Interfacial Charge Accumulation Research Articles

Perovskite Solar Cells Yielding Reproducible Photovoltage of 1.20 V.

High photovoltages and power conversion efficiencies of perovskite solar cells (PSCs) can be realized by controlling the undesired nonradiative charge carrier recombination. Here, we introduce a judicious amount of guanidinium iodide into mixed-cation and mixed-halide perovskite films to suppress the parasitic charge carrier recombination, which enabled the fabrication of >20% efficient and operationally stable PSCs yielding reproducible photovoltage as high as 1.20 V. By introducing guanidinium iodide into the perovskite precursor solution, the bandgap of the resulting absorber material changed minimally; however, the nonradiative recombination diminished considerably as revealed by time-resolved photoluminescence and electroluminescence studies. Furthermore, using capacitance-frequency measurements, we were able to correlate the hysteresis features exhibited by the PSCs with interfacial charge accumulation. This study opens up a path to realize new record efficiencies for PSCs based on guanidinium iodide doped perovskite films.

Conductivity and interface charge accumulation between XLPE and SIR for HVDC cable accessory

Interface charge accumulation between cross linked polyethylene (XLPE) and silicone rubber (SIR) is the key factor causing discharge failure for high voltage direct current (HVDC) cable accessory. However, the properties and mechanisms of interface charge behaviors in double-layer dielectric are not detailed. In the present work, dielectric performance and conductivity properties related to interface charge have been measured, and interface charge distributions between XLPE and SIR have been calculated based on Maxwell–Wagner polarization model. The experimental results indicate that charge conduction of SIR is larger than that of XLPE at room temperature, and the conductivity of SIR has a weak dependence on the temperature. By contrast, the conductivity of XLPE increases by three orders when the temperature increases from 25 °C to 90 °C, result in that the conductivity of XLPE exceed that of SIR at a certain temperature. The mismatch of conductivity properties for the two materials will cause charge accumulation at the interface. The interface charge density firstly decreases and then increases with the increasing temperature, and the polarity of charges are different for different temperature regions. Under 10 kV/mm, the transition temperature is about 40 °C. When the temperature exceeds the transition temperature, electric field in the two materials will reverse, and the polarity of interface charge will be changed accordingly.

Improving photovoltaic performance of inverted planar structure perovskite solar cells via introducing photogenerated dipoles in the electron transport layer

Organic-inorganic hybrid perovskites have attracted great attentions for photovoltaic applications due to impressive energy conversion efficiency and low cost fabrication. This article reports the effect of photogenerated dipoles in the electron transport layer (ETL) on the photovoltaic actions in perovskite solar cells (PSCs). A nonfullerene material ITIC with optically induced dipoles was doped into PCBM acting as ETL in inverted planar structure PSCs (ITO/PEDOT:PSS/MAPbI3-xClx/ETL/PEI/Ag). The power conversion efficiency (PCE) of PSCs increases from 13.48% to 14.52% with introducing ITIC into ETL. The capacitance measurements indicate that optically induced dipoles can decrease interfacial charge accumulation. Furthermore, impedance spectroscopy suggests the charge recombination as well as ionic migration are reduced with ITIC doped ETL. Steady and transient PL spectra reveal that the electron extraction is more efficient with optically induced dipoles in the ETL. These results provide unique design of ETL in perovskite solar cells.

Multiple modulations of strain- and charge-mediated converse magnetoelectric coupling effects in a STO/Fe3O4/Au/PZT multiferroic heterostructure

The multiple modulations of strain- and charge-mediated converse magnetoelectric coupling effects have been achieved in a multiferroic heterostructure of SrTiO3(STO)/Fe3O4/Au/PbZr0.52Ti0.48O3(PZT) multilayers grown on a Nb:SrTiO3 substrate. By altering the position of the applied electric field, the heterostructure is divided into three structure parts, i.e., Fe3O4/Au/PZT, STO/Fe3O4, and STO/Fe3O4/Au/PZT. In such an optimized heterostructure, the strain and charge effects can be directly separated, quantified, and co-regulated and the pure strain, pure charge, and the combined strain and charge effects can thus be obtained, respectively. The in-plane magnetization variation behaviors induced by electric fields are different for the three individual modulations, which are closely related to the interfacial strain propagation and interfacial charge accumulation. It is also found that the strain and charge effects can interact with each other as the two interfacial effects coexist.

