Organic Device Simulation Using Silvaco Software Silvaco Taiwan September 2005
Organic Devices Simulation: Contents 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: Silvaco s TCAD Software ATHENA - 2D Process Simulator ATLAS Device Simulator SPisces 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: Transport Mechanisms 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: Organic Transport TheoryFor Simulation 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: Classical Theory Of Charge Transport Drift Diffusion Model Poisson Equation Current Continuity Equations Drift Diffusion Equations
Organic Devices Simulation: Density Of State & Trapped Charge Organic Defects Density Of States (DOS) Trapped Charge
Organic Devices Simulation: Organic Defects Probability of Occupation Steady State: Recombination/Generation (SRH)
Organic Devices Simulation: Poole-Frenkel Mobility Models
Organic Devices Simulation: Langevin Recombination Rate & Exciton Rate Equations Langevin Radiative Rate Singlet Exciton
Organic Devices Simulation: Langevin Recombination Rate & Exciton Rate Equations (con t) Triplet Exciton where
ATLAS Organic Device Simulation: Mobility Simulation Time-of-flight(TOF) method SCLC method Field Effect Transistor(FET) method
ATLAS Organic Device Simulation Measurement vs. Simulation p -0.62 Density of States
ATLAS Organic TFT Device Simulation Transfer curve: linear & sqrt(ids)
ATLAS Organic LED Device Simulation: OLED Example Metal/Organic Interface injection I.D. Parker J.Appl. Phys. 75(3),1 Feb 1994, p.1656
ATLAS Organic LED Device Simulation Injection - Calcium Ca(2.9eV) is better than other cathode metal. Simulated Measured I.D. Parker J.Appl. Phys. 75(3),1 Feb 1994, p.1656
ATLAS Organic LED Device Simulation: High- Efficient Amorphous OLED Fraction of injected charge that form excitons
ATLAS Organic LED Device Simulation: Bilayer TPD/ Alq3 OLED Example: Singlet Exciton Density Profile Exciton Profile
ATLAS Organic LED Devices Simulation: Bilayer TPD/Alq3 OLED Example: IL & Internal Efficiency IL curve Internal Efficiency
ATLAS Organic LED Device Simulation: Bilayer TPD/Alq3 OLED Example: Optical Output Coupling n=1.5 n=1.9 n=1.8
Organic Device Simulation Transient Simulation of OLED Pixel
Organic Devices Simulation: Basic OLED Equivalent Circuit 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 R ON, it forms a circuit with its speed limited by the RC time constant
Organic Devices Simulation: Corresponding OLED Pixel Structure 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: OLED Pixel Simulation 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 Experiment 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: Exciton Simulation 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: Experimental EL Curve* 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: OLED Langevin Recombination Zone (2D plot)
Organic Devices Simulation: Langevin Recombination and Exciton Density Calculation of transient OLED Langevin recombination and exciton density based on 3 pulses
Organic Devices Simulation: PPV OLED Exciton Density (2D plot)
Organic Devices Simulation: Extraction of OLED Internal Efficiency IV-Curve Internal Efficiency Curve
Summary 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