OEM Research Group

OLEDs, the dawn of ultra efficient lighting	

Prof Monkman, OEM Research Group
 	

Department of Physic...
Themes
OLEDs, the dawn of ultra efficient lighting



General considerations of making OLED lighting

Problems facing blue ...
What does OLED lighting offer?
Better design esthetics, better quality of lighting, cheaper to run
‘off grid’ lighting for...
What does OLED lighting offer?
Design No No’s
impractical/fanciful

more of the same
welcome to the mental hospital
What does OLED lighting offer?

Better Design	


Introducing dimensionality
to break up the ceiling space
What does OLED lighting offer?
This is now much better Design
Small panels give much higher resolution to the shapes
withi...
FP7-224122 – OLED100.eu

D5.7 - Second set of psycho-physiological perception case st

What does OLED lighting offer?
4.2....
What does OLED lighting offer?
OLED ‘lamps’ being large area surface
emitters have lower glare and so do not
require a lum...
Inside an OLED 

Simple OLED pixel structure

Al (100 nm)
LiF (1 nm)
TPBI (30 nm)
Emissive layer (ca. 70 nm)
PEDOT:PSS (ca...
Inside an OLED 
Simple OLED pixel structure

hole injection layer ‘HIL’

	


Both HTL and ETL
are thick layers via
the use...
Inside an OLED 

Best commercial white panels from LG Chem
10 cm x 10 cm, 80 lm/W 

Complex OLED lighting structure

	


O...
Inside

prising
mal IL
an OLED 
minim
the CG
J. Appl.
p-doped
n-doped
p-doped
n-doped Phys. 111, 103107
CGL technology	

t...
What challenges remain in OLED lighting?
There are two major current challenges

1)  Improve the out-coupling of light fro...
Charge Carrier Recombination

e-	


h+	


recombination	


↓	


↓	


↑	


↑	


25 % 1P+P-	


75 % 3P+P-	


↑	


↑

25 % Si...
Light perception

The ‘average’ human eye has poor
sensitivity to blue so it will be hardest
to make a ‘bright’ blue-white...
Why is the current blue not good enough?


with FIrpic we will not make
4000K and CRI>80, so its a
non starter.

here, muc...
Why is the current blue not good enough?

LG Chem say they will mass produce 100x100 mm glass 80 lm/W	

	

4th quarter 201...
Why is the current blue not good enough?


Further, FIrpic is unstable to loose of the fluorine atoms, both
on deposition a...
Host degradation
Part II
CBP phosphorescence
We did measurements another
Usually people only consider emitter degradation ...
Basic Poly(N-vinylcarbazole) 

triplet energy measurements

Mw PVK > 106, high purity

In film ET ~ 2.9 eV

The phosphoresc...
Temperature dependence of

phosphorescence

1.0

Intensity, A.U.

0.8

14 K	


0.6

0.4

0.2

ET=2.89 eV	


0.0
400

450

...
Temperature dependence of

phosphorescence

1.0

Intensity, A.U.

0.8

14 K	

44 K	


0.6

0.4

0.2

ET=2.55 eV	


0.0
400...
Temperature dependence of

phosphorescence

1.0

Intensity, A.U.

0.8

14 K	

44 K	

85 K	


0.6

0.4

0.2

ET=2.41 eV	


...
dilute solution state phosphorescence

Temperature dependence of

phosphorescence
14 K	

44 K	

14K - 44K	


Pina et al Ch...
in toluene	


450 nm excitation	


0 .04 m g /m l
0 .4 m g /m l
4 m g /m l
3 8 m g /m l

0 .2

0 .1

1.0

ET=2.41 eV	


0....
Evidence for two ground state species
Excitation 450 nm
14 K
44 K
85 K
100 K
295 K

Intensity, A.U.

1

6650 ns and 22520 ...
What form has the dimer?

½ co-facial dimer	


Benten et al, J.Phys.Chem.B	

2007, 111, 10905	


Full co-facial dimer	

Th...
What could replace Ir ?

We need new blue emitters
that combine the stability of a
fluorescence molecule with
the an abilit...
E-type delayed fluorescence to harvest triplet excitons

TADF = E-type DF 	

!"##$%& $'( )"*+,,-

e-
:
h+

Thermally activa...
Introduction
&'()*+%,-$'%./,%("#$$%!EST%
Basic theory of CT states	

!"#$!%&!'()*+
!,#$!%&!'(&*

&'()*+%,-$'%./,%("#$$%!th...
TADF
New family of D-A-D molecules incorporating para and meta D-A coupling so we call
systems containing 1 ‘linear’ and 2...
TADF

Intramolecular Charge Transfer States
(ICT)
D

2b

D

A
carbazole donors

dibenzothiophene-S,S-dioxde
acceptor

HOMO...
Delayed Fluorescence measurements
8

Intensity, A.U.

