Improved Carrier Mobility Research Articles

High response and broadband photodetection by monolayer MoSe2 with vanadium doping and Mo vacancies

Recently, the design of broadband photodetectors based on 2D materials has been a subject of great attention. However, it remains a challenge to achieve both broadband detection and high responsivity for monolayer transition metal dichalcogenide based photodetectors due to the larger band gaps and limited absorption at monolayer thickness. Based on density functional theory calculations, we confirmed that a combination of vanadium substitution doping and Mo vacancies not only decrease the band gap but also enhance the light absorption of monolayer molybdenum diselenide. We also synthesized MoSe2 with both V substitution doping and Mo vacancies by chemical vapor deposition and investigated the electrical and photoelectrical properties. Field effect transistor devices based on the materials showed a transition from n- to p-type characteristics and improved carrier mobility with increasing V concentration. Moreover, photodetectors based on monolayer MoSe2 with 6% V and Mo vacancies displayed a broadband spectral response from 365 nm to 2240 nm, and the responsivity reached 9700 mA W−1 and 2800 mA W−1 at 520 nm and 2240 nm, respectively. Our findings may be useful for designing monolayer transition metal dichalcogenide photodetectors with high responsivities in the infrared region.

Contrasting Cu Roles Lead to High Ranged Thermoelectric Performance of PbS

AbstractTo obtain high‐performance PbS‐based thermoelectric materials, this study introduces Cu with different contrasting roles in p‐type PbS, which can effectively decrease the lattice thermal conductivity and simultaneously optimize the electrical transport properties. Experimental results illustrate that Cu substitutions and Cu interstitials can improve carrier mobility through lowering effective mass (m*) and carrier concentration (nH) in a low temperature range (300–450 K), and further optimize temperature‐dependent nH in a high temperature range (450–823 K). Both decreased m* and nH shift the peak power factor to low temperature range, leading to an ultrahigh power factor ≈23 µW cm−1 K−2 at 423 K for Pb0.99Cu0.01S‐0.01Cu. Additionally, the special dynamic‐doping behaviors of Cu can continuously promote nH to approach the temperature‐dependent relationship of (nH, opt) ≈ (m*T)1.5, which brings about an eminent average power factor (PFave) ≈ 18 µW cm−1 K−2 among 300–823 K in Pb0.99Cu0.01S‐0.01Cu. Furthermore, the microstructure characterizations unclose that the atomic and nanoscale Cu‐containing defects can effectively intensify the phonon scattering and suppress the lattice thermal conductivity. Consequently, both high ZT (≈0.2 at 300 K) and peak ZT (≈1.2 at 773 K) result in a record‐high average ZT (ZTave) of ≈0.79 at 300–823 K for Pb0.99Cu0.01S‐0.01Cu.

Role of MoTi diffusion barrier in amorphous indium-gallium-zinc-oxide thin-film transistors with a copper source/drain electrode

This paper reports why the Cu/MoTi/In-Ga-Zn-O(IGZO) contact schematic exhibited acceptable electrical properties and stability in an IGZO transistor that is used in a state-of-the-art active-matrix organic-light-emitting-diodes television as the switcher and driver. For a comparative study, different Cu/diffusion barrier(DB)/IGZO contact schemes (DB = Mo, Ti, and MoTi) were applied in IGZO transistors to evaluate the suitability of different diffusion barriers. The IGZO transistor with the Cu/(Mo, Ti)/IGZO contact exhibited improved carrier mobility and a lower gate swing compared to those of the control device with direct Cu/IGZO contact. The role of the Mo DB was to effectively prevent in-diffusion of Cu atoms during thermal annealing, which is responsible for its superior performance. In contrast, the Ti DB was found to be suitable for low-ohmic contact due to its higher oxidation power, leading to enhanced transport properties. a-IGZO thin-film transistors with the Cu/MoTi/IGZO stack showed the best transport properties in terms of the mobility, gate swing, and contact resistance. Our findings are discussed on the basis of the synergic effect of the MoTi alloy system based on various structural and chemical analyses.

