High-mobility Molecular Semiconductors Research Articles

Crystal-Structure Control of Molecular Semiconductors by Methylthiolation: Toward Ultrahigh Mobility.

ConspectusThe crystal structure of organic semiconductors has been regarded as one of the crucial factors for realizing high-performance electronic devices, such as organic field-effect transistors. However, although the control of crystal structures of organic semiconductors has been examined in the last two decades of intensive efforts of the development of organic semiconductors, active measures to control crystal structures enabling high carrier mobility are still limited. In 2016, our research group noticed that regioselective methylthiolation could provide a selective crystal structure change from an ordinary herringbone structure to a pitched π-stacking structure, similar to the crystal structure of rubrene, in the benzo[1,2-b:4,5-b']dithiophene (BDT) system. Following this serendipitous finding, our group systematically investigated the relationship between the molecular and crystal structures of a range of methylthiolated aromatic and heteroaromatic compounds.This Account provides a comprehensive overview of our research efforts and advancements in the development of methylthiolated small-molecule-based organic semiconductors (molecular semiconductors). We first describe the outline of the past development of molecular semiconductors, focusing on the types of crystal structures of high-performance molecular semiconductors. Then, we describe our findings on the drastic crystal structure change in the BDT system upon methylthiolation, detailing the causes of the change in terms of the intermolecular contacts and intermolecular interaction energies. This is followed by the confirmation of the generality of the crystal-structure change by methylthiolation of a series of acene and heteroacenes, where the herringbone structure in the parent system is unexceptionally transformed into the pitched π-stacking structure, a promising crystal structure for high-mobility molecular semiconductors well exemplified by the prototypical molecular semiconductor, rubrene. In fact, the methylthiolated anthradithiophene afforded comparable high mobility to rubrene in single-crystal field-effect transistors. Then, we demonstrate that the sandwich herringbone structures of peri-condensed polycyclic aromatic hydrocarbons, including pyrene, perylene, and peropyrene, change into brickwork crystal structures upon methylthiolation and that, among these compounds, very promising molecular semiconductors, methylthiolated pyrene and peropyrene, showing ultrahigh mobility of 30 cm2 V s-1, are realized.Through the studies, by gaining insights into the underlying mechanisms driving the crystal structure changes, we lay a strong foundation for tackling challenges related to controlling the crystal structures and developing high-performance molecular semiconductors. This will be a distinct approach from the past activities in the development of molecular semiconductors that mainly focused on molecules themselves, including their synthesis, properties, and characterization. We thus anticipate that our findings and the present Account will open the door to a new era of the development of molecular semiconductors.

Backbone-shape engineering of fused-ring electron-deficient molecular semiconductors for unipolar n-type organic transistors: synthesis, conformation changes, and structure–property correlations

Three n-type fused-ring small molecules with different shapes are synthesized. Changing the backbone shape is found to affect the energy levels, molecular conformation and packing ordering, leading to different charge transport properties.

Modeling charge transport in high-mobility molecular semiconductors: Balancing electronic structure and quantum dynamics methods with the help of experiments.

Computing the charge mobility of molecular semiconductors requires a balanced set of approximations covering both the electronic structure of the Hamiltonian parameters and the modeling of the charge dynamics. For problems of such complexity, it is hard to make progress without independently validating each layer of approximation. In this perspective, we survey how all terms of the model Hamiltonian can be computed and validated by independent experiments and discuss whether some common approximations made to build the model Hamiltonian are valid. We then consider the range of quantum dynamics approaches used to model the charge carrier dynamics stressing the strong and weak points of each method on the basis of the available computational results. Finally, we discuss non-trivial aspects and novel opportunities related to the comparison of theoretical predictions with recent experimental data.

Chasing the "Killer" Phonon Mode for the Rational Design of Low-Disorder, High-Mobility Molecular Semiconductors.

Molecular vibrations play a critical role in the charge transport properties of weakly van der Waals bonded organic semiconductors. To understand which specific phonon modes contribute most strongly to the electron-phonon coupling and ensuing thermal energetic disorder in some of the most widely studied high-mobility molecular semiconductors, state-of-the-art quantum mechanical simulations of the vibrational modes and the ensuing electron-phonon coupling constants are combined with experimental measurements of the low-frequency vibrations using inelastic neutron scattering and terahertz time-domain spectroscopy. In this way, the long-axis sliding motion is identified as a "killer" phonon mode, which in some molecules contributes more than 80% to the total thermal disorder. Based on this insight, a way to rationalize mobility trends between different materials and derive important molecular design guidelines for new high-mobility molecular semiconductors is suggested.

A map of high-mobility molecular semiconductors.

The charge mobility of molecular semiconductors is limited by the large fluctuation of intermolecular transfer integrals, often referred to as off-diagonal dynamic disorder, which causes transient localization of the carriers' eigenstates. Using a recently developed theoretical framework, we show here that the electronic structure of the molecular crystals determines its sensitivity to intermolecular fluctuations. We build a map of the transient localization lengths of high-mobility molecular semiconductors to identify what patterns of nearest-neighbour transfer integrals in the two-dimensional (2D) high-mobility plane protect the semiconductor from the effect of dynamic disorder and yield larger mobility. Such a map helps rationalizing the transport properties of the whole family of molecular semiconductors and is also used to demonstrate why common textbook approaches fail in describing this important class of materials. These results can be used to rapidly screen many compounds and design new ones with optimal transport characteristics.

Tuning the effective spin-orbit coupling in molecular semiconductors

The control of spins and spin to charge conversion in organics requires understanding the molecular spin-orbit coupling (SOC), and a means to tune its strength. However, quantifying SOC strengths indirectly through spin relaxation effects has proven difficult due to competing relaxation mechanisms. Here we present a systematic study of the g-tensor shift in molecular semiconductors and link it directly to the SOC strength in a series of high-mobility molecular semiconductors with strong potential for future devices. The results demonstrate a rich variability of the molecular g-shifts with the effective SOC, depending on subtle aspects of molecular composition and structure. We correlate the above g-shifts to spin-lattice relaxation times over four orders of magnitude, from 200 to 0.15 μs, for isolated molecules in solution and relate our findings for isolated molecules in solution to the spin relaxation mechanisms that are likely to be relevant in solid state systems.

Multi-Scale Approach to Non-Adiabatic Charge Transport in High-Mobility Organic Semiconductors.

A linear scaling QM/MM model for studying charge transport in high-mobility molecular semiconductors is presented and applied to an anthracene single crystal and a hexabenzocoronene derivative in its liquid crystalline phase. The model includes both intra- and intermolecular electron-phonon couplings, long-range interactions with the environment, and corrections to the self-interaction error of density functional theory. By performing Ehrenfest simulations of the cationic system, hole mobilities are derived and compared to the experiment. A detailed picture of the charge carrier dynamics is given, and the performance of our method is discussed.