Inhibiting Low-Frequency Vibrations Explains Exceptionally High Electron Mobility in 2,5-Difluoro-7,7,8,8-tetracyanoquinodimethane (F₂-TCNQ) Single Crystals

Abstract

Organic electronics requires materials with high charge mobility. Despite decades of intensive research, charge transport in high-mobility organic semiconductors has not been well understood. In this Letter, we address the physical mechanism underlying the exceptionally high band-like electron mobility in F-TCNQ (2,5-difluoro-7,7,8,8-tetracyanoquinodimethane) single crystals among a crystal family of similar compounds F-TCNQ (n = 0, 2, 4) using a combined experimental and theoretical approach. While electron transfer integrals and reorganization energies did not show outstanding features for F-TCNQ, Raman spectroscopy and solid-state DFT indicated that the frequency of the lowest vibrational mode is nearly twice higher in the F-TCNQ crystal than in TCNQ and F-TCNQ. This phenomenon is explained by the specific packing motif of F-TCNQ with only one molecule per primitive cell so that electron-phonon interaction decreases and the electron mobility increases. We anticipate that our findings will encourage investigators for the search and design of organic semiconductors with one molecule per primitive cell and/or the poor low-frequency vibrational spectrum.

Molecular Semiconductors for Logic Operations: Dead‐End or Bright Future?

“…Sosorev et al have investigated the case of F2‐TCNQ 19 , which exhibits a band‐like electron transport with record‐high mobility, whereas structurally similar tetracyanoquinodimethane derivatives, such as TCNQ and F4‐TCNQ, have much more modest performances . They have put forward that the specific packing motif of F2‐TCNQ 19 , with one molecule per primitive cell, decreases the electron–phonon coupling . Intermolecular phonon modes, which are polymorph specific, definitively play a key role .…”

Impact of Low‐Frequency Vibrations on Charge Transport in High‐Mobility Organic Semiconductors

“…The primitive cells of the crystals are depicted in Figure a. While the calculated LF vibrational frequencies are similar for single molecules of the investigated compounds, they are substantially different for their crystals as shown in Figure b . Specifically, the lowest vibrational frequency, ω 0 , was found ca.…”

“…In ref. , this difference was attributed to the different crystal packing, namely the number of molecules in the primitive (reduced) cell, Z red , for the two polymorphs. Specifically, in the LT phase, the primitive unit cell contains Z red = 4 molecules, while in the HT phase, Z red = 2.…”

“…A similar effect of Z red is observed for the polymorphs of 9,10‐diphenylanthracene: in the α‐phase Z red = 2 in contrast to β and γ phases possessing Z red = 4. This means that the symmetry of the crystal dramatically affects the LF vibrations, which can in turn significantly alter electron–phonon interaction (see Section 5.1): increase of the lowest vibrational frequency and decrease of the number of vibrational modes due to higher crystal symmetry were suggested to result in higher µ …”

“…In refs. , a crystal series consisting of TCNQ and its fluorinated derivatives, F 2 ‐TCNQ and F 4 ‐TCNQ, was investigated using solid‐state DFT and LF Raman spectroscopy. The primitive cells of the crystals are depicted in Figure a.…”