lifetime (τ) Förster radius (R F ) τ = 3.3 ns decreasing doping (5% down to 0.5%) Time [ ns ] Assign value for allowed transfers: Γ F 6 1 RF τ R 0 E excitonic density of states (g ex (E)) Assume Gaussian shape of width, w DOS Center at peak of initial bulk PL spectrum Molecular PL spectrum implied ormalized Intensity [ arb. units ] Wavelength [ nm ] FWHMs S init (E) : 0.27 ev S mol (E) : 0.22 ev g ex (E) : 0.15 ev Energy [ ev ] 14

15 Fitting Simulation to Experiment Doped Films Energy [ ev ] % DCM2:Alq 3 R F =0.88 w DOS =0.185 ev R F =0.98 w DOS =0.161 ev R F =1.08 w DOS =0.146 ev R F =1.18 w DOS =0.136 ev R F =1.28 w DOS =0.130 ev Time [ ns ] Wavelength [ nm ] Energy [ ev ] Peak PL of x% DCM2:Alq % 640 1% 650 2% 660 5% Time [ ns ] Wavelength [ nm ] Good fits possible for all data sets R F decreases with increasing doping, falling from 52 Å to 22 Å w DOS also decreases with increasing doping, ranging from ev to ev 15

16 2.40 Fitting Simulation eat Films Energy [ ev ] Ir(ppy) 3 w DOS ~0.21 ev Time [ ns ] Spectral shift observed in each material system Molecular dipole and w DOS are correllated: lower dipoles 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 Energy [ ev ] Alq 3 w DOS ~0.15 ev Energy [ ev ] PD w DOS ~0.10 ev Time [ ns ] Time [ ns ] 16

17 Temporal Solid State Solvation upon excitation both magnitude and direction of lumophore dipole moment can change FOR EXAMPLE for DCM: µ 1 µ 0 > 20 Debye! ~ from 5.6 D to 26.3 D ~ µ 0 µ 1 µ 1 t < 0 E loc t = 0 E loc E loc t ~ 1 ns following the excitation the environment surrounding the excited molecule will reorganize to minimize the overall energy of the system (maximize µ E loc ) 17

18 Exciton Distribution in the Excited State (S 1 or T 1 ) ~ Time Evolved Molecular Reconfiguration ~ Energy E DIPOLE-DIPOLE ITERACTIO LEADS TO EERGY SHIFT I DESITY OF EXCITED STATES log(time) 18

19 Efficient Fusion of Two Material Sets Organic Semiconductors Flexible Absorption [a.u.] Emissive Tunable Colloidal QDs Photo by F. Frankel anoscale Wavelength [nm] Hybrid devices could enable LEDs, Solar Cells, Photodetectors, Modulators, and Lasers which utilize the best properties of each individual material. Fabrication of rational structures has been the main obstacle to date. 19

20 Inorganic anocrystals Quantum Dots E CdSe ZnS D Quantum Dot SIZE Lowest allowed energy level Absorption [a.u.] D= 120Å 80Å 72Å 55Å 45Å 33Å 29Å 20Å 17Å Wavelength [nm] Photo by F. Frankel Synthetic route of Murray et al, J. Am. Chem. Soc. 115, 8706 (1993). 20

21 Fusion of Two Material Sets Quantum Dots Organic Molecules Absorption Spectra EMISSIVE AOSCALE TUABLE Alq 3 O Al O TAZ O CH 3 TPD H 3 C PD CdSe(ZnS) PbSe Wavelength [nm] Wavelength [µm] 21

22 ZnS overcoating shell (0 to 5 monolayers) Synthetic routes of Murray et al, J. Am. Chem. Soc. 115, 8706 (1993) and Chen, et al, MRS Symp. Proc. 691,G10.2. Alq 3 O Al O Tris(8-hydroxyquinoline) Aluminum (III) O Integration of anoscale Materials Quantum Dots and Organic Semiconductors PbSe or CdSe Core D = Å (~50Å pictured) Oleic Acid or TOPO caps Trioctylphosphine oxide HO O Oleic Acid CH 3 10Å TPD H 3 C TAZ,'-Bis(3-methylphenyl)-,'-bis-(phenyl)-benzidine 3-(4-Biphenylyl)-4-phenyl-5- tert-butylphenyl-1,2,4-triazole PD,'-Bis(naphthalen-1-yl)-,'-bis(phenyl)benzidine poly-tpd Differences in: Molecular Organics Quantum Dots Chemistry Aromatic Aliphatic Caps Phase Segregation Size Small Big 22