Cation-Assisted Restraint of a Wide Quantum Well and Interfacial Charge Accumulation in Two-Dimensional Perovskites

Two-dimensional (2D) layered perovskite compounds hold promise to solve stability issues for photovoltaic devices. However, the naturally formed multiple quantum well structure contributes to a large distribution of optical band gaps in 2D perovskite materials. Therefore, intensive efforts have been devoted to control the quantum well width into an extremely small distribution and optimize the charge-transport characteristic. Herein, we report that a cation-induced recrystallization process (CIRP) can form well alignment orientation and increase the crystallinity of 2D perovskite (BA)2(CH3NH3)3Pb4I13 (where BA is butylammonium) via an intermediate catalytic effect. More importantly, CIRP treatment enables a homogeneous distribution of organic cations, narrows the distribution of the quantum well width, and reduces charge accumulation as well. The 2D perovskite devices with CIRP treatment using a metal oxide TiO2/Al2O3/NiO/C framework exhibited an impressive power conversion efficiency of 8.2%, which is th...

Enhancement of superconducting transition temperature in FeSe electric-double-layer transistor with multivalent ionic liquids

Electric-double-layer transistors (EDLTs), which consist of ionic liquids (IL) as gate dielectrics, have attracted keen interest owing to dramatic modulation of physical properties by high-density carrier accumulation and tuning. In this study, EDLTs based on the superconducting ultrathin FeSe film with multivalent ILs exhibit a great potential for improvement of physical properties at the interface charge accumulation layer. The superior superconducting property found in high-Tc FeSe-EDLT with divalent IL sheds new light on the importance of interface engineering via appropriate selection of ILs.

Influence of Charge Transport Layers on Open-Circuit Voltage and Hysteresis in Perovskite Solar Cells

Summary Perovskite materials have experienced an impressive improvement in photovoltaic performance due to their unique combination of optoelectronic properties. Their remarkable progression, facilitated by the use of different device architectures, compositional engineering, and processing methodologies, contrasts with the lack of understanding of the materials properties and interface phenomena. Here we directly target the interplay between the charge-transporting layers (CTLs) and open-circuit potential ( V OC ) in the operation mechanism of the state-of-the-art CH 3 NH 3 PbI 3 solar cells. Our results suggest that the V OC is controlled by the splitting of quasi-Fermi levels and recombination inside the perovskite, rather than being governed by any internal electric field established by the difference in the CTL work functions. In addition, we provide novel insights into the hysteretic origin in perovskite solar cells, identifying the nature of the contacts as a critical factor in defining the charge accumulation at its interface, leading to either ionic, electronic, or mixed ionic-electronic accumulation.

Surface potential decay on silicon rubber samples at reduced gas pressure

Accumulation of interfacial charges is an inherent feature of HVDC insulation based on solid and gaseous media. The collected surface charges can alter the geometrical electric field leading to undesirable phenomena such as partial discharges and even unexpected flashovers. In the present paper, surface potential decay on silicone rubber samples is analyzed at reduced pressures of ambient air that allows for elimination of surface charge neutralization by gas ions. Thus, influences imposed by bulk and surface conduction in the solid material are studied by means of computer simulations and experimental measurements. The results allow for identifying levels of bulk and surface conductivities above which the corresponding charge decay mechanism becomes dominant. It is shown that with a negligible space charge effect and significant surface leakage, there exists a notable spread of charge along gas-solid interface yielding visible crossover phenomenon in charge decay characteristics. It is also demonstrated that the effect of space charge in the material bulk on surface potential decay can only be significant within layers of material finer than ca. 100 μm.

Charge behavior at interfaces involving nanocomposites

A major disadvantage of polymeric insulation is space charge accumulation at high dc electric fields. Space charge is associated with degradation and premature failure of polymeric insulation through distortion of electric fields. Polymer nanocomposites or nanodielectrics have emerged as a class of novel dielectric materials with improved material properties. We are therefore, looking at an increased use of nanocomposites in the near future. Additionally, we expect nanocomposites to be used in conjunction with conventional insulating materials, thus giving rise to interfaces. Interfaces are important because they often cause accumulation of interfacial charge or space charge with its attendant issues. The present work studies space charge behavior in nanocomposites and its interfaces with other materials. The space charge distribution is mapped using a Pulsed Electro-Acoustic (PEA) system. Polarization-depolarization current measurements are also performed. Measurements are performed under different experimental conditions (electric field, aging etc.). Experimental data indicate that nanoparticles affect charge injection as well as charge transport through change in interfacial and bulk properties. Further, thermal aging causes significant increase in charge injection and charge transport through the material. Experiments with as-received and surface-treated nanoparticles indicate that surface treatment affects the interphase significantly.

