Phase transition thermodynamics of organic semiconductors: N,N,N’,N’-tetraphenylbenzidine and 4,4′-bis(N-carbazolyl)-1,1′-biphenyl

Abstract

In this work, we report a comprehensive study of the phase change thermodynamics of two compounds, N,N,N’,N’-tetraphenylbenzidine (TPB) and 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP), used in the production of organic light-emitting diodes and photovoltaics. The vapor pressures and phase transition enthalpies of these compounds have been previously studied at the elevated temperatures which may not be relevant to the conditions during fabrication and use TPB- and CBP-based materials. In this work, a combination of experimental and computational methods was employed to establish the reliable temperature dependences of the sublimation, fusion, and vaporization enthalpies of TPB and CBP in a wide temperature range. The vapor pressures of both compounds over solid and liquid phases were measured using fast scanning calorimetry. The fusion enthalpies at the melting point were determined by DSC. The isobaric heat capacities of the compounds in crystalline and liquid phases, including the supercooled liquids, were measured using scanning calorimetry. For the first time, the heat capacities of the ideal gas TPB and CBP were computed, considering internal rotation effects. The solution enthalpies in benzene at 298.15 K were measured, providing an independent estimate of the fusion enthalpy temperature dependence. Its consistency with the data on the fusion enthalpies was confirmed using Hess’s and Kirchhoff’s laws of Thermochemistry. Using the obtained results, the available literature data were critically evaluated. The obtained thermodynamic parameters may facilitate optimization of vapor deposition conditions and description of the thermodynamic and kinetic stability of the glassy state of TPB and CBP.