Study On The Performance Of High Efficiency Tandem Organic Light-emitting Devices

The tandem organic light-emitting diodes?TOLEDs?have a great prospect and practical value in the field of solid-state semiconductor's light-emitting.At the same current density,the luminance and current efficency of TOLED are much higher than that of the traditional OLED because of its multiple emitting units.However,there are also some problems need to be resolved in TOLED at present.For example,the typical active metal like Li and Cs are often introduced into the charge generation layer as an n-type dopant.However,the instability of chemical properties and long diffusion distance of metal can result in the exciton's quenching.Meanwhile the ability of charge separation and transport in the charge generation layer need to be further improved to enhance the carrier injection into the adjacent emitting unit.In addition,the current efficiency of TOLEDs will increase exponentially with the increases of the emitting units theoretically.But the improvement of power efficiency is not obvious because of the increased driving voltage with the increasing of thickness of TOLED.Therefore,it is critical to design the charge generation layer with efficient charge separation and injection for the preparation of lower driving voltage and higher efficiency TOLED.In this work,two kind of novel charge generation layer are designed by doping and non-doping method,and the efficient TOLEDs are fabricated.The main research contents are listed as follows:Firstly,we prepared a non-doped structure Cs₂CO₃/Ag/MoO₃/NPB as a charge generation layer,which exhibits a better charge separation and transport ability.By optimizing the thickness of Cs₂CO₃?0.6 nm?and Ag?6 nm?,the tandem OLED obtains an optimal electroluminescent performance.Compared with the traditional device,the maximum current efficiency of TOLED reaches to 11.2 cd/A,and the maximum power efficiency is 5 lm/W,which are about 1.7 times and 1.5 times than those of the traditional devices.It is attributed to the efficient separation of charge and carriers' injection to the adjacent emitting units respectively.Thereby the driving voltage has been reduced to 5 V.Finally,the current efficiency and power efficiency have been improved obviously.Secondly,in single emitting unit OLED,rubrene:MoO₃ is used as a hole buffer layer to improve the hole injection and transport ability.By optimizing the thickness and doping ratio of rubrene:MoO₃?thickness of 15 nm and doping concentration of 50%?,the device achieves the optimized performance.The maximum current efficiency and power efficiency are 13.6 cd/A and 7.8 lm/W,respectively.Based on the above mentioned hole buffer layer,efficient blue TOLED based on C60/rubrene:MoO₃/NPB as the charge generation layer is prepared.With Cs₂CO₃/Ag as an electron injection layer,the maximum power efficiency and current efficiency of the optimal TOLED are 16.4 lm/W and 42.8 cd/A,respectively.Meanwhile the turn-on voltage and drive voltage of the device are 5.6V and 11.5V,respectively.The experimental results indicate that C60/rubrene: MoO₃/NPB has an excellent charge separation and transport ability,and Cs₂CO₃/Ag further enhances the electron injection ability.Based on the mechanism analysis,It is found that the insertion of thin Ag layer not only reduced the work function of C60 but also smoothed the film surface.As a result,the efficient TOLED is obtained.