有机电致发光器件中界面激基复合物的形成

Exciplex Formation at the Interface in Organic Electroluminescent Device

在ITO/TPD/:PBD/Al双层器件中,电致发光光谱相对于TPD和PBD的荧光光谱有明显的红移。根据对TPD、PBD和TPD∶PBD等量混蒸膜的PL激发光谱及电致发光光谱的研究,认为在TPD和PBD的界面产生了激基复合物。另外,该器件发射光谱既有TPD的成分也有激基复合物的成分,并且随着电流的改变,两者的相对强度发生变化。在电流密度增大时,峰值波长向长波方向移动,器件发光颜色由白光变为黄绿光。根据器件能级关系,说明了相关的现象。

Exciplex formation at the organic interface and its potential application for emission color tuning have been investigated. The bilayer electroluminescent devices using N, N '-bis( 3-methylphenyl)-N, N'- diphenyl-[1, 1'-biphenyl]-4-4' diamine(TPD) as an hole-transport layer and 2-(4-tert-butylphenyl)-5-(4- biphenylyl)-1,3, 4-oxadiazole(PBD) as an electron-transport layer was prepared. The thickness of the TPD and PBD were both 60nm, and the cathode material is air stable aluminum. The EL spectra of this device were different from the PL spectra of TPD and PBD, showing a new band in the wavelength region of visible light together with weak emission in the wavelength of 400-430nm originating from the emission of TPD. The exciplex formation was evidenced by measurement and comparison of the photoluminescence spectra of the TPD, PBD and TPD:PBD equimolar vacuum evaporated thin films. The EL spectra were changed with different driving current densities. With an increase of the current density, the emission resulting from TPD became stronger relative to that from exciplex, when the current density was larger than 12mA/cm2, the strength of emission from TPD became smaller relative to that from exciplex. Meanwhile, the peak of the EL spectra shifted towards longer wavelength as the current density was increased. The change of EL spectra with the increase of the current density leads to change of emission color from white with the current density more than 2mA/cm2 to yellowish green at current density of 40mA/cm2. Consideration of the energy levels of TPD(LUMO:2.4eV;HOMO:5.5eV) and PBD(LUMO:2.8eV, HOMO:6.2eV) suggests that the energy barrier for the electrons injection from the PBD layer into the TPD layer is much lower than that for the holes injection from the TPD layer to the PBD layer. Therefore, it is suggested that the exciplex is formed by the interaction between the electronically excited singlet-state generated by recombination of holes and electrons in the bulk of TPD layer and the ground-state of PBD. The maximum luminance of this bilayer device is 149cd/m2 at a bias voltage of 18.6V.