Organic LEDs: Today displays, lighting tomorrow? Lieven Penninck, Stephane Altazin, Beat Ruhstaller , summer school, Belgium

Transcription

1 Organic LEDs: Today displays, lighting tomorrow? Lieven Penninck, Stephane Altazin, Beat Ruhstaller , summer school, Belgium

2 Content OLED applications & back ground Operation principles From charge to photon Light extraction Conclusion

3 Simulation Software Measurement Hardware Who we are Research on OLED and OPV

4 Setfos 4.1 Software Modules:

5 All-in-One!

6 Fluxim Distribution partners: Research partners:

7 Fluxim s team Distribution partners: Research partners:

8 Content OLED applications & background Operation principles From charge to photon Light extraction Conclusion

9 rganic and Large Area Electronics & Photovoltaics OLED lamps Lumiblade by Philips Tridonic, Austria 55 inch OLED TV from LG, Korea OLED flexible display, Samsung, Korea Organic solar cells by Konarka, USA

10 OLED displays

11 OLED vs. LCD display

12 OLED lighting High CRI & warm white Dimmable Large area (not blinding) Efficient Flexible/shatterproof/transparent? Low production cost (roll to roll)

13 OLED vs. other lighting

14 OLED: lighting 14

15 Organic LEDs NOT small size lab sample Panasonic SID

16 OPV: organic photovoltaics Low cost ~ less efficient than conventional PV Flexible Free form shapes Organic solar cells by Konarka, USA

17 OLED: structure Cathode ETL HBL EML EBL Organic semiconductor + metal/ito electrodes Stack of thin layers (total ~200nm) HTL Anode Glass/PET/Steel Electron/hole transport, emitting layers Deposited by evaporation/ spinning/ printing,... - V + Cathode ETL HBL EML EBL HTL Anode Substrate 17

18 OLED Aluminium/ Silver ETL HBL EML EBL HTL ITO Substrate Bottom emission Thick metal=mirror Al/Ag >100nm Transparent anode ITO/ PEDOT Top emission Aluminium/Silver ETL HBL EML EBL HTL Aluminium/Silver Substrate Thin metal=(semi-) transparent Al/Ag ~20nm Thick metal=mirror Al/Ag >100nm 18

19 Aluminium/ Silver ETL R G B HTL ITO Substrate

20 Aluminium/ Silver ETL R HTL CGL ETL G HTL CGL ETL B HTL ITO Substrate

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23 OLED Challenges Optimize EQE, Color... Monitor & understand Design panels/displays, Minimize losses Extract charge mobility Nowy et al. JAP (2010) C(f) for fresh & aged OLED Simulated potential with metal grid & shunts

24 Content OLED applications & back ground Operating principles From charge to photon Light extraction Conclusion

25 OLED: basic principle Cathode ETL - EBL EML - HBL HTL Anode Electron/hole injection Charge transport LUMO Recombination Light emission HOMO + + Light extraction 25

26 Efficiency definition EQE cb st rad out

27 lm hc LCE 683 ( ) ( ) V ( ) S0( ) d e air cb st rad out W 0 LE air LCE V air *S. Mladenovski et. al. Journal of Applied Physics 109, (2011) 28

28 Content OLED applications & back ground Operating principles From charge to photon Light extraction Conclusion

29 Organic semiconductor ETL EML HTL - - LUMO HOMO

30 Actual charge transport

31 Gaussian Disorder Model (GDM) ( ) *exp 0 T C kt B 2

32 - Charge balance ETL EML HTL - LUMO EQE cb st rad out HOMO

33 «Current Balance» & Recombination in OLEDs Perfect balance = all charges recombine Jrec cb J

34 Blocking layers ETL HBL EML HTL - LUMO - HOMO

35 Blocking layers 1 cb 1 cb 37

36 Electron + hole recombine Not all excitons CAN emit light Spin state: 25% singlet, 75% triplet Fluorescent emitters traditional OLED materials: AlQ 3, EQE cb st rad out Fluorescent: only singlets η st =25% 44

37 Phophorescent emitters: triplet decay ISC: Singlets-> triplets Guest/host system Ir(ppy)3, Contain Ir or Pt -> Expensive Phosphorescent emitters: triplets+singlets η st =100% EQE cb st rad out 45

38 Hot topic : TADF Thermally activated delayed fluorescence reverse ISC: triplets<->singlet IQE depends on T No Ir: high efficiency at low cost EQE cb st rad out 46

