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AC, DC and Transient Characterization of OLEDs
M.T. Neukom1, S. Züfle2, J. Sastre1, A. Gentsch1, S. Altazin1, B. Ruhstaller1,2
1Fluxim AG, Winterthur, Switzerland2Zurich University of Applied Sciences, Institute of Computational Physics, Winterthur, Switzerland
TADF OLED Summer School, May 2017, Krutyn, Poland
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• 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
R&D Tools for OLEDs & Next Gen PV
www.fluxim.comContent Talk 2
• All‐in‐one measurement platform• Overview of AC, DC and transient measurement techniques for OLEDs
• Case studies with simulation or fitting(polar OLED, MIS‐CELIV, pip)
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www.fluxim.comPaios Research Cycle
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Measured fully automated within a few minutes…
www.fluxim.compaios Measurement Techniques
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Impedance Spectroscopy Capacitance‐Voltage Dark Injection Transients
Transient Photocurrent
Photo‐CELIV
Transient Photovoltage Transient ELIV‐CurvesCharge Extraction
IMPS IMVS
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Basic postprocessing (1) in Paios GUI: Simple IV Curve Analysis
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Charge Carrier Mobility from Mott‐Gurney Fit One‐Diode Model Fit
(+ Fitting with equivalent circuits)
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Basic postprocessing (2):CELIV mobility
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Use various post-processing routines to extract parameters
Example shown:Extraction of hole-mobility from CELIV
www.fluxim.comsetfos‐paios‐integration
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• Drift-diffusion modeling for direct comparison with experimental data.
• Parameter extraction with global fitting!
SimulationMeasurement
setfos
paios
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setfos‐paios‐integration:Global Parameter Fitting
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Use parameter fitting to extract
material and device parameters
from your experiments.
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OPV Application Example: Global Fitting of Multiple Curves!
M.T. Neukom, S. Züfle, B. Ruhstaller, Organic Electronics 13 (2012)
IV curves Light pulse
Dark CELIV Photo‐CELIV
Extracted Parameters:(setfos Drift‐diffusion)
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Reduced Parameter Correlation: more curves => more info!
Basic fit ofIV-curves
Improved fit involving JV, light pulse andphoto-CELIV
M.T. Neukom, S. Züfle, B. Ruhstaller, Organic Electronics 13 (2012)
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Capacitance Voltage
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• Paios screenshot• Charge density 5e23 m‐3 (too low)
measured
simulated
Parameter sliders
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Capacitance Voltage• Matching simulation:Charge density 7e23 m‐3
• Measurement – simulation fit
C (nF/cm
2)V (V)
www.fluxim.comImpedance Spectroscopy
t
Voltage
t
Current
V(t) V0 Vac eit
Applying small oscillating voltage with frequency ω
Phase
Y 1Z Jac
Vac
Measure current and calculate admittance
Y G i CConductance Capacitance
E. Knapp and B. Ruhstaller, Appl. Phys. Lett. 99, 093304 (2011)E. Knapp and B. Ruhstaller, J. Appl. Phys. (2012)
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New: Modulated EL Spectroscopy(MELS)
• Vary the voltage pulse frequency applied to theOLED, detect the light output
Characteristic (inflection) frequency is related to EL turn‐on delaytime and thus carrier mobility
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www.fluxim.comCapacitance‐Voltage C‐V
Offset-Voltage
CapacitanceConstant frequency –varied offset voltage
Cgeom
0
Vpeak
Mott-Shottky1/C2
PV: Extracting doping density (only for thick and highly doped devices)
Negative capacitance due to recombination
Vpeak is sensitive on barriers and Vbi
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Aging study: S1: fresh L=100 %, S2: aged L=90 %, S3: aged L=75%)
Capacitance C(f) Capacitance C(V)
OLED Degradation: Impedance Spectra
C(f) is in agreement with OLED aging study of Nowy et al. J. Appl. Phys. 2010
Voltage shift and peak reduction in C(V) indicates reduced charge injection
unpublished
www.fluxim.comDark Injection Transients DIT
t
Voltage
t
Current
tmax
• Performed on mono‐polardevices in space charge limited regime (sometimes OLEDs or solar cells work)
• Ohmic contacts required
0.786 d2
tmax (V Vbi )
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• Zoomed DIT currents
• Peak shifts to shorter times a higher voltages
• Mobility‐dependence on electric field
Dark‐injection Transients
tmax
tmax
tmax
inc voltage
www.fluxim.comDIT: Measurement and Simulation
MTR model with slow trapping reproduces measurement features of hole‐only device
high bias
low bias
(Simulation, E. Knapp, ZHAW)
a-NPD single layer hole-only device
www.fluxim.comEL turn‐on
t
Voltage
• Determine mobilities with onset delay
• Determine both charge carrier mobilities at EL rise
t
ELrise τ
onset delay td
2
( )hbi d
LV V t
www.fluxim.comTEL Rise
Increasing V
Time (us)
Intensity
(norm.)
