Ferdinand C. Grozema

Towards high charge-carrier mobilities by rational design of the shape and periphery of discotics

Discotic liquid crystals are a promising class of materials for molecular electronics thanks to their self-organization and charge transporting properties. The best discotics so far are built around the coronene unit and possess six-fold symmetry. In the discotic phase six-fold-symmetric molecules stack with an average twist of 30 degrees, whereas the angle that would lead to the greatest electronic coupling is 60 degrees. Here, a molecule with three-fold symmetry and alternating hydrophilic/hydrophobic side chains is synthesized and X-ray scattering is used to prove the formation of the desired helical microstructure. Time-resolved microwave-conductivity measurements show that the material has indeed a very high mobility, 0.2 cm(2) V(-1) s(-1). The assemblies of molecules are simulated using molecular dynamics, confirming the model deduced from X-ray scattering. The simulated structures, together with quantum-chemical techniques, prove that mobility is still limited by structural defects and that a defect-free assembly could lead to mobilities in excess of 10 cm(2) V(-1) s(-1).

Charge transport in columnar stacked triphenylenes: Effects of conformational fluctuations on charge transfer integrals and site energies

Values of charge transfer integrals, spatial overlap integrals and site energies involved in transport of positive charges along columnar stacked triphenylene derivatives are provided. These parameters were calculated directly as the matrix elements of the Kohn–Sham Hamiltonian, defined in terms of the molecular orbitals on individual triphenylene molecules. This was realized by exploiting the unique feature of the Amsterdam density functional theory program that allows one to use molecular orbitals on individual molecules as a basis set in calculations on a system composed of two or more molecules. The charge transfer integrals obtained in this way differ significantly from values estimated from the energy splitting between the highest occupied molecular orbitals in a dimer. The difference is due to the nonzero spatial overlap between the molecular orbitals on adjacent molecules. Calculations were performed on unsubstituted and methoxy- or methylthio-substituted triphenylenes. Charge transfer integrals and site energies were computed as a function of the twist angle, stacking distance and lateral slide distance between adjacent molecules. The variation of the charge transfer integrals and site energies with these conformational degrees of freedom provide a qualitative explanation of the similarities and differences between the experimental charge carrier mobilities in different phases of alkoxy- and alkylthio-substituted triphenylenes. The data obtained from the present work can be used as input in quantitative studies of charge transport in columnar stacked triphenylene derivatives.

Mechanism of charge transport in self-organizing organic materials

Currently there is great interest in the use of organic materials as the active component in opto-electronic devices such as field-effect transistors, light-emitting diodes, solar cells and in nanoscale molecular electronics. Device performance is to a large extent determined by the mobility of charge carriers, which strongly depends on material morphology. Therefore, a fundamental understanding of the relation between the mechanism of charge transport and chemical composition and supramolecular organization of the active organic material is essential for improvement of device performance. Self-assembling materials are of specific interest, since they have the potential to form well defined structures in which molecular ordering facilitates efficient charge transport. This review gives an overview of theoretical models that can be used to describe the mobility of charge carriers, including band theory for structurally ordered materials, tight-binding models for weakly disordered systems and hopping models for localized charges in strongly disordered materials. It is discussed how the charge transport parameters needed in these models; i.e. charge transfer integrals, site energies and reorganization energies, can be obtained from quantum chemical calculations. Illustrative examples of application of the theoretical methods to charge transport in self-assembling materials are discussed: columns of discotic molecules, stacks of oligo(phenylene-vinylene) molecules and strands of DNA base pairs. It is argued that the mobility of charge carriers along stacks of triphenylene and oligo(phenylene-vinylene) molecules can be significantly enhanced by improvement of molecular organization. According to calculations, the mobility of charge carriers along DNA strands is strongly limited by the large charge induced structural reorganization of the nucleobases and the surrounding water.

Direct–indirect character of the bandgap in methylammonium lead iodide perovskite

Metal halide perovskites such as methylammonium lead iodide (CH3NH3PbI3) are generating great excitement due to their outstanding optoelectronic properties, which lend them to application in high-efficiency solar cells and light-emission devices. However, there is currently debate over what drives the second-order electron-hole recombination in these materials. Here, we propose that the bandgap in CH3NH3PbI3 has a direct-indirect character. Time-resolved photo-conductance measurements show that generation of free mobile charges is maximized for excitation energies just above the indirect bandgap. Furthermore, we find that second-order electron-hole recombination of photo-excited charges is retarded at lower temperature. These observations are consistent with a slow phonon-assisted recombination pathway via the indirect bandgap. Interestingly, in the low-temperature orthorhombic phase, fast quenching of mobile charges occurs independent of the temperature and photon excitation energy. Our work provides a new framework to understand the optoelectronic properties of metal halide perovskites and analyse spectroscopic data.

Effect of structural dynamics on charge transfer in DNA hairpins.

