Analyzing Bipolar Carrier Transport Characteristics of Diarylamino-Substituted Heterocyclic Compounds in Organic Light-Emitting Diodes by Probing Electroluminescence Spectra

We analyzed bipolar carrier transport characteristics of diarylamino-substituted heterocyclic compounds (DAHCs) having benzene, pyridine, pyrimidine, or triazine electron-accepting core units and carbazole or β-naphthylamine as electron-donating substituents. The highest occupied molecular and lowest unoccupied molecular orbital levels, carrier injection, and transport characteristics were systematically controlled by changing combinations of the core and substituent units. By analyzing electroluminescence spectra as a probe in four kinds of organic light emitting diode (OLED) structures, we found that the carrier transport characteristics of DAHCs significantly change depending on device structure. We concluded that all DAHCs have bipolar carrier transport characteristics, that is, DAHCs intrinsically possess both hole and electron mobilities that are based on unique molecular structures having both electron-donating and -accepting units. We also demonstrated that carrier injection barriers between DAHCs and adjacent carrier transport layers virtually control the appearance of bipolar characteristics in OLEDs.