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Polymer composite materials require softening to reduce their glass transition temperature and improve processability. To this end, plasticizers, which are small organic molecules, are added to the polymer matrix. The miscibility of these plasticizers has a large impact on their effectiveness and therefore their interactions with the polymer matrix must be carefully considered. Many plasticizer characteristics, including their size, topology and flexibility, can impact their miscibility and, because of the exponentially large numbers of plasticizers, the current trial-and-error approach is very ineffective. In this work we show that using molecular simulations of a small dataset of 48 plasticizers, it is possible to identify topological and thermodynamic descriptors that are proxy for their miscibility. Using ad-hoc molecular dynamics simulation set-ups that are relatively computationally inexpensive, we establish correlations between the plasticizers' topology, internal flexibility, thermodynamics of aggregation and their degree of miscibility and use these descriptors to classify the molecules as miscible or immiscible. With all available data we also construct a decision tree model which achieves a F1 score of 0.86 +/- 0.01 with repeated, stratified 5-fold cross-validation, indicating that this machine learning method is a promising route to fully automate the screening. By evaluating the individual performance of the descriptors, we show this procedure enables a 10-fold reduction of the test space and provides the basis for the development of workflows which can efficiently screen thousands of plasticizers with a variety of features., Comment: 49 pages, 16 figures

We present a first-principle numerical study of charge transport in a realistic two-dimensional tight-binding model of organic molecular semiconductors. We use the Hybrid Monte Carlo (HMC) algorithm to simulate the full quantum dynamics of phonons and either a single or multiple charge carriers without any tunable parameters. We introduce a number of algorithmic improvements, including efficient Metropolis updates for phonon fields based on analytic insights, which lead to negligible autocorrelation times and allow to reach sub-permille precisions at small computational cost of $O(1)$ CPU-hour. Our simulations produce charge mobility estimates that are in good agreement with experiment and that also justify the phenomenological Transient Localisation approach., Comment: 6 + 15 pages, 10 figures; v2: published version

Organic solar cells (OSCs) are uniquely suited for semitransparent applications due to their adjustable absorption spectrum. However, most high-performance semitransparent cells reported to date are based on materials that have shown high power conversion efficiency for opaque devices. We therefore present a model to assess the optimum efficiency and transparency for a specific donor and acceptor band gap. The absorption characteristics of both donor and acceptor are modeled with spectral data of typical absorber materials from the literature which are adjusted to achieve the desired band gap value. The results show three distinct regions of high light utilization efficiency if the photopic curve is employed as a weighting function (corresponding to window applications), and a broad maximum for the plant action spectrum as a weighting function (corresponding to greenhouse applications). When comparing these findings to reported experimental values, it is evident that the band gaps of the materials used for the experimental studies do not correspond to the maxima identified by our simulation model. The analysis of the energy levels of molecules recorded in the literature confirms that all band gaps and therefore all LUE maxima are chemically feasible so that the performance of semitransparent OSCs can be further improved by designing materials with optimized absorption spectra., Comment: 20 pages and 10 figure in the main manuscript, aditionally 6 pages and 4 figures in the supporting information

Photovoltaics (PVs) are a critical technology for curbing growing levels of anthropogenic greenhouse gas emissions, and meeting increases in future demand for low-carbon electricity. In order to fulfil ambitions for net-zero carbon dioxide equivalent (CO2eq) emissions worldwide, the global cumulative capacity of solar PVs must increase by an order of magnitude from 0.9 TWp in 2021 to 8.5 TWp by 2050 according to the International Renewable Energy Agency, which is considered to be a highly conservative estimate. In 2020, the Henry Royce Institute brought together the UK PV community to discuss the critical technological and infrastructure challenges that need to be overcome to address the vast challenges in accelerating PV deployment. Herein, we examine the key developments in the global community, especially the progress made in the field since this earlier roadmap, bringing together experts primarily from the UK across the breadth of the photovoltaics community. The focus is both on the challenges in improving the efficiency, stability and levelized cost of electricity of current technologies for utility-scale PVs, as well as the fundamental questions in novel technologies that can have a significant impact on emerging markets, such as indoor PVs, space PVs, and agrivoltaics. We discuss challenges in advanced metrology and computational tools, as well as the growing synergies between PVs and solar fuels, and offer a perspective on the environmental sustainability of the PV industry. Through this roadmap, we emphasize promising pathways forward in both the short- and long-term, and for communities working on technologies across a range of maturity levels to learn from each other., Comment: 160 pages, 21 figures

