Evolution of Performance Parameters of Perovskite Solar Cells with Current–Voltage Scan Frequency
Enrique H. Balaguera, Juan Bisquert
© 2025 American Chemical Society, February 9, 2025, https://doi.org/10.1021/acs.energyfuels.4c04088
Abstract:
Current–voltage measurements are a standard testing protocol to determine the efficiency of any solar cell. However, perovskite solar cells display significant kinetic phenomena that modify the performance at several time scales, due to hysteresis, internal capacitances, and related mechanisms. Here, we develop a method to analyze the current–voltage curves by using large amplitude sinusoids as the excitation waveforms, specifically addressed to determine the influence of cycling frequency on relevant performance parameters. We solve a system of equations representative of charge collection and recombination, that provide the frequency-dependent dynamical behavior of the internal ion-controlled surface recombination processes that cause open-circuit voltage variations often observed in high performance devices. We analyze several reported experimental data pertaining to state-of-art devices, and we showcase the key parameters governing the evolution of hysteresis phenomena as the scan speed is increased in relation to Impedance Spectroscopy.
URL: https://doi.org/10.1002/smll.202409534
Evolution of Performance Parameters of Perovskite Solar Cells with Current–Voltage Scan Frequency. Enrique H. Balaguera, Juan Bisquert
Anis Allagui, Enrique H. Balaguera
On the semi-infinite distributed resistor-constant phase element transmission line
Electrochimica Acta, 17 November 2024, https://doi.org/10.1016/j.electacta.2024.145344
Abstract:
Under a particular geometrical arrangements of impedances of the type resistors and capacitors for the modeling of a transmission line, the voltage and current along the line are known to follow the standard partial differential equation of diffusion. In this work we propose a generalization of this circuit network by considering the non-ideal fractional capacitive element, also known as constant phase element (CPE), as the energy storage component. The CPE’s impedance is given by Zc(s)=1/(Cαs^α), where Cα>0 and 0<α⩽1, and offers an extra degree of freedom compared to the ideal capacitor of impedance Z=1/(Cs). This leads to an anomalous time-fractional diffusion equation that we solve considering the Caputo fractional derivative definition for the case of one-dimensional, semi-infinite propagation under a constant voltage excitation at x=0. The voltage and current responses are found analytically in terms of the Fox’s H-function. We discuss the implications of the dispersive nature of the CPE on the time-domain response along the transmission line system, as well as on the frequency-domain input impedance.
URL: https://doi.org/10.1016/j.electacta.2024.145344
On the semi-infinite distributed resistor-constant phase element transmission line. Anis Allagui, Enrique H. Balaguera
Enrique H. Balaguera, Anis Allagui
Time delay in the charge/discharge of fractional-order capacitive energy storage devices
Journal of Power Sources, 235094, November 2024, https://doi.org/10.1016/j.jpowsour.2024.235094
Abstract:
Electrical energy storage devices exhibit dispersive properties that control their charge and discharge processes. To get a deeper understanding of these anomalous phenomena, it is essential to go beyond static viewpoints of circuit theory in order to accurately characterize the complex interplay of internal mechanisms. Specifically, the (dis)charging time of resistive-capacitive networks is commonly estimated as four times the product of the Thévenin resistance and the capacitance itself by assuming ideal exponential relaxations in spite of the intrinsic fractional dynamics of the real energy storage materials, leading to inaccurate and erroneous characterization protocols. The purpose of this work is to provide recommended practices to find the steady-state operation of such type of devices from time-domain data with a decelerated behavior of the Mittag-Leffler function at long time scales, introducing the concept of “incremental capacitance” in the transition from ideal to fractional-order capacitor and thus, an estimation of the charge/discharge time delay. Our theoretical analysis is validated by providing a representative example of experimental application, based on an electrochemical power source, such as supercapacitors under switching-type operation. We hope to bring such study to the attention of multidisciplinary readers, both from academia and industry, focused on energy storage device research.
Anis Allagui, Ahmed Elwakil, Enrique H. Balaguera
Journal of Power Sources, 234907, September 2024, https://doi.org/10.1016/j.jpowsour.2024.234907
Abstract:
A resistance ?? in series with a constant phase element (CPE) of frequency-dependent impedance given by the power law function ??(?) = 1∕(????) is commonly used for the analysis of steady-state frequency response data exhibiting non-purely capacitive behavior. This is the case in most (bio)(electro)chemical systems including dielectrics, batteries, supercapacitors, capacitive deionization units, biological tissues and bioelectrodes. Passing to the time domain, the current, voltage and charge of the system are governed by differential equations with non-integer, fractional-order operators. The purpose of this study is to provide the exact analytical expressions for the electrical response of an ??-CPE model under linear sweep voltammetry with the use of the Laplace transform method. The electrical variables are expressed in terms of special functions regularly encountered with fractional calculus such as the Fox’s ?-function and the Mittag-Leffler function, and can be used for modeling non-ideal devices as well as extracting their characteristic parameters.
