Interfacial Doping Research Articles

Effect of ZnO nanoparticles doping on structural, dielectric and interfacial behavior of polyoxyethylene (20) sorbitan/ethylene glycol lyotropic phases

ABSTRACT We are reporting on the interaction of zinc oxide (ZnO) nanoparticles (NPs) with the lyotropic phase comprises of Polyoxyethylene (20) sorbitan monolaurate and protic solvent ethylene glycol. The concentration of the NPs has been varying from 0.05 to 0.5 wt%. Multiwall lamellar and inverse phases have been observed at lower and higher concentration of ZnO NPs doping. Interestingly, the organization of ZnO NPs on the periphery and inside the periphery of ring-like structures has been observed at lower and higher concentration of the dopant, respectively. Such organization of the NPs can be explained considering interfacial interaction amid host and dopant and may also attribute to the adsorption mechanisms of surfactant. Effects of NPs doping on the dielectric dynamics has also been examined. About 32.6% decrease in the dielectric permittivity has been noticed at higher NPs doping. Such decrement in permittivity could be a result of the screening of the ZnO NPs dipole moment by the adsorption of surfactant molecules on their surface. Relaxation and optical parameters of the non-doped and doped mixtures have also been discussed.

Thermoelectric composite with enhanced figure of merit via interfacial doping

In order to improve the thermoelectric conversion efficiency and figure of merit, ZT, composite materials of organic or inorganic constituents often are considered. The limitation of this approach is set by the effective medium theory, which states that the ZT in a composite material cannot exceed the greatest value of any single constituent, if the constituents do not interact. Here, we describe a method that circumvents this limit, based on the introduction of interfacial doping. An electrically and thermally insulating medium is distributed into a conventional thermoelectric host material but is coated with an aliovalent acceptor that is allowed to diffuse locally into the host matrix, thereby doping it locally. The thermal conductivity decreases when the insulating material is added, but the more electrically conducting region around the insulator prevents an equally large increase in electrical resistivity. Employing this method in p-type (Bi1-xSbx)2Te3 compounds results in a maximum figure of merit zT = 1.3, an over 10% improvement compared to the host material alone. We report synthesis and measurement techniques in addition to thermoelectric transport properties. While we report on one material system, the concept is not specific to that system and may be used to provide functionality in other thermoelectric composites.

Unraveling Doping Capability of Conjugated Polymers for Strategic Manipulation of Electric Dipole Layer toward Efficient Charge Collection in Perovskite Solar Cells

AbstractDeveloping electrical organic conductors is challenging because of the difficulties involved in generating free charge carriers through chemical doping. To devise a novel doping platform, the doping capabilities of four designed conjugated polymers (CPs) are quantitatively characterized using an AC Hall‐effect device. The resulting carrier density is related to the degree of electronic coupling between the CP repeating unit and 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4‐TCNQ), and doped PIDF‐BT provides an outstanding electrical conductivity, exceeding 210 S cm−1, mainly due to the doping‐assisted facile carrier generation and relatively fast carrier mobility. In addition, it is noted that a slight increment in the electron‐withdrawing ability of the repeating unit in each CP diminishes electronic coupling with F4‐TCNQ, and severely deteriorates the doping efficiency including the alteration of operating doping mechanism for the CPs. Furthermore, when PIDF‐BT with high doping capability is applied to the hole transporting layer, with F4‐TCNQ as the interfacial doping layer at the interface with perovskite, the power conversion efficiency of the perovskite solar cell improves significantly, from 17.4% to over 20%, owing to the ameliorated charge‐collection efficiency. X‐ray photoelectron spectroscopy and Kelvin probe analyses verify that the improved solar cell performance originates from the increase in the built‐in potential because of the generation of electric dipole layer.

