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Polythiophenes bearing glycolated side chains have rapidly surged as the highest performing materials for organic electrochemical transistors (OECTs) because of their ability to conjugate volumetri...

Achieving efficient polymer solar cells (PSCs) requires a structurally optimal donor-acceptor heterojunction morphology. Here we report the combined experimental and theoretical characterization of a benzodithiophene-benzothiadiazole donor polymer series (PBTZF4-R; R = alkyl substituent) blended with the non-fullerene acceptor ITIC-Th, and analyse the effects of substituent dimensions on blend morphology, charge transport, carrier dynamics, and PSC metrics. Varying substituent dimensions has a pronounced effect on the blend morphology with a direct link between domain purity, to some extent domain dimensions, and charge generation and collection. The polymer with the smallest alkyl substituent yields the highest PSC power conversion efficiency (PCE, 11%), reflecting relatively small, high-purity domains, and possibly benefiting from “matched” donor polymer - small molecule acceptor orientations. The distinctive morphologies arising from the substituents are investigated using molecular dynamics (MD) computation which reveals that substituent dimensions dictate a well-defined set of polymer conformations, in turn driving chain aggregation, and ultimately, the various film morphologies and mixing with acceptor small molecules. A straightforward energetic parameter explains the experimental polymer domain morphological trends, hence PCE, and suggests strategies for substituent selection to optimize PSC materials morphologies.

Indacenodithienothiophene (IDTT)-based postfullerene electron acceptors, such as ITIC (2,2'-[[6,6,12,12-tetrakis(4-hexylphenyl)-6,12-dihydrodithieno[2,3- d:2',3'- d']-s-indaceno[1,2- b:5,6- b']dithiophene-2,8-diyl]-bis[methylidyne(3-oxo-1 H-indene-2,1(3 H)-diylidene)]]bis[propanedinitrile]), have become synonymous with high power conversion efficiencies (PCEs) in bulk heterojunction (BHJ) polymer solar cells (PSCs). Here we systematically investigate the influence of end-group fluorination density and positioning on the physicochemical properties, single-crystal packing, end-group redistribution propensity, and BHJ photovoltaic performance of a series of ITIC variants, ITIC- nF ( n = 0, 2, 3, 4, and 6). Increasing n from 0 → 6 contracts the optical bandgap, but only marginally lowers the LUMO for n > 4. This yields enhanced photovoltaic short-circuit current density and good open-circuit voltage, so that ITIC-6F achieves the highest PCE of the series, approaching 12% in blends with the PBDB-TF donor polymer. Single-crystal diffraction reveals that the ITIC- nF molecules cofacially interleave with ITIC-6F having the shortest π-π distance of 3.28 A. This feature together with ZINDO-level computed intermolecular electronic coupling integrals as high as 57 meV, and B3LYP/DZP-level reorganization energies as low as 147 meV, rival or surpass the corresponding values for fullerenes, ITIC-0F, and ITIC-4F, and track a positive correlation between the ITIC- nF space-charge limited electron mobility and n. Finally, a heretofore unrecognized solution-phase redistribution process between the 2-(3-oxo-indan-1-ylidene)-malononitrile-derived end-groups (EGs) of IDTT-based NFAs, i.e., EG1-IDTT-EG1 + EG2-IDTT-EG2 ⇌ 2 EG1-IDTT-EG2, with implications for the entire ITIC PSC field, is identified and mechanistically characterized, and the effects on PSC performance are assessed.

none 10 si Organic semiconductors are intensively studied as promising materials for the realisation of low-cost flexible electronic devices. The flexibility requirement implies either performance stability towards deformation, or conversely, detectable response to the deformation itself. The knowledge of the electromechanical response of organic semiconductors to external stresses is therefore not only interesting from a fundamental point of view, but also necessary for the development of real world applications. To this end, in this work we predict and measure the variation of charge carrier mobility in rubrene single crystals as a function of mechanical strain, applied selectively along the crystal axes. We find that strain induces simultaneous mobility changes along all three axes, and that in some cases the response is higher along directions orthogonal to the mechanical deformation. These variations cannot be explained by the modulation of intermolecular distances, but only by a more complex molecular reorganisation, which is particularly enhanced, in terms of response, by π-stacking and herringbone stacking. This microscopic knowledge of the relation between structural and mobility variations is essential for the interpretation of electromechanical measurements for crystalline organic semiconductors, and for the rational design of electronic devices. mixed Matta, Micaela; Pereira, Marco José; Gali, Sai Manoj; Thuau, Damien; Olivier, Yoann; Briseno, Alejandro; Dufour, Isabelle; Ayela, Cedric; Wantz, Guillaume; Muccioli, Luca Matta, Micaela; Pereira, Marco José; Gali, Sai Manoj; Thuau, Damien; Olivier, Yoann; Briseno, Alejandro; Dufour, Isabelle; Ayela, Cedric; Wantz, Guillaume; Muccioli, Luca

