New electroluminescent bipolar compounds for balanced charge-transport and tuneable colour in organic light emitting diodes: triphenylamine oxadiazole-fluorene triad molecules

This work describes bipolar 2,5-diaryl-1,3,4-oxadiazole-fluorene hybrids which incorporate triphenylamine or carbazole units within the pi-electron system, viz. compounds 7, 8, 14 and 16. A related bipolar bis(oxadiazolyl) pyridine system 20 is reported. The syntheses of these five new materials are discussed, along with their optoelectronic absorption and emission properties, and their solution electrochemical redox properties. Anodic electropolymerisation of 20 was observed. Calculations using DFT (density functional theory) establish that they all possess a significantly higher HOMO energy level ( by 0.60 - 1.02 eV) than 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5- yl] benzene (OXD-7) due to the presence of electron-rich amine moieties and increased conjugation lengths, thereby leading to more balanced charge-transport characteristics. Devices were fabricated by spin-coating techniques using the bipolar compounds as the emitters in the simple device architecture ITO: PEDOT-PSS: X: Ca/Al ( X = 7, 8, 14, 16 or 20). The turn-on voltages were 2.9, 5.5, 3.6, 4.5 and 3.4 V for the devices incorporating 7, 8, 14, 16 and 20, respectively. The highest external quantum efficiency (EQE) was observed for compound 7: viz. EQE 0.36%; current efficiency 1.00 cd A(-1); power efficiency 0.56 lm W-1 at 5.7 V. The EQE of the device fabricated from 8 was considerably lower than for devices using other materials due to low light emission. The EL emission peaked at lambda(max) 430, 487, 487 and 521 nm for 8, 14 and 16, and 7, respectively. For the 20 device lambda(max) = 521 nm and 564 nm. Thus the HOMO - LUMO gap has been modified, allowing the colour of the emitted light to vary from light blue through to green by the systematic chemical modification of the molecular subunits. The high chemical and thermal durability of these materials combined with the simplicity of the device structure and low turn-on voltages offers considerable potential for OLED applications.