A. Fischer, S. Forget, S. Chénais, M.-C. Castex, Lab. de Physique des Lasers, Univ. Paris Nord, France Highly efficient multilayer organic pure-blue- light emitting diodes with substituted carbazole compounds in the emitting layer. D. Adès, A. Siove, Lab. Biomateriaux et Polymères de Spécialité, Univ. Paris Nord, France C. Denis, P. Maisse and B. Geffroy Lab. Cellules et Composants, CEA Saclay, France

CLEO ’06 – Long Beach (USA) 2 Outline Introduction : why BLUE oleds ? Two new carbazolic compounds : PMC (Pentamethylcarbazole) and DEC (Dimer of N-ethylcarbazole) Devices using neat films of PMC and DEC in single layer and multilayer structures Devices using doped films of PMC:DPVBi and DEC:DPVBi Conclusion

CLEO ’06 – Long Beach (USA) 3 Introduction Organic Light Emitting Diodes : Ultrathin light sources, lightweight High brightness and viewing angle > 160° Low drive voltage (3-10 V) and low power consumption Extremely rich diversity of materials : All visible colors available (≠ inorganic LEDs), including saturated colors Potentially flexible Long lifetimes (> h reported) Low cost potential for mass production Applications : flat-panel RGB DISPLAYS, solid-state lighting,...

CLEO ’06 – Long Beach (USA) 4 needs efficient blue emitters Why BLUE ? Why Blue OLEDs with high efficiencies are needed ? different approaches for multi-color emission : RGB emitters + : power efficient, mature - : different aging and optimization needs efficient blue emitters (efficient R, G already exist) White emitters + Filters + : homogeneous aging - : not efficient (filters) needs efficient blue emitters to achieve bright white Color changing media + : homogeneous aging - : not efficient (photoconversion)

CLEO ’06 – Long Beach (USA) 5 OLEDs materials Requirements for an efficient blue material : Chemical stability and Electrochemical stability High T g High quantum yield of photoluminescence in the solid state Chromaticity coordinates approaching the spectrum locus (saturated color) Active research for new blue-emitting organic materials (both fluorescent and phosphorescent) CIE 1931

CLEO ’06 – Long Beach (USA) 6 OLEDs materials Carbazolic derivatives PMC DEC Carbazole unit : penta-methyl carbazole Dimer of N -Ethyl carbazole Chemically and thermally stable (up to 430 °C) T g = 75°C Polaronic transport levels measured by cyclic voltammetry (eV) : - Blue emitters: Carbazole- substituted Distyrylarylenes (DSA) - Hole Transport materials : PVK - Host material for triplet emitters: CBP Vacuum level L owest U noccupied M olecular O rbital H ighest O ccupied M olecular O rbital PMC DEC Already used as… new

CLEO ’06 – Long Beach (USA) 7 OLEDs structures 1st DEC-based diode : single layer Drawbacks: Low ext. quantum efficiency  ext. = % High operating voltage (20 V), crystallization during operation (short-circuit) DEC ITO Al h V D. Romero, A. Siove et al., Adv. Mater. 9, 1158 (1997) This work : Use of DEC (and PMC) in a multilayer OLED structure with both neat films and doped films configurations: efficient deep-blue organic emitter Bad performance due to recombination and quenching of excitons at Al/DEC interface, poor charge injection

CLEO ’06 – Long Beach (USA) 8 Device a : OLED with NEAT film of DEC Anode ITO nm Cathode LUMO HOMO CuPc 10nm ETL NPB 50 nm Alq 3 10nm LiF / Al 1.2 / 100nm HIL HTL HBL BCP 10nm DEC 50 nm CuPc NPB holes electrons

CLEO ’06 – Long Beach (USA) 9 Device a : OLED with neat film of DEC Anode Cathode LUMO HOMO ETL HIL HTL HBL holes electrons Main recombination zone η ext = 1.5 % (optical design not optimized) ITO nm CuPc 10nm NPB 50 nm Alq 3 10nm LiF / Al 1.2 / 100nm BCP 10nm DEC 50 nm

CLEO ’06 – Long Beach (USA) 10 Anode Cathode LUMO HOMO ETL HIL HTL HBL NPB holes electrons Device a : OLED with neat film of PMC PMC OLED η ext = 0.6 % → attributed to bad electron transport properties of PMC / electron barrier of BCP ITO nm CuPc 10nm NPB 50 nm Alq 3 10nm LiF / Al 1.2 / 100nm BCP 10nm PMC 50 nm

