Excited state interaction in P-OLEDS implications for efficiency and lifetime

Transcription

1 Excited state interaction in P-OLEDS implications for efficiency and lifetime M. Roberts, S.M. King, M.Cass, M. Pintani, C.Coward N. Akino, H. Nakajima, M. Anryu SID 2011 Session 56: OLED Device I (Paper Number 56.1) Los Angeles Convention Center, Petree Hall C Thursday, May 19, 2011, 1:30 PM - 1:50 PM

2 10% EQE deep blue with long lifetime Efficiency Lifetime Strategies Current Performance 2

3 Excited state interaction in POLEDs Efficiency Lifetime Strategies Current Performance 3

4 Expected EQE of fluorescent OLED charge balance S:T ratio PLQE outcoupling EQE = exciton formation x singlet formation x photon emission x photon escape 100% 25% 70% 30% ~ 5% Cathode LEP IL HIL ITO Glass Model device / materials F8 PFB F8 TFB N N N C 8 H 17 C 8 H 17 n C 8 H 17 C 8 H 17 n

5 EL Intensity EQE (%) cd/a 10% EQE at CIEy= cd/m CIE (0.14, 0.13) 0.2 Cathode LEP IL HIL ITO Glass nm

6 How can we explain 10% EQE? charge balance Singlet Yield PLQE outcoupling EQE = exciton formation x singlet formation x photon emission x photon escape 100% 25% 70% 30% ~ 5% Most likely cause of discrepancy S:T ratio > 25% for polymers? Triplet harvesting via TTA? 6

7 EL intensity (normalised) Delayed Electroluminescence Fast decay ~24ns (Device RC limited) V reverse bias applied Residual Delayed EL unchanged by reverse bias pulse 1E x x x x x10-6 Time /s Delayed fluorescence does not originate from trapped charges

8 EL intensity / dt/t(780nm) (normalised) Triplet Triplet Annihilation (TTA) 1 Intercept = ~ [ T ] ~ [ T ] 2 Triplet Density (from transient absorption at 780nm) (Triplet Density) E x x x x x10-6 Time /s ~ [ T ] 2 Delayed EL Origin of delayed fluorescence is TTA : T 1 + T 1 S 1 + S 0 Triplets make up 20-30% of efficiency

9 Including TTA in EQE expression charge balance Singlet+Triplet Yield PLQE outcoupling EQE = exciton formation x singlet formation x photon emission x photon escape 100% 70% 30% ~10% 50% F s:t + (1- F s:t ) c t F s:t = S:T ratio c t = triplet yield 2 strategies for improving EQE in fluorescent systems

10 Review 10% CIEy=0.13 demonstrated TTA process is key to achieving this EQE

11 Excited state interaction in POLEDs Efficiency Lifetime Strategies Current Performance 11

12 Brightness Cd/m2 Counts Lifetime testing of P-OLED devices 1000 Constant current density Driven pixels T50 hrs PL efficiency drop is a major cause of efficiency loss How stable is the TTA contribution? 0 undriven driven nm

13 PL Counts Triplet Density (dt/t) Driving Effect on Triplet Density undriven Singlets Pristine quenched Driven by to 30% T50 at T50 1.2x x10-5 Triplets quenched by 85% at T50 V T0 undriven T50 4.0x10-5 T V T Wavelength /nm V Triplets are quenched very effectively in a driven device

14 Cd/m2 Normalised Luminance TTA component is unstable Total luminance T100 T90 T80 T70 T Non TTA component x x x x x10-6 Time /s time TTA component TTA contribution can limit lifetime and give rapid initial decay 14

