Can hydrogen bonds improve the hole-mobility in amorphous organic semiconductors? Experimental and theoretical insights

Abstract

Five hole-transporting triphenylamine derivatives containing methoxy and methyl groups are synthesized and investigated. The hole-mobility increases in the presence of methyl and methoxy substituents, exceeding 10⁻² cm² V⁻¹ s⁻¹ in the case of methyl groups. Quantum mechanical calculations on these compounds indicate very different dipole moments and intermolecular interaction strengths, with intriguing correlations with the trend in hole-mobility. Temperature dependent hole-mobility measurements indicate disorder dominated hole transport. The values of the energetic disorder parameter (σ) decrease upon methyl and methoxy substitutions despite the increase in dipole moments. This trend is discussed as a function of the interaction energy between adjacent molecules, the dipole moment, the molecular polarizability, and the conformational degree of freedom. Our results indicate that the global decrease of σ upon methyl and methoxy substitutions is dominated by the larger decrease in the geometrical randomness component of the energetic disorder. A direct correlation is established between the decrease in geometrical randomness and the increase in intermolecular interaction energies, mainly stemming from the additional C–H⋯π, O, N hydrogen bonds induced by methyl and methoxy groups.