Two hole transport materials with high glass transition temperatures (T-g similar to 200 degrees C) have been synthesized by replacing the phenyl groups of 4,4'-bis[N-(1-naphthyl-1)-N'-phenyl-amino]-biphenyl (alpha-NPD) with the bulkier phenanthrene (N,N'-di(naphthalene-1-yl)-N,N'-di(phenanthrene-9-yl)biphenyl-4,4'-diamine, NPhenD) or anthracene (N,N'-di(anthracene-9-yl)-N,N'-di(naphthalene-1-yl)biphenyl-4,4'-diamine, NAD). The organic light-emitting diodes (OLEDs) using these hole transport materials exhibited stable operation at high temperatures up to 420 K, improved device lifetimes, and reduced operating voltage changes compared to the conventional hole transport materials owing to their high T-g. Although NAD has quite small bandgap as a hole transport material, superior thermal properties of NPhenD and NAD suggest that they can be promising materials for highly stable and high temperature-durable OLEDs and other organic optoelectronic devices.