URL: https://doi.org/10.1016/j.jpowsour.2024.234907
Exact solution for the electrical response of a constant phase element with a series resistance to linear voltage sweep – ScienceDirect
Silvia Delgado-Rodríguez, Eva Jaldo Serrano, Mahmoud H. Elshorbagy, Javier Alda, Gonzalo del Pozo and Alexander Cuadrado
Nanomaterials, 2400067, September, (2024), https://doi.org/10.3390/nano14191582
Abstract:
Perovskite solar cells are considered to be one of the most promising solar cell designs in terms of photovoltaic efficiency. However, their practical deployment is strongly affected by their short lifetimes, mostly caused by environmental conditions and UV degradation. In this contribution, we present a metasurface made of silver nanoparticles used as a UV filter on a perovskite solar cell. The UV-blocking layer was fabricated and morphologically and compositionally analyzed. Its optical response, in terms of optical transmission, was also experimentally measured. These results were compared with simulations made through the use of a well-proven computational electromagnetism model. After analyzing the discrepancies between the experimental and simulated results and checking those obtained from electron beam microscopy and electron dispersion spectroscopy, we could see that a residue from fabrication, sodium citrate, strongly modified the optical response of the system, generating a redshift of about 50 nm. Then, we proposed and simulated the optical behavior of core–shell nanoparticles made of silver and silica. The calculated spectral absorption at the active perovskite layer shows how the appropriate selection of the geometrical parameters of these core–shell particles is able to tune the absorption at the active layer by removing a significant portion of the UV band and reducing the absorption of the active layer from 90% to 5% at a resonance wavelength of 403 nm.
Perovskite solar cells are considered to be one of the most promising solar cell designs in terms of photovoltaic efficiency. However, their practical deployment is strongly affected by their short lifetimes, mostly caused by environmental conditions and UV degradation. In this contribution, we present a metasurface made of silver nanoparticles used as a UV filter on a perovskite solar cell. The UV-blocking layer was fabricated and morphologically and compositionally analyzed. Its optical response, in terms of optical transmission, was also experimentally measured. These results were compared with simulations made through the use of a well-proven computational electromagnetism model. After analyzing the discrepancies between the experimental and simulated results and checking those obtained from electron beam microscopy and electron dispersion spectroscopy, we could see that a residue from fabrication, sodium citrate, strongly modified the optical response of the system, generating a redshift of about 50 nm. Then, we proposed and simulated the optical behavior of core–shell nanoparticles made of silver and silica. The calculated spectral absorption at the active perovskite layer shows how the appropriate selection of the geometrical parameters of these core–shell particles is able to tune the absorption at the active layer by removing a significant portion of the UV band and reducing the absorption of the active layer from 90% to 5% at a resonance wavelength of 403 nm.
Anis Allagui, Ahmed Elwakil, Enrique H. Balaguera
Advanced Electronic Materials, 2400067, July, (2024), https://doi.org/10.1016/j.jpowsour.2024.234907
Abstract:
A resistance ?? in series with a constant phase element (CPE) of frequency-dependent impedance given by the power law function ??(?)=1/(????) is commonly used for the analysis of steady-state frequency response data exhibiting non-purely capacitive behavior. This is the case in most (bio)(electro)chemical systems including dielectrics, batteries, supercapacitors, capacitive deionization units, biological tissues and bioelectrodes. Passing to the time domain, the current, voltage and charge of the system are governed by differential equations with non-integer, fractional-order operators. The purpose of this study is to provide the exact analytical expressions for the electrical response of an ??-CPE model under linear sweep voltammetry with the use of the Laplace transform method. The electrical variables are expressed in terms of special functions regularly encountered with fractional calculus such as the Fox’s ?-function and the Mittag-Leffler function, and can be used for modeling non-ideal devices as well as extracting their characteristic parameters.
