Correlation of interfacial electronic structures and transport properties in organic light emitting diodes

摘要

Turn on voltage in the current density-voltage characteristics is one of the important factors to evaluate the performance of organic light emitting diodes (OLEDs). In this paper, we report investigation of the origins of turn-on voltage, defined at where log J (current density) has a sharp rise and starts to increase dramatically. In OLEDs with NPB as the hole transport layer (HTL) and Alq_3 as the electron transport layer (ETL), we find that the turn on voltage is always at 2V, regardless the cathode structures, such as Ca, Al, LiF/Al, and Cs_2CO_3/Al, being used. The turn on voltage is also independent on the thickness of organic layers. Beside NPB and Alq_3, we also study the J-V characteristics on OLEDs with various combinations of HTLs and ETLs. In all the devices investigated, the turn on voltage just equals to the difference between the LUMO of ETL and the HOMO of HTL, taking into consideration of vacuum level shift at organic interfaces measured from the ultraviolet photoemission spectroscopy (UPS). Combined with J-V characteristics of OLEDs and UPS measurement, we propose that the turn on voltage of organic light emitting devices is determined by the difference between LUMO of ETL and HOMO of HTL and is independent of the cathode and thickness of organic layers. We also found that the charge transfers at the interface of ETL/HTL play an important role to the turn on voltage of OLEDs.