E L E C T R O P H O S P H O R E S C E N C E

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1 Organic LEDs part 4 E L E C T R O P H O S P H O R E S C E C E. OLED efficiency 2. Spin 3. Energy transfer 4. Organic phosphors 5. Singlet/triplet ratios 6. Phosphor sensitized fluorescence 7. Endothermic triplet energy transfer Lecture by Prof. Marc Baldo April, 2003 Organic Optoelectronics - Lecture 7

2 POWER EFFICIECY The eye s response is wavelength dependent. The unit of perceived optical power is defined as the lumen (lm). Power efficiency of LED s is therefore measured in lumens/watt. The maximum photopic response is: Φ ~ 20 lm (blue) Φ ~ 680 lm (green) Φ ~ 0 lm (red) Photopic response (lm/w) For an OLED of given color, we must maximize: Wavelength (nm) - quantum efficiency (η Q = photons out/electrons in) - electrical efficiency (V λ /V = photon energy/operating voltage) η P L = =Φ VI η Q V λ V Power efficiency = photopic response of eye x quantum efficiency x electrical efficiency 2

$η = χη φ Q out PL φ PL is fundamental luminescence efficiency of organic material. η out is output coupling fraction reduced by absorption losses and wave guiding within the device and its substrate.$

3 Quantum efficiency is the measure of a luminescent dye s performance. η Q = the number of photons emitted per electron injected. η = χη φ Q out PL φ PL is fundamental luminescence efficiency of organic material. η out is output coupling fraction reduced by absorption losses and wave guiding within the device and its substrate. χ is fraction of luminescent molecular excitations (defined by spin selection rules) typically ~ 25% remaining energy is wasted OLED glass Imposes a fundamental limit to OLED efficiencies 3

4 EXCITO SPI AD SYMMETRY Molecular excited states (or excitons) are typically mobile, with binding energy ~ ev, and spin S = 0, Molecular ground state (spatially symmetric under exchange of electrons) E Lowest unoccupied (LUMO) molecular orbital st excited state (spatially anti-symmetric (triplet) or spatially symmetric (singlet)) E S=0, singlet LUMO HOMO ψ ψ ψ ψ Highest occupied (HOMO) molecular orbital } S=, triplet S=0, singlet 4

5 Fluorescence and Phosphorescence Intersystem crossing (ISC) S T Fluorescence Triplet has lower energy because it is spatially antisymmetric under exchange of electrons. (e - -e - repulsion lower). Phosphorescence S 0 Molecular ground state Fluorescence: Decay from singlet allowed by symmetry: fast ( 9 s - ) and often efficient. Phosphorescence: Decay from triplet disallowed by symmetry: slow ( > s - ) and inefficient. 5

Efficient phosphorescence eed to mix singlet and triplet states: - make both singlet and

Use metal-organic complexes with heavy transition metals: Pt, Ir, etc.

number Type I phosphor Exciton localized on organic ligand exciton + - Pt Z 4 Type II

6 Efficient phosphorescence eed to mix singlet and triplet states: - make both singlet and triplet decay allowed. Use metal-organic complexes with heavy transition metals: Pt, Ir, etc... Ligand molecular orbital Spin orbit coupling mixes states: proportional to atomic number Type I phosphor Exciton localized on organic ligand exciton + - Pt Z 4 Type II phosphor Metal-ligand charge transfer exciton MLCT exciton + - Ir metal ligand PtOEP less mixing ~ 0µs triplet lifetime metal ligand most mixing ~ µs triplet lifetime 3 Ir(ppy) 3 6

7 Structure and operation of OLEDs - must transfer energy from host material to luminescent dopant. This determines the quantum efficiency of luminescence. A heterostructure OLED Light holes Hole transport layer Luminescent dopant molecules electrons Electron transport layer Cathode Transparent Anode Assume balanced currents: every hole combines with an electron Excitons form in host at interface. Ideally energy is transferred to luminescent molecules 7

8 Energy transfer (a) Förster energy transfer Long range (up to ~ 0Å) Donor * Acceptor Donor Acceptor * Donor Acceptor (dye ) Exciton non-radiatively transferred by dipole-dipole coupling if transitions are allowed (usually singlet-singlet). (b) Dexter energy transfer Donor Acceptor (dye ) Donor * Acceptor Donor Acceptor * Exciton hops from donor to acceptor. Short range ~ Å 8

ntum efficiency in Alq 3 ~ 4% Pt PtOEP Intensity (a.u.) 0 Emission spectrum 400 500 600 700 800 900 Wavelength (nm) Device structure

