Density Of Mobile Ions Research Articles

Multi‐Scale Simulation of Reverse‐Bias Breakdown in All‐Perovskite Tandem Photovoltaic Modules under Partial Shading Conditions

Herein, a multi‐scale simulation approach to quantify the impact of nonuniformities in cell‐level performance on the photovoltaic characteristics of monolithically interconnected large‐area all‐perovskite tandem modules under partial shading conditions is presented, addressing a crucial aspect of the up‐scaling challenge for this promising photovoltaic technology. To this end, current–voltage characteristics of small‐area all‐perovskite tandem solar cells are obtained for dark and illuminated cases from a calibrated optoelectronic device model using drift–diffusion simulation coupled to a quantum transport formalism for the band‐to‐band tunneling underlying the Zener breakdown. These current–voltage curves are computed for varying density of mobile ions and subsequently used as local 1D coupling laws connecting the 2D electrodes in a quasi‐3D large‐area finite‐element simulation approach that then provides the module characteristics under consideration of spatial variation in active area quality related to mobile ion density. The simulation reveals the appearance of localized current hot spots for the case where the shaded cell is strongly reverse biased.

Direct quantification of ion composition and mobility in organic mixed ionic-electronic conductors.

Ion transport in organic mixed ionic-electronic conductors (OMIECs) is crucial due to its direct impact on device response time and operating mechanisms but is often assessed indirectly or necessitates extra assumptions. Operando x-ray fluorescence (XRF) is a powerful, direct probe for elemental characterization of bulk OMIECs and was used to directly quantify ion composition and mobility in a model OMIEC, poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS), during device operation. The first cycle revealed slow electrowetting and cation-proton exchange. Subsequent cycles showed rapid response with minor cation fluctuation (~5%). Comparison with optical-tracked electrochromic fronts revealed mesoscale structure-dependent proton transport. The calculated effective ion mobility demonstrated thickness-dependent behavior, emphasizing an interfacial ion transport pathway with a higher mobile ion density. The decoupling of interfacial effects on bulk ion mobility and the decoupling of cation and proton migration elucidate ion transport in conventional and emerging OMIEC-based devices and has broader implications for other ionic conductors writ large.

Ion-induced field screening as a dominant factor in perovskite solar cell operational stability

AbstractThe presence of mobile ions in metal halide perovskites has been shown to adversely affect the intrinsic stability of perovskite solar cells (PSCs). However, the actual contribution of mobile ions to the total degradation loss compared with other factors such as trap-assisted recombination remains poorly understood. Here we reveal that mobile ion-induced internal field screening is the dominant factor in the degradation of PSCs under operational conditions. The increased field screening leads to a decrease in the steady-state efficiency, often owing to a large reduction in the current density. Instead, the efficiency at high scan speeds (>1,000 V s−1), where the ions are immobilized, is much less affected. We also show that the bulk and interface quality do not degrade upon ageing, yet the open-circuit voltage decreases owing to an increase in the mobile ion density. This work reveals the importance of ionic losses for intrinsic PSC degradation before chemical or extrinsic mechanical effects manifest.

PIC simulations of stable surface waves on a subcritical fast magnetosonic shock front

