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While it is known that the chain length strongly affects the properties of π-conjugated polymers, the effects of chain length on the molecular structure, chain conformation, and oxidation state pro...

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.

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.

A carbon side-chain analogue to the high-performance organic semiconductor triethylsilylethynyl difluoroanthradithiophene has been synthesised and characterized. Atomic substitution of carbon for silicon results in subtle changes to opto-electronic properties, which are rationalised by density functional theory and balance of electron donating and withdrawing effects. Larger differences are observed in photostability and solid-state packing of the new material in comparison to known silicon and germanium derivatives. Comparison of the group 14 elements teaches us about the newly synthesised system, but also how the silylethynyl substituents used for the last two decades contribute to successful employment of functionalised polycyclic aromatic hydrocarbons as organic semiconductors.

Derived from the lateral fusion of benzene rings, acenes are a class of π-conjugated molecules containing a single aromatic sextet, where system size is inversely correlated with chemical stability. In the pursuit of creating graphene nanoribbons/nanowires, several extended-ring structures have been synthesized through linear combinations of azaacenes and pyrene. Importantly, these extended systems demonstrate enhanced chemical stability and allow for the construction of macromolecular-sized structures. Here, we present a combined quantum-chemical and experimental study to reveal the cost of these improved characteristics in fully carbon-based systems. The results clearly show that pyrene moieties inserted among acene units do not result in long acene-like structures, rather the pyrene-inserted acene is, electronically, a series of (nearly) isolated acenes. The origin of pyrene's electronic blocking effect and implications on oxidized and photoexcited states of these extended-ring systems are detailed. The results of this investigation definitively show that coupling pyrene in an orthogonal orientation (through the 4, 5/9, 10 positions or e/l faces) to acenes should be eschewed if nanographene-/nanowire-like structures are desired.

Layered double hydroxides (LDH) demonstrate significant potential across a range of applications, including as catalysts, delivery vehicles for pharmaceuticals, environmental remediation, and supercapacitors. Explaining the mechanism of LDH action at the atomic scale in these and other applications is challenging, however, due to the difficulty in precisely defining the bulk and surface structure and chemical compositions. Here, we focus on the determination of the structure of lithium-aluminum (Li-Al) LDH, which has shown promise in the catalytic depolymerization of lignin, both directly as the catalyst and as a support for gold nanoparticles. While the relative positions of the Li and Al metals are generally well resolved by X-ray crystallography, it is the structures of the anionic layers, consisting of water and carbonate, that are less well established. Combinatorial analyses of all possible positions and rotations of the water and carbonate in the three-layered Li-AL LDH polytope reveals that the phase space is much too large to examine in any reasonable time frame in a one-by-one structure exploration. To overcome this limitation, we develop and deploy a genetic algorithm (GA) wherein fitness is determined by matching a calculated X-ray diffraction (XRD) pattern for a given structure to the known experimental XRD pattern. The GA approach results in structures of high fitness that portend the bulk Li-Al LDH structure. Importantly, the GA approach offers the potential to determine the structures of other LDH, and more generally layered materials, which are generally difficult to describe given the large chemical and structural space to be explored.

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.

The design of molecules and polymers for solution-deposited organic semiconducting materials generally considers the chemical modulation of (i) the π-conjugated backbone to modify the electronic and optical characteristics and (ii) the alkyl side chains to govern solubility. As the solid-state material forms, physical interactions among these constituents play an important, yet not well understood, role in directing the molecular-scale packing arrangements that in part determine the final material properties. In this presentation we will discuss how the dynamics of these moieties under different conditions, including the potential for conformational disorder among various points of torsion within the π-conjugated backbone, can impact aggregate formation and resulting solid-state morphology. The chemical insight developed through these investigations is beginning to refine and offer novel understanding essential to the development of next generation organic semiconducting active layers.

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...

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...

