Density Functional Theory Study of the Structural, Electronic, Non-Linear Optical and Thermodynamic Properties of Poly (3-Hexylthiophene-2, 5 - Diyl) in Gas Phase and in Some Solvents
Authors: B. Suleiman A. , Maigari Abubakar , S. Gidado A. , E. Ndikilar Chifu 
Published: August 2022

Poly(3-hexylthiophene-2,5-diyl) (P3HT) and its derivatives are polymer based materials with π conjugation framework. P3HT is useful photoelectric material and can be used in organic semiconductor devices such as PLED, OLED, Nonlinear optical devices and solar cells. In this work, a theoretical study of P3HT in the gas phase and in some solvents (methanol, thiophene, chloroform, toluene, and acetone) were investigated and reported based on Density Functional Theory (DFT) as implemented in Gaussian 09 package using B3LYP/6-31++G (d, p) basis set. Structural properties such as bond lengths and bond angles as well as the HUMO, LUMO, energy gap, global chemical index, thermodynamic properties, NLO and DOS of the P3HT molecule in order to determine the reactivity and stability of the molecule were obtained. The results obtained showed that the solvents have effects on the structural, electronic and non-linear-optical properties of the molecule. The optimized bond length revealed that the molecule has a stronger bond in methanol with smallest bond length of about 1.0840Å (C28-H35) than in gas phase and the rest of the solvents. It was observed that the molecule is more stable in methanol with HOMO-LUMO energy gap and chemical hardness of 3.8338eV and 1.9169eV respectively. This indicates that the energy gap and chemical hardness of P3HT molecule increase with the increase in polarity and dielectric constants of the solvents. The energy gap obtained is compared with the one in literature \((3.10 \mathrm{eV})\). This indicates that the reported energy-gap leads by about \(0.7 \mathrm{eV}\). The calculations of thermodynamic properties indicate that P3HT molecule has the highest value of specific heat capacity \((\mathrm{CV})\), that is \(152.307 \mathrm{Cv}(\mathrm{Kcal} / \mathrm{Mol})\) in methanol, toluene has the highest value of entropy as \(266.960\) (Kcal/Mol), and thiophene has the highest value of zero-point vibrational energy (ZPVE) as \(455.37486 \mathrm{Kcal} / \mathrm{Mol}\). The NLO properties show that methanol has the highest value of total dipole moment ( \(\mu_{\text {tat }}\) ) as \(1.01764\) a u while acetone has the highest value of first order hyperpolarizability ( ( tot) as \(4.4447 \times 10^{-30}\) esu than the rest of the solvents. The values obtained for the first order hyperpolarizability for P3HT in acetone is about 12 times than that of the urea molecule, \(\left(0.3728 \times 10^{-30} \mathrm{esu}\right)\) which is commonly used for the comparison of NLO properties The results of the \(I R\) spectra show that the studied molecule was stable in both the gas phase and in solvents since no imaginary frequency was observed. It was also observed that the most intense frequency was found to be \(3024.9421 \mathrm{~cm}^{-1}\) at an intensity of \(140.2464 \mathrm{~km} / \mathrm{mol}\) in methanol and \(3088.6908\) \(\mathrm{cm}^{-1} at intensity of 77.6119 \mathrm{~km} / \mathrm{mol}\) in gas phase. The theoretical values of the open circuit voltage were found to be \(1.635 \mathrm{eV}, 1.614 \mathrm{eV}, 1.605 \mathrm{eV}, 1.576 \mathrm{eV}, 1514 \mathrm{eV}\), and \(1.445 \mathrm{eV}\) for methanol, toluene, acetone, chloroform, thiophene and the gas phase respectively. These changes in the properties of the molecule are observed due to the differences in the dielectric constants of the solvents. The results show that careful selection of the solvents can enhance the properties of the molecule for Organic solar cells (OSC)s applications.