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First-principles thermodynamics enables the description of the surface chemistry of inorganic materials as a function of temperature and partial pressures of atmospheric gases, providing a framework to connect atomistic simulations with macroscopic materials properties. Here we re-examine the surface chemistry of LiFePO4 (LFP), a widely studied material for use as the cathode in Li-ion batteries. Our results reveal that at room temperature and under standard pressures the LFP (010) surface is covered with water. At elevated temperatures and reduced H2 partial pressure, one water molecule loses a hydrogen atom and the preferred binding moieties are OH and H2O; while further reducing the H2O partial pressure results in the desorption of water leaving only the OH behind. This work also shines new light on the configuration, and resulting electronic properties, of the LFP (010) surface when molecular oxygen (O2) is adsorbed. The molecular adsorbates are also shown to have an impact on the LFP surface potentials and magnetic properties. These simulations provide an enhanced picture of the LFP surface chemistry and the potential impact of these adsorbates on understanding the characteristics of LFP in different materials applications.

Summary 1,3-Dimethyl-2-(4-(dimethylamino)phenyl)-2,4-dihydro-1H-benzoimidazole, N-DMBI-H, is widely used as an air-stable n-dopant for organic semiconductors. Here, its reactivity is investigated with a variety of imide- and amide-containing semiconductor molecules that have reduction potentials in the range −0.54 to −1.10 V versus ferrocene. Reaction rates correlate poorly with these potentials. The more reactive of the imides form the corresponding radical anions cleanly, but kinetic isotope studies using N-DMBI-D indicate that the reaction proceeds via an initial hydride- or hydrogen-transfer step. For an amide- and ester-rigidified bis(styryl)benzene derivative the hydride-reduced product is stable under an inert atmosphere and can be observed directly; the radical anion can only be obtained in sub-stoichiometric yield and under certain reaction conditions. On the other hand, (N-DMBI)2 rapidly reduces all the imides and amides examined to the corresponding radical anions. The implications of these findings for dopant selection and use are discussed.

Rapid increase in the power conversion efficiency of organic solar cells (OSCs) has been achieved with the development of non-fullerene small-molecule acceptors (NF-SMAs). Although the morphological stability of these NF-SMA devices critically affects their intrinsic lifetime, their fundamental intermolecular interactions and how they govern property–function relations and morphological stability of OSCs remain elusive. Here, we discover that the diffusion of an NF-SMA into the donor polymer exhibits Arrhenius behaviour and that the activation energy Ea scales linearly with the enthalpic interaction parameters χH between the polymer and the NF-SMA. Consequently, the thermodynamically most unstable, hypo-miscible systems (high χ) are the most kinetically stabilized. We relate the differences in Ea to measured and selectively simulated molecular self-interaction properties of the constituent materials and develop quantitative property–function relations that link thermal and mechanical characteristics of the NF-SMA and polymer to predict relative diffusion properties and thus morphological stability. Studies on the morphology stability of polymer donor–small-molecule acceptor blends relevant to solar cell stability reveal relationships between their intermolecular interactions and the thermodynamic, kinetic, thermal and mechanical properties.

Despite recent literature reporting the remarkable electrochemical CO2 reduction reaction (CO2RR) performance of nitrogen-doped graphitic carbon materials (sp2-carbon) and nitrogen-doped diamond materials (sp3-carbon), no systematic studies have been conducted on the catalytic activities of hybrid carbon nanomaterials between diamond and graphitic extremes. In this study, nitrogen-doped ultra-nanocrystalline diamond thin films were prepared by a microwave-assisted chemical vapor deposition technique. The ratio of sp2-carbon phase to sp3-carbon phase was controlled by varying growth conditions. Our results confirm that nitrogen-doped sp2-carbon (graphitic) rich electrodes have better selectivity for the CO2RR products over the nitrogen-doped sp3-carbon rich electrodes, indicating that the host structure of nitrogen dopants is crucial for the catalytic activity. Nitrogen-doped sp2-carbon electrodes present Faradaic efficiency for CO production up to 82% with excellent activity and selectivity. The vital role of the host structure and the potential catalytic sites were detailed by density functional theory (DFT) calculations.

The alcohol and water-based processing of a perylene diimide (PDI) organic semiconductor into large area and solvent resistant films is reported. The compound, PDIN-H, is an N-annulated PDI dye with a pyrrolic NH functional group that can be deprotonated to render the material soluble in polar solvents. Addition of NaOH to mixtures of PDIN-H in 1-propanol results in a progressive color change from orange/red to purple with increasing equivalents of base. Use of 1 molar equivalent of NaOH was found to fully dissolve the PDIN-H in alcohol solvents up to a concentration of 10 mg mL−1. Primary alcohols 1-propanol to 1-hexanol as well as 2-propanol were used. All solutions were readily spin-coated or slot-die coated into uniform thin films. Solutions in 1-propanol could be coated with concentrations up to 50 mg mL−1. All films were red in color and characterized by optical absorption spectroscopy confirming the existence of the parent PDIN-H species in the film. Single crystal X-ray diffraction was used to determine the molecular packing of PDIN-H. Films showed no signs of dewetting, swelling, or dissolution upon exposure to 2-propanol, water, or o-xylene via coating or dipping indicating they were solvent resistant. To exploit the semiconducting properties of these PDIN-H films, organic photovoltaic devices were fabricated using the films as electron transport layers in inverted type P3HT:PC61BM bulk heterojunction devices. Moreover, the PDIN-H films changed from red to purple upon exposure to butylamine vapors which prompted the investigation of hydroxide free processing. Indeed, films of PDIN-H were easily formed by processing with 1-propanol/butylamine or water/butylamine solutions.