Functionalization of organic semiconductors through the attachment of bulky side groups to the conjugated core has imparted solution processability to this class of otherwise insoluble materials. A consequence of this functionalization is that the bulky side groups impact the solid-state packing of these materials. To examine the importance of side-group electronic character on accessing the structural phase space of functionalized materials, germanium was substituted for silicon in triisopropylsilylethynylpentacene (TIPS-Pn) to produce triisopropylgermanylethynylpentacene (TIPGe-Pn), with the TIPGe side group comparable in size to TIPS, but higher in electron density. We find TIPGe-Pn single crystals exhibit slip-stack, herringbone, and brickwork packing motifs depending on growth conditions, a stark contrast to TIPS-Pn, which accesses only the brickwork packing motif in both single crystals and thin films. Polycrystalline thin films of TIPGe-Pn exhibit two new, unidentified polymorphs from spin-coating ...

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.

Herein, we describe the design and synthesis of a suite of molecules based on a benzodithiophene “universal crystal engineering core”. After computationally screening derivatives, a trialkylsilylethyne-based crystal engineering strategy was employed to tailor the crystal packing for use as the active material in an organic field-effect transistor. Electronic structure calculations were undertaken to reveal derivatives that exhibit exceptional potential for high-efficiency hole transport. The promising theoretical properties are reflected in the preliminary device results, with the computationally optimized material showing simple solution processing, enhanced stability, and a maximum hole mobility of 1.6 cm2 V−1 s−1.

Rigid three-dimensional (3D) polycyclic aromatic hydrocarbons (PAHs), in particular 3D nanographenes, have garnered interest due to their potential use in semiconductor applications and as models to study through-bond and through-space electronic interactions. Herein we report the development of a novel 3D-symmetric rylene imide building block, triperyleno[3,3,3]propellane triimides (6), that possesses three perylene monoimide subunits fused on a propellane. This building block shows several promising characteristics, including high solubility, large π-surfaces, electron-accepting capabilities, and a variety of reactive sites. Further, the building block is compatible with different reactions to readily yield quasi-D3h symmetric nanostructures (9, 11, and 13) of varied chemistries. For the 3D nanostructures we observed red-shift absorption maxima and amplification of the absorption coefficients when compared to the individual subunits, indicating intramolecular electronic coupling among the subunits. In addition, the microplates of 9 exhibit comparable mobilities in different directions in the range of 10−3 cm2 V−1 s−1, despite the rather limited intermolecular overlap of the π-conjugated moieties. These findings demonstrate that these quasi-D3h symmetric rylene imides have potential as 3D nanostructures for a range of materials applications, including in organic electronic devices.

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.

We use organic field-effect transistor (OFET) measurements to perform a systematic study of the generation/annihilation of traps in organic semiconductors deliberately exposed to different stimuli. The first study involves the generation of a deep trap during repetitive transistor operation in ambient. The second study focuses on the dynamics of traps due to changes induced in the film microstructure in small molecule OFETs during exposure to solvents. Lastly, water-related traps in polymer films are investigated via real-time monitoring of the trap DOS spectrum during removal of water using a desiccant. A discrete peak responsible for threshold voltage instabilities was established upon removal of water from the SiO2 surface. The trap DOS spectrum provided a solid platform to understand the dynamics of trap formation/annihilation, which led to complete suppression of the discrete peak through a new surface passivation route that led to a remarkable enhancement in device performance.

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...

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.

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.

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.

This chapter devotes to the progress achieved in the theoretical understanding of how the chemical nature and molecular architecture of p-conjugated chromophores determine: the intermolecular noncovalent interactions that impact the molecular packing arrangements in thin films or crystals; and polarization phenomena in the solid state, both of which govern the material electronic and optical properties. It present a brief overview of recent advances in the density functional theory (DFT) description of organic electronic systems, as well as a discussion of the most pressing challenges and of important caveats. Ground-state DFT calculations can be used to calculate a number of important parameters, including molecular geometries, vibrational frequencies, electronic band-structures, electronic densities, and density differences, or dipole moments. DFT-based methodologies have been widely applied to characterize the geometric and electronic structures of various metal and conducting oxide surfaces and their interfaces with organic molecular layers of relevance to opto-electronic device applications.

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 ...

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.

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.

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.

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.

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.

