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With the development of novel semiconductors for optoelectronic applications, new device functionalities utilizing unique characteristics of emerging materials can be particularly appealing. Here, we demonstrate a reversible control of photoluminescence (PL) emission from lead-halide perovskites achieved in perovskite electric-double-layer transistors. PL in several prototypical lead-halide perovskite compounds is shown to be reversibly tuned by a small gate voltage in the range ±1.2 V applied to the ionic-liquid gel on the perovskite surface, with the intensity modulation that can reach one to two orders of magnitude. This effect may be mediated by a reversible migration of oxygen ions affecting the crystal region near the interface with the ion gel. The resulting passivation (or activation) of non-radiative recombination centers (traps) by oxygen ions would then modulate the population of mobile photogenerated electrons and holes that give rise to PL, which is thus tuned with an electric “knob” (the gate) in these devices.

Online Version of Record before inclusion in an issue 2101533 Funding Information: A.G. and A.B. contributed equally to this work. The authors thank Andrey Starkov for the help with illustrations and Dr. Ivan Shuklov for providing chemical reagents. A.G., K.M., V.D. thank the late Vsevolod Gantmakher for the inspiration and support provided at the beginning of this study. The development concept was designed under financial support by Russian Science Foundation under grant 19‐73‐10154. The experiments of the model developer investigation were supported by Russian Science Foundation under grant No. 18‐72‐10135. XPS was carried out with the support of the Research Facility Center at the Institute of Solid State Physics (RAS) (R.N.M. and A.M.I). This work was performed using equipment of MIPT Shared Facilities Center. F.S.F. acknowledges Russian Foundation for Basic Research grant no. 19‐07‐00300. V.P. acknowledges support from the National Science Foundation under the grant ECCS‐1806363. B.P.R. and J.T.D. acknowledge support from the National Science Foundation Award No. ECCS‐1709222. Section on the synthesis of chitosan acetate with various molecular weights was supported by Ministry of Science and Higher Education of the Russian Federation. A.I. expresses gratitude to the Russian Foundation for Basic Research (project no. 19‐29‐03021 mk) for supporting his work. A.K.G. acknowledge RFBR grant 19‐32‐90143. A.G.N. acknowledges the Russian Science Foundation (project No 17‐19‐01787‐synthesis of carbon nanotubes and project No 21‐72‐20050‐SWCNT transistor fabrication and characterisation). Publisher Copyright: © 2021 Wiley-VCH GmbH Copyright: Copyright 2021 Elsevier B.V., All rights reserved. A variety of unconventional materials, including biological nanostructures, organic and hybrid semiconductors, as well as monolayer, and other low-dimensional systems, are actively explored. They are usually incompatible with standard lithographic techniques that use harsh organic solvents and other detrimental processing. Here, a new class of green and gentle lithographic resists, compatible with delicate materials and capable of both top-down and bottom-up fabrication routines is developed. To demonstrate the excellence of this approach, devices with sub-micron features are fabricated on organic semiconductor crystals and individual animal's brain microtubules. Such structures are created for the first time, thanks to the genuinely water-based lithography, which opens an avenue for the thorough research of unconventional delicate materials at the nanoscale.

High-performance, benchmark organic semiconductor rubrene is investigated by modulated-photocurrent (MPC) spectroscopy. In this technique, periodically modulated and steady-state photocarrier populations are simultaneously generated in the sample by two independent light beams, with the resultant photocurrent measured by a lock-in amplifier. The technique allows identifying a bandlike carrier motion (as opposed to incoherent hopping) as the dominant charge transport mechanism in this highly crystalline molecular semiconductor, in agreement with prior studies, yet without the need for fabrication of complex transistor devices. Moreover, MPC spectroscopy is used to determine the important parameters, such as the density of states for traps, their carrier capture coefficient, and attempt to escape frequency, as well as the quantum efficiency of photocarrier generation and the photocarrier mobility. The MPC spectroscopy is also shown to be able to discriminate charge transport at the surface of the crystal from that occurring in the bulk. The wealth of the thus obtained information is essential for our better understanding of the microscopic mechanisms governing photoconductivity in this class of materials.

