Abstract
Organic small molecules, especially oligoacene derivatives, have been used for electronic devices, such as field-effect transistors (FET), because of their high solubility in organic solvents and chemical stability. Blends of acene derivatives within a polymer matrix have generated interest as a means to combine the carrier mobility of small molecules with the attractive properties of polymers. This blend system provides high performance with ease of processing and mechanical robustness. Whereas most previous work has considered acenes for transistors, this current research investigates the properties of acene/polymer blends as candidate materials for lasers. Specifically, we correlate the photoluminescence (PL) of blends of acene and glassy polymers with microstructure, as it is influenced by the selection of solvent and processing conditions. 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) and polystyrene were blended in an organic solvent, tetralin with one of two different grades: anhydrous and reagent-grade. It was discovered that the grade of the solvent and the age of the solution prior to film deposition have a pronounced impact on the degree of TIPS-pentacene crystallinity. Film deposition from solutions in the anhydrous solvent resulted in large, needle-like TIPS-pentacene crystals. On the other hand, the use of the reagent grade-solvent leads to a loss in crystallinity. The size and amount of crystals decrease as the reagent-grade solution ages over time. Specifically, the PL intensity from films cast from reagent-grade solutions (aged for 22 hours) was significantly stronger (by a factor of 100) than a solution of the same age cast from anhydrous solvent. 9,10-bis(triethylsilylethynyl) anthracene (TES-anthracene) and TES-anthracene/polystyrene blends were investigated for the first time in this research. It was discovered that the preparation method and the processing conditions have a significant influenced on the quantum efficiency (QE) of the blend films. The polymer dynamics adjusted by the PS Mw do not have much effect on QE. Solvent-cast films present dendritic crystals that exhibit a QE of ca. 10-15%. In contrast, after a newly-developed melt-processing method, films exhibit more dense crystals with a lack of order in TES-anthracene crystal arrangement. Interestingly, films made with low Mw polystyrene by melt-processing (160°C with 5 min. hold time) show a disordered arrangement of crystals and exhibit the strongest QE of 20%. These results provide a means to adjust the microstructure of small molecule/polymer blends and thereby to adjust their performance in lasing applications.