Solution Processable LEDs Merck KGaA Anna Hayer EuroDisplay 2013
Content 1 Introduction 2 LED Basics 3 Challenges for Solution Processing 4 Current Results 5 Summary 2 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs
Why LED? ultra-thin & light-weight wide viewing angle high contrast / true black fast switching large area flat light source new design opportunities arbitrary 2D shape flexible transparent high energy efficiency 3 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs
Why solution-processing? evaporation-based LEDs already in mass production solution-processed LEDs offer the advantages of greater ease of processing reduced manufacturing costs very large area application variety of processing techniques available 4 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs
LED: What is the buzz about? rganic Light Emitting Diode = Light Generation Liquid Crystal = Light Modulation 3 6 V Back Light Back Light LED: A fully controllable light bulb with the correct colour in every pixel. 5 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs
Content 1 Introduction 2 LED Basics 3 Challenges for Solution Processing 4 Current Results 5 Summary 6 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs
Working principle of simple LEDs 3 6 V LUM E F - eu E F + HM Anode Emissive Layer Cathode Anode Emissive Layer Cathode 7 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs
Device setup: Simple stack 2 1 Basic Processes 3 Charge injection (1) Charge transport (2) hνν 4 Exciton formation (3) Recombination (4) 3 Drawbacks of single layer device Large injection barriers 1 Anode 2 Emissive layer (EML) Cathode Holes und electrons not balanced Leakage current 8 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs
Device setup: Improved stack 2 3 hν 4 1 Improvements via multilayer: Additional injection layer Lower voltage Additional blocking layer Reduced leakage 1 Anode 2 Hole injection layer HTL 3 EML ETL Electron injection layer Cathode Specialization of materials High mobilities High quantum efficiencies (HIL) (EIL) 9 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs
LED characterisation: Initial performance apply drive voltage; increase in steps measure current measure brightness measure spectrum 100 100000 1.2 current d density [ma/cm²] 10 1 0.1 0.01 0.001 0.0001 0.00001 lumin nance [cd/m²] 10000 1000 100 10 EL norm m. [arb. units] 1 0.8 0.6 0.4 0.2 0.000001 0 5 10 voltage [V] 1 0 5 10 voltage [V] 0 400 500 600 700 800 wavelength [nm] efficiency [cd/a] 10 80 70 60 50 40 30 20 10 0 0 2000 4000 luminance [cd/m²] EuroDisplay 2013 Hayer - Merck Solution Processable LEDs calculate efficiency in EQE photons per electron cd/a visible light per current lm/w optical power per electrical power calculate colour coordinates CIEx, CIEy Y 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 520 530 540 550 560 570 580 590 600 610 630 680 490 480 470 460 0 0.5 X
LED characterisation: Long term stability apply constant current measure light output over time 12000 10000 measure voltage increase over time 6 5 cd/m²] luminance [ 8000 6000 4000 2000 [V] voltage [ 4 3 2 1 0 0 200 400 600 800 time [h] 0 0 200 400 600 800 time [h] Lifetime LT50 : time until the brightness is reduced to 50% LT80, LT95, LT97 etc. accordingly 11 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs
LED Discovery 1987: Tang & van Slyke @Eastman Kodak LED from evaporated small molecules 1990: Burroughes, Bradley, Brown, Marks, Mackay, Friend, Burn, Holmes @University of Cambridge polymer LED from solutionprocessed precursor 12 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs
LED preparation: Vacuum evaporation RGB Pixels Substrate Cathode separator Source Shadow Mask Advantages: already in mass production high performance easy fabrication of multilayer stacks Drawbacks: relatively low material utilisation scaling to very large areas challenging (fine metal masks,...) 13 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs
LED preparation: Solution Processing Inkjet printing Examples: other printing methods Already in mass production in LC industry: used for colour filters up to Gen 8 (glass size : ~ 2.2 x 2.5 m²) to be adapted to functional layers Spin-coating gravure printing flexographic printing Fast and easy process ideal for material evaluation unstructured not for polychrome displays slot die coating (unstructured, for wide area) 14 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs
Content 1 Introduction 2 LED Basics 3 Challenges for Solution Processing 4 Current Results 5 Summary 15 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs
Solubility Examples for low solubility in common organic solvents: standard evaporable material: not soluble Ir Ir(ppy) 3 a green emitter 3 PB a hole transport material often purely aromatic compounds soluble LED material one approach to solubilise: long alkyl chains * n * octyl-pb TFB 16 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs
Film Formation Examples: What it should not look like What is needed during coating: low aggregation & crystallisation tendency low phase separation (in a mixture) good wetting during solvent removal: high glass transition temperature low aggregation & crystallisation tendency 17 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs
Materials: Polymers vs. SM Polymers Soluble small molecules Film formation very good crystallisation & aggregation can be an issue Reproducibility in synthesis challenging to reproduce molecular weight & polydispersity impact on processing & device performance very reproducible Purification challenging broad range of established methods (including sublimation) 18 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs
Multilayer from solution: Challenge Cathode ETL HTL achieve EML HTL achieve EML HTL IT IT IT avoid EML IT EML IT EML/ HTL IT avoid ETL/ EML/ HTL washed off reduced mixed mixed IT Avoid washing off or redisolving previous layer during deposition of next layer 19 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs
Multilayer approaches: rthogonal solvents ETL EML EML HTL IT HTL IT HTL IT example: Ma et al., Adv.Mat. 2005, 17, 274 first layer from water second layer from organic solvent (e.g. xylene) third layer from alcohol (e.g. methanol) * n * I- + + I- * S n * 20 S 3 - PEDT/PSS PF PF + (CH 3 ) 3 I - -PBD EuroDisplay 2013 Hayer - Merck Solution Processable LEDs * * n * n *
Multilayer approaches: Cross-linking EML hν or T ETL EML deposition of first layer from solution IT curing next layer can be polymerisation deposited from solution layer becomes insoluble (even from same solvent) IT Al for a review, see Zuniga et al., Chem.Mat. 2011, 23, 658 example: Du et al., Macromol. Rapid Commun. 2006, 27, 412: Al photoinitiator, RT Al n Al m Al l 21 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs
Content 1 Introduction 2 LED Basics 3 Challenges for Solution Processing 4 Current Results 5 Summary 22 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs
Limitations of simple device Lif fetime [kh] 125 100 75 50 25 Triplet Green not accessible simple device, no evaporated layers Ba/Al EML HTL Lif fetime [kh] 60 50 40 30 20 10 Triplet Red not accessible 0 0 25 50 75 efficiency [cd/a] HIL IT 0 0 5 10 15 efficiency [cd/a] Efficiency in simple device is limited cathode quenching non-optimal emission zone non-tunable electron injection 23 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs How to overcome limitation?
