Average Hole Mobility Research Articles

Growth of ultra-uniform graphene using a Ni/W bilayer metal catalyst

We investigated a bilayer catalyst system consisting of polycrystalline Ni and W films for growing mono-layer graphene over large areas. Highly uniform graphene was grown on Ni/W bilayer film with 100% coverage. The graphene grown on Ni/W bilayer film and transferred onto an insulating substrate exhibited average hole and electron mobilities of 727 and 340 cm2V−1s−1, respectively. A probable growth mechanism is proposed based on X-ray diffractometry and transmission electron microscopy, which suggests that the reaction between diffused carbon and tungsten atoms results in formation of tungsten carbides. This reaction allows the control of carbon precipitation and prevents the growth of non-uniform multilayer graphene on the Ni surface; this has not been straightforwardly achieved before. These results could be of importance in better understanding mono-layer graphene growth, and suggest a facile fabrication route for electronic applications.

Exploring SiSn as a performance enhancing semiconductor: A theoretical and experimental approach

We present a novel semiconducting alloy, silicon-tin (SiSn), as channel material for complementary metal oxide semiconductor (CMOS) circuit applications. The material has been studied theoretically using first principles analysis as well as experimentally by fabricating MOSFETs. Our study suggests that the alloy offers interesting possibilities in the realm of silicon band gap tuning. We have explored diffusion of tin (Sn) into the industry's most widely used substrate, silicon (100), as it is the most cost effective, scalable and CMOS compatible way of obtaining SiSn. Our theoretical model predicts a higher mobility for p-channel SiSn MOSFETs, due to a lower effective mass of the holes, which has been experimentally validated using the fabricated MOSFETs. We report an increase of 13.6% in the average field effect hole mobility for SiSn devices compared to silicon control devices.

Electrical responses of short-channel organic transistor prepared by solution-processed organic crystal wire mask

We report a formation of a solution-grown single crystal wire mask for the fabrication of short-channel organic field-effect transistor with enhanced dynamic response time. The various channel length, ranging from submicrometer to a few micrometers, were obtained by controlling the concentration of solution and processing conditions. We fabricated p- and n-channel bottom-contact organic field-effect transistors using pentacene and PTCDI-C13, respectively, and static and dynamic electrical characteristics of the devices were investigated. The highest and average field-effect hole mobility values were found to be 0.892cm2/Vs and 0.192cm2/Vs, respectively. The load type inverter based on the short-channel transistor connected with a 2MΩ resistor showed a clear switching response when square wave input signals up to 1kHz were applied at VDD=−60V.

Study of 6,13-bis(tri-isopropylsilylethynyl) pentacene (TIPS-pentacene crystal) based organic field effect transistors (OFETs)

Organic field-effect transistors are fabricated with the help of an active organic material 6,13-bis(tri-isopropylsilylethynyl) pentacene (TIPS-pentacene). The conduction parameters such as mobility due to electrons, holes, threshold voltage, sub-threshold swing, the maximum density of surface states and standard deviation are calculated. The average mobility due to electron in the single crystal having concentrations 0.8mg/ml, 0.4mg/ml is 0.206cm2/Vs and 2.452cm2/Vs. The average hole mobility of a crystal having concentrations 0.8mg/ml, 0.4mg/ml is 0.25cm2/Vs and 2.42cm2/Vs. The performance of the devices is investigated with respect to surface morphology. The crystals are prepared by the droplet pinned crystallization (DPC) method.

Naphthalene Diimide Copolymers with Oligo(p-phenylenevinylene) and Benzobisoxazole for Balanced Ambipolar Charge Transport

