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

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.


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Dielectric curves in modulus and permittivity representation  T dep (K) Figure S4. Modulus'' spectra of a glass deposited at 306 K, with order parameter close to 0, remarking the presence of two contributions in the relaxation process. We also show the spectra of a glass deposited at 292 K (negative order parameter) and 311 K (positive order parameter) to compare.

About possible effect of water absorbed in the sample
The grown samples were removed from the UHV chamber and immediately stored in vacuum bags to reduce to minimum level any possible water interaction. The samples were then removed from the vacuum bags prior to the measurement. Throughout all the process (from production to measurement) samples are exposed to ambient air only for few minutes. However, since water absorption is a delicate issue when dealing with organic samples, we provide several reasons to suggest that water is not affecting the characteristics of the measured samples: -TPD is not hydrophilic and we do not expect water absorption. The same applies to most of the molecules used in the large-scale electronic industry. Also, the S5 structure is not porous, as in many reported works dealing with the effect of water in organic systems.
-Typically, absorbed water in organic glasses (pharmaceutically glasses, mainly, which are, in most of the cases, highly hydrophilic) acts as a plasticizer, decreasing the associated relaxation time. In a calorimetric scan, absorbed water is reflected with a decrease of the onset of devitrification, with the widening of the transformation peak or, in some cases, with the appearance of additional transformation peaks in the calorimetric scan (related to different areas in the glass structure, depending on their water content). In the case of indomethacin, a pharmaceutical glass former, it was reported that each 1% of water induced a reduction of Ton of around 10 K (Journal of Pharmaceutical Sciences, 86 (3)   -Glasses deposited between Tg and around 0.8Tg are denser than the ordinary glass produced by cooling from the liquid state. In this deposition temperature range, the closer the temperature to 0.8Tg, the denser the glass (as manifested also in the previous figure with DSC scans, where lower Ton is associated with lower density). As a consequence, these glasses absorb less water than the ones prepared by cooling from the liquid. An example of this was reported for an ultrastable glass of indomethacin, a pharmaceutical material which does absorb water S7 ("Highly Stable Indomethacin Glasses Resist Uptake of Water Vapor", J. Phys. Chem. B 2009Chem. B , 113, 2422Chem. B -2427. Therefore, in the unexpected case of water absorption in TPD glasses, we would expect a larger effect in the ordinary glass rather than in the ultrastable glass (deposited at 0.86Tg, in the case of this work).
Since absorbed water increase the conductivity of the sample, its effect would go in an opposite direction as the reported in this work, i.e. glasses deposited closer to Tg, with more absorbed water, would exhibit larger values of conductivity, and the closer the Tdep to 0.85Tg, the lower the conductivity. We observe, however, the opposite result. Furthermore, in our work we also observed that an ordinary glass prepared by cooling from the liquid at ambient conditions does not show any dielectric process in the glassy state.
Furthermore, as observed in the case of indomethacin ("Highly Stable Indomethacin Glasses Resist Uptake of Water Vapor", J. Phys. Chem. B 2009Chem. B , 113, 2422Chem. B -2427, which is a pharmaceutical compound and susceptible of water absorption, the ordinary glass absorbs around 1% of water (mass %), while the ultrastable absorbs only around 0.1%.
In the case of TPD, which is not hydrophilic and not expected to interact with water, we expect much lower values (even negligible, as commented above). In some reported works concerning the effect of absorbed water on the electrical conduction properties of glasses, the samples are intentionally hydrated (several days at ambient conditions) reaching in some cases values of 20% of water in mass%, or even more. For example, in J. Phys. Chem. C, 2015, 119 (1), pp 685-694, the authors study the behaviour of hydroscopic materials with water content of around 20%, and report an enhancement of DC conductivity of 2 to 4 orders of magnitude.
As a final remark, we note that it is a general trend that vapour deposited glasses exhibit similar properties, i.e. increased density and slight molecular orientation, which depends on the deposition conditions. If the observed results were strongly affected by water, or if water were key to explain the observed results, then we would expect the same for other vapour deposited glasses, which is not the case. In particular, we do not observe DC conductivity in other vapour deposited glasses as a consequence of any possible water inclusion.