10

pe 1
- slo
PF

7

10

1
lope
P H -s

6

10

DF

5

10

a)

Laser...
TADF

Enhanced CT character for bent materials (2’s)

2c
D

D

A

	
  

ΔE /Δf= 0.91

ΔE 	
  (e V )

	
  

0 .8

hν

0 .4
...
Fluorescence lifetimes
8

1d
he x a ne
λx :3 6 3 nm 	
   λx :4 2 0 nm

6

he x a ne
λx :	
  3 6 3 	
  nm 	
   λe m :	
  4 ...
TADF

Properties of delayed fluorescence (DF)

4a

	
  

2

4a

barrier to viscous flow
in ethanol is 0.15 eV

6

D F 	
  in...
Properties of delayed fluorescence (DF)
2.95 eV

1CT*

2.74 eV

3ππ*

ΔEST=0.63 eV

3CT*

ΔEST=0.63 eV
2.11 eV

1

-­‐7

10...
TADF

Properties of delayed fluorescence (DF)

2e

	
  

ΔEST=0.48 eV

3CT*

ΔEST=0.48 eV
2.39 eV

0 .8

0 .4

0 .0
400

	
...
TADF

Constant acceptor

1e

2e

2d

	
  

ΔE

ΔE S T = 0.48 ±0.05	
  e V

= 0.63 ±0.06	
  e V
ST
E a = 0.26 ±0.02e V

ΔE ...
TADF

Constant donor

2d

3d

4d

	
  

	
  

ΔE S T = 0.57 ±0.05e V

ΔE S T = 0.35 ±0.04	
  e V

E a = 0.83 ±0.02	
  e V
...
TADF
We require another energy level to explain the temperature dependent behaviour of the DF
1e
2e

1ππ*

1CT*

1ππ*

2.9...
TADF

We introduce the “little known” nπ* orbital 

Dias and Monkman et al., Adv. Mater. 2013, 25, 3707

Monkman

OEM Rese...
How efficient is TADF even with a gap of 0.36 eV

10

DF/PF ratio∼ 5%

400

	
  

2

	
  T A D F
	
  s te a dy-­‐s ta te 	
...
0.8

Neat 2d Film

0.4

1.0
0.8
0.6
0.4
0.2

400 450 500 550 600 650 700
Wavelength, nm

500
600
Wavelength, nm

Excitatio...
TADF in solid state

Critical role of host triplet energy

Normalized intensity, A.U.

In TAPC no 1,3CT host quenching 
TA...
TADF devices

How do OLEDs performed

based on TADF emitters?

Monkman

OEM Research Group
Phosphorescence

TADF devices
b

N

10

CN

CN 300 K
200 K
100 K
N

NC

0

N

R
1

300 K N

R

N

N

R

CN
R

4CzIPN

10

...
OLED Lighting, a bright future
Cost no object
Designed by
Kardorff
Deutsche Bank
Berlin
OLED Lighting, a bright future
Cost no object
Designed by
Kardorff
Deutsche Bank
Berlin
OEM Research Group

Blue is always a problem for any system
Phosphorescent emitters are difficult to push blue enough
High ...
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OLEDs,the dawn of ultra efficient lighting

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Palestra plenária do XII Encontro da SBPMat (Campos do Jordão, setembro/outubro de 2013). Palestrante: Andy Monkman - Durham University (Reino Unido).

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OLEDs,the dawn of ultra efficient lighting