Double Free: A Promising Route toward Moisture-Stable Hypotoxic Hybrid Perovskites

Double Free: A Promising Route toward Moisture-Stable Hypotoxic Hybrid Perovskites

Morphology Studies of SWCNT Dispersed in Conducting Polymer as Potential Sensing Materials

Novel electronic nanomaterial, the carbon nanotube (CNT) has emerged in many sensor applications as such its state dispersion has considerable importance to ensure the sustainability of its electronic properties. In this paper, we reported a state of art conductivity mapping on nanostructure surface of single walled carbon nanotubes (SWCNT) and poly(3-hexylthiophene-2,5-diyl), (P3HT) as potential sensing film. This composite is proposed to give selective analyte anchoring across the film as well as improved carrier mobility. The easy solution processing method was chosen to produce non-covalently wrapped conducting polymer onto the surface of SWCNT. We successfully observed high resolution images of the SWCNT walls that indicated increase of the thickness due to polymer wrapping. The image obtained from conductivity atomic force microscopy (CAFM) show the film’s electrical distribution that correlated with the observed nanostructure of film. Supporting optical characteristics of the nanocomposite obtained from UV-Vis spectroscopy and Raman spectroscopy discussed the morphology of the polymer wrapping and the state of dispersion of the polymer and the nanotubes. It is hypothesized the filament structures made by P3HT/SWCNT can give better sensing performance due to modification of π-π electronic band of SWCNT.

Gradient rhenium doping enabled tunable anisotropic valleytronic material based on monolayer molybdenum disulfide

Two-dimensional molybdenum disulfide (MoS2) possesses premium electrical, optoelectrical and valleytronic properties to develop future nano-electronic and optoelectronic devices. Valleytronic material platform with anisotropic response is highly desired to develop polarization sensitive valleytronic devices. In this work, we acquire monolayer MoS2 with both valley degree of freedom and anisotropy by introducing Re doping during chemical vapor deposition growth. The atomic substitution with Re element induces prominent n-type doping. Electrical tests under both dark and light illumination show that 5 at% Re-doped MoS2 achieves the optimum electrical and optoelectrical performances, corresponding to 27 and 35 times improved carrier mobility and photoresponsivity than pristine MoS2, respectively. Second harmonic generation (SHG) and polarization Raman spectroscopy (PRS) indicate increased anisotropic optical response of Re doped MoS2 as Re concentration increases: maximum anisotropic ratios of 1.6 and 1.7 have been achieved for SHG and PRS measurements at different doping concentrations, respectively. Circularly polarized PL measurement demonstrates that the Re-doped MoS2 preserves valley dependent optical response. Our theoretical calculations show that the anisotropic response is due to the strain induced by the dopants, so that the 2H lattice structure optical helicity dependent valley selection rules are largely preserved. The work opens promising venue toward polarization sensitive valleytronic material platform.

Charge Carrier Mobility Improvement in Diketopyrrolopyrrole Block-Copolymers by Shear Coating.

Shear coating is a promising deposition method for upscaling device fabrication and enabling high throughput, and is furthermore suitable for translating to roll-to-roll processing. Although common polymer semiconductors (PSCs) are solution processible, they are still prone to mechanical failure upon stretching, limiting applications in e.g., electronic skin and health monitoring. Progress made towards mechanically compliant PSCs, e.g., the incorporation of soft segments into the polymer backbone, could not only allow such applications, but also benefit advanced fabrication methods, like roll-to-roll printing on flexible substrates, to produce the targeted devices. Tri-block copolymers (TBCs), consisting of an inner rigid semiconducting poly-diketo-pyrrolopyrrole-thienothiophene (PDPP-TT) block flanked by two soft elastomeric poly(dimethylsiloxane) (PDMS) chains, maintain good charge transport properties, while being mechanically soft and flexible. Potentially aiming at the fabrication of TBC-based wearable electronics by means of cost-efficient and scalable deposition methods (e.g., blade-coating), a tolerance of the electrical performance of the TBCs to the shear speed was investigated. Herein, we demonstrate that such TBCs can be deposited at high shear speeds (film formation up to a speed of 10 mm s−1). While such high speeds result in increased film thickness, no degradation of the electrical performance was observed, as was frequently reported for polymer−based OFETs. Instead, high shear speeds even led to a small improvement in the electrical performance: mobility increased from 0.06 cm2 V−1 s−1 at 0.5 mm s−1 to 0.16 cm2 V−1 s−1 at 7 mm s−1 for the TBC with 24 wt% PDMS, and for the TBC containing 37 wt% PDMS from 0.05 cm2 V−1 s−1 at 0.5 mm s−1 to 0.13 cm2 V−1 s−1 at 7 mm s−1. Interestingly, the improvement of mobility is not accompanied by any significant changes in morphology.

