Transient Behavior And Infrared Emission Properties Of Organic Light-Emitting Diodes Based On Rare Earth Complexes

Rare earth (RE) complexes used in the field of organic light-emitting diodes (OLED) have attracted much attention because of their narrow-band emissions and miscellaneous luminous properties. Given the complete intersystem crossing from the siglet to triplet and efficient intramolecular energy transfer from triplet to center ions, one can expect excellent performance of RE-complex-containing devices. Therefore, the application of RE complexes into OLED display technology shows satisfied narrow-band emissions covering the whole visible range. Moreover, since the abundant emissions of RE ions in the infrared range, RE complexes become the candidate materials for optical communicationsin the future.For better understanding of the mechanism in the RE complexes-based devices, we investigated the transient behaviors of the common RE complexes devices. On the other hand, electroluminescence (EL) and photoluminescence (PL) properties of new infrared emissive materials, including Tm³+- and Ho³+- complexes, were also explored.The electron mobility of tris-(dibenzoylmethanato)-mono-(bathophenanthroline or 1,10-phenonthroline) RE (RE(DBM)₃bath), i.e., 8×10^-5 cm²/Vs under electric field of 1 MV/cm, was reported for the first time using transient EL measurement. Meanwhile, it was found that the electron mobility of Eu(DBM)3bath was higher than that of Gd(DBM)3bathat same electric field. The superior electron mobility of Eu complex suggested that there existed a considerable part of recombination taking place in bulk layer of RE(DBM)₃bath film which leaded to higher electron mobility which was observed from Eu(DBM)3bath, although principly the mobility of RE(DBM)3bath of different central ions should be same.Besides, we observed a strong EL spike at the end of driving pulse,which was attributed to recombination of the remaining space charges. This phenomenon will be helpful to realize the electrical pumped Eu(DBM)3bath laser diodes.EL and PL in both visible and near infrared spectral range were observed from a holmium complex, Ho(DBM)3bath, for the first time. Five peaks at 580 nm, 660 nm, 980 nm, 1200 nm and 1500 nm were attributed to the internal 4f electronical transitions of the Ho3+ ion. The 1500 nm infrared emission corresponding to the 5F5—5I6 transition suggests Ho(DBM)3bath a potential candidate for the optical communications.Another type of near infrared OEL devices was also fabricated employing Tm complexes as emitting materials. The EL peaks at 1.4 um and 0.8 um were observed from the devices based on Tm(DBM)3bath or Tm(DBM)3phen at room temperature and were assigned to 3F4—3H4 and 3F4—3H6 transitions of Tm3+ ions, respectively. By comparison with the NIR emissions of four Tm-complexes with different ligands, it was found that the first ligand played a more important role for the Tm3+ ion emissions rather than the second one. Furthermore, in order to meet the requirement of optical communication, both Tm(DBM)3bath and Er(DBM)3bath were incorporated into EL devices so that a broadened EL emission band ranging from 1.4 um to 1.6 um was obtained, showing the potential application of Tm-complexes for optical communication systems.A novel type ofvoltage-tunable infrared emitting device was proposed. There existed two emissive centers in the emission layer and performed an interesting voltage-controlled infrared emission, which could be ascribed to the non-uniform distribution of excitons in the EL device.