www.fluxim.com
Modeling Electronic and ExcitonicProcesses in OLED Devices
Beat Ruhstaller1,2
1Fluxim AG, Switzerland2Zurich Univ. of Applied Sciences, Inst. of Computational Physics,
Switzerland
TADF Summer School in Krutyn, PLMay, 2017
www.fluxim.comAbout us: ZHAW vs. Fluxim AG
2
Research group of Prof. Ruhstaller on Organic Electronics and Photovoltaics (OEPHO) www.zhaw.ch/icp/oehpo
Numerical algorithms / device fabrication & characterization
Commercial R&D tools for OLEDs and solar cells
DE
CH
in Winterthur, SwitzerlandSpin‐off in 2006
www.fluxim.comMotivation for OLED Modeling?
3
BUT:‐ Efficiency roll‐off at high current densities
Nowadays: State‐of‐the‐art OLEDs with high (EQ) efficiencies of > 30%
T. Tsutsui and N. Takada; Jpn. J. Appl. Phys. 52 (2013) 110001
‐ Degradation during prolonged operation
Therefore, to find out what’s going on we need sound physical models and reliable, comprehensive measurement techniques!
www.fluxim.comMulti‐scale, Multi‐physics OLED Modeling
cm
mm
um
nm
Length Scale
Electrical Optical
electro-(thermal) FEM model
Drift-diffusion model(1D vertical)
Monte-Carlo,MD, DFT
3D Ray-tracing
statistical microtexture
Dipole emission & thin film optics
Full-wavem
acro
nano
mic
ro
Thermal
www.fluxim.com
• 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
Fluxim’s R&D Tools
www.fluxim.comsetfos‐paios‐integration
6
• Drift-diffusion modeling for direct comparison with experimental data.
• Parameter extraction with global fitting!
SimulationMeasurement
setfos
paios
www.fluxim.comOverview of talks
1. Modeling Electronic and Excitonic Processesin 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
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‐ Monday:
‐ Dinner break
‐ Tuesday:
www.fluxim.comContent Talk 1
• Drift‐diffusion model• Charge transport, trapping and recombination• Exciton dynamics (e.g. TADF, TPQ, TTA)• Simulation examples
10
www.fluxim.comContent Talk 2
• Overview on characterization techniques(AC, DC, transient)
• Exp. vs. simulation (Setfos – Paios Integration)• Features of Paios measurement platform
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www.fluxim.comOLED Device Physics
Anode CathodeEML
1
2
1
2
3
3
h 4
ETLHTLHIL EIL
• Processes:› Charge injection (1)› Charge transport (2)› Exciton formation, transfer &
diffusion (3)› Light outcoupling (4)
• Multilayer design:› facilitates injection› improves confinement› reduces leakage
www.fluxim.com
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Efficiency definition
cb st rad outEQE
% of injected charges that recombine
% of electron hole pairs in a state that can emit light
% radiativerecombinationvs. non‐radiative processes
% of generated photons that leaves the device
www.fluxim.com«Current Balance» & Recombination in OLEDs
Scott et al., J. Appl. Phys. (1997)Tsutsui, J. J. Appl. Phys. (2013)
Current balance Recombination efficiency <= 1
www.fluxim.comSetfos Drift‐diffusion Simulation
OLED stack / energy diagram
Structure• Layer thickness
Material properties• HOMO/LUMO level• Mobility e-/h+• Doping/traps
Input Output
J(mA/cm
2)
U(V)
Density
x(nm)
www.fluxim.comAnalyzing charge densities
• Charge pile up @ internal energy barrier• Decrease after barrier
• Recombination zone
LUMO difference
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Transient Electroluminescence (EL) of Traditional Bilayer OLED
TPDAlq
mobilities (TOF):e,Alq ~ 10-6 cm2V-1s-1
h,Alq = 0.1 e,Alq
h,TPD ~ 10-3 cm2V-1s-1
Applied Pulse:
Experiment:
Simulation:
-25
0
25
50
75
-1
-0.5
0
0.5
1
0 2 4 6 8 10
curr
ent d
ensi
ty (m
A/c
m2 ) recom
bination rate density (10
22s -1cm-3)
time ( s)
8 V
7 V6 V
-0.5
0
0.5
1
1.5
2
curr
ent d
ensi
ty (A
/cm
2 )
light output (a.u.)
