Increase In Carrier Mobility Research Articles

High performance NiO/Ag/NiO transparent conducting electrodes for p-Si/n-ZnO heterojunction photodiodes

A NiO/Ag/NiO (NAN) transparent conducting electrode (TCE) was fabricated using a magnetron-sputtering system, and the NAN TCE was applied to p-Si/n-ZnO heterojunction photodiodes (HPDs). The NAN TCE exhibits a 50% higher transmittance than the Al electrode (less than 1%). However, a very low sheet resistance of 0.27 Ω/sq is obtained in Al electrode than in NAN TCE (6.5 Ω/sq). The NAN TCE could effectively enhance the photo response of p-Si/n-ZnO HPDs for both ultraviolet (UV) and visible bands as compared to the p-Si/n-ZnO HPDs with the traditional Al electrode. Compared to the traditional p-Si/n-ZnO HPDs using the Al electrode, for the p-Si/n-ZnO HPDs using NAN TCE, the 500 nm photo response is increased by approximately 10 times and the 280 nm photo response is significantly enhanced by approximately 100 times at a reverse-bias voltage of 1 V. The dark current of p-Si/n-ZnO HPDs with NAN TCE is two orders of magnitude lower than that of p-Si/n-ZnO HPDs with an Al electrode. The improved performance enhances the photo (500 nm) to dark current ratio from 2 for the p-Si/n-ZnO HPDs with Al electrode to 1.4 × 103 for the one with NAN TCE. The photo (280 nm) to dark current ratio is enhanced from 8.5 × 102 to 6.8 × 105. The mechanism results from the high transmittance in the NAN TCE and Ni diffusion in ZnO. The Ni diffusion in ZnO suppresses its defects and hence decreases electron scattering from crystallites/grains, thereby increasing carrier mobility.

Thermal treatment for enhancing performance of NiO/Ag/NiO transparent conducting electrode fabricated via magnetron radio-frequency sputtering

A NiO/Ag/NiO (NAN) transparent conducting electrode (TCE) is fabricated using a low-cost magnetron-sputtering system. The effects of various thicknesses of the Ag intermediate layer and different annealing temperatures on the optical and electrical properties of the prepared TCEs are investigated comprehensively. A NAN TCE with a thin (8 nm) Ag layer demonstrates a slightly lower average transmittance than that with a 10 nm Ag layer because the thinner Ag layer appears as an isolated island structure, resulting in higher optical absorption. As the Ag thickness increases to 10 nm, the isolated islands become connected to each other and form a continuous thin film. As the Ag thickness increases, the carrier concentration and mobility increase and the formation of a continuous dense film, which decrease the resistivity and sheet resistance of the NAN TCE. The NAN TCE with a 14-nm intermediate Ag layer exhibits a sheet resistance of 3.67 Ω/sq, which is much lower than that (34 Ω/sq) of a single ITO electrode. The NAN TCE with a 10-nm Ag exhibits the highest figure-of-merit (FOM) than others. The annealing significantly increases the FOM from 2.5 × 10–4 to 1.2 × 10–2 Ω−1, respectively, for the as-deposited NAN TCE with a 10-nm Ag layer and the NAN TCE annealed at 400 °C. However, the performance of NAN TCE degrades at higher annealing temperature (500 °C).

Electrical Loss Management by Molecularly Manipulating Dopant-free Poly(3-hexylthiophene) towards 16.93 % CsPbI2 Br Solar Cells.

Inorganic cesium lead halide perovskites offer a pathway towards thermally stable photovoltaics. However, moisture-induced phase degradation restricts the application of hole transport layers (HTLs) with hygroscopic dopants. Dopant-free HTLs fail to realize efficient photovoltaics due to severe electrical loss. Herein, we developed an electrical loss management strategy by manipulating poly(3-hexylthiophene) with a small molecule, i.e., SMe-TATPyr. The developed P3HT/SMe-TATPyr HTL shows a three-time increase of carrier mobility owing to breaking the long-range ordering of "edge-on" P3HT and inducing the formation of "face-on" clusters, over 50 % decrease of the perovskite surface defect density, and a reduced voltage loss at the perovskite/HTL interface because of favorable energy level alignment. The CsPbI2 Br perovskite solar cell demonstrates a record-high efficiency of 16.93 % for dopant-free HTL, and superior moisture and thermal stability by maintaining 96 % efficiency at low-humidity condition (10-25 % R. H.) for 1500 hours and over 95 % efficiency after annealing at 85 °C for 1000 hours.