Replacing the alkyl side chains on conventional semiconducting polymers with ethylene glycol (EG)-based chains is a successful strategy in the molecular design of mixed conduction materials for bioelectronic devices, including organic electrochemical transistors (OECTs). Such polymers have demonstrated the capability to conduct both ionic and electronic charges and can offer superior performance compared to the most commonly used active material, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate). While many research efforts have been dedicated to optimizing OECT performance through the engineering of the semiconducting polymers’ conjugated backbones, variation of the EG chain length has been investigated considerably less. In this work, a series of glycolated polythiophenes with pendant EG chains spanning two to six EG repeat units was synthesized and the electrochemical and structural characteristics of the resulting films were characterized by experimental means and molecular dynamics simulations. OECTs were fabricated and tested, and their performance showed a strong correlation to the the EG side chain length, thereby elucidating important structure–property guidelines for the molecular design of future channel materials. Specifically, a careful balance in the EG length must be struck during the design of EG-functionalized conjugated polymers for OECTs. While minimizing the EG side chain length appears to boost both the capacitive and charge carrier transport properties of the polymers, the chosen EG side chain length must be kept sufficiently long to induce solubility for processing, and allow for the necessary ion interactions with the conjugated polymer backbone.

Eumelanins are a family of natural and synthetic pigments obtained by oxidative polymerization of their natural precursors: 5,6 dihydroxyindole and its 2-carboxy derivative (DHICA). The simultaneous presence of ionic and electronic charge carriers makes these pigments promising materials for applications in bioelectronics. In this computational study we build a structural model of DHICA melanin considering the interplay between its many degrees of freedom, then we examine the electronic structure of representative oligomers. We find that a non-vanishing dipole along the polymer chain sets this system apart from conventional polymer semiconductors, determining its electronic structure, reactivity toward oxidation and localization of the charge carriers. Our work sheds light on previously unnoticed features of DHICA melanin that not only fit well with its radical scavenging and photoprotective properties, but open new perspectives towards understanding and tuning charge transport in this class of materials.

none 3 no Electric field; Density functional theory; organic field effect transistors; dimerization process Density functional theory (DFT) calculations were used to assess the effect of electric fields of varying magnitudes and directions on the molecular structure of pentacene and other acenes. The aim is to understand the response of acenes in organic field effect transistors, specifically the structure of the first monolayer(s) deposited on the gate dielectric, where the transversal electric field and the charge carrier density are largest and charge transport occurs. Pentacene cycloaddition can be enhanced by the application of electric fields oriented along the direction of the forming bonds. Dimerization is likely to occur in low-density, disordered domains, such as grain boundaries or terrace edges. Together with other factors, dimerization could affect device performance leading to an irreversible decrease of mobility due to the creation of new trap states. Matta, Micaela; Biscarini, Fabio; Zerbetto, Francesco Matta, Micaela; Biscarini, Fabio; Zerbetto, Francesco

Micro-electromechanical systems (MEMS) made of organic materials have attracted efforts for the development a new generation of physical, chemical, and biological sensors, for which the electromechanical sensitivity is the current major concern. Here, we present an organic MEMS made of a rubrene single-crystal air-gap transistor. Applying mechanical pressure on the semiconductor results in high variations in drain current: an unparalleled gauge factor above 4000 has been measured experimentally. Such a high sensitivity is induced by the modulation of charge injection at the interface between the gold electrode and the rubrene semiconductor as an unusual transducing effect. Applying these devices to the detection of acoustic pressure shows that force down to 230 nN can be measured with a resolution of 40 nN. This study demonstrates that MEMS based on rubrene air-gap transistors constitute a step forward in the development of high-performance flexible sensors.

New organic semiconductors are essential for developing inexpensive, high-efficiency, solution-processable polymer solar cells (PSCs). PSC photoactive layers are typically fabricated by film-casting a donor polymer and a fullerene acceptor blend, with ensuing solvent evaporation and phase separation creating discrete conduits for photogenerated holes and electrons. Until recently, n-type fullerene acceptors dominated the PSC literature; however, indacenodithienothiophene (IDTT)-based acceptors have recently enabled remarkable PSC performance metrics, for reasons that are not entirely obvious. We report two isomeric IDTT-based acceptors 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-benz-(5, 6)indanone))-5,5,11,11-tetrakis(4-nonylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]di-thiophene (ITN-C9) and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-benz(6,7)indanone))-5,5,11,11-tetrakis(4-nonylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITzN-C9) that shed light on the exceptional IDTT properties vis-à-vis fullerenes. The neat acceptors and blends with fluoropolymer donor poly{[4,8-bis[5-(2- ethylhexyl)-4-fluoro-2-thienyl]benzo[1,2-b:4,5-b′]dithiophene2,6-diyl]-alt-[2,5-thiophenediyl[5,7-bis(2-ethylhexyl)-4,8-dioxo4H,8H-benzo[1,2-c:4,5-c′]dithiophene-1,3-diyl]]} (PBDB-TF) are investigated by optical spectroscopy, cyclic voltammetry, thermogravimetric analysis, differential scanning calorimetry, single-crystal X-ray diffraction, photovoltaic response, space-charge-limited current transport, atomic force microscopy, grazing incidence wide-angle X-ray scattering, and density functional theory-level quantum chemical analysis. The data reveal that ITN-C9 and ITzN-C9 organize such that the lowest unoccupied molecular orbital-rich end groups have intermolecular π−π distances as close as 3.31(1) Å, with electronic coupling integrals as large as 38 meV, and internal reorganization energies as small as 0.133 eV, comparable to or superior to those in fullerenes. ITN-C9 and ITzN-C9 have broad solar-relevant optical absorption, and, when blended with PBDB-TF, afford devices with power conversion efficiencies near 10%. Performance differences between ITN-C9 and ITzN-C9 are understandable in terms of molecular and electronic structure distinctions via the influences on molecular packing and orientation with respect to the electrode.