Controlling hierarchical structural development in organic semiconductors, and across synthetic materials more broadly, is critical to the performance of the material in device applications. Such r...

Alkyl chains of varied length and steric bulk are generally appended to π-conjugated chromophores to increase solubility. These alkyl chains also regulate many aspects of the solid-state packing an...

Anthropogenic greenhouse gas emissions, such as CO2 from fossil fuel combustion, are a global environmental, health, and economic concern. Aqueous amine-based CO2 capture processes offer a technologically mature and relevant approach to CO2 sequestration, although cost reduction strategies are still necessary for widespread deployment. Inspired by the metalloenzyme carbonic anhydrase (CA), we report the design, synthesis, and activity testing of zinc(II) complexes [ZnII(PSAAMP)Cl2] (1) and [ZnII(PSAMEA)Cl2] (2) as CO2 hydration catalysts in aqueous amine solutions. The novel multifunctional ligand environment includes features in the primary and secondary coordination spheres that result in enhanced CO2 mass transfer in industrially relevant carbon capture solvents and stability towards harsh industrial process conditions. Complexes that lack these key features do not show enhanced CO2 absorption. Density functional theory (DFT) calculations that assess the catalytic pathway demonstrate how 1 and 2 catalyze CO2 hydration analogous to CA. These catalysts increase mass transfer by 20–55% in lab scale experiments, offering the potential to reduce the cost of amine-based CO2 capture processes without significantly altering industrial-scale system design, making rapid deployment of this critical bridge technology a viable strategy to reduce global greenhouse gas emissions.

Charge-carrier transport in thin-film organic semiconductors is strongly related to the molecular structure and the solid-state packing, which in turn are dependent on materials processing and devi...

Aryl sulfones and phosphine oxides are widely used as molecular building blocks for host materials in the emissive layers of organic light-emitting diodes. In this context, the chemical stability of such molecules in the triplet state is of paramount concern to long-term device performance. Here, we explore the triplet excited-state (T1) chemical stabilities of aryl sulfonyl and aryl phosphoryl molecules by means of UV absorption spectroscopy and density functional theory calculations. Both the sulfur–carbon bonds of the aryl sulfonyl molecules and the phosphorus–carbon bonds of aryl phosphoryl derivatives are significantly more vulnerable to dissociation in the T1 state when compared to the ground (S0) state. Although the vertical S0 → T1 transitions correspond to nonbonding → π-orbital transitions, geometry relaxations in the T1 state lead to σ–σ* character over the respective sulfur–carbon or phosphorus–carbon bond, a result of significant electronic state mixing, which facilitates bond dissociation. B...

2,7-Bis(3,3′′,6,6′′-tetra(tert-butyl)-9′H-9,3′:6′,9′′-tercarbazol-9-yl)-4,5,9,10-tetrahydropyrene (3) and 2,7-bis(3,3′′,6,6′′-tetra(tert-butyl)-9′H-9,3′:6′,9′′-tercarbazol-9-yl)pyrene (4), along with a model compound, 3,3′′,6,6′′-tetra(tert-butyl)-9′-(4-(tert-butyl)phenyl)-9′H-9,3′:6′,9′′-tercarbazole (6), have been synthesized using microwave-assisted palladium-catalyzed coupling and compared to analogous 3,6-di(tert-butyl)carbazol-9-yl species (1, 2, and 5). Time-dependent density functional theory (TDDFT) calculations reveal absorption with quadrupolar (ter)carbazole-to-bridge CT character for 1–4. Compound 4 is unusual in showing dual fluorescence in a number of solvents; the longer wavelength feature of which is markedly more solvatochromic than the bands of the other compounds. Following the observed three oxidations on the TCz model compound 6, compounds 3 and 4 can be electrochemically reversibly oxidized to hexacations with four oxidation and three oxidation steps in a 2 : 2 : 1 : 1 and a 2 : 2 : 2 current ratio, respectively. Tercarbazole derivatives 3 and 4 have been used as the emissive layers of simple solution-processed few-layer organic light-emitting diodes.

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.

Solution processed organic field effect transistors can become ubiquitous in flexible optoelectronics. While progress in material and device design has been astonishing, low environmental and operational stabilities remain longstanding problems obstructing their immediate deployment in real world applications. Here, we introduce a strategy to identify the most probable and severe degradation pathways in organic transistors and then implement a method to eliminate the main sources of instabilities. Real time monitoring of the energetic distribution and transformation of electronic trap states during device operation, in conjunction with simulations, revealed the nature of traps responsible for performance degradation. With this information, we designed the most efficient encapsulation strategy for each device type, which resulted in fabrication of high performance, environmentally and operationally stable small molecule and polymeric transistors with consistent mobility and unparalleled threshold voltage shifts as low as 0.1 V under the application of high bias stress in air., Electrical instability of organic field-effect transistors (OFETs) during operation remains a challenge that limits the device’s real-world technological viability. Here, the authors report a method for diagnosing and suppressing bias stress in solution-processed OFETs operated in air.

