Small organic semiconductor molecules are essential components of thin-film organic electronic devices, including organic light-emitting diodes (OLEDs) and organic photovoltaics (OPVs). In many cases, the intrinsic non-planarity of these molecules inhibits crystallization, resulting in glass formation. While conventional organic glasses are typically produced by cooling a liquid to bypass crystallization, alternative fabrication methods, such as thermal evaporation, enable the creation of a novel class of denser, highly stable glasses. These so-called ultrastable glasses (UGs) can be prepared within minutes, yet they achieve thermodynamic stabilities comparable to liquid-cooled glasses aged for millions of years.
UGs exhibit exceptional properties, including the suppression of two-level systems, a reduction in the boson peak at low temperatures, anomalous melting behavior, enhanced OLED stability, and slower crystallization. Additionally, because many organic semiconductor molecules are anisotropic and vapor deposition is inherently directional, vapor-deposited glasses often exhibit anisotropic molecular orientations. This anisotropy provides an additional degree of freedom for tuning material properties, influencing characteristics such as electronic and thermal transport. Moreover, for molecules with intrinsic permanent dipole moments, anisotropic alignment can generate surface potentials that profoundly impact device performance.
In this talk, I will present recent advances in our group in Barcelona on ultrastable glasses highlighting the use of experimental techniques such as quasi-adiabatic nanocalorimetry and in-operando AFM/KPFM for their characterization. Special emphasis will be placed on the relaxation dynamics of UGs as they transition into the supercooled liquid state following temperature jumps above the glass transition temperature, offering insights into their exceptional stability and transformation behavior.