Electrical doping of organic semiconductors increases conductivity and reduces injection barriers from electrode materials, both of which effects can improve the performance of organic light-emitting diodes (OLEDs). However, the low electron affinities of typical OLED electron-transport materials make the identification of suitable n-dopants particularly challenging; electropositive metals such as the alkali metals are not easily handled and form monoatomic ions that are rather mobile in host materials, whereas molecular dopants that operate as simple one-electron reductants must have low ionization energies, which leads to severe air sensitivity. This presentation will discuss approaches to circumventing this issue by coupling electron transfer to other chemical reactivity. In particular, dimers formed by certain highly reducing organometallic sandwich compounds and organic radicals can be handled in air, yet have effective reducing potentials, corresponding to formation of the corresponding monomeric cations and contribution of two electrons to the semiconductor, of ca. –2.0 V vs. ferrocene. These values fall a little short of what is required for typical OLED materials; approaches to further extending the doping reach of these dimers will be described. One such approach involving photoirradiation of a dimer:semiconductor blend leads to metastable doping of a material with a redox potential of –2.24 V, which allows the fabrication of efficient OLEDs in which even high-workfunction electrodes, such as indium tin oxide, can be used as electron-injection contacts.

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...

In the quest to improve the performance of organic bulk heterojunction solar cells, many recent efforts have focused on developing molecular and polymer alternatives to commonly used fullerene acceptors. Here, molecular dynamics simulations are used to investigate polymer:molecule blends comprised of the polymer donor poly(3-hexylthiophene) (P3HT) with a series of acceptors based on trialkylsilylethynyl-substituted pentacene. A matrix of nine pentacene derivatives, consisting of systematic chemical variation both in the nature of the alkyl groups and electron-withdrawing moieties appended to the acene, is used to draw connections between the chemical structure of the acene acceptor and the nanoscale properties of the polymer:molecule blend, which include polymer and molecular diffusivity, donor–acceptor packing and interfacial (contact) area, and miscibility. The results point to the very significant role that seemingly modest changes in chemical structure play during the formation of polymer:molecule ble...

Quantifying cohesion and understanding fracture phenomena in thin-film electronic devices are necessary for improved materials design and processing criteria. For organic photovoltaics (OPVs), the cohesion of the photoactive layer portends its mechanical flexibility, reliability, and lifetime. Here, the molecular mechanism for the initiation of cohesive failure in bulk heterojunction (BHJ) OPV active layers derived from the semiconducting polymer poly(3-hexylthiophene) [P3HT] and two monosubstituted fullerenes is examined experimentally and through molecular-dynamics simulations. The results detail how, under identical conditions, cohesion significantly changes due to minor variations in the fullerene adduct functionality, an important materials consideration that needs to be taken into account across fields where soluble fullerene derivatives are used.

Due to their inherent mechanical flexibility and stretchability, organic-based electronic devices have garnered a great deal of academic and industrial interest. Here, molecular-dynamics simulations are used to examine the molecular-scale details that govern the relationships among molecular weight, chain entanglement, persistence length, and the elastic characteristics of the widely studied π-conjugated polymer poly-(3-hexyl thiophene), P3HT. Oligomers containing at least 50 monomer units are required in the simulations to observe elastic behavior in P3HT, while much longer chains are required to ensure description of appropriate levels of entanglement: only when the molecular weight is greater than 50 kDa, that is, oligomers with approximately 400 monomer units, is truly entangled behavior observed. Interestingly, results from primitive path analysis of amorphous P3HT matches well with the observed onsets of inter-chain excitonic coherence with increased molecular weight. The simulations also indicate that the P3HT modulus saturates at 1.6 GPa for chain lengths of 50–100 monomers, a result that compares well with experimental results. This work highlights the care that needs to be taken to accurately model P3HT morphologies in relation to experimental measurements. © 2015 The Authors. Journal of Polymer Science Part B: Polymer Physics Published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 934–942