Reversible hydrogenation restores defected graphene to graphene

Graphene as a two-dimensional material is prone to hydrocarbon contaminations, which can significantly alter its intrinsic electrical properties. Herein, we implement a facile hydrogenation-dehydrogenation strategy to remove hydrocarbon contaminations and preserve the excellent transport properties of monolayer graphene. Using electron microscopy we quantitatively characterized the improved cleanness of hydrogenated graphene compared to untreated samples. In situ spectroscopic investigations revealed that the hydrogenation treatment promoted the adsorption ofytyt water at the graphene surface, resulting in a protective layer against the re-deposition of hydrocarbon molecules. Additionally, the further dehydrogenation of hydrogenated graphene rendered a more pristine-like basal plane with improved carrier mobility compared to untreated pristine graphene. Our findings provide a practical post-growth cleaning protocol for graphene with maintained surface cleanness and lattice integrity to systematically carry a range of surface chemistry in the form of a well-performing and reproducible transistor.

Rational Electronic and Structural Designs Advance BiCuSeO Thermoelectrics

AbstractIn this work, a record high thermoelectric figure‐of‐merit ZT of 1.6 ± 0.2 at 873 K in p‐type polycrystalline Bi0.94Pb0.06CuSe1.01O0.99 by a synergy of rational band manipulation and novel nanostructural design is reported. First‐principles density functional theory calculation results indicate that the density of state at the Fermi level that crosses the valence band can be significantly reduced and the measured optical bandgap can be enlarged from 0.70 to 0.74 eV by simply replacing 1% O with 1% Se, both indicating a potentially reduced carrier concentration and in turn, an improved carrier mobility and a boosted power factor up to 9.0 µW cm−1 K−2. Meanwhile, comprehensive characterizations reveal that under Se‐rich condition, Cu2Se secondary microphases and significant lattice distortions triggered by Pb‐doping and Se‐substitution can be simultaneously achieved, contributing to a reduced lattice thermal conductivity of 0.4 W m−1 K−1. Furthermore, a unique shear exfoliation technique enables an effective grain refinement with higher anisotropy of the polycrystalline pellet, leading to a further improved power factor up to 10.9 µW cm−1 K−2 and a further reduced lattice thermal conductivity of 0.30 W m−1 K−1, which gives rise to record high ZT.

Solution-Processed Organic and ZnO Field-Effect Transistors in Complementary Circuits

The use of high κ dielectrics lowers the operating voltage in organic field-effect transistors (FETs). Polymer ferroelectrics open the path not just for high κ values but allow processing of the dielectric films via electrical poling. Poled ferroelectric dielectrics in p-type organic FETs was seen to improve carrier mobility and reduce leakage current when compared to unpoled devices using the same dielectric. For n-type FETs, solution-processed ZnO films provide a viable low-cost option. UV–ozone-treated ZnO films was seen to improve the FET performance due to the filling of oxygen vacancies. P-type FETs were fabricated using the ferroelectric polymer poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) as the dielectric along with a donor–acceptor polymer based on diketopyrrolopyrrole (DPP-DTT) as the semiconductor layer. The DPP-DTT FETs yield carrier mobilities upwards of 0.4 cm2/Vs and high on/off ratios when the PVDF-TrFE layer is electrically poled. For n-type FETs, UV–ozone-treated sol–gel ZnO films on SiO2 yield carrier mobilities of 10−2 cm2/Vs. DPP-DTT-based p- and ZnO-based n-type FETs were used in a complementary voltage inverter circuit, showing promising characteristic gain. A basic inverter model was used to simulate the inverter characteristics, using parameters from the individual FET characteristics.