José Carlos Pérez-Martínez, Diego Martín-Martín, Belén Arredondo and Beatriz Romero
Unraveling Conductive Filament Formation in High Performance Halide Perovskite Memristor
Advanced Electronic Materials, 2400067, July, (2024), https://doi.org/10.1002/aelm.202400067
Abstract:
Halide perovskites (HPs) are promising materials for memristor devices because of their unique characteristics. In this study, nonvolatile resistive switching memory devices based on thick MAPbI3 perovskite (800 nm) films with structure FTO/MAPbI3/polymethyl methacrylate (PMMA)/Ag are presented. Reproducible and reliable bipolar switching characteristics are demonstrated with an ultra-low operating voltage (−0.1 V), high ON/OFF ratio (106), endurance (>2 × 103 times) and a record retention time (>105 s). The I–V curve of the first cycle exhibits self-formed conductive filaments. These are attributed to the presence of metallic Pb resulting from an excess of PbI2 in the perovskite film. The subsequent activation process involves the formation of conductive filaments, consisting of either iodide vacancies or migrated charged metals. Numerical simulations are then carried out to understand the nature of these conductive filaments and the role of the internal electric field in the migration of iodide ions, iodide vacancies, and Ag cations. Finally, an exhaustive model is proposed that explains the set and reset processes governing the first voltage cycle and the steady state, at different voltage ranges. In summary, this work offers a novel and thorough perspective of the complete resistive switching (RS) behavior in a MAPbI3/buffer/Ag memristor, supported by numerical simulations.
Enrique H. Balaguera and Anis Allagui
Limit capacitance of the constant phase element
Journal of Energy Storage 90, Part A, 111801 (2024), https://doi.org/10.1016/j.est.2024.111801
Abstract:
The constant-phase element (CPE) is a universal electrical model widely used to describe the intricate nature of a multitude of materials and processes under real-world conditions. The physical interpretation of the corresponding anomalous phenomenology is a challenging task, which traditionally relies on calculating an effective capacitance in the sense of a classical charge accumulation. However, a picture of this electrical element is not yet complete for cases of practical interest, and many questions remain open in relation to the intrinsic characteristics that makes it “unphysical” at long time scales. In this work, we derive mathematical formulas for estimating the limit capacitance of the CPE associated with surface and normal time-constant distributions. For this purpose, we obtain the transient responses, in term of multi-exponential relaxation patterns, attributable to the charge processes of micro-capacitances that constitute the “macroscopic CPE” with a dynamical behavior described in terms of the Mittag-Leffler function. As both transient dynamics can be considered negligible in practice from a certain time instant, we subsequently find the limit capacitance from a direct comparison of both steady-state times in the style of CPE reference works. Simulations are used to show that the obtained limit capacitance yields reasonable values for cases of multidisciplinary interest. Our study contributes to the advanced understanding of the pervasive presence of the CPE in natural and engineering contexts, shedding light on the problem of infinite charge and energy in complex systems.
Enrique H. Balaguera and Juan Bisquert
Accelerating the Assessment of Hysteresis in Perovskite Solar Cells
ACS Energy Letters 9, 478-486 (2024), https://doi.org/10.1021/acsenergylett.3c02779
Abstract:
Halide perovskite materials have reached important milestones in the photovoltaic field, positioning them as realistic alternatives to conventional solar cells. However, unavoidable kinetic phenomena have represented a major concern for reliable steady-state performance assessment from standard current–voltage measurements. In particular, the dynamic hysteresis of current–voltage curves requires relatively long stabilization to achieve a credible figure for the power conversion efficiency. Hysteresis is caused by complex current transient phenomena that become active during staircase voltammetry. Here, we address the root of this problem. We pinpoint the dynamic characteristics behind the slow transient responses to strategically predict a minimum time delay and thus estimate the power conversion efficiency under steady-state conditions. Circuit-element analysis and impedance spectroscopy confirm our predictions based on an advanced transient study. Our results fundamentally explore the possibility of reducing data time acquisition in a reliable performance assessment, providing disruptive solutions and perspectives toward systematic production of photovoltaic perovskites.
Dayron Armas, Ignacio R. Matías, M. Carmen López-González, Carlos Ruiz Zamarreño, Pablo Zubiate, Ignacio del Villar and Beatriz Romero
Generation of Lossy mode resonances (LMR) using perovskite nanofilms
Opto-Electron Adv 7, 230072 (2024), https://doi.org/10.29026/oea.2024.230072
Abstract:
The results presented here show for the first time the experimental demonstration of the fabrication of lossy mode resonance (LMR) devices based on perovskite coatings deposited on planar waveguides. Perovskite thin films have been obtained by means of the spin coating technique and their presence was confirmed by ellipsometry, scanning electron microscopy, and X-ray diffraction testing. The LMRs can be generated in a wide wavelength range and the experimental results agree with the theoretical simulations. Overall, this study highlights the potential of perovskite thin films for the development of novel LMR-based devices that can be used for environmental monitoring, industrial sensing, and gas detection, among other applications.
