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Device instability and limited lifetime have been the hurdles to commercialization of organic electronics. Through electrical characterizations and microscopy techniques, much progress has been made in understanding gate bias stress that limits the stability of organic field-effect transistors. The kinetics and mechanisms of charge trapping in organic semiconductors are examined to explain the bias-stress behaviors. The external processing factors, such as dielectric treatments and environmental conditions that affect the severity of bias stress, are also investigated to enable controllable and reproducible device fabrication. © 2007 IEEE.

Organic semiconductors have emerged as an attractive class of materials for neuromorphic computing applications, particularly in crossbar arrays for artificial neural network (ANN) accelerators. Here, one of the last persistent challenges facing organic materials for adoption in these applications is addressed by developing a fabrication process capable of lithographically patterning vertical, three‐terminal electrochemical random‐access memories (ECRAMs). Central to the realization of this device architecture is the development of a hybrid electrolyte system: a porous inorganic matrix permeated with an ionic liquid, which enables vertical stacking of the organic semiconductor channel and gate. The resulting stacked hybrid organic/inorganic ECRAMs (SHOEs) exhibit superior dynamic range to comparable lateral devices (>2×), exceptional cycling endurance (>109 Write–Read Cycles), low energy switching (2.7 pJ), and can be fabricated with dimensions limited by lithographic resolution. The fabrication process developed allows for independent control over device channel, electrolyte, and gate dimensions, and by reducing channel lengths down to a single micron, the fabricated devices can operate (Write+Read) at >MHz speeds. Further, the hybrid electrolyte design provides an effective means to confine an ionic liquid for use in other electrolyte‐gated devices. [ABSTRACT FROM AUTHOR]

In organic mixed ionic–electronic conductors (OMIECs), it is critical to understand the motion of ions in the electrolyte and OMIEC. Generally, the focus is on the movement of net charge during gating, and the motion of neutral anion–cation pairs is seldom considered. Uptake of mobile ion pairs by the semiconductor before electrochemical gating (passive uptake) can be advantageous as this can improve device speed, and both ions can participate in charge compensation during gating. Here, such passive ion pair uptake in high‐speed solid‐state devices is demonstrated using an ion gel electrolyte. This is compared to a polymerized ionic liquid (PIL) electrolyte to understand how ion pair uptake affects device characteristics. Using X‐ray photoelectron spectroscopy, the passive uptake of ion pairs from the ion gel into the OMIEC is detected, whereas no uptake is observed with a PIL electrolyte. This is corroborated by X‐ray scattering, which reveals morphological changes to the OMIEC from the uptake of ion pairs. With in situ Raman, a reorganization of both anions and cations is then observed during gating. Finally, the speed and retention of OMIEC‐based neuromorphic devices are tuned by controlling the freedom of charge motion in the electrolyte. [ABSTRACT FROM AUTHOR]

Self‐assembled monolayers (SAMs) are widely employed in organic field‐effect transistors to modify the surface energy, surface roughness, film growth kinetics, and electrical surface potential of the gate oxide to control the device's operating voltage. In this study, amino‐functionalized SAM molecules are compared to pure alkylsilane SAMS in terms of their impact on the electrical properties of organic field‐effect transistors, using the n‐type polycrystalline small molecule semiconductor material N,N′‐dioctyl‐3,4,9,10‐perylenedicarboximide (PTCDI‐C8). In order to understand the electronic impact of the amino groups, the effect of both the number of amino‐containing functional groups and the SAM molecular length are systematically studied. Though amino‐functionalized SAM materials have been studied previously, this study is, for the first time, able to shed light on the nature of the doping effect that occurs when the gate oxide is treated with polar aminosilane materials. By a comprehensive theoretical study of the interface on the molecular level, it is shown that the observed shift in the threshold voltage is caused by free charges, which are attracted to the PTCDI‐C8 and are stabilized there by protonated aminosilanes. This attraction and the voltage shift can be systematically tuned by varying the length of the neutral terminal chain of the aminosilane. [ABSTRACT FROM AUTHOR]

