A multi-step simulation of electron mobility in fluorene–benzothiadiazole conjugated polymer – Case study

Abstract

Recently, organic solar cells have attracted considerable attention in the field of photovoltaic devices. Charge mobility of conjugated polymer materials is one of the important factors that determine the efficiency of organic solar cells. We investigate charge transport characteristics of conjugated polymers using computational means. We employ a multi-step approach that involves the use of the density functional theory (DFT), semiempirical (ZINDO) and Monte Carlo (MC) theoretical methods within the context of a hopping (Marcus–Hush) model to simulate the electron mobility of poly(9,9′-di-n-octylfluorene-alt-benzothiadiazole) (F8BT). Taking into account the three dimensional (3D) structure of the annealed F8BT as obtained from the X-ray diffraction experiments we illustrate that the multi-step computational approach can give electron mobility values that are in good agreement with the experimental data for the ordered (crystalline) system provided the planes of the side-by-side polymer chains are tilted relative to the bottom plane (which is parallel to the substrate) of the unit cell and the π-chains stacked in a direction perpendicular to the substrate have an “alternating” structure. We also find that the inclusion of an orientational disorder in the crystal, in general, tends to reduce the electron mobility of F8BT. The proposed research is a case study. It illustrates that the use of computations/simulations based on a chemical structure of a monomer and known crystal structure of an organic semiconductor is one possible approach to studying charge mobility in organic polymers.