Whether or not specific ion effects determine the charge storage properties of aqueous graphene and graphite-based supercapacitors remains a highly debated topic. In this work we present a multiscale quantum mechanics classical molecular dynamics investigation of aqueous mono- and divalent salt electrolytes in contact with fully polarizable charged graphene sheets. By computing both the electrochemical double layer and quantum capacitance we observe a constant electrode specific capacitance with cationic radii and charge. Counterintuitively, we determine that a switch in the cation adsorption mechanism from inner to outer Helmholtz layers leads to negligible changes to the EDL capacitance, this appears to be due to the robust electronic structure of the graphene electrodes. However, the ability of ions (such as K+) with a relatively low hydration free energy to penetrate the inner Helmholtz plane and adsorb directly on the electrode surface is found to slow their diffusion parallel to the interface. Ions in the outer Helmholtz layer are found to have higher diffusivity at the surface due to their position in ion channels between water layers. Our results show that surface effects such as the surface polarization and the partial dehydration and local structuring of ions on the surface underpin the behaviour of cations at the interface and add a vital new perspective on trends in ion mobilities seen under confinement., Comment: 23 pages, 5 figures 4 tables

We report a new Quantum Mechanical/Molecular Dynamics (QM/MD) simulation loop to model the coupling between the electron and atom dynamics in solid/liquid interfacial systems. The method can describe simultaneously both the quantum mechanical surface polarizability emerging from the proximity to the electrolyte, and the electrolyte structure and dynamics. In the current set up Density Functional Tight Binding calculations for the electronic structure calculations of the surface are coupled with classical molecular dynamics to simulate the electrolyte solution. The reduced computational cost of the QM part makes the coupling with a classical simulation engine computationally feasible and allows simulation of large systems for hundreds of nanoseconds. We tested the method by simulating both a non-charged graphene flake and a non-charged and charged infinite graphene sheet immersed in an NaCl electrolyte solution. We found that, when no bias is applied, ions preferentially remained in solution and only cations are mildly attracted to the surface of the graphene. This preferential adsorption of cations vs anions seems to persist also when the surface is moderately charged and rules out any substantial ions/surface charge transfer., Comment: 22 pages, 8 figures

We study a large number of physically-plausible arrangements of chromophores, generated via a computational method involving stochastic real-space transformations of a naturally occurring `reference' structure, illustrating our methodology using the well-studied Fenna-Matthews-Olson complex (FMO). To explore the idea that the natural structure has been tuned for efficient energy transport we use an atomic transition charge method to calculate the excitonic couplings of each generated structure and a Lindblad master equation to study the quantum transport of an exciton from a `source' to a `drain' chromophore. We find significant correlations between structure and transport efficiency: High-performing structures tend to be more compact and, among those, the best structures display a certain orientation of the chromophores, particularly the chromophore closest to the source-to-drain vector. We conclude that, subject to reasonable, physically-motivated constraints, the FMO complex is highly attuned to the purpose of energy transport, partly by exploiting these structural motifs., Comment: 7+6pp, 3+10 figures. Accepted version. Data available at http://doi.org/10.6084/m9.figshare.c.3784874.v1

We develop a series of propensity rules for interpreting Inelastic Electron Tunneling (IET) spectra of single-molecule transport junctions. IETS has no selection rules, such as those seen in optical, infrared and Raman spectra, because IETS features arise not from the field-dipole interaction characterizing these other spectroscopies, but from vibronic modification of the electronic levels. Expansion of the Landauer-Imry formula in Taylor series in molecular normal coordinates gives a convenient, accurate perturbation-type formula for calculating both frequency and intensity of the IETS spectrum. Expansion in a Dyson-like form permits derivation of propensity rules, both symmetry-based and pathway-deduced, allowing correlation of structure and coupling geometry with the IETS spectrum. These propensity rules work very well for the calculated spectrum of four typical molecular bridges., Comment: 10 pages

The effect of thermal structural fluctuations on the modulation of the transfer integrals between close molecules is studied using a combination of molecular dynamics simulations and quantum chemical calculations of the transfer integral. We have found that the fluctuations of the transfer integrals are of the same order of magnitude of their average value for two common organic semiconductors an observation that puts into question the currently adopted models for charge transport in organic semiconductors and identifies in the dynamic electronic disorder the limiting factor for the charge carrier mobility., Comment: 12 pages, 2 figures