Linear-scaling density functional simulations of the effect of crystallographic structure on the electronic and optical properties of fullerene solvates

Aggregate-State Effects in the Atomistic Modeling of Organic Materials for Electrochemical Energy Conversion and Storage Devices: A Perspective

Development of new functional materials for novel energy conversion and storage technologies is often assisted by ab initio modeling. Specifically, for organic materials, such as electron and hole transport materials for perovskite solar cells, LED (light emitting diodes) emitters for organic LEDs (OLEDs), and active electrode materials for organic batteries, such modeling is often done at the molecular level. Modeling of aggregate-state effects is onerous, as packing may not be known or large simulation cells may be required for amorphous materials. Yet aggregate-state effects are essential to estimate charge transport rates, and they may also have substantial effects on redox potentials (voltages) and optical properties. This paper summarizes recent studies by the author's group of aggregation effects on the electronic properties of organic materials used in optoelectronic devices and in organic batteries. We show that in some cases it is possible to understand the mechanism and predict specific performance characteristics based on simple molecular models, while in other cases the inclusion of effects of aggregation is essential. For example, it is possible to understand the mechanism and predict the overall shape of the voltage-capacity curve for insertion-type organic battery materials, but not the absolute voltage. On the other hand, oligomeric models of p-type organic electrode materials can allow for relatively reliable estimates of voltages. Inclusion of aggregate state modeling is critically important for estimating charge transport rates in materials and interfaces used in optoelectronic devices or when intermolecular charge transfer bands are important. We highlight the use of the semi-empirical DFTB (density functional tight binding) method to simplify such calculations.

Promoting Noncovalent Intermolecular Interactions Using a C60 Core Particle in Aqueous PC60s-Covered Colloids for Ultraefficient Photoinduced Particle Activity

Noncovalent intermolecular interactions in nanomaterials, such as van der Waals effects, allow adjustment of the nanoscopic size of compounds and their conformation in molecular crystal regimes. These strong interactions permit small particle sizes to be maintained as the crystals grow. In particular, these effects can be leveraged in the confined/reinforcing phase of molecules. With this in mind, we used C₆₀ molecules as a core particle in single-PC60 surfactant-covered colloid in a water-processable system. Compared with our previous results based on a PC₆₁BM core-PC60 shell particle, the PC60-C₆₀ colloid had a considerably smaller spherical structure due to the increased intermolecular interactions between C₆₀ (fullerene) molecules. Interestingly, the conformation of C₆₀ aggregates was altered depending on the mixed solvents and their volume fraction in the organic phase, which strongly affected the structural properties of the PC60-C₆₀ colloids. The particle facilitated strong interactions with a p-type core sphere when it was introduced as the shell part of a p-n heterojunction particle. This direct interaction provided effective electronic communication between p- and n-type particles, resulting in ultraefficient photonic properties, particularly in charge separation in aqueous heterostructured colloids. This enabled the development of an extremely efficient photovoltaic device with a 6.74% efficiency, which could provide the basis for creating high-performance water-processable solar cells based on p-n heterostructured NPs.

Effect of organic cation states on electronic properties of mixed organic–inorganic halide perovskite clusters

We study the effect of organic cation-centered states in mixed organic–inorganic halide perovskite clusters on the bandstructure and optical properties.

Transuranic Computational Chemistry

Recent developments in the chemistry of the transuranic elements are surveyed, with particular emphasis on computational contributions. Examples are drawn from molecular coordination and organometallic chemistry, and from the study of extended solid systems. The role of the metal valence orbitals in covalent bonding is a particular focus, especially the consequences of the stabilization of the 5f orbitals as the actinide series is traversed. The fledgling chemistry of transuranic elements in the +II oxidation state is highlighted. Throughout, the symbiotic interplay of experimental and computational studies is emphasized; the extraordinary challenges of experimental transuranic chemistry afford computational chemistry a particularly valuable role at the frontier of the periodic table.