OCELOT: An infrastructure for data-driven research to discover and design crystalline organic semiconductors.

Materials design and discovery are often hampered by the slow pace and materials and human costs associated with Edisonian trial-and-error screening approaches. Recent advances in computational power, theoretical methods, and data science techniques, however, are being manifest in a convergence of these tools to enable in silico materials discovery. Here, we present the development and deployment of computational materials data and data analytic approaches for crystalline organic semiconductors. The OCELOT (Organic Crystals in Electronic and Light-Oriented Technologies) infrastructure, consisting of a Python-based OCELOT application programming interface and OCELOT database, is designed to enable rapid materials exploration. The database contains a descriptor-based schema for high-throughput calculations that have been implemented on more than 56 000 experimental crystal structures derived from 47 000 distinct molecular structures. OCELOT is open-access and accessible via a web-user interface at https://oscar.as.uky.edu.

Determination of the Free Energies of Mixing of Organic Solutions through a Combined Molecular Dynamics and Bayesian Statistics Approach.

As new generations of thin-film semiconductors are moving toward solution-based processing, the development of printing formulations will require information pertaining to the free energies of mixing of complex mixtures. From the standpoint of in silico material design, this move necessitates the development of methods that can accurately and quickly evaluate these formulations in order to maximize processing speed and reproducibility. Here, we make use of molecular dynamics (MD) simulations, in combination with the two-phase thermodynamic (2PT) model, to explore the free energy of mixing surfaces for a series of halogenated solvents and high-boiling point solvent additives used in the development of thin-film organic semiconductors. Although the combined methods generally show good agreement with available experimental data, the computational cost to traverse the free-energy landscape is considerable. Hence, we demonstrate how a Bayesian optimization scheme, coupled with the MD and 2PT approaches, can drastically reduce the number of simulations required, in turn shrinking both the computational cost and time.

Oxidation Pathways Involving a Sulfide-Endcapped Donor-Acceptor-Donor π-Conjugated Molecule and Antimony(V) Chloride.

The oxidation pathways and products of a discrete, sulfide-endcapped donor-acceptor-donor (D/A/D) molecule, namely, propylenedioxythiophene-benzothiadiazole-propylenedioxythiophene, are investigated. The electrochemical and chemical oxidations proceed by two distinct routes. Specifically, electrochemical oxidation undergoes a sequential two-step, one-electron (1e-) oxidation route with a 117 mV difference between consecutive half-wave potentials. In contrast, chemical oxidation by antimony(V) chloride (SbCl5) causes the generation of four different oxidized species: (a) the 1e- oxidation state, (b) a decomposition product, (c) the 2e- oxidation state, and (d) a chloride adduct of the 2e- oxidation state. The decomposition product is generated by the reaction of the 1e- oxidation state with residual water, resulting in nucleophilic aromatic substitution at the sulfide group terminal positions. This reaction leads to the formation of a 2e- oxidized, oxygen atom (ketone) terminated decomposed molecule. The chloride adduct is determined to be produced by electrophilic chloronium ion (2e-) oxidation by the SbCl4+ complex, which is a product of SbCl5 ligand disproportionation. The formation of the 2e- oxidized chlorine adduct shows to be linearly dependent on the molarity of SbCl5 in dichloromethane, giving new insight into the concentration dependent reactivity of SbCl5 as a 2e- oxidant. The electronic, optical, and magnetic properties and geometric structures of the 1e- and 2e- oxidized hexachloroantimonate salts are fully characterized by a combination of electrochemistry, X-ray crystallography, UV-vis-NIR, electron paramagnetic resonance, NMR spectroscopies, and density functional theory calculations. The aim of this study is to provide a thorough understanding of the redox pathways of a D/A/D π-conjugated organic molecule for potential application in organic electrochromic devices.

Deconstructing the behavior of donor-acceptor copolymers in solution & the melt: the case of PTB7.

For organic semiconductors, the solid-state packing of the π-conjugated molecules or polymers dictate the material electronic, optical, and mechanical characteristics. Combinations of solution and solid-state investigations are often used to establish structure-function relationships, though these connections are often loosely correlated, and experiments in different laboratories can lead to widely variable interpretations. Hence, there remains a need to develop a deeper, more robust understanding of the connections between molecular and polymer chemistry, structure, processing, solid-state order, and materials properties to enable judicious materials design principles. Towards this goal, we employ fully-atomistic molecular dynamics (MD) simulations of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl] (PTB7), a donor-acceptor copolymer that has been widely investigated in the organic solar cell literature, to unravel some of these associations. The MD simulations make use of polymer lengths (molecular weights) and solution concentrations that are consistent with those used in experiment, allowing for a detailed picture to arise as to how variations in the polymer environment can direct polymer structure. Comparisons between experiment and theory suggest that processing history can be an important factor in the polymer structures presumed experimentally that are used to interpret optical and electronic responses. The results of these simulations provide specific information into the behavior of PTB7 under different conditions, and showcase how atomistic MD simulations that approach experimentally relevant sizes can be used to develop broader chemical insight that can aid in the design, processing, and characterization of polymer-based organic semiconductors.

