Revealing the interplay between the structural complexity of triphenylamine redox derivatives and their charge transport processes via computational modeling

Abstract

Triphenylamine derivatives (TPAs) are organic functional materials well known for their semiconducting charge transport and redox properties. These features characterize their applications in the field of organic electronics, for instance as hole transport layers for organic light emitting diodes (OLEDs), and perovskite-based solar cells (PSCs), as well as organic cathodes for electrochemical energy storage (EES) devices (e.g. organic batteries). Despite a large number of experimental and computational investigations, some structure–property relationships still remain elusive. Here, we explore through a bottom-up computational approach the molecular and solid state structures, as well as the charge transport processes in amorphous and single crystalline phases of four different redox active TPAs, characterized by increased molecular structure complexity. The TPAs considered feature one-, two- or four redox centers, namely (i) a single TPA unit, two TPAs linked via (ii) a flexible diphenyl bridge (TPD) or (iii) a rigid fluorene bridge (FTPD), and (iv) four TPAs connected via a spiro-center (spiro-OMeTAD). A combination of density functional theory, semiempirical and molecular dynamics methods is used to analyse the experimental crystalline structures, to generate the amorphous morphologies, and to calculate the charge transport parameters and hole mobility. Our results show that short- and long-range structural order in condensed phases is strongly influenced by the molecular architecture. Furthermore, charge transport parameters, such as site energies, reorganization energies and coupling integrals, are intimately coupled with the number of redox centers and the way they are connected. The charge transport is differently characterized depending on the degree of morphological disorder, namely reorganization energy-controlled transport in the crystalline phase and site-energy static-disorder controlled transport in the amorphous phase. The computed hole bulk mobilities for both single crystal and amorphous cases are in good agreement with the available experimental literature data.