The charge transport properties of ten group 14 phthalocyanine crystals are investigated by means of kinetic Monte Carlo simulations based on experimental X-ray structures. All investigated materials are predicted to exhibit an ambipolar semiconducting behavior, with hole and electron mobilities lying in the range 0.1–1 cm2 V–1 s–1, showing their potential for organic electronic devices. The simulations also provide evidence that the dimensionality of charge transport in these materials can be finely tuned by substituting the phenoxy axial groups with fluorine atoms and by varying their number and positions; a complete substitution gives rise to two-dimensional transport for both electrons and holes. Most remarkably, one of the investigated compounds that incorporates iodine-phenoxy groups as axial substituents exhibits the largest mobilities for both electrons and holes, owing to large intermolecular couplings and low reorganization energies, and thus emerges as a highly promising one-dimensional semiconductor.

In this work we present friction-force microscopy (FFM) lattice-resolved images acquired on the (100) facet of the semiconductor organic oligomer ( 2 Z , 2 ′ Z ) − 3 , 3 ′ -(1,4-phenylene)bis(2-(4-butoxyphenyl)acrylonitrile) ( β -DBDCS) crystal in water at room temperature. Stick-slip contrast, lateral contact stiffness, and friction forces are found to depend strongly on the sliding direction due to the anisotropic packing of the molecular chains forming the crystal surface along the [010] and [001] directions. The anisotropy also causes the maximum value of the normal force applicable before wearing to increase by a factor of 3 when the scan is performed along the [001] direction on the (100) face. Altogether, our results contribute to achieving a better understanding of the molecular origin of friction anisotropy on soft crystalline surfaces, which has been often hypothesized but rarely investigated in the literature., This work was supported by the Spanish Ministry of Economy and Competitiveness (MINECO) (Projects No. MAT2012-38810 and No. CTQ2014-58801), by the Comunidad de Madrid (project Mad2D, Grant No. S2013/MIT- 3007), by the Campus of International Excellence (CEI) UAM+CSIC, and by theNational Research Foundation ofKorea (NRF) through a grant funded by the Korean government (MSIP; Grant No. 2009-0081571).

With the development of new technologies, researchers have recently focused their interest on flexible electronics to improve the interaction between users and devices. In this context, integrated ultra-low-pressure sensors remain a technological challenge. Here, we show that organic single crystal-based air-gap transistors fabricated by soft lithography are extremely sensitive to mechanical strain and pressure. When low forces are applied to the suspended semiconducting crystal (rubrene), the gated transistors show that drain currents as output signals are extremely sensitive to the deformations. Incredibly high gauge factors, over 4000, are measured using this technique. Further investigation shows that the contact resistance at the rubrene-gold interface is responsible for the high sensing performances. Such a platform is also able to detect sound waves with high sensitivity, making these simple devices suitable for applications in pressure sensing.

© 2015 American Chemical Society. Thin films of organic semiconductors have been widely studied at different length scales for improving the electrical response of devices based on them. Hitherto, a lot of knowledge has been gained about how molecular packing, morphology, grain boundaries, and defects affect the charge transport in organic thin film transistors. However, little is known about the impact of an electric field on the organic semiconductor microstructure and the consequent effect on the device performances. To fill this gap, we investigated the evolution of the structure of pentacene thin film transistors during device operation by in situ real time X-ray diffraction measurements and theoretical calculations. We observed for the first time the occurrence of a reversible structural strain taking place during the bias application mainly due to reorientation at the terrace edges of monolayer islands under the effect of electrical field. Strain exhibits the same trend of the threshold voltage hinting to the existence of a direct correlation between the phenomenon of bias stress and the structural modification., The authors acknowledge A. Shehu for his collaboration at the early stage of the project, F. Borgatti for the fruitful discussion, J. R. Plaisier, G. Zerauschek and A. Lausi for optimizing the experimental setup at the MCX-ELETTRA beamline, and V. L.R. Jacques and P. Evans for technical support at the ID01-ESRF beamline. Financial support for this research was by the National Project N−CHEM, Flagship NANOMAX, by the DGI (Spain) with Project BE-WELL CTQ2013-40480-R, Generalitat de Catalunya (2014-SGR-17), the ERC StG 2012-306826, and by the Networking Research Center on Bioengineering, Biomaterials and Nanomedicine (CIBERBBN).