Presence of Short Intermolecular Contacts Screens for Kinetic Stability in Packing Polymorphs.

Polymorphism is pervasive in molecular solids. While computational predictions of the molecular polymorphic landscape have improved significantly, identifying which polymorphs are preferentially accessed and experimentally stable remains a challenge. We report a framework that correlates short intermolecular contacts with polymorphic stability. The presence of short contacts between neighboring molecules prevents structural rearrangement and stabilizes the packing arrangement, even when the stabilized polymorph is not enthalpically favored. In the absence of such intermolecular short contacts, the molecules have added degrees of freedom for structural rearrangement, and solid-solid polymorphic transformations occur readily. Starting with a series of core-halogenated naphthalene tetracarboxylic diimides, we establish this framework with the packing polymorphs of more than 20 compounds, ranging from molecular semiconductors to pharmaceutics and biological building blocks. This framework, widely applicable across molecular solids, can help refine computational predictions by identifying the polymorphs that are kinetically stable.

Polarization Energies at Organic-Organic Interfaces: Impact on the Charge Separation Barrier at Donor-Acceptor Interfaces in Organic Solar Cells.

We probe the energetic landscape at a model pentacene/fullerene (C60) interface to investigate the interactions between positive and negative charges, which are critical to the processes of charge separation and recombination in organic solar cells. Using a polarizable force field, we find that polarization energy, i.e., the stabilization a charge feels due to its environment, is larger at the interface than in the bulk for both a positive and a negative charge. The combination of the charge being more stabilized at the interface and the Coulomb attraction between the charges results in a barrier to charge separation at the pentacene/C60 interface that can be in excess of 0.7 eV for static configurations of the donor and acceptor locations. However, the impact of molecular motions, i.e., the dynamics, at the interface at room temperature results in a distribution of polarization energies and in charge separation barriers that can be significantly reduced. The dynamic nature of the interface is thus critical, with the polarization energy distributions indicating that sites along the interface shift in time between favorable and unfavorable configurations for charge separation.

Ionization Energies, Electron Affinities, and Polarization Energies of Organic Molecular Crystals: Quantitative Estimations from a Polarizable Continuum Model (PCM)-Tuned Range-Separated Density Functional Approach.

We propose a new methodology for the first-principles description of the electronic properties relevant for charge transport in organic molecular crystals. This methodology, which is based on the combination of a nonempirical, optimally tuned range-separated hybrid functional with the polarizable continuum model, is applied to a series of eight representative molecular semiconductor crystals. We show that it provides ionization energies, electron affinities, and transport gaps in very good agreement with experimental values, as well as with the results of many-body perturbation theory within the GW approximation at a fraction of the computational costs. Hence, this approach represents an easily applicable and computationally efficient tool to estimate the gas-to-crystal phase shifts of the frontier-orbital quasiparticle energies in organic electronic materials.

Impact of Molecular Orientation and Packing Density on Electronic Polarization in the Bulk and at Surfaces of Organic Semiconductors.

The polarizable environment surrounding charge carriers in organic semiconductors impacts the efficiency of the charge transport process. Here, we consider two representative organic semiconductors, tetracene and rubrene, and evaluate their polarization energies in the bulk and at the organic-vacuum interface using a polarizable force field that accounts for induced-dipole and quadrupole interactions. Though both oligoacenes pack in a herringbone motif, the tetraphenyl substituents on the tetracene backbone of rubrene alter greatly the nature of the packing. The resulting change in relative orientations of neighboring molecules is found to reduce the bulk polarization energy of holes in rubrene by some 0.3 eV when compared to tetracene. The consideration of model organic-vacuum interfaces highlights the significant variation in the electrostatic environment for a charge carrier at a surface although the net change in polarization energy is small; interestingly, the environment of a charge even just one layer removed from the surface can be viewed already as representative of the bulk. Overall, it is found that in these herringbone-type layered crystals the polarization energy has a much stronger dependence on the intralayer packing density than interlayer packing density.

Impact of molecular packing on electronic polarization in organic crystals: the case of pentacene vs TIPS-pentacene.

Polarization energy corresponds to the stabilization of the cation or anion state of an atom or molecule when going from the gas phase to the solid state. The decrease in ionization energy and increase in electron affinity in the solid state are related to the (electronic and nuclear) polarization of the surrounding atoms and molecules in the presence of a charged entity. Here, through a combination of molecular mechanics and quantum mechanics calculations, we evaluate the polarization energies in two prototypical organic semiconductors, pentacene and 6,13-bis(2-(tri-isopropylsilyl)ethynyl)pentacene (TIPS-pentacene). Comparison of the results for the two systems reveals the critical role played by the molecular packing configurations in the determination of the polarization energies and provides physical insight into the experimental data reported by Lichtenberger and co-workers (J. Amer. Chem. Soc. 2010, 132, 580; J. Phys. Chem. C 2010, 114, 13838). Our results underline that the impact of packing configurations, well established in the case of the charge-transport properties, also extends to the polarization properties of π-conjugated materials.