Electrical Loss Management by Molecularly Manipulating Dopant‐free Poly(3‐hexylthiophene) towards 16.93 % CsPbI2Br Solar Cells

AbstractInorganic cesium lead halide perovskites offer a pathway towards thermally stable photovoltaics. However, moisture‐induced phase degradation restricts the application of hole transport layers (HTLs) with hygroscopic dopants. Dopant‐free HTLs fail to realize efficient photovoltaics due to severe electrical loss. Herein, we developed an electrical loss management strategy by manipulating poly(3‐hexylthiophene) with a small molecule, i.e., SMe‐TATPyr. The developed P3HT/SMe‐TATPyr HTL shows a three‐time increase of carrier mobility owing to breaking the long‐range ordering of “edge‐on” P3HT and inducing the formation of “face‐on” clusters, over 50 % decrease of the perovskite surface defect density, and a reduced voltage loss at the perovskite/HTL interface because of favorable energy level alignment. The CsPbI2Br perovskite solar cell demonstrates a record‐high efficiency of 16.93 % for dopant‐free HTL, and superior moisture and thermal stability by maintaining 96 % efficiency at low‐humidity condition (10–25 % R. H.) for 1500 hours and over 95 % efficiency after annealing at 85 °C for 1000 hours.

Realizing N-type SnTe Thermoelectrics with Competitive Performance through Suppressing Sn Vacancies.

Due to the intrinsically plentiful Sn vacancies, developing n-type SnTe thermoelectric materials is a big challenge. Herein, n-type SnTe thermoelectric materials with remarkable performance were successfully synthesized through suppressing Sn vacancies, followed by electron-doping. Pb alloying notably depressed the Sn vacancies via populating Sn vacancies in SnTe (supported by transmission electron microscopy), and the electrical transports were shifted from p-type to n-type through introducing electrons using I doping. In the n-type SnTe, we found that the electrical conductivity could be enhanced by increased carrier mobility through sharpening conduction bands after alloying Pb, while the lattice thermal conductivity could be reduced via strong phonon scattering after introducing defects by Pb alloying and I doping. Resulting from these enhancements, the n-type Sn0.6Pb0.4Te0.98I0.02 achieves a notably high ZTmax ∼ 0.8 at 573 K and a remarkable ZTave ∼ 0.51 at 300-823 K, which can match many excellent p-type SnTe. This work indicates that n-type SnTe could be experimentally acquired and is a promising candidate for thermoelectric generation, which will stimulate further research on n-type SnTe thermoelectric materials and even devices on the basis of both n- and p-type SnTe legs.

III-V-on-Si transistor technologies: Performance boosters and integration

In this work, we review progress in III-V transistor technologies. Key approaches for silicon integration are described, with a distinction being made between large area layer transfer and selective growth techniques. We show how the integration approach must be tailored for the intended application to maximize performance, functionality and minimize cost. We also highlight performance boosters such as heterostructure channel stacks, which offer increased carrier mobility towards improved RF and RF-CMOS performance. Recent progress in tunneling field-effect transistors (TFETs) have enabled the TFET architecture as a process module, allowing for dense integration with MOSFETs on the same chip towards an extremely low-power CMOS technology. Finally, we highlight emerging applications for III-V devices at cryogenic temperatures, where there is a rising need for low-power electronics to support the scaling of quantum computers. The unique properties of III-V, their high mobility and band gap engineering make them highly suitable for this application.

