Efficient Electroluminescence from a Quinacridone sub-monolayer inserted in a narrow exciton formation zone confined by a blocking layer

Efficient electroluminescence (EL) was obtained from quinacridone (QA) sub-monolayer inserted in a tris(8-quinolinolato) aluminum (ALQ) layer which was confined by a blocking layer 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP). The thickness of the confined ALQ layer was optimized and the highest EL current efficiency of 7.2 cd/A was obtained by inserting a 0.02 nm-thick QA layer in the 10-nm thick ALQ layer, we attribute it to the narrow exciton formation zone, in which both electron and hole carriers were confined and resulted in efficient collision capture in the exciton formation process. In addition, exciton confinement led to improved energy transfer from ALQ to QA and weaken the emission from the ALQ itself. The two factors are simultaneously operating in efficiency enhancement.