Temperature Performance of Doping‐Free Top‐Gate CNT Field‐Effect Transistors: Potential for Low‐ and High‐Temperature Electronics

High‐performance top‐gate carbon nanotube (CNT) field‐effect transistors (FETs) have been fabricated via a doping‐free fabrication process in which the polarity of the CNT FET is controlled by the injection of carriers from the electrodes, instead of using dopants. The performance of the doping‐free CNT FETs is systemically investigated over a wide temperature range, from very low temperatures of down to 4.3 K up to 573 K, and analyzed using several temperature‐dependent key device parameters including the ON/OFF state current and ratio, carrier mobility, and subthreshold swing. It is demonstrated that for ballistic and quasi‐ballistic CNT FETs, the operation of the CNT FETs is largely independent of the presence of dopant, thus avoiding detrimental effects due to dopant freeze‐out at low temperature and dopant diffusion at high temperature, and making it possible to use doping‐free CNT FETs in both low‐ and high‐temperature electronics. A new method is also proposed for extracting the band‐gap and diameter of a semiconducting CNT from the temperature dependent OFF‐state current and shown to yield results that are consistent with AFM measurements.     

Control of Current Hysteresis of Networked Single‐Walled Carbon Nanotube Transistors by a Ferroelectric Polymer Gate Insulator

Films made of 2D networks of single‐walled carbon nanotubes (SWNTs) are one of the most promising active‐channel materials for field‐effect transistors (FETs) and have a variety of flexible electronic applications, ranging from biological and chemical sensors to high‐speed switching devices. Challenges, however, still remain due to the current hysteresis of SWNT‐containing FETs, which has hindered further development. A new and robust method to control the current hysteresis of a SWNT‐network FET is presented, which involves the non‐volatile polarization of a ferroelectric poly(vinylidene fluoride‐trifluoroethylene) (P(VDF‐TrFE)) gate insulator. A top‐gate FET with a solution‐processed SWNT‐network exhibits significant suppression of the hysteresis when the gate‐voltage sweep is greater than the coercive field of the ferroelectric polymer layer (≈50 MV m^?1). These near‐hysteresis‐free characteristics are believed to be due to the characteristic hysteresis of the P(VDF‐TrFE), resulting from its non‐volatile polarization, which makes effective compensation for the current hysteresis of the SWNT‐network FETs. The onset voltage for hysteresis‐minimized operation is able to be tuned simply by controlling the thickness of the ferroelectric film, which opens the possibility of operating hysteresis‐free devices with gate voltages down to a few volts.     

Non‐Volatile Organic Memory Elements Based on Carbon‐Nanotube‐Enabled Vertical Field‐Effect Transistors

High‐performance non‐volatile memory elements based on carbon‐nanotube‐enabled vertical field‐effect transistors (CN‐VFETs) are demonstrated. A thin crosslinking polymer layer, benzocyclobutene (BCB), on top of the gate dielectric acts as the charge storage layer. This results in a large, fully gate sweep programmable, hysteresis in the cyclic transfer curves exhibiting on/off ratios >4 orders of magnitude. The carbon nanotube random network source electrode facilitates charge injection into the charge storage layer, realizing the strong memory effect without sacrificing mobility in the vertical channel. Given their intrinsically simple fabrication and compact size CN‐VFETs could provide a path to cost‐effective, high‐density organic memory devices.     

Sources of Hysteresis in Carbon Nanotube Field‐Effect Transistors and Their Elimination Via Methylsiloxane Encapsulants and Optimized Growth Procedures

The origins of gate‐induced hysteresis in carbon nanotube field‐effect transistors are explained and techniques to eliminate this hysteresis with encapsulating layers of methylsiloxane and modified processes for nanotube growth are reported. A combined experimental and theoretical analysis of the dependence of hysteresis on the gate voltage sweep‐rate reveals the locations, types, and densities of defects that contribute to hysteresis. Devices with designs that eliminate these defects exhibit more than ten times reduction in hysteresis compared to conventional layouts. Demonstrations in individual transistors that use both networks and arrays of nanotubes, and in simple logic gates built with these devices, illustrate the utility of the proposed approaches.     

