Ambipolar Charge Transport in Organic Semiconductors: How Intramolecular Reorganization Energy Is Controlled by Diradical Character

Abstract

The charged forms of π–conjugated chromophores are relevant in the field of organic electronics as charge carriers in optoelectronic devices, but also as energy storage substrates in organic batteries. In this context, intramolecular reorganization energy plays an important role in controlling material efficiency. In this work, we investigate how the diradical character influences the reorganization energies of holes and electrons by considering a library of diradicaloid chromophores. We determine the reorganization energies with the four-point adiabatic potential method using quantum–chemical calculations at density functional theory (DFT) level. To assess the role of diradical character, we compare the results obtained, assuming both closed-shell and open-shell representations of the neutral species. The study shows how the diradical character impacts the geometrical and electronic structure of neutral species, which in turn control the magnitude of reorganization energies for both charge carriers. Based on computed geometries of neutral and charged species, we propose a simple scheme to rationalize the small, computed reorganization energies for both n-type and p-type charge transport. The study is supplemented with the calculation of intermolecular electronic couplings governing charge transport for selected diradicals, further supporting the ambipolar character of the investigated diradicals.

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

“…Small-molecule organic semiconductors (SMOS) with salient charge-transport (CT) efficiencies are candidate materials for organic field-effect transistors, − organic solar cells, , etc. − According to the semiclassical Marcus theory, in silico predictions or diagnoses of CT efficiencies in SMOS require the accurate computation of CT parameters, for instance, the transfer integral ( t ) and the internal charge reorganization energy (λ). Along with the recent advances on the computation of t that well addresses the electromechanical response and the anisotropy of the measured carrier mobility for SMOS, increasing focus has also been placed on the assessment of λ, − which influences the upper bound of the carrier mobility and is extensively used for novel SMOS design. − …”