Field-effect Hole Mobility Research Articles

Heterostructured semiconductor single-walled carbon nanotube films for solution-processed high-performance field-effect transistors

In this paper, we report a simple and effective method to simultaneously achieve a high charge-carrier mobility and low off current in conjugated polymer-wrapped semiconducting single-walled carbon nanotube (s-SWNT) transistors by applying a SWNT bilayer. To achieve the high mobility and low off current, highly purified and less purified s-SWNTs are successively coated to form the semiconducting layer consisting of poly (3-dodecylthiophene-2,5-diyl) (P3DDT)-wrapped high-pressure carbon mono oxide (HiPCO) SWNT (P3DDT-HiPCO) and poly (9, 9-di-n-dodecylfluorene) (PFDD)-wrapped plasma discharge (PD) SWNT (PFDD-PD). The SWNT transistors with bilayer SWNT networked film showed highly improved hole field-effect mobility (6.18 ± 0.85 cm2V−1s−1 average), on/off current ratio (107), and off current (∼1 pA). Thus, the combination of less purified PFDD-PD (98%–99%) charge-injection layer and highly purified s-P3DDT-HiPCO (>99%) charge-transport layer as the bi-layered semiconducting film achieved high mobility and low off current simultaneously.

A High-Performance Top-Gated Graphene Field-Effect Transistor with Excellent Flexibility Enabled by an iCVD Copolymer Gate Dielectric.

A high-performance top-gated graphene field-effect transistor (FET) with excellent mechanical flexibility is demonstrated by implementing a surface-energy-engineered copolymer gate dielectric via a solvent-free process called initiated chemical vapor deposition. The ultrathin, flexible copolymer dielectric is synthesized from two monomers composed of 1,3,5-trimethyl-1,3,5-trivinyl cyclotrisiloxane and 1-vinylimidazole (VIDZ). The copolymer dielectric enables the graphene device to exhibit excellent dielectric performance and substantially enhanced mechanical flexibility. The p-doping level of the graphene can be tuned by varying the polar VIDZ fraction in the copolymer dielectric, and the Dirac voltage (VDirac ) of the graphene FET can thus be systematically controlled. In particular, the VDirac approaches neutrality with higher VIDZ concentrations in the copolymer dielectric, which minimizes the carrier scattering and thereby improves the charge transport of the graphene device. As a result, the graphene FET with 20 nm thick copolymer dielectrics exhibits field-effect hole and electron mobility values of over 7200 and 3800 cm2 V-1 s-1 , respectively, at room temperature. These electrical characteristics remain unchanged even at the 1 mm bending radius, corresponding to a tensile strain of 1.28%. The formed gate stack with the copolymer gate dielectric is further investigated for high-frequency flexible device applications.

Ambipolar charge transport of diketopyrrolepyrrole-silole-based copolymers and effect of side chain engineering: Compact model parameter extraction strategy for high-voltage logic applications

Abstract The copolymers P24DPP-silole and P29DPP-silole, each composed of diketopyrrolopyrrole (DPP) and silole derivatives, were synthesized using a Stille coupling reaction, and their electrical performances in organic field-effect transistors (OFETs) and circuits were investigated. While both the as-spun OFETs exhibited quite low field-effect hole mobility values, the OFETs subjected to thermal annealing at 150 °C exhibited typical ambipolar transport characteristics with average hole and electron mobility values of 1 × 10−1 cm2/(V s) and 2 × 10−3 cm2/(V s). Because the compact model was necessary to perform circuit design with the synthesized OFETs, a strategy for extracting compact model parameters was proposed for high-voltage logic circuit applications by using the industry standard compact Berkeley short-channel IGFET model (BSIM).

Linear Conjugated Polymer Backbones Improve Alignment in Nanogroove-Assisted Organic Field-Effect Transistors.

