Novel Regiospecific MDMO-PPV Polymers with Improved Charge Transport Properties for Bulk Heterojunction Solar Cells

A series of novel regiospecific MDMO-PPV polymers have been synthesized by the copolymerization of the mixture of the two isomers of the asymmetrically substituted monomer via the sulphinyl precursor route. As the weight percent of either one of the two isomers is increased above ∼80 percent, the solubility of the resulting polymer is reduced. This is attributed to enhanced interchain ordering in the solid as evidenced by X-ray diffraction measurements. The hole mobility of the polymer prepared by the copolymerization of the 70:30 weight percent mixture of the two isomers (70:30 RS-MDMO-PPV) is found to be a factor of ∼3.5 higher at all measured electric fields at room temperature as compared to the regiorandom MDMO-PPV (RRa-MDMO-PPV). The electric field and temperature dependence of the hole mobility is discussed in the framework of disorder formalism and reveals an interplay between the one order of magnitude higher prefactor mobility and the slightly increased energetic disorder. Such behavior is attributed to the presence of ordered regions embedded in an otherwise amorphous matrix. The mobility is expected to increase within the ordered regions due to better electronic coupling. On the other hand, such regions may act as traps for the charge carriers, as it is supported by thermally stimulated luminescence measurements. Finally, bulk heterojunction photovoltaic devices based on the blends of 70:30 MDMO-PPV and the methanofullerene PCBM with improved power conversion efficiency and a high (0.71) filling factor have been fabricated.