Low temperature carrier transport mechanism and photo-conductivity of WSe2

This work reports the electrical-transport and temperature-dependent photoconductivity in tungsten diselenide (WSe2) thin films. Temperature-dependent electrical conductivity has been demonstrated using different models. At lower temperatures (< 190 K), carriers become localized to small regions in the film due to Mott’s hopping mechanism. The middle-temperature region (190–273 K) follows Seto’s parameters and obtained low barrier height (0.0873 eV) may be responsible for the improved carrier mobility. At higher temperature (> 273 K) region, thermally activated conduction is dominated with two activation energies of ~138 meV and 98 meV. The peaks obtained in photoluminescent analysis attributes to the presence of mid-bandgap states or defect states which play an important role in the photoconductivity of WSe2. The transient photoconductivity measurements show consistent temperature-dependent behaviour. The effect of light intensity and wavelength variation on the photoconductivity of WSe2 thin films is also discussed. The photo-current is 1.19 * 10−5 A at 125 K, while at 350 K was observed to be 3.12 * 10−4 A. The light-on/off current cycles show that the current can recover to its initial state after various cycles, which points to the WSe2 thin-film device's stable and reversible properties that can be used in optoelectronic applications.

Probing the effects of surface roughness and barrier layer thickness in InAlGaN/GaN HEMTs to improve carrier mobility

In this study, we investigated the effects of the thickness and surface roughness of InAlGaN barrier layers on the electron mobility of InAlGaN/GaN high-electron-mobility transistors (HEMTs) with low sheet resistance for applications at high frequencies. The results indicate that the carrier electron mobility of InAlGaN/GaN HEMTs decreases with barrier thickness. This is mainly due to the surface roughness of the InAlGaN barrier layer, which is significantly higher than that of the AlGaN barrier surface. In our experiments, we revealed that a thin GaN cap layer led to a decrease in the surface roughness of the InAlGaN barrier layer, thereby improving the electron mobility.

Modulation of the Hydrophilicity on Asymmetric Side Chains of Isoindigo-Based Polymers for Improving Carrier Mobility–Stretchability Properties

To realize high-performance and intrinsically stretchable materials for field-effect transistor (FET) devices, a plethora of approaches about structure design have been explored. Herein, we report a new approach to control the carrier mobility–stretchability properties of the polymers by tuning the hydrophilicity and asymmetric side-chain combination. A series of isoindigo–bithiophene (II2T)-based semiconducting polymers with three kinds of side chains including carbosilane side chain, semifluorinated side chain, and oligoether side chain were synthesized for investigating the structure–mobility and structure–stretchability relationships. The experimental results showed that the molecular stacking pattern and orientation of the derived polymers could be controlled by altering the hydrophilicity and asymmetric side-chain combination. The side chains of carbosilane and oligoether and a semifluorinated side chain could provide an order edge-on stacking, conformability and backbone aggregation, and an irregular solid-state aggregation, respectively. Among them, P(Si–O) with oligoether and a carbosilane side chain exhibited an enhanced μₕ of 0.56 cm² V–¹ s–¹, edge-on stacking, and aggregation behavior owing to the favorable intermolecular interaction between the oligoether side chain and the asymmetric side chain to mitigate the steric hindrance. Also, P(Si–O) possessed a remarkable stretchability of (92%,⊥, 82%,∥) orthogonal μₕ retention under 100% strain and almost unchanged μₕ was observed after 1000 stretching–releasing cycles at 60% strain. The experimental results suggested that the combination and hydrophilicity of side chain played a pivotal role in developing semiconducting polymers with a high performance and an intrinsic stretchability.