Three cyclopentadithiophene-difluorophenylene copolymers (named PhF2,3, PhF2,5, and PhF2,6), which differ by the arrangement of fluorines on the phenylene structural unit, were designed and synthesized for the fabrication of organic field-effect transistors (OFETs). Single crystal structures of model compounds representative of the backbone and density functional theory (DFT) were used to estimate the backbone shape for each copolymer. The different substitution arrangements impact the backbone secondary structure through different nonbonding F···H interactions. PhF2,5 and PhF2,6 assumed more linear backbones relative to PhF2,3, which in turn impacts self-assembly and polymer chain alignment on nanogrooved (NG) substrates. A larger improvement of charge carrier mobility for the more linear backbones was achieved when using NG substrates. Among the three polymers, PhF2,6 achieved the highest average field-effect hole mobility (5.1 cm2 V-1 s-1). As evidenced by grazing incidence wide-angle X-ray scattering (GIWAXS), thin films of PF2,5 and PF2,6 exhibited a higher degree of anisotropic alignment, relative to the more curved PF2,3 counterpart, consistent with the higher hole mobilities. This work gives insight into how nonbonding interactions can influence charge carrier mobility through changes in secondary structure and suggests that polymers with more linear shapes might be preferred for achieving greater levels of alignment within the confines of a NG environment.

A new electrode design for ambipolar injection in organic semiconductors

Organic semiconductors have attracted much attention for low-cost, flexible and human-friendly optoelectronics. However, achieving high electron-injection efficiency is difficult from air-stable electrodes and cannot be equivalent to that of holes. Here, we present a novel concept of electrode composed of a bilayer of tetratetracontane (TTC) and polycrystalline organic semiconductors (pc-OSC) covered by a metal layer. Field-effect transistors of single-crystal organic semiconductors with the new electrodes of M/pc-OSC/TTC (M: Ca or Au) show both highly efficient electron and hole injection. Contact resistance for electron injection from Au/pc-OSC/TTC and hole injection from Ca/pc-OSC/TTC are comparable to those for electron injection from Ca and hole injection from Au, respectively. Furthermore, the highest field-effect mobilities of holes (22 cm2 V–1 s–1) and electrons (5.0 cm2 V–1 s–1) are observed in rubrene among field-effect transistors with electrodes so far proposed by employing Ca/pc-OSC/TTC and Au/pc-OSC/TTC electrodes for electron and hole injection, respectively.

Appearance of the p-channel performance of poly-Si TFTs with a metal S/D electrode using BLDA aiming for low-cost CMOS

ABSTRACTProposed in this study and fabricated on a glass substrate without adopting impurity doping were p-channel polycrystalline silicon (Si) thin-film transistors (TFTs) with a metal source/drain (S/D) electrode. The amorphous 50-nm-thick Si films deposited on a glass substrate via plasma-enhanced chemical vapor deposition were polycrystallized using blue laser diode annealing. Gold (Au), a high-work-function metal, was evaporated for the S/D electrode directly onto the Si channel layer. As a result of the TFT formation, the typical Id–Vg characteristics of the p-channel TFT were successfully obtained. In addition, after hydrogenation at 200°C, the drain current drastically increased. The 14 cm2/Vs effective field effect hole mobility was deduced at the drain voltage of −1 V.

Anomalous Ambipolar Transport of Organic Semiconducting Crystals via Control of Molecular Packing Structures.

Organic crystals deposited on 2-dimensional (2D) van der Waals substrates have been widely investigated due to their unprecedented crystal structures and electrical properties. van der Waals interaction between organic molecules and the substrate induces epitaxial growth of high quality organic crystals and their anomalous crystal morphologies. Here, we report on unique ambipolar charge transport of a "lying-down" pentacene crystal grown on a 2D hexagonal boron nitride van der Waals substrate. From in-depth analysis on crystal growth behavior and ultraviolet photoemission spectroscopy measurement, it is revealed that the pentacene crystal at the initial growth stage have a lattice-strained packing structure and unique energy band structure with a deep highest occupied molecular orbital level compared to conventional "standing-up" crystals. The lattice-strained pentacene few layers enable ambipolar charge transport in field-effect transistors with balanced hole and electron field-effect mobilities. Complementary logic circuits composed of the two identical transistors show clear inverting functionality with a high gain up to 15. The interesting crystal morphology of organic crystals on van der Waals substrates is expected to attract broad attentions on organic/2D interfaces for their electronic applications.