  1. 1. OEM Research Group OLEDs, the dawn of ultra efficient lighting Prof Monkman, OEM Research Group   Department of Physics, Durham University http://www.dur.ac.uk/OEM.group a.p.monkman@dur.ac.uk
  2. 2. Themes OLEDs, the dawn of ultra efficient lighting General considerations of making OLED lighting Problems facing blue phosphorescence The brave new world of TADF! Deutsche Bank Berlin Monkman OEM Research Group
  3. 3. What does OLED lighting offer? Better design esthetics, better quality of lighting, cheaper to run ‘off grid’ lighting for the third world We must get away from this 1953 2013 UK strongly engaged with lighting designers and architects and lighting industry in major UK funded R&D projects TOPLESS and TOPDRAWER projects
  4. 4. What does OLED lighting offer? Design No No’s impractical/fanciful more of the same welcome to the mental hospital
  5. 5. What does OLED lighting offer? Better Design Introducing dimensionality to break up the ceiling space
  6. 6. What does OLED lighting offer? This is now much better Design Small panels give much higher resolution to the shapes within a small vertical distance. Very important for design vital for production yields. We do not want 1m x 1m panels And it will be hybrid OLED/LED
  7. 7. FP7-224122 – OLED100.eu D5.7 - Second set of psycho-physiological perception case st What does OLED lighting offer? 4.2.2.2 “Crater” Gradient Fabrication/manufacture !"#$%&%'()$%)'*+#3 !"#$%&%'()$%)'*+#, minimal patterning required Figure 4.10: Real picture of the light box (right) and luminance distribution pattern with indicated lu solution/vacuum deposition hybrid processing profile cross-sections thus slot die coating for soluble layers '(% &#!! -./&0(/-)0/&%12(/1( &"!! '() pre patterned TCO with bus bars &!!! '($ potentially ink jet/electroless plating %!! '(* $!! '(# #!! hybrids enable multilayer and '(+ "!! p-i-n structures '(" ! Small panels, improve yield '(% -./&0(/-)*$&45%&6 &*!! reduced cost of bus bars on TCO '() high compatibility with hybrid &!!! '($ '(* *!! ! '(# '(+
  8. 8. What does OLED lighting offer? OLED ‘lamps’ being large area surface emitters have lower glare and so do not require a luminaire to control brightness Fluorescent T5 lamps inside a metal reflector luminaire to control brightness The luminaire introduces typical 40% loose of light output, so a 90 lm/W tube effectively gives a light sources of less than 55 lm/W
  9. 9. Inside an OLED Simple OLED pixel structure Al (100 nm) LiF (1 nm) TPBI (30 nm) Emissive layer (ca. 70 nm) PEDOT:PSS (ca. 50 nm) ITO (125 nm) Glass low work function composite metal cathode electron transport layer ‘ETL’ combined emissive and hole transport layer HTL transparent anode ‘TCO’ hole injection layer ‘HIL’
  10. 10. Inside an OLED Simple OLED pixel structure hole injection layer ‘HIL’ Both HTL and ETL are thick layers via the use of doping i.e. p-i-n structure Schematic energy level diagram of an OLED under bias
  11. 11. Inside an OLED Best commercial white panels from LG Chem 10 cm x 10 cm, 80 lm/W Complex OLED lighting structure OLED Stack 1 Charge regeneration layer MoO3 or TPBi:Cs3PO4 OLED Stack 2 Two stacked OLEDs running at half effective brightness, thus half drive current required at twice the applied voltage of a single stack. This improves lifetime by a factor of 4 at high brightness.
  12. 12. Inside prising mal IL an OLED minim the CG J. Appl. p-doped n-doped p-doped n-doped Phys. 111, 103107 CGL technology time m IL IL good f (c.) no external bias, (d.) under reverse bias, Anode lifetim with Al O IL NPD:MoOwith Al O IL 3 withou cussed ferent alignm ings. F p-doped n-doped p-doped n-doped chemic IL IL leading of the (e.) no external bias, (f.) under reverse bias, Cathode with CuPc IL with CuPcBCP:Cs3PO4 IL the ene interfa FIG. 11. Simplified model of the energy-level alignment for a CGL device The O3 IL ((c) and (d)) and with CuPc IL electronduced without IL ((a) and (b)), with Al2CGL under reverse bias generates ((e) hole bias, (b), (d), and (f.) under by a bias and and (f)): (a), (c), and (e) no FIG. 4.pairs which are separated reverseconstant current den externalVoltage dependency over time atthe bias. field increas 2 he non-stacked (a) reference OLED and the 3 2 3 10 mA/cmintoCGL test devices. The OLED stacks. was varied b moves 2 of the two adjacent CuPc IL thickness 0 nm and 8 nm. Diez et al, J. Appl. Phys., 11, 103107, 2012 The stability of the CGL test device comprisin
  13. 13. What challenges remain in OLED lighting? There are two major current challenges 1)  Improve the out-coupling of light from the OLED Only 22-25% of light escapes from the OLED, current out-coupling techniques can increase this to nearly 50%. Thus there is still half the light being lost, getting that light out is a major challenge 2) BLUE – a real materials challenge The current blue emitters are not blue enough or stable enough. Host materials have to be extremely high triplet energy which causes many further problems New emitter materials and new ways of generating light are needed.
