Fundamental Understanding Of Charge Transport Research Articles

High-Density, Nonvolatile, Flexible Multilevel Organic Memristor Using Multilayered Polymer Semiconductors.

Nonvolatile organic memristors have emerged as promising candidates for next-generation electronics, emphasizing the need for vertical device fabrication to attain a high density. Herein, we present a comprehensive investigation of high-performance organic memristors, fabricated in crossbar architecture with PTB7/Al-AlOx-nanocluster/PTB7 embedded between Al electrodes. PTB7 films were fabricated using the Unidirectional Floating Film Transfer Method, enabling independent uniform film fabrication in the Layer-by-Layer (LbL) configuration without disturbing underlying films. We examined the charge transport mechanism of our memristors using the Hubbard model highlighting the role of Al-AlOx-nanoclusters in switching-on the devices, due to the accumulation of bipolarons in the semiconducting layer. By varying the number of LbL films in the device architecture, the resistance of resistive states was systematically altered, enabling the fabrication of novel multilevel memristors. These multilevel devices exhibited excellent performance metrics, including enhanced memory density, high on-off ratio (>108), remarkable memory retention (>105 s), high endurance (87 on-off cycles), and rapid switching (∼100 ns). Furthermore, flexible memristors were fabricated, demonstrating consistent performance even under bending conditions, with a radius of 2.78 mm for >104 bending cycles. This study not only demonstrates the fundamental understanding of charge transport in organic memristors but also introduces novel device architectures with significant implications for high-density flexible applications.

Nanoscale Operando Characterization of Electrolyte-Gated Organic Field-Effect Transistors Reveals Charge Transport Bottlenecks.

Charge transport in electrolyte-gated organic field-effect transistors (EGOFETs) is governed by the microstructural property of the semiconducting thin film that is in direct contact with the electrolyte. Therefore, a comprehensive nanoscale operando characterization of the active channel is crucial to pinpoint various charge transport bottlenecks for rational and targeted optimization of the devices. Here, the local electrical properties of EGOFETs are systematically probed by in-liquid scanning dielectric microscopy (in-liquid SDM) and a direct picture of their functional mechanism at the nanoscale is provided across all operational regimes, starting from subthreshold, linear to saturation, until the onset of pinch-off. To this end, a robust interpretation framework of in-liquid SDM is introduced that enables quantitative local electric potential mapping directly from raw experimental data without requiring calibration or numerical simulations. Based on this development, a straightforward nanoscale assessment of various charge transport bottlenecks is performed, like contact access resistances, inter- and intradomain charge transport, microstructural inhomogeneities, and conduction anisotropy, which have been inaccessible earlier. Present results contribute to the fundamental understanding of charge transport in electrolyte-gated transistors and promote the development of direct structure-property-function relationships to guide future design rules.

Molecular Thermoelectricity in EGaIn-Based Molecular Junctions.