Self-organized 〈1 0 0〉 direction growth of germanium film on insulator obtained by high speed continuous wave laser annealing

To realize next-generation electronics, germanium crystal growth on insulating substrates is attracting attention due to its high carrier mobility, and further improvement of carrier mobility is anticipated by applying tensile strain to germanium. We have previously realized highly tensile-strained germanium films on insulating substrates using a high speed continuous wave laser annealing process. This letter reports how we characterized the grown films using electron diffraction backscatter measurements and discovered an interesting self-organized 〈100〉 direction type of growth that results from scanning the annealing laser at high speed. Dominant {110} oriented films were obtained in this way. This phenomenon is discussed below from the viewpoint of surface energy.

Rapid Effective Reduction by Microwave-Irradiated Thermal Reaction for Large-Scale Production of High-Quality Reduced Graphene Oxide

ABSTACT In this study, we suggest a simple and effective one-pot hybrid reduction process for the mass production of high-quality reduced graphene oxide (rGO) by simultaneously doing deoxygenation and healing reactions. During the microwave-irradiated thermal reduction, intercalated benzene in the GO easily generates acetylene by pyrolysis; the released acetylene react with surrounding defect sites in the GO surface to successfully form new C–C bonds. As a result of the newly formed sp2-hybridized C–C bond in the rGO surface, the defect-repaired rGO (rGO-B) shows remarkably enhanced crystallinity (ID/IG ratio: rGO-B, 0.63; rGO-T, 1.08), thermal stability, and electrical properties over that of rGO prepared without a carbon-source supplement (rGO-T). Especially, compared to the rGO-T, the rGO-B had 4.4 times more carrier density and 18 times increased carrier mobility because of the restoration of defect sites in the rGO-B surface. The rGO-B exhibited six times higher electrical conductivity than did rGO-T because of the improved carrier mobility. These results obviously suggest that the reduction of GO by means of microwave-irradiated thermal reduction with a carbon-source supplement could be a powerful approach for commercial mass production of high-quality rGO because of its easy manufacturing approach.

A Scalable Integrated Dopant‐Free Heterostructure to Stabilize Perovskite Solar Cell Modules

AbstractPerovskite solar cell (PSC) modules employing a hole transport layer (HTL) without unstable dopants possess high potential for improving operational stability. However, the low efficiencies of the devices greatly limit their commercial applications owing to the lower efficacy of the dopant‐free HTL, introduced by the unintentional n‐doping effect of volatile ions from the halide‐rich perovskite surface. Here, a scalable heterostructure integrated by a methylammonium‐free perovskite film with an iodide‐rich surface, an ultrathin interlayer of bridge‐jointed graphene oxide nanosheets (BJ‐GO), and an HTL without additional ionic dopants is developed. In this heterostructure, the iodide ions are physically immobilized by the compact 2D network, and lead defects are chemically passivated by multiple coordination bonds. Moreover, the BJ‐GO with tunable surface energy enables a highly ordered HTL a considerably improved carrier mobility by an order of magnitude. Finally, the PSC module with an area of 35.80 cm2 employing this heterostructure shows a certified efficiency of 15.3%. The encapsulated PSC modules retain over 91% of initial efficiency after the damp heat test at 85 °C and ≈85% relative humidity for 1000 h, while maintaining 90% of the initial value for 1000 h at the maximum power point under continuous 1‐Sun illumination at 60 °C.

Enhanced Thermoelectric Performance of Ba8Ga16Ge30 Clathrate by Modulation Doping and Improved Carrier Mobility

