H. Choukri, A.Fischer, S. Forget, S. Chénais, M.-C. Castex, Lab. de Physique des Lasers, Univ. Paris Nord, France Color-control (including White) in OLEDs by shifting the position of an ultrathin yellow layer in a blue matrix. D. Adès, A. Siove, Lab. Biomateriaux et Polymères de Spécialité, Univ. Paris Nord, France N.Lemaitre, B. Geffroy Lab. Cellules et Composants, CEA Saclay, France

CLEO ’06 – Long Beach (USA) 2 WHITE oleds WHITE oleds applications Solid-state LIGHTING : efficient and energy-saving large areas (≠ inorganic leds) and potentially flexible devices : new paradigm in lighting Pixels or backlight panels for DISPLAYS Demanding requirements to compete with fluorescent and incandescent light bulbs : 1) 100% internal quantum efficiency (phosphorescent materials ?) 2) Low operational voltages and power consumption ( p and n-doped injection layers ?) 3) efficient and reliable color-mixing schemes to achieve white

CLEO ’06 – Long Beach (USA) 3 INTRODUCTION Getting white from OLEDs : Down-Conversion (Blue OLED + Phosphors) Using exciplex/excimer emission Mixing basic colors in a single layer (« doping ») Mixing colors obtained in separate regions engineering of the recombination zone microcavity effects

CLEO ’06 – Long Beach (USA) 4 INTRODUCTION : underlying concepts Mixing two complementary colors in appropriate proportions : blue + yellow « doping strategy » = mixing a small proportion (typ. <5%) of yellow emitter in a blue matrix →  accurate control of weak doping levels difficult ð Alternative : Including an ultrathin yellow layer in a blue matrix : better control of the thin film deposition Varying different parameters (thickness, position) to finely tune the color

CLEO ’06 – Long Beach (USA) 5 STRUCTURE of the OLEDs + Glass substrate Anode ITO HIL HTL ETL Cathode (Al+LiF) EL (blue) EL (yellow) Thin film deposition by thermal evaporation under high vacuum (10 -7 torr) Anode ITO nm Cathode LUMO HOMO CuPc 10 nm ETL NPB 50 nm DPVBi : Rubrène 60-e : e nm Alq 3 10nm LiF / Al 1.2 / 100nm HIL HTL Blue Emitter Yellow Emitter

CLEO ’06 – Long Beach (USA) 6 absorption Photoluminescence CHEMISTRY Well-known materials : CuPc Alq 3 NPB DPVBi Rubrene Transporting layers Emitting layers Anode ITO nm Cathode LUMO HOMO CuPc 10 nm ETL NPB 50 nm DPVBi : Rubrène 60-e : e nm Alq 3 10nm LiF / Al 1.2 / 100nm HIL HTL Blue Emitter Yellow Emitter + Efficient Förster energy transfer (Blue) (Yellow)

CLEO ’06 – Long Beach (USA) 7 WHITE OLEDs : principle Anode ITO nm Cathode LUMO HOMO CuPc 10nm ETL NPB 50 nm DPVBi : Rubrène 60-e : e nm Alq 3 10nm LiF / Al 1.2 / 100nm HIL HTL Blue Emitter Yellow Emitter Exciton diffusion + Förster transfer Exciton formation Yellow + Blue = White

CLEO ’06 – Long Beach (USA) 8 Influence of the total thickness Design of the OLEDs - part 1 Taking into account microcavity effects = locating the recombination zone at an antinode for blue and yellow eigenmodes ITO[150] CuPc[10] NPB[45.5] Ru[1] NPB[3.5] DPVBi[x] Alq 3 [10] Al[100] Balanced white Al mirror ITO/glass interface blue yellow

CLEO ’06 – Long Beach (USA) 9 Experimental determination of optimum thickness ETFOS © software 60 nm 140 nm 230 nm Etfos© software

CLEO ’06 – Long Beach (USA) 10 Influence of the rubrene thickness Design of the OLEDs - part 2 Variation of the rubrene thickness e [1-10 nm] NPB [50 nm] Ru [e nm] DPVBi + Ru [60 nm] [5 nm] [(55 – e) nm] Optimal Rubrene thickness : ~1 nm → « monolayer » 17 Å 7 Å Rubrene molecule 14 Å

CLEO ’06 – Long Beach (USA) 11 Anode ITO nm Cathode LUMO HOMO CuPc 10nm ETL NPB 50 nm DPVBi : Rubrène 60-e : e nm Alq 3 10nm LiF / Al 1.2 / 100nm Energy HIL HTL Blue Emitter Yellow Emitter Possible explanations : 1 ) electrons are trapped when rubrene thickness e exceeds the monolayer width  poor e - /holes balance at the interface 2) Concentration Quenching of excitons in neat rubrene - Influence of the rubrene thickness e

CLEO ’06 – Long Beach (USA) 12 Influence of the rubrene position Design of the OLEDs - part 3 Variation of the rubrene position d [-10 ► +20 nm] NPB [50 nm] DPVBi [60 nm] ● d = Ru [1 nm] ▲ Variation of the OLED color from pure yellow (Rubrene) to pure blue (DPVBi) via white

