Electronic Structure, Charge-transfer and Optical Properties of Neutral, Anionic and Cationic Poly(3-hexylthiophene-2,5-diyl) (P3HT) Using DFT and TD-DFT Quantum Mechanical Methods

Abstract

Poly(3-hexylthiophene-2,5-diyl) (P3HT) is a semiconducting polymer that is  useful in photoelectric material and can be used as a promising material for applications in solar cells, light-emitting diodes, displays, or other optoelectronic devices In this work, a theoretical study of Poly(3-hexylthiophene-2,5-diyl) (P3HT) was carried out based on density functional theory (DFT) and its derivative time dependent-density functional theory (TD-DFT) as implemented in Gaussian 09 package using B3LYP/6-31++G (d, p) and B3LYP/6-31+G(d) basis sets. The properties of the molecule such as electronic properties, charge transfer properties such as frontier molecular orbitals (FMOs), ionization potential (IP), and electron affinity (EA), and optical properties such as the maximum absorption (λmax) along with oscillator strengths (f) at the excited states in vacuum and solvents were calculated and reported for the neutral, anionic, and cationic forms of the  molecule. The energy band gap  of the material was best obtained at the anionic form with a value of  using the 6-31+G(d) basis set. This small band gap of the material eases the transporting of electrons from HOMO level to LUMO level when the material absorbs the wavelength light. The NLO properties show that anionic has the highest value of total dipole moment (μtot) as 5.84116 a.u while cationic has the highest value of first order hyperpolarizability (βtot) as  (2.623x10-30 esu using 6-311++G(d,p) basis. This value is found to be seven times more than that of urea (0.3728 x 10-30 esu) which is commonly used for the comparison of NLO properties. The oscillator strength expresses the probability of absorption of electromagnetic radiation, larger oscillator strength represents larger absorption coefficient. The UV-Vis results show that thiophene solvent at excited state   corresponds to the strongest absorption at    with excitation energy of  using 6-311++G(d,p) basis set. The theoretical values of the open circuit voltage were found to be 1.685 eV for neutral, 2.729 eV for cationic, and 0.576 eV for anionic. The ir spectra results confirm the stability of the molecule with the most intense frequency of 3023cm-1 at an intensity of 91.2256km/mol for neutral molecule and the least intense frequency was found to be 1112.9091cm-1 at an intensity of 158.8877km/mol for the cationic molecule. This work could be applied in the design of  more efficient functional photovoltaic organic materials.