Ionic liquids are increasingly employed as dielectrics to generate high charge densities and enable low-voltage operation with organic semiconductors. However, effects on structure and morphology of the active material are not fully known, particularly for permeable semiconductors such as conjugated polymers, in which ions from the ionic liquid can enter and electrochemically dope the semicrystalline film. To understand when ions enter, where they go, and how they affect the film, thin films of the archetypal semiconducting polymer, poly(3-hexylthiophene), are electrochemically doped with 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, the archetypal ionic liquid. High-resolution, ex situ X-ray diffraction measurements and complete pole figures reveal changes with applied voltage, cycling, and frequency in lattice spacing, crystallite orientation, and crystallinity in the bulk and at the buried interface. Dopant ions penetrate the film and enter the crystallites at sufficiently high voltages and low frequencies. Upon infiltrating crystallites, ions permanently expand lamellar stacking and contract pi-stacking. Cycling amplifies these effects, but higher frequencies mitigate the expansion of bulk crystallites as ions are hindered from entering crystallites. This mechanistic understanding of the structural effects of ion penetration will help develop models of the frequency and voltage impedance response of electrochemically doped conjugated polymers and advance electronic applications. [ABSTRACT FROM AUTHOR]

Isotopic substitution is a useful method to study the influence of nuclear motion on the kinetics of charge transport in semiconductors. However, in organic semiconductors, no observable isotope effect on field‐effect mobility has been reported. To understand the charge transport mechanism in rubrene, the benchmark organic semiconductor, crystals of fully isotopically substituted rubrene, 13C‐rubrene (13C42H28), are synthesized and characterized. Vapor‐grown 13C‐rubrene single crystals have the same crystal structure and quality as native rubrene crystals (i.e., rubrene with a natural abundance of carbon isotopes). The characteristic transport signatures of rubrene, including room temperature hole mobility over 10 cm2 V−1 s−1, intrinsic band‐like transport, and clear Hall behavior in the accumulation layer of air‐gap transistors, are also observed for 13C‐rubrene crystals. The field‐effect mobility distributions based on 74 rubrene and 13C‐rubrene devices, respectively, reveal that 13C isotopic substitution produces a 13% reduction in the hole mobility of rubrene. The origin of the negative isotope effect is linked to the redshift of vibrational frequencies after 13C‐substitution, as demonstrated by computer simulations based on the transient localization (dynamic disorder) scenario. Overall, the data and analysis provide an important benchmark for ongoing efforts to understand transport in ordered organic semiconductors. [ABSTRACT FROM AUTHOR]

The energy landscape in organic semiconducting materials greatly influences charge and exciton behavior, which are both critical to the operation of organic electronic devices. These energy landscapes can change dramatically depending on the phases of material present, including pure phases of one molecule or polymer and mixed phases exhibiting different degrees of order and composition. In this work, ultraviolet photoelectron spectroscopy measurements of ionization energies (IEs) and external quantum efficiency measurements of charge-transfer (CT) state energies ( ECT) are applied to molecular photovoltaic material systems to characterize energy landscapes. The results show that IEs and ECT values are highly dependent on structural order and phase composition. In the sexithiophene:C60 system both the IEs of sexithiophene and C60 shift by over 0.4 eV while ECT shifts by 0.5 eV depending on molecular composition. By contrast, in the rubrene:C60 system the IE of rubrene and C60 vary by ≤0.11 eV and ECT varies by ≤0.04 eV as the material composition varies. These results suggest that energy landscapes can exist whereby the binding energies of the CT states are overcome by energy offsets between charges in CT states in mixed regions and free charges in pure phases. [ABSTRACT FROM AUTHOR]

The article focuses on use of polymer blends is also an effective way to explore charge transport in organic semiconductors, with blends usually exhibiting unique behaviors. It mentions amorphous polymer blend as the semiconducting layer and thin film is polycrystalline and charge density is high enough to enter the amorphous region. It also mentions charge accumulation spectroscopy and charge transport behavior and flexibility.

