Enhanced Electron Mobility Research Articles

One-step facile synthesis of poly(N-vinylcarbazole)-polypyrrole/graphene oxide nanocomposites: enhanced solubility, thermal stability and good electrical conductivity

Poly (N-vinylcarbazole)-polypyrrole/graphene oxide (PNVC-Ppy/GO) nanocomposites have been successfully prepared by one-step chemical oxidative polymerization using ferric chloride hexahydrate in the presence of dodecyl benzene sulfonic acid. The composite formation, morphology and the crystallinity of the composite have been characterized by FTIR spectroscopy, FESEM, and XRD, respectively. The incorporation of graphene oxide into the PNVC-Ppy matrix induces interaction between graphene oxide and PNVC-Ppy via hydrogen bonding and π–π* stacking. This π–π* stacking between the GO layers and PNVC-Ppy produces longer conjugation length leading to a higher solubility in organic solvents and enhanced electron mobility. The information of conjugation chain length and charge transfer capacity at the interface of the composite has been obtained from the Raman spectroscopy and photolumincience spectroscopy. The improved thermal stability and electrical d.c. conductivity (0.123 S/cm) of the resulting PNVC-Ppy/GO composite compared to the PNVC–Ppy copolymer (0.08 S/cm) is attributed to the incorporation of graphene oxide in the composite.

Oxygen annealing of the ZnO nanoparticle layer for the high-performance PbS colloidal quantum-dot photovoltaics

Though numerous researches regarding the influence of annealing atmospheric condition of ZnO have been carried out, the impact of annealing atmosphere on the carrier transporting properties and the performance of the ZnO-based optoelectronics has not been well-established. Here, the effects of annealing atmosphere (i.e., N2, ambient air, and O2) used to generate ZnO nanoparticle (NP) layers are elucidated. The chemical nature of ZnO layers, especially the amount of oxygen vacancies in ZnO NPs, is modulated by the annealing atmosphere. As the composition of O2 gas increases in the annealing atmosphere, a notable reduction of oxygen vacancies of ZnO NPs and electron mobility enhancement are observed, indicating that O2 gas contributes to a reduction of surface defects on ZnO NPs during the annealing process. In addition, trap-filling by reduced oxygen vacancies of air- and O2-annealed ZnO layers, induces the enhanced built-in potential in colloidal quantum-dot photovoltaic (CQDPV) devices. As expected, PbS CQDPVs with an air- and O2-annealed ZnO layer demonstrate significantly improved power conversion efficiencies than CQDPVs with an N2-annealed ZnO layer. Further analysis shows that the interfacial recombination is reduced for CQDPVs with an air- and O2-annealed ZnO layer due to the reduced trap states of ZnO NPs.

Efficient Thick-Film Polymer Solar Cells with Enhanced Fill Factors via Increased Fullerene Loading.

Developing effective methods to make efficient bulk-heterojunction polymer solar cells at roll-to-roll relevant active layer thickness is of significant importance. We investigate the effect of fullerene content in polymer:fullerene blends on the fill factor (FF) and on the performance of thick-film solar cells for four different donor polymers PTB7-Th, PDPP-TPT, BDT-FBT-2T, and poly[5,5′-bis(2-butyloctyl)-(2,2′-bithiophene)-4,4′-dicarboxylate-alt-5,5′-2,2′-bithiophene] (PDCBT). At a few hundreds of nanometers thickness, increased FFs are observed in all cases and improved overall device performances are obtained except for PDCBT upon increasing fullerene content in blend films. This fullerene content effect was studied in more detail by electrical and morphological characterization. The results suggest enhanced electron mobility and suppressed bimolecular recombination upon increasing fullerene content in thick polymer:fullerene blend films, which are the result of larger fullerene aggregates and improved interconnectivity of the fullerene phases that provide continuous percolating pathways for electron transport in thick films. These findings are important because an effective and straightforward method that enables fabricating efficient thick-film polymer solar cells is desirable for large-scale manufacturing via roll-to-roll processing and for multijunction devices.

