Direct evaluation of effective hole mobility in multilayer organic light-emitting diodes using the MIS-CELIV technique

Abstract To optimize carrier balance in organic light-emitting diodes (OLEDs), it is crucial to gather data on the charge transport properties of organic semiconductors as measured within a functioning device. In this study, we directly investigated the hole mobility of the N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine ( α -NPD) layer in multilayer OLEDs with various hole injection layers (HILs). This was achieved using the metal–insulator–semiconductor charge extraction by linearly increasing the voltage (MIS-CELIV) technique, which utilized hole accumulation at the α-NPD/8-hydroxyquinoline aluminum (Alq 3 ) interface. Our findings confirmed that hole mobilities evaluated via MIS-CELIV with sufficiently large offset voltages were more reliable, as the space-charge-limited extraction was realized through adequate hole accumulation at the α -NPD/Alq 3 interface. Moreover, the hole mobility in the multilayer OLED configuration, as assessed by MIS-CELIV, reflects not only the hole transport properties of the α -NPD layer but also the hole injection properties at the HIL/ α -NPD interface.