  14. 14. Charge Carrier Recombination e- h+ recombination ↓ ↓ ↑ ↑ 25 % 1P+P- 75 % 3P+P- ↑ ↑ 25 % Singlet Excitons ↓ ↓ 75 % Triplet Excitons None emissive states in purely organic emitters
  15. 15. Light perception The ‘average’ human eye has poor sensitivity to blue so it will be hardest to make a ‘bright’ blue-white lamp. The eye response shifts to the blue as intensity diminishes as rods dominate which are responsive to blue-green only
  16. 16. Why is the current blue not good enough? with FIrpic we will not make 4000K and CRI>80, so its a non starter. here, much more blue required, much less red (relative) so power efficiency must go down Even a fluorescent tube looses about 40% efficiency to achieve 6200K because of poor blue eye response Monkman OEM Research Group
  17. 17. Why is the current blue not good enough? LG Chem say they will mass produce 100x100 mm glass 80 lm/W 4th quarter 2013, 3000K, 75 lm output, 20,000 hrs note typical fluorescent luminaire is a 3500 lm package The lighting industry considers LT 90 as standard Data from UDC Monkman OEM Research Group
  18. 18. Why is the current blue not good enough? Further, FIrpic is unstable to loose of the fluorine atoms, both on deposition and during operation. This is also true for any Ir based complex containing fluorine electron withdrawing groups to blue shift the emission. Also, for high SOC, the HOMO must contain a significant Ir dorbital content. As the MLCT gap widens (giving bluer emission) it becomes easier to also populate the dd* states which very efficiently quench emission. This there are intrinsic limits on how blue an Ir complex can be pushed whilst retaining high PLQY What then might be able to replace an Ir based phosphorescent blue emitter? Monkman OEM Research Group
  19. 19. Host degradation Part II CBP phosphorescence We did measurements another Usually people only consider emitter degradation butofthe sample which was evaporated at the rate between 30-70 angstroms Very commonly used host per sec i.e. very high evaporation rate. Normalized delayed fluorescence and phosphorescence Structure: hosts, especially high triplet energy host are a various temperatures are shown in figure 15. There are four features of phosphorescence at problem spectra at low temperature and two of them (497nm and 532nm) become more and more insignificant with increase of temperature as crest at 607 steadily peaks out in relation to the crest at 560. CBP d200ms i50ms just after evaporation d200ms i300ms couple of weeks after evaporation d200ms i300ms after annealing in air Other info: d20msi10ms100K not annealed, evaporation rate 30 to 70 a per sec, measured just after evaporation d20msi50ms112K OSA3251 DCBP, CBP d20msi70ms110K Name: 4,4'-Bis(carbazol-9-yl)biphenyl; sublimed d10msi50ms120K d20msi10ms135K Application: OLED-Hole Transport d20msi50ms142K Abs.Max: (nm) 318 nm (Methylene Chloride)14K 0.8 d20ms i10ms Melting Point(C): 281 (DSC) d20msi70ms270K DF 0.4 Ph Intensity, A.U. Intensity, A.U. 0.8 0.0 400 500 Wavelength, nm 600 700 Formula: Appearance: Abbreviation: 0.4 C36H24N2 Odorless pale yellow powder DCBP, CBP Sample Preparation: CBP was evaporated on quartz substrate which before it was rinsed for 6 minutes with soaped water, deionized bath. Then it was cured in UV-ozone atmosphere for Lesker OLED evaporator at the controllable rate. Total 0.0 Spectra recording Use triplets because they live so long Gated luminescence measurements were made at the te that they can sample a large volume and 400 500 600 700 as indicated Change of spectra without annealing Wavelength, nm in the figures. Samples were excited with 355nm, at 45 degree angle to the substrate plane. Energ find the defects, especially at room falling on the sample was about 1 cm. With the sides the above changes in phosphorescence spectra we were able to observe the shifted triplet energy reduces markedly oversensitive CCD cam luminescence was collected with the time Figure 15 temperature osphorescence with the peaks only at 560 and 607 without annealing the film just after aporation (not later than two hours). The results are shown in Figures 19-21. The spectra of this whilst TF increases showing better hopping Part I gure 18 Then, after mple is very similar to the spectra at ~16K of heated films (Figure 17,18), at ~16K of films which couple of weeks (about 9/10 of time sample was held in the compartment near the glove box the ectra were recorded after considerable time of evaporation (Figure 12,13,17,18), as well toi.e. it had some exposure to oxygen) the same sample of allmeasure again.fluorescence and shown First was we measured The results are phosphoresc CBP ambient evaporation speed hours at 2.5 ectra of unheated films at room temperature (Figure 15). We think this might have somein fig 16-18, red curves. As well this sample was heated infilms. Theatmosphere for twowas about108 angs relation were recorded is as shown hours 16-18, green degrees Celsius and effect on photophysics of heating (annealing)not later than 2in fig after evaporation. Monkman the conformation of film. The structure (conformation) which is formed on substrate during /after OEM Research Group curves. aporation should depend on the rate material is cooled. This depends on temperature difference
