**2.2. Side chain engineering**

Side chains are attached on the rigid aromatic π-backbones to form "hairy rod" type polymers with suitable solubility to allow solution based processing techniques viable. In fact, the role of the side chains are far beyond the solubility concerns. Other physical properties of the polymer, such as absorption, emission, energy levels, molecular packing, charge transport and the morphology of the thin film are critically affected by the side chains attached in many cases. Commonly used side chains include linear alkyl side chains (n-CnH2n+1), branched alkyl side chains, electron donating side chains (-OR,-SR,-NHR, etc.), electron withdrawing side chains (-C(=O)R,-SO2R, etc.), aromatic side chains (4-alkoxyphenyl, etc.), functional side chains (e.g. with cleavable groups on the side chains), ionic side chains, oligoether side chains, fluorinated side chains and so on. A comprehensive discussion on various types of side chains has been presented by Mei and Bao [87]. Side chain engineering has become a routinely used strategy to modify the physical properties, especially the self-assembly of the materials in the thin film, and thus to optimize the light absorbing materials in the OPV device. Given the importance of side chain engineering, it should be noted that as the side chains do not contribute to the light absorption or charge transport in the thin films, a trade-off between the solubility and the performance of the final polymer must be made.
