Hole-Transporting Materials Based on a Fluorene Unit for Efficient Optoelectronic Devices

Solution-processable hole-transporting materials (HTMs) that form highly soluble films and thermally stable amorphous states are essential for advancing optoelectronic devices. However, the currently commercialized HTM, N,N-bis(3-methylphenyl)-N,N0-bis(phenyl)benzidine (TPD), exhibits poor solubility and limited carrier transport when spin-coated into thin films. Herein, to address these issues, a fluorenyl group was ingeniously incorporated into a series of molecules structurally similar to TPD. The resulting compounds, namely, 2,7-di-(N,N-diphenylamino)-9,9-dimethyl-9H-fluorene (DDF), 2,7-di-p-tolyl-(N,N-diphenylamino)-9,9-dimethyl-9H-fluorene (2M-DDF), and 2,7-di-tetra-p-tolyl-(N,N-diphenylamino)-9,9-dimethyl-9H-fluorene (4M-DDF), offered tunable energy levels, carrier transport, crystallinity, and steric configuration via adjustment of the number of terminal methyl groups. Owing to its satisfactory performance, 2M-DDF can serve as an effective alternative to TPD in OLED devices as well as a guest molecule in host-guest systems for long-afterglow materials. Devices incorporating 2M-DDF as the HTM, with an Alq₃ emitter, achieved a maximum CE of 4.78 cd/A and a maximum <i>L</i> (<i>L</i>_max) of 21,412 cd m^-2, with a turn-on voltage (<i>V</i>_on) of 3.8 V. The luminous efficiency of 2M-DDF was approximately five times that of TPD (4106 cd m^-2). Furthermore, when 2M-DDF and TPD were utilized as guest molecules in afterglow materials, the afterglow duration of 2M-DDF (10 s) was 2.5 times that of TPD (4 s). This study provides a theoretical basis for the development of high-performance HTMs and long-afterglow materials, establishing a framework for the application of fluorene-based compounds in emerging fields such as long-afterglow materials.