Increasing the acceptor strength of the widely used acceptor benzothiadiazole (BT) by extending the heterocyclic core is a promising strategy for developing new and stronger acceptors for materials in organic electronics and photonics. In recent years, such heteroannulated BT acceptors have been incorporated into a wide variety of materials that have been used in organic electronic and photonic devices. This review critically assesses the properties of these materials. Although heteroannulation to form acceptors, such as benzo[1,2-c:4,5-c′]bis[1,2,5]thiadiazole (BBT), does result in materials with significantly higher electron affinity (EA) relative to BT, in many cases the extended BT systems also exhibit lower ionization energy (IE) than BT. Both the significantly higher EA and lower IE limit the efficacy of these materials in applications such as bulk heterojunction organic photovoltaics (BHJ-OPV) based on C60. Although the relatively high EA may enable some applications such as air stable organic field effect transistors (OFET), more widespread use of heteroannulated BT acceptors will likely require the ability to moderate or retain the high EA while increasing IE.

We review some of the computational methodologies used in our research group to develop a better understanding of the geometric and electronic structures of organic-organic interfaces present in the active layer of organic solar cells. We focus in particular on the exciton-dissociation and charge-transfer processes at the pentacene-fullerene interface. We also discuss the local morphology at this interface on the basis of molecular dynamics simulations.

Judicious substitution of chalcogenopyrylium-terminated polymethine dyes with sterically demanding groups ameliorates the deleterious effects of aggregation on the optical properties of these materials in the solid state, facilitating high-number-density films that exhibit an unprecedented combination of nonlinear optical properties with low linear and nonlinear losses.

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.

Organic solar cells based on the combination of squaraine dyes (as electron donors) and fullerenes (as electron acceptors) have recently garnered much attention. Here, molecular dynamics simulations are carried out to investigate the evolution of a squaraine–C60 bilayer interface as a function of the orientation and order of the underlying squaraine layer. Electronic couplings between the main electronic states involved in exciton dissociation and charge (polaron pair) recombination are derived for donor–acceptor complexes extracted from the simulations. The results of the combined molecular-dynamics−quantum-mechanics approach provide insight into how the degree of molecular order and the dynamics at the interface impact the key processes involved in the photovoltaic effect.

Charge-carrier transport in thin-film organic field-effect transistors takes place within the first (few) molecular layer(s) of the active organic material in contact with the gate dielectric. Here, we use atomistic molecular dynamics simulations to evaluate how interactions with bare amorphous silica surfaces that vary in terms of surface potential influence the molecular packing and dynamics of a monolayer pentacene film. The results indicate that the long axis of the pentacene molecules has a non-negligible tilt angle away from the surface normal. Grazing-incidence X-ray diffraction patterns for these models are calculated, and we discuss notable differences in the shapes of the Bragg rods as a function of the molecular packing, also in relation to previously published experimental reports. Intermolecular electronic couplings (transfer integrals) evaluated for the monolayers show marked differences compared to bulk crystal calculations, a result that points to the importance of fully considering the molecular packing environment in charge-carrier mobility models for organic electronic materials.

Donor–acceptor π-conjugated copolymers are of interest for a wide range of electronic applications, including field-effect transistors and solar cells. Here, we present a density functional theory (DFT) study of the impact of varying the conjugation pathway on the geometric, electronic, and optical properties of donor–acceptor systems. We consider both linear (“in series”), traditional conjugation among the donor–acceptor moieties versus structures where the acceptor units are appended orthogonally to the linear, donor-only conjugated backbone. Long-range-corrected hybrid functionals are used in the investigation with the values of the tuned long-range separation parameters providing an estimate of the extent of conjugation as a function of the oligomer architecture. Considerable differences in the electronic and optical properties are determined as a function of the nature of the conjugation pathway, features that should be taken into account in the design of donor–acceptor copolymers.

All-optical switching—controlling light with light—has the potential to meet the ever-increasing demand for data transmission bandwidth. The development of organic π -conjugated molecular materials with the requisite properties for all-optical switching applications has long proven to be a signifi cant challenge. However, recent advances demonstrate that polymethine dyes have the potential to meet the necessary requirements. In this review, we explore the theoretical underpinnings that guide the design of π -conjugated materials for all-optical switching applications. We underline, from a computational chemistry standpoint, the relationships among chemical structure, electronic structure, and optical properties that make polymethines such promising materials.