Organic Light-emitting Diodes Prepared by All-solution Processing and the Effect of PEIE Concentration on Photoelectric Properties of Devices

Organic Light-emitting Diode (OLED) has attracted extensive attention in display and lighting due to its low power consumption and high contrast. Since a high-efficiency and low-voltage electroluminescent device firstly came out in 1987, there has been a surge of research on the performance improvement of electroluminescent devices. With the progress of processing technology, the performance of OLED has been dramatically improved. However,the production of these OLED is highly dependent on vacuum evaporation technology and requires high thermal stability of their materials. OLED prepared by solution method can overcome these limitations,while it can also present the characteristics of low cost and easy large-scale production,which can meet the demand of the future market for OLED products. Solution treatment of functional layer and matching of energy levels are the key factors to realize the luminescence of devices. However, how to achieve all-solution processed OLED has been a challenge. In the preparation of all-solution processed OLED,the unbalance of electron and hole transport will seriously affect the efficiency of light-emitting devices. Moreover, the electron mobility of most luminescent polymers is lower than the hole mobility,so electron injection is more important than hole injection. At present,the most commonly used material of the electron injection is LiF,but LiF layer is an ultra-thin film layer deposited frequently on the luminescent layer by the evaporation method,which can not be processed into a solution. Ethoxylated Polyethyleneimine (PEIE), as a widely used electron transport material with good solubility,can be processed at a relatively low temperature in ambient air and shows a suitable energy level,which is able to be a substitute for LiF. However,due to its insulating properties, the high concentration of electrons will not be able to transmit from the cathode to the luminescent layer, so exploring the appropriate concentration is a major concern to achieve efficient luminescent devices. In addition, the preparation of transparent electrodes is indispensable for the fabrication of light-emitting devices by the all-solution process. Conductive polymer poly (3,4-ethylenedioxythiophene)-polystyrene sulfonic acid (PEDOT:PSS) is one of the most important transparent conductive materials. It has great advantages that other conductive polymers cannot compare with, and is expected to become the next generation of transparent electrode materials. The conductive film prepared with PEDOT:PSS often displays high transparency,good mechanical flexibility and excellent thermal stability,which also can be dissolved well in water and polar solvents. PEDOT:PSS,however,presents a very low conductivity of less than 1 S.cm(-1), so it is necessary to improve its conductivity by resorting to post-treatment, such as an acid post-treatment onto the prepared PEDOT:PSS film. In this paper, aiming at the formation of functional layers for building an all-solution processed OLED, the emitting layer by spin-coating with a rotation speed of 1000 rpm/min shows an optimal effect of film forming, confirmed by morphology features based on ultra-deep field three-dimensional images and Atomic Force Microscope (AFM) images. Besides,due to the insulating properties of PEIE, different concentrations of PEIE as an electron transport layer can impact on the brightness and efficiency of obtained OLEDs. The measured results have shown that the brightness and efficiency of the device in the low concentrations ranging from 0.5 mg/mL to 2 mg/mL can be enhanced with the increase of concentrations, as the thin PEIE layer formed at a very low concentration is weak for the hole blocking,and cannot prevent hole leakage effectively. However,these characteristics at the high concentrations ranging from 2 mg/mL to 4 mg/mL are decreased as the concentration of PEIE increases. This is because a relatively thick PEIE layer prevents not only hole leakage but also electron injection as well at high concentrations. When the concentration of PEIE is 2 mg/mL, the hole leakage can be effectively prevented and the maximum injected electrons can enhance the balance of carrier transport. Additionally, the sheet resistance of prepared PEDOT:PSS film can be deceased by post-treatment with sulfonic acid,reaching as low as 94 Omega/square, only slightly higher than that of Indium Tin Oxide(ITO),but the transmittance of PEDOT:PSS can reach the same level with ITO electrode. Finally, on the basis of these optimal parameters for the formation of functional layers including emitting layer, electron transport layer and electrode layer,a full-solution processed OLED is prepared in this paper. The experimental results show that the maximum current efficiency is 1.441 cd/A, which are 50 times higher than that of OLEDs prepared with ITO electrode. Therefore,it is also a quite feasible to fabricate the OLEDs by all-solution method except for the vacuum evaporation technology.