High efficiency organic light-emitting diodes based on HAT-CN/TAPC heterojunction charge generation layer as charge injectors

Abstract

High efficiency organic light-emitting diodes (OLEDs) strongly depend on the balanced injection and transport of charge carriers from electrodes. Here, an efficient method that utilizes a charge generation layer composed of 1,4,5,8,9,11-hexaazatriphenylene- hexacarbonitrile (HAT-CN)/1,1-bis[(di-4-tolyamino)phenyl]cyclohexane organic heterojunction (OHJ) simultaneously as electron and hole injectors was proposed. Attractively, the fabricated green, blue and red OLEDs show higher efficiency compared with the normal OLEDs without OHJ as charge injectors. The improvement in efficiency benefits from the more balanced generation and injection of electrons and holes from the OHJs. The capacitance–frequency and current density–voltage characteristic measurements were used to well demonstrate the injection properties of electrons generated from the OHJ and the effects of the introduced ultra-thin Al layer between HAT-CN and electron transport layer. It can be seen that the injection way of charge carriers generated by OHJ, only depending on the external electric field, is completely different from that of direct injection via electrodes. This is not only beneficial to the charge balance, but also far away from the unstable metal electrode interfaces, therefore, the high efficiency and long lifetime OLEDs could be expected.