Electroluminescence-detected magnetic resonance study of OLEDs containing a mixed N,N'-diphenyl-N,N'-bis(1-naphthylphenyl)-1,1'-biphenyl-4,4'-diamine (NPB) / tris(quinolinolate) Al (Alq_3) layer

摘要

The electroluminescence (EL)-detected magnetic resonance (ELDMR) of abrupt junction and mixed layer N,N'- diphenyl-N,N'-bis(1-naphthylphenyl)-1,1'-biphenyl-4,4'-diamine (NPB) / tris(quinolinolate) Al (Alq_3) OLEDs was measured at room temperature and at 20K, at current densities 0.83 ≤ J ≤ 83 mA/cm~2. The abrupt junction and mixed layer devices were indium tin oxide (ITO) / 5 nm copper phthalocyanine (CuPc) / 50 nm NPB / 40 rim Alq_3 / 1 nm CsF / Al, and ITO / 5 nm CuPc / 40 nm NPB / 20 nm 1:1 NPB:Alq_3 / 30 nm A1q_3 / 1 nm CsF / Al, respectively. As expected, the devices exhibited a positive (EL-enhancing) spin 1/2 resonance at T = 20 K and a negative (EL-quenching) spin 1 /2 resonance at room temperature. It was found that the positive and negative resonance was stronger in the abrupt junction and in the mixed layer devices, respectively. The results are discussed in relation to the mechanisms responsible for these resonances, namely reduced quenching of singlet excitons by polarons and triplet excitons, and enhanced quenching by trions, respectively.