AbstractType‐I inorganic clathrates are promising high temperature thermoelectric materials. They are known for their intrinsic low thermal conductivity, but a moderate power factor leaves room for further improvement. In this paper, a new route for improving the power factor by enhanced carrier mobility achieved via modulation doping is reported. A series of clathrates with composition Ba8(AlxGa1−x)16Ge30 are synthesized through ball milling and spark plasma sintering of mixtures of Ba8Al16Ge30 and Ba8Ga16Ge30. Among the materials with x = 0.20, 0.23, and 0.25, it is found that the electrical conductivity is significantly enhanced with increasing x, while the Seebeck coefficient decreases slightly. It is further revealed that the carrier mobility of the sintered sample x = 0.25 is greatly increased, reaching a value that exceeds that for a single crystal. Electron microscopy analysis reveals that the material consists of a heterostructure and is composed of a Ga‐rich clathrate matrix phase and Al particles, suggesting that the power factor enhancement is due to modulation doping. As a result, the highest power factor is achieved for Ba8(Al0.25Ga0.75)16Ge30, with a value of 1.89 mW m−1 K−2 at 800 °C. Consequently, the maximum zT of sample x = 0.25 reaches 0.93 at 800 °C.

Enhancement of thermoelectric performance of n-type AgBi1+xSe2 via improvement of the carrier mobility by modulation doping

High charge carrier mobility with low lattice thermal conductivity is one of the key factors for the design of a good thermoelectric material. Recent studies show that n-type Te-free AgBiSe2 is promising compound for thermoelectric energy conversion due to intrinsically low lattice thermal conductivity. However, low charge carrier mobility in AgBiSe2 is the constraint for enhancement of its power factor. In the present study, we use a chemical modification way to realize modulation doping in AgBiSe2. The addition of 2–6 mol% excess Bi in AgBiSe2 results in the formation of Bi-rich modulation-doped microstructures of topological semimetal, Bi4Se3 in AgBiSe2 matrix. We show that due to facile carrier transport via semi-metallic Bi4Se3 microstructure results in overall improvement of carrier mobility without compromising Seebeck coefficient in AgBiSe2 system, which in turn results in a remarkable improvement in the power factor (σS2) value. A highest σS2 value of ~6.35 μW cm−1 K−2 at 800 K has been achieved in AgBiSe2-3% Bi excess sample, which is higher than previously reported metal ion and halogen-doped AgBiSe2.

Binary Additive–Induced Performance Improvement of PM7:PC71BM Organic Solar Cells with High Open‐Circuit Voltage and Enhanced Current Intensity

Adding a solvent additive into the active layer is a simple and effective strategy to improve active layer morphology and performance of bulk heterojunction organic solar cells (OSCs). Herein, poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl‐3‐chloro)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione)] (PM7) is used as donor and [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM) as acceptor, and 1,8‐diiodooctane (DIO) and 1‐chloronthalene (CN) are utilized as solvent additives to prepare OSCs. The impact of binary additives on the photoelectric performance of OSCs is studied. The binary additives restrict charge recombination and improve carrier mobility effectively, which is proved by the transient photovoltage/transient photocurrent measurement and photoinduced charge carrier extraction by linearly increasing the voltage technology. With the synergetic effect of DIO and CN, the binary additive devices exhibit a high open‐circuit voltage of 0.96 V. Apart from the improved short‐circuit current density and fill factor, the power conversion efficiency reaches to 9.85%, much higher than that of the pristine device without additive (3.79%).

Enhancing room-temperature thermoelectric performance of n-type Bi2Te3-based alloys via sulfur alloying

Bismuth-telluride-based alloys are the best thermoelectric materials used in commercial solid-state refrigeration near room temperature. Nevertheless, for n-type polycrystalline alloys, their thermoelectric figure of merit (zT) values at room temperature are often less than 1.0, due to the high electron concentration originating from the donor-like effect induced by the mechanical deformation process. Herein, carrier concentration for better performance near room temperature was optimized through manipulating intrinsic point defects by sulfur alloying. Sulfur alloying significantly decreases antisite defects concentration and suppresses donor-like effect, resulting in optimized carrier concentration and reduced electronic thermal conductivity. The hot deformation process was also applied to improve carrier mobility due to the enhanced texture. As a result, a high zT value of 1 at 300 K and peak zT value of 1.1 at 350 K were obtained for the twice hot-deformed Bi2Te2.7Se0.21S0.09 sample, which verifies sulfur alloying is an effective method to improve thermoelectric performance of n-type polycrystalline Bi2Te3-based alloys near room temperature.