The first experimental realization of the intrinsic (not dominated by defects) charge conduction regime in lead-halide perovskite field-effect transistors (FETs) is reported. The advance is enabled by: i) a new vapor-phase epitaxy technique that results in large-area single-crystalline cesium lead bromide (CsPbBr

Triplet–triplet exciton annihilation after sensitization of the triplet states by a near-infrared (NIR)-absorbing sensitizer enables rubrene to function as a photon upconversion (UC) material. In t...

Photoinduced charge transfer between semiconductors and gate dielectrics can occur in organic field-effect transistors (OFETs) operating under illumination, leading to a pronounced bias-stress effect in devices that are normally stable while operating in the dark. Here, we report an observation of a polarization-dependent photoinduced bias-stress effect in two prototypical single-crystal OFETs, based on rubrene and tetraphenylbis(indolo{1,2-a})quinolin. We find that the decay rate of the source-drain current in these OFETs under illumination is a periodic function of the polarization angle of incident photoexcitation with respect to the crystal axes, with a periodicity of π. The angular positions of maxima and minima of the bias-stress rate match those of the optical absorption coefficient of the corresponding crystals. The analysis of the effect shows that it stems from a charge transfer of "hot" holes, photogenerated in the crystal within a very short thermalization length (≪μm) from the semiconductor-dielectric interface. The observed phenomenon is a type of intrinsic structure-property relationship, revealing how molecular packing affects parameter drift in organic transistors under illumination. We also demonstrate that a photoinduced charge transfer in OFETs can be used for recording rewritable accumulation channels with an optically defined geometry and resolution, which can be used in a number of potential applications.

Doping is one of the most fundamental building blocks for semiconductor processing, and enables adaptation to both charge concentration and electrical conductivity. Despite attempts to investigate various doping and de-doping methods for organic semiconductors over many years, their use has been limited to date. Here, we present a method to precisely control the degree of doping in dopant-implanted thiophene-based polymeric conductors. A simple post annealing at a relatively low temperature of approximately 110 °C can significantly diminish the charge carrier concentration with precision, where the controllability is evaluated comprehensively using electron spin resonance and optical absorption techniques. At higher doping regimes, a band-like charge transport with an ideal Hall effect and weak localization is confirmed.

Utilizing the intrinsic mobility–strain relationship in semiconductors is critical for enabling strain engineering applications in high‐performance flexible electronics. Here, measurements of Hall effect and Raman spectra of an organic semiconductor as a function of uniaxial mechanical strain are reported. This study reveals a very strong, anisotropic, and reversible modulation of the intrinsic (trap‐free) charge carrier mobility of single‐crystal rubrene transistors with strain, showing that the effective mobility of organic circuits can be enhanced by up to 100% with only 1% of compressive strain. Consistently, Raman spectroscopy reveals a systematic shift of the low‐frequency Raman modes of rubrene to higher (lower) frequencies with compressive (tensile) strain, which is indicative of a reduction (enhancement) of thermal molecular disorder in the crystal with strain. This study lays the foundation of the strain engineering in organic electronics and advances the knowledge of the relationship between the carrier mobility, low‐frequency vibrational modes, strain, and molecular disorder in organic semiconductors., This paper reports Hall effect and Raman measurements of organic semiconductors as a function of uniaxial mechanical strain. It reveals a strong, anisotropic modulation of the intrinsic charge carrier mobility of single‐crystal rubrene transistors with strain, showing that the performance of organic circuits can be enhanced by up to 100% with only 1% of compessive strain.

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.

There has been increased interest in organic semiconductors over the last decade because of their unique properties. Of these, 5, 6, 11, 12-tetraphenylnaphthacene (rubrene) has generated the most interest because of its high charge carrier mobility. In this work, large single crystals with a volume of ~1 cm3 were grown from solution by a temperature reduction technique. The faceted crystals had flat surfaces and cm-scale, visually defect-free areas suitable for physical characterization. X-ray diffraction analysis indicates that solvent does not incorporate into the crystals and photoluminescence spectra are consistent with pristine, high-crystallinity rubrene. Furthermore, the response curve to pulsed optical illumination indicates that the solution grown crystals are of similar quality to those grown by physical vapor transport, albeit larger. The good quality of these crystals in combination with the improvement of electrical contacts by application of conductive polymer on the graphite electrodes have led to the clear observation of alpha particles with these rubrene detectors. Preliminary results with a 252Cf source generate a small signal with the rubrene detector and may demonstrate that rubrene can also be used for detecting high-energy neutrons.