URL: https://www.oejournal.org//article/doi/10.29026/oea.2024.230072
Balasubramanian, S, Leon-Luna, MA, Romero, B, Madsen, M, Turkovic, V
Vitamin C for Photo-Stable Non-fullerene-acceptor-Based Organic Solar Cells
ACS Appl. Mater. Interfaces 2023, 15, 33, 39647–39656, https://doi.org/10.1021/acsami.3c06321
Abstract:
The recent advent of the new class of organic molecules, the so-called non-fullerene acceptors, has resulted in skyrocketing power conversion efficiencies of organic solar cells. However, rapid degradation occurs under illumination, particularly when photocatalytic metal oxide electron transport layers are used in these devices. We introduced vitamin C (ascorbic acid) into the organic solar cells as a photostabilizer and systematically studied its photostabilizing effect on inverted PBDB-T:IT-4F devices. The presence of vitamin C as an antioxidant layer between the ZnO electron transport layer and the photoactive layer strongly suppressed the photocatalytic effect of ZnO that induces NFA photodegradation. Upon 96 h of exposure to AM 1.5G 1 Sun irradiation, the reference devices lost 64% of their initial efficiency, while those containing vitamin C lost only 38%. The UV–visible absorption, impedance spectroscopy, and light-dependent voltage and current measurements reveal that vitamin C reduces the photobleaching of NFA molecules and suppresses the charge recombination. This simple approach using a low-cost, naturally occurring antioxidant, provides an efficient strategy for improving photostability of organic semiconductor-based devices.
Mari Carmen López-González, Gonzalo del Pozo, Belén Arredondo, Silvia Delgado, Diego Martín-Martín, Marina García-Pardo, Beatriz Romero.
Organic Electronics, Volume 120, 2023, 106843, https://doi.org/10.1016/j.orgel.2023.106843
Abstract:
In our study, we show that compositional engineering of the “A” site cation of ABX3 perovskite structure formed by a mix of organic and inorganic cations is an effective route to improve the thermal stability of perovskite solar cells (PSCs). In this work, mixed-cation mixed-halide PSCs have been fabricated and characterized with temperature, from 253 up to 333 K. The active layer based on CsRbFAMAPb(IBr)3 results in a more stable device compared to standard MAPbI3 devices. Electrical characterization reveals a decrease of the solar cell parameters with temperature. Using Impedance Spectroscopy (IS) characterization, we have estimated an activation energy for the halide ion migration of 0.63 ± 0.08 eV, an ion diffusion coefficient of 10−14 cm2 s−1, and a defect density of 7.27·1015 cm−3. To our knowledge, this is the first time that these parameters have been calculated in CsRbFAMAPb(IBr)3 based devices, resulting in improved values compared to MAPbI3 devices. The worsening of device performance for temperatures above 300 K is attributed to a decrease of the spiro-OMeTAD conductivity and the degradation of the perovskite/spiro-OMeTAD interface. It is shown that for low temperatures (from 253 to 323 K), Shockley-Red-Hall (SRH) recombination in the bulk governs, while for temperatures above 323 K the increase in surface recombination becomes dominant due to the presence of non-selective contacts.
Numerical simulations using SILVACO ATLAS corroborate the role of SRH in the perovskite active layer for low and medium temperatures, and the crucial influence of spiro-OMeTAD transport properties in the device performance parameters.
URL:https://www.sciencedirect.com/science/article/pii/S156611992300099X?via%3Dihub
Hernández-Balaguera, E., Bisquert, J.
