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Thermally Activated Delayed Fluorescence (TADF)
The document discusses thermally activated delayed fluorescence (TADF) for organic light emitting diodes. It begins with an introduction to TADF and the mechanisms allowing conversion of triplet excitons to light. Key parameters for efficient TADF emitters include a small singlet-triplet energy gap, high photoluminescence quantum yield, and high oscillator strength. Molecular design strategies aim to engineer these parameters, such as introducing strong donor-acceptor motifs or using spiro linkers to separate orbitals. The document concludes that TADF could harvest normally wasted triplet excitons, improving light emission efficiency.
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Thermally Activated Delayed Fluorescence (TADF)
- 1. Presentation By Biswajit Kumar Barman HarnessingTriplet Excitons for Organic Light Emitting Devices by Thermally Activated Delayed Fluorescence Harnessing Triplet Excitons for Organic Light Emitting Devices by Thermally Activated Delayed Fluorescence 1
- 2. ContentContent ❑ Introduction-Thermally ActivatedDelayed Fluorescence (TADF) ❑ Detailed Mechanism and Factors affecting TADF ❑ Molecular Design strategy and introduction to present work 2 ✓ Radiative process is from singlet state ✓ Thermally activated from triplet state ✓ Life time higher than prompt one ,so delayed ✓ Triplet state to Singlet state and Singlet to ground state radiative transition ✓ Energetics and structural feature to enhance the TADF process
- 3. Organic Light EmittingDiode - comprises of doped/ non-doped organic electroluminescent material as the active layer with appropriate transport layer and injection contacts. Organic Light Emitting Diode (OLED)Organic Light Emitting Diode (OLED) 3 Advantages of OLEDs ➢ Flexible lighting ➢ Large panel displays ➢ Low Power consumption Disadvantages of OLED ➢ Low stability
- 4. Fluorescent OLEDFluorescent OLED Conversionof triplet excitons into light is necessary. Two popular ways 4 Electron from cathode Hole from anode Exciton in Light emitting layer Recombination 25% singlet exciton 75% triplet exciton Spin –Statistics rule Phosphorescence channel - IQE =100% TADF channel - IQE =100% Maximum theoretical IQE=25%
- 5. Effect of heavymetal atom Phosphorescent emitters have high cost, toxic and triplet-polaron annihilation, triplet-triplet annihilation effect that decreases device efficiency. Problems Phosphorescence channelPhosphorescence channel Iridium, Platinum ⇒High atomic number ⇒Large spin–orbit coupling constant S1 T1 S0 ISC Phosphorescence λ = spin orbit coupling constant Z = atomic number l = orbital angular momentum n = principle quantum number In phosphorescent emitter, Triplet excitons go to ground state radiatively.Lifetime of this process is around ms to s . 5
- 6. S1 S0 T1 Reverse Inter SystemCrossing (RISC) Fluorescence Delayed fluorescence(TADF) Electrical excitation Electrical excitation 25% 75% TADF channelTADF channel In TADF, reverse intersystem crossing (RISC) from the lowest triplet state (T1) to the lowest excited singlet state (S1), and followed by radiative decay from S1 to the ground state (S0). 6
- 7. S1 S1 S1 T1 T1 T1 S0 S0S0 Radiative phosphorescence TADF ISC ISC RISC Radiative Nonradiative Radiative Fluorescence 1-st: Fluorescent emitter 2-nd: Phosphorescent emitter 3-rd:TADF emitter Three generations of OLEDThree generations of OLED Electrical excitation Electrical excitation Electrical excitation Electrical excitation Electrical excitation Electrical excitation 7
- 8. Mechanism of TADFprocessMechanism of TADF process Electroluminescence process Electrical excitation Electrical excitation S1 S0 T1 8Ref: Chem. Mater. 2017, 29, 1946−1963 Thermal energy = 25.9 meV at 298K.
- 9. Key Parameters ofTADF Emitters Small Singlet−Triplet Energy Gap High PL Quantum Yield High Oscillator strength from S1 to S0 electronic state 9 If energy gap is low,then rate of reverse inter system crossing is high If the oscillator strength is high,then radiative rate of transition is quite high PLQY = Quantum efficiency increasing with increase PLQY .