Halogenation, particularly fluorination, is commonly used to manipulate the energetics, stability, and morphology of organic semiconductors. In the case of organic photovoltaics (OPVs), fluorination of electron donor molecules or polymers at appropriate positions can lead to improved performance. In this contribution, we use ultraviolet photoelectron spectroscopy, external quantum efficiency measurements of charge-transfer (CT) states, and density functional theory calculations to systematically investigate the effects of halogenation on the bulk solid-state energetics of model anthradithiophene (ADT) materials, their interfacial energetics with C60, and the energetics of various ADT:C60 blend compositions. In agreement with previous work, nonhalogenated ADT molecules show higher energy CT states in blends with C60 and lower energy CT states in the ADT/C60 bilayers. However, this trend is reversed in the halogenated ADT/C60 systems, wherein the CT state energies of ADT:C60 blends are lower than those in t...

Carbon nanoribbons offer the potential of semiconducting materials that maintain the large charge-carrier mobilities of graphene. Here, starting with polyacene as a reference, we present a theoretical investigation as to how polycyclic aromatic hydrocarbons inserted into the polymer structure modulate the edge topology of the zigzag polyacene. The variations in edge topology, in turn, produce nanoribbon structures that have electronic properties that span insulators to narrow-gap semiconductors. Clear connections are made among foundational models in aromatic chemistry, namely, descriptions in terms of Clar formulas and bond-length alternation patterns, and the nanoribbon electronic, phonon, and charge-carrier mobility characteristics. These relationships, for systems that are synthetically feasible from bottom-up, solution-based approaches, offer a priori and rational design paradigms for the creation of new nanoribbon architectures.

Conformational diversity due to different orientations of structural subunits has a complex impact on morphological disorder of organic semiconductors. Here, we isolate the impact of a specific structural change: replacing bithiophene (biTh) units with thieno[3,2-b]thiophene (TT). We compare four molecules with an alternating donor–acceptor structure (D′–A–D–A–D′) composed of a central, electron-rich dithienosilole (DTS) unit flanked by pyridyl-[2,1,3]thiadiazole (PT) or fluorinated benzo[c][1,2,5]thiadiazole (FBT) and end-capped with bithiophene biTh or TT groups. We find that using TT instead of biTh results in an increased degree of order within films cast directly from solution by influencing the self-assembly tendencies of the different molecules. Unlike switching the acceptor subunit, such as FBT for PT, the TT for biTh structural change has little impact on the electronic structure of these molecular semiconductors. Instead, these morphological effects can be understood within the context of the predicted conformational diversity. TT units limit the number of rotational conformations (rotamers) available within this molecular architecture; low rotamer dispersity facilitates self-assembly into ordered domains. As a practical illustration of this greater drive toward self-assembly, we use the TT-containing molecules as donors in bulk heterojunction solar cells with PC70BM. Devices with TT-containing molecules show improved photovoltaic performance compared to their previously characterized biTh analogs (d-DTS(PTTh2)2 and p-DTS(FBTTh2)2) in both as-cast and optimized conditions, with efficiencies up to 6.4% and 8.8% for PT-TT and FBT-TT, respectively. The TT subunit and, more broadly, the strategy of limiting conformational diversity can be readily applied toward the design of solution-processable organic semiconductors with increased as-cast order.

Thermal analyses provide macroscopic measures as to how molecular-scale features impact the solid-state packing of organic semiconductors. Here, we make use of molecular dynamics simulations to explore the phase transitions of a series of anthradithiophene-based molecular materials. Various models are explored to overcome superheating effects that are typically associated with the simulated annealing of crystalline materials using periodic boundary conditions. Slab models, in particular, are shown to provide good agreement with melt temperatures determined experimentally, especially for unsubstituted anthradithiophenes. Importantly, the simulations provide atomic-scale details regarding solid–solid and solid–liquid phase transitions that deliver key insights into how variations in the anthradithiophene chemistry impact the nature of the molecular packing and, in turn, can be used to enhance the rational engineering of crystal packing in these organic semiconductors.

Here we present the synthesis and characterization of a hybrid vanadium-organic coordination polymer with robust magnetic order, a Curie temperature TC of ∼110 K, a coercive field of ∼5 Oe at 5 K, and a maximum mass magnetization of about half that of the benchmark ferrimagnetic vanadium(tetracyanoethylene)∼2 (V·(TCNE)∼2). This material was prepared using a new tetracyano-substituted quinoidal organic small molecule 7 based on a tricyclic heterocycle 4-hexyl-4H-pyrrolo[2,3-d:5,4-d′]bis(thiazole) (C6-PBTz). Single crystal X-ray diffraction of the 2,6-diiodo derivative of the parent C6-PBTz, showed a disordered hexyl chain and a nearly linear arrangement of the substituents in positions 2 and 6 of the tricyclic core. Density functional theory (DFT) calculations indicate that C6-PBTz-based ligand 7 is a strong acceptor with an electron affinity larger than that of TCNE and several other ligands previously used in molecular magnets. This effect is due in part to the electron-deficient thiazole rings and extended delocalization of the frontier molecular orbitals. The ligand detailed in this study, a representative example of fused heterocycle aromatic cores with extended π conjugation, introduces new opportunities for structure–magnetic-property correlation studies where the chemistry of the tricyclic heterocycles can modulate the electronic properties and the substituent at the central N-position can vary the spatial characteristics of the magnetic polymer.

