Glasses are amorphous materials that have been widely used in organic electronics. One important issue for organic glasses is photochemical stability, as photodegradation can cause the failure of organic electronics in both display and light harvesting technologies. Here we demonstrate that physical vapor deposition can substantially improve the photostability of organic glasses. These results provide insights that may assist in the design of organic photovoltaics and light emission devices with longer lifetimes.
Recently, physical vapor deposition (PVD) has been used to prepare glasses with exceptional properties that are not accessible by any other preparation method, including high kinetic stability and high density. In this work, we test whether the extraordinary kinetic stability and high density of PVD glasses also lead to extraordinary photostability. As a model system, we investigate the photostability of vapor-deposited and liquid-cooled glasses of Disperse Orange 37 (or DO37), an azobenzene derivative. Azobenzenes can undergo trans->cis photoisomerization reactions when irradiated by light. Photostability is assessed through changes in the density and molecular orientation of glassy thin films, as monitored by spectroscopic ellipsometry during light irradiation
By optimizing the substrate temperature used for deposition, we can increase photostability by a factor of 50 relative to the liquid-cooled glass. The density of the liquid-cooled glass decreases immediately after irradiation begins and reaches steady-state in tens of seconds. In contrast, PVD glasses can maintain their original density for hundreds to thousands of seconds, depending on the substrate temperature at which the sample was deposited.
Our collaborators, de Pablo group from the University of Chicago, performed molecular simulations of the vapor deposition and photoisomerization processes to help us understand the mechanism of enhanced photostability in PVD glasses. Their results also indicate that glasses with higher density have substantially increased photostability. Furthermore, simulations provide a molecular level explanation for the experiments, and demonstrate that vapor-deposited glasses are more photostable as it can prevent the photoreaction at least at the early stage of irradiation.