Thickness dependent modulus of vacuum deposited organic molecular glasses for organic electronics applications

The thin film mechanical properties of a series of glassy triarylamines [4,4'-N,N'-dicarbazole-biphenyl (CBP), N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD) and N,N'-di-(1-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4-4'-diamine (NPD)], which are commonly used in organic light emitting devices (OLEDs), are examined using surface wrinkling to elucidate their elastic moduli at ambient temperature. As mechanical properties are closely associated with molecular packing and morphology, these measurements provide an effective route to study the correlation between the structure and the related thickness dependent morphology of organic thin films. For all molecular glasses examined in this article, the modulus remains statistically invariant for films thicker than 20 nm. However, a large variation in the modulus is observed for films thinner than 20 nm. This behaviour is found to be correlated with the bulk glass transition temperature (T-g) of the materials. For both CBP and TPD with T-g of approximately 60 degrees C, the modulus decreases by a factor of two when the thin film thickness is approximately 10 nm. Conversely, the modulus of NPD with T-g of 95 degrees C increases by nearly a factor of two with the decrease of film thickness to 20 nm or less, similar to the trend observed for tris(8-hydroxyquinolinato) aluminum (T-g = 175 degrees C). This work provides critical information to consider for the applications of organic electronic devices, which frequently use organic thin films with sub-50 nm thickness.