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Doped organic semiconductors are critical to emerging device applications, including thermoelectrics, bioelectronics, and neuromorphic computing devices. It is commonly assumed that low conductivities in these materials result primarily from charge trapping by the Coulomb potentials of the dopant counterions. Here, we present a combined experimental and theoretical study rebutting this belief. Using a newly developed doping technique based on ion exchange, we prepare highly doped films with several counterions of varying size and shape and characterize their carrier density, electrical conductivity, and paracrystalline disorder. In this uniquely large data set composed of several classes of high-mobility conjugated polymers, each doped with at least five different ions, we find electrical conductivity to be strongly correlated with paracrystalline disorder but poorly correlated with ionic size, suggesting that Coulomb traps do not limit transport. A general model for interacting electrons in highly doped polymers is proposed and carefully parametrized against atomistic calculations, enabling the calculation of electrical conductivity within the framework of transient localization theory. Theoretical calculations are in excellent agreement with experimental data, providing insights into the disorder-limited nature of charge transport and suggesting new strategies to further improve conductivities.

Inelastic neutron scattering (INS) provides a weighted density of phonon modes. Currently, INS spectra can only be interpreted for perfectly crystalline materials because of high computational cost for electronic simulations. INS has the potential to provide detailed morphological information if sufficiently large volumes and appropriate structural variety are simulated. Here, we propose a method that allows direct comparison between INS data with molecular dynamics simulations, a simulation method that is frequently used to simulate semicrystalline/amorphous materials. We illustrate the technique by analyzing spectra of a well-studied conjugated polymer, poly(3-hexylthiophene-2,5-diyl) (P3HT) and conclude that our technique provides improved volume and structural variety, but that the classical force field requires improvement before the morphology can be accurately interpreted.

Doping-induced solubility control (DISC) is a recently introduced photolithographic technique for semiconducting polymers, which utilizes reversible changes in polymer solubility upon doping to allow the polymer to function as its own photoresist. Central to this process is a wavelength sensitive optical dedoping reaction, which is poorly understood but generates subdiffraction-limited topographic features and provides optical control of the polymer doping level. Here, we examine the mechanism of optical dedoping in the semiconducting polymer poly-3-hexylthiophene (P3HT) doped by 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), via a combination of ultrafast and steady-state spectroscopy, ab initio calculations, and multidimensional NMR. A simple photoinduced back electron transfer mechanism from reduced F4TCNQ to oxidized P3HT does not explain the observed photophysics. Instead, photoexcited F4TCNQ* reacts with THF solvent molecules to form a neutral, nondoping, and highly soluble F4TCNQ-THF complex. Hence, ionized F4TCNQ is removed from the P3HT indirectly by depletion of the neutral F4TCNQ. Because the reaction involves only the dopant and similar photoreactivity would expected for most other dopant molecules, we expect optical DISC patterning should be generalizable to a wide range of polymer:dopant systems.

© The Royal Society of Chemistry. Recent theories suggest that low frequency dynamic intramolecular and intermolecular motions in organic semiconductors (OSCs) are critical to determining the hole mobility. So far, however, it has not been possible to probe these motions directly experimentally and therefore no unequivocal and quantitative link exists between molecular-scale thermal disorder and macroscale hole mobility in OSCs. Here we use inelastic neutron scattering to probe thermal disorder directly by measuring the phonon spectrum in six different small molecule OSCs, which we accurately reproduce with first principles simulations. We use the simulated phonons to generate a set of electron-phonon coupling parameters. Using these parameters, the theoretical mobility is in excellent agreement with macroscopic measurements. Comparison of mobility between different materials reveals routes to improve mobility by engineering phonon and electron-phonon coupling.

When an organic semiconductor (OSC) is blended with an electron acceptor molecule that can act as a p-type dopant, there should ideally be complete (integer) transfer of charge from the OSC to the dopant. However, some dopant–OSC blends instead form charge transfer complexes (CTCs), characterized by fractional charge transfer (CT) and strong orbital hybridization between the two molecules. Fractional CT doping does not efficiently generate free charge carriers, but it is unclear what conditions lead to incomplete charge transfer. Here we show that by modifying film processing conditions in the semiconductor–dopant couple poly(3-hexylthiophene):2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (P3HT:F4TCNQ), we can selectively obtain nearly pure integer or fractional CT phases. Fractional CT films show electrical conductivities approximately 2 orders of magnitude lower than corresponding integer CT films, and remarkably different optical absorption spectra. Grazing incidence wide-angle X-ray diffraction (GIXD) reveals that fractional CT films display an unusually dense and well-ordered crystal structure. These films show lower paracrystallinity and shorter lamellar and π-stacking distances than undoped films processed under similar conditions. Using plane-wave DFT we obtain a structure with unit cell parameters closely matching those observed by GIXD. This first-ever observation of both fractional and integer CT in a single OSC–dopant system demonstrates the importance of structural effects on OSC doping and opens the door to further studies.

© 2017 Informa UK Limited, trading as Taylor & Francis Group. Organic electronics is a popular and rapidly growing field of research. The optical, electrical and mechanical properties of organic molecules and materials can be tailored using increasingly well controlled synthetic methods. The challenge and fascination with this field of research is derived from the fact that not only the chemical identity, but also the spatial arrangement of the molecules critically affects the performance of the material. Thus synthetic, fabrication, characterisation and computational scientists need to work closely to relate a materials performance in a device to the molecular details that cause and optimise that performance. For computational scientists in particular, the need to relate macroscopic device performance to details of molecular electronic structure brings challenges in methodology due to the need to bridge many orders of time and length scales. This article provides a survey of computational methods applied to multiple-length and time scale problems in organic electronic materials. Here we seek to highlight a few particular approaches that expand the simulation toolbox.