ConspectusUnderstanding the thermoelectric effects that convert energy between heat and electricity on a molecular scale is of great interest to the nanoscience community. As electronic devices continue to be miniaturized to nanometer scales, thermoregulation on such devices becomes increasingly critical. In addition, the study of molecular thermoelectricity provides information that cannot be accessed through conventional electrical conductance measurements. The field of molecular thermoelectrics aims to explore thermoelectric effects in electrode-molecule-electrode tunnel junctions and draw inferences on how the (supra)molecular structure of active molecules is associated with their thermopower. In this Account, we introduce a convenient and useful junction technique that enables thermovoltage measurements of one molecule thick films, self-assembled monolayers (SAMs), with reliability, and discuss the atomic-detailed structure-thermopower relations established by the technique. The technique relies on a microelectrode composed of non-Newtonian liquid metal, eutectic gallium-indium (EGaIn) covered with a native gallium oxide layer. The EGaIn electrode makes it possible to form thermoelectric contacts with the delicate structure of SAMs in a noninvasive fashion. A defined interface between SAM and the EGaIn electrode allows time-effective collection of large amounts of thermovoltage data, with great reproducibility, efficiency, and reliable interpretation and statistical analysis of the data. We also highlight recent efforts to utilize the EGaIn technique for probing molecular thermoelectricity and structure-thermopower relations. Using the technique, it was possible to unravel quantum-chemical mechanisms of thermoelectric functions, based on the Mott formula, in SAM-based large-area junctions, which in turn led us to set various hypotheses to boost the Seebeck coefficient. By validating the hypotheses again with the EGaIn technique, we revealed that the thermopower of junction increases through the reduction of the energy offset between accessible molecular orbital energy level and Fermi level or the tuning of broadening of the orbital energy level. Such alterations in the shape of energy topography of junction could be achieved through structural modifications in anchoring group and molecular backbone of SAM, and the bottom electrode. Molecular thermoelectrics offers a unique opportunity to build a well-defined nanoscale system and isolate an effect of interest from others, advancing fundamental understanding of charge transport across individual molecules and molecule-electrode interfaces. In the Account, we showed our recent work involving carefully designed molecular system that are relevant to answering the question of how thermopower differs between the tunneling and thermal-hopping regimes. The field of molecular thermoelectrics needs to address practical application-related issues, particularly molecular degradation in thermal environments. In this regard, we summarized the results highlighting the thermal instability of SAM-based junctions based on a traditional thiol anchor group and how to circumvent this problem. We also discussed the power factor (PF)─a practical parameter representing the efficiency for converting heat into electricity─of SAMs, evaluated using the EGaIn technique. In the Conclusion section of this Account, we present future challenges and perspectives.

Band transport by large Fröhlich polarons in MXenes

MXenes are emerging layered materials that are promising for electrochemical energy storage and (opto-)electronic applications. A fundamental understanding of charge transport in MXenes is essential for such applications, but has remained under debate. While theoretical studies pointed to efficient band transport, device measurements have revealed thermally activated, hopping-type transport. Here we present a unifying picture of charge transport in two model MXenes by combining ultrafast terahertz and static electrical transport measurements to distinguish the short- and long-range transport characteristics. We find that band-like transport dominates short-range, intra-flake charge conduction in MXenes, whereas long-range, inter-flake transport occurs through thermally activated hopping, and limits charge percolation across the MXene flakes. Our analysis of the intra-flake charge carrier scattering rate shows that it is dominated by scattering from longitudinal optical phonons with a small coupling constant (α ≈ 1), for both semiconducting and metallic MXenes. This indicates the formation of large polarons in MXenes. Our work therefore provides insight into the polaronic nature of free charges in MXenes, and unveils intra- and inter-flake transport mechanisms in the MXene materials, which are relevant for both fundamental studies and applications.

Mechanical single-molecule potentiometers with large switching factors from ortho-pentaphenylene foldamers

Molecular potentiometers that can indicate displacement-conductance relationship, and predict and control molecular conductance are of significant importance but rarely developed. Herein, single-molecule potentiometers are designed based on ortho-pentaphenylene. The ortho-pentaphenylene derivatives with anchoring groups adopt multiple folded conformers and undergo conformational interconversion in solutions. Solvent-sensitive multiple conductance originating from different conformers is recorded by scanning tunneling microscopy break junction technique. These pseudo-elastic folded molecules can be stretched and compressed by mechanical force along with a variable conductance by up to two orders of magnitude, providing an impressively higher switching factor (114) than the reported values (ca. 1~25). The multichannel conductance governed by through-space and through-bond conducting pathways is rationalized as the charge transport mechanism for the folded ortho-pentaphenylene derivatives. These findings shed light on exploring robust single-molecule potentiometers based on helical structures, and are conducive to fundamental understanding of charge transport in higher-order helical molecules.