15 Review 10% CIEy=0.13 demonstrated TTA process key to achieving this EQE TTA contribution can be relatively unstable

16 Excited state interaction in POLEDs Efficiency Lifetime Strategies Current Performance 16

17 Triplet density dt/t (780nm) Strategy 1 : Remove triplets Triplet Quenching unit Efficiently removes triplets from system 1.2x10-4 F8-PFB+Triplet Quencher 1.0x10-4 F8-PFB No TQ 8.0x10-5 dpvbi (1% mol ratio) 6.0x x x With TQ S1=2.65eV S1= 2.75eV V T1= 2.1eV F8-PFB dpvbi T1= 2.0eV Triplets quenched to S0

18 External Quantum Efficiency Strategy 1 : Remove triplets 0.06 Without Triplet Quencher 0.04 With Triplet Quencher V 20% drop in EQE consistent with loss of TTA contribution

19 Normalised Luminance Efficiency Cd/A Strategy 1 : Remove triplets Without Triplet Quencher With Triplet Quencher Time /h Triplet quenchers can significantly improved T90 and T50 but at expense of efficiency

20 Strategy 2 : Stabilise TTA TTA stabilising unit Maintain TTA efficiency boost Prevent triplets from being quenched S1=2.65eV S1> 2.65eV TTA T1= 2.1eV T1< 2.1eV F8-PFB TTA stabiliser X Triplets not quenched to S0

21 Strategy 2 : Stabilise TTA Stable TTA units allow both high efficiency and long lifetime

22 EQE [%] Normalized Luminance EQE and lifetime in 10% EQE system LEP only LEP+TTA LEP+TQ 100% 90% 80% 70% 60% 50% Current Density [ma/cm2] 5000cd/m2 start LEP only LEP+TTA LEP+TQ A A A A A A Time [h] Long lifetime + High efficiency can be achieved by stabilising the TTA contribution

23 Review 10% CIEy=0.13 demonstrated TTA process key to achieving this EQE Stable TTA units can give combination of good lifetime and high efficiency

24 Excited state interaction in POLEDs Efficiency Lifetime Strategies Current Performance 24

25 Latest PLED performance data 2011/Spring Spin/BE Red Green Blue High efficiency + good lifetime Efficiency [cd/a] Colour (C.I.E.) x=0.67 y=0.32 x=0.65 y=0.35 x=0.63 y=0.37 x=0.35 y=0.60 x=0.31 y=0.63 x=0.14 y=0.22 x=0.15 y=0.13 x=0.15 y=0.12 x=0.14 y=0.14 Lifetime [hrs] 200k 170k 350k 200k 190k 34k 21k 14k 15k Vd [V] Device structure ITO (45nm)/ spin-coated HIL (50-65nm)/ Interlayer (20nm)/ LEP (60-75nm)/ low-wf cathode RGB common and simple layer structure Organics are fully solution-processed

26 Appendix 26

27 TTA yield TTA PROCESSES AVAILABLE SINGLET YIELD FROM TTA INTERNAL QE FOR 25% S:T INTERNAL QE FOR 50% S:T 5 x (S 0 + Q) T 1 + T 1 3 x (S 0 + T n ) 1 x (S 0 + S 1 ) 1/18 + 3/18x1/18 + 3/18x3/18x1/18 +..~ 5% Ch. E. Swenberg and N. E. Geacintov, in Organic Molecular Photophysics, edited by J. B. Birks ~ John Wiley and Sons, NY, 1973, p %+0.05*75% = 30% 50%+0.05*50% = 52.5% 3 x (S 0 + T n ) T 1 + T 1 1 x (S 0 + S 1 ) 1/8 + 3/8x1/8 + 3/8x3/8x1/8 + ~20% 25%+0.2*75% = 40% Kondakov, J. Soc. Inf. Disp. 17, 137 (2009) 50%+0.2*50% = 60% T 1 + T 1 1 x (S 0 + S 1 ) 1/2 = 50% 25%+0.5*75% = 62.5% Kondakov JAP 106, (2009) 50%+0.5*50% = 75% Combination of S:T>25% and high TTA yields now established as promising routes to higher efficiency fluorescent POLEDs