Mixed cocrystals derived from electron-rich donor (D) and electron-deficient acceptor (A) molecules showcase electronic, optical, and magnetic properties of interest for a wide range of applications. We explore the structural and electronic properties of a cocrystal synthesized from dithieno[3,2-a:2',3'-c]phenazine (DTPhz) and 7,7,8,8-tetracyanoquinodimethane (TCNQ), which has a mixed-stack packing arrangement of the (π-electronic) face-to-face stacks in a 2:1 D:A stoichiometry. Density functional theory investigations reveal that the primary electronic characteristics of the cocrystal are not determined by electronic interactions along the face-to-face stacks, but rather they are characterized by stronger electronic interactions orthogonal to these stacks that follow the edge-to-edge donor-donor or acceptor-acceptor contacts. These distinctive electronic characteristics portend semiconducting properties that are unusual for semiconducting mixed cocrystals and suggest further potential to design organic semiconductors with orthogonal transport characteristics for different charge carriers.

Cyanine dyes in dilute solution have demonstrated the required third-order nonlinear optical properties necessary for all-optical switching (AOS) applications. However, in the solid state, interactions between the cyanine and its counterion typically induce significant geometric and electronic changes. When the counterion localizes toward one end of the cyanine, symmetry is broken and the figure-of-merit for AOS is dramatically reduced. Here, we use quantum-chemical methods to assess zwitterionic cyanines based on the heptamethine thiopyrylium dye as a means to eliminate untethered counterion interactions. In these zwitterionic systems, rigid π-conjugated substituents oppositely charged to the cyanine backbone are added to the thiopyrylium central carbon atom. For substituents that are unable to retain their anionic charge, and thus have a lower degree of zwitterionic character, electronic coupling between the substituent and cyanine backbone leads to profound geometry modifications and the thiopyrylium e...

Stable electron-donating organic compounds are of interest for numerous applications that require reversible electron-transfer reactions. Although many organic compounds are stable one-electron donors, removing a second electron from a small molecule to form its dication usually leads to rapid decomposition. For cost-effective electrochemical energy storage utilizing organic charge-storage species, the creation of high-capacity materials requires stabilizing more charge whilst keeping molecular weights low. Here we report the simple modification of N-ethylphenothiazine, which is only stable as a radical cation (not as a dication), and demonstrate that introducing electron-donating methoxy groups para to nitrogen leads to dramatically improved stability of the doubly oxidized (dication) state. Our results reveal that this derivative is more stable than an analogous compound with substituents that do not allow for further charge delocalization, rendering it a promising scaffold for developing atom-efficient, two-electron donors.

This study reports on the design and synthesis of an unsymmetrical π-conjugated organic molecule composed of perylene diimide, thienyl diketopyrrolopyrrole, and indoloquinoxaline pieced together using direct heteroarylation. This material demonstrates unprecedented response in the thin-film upon post-deposition solvent vapor annealing, resulting in dramatic red-shifts in optical absorption. Such changes were utilized to enhance photocurrent generation in P3HT based organic solar cells.

It is commonly assumed that charge-carrier transport in doped π-conjugated polymers is dominated by one type of charge carrier, either holes or electrons, as determined by the chemistry of the dopant. Here, through Seebeck coefficient and Hall effect measurements, we show that mobile electrons contribute substantially to charge-carrier transport in π-conjugated polymers that are heavily p-doped with strong electron acceptors. Specifically, the Seebeck coefficient of several p-doped polymers changes sign from positive to negative as the concentration of the oxidizing agents FeCl3 or NOBF4 increase, and Hall effect measurements for the same p-doped polymers reveal that electrons become the dominant delocalized charge carriers. Ultraviolet and inverse photoelectron spectroscopy measurements show that doping with oxidizing agents results in elimination of the transport gap at high doping concentrations. This approach of heavy p-type doping is demonstrated to provide a promising route to high-performance n-type organic thermoelectric materials. A broad range of characterization techniques is used to understand the dominant electron conduction in various p-type doped π-conjugated polymers, which show p-type and n-type thermoelectric power factors depending on the dopant concentration.

Using a simple -conjugated trimer, EDOT-phenylene-EDOT (where EDOT = 3,4-ethylenedioxythiophene), we evaluate the effect that fluorine substituents have upon changes in conformation, conjugation and oxidation potentials in -conjugated structures. These variations are assessed as a function of the fluorine atom’s propensity to feature in hydrogen and/or halogen bonding with other heteroatoms. The molecular motif was chosen because the EDOT unit presents the possibility of competing O···X or S···X non-covalent contacts (where X = H or F). Such non-bonding interactions are acknowledged to be highly influential in dictating molecular and polymer morphology and inducing changes in certain physical properties. We have studied four compounds, beginning with an unsubstituted bridging phenylene ring and then adding one, two or four fluorine units to the parent molecule. Our studies involve single crystal XRD studies, cyclic voltammetry, absorption spectroscopy and density functional theory calculations to identify the dominant non-covalent interactions and elucidate their effects on the molecules described. Experimental studies have also been carried out on the corresponding electrochemically synthesized polymers to confirm that these non-covalent interactions and their effects persist in polymers. Our findings show that hydrogen bonding and halogen bonding feature in these molecules and their corresponding polymers. ABSTRACT: Using a simple -conjugated trimer, EDOT-phenylene-EDOT (where EDOT = 3,4-ethylenedioxythiophene), we evaluate the effect that fluorine substituents have upon changes in conformation, conjugation and oxidation potentials in -conjugated structures. These variations are assessed as a function of the fluorine atom’s propensity to feature in hydrogen and/or halogen bonding with other heteroatoms. The molecular motif was chosen because the EDOT unit presents the possibility of competing O···X or S···X non-covalent contacts (where X = H or F). Such non-bonding interactions are acknowledged to be highly influential in dictating molecular and polymer morphology and inducing changes in certain physical properties. We have studied four compounds, beginning with an unsubstituted bridging phenylene ring and then adding one, two or four fluorine units to the parent molecule. Our studies involve single crystal XRD studies, cyclic voltammetry, absorption spectroscopy and density functional theory calculations to identify the dominant non-covalent interactions and elucidate their effects on the molecules described. Experimental studies have also been carried out on the corresponding electrochemically synthesized polymers to confirm that these non-covalent interactions and their effects persist in polymers. Our findings show that hydrogen bonding and halogen bonding feature in these molecules and their corresponding polymers.

