Synergistic tuning of the optical and electrical performance of AIEgens with a hybridized local and charge-transfer excited state

Abstract

The reported organic light-emitting diodes (OLEDs) based on hybridized local and charge-transfer (HLCT) state emitters mostly exhibit low photoluminescence quantum yields (PLQYs) in aggregates, failing to achieve optimal device efficiency although they exhibit good exciton utilization efficiencies (EUEs). In this work, by introducing an aggregation-induced emission (AIE) moiety (tetraphenylethene, TPE) and a cyano group (CN) to the HLCT-typed core, phenanthroimidazole (PI), six luminescent compounds with different conjugation patterns at the C2 and N1 substituent positions were obtained, and their excited states were regulated effectively. Based on systematic photophysical analysis, the impacts of molecular conjugation patterns on the regulation of the locally excited (LE) and charge-transfer (CT) components are disclosed, and their AIE characteristics that ensure the high PLQYs of these compounds in aggregates were observed. Exciton conversion channels from triplets to singlets via the tuning of excited states were proposed based on theoretical calculations. The non-doped OLED based on ppCTPI compounds exhibited excellent performance with a maximum luminance, current efficiency, and external quantum efficiency of up to 31 070 cd m⁻², 18.46 cd A⁻¹, and 7.16%, respectively, and a very small efficiency roll-off of 4.0% at 1000 cd m⁻² luminance. The successful design of these HLCT-based AIEgens not only provides more optimization choices for OLED emitters, but also demonstrates that the strategy of reasonably superposing the AIE unit onto HLCT emitters is feasible in materials design.