(Invited) Carbon Nanotubes As an Effective and Stable p-Type Contact for Perovskite Solar Cells

The enormous strides researchers have made in advancing perovskite-based photovoltaics in the past few years are fueling the dream of a rapid entry of this highly promising technology into the global energy market. This seems like serendipitous timing since solar power has gained an enormous momentum over the past decade exceeding all projections. This expansion in solar power is driven by an increasing understanding that climate change requires immediate action, in particular with regards to power generation; and simultaneously, the price for photovoltaic modules has remarkably quickly fallen to make solar power generation competitive vis-à-vis conventional technologies.Compositional tuning of the perovskite absorber has led to immense improvements in stability and efficiency of perovskite solar cells leading to a certified efficiency of over 25% for a single-junction laboratory device. The successful transition of a photovoltaic technology from the lab into the field requires, however, two additional important parameters aside from efficiency which are essential for real-world deployment: stability and scalability.We propose using single-walled carbon nanotubes (SWCNTs) as an alternative p-type layer for future application in perovskite solar cells.Carbon nanotubes combine several highly attractive characteristics such as chemical inertness and mechanical resilience with intrinsically high charge-carrier mobilities endowing them with a unique potential for a much more stable dopant-free charge-selective contact. Furthermore, the SWCNT deposition techniques for large-scale use, such as spray coating, are already well-established opening clear avenues for direct implementation in a real-world industrial setting.We demonstrate that a cascade-like transfer extraction through SWCNTs can minimize photovoltage losses by suppressing non-radiative recombination. By further improving the absorber quality, the voltage losses in solar cells can be reduced to around 340 mV which is comparable to conventional III-V semiconductor-based photovoltaics.By further enhancing the selective charge extraction at the perovskite-SWCNT interface through interfacial charge transfer doping, we achieve steady-state efficiencies approaching 22% for alloyed narrow bandgap perovskites, illustrating the versatility and excellent performance of SWCNTs as contact material.

Sequentially Processed P3HT/CN6‐CP•−NBu4+ Films: Interfacial or Bulk Doping?

AbstractDerivatives of the hexacyano‐[3]‐radialene anion radical (CN6‐CP•−) emerge as a promising new family of p‐dopants having a doping strength comparable to that of archetypical dopant 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyano‐quinodimethane (F4TCNQ). Here, mixed solution (MxS) and sequential processing (SqP) doping methods are compared by using a model semiconductor poly(3‐hexylthiophene) (P3HT) and the dopant CN6‐CP•−NBu4+ (NBu4+ = tetrabutylammonium). MxS films show a moderate yet thickness‐independent conductivity of ≈0.1 S cm−1. For the SqP case, the highest conductivity value of ≈6 S cm−1 is achieved for the thinnest (1.5–3 nm) films whereas conductivity drops two orders of magnitudes for 100 times thicker films. These results are explained in terms of an interfacial doping mechanism realized in the SqP films, where only layers close to the P3HT/dopant interface are doped efficiently, whereas internal P3HT layers remain essentially undoped. This structure is in agreement with transmission electron microscopy, atomic force microscopy, and Kelvin probe force microscopy results. The temperature‐dependent conductivity measurements reveal a lower activation energy for charge carriers in SqP samples than in MxS films (79 meV vs 110 meV), which could be a reason for their superior conductivity.

Synergetic effect of interface barrier and doping on the thermoelectric transport properties of tellurium

Energy filtering effect is helpful to decouple Seebeck coefficient and electric conductivity, and dopant is helpful for improving conductivity; in this work, synergetic function of both is supposed to enhance zT values of tellurium. In bulk Te and Sb0.003Te0.997, the Ni/Te heterojunction was built by electroless plating combined with spark plasma sintering. The Ni/Te interface barrier successfully decoupled the relationship between the Seebeck coefficient and the electrical resistivity. The Seebeck coefficient and resistivity of Ni/Te samples increased by 37.9% and decreased by 28.1%, respectively. Meanwhile, the thermal conductivity of the Ni/Te samples decreased by 25.8% (T = 300 K), and the zT value was increased by 36.4% compared to the pure Te sample. Consequently, a peak zT of 0.96 at 550 K is achieved in 18wt %Ni/Sb0.003Te0.997 sample,which is 50% higher than Sb0.003Te0.997 samples. This work indicates that electroless plating combined with spark plasma sintering is an effective method to construct heterojunction in bulk materials. This method should be widely applicable to various materials for achieving better thermoelectric properties.

