Colour tuning of blue electroluminescence using bipolar carbazole–oxadiazole molecules in single-active-layer organic light emitting devices (OLEDs)

Abstract

A synthetically versatile strategy has been employed for luminescence colour tuning in a new series of bipolar carbazole–2,5-diaryl-1,3,4-oxadiazole hybrid molecules 1–7. Their syntheses, solution absorption and emission properties and cyclic voltammetric data are reported. Calculations using DFT (density functional theory) establish that they possess molecular orbitals which favour bipolar charge-transport. Single-active-layer organic light emitting devices (OLEDs) have been fabricated by thermal evaporation using the bipolar compounds as the emitters in the architecture ITO:PEDOT-PSS:X:Ca/Al (X = 1–7). The structure–property relationships within the series of compounds are assessed with emphasis on the OLED performance and emission colour. The HOMO–LUMO gap has been varied by systematic modifications of the molecular subunits of 1–7, allowing the colour of the electroluminescence to be tuned from deep blue (CIE x,y 0.157, 0.079) through to green (CIE x,y 0.151, 0.096). These materials are very attractive for further development due to the combination of good processability of the molecules, their bipolar structure, colour tunability and efficient performance of OLEDs using a simple device architecture.