Senior Thesis Presentation
Thursday, June 4, 2020 - 14:45 to 15:15
Zoom (contact Oksana or see PH403 Canvas site)
Event Speaker: 
Ryan Tollefsen
Local Contact: 

Semiconductors have earned their place in history as the keystone to digital technology. With no foreseeable end to their employment, there remains an exhaustive search for new semiconducting materials with improved utility and ease of fabrication. This thesis investigates a promising class of solids called organic semiconductors whose unique ma- terial properties lead many to believe they can carve out a niche role in the electronics industry. However, degradation in the presence of light and oxygen currently prevents the widespread adoption of highly conductive organic molecules.

All prevailing commercial applications of organic electronics rely on encapsulation to minimize oxygen penetration. This technique has lead to many successful products, such as flexible organic light emitting diode screens and cheap organic photovoltaic solar cells. However, previous research suggests that complete encapsulation might not be necessary, as the lifespan of organic semiconductors can be dramatically improved by molecular design and tuning the local nanoenvironment.

This work aims to broaden use of the organic semiconductor pentacene (Pn) by means of improving its oxidation and photodimerization stability. In particular, we study the photophysical properties of functionalized Pn derivatives with side groups R in amor- phous thin film, referred to as Pn-R-F8 when fluorinated. Using the optics experiments presented in the following chapters, we found that side groups (tricyclohexylsilyl)-ethynyl (TCHS) are more effective at preventing degradation under visible light than side groups (triisopropylsilyl)-ethynyl (TIPS). We also demonstrate how host polymers with low oxy- gen permeability, such as poly-(methyl methacrylate) (PMMA) and poly-(vinylidene di- fluoride) (PVDF), can be used to protect organic semiconductors without complicating sample preparation. Additionally, we explore how the average intermolecular spacing of Pn-R-F8 in the host polymer matrix affects the film’s photoluminescence (PL) spectrum and photodegradation dynamics. We also found that the wavelength of incident light changes the primary chemical pathway for Pn-R-F8 decomposition, as ultraviolet (355 nm wavelength) light favors photodimerization while visible (633 nm wavelength) light catalyzes endoperoxide (EPO) formation. Finally, we demonstrate that thermolysis can reverse EPO formation in Pn-R-F8 films and restore lost functionality.