Interface Heteroatom-doping: Emerging Solutions to Silicon-based Anodes.

Silicon-based composites have been recognized as a promising anode material for high-energy lithium-ion batteries (LIBs). However, the intrinsically low conductivity and the huge volume expansion during lithiation/delithiation progresses impede its further practical applications. In the past decades, numerous efforts have been made for surface and interface modification of Si-based anodes. Among these, doping of active materials with heteroatoms is one promising method to endow silicon many unmatched electrochemical properties. In this review, we focus on the effects of heteroatom doping on the interfacial properties of Si-based anodes, and some typical strategies for the interface doping are highlighted. We aim to give some reference for interfacial doping of Si-based anodes in LIBs.

Embedding physics domain knowledge into a Bayesian network enables layer-by-layer process innovation for photovoltaics

Process optimization of photovoltaic devices is a time-intensive, trial-and-error endeavor, which lacks full transparency of the underlying physics and relies on user-imposed constraints that may or may not lead to a global optimum. Herein, we demonstrate that embedding physics domain knowledge into a Bayesian network enables an optimization approach for gallium arsenide (GaAs) solar cells that identifies the root cause(s) of underperformance with layer-by-layer resolution and reveals alternative optimal process windows beyond traditional black-box optimization. Our Bayesian network approach links a key GaAs process variable (growth temperature) to material descriptors (bulk and interface properties, e.g., bulk lifetime, doping, and surface recombination) and device performance parameters (e.g., cell efficiency). For this purpose, we combine a Bayesian inference framework with a neural network surrogate device-physics model that is 100× faster than numerical solvers. With the trained surrogate model and only a small number of experimental samples, our approach reduces significantly the time-consuming intervention and characterization required by the experimentalist. As a demonstration of our method, in only five metal organic chemical vapor depositions, we identify a superior growth temperature profile for the window, bulk, and back surface field layer of a GaAs solar cell, without any secondary measurements, and demonstrate a 6.5% relative AM1.5G efficiency improvement above traditional grid search methods.

Stabilization of ε-CL-20 crystals by a minor interfacial doping of polydopamine-coated graphene oxide

Hexanitrohexaazaisowurtzitane (CL-20) is a well-known high energy density nitramine, but it is of high sensitivity and has the problem of polymorphic transition. In order to reduce the sensitivity of ε-CL-20 and improve the stability of its crystal structure, the polydopamine (PDA)-coated graphene oxide (GO) was used to dope ε-CL-20 crystals by strategy of in-situ coating followed with a solvent-non-solvent crystallization process. It has been shown that modified CL-20 crystals with the best performances are of polygon shape with a smooth surface and with the average diameter of 16 μm, much smaller than that of raw ε-CL-20 (140 μm). In addition, the doping can either improve the polymorphic transition temperature up to 19.0 °C or exclude such a transition due to formation of completely new crystal phase with better stability, depending on the type and content of the dopants. The density of a certain GO@PDA modified CL-20 is even slightly higher than that of pristine ε-CL-20 due to better assembling of the molecules under the nucleation of the dopants. CL-20/PDAG-2 does not show any polymorphic transition peak. Moreover, the XRD spectrum of CL-20/PDAG-2 also demonstrates that it is a completely new crystal phase better than the other four reported ones in terms of thermal stability.

Molecular materials as interfacial layers and additives in perovskite solar cells.