- 10. Design principleDesign principle Strongdonor/acceptor Phenyl linker HOMO dispersion Dual emitting core Frozen donor/acceptor Rigid acceptor High BDE 10Ref: Chem. Mater. 2017, 29, 1946−1963 Distortion between Donors and Acceptors Small energy gapSmall energy gap High PLQYHigh PLQY Narrow emissionNarrow emission Stable structureStable structure
- 11. Small Singlet−Triplet EnergyGap(ΔEST) 1 Minimal overlap between the HOMO and LUMO Minimum overlap Low exchange energy That reduce energy gap between S1 and T1 E= orbital energy K=electron repulsion energy J =electron exchange energy S1 state have energy ES=E+K+J T1 state have energy ET=E+K –J ΔEST=ES – ET ΔEST= 2J Ref: Chem. Soc. Rev., 2017,46,915-1016 & Adv. Mater. 2014, 26, 7931–7958
- 12. 1. Separation ofHOMO - LUMO , (small ΔEST) (a) Introduction of steric hindrance (b) Spiro linker, physical separation of donor and acceptor units (c) X shaped molecular structure HOMO-LUMO SeparationHOMO-LUMO Separation Ref : Chem. Soc. Rev., 2017,46,915-1016 Methods: Donor- Acceptor backbone 1
- 13. oBFCzTrz mBFCzTrz pBFCzTrz CompoundEnergy of S1(eV) Energy of T1(eV) ΔEST(eV) Life time(τ),μs oBFCzTrz 2.78 2.73 0.05 5.4 mBFCzTrz 2.79 2.68 0.11 29.6 pBFCzTrz 2.89 2.64 0.25 31.2 Short LifetimeShort Lifetime Ref: Appl. Mater. Interfaces, 2016, 8, 23190−23196 The increased CT character due to geometrical distortion Short conjugation length of oBFCzTrz, that increase T1 value 13
- 14. PLQY: HOMO dispersionPLQY:HOMO dispersion Compound (in 94% DPEPO film) PLQY EQE Compound 1 100 22.4 Compound 2 95 18.5 Compound 3 93 15.6 Compound 1 Compound 2 Compound 3 Ref: Nat. Mater. 2015, 14, 330−336 Increase HOMO-LUMO overlap 14
- 15. DDCzIPNDCZIPN PLQY: Dual emittingcorePLQY: Dual emitting core Absorption coefficient = 1.1 X 105 M-1 cm-1 PLQY = 67% EQE = 16.4% Absorption coefficient = 3.7 X 105 M-1 cm-1 PLQY = 91% EQE = 18.9% The UV/Vis absorption of DDCzIPN was stronger than that of DCzIPN Ref: Angew. Chem. Int. Ed. 2015, 54, 5201–5204 15
- 16. Narrow EmissionNarrow Emission Tofreeze the donor−acceptor based core structure by fused structure or sterically hindered structure. To adopt a rigid acceptor structure to minimize vibrational motion of the TADF emitters DABNA 1 FWHM=33nm DABNA 2 FWHM=34nm Large steric hindrance Ref: Adv. Mater. 2016, 28, 2777–2781 Rigid π-conjugated framework CNBPCz FWHM=76nm CzBPCN FWHM=48nm 16
- 17. Role of sterichindranceRole of steric hindrance 1. Dihedral angle = 470 2. ΔEST = 0.10 eV 3. EQE = 14.4% 1. Dihedral angle = 600 2. ΔEST = 0.03 eV 3. EQE = 20.1% Ref: J. Mater. Chem. C, 2017, 5, 4797--4803 17 DCz-TRCz-TR
- 18. Role of DonorpartRole of Donor part Ref: Org. Electron, 50 (2017) ,70-76 18 1 (TADF inactive) 2 (TADF active) 1. S1 = 3.809 eV 2. T1 = 3.108 eV 3. ΔEST = 0.701 eV 1. S1 = 3.526 eV 2. T1 = 3.100 eV 3. ΔEST = 0.426 eV
- 19. HIL HTL EBL Anode EML HBL ETL EIL Cathode Device structureDevice structure ITO= 4.6 eV Al = 4.28 eV PEDOT:PSS 5.2 ,2.4 Ref: J. Mater. Chem. C, 2013, 1, 1739–1744 19 PPV= hole transporting materials CN-PPV= electron transporting materials Where, HIL = hole injection layer HTL = hole transport layer EBL = electron blocking layer EML= emissive layer HBL = hole blocking layer ETL = electron transport layer EIL = electron injection layer
- 20. Electroluminescence propertyElectroluminescence property DeviceHOMO(eV) LUMO(eV) Turn on voltage (V) EQE (%) 1 5.2 2.6 8.1 5.6 2 5.0 2.6 6.7 8.3 Host= mCP (5.9,2.4) 1 2 Device 1 Device 2 Turn on voltage – 1. Mobility of charges/carrier 2. Hole injection barrier Fig: Current density vs. Voltage plot Ref: Dyes and Pigments, 2017,141, 83-92 20
- 21. Electroluminescence propertyElectroluminescence property Triplet–triplet anihilation,Triplet –polaron annihilation process at high current density Fig: EQE vs. Current density plot Ref: Adv. Mater. 2014, 26, 7931–7958 21
- 22. Photophysical characteristicsPhotophysical characteristics Fig:Photoluminescence quantum yield vs. Emitter concentration plot Fig: (a) Molecular structures of the emitters. Ref: Chem. Commun., 2015, 51, 9443—9446 ➢ Bridging sp3 carbon ➢ Concentration quenching in emitter 4, due to planer structure 22
- 23. Ref: Chem. Commun.,2015, 51, 9443—9446 Photophysical characteristicsPhotophysical characteristics Fig: Normalized intensity vs. Wavelength plot Conclusion = Delayed transition from S1 to S0 state like prompt flurescence one 23
- 24. ConclusionConclusion 24 ❖ To harvesttriplet excitons for light emission. ❖ Preparation of TADF molecules with a high efficiency.
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