Charge carrier mobility is at the center of organic electronic devices. The strong couplings between electrons and nuclear motions lead to complexities in theoretical description of charge transport, which pose a major challenge for the fundamental understanding and computational design of transport organic materials. This tutorial review describes recent progresses in developing computational tools to assess the carrier mobility in organic molecular semiconductors at the first-principles level. Some rational molecular design strategies for high mobility organic materials are outlined.

Computational methods for design of organic materials with high charge mobility

We report a simple first-principles-based simulation model (combining quantum mechanics with Marcus−Hush theory) that provides the quantitative structural relationships between angular resolution anisotropic hole mobility and molecular structures and packing. We validate that this model correctly predicts the anisotropic hole mobilities of ruberene, pentacene, tetracene, 5,11-dichlorotetracene (DCT), and hexathiapentacene (HTP), leading to results in good agreement with experiment.

First-Principles Investigation of Anistropic Hole Mobilities in Organic Semiconductors

Optical band gaps of organic semiconductor materials

Carrier mobility is the most important parameter for assessing charge carrier transfer. This protocol describes a tool for predicting the charge carrier mobilities of π-stacked systems such as organic semiconductors and the DNA double helix. This protocol is intended to provide chemists and physicists with a tool for predicting the charge carrier mobilities of π-stacked systems such as organic semiconductors and the DNA double helix. An experimentally determined crystal structure is required as a starting point. The simulation involves the following operations: (i) searching the crystal structure; (ii) selecting molecular monomers and dimers from the crystal structure; (iii) using density function theory (DFT) calculations to determine electronic coupling for dimers; (iv) using DFT calculations to determine self-reorganization energy of monomers; and (v) using a numerical calculation to determine the charge carrier mobility. For a single crystal structure consisting of medium-sized molecules, this protocol can be completed in ∼4 h. We have selected two case studies (a rubrene crystal and a DNA segment) as examples of how this procedure can be used.

http://www-nature-com-443.webvpn.bjmu.tsg211.com/articles/nprot.2015.038.pdf

Quantitative prediction of charge mobilities of π-stacked systems by first-principles simulation

This review introduces the development and application of a multiscale approach to assess the charge mobility for organic semiconductors, which combines quantum chemistry, Kinetic Monte Carlo (KMC), and molecular dynamics (MD) simulations. This approach is especially applicable in describing a large class of organic semiconductors with intermolecular electronic coupling (V) much less than intramolecular charge reorganization energy (λ), a situation where the band description fails obviously. The charge transport is modeled as successive charge hopping from one molecule to another. We highlight the quantum nuclear tunneling effect in the charge transfer, beyond the semiclassical Marcus theory. Such an effect is essential for interpreting the “paradoxical” experimental finding that optical measurement indicated “local charge” while electrical measurement indicated “bandlike”. Coupled MD and KMC simulations demonstrated that the dynamic disorder caused by intermolecular vibration has negligible effect on the carrier mobility. We further apply the approach for molecular design of n-type materials and for rationalization of experimental results. The charge reorganization energy is analyzed through decomposition into internal coordinates relaxation, so that chemical structure contributions to the intramolecular electron–phonon interaction are revealed and give helpful indication to reduce the charge reorganization energy.

From charge transport parameters to charge mobility in organic semiconductors through multiscale simulation

Both crystal packing and molecular size have strong influences on the charge mobility for organic semiconductors. The crystal structures for oligothiophene (nT) can be roughly classified into two types: the Z = 2 (two molecules in one unit cell) or high temperature (HT) phase and the Z = 4 or low temperature (LT) phase. Through first-principles calculations within the Marcus electron transfer theory coupled with random walk simulation for room temperature charge diffusion constants, we found that the hole mobility of the HT phase is about 3–4 times larger than that of the LT phase because the molecular packing in the HT phase favors the hole transfer (i.e., the frontier orbital wave function phases of the dimer are constructive, which tends to maximize the overlap), while for the LT phase, the molecules are packed in a position that reduces the intermolecular orbital overlap due to phase cancellation. As the molecular size increases from 2T to 8T, the hole mobility tends to increase because the reorganiza...

Influences of Crystal Structures and Molecular Sizes on the Charge Mobility of Organic Semiconductors: Oligothiophenes

Theoretical comparative studies of charge mobilities for molecular materials: Pet versus bnpery

The preparation of N-doped carbon with well-defined and well-controlled doping of N species is a challenging work, especially with supported Pt atoms. Here, four kinds of N-doped carbons with well-controlled N species were synthesized using Friedel–Crafts reaction or thermal pyrolysis. It was found that the strong interaction between Pt and N in C–N═C and bridged −NH at the edge of heptazine rings in g-C3N4 improved the performance of Pt/g-C3N4 for nitrobenzene hydrogenation. The calculated turnover frequency of each Pt atom in Pt/g-C3N4 reached 42,865 h–1 at 50 °C, and it was stable for five cycles without agglomeration or leaching of Pt species. These findings provide a promising way for the rational design of the hydrogenation catalysts and the development of well-controlled N-doped carbon materials. 

Structure–Activity Relationship for the Catalytic Hydrogenation of Nitrobenzene by Single Platinum Atoms Supported on Nitrogen-Doped Carbon

Polylactic acid (PLA) is popularly used to replace some petroleum-based polymers due to its biodegradability. However, biodegradation of waste PLA requires specific bacteria, stringent conditions and long time. Catalytic recycling of waste PLA to valuable added products rather than composting is a sustainable green routine. In this work, catalytic alcoholysis of waste PLA to methyl lactate (MLA) was carried out over a SiO2 @UiO-66 derived solid acid (SO2–4/ZrO2/SiO2). Characterizations indicated that SO2–4/ZrO2/SiO2 processed large surface area and homogenously dispersed acid sites, and it exhibited excellent performance for the alcoholysis of waste PLA under mild conditions. At 140 oC, the yield of MLA is 92.7% within 5 h, and SO2–4/ZrO2/SiO2 could be recycled at least five times without obvious loss of activity. The specific yield of MLA over SO2–4/ZrO2/SiO2 reached 2678 mg-MLA/g-cat/h, which was higher than that of H3PW12O40, Amberlyst-45, Hβ, HZSM-5 and α-ZrP. At the same time, SO2–4/ZrO2/SiO2 was also capable of the alcoholysis of waste PLA-based products. A possible pathway of PLA alcoholysis over SO2–4/ZrO2/SiO2 catalyst was proposed. 

Caili Wang

Papers

Influences of Crystal Structures and Molecular Sizes on the Charge Mobility of Organic Semiconductors: Oligothiophenes

Theoretical comparative studies of charge mobilities for molecular materials: Pet versus bnpery

Structure–Activity Relationship for the Catalytic Hydrogenation of Nitrobenzene by Single Platinum Atoms Supported on Nitrogen-Doped Carbon

Alcoholysis of waste PLA-based plastics to methyl lactate over sulfated ZrO2/SiO2 catalyst