Mechanistic Multiscale Simulations and Charge Transport Properties of Amorphous and Crystalline α-NPD Molecular Conformations: Insights From Molecule to Material Level

Abstract

The optoelectronic and charge transfer integral properties of N,N′-di(1-naphthyl)-N,N′-diphenyl-4,4′-diamine (α-NPD) organic light-emitting diode (OLED) in amorphous and crystalline structures is studied based on the Marcus–Levitch–Jortner theory and quantum chemistry calculations. The charge transfer integral simulations have been investigated through hole-hopping regime from molecule-to-molecule in dimers molecules and are determined by $\text{HOMO}\to \text{LUMO}$ frontier molecular orbitals (FMOs) for hole and electron transport. Quantum approaches with TD/DFT and DFT have been used to describe the most relevant electronic transitions of α-NPD, which present $\pi \to {\pi }^{*}$ character in harmony with the solvent states. Furthermore, the obtained results reveal that geometric deformations have been relied to naphthalene form and benzene rings in α-NPD structures, and the charge transfer integral in amorphous state shows $t_{\text{hole}}=4.46\mathrm{meV}$ and $t_{\text{elec}}=3.18\mathrm{meV}$, and in the crystalline state, it shows $t_{\text{hole}}=4.25\mathrm{meV}$ and $t_{\text{elec}}=3.95\mathrm{meV}$. Comparing the transfer integrals average of hole/electron in the both amorphous and crystalline states, a higher value of hole transfer is explored in the amorphous form. The charge transfer transition obtained from FMO states and density of states (DOS), as well as reorganization energies values, indicates that α-NPD would be an effective organic electronic hole transport material.