**1. Introduction**

414 Solar Cells – New Aspects and Solutions

Yan, B.; Yue, G.; Xu, X.; Yang, J.; and Guha, S. (2010). High efficiency amorphous and

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nanocrystalline silicon solar cells, Physica Status Solidi A, Vol.207, No. 3, pp.671–

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*-phenylenevinylene)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; Günes 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

Organic Bulk Heterojunction Solar Cells Based on Poly(*p*-Phenylene-Vinylene) Derivatives 417

Padinger *et al.* (Padinger et al., 2003) presented a further increase in the power conversion efficiency by using a blend, which is nowadays the best investigated organic solar cell system: a poly(3-hexyl thiophene) donor (P3HT) in conjunction with PCBM. It was shown that annealing at a temperature above the glass transition of the polymer enabled an

In the following years, the power conversion efficiency could be increased steadily. This is, to a large fraction, due to the considerable amount of time that has been spent by many laboratories around the world on the optimization of bulk heterojunction solar cells—many of them using P3HT:PCBM—but also by new approaches. Additives have been used in order to allow an increased control of the phase segregation during film formation of a copolymer–fullerene blend (Park et al., 2009; Peet et al., 2007), thus yielding efficiencies of up to 6%. The process additive is a solvent for the fullerene, but not the polymer, thus allowing the PCBM an extended time for self-organization during the drying process. A positive effect by heating the solvent before the film application could also be shown (Bertho et al., 2009). Today, up to 8% power conversion efficiency are reported in this kind of

The sequential process involved in the light into electricity conversion can be summarized by the following steps: First, incident light is absorbed within the photoactive layer leading to the created of a bound electron-hole pairs (singlet excitons); the created excitons start to diffuse within the donor phase leading to charge separation; the separated charge carriers

Fig. 1. (a) Schematic device structure and (b) energy diagram for an organic bilayer solar cell Figure 1 (a) shows the simplest structure of an organic bilayer solar cell appears to be the superposition of donor and acceptor materials on top of each other, providing the interface needed to ensure the charge transfer. The schematic energy diagram of such an organic bilayer solar cell is depicted in Figure 1 (b). The excitons photogenerated in the donor or in the acceptor can diffuse to the interface where they are dissociated. According to the Onsager theory (Onsager, 1938) that can be invoked as a first approximation in organic semiconductors, photoexcited electrons and holes, by virtue of the low dielectric constant intrinsic to conjugated polymers, are coulombically bound. Due to the related exciton binding energy, which with around 0.5 eV is much larger than the thermal energy, the photogenerated excitons are not easily separated. Once excitons have been generated by the

enhancement of the efficiency from 0.4% to 3.5%.

organic solar cells (Park et al., 2009; Green et al., 2010).

**2.2 Organic bulk heterojunction solar cells** 

are transported to the corresponding electrodes.

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.