  20. 20. Basic Poly(N-vinylcarbazole) triplet energy measurements Mw PVK > 106, high purity In film ET ~ 2.9 eV The phosphorescence spectra does not shift with time implying that the triplet does not migrate. Triplet lifetime is also >10 s at 14 K. Compare to our original pulse radiolysis energy transfer measurements made in benzene in solution, ET ~ 3 eV. Pina et al Chemical Physics letters    400, 2004, 441-445    Monkman OEM Research Group
  21. 21. Temperature dependence of phosphorescence 1.0 Intensity, A.U. 0.8 14 K 0.6 0.4 0.2 ET=2.89 eV 0.0 400 450 500 550 600 650 700 Wavelength, nm Monkman and Jankus PHOTONIC MATERIALS CENTRE 70 ms delay 355 nm excitation 150 ps pulse
  22. 22. Temperature dependence of phosphorescence 1.0 Intensity, A.U. 0.8 14 K 44 K 0.6 0.4 0.2 ET=2.55 eV 0.0 400 450 500 550 600 650 700 Wavelength, nm Monkman OEM Research Group 70 ms delay 355 nm excitation 150 ps pulse
  23. 23. Temperature dependence of phosphorescence 1.0 Intensity, A.U. 0.8 14 K 44 K 85 K 0.6 0.4 0.2 ET=2.41 eV 0.0 400 450 500 550 600 650 700 Wavelength, nm Monkman OEM Research Group 70 ms delay 355 nm excitation 150 ps pulse
  24. 24. dilute solution state phosphorescence Temperature dependence of phosphorescence 14 K 44 K 14K - 44K Pina et al Chemical Physics letters    400, 2004, 441-445 Monomeric ET=2.88 eV Dimeric ET=2.4 eV Jankus and Monkman Adv Func Mat 21, 3350, 2011 Monkman OEM Research Group
  25. 25. in toluene 450 nm excitation 0 .04 m g /m l 0 .4 m g /m l 4 m g /m l 3 8 m g /m l 0 .2 0 .1 1.0 ET=2.41 eV 0.8 Inte ns ity, A .U . O pt c a l de ns ity Is this a ground state or excited state species? 0.6 0.4 0.2 0 .0 350 400 45 0 W a veleng th, nm 500 Clear concentration dependent ground state absorption. Monkman 400 450 500 5 50 6 00 W a veleng th, nm 6 50 Emission at 12us delay, 450 nm excitation at 14 K, identical to dimeric species emission NB, good vibronic structure OEM Research Group
  26. 26. Evidence for two ground state species Excitation 450 nm 14 K 44 K 85 K 100 K 295 K Intensity, A.U. 1 6650 ns and 22520 ns 4980 ns and 8850 ns 2640 ns 2190 ns 980 ns Very clearly these species are monomer and dimer they are NOT ‘excimer’ 0.1 0.01 0 10000 20000 30000 Time, ns Two distinct lifetime components which are highly temperature dependent Monkman OEM Research Group
  27. 27. What form has the dimer? ½ co-facial dimer Benten et al, J.Phys.Chem.B 2007, 111, 10905 Full co-facial dimer The closer the co-facial interaction between neighboring carbazole units the larger the shift in the phosphorescence. Further, the shift is so large that these dimers must have substantial charge transfer character. Monkman OEM Research Group
  28. 28. What could replace Ir ? We need new blue emitters that combine the stability of a fluorescence molecule with the an ability to ‘harvest’ triplet excitons very efficiently Is this fundamentally impossible? Monkman OEM Research Group
  29. 29. E-type delayed fluorescence to harvest triplet excitons TADF = E-type DF !"##$%& $'( )"*+,,- e- : h+ Thermally activated !" #"$%&'"# (' "))%*%"'+ ,-./ 0(+"1%(2 3(4%'& delayed fluorescence ./!012345 1"$62+"# %' +7+(2 89 "))%*%"'*: 7) ;<=> 25% ?'#"1 "2"*+1%*(2 "@*%+(+%7'5 #"07'$+1(+"#> Singlet  State 56789*: ;<4 A($ B%'*" +3" 072"*62(1 %$ 6'2%0%+"#5 A" 75% #"$%&'$61"2: 1"(2%D" =5.)"@C"*+ +3(+5 )61+3"1 %0C174"0"'+ A%22 >$? 5 89: 56 !6 $%&'()*+),+) 2"(#%'& +7 E<<= F9 "))%*%"'*:> Triplet  State Fluorescence <=5>?677:@ @A Excitation Delayed Fluorescence (TADF) !A Phosphorescence !"#$ !@ ΔEST 0%)+1(2+3% 04+21312' , ;9: -.'*/.'()*+),+) 57 =5>?677:@ '% % %7@CDE: B '? !3 K. Goushi, K. Yoshida, K. Sato, and C. Adachi, "Organic light-emitting diodes employing efficient reverse intersystem crossing for triplet-to!"#$%&'#(%)*+,-./012+345675+8#%9':6%(4; singlet state conversion," Nature Photonics, vol. 6, pp. 253-258, Apr 2012. Ground  State Monkman OEM Research Group