Correlating charge and thermoelectric transport to paracrystallinity in conducting polymers

The conceptual understanding of charge transport in conducting polymers is still ambiguous due to a wide range of paracrystallinity (disorder). Here, we advance this understanding by presenting the relationship between transport, electronic density of states and scattering parameter in conducting polymers. We show that the tail of the density of states possesses a Gaussian form confirmed by two-dimensional tight-binding model supported by Density Functional Theory and Molecular Dynamics simulations. Furthermore, by using the Boltzmann Transport Equation, we find that transport can be understood by the scattering parameter and the effective density of states. Our model aligns well with the experimental transport properties of a variety of conducting polymers; the scattering parameter affects electrical conductivity, carrier mobility, and Seebeck coefficient, while the effective density of states only affects the electrical conductivity. We hope our results advance the fundamental understanding of charge transport in conducting polymers to further enhance their performance in electronic applications.

Mixed Molecular Electronics: Tunneling Behaviors and Applications of Mixed Self‐Assembled Monolayers

AbstractMolecular electronics promise electronic devices that are miniaturized to molecular and atomic levels. Despite significant efforts made towards understanding and improving the electronic functions of devices based on homogeneous molecular systems, the roles, effects, and importance of molecular‐level dilution and mixing in active components for device performance have only recently begun to be understood. Recent advances in molecular electronics based on heterogeneous active components, defined as mixed molecular electronics, are reviewed, with a particular focus on molecular tunnel junctions containing mixed self‐assembled monolayers (SAMs). Through a stepwise approach, the following is provided: a) a summary of the unique (supra)molecular and electronic structural features of mixed SAMs compared to homogeneous SAMs, b) a discussion on fundamental understanding of charge transport across mixed molecular systems, c) a comprehensive summary of functional mixed‐molecular tunneling devices, and d) perspectives on the future direction of mixed molecular electronics.

Probing Charge Transport Difference in Parallel and Vertical Layered Electronics with Thin Graphite Source/Drain Contacts

In the present study, we aim to help improve the design of van der Waals stacking, i.e., vertical 2D electronics, by probing charge transport differences in both parallel and vertical conducting channels of layered molybdenum disulfide (MoS2), with thin graphite acting as source and drain electrodes. To avoid systematic errors and variable contact contributions to the MoS2 channel, parallel and vertical electronics are all fabricated and measured on the same conducting material. Large differences in the on/off current ratio, mobility, and charge fluctuations, between parallel and vertical electronics are evident in electrical performance as well as in charge transport mechanisms. Further insights are drawn from a well-constrained analysis of both temperature-dependent current-voltage characteristics and low-frequency (LF) current fluctuations. This work offers significant insight into the fundamental understanding of charge transport and the development of future layered-materials-based integration technology.

Correlating anisotropic mobility and intermolecular phonons in organic semiconductors to investigate transient localization

Developing a fundamental understanding of charge transport in organic semiconductors has been a decades-long challenge that hinders performance improvement. In particular, recent work suggests that organic semiconductors have unique charge–phonon interactions where phonons temporarily interrupt the band structure causing a transient localization of charge carriers. Despite growing evidence to support this mechanism, further understanding and control will depend on pinpointing the molecular motions that cause substantial change to the band structure. Here we combine experimental and theoretical techniques to demonstrate the phonon energies and associated molecular motions governing the charge–phonon interaction in single crystal tetracene. We investigate phonon properties using polarized Raman spectroscopy, transmission electron microscopy, and density functional theory, and correlate this with the anisotropic mobility. We find that specific phonons disrupt the band orbital in the high-mobility direction, evident in the discrepancy between measured and static calculations of the mobility anisotropy ratio in tetracene.

A Liquid-Metal-Enabled Versatile Organic Alkali-Ion Battery.