23 Phase Segregation and Self-Assembly QD Solution TPD Solution 1 2 QDs 3 TPD/solvent Substrate 4 close packed QDs TPD Substrate 1. A solution of an organic material, QDs, and solvent 2. is spin-coated onto a clean substrate. 3. During the solvent drying time, the QDs rise to the surface 4. and self-assemble into grains of hexagonally close packed spheres. ~8nm PbSe QDs Organic hosts that deposit as flat films allow for imaging via AFM, despite the AFM tip being as large as the QDs. Phase segregation is driven by a combination of size and chemistry. Flat TPD background 50nm 23

24 Monolayer Coverage QD concentration QD coverage = 20% 50% 80% 250nm 500nm 500nm 500nm 90% 100% 100nm As the concentration of QDs in the spin-casting solution is increased, the coverage of QDs on the monolayer is also increased. 24

25 QD-LED Performance 50nm Ag 50nm Mg:Ag 30nm Alq 3 10nm TAZ Current Density [A cm -2 ] Control QD-LED 40nm TPD ITO Glass CdSe(ZnS)/TOPO 1 Voltage [V] 10 PbSe/oleic acid Vacuum Level = 0 ev Electron Energy [ev] EA=2.1 TPD E F =4.7 ITO IE=5.4 QD EA=4.6 CdSe IE=6.8 EA=3.0 TAZ IE=6.5 EA=3.1 Alq 3 E F =3.7 Mg IE=5.8 ormalized counts ZnS TOPO Wavelength [nm] 25

26 Structure of a Single anocrystal Full Size Series of PbSe anocrystals from 3 nm to 10 nm in Diameter 9 peak absorption 4 nm HRTEM Jonny Steckel and Seth Coe Absorbance (arbitrary units) nm 2036 nm 1812 nm 1708 nm 1600 nm 1504 nm 1404 nm nm 8 nm AFM Ordered Monolayer of PbSe anocrystals Wavelength (nm) 1136 nm 26

27 QDs are poor charge transport materials... QD CdSe ZnS TOPO caps e - QD intersite spacing Isolate layer functions of maximize device performance. 1. Generate excitons on organic sites. 2. Transfer excitons to QDs via Förster or Dexter energy transfer. 3. QD electroluminescence. eed a new fabrication method in order to be able to make such double heterostructures: Phase Segregation. Design of Device Structures ZnS QD monolayer shell 50nm Ag 50nm Mg:Ag But efficient emitters 30nm Alq 3 Electron Energy [ev] E F =4.7 ITO Vacuum Level = 0 ev EA=2.1 TPD IE=5.4 10nm TAZ 40nm TPD ITO Glass Use organics for charge transport. QD EA=4.6 CdSe IE=6.8 ZnS TOPO EA=3.0 EA=3.1 TAZ Alq 3 E F =3.7 IE=6.5 CdSe core IE=5.8 QD Mg TOPO caps 27

28 A general method? Phase segregation occurs for different 1) organic hosts: TPD, PD, and poly-tpd. 2) solvents: chloroform, chlorobenzene, and mixtures with toluene. 3) QD core materials: PbSe, CdSe, and CdSe(ZnS). 4) QD capping molecules: oleic acid and TOPO. 5) QD core size: 4-8nm. 6) substrates: Silicon, Glass, ITO. 7) Spin parameters: speed, acceleration and 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 segregation QD-LED structures 2) Solvent 7) Spin Parameters QD monolayer shell Cathode ETL 1) Organic Host 6) Substrate/Anode 5) QD size QD 4) caps 3) core 28

29 EL Recombination Region Dependence on Current increasing current depth Cathode Alq 3 (35 nm) exciton density R F 2% DCM2 in Alq3 (5nm) ormalized Intensity J [ma/cm 2 ] Alq 3 DCM2 EL Fraction [%] DCM EL Alq 3 EL log(j[ma/cm 2 ]) TPD (40 nm) Anode Glass Wavelength [nm] Coe et al., Org. Elect. (2003) 29

30 Spectral Dependence on Current Density J [ma/cm 2 ] 380 Alq 3 TOP DOW VIEW of the QD MOOLAYER Device Area Grain Boundary QD R F Wavelength [nm] Interstitial Space Monolayer Void Exciton recombination width far exceeds the QD monolayer thickness at high current density. To achieve true monochrome emission, new exciton confinement techniques are needed. ETL electron HTL hole Cathode Anode Glass Alq 3 EL QD EL depth increasing current exciton density R F CROSS-SECTIOAL VIEW of QD-LED 30

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! QD FWHM 32nm CIE Diagram Potential QD-LED colors standard CRT color triangle x Coe et al, ature 420, 800 (2002) y