Interfacial Charge Accumulation Research Articles

Modulating Chemical Interaction to Realize Bottom‐Up Defect Passivation for Efficient and Stable Perovskite Solar Cells

The defects of perovskite and SnO2layers, interfacial energy barrier, and high grain boundary density impede further development of perovskite solar cells. Herein, a bottom‐up versatile modification strategy, which is implemented via introducing Lewis base ligand molecules containing different numbers of carbonyls (urea, propanedioic acid, and barbituric acid [BA]) into SnO2colloidal solution, is reported. All modifiers exhibit positive but different defect passivation effects for both SnO2and perovskite layers. The defect passivation effect can be rationally modulated by tuning the number of carbonyls and is directly proportional to the number of carbonyls. The enlargement sequence of the grain size is the same as that of the defect passivation effect. Meanwhile, the interfacial energy barrier, charge accumulation, and hysteresis are mitigated after modification, which is principally attributed to improved energy band alignment, reduced interfacial defects, and improved electrical properties of SnO2films. The BA‐modified device achieves an impressive efficiency up to 23.35%, which is significantly higher than 21.59% of the reference device. In addition, the modified devices demonstrate enhanced moisture and thermal stabilities.

Dielectric and ferroelectric properties of multilayer BaTiO3/NiFe2O4 thin films prepared by solution deposition technique

In this work different lead-free multilayered structures, composed of perovskite BaTiO3 and spinel NiFe2O4 thin layers, were obtained by solution deposition method. Structural characterization of the sintered thin films confirmed the well-defined layered structure with overall thickness from 160 to 600 nm, crystalline nature of perovskite BaTiO3 and spinel NiFe2O4 phases without secondary phases (after sintering below 900 °C) and grains on nanometer scale. Dielectric properties of the multiferroic multilayer BaTiO3/NiFe2O4 thin films were analyzed in temperature and frequency range from 30 °C to 200 °C and 100 Hz to 1 MHz, respectively. In comparison to the pure BaTiO3 films, the introduction of ferrite layer reduces dielectric response and increases low frequency permittivity dispersion of the multilayer thin films. The multilayer samples have shown relatively low dielectric loss with stronger contribution of conductivity at higher temperatures, and characteristic broad peak representing “relaxation” of the interface charge accumulation.

Transport Characteristics of Interfacial Charge in SiC Semiconductor-Epoxy Resin Packaging Materials.

The silicon carbide (SiC) wide bandgap (WBG) semiconductor power device has been widely applied for its excellent properties. However, the charge accumulated in the interface of SiC semiconductor-related insulation packaging may lead to serious material performance degradation and failure, threatening the reliability and operation life of power devices. In this research, interface charge accumulation characteristics of SiC–epoxy resin double-layered material were investigated, and space charge injection, transport, and accumulation mechanisms, as well as the related temperature effect for the SiC–epoxy resin under polarization and depolarization conditions, were studied by the pulsed electro-acoustic (PEA) technique. The results show that a charge peak appears between the SiC–epoxy resin interface, which shows the same polarity as the SiC side electrode. Charge injects from the SiC electrode, transports along with the SiC semiconductor bulk due to the high mobility, and is blocked by the interface barrier. In addition, under high temperature and high electrical stress conditions, obvious charge accumulation occurs inside the epoxy resin bulk, which was captured by the deep traps. The charge accumulation of the SiC-insulation packaging material can influence the operation of the power device and should attract more attention.

The Improvement of the Performance of Sky-Blue OLEDs by Decreasing Interface Traps and Balancing Carriers with PSVA Treatment.