CLEO ’06 – Long Beach (USA) 11 Device a : OLED with neat film of PMC Anode Cathode LUMO HOMO ETL HIL HTL HBL holes electrons PMC OLED η ext = 0.6 % → attributed to bad electron transport properties of PMC / electron barrier of BCP Main recombination zone ITO nm CuPc 10nm NPB 50 nm Alq 3 10nm LiF / Al 1.2 / 100nm BCP 10nm PMC 50 nm

CLEO ’06 – Long Beach (USA) 12 Device a (neat films) : Experimental results Electroluminescence spectra a Chromaticity coordinates PMC DEC Aggregates, excimers ? PMC : CIE x = ; y = DEC : CIE x = ; y = Ext. Quantum efficiency : η ext = 0.6 % (PMC) η ext = 1.5 % (DEC) Brightness L = 236 cd/m 60 mA/cm 2 (PMC) Luminous efficiency η power = 0.2 lm/W (PMC) → Bright saturated blue With PMC, but modest efficiency

CLEO ’06 – Long Beach (USA) 13 Investigating emitting mixtures (« doping ») The role of emitting mixtures (or « doping » but not in the electrical sense !) « energy transfer » doping = diluting a low-gap guest material inside a wide-gap host : Förster (and Dexter) energy transfers possible → Very efficient mechanism but not useful for blue emitters guest host other types of doping : the dopant « impurities » can enhance exciton recombination by trapping charge carriers (and diffusing excitons) guest host Ex : Barrier for electrons + trap for holes = improved recombination rate

CLEO ’06 – Long Beach (USA) 14 Device b : OLEDs with DPVBi doped with PMC (DEC) CuPc 10nm NPB 50nm DPVBi (PMC or DEC) 50nm Alq 3 10nm LiF 1.2nm/Al 100nm (b) ITO glass DEC PMC + or 5% wt. 2% wt. DPVBi 4,4’-bis(2,2’-diphenylvinyl)- 1,1’-biphenyl Vacuum level L owest U noccupied M olecular O rbital H ighest O ccupied M olecular O rbital PMC DEC DPVBI Doping by coevaporation from 2 resistively heated cells

CLEO ’06 – Long Beach (USA) 15 OLEDs with DPVBi doped with DEC Anode ITO nm Cathode LUMO HOMO CuPc 10nm ETL NPB 50 nm Alq 3 10nm LiF / Al 1.2 / 100nm HIL HTL DEC:DPVBi 50 nm holes electrons % DEC DPVBi

CLEO ’06 – Long Beach (USA) 16 OLEDs with DPVBi doped with DEC Anode Cathode LUMO HOMO ETL HIL HTL holes electrons % DEC DPVBi Recombination zone η ext = 3.3 % ITO nm CuPc 10nm NPB 50 nm Alq 3 10nm LiF / Al 1.2 / 100nm DEC:DPVBi 50 nm

CLEO ’06 – Long Beach (USA) 17 Anode Cathode LUMO HOMO ETL HIL HTL holes electrons % PMC DPVBi OLEDs with DPVBi doped with PMC Recombination zone η ext = 2.8 % ITO nm CuPc 10nm NPB 50 nm Alq 3 10nm LiF / Al 1.2 / 100nm PMC:DPVBi 50 nm

CLEO ’06 – Long Beach (USA) 18 Anode Cathode LUMO HOMO ETL HIL HTL holes electrons DPVBi Recombination zone η ext = 2.7 % Comparison point : OLEDs with DPVBi ALONE ITO nm CuPc 10nm NPB 50 nm Alq 3 10nm LiF / Al 1.2 / 100nm PMC:DPVBi 50 nm

CLEO ’06 – Long Beach (USA) 19 Device b (doping) : SUMMARY PMC:DPVBi DEC:DPVBi DPVBi Device (a) PMC Device (a) DEC Device (b) DPVBi PMC- doped (5%) Device (b) DPVBi DEC- doped (2%) Device (b) DPVBi nondoped  ext (%)  power (lm/W) 0.2… L (cd/m 2 60 mA/cm 2 236… … C.I.E. x C.I.E. y ► All spectra similar to DPVBi and NPB : which material is emitting light ? ►no shoulder in DEC spectra : suppression of aggregates by dilution

CLEO ’06 – Long Beach (USA) 20 Summary We demonstrated state-of-the-art external quantum efficiency of 3.3% with a deep-blue OLED (CIE x = 0.15 ; y = 0.17) using a DEC:DPVBi emitting mixture Close to the max 5% = 25% (singlet/triplet ratio) x 20% (extraction efficiency) Efficiency of the doping approach : DEC:DPVBi better than DPVBi alone (or DPVBI:PMC) : attributed to enhanced trapping of charged carriers PMC exhibits the most saturated color (x = 0.15 ; y= 0.10) : better efficiency would be achievable with a different design while keeping the CIE coordinates (in progress)

CLEO ’06 – Long Beach (USA) 21