  30. 30. Introduction &'()*+%,-$'%./,%("#$$%!EST% Basic theory of CT states !"#$!%&!'()*+ !,#$!%&!'(&* &'()*+%,-$'%./,%("#$$%!theST% K, E exchange energy EH :HOMO Energy EL : LUMO Energy K :Exchange Energy dependents on the spatial !EST=2K separation of the orbitals and Energy !"#$!%&!'()*+ EH ::HOMO Energy LUMO !EST=2K 1 &! (&* EL :Exchange Energy the overlap of the HOMO and !,#$!%! ' ! (1)d" d" K K = !L (1)!U (2) LUMO in this case L (2) U 1 2 r12 1 K = ## !L (1)!U (2) !L (2)!U (1)d" 1d" 2 r12 (LUMO) (HOMO) (LUMO) ## (HOMO) (HOMO) ・0#,*'%!EST (LUMO) ・0#,*'%!EST (HOMO) ・ !"#$$%!EST (LUMO) ・ !"#$$%!EST The trade off, if HOMO and LUMO have zero overlap the radiative decay rate becomes Donor X Donor state has Acceptor unless Acceptor X very small so the CT low PLQY the lifetime is long i.e. IC and NR decay !  Donor-Acceptor backbone are even weaker. !  Donor-Acceptor backbone !  X:Introduction of steric hindrance !  X:Introduction of steric hindrance !  Rather high radiative decay rate !  Rather high radiative decay rate Monkman OEM Research Group
  31. 31. TADF New family of D-A-D molecules incorporating para and meta D-A coupling so we call systems containing 1 ‘linear’ and 2 ‘bent’ based on the electron transport unit dibenzothiophene-S,S-dioxide developed in Durham Dias et al., J.Phys.Chem.B. 110, 19329, 2006 ST energy splitting from 0.35 to 0.9 eV 1d Dias and Monkman et al., Adv. Mater. 2013, 25, 3707 Monkman OEM Research Group
  32. 32. TADF Intramolecular Charge Transfer States (ICT) D 2b D A carbazole donors dibenzothiophene-S,S-dioxde acceptor HOMO hν LUMO Small singlet-triplet energy splitting in CT states Singlet state Ground and excited state orbitals are nearly independent with very little orbital overlap. This is important. But also gives low radiative decay rate 1ICT 3ICT ΔE (1CT-3CT)<100 meV Triplet state ΔEST∼ 0.3 to 0.9 eV Dias et al., J. Phys. Chem. B 2006, 110, 19329-19339 K. Moss et al., J. Org. Chem. 2010, 75, 6771–6781 Monkman OEM Research Group
  33. 33. Delayed Fluorescence measurements 8 Intensity, A.U. 10 pe 1 - slo PF 7 10 1 lope P H -s 6 10 DF 5 10 a) Laser pulse energy, µJ 1 prompt emission measured at 450 nm delayed 0 1 2 3 4 5 6 7 8 9 10 10 10 10 10 10 10 10 10 10 Time, ns Normalized intensity, A.U. Intensity, A.U. 0 10 -1 10 -2 10 -3 10 -4 10 -5 10 -6 10 -7 10 -8 10 -9 10 -10 10 -11 10 0.1 - e2 lop s 3.6 µJ 0.1 µJ 0.8 150 ps Nd:YAG (70 mJ @ 355 nm) 200 ps FWHM gated iCCD HP pulse generator FWHM 10 ns 0.4 0.0 b) 400 450 500 550 600 Wavelength, nm 650 Rothe, Monkman Phys.Rev.B (2002), Rothe et al Chem.Phys. (2002), Sinha et al Phys.Rev.Lett (2003) Monkman OEM Research Group
  34. 34. TADF Enhanced CT character for bent materials (2’s) 2c D D A   ΔE /Δf= 0.91 ΔE  (e V )   0 .8 hν 0 .4   1 .2 2e 2d Lippert-Mataga 0 .4 plots 0 .0   0 .8 HOMO ΔE /Δf= 2.47 ΔE /Δf= 1.55 0 .1 0 .2 0 .3 0 .4 0 .0 Δf Monkman ΔE /Δf= 1.01 1b   LUMO 1e   1 .2   OEM Research Group 0 .1 0 .2 0 .3 0 .4
  35. 35. Fluorescence lifetimes 8 1d he x a ne λx :3 6 3 nm   λx :4 2 0 nm 6 he x a ne λx :  3 6 3  nm   λe m :  4 1 0  nm 6 τ:  3.08  ns 4 2 1d 2d 3 C ounts x 1 0 3 8 C ounts x 1 0 TADF IR F :  2 1  ps 0 0 1 2 3 4 T im e  (ns ) 5 4 τ1 :  1.60  ns 2 0 6 2d IR F :  2 1  ps 0 2 4 T im e  (ns ) 6 8 The CT character is not responsible for the observation of strong Phosphorescence   80  290K  100K 60 RT Phosphorescence 6   40   2 .0 2d  in  z eo n ex   ΔEST=0.46 eV 6 4a Inte ns ityx 1 0  (c ps ) 200 ΔEST=1.1 eV 500 600  T 290K  T 270K  T 250K  T 200K  T 150K  T 100K  T 80K 160 120 80 N o  O 2 1 .6 1 .2 0 .8 with  O 2 0 .4 0 .0 400 450 500 550 600 wa ve le ng th  (nm ) 40 0 300 6 400   0 Inte ns ity  x 1 0  (c ps ) 20   Inte ns ityx 1 0  (c ps ) 5d Lone pairs are important 400 500 W a ve le ng th  (nm ) Monkman 600 Dias and Monkman et al., Adv. Mater. 2013, 25, 3707 OEM Research Group
  36. 36. TADF Properties of delayed fluorescence (DF) 4a   2 4a barrier to viscous flow in ethanol is 0.15 eV 6 D F  inte g ra lx 1 0  (c ounts ) 10 Pure Triplet Fusion ΔEST=1.1 eV E a = 0.16 ±0.05  e V 10 1b 1   10 0 3.0 3.2 3.4   10 2   10 E a = 0.18 ±0.05  e V 10 N orm .  Inte ns ity  A .U . 1b 6 D F  inte g ra lx 1 0  (c ounts )   3 10 -­‐7 -­‐9 1ππ* 3.5 4.0 4.5 -­‐1 10 /T  (K ) 5.0 10 3 Dias and Monkman et al., Adv. Mater. 2013, 25, 3707 Monkman 3.26 eV 1CT* 3.12 eV 3ππ* 1 3.0 P rom pt  (7 .4 1  ns ) -­‐5 10 DF I2 3 -­‐3 10 4.2 1b -­‐1 10 4.0 10 /T 1 10 10 3.8   ΔEST=0.84 eV 3.6 -­‐1 1 10 -­‐1 0 10 -­‐8 10 -­‐6 10 -­‐4 10 ΔEST=0.84 eV 2.28 eV T A D F (bi  e x pone ntia l)   378   µs  (6 7 % ),  1.8  m s  (3 3 % ) -­‐2 T im e  (ns ) OEM Research Group 3CT*
