Organic Device Simulation Using Silvaco Software · Organic Device Simulation Using Silvaco...

Organic Device Simulation Using Silvaco Software


• Introduction – Silvaco TCAD Simulator • Theory – Models • OTFT Simulation v.s Measurement • OLED Simulation v.s Measurement • Bilayer TPD/Alq3 OLED Example • Transient Simulation of OLED Pixel • Summary

Organic Devices Simulation: Contents

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• Athena- 2D Process Simulator • Atlas – Device Simulator

• S-Pisces – Silicon material Drift-Diffusion Simulator • Blaze – Hetero-interfaces (Compound Semiconductor) Materials Simulator • TFT – a-Si/poly-Si TFT Device Simulator • OTFT – Organic TFT Simulator • OLED – Organic Light Emitting Diode Simulator

Organic Devices Simulation: Silvaco’s TCAD Software

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• Metal & Semiconductors: charge transport is limited by scattering of the carriers, mainly due to thermally induced phonons and lattice deformations. Transport is limited by phonon scattering. Charge mobility decreases with temperature

• Organic materials: transport occurs by phonon assisted hopping of charges between localized states. Charge mobility increases with temperature

• General mobility model of organic material : • Poole-Frenkel field-dependent mobility

Organic Devices Simulation: Transport Mechanisms

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• Charge Injection (metal contact) • Ohmic (Dirichlet boundary condition) • Schottky contact (injection limited current) :

• thermionic emission model - tunneling • interface barrier lowering

• Transport model(bulk) • Band-like transport model (organic molecular crystals: pentacene, tetracene) at low T. • Space-Charge-Limited Current(SCLC): Poisson + Current continuity equations • Hopping transport in disordered organic semiconductor

• Density of States • Poole-Frenkel Mobility

Organic Devices Simulation: Organic Transport Theory For Simulation

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• Poisson Equation

• Current Continuity Equations

• Drift Diffusion Equations

Organic Devices Simulation: Classical Theory Of Charge Transport – Drift Diffusion Model

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• Density Of States (DOS)

• Trapped Charge

Organic Devices Simulation: Density Of State & Trapped Charge – Organic Defects

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• Probability of Occupation

• Steady State: Recombination/Generation

(SRH)

Organic Devices Simulation: Organic Defects

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Organic Devices Simulation: Poole-Frenkel Mobility Models

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• Langevin Radiative Rate

• Singlet Exciton

Organic Devices Simulation: Langevin Recombination Rate & Exciton Rate Equations

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• Triplet Exciton

where

Organic Devices Simulation: Langevin Recombination Rate & Exciton Rate Equations (con’t)

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• Time-of-flight(TOF) method • SCLC method • Field Effect Transistor(FET) method

Atlas – Organic Device Simulation: Mobility Simulation

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Atlas – Organic Device Simulation

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Measurement vs. Simulation

Density of States

µp-0.62


Transfer curve: linear & sqrt(Ids)

Atlas – Organic TFT Device Simulation

Hole concentration distribution and current flow lines in the OTFT device.

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• Metal/Organic Interface injection

Atlas – Organic LED Device Simulation: OLED Example

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I.D. Parker J.Appl. Phys. 75(3),1 Feb 1994, p.1656


• Injection - Calcium • Ca(2.9eV) is better than other cathode metal.

Atlas – Organic LED Device Simulation

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Simulated

Measured


Atlas – Organic LED Device Simulation: High-Efficient Amorphous OLED

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Fraction of injected charge that form excitons


• Exciton Profile

Atlas – Organic LED Device Simulation: Bilayer TPD/Alq3 OLED Example: Singlet Exciton Density Profile

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Atlas – Organic LED Devices Simulation: Bilayer TPD/Alq3 OLED Example: IL & Internal Efficiency

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IL curve Internal Efficiency


Atlas – Organic LED Device Simulation: Bilayer TPD/Alq3 OLED Example: Optical Output Coupling

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n=1.5

n=1.9

n=1.8

Organic Device Simulation

Transient Simulation of OLED Pixel


• A p-type poly-Si TFT AM-OLED pixel is shown

• The cathode and anode electrodes of the OLED form an intrinsic capacitance C and the resulting equivalent circuit is shown

• When it is connected to a poly-Si TFT with an on resistance RON, it forms a circuit with its speed limited by the RC time constant

Organic Devices Simulation: Basic OLED Equivalent Circuit

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• The device simulation structure of a p-type Poly-Si TFT AM-OLED pixel is shown here

• The structure is set up for device simulation and does not represent actual process steps

• More complicated OLED pixels can be simulated using Atlas MixedMode

Organic Devices Simulation: Corresponding OLED Pixel Structure

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• Curve 1: Transient current simulation results of the PPV OLED only (in blue)

• Curve 2: The combined poly-Si TFT/OLED pixel (in black) – note the effect of TFT on current level

• The rise/fall (ON/OFF) signal is coupled through the poly-Si TFT and is converted as a current spike in the OLED as shown

Organic Devices Simulation: OLED Pixel Simulation

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• The transient OLED current density response due to a 600ns square data voltage pulse of the experimental and simulation curves are characterized by: • A sharp charging spike due to the

capacitance of the device followed by a quasi-steady state

• At turn-off there is a sharp discharging spike followed by some decay

* Pinner et al, J Appl Phys 86 (9) 5116

Organic Devices Simulation: OLED Experiment

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• A simulated transient result of the exciton density is shown

• The exciton density assumes a Langevin recombination process and takes into account singlet excitons, inclusive of diffusive and decay terms

Organic Devices Simulation: Exciton Simulation

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• One can observe the fast initial EL rise followed by a slower rise, fast modulation in the turn-off, and a decaying exponential tail

• Assuming the exciton density is proportional to EL, note the similar shape of the previous exciton density simulation with the EL curve

* Pinner et al, J Appl Phys 86 (9) 5116

Organic Devices Simulation: Experimental EL Curve*

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Organic Devices Simulation: OLED Langevin Recombination Zone (2D plot)

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• Calculation of transient OLED Langevin recombination and exciton density based on 3 pulses

Organic Devices Simulation: Langevin Recombination and Exciton Density

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Organic Devices Simulation: PPV OLED Exciton Density (2D plot)

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Organic Devices Simulation: Extraction of OLED Internal Efficiency

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IV-Curve Internal Efficiency Curve


• Organic Materials: • Default Bandgap parameters. Others are defined by user-defined • Density-Of-States(DOS)

• Transport: Drift-Diffusion/Poole-Frenkel mobility model • Bimolecular Langevin Recombination • Excition Rate Equation: singlet/triplet exciton profiles

• Radiative rate for luminescence or phosphorescence

• Reverse Ray-Tracing: external efficiency (refractive index step) • Angular power plot/optical output coupling coefficient/near&far field distribution

Summary

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Organic Device Simulation Using Silvaco Software · Organic Device Simulation Using Silvaco...

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