Numerical investigations on the current conduction in bilayer organic light-emitting devices with ohmic injection of charge carriers

A numerical model for bilayer organic light-emitting diodes (OLEDs) has been developed on the basis of trapped charge limited conduction. The dependences of the current density on the operation voltage, the thickness and trap properties of the hole transport layer (HTL) and emission layer (EML) in bilayer OLEDs of the structure anode/HTL/EML/cathode have been numerically investigated. It has been found that, for given values of reduced trap depth, total trap density, and carrier mobility of HTL and EML, there exists an optimum thickness ratio of HTL to the sum of HTL and EML, by which a maximal current density, and hence maximal quantum efficiency and luminance, can be achieved. The current density decreases quickly with the mean trap density, and decreases nearly exponentially with the mean reduced trap depth.