  37. 37. Properties of delayed fluorescence (DF) 2.95 eV 1CT* 2.74 eV 3ππ* ΔEST=0.63 eV 3CT* ΔEST=0.63 eV 2.11 eV 1 -­‐7 10 2 1e 10 1 10 2 240K D F /P F  :  0 .0 0 2 8   10 E a = 0.26 ±0.02e V ΔE S T = 0.63  e V -­‐9 T A D F  (487   µs )     -­‐5 10 1e ΔE S T = 0 .6 3 ±0 .0 6  e V 10 -­‐1 1 -­‐1 0 -­‐9 -­‐8 -­‐7 -­‐6 -­‐5 -­‐4 -­‐3 -­‐2 10 10 10 10 10 10 10 10 10 10 -­‐1 700 6 10 10   -­‐3 6 10   N orm .  Inte ns ity  A .U . -­‐1 600 3 D F  inte g ra l  v s  e x c ita tion  dos e R T ,  e tO H P rom pt  (8.66  ns ) 10 500 wa ve le ng th  (nm ) 3 1e E m is s ion  Inte g ra lx 1 0  (c ounts ) 10  S S -­‐R T  T A D F -­‐3 0 0 K  T A D F -­‐2 0 0 K 400 We observe TADF with a ST energy gap of 0.63 eV!     10 1e   1ππ*   N orm a liz e d  Inte ns ity  (a .u.) 1e Mixed Triplet Fusion TADF D F -­‐inte g ra lx 1 0  (c ounts ) TADF 10 0 10 line a r  fit  with  s lope  fix e d  a t  2 (c oe f.  c orr.  0.995) 0 T im e  (s ) 10 1 10 2 10 3 10 1 P owe r  ( µJ ) Monkman OEM Research Group 2 .5 3 .0 3 .5 4 .0 4 .5 5 .0 5 .5 3 -­‐1 10 /T  (K )
  38. 38. TADF Properties of delayed fluorescence (DF) 2e   ΔEST=0.48 eV 3CT* ΔEST=0.48 eV 2.39 eV 0 .8 0 .4 0 .0 400     We observe strong TADF with an energy gap of 0.48 eV! 10 4 2e 10 -­‐4 10 -­‐6 10 3 T A D F   ( 818   µs ) -­‐8 -­‐1 0 -­‐9 -­‐8 -­‐7 -­‐6 -­‐5 -­‐4 -­‐3 -­‐2 10 10 10 10 10 10 10 10 10 10 -­‐1 700 W a ve le ng th  (nm )   4 E a = 0.28 ±0.02e V ΔE S T = 0.48  e V ΔE S T = 0 .4 8 ±0 .0 6  e V 10 3 10 2 6 10 2   10 1 240K line a r  fit  with  s lope  fix e d  a t  1 c oe f  c orre l:  0.996 D F /P F :0 .0 2 6   -­‐1 0 600   -­‐2 10 500 2e D F  Inte g ra l  a s  a  func tion of  e x c ita tion  powe r R T ,  e tO H 6 10 P rom pt  (3.07  ns )   N orm .  Inte ns ity  A .U . 0 E m is s ion  Inte g ra lx 1 0  (c ounts ) 2e 10 10 D F -­‐Inte g ra lx 1 0  (c ounts ) 10 2  S te a dy-­‐S ta te  T A D F 2e   2.87 eV N orm a liz e d  Inte ns ity  (a .u.) 1CT* 3ππ* 1 .2 3.15 eV 1ππ* 10 Dominant TADF 10 0 T im e  (s ) 10 1 10 2 10 3 10 1 2 .5 P owe r  ( µJ ) Monkman OEM Research Group 3 .0 3 .5 4 .0 3 4 .5 -­‐1 10 /T  (K ) 5 .0 5 .5
  39. 39. TADF Constant acceptor 1e 2e 2d   ΔE ΔE S T = 0.48 ±0.05  e V = 0.63 ±0.06  e V ST E a = 0.26 ±0.02e V ΔE S T = 0.35 ±0.04  e V E a = 0.28 ±0.02  e V E a = 0.28 ±0.02e V Activation energy is independent of the ST gap and is not due to viscous flow 2   3 10 2d 2e   10     6 D F -­‐inte g ra lx 10  (c ounts ) 4 1 0 1e   240  K 10 1 240  K 240  K 3 4 5 3 4 3 5 4 5 -­‐1 1 0 /T (K ) Monkman 3 Dias and Monkman et al., Adv. Mater. 2013, 25, 3707 OEM Research Group
  40. 40. TADF Constant donor 2d 3d 4d     ΔE S T = 0.57 ±0.05e V ΔE S T = 0.35 ±0.04  e V E a = 0.83 ±0.02  e V Activation energy is dependent on the donor structure ie. lone pairs   2   3 10 ΔE S T = 0.87 ±0.04  e V E a = 0.47 ±0.03e V E a = 0.28 ±0.02  e V 10 4d 3d   6 D F -­‐inte g ra lx 10   ( c ounts ) 4 1 0 2d   280  K 240  K 10 260  K 1 3 4 5 3 4 5 1 0 3 /T (K -­‐1 ) Monkman 3 4 5 Dias and Monkman et al., Adv. Mater. 2013, 25, 3707 OEM Research Group
  41. 41. TADF We require another energy level to explain the temperature dependent behaviour of the DF 1e 2e 1ππ* 1CT* 1ππ* 2.95 eV 2.74 eV 3CT* ΔEST=0.63 eV 3ππ* ΔE=0.35 eV 2.11 eV Large gap TF dominates Monkman 2.87 eV 1CT* 3CT* ΔEST=0.28 eV ΔEST=0.48 eV ΔEST=0.28 eV 3.15 eV 3ππ* ΔE=0.2 eV 2.39 eV Small gap TADF dominates OEM Research Group
  42. 42. TADF We introduce the “little known” nπ* orbital Dias and Monkman et al., Adv. Mater. 2013, 25, 3707 Monkman OEM Research Group
  43. 43. How efficient is TADF even with a gap of 0.36 eV 10 DF/PF ratio∼ 5% 400   2  T A D F  s te a dy-­‐s ta te  e m is s ion 500 600 700 10 1 10 line a r  fit  with  s lope  fix e d  a t  1. c oe f.  c orre l.:  0.995 0 10 0 10 ΦT = 0.05 ±0.01 10 10 -­‐6 2 10 600 4 400 200K 2 -­‐8 200 D F /P F :  0.048 10 3 6 800   T A D F  (230   µs ) -­‐1 330K 6 100% efficient TADF Inte ns ity x 10  (c ps ) -­‐4 9 k is c = (5 ±1)x 10  s 6 -­‐2 10 10 ΔE T A D F = 0.38 ±0.05  e V -­‐T 10 1   2d P rom pt  (4.7  ns )   N orm  Inte ns ity  A .U . 2 Triplet yield ∼ 5% 1 0 0 0   0 10 powe r  ( µJ ) 8 10 2d wa ve le ng th  (nm ) 2d   3   2d   2d 10 E m is s ion  Inte g ra l  (c ounts ) N orm a liz e d  Inte ns ity  (a .u,)    E m is s ion  Inte g ra lx 10  (c ounts ) TADF -­‐1 0 10 -­‐1 0 -­‐9 -­‐8 -­‐7 -­‐6 -­‐5 -­‐4 -­‐3 10 10 10 10 10 10 10 10 -­‐2 0 400 T im e  (s ) 500 600 700 wa ve le ng th  (nm ) Dias and Monkman et al., Adv. Mater. 2013, 25, 3707 Monkman 0 200 220 240 260 280 300 320 340 T e m pe ra ture  (K ) M. N. Berberan-Santos, J. M. M. Garcia, JACS. 1996, 118, 9391 OEM Research Group
