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