© 2017 American Chemical Society. The greatest advantage of organic materials is the ability to synthetically tune desired properties. However, structural heterogeneity often obfuscates the relationship between chemical structure and functional properties. Inelastic neutron scattering (INS) is sensitive to both local structure and chemical environment and provides atomic level details that cannot be obtained through other spectroscopic or diffraction methods. INS data are composed of a density of vibrational states with no selection rules, which means that every structural configuration is equally weighted in the spectrum. This allows the INS spectrum to be quantitatively decomposed into different structural motifs. We present INS measurements of the semiconducting polymer P3HT doped with F4TCNQ supported by density functional theory calculations to identify two dominant families of undoped crystalline structures and one dominant doped structural motif, in spite of considerable heterogeneity. The differences between the undoped and doped structures indicate that P3HT side chains flatten upon doping.

© 2016 American Chemical Society. Although doping is a cornerstone of the inorganic semiconductor industry, most devices using organic semiconductors (OSCs) make use of intrinsic (undoped) materials. Recent work on OSC doping has focused on the use of dopants to modify a material's physical properties, such as solubility, in addition to electronic and optical properties. However, if these effects are to be exploited in device manufacturing, a method for dedoping organic semiconductors is required. Here, we outline two chemical strategies for dedoping OSC films. In the first strategy, we use an electron donor (a tertiary amine) to act as competitive donor. This process is based on a thermodynamic equilibrium between ionization of the donor and OSC and results in only partial dedoping. In the second strategy, we use an electron donor that subsequently reacts with the p-type dopant to create a nondoping product molecule. Primary and secondary amines undergo a rapid addition reaction with the dopant molecule 2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (F4TCNQ), with primary amines undergoing a further reaction eliminating HCN. Under optimized conditions, films of semiconducting polymer poly(3-hexylthiophene) (P3HT) dedoped with 1-propylamine (PA) reach as-cast fluorescence intensities within 5 s of exposure to the amine, eventually reaching 140% of the as-cast values. Field-effect mobilities similarly recover after dedoping. Quantitative fluorescence recovery is possible even in highly fluorescent polymers such as PFB, which are expected to be much more sensitive to residual dopants. Interestingly, treatment of undoped films with PA also yields increased fluorescence intensity and a reduction in conductivity of at least 2 orders of magnitude. These results indicate that the process quantitatively removes not only F4TCNQ but also intrinsic p-type impurities present in as-cast films. The dedoping strategies outlined in this article are generally applicable to other p- and n-type molecular dopants in OSC films.

Computational models using metabolic reconstructions for in silico simulation of metabolic disorders such as type 2 diabetes mellitus (T2DM) can provide a better understanding of disease pathophysiology and avoid high experimentation costs. There is a limited amount of computational work, using metabolic reconstructions, performed in this field for the better understanding of T2DM. In this study, a new algorithm for generating tissue-specific metabolic models is presented, along with the resulting multi-confidence level (MCL) multi-tissue model. The effect of T2DM on liver, muscle, and fat in MKR mice was first studied by microarray analysis and subsequently the changes in gene expression of frank T2DM MKR mice versus healthy mice were applied to the multi-tissue model to test the effect. Using the first multi-tissue genome-scale model of all metabolic pathways in T2DM, we found out that branched-chain amino acids' degradation and fatty acids oxidation pathway is downregulated in T2DM MKR mice. Microarray data showed low expression of genes in MKR mice versus healthy mice in the degradation of branched-chain amino acids and fatty-acid oxidation pathways. In addition, the flux balance analysis using the MCL multi-tissue model showed that the degradation pathways of branched-chain amino acid and fatty acid oxidation were significantly downregulated in MKR mice versus healthy mice. Validation of the model was performed using data derived from the literature regarding T2DM. Microarray data was used in conjunction with the model to predict fluxes of various other metabolic pathways in the T2DM mouse model and alterations in a number of pathways were detected. The Type 2 Diabetes MCL multi-tissue model may explain the high level of branched-chain amino acids and free fatty acids in plasma of Type 2 Diabetic subjects from a metabolic fluxes perspective.

Molecular doping is a crucial tool for controlling the charge-carrier concentration in organic semiconductors. Each dopant molecule is commonly thought to give rise to only one polaron, leading to a maximum of one donor: acceptor charge-transfer complex and hence an ionization efficiency of 100%. However, this theoretical limit is rarely achieved because of incomplete charge transfer and the presence of unreacted dopant. Here, we establish that common p-dopants can in fact accept two electrons per molecule from conjugated polymers with a low ionization energy. Each dopant molecule participates in two charge-transfer events, leading to the formation of dopant dianions and an ionization efficiency of up to 200%. Furthermore, we show that the resulting integer charge-transfer complex can dissociate with an efficiency of up to 170%. The concept of double doping introduced here may allow the dopant fraction required to optimize charge conduction to be halved. Funding Agencies|Swedish Research Council [2016-06146, 2016-03979, 201605498]; Knut and Alice Wallenberg Foundation through a Wallenberg Academy Fellowship; European Research Council (ERC) [637624]; NIH/NIGMS through NSF [DMR-1332208]; US National Science Foundation [DMR-1729737]; VINNOVA [2015-04859]; US Department of Energy, Office of Basic Energy Sciences [DE-SC0010419]; Engineering and Physical Sciences Research Council [EP/G037515/1]