A series of alternating and random donor (D)–acceptor (A) copolymers based on naphthalene diimide (NDI) as the acceptor and oligo(p-phenylenevinylene) (OPV) or benzobisoxazole (BBO) as the strong and weak donor, respectively, were designed and synthesized by Suzuki coupling and Horner–Wadsworth–Emmons polymerization. The effect of the varying donor strength of OPV and BBO on the photophysical, electrochemical, and semiconducting properties of the polymers was investigated. Absorption and emission spectra recorded for dilute chloroform solution and thin film showed increased intramolecular charge transfer for NDI-alt-OPV polymer compared to NDI-alt-BBO polymer. Cyclic voltammetry studies along with DFT (density functional theory) studies at the B3LYP/6-31g* level gave insight into the energy level (HOMO/LUMO) and molecular orientation of donor and acceptor along the polymer backbone. NDI-alt-OPV polymer exhibited rigid coplanar structure with extended π-conjugation which induced backbone planarity and crystallinity to the polymer. The inherent poor solubility of the NDI-alt-BBO prevented further device characterization of this polymer. Random copolymer having maximum 30% incorporation of BBO comonomer in NDI-r-OPV/BBO was found to be soluble for further characterization. Compared to NDI-alt-OPV, lowering of both energy levels LUMO (∼0.2 eV) and HOMO (∼0.5 eV) was observed for both NDI-alt-BBO and the NDI-r-OPV/BBO. Bottom gate–top contact organic field effect transistors (OFETs) of NDI-alt-OPV exhibited balanced ambipolar charge transport with average electron and hole mobility of 3.09 × 10–3 cm2 V–1 s–1 and 2.1 × 10–3 cm2 V–1 s–1, respectively, whereas the random copolymer incorporating both OPV and BBO units NDI-r-OPV/BBO showed dominant n-type charge transport with moderate 4 × 10–4 cm2 V–1 s–1 average electron mobility. The present work thus highlights the structure–property relationship and the electronic tunability required in this class of NDI-based polymers to produce ambipolar transistors.

Improving performance of TIPS pentacene-based organic thin film transistors with small-molecule additives

This work demonstrates an effective approach to improve both charge transport and performance consistency in solution-processed organic thin-film transistors (OTFTs) by blending 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene, or TP) with a series of small-molecule additives: 4-butylbenzoic acid (BBA), 4-hexylbenzoic acid (HBA), and 4-octylbenzoic acid (OBA). These three small molecules share a benzoic acid moiety, but have different length of hydrophobic tails. The self-assembled interfacial layer of small molecules on the gate oxide surface leads to uniform deposition of TP crystal seeds and facilitates TP to grow along the tilted orientation of substrate, which results in a film of enhanced crystal orientation and areal coverage. OTFTs based on TP/small molecule blends demonstrate greatly improved average hole mobility and performance consistency, which correlates with the length of hydrophobic tail of the small-molecule additives.

Naphthalenediimide-Benzothiadiazole Copolymer Semiconductors: Rational Molecular Design for Air-Stable Ambipolar Charge Transport

Rational design of air-stable ambipolar polymeric semiconductors was achieved by covalently connecting naphthalenediimide (NDI) units with benzothiadiazole (BZ) through thiophene (T) linkers, namely, PNDI-mT(BZ)mT (m = 1, 2), in which well-coplanar mT(BZ)mT moieties as a whole act as donors rather than acceptors reported in previous studies. Decreasing the number of thiophene linkers from m = 2 to 1 lowers both LUMO and HOMO energy levels. As a result, the carriers in organic thin film transistors (OTFTs) could be switched from unipolar p-channel only to ambipolar transport. In ambient conditions, PNDI-2T(BZ)2T presents an average hole mobility of 0.07 ± 0.02 cm2 V–1 s–1, while PNDI-T(BZ)T exhibits balanced ambipolar charge transport in a bottom-gate/top-contact device architecture, the average electron and hole mobilities was 0.05 ± 0.02 (μe) and 0.1 ± 0.03 (μh) cm2 V–1 s–1, respectively. Moreover, OTFTs based on both polymer show good air-stability with negligible changes after stored in ambient over 3 ...

Environmentally Friendly Approaches Toward the Production of Processable Graphene by Exfoliation of Graphite Using Ionic Liquid

A facile, low-cost and environmentally friendly method was developed to prepare processable graphene materials by direct exfoliation of expandable graphite flakes under wet ball milling conditions in ionic liquids. The graphene nano-sheets obtained in this method were characterized by AFM, Raman spectroscopy, UV-NIR. Thin-film field-effect transistors made from this graphene sheets exhibit ambipolar effect, an average hole mobility of -0.29 cm2 V(-1) s(-1), electron mobility of -0.57 cm2 V(-1) s(-1) was obtained.

Air-flow navigated crystal growth for TIPS pentacene-based organic thin-film transistors

6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS pentacene) is a promising active channel material of organic thin-film transistors (OTFTs) due to its solubility, stability, and high mobility. However, the growth of TIPS pentacene crystals is intrinsically anisotropic and thus leads to significant variation in the performance of OTFTs. In this paper, air flow is utilized to effectively improve the TIPS pentacene crystal orientation and enhance performance consistency in OTFTs, and the resulted films are examined with optical microscopy, X-ray diffraction, and thin-film transistor measurements. Under air-flow navigation (AFN), TIPS pentacene drop-cast from toluene solution has been observed to form thin films with improved crystal orientation and increased areal coverage on substrates, which subsequently lead to a fourfold increase of average hole mobility and one order of magnitude enhancement in performance consistency defined by the ratio of average mobility to the standard deviation of the field-effect mobilities.