Air-stable n-doped black phosphorus transistor by thermal deposition of metal adatoms

In this work, a simple and effective thermal deposition method is described to enhance the stability and achieve n-doping of two-dimensional black phosphorus (BP), resulting in the successful fabrication of logic devices. By doping with Al adatoms, the stability of BP against oxidation is enhanced and the transfer characteristics are maintained for several days under ambient conditions. Furthermore, the resulting BP is n-doping showing a negative-shifted threshold voltage and enhanced electron mobility. The mechanism of Al-doping is investigated by first-principles calculation and Al induces a downward shift of the conduction band minimum and increases the Fermi level. In addition, integration of bare BP and Al-doped BP enables the fabrication of various logic devices such as inverters and p−n diodes. This work reveals a facile strategy to simultaneously enhance the stability and regulate the conductivity of BP by doping with metal adatoms boding well for device applications.

Mesopolymer synthesis by ligand-modulated direct arylation polycondensation towards n-type and ambipolar conjugated systems.

Conjugated polymers are attractive components for plastic electronics, but their structural defects, low solubility and batch-to-batch variation-mainly in terms of molecular weight and dispersity-hinder practical applications. Here, we demonstrate that these issues can be circumvented by using conjugated mesopolymers, which have the advantages of both oligomers and polymers. A diketopyrrolopyrrole monomer and a benzothiadiazole derivative react through direct arylation polycondensation, promoted by sterically hindered adamantyl ligand coordinated palladium catalysts, to form mesopolymers. The reaction is facile, environmentally benign (it does not require tin or boron reagents) and occurs in high yields. The resulting mesopolymers have a strictly alternating donor-acceptor structure, without detectable homocoupling and β-arylation defects, and exhibit number-averaged molecular weights (Mn) between 1 and 10 kDa. They also show good solution processability and have significantly enhanced electron mobilities, which makes them n-type and ambipolar semiconductors, with advantages over their polymer counterparts.

Copper/Cuprous Oxide Nanoparticles Decorated Reduced Graphene Oxide Sheets Based Platform for Bio‐Electrochemical Sensing of Dopamine

AbstractA rudimentary simple sonochemical approach has been employed for the synthesis of copper/cuprous oxide nanoparticles (Cu/Cu2O NPs) decorated graphene oxide (GO) sheets. Simply, an aqueous dispersion of GO was sonochemically treated with Cu/Cu2O NPs as obtained through a simple electrochemical dissolution method to get the Cu/Cu2O/rGO nanocomposite. The decoration of Cu/Cu2O NPs on GO sheets resulted in the excellent sensing activity of Cu/Cu2O/rGO nanocomposite towards dopamine in the pharmaceutical formulation at the electrode interface due to enhanced electron mobility in GO sheet and high catalytic activity of Cu/Cu2O NPs. The comparison has been laid out based on various characterization techniques viz. powder X‐ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infra‐red (FTIR) spectroscopy and Raman spectroscopy. The fabricated sensor depicted high sensitivity of 0.806 μA/μM for dopamine in a broad linear range of 0.25 –17 μM. Moreover, the remarkable detection limit of 2.6 nM was found for dopamine, and the developed sensor exhibited a wide separation ca. 140 mV between the respective detection peaks of dopamine and uric acid, which is a major interferent in the analysis of dopamine.

Double B←N bridged bipyridine-containing polymer acceptors with enhanced electron mobility for all-polymer solar cells

Two novel polymer electron acceptors based on double B←N bridged bipyridine with enhanced electron mobilities were synthesized for efficient all-polymer solar cells.

Progress of the study on carrier scattering mechanisms of silicon/germanium field effect transistors

