Is a Single Molecule Sufficient to Determine the Internal Charge Trapping Energy in Crystalline Organic Semiconductors?

Abstract

In silico diagnoses of charge-transfer efficiencies in organic semiconductors require the accurate computations of transport parameters. We show here that ignoring the molecular packing effects when computing the internal charge trapping energy may cause a severe deviation to the result deduced from the periodic crystal structure. This deviation can reach up to 100 meV in common organic materials. According to the semiclassical Marcus theory, this energy difference can lead to orders of magnitude change in the charge transfer rate. Studying from a total of 45 organic crystals, we find that single-molecule approximation though is adequate for rigid planar molecules yet it fails to describe the trapping energy for molecules that have inter-ring single bond(s) or are subjected to planarity changes during the transition from the isolated state to the embedded state. These results and conclusions may shed light on the removal of theory–experiment discrepancies on charge mobilities and lay the basis for the future fast and precise screening of high-performance organic semiconductors.

Electric-Field Effects on the Internal Charge Reorganization Energies of Crystalline Organic Semiconductors

“…22 We have previously shown that the above single-molecule approximation is valid only when the molecule-packing impacts are negligible. 23 In addition to molecular packing, another environmental factor that is often overlooked is the external electric field (EEF). 24 In a typical field-effect transistor, the EEF can be generated by the electrodes, and the gate voltage controls the channel conductivity.…”

“…So far, λ is mostly calculated based on the isolated molecule via either the normal-mode analysis , or the Nelsen’s four-point method . We have previously shown that the above single-molecule approximation is valid only when the molecule-packing impacts are negligible . In addition to molecular packing, another environmental factor that is often overlooked is the external electric field (EEF) .…”

“…Focusing on the effects of molecular packing, several groups ,− have demonstrated based on molecular dynamic or quantum mechanical simulations that the solid-state constraint can have a significant impact on λ. Sancho-García et al and Lu et al have investigated the λ change as a function of F for isolated SMOS molecules, assuming that the EEF is applied along the long molecular axis. Förster et al have expanded the EEF directions and studied the λ response to three axial fields grounded on one donor–acceptor-type SMOS molecule.…”