Study of different roles phosphorescent material played in different positions of organic light emitting diodes

Phosphorescent materials are crucial to improve the luminescence and efficiency of organic light emitting diodes (OLED), because its internal quantum efficiency can reach 100%. So the studying of optical and electrical properties of phosphorescent materials is propitious for the further development of phosphorescent OLED. Phosphorescent materials were generally doped into different host materials as emitting components, not only played an important role in emitting light but also had a profound influence on carrier transport properties. We studied the optical and electrical properties of the blue 4,4′-bis(2,2-diphenylvinyl)-1,1′-biphenyl (DPVBi)-based devices, adding a common yellow phosphorescent material bis[2-(4- tert -butylphenyl)benzothiazolato- N,C 2′] iridium(acetylacetonate) [( t -bt) 2 Ir(acac)] in different positions. The results showed ( t -bt) 2 Ir(acac) has remarkable hole-trapping ability. Especially the ultrathin structure device, compared to the device without ( t -bt) 2 Ir(acac), had increased the luminance by about 60%, and the efficiency by about 97%. Then introduced thin 4,4′-bis(carbazol-9-yl)biphenyl (CBP) host layer between DPVBi and ( t -bt) 2 Ir(acac), and got devices with stable white color.