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.

The electric eld distribution in organic hetero-layer light-emitting devices based on N,N 0 -diphenyl-N,N 0 -bis(1-naphtyl)-1,1 0 -biphenyl-4,4 0 -diamine (NPB) and 8-tris-hydroxyquinoline aluminium (Alq 3 ) has beeninvestigated under di erent bias conditions using capacitance-voltage measurements. Although this method yields primarily information on the di erential capacitance, the data give clear evidence for the presence of negative interfacial charges with a density o f 6 :8 10 11 e=cm 2 at the NPB/Alq 3 interface at large reverse bias. This leads to a jump of the electric eld at the interface and a non-uniform eld distribution in the hetero-layer device.A major breakthrough in the development of organic electroluminescent devices for display applications was the demonstration of high e ciency and brightness at moderate voltages by m e a n s o f a n organic multi-layer device 1,2]. The underlying idea was to separately optimize injection and transport of holes and electrons and their radiative r e c o m bination in di erent organic layers. A common starting point for device optimization is the considera-

Electron transport in tris(8-hydroxyquinoline) aluminum (Alq3) is investigated by impedance spectroscopy under conditions of space-charge limited conduction (SCLC). Existing SCLC models are extended to include the field dependence of the charge carrier mobility and energetically distributed trap states. The dispersive nature of electron transport is revealed by a frequency-dependent mobility with a dispersion parameter alpha in the range 0.4-0.5, independent of temperature. This indicates that positional rather than energetic disorder is the dominant mechanism for the dispersive transport of electrons in Alq3.

Optically pumped organic semiconductor lasers are fabricated by evaporation of a thin film of tris(8-hydroxyquinoline) aluminum (Alq(3)) molecularly doped with a laser dye on top of a polyester substrate with an embossed grating structure. We achieve low-threshold, longitudinally monomode distributed-feedback laser operation. By varying the film thickness of the organic semiconductor film, we can tune the wavelength of the surface-emitting laser over 44 nm. The low laser threshold allows the use of a very compact all-solid-state pump laser ( approximately 10 cm long). This concept opens up a way to obtain inexpensive lasers that are tunable over the whole visible range.

Transient current-voltage characteristics of organic light-emitting diodes (OLEDs) made from both conjugated polymers and low molecular-weight materials show h ysteresis e ects in the reverse bias regime depending on the direction and speed of the bias sweep. This behaviour is quantitatively investigated here for the example of devices based on N,N'-diphenyl-N,N'-bis(1naphtyl)-1,1'-biphenyl-4,4'-diamine (NPB) with Ca and indium-tin oxide as electrodes. To clarify the origin of this peculiarity n umerical simulations have been carried out supposing the existence of deep acceptor-like trap states. Typical trends are shown by systematically varying parameters such a s m e asuring conditions, trap characteristics, basic doping level, mobility and injec-