As the feature size of Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs) continues to decrease, large numbers of new problems appear. Techniques such as strain project, new channel materials and new device structures are considered by academics and industry to be effective ways to continue to improve device performance. In this paper, the scattering mechanism of carriers in the device channel is studied from three aspects: strain technique, new channel material and new structure device: (1) strain technique: Biaxial tensile strain can change carrier distribution among different energy levels, which affects Coulomb scattering and Coulomb mobility. Furthermore, from the TEM image, it is found that biaxial tensile reduces the channel surface roughness for Si nMOSFET, leading an enhancement of electron mobility. However, no such similar phenomena were observed in pMOSFET. Based on this, a new method for calculating the mobility of MOSFET surface roughness scattering using TEM image has been proposed. (2) New channel material: In the germanium (Ge) transistors with different crystal faces, the scattering mechanisms of electron under high field conditions are different. The phonon scattering dominates the Ge(100) transistor, while the surface roughness scattering dominates the Ge(110), (111) transistors. This result is quite different from Si MOSFET. Therefore, a unified model for the scattering mechanism of electron in Ge nMOSFET has been proposed. In SiGe transistors, alloy scattering mainly play a role in the region with relatively small effective electric field (<i>E</i><sub>eff</sub>). The strength of alloy scattering would be weakened in high field and could be decreased by reducing the thickness of SiGe layer. (3) New structure devices: In ultra-thin body germanium (GeOI) transistors, carrier transport is influenced by high- k /channel interfaces, as well as Ge channel/buried oxide (BOX) interface. As the Ge layer thickness decreases, carrier distribution is closer to the interfaces, which intensifies Coulomb scattering and surface roughness scattering. As a result, the mobility in GeOI transistor decreases as the thickness scaling. In addition, the distribution of electron in different energy valleys changes with the thickness decrease in Ge layer, which affects the scattering of electrons. When the Ge film is lower than 10 nm, a part of electron in the L valley will move to the Γ valley, causing the decrease of electron effective mass and increasing the electron mobility.

Optimizing one-dimensional TiO2 for photocatalytic hydrogen production from a water-ethanol mixture and other electron donors

This work is focused on synthesizing and employing one-dimensional (1D) titanium dioxide (TiO2) for hydrogen (H2) production. Based on using electron donors (EDs) (ethanol, methanol, fomic acid and 1,2,3 propanetriol), the increased H2 production, when compared to P25 TiO2 nanoparticles, was due to the large specific surface area (SSA) and enhanced electron mobility of 1D TiO2. The impact of the 1D TiO2 synthesis reaction conditions (temperature, NaOH concentration and the TiO2 precursor concentration) on the photocatalytic H2 production rate was evaluated using a 3-factor 3-level Box Behnken design (BBD). The BBD model demonstrated that the temperature and the NaOH concentration significantly affected the 1D TiO2 phase structure, crystal size, SSA, bandgap and the photocatalytic H2 production rate. The phase structure and crystal size of 1D TiO2 were key factors affecting the H2 production rate. 1D TiO2 containing an anatase phase with a mean crystal size of 20.1 ± 0.2 nm was synthesized at 126 °C, 15 M NaOH and 49 g L−1 TiO2. The maximum H2 production rate of 475 ± 12 μmol·h−1 (quantum efficiency (ε) = 20.2 ± 0.5%) for the 1D TiO2 sample was significantly enhanced when compared to commercial TiO2 P25. The H2 production rate for the optimized 1D TiO2 was significantly enhanced by decorating the structure with Pt and Au. Hydrothermal synthesized of 1D TiO2 provided an efficient and low cost method for producing H2 from ethanol, methanol, fomic acid and 1,2,3 propanetriol.

Fine Structural Tuning of Thieno[3,2-b] Pyrrole Donor for Designing Banana-Shaped Semiconductors Relevant to Organic Field Effect Transistors

On the basis of the newly synthesized banana-shaped thieno[3,2- b] pyrrole building block [Bulumulla, C.; Gunawardhana, R.; Kularatne, R. N.; Hill, M. E.; McCandless, G. T.; Biewer, M. C.; Stefan, M. C. Thieno[3,2- b] pyrrole-Benzothiadiazole Banana-Shaped Small Molecules for Organic Field Effect Transistors. ACS Appl. Mater. Interfaces 2018, 10, 11818-11825], several small molecules that can be used as organic semiconducting materials were theoretically designed. We have shown that these novel molecules with the donor-π conjugated bridge-acceptor-π conjugated bridge-donor (D-π-A-π-D) building block exhibit superior charge transport properties in organic field-effect transistors (OFETs). A variety of donors, π-bridges, and acceptors are examined, and the structural, electronic, optical, and charge transport properties of designed semiconductors are systematically investigated. The results highlight the impact of the core acceptor in improving the transport properties of the designed molecules. In particular, this work points toward the benzo-bis(1,2,5-thiadiazole) as the most promising acceptor that can be combined with thiophene π-bridge and flanked benzo-thiadiazole terminal units to produce a reasonable candidate for synthesis and for incorporating into OFET materials. For the suggested semiconductor, the small electron reorganization energy and large intramolecular coupling originating from dense π-stacking gave rise to enhanced electron mobility. This strategy can be helpful for further improving the performance of curved small molecules in field-effect devices.

