Modeling Electronic and Excitonic Processes in OLED Devices

Transcription

1 Modeling Electronic and Excitonic Processes in OLED Devices Beat Ruhstaller 1,2 1 Fluxim AG, Switzerland 2 Zurich Univ. of Applied Sciences, Inst. of Computational Physics, Switzerland TADF Summer School in Krutyn, PL May, 2017 beat.ruhstaller@zhaw.ch

2 About us: ZHAW vs. Fluxim AG in Winterthur, Switzerland DE Spin off in 2006 Research group of Prof. Ruhstaller on Organic Electronics and Photovoltaics (OEPHO) Numerical algorithms / device fabrication & characterization CH Commercial R&D tools for OLEDs and solar cells 2

3 Motivation for OLED Modeling? Nowadays: State of the art OLEDs with high (EQ) efficiencies of > 30% BUT: Efficiency roll off at high current densities Degradation during prolonged operation T. Tsutsui and N. Takada; Jpn. J. Appl. Phys. 52 (2013) Therefore, to find out what s going on we need sound physical models and reliable, comprehensive measurement techniques! 3

4 Multi scale, Multi physics OLED Modeling Length Scale electro-(thermal) FEM model Drift-diffusion model (1D vertical) Monte-Carlo, MD, DFT macro nano micro cm mm um nm 3D Ray-tracing statistical microtexture Dipole emission & thin film optics Full-wave Electrical Optical Thermal

5 Fluxim s R&D Tools Easy-to-use simulation software setfos able to simulate OLEDs and thin film PVs on the small scale/cell level. Easy-to-use all-in-one characterization platform paios to extract device and material parameters by dynamic characterization. Easy-to-use large-area simulation software laoss able to simulate OLEDs and solar cells up to the module scale. laoss

6 setfos paios integration Drift-diffusion modeling for direct comparison with experimental data. Parameter extraction with global fitting! paios Simulation Measurement setfos 6

7 Overview of talks Monday: 1. Modeling Electronic and Excitonic Processes in OLEDs 2. AC, DC and Transient Characterization of OLEDs 3. Enhancement of Light Outcoupling Efficiency in OLEDs 4. Design and Optimization of Large Area OLEDs by Electro thermal Modeling Dinner break Tuesday: 9

8 Content Talk 1 Drift diffusion model Charge transport, trapping and recombination Exciton dynamics (e.g. TADF, TPQ, TTA) Simulation examples 10

9 Content Talk 2 Overview on characterization techniques (AC, DC, transient) Exp. vs. simulation (Setfos Paios Integration) Features of Paios measurement platform 11

10 OLED Device Physics h Processes: Charge injection (1) Charge transport (2) Exciton formation, transfer & diffusion (3) Light outcoupling (4) 1 Anode 2 HIL HTL 3 EML ETL EIL Cathode Multilayer design: facilitates injection improves confinement reduces leakage

11 Efficiency definition EQE cb st rad out % of injected charges that recombine % of electron hole % of generated pairs in a state that photons that leaves can emit light the device % radiative recombination vs. non radiative processes 17

12 «Current Balance» & Recombination in OLEDs Scott et al., J. Appl. Phys. (1997) Tsutsui, J. J. Appl. Phys. (2013) Current balance Recombination efficiency <= 1

13 SetfosDrift diffusion Simulation Input Output OLED stack / energy diagram J(mA/cm2) Structure Layer thickness U(V) Material properties HOMO/LUMO level Mobility e-/h+ Doping/traps Density x(nm)

14 Analyzing charge densities LUMO difference Charge pile internal energy barrier Decrease after barrier Recombination zone

15 Transient Electroluminescence (EL) of Traditional Bilayer OLED Applied Pulse: TPD Alq Experiment: current density (A/cm 2 ) V 7 V 6 V light output (a.u.) mobilities (TOF): e,alq ~ 10-6 cm 2 V -1 s -1 h,alq = 0.1 e,alq h,tpd ~ 10-3 cm 2 V -1 s -1 Simulation: current density (ma/cm 2 ) V 75 7 V V time ( s) [Ruhstaller et al., J. Appl. Phys. 89, 4575, (2001)] recombination rate density (10 22 s -1 cm -3 )

16 Transient EL Overshoot in 4 layer OLED Anode S-TAD CuPc S-DPVBi Cathode Alq nm / 50 nm / 15 nm / 45 nm mechanism: short-lived recombination maximum due to charge accumulation at internal interfaces critical parameters: mobilities, molecular energy levels, electrodes, bias B. Ruhstaller et al., IEEE JSTQE 9, (3) 723ff, 2003

17 Overview: Device Model & Applications OLED charge drift-diffusion & recombination exciton diffusion, transfer & decay dipole emission / light-incoupling Solar Cell Optimization, Fitting, Sweeping