Porphyrins are attractive chromophores for application in dye-sensitized solar cells (DSCs), as judicious tuning of donor–acceptor properties can enable excellent near-infrared (NIR) absorption and exceptional device performance. Here, we report a porphyrin-based dye (SM85) conjugated to the planar strong electron donor, indolizine, designed to extend absorption further into the NIR region by inducing π–π interactions such as head-to-tail dye aggregation. The optoelectronic consequences of indolizine incorporation in SM85 include raising the ground-state oxidation potential and broadening and red-shifting ultraviolet–visible–NIR absorptions, along with increased molar absorptivity when compared to the dye SM315. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations confirm the push–pull character of SM85, which features an overlap of frontier occupied and unoccupied orbitals. Steady-state spectrophotometric analyses reveal the presence of solution aggregates via absorption and emission spectroscopies. Aggregate modes were probed by DFT and TD-DFT analyses, and plausible models are presented. SM85-based DSC devices demonstrate a 5.7% power conversion efficiency (PCE) at full sun (7.4% PCE at 10% sun) with an exceptional improvement to the incident photon-to-current conversion onset at ∼850 nm. Current dynamics measurements, time-correlated single photon counting, and computational analyses are used to better understand device performances. This study puts forward a novel intramolecular charge-transfer porphyrin system with a dramatic shift into the NIR region, as is needed for nonprecious metal-based sensitizers, and provides an example of controlled, donor–acceptor-mediated aggregation as a complementary strategy to traditional donor–acceptor modifications to single-molecule π-systems in accessing enhancements in long wavelength light harvesting in molecular-based optoelectronic devices.

The functionalization of oligoacenes and similar π-conjugated chromophores with trialkylsilylethynyl groups has proven to be a versatile means to enhance solubility and solution processability and engineer solid-state packing arrangements to produce organic semiconductors that demonstrate outstanding charge-carrier transport characteristics. While a general, empirical-based geometric model has been developed and implemented to direct the solid-state packing arrangements of these molecular materials, there exist numerous examples where the model falters. Here, we employ electronic structure methods to probe the noncovalent, intermolecular interactions of two closely related systems that result in two very different crystal packing configurations: triisopropylsilylethynyl (TIPS) pentacene and its triethylsilylethynyl (TES) analog. The quantum-chemical evaluation details how the slightly larger electron density contained within the volume of the TIPS moiety with respect to TES is in part responsible for the ...

The authors wish to thank Dr. Stephen Barlow, Dr. Anthony Giordano, and Prof. Seth Marder for insightful discussions. This work is based on research supported in part by the Center for Interface Science: Solar-Electric Materials (CIS:SEM), an Energy Frontier Research Center funded through the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC0001084. We also acknowledge generous support from King Abdullah University of Science and Technology; we thank the IT Research Computing Team and Supercomputing Laboratory at KAUST for providing computational and storage resources. The computational resources at Georgia Tech are funded in part by the CRIF Program of the NSF under Award Number CHE-0946869.

Fullerenes are ubiquitous as electron-acceptor and electron-transport materials in organic solar cells. Recent synthetic strategies to improve the solubility and electronic characteristics of these molecules have translated into a tremendous increase in the variety of derivatives employed in these applications. Here, we use molecular dynamics (MD) simulations to examine the impact of going from monoadducts to bis- and tris-adducts on the structural, cohesive, and packing characteristics of [6,6]-phenyl-C60-butyric acid methyl ester (PCBM) and indene-C60. The packing configurations obtained at the MD level then serve as input for density functional theory calculations that examine the solid-state energetic disorder (distribution of site energies) as a function of chemical substitution. The variations in structural and site-energy disorders reflect the fundamental materials differences among the derivatives and impact the performance of these materials in thin-film electronic devices.

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.

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.

Seven cocrystals derived from 2,7-substituted benzo[1,2-b:6,5-b′]dithiophene-4,5-diones (BDDO) and trimeric perfluoro-o-phenylene mercury (TPPM) exhibit two prominent packing motifs: parallel mixed stacks and T-shaped columnar structures. The varied packing patterns reveal an interplay of noncovalent intermolecular interactions that depend on the nature of the BDDO 2,7-substituents and on the crystallization conditions. Quantum-chemical analyses show little charge-transfer character in the mixed-stack structures, suggesting limited electronic interaction among the mixed TPPM and BDDO constituents. The variations in molecular packing with rather minimal change in chemical structure expose the ability to fine-tune the structure of these molecular cocrystals.