Time Transients with Inductive Loop Traces in Metal Halide Perovskites
Adv. Funct. Mater. 2023, 2308678. https://doi.org/10.1002/adfm.202308678
Abstract: Metal halide perovskites are archetypal ionic-electronic materials with great prospects for optoelectronic applications. Among the rich variety of physics exhibited by ionic-electronic conduction, here, those most relevant to optoelectronic devices in which ionic mechanisms introduce a kinetic delay in the electronic phenomena are analyzed. The attention is focused on the inductive loop features and a dynamical model is developed to describe the corresponding complex multiscale dynamics in the time domain under experimental conditions, finally explaining the fundamental structure of the current transient responses in halide perovskite semiconductors. Based on complex capacitive and inductive patterns extensively studied in impedance measurements, an adequate interpretation of time domain methods capable of monitoring charge-carrier dynamics is produced. Therefore, this methodology identifies the characteristic parameters of all types of transient dynamics in metal halide perovskites, providing a suitable connection of correlated techniques, including impedance and chronoamperometric experiments, toward a robust interpretation of the device response. The scope of the method is fairly general, since these counterintuitive transient effects are observable not only in metal halide perovskites, but also in multiple materials and processes, mainly in different research fields pertaining to electrochemistry and electronics.
URL: https://onlinelibrary.wiley.com/doi/full/10.1002/adfm.202308678
Enrique Hernández-Balaguera,
Fractional model of the chemical inductor
Volume 172,, 2023,, 113470, ISSN 0960-0779, https://doi.org/10.1016/j.chaos.2023.113470.
Abstract: A multitude of materials and processes of a different nature frequently exhibit, in addition to the classical capacitive arcs, inductive loops in the complex impedance plane. It is a stable and physically robust response originated, from a mathematical perspective, by the introduction of a capacitive coupling in the slow relaxation variable. Nevertheless, the dynamical behavior of such systems shows, in reality, pronounced experimental deviations from the ideal inductive behavior. For this reason, the scientific community in general claims for the development of a novel theory that helps to phenomenologically interpret this pattern change, also encompassing the associated inherent physical complexity that capitalizes on the vast majority of real-world processes. Here, we present a generalization of the classical fast-slow models to naturally explain the anomalous dynamics observed experimentally in different types of measurement techniques, such as the inductance dispersion in impedance, the fractional relaxation processes with negative spike components in the transient responses, and the inverted current-voltage hysteresis. From numerical simulations, we analyze in detail the crossover dynamics from capacitive to inductive properties from the perspective of the constant phase element. Our work devises a useful theoretical framework that explains, in frequency and time domain, the coexistence of dispersive features and the transformation of the electrical behavior in terms of an anomalous bifractional crossover. Finally, we show the dominant role of the fractional-order dynamics in the appearance of inverted hysteresis commonly found in current-voltage curves. Although in literature, everything looks, in a certain manner, simple, the reality is that many real-world materials exhibit an intricate and complex nature that leads to anomalous processes evidenced by dispersive dynamics observed through different measurement techniques. This change of pattern analyzed here is appealing as a mathematical tool to interpret the physical phenomena of many familiar systems.
URL: https://www.sciencedirect.com/science/article/pii/S0960077923003715
Juan Bisquert, Agustín Bou, Antonio Guerrero, Enrique Hernández-Balaguera
Resistance transient dynamics in switchable perovskite memristors
APL Mach. Learn. 1 September 2023; 1 (3): 036101. https://doi.org/10.1063/5.0153289
Abstract: Memristor devices have been investigated for their properties of resistive modulation that can be used in data storage and brain-like computation elements as artificial synapses and neurons. Memristors are characterized by an onset of high current values under applied voltage that produces a transition to a low resistance state or successively to different stable states of increasing conductivity that implement synaptic weights. Here, we develop a nonlinear model to explain the variation with time of the voltage and the resistance and compare it to experimental results on ionic–electronic halide perovskite memristors. We find separate experimental signatures of the capacitive discharge and inductive current increase. We show that the capacitor produces an increase step of the resistance due to the influence of the series resistance. In contrast, the inductor feature associated with inverted hysteresis causes a decrease of the resistance, as observed experimentally. The chemical inductor feature dominates the potentiation effect in which the conductivity increases with the voltage stimulus. Our results enable a quantitative characterization of highly nonlinear electronic devices using a combination of techniques such as time transient decays and impedance spectroscopy.
URL: https://pubs.aip.org/aip/aml/article/1/3/036101/2900748
Hernández-Balaguera, E.; Martin-Martin, D.