Solar cells based on organo-metal halide perovskites have gained unprecedented research interest over the last few years due to their low-cost solution processability, high power conversion efficiency, which has recently reached a certified value of 25.2%, and abundance of raw materials. Nevertheless, the best efficiencies remain below the Shockley-Queisser theoretical limit of 32.5% due to several losses arising from either defect traps present in the bulk of the perovskite absorber or at the device heterointerfaces. While bulk defects are detrimental for the device performance by mainly limiting the open circuit voltage, interfacial layers are also crucial. They dictate the charge transfer/transport from the perovskite layer to the collecting electrodes, hence influencing the device photocurrent, but also act as protective barriers against oxygen and moisture penetration. Molecular materials and additives are widely used to improve the bulk properties of perovskite absorbers through the formation of high-quality perovskite films with superior optoelectronic properties, and improved crystallinity, and also of electronically clean interfaces with minimum losses during charge transfer/transport. In this review, we analyze the predominant pathways that contribute to voltage and current losses due to poor interfaces and also due to non-radiative recombination losses arising from inferior perovskite morphology and its inherent polycrystalline and highly defective nature. We then discuss strategies for achieving interfacial organic and inorganic molecular materials for application as electron and hole transport layers in perovskite solar cells with ideal energy levels, high charge mobilities and improved thermal, photo, and structural stability. Moreover, the prerequisites for molecular additives to achieve dimensionality engineering, defect passivation, molecular cross-linking, interfacial energy alignment and electronic doping are thoroughly discussed. Finally, we examine prospects for future research directions and commercialization.

Kinetic Ionic Permeation and Interfacial Doping of Supported Graphene.

Due to its outstanding electrical properties and chemical stability, graphene finds widespread use in various electrochemical applications. Although the presence of electrolytes strongly affects its electrical conductivity, the underlying mechanism has remained elusive. Here, we employ terahertz spectroscopy as a contact-free means to investigate the impact of ubiquitous cations (Li+, Na+, K+, and Ca2+) in aqueous solution on the electronic properties of SiO2-supported graphene. We find that, without applying any external potential, cations can shift the Fermi energy of initially hole-doped graphene by ∼200 meV up to the Dirac point, thus counteracting the initial substrate-induced hole doping. Remarkably, the cation concentration and cation hydration complex size determine the kinetics and magnitude of this shift in the Fermi level. Combined with theoretical calculations, we show that the ion-induced Fermi level shift of graphene involves cationic permeation through graphene. The interfacial cations located between graphene and SiO2 electrostatically counteract the substrate-induced hole doping effect in graphene. These insights are crucial for graphene device processing and further developing graphene as an ion-sensing material.

S-Shaped Current–Voltage Characteristics in Solar Cells: A Review

S-shaped current–voltage ( I – V ) characteristics are a frequently occurring hurdle in the development of new solar cell material combinations and device architectures. Their presence points to the existence of a charge transport bottleneck that needs to be removed in order to unlock high fill factors and power conversion efficiencies. In this review, examples of studies in which s-shaped I – V curves have appeared are presented, and the cause and mitigation are discussed. Different solar cell material systems are often treated by separate communities, thereby, also the physics of s-shaped I – V curves have been treated separately. This review covers the main solar cell technologies—silicon, thin film, organic, hybrid—with the aim to provide an overarching picture of the common mechanisms and universal guidelines for mitigation of s-shaped I – V characteristics in emerging solar cell technologies. Except for a few studies on organic solar cells, s-shaped I – V curves are reported to result from charge transport barriers at one of the (selective) contact layers that can be overcome by interface engineering and doping.

Optimizing the Performance of CsPbI3-Based Perovskite Solar Cells via Doping a ZnO Electron Transport Layer Coupled with Interface Engineering

HighlightsDevice simulations and first-principle calculations are employed to derive a guideline for the optimization of CsPbI3-based perovskite solar cells (PSCs).The open voltage and power conversion efficiency of the PSCs are, respectively, improved to 1.31 V and 21.06% by simultaneously introducing an ultra-thin TiO2 buffer layer and increasing the doping concentration of the ZnO electron transport layer.The influence of the interfacial buffer layer and doping of the CsPbI3/ZnO interface on PSC performance is discussed.

