Cigdem Yumusak12 and Daniel A. M. Egbe2 1Department of Physics, Faculty of Arts and Sciences, Yildiz Technical University,
Davutpasa Campus, Esenler, Istanbul, 2Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University of Linz, Linz,
Since the discovery of electrical conductivity in chemically doped polyacetylene (Shirakawa et al., 1977; Chiang et al., 1977; Chiang et al., 1978), enormous progress has been made in the design, synthesis and detailed studies of the properties and applications of ^-conjugated polymers (Yu et al., 1998; Skotheim et al., 1998; Hadziioannou et al., 1998). The award of the Nobel prize in Chemistry three decades later in the year 2000 to Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa for the abovementioned discovery and development of semiconducting polymers, was greeted worldwide among researchers as a recognition for the intensified research, which has been going on in the field of organic ^-conjugated polymers (Shirakawa, 2001). Such polymers are advantageous compared to inorganic semiconductors due to their low production cost, ease of processability, flexibility as well as tenability of their optical and electronic properties through chemical modifications. These outstanding properties make them attractive candidates as advanced materials in the field of photonics and electronics (Forrest, 2004; Klauk, 2006; Bao & Locklin, 2007; Sun & Dalton, 2008; Moliton, 2006; Hadziioannou & Mallarias, 2007; Shinar & Shinar, 2009; Nalwa, 2008).
Among the most used polymers in optoelectronic devices are the poly(p-phenylene- vinylene)s (PPV), polyfluorenes, polythiophenes and their derivatives. The insertion of side-chains in these polymers reduces the rigidity of the backbone, increases their solubility and enables the preparation of films through inexpensive, solution-based methods, such as spin-coating (Akcelrud, 2003). Besides, these ramifications can also be used to tune the photophysical and electrochemical properties of these polymers using a variety of routes.
Solar cells based on solution-processable organic semiconductors have shown a considerable performance increase in recent years, and a lot of progress has been made in the understanding of the elementary processes of photogeneration (Hoppe & Sariciftci, 2004; Mozer & Sariciftci, 2006; Gunes et al., 2007). Recently, organic bulk heterojunction solar cells with almost 100% internal quantum yield were presented, resulting in up to almost 8% power conversion efficiency (Park et al., 2009; Green et al., 2010). This device concept has
been shown to be compatible with solution-processing at room temperature, for instance, by high-throughput printing techniques. Processing on flexible substrates is possible, thus allowing for roll-to-roll manufacturing as well as influencing the properties of the finished electronic devices. The recent considerable achievements in terms of power conversion efficiency have been made possible now by more than 15 year long research and development on solution-processed organic solar cells. Nevertheless, in order to let the scientific progress be followed by a commercial success, further improvements in term of efficiency and device lifetime have to be made.
In this chapter, we will briefly introduce the basic working principles of organic solar cells and present an overview of the most often studied PPV-type materials as applied within the photoactive layer.