Enhanced Photoluminescence and Photoresponsiveness of Eu3+ Ions‐Doped CsPbCl3 Perovskite Quantum Dots under High Pressure

AbstractMetal halide perovskite quantum dots (QDs) have garnered tremendous attention in optoelectronic devices owing to their excellent optical and electrical properties. However, these perovskite QDs are plagued by pressure‐induced photoluminescence (PL) quenching, which greatly restricts their potential applications. Herein, the unique optical and electrical properties of Eu3+‐doped CsPbCl3 QDs under high pressure are reported. Intriguingly, the PL of Eu3+ ions displays an enhancement with pressure up to 10.1 GPa and still preserves a relatively high intensity at 22 GPa. The optical and structural analysis indicates that the sample experiences an isostructural phase transition at approximately 1.53 GPa, followed by an amorphous state evolution, which is simulated and confirmed through density functional theory calculations. The pressure‐induced PL enhancement of Eu3+ ions can be associated with the enhanced energy transfer rate from excitonic state to Eu3+ ions. The photoelectric performance is enhanced by compression and can be preserved upon the release of pressure, which is attributed to the decreased defect density and increased carrier mobility induced by the high pressure. This work enriches the understanding of the high‐pressure behavior of rare‐earth‐doped luminescent materials and proves that high pressure technique is a promising way to design and realize superior optoelectronic materials.

Amorphous-quantized WO3·H2O films as novel flexible electrode for advanced electrochromic energy storage devices

The existing synthetic approaches for high-performance WO3-based electrodes require energy-intensive instrumentation and complex processing, which hinder the development of flexible electrochromic (EC) energy storage devices. Herein, new low-temperature-synthesized amorphous (a)-quantized WO3·H2O films for application in EC energy storage devices are proposed. The WO3·H2O films are fabricated by the spontaneous hydrolysis of a spin-coated WCl6 solution by the water molecules in the surrounding atmosphere followed by annealing at 80 °C. This is an original and unique concept in that the induced quantization of a-WO3 increases the number of electroactive sites, provides abundant oxygen vacancies, and widens the band gap, while the intercalated water molecules stabilize the structure, resulting in efficient charge transfer and stress alleviation during electrochemical reactions. Consequently, the transmittance (53.8% at 633 nm) and specific capacitance (78.5 F g−1 at 1 A g−1) of the flexible electrodes improves. Additionally, the carrier concentration and mobility increase due to a-WO3 quantization, thereby increasing the electrical conductivity, resulting in the rapid switchability (3.7 s for coloration and 2.9 s for bleaching) and high rate capability of the flexible electrodes. The flexible solid-state devices light a 1.5 V white-light-emitting diode and maintain their good EC energy storage performance (76.1% transmittance modulation and 73.1% specific capacitance) even after 300 bending cycles at a bending radius of 1.3 cm. Therefore, the proposed a-quantized WO3·H2O films are promising active electrodes for flexible EC energy storage devices.

The Quinonoid Zwitterion Interlayer for the Improvement of Charge Carrier Mobility in Organic Field-Effect Transistors.

The interface between the semiconductor and the dielectric layer plays a crucial role in organic field-effect transistors (OFETs) because it is at the interface that charge carriers are accumulated and transported. In this study, four zwitterionic benzoquinonemonoimine dyes featuring alkyl and aryl N-substituents were used to cover the dielectric layers in OFET structures. The best interlayer material, containing aliphatic side groups, increased charge carrier mobility in the measured systems. This improvement can be explained by the reduction in the number of the charge carrier trapping sites at the dielectric active layer interface from 1014 eV−1 cm−2 to 2 × 1013 eV−1 cm−2. The density of the traps was one order of magnitude lower compared to the unmodified transistors. This resulted in an increase in charge carrier mobility in the tested poly [2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno [3,2-b]thiophene)] (DPPDTT)-based transistors to 5.4 × 10−1 cm2 V−1 s−1.

