Effect of Electronic Polarization on Charge-Transport Parameters in Molecular Organic Semiconductors

Abstract

Theoretical investigations of charge transport in organic materials are generally based on the "energy splitting in dimer" method and routinely assume that the transport parameters (site energies and transfer integrals) determined from monomer and dimer calculations can be reliably used to describe extended systems. Here, we demonstrate that this transferability can fail even in molecular crystals with weak van der Waals intermolecular interactions, due to the substantial (but often ignored) impact of polarization effects, particularly on the site energies. We show that the neglect of electronic polarization leads to qualitatively incorrect values and trends for the transfer integrals computed with the energy splitting method, even in simple prototypes such as ethylene or pentacene dimers. The polarization effect in these systems is largely electrostatic in nature and can change dramatically upon transition from a dimer to an extended system. For example, the difference in site energy for a prototypical "face-to-edge" one-dimensional stack of pentacene molecules is calculated to be 30% greater than that in the "face-to-edge" dimer, whereas the site energy difference in the pentacene crystal is vanishingly small. Importantly, when computed directly in the framework of localized monomer orbitals, the transfer integral values for dimer and extended systems are very similar.

Charge transport and exciton dissociation in organic solar cells consisting of dipolar donors mixed withC70

“…The constrained DFT approach 19 minimizes errors that occur due to neglect of polarization in the energy splitting in the dimer method 37 by assigning charge to a single molecule within the pair when calculating the wavefunction. Moreover, Bredas, et.…”

“…al., have shown that the principal error introduced by considering the dimer only, rather than the complete crystals, is in the difference in site energies while the transfer integral is nearly the same for the dimer and a larger system that takes even more molecules into consideration. 37 The The hole coupling and transfer rates between CPB, CTB, and DBP molecules were theoretically estimated as described in the Experimental Methods section, and are given in Table II. Our initial approach was to base our calculations on each donor dimer in the gas phase, with the dimer coordinates taken from the reported crystal structures.…”

Rubrene: The Interplay between Intramolecular and Intermolecular Interactions Determines the Planarization of Its Tetracene Core in the Solid State

“…Transfer integrals, ‫ݐ‬ , for molecular dimers selected from the crystal structures were evaluated using a fragment orbital approach in combination with a basis set orthogonalization procedure at the B3LYP/cc-pVDZ level. 32,33 For the long-range corrected hybrid ωB97 functional, 29 the optimal range-separation parameter ω (i.e., the ω value minimizing the many-electron self-interaction error) was determined following a non-empirical IP-tuning procedure, 34 where the difference between the highest occupied molecular orbital (HOMO) eigenvalue and the computed vertical ionization potential was minimized through the relation:…”

BODIPY derivatives as n-type organic semiconductors: Isomer effect on carrier mobility

“…where H is the system Kohn-Sham Hamiltonian of the dimer system, and W i(j) means the monomer HOMOs (for hole transport) or LUMOs (for electron transport) with Löwdin's symmetric transformation, which can be used as the orthogonal basis set for calculation [61]. Then the intermolecular electronic coupling V ij can be written as…”