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Modelling of electroluminescence in polymers under ac stressJunwei Zhao, David H. Mills, George Chen and Paul L. Lewin19th January 2011
Electroluminescence (EL) in polymers• Origin: light emission from the recombination of
opposite polarity charge carriers
− What are charge carriers?
− Where do the charge carriers come from?
− What mechanism is behind the existence of these charges?
• Indication: storage, transport and interaction of charge carriers within insulation materials
• Implication: the effects of degradation or ageing on polymeric materials
Experiment
Experimental setup• CCD camera and
triggering system allows EL emission synchronised with applied field
• Uniform electrode arrangement with semitransparent gold coated 100 µm LDPE
EL experimental setup
0 45 90 135 180 225 270 315 360
-6
0
6
App
lied
volta
ge (
kV)
angle ( )
TriangularSinusoidalSquare
Measured EL emission
Measured EL under 50 Hz, 6 kVpk, ac voltage of various waveforms
(a) Applied voltage (b) EL intensity
Positive half cycle Negative half cycle
0 45 90 135 180 225 270 315 3600
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2x 104
EL
inte
nsity
angle ( )
EL-TriangularEL-SinusoidalEL-Square
Modelling
EL can be described by the total recombination rate (TRR)
Si is the recombination coefficients
Bipolar charge transport model• Model description: - Injection and extraction of charge
carriers (electron and hole) at boundaries
- Charge transport by a field dependent mobility
- Deep trapping for electrons and holes
- Recombination of electrons and holes
x=0 x=dPolymeric film
Discretization of polymeric film
0 1 2 3TRR = ht et ht e et h h eS n n S n n S n n S n n
Trapping and recombination of bipolar charges
Modelled EL under sinusoidal voltage
Electroluminescence per cycle (6 kV 50 Hz)
Comparison of simulation and experiment
0 50 100 150 200 250 300 3501400
1450
1500
1550
1600
Tot
al r
ecom
bina
tion
rate
(C
m-3
s-1)
Angle ( )
0 50 100 150 200 250 300 350-10
-5
0
5
10
App
lied
volta
ge (
kV)
Total recombination rate Applied voltage(6kV 50Hz)
0 45 90 135 180 225 270 315 3600
1
2
3
4
5
6
Nor
mal
ized
EL
inte
nsity
angle ( )
Simulation 6kV Experiment 6kV
0 50 100 150 200 250 300 3502000
2500
3000
De
nsi
ty o
f m
ob
ile e
lect
ron
(C
/m3)
Angle ( )
0 50 100 150 200 250 300 350300
350
400
De
nsi
ty o
f tr
ap
pe
d h
ole
(C
/m3)
Mobile electron Trapped hole
Contribution of charge carriers
Density of charge carriers per cycle (6 kV)
0 1 2 3TRR = ht et ht e et h h eS n n S n n S n n S n n
Distorted injection flux at boundaries
(a) Distorted injection field
(b) Injection current density
(c) Conduction current density
0 50 100 150 200 250 300 350-2
-1
0
1
2x 10-11
Co
nd
uct
ion
cu
rre
nt
de
nsi
ty (
Am
m-2
)
Angle ( )
0 50 100 150 200 250 300 350-10
-5
0
5
10
Ap
plie
d v
olta
ge
(kV
)
Conduction current densityApplied voltage(6kV 50Hz)
0 50 100 150 200 250 300 350-40
-30
-20
-10
0
10
20
30
40
Inje
ctio
n f
ield
at
the
ele
ctro
de
(kV
/mm
)
Angle ( )
0 50 100 150 200 250 300 350-8
-6
-4
-2
0
2
4
6
8x 10-7
Inje
ctio
n c
urr
en
t d
en
sity
(A
mm
-2)
Injection field at the electrodeInjection current density
0 50 100 150 200 250 300 350-40
-20
0
20
40
Inje
ctio
n f
ield
at
the
ele
ctro
de
(kV
/mm
)
Angle ( )
0 50 100 150 200 250 300 350-10
-5
0
5
10
Ap
plie
d v
olta
ge
(kV
)
Injection field at the electrodeApplied voltage(6kV 50Hz)
0 45 90 135 180 225 270 315 3600
20
40
60
80
100
120
140
Tot
al r
ecom
bina
tion
rate
(C
m-3
s-1)
angle ( )
Sine 6kV 10HzSine 6kV 20HzSine 6kV 30HzSine 6kV 40HzSine 6kV 50HzSine 6kV 60HzSine 6kV 70HzSine 6kV 80HzSine 6kV 90Hz
0 45 90 135 180 225 270 315 3600
50
100
150
200
250
To
tal r
eco
mb
ina
tion
ra
te (
Cm-3
s-1)
angle ( )
Sine 5kV 50HzSine 6kV 50HzSine 7kV 50HzSine 8kV 50HzSine 9kV 50Hz
Modelled EL under sinusoidal voltage
Electroluminescence at increased applied field
Electroluminescence at increased frequency
Modelled EL under triangular & square voltage
(a) simulation
(b) experiment
Comparison between simulation and experimental results
0 45 90 135 180 225 270 315 3600
0.05
0.1
0.15
0.2
0.25
Nor
mal
ized
TR
R
angle ( )
Sine 6kV 50HzTriangle 6kV 50HzSquare 6kV 50Hz
0 45 90 135 180 225 270 315 3600
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2x 104
EL
inte
nsity
angle ( )
EL-TriangularEL-SinusoidalEL-Square
Conclusions• Satisfying EL simulation results have been
achieved using a bipolar charge transport model; Two typical peaks which occur prior to the voltage peak in each cycle are reproduced
• Charge carriers from the injection at the boundaries contribute more than that from the conduction process to the resultant EL
• Injection current and conduction current are both distorted from the sinusoidal form due to the presence of space charge