Highly Efficient Green Phosphorescent Organic Light-Emitting Devices Based On P-type Doping Hole Transporting Layer

In recent years,organic light-emitting devices?OLEDs?exhibit many advantages such as lightness,flexibility,low-consumption and area source etc.,which will act as one of best candidates for the next generation full color display and solid light source.In OLEDs,the anode and cathode are sandwiched with organic functional layers which are mainly composed of the hole transporting layer?HTL?,emitting layer?EML?and electron transporting layer?ETL?.As one of the key components of functional layers,the hole transporting property of HTL determines the process of hole injection and transportation and to emitting layer directly,and then influences the carriers balance in the recombination process which will affect the performance of OLEDs ultimately.Due to the shortcomings of organic materials of HTL such as low carrier mobility,low conductivity and poor contact with metal electrode leading to high hole injection barrier etc.,the improvement of device performance are restricted.In order to solve the above issues,the p-type doping HTL which is fabricated by doping HTL materials with p-type dopants have been utilized widely owing to its remarkable improvement of transporting property.In this thesis,different structures of p-type hole transporting layers have been fabricated to realize better performance for green phosphorescent OLEDs based on Ir?ppy?3.The main works of this thesis are as follows:1.Green phosphorescent OLEDs based on the p-type co-doping HTL?CBP:MoO₃/CBP?with co-evaporation structure?CS?and mixed structure?MS?were fabricated.The thickness and concentration of p-type co-doping HTL were optimized firstly.For the CS device,the optimized thickness and concentration are 20 nm and 50%,and the device with the above co-doping HTL exhibits superior performance,the maximum luminance and maximum current efficiency are 97880 cd/m2 and 22 lm/W,respectively,which are 52 times and 1.3 times as large as those of the reference device;For the MS device,the optimized thickness and concentration are 30 nm and 50%,respectively,and the device with the above co-doping HTL exhibits superior performance.The maximum luminance and maximum current efficiency are 152600 cd/m2 and 49 lm/W,which were improved by 51% and 26% compared with reference device.By the characterization of corresponding hole-only devices.The enhancement of device performance can be attributed to the co-doping HTL,which can improve the hole transporting property and thus the balanced recombination of hole and election,leading to higher luminance and efficiency of devices.2.The periodic p-type interfacial doping HTL composed of [Mo O₃?3 nm?]/CBP]n have been fabricated which not only decrease the onset voltage but also improve the luminance and power efficiency of the device.Firstly,the p-type interfacial doping HTL have been optimized.Then,the enhancement of luminance and efficiency of devices are attributed to the improved hole transporting property of the p-type interfacial doping HTL.Compared with other devices?D-1,D-2 and D-4?,the three periodic structure device D-3 has the most outstanding performance,which exhisbits the best current efficiency of 56 cd/A,EQE of 15.6% and power efficiency of 45 lm/W,and at the same time,the lowest roll-off of efficiency and onset voltage of 3.2 V.The performance of D-3 can be attributed to the optimization of doping concentration and spacial distribution of Mo O₃ in HTL.Furthermore,compared with the reference device based on co-doping HTL,the maximum power efficiency of device D-3 is increased by 18%,indicating that periodic p-type interfacial HTL is better than co-doping structure HTL.Finally,the p-type doping mechanism of periodic p-type doping HTL were investigated.3.Highly efficient green phosphorescent OLEDs were fabricated based on semiconductor heterojunctions of C₆₀ and Cu Pc as the anode modified layer.Compared with the reference device based on MoO₃ as the anode modified layer,the maximum current efficiency and EQE for C₆₀?5 nm?/CuPc?25 nm?planar heterojunction modified device were improved by 12% and 11%,reaching up to 60 cd/A and 16.8%,respectively;and the values for CuPc:C₆₀?50%,30 nm?bulk heterojunction-based device were increased by 26% and 27%,arriving at 67cd/A and 19.3%,respectively.On one hand,the superior device efficiency based on heterojunction modified layer can be attributed to efficient dissociation and hole injection of accumulated charges generated in the heterojunction interfaces driven by the external electrical field;On the other hand,the higher efficiency is also attributed to the photovoltaic effect of heterojunction which can utilize the green light to produce photon-generated carriers.Owing to the more efficient charge accumulation and more proper carrier transport property attaining more balanced recombination of carriers in addition to its better photovoltaic effect,the bulk heterojunction modified device shows higher efficiency compared with the planar heterojunction modified device.