39 IQE S T 1 T S T T st IQE st rad

40 1,2 2 k k k k k k k k 4 k k k k k k 2 r T nr S r T nr S r nr nr T S r

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42 EQE=η cb η st η rad η out * rad r r nr r [1/ µs] F tot r nr ** F=total emitted power constant Depends on microcavity rad F r r F nr *S. Mladenovski et al.; J. App. Phys. 109; (2011) **S. Mladenovski et al.; Opt. Lett. 34; (2009) 50

43 Investigating η rad : phosphorescence lifetime 1/ L Lifetime in different devices Time resolved luminescence Fit F rad ~ exp( t/ ) Mladenowski, Reineke, Neyts (2009) Penninck, Steinbacher, Krause, Neyts (2012) 51

44 Start from Drift-diffusion equations. Excitons continuity equation is then solved accounting the optical feedback (quenching & recombination). Every radiatively decaying exciton is considered as an emitting dipole in the optical model.

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46 rad F r ( I) F n N r nr TPQ TTA rad rad

47 Content OLED applications & back ground Operation principles From charge to photon Light extraction Conclusion

48 EQE=η cb η st η rad η out Light emitted in all directions 2 barriers in OLED: Organic: n=1.8 Glass/plastic: n=1.5 Air n=1 η out =15-20% is outcoupled Escape cone: 56 Escape cone: 42 57

49 Ways to improve light outcoupling: OLED Stack design Outcoupling layers Oriented emitters q=1 q=0.7

50 Emission inside OLED Dipole antenna in planar 1D layer stack Interference between reflections on layer boundaries TE & TM polarization R TM RTE E TM Modified emitted field E TE E cav,te/tm E a E,TE/TM TE / TM,TE/TM 1 a TE / TM a TE / TM 60

51 Metal electrodes = mirror surface Interference increases/reduces emission for certian wavelengths & angles Distance to cathode modifies spectrum & efficiency Aluminium(100nm) ETL (varied) HBL (10nm) EML(20nm) EBL(10nm) HTL(60nm) ITO(90nm) Substrate 61

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53 Optical Model: Validation Bottom (through glass) and top emission due to semitransparent electrodes bottom emission Device: Al(12 nm)/pedot(60 nm)/ [76% PVK+19% PBD+5% CGR-Red(x nm)] / Ba(1 nm)/al (12 nm) top emission Emitted radiance well reproduced!

54 η out (%) Air Substrate d_etl(nm) 64

55 Ways to improve light outcoupling: OLED Stack design Outcoupling layers Oriented emitters q=1 q=0.7

56 Structured layers for outcoupling: Redirect trapped light External outcoupling: from glass to air Internal outcoupling: from organic to glass & glass to air 66

57 Light extracted in many passes Competition between scattering & absorption Maximum outcoupling: -effective scattering -high reflectivity 60% trapped in organic layers Internal > external But difficult Internal & external combined 67

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59 Scattering simulation workflow OLED-stack Dipole radiation Net Radiation Model Scattering Mie Theory Measurement BSDF Semi-analytical dipole emission model R & T Ray tracing Analytical scattering Full wave simulation 69

60 Design Challenges for Scattering OLEDs Scattering properties? Ideal haze? Angular width? OLED stack properties? OLED reflectivity? Absorption in OLED layers??

61 Emitted power (W.m -2 ) Impact of Haze on Emitted Power How strong should scattering be? (amount of haze?) BSDF assumed Lambertian(cos(θ)) R & T = flat case Scattering case Flat case Haze Emitted power in air increases with the Haze. Even with 50% haze the emitted power almost doubles!

62 Radiance (W.m -2.sr -1 ) Emitted power (W) Impact of the Scattering Function on the outcoupling Efficiency of a White OLED : Phong factor l = 1 : Phong factor l = 0.1 : Phong factor l = Angle a (degree) Sub- Lambertian Super- Lambertian Phong Factor l Scattering function: cos l (θ) maximum out-coupling efficiency l >1 Too broad scattering functions (l <1) has a negative impact on the outcoupling efficiency of the OLED.

