Research On Organic Light-emitting Devices Based On Bismuth Alloy As Cathode

In recent years,the field of organic light-emitting devices(OLEDs)has developed vigorously,and has been widely used in display and lighting fields,such as tablet computers and TVs,due to its series of advantages.It can be seen from the process of OLED industrialization that the top emission device using metal microcavity structure is the best solution to realize the flexible display of OLED.Therefore,the top electrode with high transmittance,high conductivity and high stability has become a research hotspot in this field.In this paper,the half-metal bismuth(Bi),which is compatible with the OLED preparation process and chemically inactive,is used as the core material,and a simple vacuum thermal evaporation technique is used to dope bismuth into metallic silver or metallic magnesium to form an alloy cathode.By adjusting the thickness of the electron transport layer(ETL)and hole transport layer(HTL),high-performance top-emitting devices were fabricated,and the device lifetime was significantly improved compared with conventional magnesium-silver alloy cathode devices of the same structure.First,we designed a top-emitting green OLED structure based on the existing basics in the laboratory,and doped bismuth in silver to form a silver-bismuth alloy,which was used as the semi-transparent cathode of the device.Adjust the doping ratio of the metal to make the electrode have both good conductivity and transmittance,and then adjust the thickness of the ETL and HTL to change the cavity length to optimize the performance of the device.The optimized silver-bismuth alloy cathode-based unencapsulated top-emitting green OLED has a LT50 lifetime(the time for the brightness to drop to 50%of the initial brightness of 1000 cd/m~2)of up to 120 hours and a maximum current efficiency of 62.6 cd/A.Then,under the condition of controlling other functional layers unchanged,we use magnesium-bismuth alloy as the semi-transparent cathode of the device,and adjust the thickness of the hole transport layer.After optimization,the LT50 lifetime of the top-emitting device can reach 250 hours,and the maximum current efficiency can reach65.9 cd/A.Furthermore,the angular stability of the device spectrum is improved by adding a light extraction layer.Finally,without changing other functional layers,we prepared a conventional magnesium-silver alloy cathode OLED.The LT50 life of the device was 94 hours and the maximum current efficiency was 57.8 cd/A.By comparing the optical properties,electrical properties and mechanical properties of the three electrodes,the mechanism of improving the performance of top-emitting devices by bismuth alloy cathodes is discussed.