Optical, Photoluminescent, and Photoconductive Properties of Novel High-Performance Organic Semiconductors

Organic semiconductors have been investigated as an alternative to inorganic semiconductors due to their low cost, ease of fabrication, and tunable properties (Forrest, 2004). Applications envisioned for organic semiconductors include xerography, thin-film transistors, light-emitting diodes, solar cells, organic lasers, and many others (Peumans et al., 2003; Murphy & Frechet, 2007; Samuel & Turnbull, 2007). Since most of these applications rely on the conductive and photoconductive properties of the materials, it is important to understand physical mechanisms of charge photogeneration, transport, trapping, and recombination. However, despite numerous theoretical and experimental studies of the optical and electronic properties of organic solids, these mechanisms are not well understood and are still the subject of debate in the literature (Sariciftci, 1997; Podzorov et al., 2004; Moses et al., 2006; Troisi & Orlandi, 2006; Coropceanu et al., 2007; Cheng & Silbey, 2008; Laarhoven et al., 2008). Indeed, it is a complicated task to reveal and utilize the intrinsic properties of organic materials, since they are often masked by the influence of impurities, the presence of which is sensitive to the methods of material purification and device fabrication. As a result, measurements performed in the same material using different experimental techniques often provide conflicting results (Nelson et al., 1998; Hegmann et al., 2002; Podzorov et al., 2003; Jurchescu et al., 2004; Lang et al., 2004; Thorsmolle et al., 2004; Ostroverkhova et al., 2005a; Ostroverkhova et al., 2006a; Koeberg et al., 2007; Laarhoven et al., 2008; Najafov et al., 2008; Marciniak et al., 2009). Experimental methods that probe charge carrier dynamics on picosecond (ps) time-scales after a 100femtosecond (fs) pulsed photoexcitation have had most success in revealing intrinsic properties of organic semiconductors (Hegmann et al., 2002; Thorsmolle et al., 2004; Hegmann et al., 2005; Ostroverkhova et al., 2005a; Ostroverkhova et al., 2005b). In contrast, techniques that probe equilibrium charge transport are much more sensitive to extrinsic effects (Nelson et al., 1998; Knipp et al., 2003; de Boer et al., 2004; Jurchescu et al., 2004). However, since properties under equilibrium conditions are relevant for most devices, it is necessary to understand how they are related to intrinsic properties and find ways to improve materials and device fabrication techniques in order to minimize extrinsic effects. Therefore, one of the emphases of this chapter is on photoexcited charge carrier dynamics from sub-ps (non-equilibrium) to many seconds (equilibrium) after photoexcitation in a variety of organic crystals and thin films (Sections 3.3-3.4).