Nondoped-type white organic electroluminescent devices utilizing complementary color and exciton diffusion

We fabricated nondoped white organic electroluminescent devices using vacuum-deposited thin films of blue-emitting ${\text{4,4}}^{\prime }-\mathrm{bis}\text{[N-1-}\mathrm{napthyl}\mathrm{-N-}\mathrm{phenyl-amino}]\mathrm{biphenyl}$ (α-NPD) and orange-emitting $\text{4-(}\mathrm{dicyanomethylene}\text{)-2-}\mathrm{metyl}\text{-6-(p-}\mathrm{dimethyl\hspace{0.17em}aminostyryl}\text{)-4H-}\mathrm{pyran}$ (DCM), a hole-blocking layer of $\text{2-(4-}\mathrm{biphenyl}\text{)-5-(p-tert-}\mathrm{butylphenyl}\text{)-1,3,4-}\mathrm{oxadiazole}$ $\mathrm{(t}\mathrm{Bu-PBD})$ and electron-transporting tris(8-quinolinolato) aluminum (III). Excitons formed at the $\mathrm{\alpha -}\mathrm{NPD}\mathrm{/t}\mathrm{Bu-PBD}$ interface sequentially transfer their energy to α-NPD via the Förster mechanism. The exciton is captured by an ultrathin DCM layer located within the pure α-NPD layer. The position of the DCM determines the device spectrum, and enables a white emission to be achieved. The spectrum is not sensitive to the voltage applied, and the devices show maximum luminance of about $\text{1000\hspace{0.17em}}{\mathrm{cd/m}}^2.$