Giant Temperature Coefficient of Resistance in Carbon Nanotube/Phase‐Change Polymer Nanocomposites

The temperature coefficient of resistance of a carbon nanotube nanocomposite with the non‐conductive phase‐change hydrogel Poly(N‐isopropylacrylamide) is studied. This nanocomposite is found to achieve the largest reported temperature coefficient of resistance, ≈?10%/°C, observed in carbon nanotube‐polymer nanocomposites to date. The giant temperature coefficients of resistance results from a volume‐phase‐transition that is induced by the humidity present in the surrounding atmosphere and that enhances the temperature dependence of the resistivity via direct changes in the tunneling resistance that electrons experience in moving between nearby carbon nanotubes. The bolometric photoresponses of this new material are also studied. The nanocomposite's enhanced responses to temperature and humidity give it great potential for sensor applications and uncooled infrared detection.     

Organic Field‐Effect Devices as Tool to Characterize the Bipolar Transport in Polymer‐Fullerene Blends: The Case of P3HT‐PCBM

In organic bulk heterojunction solar cells (oBHJ) the blend morphology in combination with the charge transport properties of the individual components controls the extracted photocurrent. The organic field‐effect transistor (OFET) has been proved as a powerful instrument to evaluate the unipolar carrier transport properties in a wide range of cases. In our work we extend the OFET concept to the evaluation of the bipolar transport properties in polymer‐fullerenes blends and propose a method to improve the accuracy of the evaluation. The method is based on capacitance–voltage (C–V) measurements on MOS structures prepared on the same blends and delivers complementary information on the bulk heterojunction to the one obtained with FETs. The relevance for photovoltaic applications is investigated through the correlation between the current–voltage behavior of solar cells and the bipolar mobility for composites with varying polymer molecular weight and processed from different solvents. In particular the transport features of solar cells produced from o‐Xylene (oX), a non chlorinated solvent more suitable to production requirements, have been compared to the one of devices cast from Chlorobenzene (CB) solution. For the P3HT‐PCBM blend a consistent correlation between the mobility and the electrical fill factor and power performance was found. A significant asymmetry in the bipolar carrier mobility, together with low electron mobility dependent on the M_w value, affects the performances of thick o‐Xylene cast devices. In the case of devices processed from Chlorobenzene the slower carrier has higher mobility and the small electrical losses detected are eventually more related to the formation of space‐charge and eventually to surface recombination. This results in an efficient charge collection that is almost thickness independent. We report a dependence of the slow‐carrier type (electrons or holes) and their mobility on the specific combination of molecular weight and solvent. The mobility data and the solar cell performance coherently fit to the prediction of a device model only based on the drift of carriers under the built‐in electric field originated in the donor‐acceptor oBHJ.     

The Effect of Gate‐Dielectric Surface Energy on Pentacene Morphology and Organic Field‐Effect Transistor Characteristics

The effects of the surface energy of polymer gate dielectrics on pentacene morphology and the electrical properties of pentacene field‐effect transistors (FETs) are reported, using surface‐energy‐controllable poly(imide‐siloxane)s as gate‐dielectric layers. The surface energy of gate dielectrics strongly influences the pentacene film morphology and growth mode, producing Stranski–Krastanov growth with large and dendritic grains at high surface energy and three‐dimensional island growth with small grains at low surface energy. In spite of the small grain size (≈ 300 nm) and decreased ordering of pentacene molecules vertical to the gate dielectric with low surface energy, the mobility of FETs with a low‐surface‐energy gate dielectric is larger by a factor of about five, compared to their high‐surface‐energy counterparts. In pentacene growth on the low‐surface‐energy gate dielectric, interconnection between grains is observed and gradual lateral growth of grains causes the vacant space between grains to be filled. Hence, the higher mobility of the FETs with low‐surface‐energy gate dielectrics can be achieved by interconnection and tight packing between pentacene grains. On the other hand, the high‐surface‐energy dielectric forms the first pentacene layer with some voids and then successive, incomplete layers over the first, which can limit the transport of charge carriers and cause lower carrier mobility, in spite of the formation of large grains (≈ 1.3 μm) in a thicker pentacene film.     