Experimental Analyses on Multiscale Structural and Mechanical Properties of ε-Si/GeSi/C-Si Materials

Strained silicon (ε-Si) is a promising material that could extend Moore’s law by enhancing electron mobility. A ε-Si material is usually composed of multiscale, multilayer heterostructures, where the strained-silicon film or strap is tens-of-nanometers thick, and its buffer layers are of the micrometer scale. The structural properties determine the electrical performance and reliability of ε-Si-based devices. Inhomogeneous residual stress is induced during the preparation, which induces ε-Si structure failure. In this work, biaxial strained-silicon films that contain graded and relaxed germanium-silicon buffer layers were prepared on monocrystalline silicon wafers through reduced-pressure chemical-vapor epitaxy. The layer components and thicknesses were measured using energy-dispersive spectroscopy and scanning-electron microscopy. Crystal and lattice characters were observed by using high-resolution transmission-electron microscopy and micro-Raman spectroscopy. The residual stress distribution along cross-sections of the ε-Si multilayer structures was examined by using micro-Raman mapping. The experimental results showed that, with a gradual increase in germanium concentration, the increasing residual stress was suppressed owing to dislocation networks and dislocation loops inside the buffer layers, which favored the practical application.

Oxygen Electromigration and Energy Band Reconstruction Induced by Electrolyte Field Effect at Oxide Interfaces.

Electrolyte gating is a powerful means for tuning the carrier density and exploring the resultant modulation of novel properties on solid surfaces. However, the mechanism, especially its effect on the oxygen migration and electrostatic charging at the oxide heterostructures, is still unclear. Here we explore the electrolyte gating on oxygen-deficient interfaces between SrTiO_{3} (STO) crystals and LaAlO_{3} (LAO) overlayer through the measurements of electrical transport, x-ray absorption spectroscopy, and photoluminescence spectra. We found that oxygen vacancies (O_{vac}) were filled selectively and irreversibly after gating due to oxygen electromigration at the amorphous LAO/STO interface, resulting in a reconstruction of its interfacial band structure. Because of the filling of O_{vac}, the amorphous interface also showed an enhanced electron mobility and quantum oscillation of the conductance. Further, the filling effect could be controlled by the degree of the crystallinity of the LAO overlayer by varying the growth temperatures. Our results reveal the different effects induced by electrolyte gating, providing further clues to understand the mechanism of electrolyte gating on buried interfaces and also opening a new avenue for constructing high-mobility oxide interfaces.

Physical Adsorption and Photocatalytic Activity of Titanium Dioxide Nanotube and Graphene Oxide Composite

In this study, we synthesized graphene oxide (GO) by modified low temperature Hummer’s method and in situ hydrothermally grown TiO­2 nanotube (TNT) onGO sheet. Transmission electron microscopic (TEM) images showed the homogeneous formation of TNT with the mean diameter of ~8 nm and the co-existence of TNT and GO in the composite sample. X-ray differaction pattern of GO indicated the successful fabrication. The UV-vis measurement with methylene blue indicated the improvement of physical adsorption of the composite samples.
 Keywords
 TNTs, GO, physical adsorption, composite
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One-Dimensional-Like Titania/4'-Pentyl-4-Biphenylcarbonitrile Composite Synthesized Under Magnetic Field and its Structure-Photocatalytic Activity Relationship.

