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

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.