A red‐light emitting europium complex(see Figure) is investigated here as an alternative to traditional fluorescent materials, which have an inherent electroluminescence (EL) efficiency limited to 25 % of their photoluminescence (PL) efficiency. It is shown that by optimizing the structure of Eu complex–based devices significant improvements in the EL efficiency and stability can be obtained.

High-performance phosphorescent organic light emitting devices with reduced efficiency roll-off at ultrahigh current densities were realized. The devices were Ir(ppy)3-based phosphorescent organic light emitting diodes that employed 1,3-bis[2-(2,2′-bipyridine-6-yl)-1,3,4-oxadiazo-5-yl]benzene as a high mobility electron transfer layer. The device’s brightness was enhanced while the efficiency roll-off was reduced. The device exhibits high current efficiency (21cd∕A) at high brightness (80000cd∕m2), with a maximum luminescence of 136000cd∕m2 at over 1A∕m2 (with an efficiency of 13cd∕A). This reduction in efficiency roll-off is attributed to the suppression of the triplet-polaron quenching rate through balancing the charge carrier ratio in the device.

Near-infrared (NIR) electroluminescence (EL) devices have been fabricated employing thulium complexes as emitting materials. The EL emissions at 1.4 and 0.8 μm were observed from the devices of tris-(dibenzoylmethanato)-mono-(bathophenanthroline or 1,10-phenonthroline) thulium [Tm(DBM)3bath or Tm(DBM)3phen] at room temperature and assigned to F34→H34 and F34→H36 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. In order to meet the requirement of optical communication, both Tm(DBM)3bath and erbium [Er] (DBM)3bath were incorporated into EL devices so that a broadened EL emission band ranging from 1.4 to 1.6 μm was obtained, showing the potential application of Tm complexes for optical communication systems.

Electroluminescence (EL) and photoluminescence in both the visible and near-infrared spectral range were observed from a holmium(dibenzoylmethanato)3(bathophenanthroline) [Ho(DBM)3bath]. Five peaks at 580nm, 660nm, 980nm, 1200nm, and 1500nm, respectively, were attributed to the internal 4f electronical transitions of the Ho3+ ions. Except for the emissions of the Ho3+ ions, a broadband exciplex emission from 480nmto670nm appeared in the EL cases. The emission intensity of the exciplex at organic interface showed a tendency to saturation beyond a certain driving voltage, while the emissions of the Ho3+ ions kept increasing. This evolution of visible EL spectra was discussed in terms of the extension of the charge recombination zone. The 1500nm emission corresponding to the F55→I65 transition suggests that the Ho(DBM)3bath is a potential candidate for optical communications.