Optical characterization of organic semiconducting single crystals

Over the last decade interest in the possibility of "plastic electronics," semi-conducting organic materials that hold the potential for display devices with improved characteristics, has increased. To date, the majority of research has focused on material development and device design. However, if scientists are able to understand the connection between the structural and electronic properties of molecules, they can tailor new compounds with desired physical characteristics. Optical characterization is the first step to understanding the structural properties of materials. This work focuses on characterizing single crystals of two specific molecules: α-hexathiophene and 5,6,11,12-tetraphenyl tetracene (rubrene). While there is plenty of information currently available in the literature about α-hexathiophene, the resonant Raman experiments I describe in Chapter 3 have uncovered the lowest experimentally-recorded excited electronic levels. I have tentatively identified these states as either a triplet Frenkel exciton or a bound singlet Frenkel exciton. Chapter 4 details how I used Raman, infrared and photoluminescence spectroscopy, as well as computer simulations, to characterize rubrene. Studies of tetracene single crystals helped guide the understanding of the structural and electronic properties of rubrene. Although all molecular crystals have low intermolecular coupling, my analysis showed that rubrene has particularly low coupling, even for a molecular crystal. This result goes against commonly held belief that the best molecules for "plastic electronic" devices should have strong π-electron overlap. The temperature dependent photoluminescence spectrum clearly reveals emission from free carriers as well as free charge transfer excitons, free Frenkel excitons and deep impurity and defect levels