9 A red phosphor: Platinum octaethylporphyrin Triplet lifetime ~0ms Peak external quantum efficiency in Alq 3 ~ 4% Pt PtOEP Intensity (a.u.) 0 Emission spectrum Wavelength (nm) Device structure Mg:Ag cathode PtOEP in Alq 3 a-pd Indium tin oxide Quantum Efficiency (%) Luminance of 6% device (cd/m 2 ) % PtOEP 6% PtOEP 20% PtOEP efficiency roll-off due to triplet-triplet annihilation triplet triplet singlet ground state Loss proportional to square of triplet lifetime Current Density (ma/cm 2 ) 9

DOES PTOEP CAPTURE ALQ 3 TRIPLETS? Put fluorescent dye DCM2 in exciton formation zone..8 Ag PtOEP Ag.6 Mg/Ag Mg/Ag Intensity (arb. units).4.2.0 0.8 0.6 0.4 0.

10 DOES PTOEP CAPTURE ALQ 3 TRIPLETS? Put fluorescent dye DCM2 in exciton formation zone..8 Ag PtOEP Ag.6 Mg/Ag Mg/Ag Intensity (arb. units) Å Alq 3 0Å DCM2/Alq 3 350Å a-pd 60Å CuPc ITO Glass Device Device 2 Alq 3 DCM2 500Å Alq 3 0Å PtOEP/Alq 3 0Å Alq 3 0Å DCM2/Alq 3 350Å a-pd 60Å CuPc ITO Glass Wavelength (nm)

11 The singlet/triplet ratio in Alq 3 Device emits from singlets only Device 2 emits from singlets and triplets Ag Mg/Ag 200Å Alq 3 Ag Mg/Ag 200Å Alq 3 400Å DCM2/Alq 3 400Å PtOEP/Alq 3 450Å a-pd 60Å CuPc ITO Glass 450Å a-pd 60Å CuPc ITO Glass Compare ratio of EL emission from devices to get singlet fraction. After correction for PL efficiency of DCM2 and PtOEP, get: χ=(22±3)%

12 IMPROVED PHOSPHORS Must reduce triplet lifetime to minimize T-T annihilation. Increase MLCT component of excited state. The first high efficiency green phosphor was iridium tris(2-phenylpyridine) Luminance (cd/m 2 ) Voltage (V) Power efficiency (lm/w) Ir Ir(ppy) 3 3 metal MLCT exciton Broad green emission at λ ~ 55 nm. Phosphorescent lifetime, τ ~ ms. + - ligand 2

13 TRIPLET-SIGLET EERGY TRASFER There are many more fluorescent than phosphorescent materials. Can we get high efficiency from a fluorescent dye? exciton formation exciton formation host S T? fluorescent dye S T eed to prevent exciting triplet state in fluorescent dye. Want mechanism for triplet-singlet energy transfer. 3

14 Phosphor sensitized fluorescence. Triplet-singlet hopping transfer is disallowed 2. Triplet-singlet Förster transfer permitted if triplet relaxation on donor is allowed i.e. triplet-singlet transfer is possible from a phosphorescent donor Predicted by Förster in 959 ( ) Observed by Ermolaev and Sveshnikova in 963 ( ) e.g. for triphenylamine as the donor and chrysoidine as the acceptor, in rigid media at 77K or 90K the interaction length is 52Å ( ) Förster, Th. Dicussions of the Faraday Society 27, 7-7 (959). ( ) Ermolaev, V.L. & Sveshnikova, E.B. Doklady Akademii auk SSSR 49, (963). 4

15 IMPLEMETATIO OF TRIPLET-SIGLET EERGY TRASFER host S phosphorescent sensitizer S ISC fluorescent dye S T T T Ir DCM2 absorbs in the green O CH 3 CBP Ir(ppy) 3 3 C C 5

16 DCM2 fluorescence sensitized by Ir(ppy) 3 External quantum efficiency (%) % DCM2 and 8% Ir(ppy) 3 in CBP ote: factor of 4 difference % DCM2 in CBP EL Intensity (arbitrary units) Ir(ppy) 3 DCM Current density (ma/cm 2 ) Wavelength (nm) Roll-off in efficiency is due to charge trapping on DCM2 molecules early complete energy transfer from Ir(ppy) 3 to DCM2 6

17 OLED TRASIET RESPOSE Intensity (arbitrary units) 0 DCM2 Ir(ppy) 3 Examine transient response of different spectral components of OLED luminescence. atural lifetime of DCM2 only ~5ns Time (ns) Delayed DCM2 fluorescence confirms sensitizing action of Ir(ppy) 3 7

B L U E Problem: the most energetic charge transport hosts currently available have blue-green triplets. So to transfer energy to a blue phosphor, exciton transfer must be endothermic.

