Energy transfer from exciplexes to dopants and its effect on efficiency of organic light-emitting diodes

We report that an exciplex is formed at the interface between the N,N′-dicarbazolyl-4-4′-biphenyl (CBP) and the bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine (B3PYMPM), which are widely used as an emitting layer (EML) host and an electron transporting layer (ETL) for high efficiency, green phosphorescent, organic light-emitting diodes (OLEDs), respectively. The intensity of the exciplex emission is almost proportional to the inverse square of the fac-tris(2-phenylpyridine) iridium [Ir(ppy)3] concentration of the EML. Meanwhile, the efficiency of the OLEDs increases as the concentration of the Ir(ppy)3 increases. This enhancement of the efficiency and the decrease of the exciplex emission originates from the increase in the energy transfer rate from the exciplex to the dopants, due to the decrease in the distance between the exciplex and the dopant. The energy transfer processes were successfully analyzed using the Förster energy transfer mechanism. The high-efficiency OLEDs were obtained through the energy transfer from the exciplex to the dopant at the EML/ETL interface. The external quantum efficiency of the OLED reached 20.1% when the concentration of the Ir(ppy)3 is 6 mol. %. In addition, we investigated the relationship between the efficiency roll-off of the OLEDs and the energy transfer from the exciplex to the dopant by inserting a thin, undoped CBP layer at the EML/ETL interface.