Study of Electronic and Molecular Properties of Poly (3-Octylthiophene-2,5 diyl) Polymer using Density Functional Theory (DFT) And Time-Dependant Density Functional Theory (TD-DFT)

Poly(3-octylthiophene-2,5-diyl) denoted as P3OT with a chemical formula (C12H18S)n is an alkylthiophene based conductive polymer with high electroluminescence that is soluble in most of the organic solvents. P3OT is a promising material for applications in solar cells, light-emitting diodes, displays, and other optoelectronic devices due to their processability, flexibility, and low production cost. In this work, DFT and TD-DFT methods were used to investigate the molecular and electronic properties of P3OT in a vacuum (gas phase). The effect of solvent was also investigated using different models such as Polarisable Continuum Model (PCM), Continuum polarisable continuum model (C-PCM), and Integral Equation Fourier Polarisable continuum model (IEFPCM) with Chloroform, Tetrahydrofuran (THF), and Toluene as solvents. Gaussian 09 program was used for all the calculations at the DFT/B3LYP/6-311+G(d) level of theory. The results obtained for the molecular geometry show that the lowest average value of bond length in the vacuum and the medium are 1.08311Å and 1.0830Å respectively. The HOMO, LUMO and Energy-gap calculated in the vacuum are obtained as -6.44eV,-3.63eV and 2.81eV respectively while the results obtained in medium show a little effect on the HOMO, LUMO and energy gap for all the solvents. The energy gap obtained is compared with the one in literature (2.51eV).This indicates that the reported energy-gap leads by 0.3eV. From the UV-VIS spectrum, the excitation energy and oscillator strength were found to be 948.46nm and 0.0015 in the vacuum, while in the medium, the presence of solvents increase the excitation energy in the range 950.10-956.07nm and oscillator strength in the range 0.0020-0.0022. The open-circuit voltage was calculated theoretically. The result shows that the open-circuit voltage was found to be 2.711eV, this value is sufficient for possible electron injection from the molecule to the conduction band of ICBA(Indene-C60-bisadduct). Thus, it can be concluded that P3OT shows high stability and low reactivity in a chemical reaction, and due to its strong absorption properties in both a vacuum and a medium, it can be used for solar cell application.