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.

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.

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Current-voltage characteristics of single and hetero-layer light-emitting devices with an aromatic diamine TPD as hole transport material and tris-8-hydroxyquinoline aluminum Alq as electron transport material and emitter have been investigated over a wide 3 temperature range and for various film thickness in order to identify the limiting mechanism: charge carrier transport or injection. From the observed thickness and temperature dependence, pure injection limitation can be ruled out as dominant mechanism. Instead, the m 1 voltage dependence of the current density can be well described by power laws j V with V corrected by the built-in potential with temperature dependent exponents m ranging from 4 to 25. This can be interpreted in terms of space charge limited currents SCLC in Alq with an exponential energetic distribution of traps where m is given by m E kT. A reasonable trap energy of 0.15-0.2 eV is 3 t obtained by a temperature dependent analysis of the I-V characteristics. However, the thickness dependence cannot be satisfactorily explained by the simple SCLC-model. This indicates that more sophisticated models are required.