Despite the high specific capacity and low redox potential of alkali metals, their practical application as anodes is still limited by the inherent dendrite-growth problem. The fusible sodium-potassium (Na-K) liquid metal alloy is an alternative that detours this drawback, but the fundamental understanding of charge transport in this binary electroactive alloy anode remains elusive. Here, comprehensive characterization, accompanied with density function theory (DFT) calculations, jointly expound the Na-K anode-based battery working mechanism. With the organic cathode sodium rhodizonate dibasic (SR) that has negligible selectivity toward cations, the charge carrier is screened by electrolytes due to the selective ionic pathways in the solid electrolyte interphase (SEI). Stable cycling for this Na-K/SR battery is achieved with capacity retention per cycle to be 99.88% as a sodium-ion battery (SIB) and 99.70% as a potassium-ion battery (PIB) for over 100 cycles. Benefitting from the flexibility of the liquid metal and the specially designed carbon nanofiber (CNF)/SR layer-by-layer cathode, a flexible dendrite-free alkali-ion battery is achieved with an ultrahigh areal capacity of 2.1 mAh cm-2 . Computation-guided materials selection, characterization-supported mechanistic understanding, and self-validating battery performance collectively promise the prospect of a high-performance, dendrite-free, and versatile organic-based liquid metal battery.

Identifying Charge Transfer Mechanisms across Semiconductor Heterostructures via Surface Dipole Modulation and Multiscale Modeling.

The design and fabrication of stable and efficient photoelectrochemical devices requires the use of multifunctional structures with complex heterojunctions composed of semiconducting, protecting, and catalytic layers. Understanding charge transport across such devices is challenging due to the interplay of bulk and interfacial properties. In this work, we analyze hole transfer across n-Si(111)- R|TiO2 photoanodes where - R is a series of mixed aryl/methyl monolayers containing an increasing number of methoxy units (mono, di, and tri). In the dimethoxy case, triethylene glycol units were also appended to substantially enhance the dipolar character of the surface. We find that hole transport is limited at the n-Si(111)- R|TiO2 interface and occurs by two processes- thermionic emission and/or intraband tunneling-where the interplay between them is regulated by the interfacial molecular dipole. This was determined by characterizing the photoanode experimentally (X-ray photoelectron spectroscopy, voltammetry, impedance) with increasingly thick TiO2 films and complementing the characterization with a multiscale computational approach (first-principles density functional theory (DFT) and finite-element device modeling). The tested theoretical model that successfully distinguished thermionic emission and intraband tunneling was then used to predict the effect of solution potential on charge transport. This prediction was then experimentally validated using a series of nonaqueous redox couples (ferrocence derivatives spanning 800 mV). As a result, this work provides a fundamental understanding of charge transport across TiO2-protected electrodes, a widely used semiconductor passivation scheme, and demonstrates the predictive capability of the combined DFT/device-modeling approach.

