Enhancing electron transport of 7-bis(diphenylphosphoryl)-9,9′-spirobifluorene through polarity reduction of polar covalent bonds using organic molecular additives

Abstract

In the electron-transporting organic semiconductor, polar covalent bonds are a necessary design to deepen the Lowest Occupied Molecular Orbitals for effective charge injection. However, these bonds can increase molecular dipole moment, which can impede charge transport. In this study, molecular additives including Tetrathiafulvalene (TTF), 4-(2,3-Dihydro-1,3-dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzenamine (N-DMBI), and tetrakis(dimethylamno)ethylene (TDAE) were investigated. Despite being electron donors, no charge transfer occurred between the molecular additives and the π conjugated core of 7-bis(diphenylphosphoryl)-9,9′-spirobifluorene (SPPO13). However, the addition of 2% TTF resulted in a higher current density, while N-DMBI and TDAE led to a decrease. X-ray photoelectron spectroscopy, density functional theory, and reduced density gradient analysis indicated that a marginal charge transfer occurred from phosphorus atoms to TTF, forming a strong attractive bond between P = O and TTF, which was absent in N-DMBI and TDAE. The low molecular polarity and lack of polar covalent bonds are prerequisites for polarity reduction of the P = O bond in SPPO13, as demonstrated by the 2% addition of TTF resulting in higher electron transport current.