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.

1. Presentation
By
Biswajit Kumar Barman
Harnessing Triplet Excitons for Organic Light Emitting
Devices by Thermally Activated Delayed Fluorescence
Harnessing Triplet Excitons for Organic Light Emitting
Devices by Thermally Activated Delayed Fluorescence
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2. ContentContent
❑ Introduction-Thermally Activated Delayed Fluorescence (TADF)
❑ Detailed Mechanism and Factors affecting TADF
❑ Molecular Design strategy and introduction to present work
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✓ 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 Emitting Diode - 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)
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Advantages of OLEDs
➢ Flexible lighting
➢ Large panel displays
➢ Low Power consumption
Disadvantages of OLED
➢ Low stability

4. Fluorescent OLEDFluorescent OLED
Conversion of triplet excitons into
light is necessary.
Two popular ways
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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 heavy metal 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 .
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6. S1
S0
T1
Reverse Inter System Crossing (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).
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7. S1 S1
S1
T1
T1
T1
S0 S0 S0
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
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8. Mechanism of TADF processMechanism 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 of TADF Emitters
Small Singlet−Triplet Energy Gap
High PL Quantum Yield
High Oscillator strength from S1 to S0 electronic state
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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
Strong donor/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 Energy Gap(ΔEST)
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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 of HOMO - 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
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13. oBFCzTrz mBFCzTrz pBFCzTrz
Compound Energy 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
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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
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15. DDCzIPNDCZIPN
PLQY: Dual emitting corePLQY: 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
To freeze 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
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17. Role of steric hindranceRole 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 Donor partRole 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
Device HOMO(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
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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
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24. ConclusionConclusion
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❖ To harvest triplet excitons for light emission.
❖ Preparation of TADF molecules with a high efficiency.

25. Thank YouThank You
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