Fractal Fract. 2023, 7(7), 516; https://doi.org/10.3390/fractalfract7070516
Abstract: One of the most promising emerging photovoltaic technologies is represented by perovskite materials essentially due to their outstanding performance. However, the complex fundamental understanding of relevant device physics is challenging, making it harder to obtain correlations with efficiency and long-term stability, and thus definitely transforming the landscape of solar energy. In electrical terms, perovskite solar cells often show different types of experimental behaviors at long timescales (light-enhanced capacitance and chemical inductor) in separate voltage domains, but with permanent deviations from the ideal pattern (Cole–Cole relaxation processes, fractional dynamics, and beyond). Here, we reevaluate the dynamical behavior of a photovoltaic perovskite model that leads to the two versions of constant-phase element behavior in the impedance response. Our general theory is, therefore, able to explain naturally the vast majority of results concerning the nonlinear polarization mechanisms of perovskite solar cells, extending the mathematical framework from the perspective of fractional-order electrical circuits. In this context, we discover a novel property that reveals the anomalous electrical coupling of memory effects in photovoltaic perovskites. We hope that this work can provide a useful tool for modeling experts and device physicists belonging to the photovoltaic community, moving forward toward addressing the outstanding challenges in this fast-developing field.
Enrique Hernández-Balaguera, Laura Munoz-Díaz, Agustín Bou, Beatriz Romer1, Baurzhan Ilyassov, Antonio Guerrero and Juan Bisquert
Enrique Hernández-Balaguera et al 2023 Neuromorph. Comput. Eng. 3 024005, DOI 10.1088/2634-4386/accec4
Abstract: Perovskite memristors have emerged as leading contenders in brain-inspired neuromorphic electronics. Although these devices have been shown to accurately reproduce synaptic dynamics, they pose challenges for in-depth understanding of the underlying nonlinear phenomena. Potentiation effects on the electrical conductance of memristive devices have attracted increasing attention from the emerging neuromorphic community, demanding adequate interpretation. Here, we propose a detailed interpretation of the temporal dynamics of potentiation based on nonlinear electrical circuits that can be validated by impedance spectroscopy. The fundamental observation is that the current in a capacitor decreases with time; conversely, for an inductor, it increases with time. There is no electromagnetic effect in a halide perovskite memristor, but ionic-electronic coupling creates a chemical inductor effect that lies behind the potentiation property. Therefore, we show that beyond negative transients, the accumulation of mobile ions and the eventual penetration into the charge-transport layers constitute a bioelectrical memory feature that is the key to long-term synaptic enhancement. A quantitative dynamical electrical model formed by nonlinear differential equations explains the memory-based ionic effects to inductive phenomena associated with the slow and delayed currents, invisible during the ‘off mode’ of the presynaptic spike-based stimuli. Our work opens a new pathway for the rational development of material mimesis of neural communications across synapses, particularly the learning and memory functions in the human brain, through a Hodgkin–Huxley-style biophysical model.
URL: https://onlinelibrary.wiley.com/doi/10.1002/adfm.202305211
J. C. Pérez-Martínez, D. Martín-Martín, G. d. Pozo, B. Arredondo, A. Guerrero and B. Romero
IEEE Electron Device Letters, vol. 44, no. 8, pp. 1276-1279, Aug. 2023, doi: 10.1109/LED.2023.3288298.
Abstract: Bias voltage scan rate and mobile ion concentration have a strong influence in J-V curves of metal halide perovskite-based memristors. In addition to hysteresis, in some cases J-V curves also show an anomalous drop in current known as negative differential resistance. This feature is usually related to electrochemical reactions between the reactive metal and I− ions, and to air exposure. However, in devices with low-reactive electrodes, its origin is still under debate. In this work, we propose a theoretical model based on ionic-electronic drift-diffusion. This model sheds light into the ionic-electronic processes that shape hysteresis, and it helps to explain the appearance and evolution of a negative resistance in memristors with low-reactive contacts and capacitive hysteresis. Finally, experimental J-V curves are presented to validate the proposed model.
URL: https://onlinelibrary.wiley.com/doi/10.1002/adfm.202305211
Pérez-Martínez, J. C., Berruet, M., Gonzales, C., Salehpour, S., Bahari, A., Arredondo, B., Guerrero, A.
