Advanced Initialization

The disordered nature of typical OLED materials gives rise to a diversity in molecular environments. This diversity is accounted for by including e.g. energy level broadening and is an important feature in describing charge transport inside the device.

The description of the molecular disorder can have a strong impact on the simulated performance. Various settings are provided to customize the disorder character.

Charge Carrier Initialization

By default, devices are initialized without charge carriers. Polaron injection instead occurs at the electrode interface. For bulk materials, a minimum charge carrier density is specified to populate the device without direct electrode contacts.

Additional charges can be distributed throughout the device. This can be utilized to accelerate device equilibration, to simulate specific off-equilibrium conditions or to investigate dopants and trap states. Initial carrier concentrations are configured in the stack editor by specifying the fraction of sites that is occupied by a given type of polar or exciton.

Note

The cost of the kinetic Monte Carlo simulations scales directly with the number of carriers in the device, as process rates have to be evaluated for each individual particle. High carrier densities can result in long simulation times.

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Fig. 71 Dopant editor

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Fig. 72 Dopant configuration in the stack editor

Consistency in HOMO, LUMO and Exciton Levels

Energy level broadening can be enabled for the OLED materials.

By default, the energy level shifts for the HOMO and LUMO are treated as being uncorrelated. The HOMO shift and the LUMO shift are then computed independently from one another.

A correlated HOMO-LUMO shift can be requested in the parameter set. This preserves the band gap of the material.

Anti-correlation is also supported, such that the shift in the HOMO level is opposite that of the LUMO.

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Fig. 73 Configuration of energy level shifts in the parameter set

Exciton energies can be correlated to the HOMO-LUMO shifts. The change in exciton binding energy is then scaled to be proportional to the change in band gap.

Dipole Orientation

The polaron and exciton energy level broadening can be modified by specifying a dipole field.

A random arrangement of dipoles can be generated by selecting the Correlated Disorder option in the Energy Landscape settings in the parameter set. The dipolar coupling at each gridpoint will then be evaluated to determine the spatial energy level distribution.

Alternatively, biased dipole fields can be specified as part of the dipole settings of the materials. Quantile functions are then used to sample three-dimensional vector orientations. Dipole strengths are provided for each material to allow scaling of the interactions based on the composition morphology.

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Fig. 74 Material dipole settings

The non-uniform dipole fields must be enabled in the Dipole-induced Disorder section of ‘Energy Landscape’ tab in the parameter set. This will override the random dipole field if both are selected.

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Fig. 75 Dipole field generator in the parameter set

By default, interactions across the periodic boundaries are accounted for when calculating the dipolar coupling.

It is possible to override the material-specific quantile functions to eliminate field discontinuities at the material interfaces. Differences in the dipole strength of the materials is still accounted for in this consistent field, such that the material-correlation is preserved.