  44. 44. 0.8 Neat 2d Film 0.4 1.0 0.8 0.6 0.4 0.2 400 450 500 550 600 650 700 Wavelength, nm 500 600 Wavelength, nm Excitation pulse energy in microjouls 2d in TPBi 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1 700 0.130mkJ 1.061mkJ 2.32mkJ 7.36mkJ 7.94mkJ 2d in TAPC Excitation pulse energy in microjouls 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1 0.063mkJ 0.205mkJ 1.09mkJ 4.16mkJ 350 400 450 500 550 600 650 700 Wavelength, nm 350 400 450 500 550 600 650 700 Wavelength, nm 2d in TAPC - TADF dominated 1,3CT In TAPC no host quenching TADF PLQY 0.5 In TPBi 2d TF dominates with a small TADF component Integrated intensity, A.U. 2 Critical role of host triplet energy 10 1 10 0 10 -1 10 -2 10 -3 10 -4 10 -5 10 -6 10 2d in TPBi red edgesmaller TF influence 1.29 1.17 1.05 1.04 neat 2d red edge TADF dominated -2 Monkman 2d in TPBi blue edgestrong TF influence 1.09 10 Jankus et al,. PRL in press Excitation pulse energy in microjouls 0.170mkJ 0.776mkJ 1.2mkJ 4.75mkJ 9.1mkJ 15.6 50mkJ 0.0 0.0 400 Normalized intensity, A.U. 2d neat in TPBi in TAPC Normalized intensity, A.U. Normalized intensity, A. U. 14 % 8% 50% Normalized intensity, A.U. TADF in solid state PLQY 1.87 -1 10 0 neat 2d blue edge TF dominated 1 10 10 Pulse fluence, µJ OEM Research Group 2 10
  45. 45. TADF in solid state Critical role of host triplet energy Normalized intensity, A.U. In TAPC no 1,3CT host quenching TADF PLQY 0.5 So TAPC is special as all excitations remain trapped on the 2d thus we find much longer lifetimes of excited states 433 ns for the prompt emission 25 us for TADF and 165 us for phosphorescence We also find that the first and last decay components decrease with increasing temperature whereas the middle one increases with increasing temperature DF/PF~0.082 2d neat film 7pc 2d in TAPCDF/PF~1.8 7pc 2d in TPBi DF/PF~0.15 2d in solution DF/PF=0.048 0 10 -2 10 -4 10 -6 10 -8 10 0 10 1 10 2 3 4 10 10 10 Time, ns 5 10 6 DF/PF ratio of  1.8 the triplet yield is 64% and the fluorescence yield is 50% This signifies long CT lifetimes yielding high triplet CT production, energy cycling between the iso energetic CT states and thermal activation from the ππ* triplet to the 3CT 10 Monkman OEM Research Group
  46. 46. TADF devices How do OLEDs performed based on TADF emitters? Monkman OEM Research Group
  47. 47. Phosphorescence TADF devices b N 10 CN CN 300 K 200 K 100 K N NC 0 N R 1 300 K N R N N R CN R 4CzIPN 10 Prompt R Delayed N N 10–1 bazolyl –2 R R 4CzTPN: R = H 4CzTPN-Me: R = Me 4CzTPN-Ph: R = Ph Photoluminescence quantum efficiency gy 10 diagram and molecular structures0of CDCBs. a, Energy 400 500 600 0 10 20 30 40 ventional organic molecule. b, Molecular structures of CDCBs. Time (μs) phenyl. c d –3 14.2 1.0 ΔEST = 83 Combined n. 2International Institute for Carbon Neutral Energy Research (WPI-I2CNER), meV 0.9 14.0 Prompt Delayed 0.8 101 100 10–1 Normalized electroluminescence intensity (a.u.) Normalized photoluminescence intensity a R External electroluminescence quantum efficiency (%) LETTER RESEARCH 1 4CzIPN 4CzTPN-Ph 2CzPN 0 400 10–2 10–3 500 600 Wavelength (nm) 10–2 10–1 700 100 101 102 For a green device 13.6 using 4CzTPN Current density (mA cm ) 0.6 devices having EQE of 19% have been Figure 5 | Performance of OLEDs containing CDCB derivatives. External 13.4 reserved 0.5 13.2 electroluminescence quantum efficiency as a function of current density for reported. This equates to an internal 0.4 OLEDs containing 4CzIPN (green circles; error within 1.5%), 4CzTPN-Ph (red 13.0 0.3 QE approaching 85%, as good as phosphorescent 1.0%) and 2CzPN (blue triangles; error within 1.0%) as triangles; error within 12.8 0.2 emitters. Inset, electroluminescence spectra of the same OLEDs (coloured 12.6 OLED all from TADF accordingly) at a current density of 10 mA cm22. 0.1 13.8 –2 In(kRISC) 0.7 0.0 12.4 0 50 100 150 200 Temperature (K) 250 300 3.5 4.0 4.5 –3/T (K–1) 10 5.0 Figure 4 | Temperature dependence of photoluminescence characteristics of a 5 6 1 wt% 4CzIPN:CBP film. a, Photoluminescence decay curves of a wt% 4CzIPN:CBP film at 300 K (black line), 200 K (red line) and 100 K (blue Monkman ine). The photoluminescence decay curves show integrated 4CzIPN emission. The excitation wavelength of the films was 337 nm. b, Photoluminescence Adachi et al, Nature, 492, 237, 2012 11.2 6 1% and 8.0 6 1%, respectively, which are also higher than those of conventional fluorescence-based OLEDs. Finally, we consider the mechanism that drives such efficient reverse ISC without heavy metals. It is generally accepted that the introduction of spin–orbit coupling provided by heavy atoms is required for both OEM Research Group ISC and reverse ISC to be efficient. Thus, metal complexes containing
  48. 48. OLED Lighting, a bright future Cost no object Designed by Kardorff Deutsche Bank Berlin
  49. 49. OLED Lighting, a bright future Cost no object Designed by Kardorff Deutsche Bank Berlin
  50. 50. OEM Research Group Blue is always a problem for any system Phosphorescent emitters are difficult to push blue enough High triplet hosts suffer major degradation problem Triplet Fusion not good enough, 62.5% maximum not attainable TADF promising, true 100% triplet harvesting ICT again has host problem DA exciplex being a self ambipolar host is really exciting The TOPLESS lamp. Thank you http://www.dur.ac.uk/OEM.group a.p.monkman@dur.ac.uk

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