Why porphyrin? Low-cost technology High-selective gas sensors Light emitting devices

Why porphyrin?

• Low-cost technology

• High-selective gas sensors

• Light emitting devices

• deposition technique• plastic substrate

Electronic nose

Multi-colour display

Organic Transistors: design, fabrication and characterization

S. Salvatori, C. Calcavento,

M. C. Rossi, G. Conte

M. Berliocchi, M. Manenti, A.

Bolognesi, F. Brunetti, P. Lugli, A. Di

Carlo, R.Paolesse, C. Di Natale, A. D’

Amico

A. Sassella,

A. Borghesi

2003PAIS-2001 “TRANS”

d

Contact geometry for test-structures

H2TPP deposition by spray casting or OMBE

Photolitographic definition of contacts

PVD of metal layer on CORNING 7059

substrate 10 m0 50

5

10 m

0

5

0

10m0

5

10 m

Film morphology

OMBE deposition

Band diagram

LUMO

Vacuum level

EF

= 4.8eV = 3.4eV

2.0 eV

0.7 eV

HOMO 0.7 eV

Structure

H2TPP

N N

N N

H

H

Energy (eV)

Inte

nsi

ty (

a.u.

) Luminescence

0.2

0.3

0.4

0.5

0.6

1.6 2 2.4 2.8 3.2

Energy (eV)

Op

tica

l Den

sity Absorption

1.4 1.6 1.8 2 2.2

Spray-deposited films show charge carrier transport which reflects a thermal activated Hopping between homogeneously-dispersed molecules on the surface

50 m

1000/T (K

10-14

10-13

10-12

10-11

5 10 15 20 25-1

)

G (

A V

-1) EA 0.78 eV

EA 2.2 meV

10-14

10-13

10-12

10-11

10-10

10-9

10 100

340 K350 K

325 K

300 K250 K50 K

I=G(T) V

Voltage (V)

Cu

rren

t (A

)

0 100

4 103

8 103

1.2 104

600 800 1000 1200 1400 1600

X 30

Raman shift (cm-1)

Inte

nsi

ty (

a.u

.)

ConclusionsCharge carrier transport properties of metal-free tetra-phenyl-porphyrin samples have been investigated in vacuum condition (5x10-3 Pa) in the 40-340 K temperature range.

We have used two dimensional drift diffusion simulations to calculate the electrical properties of bottom contact pentacene based organic thin film transistor, taking into account field-dependent mobility and interface or bulk trap states.

For the first time a thin film transistor based on a porphyrin (octamethylporhyrin (OMP)) has been realized.

Metal Semiconductor Metal

EF

Va

70 nm

150 nm

(Cr)(Cr)

Multifingers on glass

Drain and Source (Au)

Gate

Insulating layer

Active layer

Si--

Drain and Source contacts:

Multifingers: Cr/Au

Spacing:12 mm

W/L=140

Vth= 0V

Thin Film Transistor structure

0 10 20 30 40-100,0p

-50,0p

0,0

50,0p

100,0p

150,0p

200,0p

250,0p

300,0p

Vg=+20

Vg=-40

-Vds (V)-40 -30 -20 -10 0 10 20

50,0p

100,0p

150,0p

200,0p

250,0p

300,0p

- Id

s (A

)

Vgs (V)

Output Characteristic Transfer Characteristic OMP

We simulate organic devices using an industry standard device simulation tool, namely ISETCADTM, a package able to resolve the standard drift-diffusion equations coupled with Poisson’s equation in two and three dimensions.

ISETCADTM is not capable to handle charge transport in organic material, so we have implemented a new model based on the following properties:

•Organic/Inorganic Band Alignment

•Density of States (equal to molecule density)

•Monte Carlo extracted Field dependent Mobility (see panel above)

•Trap states at the interface between organic material and silicon oxide (see panel below).

Pentacene TFT:Simulation Model and Experimental Device

Geometric Characteristics Measured and simulatedoutput characteristic

Measured transfer characteristic @ Vds=-30 V

All measurements were carried out in vacuum atmosphere at room temperature with Agilent Semiconductor Analyzer 4155C.

Experimental Thin Film Transistor

Vacuum Level

LOMO

HOMO

2.6 eV

2.4 eV

Pentacene Band Structure

From square root of IDSS is possible to calculate pentacene mobility using the expression:

Square root of saturation corrent

)(1

)(1

)()( 0

RGJqt

p

RGJqt

n

NNnpq

pqDqJ

nqDqJ

p

n

ADr

ppp

nnn

Drift Diffusion Equation

We apply this model to the simulation of pentacene organic thin film transistor. In particular we simulate a device with a channel length of 12 m and oxide thickness of 250 nm. The same device was realized and measured from our group.

Source/DrainMetal

0.1 eV

OMP TFT:Experimental Device

Temperature dependence of the dark current of film on Cr multifinger. The bias was 100 V.