Cyano-substituted poly(p-phenylene-vinylene)s (CN-PPV) with electron deficient cyano groups on the vinyl units are synthesized by Knoevenagel polycondensation polymerization of terephthaldehyde and 1,4-bis(cyanomethyl)benzene in the presence of the base f-BuOK (Figure 12). Hence, the LUMO and HOMO levels of PPV derivatives can also be tuned by incorporating electronic substituent into the vinylene bridges (Cheng et al., 2009).
CN-PPVs show high electron affinity to reduce the barrier to electron injection and good electron-transport properties as a result of the electron-withdrawing effect of the cyano side group and suitable electron acceptors in organic photovoltaic devices (Granstrom et al., 1998; Halls et al., 1995; Gupta et al., 2007). To effectively reduce the band gap of CN-PPV below 2 eV, electron-rich thiophene units with lower aromaticities have been incorporated into the main chain to form a D-A arrangement. A series of copolymers based on the bis(1- cyano-2-thienylvinylene)pheniylene structures with different alkyl or alkoxy side chains on the thiophene rings were reported by Vanderzande et al. (Colladet et al., 2007) (Figure 13).
These monomers were all prepared by Knoevenagel condensations to construct cyanovinylene linkages. The electron-rich nature of thiophene units in these polymers makes them good candidates to serve as electron donors in organic bulk heterojunction solar cells. For example, both P11/PCBM – and P12/PCBM-based solar cells achieved a PCE of around 0.14%. Optimization of these devices by thermal annealing showed a slight increase of PCE to 0.19%. Reynold et al. also reported synthesizing a range of CN-PPV derivatives (P14-P17) containing dioxythiophene moieties in the polymer main chain (Figure 14) (Thompson et al., 2005; Thompson et al., 2006). The best photovoltaic device based on these CN-PPV derivatives with PCBM as the active layer achieved a PCE of 0.4%.