Organic Electronics is one of the most active areas of interdisciplinary research, targeting implementation of organic semiconductors in electronic applications ranging from low-cost, light-weight, fl exible and wearable electronics, to light-emitting diodes, sensors, and solar cells. [ 1 ] In order to provide a stronger fundamental support to this thriving applications-driven fi eld, a deeper understanding of the basic principles governing the operation of organic materials and devices is necessary. Such understanding would help to design better semiconductors, improve device performance, and invent novel device concepts and prototypes. However, this task is very challenging because of the complexity and enormous variety of organic-molecule semiconductors, as well as the multitude of factors and microscopic phenomena that defi ne their electronic properties.

Pyromellitic diimides (PyDIs) are π-conjugated electron-transport materials based on an unusually small aromatic core (benzene), which provides low temperature processing and transparency in much of the visible range. We synthesized PyDI derivatives with a systematic series of fluoroalkyl side chains and investigated their film structures and electrical performances in thin-film transistors. The effect of the length of the fluorinated segment in fluoroalkylmethylene side chains was examined. Shorter side chains within this series induce higher electron mobilities, with a maximum of 0.026 cm2 V−1 s−1 achieved with the perfluorobutylmethyl side chain. Atomic force microscopy images and X-ray diffraction peak widths were used as indications of crystallinity correlating with the mobility trend. The perfluorobutylmethyl side chain, when attached to 3,6-dibromo PyDI using a total of three synthetic steps, allowed nearly parallel PyDI cores and an exceptional mobility of 0.2 cm2 V−1 s−1, accompanied by a correspondingly excellent morphology and effective intermolecular packing illustrated by a single crystal X-ray structure. This is the highest PyDI mobility yet reported, and is an unusually high mobility for a compound with such a small core, having such low visible range absorbance, and requiring so few synthetic steps.

Mobility is an important charge-transport parameter in organic, inorganic and hybrid semiconductors. We outline some of the common pitfalls of mobility extraction from field-effect transistor (FET) measurements and propose practical recommendations to avoid reporting erroneous mobilities in publications.

The authors are grateful to the following programs for the financial support of this work. V.P. and H.H.C. acknowledge support from the National Science Foundation under the grant ECCS-1806363 and the Rutgers Energy Institute (REI). K.C. and H.H.C. acknowledge support from the Center for Advanced Soft-Electronics at Pohang University of Science and Technology funded by the Republic of Korea's Ministry of Science, ICT and Future Planning as Global Frontier Project (CASE-2011-0031628). M.A.F. and B.P.R. acknowledge support from the National Science Foundation Award No. ECCS-1709222. A.F.P., J.P., M.H., and T.D.A. acknowledge financial support from the Engineering and Physical Sciences Research Council (EPSRC) (Grant No. EP/G037515/1) and from the European Research Council (ERC) AMPRO Project No. 280221. T.D.A. and A.F.P. acknowledge the support from King Abdullah University of Science and Technology (KAUST). O.S. acknowledge the support of the Center for Absorption in Science of the Ministry of Immigrant Absorption in Israel under the framework of the KAMEA Program.

Fundamental studies of intrinsic charge transport properties of organic semiconductors are often hindered by charge traps associated with static disorder present even in optimized single-crystal devices. Here, we report a method of surface functionalization using an inert non-conjugated polymer, perfluoropolyether (PFPE), deposited at the surface of organic molecular crystals, which results in accumulation of mobile holes and a 'trap healing' effect at the crystal/PFPE interface. As a consequence, a remarkable ultralow-noise, trp-free conduction regime characterized by intrinsic mobility and transport anisotropy emerges in organic single crystals, and Hall effect measurements with an unprecedented signal-to-noise ratio are demonstrated. This general method to convert trap-dominated organic semiconductors to intrinsic systems may enable the determination of intrinsic transport parameters with high accuracy and make Hall effect measurements in molecular crystals ubiquitous.

