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-phenyleneethynylene)-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 L 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 pi T 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, mu hole (measured using CELIV technique) for MEH- (2.8 x 10(-6) cm(2)/V center dot s) than DO-ST-PPE2-PPV2 (1.5 x 10(-6) cm(2)/V center dot s). Both values are, however, at least 2-fold lower than that of O-8-PPE2-PPV2 (5.5 x 10(-6) cm(2)/N center dot s) due to the twist in their conjugated backbone caused by the styryl side groups. Nevertheless, nonoptimized bulk heterojunction solar cells Of eta(AM1.5) around 1% were readily designed using MEH-ST-PPE2-PPV2 as donor and PCBM as acceptor in a 1:3 weight ratio.