Emergence of a substrate-temperature-dependent dielectric process in a prototypical vapor deposited hole-transport glass

Abstract

Since the discovery of ultrastability, vapor deposition has emerged as a relevant tool to further understand the nature of glasses. By this route, the density and average orientation of glasses can be tuned by selecting the proper deposition conditions. Dielectric spectroscopy, on the other hand, is a basic technique to study the properties of glasses at a molecular level, probing the dynamics of dipoles or charge carriers. Here, and for the first time, we explore the dielectric behavior of vapor deposited N,N-Diphenyl-N,N’bis(methylphenyl)-1,1′-biphenyl-4,4′-diamines (TPD), a prototypical hole-transport material, prepared at different deposition temperatures. We report the emergence of a new relaxation process which is not present in the ordinary glass. We associate this process to the Maxwell-Wagner polarization observed in heterogeneous systems, and induced by the enhanced mobility of charge carriers in the more ordered vapor deposited glasses. Furthermore, the associated activation energy establishes a clear distinction between two families of glasses, depending on the selected substrate-temperature range. This finding positions dielectric spectroscopy as a unique tool to investigate the structural and electronic properties of charge transport materials and remarks the importance of controlling the deposition conditions, historically forgotten in the preparation of optoelectronic devices.

Depletion of two-level systems in highly stable glasses with different molecular ordering

“…A signi cant linear coe cient on the speci c heat at the lowest temperatures, ascribed to the "universal" presence of TLS in glasses, was found to persist invariant in geologically hyperaged glasses of amber (a typical polymer glass) [30−32], when comparing the pristine samples (up to 110 My aged) to the rejuvenated samples. One possible difference to take into account between standard glasses and PVD glasses is that the former are essentially homogeneous and isotropic, whereas the latter typically exhibit anisotropic packing and layered structures [33][34][35][36] that can play a role in different properties [37][38][39][40][41][42]. In fact, the anisotropy of IMC ultrastable glasses was argued in Ref.…”

Real-time microscopy of the relaxation of a glass

“…1b to allow for equilibration of the AFM setup (Methods). At 349 ± 1 K, the transformation time of TPD into the SCL is around 270 min (t trans ≅ 2 × 10 6 τ α , where τ α is the relaxation time of the equilibrated liquid at that temperature) 38 , taking t = 0 s as the time at which this temperature is reached. This time interval offers a convenient scale to follow the kinetics of the transformation in real time, given that each AFM scan takes around 4 min to complete (Methods).…”