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In this study, the authors show an empirical guideline for designing hole transport materials (HTMs) that suppress rises in driving voltage even with a few hundred nanometer thick film in the organic light emitting diodes (OLEDs). In a device structure of indium tin oxide (110nm)/hole transport layer (HTL) (Xnm)∕4,4′-N,N′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (10nm)/tris-(8-hydroxyquinoline)aluminum (Alq3) (50nm)∕MgAg (100nm)∕Ag (10nm), the authors compared electroluminescence characteristics of the OLEDs having a thin-film HTL (X=50nm) and a thick-film HTL (X=300nm) using 13 kinds of HTMs. They observed a closed correlation between suppression of the driving voltage and the HTMs’ thermal characteristics. Highly thermally stable HTMs resulted in a small increase in the driving voltage.

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