Electric Dipole Layer Research Articles

Largely‐Tuned Effective Work‐Function of Al/Graphene/SiO2/Si Junction with Electric Dipole Layer at Al/Graphene Interface

AbstractThe effective work‐function of metal electrode is one of the major factors to determine the threshold voltage of metal/oxide/semiconductor junction. In this work, it is demonstrated experimentally that the effective work‐function of the Aluminum (Al) electrode in Al/SiO2/n‐Si junction increases significantly by ≈1.04 eV with the graphene interlayer inserted at Al/SiO2 interface. The device‐physical analysis of solving Poisson equation analytically is provided when the flat‐band voltage is applied to the junction, supporting that the large tuning of Al effective work‐function may originate from the electric dipole layer formed by the off‐centric distribution of electron orbitals between Al and graphene layer. Our work suggests the feasibility of constructing the dual‐metal gate CMOS circuitry just by using Al electrodes with area‐specific underlying graphene interlayer.

First Principle Study on Schottky Barrier at Ni/Graphene/4H-SiC Interface

High stability 4H-SiC ohmic contact is currently a key technical challenge that silicon carbide devices urgently need to overcome. It is important to reduce the Schottky barrier height (SBH) at the Ni/4H-SiC interface to optimize ohmic contact. In this paper, the mechanisms of graphene layer changing Ni/4H-SiC interface Schottky barrier height (SBH) are studied based on the first-principles method within the local density approximation. Theoretical studies have shown that graphene intercalation can reduce the SBH of Ni and 4H-SiC interfaces. The reason of SBH reduction may be that the graphene C atoms saturate the dangling bonds on the 4H-SiC surface and the influence of the metal-induced energy gap state at the interface is reduced. In addition, the new phase formed at the interface of graphene and silicon carbide has a lower work function. Furthermore, an interfacial electric dipole layer may be formed at the 4H-SiC/graphene interface which may also reduce the SBH. These results make them to be promising candidates for future radiation resistant electronics.

Heterojunction‐Accelerating Lithium Salt Dissociation in Polymer Solid Electrolytes

AbstractThe practical application of solid‐state lithium‐metal batteries (SSLMBs) based on polymer solid electrolytes has been hampered by their low ion conductivity and lithium‐dendrite‐induced short circuits. This study innovatively introduces 1D ferroelectric ceramic‐based Bi4Ti3O12‐BiOBr heterojunction nanofibers (BIT‐BOB HNFs) into poly(ethylene oxide) (PEO) matrix, constructing lithium‐ion conduction highways with “dissociators” and “accelerating regions.” BIT‐BOB HNFs, as 1D ceramic fillers, not only can construct long‐range organic/inorganic interfaces as ion transport pathways, but also install “dissociators” and “accelerating regions” for these pathways through the electric dipole layer and built‐in electric field of BIT‐BOB HNFs, promoting the dissociation of lithium salts and the transfer of lithium ions. The working mechanisms of BIT‐BOB HNFs in the polymer matrix are verified by experimental tests and density functional theory calculations. The obtained composite solid electrolytes exhibit excellent lithium‐ion conductivity and migration number (6.67 × 10−4 S cm−1 and 0.54 at 50 °C, respectively). The assembled lithium symmetric battery achieves good cycling stability of over 4500 h. The LiFePO4||Li full battery delivers a high Coulombic efficiency (>99.9%) and discharge capacity retention rate (>87%) after 2200 cycles. In addition, the prepared composite polymer solid electrolyte demonstrates good practical application potential in flexible pouch batteries.

Hydrophobic PbS QDs layer decorated ZnO electron transport layer to boost photovoltaic performance of perovskite solar cells

Zinc oxide (ZnO) was traditional cathode material for solar cells due to excellent photoelectric properties. However, when ZnO is used as electron transport layers (ETLs) in perovskite solar cells (PSCs), the occurred deprotonation reactions can easily lead to the decomposition of perovskite layer. Herein, we reported a simple and controllable low-temperature strategy to overcome above problem for achieving high-efficient and stable ZnO based planar PSCs, i.e. the colloidal PbS QDs and tetrabutylammonium iodide (TBAI) was sequentially spin-coated on ZnO ETLs and then mild post-annealing process was carried out to obtaining ZnO/PbS-TBAI compact layer. Due to the iodine ligands from TBAI, the PbS-TBAI films act as both surface wetting control layer and electric dipole layer to adjust the crystal growth of perovskite films and charge behavior within devices. The better non-wetting surface of ZnO/PbS-TBAI ETLs improves the crystallization of the perovskite films by suppressing heterogeneous nucleation. And the unique energy level alignment of ZnO/PbS-TBAI induced by tunable surface dipole moment of TBAI boosted hetero-interface charge extraction of PSCs. Consequently, the optimized ZnO/PbS-TBAI based PSCs exhibits the power conversion efficiency (PCE) of 20.53 %, which is the champion PCE of ZnO PSCs with MAPbI3 as the absorption layer. Stability tests also proved that the PCE of ZnO/PbS-TBAI based PSCs remains more than 86 % after exposure to 40 % humidity for 30 days, which is much more superior to that of control device. We firstly used iodination PbS QD films in this work to achieve higher photovoltaic performance in ZnO based PSCs, which prompts the further application of QDs materials within optoelectronic devices.