Organic optoelectronics is an emerging field that exploits the unique properties of conjugated organic materials to develop new applications that require a combination of performance, low cost, light weight, and processability. For instance, disposable or wearable electronics, light-emitting diodes, smart tags, sensors, and solar cells all fall into this active area of research. Single crystals of conjugated organic molecules are, undoubtedly, the materials with the highest degree of order and purity among the variety of different forms of organic semiconductors. Electronic devices comprising these materials, such as single-crystal transistors and photoconductors developed during the last decade, are by far the best performers in terms of the fundamental parameters such as charge-carrier mobility, exciton diffusivity, concentration of defects, and operational stability. Extremely low density of defects and the resultant remarkable electrical characteristics of some of the organic single-crystal devices allow experimental access to the intrinsic charge transport properties not dominated by charge scattering and trapping. This enables basic studies of the physics of organic semiconductors, including examining the intrinsic structure-property relationship, thus providing a test bed for charge and energy transport theories. The goal of this issue of MRS Bulletin is to provide a broad overview of the state of the art of the field of organic semiconductor single-crystal materials, devices, and theory.

Impressive performance of hybrid perovskite solar cells reported in recent years still awaits a comprehensive understanding of its microscopic origins. In this work, the intrinsic Hall mobility and photocarrier recombination coefficient are directly measured in these materials in steady-state transport studies. The results show that electron-hole recombination and carrier trapping rates in hybrid perovskites are very low. The bimolecular recombination coefficient (10−11 to 10−10 cm3 s−1) is found to be on par with that in the best direct-band inorganic semiconductors, even though the intrinsic Hall mobility in hybrid perovskites is considerably lower (up to 60 cm2 V−1 s−1). Measured here, steady-state carrier lifetimes (of up to 3 ms) and diffusion lengths (as long as 650 μm) are significantly longer than those in high-purity crystalline inorganic semiconductors. We suggest that these experimental findings are consistent with the polaronic nature of charge carriers, resulting from an interaction of charges with methylammonium dipoles., Hybrid perovskites exhibit long carrier diffusion lengths and lifetimes. Here, Chen et al. show experimentally that carrier recombination in perovskites is far from Langevin and closer to the best direct-bandgap semiconductors, which can be explained by the dipolar polaronic nature of charge carriers.

The origin of the bias stress effect related only to semiconductor properties is investigated in "air-gap" organic field-effect transistors (OFETs) in the absence of a material gate dielectric. The effect becomes stronger as the density of trap states in the semiconductor increases. A theoretical model based on carrier trapping and relaxation in localized tail states is formulated. Polar molecular vapors in the gap of "air-gap" OFETs also have a significant impact on the bias stress effect via the formation of bound states between the charge carriers and molecular dipoles at the semiconductor surface.

Commonly observed variations in photoluminescence (PL) spectra of crystalline organic semiconductors, including the appearance or enhancement of certain PL bands, are shown to originate from a small amount of structural disorder (e.g., amorphous inclusions embedded in a crystal), rather than be necessarily related to chemical impurities or material oxidation. For instance, in rubrene, a minute amount of such disorder can lead to the appearance of a dominant PL band at 650 nm as a result of triplet excitons captured and fused at these sites, with a subsequent emission from the amorphous phase.

We describe new Rutherford Backscattering Spectroscopy (RBS) measurements to explore the surface chemistry associated with the growth of self-assembled monolayer (SAM) molecules on conducting organic films. The report includes a description of the optimization of both substrates and RBS scattering parameters to appropriately analyze these novel and damage susceptible structures. Our RBS measurements reveal that the final surface stoichiometry is consistent with a specific model of hydrolyzed and crosslinked trichlorosilanes that form a dense two-dimensional network (a monolayer) at the surface of small-molecule organic semiconductors and a bulk SAM network in the case of conjugated polymer films. Organic semiconductors used in this study are thin films of rubrene (a small molecule semiconductor (C42H28)) and poly(3-hexyl)thiophene (P3HT) (a conjugated polymer (C10H18S)n). As a substrate we used a thick (1 μm) film of parylene (a non-conjugated polymer (C8H8)n) deposited on Si (1 0 0) wafers. The SAM molecules used to functionalize the organic semiconductor films are fluoroalkyl trichlorosilane (FTS) (C8(H4F13)SiCl3) and octyltrichlorosilane (OTS) (C8H17SiCl3). Quantitative detection of medium and small-mass elements, such as O, F, Si, S and residual Cl is demonstrated and used to elucidate the surface chemistry in these novel organic systems.