We present a theoretical study of the positive charge transfer in stilbene-linked DNA hairpins containing only AT base pairs using a tight-binding model that includes a description of structural fluctuations. The parameters are the charge transfer integral between neighboring units and the site energies. Fluctuations in these parameters were studied by a combination of molecular dynamics simulations of the structural dynamics and density functional theory calculations of charge transfer integrals and orbital energies. The fluctuations in both parameters were found to be substantial and to occur on subpicosecond time scales. Tight-binding calculations of the dynamics of charge transfer show that for short DNA hairpins (<4 base pairs) the charge moves by a single-step superexchange mechanism with a relatively strong distance dependence. For longer hairpins, a crossover to a fluctuation-assisted incoherent mechanism was found. Analysis of the charge distribution during the charge transfer process indicates that for longer bridges substantial charge density builds up on the bridge, but this charge density is mostly confined to the adenine next to the hole donor. This is caused by the electrostatic interaction between the hole on the AT bridge and the negative charge on the hole donor. We conclude both that the relatively strong distance dependence for short bridges is mostly due to this electrostatic interaction and that structural fluctuations play a critical role in the charge transfer, especially for longer bridge lengths.

Signatures of Quantum Interference Effects on Charge Transport Through a Single Benzene Ring

Inthis description, the width and height of the energy barrier,and the electronic coupling between the molecule and theelectrodes are the main parameters that characterize theefficiency of charge transport. Electron transfer througha molecule then depends exponentially on the length of theconductance pathway and this has indeed been observed inmany experiments.

Large negative differential conductance in single-molecule break junctions

Molecular electronics aims at exploiting the internal structure and electronic orbitals of molecules to construct functional building blocks. To date, however, the overwhelming majority of experimentally realized single-molecule junctions can be described as single quantum dots, where transport is mainly determined by the alignment of the molecular orbital levels with respect to the Fermi energies of the electrodes and the electronic coupling with those electrodes. Particularly appealing exceptions include molecules in which two moieties are twisted with respect to each other and molecules in which quantum interference effects are possible. Here, we report the experimental observation of pronounced negative differential conductance in the current-voltage characteristics of a single molecule in break junctions. The molecule of interest consists of two conjugated arms, connected by a non-conjugated segment, resulting in two coupled sites. A voltage applied across the molecule pulls the energy of the sites apart, suppressing resonant transport through the molecule and causing the current to decrease. A generic theoretical model based on a two-site molecular orbital structure captures the experimental findings well, as confirmed by density functional theory with non-equilibrium Greens functions calculations that include the effect of the bias. Our results point towards a conductance mechanism mediated by the intrinsic molecular orbitals alignment of the molecule.

Understanding Structure-Mobility Relations for Perylene Tetracarboxydiimide Derivatives

Discotic mesophases are known for their ability to self-assemble into columnar structures and can serve as semiconducting molecular wires. Charge carrier mobility along these wires strongly depends on molecular packing, which is controlled by intermolecular interactions. By combining wide-angle X-ray scattering experiments with molecular dynamics simulations, we elucidate packing motifs of a perylene tetracarboxdiimide derivative, a task which is hard to achieve by using a single experimental or theoretical technique. We then relate the charge mobility to the molecular arrangement, both by pulse-radiolysis time-resolved microwave conductivity experiments and simulations based on the non-adiabatic Marcus charge transfer theory. Our results indicate that the helical molecular arrangement with the 45 degrees twist angle between the neighboring molecules favors hole transport in a compound normally considered as an n-type semiconductor. Statistical analysis shows that the transport is strongly suppressed by structural defects. By linking molecular packing and mobility, we eventually provide a pathway to the rational design of perylenediimide derivatives with high charge mobilities.

Biosupramolecular Nanowires from Chlorophyll Dyes with Exceptional Charge‐Transport Properties

Conductive tubes: Self-assembled nanotubes of a bacteriochlorophyll derivative are reminiscent of natural chlorosomal light-harvesting assemblies. After deposition on a substrate that consists of a non-conductive silicon oxide surface (see picture, brown) and contacting the chlorin nanowires to a conductive polymer (yellow), they show exceptional charge-transport properties.

Excited state polarizabilities of conjugated molecules calculated using time dependent density functional theory

In this paper, time-dependent density functional theory (TDDFT) calculations of excited state polarizabilities of conjugated molecules are presented. The increase in polarizability upon excitation was obtained by evaluating the dependence of the excitation energy on an applied static electric field. The excitation energy was found to vary quadratically with the field strength. The excess polarizabilities obtained for singlet excited states are in reasonable agreement with the experimental results for the shorter oligomers, particularly if the experimental uncertainties are considered. For longer oligomers the excess polarizability is considerably overestimated, similar to DFT calculations of ground state polarizabilities. Excess polarizabilities of triplet states were found to be smaller than those for the corresponding singlet state, which agrees with experimental results that are available for triplet polarizabilities. Negative polarizabilities are obtained for the lowest singlet Ag states of longer oligomers. The polarizability of the lowest Bu and Ag excited states of the conjugated molecules studied here are determined mainly by the interaction between these two states. Upon application of a static electric field a quadratic Stark effect is observed in which the lower Bu state has a positive excess polarizability and the upper Ag state exhibits a decrease in polarizability upon excitation. All results are explained in terms of a sum-over-states description for the polarizability.