Stacked OLEDs for Lighting Applications - Improvement of the yellow building block

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1 Stacked OLEDs for Lighting Applications Improvement of the yellow building block 13/12/2010 Carola Diez Osram Opto Semiconductors GmbH and University of Augsburg

2 OLED Lighting White organic light emitting diodes Lighting applications require White emission High brightness High efficiency Lifetime Low current densities Cathode Electron Transport Layer Hole Blocking Layer Blue Dopand in Matrix Green Dopand in Matrix Red Dopand in Matrix Electron Blocking Layer Hole Transport Layer ITO Anode PirOLED by OSRAM K. Heuser, Plastic Electronics 2010 Stacked OLEDs for Lighting Applications Improvement of the yellow building block 13/12/2010 Page 2 Carola Diez

3 Stacking of OLEDs V blue rgb stacking Charge generation layers (CGLs) connect OLEDs in series. Multiple photons per injected electron hole pair. V CGL green red Forrest et al., JAP, 86, No 8 (1999). Liao et al., Appl. Phys. Lett. 92, (2008). Chen et al., Appl. Phys. Lett. 93, (2008). Boosts current efficiency (cd/a). Longer lifetime Automatic fluorescent / phosphorescent separation. Individual cavity optimization for each unit. Stacked OLEDs for Lighting Applications Improvement of the yellow building block 13/12/2010 Page 3 Carola Diez

4 Charge Generation Layers (CGLs) p/ndoped organic interface:tunneling model CGL pdoped organic layer ndoped organic layer Band diagram V V Tunneling of electrons occurs from the HOMO of the pdoped part to the LUMO of the ndoped part. Electrons are driven away from the interface by an external electric field. Holes left in the HOMO state are injected into the adjacent layer by the electric field. Liao et al., Appl. Phys. Lett. 84, 167 (2004). Kröger et al., Phys. Rev. B 75, (2007). Stacked OLEDs for Lighting Applications Improvement of the yellow building block 13/12/2010 Page 4 Carola Diez

5 White OLEDs and color mixing c y green yellow red Metal EIL ETL / HBL Blue HTL / EBL CGL ETL / HBL Green Red HTL / EBL HIL } 1st emission unit (blue building block) }2nd emission unit (yellow building block) ITO blue Black body line White spectrum is as a linear combination of a blue and a yellow spectrum. c x Investigation and optimization of both emission units have to be done separately. Stacked OLEDs for Lighting Applications Improvement of the yellow building block 13/12/2010 Page 5 Carola Diez

6 The yellow building block c y green Metal EIL ETL / HBL Green Red HTL / EBL HIL red ITO The yellow building block can be formed by a red and green phosphorescent emitting layer (EML). c x Investigation of the yellow building block necessary for efficient white OLEDs. Stacked OLEDs for Lighting Applications Improvement of the yellow building block 13/12/2010 Page 6 Carola Diez

7 Investigation of the yellow building block Band diagram of the rgunit E LUMO HIL HTL,EBL HTL red ETL green Electron ETL,HBL EIL Metal ITO HOMO Hole Stacked OLEDs for Lighting Applications Improvement of the yellow building block 13/12/2010 Page 7 Carola Diez

8 Investigation of the yellow building block E LUMO HIL HTL,EBL HTL red Electron ETL green ETL,HBL EIL Metal ITO HOMO Hole Mechanisms at the red and green EML Shift of recombination zone due to a nonoptimal charge balance. TripletPolaronQuenching (TPQ) TripletTripletAnnihilation (TTA) Exciton diffusion Förster and Dexter transfer for fluorescent and phosphorescent emitters, respectively. Stacked OLEDs for Lighting Applications Improvement of the yellow building block 13/12/2010 Page 8 Carola Diez

9 Mechanisms at the red and green EML N N 2 Ir O O N N Ir N Energy transfer and TripletTriplet Annihilation Phosphorescent red and green emitter. ADS076 Ir(ppy)3 T 1 of the host and blocker materials > T 1 of the emitters. E Transfer of triplets T 1(Irppy) > T 1(ADS) Dexter transfer from Irppy to ADS possible. T 1 (Irppy) T 1 / ev T1 (ADS) S 0 The higher the emitter concentration, the higher the probability for Dexter transfer. reduction of green emission. The higher the emitter concentration, the higher the probability for Triplet Triplet quenching. Variation of the emitter concentration Stacked OLEDs for Lighting Applications Improvement of the yellow building block 13/12/2010 Page 9 Carola Diez

10 Variation of the emitter concentration Dexter transfer at the interface HTL ADS ETL Irppy Electron Radiance (W/(sr m² nm)) % ADS076 5% ADS076 8% 3mA/cm² Wavelength (nm) Hole Constant Irppy concentration The higher the concentration of the red emitter, the lower the green emission. More ADS molecules at the interface Dexter transfer and TTA. Recombination at the interface (Dexter transfer ~ 1nm). Stacked OLEDs for Lighting Applications Improvement of the yellow building block 13/12/2010 Page 10 Carola Diez

11 Variation of the emitter concentration Dexter transfer at the interface HTL ADS ETL Irppy Electron EQE [%] Current density [ma/cm²] 3% ADS076 5% ADS076 8% ADS076 Hole Constant Irppy concentration TripletPolaron Quenching (TPQ) at high current densities results in reduced EQE. TTA occurs the higher the ADS076 concentration, the lower the EQE. Stacked OLEDs for Lighting Applications Improvement of the yellow building block 13/12/2010 Page 11 Carola Diez