The authors are grateful to the following programs for the financial support of this work: the National Science Foundation under the grant DMR-1506609, the Rutgers Energy Institute (REI), the Center for Advanced Soft-Electronics funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project (CASE-2011-0031628), and the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST «MISiS» (№ K3-2016-004), implemented by a governmental decree dated 16th of March 2013, N 211. T.D.A., and A.F.P, acknowledge the King Abdullah University of Science and Technology (KAUST) for financial support.

Under certain conditions, self-assembling molecules preferentially bind to molecular steps at the surface of crystalline organic semiconductors, inducing a strong local doping effect. This creates macroscopically long conducting paths of nanoscale width (a single crystalline analogue of organic nanowires) that can span distances of up to 1 cm between electrical contacts. The observed effect of molecular step decoration opens intriguing possibilities for visualization, passivation, and selective doping of surface and interfacial defects in organic electronic devices and provides a novel system for research on nanoscale charge transport in organic semiconductors. In addition, this effect sheds light on the microscopic origin of nucleation and growth of self-assembled monolayers at organic surfaces. It can also have implications in electronic patterning, nanoscale chemical sensors, integrated interconnects and charge-transfer interfaces in organic transistors and solar cells.

A strong modification of the electronic properties of solution-processable conjugated polythiophenes by self-assembled silane molecules is reported. Upon bulk doping with hydrolized fluoroalkyl trichlorosilane, the electrical conductivity of ultrathin polythiophene films increases by up to six orders of magnitude, reaching record values for polythiophenes: (1.1 ± 0.1) × 103 S cm−1 for poly(2,5-bis(3-tetradecylthiophen -2-yl)thieno[3,2-b]thiophene) (PBTTT) and 50 ± 20 S cm−1 for poly(3-hexyl)thiophene (P3HT). Interband optical absorption of the polymers in the doped state is drastically reduced, making these highly conductive films transparent in the visible range. The dopants within the porous polymer matrix are partially crosslinked via a silane self-polymerization mechanism that makes the samples very stable in vacuum and nonpolar environments. The mechanism of SAM-induced conductivity is believed to be based on protonic doping by the free silanol groups available within the partially crosslinked SAM network incorporated in the polythiophene structure. The SAM-doped polythiophenes exhibit an intrinsic sensing effect: a drastic and reversible change in conductivity in response to ambient polar molecules, which is believed to be due to the interaction of the silanol groups with polar analytes. The reported electronic effects point to a new attractive route for doping conjugated polymers with potential applications in transparent conductors and molecular sensors.

The Hall effect would be a powerful probe for studying delocalized (bandlike) transport in semiconductors, because localized carriers do not contribute significantly to the effect. Unfortunately, the Hall voltage is typically very small in organic semiconductors. The authors develop a technique for high-resolution measurements of this effect in organic field-effect transistors (OFETs), utilizing a small ac magnetic field ($l$0.25 T). This method extends studies of intrinsic charge transport to systems with very low carrier mobilities, providing a significant step forward that should improve the design and optimization of OFETs, and other devices.

Hall effect measurements are important for elucidating the fundamental charge transport mechanisms and intrinsic mobility in organic semiconductors. However, Hall effect studies frequently reveal an unconventional behavior that cannot be readily explained with the simple band-semiconductor Hall effect model. Here, we develop an analytical model of Hall effect in organic field-effect transistors in a regime of coexisting band and hopping carriers. The model, which is supported by the experiments, is based on a partial Hall voltage compensation effect, occurring because hopping carriers respond to the transverse Hall electric field and drift in the direction opposite to the Lorentz force acting on band carriers. We show that this can lead in particular to an underdeveloped Hall effect observed in organic semiconductors with substantial off-diagonal thermal disorder. Our model explains the main features of Hall effect in a variety of organic semiconductors and provides an analytical description of Hall mobility, carrier density and carrier coherence factor., Submitted to Scientific Reports 01/13/2016

Hall effect measurements in CH3 NH3 PbBr3 single crystals reveal that the charge-carrier mobility follows an inverse-temperature power-law dependence, μ ∝ T(-) (γ) , with the power exponent γ = 1.4 ± 0.1 in the cubic phase, indicating an acoustic-phonon-dominated carrier scattering, and γ = 0.5 ± 0.1 in the tetragonal phase, suggesting another dominant mechanism, such as a piezoelectric or space-charge scattering.