The demonstration of the structure–properties relationship of shape-dependent photocatalysts remains a challenge today. Herein, one-dimensional (1-D)-like titania (TiO2), as a model photocatalyst, has been synthesized under a strong magnetic field in the presence of a magnetically responsive liquid crystal as the structure-aligning agent to demonstrate the relationship between a well-aligned structure and its photocatalytic properties. The importance of the 1-D-like TiO2 and its relationship with the electronic structures that affect the electron–hole recombination and the photocatalytic activity need to be clarified. The synthesis of 1-D-like TiO2 with liquid crystal as the structure-aligning agent was carried out using the sol–gel method under a magnetic field (0.3 T). The mixture of liquid crystal, 4′-pentyl-4-biphenylcarbonitrile (5CB), tetra-n-butyl orthotitanate (TBOT), 2-propanol, and water, was subjected to slow hydrolysis under a magnetic field. The TiO2–5CB took a well-aligned whiskerlike shape when the reaction mixture was placed under the magnetic field, while irregularly shaped TiO2–5CB particles were formed when no magnetic field was applied. It shows that the strong interaction between 5CB and TBOT during the hydrolysis process under a magnetic field controls the shape of titania. The intensity of the emission peaks in the photoluminescence spectrum of 1-D-like TiO2–5CB was lowered compared with the TiO2–5CB synthesized without the magnetic field, suggesting the occurrence of electron transfer from 5CB to the 1-D-like TiO2–5CB during ultraviolet irradiation. Apart from that, direct current electrical conductivity and Hall effect studies showed that the 1-D-like TiO2 composite enhanced electron mobility. Thus, the recombination of electrons and holes was delayed due to the increase in electron mobility; hence, the photocatalytic activity of the 1-D-like TiO2 composite in the oxidation of styrene in the presence of aqueous hydrogen peroxide under UV irradiation was enhanced. This suggests that the 1-D-like shape of TiO2 composite plays an important role in its photocatalytic activity.

Strained SixGe1−x-Ge-Si core-double-shell nanowire heterostructures for simultaneous hole and electron mobility enhancement

We report the growth, structural, and electrical characterization of epitaxial, strained SixGe1−x-Ge-Si core-double-shell nanowire heterostructures designed to provide quantum confinement of holes and electrons in the compressively strained Ge and tensile-strained Si shells, respectively. The growth utilizes the vapor-liquid-solid growth mechanism for the SixGe1−x core, followed by a sequence of in-situ ultra-high-vacuum chemical vapor deposition for the epitaxial Ge and Si shell growth. Using a combination of micro-Raman spectroscopy on individual nanowires and lattice dynamic theory, we determine a large compressive (tensile) hydrostatic strain of up to −0.9% (0.67%) in the Ge (Si) shell. We demonstrate p- and n-type metal-oxide-semiconductor field-effect transistors using SixGe1−x-Ge-Si core-double-shell nanowires as channel and observe a 500% (20%) enhancement of the average hole (electron) mobility compared to control devices using Si nanowires, due to an increased hole (electron) mobility in the compressively strained Ge (tensile strained Si) shell. An analysis of the hole transport provides the valence band offset in the core-double-shell nanowire heterostructures.

All-Polymer Solar Cells with 9.4% Efficiency from Naphthalene Diimide-Biselenophene Copolymer Acceptor

We report herein the development of high performance all-polymer solar cells (all-PSCs) based on a naphthalene diimide-biselenophene copolymer acceptor bearing 2-octyldodecyl side chains (PNDIBS). Inverted all-PSCs with up to 9.4% power conversion efficiency, high photocurrent (18.32 mA/cm2), and remarkably low optical band-gap energy loss (0.53 eV) were achieved by blending high molecular weight (Mn = 53.7 kDa) PNDIBS with a donor polymer PBDB-T that has a complementary absorption spectrum. In contrast, the corresponding lower Mn (28.4 kDa) PNDIBS devices had a maximum efficiency of 7.9%. The enhanced performance of the high-Mn devices is correlated with improved face-on molecular orientation, enhanced electron mobility, and reduced domain sizes of PNDIBS and balanced charge transport in the blend active layers. These findings demonstrate that PNDIBS acceptor polymer is promising for developing highly efficient all-PSCs while providing insights on the critical roles of size of side chains, molecular weig...