The influence of the resistance layer dimension and interface doping on the electrical properties of aGNR/NiO/aGNR-structured resistive random access memory

A first-principles method based on density functional theory is used to study the effects of the resistance layer dimension and interface doping on the electrical properties of aGNR/NiO/aGNR-structured resistive random access memory. First, calculations of the interface adhesion energy and Mulliken mean populations show that the aGNR/unilaminar NiO interface has good interface stability. Second, the current–voltage characteristics and the micro formation process of conducting filaments of graphene/NiO/graphene resistive random access memory with resistance layers of different widths are studied. The comprehensive performance of the device is fully exploited, and its storage window can satisfy the requirement of data storage when the width of the resistance layer is 17.6 Å. Finally, an increase in the concentration of excess oxygen causes an increase in power consumption and a decline in the current drive capacity, while the stability of the device is improved; moreover, the size of the storage window decreases but still meets the requirement of data storage. In summary, the storage performance is effectively improved by excess oxygen concentration of 3.82%. These results may provide a theoretical basis for rational design and device optimization regarding the resistance layer dimension and excess oxygen.

(Invited) Investigation of Interfacial Charge Transfer Doping of 2D MoS2

Layered two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) have attracted a great deal of attention because of their exciting electrical and optical properties and their potential applications in electronic and optoelectronic devices. Because of ultra low volume and chemically inert surface of 2D materials, it is challenging to achieve controlled doping of 2D materials. Recently, interfacial charge transfer induced doping of 2D materials have gained significant attention. In this talk, I will present electrical transport studies of 2D molybdenum disulfide (MoS2) field effect transistors decorated with zero dimensional (0D) gold (Au) nanoislands. I will discuss how the Au nanoisland coverage affects the interfacial charge transfer based doping and tailor the electrical properties of the host material. From the detailed low temperature electron transport measurements, I will elucidate the role of 2D-0D interface on the transport properties of MoS2 FETs.

Toward high thermoelectric performance for polypyrrole composites by dynamic 3-phase interfacial electropolymerization and chemical doping of carbon nanotubes

Polymer thermoelectric composites have received significant attention in recent years. To achieve high thermoelectric performance is one main aim, especially when being compared with their inorganic counterparts. Here, we report high-performance thermoelectric composites based on single-walled carbon nanotube (SWCNT) and polypyrrole (PPy), which are prepared by dynamic 3-phase interfacial electropolymerization and subsequent physical mixing. We found that both SWCNT content and chemical doping of SWCNT dramatically affected the composite thermoelectric performance. The maximum power factor at room temperature of the PPy composite reaches as high as 240.3 ± 5.0 μW m−1 K−2, which may be the highest value for PPy and its composites reported so far. The present study demonstrates that the dynamic 3-phase interfacial electropolymerization combining SWCNT chemical doping is effective to fabricate high-performance polymer thermoelectric composites.

Interface intrinsic strengthening mechanism on the tensile properties of Al2O3/Al composites

Interface plays a crucial role to enhance the mechanical properties of metal-matrix composites. Here, we investigated and modulated the intrinsic characteristics of Al2O3/Al interface through interfacial doping by the first-principles calculations. The Al-terminated Al2O3 (0 0 0 1)/Al (1 1 1) interface with OT stacking was found to be the most stable structure by calculating works of adhesion and interface energies. Then, the first principles tensile experiments were performed on the stable interfacial structure, and the tensile strength and elongation were calculated. It is revealed that there is no direct correlation between tensile properties and the work of adhesion or the interface energy. Systematical analysis on electronic structure indicates that the incorporation of doping elements will redistribute the electrons at the interfaces, and there is the interplay between the uniform electron distribution and the tensile properties, especially the elongation. It is found that Mg-doped and Cu-doped interface systems with more uniform electron distribution show better mechanical properties. Therefore, the interface intrinsic strengthening mechanism caused by the interfacial microstructure has a significant impact on the mechanical properties of the composite materials, and complements the load transfer mechanism in metal matrix composites.