Spatially Resolved STM Spectroscopy of Charge Injection at the Ladder‐Type Poly(para‐phenylene)/Au(111) Interface

The effect of the morphology on charge‐carrier injection into methyl‐substituted ladder‐type poly(para‐phenylene) (Me‐LPPP) thin films deposited on a Au(111) substrate has been studied by scanning‐tunneling‐microscope‐based spectroscopy. We find that the charge‐carrier injection barrier as well as the single‐particle bandgap, E_gsp, of the polymer show significant variations at different locations of the sample surface. Normally, we find that the values of E_gsp are larger than the optical absorption edge, the energy difference being attributed to the exciton binding energy. In some regions of the sample, however, E_gsp appears to be close to or below the optical absorption edge, pointing to the effect of aggregates within the polymer film which act as hole‐trapping centers with a depth of a few 100 meV. Density functional calculations are used to elucidate the dependence of the electronic states on the polymer packing density. Our results show that in this polymer morphological inhomogeneities strongly influence the charge carrier injection and transport properties. This points to a common behavior of materials exhibiting a tendency to form aggregates. In addition, the exciton binding energy of Me‐LPPP is determined to be approx. 0.85 eV. Moreover, the comparison between the charge‐injection energy gap and the photocurrent action spectrum indicates that the photoionization threshold is not directly related to the exciton binding energy.     

Single‐Walled Carbon Nanotube Networks: The Influence of Individual Tube–Tube Contacts on the Large‐Scale Conductivity of Polymer Composites

Over two decades after carbon nanotubes started to attract interest for their seemingly huge prospects, their electrical properties are far from being used to the maximum potential. Composite materials based on carbon nanotubes still have conductivities several orders of magnitude below those of the tubes themselves. This study aims at understanding the reason for these limitations and the possibilities to overcome them. Based on and validated by real single‐walled carbon nanotube (SWCNT) networks, a simple model is developed, which can bridge the gap between macroscale and nanoscale down to individual tube–tube contacts. The model is used to calculate the electrical properties of the SWCNT networks, both as‐prepared and impregnated with an epoxy‐amine polymer. The experimental results show that the polymer has a small effect on the large‐scale network resistance. From the model results it is concluded that the main contribution to the conductivity of the network results from direct contacts, and that in their presence tunneling contacts contribute insignificantly to the conductivity. Preparing highly conductive polymer composites is only possible if the number of direct, low‐resistance contacts in the network is sufficiently large and therefore these direct contacts play the key role.     

Ultraviolet‐Light‐Induced Reversible and Stable Carrier Modulation in MoS2 Field‐Effect Transistors

The tuning of charge carrier concentrations in semiconductor is necessary in order to approach high performance of the electronic and optoelectronic devices. It is demonstrated that the charge‐carrier density of single‐layer (SL), bilayer (BL), and few‐layer (FL) MoS₂ nanosheets can be finely and reversibly tuned with N₂ and O₂ gas in the presence of deep‐ultraviolet (DUV) light. After exposure to N₂ gas in the presence of DUV light, the threshold voltages of SL, BL, and FL MoS₂ field‐effect transistors (FETs) shift towards negative gate voltages. The exposure to N₂ gas in the presence of DUV light notably improves the drain‐to‐source current, carrier density, and charge‐carrier mobility for SL, BL, and FL MoS₂ FETs. Subsequently, the same devices are exposed to O₂ gas in the presence of DUV light for different periods and the electrical characteristics are completely recovered after a certain time. The doping by using the combination of N₂ and O₂ gas with DUV light provides a stable, effective, and facile approach for improving the performance of MoS₂ electronic devices.     