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«basic post‐processing» of TEL for mobility estimation
Basic Postprocessing (3): Carrier mobility from TEL delay time
www.fluxim.comEL turn‐off
t
Voltage
• Decay rate extraction• Exciton dynamics‐IQE‐Roll‐off mechanisms‐emitter orientation
t
ELrise τ
onset delay
www.fluxim.comEL Decay
Exponential decayΤ=~2μs
www.fluxim.comEL Turn‐off
• Exponential decay ->phosphorescence lifetime• Decay time varies with optical cavity (Purcell factor)
< 36
Mladenowski, Reineke, Neyts (2009)Penninck, Steinbacher, Krause, Neyts (2012)
1
r nrk k
OLED degradation state (LT 100, 90 75)
www.fluxim.comTEL Decay Temperature Dependent
OLED 2 (T: 247 K – 322 K)Temperature dependent decay
IncreasingTemperature
OLED 1 (T: 230 K – 300 K)Temperature independent decay
www.fluxim.comMotivation: Polar OLED Materials• Decreasing the operating voltage in OLEDs is of primary importance.• This requires high mobility/conductivity ETL layers and well‐matched
energy levels.• Many ETL materials are known to be polar and exhibit spontaneous
molecular dipole orientation1,2,3: Bphen, BCP, Alq3 ... • This leads to charged layer boundaries in the device:
Questions: • Can we determine this
interfacial charge density?• How are the electrical
properties impacted?
1T. D. Schmidt et al., JAP 117, 215502 (2015)2T. Miyamae et al., Chem. Phys. Lett. 616‐617, 86‐90 (2014)3S. Nowy et al., Proc. of SPIE, 7415 (2009)
www.fluxim.comExperiment vs. Simulation
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-10 -8 -6 -4 -2 0 20.5
1
1.5
2
2.5
-10 -8 -6 -4 -2 0 20.5
1
1.5
2
2.5
Voltage (V)
Nor
mal
ized
cap
acita
nce
: 60 nm
: 120 nm
: 240 nm
Alq thickness:
Voltage (V)
Nor
mal
ized
cap
acita
nce
: 60 nm
: 120 nm
: 240 nm
Alq thickness:
SimulationExperiments from [2](a) (b)
CAlq3
Cgeo
CAlq3CAlq3
CAlq3
CAlq3CAlq3
Cgeo
[2] Brütting, W., Berleb, S., Mückl, A. G., “Device physics of organic light‐emitting diodes based on molecular materials”, Org. Elec., 2 (1), 1‐36 (2001).
setfos
S. Altazin et al., Organic Electronics. 39 244‐249 (2016)
www.fluxim.comSimulated OLED Band Diagram
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V<Vt Vt<V<Vbi V>Vbi
setfos
www.fluxim.comStability of (Polar) OLEDs• C‐V responses of polar OLEDs evolve during degradation.• A similar trend can be observed with setfos simulations
diminishing the interface charges.
Interpretation: Orientation polarization isreduced during degradation!
Degradation
C-V measured with PAIOS (TPD/Alq3 bilayer OLED)
C-V simulated with SETFOSfor varying interface charges
Reduction of orientationpolarization
Interfacial charge:
www.fluxim.comStress‐Testing for Degradation Analysis
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Cell C
urre
nt (I
) (m
A/cm
2)
1.4
-17.3-16
-14
-12
-10
-8
-6
-4
-2
0
Time (t) (us)90.3-6.83 0 10 20 30 40 50 60 70 80
Capa
citan
ce (C
) (F)
2.97E-9
7.81E-10
1E-9
1.2E-9
1.4E-9
1.6E-9
1.8E-9
2E-9
2.2E-9
2.4E-9
2.6E-9
2.8E-9
Offset Voltage (V_offset) (V)2.14-1.15 -1 -0.5 0 0.5 1 1.5 2
Perform automated systematic degradation studiesUse current-stress or light-stress
degradation
degradation
S. Züfle et al. Adv. En. Materials (2015) “Areal degradation of OPV cells – experiment & modeling”
photocurrent response to light pulse
un‐encapsulated OPV placed in climate chamber at high T, repeated Paios measurements
www.fluxim.comMobility extraction using CELIV
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t
Voltage
Vtr<V<Vbi
extraction of holes
holes pile up at the HTL/ETL
interface
Polar Alq3 layer prevents electron injection in some voltage range
Using CELIV we are able to determine thehole mobility of the HTL material!
S. Züfle et al., J. Appl. Phys., 2017
www.fluxim.com
63
t
Voltage
t
Current
Extract charges with a voltage ramp
Measure mobility, and charge concentration
Mobility extraction using CELIV
tmax: related to the mobility
Extracted charges
0
2
max
2
36.011
32
jjtAd
available with the platform
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Hole Mobility from CELIV withpolar OLED
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S. Züfle et al., J. Appl. Phys., 2017“The use of charge extraction by linearly increasing voltage in polar organic light‐emitting diodes”
Consistent with simulation
Determine sheet charge Q
CELIV formulafor Bilayer OLED
www.fluxim.comCELIV with polar layer vs. MIS‐CELIV
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S. Züfle et al., J. Appl. Phys., 2017“The use of charge extraction by linearly increasing voltage in polar organic light‐emitting diodes”
OLED with polar ETL:
MIS structure:
In both cases we have carrier selectivity!
a‐NPD(80 nm)/Alq3(60 nm)
a‐NPD(80 nm)/Ins.(60 nm)
www.fluxim.comLow Temperature Module
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Perform all experiments at temperatures down to 150 Kelvin
www.fluxim.comLow Temperature Module
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Paios Measurement C-V Setfos Calculation C-V
Compare temperature dependent measurement with setfos simulation.
www.fluxim.comThermal Activation in C‐f?
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setfospaios
fr depends on the temperature, activation energy 0.45eV setfos simulation reproduces the paios measurement Injection + transport in HTL is thermally activated
fr=1/RC1
TPD
Alq3
C2
C1
www.fluxim.comSummary
• All‐in‐one comprehensive measurements arepossible and needed for reliable parameterestimation and model validation
Paios Software demo?Next talk: OLED light outcoupling!
Thank you for your attention!
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