Device Equilibration

When an OLED circuit is connected, the voltage over the device gradually increases until steady-state conditions are reached. This voltage ramp can be included as part of the OLED simulation.

Circuit Closure

We will investigate circuit closure for the phosphorescent OLED constructed in the exciton tutorial.

We create a new voltage-point parameter set. We select the phosphorescent stack and choose a default voltage of 5 V.

In the Termination Criteria tab, we set the target convergence to 0.05. This will automatically terminate the simulation once the system is within 5% of the steady-state current.

The Annealing and Equilibration tab allows configuration of the voltage ramp. We set the starting voltage to 0 V.

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Fig. 69 Voltage ramp configuration in the parameter set

The ramp implementation increments the voltage over a fixed number of intervals, resulting in a stepped gradient. Here, we set the number of steps to 10.

The total duration of the voltage ramp is specified in terms of the number of Monte Carlo steps. We will use the first 2 output intervals (as defined in the output settings).

Separate report and output intervals can be specified during the voltage ramp, allowing you to use different parameters compared to the rest of the simulation. In order to obtain output at the end of every ramp stage, we set both the report and output intervals equal to 1/10th of the ramp duration.

After the simulation has concluded, transient current profiles can be viewed in the Transient OLED Response section of the Multibox Report.

Circuit Disconnect

When the OLED circuit is disconnected, the external voltage drops near-instantly. This behavior can be investigated using the transient response feature.

During this process, we first want the device to be operating at steady-state conditions, i.e. having a stable internal field. A pre-equilibration stage may be specified in the simulation settings to allow the system to de-correlate from the initial state, approaching the device equilibrium. Simulation statistics will then only be generated for the samples obtained after pre-equilibration.

We create a new voltage-point parameter set. The starting voltage is kept at 5 V. The target convergence is again set at 5%.

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Fig. 70 Pre-equilibration stage in the parameter set

The pre-equilibration is configured in the Annealing and Equilibration tab. We set a number of equilibration steps equal to 5 times the output interval. As with the voltage ramp, custom report and output intervals may be specified during the pre-equilibration stage. As we are interested in the dynamic behavior here, we will use the regular simulation intervals.

We will move to the Transient Parameters tab to configure the voltage switch. We create a new checkpoint at 0.001 seconds, at which point the voltage will be set to 0 V.

We can now run the simulation and analyze the transient current profiles.