Measuring Aging Stability of Perovskite Solar Cells

Michael Saliba is a Group Leader at the Adolphe Merkle Institute in Fribourg, Switzerland. His group studies novel materials focusing on perovskites for a sustainable energy future. Michael was a Marie Curie Fellow at EPFL. He obtained his PhD at Oxford University, an MSc with the Max Planck Institute for Solid State Research, and BSc degrees in mathematics and physics from Stuttgart University. Michael was awarded the Young Scientist Award of the German University Association, was named one of the World's 35 Innovators Under 35 by the MIT Technology Review, and is a Member of the Global Young Academy. Martin Stolterfoht is a postdoctoral research fellow in the Soft Matter Physics group at the University of Potsdam, Germany. He completed his Master degree in Physics at the University of Graz and obtained his PhD at the University of Queensland Australia in 2016 before moving to Potsdam. His research is focused on improving device efficiency through a fundamental understanding of charge transport and recombination processes in perovskite and organic solar cells. Christian M. Wolff obtained an MSc in Physics from Ludwig-Maximilians- Universitat Munchen with a focus on spectroscopy on semiconductor nanomaterials for solar-to-fuel conversion with Prof. J. Feldmann. He joined Prof. Neher's group in 2015 investigating halide perovskites and loss mechanisms in single and multijunction solar cells. Dieter Neher is a full professor of Soft Matter Physics at the University of Potsdam, Germany. He received his Diploma in Physics and his PhD in Chemical Physics at the Johannes Gutenberg University in Mainz, Germany. He worked as a postdoctoral research fellow at the Optical Science Center in Tuscon and the CREOL in Orlando, US. His current research focuses on the understanding and improvement of the electrical and optoelectronic properties of organic conjugated materials, hybrid organic/inorganic systems and organometallic perovskite semiconductors, and the implementation of such materials into highly efficient devices. Antonio Abate is a group leader at the Helmholtz-Zentrum Berlin in Germany and Visiting Professor at Fuzhou University in China. His group is currently researching novel active materials and interfaces to make stable perovskite solar cells. Before his move to the Helmholtz, Antonio was leading the solar cell research at the Adolphe Merkle Institute in Switzerland. After his PhD in Politecnico di Milano in Italy, he was a Marie Sklodowska-Curie Fellow at Ecole Polytechnique Federale de Lausanne and postdoctoral researcher at the University of Oxford.

Role of Disorder Induced by Doping on the Thermoelectric Properties of Semiconducting Polymers

A fundamental understanding of charge transport in polymeric semiconductors requires knowledge of how the electrical conductivity varies with carrier density. The thermopower of semiconducting polymers is also a complex function of carrier density making it difficult to assess structure–property relationships for the thermoelectric power factor. We examined the thermoelectric properties of poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (pBTTT-C14) by measurements of an electrochemical transistor using a polymeric ionic liquid (PIL) gate dielectric that can modulate the carrier concentration from 4 × 1018 to 3 × 1020 cm–3. As carrier density increases, so does the concentration of associated counterions, leading to a greater degree of energetic disorder within the semiconductor. Using thermopower measurements, we show experimentally that the electronic density-of-states broadens with increasing carrier density in the semiconducting polymer. The origin of a commonly observed power law relati...

Thin Film Ion Transport and Morphology of Poly(ethylene oxide) and Lithium Salt Mixtures

