Interface engineering for low-voltage operation of organic light-emitting diodes

Organic light-emitting diodes (OLEDs) have been studied intensively, and their practical applications are advancing. The efficiency of light-emitting materials has been improved significantly through the understanding of their emission mechanisms. However, the correlation between the bandgap of the emitter and the operating voltage in OLEDs remains unclear, because OLEDs require a complex multilayer configuration that includes many materials other than the emitter. It is difficult to investigate the exact energy diagram for OLEDs, which have many interfaces, and many uncertainties remain regarding the mechanisms of charge injection and recombination. In this review, we introduce both the charge injection and recombination mechanisms in OLEDs and the interface control technology effective for lowering their operating voltage. We explain the electron injection mechanism at organic/cathode interfaces, which is clarified by using organic bases as the electron injection layers. The hole injection mechanism in OLEDs, which is clarified by investigating the correlation between the characteristics of OLEDs and the actual energy levels at organic/anode interfaces, is also introduced. With the elucidation of the charge injection mechanism, holes and electrons can now be injected into various organic materials. These charge injection techniques minimize the voltage required for charge recombination. The correlation between the bandgap of the emitter and the minimum voltage required for OLED operation is clarified by controlling the energy levels at organic/organic interfaces. Understanding this correlation enables the design of molecules for ultralow-voltage OLEDs, thereby realizing blue OLEDs with an extremely low turn-on voltage of about 1.5 V.