Ultralow voltage operation of biologically assembled all carbon nanotube nanomesh transistors with ion-gel gate dielectrics

The demonstration of field-effect transistors (FETs) based entirely on single-walled carbon nanotubes (SWNTs) would enable the fabrication of high-on-current, flexible, transparent and stretchable devices owing to the excellent electrical, optical, and mechanical properties of SWNTs. Fabricating all-SWNT-based FETs via simple solution process, at room temperature and without using lithography and vacuum process could further broaden the applicability of all-SWNT-FETs. In this work, we report on biologically assembled all SWNT-based transistors and demonstrate that ion-gel-gated network structures of unsorted SWNTs assembled using a biological template material enabled operation of SWNT-based transistors at a very low voltage. The compatibility of the biologically assembled SWNT networks with ion gel dielectrics and the large capacitance of both the three-dimensional channel networks and the ion gel allowed an ultralow operation voltage. The all-SWNT-based FETs showed an Ion/Ioff value of >102, an on-current density per channel width of 2.16 × 10−4 A/mm at VDS = 0.4 V, and a field-effect hole mobility of 1.12 cm2/V · s in addition to the low operation voltage of <−0.5 V. We envision that our work suggests a solution-based simple and low-cost approach to realizing all-carbon-based FETs for low voltage operation and flexible applications.

High-performance multilayer WSe2 field-effect transistors with carrier type control

In this study, high-performance multilayer WSe2 field-effect transistor (FET) devices with carrier type control are demonstrated via thickness modulation and a remote oxygen plasma surface treatment. Carrier type control in multilayer WSe2 FET devices with Cr/Au contacts is initially demonstrated by modulating the WSe2 thickness. The carrier type evolves with increasing WSe2 channel thickness, being p-type, ambipolar, and n-type at thicknesses 5 nm, respectively. The thickness-dependent carrier type is attributed to changes in the bandgap of WSe2 as a function of the thickness and the carrier band offsets relative to the metal contacts. Furthermore, we present a strong hole carrier doping effect via remote oxygen plasma treatment. It non-degenerately converts n-type characteristics into p-type and enhances field-effect hole mobility by three orders of magnitude. This work demonstrates progress towards the realization of high-performance multilayer WSe2 FETs with carrier type control, potentially extendable to other transition metal dichalcogenides, for future electronic and optoelectronic applications.

Highly soluble small-molecule organic semiconductor with trihexylsilyloxy side chain for high-performance organic field-effect transistors with mobility of up to 3.10 cm2 V−1 s−1

A donor–acceptor type small molecule organic semiconductor with a trihexylsilyloxy bulky side chain, coded LGC-D127, was synthesized, and its electronic, electrochemical, and electrical properties were investigated for use as the active layer of solution-processable organic field-effect transistors. LGC-D127 consisted of a phenylene–dithiophene moiety with a bulky trihexylsilyloxy side chain as the electron-donating core, diketopyrrolopyrrole as the electron-accepting linker, and octylrhodanine as the electron-accepting end group. In spite of bulky trihexylsilyloxy side chains, LGC-D127 film was highly crystalline. The charge-carrier transport properties of the LGC-D127 was investigated through the fabrication and characterization of field-effect transistor via solution process. LGC-D127 showed significantly high field-effect hole mobility of 3.16 cm2 V−1 s−1 after thermal annealing due to the large crystalline nanostructure and the small grain boundaries. In particular, LGC-D127 had good solubility in the environmentally friendly solvent such as 2-methyltetrahydrofuran due to the bulky trihexylsilyloxy side chain, and its high hole mobility (max. 3.06 cm2 V−1 s−1) was sustained from the LGC-D127 solution in 2-methyltetrahydrofuran.

Engineering Thin Films of a Tetrabenzoporphyrin toward Efficient Charge-Carrier Transport: Selective Formation of a Brickwork Motif.

