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Phosphorescence from an aluminum tris(quinoline-8-olate) (Alq3)-based organic light-emitting diode was observed by measuring the delayed electroluminescence at low temperatures. The spectra as well as the transient intensities were investigated. Vibronic progressions of the phosphorescence (T1→S0 transition) were resolved and the triplet energy of Alq3 was measured to be 2.05±0.1 eV. The lifetimes of the triplet state were determined from the delayed fluorescence as well as from the phosphorescence decay to be 8.1±0.5, 7.0±0.5, and 5.6±0.5 ms at 30, 50, and 80 K, respectively.

Joule heating and its impact on the brightness homogeneity are investigated since the luminance distribution is a key issue in large area organic light emitting diodes. In addition to previous reports, it is also important to consider the device temperature as a main factor for determining the luminance homogeneity. At a luminance of 1000cd∕m2 the active area reaches an average temperature of 40.6°C and a peak of 46.2°C. The increased device temperature is leading to higher local current densities resulting in a reduced brightness homogeneity. Modeling confirms these results and can be used for further device layout optimization.

This paper presents the characterization of the triplet state in tris-(8-hydroxyquinoline)aluminum (Alq3). An emission spectrum of Alq3, namely, the phosphorescence, is presented for crystalline and amorphous Alq3, which enables direct investigation of the T1→S0 transition. The assignment of this spectrum to the phosphorescence is further confirmed by temperature-dependent measurements and comparison with the delayed fluorescence. The triplet energies of the meridional and facial isomer (in α- and δ-Alq3) are determined from the well-resolved vibronic progressions of the phosphorescence as 2.11±0.1 and 2.16±0.1eV, respectively. Furthermore, the lifetime of the triplet state is measured for a temperature range from 6to150K. These temperature-dependent measurements also identify a so far unknown phase transition of Alq3 at about 50K. Optically detected magnetic resonance at a zero field is used to measure the characteristic zero-field splitting parameters (∣E∣=0.0114cm−1 and ∣D∣=0.0630cm−1), and these are discussed in terms of a molecular symmetry and contribution of different ligands and suggest a mini-exciton-like behavior of the triplet state on the three ligands of the Alq3 molecule. All these measurements are performed on different crystalline phases (α- and δ-Alq3) and on evaporated amorphous films. The differences observed for the δ-phase are consistent with the reduced intersystem crossing and thus with the facial isomer in this phase.

The effect of a magnetic field on the triplet emission in Alq3 was investigated by measuring the delayed electroluminescence (EL) and the phosphorescence in Alq3-based organic light-emitting diodes and amorphous films, respectively. The change of the delayed EL intensity in amorphous Alq3 is explained by Merrifield’s theory of mutual triplet-triplet annihilation known for molecular crystals. The intensity of the instantaneous EL shows an increase of up to +4.8% depending on the magnetic-field strength, which is due to the influence of the magnetic-field on the charge balance factor and thus on the injection of minority charge carriers.