Synthesis and characterization of ZnO/ZnAl2O4/Zn2TiO4 composite films by Ar–O2 mixture hollow cathode glow discharge

In this study, ZnO/ZnAl 2 O 4 /Zn 2 TiO 4 composite nanostructured thin films are deposited assisted with hollow-cathode glow discharge (HCD). The films are deposited using various argon-oxygen gases mixture (0–50% O 2 ), and its effect on film quality, optical and electrical properties is examined. Remarkably, instead of metallic targets, ZnO, TiO 2, and Al 2 O 3 powders filled in hollow-cathode are used for deposition, and thus no specific metallic targets are required in this technique. The deposited films consist of hexagonal wurtzite ZnO structure, ZnAl 2 O 4, and Zn 2 TiO 4 phases, and the crystalline size of films increases for higher oxygen contents. The thickness of films reduced by increasing oxygen contents, and all deposited films exhibit transmittance higher than 82%. The carrier concentration and mobility increase, whereas resistivity decreases when a higher amount of oxygen is added. This study shows that HCD can effectively synthesize the composite film with good optical and electrical properties. Additionally, the film's properties and elemental composition can be tuned by changing the gas composition, and thus, no separate target metallic materials with the specific composition are required.

Solution-processed near-infrared Cu(In,Ga)(S,Se)2 photodetectors with enhanced chalcopyrite crystallization and bandgap grading structure via potassium incorporation

Although solution-processed Cu(In,Ga)(S,Se)2 (CIGS) absorber layers can potentially enable the low-cost and large-area production of highly stable electronic devices, they have rarely been applied in photodetector applications. In this work, we present a near-infrared photodetector functioning at 980 nm based on solution-processed CIGS with a potassium-induced bandgap grading structure and chalcopyrite grain growth. The incorporation of potassium in the CIGS film promotes Se uptake in the bulk of the film during the chalcogenization process, resulting in a bandgap grading structure with a wide space charge region that allows improved light absorption in the near-infrared region and charge carrier separation. Also, increasing the Se penetration in the potassium-incorporated CIGS film leads to the enhancement of chalcopyrite crystalline grain growth, increasing charge carrier mobility. Under the reverse bias condition, associated with hole tunneling from the ZnO interlayer, the increasing carrier mobility of potassium-incorporated CIGS photodetector improved photosensitivity and particularly external quantum efficiency more than 100% at low light intensity. The responsivity and detectivity of the potassium-incorporated CIGS photodetector reach 1.87 A W−1 and 6.45 times 1010 Jones, respectively, and the − 3 dB bandwidth of the device extends to 10.5 kHz under 980 nm near-infrared light.

Effect of Change in Valence State of Ga During Annealing on the Structural, Optical, and Electrical Properties of GZO Crystals

AbstractIn this work, the ZnO:Ga (GZO) single crystals are grown by the chemical vapor transport (CVT) method. The as‐grown crystals are annealed under an oxygen atmosphere at different temperatures. The GZO crystal's structure and its optical and electrical properties are characterized by X‐ray photoelectron spectroscopy (XPS), Raman spectroscopy, X‐ray diffraction (XRD), UV‐VIS spectrophotometry, and variable‐temperature Hall‐effect measurement. The XPS results indicate that the valence states of the majority of gallium atoms in the GZO crystals undergo transition from the metallic (Ga0) to the non‐metallic state (Gax+) with increasing annealing temperature. The Raman and XRD results show that the compressive stress along the biaxial c‐axis on GZO crystals increases gradually with annealing temperature. Meanwhile, the GZO crystal's transmittance within the range of 300 to 1000 nm is improved significantly from being opaque to about 57%. The GZO crystals exhibit a decrease in free‐carrier concentration (1020–1019 cm−3), an increase in carrier mobility (77.8–87.9 cm2/V−1s−1) and resistivity (10−4–10−2 Ω·cm). The annealed GZO crystal's carrier concentration is practically independent of temperature (90–300 K). These results show that the free‐carrier concentrations are affected by the change of valence states in gallium atoms present in the GZO crystal.

