Electrical transport in single-and hetero-layer organic light-emitting diodes (OLEDs) based on aromatic amines like TPD (N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'biphenyl-4,4'-diamine) or NPB (N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'-biphenyl-4,4'diamine) and the aluminium chelate complex Alq (tris(8-hydroxyquinolato)aluminium) has been investigated as a function of temperature and organic layer thickness. It is shown that the thickness dependence of the current-voltage (I ; V ) c haracteristics provides a unique criterion to discriminate between (1) injection limited behaviour, (2) trap-charge limited conduction with an exponential trap distribution and a eld-independent mobility, and (3) trap-free space-charge limited conduction (SCLC) with a eld and temperature dependent mobility.The I;V characteristics of NPB-based hole-only devices with indium-tin oxide anodes are neither purely injection nor purely space-charge limited, although the current s h o ws a square-law dependence on the applied voltage. In Al/Alq/Ca electron-only devices with Alq thickness in the range 100 to 350nm the observed thickness and temperature dependent I ; V characteristics can be described by SCLC with a hopping-type charge carrier mobility. Additionally, trapping in energetically distributed trap states is involved at low voltages and for thick l a yers. The electric eld and temperature dependence of the charge carrier mobility in Alq has been independently determined from transient electroluminescence. The obtained values of the electron mobility are consistent with temperature dependent I;V characteristics and can be described by both the phenomenological Poole-Frenkel model with a zero-eld activation energy E = 0 :4;0:5eV and the Gaussian disorder model with a disorder parameter = 100meV. Measurements of the bias-dependent capacitance in NPB/Alq hetero-layer devices give clear evidence for the presence of negative charges with a density of about 6:8 10 11 cm ;2 at the organic-organic interface under large reverse bias. This leads to a non-uniform electric eld distribution in the hetero-layer device, which has to be considered in device description.

Organic light-emitting diodes with 30% external quantum efficiency based on a horizontally oriented emitter

Organic light‐emitting diodes (OLEDs) are efficient large‐area light sources facing their market entry. Still, the development of stable and more efficient blue emitters and the enhancement of light outcoupling remain challenges for further device improvements. Here, we review the working principles of OLEDs and highlight ongoing efforts to improve their efficiency, in particular by coupling out more light.

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The distinct preferential alignment, i.e., horizontal orientation with respect to the substrate plane, of the optical transition dipole moment vectors (TDMVs) of organic dye molecules is of paramount importance for extracting the internally generated power of organic light-emitting diodes (OLEDs) to the outside world. This feature is one of the most promising approaches for the enhancement of the electrical efficacy in stateof-the-art OLEDs, as their internal quantum efficiencies are already close to the ultimate limit. If one can achieve complete horizontal orientation of the TDMVs, it is possible to increase the efficiency by at least 50% because alignment strongly influences the power dissipation into the different optical modes present in such a thin-film device. Thus, this feature of organic light-emitting molecules can lead to advanced performance for future applications. Therefore, we present here a review of recent achievements, ongoing research, and future tasks in this particular area of organic electronics.