Influences Of Double Electron Transport Layer On The Performance Of Solution-processed Blue PhOLEDs

Compared with inorganic electroluminescence,Organic light-emitting diodes?OLEDs?have the advantages of high color purity,wide range of material selection,fast response,relative simple preparation process and flexible preparation.Therefore,OLEDs have been developed rapidly in the past few years and applied in many fields,such as smart phones,TVs,solid-state lighting,etc.However,the properties of blue OLEDs need to be improved further to achieve the application requirements,especially the solution processed blue OLEDs.In this thesis,the host material 3,3?-bis?N-carbazolyl?biphenyl?mcp?and blue phosphorescent material bis[?4,6-difluorophenyl?-pyridina-to-N,C2]?picolinate?iridium?III??Firpic?are used as the emitting layer?EML?to prepare blue phosphorescent OLEDs?PhOLEDs?with high luminance and low turn-on voltage.Through a variety of testing methods,the internal mechanism of the improved performance of the device is explored.The main contents are as follows:?1?Based on the commonly used host material mcp and the blue phosphorescent material Firpic,PhOLEDs with the dopant emitting layer of mcp:Firpic are prepared by the solution-process method.The effects of the proportion of the guest dopant,the thickness of the electron transport layer?ETL?,the spin coating speed of the hole injection layer?HIL?and the emitting layer on the turn-on voltage,luminance,current density and current efficiency of the device are investigated based on the device structure and material mobility;?2?A set of blue PhOLEDs with 1,3,5-tris?1-phenyl-1H-benzimidazol-2-yl?benze-ne?TPBi?and Tris-?8-hydroxyquinoline?alu-minum?Alq₃?as double electron transport layers are prepared.The maximum current efficiency(CE_max)and the maximum luminance(L_max)of the improved PhOLEDs are 11.4cd/A and 13054cd/m²,respectively,which is significantly higher than that of the device with a single electron transport layer.Then,a group of devices based on holes and electrons are fabricated to confirm the hypothesis that the second electron transport layer optimizes the carrier balance.In order to further determine the internal mechanism of the improved performance,the transient electroluminescence measurement is carried out.The results show that the device with Alq₃ as the second electron transport layer can achieve the steady-state emission faster,and the instantaneous overshoot peak is significantly reduced.Combined with the energy level structure of the device,we believe that the instantaneous overshoot peak is mainly due to the release of electrons accumulated at the interface between TPBi and cathode after the driving voltage is closed.The addition of Alq₃ provides an energy level gradient for electron injection and reduces the injection barrier from cathode to TPBi.Therefore,the interface electron accumulation is suppressed.The photoluminescence?PL?decay experiments show that the interface introduced in the double electron transport layer device has an adverse effect on the exciton,which is the main reason for the more serious efficiency roll off of TPBi/Alq₃device.Therefore,it is necessary to optimize the device structure and improve the interface effect.In conclusion,the parameters and structure of blue PhOLEDs are optimized in this thesis,and the influence of double electron transport layer structure on carrier injection and transmission is verified by single carrier experiments.The mechanism of double electron transport layer structure is deeply explored by transient electroluminescence experiment,and the influence of new interface introduced by double electron transport layer structure is confirmed by PL decay experiments,which provides a theoretical basis to the double electron transport layer system with multiple perspectives.