| Organic light-emitting diodes（OLEDs）have promising applications in the fields of flat-panel displays and lighting,due to their advantages of light weight,fast response speed,self-emission,flexibility,high contrast,etc.In order to improve device performance,researchers have adopted various strategies,such as synthesizing new functional materials,designing new device structures,and studying device physics,etc.In particular,the exciton formation zone is an important factor of affecting the device performance.In OLEDs,the area of emissive layer where holes and electrons meet to form the excitons is called the exciton formation zone.The wide exciton formation zone helps alleviate triplet-triplet annihilation and triplet-polaron quenching and increase the stability of the device.In this thesis,the factors to affect the exciton formation zone in OLEDs have been studies and the relationship between the width of exciton formation zones and device performance has been investigated,helpful to guide the delicate designs of the OLED materials and structures.The main content of the thesis is as follows:（1）Explore the influences of the width of exciton formation zone on the stability of phosphorescent OLEDs.It is found that the width of exciton formation zone is increased by reducing the transit time for holes（electrons）from anode（cathode）into emissive layer via decreasing the thickness of hole（electron）transport layer.The increase of emissive layer’s thickness also widens the exciton formation zone.The efficiency roll-off of device gets relieved when the width of exciton formation zone increases.The stability of device is found directly proportional to the product of the width of exciton formation zone and electron transport layer thickness.The thicker electron transport layer better suppresses the diffusion of metal cathode into emissive layer,helpful for increasing device stability,but simultaneously increases the transit time for electrons from cathode into emissive layer,decreasing the width of exciton formation zone and thereby device stability.（2）Fabricate phosphorescent OLEDs based on organic p-doped thin films,NPB:Mo O3and CBP:Mo O3.The device using NPB:Mo O320 nm/CBP:Mo O310 nm shows increased width of exciton formation zone and thereby relieved efficiency roll-off than the one using 30 nm CBP:Mo O3.This is mostly because the NPB:Mo O3/CBP:Mo O3reduces the transit time for holes from anode to emissive layer than the CBP:Mo O3,as a result of the higher hole mobility of NPB:Mo O3than that of CBP:Mo O3.Despite markedly affecting device current,the energy barrier from NPB:Mo O3to CBP:Mo O3barely alters the efficiency roll-off and width of exciton formation zone,mostly ascribed to that it offers little effect on the transit time for holes from anode to emissive layer.（3）Investigate the influences of charge blocking layers on the performance of blue thermally activated delayed fluorescence OLEDs.It is found that the device efficiency increases with the exciton blocking capability of electron blocking layer increasing.But the device stability decreases concomitantly,mostly because the hole mobility of electron blocking layer decreases with exciton blocking capability increasing,leading to the smaller width of exciton formation zone.It is also found that the device stability increases with increasing the electron mobility of hole blocking layer,regardless of the exciton blocking capabilities.It is mostly ascribed to that the width of exciton formation zone increases with the electron mobility of hole blocking layer increasing.The width of exciton formation zone is understood based on the transit times for holes from anode to exciton formation zone and for electrons from cathode to exciton formation zone. |