Plasmonic Electrically Functionalized TiO2 for High‐Performance Organic Solar Cells

Optical effects of the plasmonic structures and the materials effects of the metal nanomaterials have recently been individually studied for enhancing performance of organic solar cells (OSCs). Here, the effects of plasmonically induced carrier generation and enhanced carrier extraction of the carrier transport layer (i.e., plasmonic‐electrical effects) in OSCs are investigated. Enhanced charge extraction in TiO₂ as a highly efficient electron transport layer by the incorporation of metal nanoparticles (NPs) is proposed and demonstrated. Efficient device performance is demonstrated by using Au NPs incorporated TiO₂ at a plasmonic wavelength (560–600 nm), which is far longer than the originally necessary UV light. By optimizing the concentration ratio of the Au NPs in the NP‐TiO₂ composite, the performances of OSCs with various polymer active layers are enhanced and efficiency of 8.74% is reached. An integrated optical and electrical model, which takes into account plasmonic‐induced hot carrier tunneling probability and extraction barrier between TiO₂ and the active layer, is introduced. The enhanced charge extraction under plasmonic illumination is attributed to the strong charge injection of plasmonically excited electrons from NPs into TiO₂. The mechanism favors trap filling in TiO₂, which can lower the effective energy barrier and facilitate carrier transport in OSCs.     

A General Approach to Probe Dynamic Operation and Carrier Mobility in Field‐Effect Transistors with Nonuniform Accumulation

Revealing the intrinsic electrical properties is the basis of understanding new functional materials and developing their applications. However, in nonideal field‐effect transistors (FETs), conventional current–voltage characterizations do not accurately probe charge transport, particularly for newly developed semiconductors. Here, a generalized gated four‐probe (G‐GFP) technique is developed, which detects dynamic changes in carrier accumulation and transport. The technique is suitable for exploring the intrinsic properties of semiconductors in FETs with arbitrary contacts and in any operational regimes above the threshold. Application to simulated transistors confirms its accuracy in probing the evolution of channel potential, drift field, and gate‐dependent carrier mobility for devices with a contact‐limited operation and disordered semiconductors. Comparative experiments are performed based on FETs with various materials, device structures, and operational temperatures. The G‐GFP technique proves to exclude the various injection properties, to detect in situ how carriers are accumulated, and to clarify carrier mobility of the semiconductors. In particular, the well‐known “double‐slope” features in the current–voltage relations are controllably generated and their origins are identified. The approach could be used to explore electronic properties of newly developed materials such as organic, oxide, or 2D semiconductors.     

Tunneling Current in Polycrystalline Organic Thin‐Film Transistors

Several recent papers have demonstrated that charge‐carrier mobility in organic field‐effect transistors made of vacuum‐evaporated films may become temperature‐independent at low temperature. To account for this behavior, we developed a model based on the polycrystalline nature of these films, where charge transport is mostly limited by grain boundaries. The free‐carrier density in the intergrain regions is controlled by traps, which leads to the formation of back‐to‐back Schottky barriers at each side of the grain boundaries. The height and width of these barriers is estimated from solving Poisson’s equation using the graded‐channel approximation. It is shown that in most cases the barrier width is negligibly small as compared to the physical size of the grain boundaries. In the high‐temperature regime, the conducting channel can be simply described by grains and grain boundaries connected in series, so that the overall resistance reduces to that of the grain boundaries. At low temperatures, tunneling through the barrier becomes predominant, leading to temperature‐independent mobility. A complete two‐dimensional model for charge tunneling through the barriers is developed. A quantitative check of the model is made by least‐squares fitting of the gate voltage‐dependent current measured on an octithiophene transistor at low temperature, which gives a reasonable determination of the trap density and size of the grain boundaries.     