With the ability to tune ion transport and mechanical properties, polymer electrolytes have demonstrated promise as solid electrolyte materials for lithium-metal anode batteries. In particular, polyethylene oxide (PEO) and lithium salt mixtures have been the most widely studied polymer electrolyte system.1 Moreover, block copolymer electrolytes (e.g. block polystyrene-block-PEO) have yielded more advanced nanostructured architectures with increased mechanical robustness and efficient ion transport.2 Driven by their potential applications, enormous efforts have been devoted to elucidating mechanisms for ion transport and the formation of lithium dendrites. However, majority of reported studies have focused only on thick bulk samples (100’s of microns thick films). This is significant to note as interfaces3 play a critical role in the performance of batteries, but researchers have mostly inferred interfacial effects using bulk properties. As a consequence, we focus on investigating polymer electrolytes in the (ultra)-thin film regime (5-500 nm), which can be used as a platform to directly study fundamental interfacial effects. In addition, structural characteristics limiting charge transport in polymer electrolytes such as domain orientation and interconnectivity can be more easily probed using thin films.4 Ultimately, investigating polymer electrolyte in the context of thin films will lead to better fundamental understanding of charge transport in polymer electrolytes and the charge transfer limitations at the electrode/electrolyte interfaces. Here, we report on our initial study on charge transport and morphology of a model polymer electrolyte system (mixtures of PEO and LiTFSI, lithium bis(trifluoromethanesulfonyl)imide)) in the thin film regime (<200 nm). We specifically probe the structure-property relationships as function of salt concentration, temperature, and film thickness. We utilized synchrotron-based grazing-incidence wide angle X-ray scattering (GIWAXS) to probe the effects of salt concentration and film thickness on the semicrystalline morphology. At room temperature, the PEO crystallites become more ordered, but the relative degree of crystallinity (rDoC) of PEO gradually decreases with increasing salt concentration. The film becomes completely amorphous at the highest salt concentration r = 0.15, where r is the molar ratio of lithium ions to ethylene oxide repeat units. This trend is consistent with the established theories that adding Li salt disrupts the crystallite structure of PEO and reduces the overall degree of crystallinity. Ionic transport measurements were performed using a.c. impedance spectroscopy on PEO-LiTFSI thin films on custom-designed interdigitated electrode devices (IDE). Above the melting point of PEO (≈ º60 C), the temperature dependence of ionic conductivity of all samples can be well-described using Vogel-Tammann-Fulcher (VTF) model. This suggests that segmental motion of PEO chains facilitate ionic transport in PEO-LiTFSI thin films. Ionic conductivity is found to increase with increasing salt concentration at first but decreases at high salt concentration. This is likely due to the reduced dissociation rate and increased glass transition temperature at high concentration, similar to the behavior of bulk samples.1,2 Our results have demonstrated the first successful fabrication and characterization of ionic transport in PEO-Li salt thin films. This provides an important step toward exploiting thin film configuration to reveal many morphological and interfacial effects on ion conduction mechanism of polymer electrolytes and will be subject of investigations in our future studies. REFERENCES S. Lascaud et al., Macromolecules, 27, 7469–7477 (1994).D. T. Hallinan and N. P. Balsara, Annu. Rev. Mater. Res., 43, 503–525 (2013) http://www.annualreviews.org/doi/abs/10.1146/annurev-matsci-071312-121705.E. Peled and S. Menkin, J. Electrochem. Soc., 164, A1703–A1719 (2017).Y. Kambe, C. G. Arges, S. N. Patel, M. P. Stoykovich, and P. F. Nealey, ECS Interface (2017).

Multilevel Investigation of Charge Transport in Conjugated Polymers.

Conjugated polymers have attracted the world's attentions since their discovery due to their great promise for optoelectronic devices. However, the fundamental understanding of charge transport in conjugated polymers remains far from clear. The origin of this challenge is the natural disorder of polymers with complex molecular structures in the solid state. Moreover, an effective way to examine the intrinsic properties of conjugated polymers is absent. Optoelectronic devices are always based on spin-coated films. In films, polymers tend to form highly disordered structures at nanometer to micrometer length scales due to the high degree of conformational freedom of macromolecular chains and the irregular interchain entanglement, thus typically resulting in much lower charge transport properties than their intrinsic performance. Furthermore, a subtle change of processing conditions may dramatically affect the film formation-inducing large variations in the morphology, crystallinity, microstructure, molecular packing, and alignment, and finally varying the effective charge transport significantly and leading to great inconsistency over an order of magnitude even for devices based on the same polymer semiconductor. Meanwhile, the charge transport mechanism in conjugated polymers is still unclear and its investigation is challenging based on such complex microstructures of polymers in films. Therefore, how to objectively evaluate the charge transport and probe the charge transport mechanism of conjugated polymers has confronted the world for decades. In this Account, we present our recent progress on multilevel charge transport in conjugated polymers, from disordered films, uniaxially aligned thin films, and single crystalline micro- or nanowires to molecular scale, where a derivative of poly(para-phenylene ethynylene) with thioacetyl end groups (TA-PPE) is selected as the candidate for investigation, which could also be extended to other conjugated polymer systems. Our systematic investigations demonstrated that 3-4 orders higher charge transport properties could be achieved with the improvement of polymer chain order and confirmed efficient charge transport along the conjugated polymer backbones. Moreover, with downscaling to molecular scale, many novel phenomena were observed such as the largely quantized electronic structure for an 18 nm-long TA-PPE and the modulation of the redox center of tetrathiafulvalene (TTF) units on tunneling charge transport, which opens the door for conjugated polymers used in nanometer quantum devices. We hope the understanding of charge transport in PPE and its related conjugated polymer at multilevel scale in this Account will provide a new method to sketch the charge transport properties of conjugated polymers, and new insights into the combination of more conjugated polymer materials in the multilevel optoelectronic and other related functional devices, which will offer great promise for the next generation of electronic devices.

