Cyano-substituted poly(p-phenylene-vinylene)s

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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).

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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).

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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%.

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