Recombination Zone Modeling in Organic Light Emitting Field Effect Transistors

Molecular Materials and Nanosystems

Dimitri Charrier Martijn Kemerink Simon Mathijssen Edsger Smits Dago de Leeuw René Janssen

Organic Transistor organic semiconductor drain (Au) source (Au) dielectric (SiO2) gate (Si-n doped)

source-drain current

p

n

on

off

gate voltage 0V

unipolar 0V

++++++++++++ ________ -5V

ambipolar 1V

0V

++++++

____

_ _ _ _ +++++ -5V

- 10 V

Theoretical Predictions

0.01 -2

n p recombination

1E-4

-4

1E-6 -6 1E-8 -8 1E-10 0

2

4

6

8

10

position ( m) D.L. Smith et al, J. Appl. Phys. 101, 084503 (2007)

Numerical drift/diffusion model vs. Analytical drift-only model (recombination according to Langevin) Martijn Kemerink

Full Width at Half Maximum FWHM ~ 20 - 200 nm

surface potential (V)

n, p, recombination

0

Experimental Results Scanning Kelvin Probe Microscope NiDT

Confocal microscope / High fields PPV

J.S. Swensen et al, J. Appl. Phys. 102, 013103 (2007)

E.C.P. Smits et al, Phys. Rev. B 76, 125202 (2007)

FWHM ~ 2 µm

Scanning Kelvin Probe Microscope = SKPM

10

-Atomic Force Microscope in tapping mode -Surface potential at Lift Height ZL

8

80

Vsd

40

6 4

0

2

voltage

Interleave mode

height (nm)

120

-40 0 0 2 4 6 8 10 12 14 16

position ( m)

lever cone

Au

Au SiO2

Vsd

Principle: First Harmonic Force Microscope

V 2 dC F = force between tip and sample V = tip-sample voltage difference F  2 dz C = capacity between tip and sample V  Vdc  Vac sin(t )  Vcpd

F  Fdc  F sin t  F2 cos 2t F 

Vdc = tip voltage Vac = amplitude voltage Vcpd = contact potential difference

dC Then Vcpd = Vdc Vac (Vcpd  Vdc ) For F =0  dz 8V lever cone

apex source

channel

drain

0V

orthogonal parallel

10

40

8

30

6 20 4 10

2

0

0 2

4

6

8

10

position ( m)

12

14

height (nm)

surface potential (V)

Instrumental Problem: SKPM Response

Problems: •Asymmetry •Rounding •Magnitude

SKPM Response 3D Model

Simulated with Finite Element Program (COMSOL)

0

force (nN)

force (nN)

10

1

0.1

-2 -4 -6 -8

10

100 1000 tip-to-sample distance (nm)

Scattering = meshing limitation

0

1

2

3

4

tip voltage (V)

5

6

orthogonal parallel theoritical

10

40

8

30

6 20 4 10

2

0

0 2

height (nm)

surface potential (V)

Calculated SKPM Response

4

6

8

10

position ( m)

12

14

Reproduced features: •Asymmetry •Rounding •Magnitude

D.S.H. Charrier et al, ACS Nano 2, xxx-xxx (2008)

surface potential (V)

Tip = Apex + Cone +Lever

10

apex cone

8

apex

6 lever

4

cone apex

2

orthogonal Apex + Cone + Lever Apex + Cone Apex

0 -2

0

2

4

position ( m)

6

•Apex = perfect probe

8

•Cone = magnitude + rounding •Lever = asymmetry

D.S.H. Charrier et al, ACS Nano 2, xxx-xxx (2008)

SKPM Response for FET

Theoretical predictions (drift) from Smits = input of SKPM modeling

Assumption: FWHM = “0” nm

SKPM experiments + modeling

‘real’ FWHM < 0.5 micron

Note: We checked that the SKPM probe influence only few % the source drain current. M. Kemerink, D.S.H. Charrier et al, in preparation

Conclusions •Identified the full problem of SKPM response: Developed a numerical model to predict the SKPM response from any theoretical potential.

•FWHM recombination: •theoretical (Langevin) ~ 200 nm •experimental SKPM response •raw ~ 2.1 µm •difference with model < 0.5 µm

•SKPM is not optimal for investigating the recombination width.

Thanks to

Molecular Materials and Nanosystems Group Martijn Kemerink René Janssen Simon Mathijssen

Clean room facilities

For discussions and data

Barry Smalbrugge Tjibbe de Vries Erik-Jan Geluk

Reinder Coehoorn Edsger Smits Dago de Leeuw