Abstract
Theoretical understanding of charge transport in organic semiconductors is exclusively important for organic electronics, but still remains a subject of debate. The recently discovered record-high band-like electron mobility in single crystals of 2,5-difluoro-7,7,8,8-tetracyanoquinodimethane (F2-TCNQ) is challenging from the theoretical viewpoint. First, the very small size of the F2-TCNQ molecule implies high reorganization energy that seems incompatible with efficient charge transport. Second, it is not clear why the crystals of a similar compound, 7,7,8,8-tetracyanoquinodimethane (TCNQ), show an inefficient hopping electron transport mechanism. To address these issues, we apply DFT and QM/MM calculations to the Fn-TCNQ (n = 0,2,4) crystal series. We show that multidimensional intermolecular charge delocalization is of key importance for efficient charge transport in materials consisting of small-sized molecules, and commonly used guidelines for the search for high-mobility organic semiconductors are to be corrected.
Highlights
Organic electronics requires materials with efficient charge transport, i.e. high charge mobility m
We show that while l is large in the Fn-TCNQ isolated molecules, in the TCNQ and F2-TCNQ crystals it is strongly reduced due to intermolecular charge delocalization, making efficient charge transport possible
To estimate the impact of the intermolecular charge delocalization on l, we propose the following original approach based on quantum mechanical/molecular mechanical method (QM/molecular mechanics (MM))
Summary
Organic electronics requires materials with efficient charge transport, i.e. high charge mobility m. Only several organic semiconductors (OSCs) with m 4 1 cm[2] (V s)À1 have been discovered.[1] It is commonly considered that charge transport in OSCs is determined by the interplay of two factors.[2,3,4] The first factor is charge carrier delocalization over several molecules (‘‘sites’’) due to the electronic coupling between them; this coupling is commonly described with transfer integrals J. The opposite factor is charge localization at one site due to the electron–phonon coupling. In high-mobility OSCs, non-local electron–phonon coupling related to the modulation of J by vibrations plays a significant role.[3] Delocalization can enable coherent band-like charge transport with high m.2,3 In high-mobility OSCs, non-local electron–phonon coupling related to the modulation of J by vibrations plays a significant role.[3]
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