ZnO/graphene ambipolar transistor with low sub-threshold swing

We reported on enhanced device performance of ambipolar thin-film transistors (TFTs) with hybrid channel of Zinc oxide (ZnO) and multi-layer graphene (MLG), especially in reduced sub-threshold swing characteristics and increased carrier mobilities for the ambipolar conduction. The Raman spectroscopy and x-ray photoelectron spectroscopy (XPS) showed that the single-layer graphene could be damaged by oxidation during the ZnO growth process. In MLG, we observed that the graphene layers distant from the interface of ZnO/graphene could be protected, leading to enhanced electrical properties in ZnO/graphene hybrid TFTs. These results showed that the ZnO/MLG hybrid structure is a suitable building block to realize advanced TFTs with low power consumption and high switching speed.

Growth of Bi2Se3/graphene heterostructures with the room temperature high carrier mobility

Heterostructures of Bi2Se3 topological insulators were epitaxially grown on graphene by means of the physical vapor deposition at 500 °C. Micrometer-sized flakes with thickness 1 QL (quintuple layer ~ 1 nm) and films of millimeter-scale with thicknesses 2–6 QL had been grown on CVD graphene. The minimum thickness of large-scaled continuous Bi2Se3 films was found to be ~ 8 QL for the regime used. The heterostructures with a Bi2Se3 film thickness of > 10 QL had resistivity as low as 200–500 Ω/sq and a high room temperature carrier mobility ~ 1000–3400 cm2/Vs in the Bi2Se3/graphene interface channel. Moreover, the coexistence of a p-type graphene-related conductive channel, simultaneously with the n-type conductive surface channel of Bi2Se3, was observed. The improvement of the bottom Bi2Se3/graphene interface with the increase in the growth time clearly manifested itself in the increase of conductivity and carrier mobility in the grown layer. The grown Bi2Se3/G structures have lower resistivities and more than one order of magnitude higher carrier mobilities in comparison with the van der Waals Bi2Se3/graphene heterostructures created employing exfoliation of thin Bi2Se3 layers. The grown heterostructures demonstrated the properties that are perspective for new functional devices, for a variety of signal processing and logic applications.

Defect-Free and Annealing Influences in P3HT Organic Field-Effect Transistor Performance

Organic field effect transistors were fabricated using photolithography and plasma etching technique with defect free Poly(3-hexylthiophene) (P3HT) and P3HT 90 % regioregular with different temperatures of annealing at normal ambient. PVA (Poly - vinyl Alcohol) was used as gate dielectric and Ni contacts formed the source and drain electrodes. Improvement of transistor stability and increase of carrier mobility with use of defect free P3HT was reached. The stability enhancement permitted an higher ION current during stress. The auto-encapsulation process involved in the process of fabrication permits an ambient stability, in which the transistors do not degrade due to moisture and oxygen. All the transistors operate at low voltages, due to the high dielectric constant of PVA, being adequate to actual applications. P3HT films deposited in the same conditions as in the active area of the transistors were analyzed by photoluminescence and XRR measurements and it was observed an improvement in crystallinity for the defect free P3HT annealed at lower temperatures, which corroborates the results observed in the transistors characteristics.

Disentangling Bulk and Interface Phenomena in a Molecularly Doped Polymer Semiconductor

AbstractDoping the electron‐transport polymer poly{[N,N′‐bis(2‐octyldodecyl)naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)} [P(NDI2OD‐T2)] with the bulky, strongly reducing metallocene 1,2,3,4,1′,2′,3′,4′‐octaphenylrhodocene (OPR) leads to an increased bulk conductivity and a decreased contact resistance. While the former arises from low‐level n‐doping of the intrinsic polymer and increased carrier mobility due to trap‐filling, the latter arises from a pronounced accumulation of dopant molecules at an indium tin oxide (ITO) substrate. Electron transfer from OPR to ITO leads to a work function reduction, which pins the Fermi level at the P(NDI2OD‐T2) conduction band and thus minimizes the electron injection barrier and the contact resistance. The results demonstrate that disentangling the effects of electrode modification by the dopant and bulk doping is essential to comprehensively understand doped organic semiconductors.

