Development and Testing of an All-Atom Force Field for Diketopyrrolopyrrole Polymers with Conjugated Substituents

Kinetic processes of interfacial transport of reactive species across plasma-water interfaces: the effect of temperature

This work quantifies, through use of molecular dynamics (MD) simulations, the kinetic rates of physical surface processes occurring at a plasma-water interface. The probabilities of adsorption, absorption, desorption and scattering were computed for O3, N2O, NO2, NO, OH, H2O2, HNO2, HNO3, and N2O5 as they interact with the interface at three water temperatures: 298 K, 323 K, and 348 K. Species are categorised into the short-residence group (O3, N2O, NO2, and NO) and the long-residence group (OH, H2O2, HNO2, HNO3, and N2O5) based on their mean surface residence time. It is reported that the most probable process for the short-residence group is desorption, which limits their characteristic residence time at the interface to less than 100 ps, while the long-residence species experience a mixture of absorption and desorption, with a characteristic residence time exceeding 200 ps for many species in this group. With increasing water temperature, a universal decline in characteristic surface residence time is observed. It is found that the short-residence group experience a reduction in probability of desorption in favour of scattering, whereas the long-residence group experience a reduction in probability of adsorption in favour of absorption and desorption. The data reported in this work facilitate the development of a basic surface kinetic model, which was used to find that tuning the plasma toward the production of HNO3 will result in an increase in the rate of uptake of reactive nitrogen species by a factor of 250%.

Quantum-Quantum and Quantum-Quantum-Classical Schemes for Near-Gap Excitations with Projection-Based-Embedded GW-Bethe-Salpeter Equation

We present quantum-quantum and quantum-quantum-classical schemes based on many-body Green's functions theory in the GW approximation with the Bethe-Salpeter equation (GW-BSE) employing projection-based-embedding (PbE). Such approaches allow defining active and inactive subsystems of larger, complex molecular systems, with only the smaller active subsystem being explicitly treated by GW-BSE offering significant computational advantages. However, as PbE can modify the single-particle states in the Kohn-Sham (KS) ground state calculation and screening effects from the inactive region are not automatically included in GW-BSE, results from such PbE-GW-BSE calculations can deviate from a full-system reference. Here, we scrutinize in detail, e.g., the individual and combined effects of different choices of active regions, the influence of omitting the screening from the inactive region, and strategies for basis set truncation on frontier orbital and near-gap electron-hole excitation energies. As prototypical systems, we consider a diketopyrrolopyrrole bicyclic ring including side-chains, a polarity-sensitive dye (prodan) in aqueous environment, and a π-stacked dimer of benzene and tetracyanoethylene in water, respectively, covering a variety of excitation characters in molecular systems with complex chemical environments and photoinduced processes. Our results suggest that to obtain agreement of approximately 0.1 eV between near-gap excitation energies from embedded and full calculations, the active region should be chosen based on the Mulliken population of the full highest-occupied molecular orbital and that careful benchmarking should be done on the KS level before the actual GW-BSE steps when basis set truncation is used. We find that PbE-GW-BSE offers significant reductions in computation times and, more importantly, memory requirements, making calculations for considerably larger systems tractable.

Effect of Solvent Removal Rate and Annealing on the Interface Properties in a Blend of a Diketopyrrolopyrrole-Based Polymer with Fullerene

We study the effect of solvent-free annealing and explicit solvent evaporation protocols in classical molecular dynamics simulations on the interface properties of a blend of a diketopyrrolopyrrole (DPP) polymer with conjugated substituents (DPP2Py2T) and PCBM[60]. We specifically analyze the intramolecular segmental mobility of the different polymer building blocks as well as intermolecular radial and angular distribution functions between donor and acceptor. The annealing simulations reveal an increase of the glass-transition temperature of 45 K in the polymer–fullerene blend compared to that of pure DPP2Py2T. Our results show that the effective solvent evaporation rates at room temperature only have a minor influence on the segmental mobility and intermolecular orientation, characterized in all cases by a preferential arrangement of PCBM[60] close to the electron-donating substituents in DPP2Py2T. In contrast, solvent-free annealing from a liquid yields clustering of the fullerene close to the electron-withdrawing DPP, generally considered to be detrimental for application in organic solar cells. We find that the difference can be attributed to differences in the behavior of 2-hexyldecyl side-chains, which collapse toward DPP when solvent is explicitly removed, thereby blocking access of PCBM[60].