Growth of large-size-two-dimensional crystalline pentacene grains for high performance organic thin film transistors

New approach is presented for growth of pentacene crystalline thin film with large grain size. Modification of dielectric surfaces using a monolayer of small molecule results in the formation of pentacene thin films with well ordered large crystalline domain structures. This suggests that pentacene molecules may have significantly large diffusion constant on the modified surface. An average hole mobility about 1.52 cm2/Vs of pentacene based organic thin film transistors (OTFTs) is achieved with good reproducibility.

Improvement of efficiency of polymer solar cell by incorporation of the planar shaped monomer in low band gap polymer

A new π-conjugated polymer (PFDTBT-BTBT) with planar core based on 4,7-dithiophen-2-yl-benzo(2,1,3)thiadiazole (BT), [1]benzothieno-[3,2-b][1]benzothiophene (BTBT) and 9,9-didecylfluorene was synthesized by the Suzuki coupling reaction. In order to improve the photovoltaic performance, we introduce planar and rigid shaped BTBT in the polymer backbone. We also synthesized alternating copolymer of 9,9-didecylfluorene and BT (PFDTBT) to compare with optical, electrochemical and photovoltaic properties. The HOMO and LUMO energy levels of PFDTBT-BTBT were −5.67 and −3.75eV, respectively, which were very similar to those of PFDTBT (−5.65 and −3.75eV). The power conversion efficiency (PCE) of the device with a structure of ITO/PEDOT:PSS/PFDTBT-BTBT:PCBM (1:3)/Al was 2.08%, which is higher than that of PFDTBT:PCBM (1:3) (1.66%). The root-mean-square (RMS) roughness of PFDTBT-BTBT:PCBM (1:3) blend was 0.820nm, which was smaller than that of PFDTBT:PCBM (1:3) blend (1.75nm). PFDTBT-BTBT:PCBM showed smaller domain size and more well-penetrated morphology than PFDTBT:PCBM. The average field-effect hole mobility of the PFDTBT-BTBT was 6.2×10−4cm2/Vs, much larger than that of PFDTBT (2.3×10−4cm2/Vs). The results strongly support that polymer solar cell based on PFDTBT-BTBT shows better performance than that of the device based on PFDTBT.

Complementary-like inverters based on an ambipolar solution-processed molecular bis(naphthalene diimide)-dithienopyrrole derivative

We report on high-mobility top-gate organic field-effect transistors (OFETs) and complementary-like inverters fabricated with a solution-processed molecular bis(naphthalene diimide)-dithienopyrrole derivative as the channel semiconductor and a CYTOP/Al2O3 bilayer as the gate dielectric. The OFETs showed ambipolar behavior with average electron and hole mobility values of 1.2 and 0.01cm2V−1s−1, respectively. Complementary-like inverters fabricated with two ambipolar OFETs showed hysteresis-free voltage transfer characteristics with negligible variations of switching threshold voltages and yielded very high DC gain values of more than 90V/V (up to 122V/V) at a supply voltage of 25V.

Dithieno-naphthalimide based copolymers for air-stable field effect transistors: synthesis, characterization and device performance

A new thiophene-fused naphthalimide building block, 9-(2-decyltetradecyl)-8H-dithieno-[3′,2′:3,4; 2′′,3′′:5,6]benzo[1,2-f]isoindole-8,10(9H)-dione (4), was synthesized and three novel conjugated copolymers P1–P3 based on this building block and electron-rich units such as 4,4′-didodecyl-2,2′-bithiophene (for P1), benzo[1,2-b:4,5-b′]dithiophene (for P2) and 3,6-didodecylthieno[3,2-b]thiophene (for P3) were prepared by Stille coupling polymerization. The optical, electrochemical and thermal properties of polymers P1–P3 were studied and their applications in organic field effect transistors (OFETs) were also investigated. These new polymers have good solubility in common organic solvents and exhibit good thermal stability. Polymers P1 and P3 entered nematic liquid crystalline phases above 213 and 313 °C, respectively. The average field-effect hole mobilities were evaluated to be 0.15, 3 × 10−3 and 0.05 cm2 V−1 s−1 for P1, P2 and P3, respectively, under ambient conditions, with a high on/off ratios of 105–107. All the OFETs based on these polymers showed good environmental stability when stored in air.