Tetrabenzoporphyrin (BP) is a p-type organic semiconductor characterized by the large, rigid π-framework, excellent stability, and good photoabsorption capability. These characteristics make BP and its derivatives prominent active-layer components in organic electronic and optoelectronic devices. However, the control of the solid-state arrangement of BP frameworks, especially in solution-processed thin films, has not been intensively explored, and charge-carrier mobilities observed in BP-based materials have stayed relatively low as compared to those in the best organic molecular semiconductors. This work concentrates on engineering the solid-state packing of a BP derivative, 5,15-bis(triisopropylsilyl)ethynyltetrabenzoporphyrin (TIPS-BP), toward achieving efficient charge-carrier transport in its solution-processed thin films. The effort leads to the selective formation of a brickwork packing that has two dimensionally extended π-staking. The maximum field-effect hole mobility in the resulting films reaches 1.1 cm2 V-1 s-1, which is approximately 14 times higher than the record value for pristine free-base BP (0.070 cm2 V-1 s-1). This achievement is enabled mainly through the optimization of three factors; namely, deposition process, cast solvent, and self-assembled monolayer that constitutes the dielectric surface. On the other hand, polarized-light microscopy and grazing-incident wide-angle X-ray diffraction analyses show that there remains some room for improvement in the in-plane homogeneity of molecular alignment, suggesting even higher charge-carrier mobilities can be obtained upon further optimization. These results will provide a useful basis for the polymorph engineering and morphology optimization in solution-processed organic molecular semiconductors.

Spatially Uniform Thin-Film Formation of Polymeric Organic Semiconductors on Lyophobic Gate Insulator Surfaces by Self-Assisted Flow-Coating

Surface hydrophobization by self-assembled monolayer formation is a powerful technique for improving the performance of organic field-effect transistors (OFETs). However, organic thin-film formation on such a surface by solution processing often fails due to the repellent property of the surface against common organic solvents. Here, a scalable unidirectional coating technique that can solve this problem, named self-assisted flow-coating, is reported. Producing a specially designed lyophobic-lyophilic pattern on the lyophobic surface enables organic thin-film formation in the lyophobic surface areas by flow-coating. To demonstrate the usefulness of this technique, OFET arrays with an active layer of poly(2,5-bis(3-hexadecylthiophene-2-yl)thieno[3,2-b]thiophene) are fabricated. The ideal transfer curves without hysteresis behavior are obtained for all OFETs. The average field-effect hole mobility in the saturation regime is 0.273 and 0.221 cm2·V-1·s-1 for the OFETs with the channels parallel and perpendicular to the flow-coating direction, respectively, and the device-to-device variation is less than 3% for each OFET set. Very small device-to-device variation is also obtained for the on-state current, threshold voltage, and subthreshold swing. These results indicate that the self-assisted flow-coating is a promising coating technique to form spatially uniform thin films of polymeric organic semiconductors on lyophobic gate insulator surfaces.

Fabrication of tensile-strained single-crystalline GeSn on transparent substrate by nucleation-controlled liquid-phase crystallization

We developed a method of forming single-crystalline germanium-tin (GeSn) alloy on transparent substrates that is based on liquid-phase crystallization. By controlling and designing nucleation during the melting growth process, a highly tensile-strained single-crystalline GeSn layer was grown on a quartz substrate without using any crystal-seeds or catalysts. The peak field-effect hole mobility of 423 cm2/V s was obtained for a top-gate single-crystalline GeSn MOSFET on a quartz substrate with a Sn content of 2.6%, indicating excellent crystal quality and mobility enhancement due to Sn incorporation and tensile strain.

Template-Assisted Electrochemical Synthesis of p-Type InSb Nanowires

We report on the growth of Sb-rich indium antimonide (InSb) nanowires fabricated by dc electrodeposition in the pores of a gold-coated nanoporous anodic alumina oxide (AAO) template. An explanation of the various mechanisms of mass transport during successive stages of the growth process is also presented with experimental results confirming these growth stages. The 100 nm thick InSb nanowires that were grown using an electrolyte of pH 1.7 were found to be rich in antimony (Sb). The electrical properties of a single InSb nanowire was investigated by connecting the nanowire in a field-effect-transistor type configuration. The nanowires showed p-type conduction with a hole concentration of ∼ 1.9 × 1016 cm− 3 and field effect hole mobility of ∼ 507 cm2V− 1s− 1. The device had a high on-off current ratio of the order of 103. Temperature-dependent transport measurements showed thermally activated Arrhenius conduction in the temperature range from 200–325 K, yielding an activation energy of 0.1 eV. The ability to obtain high density of p-type InSb nanowires without addition of any dopants opens up new opportunities for using these nanowires in fabrication of electronic devices.