Polymer:fullerene solar cells depend heavily on the electronic coupling of the polymer and fullerene molecular species from which they are composed. The intermolecular interaction between the polymer and fullerene tends to be strong in efficient photovoltaic systems, as evidenced by efficient charge transfer processes and by large changes in the energetics of the polymer and fullerene when they are molecularly mixed. Despite the clear presence of these strong intermolecular interactions between the polymer and fullerene, there is not a consensus on the nature of these interactions. In this work, we use a combination of Raman spectroscopy, charge transfer state absorption, and density functional theory calculations to show that the intermolecular interactions do not appear to be caused by ground state charge transfer between the polymer and fullerene. We conclude that these intermolecular interactions are primarily van der Waals in nature.

Although bis(diarylamino) mixed-valence radical cations have been quite extensively studied, their bis(carbazolyl) analogues have not, even though the hole-transporting properties of species such as of 4,4′-bis(9H-carbazol-9-yl)-1,1′-biphenyl, CBP, are widely exploited in organic light-emitting diodes. This work reports the generation by chemical oxidation of the radical cations of 4,4′-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-1,1′-biphenyl (a model for the unstable radical cation of CBP), 2,7-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-4,5,9,10-tetrahydropyrene, and 2,7-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)pyrene. The visible and near-IR spectra of these cations have been compared to those of the corresponding dication spectra, to the spectrum of the 3,6-di-tert-butyl-9-(4-(tert-butyl)phenyl)-9H-carbazole radical cation, and to the results of time-dependent density-functional calculations. The biphenyl- and pyrene-bridged species are found to be localized (class-II) mixed-valence compounds, whereas stronger...

Establishing fundamental relationships between strain and work function (WF) in organic semiconductors is important not only for understanding electrical properties of organic thin films, which are subject to both intrinsic and extrinsic strains, but also for developing flexible electronic devices. Here we investigate tensile and compressive strain effects on the WF of rubrene single crystals. Mechanical strain induced by thermal expansion mismatch between the substrate and rubrene is quantified by X-ray diffraction. The corresponding WF change is measured by scanning Kelvin probe microscopy. The WF of rubrene increases (decreases) significantly with in-plane tensile (compressive) strain, which agrees qualitatively with density functional theory calculations. An elastic-to-plastic transition, characterized by a steep rise of the WF, occurs at ∼0.05% tensile strain along the rubrene π-stacking direction. The results provide the first concrete link between mechanical strain and WF of an organic semiconductor and have important implications for understanding the connection between structural and electronic disorder in soft organic electronic materials., The understanding of strain effect on electronic properties of organic semiconductors is crucial for the designs of flexible electronics. Here, Wu et al. characterize the tensile and compressive strain effects on the work function of rubrene single crystals as a benchmark system.

Non-aqueous redox flow batteries (NAqRFBs) employing redox-active organic molecules show promise to meet requirements for grid energy storage. Here, we combine the rational design of organic molecules with flow cell engineering to boost NAqRFB performance. We synthesize two highly soluble phenothiazine derivatives, N-(2-methoxyethyl)phenothiazine (MEPT) and N-[2-(2-methoxyethoxy)ethyl]phenothiazine (MEEPT), via a one-step synthesis from inexpensive precursors. Synthesis and isolation of the radical-cation salts permit UV-vis decay studies that illustrate the high stability of these open-shell species. Cyclic voltammetry and bulk electrolysis experiments reveal the promising electrochemical properties of MEPT and MEEPT under dilute conditions. A high performance non-aqueous flow cell, employing interdigitated flow fields and carbon paper electrodes, is engineered and demonstrated; polarization and impedance studies quantify the cell's low area-specific resistance (3.2–3.3 Ω cm2). We combine the most soluble derivative, MEEPT, and its tetrafluoroborate radical-cation salt in the flow cell for symmetric cycling, evincing a current density of 100 mA cm−2 with undetectable capacity fade over 100 cycles. This coincident high current density and capacity retention is unprecedented in NAqRFB literature.

Many high performing organic semiconductor materials contain heteroaromatic rings in order to control the molecular packing and material electronic properties. Here we use a combination of density functional theory and symmetry-adapted perturbation theory calculations to explore the intermolecular noncovalent interactions, which guide solid-state molecular packing, and electronic couplings in a series of benzodithiophene-based dimer models. A novel concept, termed the disordermer, is introduced to delineate how the reduced molecular symmetry of benzodithiophene, when compared to the more highly symmetric anthracene molecule, can present intermolecular isomerism in the solid state that results in a wide range of available molecular packing arrangements that in turn influence the magnitudes of the electronic couplings. The insight developed through the investigation of these disordermers is demonstrated to hold important implications in the design of new generations of organic semiconductor materials.

Electron-withdrawing substituents are introduced onto the phenothiazine core to raise its oxidation potential for use as a redox shuttle in high-voltage lithium-ion batteries. A perfluorinated derivative oxidizes at 4.3 V vs. Li+/0, and functions for ca. 500 h of 100% overcharge in LiNi0.8Co0.15Al0.05O2/graphite coin cells at a charging rate of C/10.

