ABSTRACT
In this chapter, two critical components of an organic light-emitting diode (OLED) device that determine the current injection process, namely, anode hole injection layer and cathode electron injection layer, are first presented in detail. An effective carrier injection layer ensures an Ohmic contact that minimizes the amount of undesirable charge accumulation at either electrode, which would contribute to a waste of electrical energy input. The transport across barriers formed by different organic materials will then be discussed. Figure 3.1 illustrates the sequential processes that carriers experience in an OLED including injection, migration, collision, and recombination. Charge carrier transport and injection in an OLED are two key factors that dictate the performance of a device. For example, a poor charge injection at the electrode and/or a slow charge transport in the organic layers will result in a higher driving voltage, which means a higher power is required to generate sufficient amount of excitons in the host to produce a desired brightness. On the cathode end, a low-work-function metal such as Al in conjunction with a dielectric electron injection layer such as LiF, Liq, or CsCO3 are used to achieve sufficient electron injection. On the anode end, a transparent conducting material such as indium tin oxide (ITO) combined with a high-work-function hole injection layer such as MoO3,34 poly(3,4ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS)35 and 1,4,5,8,9,11-hexa-azatriphenylene hexa-carbonitrile (HATCN)36
Figure 3.1 Simplified charge carrier energy state diagram at different stages of OLED operation. Reproduced with permission from Ref. [5]. Copyright 2014, Royal Society of Chemistry.
