1. PAIS-2001 “TRANS”
Organic Transistors: design, fabrication and characterization

2. Summary: Theoretical model and simulations
OTFTs (Organic Thin Film Transistor): different geometries and different molecules
Plastic TFTs
Circuits
Theoretical model and simulations

3. OTFT Pentacene (sublimated active layer) ALDRICH
Drain and Source
Gate
Insulating layer
Active layer
Si--
Drain and Source contacts (evaporated):
Spacing: 5, 10, 20, 50 mm
Thickness: 50Å Cr / 450Å Au
W/L= 2  20
Mask for optical litography

4. OTFT: output characteristics
20μm
Channel lenght
Ion/Ioff
Mobility (cm2/Vs)
20μm
103 m=
50μm
m=10-2

5. OTFT: morphological analysis
Active layer evaporation:
deposition rate: 0,1 Å/s
vacum: 3 *10-7 Torr

6. OTFT
To improve W/L we have used a mltifinger geometry for the drain-source contacts
Drain and Source contacts (evaporated):
Spacing: 12, 20, 24 mm
Thickness: 50Å Cr / 450Å Au
W/L= 70, 140, 16000

7. OTFT 1
Vds=-40V
m=4*10-3 cm2/Vs
W/L=140
12mm
Ion/Ioff=104
Vth=0V

8. OTFT 2
m=3*10-3 cm2/Vs
W/L=16000
20mm
Ion/Ioff=102
Vth=10V

9. OTFT: morphological analysis
Active layer evaporation:
deposition rate: 0,2 Å/s
vacum: 6 *10-6 Torr

10. OTFT based on OMP Vds=-40V m=3*10-8 cm2/Vs W/L=140 12 mm Ion/Ioff=10
-Ids (A)
Vds=-40V
m=3*10-8 cm2/Vs
W/L=140
12 mm
Ion/Ioff=10
Vth= 0V
octamethylporhyrin (OMP)

11. Inverter 1 Vcc Rc Vout Vin Vin: 250 mHz; Vpp=20 V Vcc: -20V Rc: 30 MΩ
Channel lenght: 12 μm
Pentacene

12. Inverter 2 Vcc Rc Vout Vin Vin: 200 mHz; Vpp=20 V Vcc: -20V Rc: 10 MΩ
Channel lenght: 20 μm
Pentacene

13. Plastic TFT Top gate devices
Polyimid
PEDOT:PSS
Pentacen
PVA
Top gate devices
Flexible substrate : Polymid depositated by casting and removed by peeilng off
Drain/Source contact deposited by electropolimerization
Active layer: pentacene
Insulating layer: PVA depositated by spin coating
Pedot Gate contact by ink-jet deposition

14. Theoretical model and simulations
Drift-diffusion model
Charge transport model
Simulations

15. Drift-diffusion equations
Drift-Diffusion model
Drift-diffusion equations
Poisson equations
Continuity equations
Numerical solutions

16. l.c.m. ≈ intramolecular distance
Charge transport model
Van der Walls bounds
Desordered structure
l.c.m. ≈ intramolecular distance
Inchoerent transport
hopping
μ=f(E)

17. Field Dependent mobility: Monte Carlo results
In order to calculate the field dependent mobility we apply
the model first introduced by Bassler1
Rate of hopping is described by Miller-Abrahams expression
1Ref. H.Bassler, PRB 1999, vol 59, n. 11, 7507

18. Conduction in OTFT Fixed charge and traps at organic/oxide interface
10nm
Fixed charge and traps at organic/oxide interface
Transport in the first layers

19. Output characteristic
Pentacene Band Structure
2.6 eV
Source/Drain Metal
LOMO
2.4 eV
0.1 eV
HOMO

20. Conclusions
We have designed, fabricated and characterized single organic transistors and basic circuits like inverters using commercial Pentacene.
For the first time a thin film transistor based on a porphyrin (octamethylporhyrin (OMP)) has been realized.
Completely plastic transitors have been realized
We have simulated organic devices using an industry standard device simulation tool, namely ISETCADTM. This package is able to resolve the standard drift-diffusion equations coupled with Poisson’s equation in two and three dimensions.
We settled the basis for the development of a complete family of circuits like three stages ring oscillators.