Effects of variation in phenylpyridinyl and di-tert-butyl-carbazolyl substituents of benzene on the performance of the derivatives in colour-tuneable white and exciplex-based sky-blue light-emitting diodes

Probing a multiple substituent approach, three derivatives of benzene substituted by phenylpyridinyl and di-tert-butyl-carbazolyl moieties are exploited with the aim to develop efficient emitters for organic light emitting diodes. The impact of the number and the nature of the substituents of benzene on the properties of the target compounds is discussed. The compound containing four tert-butyl-carbazole moieties forms a molecular glass with a very high glass transition temperature of 211 °C. The ionization potentials of the compounds range from 5.48 to 5.62 eV. The compounds show bipolar or unipolar charge transport with hole mobility reaching 4 × 10⁻⁴ cm² V⁻¹ s⁻¹ at an electric field of 9 × 10⁵ V cm⁻¹. The compounds show deep-blue fluorescence with quantum efficiency of the solid samples of up to 33%. It is shown that the emission of the compounds arises from the relaxation of hybridised local and charge transfer states. This property allows the development of colour-tuneable white organic light-emitting diodes. White electroluminescence is achieved due to overlapping of the emissions from the compounds and the different intensities of electroplex emission of the hole-transporting layer at different applied voltages. The fabricated devices reach an external quantum efficiency of 5.2%. They show an adjustable colour temperature of electroluminescence from 3339 to 6562 K. Sky-blue organic light-emitting diodes exploiting the combination of exciplex-forming properties of the compounds and thermally activated delayed fluorescence with an external quantum efficiency of 4.1% are demonstrated.