We study with particle-in-cell (PIC) simulations the stability of fast magnetosonic shocks. They expand across a collisionless plasma and an orthogonal magnetic field that is aligned with one of the directions resolved by the 2D simulations. The shock speed is 1.6 times the fast magnetosonic speed when it enters a layer with a reduced density of mobile ions, which decreases the shock speed by up to 15% in 1D simulations. In the 2D simulations, the density of mobile ions in the layer varies sinusoidally perpendicularly to the shock normal. We resolve one sine period. This variation only leads to small changes in the shock speed evidencing a restoring force that opposes a shock deformation. As the shock propagates through the layer, the ion density becomes increasingly spatially modulated along the shock front and the magnetic field bulges out where the mobile ion density is lowest. The perturbed shock eventually reaches a steady state. Once it leaves the layer, the perturbations of the ion density and magnetic field oscillate along its front at a frequency close to the lower-hybrid frequency; the shock is mediated by a standing wave composed of obliquely propagating lower-hybrid waves. We perform three 2D simulations with different box lengths along the shock front. The shock front oscillations are aperiodically damped in the smallest box with the fastest variation of the ion density, strongly damped in the intermediate one, and weakly damped in the largest box. The shock front oscillations perturb the magnetic field in a spatial interval that extends by several electron skin depths upstream and downstream of the shock front and could give rise to Whistler waves that propagate along the shock’s magnetic field overshoot. Similar waves were observed in hybrid and PIC simulations and by the MMS satellite mission.

Reduced Barrier for Ion Migration in Mixed-Halide Perovskites.

Halide alloying in metal halide perovskites is a useful tool for optoelectronic applications requiring a specific bandgap. However, mixed-halide perovskites show ion migration in the perovskite layer, leading to phase segregation and reducing the long-term stability of the devices. Here, we study the ion migration process in methylammonium-based mixed-halide perovskites with varying ratios of bromide to iodide. We find that the mixed-halide perovskites show two separate halide migration processes, in contrast to pure-phase perovskites, which show only a unique halide migration component. Compared to pure-halide perovskites, these processes have lower activation energies, facilitating ion migration in mixed versus pure-phase perovskites, and have a higher density of mobile ions. Under illumination, we find that the concentration of mobile halide ions is further increased and notice the emergence of a migration process involving methylammonium cations. Quantifying the ion migration processes in mixed-halide perovskites shines light on the key parameters allowing the design of bandgap-tunable perovskite solar cells with long-term stability.

Effect of CdSe/ZnS quantum dots doping on the ion transport behavior in nematic liquid crystal

The tunability of ion transport behavior in the liquid crystal (LC) system doped with nanomaterials offers a stable and potential technology for designing the next generation application devices. Earlier research efforts, including theoretical and experimental approaches, explore ion density and the effect on mesomorphic properties of several LC nanocomposite system using different variables such as doping concentration, ionic contamination and type of material. In the present work, considering the simplest nematic 5CB LC and core–shell type CdSe/ZnS quantum dots (QDs), the temperature-dependent ion transport behavior and electrical properties have been experimentally measured with varying QD concentration by dielectric spectroscopy technique. The mobile ion concentration and diffusion constant are effectively analyzed using two different approaches: modified Sawada model and Uemura formalism. Utilizing the experimental dielectric data in the pure and doped nematic LC systems, we described the ion trapping and releasing nature of QDs with respect to concentration level and observed the significant contribution of the two types of mobile ions differing in size and mobility. The study also demonstrates that the doping of QDs effectively traps two kinds of ions while the ion releasing phenomenon occurs only for one type of ion with high doping concentration. The effective interaction between QD ligands and molecules creates a tactoid-like ellipsoidal shape of the nanoparticles which in turn enhances the mobile ion density in the higher doped samples. Additionally, the diffusive motion of ions varies linearly with temperature. We have investigated the two plateau regions in the frequency-dependent real part of conductivity with Jonscher power law and observed the significant contribution of the first type of ion to the conduction process in all the studied samples. These experimental findings in this studied system will facilitate the advanced understanding of ion interaction for nanomaterials in other complex LC matrix.

