Device physics of organic light-emitting diodes based on molecular materials

Electrical transport in single- and hetero-layer organic light-emitting diodes based on aromatic amines like N,N'-diphenyl-N,N'-bis(3-methylpheny1)-1,1' -biphenyl-4,4'-diamine (TPD) or N,N'-diphenyl-N,N'-bis (1-naphthyl)-1,1'-bipheny1-4,4'-diamine (NPB) 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 (/-V) characteristics provides a unique criterion to discriminate between (1) injection limited behaviour, (2) trap-charge limited conduction with an exponential trap distribution and a field-independent mobility, and (3) trap-free space-charge limited conduction (SCLC) with a field and temperature dependent mobility. The /-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 shows a square-law dependence on the applied voltage. In Al/Alq/ Ca electron-only devices with Alq thickness in the range 100-350 nm the observed thickness and temperature dependent /-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 layers. The electric field 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 /-V characteristics and can be described by both the phenomenological Poole-Frenkel model with a zero-field activation energy AE = 0.4-0.5 eV and the Gaussian disorder model with a disorder parameter o- = 100 meV. 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 x 1011 cm-2 at the organic-organic interface under large reverse bias. This leads to a non-uniform electric field distribution in the hetero-layer device, which has to be considered in device description. 0 2001 Elsevier Science B.V. All rights reserved.