Siloxane-Terminated Solubilizing Side Chains: Bringing Conjugated Polymer Backbones Closer and Boosting Hole Mobilities in Thin-Film Transistors

We introduce a novel siloxane-terminated solubilizing group and demonstrate its effectiveness as a side chain in an isoindigo-based conjugated polymer. An average hole mobility of 2.00 cm(2) V(-1) s(-1) (with a maximum mobility of 2.48 cm(2) V(-1) s(-1)), was obtained from solution-processed thin-film transistors, one of the highest mobilities reported to date. In contrast, the reference polymer with a branched alkyl side chain gave an average hole mobility of 0.30 cm(2) V(-1) s(-1) and a maximum mobility of 0.57 cm(2) V(-1) s(-1). This is largely explained by the polymer packing: our new polymer exhibited a π-π stacking distance of 3.58 Å, while the reference polymer showed a distance of 3.76 Å.

Electrical characterization of wafer-bonded Ge(111)-on-insulator substrates using four-point-probe pseudo-metal-oxide-semiconductor field-effect transistor method

The electrical properties of wafer-bonded n-type Ge(111)-on-insulator (Ge(111)-OI) substrates were characterized using a four-point-probe pseudo-metal-oxide-semiconductor field-effect transistor (pseudo-MOSFET) method. Average electron and hole mobilities in the Ge(111)-OI channel were measured to be ~1000cm2/Vs in accumulation mode and ~310cm2/Vs in inversion mode, respectively. The measured mobility strongly depended on the sample position, due to the spatially inhomogeneous distribution of the interface states. Despite the existence of interface states, the carrier mobility exhibited a high value demonstrating the prospect of wafer-bonded Ge(111)-OI as a channel material in MOSFETs.

Benzothiadiazole-Dithienopyrrole Donor–Acceptor–Donor and Acceptor–Donor–Acceptor Triads: Synthesis and Optical, Electrochemical, and Charge-Transport Properties

2,2′-(Benzo[c][1,2,5]thiadiazol-4,7-diyl)-4,4′-dialkyl-bis(4H-dithieno[3,2-b:2′,3′-d]pyrrole) (DTP-BTD-DTP) donor–acceptor–donor (D-A-D) and 4-alkyl-2,6-bis(benzo[c][1,2,5]thiadiazol-4-yl)-4H-dithieno[3,2-b:2′,3′-d]pyrrole (BTD-DTP-BTD) acceptor–donor–acceptor (A-D-A) triads, with or without additional alkylation in the DTP 6- or BTD 7-positions, respectively, have been synthesized using Stille coupling reactions, characterized using UV–vis absorption spectroscopy and electrochemistry, modeled using density functional theory calculations, and used as charge-transport materials in field-effect transistors. The choice of alkyl substitution pattern has only minor effects on the optical and redox behavior but can be used to modify the thermal properties and solubility of these compounds. The D-A-D and A-D-A triads show long-wavelength absorption maxima at 566–588 and 517–521 nm, respectively, in solution. These transitions are attributed to excitation from a delocalized HOMO to a BTD-localized LUMO and, accordingly, are bathochromically shifted from those of analogous compounds in which the BDT moieties are replaced by benzene rings (393–417 nm). The triads are oxidized at potentials of +0.01 to +0.37 V vs ferrocenium/ferrocene and are reduced at potentials of −1.95 to −1.74 V, with the D-A-D species being both the most easily oxidized and most easily reduced. Field-effect transistors based on solution-processed films of some of the triads showed p-channel behavior; the highest average hole mobility value measured was 5.9 × 10–3 cm2 V–1 s–1 for 2,2′-(benzo[c][1,2,5]thiadiazol-4,7-diyl)-4,4′-di-n-dodecyl-bis(4H-dithieno[3,2-b:2′,3′-d]pyrrole) and was accompanied by an on/off current ratio of ca. 103.

Performance comparison of pentacene organic field-effect transistors with SiO 2 modified with octyltrichlorosilane or octadecyltrichlorosilane

Performance of pentacene organic field-effect transistors (OFETs) is significantly improved by treatment of SiO 2 with octyltrichlorosilane (OTS-8) compared to octadecyltrichlorosilane (OTS-18). The average hole mobility in these OFETs is increased from 0.4 to 0.8 cm 2/Vs when treating the dielectric with OTS-8 versus OTS-18 treated devices. The atomic force microscope (AFM) images show that the OTS-8 treated surface produces much larger grains of pentacene (∼500 nm) compared to OTS-18 (∼100 nm). X-ray diffraction (XRD) results confirmed that the pentacene on OTS-8 is more crystalline compared to the pentacene on OTS-18, resulting in higher hole mobility.