Understanding the effect of chlorobenzene and isopropanol anti-solvent treatments on the recombination and interfacial charge accumulation in efficient planar perovskite solar cells

The role of anti-solvent treatment on the morphological and optoelectronic properties of multiple-cation and mixed-halide perovskite solar cells have been studied.

Photoinduced Bulk Polarization and Its Effects on Photovoltaic Actions in Perovskite Solar Cells.

This article reports an experimental demonstration of photoinduced bulk polarization in hysteresis-free methylammonium (MA) lead-halide perovskite solar cells [ITO/PEDOT:PSS/perovskite/PCBM/PEI/Ag]. An anomalous capacitance-voltage (CV) signal is observed as a broad "shoulder" in the depletion region from -0.5 to +0.5 V under photoexcitation based on CV measurements where a dc bias is gradually scanned to continuously drift mobile ions in order to detect local polarization under a low alternating bias (50 mV, 5 kHz). Essentially, gradually scanning the dc bias and applying a low alternating bias can separately generate continuously drifting ions and a bulk CV signal from local polarization under photoexcitation. Particularly, when the device efficiency is improved from 12.41% to 18.19% upon chlorine incorporation, this anomalous CV signal can be enhanced by a factor of 3. This anomalous CV signal can be assigned as the signature of photoinduced bulk polarization by distinguishing from surface polarization associated with interfacial charge accumulation. Meanwhile, replacing easy-rotational MA+ with difficult-rotational formamidinium (FA+) cations largely minimizes such anomalous CV signal, suggesting that photoinduced bulk polarization relies on the orientational freedom of dipolar organic cations. Furthermore, a Kelvin probe force microscopy study shows that chlorine incorporation can suppress the density of charged defects and thus enhances photoinduced bulk polarization due to the reduced screening effect from charged defects. A bias-dependent photoluminescence study indicates that increasing bulk polarization can suppress carrier recombination by decreasing charge capture probability through the Coulombic screening effect. Clearly, our studies provide an insightful understanding of photoinduced bulk polarization and its effects on photovoltaic actions in perovskite solar cells.

MoS2@VS2 Nanocomposite as a Superior Hybrid Anode Material.

Using density functional theory, MoS2@VS2 nanocomposite is reported as a hybrid anode with upgraded electronic conductivity and Li/Na storage capacity. The chemically active monolayer VS2 can be stabilized in energy and phonon vibrations by using the monolayer MoS2 as a substrate. The stability of the chemically active monolayer VS2 is attributed to the interfacial charge accumulation between the monolayer MoS2 and VS2. The maximum specific capacity of the nanocomposite has been enhanced to 584 mAh/g both for Li and for Na storage. We attribute the high enhancement in the Li/Na storage capacity of MoS2@VS2 nanocomposite to the charge redistribution in the formation of the nanocomposite. The lithiation/sodiation open-circuit voltage range of the nanocomposite is quite feasible to be used as anode. Diffusion barriers of Li/Na ions on the surfaces of the nanocomposite are comparable to the barriers on corresponding monolayers, while at the interface the barriers are lower than that for bulk MoS2. This study utilizes different aspects of the two different materials in a hybrid anode with highly enhanced electrochemical performance.