The mitigation of interfacial charge accumulation in solution-processed organic light-emitting diodes (s-OLEDs) is an effective method to improve device performance. In this study, the polar solvent vapor annealing (PSVA) method was used to treat two layers in s-OLED, PEDOT:PSS and mCP:DMAC-DPS emitting layers, separately, to optimize the carrier transmission and balance. After the double-layer PSVA treatment, the current efficiency increased, the lifetime of the device is improved, the efficiency roll-off alleviated from 33.3% to 26.6%, and the maximum brightness increased by 31.3%. It is worth mentioning that the work function of the EML interface reduced by 0.36 eV, and the initial injection voltage of the electrons also reduced. Simulating the solubility of the LUMO and HOMO molecule parts of the mCP and DMAC-DPS, it was found that the LUMO parts had stronger polarity and higher solubility in polar solution than the HOMO parts. By comparing the untreated luminescent layer films, it was found that the PSVA treatment improved the uniformity of the film morphology. We may infer that a more ordered molecular arrangement enhances carrier transport as the LUMO parts tend to be close to the surface and the reduced local state traps on the EML surface promote electron injection. According to the experimental results, the injection of holes and electrons is enhanced from both sides of the EML, respectively, and the charge accumulated at the interface of s-OLEDs is significantly reduced due to the improvement of carrier-transported characteristics.

Room-temperature processed hole-transport layer in flexible inverted perovskite solar cell module

Metal oxide-based electrodes are considered to have a key role in the future production of perovskite solar cells (PSCs). Here, we report a high-quality hole transport material deposited by solution-processed vanadium (V) substituted NiO nanomaterial at room-temperature, which holds better electrical conductivity and hole extraction ability as a reference sample. The smaller crystallite size of vanadium-substituted NiO than that of pristine NiO, decreased the surface roughness and significantly improved the surface wetting of the perovskite film. Therefore, vanadium-substitution improved the quality of the perovskite film. Furthermore, the presence of V5+ effectively enhanced the electrical conductivity of the NiO film as determined from the photovoltaic properties of the inverted PSC. The increase in electrical conductivity promoted charge transport and reduced interfacial charge accumulation. These enhanced performances and room-temperature deposition method validate the feasibility of the V-incorporated NiO for flexible PSC module with promising efficiency.

Structure – properties relationships of graphene and spinel nickel ferrites based poly(vinylidene fluoride) hybrid polymer nanocomposites for improved dielectric and EMI shielding characteristics

Hybrid polymer nanocomposites with poly(vinylidene fluoride) are prepared based on graphene and spinel nickel ferrites (nFE) nanoparticles to investigate their structure-properties relationships. X-ray diffraction and differential scanning calorimetry confirmed the co-existence of α- and β-crystal phases of PVDF by the incorporation of graphene and nFE whereas solely α- crystal phase formed in neat PVDF. The influence of graphene and nFE on the mechanical properties of PVDF is concentration dependant. The modulus of PVDF is enhanced ∼ 128% from 587 to 1340 MPa at the expense of reduction in failure strain from 18% to 6% in PVDF/graphene nanocomposite whereas in PVDF/graphene/nFE hybrid nanocomposites, the mechanical properties are reduced owing to the embrittlement. On the contrary, graphene-nFE-polymer interfacial space charge accumulation has caused the enhancement of dielectric and EMI shielding characteristics as a function of frequency (1kHz-5 MHz). In PVDF/graphene nanocomposites at 3 wt%, the dielectric constant (ε/) value is increased 16-fold while keeping the dielectric loss (ε//) values equivalent to neat PVDF. Whereas, both ε/ and ε// are increased enormously in PVDF/graphene/nFE hybrid nanocomposites along the whole frequency range. Similarly, EMI shielding (SET) of PVDF/graphene and PVDF/graphene/nFE is ∼ 16 and 22 dB, respectively for a film thickness of ∼0.2 mm. Comparatively, PVDF/graphene nanocomposites exhibited the synergistic balance in mechanical, dielectric and EMI shielding properties over PVDF/graphene/nFE hybrid nanocomposites.

