2, Korea Institute of Science and Technology, Seoul, , Korea (the Republic of)
3, Ulsan National Institute of Science and Technology, Ulsan, , Korea (the Republic of)
Quantum dots(QDs) have been investigated as lighting material because their inherent properties, such as controllable color depending on the size, narrow emission bandwidth and compatibility with solution processing. Recently, quantum dot light emitting diode (QLED), which uses such QDs as emissive layer (EML), has exhibited high brightness and wide color gamut, which is comparable to those of the commercialized organic light emitting diode (OLED). Despite the excellent electro-optical characteristics of QLED, it is still necessary to improve its efficiency and lifetime for commercialization. In a conventional QLED structure, which has a structure of transparent anode electrode/hole injection layer (HIL)/hole transport layer (HTL)/QD EML/electron transport layer (ETL)/cathode electrode with high reflectance and low work function, one of the main causes for the decrease in efficiency is exciton quenching due to metal diffusion from the electrode to the emissive layer. Especially, when we use typical combination of indium-tin oxide (ITO) anode and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) HIL, the metal ion diffusion into the polymer layer is initiated during the conventional device fabrication process because sulfur contained in PEDOT:PSS etches the ITO anode. The metal ions, such as indium or tin, easily migrate into EML by high electric field on the operation of QLED.
In this work, we report the enhanced efficiency of conventional structured QLED device by inserting a Al2O3 layer between PEDOT:PSS HIL and ITO anode to prevent reaction of PEDOT:PSS with ITO. Because Al2O3 has a very large band gap and a deep valence band level, the thickness of Al2O3 should be minimized not to hinder the hole injection while the Al2O3 film should uniformly cover ITO substrate in order to block the reaction of PEDOT:PSS with ITO. So we adopted atomic layer deposition (ALD) process, which makes it possible to control the thickness of Al2O3 at a monolayer level by using trimethyl aluminium (TMA) and H2O precursors at low temperatures. In order to optimize the thickness of Al2O3 barrier layer, we fabricated the conventional structured QLED devices with various thickness of Al2O3 barrier layer. As expected, the device with 1 monolayers of Al2O3, which is formed by 2 cycles of ALD process, showed the highly enhanced current efficiency and the efficiency gradually decreased as we increase the thickness of Al2O3 layer. The maximum value of the current efficiency was measured to be 33 cd/A, which is enhanced by 80 % compared with the device without Al2O3 barrier layer. We believe this result would contribute to commercialization of QLED by enhancing efficiency and improving lifetime of QLED.