Factors Influencing the Crystallization of Rubrene Thin Films

Abstract

A challenge in improving the performance and applicability of organic thin-film devices is how to achieve a high degree of crystallinity in the semiconducting thin film. This project investigates the annealing method of abrupt heating used to transform as-deposited amorphous rubrene films into polycrystalline films with low grain boundary density. It involves overcoming both rubrene films’ tendency for spherulitic growth upon annealing and incomplete crystallization of the entire film if the annealing temperature is too low. The project investigates a previously observed phenomenon that a 5 nm under-layer of the organic small-molecule substance TPTPA drastically improves the grain size and uniformity of the annealed rubrene film. A theory explaining spherulitic growth is borrowed to explain the role of the under-layer in terms of the additional rotational mobility the under-layer molecules can provide to the rubrene molecules. To verify this explanation, four other organic small-molecule substances were used as under-layer for rubrene, and their glass transition temperature Tg and surface energy were measured. Observation of the resulting rubrene films suggests that for a material to act as a good under-layer in facilitating rubrene crystallization, its surface energy has to be around or below 30 erg cm􀀀2 and the Tg has to be within a range of about 20 C that is significantly below the annealing temperature.