Ewa Dobruchowska

Poly(N-vinylcarbazole) doped with a pyrazoloquinoline dye: A deep blue light-emitting composite for light-emitting diode applications

We investigated the spectral properties of light-emitting diodes based on a deep blue-emitting pyrazoloquinoline dye doped into a poly(N-vinylcarbazole)-based matrix. Even though the electroluminescence (EL) of the host is redshifted and broadened with respect to the emission of the dye, the EL spectrum becomes fully dominated by the dye emission at concentrations of ca. 2wt%. This is attributed to a competition of exciplex formation on the matrix and exciton formation on the dye.

Thermoluminescence and electroluminescence of annealed polyfluorene layers

As-prepared and annealed layers of polyfluorene were studied by thermoluminescence (TL), photoluminescence (PL) and electroluminescence (EL). Upon annealing at the transition to the liquid-crystalline phase, the green emission in PL and EL becomes more pronounced. At the same time the TL spectrum broadens towards higher temperatures, indicating the introduction of additional trapping sites. In contrast, photooxidation does not broaden the TL spectrum, even so it significantly affects the emission spectra. We conclude that annealing causes the formation of non-/weakly emitting aggregates which compete with the undisturbed chains with respect to charge carrier capture and exciton recombination.

2,9-Di-3-pentyl­anthra[1,9-def:6,5,10-d′e′f′]diisoquinoline-1,3,8,10-tetrone

The asymmetric unit of the title compound, C34H30N2O4, contains four independent half-molecules, the complete molecules being generated by inversion symmetry. The molecules each have planar (within 4σ) perylenetetracarboxylic diimide fragments with bent side chains. In one of the independent molecules, each 3-pentyl fragment is disordered over two conformations in a 7:3 ratio. In the crystal, π–π interactions link molecules into stacks propagated in [010]. The crystal packing also exhibits weak intermolecular C—H⋯O hydrogen bonds.

Energy and charge transfer in polymers as seen by spectrally resolved thermoluminescence

Several applications of the thermoluminescence (TL) technique for investigations of mechanisms of charge carrier trapping and recombination in electroluminescent polymers which are used for construction of various opto-electronic devices are described. The presented TL studies were focused on polymers emitting blue light, because such emissive materials are requested to realize the white-light-emitting OLEDs or RGB-displays. We have investigated wide band-gap conjugated polymers and also non-conjugated polymers: series of polyfluorenes (homo- and copolymers) and poly(9- vinylcarbazole), neat and doped with low-molecular-weight dopants. The dopants were introduced in order to improve the charge carrier transport and also the to modify the recombination processes. It is shown, that the spectrally resolved thermoluminescence studies, especially when performed in a broad temperature range allow to characterise the nature of the trapping sites and of the recombination centres and to identify the mechanisms responsible for transfer of energy in the investigated materials.