Impact of Molecular Orientation and Packing Density on Electronic Polarization in the Bulk and at Surfaces of Organic Semiconductors

Abstract

The polarizable environment surrounding charge carriers in organic semiconductors impacts the efficiency of the charge transport process. Here, we consider two representative organic semiconductors, tetracene and rubrene, and evaluate their polarization energies in the bulk and at the organic-vacuum interface using a polarizable force field that accounts for induced-dipole and quadrupole interactions. Though both oligoacenes pack in a herringbone motif, the tetraphenyl substituents on the tetracene backbone of rubrene alter greatly the nature of the packing. The resulting change in relative orientations of neighboring molecules is found to reduce the bulk polarization energy of holes in rubrene by some 0.3 eV when compared to tetracene. The consideration of model organic-vacuum interfaces highlights the significant variation in the electrostatic environment for a charge carrier at a surface although the net change in polarization energy is small; interestingly, the environment of a charge even just one layer removed from the surface can be viewed already as representative of the bulk. Overall, it is found that in these herringbone-type layered crystals the polarization energy has a much stronger dependence on the intralayer packing density than interlayer packing density.

Concentration dependent energy levels shifts in donor-acceptor mixtures due to intermolecular electrostatic interaction

“…While such models are widely used to predict properties of single isolated molecules they still struggle with the accurate prediction of bulk properties of ordered and disordered materials or material interfaces. QM/MM models were used to quantify systematic energy level difference between isolated molecules in vacuum and bulk materials 21–25 , while other studies focused on the energy level alignment in mixed materials and crystalline interfaces 15,20 . In some scenarios the polarizability of molecules was shown to be the main reason for energy level differences between molecules in vacuum and bulk materials 22,23 while electrostatic properties such as quadrupole moments dominate energy level shifts at ordered interfaces 20 .…”

On the Physical Origins of Charge Separation at Donor–Acceptor Interfaces in Organic Solar Cells: Energy Bending versus Energy Disorder

“…The choice of the exponential parameter in Equation (1) was based on the assumption that the energy bending levels up within few (roughly three-five) layers from the D-A interface. [25b,25c,36] This expression aimed at reproducing a gradual decrease of energy as charges moved away from the interface, based on the following: i) space extension of the energy bending was limited to only few (roughly three-five) layers near the D-A interface; [36,37] ii) the assumption that the dominant part of the EB (70-75%) occurred between the first two layers from the interface. [25c] This assumption can be justified by two observations, indicating that two main sources of the EB, such as the geometrical deformations [25b,25c] and the DCP, [25c] appear principally on the interface donor-and acceptor layers, and much less on the few following ones.…”

Solid state structure and properties of phenyl diketopyrrolopyrrole derivatives

“…[83][84][85] Tetracene is planar in structure and displays moderate charge mobility, packing in a classic herringbone fashion. 86,87 Introduction of peripheral phenyl groups results in twisting of the rings to minimise steric clashing and a slipped-co-facial packing of the π-conjugated tetracene backbone. This introduction of side peripheral groups, as observed for rubrene, results in closer π⋯π interactions and columnar stacks in two directions.…”