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.

Identifying the Molecular Origins of High-Performance in Organic Photodetectors Based on Highly Intermixed Bulk Heterojunction Blends

A bulk-heterojunction (BHJ) structure of organic semiconductor blend is widely used in photon-to-electron converting devices such as organic photodetectors (OPD) and photovoltaics (OPV). However, the impact of the molecular structure on the interfacial electronic states and optoelectronic properties of the constituent organic semiconductors is still unclear, limiting further development of these devices for commercialization. Herein, the critical role of donor molecular structure on OPD performance is identified in highly intermixed BHJ blends containing a small-molecule donor and C60 acceptor. Blending introduces a twisted structure in the donor molecule and a strong coupling between donor and acceptor molecules. This results in ultrafast exciton separation (<1 ps), producing bound (binding energy ∼135 meV), localized (∼0.9 nm), and highly emissive interfacial charge transfer (CT) states. These interfacial CT states undergo efficient dissociation under an applied electric field, leading to highly efficient OPDs in reverse bias but poor OPVs. Further structural twisting and molecular-scale aggregation of the donor molecules occur in blends upon thermal annealing just above the transition temperature of 150 °C at which donor molecules start to reorganize themselves without any apparent macroscopic phase-segregation. These subtle structural changes lead to significant improvements in charge transport and OPD performance, yielding ultralow dark currents (∼10-10 A cm-2), 2-fold faster charge extraction (in μs), and nearly an order of magnitude increase in effective carrier mobility. Our results provide molecular insights into high-performance OPDs by identifying the role of subtle molecular structural changes on device performance and highlight key differences in the design of BHJ blends for OPD and OPV devices.

Temperature dependence of the electrical characteristics of ZnO thin film transistor with high-k NbLaO gate dielectric

ZnO thin film transistor with high-k NbLaO/SiO2 bilayer gate dielectric was fabricated by sputtering, and the temperature dependence of the electrical properties of the device was investigated in the temperature range of 293–353 K for clarifying thermally activated carrier generation and carrier transport mechanisms in the conducting channel. With the increase in the temperature, the transfer curve shifts toward the negative gate voltage direction with a negative shift of the threshold voltage, an increase in the off-state current and the subthreshold slope, and a significant increase in carrier mobility. The decrease in the threshold voltage is originated from the formation of oxygen vacancy and the release of free electrons in the ZnO channel, and the formation energy can be estimated to be approximately 0.3 eV. In both subthreshold and above-threshold regimes, the temperature dependence of the drain current shows Arrhenius-type dependence, and the activation energy is around 0.94 eV for a gate voltage of 2 V, reducing with the increase in the gate voltage. The temperature dependence of the ZnO film resistance also exhibits an Arrhenius-type behavior, indicating that the thermal activation conduction process is the dominant conduction mechanism in the ZnO film. Two types of thermal activation conduction processes are observed in the 303–373 K temperature range. This is explained in terms of the existence of two types of deep donors that are consecutively excited to the conduction band as the temperature increases.

Scalable Integration of Coplanar Heterojunction Monolithic Devices on Two-Dimensional In2Se3.

The formation of lateral heterojunction arrays within two-dimensional (2D) crystals is an essential step to realize high-density, ultrathin electro-optical integrated circuits, although the assembling of such structures remains elusive. Here we demonstrated a rapid, scalable, and site-specific integration of lateral 2D heterojunction arrays using few-layer indium selenide (In2Se3). We use a scanning laser probe to locally convert In2Se3 into In2O3, which shows a significant increase in carrier mobility and transforms the metal-semiconductor junctions from Schottky to ohmic type. In addition, a lateral p-n heterojunction diode within a single nanosheet is demonstrated and utilized for photosensing applications. The presented method enables high-yield, site-specific formation of lateral 2D In2Se3-In2O3-based hybrid heterojunctions for realizing nanoscale devices with multiple advanced functionalities.