Electron injection effects in electroluminescent devices using polymer blend thin films

In this paper, we discuss the effects of electron injection on eleotroluminescence (EL) efficiency in single-layer EL devices using blends of poly(N-vinylcarbazole) (PVK) and poly(3-n-butyl-p-pyridyl vinylene) (Bu-PPyV). Pure Bu-PPyV thin films suffer from formation of excimers due to the strong interchain interaction. Diluting Bu-PPyV with the high-energy-gap and hole-transporting polymer PVK suppresses excimer formation and substantially raises both photoluminescence (PL) and EL efficiencies. The emission color is converted from red to green, indicating a transition from excimer emission to monomer emission of Bu-PPyV. The electron injection and EL efficiency can be further improved by adding small-molecule oxadiazoles into the blends as electron-transport materials. In optimized devices, an external EL quantum efficiency (backside emission only) as high as 0.8% photon/electron and practical brightnesses of 100 cd/m² and 1000 cd/m² can be achieved at approximately 9 V and approximately 13.5 V, respectively.