Evaporated layers for higher efficiency 14 12 RED Ba/Al EML IL Buffer IT Al ETL HBL EML HTL HIL IT Introduce evaporated HBL + ETL to overcome efficiency limit evaporated solution- processed solutionprocessed EQE [%] EQE [%] 10 8 6 4 2 0 0 5000 luminance [cd/m²] 16 14 GREE 12 10 8 6 4 2 0 0 5000 luminance [cd/m²] 24 Evaporated layers improve efficiency by ca. 40% EuroDisplay 2013 Hayer - Merck Solution Processable LEDs
Hybrid device for high efficiency Ba/Al EML IL Buffer IT Al ETL HBL solutionprocessed evaporated Lifetim me [kh] 60 50 40 30 20 10 0 Triplet Red with ETL simple device 0 5 10 15 Lifetim me [kh] 150 125 100 75 50 25 0 Triplet Green with ETL simple device 0 25 50 75 EML HTL solutionprocessed efficiency [cd/a] efficiency [cd/a] HIL IT 25 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs With ETL: Efficiency limitation overcome
Performance progess via material and stack optimisation Al ETL HBL EML HTL HIL IT evaporated solutionprocessed improved materials optimised material combinations adapted layer thickness LT50 [kh]@ @1000 cd/m² 350 300 250 200 150 100 50 0 with ETL simple device 0 25 50 75 eff. [cd/a]@1000 cd/m² With optimised materials: strong increase in lifetime as well as efficiency 26 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs
Example: Mixture optimisation 25% 20% EQE 15% 10% 5% 0% more e e-transporting component more h high low amount of h-transporting component LT80 more e e-transporting component EML mixture with 3 components: electron-transporting host co-host triplet emitter Balance optimisation: how much electron-transporting hole-transporting component is needed for best performance? In this In this case, both efficiency & lifetime increase when when balance balance is tuned is tuned towards towards more more holes holes initial mix was much too electron-dominated 27 EuroDisplay 2013 Hayer - Merck Solution Processable LEDsh
More h-injecting HTL for high lifetime curren nt density [ma/cm²] 1000 100 10 1 0.1 0.01 1E-3 HTL 1 HTL 2 HTL 3 hole-only device 0 2 4 6 voltage [V] lum minance [cd/m²] 10000 9500 9000 8500 8000 bipolar device HTL 1 HTL 2 HTL 3 0 200 400 600 800 1000 time [h] >900h @ 10 000 cd/m² If hole transport is so important for this mixture: 300 000h @ 1 000 cd/m² Improve best mix even further by also increasing hole injection With new HTL: 300 000h lifetime achieved 28 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs
Content 1 Introduction 2 LED Basics 3 Challenges for Solution Processing 4 Current Results 5 Summary 29 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs
Conclusion LEDs for ultra-thin lighting tiles and displays with wide viewing angle, high efficiency & contrast and new design opportunities solution-processing for very large area applications, greater ease of processing and reduced manufacturing costs key parameters for solution processing: solubility & film formation multilayer LED stacks from solution: e.g. via use of orthogonal solvents or via cross-linking high efficiency in solution-processed LED achieved with evaporated ETL Lifetime [kh] 150 with ETL 125 simple 100 device 75 50 25 0 ETL/ EML/ HTL IT improved materials, optimised material combinations and adapted layer thickness for very high performance: 300 000h lifetime and 80cd/A / 21% EQE in soluble green 0 25 50 75 efficiency [cd/a] LT50 [kh]@1000 cd/m² 350 with ETL 300 250 200 150 simple 100 device 50 0 0 25 50 75 eff. [cd/a]@1000 cd/m² 30 EuroDisplay 2013 Hayer - Merck Solution Processable LEDs
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