**1. Introduction**

452 Solar Cells – New Aspects and Solutions

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Energy shortage has become a worldwide issue in the 21st century (Lior, 2008). The urge to look for renewable energy to replace fossil fuel has driven substantial research effort into the energy sector (Hottel, 1989). The solar energy has enormous potential to take the place due to its vast energy stock and availability worldwide (Balzani et al., 2008). Conventional solar energy conversion device is based on silicon technology. However, wide use of silicon based solar cell technology is limited by its high power conversion cost (Wöhrle & Meissner, 1991). To address this issue, solution-processing based organic solar cell has been developed to replace Si-solar cell (Tang, 1986). Compared with conventional Si-based solar cell, conjugated polymer based solar cell (PSC) has several important advantages: 1) solution processability by spin-coating, ink-jet printing and roll-to-roll processing to reduce manufacturing cost; 2) tunable physical properties; and 3) mechanical flexibility for PSC application on curved surfaces (Sariciftci, 2004).

During the last decade, the power conversion efficiency (PCE) of organic based solar cell has increased from *ca.* 1% (Tang, 1986) to more than 7% (H. –Y. Chen et al., 2009) with the bulk heterojunction (BHJ) concept being developed and applied. During the pursuit of high efficiency, the importance of the structure-property relationship of the conjugated polymer used in the solar cell has been disclosed (J. Chen & Cao, 2009). It might be helpful to systematically summarize this structure-property relationship to guide polymer design and further improvement of the power conversion efficiency of PSCs in the future.

This chapter will be organized as follows. Firstly, we will discuss about the general criteria for a conjugated polymer to behave as an efficient sunlight absorbing agent. Secondly, we will summarize the properties of common monomer building blocks involved for construction of solar cell polymers. Only representative polymers based on the common building blocks will be discussed due to the space limit. More quality reviews and texts are directed to interested readers (C. Li, 2010; Günes et al., 2007; Sun & Sariciftci, 2005; Cheng et al., 2009).
