New Hole Transporting Materials Based on Di- and Tetra-Substituted Biphenyl Derivatives for Organic Light-Emitting Diodes

This study aimed to synthesize novel hole transporting materials (HTMs) with biphenyl derivatives that are di- or tetra-substituted with naphthylphenyl amine groups and/or methoxy groups, and to examine systematically the variations of the properties of the HTMs with the number and location of the substituents. The tetranaphthylphenyl amine-substituted biphenyl-based HTMs T1N and T2N were observed to have better thermal properties than the commercial HTM NPB, with decomposition temperatures above 500 degrees C, and a 10 degrees C higher T(g). In EL devices with ITO/2-TNATA-(60 nm)/HTM(15 nm)/Alq3(70 nm)/LiF(1 nm)/Al structures, the disubstituted biphenyl-based HTMs with an asymmetric molecular structure D1N and D2N were found to have inferior luminescence efficiencies when compared to NPB, which has a symmetric molecular structure. However, M1N, which is substituted with a further two methoxy groups, was found to exhibit excellent luminance and power efficiencies, 4.88 cd/A and 1.36 Im/W respectively at 100 mA/cm2, which are higher by about 147% and 127% respectively than those of NPB (3.30 cd/A and 1.07 Im/W at 100 mA/cm2), due to better charge balance.