8 V
7 V
6 V
[Ruhstaller et al., J. Appl. Phys. 89, 4575, (2001)]
www.fluxim.comTransient EL Overshoot in 4‐layer OLED
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
CathodeAnode
5.6
2.6
5.5
3.6
5.3
5.7
2.83.0
5 nm / 50 nm / 15 nm / 45 nm
S-TADCuPc
S-DPVBiAlq3
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Optimization, Fitting, Sweeping
Overview: Device Model & Applications
charge drift-diffusion& recombination
excitondiffusion, transfer & decay
dipole emission /light-incoupling
OLE
DS
olar Cell
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Poisson Equation
Charge (drift-diffusion) Current
Charge Continuity
Exciton CurrentExciton Continuity
0
( ) ( ) ( ) ( ) ( )t tE x e p x n x p x n x
x
( )( ) ( , ) ( ) ( ) ( )n nn xJ x e x E n x E x D
x
( ) 1 ( ) ( ) ( ) ( )nn x J x r x p x n xt e x
( )( )S SS xJ x D
x
Physical Model Overview
2
1. ( ). . . .
exc
i i i i
ni
i s rad nonrad i annihilation i ji j ij ij
dS G R J k k S k S k S k Sdt
Light-emission (from dipoles) & Light-incoupling
Electro-optical Coupling
Pho
non
Pho
ton
Exc
iton
Ele
ctro
n
Electro-thermal Coupling
Charge-exciton Coupling
www.fluxim.comEGDM & Charge Injection
metal organicmetal organic
Density at contact depends on position of Gaussian DOS
LUMO
Knapp et al., J. Appl. Phys. 108, 054504 (2010)
Extended gaussian disorder model (EGDM)
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Rate equation for electron traps:
escape rate en linked to capture rate cn and trap depth Et:
Note: deep electron traps can act as p‐dopants
Similar equations for hole traps
Charge Trapping
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Multiple Trapping and Release (MTR) Model vs. EGDM
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Gaussian Disorder Model (GDM)
free charge carriers
trapped charge carriers
www.fluxim.comTransient Current Response & the Role of (Hole) Traps
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)
Voltage stepat t=0 s
(slow vs. fast, depending on capture rate c)
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‐ LangevinProportional to carrier mobility and electron (n) and hole (p) density
‐ Shockley Read Hall (recombination with traps)
Charge Recombination
electron getstrapped trapped electron
recombines withhole
www.fluxim.com
Impacts the dark current (traps behave like generation center!)relevant for photodiodes!
Shockley Read Hall Recombination:OPV simulation example
Impacts the slope before Vbi as already experimentally observed.
Dark current
Illuminated
Voc, FF and Jsc strongly impacted by SRH recombinations
(from Setfos 4.3)
www.fluxim.comImpedance Spectroscopy (IS)
t
Voltage
t
Current
tiac eVVtV
0)(
Applying small oscillating voltage with frequency ω
Phase Y 1Z Jac
Vac
Measure current and calculate admittance Y
Y G i CConductance
Capacitance (~phase)
Charge traps may lead to increase of capacitance at low frequency!E. Knapp and B. Ruhstaller, Appl. Phys. Lett. 99, 093304 (2011)E. Knapp and B. Ruhstaller, J. Appl. Phys. (2012)
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Impedance Simulation –a powerful method for OLED R&D
Self‐heating(Joule) andtrapping arecompetingprocesses!
E. Knapp, B. Ruhstaller, J. Appl. Phys. 117, 135501 (2015)E. Knapp, B. Ruhstaller, SID Symposium Digest of Technical Papers 46 (1), 778‐781, (2015)E. Knapp, B. Ruhstaller, SPIE Organic Photonics+ Electronics, 95660X‐95660X‐7, (2015)
~ fSCLC=(transit time)‐1
www.fluxim.comC-V simulation
Geometrical capacitance
SCLC capacitance
‐C‐V Signal Insight into device!
www.fluxim.comInterface Model for Stacked Devices
Device 1 Device 2anode cathode
S. Altazin (Fluxim), E. Knapp (ZHAW)
Recombination(tandem solar cell) Generation
(stacked OLED)
www.fluxim.comComprehensive 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), 1250‐1256 (2011).[2] All models included in setfos, Fluxim AG, www.fluxim.com
Udevice≠Usource
DC, Transient andAC reponse are affected !Needed for comparisonto exp. data
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TADF(Thermally Activated Delayed Fluorescense)
and more Exciton Physics
Theory & Simulation Examples with Setfos
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www.fluxim.com
TADF: thermally activated delayed fluorescence
kr,f knr,f
kS
kT
Assumptions
Fluorescence (singlet emission): 100 % efficientknr,f=0
Phosphorescence (triplet emission): 0% efficientkr,ph=0
Fluorescence Phosphorescence
kS=kRISCkT=kISC
(simplest TADF system!)