Shift of the Branching Point of the Side‐Chain in Naphthalenediimide (NDI)‐Based Polymer for Enhanced Electron Mobility and All‐Polymer Solar Cell Performance

AbstractThe branching point of the side‐chain of naphthalenediimide (NDI)‐based conjugated polymers is systematically controlled by incorporating four different side‐chains, i.e., 2‐hexyloctyl (P(NDI1‐T)), 3‐hexylnonyl (P(NDI2‐T)), 4‐hexyldecyl (P(NDI3‐T)), and 5‐hexylundecyl (P(NDI4‐T)). When the branching point is located farther away from the conjugated backbones, steric hindrance around the backbone is relaxed and the intermolecular interactions between the polymer chains become stronger, which promotes the formation of crystalline structures in thin film state. In particular, thermally annealed films of P(NDI3‐T) and P(NDI4‐T), which have branching points far away from the backbone, possess more‐developed bimodal structure along both the face‐on and edge‐on orientations. Consequently, the field‐effect electron mobilities of P(NDIm‐T) polymers are monotonically increased from 0.03 cm2 V−1 s−1 in P(NDI1‐T) to 0.22 cm2 V−1 s−1 in P(NDI4‐T), accompanied by reduced activation energy and contact resistance of the thin films. In addition, when the series of P(NDIm‐T) polymers is applied in all‐polymer solar cells (all‐PSCs) as electron acceptor, remarkably high‐power conversion efficiency of 7.1% is achieved along with enhanced current density in P(NDI3‐T)‐based all‐PSCs, which is mainly attributed to red‐shifted light absorption and enhanced electron‐transporting ability.

Polyoxometalate doped tin oxide as electron transport layer for low cost, hole-transport-material-free perovskite solar cells

An electron transport layer (ETL) is essential for charge separation and electron extraction in perovskite solar cells (PSCs). In this work, we develop an efficient electron transport layer material SnO2-PMo12 composite for the fully printable hole transport material (HTM)-free PSCs, in which PMo12 denotes the Keggin-type polyoxometalate H3PMo12O40. Benefitting from the synergistic effect of polyoxometalates, SnO2-PMo12 shows enhanced electron mobility, faster electron extraction and proper downward shift of the conduction band minimum, which boosts the injection and transfer of electrons from the perovskite to the ETL and effectively suppresses charge recombination. In addition, the hysteresis-free behavior appears in the SnO2-PMo12 device, suggesting a remarkable improvement in photovoltaic performance and device stability. Compared to the conventional TiO2 ETL, the average power conversion efficiency of the device with SnO2-PMo12 ETL is boosted from 10.1% to 12.6%. This work brings a new insight into the promoting photovoltaic performance and stability of the fully printable HTM-free PSCs.

Development of Tri‐Layered s‐Si/s‐SiGe/s‐Si Channel Heterostructure‐on‐Insulator MOSFET for Enhanced Drive Current

Incubation of strain technology in the Metal Oxide Semiconductor Field Effect Transistor (MOSFET) arena by developing heterostructure layers combined of Si and SiGe layers within the channel is employed widely. Development of a fully depleted novel device featuring a distinguished tri‐layer heterostructure channel consisting of mobility enriched double strained Si (s‐Si) layers sandwiching strained SiGe (s‐SiGe) in between has been the crux of this paper. The channel with s‐Si/s‐SiGe/s‐Si significantly enhances electron mobility in comparison to conventional device of s‐Si on relaxed SiGe channel, leading to improved device performance without implementing additional scaling. The tri‐layered channel and the conventional device are compared and eloquent drain current enhancement of 49% attained with minimal value of threshold voltage roll‐off with increased strain infusion within the channel region, which augments electron mobility in the s‐Si layers. Augmented maximum transconductance for sub‐50 nm channel length due to velocity overshoot enhancement of 119% in comparison to conventional device acquired lead to ≈25% decrease in threshold voltage by band structure lowering with the instigation of the strain technology in the short channel device attained. Hence, with higher carrier mobility and improved drive current double strained Si heterostructure on insulator (HOI) MOSFET device is designed and developed.

Effect of Core Size on Performance of Fused-Ring Electron Acceptors

We report 4 fused-ring electron acceptors (FREAs) with the same end-groups and side-chains but different cores, whose sizes range from 5 to 11 fused rings. The core size has considerable effects on the electronic, optical, charge transport, morphological, and photovoltaic properties of the FREAs. Extending the core size leads to red-shift of absorption spectra, upshift of the energy levels, and enhancement of molecular packing and electron mobility. From 5 to 9 fused rings, the core size extension can simultaneously enhance open-circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF) of organic solar cells (OSCs). The best efficiency of the binary-blend devices increases from 5.6 to 11.7%, while the best efficiency of the ternary-blend devices increases from 6.3 to 12.6% as the acceptor core size extends.