Pathways toward 30% Efficient Single‐Junction Perovskite Solar Cells and the Role of Mobile Ions

Perovskite semiconductors have demonstrated outstanding external luminescence quantum yields, enabling high power conversion efficiencies (PCEs). However, the precise conditions to advance to an efficiency regime above monocrystalline silicon cells are not well understood. Herein, a simulation model that describes efficient p–i–n‐type perovskite solar cells well and a range of different experiments is established. Then, important device and material parameters are studied and it is found that an efficiency regime of 30% can be unlocked by optimizing the built‐in voltage across the perovskite layer using either highly doped (1019 cm−3) transport layers (TLs), doped interlayers or ultrathin self‐assembled monolayers. Importantly, only parameters that have been reported in recent literature are considered, that is, a bulk lifetime of 10 μs, interfacial recombination velocities of 10 cm s−1, a perovskite bandgap () of 1.5 eV, and an external quantum efficiency (EQE) of 95%. A maximum efficiency of 31% is predicted for a bandgap of 1.4 eV. Finally, it is demonstrated that the relatively high mobile ion density does not represent a significant barrier to reach this efficiency regime. The results of this study suggest continuous PCE improvements until perovskites may become the most efficient single‐junction solar cell technology in the near future.

Quantifying the Density and Mobility of Mobile Ions in Solid Electrolytes By Transient Current Measurements

Solid electrolytes are a key component in enabling new technological advances for rechargeable batteries by mitigating many of the challenges associated with the use of liquid organic electrolytes. One of the key properties of solid electrolytes is their ability to transport ions between the anode and the cathode. This ion migration is usually characterized by measuring the ionic conductivity by means of impedance spectroscopic measurements. However, the ionic conductivity is proportional to both the density and the mobility of mobile ions. Only the mobility represents the actual velocity of the mobile ions.Using lithium phosphorus oxynitride (LiPON), we show how measuring the temperature-dependent current transients can be used to independently quantify both the mobility and the density of mobile ions in solid electrolytes. By changing the mobile ion density in Li7La3Zr2O12 (LLZO) solid electrolyte by co-sputtering LLZO and Li2O, we show how samples that seem to have the same ionic conductivity can still differ in ionic mobility. Finally, we employ this method to quantify the effects of Al and Ga doping on the ion migration in LLZO solid electrolytes. The proposed approach in quantification of mobile ions can be extended to other solid electrolytes for a better understanding of ion migration and the influence on battery performance.

Incorporating Electrochemical Halide Oxidation into Drift‐Diffusion Models to Explain Performance Losses in Perovskite Solar Cells under Prolonged Reverse Bias

AbstractPartial shading of a solar module can induce a set of cells within the module to operate under reverse bias. Studies have shown that metal halide perovskite solar cells with a wide variety of compositions and contacts exhibit interesting behavior in reverse bias that includes both reversible performance loss and non‐reversible degradation. In this paper, an advanced drift‐diffusion approach incorporating an electrochemical term to explain the short‐circuit, open circuit and fill factor losses that are experimentally measured after prolonged reverse bias is used. It is shown that holes can tunnel into the perovskite due to sharp band bending near the contact, accumulate within the bulk of the perovskite absorber, and trigger the oxidation of halides to form neutral halogens. The density of neutral halogens is much higher in reverse bias because there are hardly any electrons available to reduce the iodine. The resulting halogens act as bulk recombination centers. While the interstitial halogen density does decay when the cell is operated in forward bias, permanent degradation can occur if the iodine diffuses out of the perovskite layer. Finally, the ways in which changing parameters such as the mobile ion density or the series resistance at the contact can influence device performance and stability are discussed.

Routes toward Long-Term Stability of Mixed-Halide Perovskites

In the recent anniversary issue of Trends in Chemistry, Brennan et al. review halide segregation in mixed-halide perovskites, discussing multiple perspectives on the underlying origins and reported routes to retard halide segregation. Here, we argue that only slowing down the segregation may be insufficient to achieve long-term stability.

