Efficient green electrophosphorescence with Al cathode using an effective electron-injecting polymer as the host

Efficient green phosphorescent polymer light-emitting diodes (PLED) based on aluminum (Al) cathode were demonstrated. The device composed of neutral aminoalkyl substituted polyfluorene, poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) as the host and an Ir complex, fac-tris(2-(4-tert-butyl)-phenylpyridine)iridium (Ir(Bu-PPy)(3)), as the dopant. High luminous efficiency of 35.7 cd/A and a luminance of more than 10 000 cd/m(2) were obtained. The device performance was comparable to those of the devices from (Ir(Bu-PPy)(3)), doped into PVK as the host with a low work-function metal as cathode (such as barium and calcium). We found out that the high-performance was due to the effective electron injection from PFN/Al bilayer cathode, improved charge balance as well as significant reduction of back-transfer of excitons from Ir complex to the low-lying PFN triplet level in the PVK/PFN interface.