Investigation of band gaps and oscillator strengths of semiconducting polymers with quantum mechanical techniques

In recent years, polymers have started to replace metals in many applications with the discovery of their conductivity. Easy preparation, low costs, corrosion resistance and light weight of polymers make them advantageous over metals. It was first discovered in the 1950s that the polymers themselves could be conductive without being directly coupled to any metal, and since then, studies focused to improve the conductivity of the polymers. Applications of conductive polymers include organic solar cells, electronic circuits, organic light emitting diodes, actuators, chemical and bio-sensors. Conductivity in polymers is explained with the help of band gap theory. If there is an energy difference of 9 eV or more between the valence band and the conductivity band, this polymer is insulating. If this energy difference ranges from 0.1 to 4 eV, it is a sign that the polymer is a semiconductor. In conductive polymers, the valence band and the conductivity band are overlapped. Conductivity properties of polymers can be studied both experimentally and theoretically. Although reliable results are obtained in experimental techniques, the synthesis and separation processes carried out in the laboratory take long time. However, with the help of iv computers and advanced softwares, a lot of information about polymers can be easily obtained by theoretical studies. Quantum mechanical methods which are an important branch of theoretical studies may be classified as Semi-Empirical, Ab Initio and Density Functional Theory (DFT). In this study, band gaps and oscillator strengths were calculated for polyacetylene (trans and cis structured), polythiophene, poly(p-phenylene) and poly(p-phenylene vinylene) by using configuration interaction singles (CIS) method with density fitting technique (DF). These values of the polymers considered were obtained by TD-DFT (time-dependent density functional theory) technique and both methods were compared with each other and with the experimental data in the literature. The results show that while the TD-DFT technique is very close to the experimental data, the values calculated by CIS method are slightly higher than both TD-DFT values and experimental results.