63 Emitted power (W.m -2.nm -1 ) Impact of the scattering layer on the emitted color Haze= Increasing Haze from 0 to 1 : ref OLED (WO high index layer) Haze= Integrated emitted spectrum on the full up space

64 Colour stability vs angle Example: Ir:ppy 3 green pixel No scattering: Colour shift with angle 0-90 toward blue/cyan With scattering: Stable colour

65 Example: optimizing scattering particles Spherical particles Distributed in host material Key parameters: size concentration refractive index (host & particle) Mie theory (1 particle) Monte Carlo simulation (many particles) 75

66 Influence of scattering particles on the outcoupling

67 What size should the particles be? Varied radius nm Highest extraction: R=640nm Particle ~1 wavelength radius

68 Emitted power (W.m -2.sr -1 ) Emitted power (W.m -2 ) Impact of Layer Absorption Assumed k 10-4 of the planarization layer Scattering case 10 Thickness from 1 to 19 um Flat case Angle θ (degree) Thickness (um) Increasing thickness of absorbing layer: a decrease of emitted power changed shape of the radiance (high absorbance at large angles)

69 OLED stack reflectance Influence of OLED Stack Reflectance How does the cathode material impact the out-coupling enhancement? Aluminum, flat Silver, flat Aluminum, rough Silver, rough P-reflectance using Aluminum P-reflectance using Silver Rough internal glass interface: RMS=100 nm wavelength (nm) In the planar case, the silver electrode leads to poorer light outcoupling than aluminum. However, the scattering structure asks for a high-reflectance OLED stack, thus silver gives higher outcoupled radiance.

70 Influence of OLED Stack Reflectance λ=530nm Perfect reflector n=0 k>0 Higher reflection= better extraction

71 Keep in mind

72 Joint optimization of stack + particle concentration Enhanced brightness: factor 1,9 1 st and 2 nd maximum shift when scattering Improved angular colour stability

73 Angle Summary of Simulation Workflow (b) 90 Haze= Wavelength (nm) 0

74 Ways to improve light outcoupling: OLED Stack design Outcoupling layers Oriented emitters q=1 q=0.7

75 Emission & outcoupling in OLEDs: oriented dipoles Different radiation for different dipole orientation Horizontal (// OLED plane) radiate into escape cone Oriented emitters: high, no structures out 85

76 Organic LEDs: decay rate Al (200nm) TPBi (x nm) TPBi:Ir(MDQ) 2 (acac)/ir(ppy) 3 (5nm) TCTA (30nm) ITO (120nm) Glass Decay rate & radiative efficiency changes with optical design & dipole orientation 2 emitters in different layer stacks Measure decay rate per device Determine decay rates (& efficiency) & dipole orientation 86

77 EL Decay Phosphorescence lifetime extraction Exponential decay ~2μs

78 Emission & outcoupling in OLEDs: oriented dipoles [1/ µs] af (1 a) F tot r nr Different radiation for different dipole orientation vertical fraction Purcell effect 88

79 Organic LEDs: Outcoupling Ir(ppy) 3 Measurement vs. calculation Ir(MDQ)2 acac Random & experiment correspond 67% horizontal 33% vertical Random & experiment don t correspond Better description: 79% horizontal 21% vertical 89

80 Organic LEDs: Outcoupling Ir(MDQ) 2 acac Alternative method Angular spectrum Detect orientation in bad layer design Same α *W. Brütting et.al. U. Augsburg 90

81 Organic LEDs: Outcoupling Al (200nm) TPBi (0-350nm) TPBi:Ir(MDQ) 2 (acac) (5nm) TCTA (30nm) ITO (120nm) Glass substrate η η air α α α=80% enhancement= α=100% enhancement=1.4 91

82 Optimization: top emitting OLED Ag(15nm) TPBi (0-350nm) TPBi:Ir(MDQ) 2 (acac)(5nm) TCTA(80nm) Al (200 nm) glass η air α α α=80% enhancement=1.21 α=100% enhancement=

83 Concluding remarks OLED display & lighting High efficiency by: -high performance emitter materials -carefull layer design -outcoupling structures Future challenges: flexible (foldable, rollabel), transparent, cost

84 Acknowledgments UGent: Kristiaan Neyts, Patrick De Viscchere, Saso Mladenowski Fluxim: Felix Müller, Martin Neukom, Benjamin Perucco, et al. ZHAW: Evelyne Knapp, Thomas Lanz, Kevin Lapagna

85 Thank you for your attention! Any questions?

86 Electrical simulation: drift diffusion Case study: a red OLED

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89 θ 1000 π Current_density Voltage =

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92 Transient Electroluminescence Voltage EL rise decay t delay

93 OLED Transient Electroluminescence: Turnon Dynamics Time t d +t 1 is related to effecitve mobility and comparable to DIT mobility (Pinner et al. 2000) EL(t) Plot 1-EL norm (t) Plot