Hole-dominated transport in InSb nanowires grown on high-quality InSb films

We have developed an effective strategy for synthesizing p-type indium antimonide (InSb) nanowires on a thin film of InSb grown on glass substrate. The InSb films were grown by a chemical reaction between Sb2S3 and In and were characterized by structural, compositional, and optical studies. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) studies reveal that the surface of the substrate is covered with a polycrystalline InSb film comprised of sub-micron sized InSb islands. Energy dispersive X-ray (EDX) results show that the film is stoichiometric InSb. The optical constants of the InSb film, characterized using a variable-angle spectroscopic ellipsometer (VASE) shows a maximum value for refractive index at 3.7 near 1.8 eV, and the extinction coefficient (k) shows a maximum value 3.3 near 4.1 eV. InSb nanowires were subsequently grown on the InSb film with 20 nm sized Au nanoparticles functioning as the metal catalyst initiating nanowire growth. The InSb nanowires with diameters in the range of 40–60 nm exhibit good crystallinity and were found to be rich in Sb. High concentrations of anions in binary semiconductors are known to introduce acceptor levels within the band gap. This un-intentional doping of the InSb nanowire resulting in hole-dominated transport in the nanowires is demonstrated by the fabrication of a p-channel nanowire field effect transistor. The hole concentration and field effect mobility are estimated to be ≈1.3 × 1017 cm−3 and 1000 cm2 V−1 s−1, respectively, at room temperature, values that are particularly attractive for the technological implications of utilizing p-InSb nanowires in CMOS electronics.

Porous Field-Effect Transistors Based on a Semiconductive Metal-Organic Framework.

Recently, the emergence of conductive metal-organic frameworks (MOFs) has given great prospects for their applications as active materials in electronic devices. In this work, a high-quality, free-standing conductive MOF membrane was prepared by an air-liquid interfacial growth method. Accordingly, field-effect transistors (FETs) possessing a crystalline microporous MOF channel layer were successfully fabricated for the first time. The porous FETs exhibited p-type behavior, distinguishable on/off ratios, and excellent field-effect hole mobilities as high as 48.6 cm2 V-1 s-1, which is even comparable to the highest value reported for solution-processed organic or inorganic FETs.

Hexa-peri-hexabenzocoronene and diketopyrrolopyrrole based D-A conjugated copolymers for organic field effect transistor and polymer solar cells

Hexa-peri-hexabenzocoronene (HBC) is a disc-shaped conjugated molecule with strong π-π stacking property, high intrinsic charge mobility and good self-assembly property. But for a long time, the organic photovoltaic (OPV) solar cells based on HBC small organic molecules demonstrated low power conversion efficiencies (PCEs). In this study, a series of polymers named as PHBCDPPC20, PHBCDPPC8, PHBCDPPF and PHBCDPPDT were designed and synthesized through copolymerization of HBC with bulky mesityl substituents and strong electron-withdrawing diketopyrrolopyrrole (DPP) with different alkyl side chains and various π-bridges. Introduction of DPP unit into the HBC derivatives broadened the absorption spectra and lowered the band gaps. Bulky mesityl substituents attached to periphery of HBC prevented polymers from self-aggregating into too large domain size in the blend films of photovoltaic devices. The different π-bridges have significant effect on the structure conformation of the polymers. The polymer PHBCDPPDT with bithiophene π-bridges demonstrated the broadest absorption for its extensive π-conjugation and more coplanar conformation compared with the thiophene π-bridge one. PHBCDPPC20, PHBCDPPC8, PHBCDPPF and PHBCDPPDT gave field-effect hole mobilities of 1.35 × 10−3, 2.31 × 10−4, 2.79 × 10−4 and 8.60 × 10−3 cm2 V−1 s−1, respectively. The solar cells based on these polymers displayed PCEs of 2.12%, 2.85%, 1.89% and 2.74%. To our knowledge, 2.85% is the highest PCE for the HBC-based photovoltaic materials till now.

