Characterisation of solution-processable organic light emitting diodes

Abstract

A range of new materials for organic light emitting diodes (OLEDs) synthesised in the Department of Chemistry at Durham University have been characterised and electroluminescent devices containing these materials have been optimised.

High triplet oxadiazole based electron transport materials were tested in devices blended with the host material poly(9-vinylcarbazole) (PVK). The materials exhibit comparable performance to standard OXD-7 in electrophosphorescent devices, while emission from exciplex devices indicate the new materials have higher LUMO energies than OXD-7.

Single layer devices containing new sky-blue iridium(III) emitters were optimised. The improved solubility of these emitters over FIrpic, the standard sky-blue emitter, resulted in improved device efficiency and brightness due to reduced aggregation, concentration quenching and self absorption in film, and higher radiative yield. Derivatives of these new emitters, with emission shifted towards a deeper blue, were characterised. Increased trapping by the PVK host led to a reduction in the device efficiency for these materials.

Two series of iridium(III) emitters with emission tuned from green to red by systematically substituted electron withdrawing or donating groups were characterised. Photophysical properties of these emitters, including the solvatochromic shift of photoluminescence spectra, correlate with theoretical values of the molecular dipole moment, thus linking changes in chemical structure with device performance.

Finally, white electroluminescence was demonstrated from single copolymers exhibiting broadened blue-green intramolecular charge transfer emission due to the interaction of fluorene (F) and dibenzothiophene-S,S-dioxide (S) units. Single layer and multilayer devices were optimised, and white emission with good spectral coverage and CIE coordinates of (0.35, 0.39) was achieved with the F/S copolymer. The emission colour varies significantly with emissive layer thickness and applied voltage. Addition of a thermally evaporated electron transport layer resulted in improvement in both device efficiency and colour stability.