Electron Transport Characteristics Of Alkali Metal-doped ZnO Films And Applications In Organic Optoelectronic Devices

Recently,organic electronics have attracted intensive and increasing attention due to the merits of low cost,flexibility,light weight,versatile chemical design and synthesis.Organic electronics including organic light-emitting diodes?OLEDs?,organic photovoltaic cells?OPVs?,organic field-effect transistors?OFETs?,organic photodetectors and organic memory,show great prospective application in the field of information technology.The electron injection and transport in organic electronics play important roles in the performance of the devices.Solution-processed zinc oxide?ZnO?has been considered as one of good candidates because of its beneficial properties such as high electron mobility and transparency in the visible wavelength range,as well as the compatibility with roll-to-roll fabrication onto the flexible substrate.It was found that doped ZnO had a higher charge carrier mobility and preferable energy level alignment,which facilitated the electron injection and transport.In this work,the effects of alkali metal ions such as lithium hydroxide?LiOH?,sodium hydroxide?NaOH?,potassium hydroxide?KOH?,and different anions in lithium salt?LiF,LiCl,LiBr,LiI?and 8-hydroxyquinolatolithium?Liq?on the optoelectrical performance of doped zinc oxide film were systematically investigated.The doped ZnO film has been successfully employed as electron injection layer?EIL?and the cathode buffer layer?CBL?in the OLEDs and OPVs,respectively.Firstly,three kinds of alkali metals including lithium?Li?,sodium?Na?,potassium?K?doped ZnO films were fabricated by a novel solution processing method.The surface morphologies,photophysical and electrical properties of the ZnO films without and with different alkali metal ions-doping were measured.We used alkali metal-doped ZnO films as an EIL on the tris?8-hydroxyquinoline?aluminum?Alq₃?based OLEDs to improve the electron injection from the cathode.The effects of the ZnO layer with different doping ratios of alkali metal on the performance of OLEDs have been studied.It was found that the current efficiency of OLEDs with 10%doping ratio was improved greatly,resulting in a better device performance compared with the neat ZnO layer.Furthermore,the OLED with a 10%K-doped ZnO EIL exhibited maximum current efficiency of6.6 cd A^-1 at luminance values of 100 cd m^-2.In addition,we employed alkali metal-doped ZnO as a CBL in inverted OSCs based on poly?3-hexylthiophene??P3HT?:[6,6]-phenyl-C₆₁-butyric acid methyl ester?PCBM?.In order to illustrate the device performance,the ZnO thin films without and with different alkali metal doping were investigated in detail.10%K-doped ZnO based OPV devices showed the best performance.Compared with the neat ZnO buffer layer,the device with 10%K-doped ZnO buffer layer enhanced the open circuit voltage?Voc?from 0.61 V to 0.62 V,the fill factor?FF?from 0.55 to 0.62 and the short circuit current(J_sc)from 8.37 mA cm^-2 to 8.50 mA cm^-2.Secondly,the photophysical properties of lithium halide including?LiF,LiCl,LiBr,LiI?doped solution-processed ZnO films were investigated.We used different lithium halide-doped ZnO films as EIL on the OLEDs to improve the electron injection ability.It found that 10%doping ratio of lithium halide in ZnO layer exhibited a relatively improved electron-injecting properties for each kind of halide ions.Compared with the LiF,LiCl and LiI-ZnO EIL,the use of the 10%LiBr-ZnO EIL achieved a maximum current efficiency of 6.0 cd A^-1 with a reduced efficiency roll off.In addition,we employed these doped ZnO film as CBL in inverted OPVs based on P3HT:PCBM.We found that 10%LiF-ZnO based OPVs devices showed the best performance with the Voc of 0.63 V,the FF of 0.62 and the J_sc of 8.31 mA cm^-2.Finally,the Li-ZnO CBL was prepared by inducing the Li ion diffusion of Liq into ZnO film through thermal annealing the bi-layer ZnO/Liq film.Based on the Li-ZnO CBL with ultraviolet?UV?treatment,the inverted OLEDs using Alq₃ as the emitting layer possessed an optimal current efficiency of 4.42 cd A^-1,which was improved by 55%than those of the devices with neat ZnO as CBL.The enhancement of the device performance could be attributed to the enhanced electron mobility and electron injection ability of the Li-ZnO EIL.This finding indicates that the low-temperature solution-processed alkali metal-doped ZnO films have great potential for the applications in organic optoelectronic devices.