Rubidium chloride doping TiO2 for efficient and hysteresis-free perovskite solar cells with decreasing traps

Adjusting the electron transport layer (ETL) is an effective method to improve the power conversion efficiency (PCE) of perovskite solar cells (PSCs) as well as suppress their hysteresis. In this study, we report that RbCl doping TiO2 can enhance the crystallinity of perovskite and reduce hysteresis as well. As RbCl crystallite has a polyhedral structure and lattice parameters similar to those of the perovskite by calculation confirmed. According to the seed-mediated growth theory, seed-mediated perovskite formation results in a dense and uniform active layer with excellent crystallinity and low trap density, which can reduce the interfacial charge accumulation and carrier recombination. Consequently, the PSCs with the RbCl-doping ETL show remarkably enhanced performance (from 18.67% to 20.75%). Impressively, negligible hysteresis, high fill factor (FF, over 80%) can be observed for the perovskite with the modified ETL. This work is expected to provide a viable path to design a simple-processing ETL for efficient PSCs.

The negative interface capacitance and its anisotropy in magnetic tunnel junctions

The capacitance of magnetic tunnel junctions (MTJs) with the tunneling magnetocapacitance (TMC) effect has drawn lots of attention but there is still a lack of research on the capacitance anisotropy. In this work, a study of the capacitance in MgO-based MTJ is performed. The negative capacitance is observed and indicates the importance of interface capacitance in the MTJ. In further research, the interface capacitance depends on the relative orientation of magnetization in the two ferromagnetic layers (FMs), exhibiting a strong anisotropy. The Debye-Fröhlich model combined with the magnetization dependence of relaxation time is introduced to explain the behavior. Theory and experiment results of the relaxation time as a function of the relative angle of the two FM magnetizations are well fitted. It reveals that the TMC originates from the interface charge accumulation and spin-dependent tunneling. All the results suggest that the interface capacitance with apparent anisotropy plays an essential role in the TMC.

Robust organic functional materials by thermally doping with metal oxide

We have investigated the failure mechanism of organic functional materials and organic light-emitting diodes (OLEDs) by annealing at high temperatures. We found that N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine (NPB) doped molybdenum oxide and 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene doped cesium carbonate can enhance the thermal stability significantly. The former composite film reveals the ions of NPB, as observed by X-ray photoelectron spectroscopy (XPS), the formation of which shows that NPB receives the electron that Mo loses. Meanwhile, it is stable for the binding energy of the element in the latter composite film from the XPS image. Through the research of carrier-only cells, the observation indicates that the thermal stability of the doped cell is better than that of the undoped cell at high temperatures. The current efficiency of the doped device is only reduced by 12% after annealing at 80℃; meanwhile the lifetime reaching 208 h is the longest among that of the devices. Simultaneously, the undoped device represents a larger decline even of about 30% with the lifetime reaching just 40 h. We assumed that the enhanced heat-resisting properties of organic materials by inorganic doping might be attributed to the decrease of energy barrier and the reduction of the interface charge accumulation phenomenon caused by high temperature. Inorganic doping paves an alternative way to substitute for synthesizing expensive functional materials with high glass transition temperature.

Study of thermoelectric properties in the PEDOT:PSS/Te double-layer thin film devices

The research on the organic hybrid composites have been regarded as important developing direction in the field of thermoelectric fields, which combine the advantages of organic thermoelectric materials and inorganic thermoelectric materials. In this paper, the thermoelectric properties of Tellurium (Te) and Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hybrid composite thin film and double-layer structured of PEDOT/Te thin film are studied, separately. We found that the PEDOT:PSS/Te double-layer structured thin film thermoelectric devices can greatly improve conductivity and Seebeck coefficient of PEDOT:PSS compared with the hybrid structure of PEDOT:PSS thermoelectric devices. We thought that the semi-metallicity of PEDOT:PSS and Te forms an ohmic contact interface, which is advantageous to charge transfer, so the double-layer structure of thin film devices possess a better carrier transport channels by the interface effect than hybrid structure devices. On the other hand, the C–V results also confirmed that the interfacial charge accumulation in the PEDOT:PSS/Te double-layered thin-film device, when the temperature rises, the Fermi level close to the conduction band as the temperature increases, leading to the number of high-energy carriers increase at the interface, which effectively improves the Seebeck coefficient of PEDOT:PSS. Our research provide experimental and theoretical support for the development of thin-film thermoelectric materials and devices.

