Structure, Morphology, and Photoluminescence of Vacuum Deposited Rubrene Thin Layers

Abstract

Among organic semiconductors, rubrene (C42H28, 5,6,11,12-tetraphenyltetracene, Rub) is extensively studied in recent years, since it exhibits interesting physical properties such as very high charge carrier mobilities in organic field-effect transistors at room temperature. Rub is used as a laser dye and as a fluorescent dopant in other organic light-emitting diodes (OLEDs) to improve the characteristics such as lifetime, stability, color, and brightness. However, there are controversies as to the Rub degradation both in the bulk state and thin layers. So, the better understanding of these phenomena is crucial for improving the performance of organic devices. In this work, we present the results on complex studies of the molecular structure, morphology, and photoluminescence (PL) of Rub layers deposited on freshly cleaved KBr(100) and Au(111) substrates by vacuum thermal evaporation. Additionally, the two-component vacuum deposited (VD) Rub-tetracene (Tc) films were studied. Special attention was paid to the susceptibility of films to the oxidation under ambient conditions. The chemical structure and molecular ordering in the obtained VD thin organic films were evaluated by FTIR spectroscopy, and the structure of the very first Rub layers on atomically smooth Au(111) surfaces was studied with scanning tunneling microscopy (STM). From the changes in the peaks position and the intensity of room-temperature PL spectra of the films with time, the degradation kinetics of the Rub emission is analyzed.

Optical Properties and Stability of Bilayer Rubrene-Alq3 Films Fabricated by Vacuum Deposition

“…Note that the amorphous structure of the fabricated single-layer VD films was also confirmed in our STM studies (not shown here) and reported elsewhere [16]. A similarity between the IR spectra of the VD Rub films and those of a polycrystalline sample implies the lacking of the formation of chemical bonds between Rub molecules and the substrate, confirming the safe vacuum deposition of Rub.…”

“…This was also confirmed for the VD Rub films in our previous paper [16]. The ratio of twisted and planar isomers in the film is determined by the technological features of the preparation of samples, in particular, by the temperature of a substrate [32].…”

“…The vibrational spectra of Rub and Alq3 a b molecules were described elsewhere in our previous papers [16][17], and the assignment of their major absorption bands to characteristic vibrations of Rub and Alq3 functional groups has been reported in various papers using experimental spectra and theoretical calculations [18][19][20]. In the present paper, we use these assignments for the analysis of the FTIR spectra of our samples.…”

“…Earlier, we investigated the influence of the protective layer of tetracene on the degradation rate of the photoluminescent properties of VD Rub films [16]. It was shown that such a layer slows down, but does not completely eliminate, the process.…”