94 Transient Electroluminescence Application example: Transient EL for extraction of phosphorescence lifetime & emitter orientation Aging study: S1: fresh L=100 %, S2: aged L=90 %, S3: aged L=75%) Bi-exponential decay? Competing decay channel? Mladenowski, Reineke, Neyts (2009) 104

95 Setfos for OLED ransient EL experiment Transient EL simulation Simulating experiments to extract material parameters Turn-on delay Phosphorescence lifetime

96 All-in-One! Impedance Spectroscopy Transient Photocurrent Transient Photovoltage Capacitance-Voltage Photo-CELIV Dark Injection Transients IV-Curves 2.0 Transient EL 106

97 Light Solar Cell Version OLED Version - + DUT Solar Cell OLED + - Light LED Photodiode 111

98 Photo-CELIV Ligh t Voltag e Current t t Determine charge carrier mobility Determine number of free charge carriers in the device Apply voltage ramp Charge carriers are generated before (by light pulse) t 112

99 Analysis of CELIV currents Current t max n j disp Charge carrier mobility m = 2 d2 3 A t 1 2 max Dj j 0 Charge carrier density t n = 1 V t 0 ò 0 Relative el. permittivity j(t)dt e r = j disp d e 0 A 113

100 Experimental CELIV with PAIOS CELIV For characterization of photo-carrier dynamics Numerical CELIV simulation with SETFOS for parameter extraction More detailed physical description than analytical formula for charge mobility 114

101 Numerical Simulation of CELIV CELIV For characterization of photo-carrier dynamics Numerical CELIV simulation with SETFOS for parameter extraction More detailed physical description than analytical formula for charge mobility: m = 2 d2 3 A t 1 2 max Dj j 0 References: - M.T. Neukom, N.A. Reinke, K.A. Brossi, and B. Ruhstaller, Proc. SPIE, Vol. 7722, 77220V (2010) - M.T. Neukom, N.A. Reinke and B. Ruhstaller, Solar Energy (2011); - M.T. Neukom, S. Züfle, B. Ruhstaller, Organic Electronics 13 (2012)

102 Dark-CELIV Ligh t Voltag e Current t t Apply voltage ramp No charge carriers are generated before Determine charge carrier mobility Determine number of free charge carriers in the device -> Doping concentration t 116

103 Doping Extraction from dark-celiv Doping level is determined for an organic solar cell at different aging conditions.

104 Impedance Spectroscopy Voltage V V ac e Applying small oscillating voltage with frequency ω V ( t) 0 i t Current Phase t Measure current and calculate admittance Y = 1 Z = J ac V ac t Y = G+i w C Conductance Capacitance E. Knapp and B. Ruhstaller, Appl. Phys. Lett. 99, (2011) E. Knapp and B. Ruhstaller, J. Appl. Phys. (2012)

105 Aging Doping Aging study with C-f Paios Measurement Setfos Simulation Doping reproduces aging effect 119

106 PEDOT:PSS HBL & ITO Why does the capacitance increase? Hole w/ doping Depletion layer close to the ZNO interface Hole w/o doping W Increased hole density in the middle of the device Schottky-Model Electrons w/o doping W 2 q N a bi V Electrons w/ doping Capacitance increases with doping 120

107 C-V on bi-layer device Wolfgang Brütting, Uni Augsburg, 2001

108 Capacitance-Voltage C-V Capacitance Mott-Shottky 1/C 2 V peak Constant frequency varied offset voltage V peak is sensitive on barriers and V bi C geom 0 Negative capacitance due to recombination Offset- Voltage Extracting doping density (only for thick and highly doped devices)

109 OLED Degradation: Dark-injection Transients Aging study: S1: fresh L=100 %, S2: aged L=90 %, S3: aged L=75%) DIT/T-SCLC peak position (i.e. transit time) is unchanged, only the current is reduced! Charge mobility is unchanged!

110 OLED aging with C-V Capacitance C(V) aging Aging study on OLED Shifted peak indicates lowered charge injection unpublished

111 C-V simulation Geometrical capacitance SCLC capacitance - 1 2

112 Light Scattering Simulation for Si Thin Film Solar Cells T. Lanz, B. Ruhstaller, C. Battaglia, and C. Ballif, "Extended light scattering model incorporating coherence for thin-film silicon solar cells", J. Appl. Phys. 110, (2011)

113 Light-scattering in Organic Solar Cells Experiment Simulation (SETFOS) Cell with textured FTO achieves +10% photocurrent (J sc )! Qualitative agreement! Hu, Zhang, Zhao, Appl. Phys. Lett. 100, (2012); doi: /