CLEO ’06 – Long Beach (USA) 13 Influence of the rubrene position NPB [50 nm] DPVBi [60 nm] d = Ru [1 nm] ▲ d = -10 nm Color SpectrumCIE x,yPerformances η ext = 3.4 % 1.2 lm/W 2275 Anode Cathode Variation of the rubrene position d [-10 ► +20 nm]

CLEO ’06 – Long Beach (USA) 14 Influence of the rubrene position NPB [50 nm] DPVBi [60 nm] d = Ru [1 nm] ▲ d = -5 nm Color SpectrumCIE x,yPerformances η ext = 1.7 % 0.9 lm/W 1700 cd/m² Anode Cathode Variation of the rubrene position d [-10 ► +20 nm]

CLEO ’06 – Long Beach (USA) 15 Influence of the rubrene position NPB [50 nm] DPVBi [60 nm] d = Ru [1 nm] ▲ d = -3.5 nm Color SpectrumCIE x,yPerformances η ext = 1.7 % 1.1 lm/W 1795 cd/A Variation of the rubrene position d [-10 ► +20 nm] Bright white : CIE coordinates x= 0.32 ; y=0.33 Color Rendering Index = 73

CLEO ’06 – Long Beach (USA) 16 Influence of the rubrene position NPB [50 nm] DPVBi [60 nm] d = Ru [1 nm] ▲ d = -3 nm Color SpectrumCIE x,yPerformances η ext = 1.4 % 0.9 lm/W 1689 cd/m² Variation of the rubrene position d [-10 ► +20 nm] Anode Cathode

CLEO ’06 – Long Beach (USA) 17 Influence of the rubrene position NPB [50 nm] DPVBi [60 nm] d = Ru [1 nm] ▲ d = 0 nm Color SpectrumCIE x,yPerformances η ext = 1.25 % 1.2 lm/W 1600 cd/m² Anode Cathode Variation of the rubrene position d [-10 ► +20 nm]

CLEO ’06 – Long Beach (USA) 18 Influence of the rubrene position NPB [50 nm] DPVBi [60 nm] d = Ru [1 nm] ▲ d = 5 nm Color SpectrumCIE x,yPerformances η ext = 2.6 % 2.5 lm/W 4067 cd/m² Anode Cathode Variation of the rubrene position d [-10 ► +20 nm]

CLEO ’06 – Long Beach (USA) 19 Influence of the rubrene position NPB [50 nm] DPVBi [60 nm] d = Ru [1 nm] ▲ d = 10 nm Color SpectrumCIE x,yPerformances η ext = 2 % 1 lm/W 1700 cd/m² Anode Cathode Variation of the rubrene position d [-10 ► +20 nm]

CLEO ’06 – Long Beach (USA) 20 Influence of the rubrene position NPB [50 nm] DPVBi [60 nm] d = Ru [1 nm] ▲ d = 20 nm Color SpectrumCIE x,yPerformances η ext = 2.8 % 1.1 lm/W 1900 cd/A Anode Cathode Variation of the rubrene position d [-10 ► +20 nm]

CLEO ’06 – Long Beach (USA) 21 Anode Cathode CuPc 10nm NPB 50 nm DPVBi : Rubrène 60-e : e nm Alq 3 10nm LiF / Al 1.2 / 100nm - Rubrene into NPB is better d = +20 nm x,y = (0.174 ; 0.151) η ext = 2.8 % d = -10 nm x,y = (0.172 ; 0.147) η ext = 3.4 % Anode Cathode CuPc 10nm NPB 50 nm DPVBi : Rubrène 60-e : e nm Alq 3 10nm LiF / Al 1.2 / 100nm - Rubrene in NPB : No hole trapping Rubrene in DPVBi : electron trapping Comparison of two BLUE diodes with identical CIE x,y :

CLEO ’06 – Long Beach (USA) 22 Influence of the rubrene position Estimation of the exciton diffusion length Simple exciton diffusion model : Peak rubrene intensity  exp(-d/L d ) ► Width of recombination zone ~ 15 nm

CLEO ’06 – Long Beach (USA) 23 Influence of the rubrene position Summary : NPB [50 nm] DPVBi [60 nm] ● d = Ru [1 nm] ▲

CLEO ’06 – Long Beach (USA) 24 Conclusion We show that we can finely control the OLED color by tuning the position and thickness of a thin layer of pure Rubrene in DPVBi We obtain very good WHITE Oled with CIE coordinates of (0.32 ; 0.33)and CRI > 70 Same kind of design with three colors (R, G, B) could pave the way toward full color control of OLED in a given chromatic gamut.

CLEO ’06 – Long Beach (USA) 25 Electrical characterization YELLOW OLED e=1nm d=0nm (DPVBi) BLUE OLED e=1 d=-10 (NPB) WHITE OLED e=1 d=-3.5 (NPB)