Fluorene based amorphous hole transporting materials for solution processed organic light-emitting diodes

Abstract

Abstract For an organic light-emitting diode (OLED), enhancing hole injection into the emissive layer for charge balance is a priority to achieve efficient device performance, while solution processing enables organic devices to be fabricated cost-effectively with a large area-size via continuous roll-to-roll manufacturing. However, a limited number of small molecules-based hole-transporting materials (HTMs) were reported with the solution process feasibility and high performance. Here, we demonstrate a series of low cost, efficient and solution processable small molecule HTMs, namely, 2,7-Di(4-fluorophenyl)-9,9-diethylfluorene (Fl-PyEyF), 2,7-Di(3,5-difluorophenyl)-9,9diethylfluorene (di-Fl-PyEyF) and 2,7-Di(2,4,6-trifluorophenyl)-9,9-diethylfluorene (tri-FlPyEyF), designed by using two fluorophenyl, difluorophenyl or trifluorophenyl fragments as common end capping groups with 9,9-Diethylfluorenes cores, respectively, for highly efficient OLEDs. The glass transition temperatures of the molecules were estimated to be higher than 80 °C, which can provide morphologically stable amorphous thin films. All the molecules possess good solubility in common organic solvents. Moreover, all the synthesized molecules have not only appropriate highest occupied molecular orbital energy levels for good hole injection ability, but also sufficient lowest unoccupied molecular orbital for electron blocking capability, adequate ionization potentials (6.15 eV) and suitable triplet energies, which make them suitable hole transporting materials. The current efficiency of a conventional iridium(III) bis(4phenylthieno[3,2-c]pyridinato-N,C-2՜) acetylacetonate based phosphorescent yellow OLED device increases from 40.2 to 61.0 cd/A, an increment of 51.7% by substituting the conventional HTM, N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB), with the tri-Fl-PyEyF counterpart. These findings suggest that the reported materials can serve as potential molecular HTMs in solution-processed OLED.