A novel nanocomposite of NH2‐MIL‐125 modified bismaleimide‐triazine resin with excellent dielectric properties

AbstractThe NH2‐MIL‐125 nanoparticles of amino‐functionalized titanium‐based metal organic frameworks were prepared by the solvothermal method and their nanocomposites with bismaleimide‐triazine resin (BT resin) were prepared. The research results showed that NH2‐MIL‐125 could significantly catalyze the curing reaction of BT resin to form triazine rings, which was attributed to the synergistic catalysis the self‐polymerization of cyanate ester by the amino group and Ti4+ in NH2‐MIL‐125. Because of the increase of triazine ring in crosslinking structures, the initial storage modulus of NH2‐MIL/BT nanocomposites improved. But the steric hindrance of NH2‐MIL‐125 brought the decrease on the crosslinking density and the increase on the free volume of nanocomposites. Meanwhile, besides the introduction of nanopores, the formation of more triazine rings and the increase of free volume, NH2‐MIL‐125 reduced the interface charge accumulation of nanocomposites through the Michael addition reaction with bismaleimide, which contributed to the reduction of dielectric constant of nanocomposites. In addition, NH2‐MIL‐125 with good dispersion played a toughening role and also improved the thermal stability and moisture resistance of nanocomposites. In this study, NH2‐MIL‐125 not only introduced nanopores but also changed crosslinking structures of BT resin, which provided a novel approach to obtain low‐dielectric materials.

A fundamental viewpoint on the hydrogen spillover phenomenon of electrocatalytic hydrogen evolution

Hydrogen spillover phenomenon of metal-supported electrocatalysts can significantly impact their activity in hydrogen evolution reaction (HER). However, design of active electrocatalysts faces grand challenges due to the insufficient understandings on how to overcome this thermodynamically and kinetically adverse process. Here we theoretically profile that the interfacial charge accumulation induces by the large work function difference between metal and support (∆Φ) and sequentially strong interfacial proton adsorption construct a high energy barrier for hydrogen transfer. Theoretical simulations and control experiments rationalize that small ∆Φ induces interfacial charge dilution and relocation, thereby weakening interfacial proton adsorption and enabling efficient hydrogen spillover for HER. Experimentally, a series of Pt alloys-CoP catalysts with tailorable ∆Φ show a strong ∆Φ-dependent HER activity, in which PtIr/CoP with the smallest ∆Φ = 0.02 eV delivers the best HER performance. These findings have conclusively identified ∆Φ as the criterion in guiding the design of hydrogen spillover-based binary HER electrocatalysts.

Increasing Stability of SnO2-Based Perovskite Solar Cells by Introducing an Anionic Conjugated Polyelectrolyte for Interfacial Adjustment.

Despite the fact that power conversion efficiency (PCE) has been greatly improved in recent years, perovskite solar cells (PSCs) need to overcome some challenges, like stability, for the commercial application. Herein, an anionic conjugated polyelectrolyte, sulfonic-containing polyfluorene (abbreviated to SPF), has been developed to modify the interface between the electron-transporting layer (ETL) SnO2 and the optoelectronic active layer MAPbI3 in the n-i-p cells. After 40 days of storage in atmospheric environment in the dark with exposure to a controlled humidity of about 10%, PCE of the SPF-modified cells with the structure of ITO/SnO2/SPF/MAPbI3/spiro-OMeTAD/Au still remained 94% of the initial value. In contrast, the control cell without SPF only remained 31.1% of its initial efficiency after 29 days. The main reason for the stability enhancement is the adjustment of interfacial energy level, the crystallinity enhancement, and the removal of the interfacial defect of the perovskite layer by introducing the hydrophobic and smooth SPF interfacial layer. Deep electrical study on the PSCs discloses that the cell has low carrier transfer resistance, low leakage current density, and minor interfacial charge accumulation. What's more, the short-circuit current density is improved, and PCE of 20.47% is achieved.

