C−H Direct Arylated 6H-Indolo[2,3-b]quinoxaline Derivative as a Thickness-Dependent Hole-Injection Layer

A novel perfluoro-1,4-phenylenyl 6H-indolo[2,3-b]quinoxaline derivative (TFBIQ) was designed and synthesized by using a C−H direct arylation method. The optoelectrical properties of the obtained TFBIQ were fully characterized by UV/Vis spectroscopy, photoluminescence spectroscopy, cyclic voltammetry, and a group of Alq₃-based green organic light-emitting diodes (OLEDs). Device A, which used 0.5 nm-thick TFBIQ as the interfacial modification layer, exhibited the five best advantages of device performance including a minimum turn-on voltage as low as 3.1 V, a maximum luminescence intensity as high as 26564 cd m⁻², a highest current density value of 348.9 mA cm⁻² at a voltage of 11 V, the smallest efficiency roll-off, as well as the greatest power efficiency of 1.46 lm W⁻¹ relative to all of the other tested devices with thicker TFBIQ and also 10 nm-thick MoO₃ as hole-injection layers (HILs). As a promising candidate for an organic HIL material, the as-prepared TFBIQ exhibited a strong thickness effect on the performance of corresponding OLEDs. Furthermore, the theoretical calculated vertical ionization potential of the fluorinated TFBIQ suggests better anti-oxidation stability than that of the non-fluorinated structure.