三重态激子在不同荧光染料掺杂体系中的湮灭过程

室温下,在红色荧光染料掺杂的有机发光器件ITO/N,N′-bis(naphthalen-1-y)-N,N′-bis(phenyl)benzidine(NPB)/tris(8-hydroxyquinolato) aluminum (Alq3):4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran(DCM)/Alq3/LiF/Al中,观察到发光随外磁场的变化(即磁致发光)表现为刚开始的快速增加,在~50mT处达到最大后,随着磁场的进一步增加,又呈现出减弱的特点(即高场效应);而且,器件的掺杂浓度越高、所加偏压越大,该高场减弱就越明显.但在另一类绿色荧光染料5,12-dihydro-5,12-dimethylquino2,3-b]acridine-7,14-dione(DMQA)的掺杂器件中,磁致发光的高场部分则是在~50mT后增加变缓并逐渐趋于饱和.分析结果表明,Frster能量转移过程占主导发射的DMQA掺杂器件,不利于染料分子上三重态激子的形成,从而,通过三重态激子对(triplet pairs)湮灭产生单重态激子(triplet-triplet annihilation,TTA)的过程不易发生;但在载流子陷阱效应参与发射的DCM掺杂器件中,室温下在染料分子上就可以形成寿命较长的三重态激子,增加了发生TTA过程的几率.因此,基于掺杂器件中两种不同的发射机制,外加磁场对有机发光中三重态激子对(T…T)的演化表现出了不同的调控作用.

Triplet-triplet annihilation process in organic light-emitting diodes doped with fluorescent dyes

At room temperature, the magnetic field dependent electroluminescence (EL) was investigated in red fluorescent dye doped organic light-emitting diodes (OLEDs) of ITO/N,N′-bis(naphthalen-1-y)-N,N′-bis(phenyl) benzidine (NPB)/ tris(8-hydroxyquinolato) aluminum (Alq3):4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM)/ Alq3/LiF/Al. It is observed that the applied magnetic field induces a sharp increase of EL in low field regime (B≤50 mT) and then a remarkable decrease at higher fields (50 mT<B≤500 mT). Furthermore, the high-fields decrease in EL will be stronger with higher doping concentration and bias voltage. While in another type of OLEDs doped with green fluorescent dye of 5,12-dihydro-5,12-dimethylquino 2,3-b]acridine-7,14-dione (DMQA), the low-fields rapid increase is also presented while the high-fields effect exhibits a slow increase and gradual saturation. These results demonstrate that the triplet-triplet annihilation (TTA) process hardly occurred in DMQA doped devices because its Förster energy transfer process is not conducive to the generation of triplet excitons; while in DCM doped devices whose EL is dominated by charge carrier trapping process, the probability of TTA process is enhanced due to the prolonged lifetime of triplet excitons. Therefore, based on two different emission mechanisms, the applied magnetic field plays different roles in controlling the evolvement of triplet pairs (T…T) in OLEDs.