Comparison of Transportation Carriers in the Insulating Materials for HVDC Cable Joint

As an interface between the different insulating polymers exists in the joints of High Voltage Direct Current (HVDC) cables, the differences in the field-and-temperature dependence of the conductivities of the materials is regarded as a problem leading to interface charge accumulation and field distortion. In this report, using blocking materials to prevent charge injection from one or both sides of the electrodes, the contributions of charge injection and ionization in low-density polyethylene (LDPE), cross-linked polyethylene with nanoparticles (nano-XLPE) and ethylene propylene diene monomer (EPDM) are distinguished and evaluated based on the results from pulsed electro-acoustic (PEA) method. It is found that the contributions of charge injection and ionization behave differently in different materials. The mechanism behind the field-and-temperature dependences of the conductivities of the three materials is explained successfully. This study helps to clarify the charge transfer mechanism, and thus supports the quantitative calculation of the space charge and electric field distribution in cable joints.

Interfacial resistive switching of Ruddlesden–Popper phase strontium titanate thin film by charge-modulated Schottky barrier

Ruddlesden–Popper (RP) phase Sr2TiO4 thin film is deposited successfully on FTO/glass substrate by spin coating. X-ray diffraction (XRD) pattern shows high-intensity (00 l) (l = 2, 4, and 6) peaks. The device shows bipolar resistive switching (RS) effect. Analysis on current–voltage (I-V) characteristic shows that conduction mechanism is dominated by Schottky emission in both low resistive state (LRS) and high resistive state (HRS). A variation (~0.12 eV) in Schottky barrier height (SBH) is observed between HRS and LRS. Thus, RS at electrode-Sr2TiO4 interface is the main mechanism. In general, variation of SBH is induced by accumulation of interfacial charges. Polarization-voltage (P-V) measurement shows hysteresis loop, which indicates the accumulation of interfacial charges. Formation and migration of charged oxygen vacancies and ions is proposed to be a possible mechanism of interfacial charges accumulation. This study paves the avenue for exploring RS application based on RP phase Sr2TiO4 perovskite oxide.

Probing Charge Accumulation at SrMnO3/SrTiO3 Heterointerfaces via Advanced Electron Microscopy and Spectroscopy.

The last three decades have seen a growing trend toward studying the interfacial phenomena in complex oxide heterostructures. Of particular concern is the charge distribution at interfaces, which is a crucial factor in controlling the interface transport behavior. However, the study of the charge distribution is very challenging due to its small length scale and the intricate structure and chemistry at interfaces. Furthermore, the underlying origin of the interfacial charge distribution has been rarely studied in-depth and is still poorly understood. Here, by a combination of aberration-corrected scanning transmission electron microscopy (STEM) and spectroscopy techniques, we identify the charge accumulation in the SrMnO3 (SMO) side of SrMnO3/SrTiO3 heterointerfaces and find that the charge density attains the maximum of 0.13 ± 0.07 e–/unit cell (uc) at the first SMO monolayer. Based on quantitative atomic-scale STEM analyses and first-principle calculations, we explore the origin of interfacial charge accumulation in terms of epitaxial strain-favored oxygen vacancies, cationic interdiffusion, interfacial charge transfer, and space-charge effects. This study, therefore, provides a comprehensive description of the charge distribution and related mechanisms at the SMO/STO heterointerfaces, which is beneficial for the functionality manipulation via charge engineering at interfaces.

Reduced trap density and mitigating the interfacial losses by placing 2D dichalcogenide material at perovskite/HTM interface in a dopant free perovskite solar cells

Abstract Solution-processed photovoltaics employing perovskite as light harvester are the next generation future energy source owing to its low manufacturing cost and potential to achieve high power conversion efficiency. Significant emphasis has been laid on the performance-related investigations, however the long-term instability under operational conditions and device reproducibility obstruct its potential commercial endeavour. Herein, by tweaking the energy level alignment between the hole-transport layer and perovskite, by the placement of a 2D-MoS2 interlayer, we demonstrate suppressed interfacial charge accumulation, fast charge extraction, and subsequently improved photovoltaic performances. Notably, with the usage of a dopant free hole transport materials, a power conversion efficiency of 18.54% with significantly upgraded open circuit voltage (Voc) and FF was recorded. The stability measurement shows that the resulting 2D-MoS2 supported dopant-free hole selective layers exhibit notable moisture stability under ambient conditions. Our study put forward the profound experimental understanding of 2D-transition metal dichalcogenides as an agent to engineering the interface, enlightening the power conversion efficiency and lifetime of the perovskite solar cells.