Fullerene Additives Convert Ambipolar Transport to p‐Type Transport while Improving the Operational Stability of Organic Thin Film Transistors

Many high charge carrier mobility (μ) active layers within organic field‐effect transistor (OFET) configurations exhibit non‐linear current–voltage characteristics that may drift with time under applied bias and, when applying conventional equations for ideal FETs, may give inconsistent μ values. This study demonstrates that the introduction of electron deficient fullerene acceptors into thin films comprised of the high‐mobility semiconducting polymer PCDTPT suppresses an undesirable “double‐slope” in the current–voltage characteristics, improves operational stability, and changes ambipolar transport to unipolar transport. Examination of other high μ polymers shows general applicability. This study also shows that one can further reduce instability by tuning the relative electron affinity of the polymer and fullerene by creating blends containing different fullerene derivatives and semiconductor polymers. One can obtain hole μ values up to 5.6 cm² V^–1 s^–1 that are remarkably stable over multiple bias‐sweeping cycles. The results provide a simple, solution‐processable route to dictate transport properties and improve semiconductor durability in systems that display similar non‐idealities.     

Preparation of Smart Polymer/Carbon Nanotube Conjugates via Stimuli‐Responsive Linkages

In this paper, we modified carbon nanotubes with the thiol‐reactive species, subsequently combined the thiol‐coupling reaction and reversible addition‐fragmentation chain transfer (RAFT) polymerization to prepare temperature‐responsive PNIPAAm (poly(N‐isopropylacrylamide))‐carbon nanotube conjugates. The prepared PNIPAAm‐carbon nanotube conjugates have temperature‐responsive PNIPAAm chain, and disulfide linkages between PNIPAAm and carbon nanotube which are sensitive to bio‐stimuli such as glutathione, therefore dual‐responsive polymer‐carbon nanotube conjugates have been prepared.     

Dielectric–Semiconductor Interface Limits Charge Carrier Motion at Elevated Temperatures and Large Carrier Densities in a High‐Mobility Organic Semiconductor

The fundamental nature of charge transport in highly ordered organic semiconductors is under constant debate. At cryogenic temperatures, effects within the semiconductor such as traps or the interaction of charge carriers with the insulating substrate (dipolar disorder or Fröhlich polarons) are known to limit carrier motion. In comparison, at elevated temperatures, where charge carrier mobility often also decreases as function of temperature, phonon scattering or dynamic disorder are frequently discussed mechanisms, but the exact microscopic cause that limits carrier motion is debated. Here, the mobility in the temperature range between 200 and 420 K as function of carrier density is explored in highly ordered perylene‐diimide from 3 to 9 nm thin films. It is observed that above room temperature increasing the gate electric field or decreasing the semiconducting film thickness leads to a suppression of the charge carrier mobility. Via X‐ray diffraction measurements at various temperatures and electric fields, changes of the thin film structure are excluded as cause for the observed mobility decrease. The experimental findings point toward scattering sites or traps at the semiconductor–dielectric interface, or in the dielectric as limiting factor for carrier mobility, whose role is usually neglected at elevated temperatures.     

Organozinc Compounds as Effective Dielectric Modification Layers for Polymer Field‐Effect Transistors

The interface between the organic semiconductor and dielectric plays an important role in determining the device performance of organic field‐effect transistors (OFETs). Although self‐assembled monolayers (SAMs) made from organosilanes have been widely used for dielectric modification to improve the device performance of OFETs, they suffer from incontinuous and lack uniform coverage of the dielectric layer. Here, it is reported that by introduction of a solution‐processed organozinc compound as a dielectric modification layer between the dielectric and the silane SAM, improved surface morphology and reduced surface polarity can be achieved. The organozinc compound originates from the reaction between diethylzinc and the cyclohexanone solvent, which leads to formation of zinc carboxylates. Being annealed at different temperatures, organozinc compound exists in various forms in the solid films. With organozinc modification, p‐type polymer FETs show a high charge carrier mobility that is about two‐fold larger than a control device that does not contain the organozinc compound, both for devices with a positive threshold voltage and for those with a negative one. After organozinc compound modification, the threshold voltage of polymer FETs can either be altered to approach zero or remain unchanged depending on positive or negative threshold voltage they have.     