Molecular self-assembly has been extensively used for surface modification of metals and oxides for a variety of applications, including molecular and organic electronics. One of the goals of this research is to learn how the electronic properties of these surfaces can be modified by self-assembled monolayers (SAM). Here, we demonstrate a new type of molecular self-assembly: the growth of organosilane SAMs at the surface of organic semiconductors, which results in a dramatic increase of the surface conductivity of organic materials. For organosilane molecules with a large dipole moment, SAM-induced surface conductivity of organic molecular crystals approaches 10^-5 S per square, which is comparable to the highest conductivity realized in organic field-effect transistors (OFETs) at ultra-high densities of charge carriers. SAM-functionalized organic surfaces are fully accessible to the environment which makes them very attractive for sensing applications. We have observed that the interaction of vapors of polar molecules with SAM-functionalized organic semiconductors results in a fast and reversible change of the conductivity, proportional to the pressure of an analyte vapor.

Fundamental understanding of photocarrier generation, transport and recombination under a steady-state photoexcitation has been an important goal of organic electronics and photonics, since these processes govern such electronic properties of organic semiconductors as, for instance, photoconductivity. Here, we discovered that photoconductivity of a highly ordered organic semiconductor rubrene exhibits several distinct regimes, in which photocurrent as a function of cw (continuous wave) excitation intensity is described by a power law with exponents sequentially taking values 1, 1/3 and ¼. We show that in pristine crystals this photocurrent is generated at the very surface of the crystals, while the bulk photocurrent is drastically smaller and follows a different sequence of exponents, 1 and ½. We describe a simple experimental procedure, based on an application of “gauge effect” in high vacuum, that allows to disentangle the surface and bulk contributions to photoconductivity. A model based on singlet exciton fission, triplet fusion and triplet-charge quenching that can describe these non-trivial effects in photoconductivity of highly ordered organic semiconductors is proposed. Observation of these effects in photoconductivity and modeling of the underlying microscopic mechanisms described in this work represent a significant step forward in our understanding of electronic properties of organic semiconductors.

Single-crystal organic semiconductors represent perfect systems not only for studying basic science associated with transport of polaronic charge carriers but also for investigating the upper limits of mobilities in thin-film organic devices for flexible displays and other emerging electronic applications. The field-effect transistor provides an important tool to explore the transport of field-induced charge carriers at the surface of organic semiconductors. [1] Special care, however, must be taken during the device fabrication to avoid degradation of the critical interface between the semiconductor and the dielectric. [2,3] High material purity, excellent crystalline quality, and nanoscale morphological smoothness are among the characteristics of this interface that are crucial to obtaining high-performance devices [4,5] and test structures for studies of intrinsic phenomena. Local surface measurements of organic semiconductors at the active interfaces of conventional transistors are difficult to perform, if not impossible, due to the inaccessibility of the semiconductor/gate dielectric interface by many classes of useful probes. [6–9] A new method for building transistors on the pristine surfaces of bulk single crystals uses free-space dielectrics (i.e., vacuum or air) to eliminate completely any contact or processing of the active surface of the semiconductor. [10] This technique, which provides a reversible ability to assemble and dissemble the devices repetitively, recently enabled observation of intrinsic transport and transport anisotropy in rubrene. [10,11] Other groups, using different techniques