First principle study on modulating of Schottky barrier at metal/4H-SiC interface by graphene intercalation

In the production of SiC electronic devices, one of the main challenges is the fabrication of good Ohmic contacts due to the difficulty in finding the metals with low Schottky barriers of wide band gap SiC. Therefore, reducing the Schottky barrier height (SBH) at the metal/SiC interface is of great importance. In this paper, the effects of graphene intercalation on the SBH in different metals (Ag, Ti, Cu, Pd, Ni, Pt)/4H-SiC interfaces are studied by combining the average electrostatic potential and local density of states calculation methods based on first-principles plane wave pseudopotential density functional theory. The calculation results show that single-layer graphene intercalation can reduce the SBH of metal/4H-SiC contact. When the two layers of graphene are inserted, the SBH are further reduced. Especially, the contact between Ni and Ti exhibits negative SBH values, inferring that good Ohmic contacts are formed. When layers of graphene continue to increase, the SBH no longer changes obviously. By analyzing the differential charge density and the local density of states of the interface, the mechanism of SBH reduction may be that the dangling bonds on the SiC surface are saturated by the graphene C atoms and the influence of the metal-induced energy gap state at the interface is reduced, thereby reducing the interface state density. In addition, graphene and the corresponding new phases at the interface have low work functions. Moreover, an interfacial electric dipole layer may be formed at the SiC/graphene interface which also contributes to barrier reduction.

Enhancing hole injection by electric dipoles for efficient blue InP QLEDs

The unbalanced carrier injection is a key factor that deteriorates the performance of blue InP quantum dot light-emitting diodes (QLEDs). Therefore, to achieve efficient blue InP QLEDs, an effective strategy that balances carrier injection through enhancing the hole injection and transport is in demand. In this study, we introduced an ultrathin MoO3 electric dipole layer between the hole injection layer and the hole transport layer (HTL) to form a pair of dipole-induced built-in electric fields with forward resultant direction to enhance hole injection and facilitate the balance of carrier injection. Meanwhile, the p-doping effect by MoO3 leads to increased carrier concentration and decreased trap density of interfacial HTL, therefore improved its effective hole mobility. Consequently, the maximal external quantum efficiency of blue InP QLEDs was enhanced from 1.0% to 2.1%. This work provides an effective method to balance carrier injection by modulating hole injection and transport, indicating the feasibility to realize high-efficiency QLEDs.

Anionic nonconjugated polyelectrolyte as an anode interfacial layer for polymer solar cells

Since the most high-performing donor polymers in polymer solar cells (PSCs) possessed the deep highest occupied molecular orbital (HOMO) level, interfacial engineering on anode contact is becoming increasingly important. Herein, we demonstrated efficient PSCs using an anionic poly(styrene sulfonate) (PSS) as an anode interfacial layer (AIL). With the formation of the dipole layer, the effective work function (WF) of indium tin oxide (ITO) electrode is significantly increased from 4.8 to 5.3 eV, providing favorable energetic alignment to the quasi-Fermi level of various donor polymers. Moreover, by incorporating cationic polyelectrolytes as a cathode interfacial layer, a pair of electric dipole layers induces a strong built-in electric field across the photoactive layer to drive efficient sweep-out of photogenerated charges. Consequently, the device with PSS AIL exhibited high power conversion efficiencies of 9.2 and 14.8% in PTB7-Th:PC71BM- and PM6:Y6-based PSCs, respectively, both of which are higher than those of the devices with PEDOT:PSS.

Influence of Anatase-Rutile Ratio on Band Edge Position and Defect States of TiO2 Homojunction Catalyst

Removal of toxic air and water dissociation in the environment has become a major challenging issue throughout the world. Mixed phase rutile-anatase titanium dioxide catalysts are very effective in photocatalysis and have been studied extensively. However, the mechanism causing this effect and band alignment of the two phases are not fully understood. Pointing to the issue, we have designed one-dimensional mixed-phase TiO2 and introduced defects near the valence band. Experimental results showed that band alignment between two phases, up-shift of the band edge, and optimum anatase percentage play a key role in the enhancement of the photocatalytic activity. We predicted shifts in band edge originating from surface electric dipole layer induced by defects.

