Application Of Alkali Chlorides And Molybdenum Trioxide In Organic Electroluminescence

1. The effect of alkali chlorides (sodium chloride, potassium chloride, rubidium chloride and cesium chloride) on the current, luminance and efficiency etc. has been systematically studied in OLED. The based structure is ITO/NPB(40nm)/Alq3(60nm)/Al. If alkali chloride is used to modify Al cathode, the injection of minor carriers (i.e. electrons) is improved and subsequently both luminance and efficiency are enhanced. And the properties of devices with four above-mentioned materials as an electron injection layer are more or less identical. According to the relationship of injection barrierΦ,injection current density J and efficiencyη: J∝exp(-Φ/kBT)(thermal electron emission model) J∝exp(-Φ3/2/F)(quantum tunneling model) it is qualitatively explained that there is a theoretical optimum thickness of alkali chlorides as an electron injection layer. In the exception of sodium chloride (NaCl), both of luminance and efficiency are enhanced for devices with other three alkali chlorides inserted inside Alq3, due to a better balance of carriers by trapped holes induced by alkali chlorides. And the performance (mainly efficiency) becomes better as the metallic property of alkali metal is improved. The model of insulator tunneling buffer layer is suggested to explain this.The details of insulator tunneling model are that:the electron injection barrier (Φ) is shifted by the gap level of insulator layer of alkali chlorides (Eg) and electron injection current density Je is shifted for J∝exp(-Φ/kBT) or J∝exp(-Φ3/2/F), consequently the efficiency η is shifted due to η∝JhJe/(Jh+Je)2The following is in details:Potassium chloride (KCl), inserted insides Alq3, results in the decrease of device efficiency is slowed and storage stability of OLED are improved. The performance of OLEDs, based on NPB/Alq3/KCl/Alq3active regions, with various electrode structures (ITO or ITO/MoO3used as an anode and Al or lithium fluoride (LiF)/Al used as a cathode) is compared. When bare Al is used as a cathode (electron injection is poor), both of luminance and efficiency are improved by the insertion of KCl inside Alq3(anode/NPB/Alq3/KCl/Alq3/Al), compared to a control device (anode/NPB/Alq3/Al). However, if the cathode is LiF/Al (electron injection is better), the performance of control device (anode/NPB/Alq3/LiF/Al) is superior to that of devices with KCl inside Alq3(anode/NPB/Alq3/KCl/Alq3/LiF/Al). The suggested mechanism is:(1) the sites induced by KCl layer trap excess holes and the balance of charge carriers is improved, since the number of holes is more than that of electrons in the only-Al cathode devices;(2) the effect of decreasing electron injection from cathode is dominant over traping hole effect due to the inserted KCl layer, which worsens the carrier balance. Additionally, wherever KCl layer is located inside Alq3, the emitting area has not been changed through a thin layer of3nm Alq3:Rubrene (100:1) substituted for a portion of Alq3as a probe emissive layer.The insulate effect is observed when1.0nm thick RbCl is inserted insides Alq3layer.And the enhanced properties of CsCl layer, inserted at the Alq3/Al interface and inside Alq3, are similar to those of KCl and RbCl. Additionally, hole blocking effect is aroused by CsCl inside NPB; the decrease of luminance and efficiency is observed due to the exciton quenching induced by the charge transfer between Cs and Alq3when CsCl is located at the NPB/Alq3interface.2. Molybdenum trioxide (MoO3) has been used to modify ITO anode, which results in an increased hole injection, a lowered operational voltage and a reduced power-consume. The operational voltage and power-consume can be further reduced by UV-ozone treated MoO3. The sheet resistance of UV-ozone treated ITO/MoO3film is smaller than that of untreated ITO/MoO3film. The spectra of X-ray photoelectron spectroscopy (XPS) show that the binding energy of Mo, O, In and C is increased in the UV-ozone treated ITO/MoO3films.The (quasi-) Ohmic contact is formed between organic material NPB and ITO/MoO3anode and the current is due to space charge-limited current (SCLC). The hole mobility is estimated in various thicknesses of NPB by using SCLC measurement. The equation of SCLC is described by The following results are shown:hole mobility of NPB is increased with the increase of thickness and reaches a saturation when the thickness of NPB is more than300nm. The proposed explanations are as follows:(1) energy disorder is decreased with the NPB thickness and saturated at300nm. Higher energy disorder can provide more trap states resulting in slower mobility.(2) the effect of interfacial trap states on mobility becomes weaker with the NPB thickness, which can be negligible when the thickness is more than300nm.