included t o a certain extent by making use of parameterized mobility functions that depend on the temperature T charge carrier concentration c, and electric field F . The analytical expressions of the mobility function used are obtained from advanced 3D modeling methods. Limitations of this approach include the proper treatment of charge carrier relaxation effects and the treatment of molecularly-scale mixed mat e rials such as used in the emissive host-guest layers in OLEDs. The limitations of 1D-DD modeling can be overcome by 3D-KMC modeling, hrst proposed by Bassler. Assuming incoherent hopping of charge carriers between local ized molecular states, 3D-KMC modeling provides a mechanistic way t o predict the macroscopic device characteristics from microscopic processes at the molecular level. This approach mimics in a realistic manner the functioning of devices by simulating the actual time and spatially resolved hopping motion of all charge carriers in a 3D system. The method is not constrained t o specihc layer structures, and has, e.g., been applied successfully t o predictively simulate the electroluminescence in complex mul tilayer white OLEDs. However, due t o the inclusion of all microscopic-scale events, 3D-KMC simulation can be computationally expensive and is under some conditions infeasible, especially when the device simulation is carried out around or below the built-in voltage , where the net device current density is due t o only a very small imbalance of the diffusive motion of charge carriers in the positive and negative field directions. Moreover, in practice, 3D-KMC simulations can also not be easily ap plied t o simulate the transient device response as measured, e.g., using impedance spectroscopy. Within master equation methods, the time-averaged instead of the actual charge carrier concentration on the molecular sites is calculated. In contrast to 1D-DD models, 1D-ME methods include in a natural way the discreteness of the molecular structure, also at internal interfaces. 3D-ME modeling can successfully bridge the gap between 1D-DD and 3D-KMC simulations. In 3D-ME device simulations, the Coulomb interaction between a charge carrier and all other charges in the device is included i an approximate manner, viz. by using the layer-averaged instead of the actual space-charge density. Recently, we demonstrated the possibility t o extend this approach by including the effects of the actual Coulomb interaction between the same type of charge carriers, which plays a role at large carrier concentrations and low electric fields. On the one hand, these 3D-ME simulations of unipolar devices still