Dithienopyrrole–quinoxaline/pyridopyrazine donor–acceptor polymers: synthesis and electrochemical, optical, charge-transport, and photovoltaic properties

Soluble alternating copolymers of N-(3,4,5-tri-n-dodecyloxyphenyl)dithieno[3,2-b:2′,3′-d]pyrrole donor groups and 2,3-di-n-decylquinoxaline, 2,3-di-n-decylpyrido[3,4-b]pyrazine, 2,3,6,7-tetrakis(n-decyloxy)benzo[a,c]phenazine, or 2,3,6,7-tetrakis(n-decyloxy)dibenzo[f,h]pyrido[4,3-b]quinoxaline acceptors were synthesised using Stille coupling reactions. Experimental absorption maxima in THF range from 645 to 770 nm. These optical data, along with the results of quantum-chemical calculations and electrochemical measurements, show that, as expected, the pyridopyrazine moiety acts as a stronger acceptor than quinoxaline and that the extended species benzophenazine and dibenzopyridoquinoxaline are stronger acceptors than quinoxaline and pyridopyrazine, respectively. Modest average hole mobilities of up to ca. 3.0 × 10−4 cm2 V−1s−1 were obtained in field-effect transistors. Bulk heterojunction photovoltaic devices made from blends of the benzo[a,c]phenazine-based polymer with 3′-phenyl-3′H-cyclopropa[1,9](C60-Ih)[5,6]fullerene-3′-butanoic acid methyl ester (1 : 3 weight ratio) exhibited average power conversion efficiencies up to 1.4%.

Synthesis, Self-Assembly, and Charge Transporting Property of Contorted Tetrabenzocoronenes

A facile route has been developed for the preparation of a new family of contorted 1.2,3.4,7.8,9.10-tetrabenzocoronenes (TBCs). A two-step cyclization reaction, i.e., oxidative photocyclization followed by FeCl(3)-mediated intramolecular cyclodehydrogenation, was carried out on the olefin precursors to obtain the final TBC compounds. These new TBC molecules have contorted conformation due to steric overcrowding as disclosed by single-crystal crystallographic analysis. Nevertheless, they showed extended π-conjugation compared with coronene and exhibited strong aggregation in solution. The thermal behavior and self-assembly of TBC-C8 in solid were studied by a combination of thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), powder X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscope (TEM). Compound TBC-C8 showed very good thermal and photostability and exhibited long-range ordered π-stacking in the bulk state. Moreover, uniform nanofibers with tens of micrometer length are formed in the drop-casted thin films. TBC-C8 also possesses a desirable HOMO energy level (-5.10 eV), which allows efficient charge injection from electrodes such as gold electrode. The charge carrier mobilities were determined by using the space-charge limited-current (SCLC) technique and high average hole mobility of 0.61 cm(2) V(-1) s(-1) was obtained for TBC-C8.

Dependence of Pentacene Crystal Growth on Dielectric Roughness for Fabrication of Flexible Field-Effect Transistors

The dependence of pentacene nanostructures on gate dielectric surfaces were investigated for flexible organic field-effect transistor (OFET) applications. Two bilayer types of polymer/aluminum oxide (Al(2)O(3)) gate dielectrics were fabricated on commercial Al foils laminated onto a polymer back plate. Some Al foils were directly used as gate electrodes, and others were smoothly polished by an electrolytic etching. These Al surfaces were then anodized and coated with poly(alpha-methyl styrene) (PAMS). For PAMS/Al(2)O(3) dielectrics onto etched Al foils, surface roughness up to approximately 1 nm could be reached, although isolated dimples with a lateral diameter of several micrometers were still present. On PAMS/Al(2)O(3) dielectrics (surface roughness >40 nm) containing mechanical grooves of Al foil, average hole mobility (mu(FET)) of 50 nm thick pentacene-FETs under the low operating voltages (|V| < 6 V) was approximately 0.15 cm(2) V(-1) s(-1). In contrast, pentacene-FETs employing the etched Al gates exhibited mu(FET) of approximately 0.39 cm(2) V(-1) s(-1), which was comparable to that of reference samples with PAMS/Al(2)O(3) dielectrics onto flat sputtered Al gates. Conducting-probe atomic force microscopy and two-dimensional X-ray diffraction of pentacene films with various thicknesses revealed different out-of-plane and in-plane crystal orderings of pentacene, depending on the surface roughness of the gate dielectrics.