Side-chain engineering is increasingly being utilized as a technique to impact the structural order and enhance the electronic properties of semiconducting polymers. However, the correlations drawn between structural changes and the resulting charge transport properties are typically indirect and qualitative in nature. In the present work, a combination of grazing incidence X-ray diffraction and crystallographic refinement calculations is used to determine the precise molecular packing structure of two thiophene-based semiconducting polymers to study the impact of side-chain modifications. The optimized structures provide high-quality fits to the experimental data and demonstrate that in addition to a large difference in interchain spacing between the two materials, there exists a significant disparity in backbone orientation as well. The calculated structures are utilized in density functional theory calculations to determine the band structure of the two materials and are shown to exhibit a dramatic disparity in interchain dispersion which accounts for the large observed difference in charge carrier mobility. The techniques presented here are meant to be general and are therefore applicable to many other highly diffracting semicrystalline polymers. [ABSTRACT FROM AUTHOR]

In this work, crystallization kinetics and aggregate growth of poly(3-ethylhexylthiophene) (P3EHT) thin films are studied as a function of film thickness. X-ray diffraction and optical absorption show that individual aggregates and crystallites grow anisotropically and mostly along only two packing directions: the alkyl stacking and the polymer chain backbone direction. Further, it is also determined that crystallization kinetics is limited by the reorganization of polymer chains and depends strongly on the film thickness and average molecular weight. Time-dependent, field-effect hole mobilities in thin films reveal a percolation threshold for both low and high molecular weight P3EHT. Structural analysis reveals that charge percolation requires bridged aggregates separated by a distance of ≈2-3 nm, which is on the order of the polymer persistence length. These results thus highlight the importance of tie molecules and inter-aggregate distance in supporting charge percolation in semiconducting polymer thin films. The study as a whole also demonstrates that P3EHT is an ideal model system for polythiophenes and should prove to be useful for future investigations into crystallization kinetics. [ABSTRACT FROM AUTHOR]

Polymer aggregation and phase separation of polymer–polymer blends are effectively tuned from self‐stratified to laterally phase‐separated by adjusting the relative solubility of the two polymers in the mixture. This is found to dramatically alter the charge transport characteristics from a preferential in‐plane to an out‐of‐plane direction, revealing the critical dependence of the resulting device performance on the film morphology and structure of the active layer. [ABSTRACT FROM AUTHOR]

Organic semiconductors are suitable candidates for printable, flexible and large-area electronics. Alongside attaining an improved device performance, to confer a multifunctional nature to the employed materials is key for organic-based logic applications. Here we report on the engineering of an electronic structure in a semiconducting film by blending two molecular components, a photochromic diarylethene derivative and a poly(3-hexylthiophene) (P3HT) matrix, to attain phototunable and bistable energy levels for the P3HT's hole transport. As a proof-of-concept we exploited this blend as a semiconducting material in organic thin-film transistors. The device illumination at defined wavelengths enabled reversible tuning of the diarylethene's electronic states in the blend, which resulted in modulation of the output current. The device photoresponse was found to be in the microsecond range, and thus on a technologically relevant timescale. This modular blending approach allows for the convenient incorporation of various molecular components, which opens up perspectives on multifunctional devices and logic circuits. [ABSTRACT FROM AUTHOR]

We have developed and have applied a mobility edge model that takes drift and diffusion currents to characterize the space-charge-limited current in organic semiconductors into account. The numerical solution of the drift-diffusion equation allows the utilization of asymmetric contacts to describe the built-in potential within the device. The model has been applied to extract information of the distribution of traps from experimental current-voltage measurements of a rubrene single crystal from Krellner et al. [Phys. Rev. B 75, 245115 (2007)] showing excellent agreement across several orders of magnitude in the current. Although the two contacts are made of the same metal, an energy offset of 580 meV between them, ascribed to differences in the deposition techniques (lamination vs evaporation) was essential to correctly interpret the shape of the current-voltage characteristics at low voltage. A band mobility of 0.13 cm² V-1 s-1 for holes is estimated, which is consistent with transport along the long axis of the orthorhombic unit cell. The total density of traps deeper than 0.1 eV was 2.2 x 1016 cm-3. The sensitivity analysis and error estimation in the obtained parameters show that it is not possible to accurately resolve the shape of the trap distribution for energies deeper than 0.3 eV or shallower than 0.1 eV above the valence-band edge. The total number of traps deeper than 0.3 eV, however, can be estimated. Contact asymmetry and the diffusion component of the current play an important role in the description of the device at low bias and are required to obtain reliable information about the distribution of deep traps. [ABSTRACT FROM AUTHOR]