Interface modulated currents in periodically proton exchanged Mg doped lithium niobate

Conductivity in Mg doped lithium niobate (Mg:LN) plays a key role in the reduction of photorefraction and is therefore widely exploited in optical devices. However, charge transport through Mg:LN and across interfaces such as electrodes also yields potential electronic applications in devices with switchable conductivity states. Furthermore, the introduction of proton exchanged (PE) phases in Mg:LN enhances ionic conductivity, thus providing tailorability of conduction mechanisms and functionality dependent on sample composition. To facilitate the construction and design of such multifunctional electronic devices based on periodically PE Mg:LN or similar ferroelectric semiconductors, fundamental understanding of charge transport in these materials, as well as the impact of internal and external interfaces, is essential. In order to gain insight into polarization and interface dependent conductivity due to band bending, UV illumination, and chemical reactivity, wedge shaped samples consisting of polar oriented Mg:LN and PE phases were investigated using conductive atomic force microscopy. In Mg:LN, three conductivity states (on/off/transient) were observed under UV illumination, controllable by the polarity of the sample and the externally applied electric field. Measurements of currents originating from electrochemical reactions at the metal electrode–PE phase interfaces demonstrate a memresistive and rectifying capability of the PE phase. Furthermore, internal interfaces such as domain walls and Mg:LN–PE phase boundaries were found to play a major role in the accumulation of charge carriers due to polarization gradients, which can lead to increased currents. The insight gained from these findings yield the potential for multifunctional applications such as switchable UV sensitive micro- and nanoelectronic devices and bistable memristors.

Complementary Semiconducting Polymer Blends for Efficient Charge Transport

Charge transport in polymeric thin films is a complicated process, which involves a multitude of coupled electronic events. Because of the growing appeal of semiconducting polymers in organic electronics, it makes the fundamental understanding of charge transport increasingly important. On the other hand, it urges the solution of the processability problem, frequently associated with high-performance polymers. In this study, we introduce complementary semiconducting polymer blends (c-SPBs), aiming to provide solutions for both the fundamental understanding of charge transport and the processability problem. The c-SPBs contain a highly crystalline matrix polymer with intentionally placed conjugation-break spacers (CBSs) along the polymer backbone, thus eliminating intrachain transport, and a tie chain polymer that is a fully conjugated polymer, restoring intrachain transport by connecting π-crystalline aggregates in the matrix polymer. The results show that the addition of as little as 1 wt % tie chain pol...

Micro-focused X-ray diffraction characterization of high-quality [6,6]-phenyl-C61-butyric acid methyl ester single crystals without solvent impurities

We report the preparation of high-quality, solvent-free [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) large single crystals (size up to 0.5 mm) by slow drying of a chlorobenzene solution at room temperature. The monoclinic structure containing four PCBM molecules per unit cell was successfully solved (R-factor = 0.0512) via micro-focused X-ray diffraction and employed as a reliable experimental model for further molecular dynamics simulations. We find that the first peak of the simulated fullerene–fullerene radial distribution function is centred at 10.05 A, giving a nearest neighbour coordination number of 7.0. The work reported herein provides the structural basis for a fundamental understanding of charge transport in this important functional material that is particularly relevant to organic solar cells.

