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

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

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

OTFT: output characteristics 20μm Channel lenght Ion/Ioff Mobility (cm2/Vs) 20μm 103 m=3.55 10-3 50μm m=10-2

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

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

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

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

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

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)

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

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

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

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

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

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)

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

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

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

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.