Electrical characteristics of the pressboard modified by nano-TiO2 for different electrical ageing stages

In order to seek the optimal mass fraction for the modified pressboards to improve electric ageing resistance ability, the samples modified by nano-TiO2 with 1, 3, 5 and 7% mass fractions were made in the lab. The electrical ageing experiments were carried on these samples. The dielectric properties tests, partial discharge (PD) and DC breakdown were executed on the samples to analyse the electrical ageing performance of the cellulose pressboards modified by nano-TiO2. The interface charge accumulation and decay characteristics of the samples were also tested. The nano-TiO2 distributions and the surface morphology for the cellulose pressboards aged 0 and 40 h were observed by scanning electron microscope (SEM) to verify the modification effect, and the trap density characteristics of unmodified and modified samples were also described. The permittivity and the conductivity properties of the modified samples are more advantageous than the unmodified ones after electrical ageing. The electrical characteristics of the modified samples are better than the unmodified ones for different electrical ageing stages. The analysis of SEM images and trap distribution give a good verification to the improvement of the modified samples for different ageing stages.

Fabrication of Ag@BaTiO3 hybrid nanofibers via coaxial electrospinning toward polymeric composites with highly enhanced dielectric performances

Here we demonstrated a remarkable improvement in the dielectric performance of poly (vinylidene fluoride) (PVDF)-based composites via incorporating distinguished hybrid nanofibers, namely, Ag nanoparticles-embedded BaTiO3 (Ag@BaTiO3, Ag@BT) nanofibers, which were fabricated by coaxial electrospinning technology followed by calcination. The Ag nanoparticles on BaTiO3 nanofibers were found to play a comprehensive role on the dielectric performances, including the better dispersion of hybrid nanofibers leading to homogeneity of electric field, the formation of charge transfer complex in the hybrid region which enhanced the interfacial polarization, and the Coulomb blockade effect of Ag nanoparticles to further suppress the interfacial charge accumulation. As a result, the PVDF/Ag@BT composites exhibited high dielectric constant of 38.87@1 kHz and low loss of 0.02@1 kHz.

Constructing m-TiO2/a-WOx hybrid electron transport layer to boost interfacial charge transfer for efficient perovskite solar cells

Recently, the rapid improvement in power conversion efficiency (PCE) of perovskite solar cells (PSCs) with surprising and efficient PSCs is considered to be the most promising alternative to the next generation photovoltaic. In PSCs, the interfacial charge transfers between perovskite and electron transporting layers (ETLs) play an important role in enhancing the PCE, hysteresis, and stability. Here, an amorphous WOx (a-WOx) interlayer is interposed between the perovskite layer and mesoporous TiO2 (m-TiO2) layer to boost the hetero-interface charge transport. The room-temperature processed a-WOx has a wide bandgap and high electron mobility, which is capable of efficiently transporting photo-generated electrons, thereby reducing interfacial charge accumulation in the PSCs. Besides, we first found that the m-TiO2/a-WOx hybrid ETLs have better non-wettability, which subsequently improves the crystallization of the perovskite films by promoting the grain boundary mobility. Consequently, under optimized MAPbI3 solar cells having m-TiO2/a-WOx hybrid ETLs exhibits the highest Voc of 1.16 V and the PCE of 20.98%. Stability tests indicate that the PCE value based on the m-TiO2/a-WOx device remains more than 91% after exposure to 45% humidity for 30 days in the dark at room temperature, much higher than the m-TiO2 based cell, which retains about 70% of the initial PCE value.