**3.3 Phenylene (benzene)**

Benzene ring is the most fundamental building block for polymer solar cell materials. A lot of chemistry and reaction carried out in this research area are rooted back to the reactivity of benzene ring. Benzene can be polymerized by direct linkage at the 1,4-position to form poly(*para*-phenylene) (Chart 2). Poly(*para*-phenylene) without any substituents has a linear rod-like structure and poor solubility in common organic solvents which limits its application as organic electronics. One strategy to increase the solubility is to introduce alkyl or alkoxyl chain on the backbone. However, the planarity of the poly(*para*-phenylene) will be disturbed due to the steric effect of the R group attached (Chart 2, P12) and therefore the effective conjugation between adjacent benzene rings will be sacrificed. To address this issue, bridges can be introduced between benzene rings, e.g., double bond in poly(phenylvinylene) (PPV)(Chart 2, P13).

Chart 2. Structures of polyphenylene and its derivatives

PPV and its derivatives have been intensively studied in organic electronics research for OLED and PSC materials due to their excellent conducting and photoluminescent properties (Burroughes et al., 1990). Poly[2-methoxy-5-((2'-ethylhexyl)oxy)-1,4- phenylenevinylene] (MEH-PPV, P14) was utilized to fabricate bilayer solar cell with C60 in the early days and it was reported that photoinduced electron transferred from electron donating MEH-PPV onto Buckminsterfullerene, C60, on a picosecond time scale (Sariciftci et al., 1992). This experiment explained one fundamental physical phenomenon present in organic photovoltaic cells and the concept developed by this study significantly inspired later research on organic solar cells.

Another derivative of PPV, poly[(2-methoxy-5-(3',7'-dimethyloctyl)oxy)-1,4-phenylene vinylene] (MDMO-PPV, Chart 2) is also widely studied for solar cells and still being used nowadays. The combination of MDMO-PPV and PCBM is used in BHJ solar cell and efficiency up to 3.1% has been reported (Tajima et al., 2008). However, the relatively low hole mobility of MDMO-PPV (5 x 10-11cm2V-1s-1) (Blom et al., 1997) is reported to limit the charge transport inside the photoactive layer. Most PPV polymers have band gap greater than 2.0 eV and have maximum absorption around 500 nm. Furthermore, PPV materials are not stable in air and vulnerable to oxygen attack. Structural defects generated either during synthesis or by oxidation will substantially degrade the performance of the device. All these factors limit the application of PPV polymers in solar cells.
