The emergence of charge coherence in soft molecular organic semiconductors via the suppression of thermal fluctuations

Abstract

Organic semiconductors are already widely used in electronics. Nevertheless, their fundamental properties are still being debated. In particular, charge transport, which determines the performance of organic light-emitting diodes, solar cells and organic field-effect transistors, has been described by a wide range of complementary but incompatible theories. These theories involve either localized charge carriers hopping from molecule to molecule, leading to incoherent charge transport, or delocalized charge carriers moving around freely in the semiconductor, leading to coherent transport. In this communication, we reveal the first experimental evidence that charge coherence can be tuned from partially to fully coherent in one and the same material -pentacene-showing a continuous transition from one transport mechanism to the other. Microscopically, the transport mechanism depends on the overlap of adjacent molecular orbitals, which in turn is sensitive to molecular thermal fluctuations. We control these fluctuations through moderate variations of pressure and temperature, leading to a mobility increase of 75%. We quantify the influence of these thermal fluctuations by estimating the critical value below which fully coherent charge transport emerges. The ability to control thermal fluctuations and therefore to effectively tune the charge coherence is an important key to improving charge transport in soft molecular materials. INTRODUCTIONHistorically, the existence of coherent electronic states in weakly van der Waals bonded crystals has been a controversial topic. 1 The weak intermolecular interaction leads to a strong thermal vibration of each molecule, resulting in the dynamic disorder of the crystal lattice. Dynamic disorder subsequently leads to a complex interaction between the soft lattice and the charge carriers, which is unique to these systems. In classical semiconductors, the charge transport mechanism can be directly determined from the temperature dependence of the mobility. In an organic semiconductor, however, the temperature dependence of the mobility does not necessarily reveal the nature of the charge transport and therefore it cannot be determined whether a system is described by the short-range hopping of localized charge carriers or band transport due to extended electronic wavefunctions. 2 For this reason, a more direct method is required to study the nature of charge transport in these systems. The Hall effect is such a method, as it is directly connected to the coherence of the charge carrier wavefunction via interaction with the applied magnetic field. The quantum mechanical origin of this interaction lies in the coupling of the wave number k of the delocalized charge carrier wavefunction with the vector potential A of the magnetic field.

Remarkably low flicker noise in solution-processed organic single crystal transistors

“…The first attempt in this work is to identify the origin of noise in the state-of-the-art organic single-crystalline semiconductor C 8 -DNBDT-NW, where the charge carrier undergoes coherent band-like transport. The band-like carrier transport in the present single crystal form of C 8 -DNBDT-NW was unambiguously verified by the Hall effect measurements and electron spin resonance spectroscopy 33,34 . Previous studies about the 1/f noise in disordered organic semiconductors have revealed that the 1/f noise in I D originates mainly from carrier number fluctuation (δn), i.e., δI ∝ e(δn)μ [23][24][25][26] , which is referred to as McWhorter's model (see "Methods" section) 35,36 .…”

“…Note that the exponent q in the power law of temperature dependence, μ ∝ T q , is estimated to be q = −0.83 ± 0.04, which is almost identical to that previously observed for C 8 -DNBDT-NW 34 . This gives confidence that at least in this temperature regime, band-like transport is realized 33,34 . At temperatures down from 215 K, the noise amplitude increases remarkably, while the mobility rapidly diminishes (Fig.…”

“…Linear and saturation mobilities are found to be identical and show a clear plateau at high gate voltage (V G ) regime (see Supplementary Fig. 2), which is an indicative of coherent band-like transport 8,33,34 .…”

High-resolution electrochemical STM of redox metalloproteins

“…STM tip and sample) can often be divided into two, non exclusive, limits [20,21] where the electron can (i) tunnel directly between the contacts but mediated by the molecule or (ii) tunnel onto the molecule and off again, but with an energy relaxation (due to inelastic phonon scattering) as it resides on the molecule. The first limit describes coherent tunnelling (maintaining phase information) while the second is incoherent.…”

Angular-Dependent EDMR Linewidth for Spin-Dependent Space-Charge-Limited Conduction in a Polycrystalline Pentacene

“…It is known that spin relaxation in organic semiconductors depends on their carrier transport mechanisms. Carrier transport in pentacene is believed to be intermediate between band-like and hopping conduction (Sakai et al, 2016;Nakayama et al, 2017), varying depending on the morphology of the material. Therefore, determination of the relation between carrier transport and morphology is a central issue for spintronic applications of pentacene.…”