Resumen de Organic vapour-deposited stable glasses: from fundamental thermal properties to high-performance organic light-emitting diodes /
Physical vapour deposition has recently emerged as an alternative route to prepare glasses that span a broad range of stabilities, together with other features. Particularly, it is possible to achieve glasses with properties that outperform conventional glasses, and that would otherwise require times from tenths to several thousands of years of slowly-cooling or ageing. For this reason, these glasses are referred as highly stable glasses or ultrastable glasses. In particular, it has been shown that for many molecular organic glass-formers, the deposition temperature plays a crucial role in determining glass properties, such as thermal stability, density or molecular orientation among others, giving the possibility to enhance the inherent instability of glasses. Vapour-deposited glasses offer new insights into the glass transition phenomenon but also potential applications in many technological processes such as in organic electronics. This work is committed to further deepen the knowledge on vapour-deposited glasses using organic semiconductor materials. We use two silicon nitride membrane-based techniques---fast-scanning quasi-adiabatic nanocalorimetry and the 3ω-Völklein method---to characterise several facets of these glasses. Firstly, we show that the most stable amorphous films are obtained when evaporated at 85 % of its corresponding glass transition temperature (Tg). Secondly, we show how vapour-deposited films transform into the supercooled liquid via a propagating growth front that starts at the highly-mobile regions (surface and interfaces). The characteristics of this mechanism are examined and rationalised regarding the different glass properties. Thirdly, we demonstrate how this heterogeneous transformation can be effectively suppressed when the high-mobility interface is capped with a lower mobility layer, gaining access to the bulk transformation. We see how the kinetic stability of the capped layers is improved using this strategy. After characterising the glass transition, we look at the thermal conductivity of these glasses. We observe how the in-plane thermal conductivity changes with the deposition temperature and we attribute this behaviour to variations in the molecular alignment. Finally, we present a simple phosphorescent organic light-emitting diode device (OLED), consisting only of two organic layers, to check the influence of the deposition temperature on the device performance. We demonstrate how its efficiency and lifetime are enhanced when its functional layers are evaporated at 0.85Tg. These results are achieved considering only the glass transition temperature and, therefore, they could be generalised to any OLED device. This work contributes to the existing knowledge of vapour-deposited glasses by providing new insights into their thermal properties and devitrification mechanisms and by exploring their potential application in the state-of-the-art OLED devices.
