Multifaceted aspects of charge transfer

Charge transfer and charge transport are by far among the most important processes for sustaining life on Earth and for making our modern ways of living possible. Involving multiple electron-transfer steps, photosynthesis and cellular respiration have been principally responsible for managing the energy flow in the biosphere of our planet since the Great Oxygen Event. It is impossible to imagine living organisms without charge transport mediated by ion channels, or electron and proton transfer mediated by redox enzymes. Concurrently, transfer and transport of electrons and holes drive the functionalities of electronic and photonic devices that are intricate for our lives. While fueling advances in engineering, charge-transfer science has established itself as an important independent field, originating from physical chemistry and chemical physics, focusing on paradigms from biology, and gaining momentum from solar-energy research. Here, we review the fundamental concepts of charge transfer, and outline its core role in a broad range of unrelated fields, such as medicine, environmental science, catalysis, electronics and photonics. The ubiquitous nature of dipoles, for example, sets demands on deepening the understanding of how localized electric fields affect charge transfer. Charge-transfer electrets, thus, prove important for advancing the field and for interfacing fundamental science with engineering. Synergy between the vastly different aspects of charge-transfer science sets the stage for the broad global impacts that the advances in this field have.

Phase Transitions and Formation of a Monolayer-Type Structure in Thin Oligothiophene Films: Exploration with a Combined In Situ X-ray Diffraction and Electrical Measurements

A combination of in situ electrical and grazing-incidence X-ray diffraction (GIXD) is a powerful tool for studies of correlations between the microstructure and charge transport in thin organic films. The information provided by such experimental approach can help optimizing the performance of the films as active layers of organic electronic devices. In this work, such combination of techniques was used to investigate the phase transitions in vacuum-deposited thin films of a common organic semiconductor dihexyl-quarterthiophene (DH4T). A transition from the initial highly crystalline phase to a mesophase was detected upon heating, while only a partial backward transition was observed upon cooling to room temperature. In situ electrical conductivity measurements revealed the impact of both transitions on charge transport. This is partly accounted for by the fact that the initial crystalline phase is characterized by inclination of molecules in the plane perpendicular to the π-π stacking direction, whereas the mesophase is built of molecules tilted in the direction of π-π stacking. Importantly, in addition to the two phases of DH4T characteristic of the bulk, a third interfacial substrate-stabilized monolayer-type phase was observed. The existence of such interfacial structure can have important implications for the charge mobility, being especially favorable for lateral two-dimensional charge transport in the organic field-effect transistors geometry.

When dendrimers are not better - rational design of nanolayers for high-performance organic electronic devices

Several generations of carbosilane dendrimers with quaterthiophene end groups were studied by X-ray scattering in small and wide angles, differential scanning calorimetry, polarizing optical and atomic force microscopy and molecular modelling. It was established that the semiconducting properties of such materials are determined by the formation of smectic structures in which aliphatic regions, possessing a low degree of the ordering, alternate with highly ordered herring-bone type crystallites formed by aromatic fragments. The presence of long aliphatic spacers in the dendrimers' structure allows easy formation of such crystallites. Such dendrimers assume flattened conformations, as a smectic mesophase is thermodynamically preferable in a wide temperature range. Only in the dendrimers of the fifth generation, as the density of periphery regions increases substantially, π-π stacking of oligothiophene groups is not enough to hold together, and the molecules take on a spherical shape. As a result, extended conducting conjugated regions do not form, and dendrimers of high generations possess comparatively low semiconducting properties. From the technological point of view, quaterthiophene based carbosilane dendrimers are able to form highly uniform functional films. However, the use of lower generation dendrimers is much more preferable, as additional synthetic steps for the production of higher generation compounds do not lead to the improvement of functional properties.

Large-Size Single-Crystal Oligothiophene-Based Monolayers for Field-Effect Transistors

High structural quality of crystalline organic semiconductors is the basis of their superior electrical performance. Recent progress in quasi two-dimensional (2D) organic semiconductor films challenges bulk single crystals because both demonstrate competing charge-carrier mobilities. As the thinnest molecular semiconductors, monolayers offer numerous advantages such as unmatched flexibility and light transparency as well  they are an excellent platform for sensing. Oligothiophene-based materials are among the most promising ones for light-emitting applications because of the combination of efficient luminescence and decent charge-carrier mobility. Here, we demonstrate single-crystal monolayers of unprecedented structural order grown from four alkyl-substituted thiophene and thiophene-phenylene oligomers. The monolayer crystals with lateral dimensions up to 3 mm were grown from the solution on substrates with various surface energies and roughness by drop or spin-casting with subsequent slow solvent evaporation. Our data indicate that 2D crystallization resulting in single-crystal monolayers occurs at the receding gas-solution-substrate contact line. The structural properties of the monolayers were studied by grazing-incidence X-ray diffraction/reflectivity, atomic force and differential interference contrast microscopies, and imaging spectroscopic ellipsometry. These highly ordered monolayers demonstrated an excellent performance in organic field-effect transistors approaching the best values reported for the thiophene or thiophene-phenylene oligomers. Our findings pave the way for efficient monolayer organic electronics highlighting the high potential of simple solution-processing techniques for the growth of large-size single-crystal monolayers with excellent structural order and electrical performance competing against bulk single crystals.

Impact of substrate temperature on the structure and electrical performance of vacuum-deposited α,α′-DH5T oligothiophene thin films

Remarkable structural changes are reported for α,α′-DH5T oligomer thin films evaporated at different substrate temperatures and directly correlated with the electrical performance where the order of π–π stacking plays a crucial role.