modeling electronic and excitonic processes in oled devices · modeling electronic and excitonic...

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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

beat.ruhstaller@zhaw.ch

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

9

‐ 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

11

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

17

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

www.fluxim.com

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

www.fluxim.com

Optimization, Fitting, Sweeping

Overview: Device Model & Applications

charge drift-diffusion& recombination

excitondiffusion, transfer & decay

dipole emission /light-incoupling

OLE

DS

olar Cell

www.fluxim.com

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)

www.fluxim.com

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

www.fluxim.com

Multiple Trapping and Release (MTR) Model vs. EGDM

30

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)

www.fluxim.com

‐ 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)

www.fluxim.com

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

www.fluxim.com

TADF(Thermally Activated Delayed Fluorescense)

and more Exciton Physics

Theory & Simulation Examples with Setfos

47

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

www.fluxim.com

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

www.fluxim.com

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

www.fluxim.com

Temperature‐dependent transient EL simulations of TADF OLED

53

200 K

300 K

EL decay

300 K

200 K

20 us 20 us

EL onset

Simulations with setfos

www.fluxim.com

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

60

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?

61

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

65

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)

71

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