Ion Dynamics in Single and Multi-Cation Perovskite

In organic-inorganic perovskites currently widely used to fabricate high-efficiency solar cells the electrical properties are to a large extent determined by the presence of mobile ions. These mobile ions are commonly held responsible for many undesirable features of perovskite solar cells, such as hysteretic behavior of electrical properties and degradation of parameters during operation. Hence, developing methods to study the properties of mobile ions and distinguish their contribution to electrical properties from the usual effects due to electronic states are essential for gaining control over the type and density of mobile ions. In this paper we show that comparison of deep levels transient spectroscopy (DLTS) measurements performed in the normal and reverse biasing/pulsing sequences provides a useful means of discriminating between the contributions of electronic traps usual for all semiconductors and the mobile ions very important in perovskites. To simplify things these experiments were performed on Schottky diodes rather than heterojunctions with organic-inorganic electron transport and hole transport layers. The results of experiments are presented and compared for single cation MAPbI3 and multication perovskites. In both cases the main features observed in DLTS could be attributed to mobile ions.

Spin polarized beam for battery materials research: µ±SR and ß-NMR

Spin polarized particles exhibiting beta decay, such as, muon and 8Li are very useful for detecting internal magnetic fields in solids. Here, I review our attempts to use these spin polarized beams for battery materials research. Due to a unique lifetime and gyromagnetic ratio of muon, a positive muon spin rotation and relaxation (µ+SR) technique is very popular for studying microscopic internal magnetic fields in condensed matters caused by electron and nuclear magnetism. Particularly in 2009, it was found that Li+ ion diffusion in LixCoO2, which is the most common cathode material in a Li-ion battery, is detectable with µ+SR even under the presence of paramagnetic Co ions (Sugiyama et al., Phys. Rev. Lett. 103, 147601, 2009), while NMR is unable to do so due to the effect of localized magnetic moments on a spin-lattice relaxation rate. Such finding opened the door for µ+SR on energy materials research. Since then, many battery materials have been investigated with µ+SR in order to determine their intrinsic diffusion coefficient (D) of Li+ and Na+ ions. Furthermore, using such intrinsic D, the other important parameters are successfully derived, such as, the reactive surface area, diffusion pathway, and density of mobile ions. In 2018, we have initiated to measure internal magnetic fields in solids with a negative muon, i.e. a µ-SR technique, in J-PARC (Sugiyama et al., Phys. Rev. Lett. 121, 087202, 2018). This will provide a new insight for microscopic internal magnetic fields from lattice sites, while µ+SR does so from interstitial sites. On the contrary, a long lifetime of 8Li (1.21 s) makes a 8Li s-NMR technique suitable for detecting both concentration and diffusion of Li in solids. Particularly in TRIUMF, since the implantation energy of 8Li is tunable from almost 0 to about 28 keV, we can measure both spin-lattice relaxation rate and spin-spin relaxation rate as a function of depth up to about 200 nm, as well as µ+SR using a low energy muon beam in PSI. These will clarify whether the space charge layer of Li+ exists at the interface between electrode and electrolyte material.

Dielectric relaxations and transport properties parameter analysis of novel blended solid polymer electrolyte for sodium-ion rechargeable batteries

A novel blended solid polymer electrolyte comprising polyethylene oxide and polyvinylpyrrolidone polymers for blending and sodium nitrate (NaNO3) as ion conducting species has been optimized via standard solution-cast technique. XRD, FESEM, and FTIR were performed to obtain the information about the structural changes, morphology, and microstructural changes (polymer–ion and ion–ion interactions) of the solid polymer electrolyte films. The electrochemical impedance spectroscopy, linear sweep voltammetry, and i–t characteristics were performed to evaluate the ionic conductivity, voltage stability window, and ion transference number. The impedance study was done in a broad temperature range (40–100 °C). The DSC and TGA were used to obtain information about the thermal transitions and thermal stability of prepared films. The ion dynamics is further investigated by analyzing the complex permittivity, loss tangent, and complex conductivity. All the plots were fitted through established theoretical model/expressions in whole frequency window to obtain dielectric strength, ion conduction path behavior, and relaxation time. Transport parameters such as number density (n), mobility (μ), and diffusion coefficient (D) of mobile ions were obtained by three methods and compared satisfactorily. Lastly, a coherent mechanism for the migration of charge transport carriers within the solid polymer composites has been proposed based on the performed experimental outcome.

