Effect of Styryl Side Groups on the Photophysical Properties and Hole Mobility of PPE−PPV Systems

This contribution reports on the synthesis of defect-free styryl-substituted poly(p-phenylene−ethynylene)-alt-poly(p-phenylene−vinylene)s, DO- and MEH-ST-PPE2-PPV2, consisting of a 2:2 triple bond/double bond ratio in the main chain. A detailed and systematic comparison of their photophysical properties with those of solely alkoxy-substituted polymer O-8-PPE2-PPV2, highlighting the effect of the styryl side groups, has been carried out. The bis(styryl) substituents act on the one hand as electron donor to the main chain conjugation system and, on the other hand, form a separate bis(styryl)phenylene chromophore system. The resulting two-dimensional and separate conjugation construct in ST-PPE-PPV polymers is confirmed by the fluorescence excitation anisotropy curves, which reach the value 0 around 360 nm corresponding to the absorption of the bis(styryl) system. Thin film PL and EL spectra of MEH-ST-PPE2-PPV2 are red-shifted relative to DO-ST-PPE2-PPV2, due to strong π−π interchain interactions resulting from the grafting of methoxy side groups, potentially explaining the better EL performance of MEH- than DO-ST-PPE2-PPV2 LED device (glass substrate/ITO/PEDOT:PSS/ST-PPE2-PPV2/Ca/Ag) as well as the higher intrinsic hole mobility value, μhole (measured using CELIV technique) for MEH- (2.8 × 10-6 cm2/V·s) than DO-ST-PPE2-PPV2 (1.5 × 10-6 cm2/V·s). Both values are, however, at least 2-fold lower than that of O-8-PPE2-PPV2 (5.5 × 10-6 cm2/V·s) due to the twist in their conjugated backbone caused by the styryl side groups. Nevertheless, nonoptimized bulk heterojunction solar cells of ηAM1.5 around 1% were readily designed using MEH-ST-PPE2-PPV2 as donor and PCBM as acceptor in a 1:3 weight ratio.