characteristics, the choice of voltage point affects the extracted ideality factor (see above) (c) In reality, there is usually a leakage current in the low voltage regime which is not included in the simulation. The leakage current is expected t o influence the ideality factor. Hence, one needs t o be careful regarding the physical interpretation of the ideality factor. In Fig. 8, the disorder energy is varied while the temperature T is hxed at 290 K. Fig. 8 part (a) shows that for large and low voltages, the current density is larger than that for the small case. This is, at least in part, due to the easier injection from the electrodes into a wider DOS. Fig. 8 part (b) shows that the ideality factor is indeed voltage dependent. For small and low voltages, the ideality factor approaches unity. In the voltage range studied, it increases with . This effect can be explained from the increasing charge carrier concentration, at any point in the device, with increasing , combined with the increase with of the diffusion coefficient enhancement factor at any given carrier concentration. In Fig. 9, the disorder energy is hxed at 0.10 eV while the temperature T is varied. We observe that the ideality factor is almost independent of temperature, which agrees with experimental results. We also hnd that for a series of symmetric 100 nm devices with varying EML layer thicknesses the ideality factor, evaluated at a hxed low voltage, is not significantly dependent on the voltage. We remark that accurate calculation of the ideality factor requires high precision in the current density-voltage calculation. To run the 3D-ME simulations with a sufficiently high precision for a current density as low as 1C 6 A/m $^{2}$, the simulation time is significantly larger than those shown in Figs. 1A-B. In this work, we have presented a bipolar 3D-ME simulation model that can be used to predictive!y model electron-hole recombination in disordered organic sem i conductor devices at low voltages. For voltages close t o or smaller than the built-in voltage $_{bi}$, the simulation method is numerically much more efficient than 3D-KMC simulations. The model has been validated by comparing the simulation results for a wide range of parameters with those obtained from 3D-KMC simulations. From the development of a theoretical expression for the recombination prefactor (Eq. (13)), IV) Conclusions and outlook