Singlet Triplet
kr,ph knr,ph
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TADF: Transient behaviour
• Transient electroluminescence (TEL) simulation with
Drift‐diffusion & Emissionmodules 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) 0 0.1
K_conversion (1/us) 1 exp(‐E/kT)
Generation (%) 25 75
www.fluxim.comTADF in steady state: T dependence
• Steady state• Device IQE/EQE rises with T• Paios cryostat range: 150‐350K
10.25 0.751
TS
T S
IQE
T
S
% of singlets that becomes a triplet
% of triplets that becomes a singlet
Singlets + ‘harvested’ triplets
‘Infinite’ conversions
Shape & slope depend on:
r
nr
EkkA
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time (microseconds)
Increasingtemperature
Increasing TADF contribution, this component becomes faster
430 K
70 K
TEL voltageturn‐off
Transient Electroluminescence (TEL) of TADF OLED simulated with Setfos
• TEL turn‐off dynamics at different temperatures
Simulation with Setfos
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Temperature‐dependent transient EL simulations of TADF OLED
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200 K
300 K
EL decay
300 K
200 K
20 us 20 us
EL onset
Simulations with setfos
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Example TADF OLED simulation: Temperature induced colour shift (exagerated)
singlettriplet
ΔE
• High‐energy (singlet) state enhanced at high temperature
200k
300k
www.fluxim.comGeneral Delayed Recombination Feature in Transient EL after Turn‐off
Voltage turn‐off
• Expect exponential decay after turn‐off, but delayed EL peak appears due torecombination among residual chargesin EML
Experiment by S. Reineke et al. phys. stat. sol. (b) 245, No. 5, 804–809 (2008)
Simulation with Setfospeak position independent of on‐voltage
www.fluxim.comEnergy and Band Diagram
At 5 V forward bias
electronaccumulation
hole accumulation
Irppy
www.fluxim.com
Transient Profiles & Spectraat EL Turn‐off
Electrons Holes
Get insight intodevice operation!
0 volts (turn off)
delayed formation & emissionof excitons
www.fluxim.comMotivation EZ Determination1. Where is the emission zone (EZ) in the EML?
LUMOHTL
HOMOHTL
HTLEML
ETL
HOMOEML
LUMOEML
HOMOHTL
LUMOETL
Cathod
e
Anod
e
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EZ position and its change is crucial to the current efficiency roll‐off.…regardless of TPQ and TTA!
M. Regnat (ZHAW)
www.fluxim.comWhat is Emission‐Zone Fitting?
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Measurement of angular & spectral emission(Paios)
λ
Dipole Spectrum
OLED stack / energy diagram
Wavelength (nm)
Ang
le (d
eg)
x
Dipole Distribution
…a non-invasive monitoring & measurement method!
Emission Zone Fitting(Setfos)
About our methods in Setfos and applications:B. Perucco et al., Optics Express, 18 S2 (2010)B. Perucco et al., Organic Electronics, 13 (2012)
www.fluxim.comOLED Emission Zone Fitting Example:
Markus Regnat62
Angular norm. EL spectraPhosphorescent OLED
OLED stack
Fitted dipole distribution
• We find a dual‐peak emission zone inside theEML.
• Emission at the HTL/EML interface isenhanced at high current density
• Same method can be used to monitor aging
TCTA CBP:Ir(ppy)2 NBPhen
See poster by Markus Regnat (ZHAW)
www.fluxim.com
totdN k Ndt
More Origins of Efficiency Roll‐off: TTA or TPQ
• Standard Exciton ( ) decay: mono‐exponential
• TTA: Non‐exponential
• TPQ: exponential
TTA examplePL experiment
2
2TTA
totkdN k N N
dt
Cd/A
V
TTA simulation example
www.fluxim.comTTA generates Singlets
• TTA leads to delayed EL in fluorescent OLEDs
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Mayr, Schmidt, Brütting, Appl. Phys. Lett. 105, 183304 (2014)
www.fluxim.comTEL decay: TTA vs. TPQ
0
5
10
15
20
25
30
0 1000 2000 3000 4000 5000 6000 7000
curren
t efficiency (cd/A)
Radiance (cd.m‐2)Cu
rrent e
fficien
cy (C
d/A)
Time (us)
Log (brig
htne
ss [cd/m2])
Efficiency roll‐off:• Triplet Triplet Annihilation (TTA), non‐exp• Triplet Polaron Quenching (TPQ), exp
Simulation with Setfos
TPQ simulation example
www.fluxim.comTriplet‐Polaron Quenching (TPQ)
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
(Simulation with Setfos 4.5)
www.fluxim.comTPQ Analysis Example
68Oyama, 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)
Idea: 1. Determine polaron (charge) density from Setfos DD fit to IV curve2. Measure PL lifetime vs. current density3. Determine rate constant for exciton quenching
Setfos
is found to be 3.7 × 10−12 cm3 s−1 (while is =4.6 × 1017 cm−3)⋯
www.fluxim.com
Host‐guest exciton energy transfer saturation
Emitted spectrum
changes with current/voltage
Exciton transfer from Host to guest, only allowed if the guest is
free
At high current levels, host starts to emit more light
Host emission Guest emission
Setfos
www.fluxim.comExciton rate equation (Setfos 4.5)
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Generation efficiency (β) Exciton diffusion
Exciton transfer(Förster)
TADF
T‐T‐Annihilation
T‐P‐Quenching
Optical generation
Exciton dissociation
Langevin recombination rate:
(from Setfos 4.5 manual)
www.fluxim.comSummary
• Electronic processes are well modeled withdrift‐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