We present an overview of recent studies of the charge transport in the field effect transistors on the surface of single crystals of organic low-molecular-weight materials. We first discuss in detail the technological progress that has made these investigations possible. Particular attention is devoted to the growth and characterization of single crystals of organic materials and to different techniques that have been developed for device fabrication. We then concentrate on the measurements of the electrical characteristics. In most cases, these characteristics are highly reproducible and demonstrate the quality of the single crystal transistors. Particularly noticeable are the small sub-threshold slope, the non-monotonic temperature dependence of the mobility, and its weak dependence on the gate voltage. In the best rubrene transistors, room-temperature values of $\mu$ as high as 15 cm$^2$/Vs have been observed. This represents an order-of-magnitude increase with respect to the highest mobility previously reported for organic thin film transistors. In addition, the highest-quality single-crystal devices exhibit a significant anisotropy of the conduction properties with respect to the crystallographic direction. These observations indicate that the field effect transistors fabricated on single crystals are suitable for the study of the \textit{intrinsic} electronic properties of organic molecular semiconductors. We conclude by indicating some directions in which near-future work should focus to progress further in this rapidly evolving area of research., Comment: Review article, to appear in special issue of Phys. Stat. Sol. on organic semiconductors

Reversible control of charge transport and magnetic properties without degradation is a key for device applications of transition metal oxides. Chemical doping during the growth of transition metal oxides can result in large changes in physical properties, but in most of the cases irreversibility is an inevitable constraint. Here we report a reversible control of charge transport, metal-insulator crossover and magnetism in field-effect devices based on ionically gated archetypal oxide system - SrRuO3. In these thin-film devices, the metal-insulator crossover temperature and the onset of magnetoresistance can be continuously and reversibly tuned in the range 90–250 K and 70–100 K, respectively, by application of a small gate voltage. We infer that a reversible diffusion of oxygen ions in the oxide lattice dominates the response of these materials to the gate electric field. These findings provide critical insights into both the understanding of ionically gated oxides and the development of novel applications.

Reversible control of charge transport and magnetic properties without degradation is a key for device applications of transition metal oxides. Chemical doping during the growth of transition metal oxides can result in large changes in physical properties, but in most of the cases irreversibility is an inevitable constraint. Here we report a reversible control of charge transport, metal-insulator crossover and magnetism in field-effect devices based on ionically gated archetypal oxide system - SrRuO3. In these thin-film devices, the metal-insulator crossover temperature and the onset of magnetoresistance can be continuously and reversibly tuned in the range 90–250 K and 70–100 K, respectively, by application of a small gate voltage. We infer that a reversible diffusion of oxygen ions in the oxide lattice dominates the response of these materials to the gate electric field. These findings provide critical insights into both the understanding of ionically gated oxides and the development of novel applications.

Single crystal rubrene is a model organic electronic material showing high carrier mobility and long exciton lifetime. These properties are detrimentally affected when rubrene is exposed to intense light under ambient conditions for prolonged periods of time, possibly due to oxygen up-take. Using photoelectron, scanning probe and ion-based methods, combined with an isotopic oxygen exposure, we present direct evidence of the light-induced reaction of molecular oxygen with single crystal rubrene. Without a significant exposure to light, there is no reaction of oxygen with rubrene for periods of greater than a year; the crystal's surface (and bulk) morphology and chemical composition remain essentially oxygen-free. Grand canonical Monte Carlo computations show no sorbtion of gases into the bulk of rubrene crystal. A mechanism for photo-induced oxygen inclusion is proposed.

We discovered an unprecedented magnitude of the 1/f noise near the Curie temperature Tc in low-Tc manganites. The scaling behavior of the 1/f noise and the resistance provides strong evidence of the percolation nature of the ferromagnetic transition in the polycrystalline samples. The step-like changes of the resistance with temperature, observed for single crystals, suggest that the size of the ferromagnetic domains depends on the size of crystallites., 4 pages, 3 figures

Organic semiconductors might enable new applications in low-cost, light-weight, flexible electronics. To build a solid foundation for these technologies, more fundamental studies of electro-mechanical properties of various types of organic semiconductors are necessary. Here we perform basic studies of charge transport in highly crystalline solution-processed organic semiconductors as a function of applied mechanical strain. As a test bed, we use small molecules crystallized on thin plastic sheets, resulting in high-performance flexible field-effect transistors. These devices can be bent multiple times without degradation to a radius as small as ~200 μm, demonstrating that crystalline solution-processed organic semiconductors are intrinsically highly flexible. This study of electro-mechanical properties suggests that solution-processable organic semiconductors are suitable for applications in flexible electronics, provided that integration with other important technological advances, such as device scalability and low-voltage operation, is achieved in the future.