Impact of Side-Chain Hydrophilicity on Packing, Swelling, and Ion Interactions in Oxy-Bithiophene Semiconductors

Exchanging hydrophobic alkyl-based side chains to hydrophilic glycol-based side chains is a widely adopted method for improving mixed-transport device performance, despite the impact on solid-state packing and polymer-electrolyte interactions being poorly understood. Presented here is a molecular dynamics (MD) force field for modeling alkoxylated and glycolated polythiophenes. The force field is validated against known packing motifs for their monomer crystals. MD simulations, coupled with X-ray diffraction (XRD), show that alkoxylated polythiophenes will pack with a "tilted stack" and straight interdigitating side chains, whilst their glycolated counterpart will pack with a "deflected stack" and an s-bend side-chain configuration. MD simulations reveal water penetration pathways into the alkoxylated and glycolated crystals-through the π-stack and through the lamellar stack respectively. Finally, the two distinct ways triethylene glycol polymers can bind to cations are revealed, showing the formation of a metastable single bound state, or an energetically deep double bound state, both with a strong side-chain length dependence. The minimum energy pathways for the formation of the chelates are identified, showing the physical process through which cations can bind to one or two side chains of a glycolated polythiophene, with consequences for ion transport in bithiophene semiconductors.

Strategies for the Development of Conjugated Polymer Molecular Dynamics Force Fields Validated with Neutron and X-ray Scattering

Conjugated polymers (CPs) enable a wide range of lightweight, lower cost, and flexible organic electronic devices, but a thorough understanding of relationships between molecular structure and dynamics and electronic performance is critical for improved device efficiencies and for new technologies. Molecular dynamics (MD) simulations offer in silico insight into this relationship, but their accuracy relies on the approach used to develop the model's parameters or force field (FF). In this Perspective, we first review current FFs for CPs and find that most of the models implement an arduous reparameterization of inter-ring torsion potentials and partial charges of classical FFs. However, there are few FFs outside of simple CP molecules, e.g., polythiophenes, that have been developed over the last two decades. There is also limited reparameterization of other parameters, such as nonbonded Lennard-Jones interactions, which we find to be directly influenced by conjugation in these materials. We further provide a discussion on experimental validation of MD FFs, with emphasis on neutron and X-ray scattering. We define multiple ways in which various scattering methods can be directly compared to results of MD simulations, providing a powerful experimental validation metric of local structure and dynamics at relevant length and time scales to charge transport mechanisms in CPs. Finally, we offer a perspective on the use of neutron scattering with machine learning to enable high-throughput parametrization of accurate and experimentally validated CP FFs enabled not only by the ongoing advancements in computational chemistry, data science, and high-performance computing but also using oligomers as proxies for longer polymer chains during FF development.

Local structuring of diketopyrrolopyrrole (DPP)-based oligomers from molecular dynamics simulations

The microscopic structure of high mobility semiconducting polymers is known to be essential for their performance but it cannot be easily deduced from the available experimental data. A series of short oligomers of diketopyrrolopyrrole (DPP)-based materials that display high charge mobility are studied by molecular dynamics simulations to understand their local structuring at an atomic level. Different analyses are proposed to compare the ability of different oligomers to form large aggregates and their driving force. The simulations show that the tendency for this class of materials to form aggregates is driven by the interaction between DPP fragments, but this is modulated by the other conjugated fragments of the materials which affect the rigidity of the polymer and, ultimately, the size of the aggregates that are formed. The main structural features and the electronic structure of the oligomers are fairly similar above the glass transition temperature and at room temperature.