Effect of Light and Voltage on Electrochemical Impedance Spectroscopy of Perovskite Solar Cells: An Empirical Approach Based on Modified Randles Circuit

The voltage (V) and light-power (P) dependences of electrochemical impedance spectroscopy (EIS) curves of perovskite solar cells (PSCs) are investigated over the entire (V, P) space rather than limited to short-circuit or open-circuit conditions. For the first time, we report that under fixed V and increasing P, the EIS curves show complicated behaviors. Employing a modified Randles circuit, we successfully fitted the data using three newly proposed empirical equations. From the behaviors of the fitting parameters in the (V, P) space, we conclude that the dynamics inside PSCs consists of two parts: (1) the migration of mobile ions/vacancies and their capturing of the free carriers at the perovskite layer boundary and (2) a classical diode involving the migration and recombination of free carriers. Our calculated diffusivity and mobility agree with those of previous reports. We further found that the mobile ion density inside the perovskite layer is proportional to light power, which could explain the inve...

Structural, electrical and ion transport properties of free-standing blended solid polymeric thin films

Blended solid polymeric thin films based on PEO–PVP complexed with LiBOB were synthesized by solution cast technique. The effect of salt on morphology, structure and electrochemical properties was examined. The XRD and FESEM analyses reveal the enhancement of amorphous content on salt addition. The FTIR spectroscopy evidences the complex formation and presence of various microscopic interactions. The ionic conductivity for the optimized system has been estimated and found to be two orders higher than the salt-free system, i.e., ~ 5.1 × 10−6 S cm−1 (@40 °C), and remains increasing with temperature i.e. 6.5 × 10−4 S cm−1 (@100 °C) for O/Li = 16. The enhancement of ionic conductivity is attributed to increase in the number density of mobile ions as concluded by the Rice and Roth model. The high tion (~ 0.99) evidences the ionic nature of complexed electrolyte. DSC analysis evidences the suppression of crystallinity and shift of glass transition and melting temperature toward lower temperature implies the enhancement of the amorphous content and forms the rubbery nature of the thin films which support the faster ion conductions. Finally, an interaction scheme is proposed for a better explanation of the ion transport on the basis of experimental findings.

Impact of ceria nanofillers on temperature dependent electrical and transport properties of PVA solid polymer electrolyte films

Thermally sensitive, ceria based proton conducting solid polymer electrolyte films were prepared by doping ammonium ceric sulfate complex salt in a PVA matrix using solution casting method. The prepared films were characterized using FTIR, FESEM, TGA, UTM and impedance analyzer in order to understand structural, morphological, thermal, mechanical and electrical properties CeO2 nanoparticle based PVA/ACS composites. The presence of spindle shaped CeO2 nanoparticle was confirmed from FESEM micrographs. The prepared films shown good thermal stability up to 200 °C. The 15 wt% ACS doped PVA electrolyte shown good conductivity, mechanical strength and thermal stability. The Rice and Roth model have been used to evaluate transport parameters and it reveals that enhancement in conductivity is due to increment in the number density of mobile ions as the increase in salt concentration.

Phase Segregation Enhanced Ion Movement in Efficient Inorganic CsPbIBr2 Solar Cells