12 Variation of the emitter concentration Hole Hopping via Irppy HTL ADS ETL Irppy Electron Radiance (W/(sr m² nm)) % Irppy 11% Irppy 15% 3mA/cm² Wavelength (nm) Constant ADS076 concentration the higher the Irppy concentration, the higher the green emission. recomb. zone in green EML shifted. Red emission is reduced due to lack of electrons in the red EML. Stacked OLEDs for Lighting Applications Improvement of the yellow building block 13/12/2010 Page 12 Carola Diez

13 Variation of the emitter concentration Hopping via Irppy HTL ADS ETL Irppy Electron EQE [%] Current density [ma/cm²] 8% Irppy 11% Irppy 15% Irppy Hole Constant ADS076 concentration TPQ at high current densities results in reduced EQE. EQE nearly independent on Irppy conc. nearly no TTA, due to reduced exciton density at the interface. Stacked OLEDs for Lighting Applications Improvement of the yellow building block 13/12/2010 Page 13 Carola Diez

14 Modification of the recombination zone Single host approach Interface between red and green Red emitter: ADS076 in HTL HTL ADS ETL Irppy Electron Green emitter: Irppy in ETL Excitons are built at the interface of the red and green emission layer. Exponential decay of the exciton generation zone. Only small region of EML used reason for degradation? Hole Stacked OLEDs for Lighting Applications Improvement of the yellow building block 13/12/2010 Page 14 Carola Diez

15 Modification of the recombination zone Hole Symmetrix mixed host approach HTLETL ADS ETL filler material Irppy Electron Interface between red and green Red emitter: ADS076 in HTL and ETL. Green emitter: Irppy in ETL and HTL. Hole in green EML via Irppy molecule. HTL (filler material) in the green EML necessary to regulate the electron. Recombination zone is broadened compared to the single host system. Probability of TTA is minimized, due to a broader recombination zone. More molecules contribute to light generation. Stacked OLEDs for Lighting Applications Improvement of the yellow building block 13/12/2010 Page 15 Carola Diez

16 Modification of the recombination zone Asymmetric mixed host approach Hole HTLETL ADS ETL Irppy Electron Stacked OLEDs for Lighting Applications Improvement of the yellow building block 13/12/2010 Page 16 Carola Diez Radiance (W/(sr m² nm)) % ETL in red 50% ETL in red Wavelength (nm) Mixed host for red 3mA/cm² The higher the fraction of the ETM in the red EML, the lower the green emission. electrons are ed into the red EML, holes are trapped by ADS076 molecules. shift of the recombination zone into the red EML.

17 Modification of the recombination zone Symmetric mixed host approach HTLETL ADS ETL filler material Irppy Electron Radiance (W/(sr m² nm)) single host mixed 3mA/cm² Wavelength (nm) Hole Single host vs. mixed host Slightly more red and green emission at const. current measurement for double mixed host system. Color coordinates nearly unchanged. recombination zone broadens equally. Stacked OLEDs for Lighting Applications Improvement of the yellow building block 13/12/2010 Page 17 Carola Diez

18 Stacked OLEDs for Lighting Applications Improvement of the yellow building block Metal EIL ETL / HBL Green Red HTL / EBL HIL ITO Results Dexter transfer and TTA could be investigated by changing the concentration of the emitter molecules. The recombination zone could be modified by introducing a mixed host system for both emitter materials. more molecules contribute to light generation higher efficiency longer lifetime before optimization after optimization 14% EQE 16% EQE ~ 600 h ~ 1500 h Stacked OLEDs for Lighting Applications Improvement of the yellow building block 13/12/2010 Page 18 Carola Diez

19 Acknowledgments TOPAS2012 BMBF FKZ 13N10474 Prof. W. Brütting and group OLED team at OSRAM in Regensburg Stacked OLEDs for Lighting Applications Improvement of the yellow building block 13/12/2010 Page 19 Carola Diez

20 OLED. Pure light. Thank you for your attention! Stacked OLEDs for Lighting Applications Improvement of the yellow building block 13/12/2010 Page 20 Carola Diez

21 Stacked OLEDs for Lighting Applications Improvement of the yellow building block 13/12/2010 Page 21 Carola Diez Backup

22 Lifetime of the yellow building block Normalized luminance L/L0 100% 90% 80% single host mixed host Normalized luminance L/L0 100% 90% 80% Each EML Each EML 20nm 70% Time (hours) 70% Time (hours) Host dependency of lifetime 45% lifetime improvement due to broader recombination zone in the mixed host system. More emitter molecules contribute to electroluminescence. Thickness dependency of lifetime Thicker emission layers lead to enhanced lifetime. Minor change in color when R and G thickness increased in similar way. Stacked OLEDs for Lighting Applications Improvement of the yellow building block 13/12/2010 Page 22 Carola Diez

23 Optimized rgb system Luminous efficacy (lm/w) Before optimization After optimization Normalized luminance L/L0 100% 95% 90% 85% 80% Before optimization After optimization Luminance (cd/m²) 70% Time (hours) Experimental 1000 cd/m² V [V] EQE [%] Lum Eff [lm/w] Cx Cy LT 70 [h] Before optimization h After optimization h Stacked OLEDs for Lighting Applications Improvement of the yellow building block 13/12/2010 Page 23 Carola Diez