Controlling the Electrical Characteristics of Charge Carrier Transporting Layers in Organic Light-Emitting Devices by the Host–Guest Structure: A Master Equation Approach

Controlling the electron and hole current is a crucial requirement in organic light-emitting devices (OLEDs) as it has a significant impact on the charge balance factor and the location of exciton generation zone. In this paper, via a self-consistent solution of the 3-D master equation approach coupled to Poisson's equation, it is shown that the charge carrier and current density can be controlled in a wide range in OLEDs by utilizing a host-guest structure in the charge carrier transporting layers of the device. To this end, first, the impact of the injection energy barrier height, trap depth, and blending ratio on the charge carrier and current density is investigated. Second, the strategy of partially blending the hole or electron transporting material with an appropriate guest material is presented, and it is shown that by employing this strategy, current density can be enhanced over two orders of magnitude for high injection energy barriers, where the charge injection is most difficult. Moreover, this strategy guarantees that the transport of charge carriers to the emissive layer is conducted by the hole or electron transporting material itself rather than the guest material which is important as hole and electron transporting materials must fulfill other requirements such as high charge carrier mobility or specific excitonic energy levels.