AbstractOrganic–inorganic hybrid perovskite solar cells with mixed cations and mixed halides have achieved impressive power conversion efficiency of up to 22.1%. Phase segregation due to the mixed compositions has attracted wide concerns, and their nature and origin are still unclear. Some very useful analytical techniques are controversial in microstructural and chemical analyses due to electron beam‐induced damage to the “soft” hybrid perovskite materials. In this study photoluminescence, cathodoluminescence, and transmission electron microscopy are used to study charge carrier recombination and retrieve crystallographic and compositional information for all‐inorganic CsPbIBr2 films on the nanoscale. It is found that under light and electron beam illumination, “iodide‐rich” CsPbI(1+x)Br(2−x) phases form at grain boundaries as well as segregate as clusters inside the film. Phase segregation generates a high density of mobile ions moving along grain boundaries as ion migration “highways.” Finally, these mobile ions can pile up at the perovskite/TiO2 interface resulting in formation of larger injection barriers, hampering electron extraction and leading to strong current density–voltage hysteresis in the polycrystalline perovskite solar cells. This explains why the planar CsPbIBr2 solar cells exhibit significant hysteresis in efficiency measurements, showing an efficiency of up to 8.02% in the reverse scan and a reduced efficiency of 4.02% in the forward scan, and giving a stabilized efficiency of 6.07%.

Structural and electrical characteristics of PVA:NaTf based solid polymer electrolytes: role of lattice energy of salts on electrical DC conductivity

Solid polymer electrolytes (SPEs) based on polyvinyl alcohol (PVA) and sodium triflate (NaTf) have been prepared by solution cast technique. X-ray diffraction was performed for structural analysis. The decrease of intensity of crystalline peaks of PVA upon addition of NaTf salt reveals the increase of amorphous domain in SPEs. Impedance plots (Zi vs. Zr) shows that the electrolyte samples have a smaller bulk resistance. The highest achieved room temperature DC conductivity is 7.39 × 10−5 S/cm for the sample incorporated with 30 wt% of NaTf. The influence of lattice energy of sodium salts on DC conductivity is discussed. It was appeared that the lattice energy of salts significantly affects the conductivity behavior of polymer electrolytes. High DC conductivity can be achieved for polymer electrolytes incorporated with low lattice energy salts. The pattern of DC ionic conductivity against 1000/T is follows Arrhenius equation. The highest conducting composition has the lowest activation energy (0.109 eV). The DC conductivities calculated from the complex impedance plots are close to those obtained from the plateau region of AC conductivity spectra. Transport parameters such as density of mobile ions (Na+), mobility, and diffusion coefficient were determined for the samples using the Rice and Roth models. The dielectric analysis suggests non-Debye type of relaxation of ions in the present work.

Transport and AC Impedance Studies of Plasticized-Chitosan Based Polymer Electrolytes

Dimethyl Carbonate (DMC)-plasticized chitosan-based polymer electrolytes were prepared in the present work. The sample 37.5 wt% chitosan - 37.5 wt% NH4CF3SO3 - 25 wt% DMC showed the highest conductivity of (1.95±0.15) x 10-6 S/cm. From calculated values obtained using the Rice and Roth model, the increase in conductivity is due to the increase in the number density of mobile ions. Universal Power Law was used to analyze the AC conductivity of the sample. Arrhenius behavior was observed in the plot of hopping frequency; ωP against temperature. The AC conductivity master curve was obtained for the highest conducting sample when scaled vertically by σ and horizontally by ω .

Ionic impurities in nematic liquid crystal doped with quantum dots CdSe/ZnS

ABSTRACTWe investigated the dielectric losses and the ionic currents in the nematic liquid crystal (NLC) doped with semiconductor quantum dots (QDs) of CdSe/ZnS core – shell type and covered with trioctylphosphine oxide (TOPO) molecules. The dielectric loss tangent of the NLC composites increased with increasing the QDs concentration from 0.1 to 0.3 wt%. The density of mobile ions in the composites increased linearly and the average values of ions mobility in the composites decreased with increasing the QDs concentration. The fast ions with the mobility of about 10–10 m2/V·s and the slow ions with the mobility of about 10–11 m2/V·s were detected in the NLC composites. The growth of the content of slow ions took place with increasing the QDs concentrations. Increasing the dielectric loss tangent was observed with increasing the duration of sonication time of the NLC composites to prepare homogeneous suspensions. The fragmentation of the CdS/ZnS shell as a result of the sonication may lead to the appearance of the slow ions in the NLC composites.