The effect of hole transporting layer in charge accumulation properties of p-i-n perovskite solar cells

The charge accumulation properties of p-i-n perovskite solar cells were investigated using three representative organic and inorganic hole transporting layer (HTL): (a) Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS, Al 4083), (b) copper-doped nickel oxide (Cu:NiOx), and (c) Copper oxide (CuO). Through impedance spectroscopy analysis and modelling, it is shown that charge accumulation is decreased in the HTL/perovskite interface, between PEDOT:PSS to Cu:NiOx and CuO. This was indicative from the decrease in double layer capacitance (Cdl) and interfacial charge accumulation capacitance (Cel), resulting in an increase to recombination resistance (Rrec), thus decreased charge recombination events between the three HTLs. Through AFM measurements, it is also shown that the reduced recombination events (followed by the increase in Rrec) are also a result of increased grain size between the three HTLs, thus reduction in the grain boundary area. These charge accumulation properties of the three HTLs have resulted in an increase to the power conversion efficiency between the PEDOT:PSS (8.44%), Cu:NiOx (11.45%), and CuO (15.3%)-based devices.

Assessment of interfacial charge accumulation in oil-paper interface in transformer insulation from polarization-depolarization current measurements

Accumulation of interfacial space charge in oil-paper interface is a critical issue in insulation diagnostics of transformers. This interfacial charge mainly accumulates due to the conductivity difference of oil and paper. Accumulation of interfacial charge leads to localized field enhancement, which further leads to partial discharges and acceleration in the aging of insulation. Therefore, from the point of view of transformer insulation diagnostics, assessment of interfacial charge is very important. However, it is not easy to estimate interfacial space charge behavior from the transformer diagnostics methods currently in use. In case of Polarization-Depolarization Current (PDC) measurement, a well known method for transformer condition monitoring, the effect of interfacial charge is reflected in the non-linearity of current response during polarization and de-polarization. During de-polarization process, a part of the interfacial charge accumulated during polarization period is absorbed by the electrodes producing a current, which is difficult to separate using conventional linear dielectric theory. In this paper, an attempt has been made to separate this current component from de-polarization current through considering charge de-trapping mechanism. Terming this current component as de-trapping current, its relationship with other parameters of transformer insulation is discussed. The developed methodology has been applied on several practical transformers. It was observed that the time constant of de-trapping current is related to the paper conductivity, oil conductivity, dissipation factor and age of the insulation.

Suppressing interface charge between LDPE and EPDM for HVDC cable accessory insulation

Space charges are likely to accumulate at the interface between insulation materials with different properties, which may lead to accelerated degradation of insulation systems. Cable accessories made of ethylene-propylene-diene terpolymer (EPDM) are considered to be the weakest part of HVDC cable system due to the existence of the interface between cable insulation and itself. Direct-fluorination can be applied to adjust the electrical properties of polymer insulation. Therefore, this paper intends to investigate the effects of direct-fluorination on the interface charge distribution between LDPE and EPDM composite insulation. The EPDM samples were fluorinated for 0, 15, 30 and 60 min, and LDPE for 0 and 30 min, respectively. The interface charge distribution were measured under 20 kV/mm. Obtained results show that the polarity of interface charge is the same with the voltage applied on EPDM sample, which can be verified by the Maxwell-Wagner-Sillars (MWS) theory. The fluorination for 30 min effectively suppresses the interface charge accumulation between LDPE and EPDM. However, excessive fluorination leads to significant accumulation of interface charge. Though fluorination of both dielectrics has the same effect with the single fluorinated EPDM on interface charge behaviors, more hetero-charges injection in the vicinity of cathode. The interface charge accumulation and dissipation process depends on not only the time constant but the surface state controlled by the fluorination time. As a consequence, the approximate direct fluorination can effectively suppress the interface charge accumulation of cable accessories and will make a potential application for the dc cable accessories.

Nonlinear conductivity and interface charge behaviors between LDPE and EPDM/SiC composite for HVDC cable accessory

Interface charges are easy to accumulate between two different dielectrics with various characteristics, which may cause accelerated degradation of insulation systems. Ethylene-propylene-diene terpolymer (EPDM) is used mainly for HVDC cable joint, which is the most vulnerable part of the cable system because of the interface. Particles with nonlinear conductivity can be doped into the polymer matrix to modify the interface charge behaviors through altering the conductivity under combined stresses. In this paper, silicon carbide (SiC) particles were dispersed into EPDM with 0, 10, 30 and 50 wt% respectively. The space charge behaviors at the interface between LDPE and EPDM filled with SiC particles was measured under 15 and 30 kV/mm. Besides, dielectric constant, dc conduction and trap distribution were introduced to elaborate the suppression mechanism with SiC doping. The SEM results show that the particles are well distributed in the EPDM. The permittivity increases with the fillgrade and the dc conductivity shows an obvious nonlinear trend under various electrical fields. SiC doping can effectively suppress the interface charge accumulation under different stresses. The suppression mechanism is attributed to the nonlinear conductivity and more shallow traps of EPDM/SiC composite. As a consequence, the approximate SiC doped EPDM can availably suppress the interface charge accumulation and offers a possible method for the improvement of cable accessory performance.

