Film thickness dependent color purity of WOLEDs in a Phenanthroimidazole derivative due to electromers

Recently, derivatives of Phenanthroimidazole (Phen-I) based small molecules have been synthesized for OLED applications. Their photophysical characterization revealed they were fluorescent with a high quantum yield, indicating their suitability as an emissive layer (EML) in OLEDs. In this paper, we report and compare the OLED performance of two Phen-I derivatives. One contains thiophene (compound 1), and the other, newly designed and synthesized, has perylene (compound 2) as a functional group. Devices of 1 with lower EML thickness (similar to 70 nm) showed bluish white emission, while devices with higher EML thickness (> similar to 100 nm) showed color tunability from bluish-white (CIE co-ordinates: (0.26, 31)) light at lower voltage to almost pure white (CIE co-ordinates: (0.29, 32)) light at higher voltages forming WOLED. The WOLED formed for 1 at all thickness of the EML is attributed to the combined monomer and electromer emission. In addition, the voltage-dependent change in colour purity observed for higher EML thickness is attributed to increased electromer species, indicating stronger intermolecular interactions due to well-connected grains in their thin films. In contrast, devices of compound 2 showed yellow-orange emission independent of the thickness of the EML. The optimized geometry for all devices was ITO/PEDOT: PSS /NPD/1 or 2 /BPhen/LiF-Al. Devices of 2 were more efficient than those of 1, indicating that forming electromers, though advantageous in giving WOLEDs, reduces the device efficiency.