Columnar self-assembly as a design strategy for naphthophenanthridine-based blue OLEDs

Abstract

Columnar materials have lately emerged as promising materials for organic light-emitting diodes (OLEDs) owing to their unidirectional charge transport and self-assembly characteristics. In the present investigation, we report the synthesis of naphthophenanthridine-based columnar dimers with varying peripheral alkyl chain lengths (CnH2n+1; n = 4, 8, and 12), emphasising a strategic design of materials for blue OLEDs. Polarised optical microscopy and X-ray diffraction studies reveal that the dimers exhibited a columnar soft-crystalline phase. This behaviour can be ascribed to the strong π-conjugation in the naphthophenanthridine, which facilitates the rigid columnar aggregation. Exhibiting charge-carrier mobility of the order of 10−5 cm2 V−1 s−1, and favourable emission characteristics, these dimers are appropriate for light-emitting applications. Using one of the homologs, blue OLEDs have been fabricated, which demonstrated an external quantum efficiency of 2.2% and a maximum luminance of 1153 cd m−2. The moderate device efficiency is attributed to the relatively deep HOMO level and suboptimal charge balance, highlighting opportunities for further molecular and device engineering to elevate OLED performance.