Surface and interface charge accumulation and decay of oil-paper insulation under DC voltage

Oil-paper insulation system is an extraordinarily essential system of converter transformer as well as HVDC bushing. This paper studies the interface charge accumulation and dissipation characteristics of oil-paper insulation system under DC voltage. In this paper, an induced electrification was used to charge the oil-paper samples. The amplitudes of the DC voltage varied from 1 to 4 kV and the paper was 0.13 mm in thickness. From the research of different voltage amplitudes and charging times of the paper, conclusions can be drawn that the decay is fast initially, and then becomes slower with the lapse of time. At last the dissipation trend tends to be flat. With the increase of the voltage amplitudes, the surface potential rises more rapidly, and higher voltage would also cause the charge to dissipate more quickly. Longer charging times would cause the charge to dissipate a bit more slowly. The decay rate of the negative voltage is a little smaller than that of the positive charge. Moreover, when it comes to the effect on the interface charge of the paper layers, the surface potential is more proportional to the increase of paper layers. When the amplitude of the voltage applied in the test is small or the charging time is short enough, the interface charge can be hardly accumulated on the bottom layer. In addition, the tdV/dt and the decay time curves under the positive voltage usually have two peak values, while there is only one peak value in most of the curves under the negative condition.

Evidence of a strong effect of defect-free metal oxide supports on Pt nanoparticles.

Influence of metal oxide (MO) supports on nanoparticle (NP) catalysts is still under investigation. Theoretical studies demonstrate that active defect sites on the surface of a MO support can affect the structure and activity of metal clusters. In the present work, we show that even defect-free surfaces of MOs can cause considerable restructuring and accumulation of interfacial charges on Pt NPs of size 1 nm (Pt55). Independent of the type of MO support, we find that supported Pt55 behaves like a conductor since the binding energy of a test adsorbate on top of it is similar to that on an intact Pt55. However, adsorption energy at binding sites close to the perimeter of the nanoparticle/support interface can vary by 1.8 eV depending on the distance between the adsorbate and surface cations (possibility of forming ionic bonds) as well as the amount and sign of charges (ionization energy) of interfacial Pt atoms.

The Influence of Structural Configuration on Charge Accumulation, Transport, Recombination, and Hysteresis in Perovskite Solar Cells

AbstractBehind the outstanding photovoltaic performance of perovskite solar cells (PSCs), the correlation between the structural configuration of the PSCs and the mechanism of photoelectric conversion is still not fully understood. In this paper, two types of PSCs with different structural configurations, that is, mesoporous (meso) and planar, were prepared, and systematic optoelectronic transient studies were performed on these devices. Although the photovoltaic performances of the tested meso‐ and planar‐PSCs were comparable, the different structural configurations resulted in distinct charge dynamics and photoelectric conversion mechanisms. By taking the dynamic results into account, the influence of a meso‐TiO2 layer on charge accumulation, transport, and recombination was elucidated. More importantly, an excessively slow temporal response component in the optoelectronic transients was observed, which was proven to be linked to specific hysteresis phenomena in the PSCs. The experimental results collectively indicated that the hysteresis, which showed strong dependence on device structure and charge dynamics, was irrelevant to charge trapping or ferroelectricity but probably resulted from interfacial charge accumulation and/or ion migration. This work provides deep insight into the structure–function relationship in PSCs and highlights the effect of hysteresis on charge dynamics.

Publisher's Note: “Equivalent circuit representation of hysteresis in solar cells that considers interface charge accumulation: Potential cause of hysteresis in perovskite solar cells” [Appl. Phys. Lett. 109, 033905 (2016)

Publisher's Note: “Equivalent circuit representation of hysteresis in solar cells that considers interface charge accumulation: Potential cause of hysteresis in perovskite solar cells” [Appl. Phys. Lett. <b>109</b>, 033905 (2016)