18 Phonon Photon Exciton Electron Physical Model Overview Poisson Equation Charge (drift-diffusion) Current Charge Continuity Exciton Current Exciton Continuity ds dt i Ex ( ) e px ( ) nx ( ) pt( x) nt( x) x 0 nx ( ) Jn( x) e n( x, E) n( x) E( x) D( ) x nx ( ) 1 Jn ( x) rx ( ) px ( ) nx ( ) t e x J ( x) D S S S( x) x GR J k k S k S k S k S nexc 2 i. s ( ).... i radi nonradi i annihilationi i ji j ij i j 1 Light-emission (from dipoles) & Light-incoupling Electro-optical Coupling Charge-exciton Coupling Electro-thermal Coupling

19 EGDM & Charge Injection Extended gaussian disorder model (EGDM) metal organic metal organic LUMO Density at contact depends on position of Gaussian DOS Knapp et al., J. Appl. Phys. 108, (2010)

20 Charge Trapping Rate equation for electron traps: escape rate e n linked to capture rate c n andtrapdepthe t : Note: deep electron traps can act as p dopants Similar equations for hole traps

21 Multiple Trapping and Release (MTR) Model vs. EGDM Gaussian Disorder Model (GDM) trapped charge carriers free charge carriers 30

22 Transient Current Response & the Role of (Hole) Traps Voltage step at t=0 s (slow vs. fast, depending on capture rate c) slow traps: current drops in the steady state limit fast traps: peak time shifts to longer time E. Knapp and B. Ruhstaller, J. Appl. Phys. 112, 2 (2012)

23 Charge Recombination Langevin Proportional to carrier mobility and electron (n) and hole (p) density Shockley Read Hall (recombination with traps) electron gets trapped trapped electron recombines with hole

24 Shockley Read Hall Recombination: OPV simulation example Dark current Impacts the dark current (traps behave like generation center!) relevant for photodiodes! Impacts the slope before V bi as already experimentally observed. Illuminated V oc, FF and J sc strongly impacted by SRH recombinations (from Setfos 4.3)

25 Impedance Spectroscopy (IS) Voltage Applying small oscillating voltage with frequency ω V ( t) V 0 V ac e i t Current Phase t Measure current and calculate admittance Y Y 1 Z J ac V ac t Y G i C Conductance Capacitance (~phase) Charge traps may lead to increase of capacitance at low frequency! E. Knapp and B. Ruhstaller, Appl. Phys. Lett. 99, (2011) E. Knapp and B. Ruhstaller, J. Appl. Phys. (2012)

26 Impedance Simulation a powerful method for OLED R&D ~ f SCLC =(transit time) 1 Self heating (Joule) and trapping are competing processes! E. Knapp, B. Ruhstaller, J. Appl. Phys. 117, (2015) E. Knapp, B. Ruhstaller, SID Symposium Digest of Technical Papers 46 (1), , (2015) E. Knapp, B. Ruhstaller, SPIE Organic Photonics+ Electronics, 95660X 95660X 7, (2015)

27 C-V simulation Geometrical capacitance SCLC capacitance C V Signal Insight into device!

28 Interface Model for Stacked Devices anode Device 1 Device 2 cathode Recombination (tandem solar cell) S. Altazin (Fluxim), E. Knapp (ZHAW) Generation (stacked OLED)

29 Comprehensive Modelling w/ Setfos Series resistance [1] and parallel R, C elements (Setfos 4.5) Drift diffusion solver for DC, AC, Transients [2] Exciton Physics Dipole emission model [2] (Emissive dipoles & Purcell, mode analysis) Advanced optics (incoherence, scattering, birefringence) [1] M.T. Neukom, N.A. Reinke, B. Ruhstaller, Solar Energy, 85(6), (2011). [2] All models included in setfos, Fluxim AG, U device U source DC, Transient and AC reponse are affected! Needed for comparison to exp. data

30 TADF (Thermally Activated Delayed Fluorescense) and more Exciton Physics Theory & Simulation Examples with Setfos 47

31 TADF: thermally activated delayed fluorescence Singlet k S Triplet Assumptions k r,f k nr,f k T k r,ph k nr,ph Fluorescence (singlet emission): 100 % efficient k nr,f =0 Fluorescence Phosphorescence Phosphorescence (triplet emission): 0% efficient k r,ph =0 (simplest TADF system!) k S =k RISC k T =k ISC

32 TADF: Transient behaviour Transient electroluminescence (TEL) simulation with Drift diffusion & Emission modules of Setfos Switching 5 V (on) => 10 V (off) TADF emitter simply modeled as 2 excitons: Singlet & triplet Temperature range: 70 K => 460 K Singlet Triplet K_rad (1/us) 10 0 K_nonrad (1/us) K_conversion (1/us) 1 exp( E/kT) Generation (%) 25 75

