Synthesis And Characterization Of Novel Poly (p-phenylenevinylene) Derivatives With Oxadiazole Substituents In The Side Chain

Conjugated polymers have attracted great attention during the past decadesfor their potential applications in polymer optoelectronic devices, such aslight-emitting diodes (LEDs), photovoltaic cells (PVCs), and thin field transistors(TFTs). These conjugated polymers possess many advantages over inorganicand organic small molecule based materials, such as low cost, facile processability,and film-forming properties. Poly(p-phenylenevinylene)s (PPVs) and theirderivatives are one of the most extensively investigated polymers employed inopto-electronic devices.Carrier mobility is one of the most important parameters to evaluate organicopto-electronic materials. For example, most of PPVs derivatives are goodhole-transport materials but with low electron mobility, resulting in an imbalanceof hole and electron transport and therefore in a decrease of the efficiency ofLEDs. There have been many attempts to balance the transport of carriers fromthe two electrodes and further to improve the efficiency of PPV derivatives basedLEDs. The effective methods that have been developed are adding an electrontransport layer between the light-emitting layer and the negative electrode, ordoping additives of electron-deficient compounds such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) into the activelayer. However, the multilayer device usually results in a conspicuous increase inturn-on voltage and requires careful selection of materials. In the latter case theefficiency of LED can be limited due to the crystallization and aggregation ofPBD. In order to solve this problem, scientists tried to introduce theelectron-transporting units such as oxadiazoles (OXD) into the main chains orside chains of PPV derivatives.In this work, we synthesized two novel soluble PPV-based copolymersPoly{[2-(3-{3,5-bis[5-(4-butoxy)phenyl]-1,3,4,-oxadiazole-2-yl}phenoxy)propoxy-5-methoxy-1,4-phenylenevinylene]-co-[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene]}(4C-OXD-PPV) and Poly{[2-(3-{3,5-bis[5-(4-dodecoxy)phenyl]-1,3,4,-oxadiazole-2-yl}phenoxy)propoxy-5-methoxy-1,4-phenylenevinylene]-co-[2-methox-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]} (12C-OXD-PPV).We introduced larger branched OXD unit into the PPV side chain, which act as anelectron transporting/hole blocking group. The design of the polymer was aimedat the enhancement of the electron mobility, the increase of the interchain distance,the decrease of formation of interchain polaron pairs and then the increase of theelectroluminescence (EL) efficiency of LEDs and the energy conversionefficiency of PVCs without affecting the intrinsic properties of PPV backbone.The branched OXD units were attached as pendants to the PPV main chainthrough a linear alkyl chain. Alkoxy substituents with different length were linkedto the OXD group at position 2 which increased the solubility and prevent thematerial from crystallization of the copolymers. We investigated the fundamentalchemical and physical properties, EL properties, photovoltaic properties as well asthe important role of OXD in the improvement of EL and photovoltaic efficienciesof the polymers.The results show that the pendants did not affect the properties of the mainchain, such as UV-vis absorption wavelength, PL and EL emission wavelength.While the thermal stability of the copolymers increased due to the introduction ofthe OXD pendants. The efficiency of the two-layer EL devices of 4C-OXD-PPVand 12C-OXD-PPV were significantly higher than that of MEH-PPV under thesame fabrication and measurement conditions. PVCs with the polymers as activelayer based on 4C-OXD-PPV and 12C-OXD-PPV showed the relatively higherenergy conversion efficiency compared to the device based on MEH-PPV. Itindicated that the OXD units play important role in facilitating the electrontransport and further increasing the efficiency in both LEDs and PVCs, and thesynthesized copolymers may have potential application in LEDs and PVCs.