Effects of Gaussian disorder on charge carrier transport and recombination in organic semiconductors

In this review, we discuss recent advances in our understanding of charge transport and exciton generation in disordered organic semiconductors with a Gaussian DOS, with a focus on applications to organic light‐emitting diodes (OLEDs). Three‐dimensional (3D) modeling shows that the actual current density in OLEDs based on materials with a Gaussian electron and hole DOS is filamentary. However, it is possible to accurately calculate the average current density by solving a one‐dimensional (1D) drift‐diffusion equation, making use of compact expressions for the temperature, electric field, and carrier density dependent mobility which have been derived from 3D‐modeling. For the cases of spatially uncorrelated energetic disorder and spatially correlated disorder due to random dipole fields, these models are called the extended Gaussian disorder model (EGDM) and extended correlated disorder model (ECDM), respectively. We discuss how the effects of trapping on guest molecules can be included, and how exciton generation is described. The application of these models to hole and electron transporting polymer and small molecule materials is discussed, with an emphasis on the modeling of the transport and emission of blue‐emitting OLEDs based on a polyfluorene‐derivative. Three‐dimensional simulation of the charge carrier transport in organic light‐emitting diodes (OLEDs) has led to the development of second generation OLED models, within which the energetic randomness of the molecular sites is taken into account. In contrast to more conventionally used first generation models, where the current density is a laterally uniform function of the position, the current density is found to be filamentary. In this Feature Article, it is discussed how within the framework of second generation modelling effective expressions for the charge carrier mobility and exciton generation may be obtained, which may be used in more efficient one‐dimensional OLED simulations. Furthermore, the results of experimental validation studies for polymer and small‐molecule devices are discussed.