An organic/inorganic hybrid sensing device is proposed based on a dual-gate organic thin film transistor architecture using polythiophenes as semiconductors and AlOx as the top dielectric. When a polar molecule adsorbs on the top dielectric, the threshold voltage of the bottom gate transistor shifts leading to several orders of magnitude increase of the current at an appropriately chosen gate voltage. The devices are tested by exposing them to a saturated water atmosphere, which leads to a four orders of magnitude current increase within one minute. This sensor design maintains some advantages of organic semiconductors such as low-temperature processing and fabrication on flexible substrates. Finally, it can be operated at low voltages with the potential for extremely low-power operation. [ABSTRACT FROM AUTHOR]

The authors suggest avoidance of kinks when measuring mobility of charge carriers in electronic devices and organic semiconductors. They comment on the measurement of carrier mobility using field-effect transistors (FET), transfer characteristics of a semiconductor device on which carrier mobility is derived, and irregularities in the characteristics of previously reported high performance organic FETs.

The article offers information on enhancement of electronic mobility with plasmonic antennas in organic semiconductors. Topics discussed include proportional relation between rate of charge carrier mobility and coupled electronic structure, use of surface plasmon resonance and analysis of electron's drift velocity.

We use the organic semiconductor tetrathiafulvalene dicarboxylate (TTFDA) to assemble PbS nanocrystals into conductive mesocrystals. Density functional theory calculations predict a size-tunable, near-resonant alignment between the PbS 1Sh state and the TTFDA HOMO with the potential to form a conductive channel for holes. We test this hypothesis with transport measurements on TTFDA-functionalized PbS nanocrystals of different sizes and find a pronounced modulation of the field-effect hole mobilities. Photothermal deflection spectroscopy reveals unchanged Urbach energies after ligand exchange, whereas further surface modification by colloidal-atomic layer deposition leads to a strong increase in the density of in-gap states. Hole transport in PbS-TTFDA is unusually robust against such surface modification. Our structural analysis of the mesocrystals suggests that TTFDA induces a defined interparticle spacing, the orientation of atomic lattices, and the angle within the mesocrystal unit cell. The results of this work pave the way toward conductive mesocrystalline assemblies of hybrid semiconductor nanostructures with size-tunable transport properties. [ABSTRACT FROM AUTHOR]

Abstract: We investigate the chemical and structural properties of solution-processed thin films of P3HT blended with p-type dopant F4TCNQ. The maximum in-plane electrical conductivity of doped films is observed at a molar doping fraction of 0.17, in agreement with the binding mechanism of F4TCNQ:P3HT complexes. Through the use of X-ray diffraction, a previously unreported crystalline phase is observed for P3HT films doped above a critical threshold concentration. This crystalline phase involves the incorporation of F4TCNQ molecules into ordered polymer regions and ultimately improves charge dissociation, leading to higher carrier density in thin film. Finally, optical absorption and X-ray diffraction reveal that the chemical state of P3HT in solution has a dramatic impact on the electrical and structural properties of the blended films. [Copyright &y& Elsevier]

The crystallite size and cumulative lattice disorder of three prototypical, high-performing organic semiconducting materials are investigated using a Fourier-transform peak shape analysis routine based on the method of Warren and Averbach (WA). A thorough incorporation of error propagation throughout the multistep analysis and a weighted fitting of Fourier-transformed data to the WA model allows for more accurate results than typically obtained and for determination of confidence bounds. We compare results obtained when assuming two types of column-length distributions, and discuss the benefits of each model in terms of simplicity and accuracy. For strongly disordered materials, the determination of a crystallite size is greatly hindered because disorder dominates the coherence length, not finite size. A simple analysis based on trends of peak widths and Lorentzian components of pseudo-Voigt line shapes as a function of diffraction order is also discussed as an approach to more easily and qualitatively assess the amount and type of disorder present in a sample. While applied directly to organic systems, this methodology is general for the accurate deconvolution of crystalline size and lattice disorder for any material investigated with diffraction techniques. [ABSTRACT FROM AUTHOR]