Voltage tunable white light generation from combined emission of monomer and electromer in phenanthroimidazole based OLED

• Benzothienyl based derivative of phenanthroimidazole (1) forms a voltage tunable WOLED. • Pyridyl based derivative of phenanthroimidazole (2) forms a green OLED. • WOLEDs of 1 are due to combined electromer and monomer emission. • Strong inter-molecular interactions in films of 1 as evidenced from its morphology. • Devices of 1 show higher efficiency than that of 2 due to higher carrier mobility. Phenanthroimidazole (Phen-I) derivatives have been studied for applications in materials science and organic electronics. We recently reported the synthesis and photophysical properties of phenanthroimidazole (Phen-I) derivatives showing strong emission in the blue region. In this work, OLED device properties of two of the Phen-I derivatives are studied and compared. In these Phen-I derivatives, the N1 position is substituted by phenyl ring and the C2 position by benzothienyl in 1 and pyridyl in 2 . Devices of 1,2 were made in the following geometry ITO/PEDOT:PSS/NPD/ 1 or 2 /BPhen/LiF-Al. The electroluminescence (EL) spectrum for the device of 1 showed two main peaks centered at 495 and 620 nm. Intensity of peak centered at 620 nm increased with an increase of applied voltage. The device of 2 showed a single electroluminescent peak at 530 nm, found to be voltage independent. The voltage dependent emission at 620 nm observed in devices of 1 is attributed to the formation of an electromer indicating strong intermolecular interactions. Strong intermolecular interaction in 1 is also evident from our morphological studies in thin films showing the formation of meso -aggregates. Devices with the geometry of ITO/PEDOT:PSS/NPD/ 1 /TPBi/LiF-Al gave the most efficient devices with a current efficiency of 4.6 Cd/A at a current density of 50 mA/cm 2 . Interestingly, devices operated at low voltage gave bluish-white emission but at 24 V it produced pure white electroluminescence having CIE coordinates of (0.30, 0.35). Thus, our results show that depending on the functional group attached to the imidazole ring, thin film forming properties change. This helps to enhance intermolecular interactions resulting in an additional electromer emission leading to white OLED devices.