The Influence Of Charge Generation Layer On The Performance Of Tandem Organic Light-emitting Diodes

In recent years, tandem organic light-emitting diodes (OLEDs) have shownexcellent potential for industrialization due to their advantages of high brightness, highefficiency, and long lifetime under low current density, which made them become one ofmain research directions. Charge generation layer (CGL), which plays an important rolein tandem OLEDs, can generate electrons and holes under an applied electrical field.The generated electrons and holes will inject into the neighboring electron transportlayer (ETL) and hole transport layer (HTL) of the individual electroluminescent (EL)units, and then recombine with holes injected from the anode side or electrons injectedfrom the cathode side for light emission. Up to now, most of the works have focused onexploiting efficient CGL and its operational mechanism, few researches care about theinfluence of stability of CGL on the tandem device performances and the effect ofillumination on the charge generation and separation process of CGL. In order to makeclear of these problems, a series of experiments have been designed in this thesis, thedetails of research work are as follows:Firstly, the doping effect of cesium-based compounds (i.e., Cs2CO3, CsN3and CsF)on carrier transport and operational stability in organic light-emitting diodes wascompared. According to interfacial electronic structures, conductivity, and stability ofdoped electron injection layers (EILs), as well as the electrical properties,capacitance-voltage characterisitics, and stability of devices which composed of then-doped layers, it is found that the OLED device using Cs2CO3as the n-type dopant forthe EIL shows a superiority in both electrical property and operational lifetime incomparison with other cesium compounds due to their different doping mechanisms.Secondly, a series of CGLs, including Mg:Alq3(BPhen, TPBi)/MoO3, Cs2CO3:Alq3 (BPhen, TPBi)/MoO3, CsN3:Alq3/MoO3, Mg (Cs2CO3):TPBi/HAT-CN andCs2CO3/Al/MoO3were designed to understand the impact of constituent materials andstability of CGL on the tandem OLEDs performances and stability. The results ofUltraviolet and X-ray photoemission spectroscopies (UPS and XPS), device electricalproperties and stability show that the performances of tandem OLEDs are highlysensitive to the constituent materials and stability of CGL. Comparing the devicescomposed of CGLs with different p-type materials, namely MoO3and HAT-CN, we findthat device performances depend on the n-type doping materials. Tandem OLEDs usingthe same n-type dopant in different ETL, i.e., Alq3, BPhen, TPBi were also explored.The device based on BPhen which has higher electron mobility (5.2×10-4cm2V-1s-1),shows higher luminance and current efficiency. To reduce the driving voltage of tandemOLEDs, the thickness of each function layer was optimized. At last, we obtain a devicewith low driving voltages of7.45and12.67V at the current density of1and100mA/cm2, the maximun current efficiency of16.08cd/A and power efficiency of6.29lm/W, respectively. Additionally, the optimized device showed long lifetime with1600h at its luminance declined to60%of the initial value. Moreover, several tandemdevices based on Cs2CO3/Al/MoO3CGL were analysed, the results revealed that theCs2CO3/Al/MoO3is a high-efficiency CGL.Finallly, inverted and double-insulated devices with (transitional metal oxide)TMO based CGL were fabricated to investigate the influence of illumination on thecharge generation and separation process of CGL and the adjustment of electrodeinterface effect on the photoresponse of CGL. Under both dark and illuminationconditions, the J-V and C-V characteristics of devices based on Mg:Alq3/MoO3, MoO3,Mg:Alq3and none revealed that MoO3can generate and separate charge carriers, whileillumination accelerated this process. When adoped LiF and MoO3adjust anode andcathode, they can lower the work function of anode and increase the work function ofcathode, which convenient for electrons injection from the anode and holes injectionfrom the cathode, enhancing the accumulation of carriers in the device, decreasing theVoc.