Improving The Host And The Morphology Of The Emission Layer For Solution-processed Red Phosphorescent Organic Light-emitting Devices

Organic light-emitting diodes(OLEDs,organic light-emitting diodes)have attracted wide attention in the fields of illuminating display and solid-state lighting due to their various excellent performance,and have great potential to become the mainstream of next-generation flat panel display devices.Solution-processed OLEDs have the unique advantages including low production cost,high material utilization ratio and simple fabrication process.Herein,in this thesis,red phosphorescent OLEDs(PhOLEDs)were fabricated by the solution process,and the fabrication conditions and the emission layer were reasonably optimized.The device performance was greatly improved.Furthermore,the underlying mechanisms for the improvements were explored.The main contents are as follows:(1)We fabricated red PhOLEDs using the solution process and optimized the doping ratio of the guest material and the thickness of the electron transport layer.With increasing the doping ratio of the guest,the quenching of triplet excitons become serious,resulting in the decreased current efficiency of the corresponding device,whereas the energy transfer between the host and the guest is gradually complete.The best doping ratio of the guest is obtained,which is 6wt%.Then we studied the effect of the thickness of different electron transport layers(ETLs)on the device performance.The device performance reached the maximum at 40nm for TPBi ETL and 45nm for Tm Py PB ETL.The OLEDs exhibit the maximum brightness of 18785 cd/m~2 and 16201 cd/m~2,and maximum current efficiency of 24.8 cd/A and 25.2 cd/A for TPBi and Tm Py PB PhOLEDs,respectively.(2)We investigated the efficiency roll-off behaviors of solution-processed red PhOLEDs with different hosts.It is proved that the carrier transport ability of different hosts has a great effect on the efficiency roll-off behaviors,while the device efficiency is related to the accumulation of electrons inside the device.The use of mixed hosts with bipolar transport capability is beneficial to reduce the efficiency roll-off.However,the addition of the additional host with electron transport ability introduces additional traps,which reduces the device efficiency.Single host with hole transport ability enables higher device efficiency due to the absence of the defects,whereas it leads to obvious efficiency roll-off.Using bipolar transport host,CBP,takes these advantages to obtain a more balanced device performance.(3)We propose the mixed solvents strategy to improve the morphology of the EML film for efficient red PhOLEDs.It is found that an appropriate mixing ratio of isopropanol(IPA)and chlorobenzene(CB)in the fabrication of the EML film can effectively improve its surface morphology,which reduces the film roughness and increase the pack density.Thereby,it facilitates the injection of electrons into the EML and reduces the current leakage in the device.Meanwhile,it also reduces the internal defect states of the device and the accumulation of electrons in the EML.Hence,the device efficiency is notably improved.The red PhOLEDs fabricated with mixed solvents achieves a maximum current efficiency of 27.1cd/A and a maximum external quantum efficiency of 17.2%.