The ability to effectively transport charge carriers is often a key determinant concerning the deployment of materials derived from π-conjugated molecules and polymers in (opto)electronic applications. Theoretical models to evaluate charge-carrier transport parameters across a range of organic materials often work under the approximation of evaluating the intermolecular electronic couplings for supermolecular complexes (i.e. dimers) in the neutral state. Here, we investigate how the explicit inclusion of the nature of the charged state (i.e. both the neutral and radical-cation states) impacts the assessment of the intermolecular electronic couplings, and how considerations of the density functionals often used to determine these couplings effect the computed magnitude. From a materials perspective, we explore the role that the dimer configuration plays in determining the magnitudes of the electronic couplings for oligoacenes. The results suggest that appropriate consideration of translational alignment along the long and short acene axes, even in configurations with near perpendicular edge-to-face interactions, can lead to molecular packing arrangements in the solid state with large electronic couplings. These results give insight into ways to fine tune solid-state molecular packing to ensure the highest possible electronic couplings.

Three molecular semiconductors are compared and evaluated in organic field-effect transistors and organic solar cells. The molecules are constructed from the dyes diketopyrrolopyrrole (DPP), perylene diimide (PDI), and N-(alkyl)benzothioxanthene-3,4-dicarboximide (BTXI). The compound PDI–DPP–PDI (1) has previously been reported and used as a nonfullerene acceptor. The compounds PDI–DPP–BTXI (2) and BTXI–DPP–BTXI (3) were synthesized using direct (hetero)arylation methods and fully identified using NMR spectroscopy and mass spectrometry. All three compounds were characterized using UV–visible spectroscopy, cyclic voltammetry, and density functional theory calculations. Increasing the BTXI content results in a progressive destabilization of the electronic energy levels. For all compounds, no significant changes in the optical absorption spectra are observed when compared to a combination of the constituent optical absorption spectra. Compound 1 exhibits electron transport characteristics and functions as an...

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.

C.R. thanks the University of Kentucky Vice President for Research and the Department of the Navy, Office of Naval Research (Award No. N00014-16-1-2985) for support. V.C. thanks the Department of the Navy, Office of Naval Research (Awards Nos. N00014-14-1-0580 and N00014-16-1-2520) for support. M.S. and D.D. acknowledge funding by the German Science Foundation through the SPP 1355 “Elementary Processes in Organic Photovoltaics.” The research data supporting this paper can be accessed at https://doi.org/10.17630/a6935caf-f7ed-48b2-b131-68ae72a26629. Fullerene-based acceptors have dominated organic solar cells for almost two decades. It is only within the last few years that alternative acceptors rival their dominance, introducing much more flexibility in the optoelectronic properties of these material blends. However, a fundamental physical understanding of the processes that drive charge separation at organic heterojunctions is still missing but urgently needed to direct further material improvements. Here we use a combined experimental and theoretical approach to understand the intimate mechanisms by which molecular structure contributes to exciton dissociation, charge separation, and charge recombination at the donor-acceptor (D-A) interface. We use model systems comprised of polythiophene-based donor and rylene diimide-based acceptor polymers and perform a detailed density functional theory (DFT) investigation. The results point to the roles that geometric deformations and direct-contact intermolecular polarization play in establishing a driving force (energy gradient) for the optoelectronic processes taking place at the interface. A substantial impact for this driving force is found to stem from polymer deformations at the interface, a finding that can clearly lead to new design approaches in the development of the next generation of conjugated polymers and small molecules. Postprint

Well-defined structure–property relationships offer a conceptual basis to afford a priori design principles to develop novel π-conjugated molecular and polymer materials for nonlinear optical (NLO) applications. Here, we introduce the bond ellipticity alternation (BEA) as a robust parameter to assess the NLO characteristics of organic chromophores and illustrate its effectiveness in the case of streptocyanines. BEA is based on the symmetry of the electron density, a physical observable that can be determined from experimental X-ray electron densities or from quantum-chemical calculations. Through comparisons to the well-established bond-length alternation and π-bond order alternation parameters, we demonstrate the generality of BEA to foreshadow NLO characteristics and underline that, in the case of large electric fields, BEA is a more reliable descriptor. Hence, this study introduces BEA as a prominent descriptor of organic chromophores of interest for NLO applications.

A selective and efficient method to afford a monobrominated benzothioxanthene (Br-BTXI) derivative is reported. Br-BTXI was extensively employed in common palladium catalyzed coupling reactions. Finally, as a proof of concept, a BTXI based molecular donor was synthesized and evaluated in bulk heterojunction solar cells.

Noncovalent intermolecular interactions, which can be tuned through the toolbox of synthetic chemistry, determine not only the molecular packing but also the resulting electronic, optical, and mechanical properties of materials derived from π-conjugated molecules, oligomers, and polymers. Here, we provide an overview of the theoretical underpinnings of noncovalent intermolecular interactions and briefly discuss the computational chemistry approaches used to understand the magnitude of these interactions. These methodologies are then exploited to illustrate how noncovalent intermolecular interactions impact important electronic properties—such as the electronic coupling between adjacent molecules, a key parameter for charge-carrier transport—through a comparison between the prototype organic semiconductor pentacene with a series of N-substituted heteropentacenes. Incorporating an understanding of these interactions into the design of organic semiconductors can assist in developing novel materials systems f...

Correction for ‘An unsymmetrical non-fullerene acceptor: synthesis via direct heteroarylation, self-assembly, and utility as a low energy absorber in organic photovoltaic cells’ by Abby-Jo Payne et al., Chem. Commun., 2017, 53, 10168–10171.