Synaptic metaplasticity of protonic/electronic coupled oxide neuromorphic transistor

In recent years, neuromorphic computing has attracted close attention. Simulating synaptic plasticity on neuromorphic devices is an important step in hardware based neuromorphic computing. Metaplasticity is a higher-order form of synaptic plasticity. It can regulate the ability of synapse to generate plasticity. Here, chitosan based electrolyte gated protonic/electronic coupled indium-tin-oxide (ITO) neuromorphic transistors are fabricated. The transistor exhibits unique interfacial ionic coupling effects and interfacial electrochemical doping abilities. Thus, metaplastic excitatory postsynaptic current and metaplastic paired-pulses response are achieved on the chitosan gated ITO neuromorphic transistor. Transitions between paired-pulse facilitation and paired-pulse depression are observed. In addition, metaplastic facilitation of long-term potentiation and metaplastic inhibition of long-term potentiation are also successfully imitated. The present work may expand the applications of solid-state electrolyte gated transistors in neuromorphic platforms.

Characterization of SiO2/4H-SiC Interfaces in 4H-SiC MOSFETs: A Review

This paper gives an overview on some state-of-the-art characterization methods of SiO2/4H-SiC interfaces in metal oxide semiconductor field effect transistors (MOSFETs). In particular, the work compares the benefits and drawbacks of different techniques to assess the physical parameters describing the electronic properties and the current transport at the SiO2/SiC interfaces (interface states, channel mobility, trapping phenomena, etc.). First, the most common electrical characterization techniques of SiO2/SiC interfaces are presented (e.g., capacitance- and current-voltage techniques, transient capacitance, and current measurements). Then, examples of electrical characterizations at the nanoscale (by scanning probe microscopy techniques) are given, to get insights on the homogeneity of the SiO2/SiC interface and the local interfacial doping effects occurring upon annealing. The trapping effects occurring in SiO2/4H-SiC MOS systems are elucidated using advanced capacitance and current measurements as a function of time. In particular, these measurements give information on the density (~1011 cm−2) of near interface oxide traps (NIOTs) present inside the SiO2 layer and their position with respect to the interface with SiC (at about 1–2 nm). Finally, it will be shown that a comparison of the electrical data with advanced structural and chemical characterization methods makes it possible to ascribe the NIOTs to the presence of a sub-stoichiometric SiOx layer at the interface.

Precious‐Metal‐Free Electrocatalysts for Activation of Hydrogen Evolution with Nonmetallic Electron Donor: Chemical Composition Controllable Phosphorous Doped Vanadium Carbide MXene

AbstractThe insufficient strategies to improve electronic transport, the poor intrinsic chemical activities, and limited active site densities are all factors inhibiting MXenes from their electrocatalytic applications in terms of hydrogen production. Herein, these limitations are overcome by tunable interfacial chemical doping with a nonmetallic electron donor, i.e., phosphorization through simple heat‐treatment with triphenyl phosphine (TPP) as a phosphorous source in 2D vanadium carbide MXene. Through this process, substitution, and/or doping of phosphorous occurs at the basal plane with controllable chemical compositions (3.83–4.84 at%). Density functional theory (DFT) calculations demonstrate that the PC bonding shows the lowest surface formation energy (ΔGSurf) of 0.027 eV Å−2 and Gibbs free energy (ΔGH) of –0.02 eV, whereas others such as P‐oxide and PV (phosphide) show highly positive ΔGH. The P3–V2CTx treated at 500 °C shows the highest concentration of PC bonds, and exhibits the lowest onset overpotential of –28 mV, Tafel slope of 74 mV dec−1, and the smallest overpotential of ‐163 mV at 10 mA cm−2 in 0.5 m H2SO4. The first strategy for electrocatalytically accelerating hydrogen evolution activity of V2CTx MXene by simple interfacial doping will open the possibility of manipulating the catalytic performance of various MXenes.