33 TADF in steady state: T dependence Steady state Device IQE/EQE rises with T Paios cryostat range: K T IQE S 1 Singlets + harvested triplets Infinite conversions T S T S % of singlets that becomes a triplet % of triplets that becomes a singlet Shape & slope depend on: E k k r A nr

34 Transient Electroluminescence (TEL) of TADF OLED simulated with Setfos TEL turn off dynamics at different temperatures 430 K Increasing TADF contribution, this component becomes faster 70 K Increasing temperature TEL voltage turn off Simulation with Setfos time (microseconds)

35 Temperature dependent transient EL simulations of TADF OLED EL onset 300 K EL decay 200 K 300 K 200 K 20 us 20 us Simulations with setfos 53

36 Example TADF OLED simulation: Temperature induced colour shift (exagerated) singlet ΔE triplet High energy (singlet) state enhanced at high temperature 200k 300k

37 General Delayed Recombination Feature in Transient EL after Turn off Expect exponential decay after turn off, but delayed EL peak appears due to recombination among residual charges in EML Voltage turn off Simulation with Setfos peak position independent of on voltage Experiment by S. Reineke et al. phys. stat. sol. (b) 245, No. 5, (2008)

38 Energy and Band Diagram At 5 V forward bias Irppy electron accumulation hole accumulation

39 Transient Profiles & Spectra at EL Turn off Get insight into device operation! delayed formation & emission of excitons 0 volts (turn off) Electrons Holes

40 Motivation EZ Determination 1. Where is the emission zone (EZ) in the EML? LUMO HTL LUMO EML LUMO ETL HTL EML ETL HOMO HTL HOMO EML Cathode Anode HOMO HTL EZ position and its change is crucial to the current efficiency roll off. regardless of TPQ and TTA! M. Regnat (ZHAW) 60

41 What is Emission Zone Fitting? OLED stack / energy diagram a non-invasive monitoring & measurement method! Measurement of angular & spectral emission (Paios) Angle (deg) Emission Zone Fitting (Setfos) Dipole Distribution Dipole Spectrum x Wavelength (nm) About our methods in Setfos and applications: B. Perucco et al., Optics Express, 18 S2 (2010) B. Perucco et al., Organic Electronics, 13 (2012) λ 61

42 OLED Emission Zone Fitting Example: Phosphorescent OLED Angular norm. EL spectra Fitted dipole distribution TCTA CBP:Ir(ppy) 2 NBPhen OLED stack We find a dual peak emission zone inside the EML. Emission at the HTL/EML interface is enhanced at high current density Same method can be used to monitor aging Markus Regnat See poster by Markus Regnat (ZHAW) 62

43 More Origins of Efficiency Roll off: TTA or TPQ Cd/A TTA example PL experiment TTA simulation example V Standard Exciton ( ) decay: mono exponential dn ktot N dt TTA: Non exponential dn ktta ktot N N dt 2 TPQ: exponential 2

44 TTA generates Singlets TTA leads to delayed EL in fluorescent OLEDs Mayr, Schmidt, Brütting, Appl. Phys. Lett. 105, (2014) 65

45 TEL decay: TTA vs. TPQ Log (brightness [cd/m2]) Simulation with Setfos Time (us) Efficiency roll off: Triplet Triplet Annihilation (TTA), non exp Triplet Polaron Quenching (TPQ), exp Current current efficiency (cd/a) (Cd/A) TPQ simulation example Radiance (cd.m 2 )

46 Triplet Polaron Quenching (TPQ) (Simulation with Setfos 4.5) In order to maximize the efficiency of an OLED, the recombination zone should be expanded as much as possible to avoid high concentration of carriers and excitons

47 TPQ Analysis Example Idea: 1. Determine polaron (charge) density from Setfos DD fit to IV curve 2. Measure PL lifetime vs. current density 3. Determine rate constant for exciton quenching Setfos is found to be cm 3 s 1 (while is = cm 3 ) Oyama, Sakai, Murata, Rate constant of exciton quenching of Ir(ppy)3 with hole measured by time resolved luminescence spectroscopy», Jap. J. Appl. Phys. 55, 03DD13 (2016) 68

48 Host guest exciton energy transfer saturation Host emission Guest emission Exciton transfer from Host to guest, only allowed if the guest is free Emitted spectrum changes with current/voltage Setfos At high current levels, host starts to emit more light

49 Exciton rate equation (Setfos 4.5) Generation efficiency (β) Langevin recombination rate: Exciton diffusion Exciton transfer (Förster) TADF T T Annihilation T P Quenching Optical generation Exciton dissociation (from Setfos 4.5 manual) 71

50 Summary Electronic processes are well modeled with drift diffusion in AC, DC and transient state Exciton dynamics in space, time and spectrum TADF is seen in EL experiments vs. t and T (not only in photophysics experiments) Setfos Software demo? Next talk: Measurement techniques! Thank you for your attention! 72