Improvement of efficiency and color purity utilizing two-step energy transfer for red organic light-emitting devices

We present organic red light-emitting devices with improved optical and electrical characteristics utilizing the two-step energy transfer. ${\mathrm{N,N}}^{\prime }$ -diphenyl-${\mathrm{N,N}}^{\prime }$ -bis(1-naphthyl)-${\text{(1,1}}^{\prime }$ -biphenyl)-${\text{4,4}}^{\prime }$ -diamine and tris-(8-hydroxyquinoline) aluminum (Alq) are used as hole and electron-transporting layers, respectively. Quinacridone (QAD) and 4-(dicyanomethylene)-2-t- butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) are codoped into the Alq emitting layer. Compared with devices where the emitting layer is only composed of Alq and DCJTB, the emitting characteristics such as emission efficiency and purity of emission color are greatly improved, and the turn-on voltage is decreased as much as 4 V. We attribute these improvements to the assistant dopant (QAD), which leads to the more efficient energy transfer from Alq to DCJTB.