N-Channel Semiconductor Materials Design for Organic Complementary Circuits

Organic semiconductors have unique properties compared to traditional inorganic materials such as amorphous or crystalline silicon. Some important advantages include their adaptability to low-temperature processing on flexible substrates, low cost, amenability to high-speed fabrication, and tunable electronic properties. These features are essential for a variety of next-generation electronic products, including low-power flexible displays, inexpensive radio frequency identification (RFID) tags, and printable sensors, among many other applications. Accordingly, the preparation of new materials based on π-conjugated organic molecules or polymers has been a central scientific and technological research focus over the past decade. Currently, p-channel (hole-transporting) materials are the leading class of organic semiconductors. In contrast, high-performance n-channel (electron-transporting) semiconductors are relatively rare, but they are of great significance for the development of plastic electronic devices such as organic field-effect transistors (OFETs). In this Account, we highlight the advances our team has made toward realizing moderately and highly electron-deficient n-channel oligomers and polymers based on oligothiophene, arylenediimide, and (bis)indenofluorene skeletons. We have synthesized and characterized a "library" of structurally related semiconductors, and we have investigated detailed structure-property relationships through optical, electrochemical, thermal, microstructural (both single-crystal and thin-film), and electrical measurements. Our results reveal highly informative correlations between structural parameters at various length scales and charge transport properties. We first discuss oligothiophenes functionalized with perfluoroalkyl and perfluoroarene substituents, which represent the initial examples of high-performance n-channel semiconductors developed in this project. The OFET characteristics of these compounds are presented with an emphasis on structure-property relationships. We then examine the synthesis and properties of carbonyl-functionalized oligomers, which constitute second-generation n-channel oligothiophenes, in both vacuum- and solution-processed FETs. These materials have high carrier mobilities and good air stability. In parallel, exceptionally electron-deficient cyano-functionalized arylenediimide derivatives are discussed as early examples of thermodynamically air-stable, high-performance n-channel semiconductors; they exhibit record electron mobilities of up to 0.64 cm(2)/V·s. Furthermore, we provide an overview of highly soluble ladder-type macromolecular semiconductors as OFET components, which combine ambient stability with solution processibility. A high electron mobility of 0.16 cm(2)/V·s is obtained under ambient conditions for solution-processed films. Finally, examples of polymeric n-channel semiconductors with electron mobilities as high as 0.85 cm(2)/V·s are discussed; these constitute an important advance toward fully printed polymeric electronic circuitry. Density functional theory (DFT) computations reveal important trends in molecular physicochemical and semiconducting properties, which, when combined with experimental data, shed new light on molecular charge transport characteristics. Our data provide the basis for a fundamental understanding of charge transport in high-performance n-channel organic semiconductors. Moreover, our results provide a road map for developing functional, complementary organic circuitry, which requires combining p- and n-channel transistors.

The Shear Flow Processing of Controlled DNA Tethering and Stretching for Organic Molecular Electronics

DNA has been recently explored as a powerful tool for developing molecular scaffolds for making reproducible and reliable metal contacts to single organic semiconducting molecules. A critical step in the process of exploiting DNA-organic molecule-DNA (DOD) array structures is the controlled tethering and stretching of DNA molecules. Here we report the development of reproducible surface chemistry for tethering DNA molecules at tunable density and demonstrate shear flow processing as a rationally controlled approach for stretching/aligning DNA molecules of various lengths. Through enzymatic cleavage of λ-phage DNA to yield a series of DNA chains of various lengths from 17.3 μm down to 4.2 μm, we have investigated the flow/extension behavior of these tethered DNA molecules under different flow strengths in the flow-gradient plane. We compared Brownian dynamic simulations for the flow dynamics of tethered λ-DNA in shear, and found our flow-gradient plane experimental results matched well with our bead-spring simulations. The shear flow processing demonstrated in our studies represents a controllable approach for tethering and stretching DNA molecules of various lengths. Together with further metallization of DNA chains within DOD structures, this bottom-up approach can potentially enable efficient and reliable fabrication of large-scale nanoelectronic devices based on single organic molecules, therefore opening opportunities in both fundamental understanding of charge transport at the single molecular level and many exciting applications for ever-shrinking molecular circuits.