18 B L U E Problem: the most energetic charge transport hosts currently available have blue-green triplets. So to transfer energy to a blue phosphor, exciton transfer must be endothermic. F F 2 Ir O FIrpic (blue phosphor) Peak wavelength 470nm Triplet lifetime µs O Inentsity (arb. units) % FIrpic in CBP FIrpic in CHCl 3 FIrpic at 460nm Wavelength (nm) CIE y coordinate % FIrpic in CBP FIrpic in CHCl 3 FIrpic at 460nm CIE x coordinate (2.56±0.) ev CBP k F k R (2.62±0.) ev FIrpic Possible to get efficient endothermic transfer if decay of host triplet disallowed. k H k G CBP Host CBP triplet lifetime ~ s. 8

19 PL intensity (arbitrary units) (a)t=300k (b)t=200k (c)t=0k (d)t= 50K Transient response of endothermic transfer PL intensity (arbitrary units) Temperature (K) (b) (c) (a) (d) Time (µs) Efficiency of luminescence decreases below T=200K as energy transfer to phosphor is frozen out. Transient response is also consistent with endothermic transfer: At T=200K, apparent transient lifetime of FIrpic (includes endothermic transfer) >> natural lifetime (~µs) EL Intensity (arb. units) PL Intensity (arb. units) Firpic in CBP % Ir(ppy) 3 :TPD 0 20 Time (µs) 0 Ir(ppy) 3 in TPD T=292K τ = (5±2)µs τ = (80±)µs T=200K τ = (5±2)µs 6% Ir(ppy) 3 :TPD Time (µs) 9

COLOR PURITY CIE y coordinate 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0. 0.0 FIrpic ppy 2 Ir(acac) btp 2 Ir(acac) PtOEP 0.0 0. 0.2 0.3 0.4 0.5 0.6 0.7 0.8 CIE x coordinate Organic materials have broad luminescent spectra.

20 COLOR PURITY CIE y coordinate FIrpic ppy 2 Ir(acac) btp 2 Ir(acac) PtOEP CIE x coordinate Organic materials have broad luminescent spectra. Overcome in red and blue by shifting toward non-visible wavelengths. 20

21 O L E D S U M M A R Y Peak power efficiency in green is 60 lm/w Peak quantum efficiency is 9% (corresponds to ~0% internal) Largest remaining problem is operating voltage J = µa/cm 2 J = ma/cm 2 phosphor host Φ (lm/w) η P (lm/w) η Q ext η Q int Vλ V η P (lm/w) η Q ext η Q int Vλ V ppy 2 Ir(acac) TAZ btpir(acac) CBP FIrpic CBP PtOEP CBP Table of phosphorescent device operating parameters 2

22 Intensity (arb. units) R E D Wavelength (nm) PHOSPHORESCET DEVICE PERFORMACE G R E E B L U E Intensity (arb. units) Wavelength (nm) Intensity (arb. units) Wavelength (nm) Quantum efficiency (%) CH 3 S O Ir O CH 3 2 btp 2 Ir(acac) btp 2 Ir(acac) in CBP Current density (ma/cm 2 ) Power efficiency (lm/w) Quantum efficiency (%) 0 CH 3 O Ir O CH 3 2 ppy 2 Ir(acac) Current density (ma/cm 2 ) Power efficiency (lm/w) Quantum efficiency (%) F F Ir O 2 FIrpic FIrpic in CBP O Power efficiency (lm/w) Current density (ma/cm 2 ) 22

23 PHOSPHORESCET STABILITY Courtesy Ray Kwong and UDC ormalized optical power cd/m 500 cd/m Time (hours) 50 cd/m 2 0 cd/m 2 Red: BtpIr(acac) in CBP ormalized optical power cd/m Time (hours) 500 cd/m 2 00 cd/m cd/m 2 Green: (ppy) 2 Ir(acac) in CBP Phosphors should improve OLED stability by rapidly removing triplet excitons and lowering drive current required. 23

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

Organic LEDs part 8. Exciton Dynamics in Disordered Organic Thin Films. Handout on QD-LEDs: Coe et al., Nature 420, 800 (2002). Organic LEDs part 8 Exciton Dynamics in Disordered Organic Thin Films Quantum Dot LEDs Handout on QD-LEDs: Coe et al., ature 420, 800 (2002). @ MIT April 29, 2003 Organic Optoelectronics - Lecture 20b