Synthesis and Characterization of Thiophene Based Conjugated Polymers for Optoelectronic Applications

Abstract

In the last three decades, a great deal of attention has been focused on the synthesis of conjugated polymers, because of their significant applications in the field of optoelectronic devices. In this context, the present research work is concentrated on design and synthesis of five new series of thiophene based donor-acceptor (D−A) conjugated polymers (P1−P12) for optoelectronic applications. The chemical structures of the polymers are designed by the selection of proper electron donor and electron acceptor units and they are synthesized using multistep synthetic routes. The newly synthesized intermediate compounds, monomers and polymers are characterized by 1H NMR and FTIR spectroscopic methods as well as using elemental analyses. All the polymers are thermally stable up to 300 oC under nitrogen atmosphere. The linear optical properties of the polymers are studied by using UVVisible absorption and fluorescence emission spectroscopic techniques. The optical band gap of the polymers is in the range of 1.70 − 2.63 eV. The electrochemical redox behavior of the polymers is investigated by cyclic voltammetry. The electrochemical studies reveal that polymers posses low-lying HOMO and low-lying LUMO energy levels. The preliminary studies on the electroluminescent properties show that polymers P5, P9, P10 and P12 emit red, green, green and white light respectively with lower threshold voltages. The photovoltaic properties of the low band gap polymers P6 and P7 are evaluated by fabricating bulk heterojunction solar cells using polymer as both electron donor and acceptor material. The devices exhibit satisfactorily good power conversion efficiency with high open circuit voltage. The nonlinear optical properties of the polymers are studied using z-scan technique. They show strong optical limiting/saturable absorption behavior. Polymers P1 and P5 are used to prepare polymer/TiO2 nanocomposite films and their nonlinear optical properties are studied. The incorporation of TiO2 nanoparticles into the polymer matrix marginally enhances the nonlinear absorption of the polymer.