Metal–Insulator–Semiconductor Coaxial Microfibers Based on Self‐Organization of Organic Semiconductor:Polymer Blend for Weavable,Fibriform Organic Field‐Effect Transistors

With the increasing importance of electronic textiles as an ideal platform for wearable electronic devices, requirements for the development of functional electronic fibers with multilayered structures are increasing. In this paper, metal–polymer insulator–organic semiconductor (MIS) coaxial microfibers using the self‐organization of organic semiconductor:insulating polymer blends for weavable, fibriform organic field‐effect transistors (FETs) are demonstrated. A holistic process for MIS coaxial microfiber fabrication, including surface modification of gold microfiber thin‐film coating on the microfiber using a die‐coating system, and the self‐organization of organic semiconductor–insulator polymer blend is presented. Vertical phase‐separation of the organic semiconductor:insulating polymer blend film wrapping the metal microfibers provides a coaxial bilayer structure of gate dielectric (inside) and organic semiconductor (outside) with intimate interfacial contact. It is determined that the fibriform FETs based on MIS coaxial microfiber exhibit good charge carrier mobilities that approach the values of typical devices with planar substrate. It additionally exhibits electrical property uniformity over the entire fiber surface and improved bending durability. Fibriform organic FET embedded in a textile is demonstrated by weaving MIS coaxial microfibers with cotton and conducting threads, which verifies the feasibility of MIS coaxial microfiber for use in electronic textile applications.     

Charge‐Carrier Density Independent Mobility in Amorphous Fluorene‐Triarylamine Copolymers

A charge‐carrier density dependent mobility has been predicted for amorphous, glassy energetically disordered semiconducting polymers, which would have considerable impact on their performance in devices. However, previous observations of a density dependent mobility are complicated by the polycrystalline materials studied. Here charge transport in field‐effect transistors and diodes of two amorphous, glassy fluorene‐triarylamine copolymers is investigated, and the results explored in terms of a charge‐carrier density dependent mobility model. The nondispersive nature of the time‐of‐flight (TOF) transients and analysis of dark injection transient results and transistor transfer characteristics indicate a charge‐carrier density independent mobility in both the low‐density diode and the high‐density transistor regimes. The mobility values for optimized transistors are in good agreement with the TOF values at the same field, and both have the same temperature dependency. The measured transistor mobility falls two to three orders of magnitude below that predicted from the charge‐carrier density dependent model, and does not follow the expected power‐law relationship. The experimental results for these two amorphous polymers are therefore consistent with a charge‐carrier density independent mobility, and this is discussed in terms of polaron‐dominated hopping and interchain correlated disorder.     

Temperature‐Dependent Charge‐Carrier Dynamics in CH3NH3PbI3 Perovskite Thin Films

The photoluminescence, transmittance, charge‐carrier recombination dynamics, mobility, and diffusion length of CH₃NH₃PbI₃ are investigated in the temperature range from 8 to 370 K. Profound changes in the optoelectronic properties of this prototypical photovoltaic material are observed across the two structural phase transitions occurring at 160 and 310 K. Drude‐like terahertz photoconductivity spectra at all temperatures above 80 K suggest that charge localization effects are absent in this range. The monomolecular charge‐carrier recombination rate generally increases with rising temperature, indicating a mechanism dominated by ionized impurity mediated recombination. Deduced activation energies E_a associated with ionization are found to increase markedly from the room‐temperature tetragonal (E_a ≈ 20 meV) to the higher‐temperature cubic (E_a ≈ 200 meV) phase adopted above 310 K. Conversely, the bimolecular rate constant decreases with rising temperature as charge‐carrier mobility declines, while the Auger rate constant is highly phase specific, suggesting a strong dependence on electronic band structure. The charge‐carrier diffusion length gradually decreases with rising temperature from about 3 μm at ?93 °C to 1.2 μm at 67 °C but remains well above the optical absorption depth in the visible spectrum. These results demonstrate that there are no fundamental obstacles to the operation of cells based on CH₃NH₃PbI₃ under typical field conditions.