Tuning the Schottky Barrier Height at the Interfaces of Metals and Mixed Conductors.

Understanding electronic and ionic transport across interfaces is crucial for designing high-performance electric devices. The adjustment of work functions is critical for band alignment at the interfaces of metals and semiconductors. However, the electronic structures at the interfaces of metals and mixed conductors, which conduct both electrons and ions, remain poorly understood. This study reveals that a Schottky barrier is present at the interface of the Nb-doped SrTiO3 metal and a LiCoO2 mixed conductor and that the interfacial resistance can be tuned by inserting an electric dipole layer. The interfacial resistance significantly decreased (by more than 5 orders of magnitude) upon the insertion of a 1 nm thick insulating LaAlO3 layer at the interface. We apply these techniques to solid-state lithium batteries and demonstrate that tuning the electronic energy band alignment by interfacial engineering is applicable to the interfaces of metals and mixed conductors. These results highlight the importance of designing positive electrode and current collector interfaces for solid-state lithium batteries with high power density.

Small grains as recombination hot spots in perovskite solar cells

Summary Non-radiative recombination in the perovskite bulk and at its interfaces prohibits the photovoltaic performance from reaching the Shockley-Queisser limit. While interfacial recombination has been widely discussed and demonstrated, bulk recombination and especially the influence of grain boundaries remain under debate. Most studies explore the role of grain boundaries on perovskite films rather than devices, making it difficult to link the film properties with those of the devices. Here, we systematically investigate the effects of grain boundaries on the performance of perovskite solar cells by two different methods. By combining experimental characterization with theoretical device simulations, we find that the recombination at grain boundaries is diffusion limited and hence is inversely proportional to the grain area to the power of 3/2. Consequently, the prevalence of small grains—which act as recombination hot spots—across the perovskite active layer dictates the photovoltaic performance of the perovskite solar cells.

Deterioration mechanism of perovskite solar cells by operando observation of spin states

Perovskite solar cells are attractive because of their remarkably improved power conversion efficiency. In view of their application, however, it is important not only to increase the power conversion efficiency, but also to elucidate the deterioration mechanism. Here, we show operando direct observation of spin states in the cells using electron spin resonance, thereby investigating the operation and deterioration mechanisms from a microscopic viewpoint. By simultaneous measurements of solar cell characteristics and electron spin resonance, the spin states in the hole transport material spiro-OMeTAD are demonstrated to change in accordance with the device performance variation under operation. These variations are ascribed to the change of hole transport and to interfacial electric dipole layers. Reverse electron transfer from TiO2 to the hole transport material layer is demonstrated under ultraviolet light irradiation, which decreases hole doping. Conducting such operando microscopic investigation will be useful to obtain further guidelines for improving the device performance and durability.

Enhanced hole injection assisted by electric dipoles for efficient perovskite light-emitting diodes

Enhanced hole injection is essential to achieve high performance in perovskite light-emitting diodes (LEDs). Here, a strategy is introduced to enhance hole injection by an electric dipole layer. Hopping theory demonstrates electric dipoles between hole injection layer and hole transport layer can enhance hole injection significantly. MoO3 is then chosen as the electric dipole layer between PEDOT:PSS (hole injection layer) and PVK (hole transport layer) to generate electric dipoles due to its deep conduction band level. Theoretical results demonstrate that strong electric fields are produced for efficient hole injection, and recombination rate is substantially increased. Capacitance-voltage analyses further prove efficient hole injection by introducing the electric dipole layer. Based on the proposed electric dipole layer structure, perovskite LEDs achieve a high current efficiency of 72.7 cd A−1, indicating that electric dipole layers are a feasible approach to enhance perovskite LEDs performance.

Photovoltage from ferroelectric domain walls in BiFeO3

We calculate the component of the photovoltage in bismuth ferrite that is generated by ferroelectric domain walls, using first-principles methods, in order to compare its magnitude to the experimentally measured photovoltage. We find that excitons at the ferroelectric domain walls form an electric dipole layer resulting in a domain-wall driven photovoltage. This is of the same order of magnitude as the experimentally measured one, but only if the carrier lifetimes and diffusion lengths are larger than previously assumed.