The substitution of sterically bulky groups at precise locations along the periphery of fused-ring aromatic systems is demonstrated to increase electrochemical oxidation potentials by preventing relaxation events in the oxidized state. Phenothiazines, which undergo significant geometric relaxation upon oxidation, are used as fused-ring models to showcase that electron-donating methyl groups, which would generally be expected to lower oxidation potential, can lead to increased oxidation potentials when used as the steric drivers. Reduction events remain inaccessible through this molecular design route, a critical characteristic for electrochemical systems where high oxidation potentials are required and in which reductive decomposition must be prevented, as in high-voltage lithium-ion batteries. This study reveals a new avenue to alter the redox characteristics of fused-ring systems that find wide use as electroactive elements across a number of developing technologies.

This work has been supported in part by King Abdullah University of Science and Technology (KAUST), the KAUST Competitive Research Grant Program, and the Office of Naval Research Global (Award N62909-15-1-2003). We acknowledge the IT Research Computing Team and Supercomputing Laboratory at King Abdullah University of Science & Technology (KAUST) for providing computational and storage resources. The work at the University of Kentucky was supported by a seed grant from the Center for Applied Energy Research (CAER) and start-up funds provided by the University of Kentucky Vice President for Research. We gratefully thank Drs. Sean Ryno, Naga Rajesh Tummala, and Chris Sutton for stimulating discussions.

International audience; Among π-conjugated polymer donors for efficient bulk-heterojunction (BHJ) solar cell applications, poly(benzo[1,2-b:4,5-b′]dithiophene–thieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) polymers yield some of the highest open-circuit voltages (VOC, ca. 0.9 V) and fill-factors (FF, ca. 70%) in conventional (single-cell) BHJ devices with PCBM acceptors. In PBDTTPD, side chains of varying size and branching affect polymer self-assembly, nanostructural order, and impact material performance. However, the role of the polymer side-chain pattern in the intimate mixing between polymer donors and PCBM acceptors, and on the development of the BHJ morphology is in general less understood. In this contribution, we show that ring substituents such as furan (F), thiophene (T) and selenophene (S)—incorporated into the side chains of PBDTTPD polymers—can induce significant and, of importance, very different morphological effects in BHJs with PCBM. A combination of experimental and theoretical (via density functional theory) characterizations sheds light on how varying the heteroatom of the ring substituents impacts (i) the preferred side-chain configurations and (ii) the ionization, electronic, and optical properties of the PBDTTPD polymers. In parallel, we find that the PBDT(X)TPD analogs (with X = F, T, or S) span a broad range of power conversion efficiencies (PCEs, 3–6.5%) in optimized devices with improved thin-film morphologies via the use of 1,8-diiodooctane (DIO), and discuss that persistent morphological impediments at the nanoscale can be at the origin of the spread in PCE across optimized PBDT(X)TPD-based devices. With their high VOC ∼1 V, PBDT(X)TPD polymers are promising candidates for use in the high-band gap cell of tandem solar cells.

Here, we investigate the effects of both tuning the range-separation parameter of long-range corrected (LRC) density functionals and including dispersion corrections on describing the local optoelectronic properties of polymer–fullerene interfaces that are critical to the performance of polymer solar cells (PSCs). Focusing on recently studied (Chen, et al., Chem. Mater., 2012, 24, 4766–4772) PSC active layers derived from poly(benzodithiophene-co-quinoxaline) and substituted fullerene PC71BM, we compare the performance of global hybrid functionals (B3LYP and B3LYP-D) alongside two LRC functionals (ωB97X and ωB97X-D) and their optimally tuned (OT) analogs (OT-ωB97X and OT-ωB97X-D). Our results confirm that OT-LRC functionals generally improve the description of the optical properties of the individual materials with respect to experiment. For electron-donor (eD)–electron-acceptor (eA) complexes used to describe the local optoelectronic properties of the material interface, PC71BM is found to preferentially settle near the quinoxaline acceptor units on the copolymer backbone, regardless of the functional, though dispersion corrections have a strong influence on the intermolecular distances and, in turn, the nature of the excited states. All functionals yield very similar descriptions of the transition maxima for the complexes, i.e. predominant local excitations on the copolymer. Importantly, tuning the range-separation parameter of the LRC functional is shown to have a profound effect, as the OT functionals allow for the description of the charge transfer states of the eD–eA complexes, while the non-tuned LRC functionals predict only local intramolecular excitations. These results hold considerable importance for deriving the appropriate physical understanding of the interfacial structure–property–function relationships of PSCs.

Conjugated donor-acceptor (D-A) copolymers show tremendous promise as active components in thin-film organic bulk heterojunction solar cells and transistors, as appropriate combinations of D-A units enable regulation of the intrinsic electronic and optical properties of the polymer. Here, the structural, electronic, and optical properties of two D-A copolymers that make use of thieno[3,4-c]pyrrole-4,6-dione as the acceptor and differ by their donor unit-benzo[1,2-b:4,5-b']dithiophene (BDT) vs the ladder-type heptacyclic benzodi(cyclopentadithiophene)-are compared using density functional theory methods. Our calculations predict some general similarities, although the differences in the donor structures lead also to clear differences. The extended conjugation of the stiff ladder-type donor destabilizes both the highest occupied and lowest unoccupied molecular orbital energies of the ladder copolymer and results in smaller gap energies compared to its smaller counterpart. However, more significant charge transfer nature is predicted for the smaller BDT-based copolymer by natural transition orbitals than for the ladder copolymer. That is, the influence of the acceptor on the copolymer properties is "diluted" to some extent by the already extended conjugation of the ladder-type donor. Thus, the use of stronger acceptor units with the ladder-type donors would benefit the future design of new D-A copolymers.