(Invited) Ambipolar Organic-Inorganic Hybrid Perovskite Tfts

: Organic-inorganic hybrid perovskites have attracted substantial attention because of their excellent physical properties, which enable them to serve as the active material in emerging hybrid solid-state solar cells. Here, we proposed a new type of thin-film field-effect transistor (TFT) with an organic-inorganic hybrid perovskite channel layer. The organic-inorganic hybrid perovskite material has advantages of a high mobility like that of an inorganic semiconductor and the easy process like an organic layer. A TFT using CH3NH3PbI3 as the semiconducting channel was demonstrated. The organic-inorganic hybrid perovskite thin fim with a grain size of 1µm and a smooth surface has been achieved as shown in fig.1. A clear electrical-field induced effect can be observed in the C-V measurements. This type of TFT shows an ambipolar performance in its transfer characteristics (as shown in fig.2 ) and the V shape of the transfer curves is similar to those of other types of ambipolar transistors. The hole and electron field effect mobilities (µFE) are derived as 5.1 cm2/Vs and 5.2cm2/Vs, respectively. The on/off current ratio is nearly 106. This Organic-inorganic hybrid perovskite TFT can be processed by cheap, low-temperature techniques, which suggest that it may be suitable for applications that require low cost, a large area, and the flexibility. Molecular engineering of the organic and inorganic components of the hybrids perovskite is expected to further improve device performance for high speed application. Figure 1

Synthesis, field-effect and photovoltaic properties of random difluorobenzothiadiazole-isoindigo electron donor-acceptor polymers

Two random electron donor-acceptor (D-A) polymers comprised of two acceptor moieties, difluorobenzothiadiazole and isoindigo, and one donor moiety (P1 and P2) have been synthesized in good yields to study the effects of donor structures on organic thin-film transistor (OTFT) and bulk-heterojunction organic solar cell (BHJ-OSC) performance. Both polymers possess reasonably good solubility in common processing solvents for organic electronics. P1, which contains a benzodithiophene donor structure, shows poor charge transport ability in OTFTs and low solar power conversion efficiency (PCE) in BHJ-OSC devices with PC71BM acceptor. On the other hand, P2, with a dithienylthieno[3,2-b]thiophene donor moiety, exhibits distinctly higher hole field-effect mobility and significantly much better PCE under similar conditions. These results have demonstrated the decisive influence of donor moiety over the optoelectronic properties of D-A polymers.

Highly Contorted 1,2,5-Thiadiazole-Fused Aromatics for Solution-Processed Field-Effect Transistors: Synthesis and Properties.

A straightforward strategy has been used to construct 1,2,5-thiadiazole-fused 12-ring π systems through twofold Stille coupling and subsequent cyclodehydrogenation by utilizing the building blocks of naphthodithiophene and 5,6-substituted benzo[b]-2,1,3-thiadidazole. Molecules 1 a and 1 b, which exhibit highly contorted π surfaces, show a butterfly-shaped conformation according to DFT calculations. Within the molecules, a plane-to-plane angle of 44.8° was found. UV/Vis absorption, thermogravimetric analysis, differential scanning calorimetry, and cyclic voltammetry (CV) were used to study their physical properties. Strong intermolecular interactions of the nonplanar molecules were also observed by concentration-dependent (1) H NMR spectroscopy measurements and thin-film XRD characterization. The low-lying LUMO and high-lying HOMO levels of the molecules are -3.73 and -5.48 eV, respectively, as estimated from CV measurements; this indicates their potential as semiconducting materials for solution-processed organic field-effect transistors (OFETS). A field-effect hole mobility of up to 0.035 cm(2) V(-1) s(-1) , a threshold voltage of 6.98 V, and a current on/off ratio of 8.65×10(5) in air for 1 a have been demonstrated with the top-contact bottom-gate field-effect transistor device structures; this represents an important step toward the solution-processed OFET application of contorted aromatics.