Structure and electronic structure of van der Waals interfaces at a Au(1 1 1) surface covered with a well-ordered molecular layer of n-alkanes

The structure and electronic structure of a system comprising molecular layers of n-alkane adsorbed on Au(111) were investigated to understand the origin of the interfacial electric dipole layer. These alkane molecules are inert and thus van der Waals (vdW) interaction is the primary adsorptive force. Although these vdW interfaces are regarded as ideal physical adsorption systems unlikely to chemically interact with the metal surface, a large interfacial electric dipole layer over 1 eV is formed. It is important to elucidate the mechanism underlying the generation of such large dipole layers to understand the electrode interface in organic semiconductor devices. Thus, this paper elucidates the details of the vdW interface, in particular between tetratetracontane (TTC) and Au(111). First, the mechanism of growth of the TTC layer up to monolayer thickness was revealed using a low-energy electron-diffraction method. Second, the interface states were newly formed, as determined using angle-resolved photoelectron spectroscopy, X-ray photoemission spectroscopy, and near-edge absorption fine-structure spectroscopy. Our findings indicate, unexpectedly, that a relatively strong inter-orbital interaction governs this vdW interface, similar to a weak chemisorption system rather than a physisorption system.

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.

Negative Fermi-Level Pinning Effect of Metal/n-GaAs(001) Junction Induced by a Graphene Interlayer.

It is demonstrated that the electric dipole layer due to the overlapping of electron wave functions at the metal/graphene contact results in a negative Fermi-level pinning effect on the region of the GaAs surface with low interface-trap density in the metal/graphene/n-GaAs(001) junction. The graphene interlayer plays the role of a diffusion barrier, preventing the atomic intermixing at the interface and preserving the low interface-trap density region. The negative Fermi-level pinning effect is supported by the decrease of the Schottky barrier with the increase of the metal work function. Our work shows that the graphene interlayer can invert the effective work function of the metal between high and low, making it possible to form both Schottky and Ohmic-like contacts with identical (particularly high work function) metal electrodes on a semiconductor substrate possessing low surface-state density.

Optimization of Electrical Properties of MoS2 Field‐Effect Transistors by Dipole Layer Coulombic Interaction With Trap States

The large negative threshold voltage is one of major electrical factors hindering potential applications of MoS2 transistors in low‐power circuit systems. Here, a strategy by forming an electric dipole layer on the surface of MoS2 is presented to optimize the threshold voltage in monolayer polycrystalline MoS2 field‐effect transistors. The electric dipole in an inert non‐conjugated polymer, perfluoropolyether (PFPE), can interact with trapped electrons in the MoS2 active layer and transfer the traps from shallow into deep states, which remarkably improves the threshold voltage. Otherwise, ab initio calculation and molecular dynamics simulation are employed to investigate the transport properties of MoS2 with PFPE. The calculated results imply that localized electrons are dominantly affected by PFPE, while free electrons are irrelevant. This method with dipole layer provides a pathway to implementation of high‐performance MoS2 transistors for future electronic applications.

Reinforcing the Built‐In Field for Efficient Charge Collection in Polymer Solar Cells

AbstractThe collection efficiency of photogenerated charges in polymer solar cells (PSCs) is strongly influenced by the built‐in field (Ein) that develops across the photoactive materials. Here, by investigating the Ein‐development regimes in PSCs by introducing two types of interlayers, electric dipole layers (EDLs) and charge transport layers (CTLs), the device architecture is optimized to result in a larger Ein. By incorporating a pair of EDLs on both sides of the photoactive layer, the Ein is modulated by shifting the vacuum energy at each metal–semiconductor interface, providing a larger Ein than that in conventional PSCs using typical CTLs, such as metal oxides and/or conducting polymers. These devices using paired EDLs exhibit an average PCE of 9.8%, which far surpasses the average PCE of ≈8.5% for paired CTLs.

Introducing paired electric dipole layers for efficient and reproducible perovskite solar cells

The paired electric dipole layers significantly intensify the built-in field across the perovskite layer, resulting in suppressed charge trapping of photogenerated charges.

How Indium Nitride Senses Water

The unique electronic band structure of indium nitride InN, part of the industrially significant III-N class of semiconductors, offers charge transport properties with great application potential due to its robust n-type conductivity. Here, we explore the water sensing mechanism of InN thin films. Using angle-resolved photoemission spectroscopy, core level spectroscopy, and theory, we derive the charge carrier density and electrical potential of a two-dimensional electron gas, 2DEG, at the InN surface and monitor its electronic properties upon in situ modulation of adsorbed water. An electric dipole layer formed by water molecules raises the surface potential and accumulates charge in the 2DEG, enhancing surface conductivity. Our intuitive model provides a novel route toward understanding the water sensing mechanism in InN and, more generally, for understanding sensing material systems beyond InN.