Role of Metal Contacts on Halide Perovskite Memristors
Adv. Funct. Mater. 2023, 33, 2305211. https://doi.org/10.1002/adfm.202305211
Abstract: Halide perovskites are promising candidates for resistive memories (memristors) due to their mixed electronic/ionic conductivity and the real activation mechanism is currently under debate. In order to unveil the role of the metal contact and its connection with the activation process, four model systems are screened on halide perovskite memristors: Nearly inert metals (Au and Pt), low reactivity contacts (Cu), highly reactive contact (Ag and Al), and pre-oxidized metal in the form of AgI. It is revealed that the threshold voltage for activation of the memory effect is highly connected with the electrochemical activity of the metals. Redox/capacitive peaks are observed for reactive metals at positive potentials and charged ions are formed that can follow the electrical field. Activation proceeds by formation of conductive filaments, either by the direct migration of the charged metals or by an increase in the concentration of halide vacancies generated by this electrochemical reaction. Importantly, the use of pre-oxidized Ag+ ions leads to very low threshold voltages of ≈0.2 V indicating that an additional electrochemical reaction is not needed in this system to activate the memristor. Overall, the effect of the metal contact is clarified, and it is revealed that AgI is a very promising interfacial layer for low-energy applications.
URL: https://onlinelibrary.wiley.com/doi/10.1002/adfm.202305211
Enrique Hernández-Balaguera, Belén Arredondo, Carlos Pereyra, Mónica Lira-Cantú,
Journal of Power Sources, Volume 560, 2023, 232614, ISSN 0378-7753, https://doi.org/10.1016/j.jpowsour.2022.232614.
Abstract:
A better characterization of the rich variety of anomalous ionic-electronic mechanisms in organic-inorganic metal halide perovskite solar cells is essential to obtain a stable and physically robust interpretation of the dynamic responses obtained. Therefore, new approaches towards light intensity-induced effects understanding are intensively searched for. Among all the mechanisms whose elucidation is locked and still under live debate, the apparent inductance phenomena stand out, which are visible not only in photovoltaic devices and optoelectronic elements, but also, for instance, in electrochemical and biological systems. Usually, the negative loops in impedance spectra are modeled through ideal elements (negative capacitance or inductance) although the results show systematic deviations (constant-phase-element behavior). In most scenarios, the influence of chemical inductance dispersion is somehow neglected, that is, ideal conditions are mimicked, omitting the practical device operation. Here we reformulate the theory that captures the slow (non-electromagnetic) inductive effects in the current-voltage curves of perovskite solar cells, deciphering the microscale behavior, consisting essentially of defects associated with deep trap states, from macroscale observations and experimental measurements. The audience is potentially huge, since many authors of multidisciplinary backgrounds are genuinely interested in adequately interpreting this behavior of general character.
URL: https://www.sciencedirect.com/science/article/pii/S0378775322015919
Beatriz Romero, Silvia Delgado, Damian Glowienka, Chen-Tsung Chang, Gonzalo del Pozo, Belén Arredondo, Diego Martín-Martín, Pedro Contreras, Yulia Galagan
Sustainable Energy & Fuels, 2023, 7, 2146-2152 , https://doi.org/10.1039/D2SE01766K
Abstract:
Mixed-cation mixed-halide perovskite solar cells have been characterized in DC at different temperatures (from −20 °C up to 50 °C) and the time evolution of the device efficiency has been assessed using different degradation protocols (indoors and outdoors). The completely planar p–i–n structure is ITO/CuNiOx/PTAA/CsFAPbIBr/PCBM/PEI/Ag. Pristine current–voltage characteristics barely show hysteresis, at any temperature. Open circuit voltage decreases with temperature at a rate of −1.5 mV °C−1, and the obtained PCE temperature coefficient is lower than −0.001% K−1, which is an outstanding value for this emerging photovoltaic technology. Cells have been degraded under different protocols: indoors using different light/dark cycles and outdoors in a high temperature and high irradiation location. Cells show no significant decrease of the efficiency after more than 350 h of indoor light cycling and the estimated T80 obtained for the sample degraded outdoors under high irradiation and high temperature conditions is ∼15 days.
J. C. Pérez-Martínez, D. Martín-Martín, G. del Pozo, B. Arredondo, E. Hernández-Balaguera and B. Romero
IEEE Transactions on Electron Devices, vol. 69, no. 10, pp. 5624-5629, Oct. 2022, doi: 10.1109/TED.2022.3198386.
Abstract: The appearance of kinks in the fourth quadrant of current–voltage curves decreases dramatically the efficiency of photovoltaic solar cells (SCs). These S-shaped J -V curves have been successfully reproduced using different lumped-parameter equivalent circuits, composed of two or more diodes. In this work, Mazhari’s three-diode equivalent circuit is used to model the S-shaped J -V curves of perovskite SCs numerically simulated with Silvaco ATLAS TCAD. The simulated cell structure is fluorine-doped tin oxide (FTO)/TiO2/MAPbI3/ 2, 2′ ,7, 7′ -tetrakis [N,N-di (4-methoxyphenyl) amino]-9, 9′ -spirobifluorene (Spiro-OMeTAD)/Au and, in order to reproduce J – V curves with a kink, perovskite carrier mobility has been decreased below 0.25 cm2/(V ⋅ s) and defect density in the active layer has been increased above 1×1014 cm −3 . For each parameter, J – V curves have been simulated for different AM1.5 irradiation levels, from dark conditions up to 2 suns. For each device, a set of circuital parameters has been obtained independent of irradiation level. The dependence of Mazhari’s circuit parameters on both carrier mobility and defect density will be discussed.
URL: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9858674&isnumber=9901396
Enrique Hernández-Balaguera and Juan Bisquert
Negative Transient Spikes in Halide Perovskite
ACS Energy Letters, 7, 2602-2610, 2022.
Abstract: The internal crossfire of ionic and electronic effects in perovskite devices forms a complex analysis problem that has not been fully solved yet. Specifically, halide photovoltaic perovskites show a photoinduced ionic inductance behavior in current transient measurements, evidenced by ubiquitous negative spikes. Here, we provide a consolidated interpretation of these observed chemical mechanisms by independent measurement routes (frequency and time domain) in order to solve an elusive topic in the development of perovskite solar cells for more than a decade. From this operational pathway, we specifically study the light-dependent negative overshoot photocurrent phenomena in the time-domain discharge of the chemical inductor, which is a transversal mechanism found in a multitude of chemical, biological, and material systems. Our results establish a general framework to understand the inductive transient effects observable in new and important applications of halide perovskites, capable of emulating the electrical activity of neurons and synapses when acting as memristors.
L. Muñoz-Díaz, A.J. Rosa, A. Bou, R.S. Sánchez, B. Romero, R.A. John, M.V. Kovalenko, A. Guerrero and J. Bisquert
Frint, Energy Research, 10, 914115, 2022, https://doi.org/10.3389/fenrg.2022.914115
Abstract: The current–voltage curves of memristors exhibit significant hysteresis effects of use for information storage and computing. Here, we provide a comparison of different devices based on MAPbI3 perovskite with different contact configurations, from a 15% efficient solar cell to a pure memristor that lacks directional photocurrent. Current–voltage curves and impedance spectroscopy give insights into the different types of hysteresis, photocapacitance, and inductance present in halide perovskites. It is shown that both halide perovskite memristors and solar cells show a large inverted hysteresis effect at the forward bias that is related to the presence of a chemical inductor component in the equivalent circuit. Based on the results, we classify the observed response according to recombination current in devices with selective contacts, to voltage-activated single-carrier device conduction in devices with symmetric contacts. These findings serve to gain an understanding of the mechanism of memristor currents in mixed ionic-electronic conductors such as halide perovskites. We establish the link in the electrical response between solar cells and memristors.
URL: https://www.frontiersin.org/articles/10.3389/fenrg.2022.914115/full
M. Berruet, J.C. Pérez-Martínez, B. Romero, C. Gonzales, A.M. Al-Mayouf, A. Guerrero and J. Bisquert
Physical Model for the Current-Voltage Hysteresis and Impedance of Halide Perovskite Memristors
ACS Energy Lett. 2022, 7, 3, 1214–1222, Publication Date:March 1, 2022
https://doi.org/10.1021/acsenergylett.2c00121
Abstract:
An investigation of the kinetic behavior of MAPbI3 memristors shows that the onset voltage to a high conducting state depends strongly on the voltage sweep rate, and the impedance spectra generate complex capacitive and inductive patterns. We develop a dynamic model to describe these features and obtain physical insight into the coupling of ionic and electronic properties that produce the resistive switching behavior. The model separates the memristive response into distinct diffusion and transition-state-formation steps that describe well the experimental current–voltage curves at different scan rates and impedance spectra. The ac impedance analysis shows that the halide perovskite memristor response contains the composition of two inductive processes that provide a huge negative capacitance associated with inverted hysteresis. The results provide a new approach to understand some typical characteristics of halide perovskite devices, such as the inductive behavior and hysteresis effects, according to the time scales of internal processes.
URL: https://pubs.acs.org/doi/full/10.1021/acsenergylett.2c00121
E. Hernández-Balaguera, L. Muñoz-Díaz, C. Pereyra, M. Lira-Cantú, M. Najafi and Y. Galagan
Materials Today Energy, 27, 10103, 2022, doi: 10.1016/j.mtener.2022.101031
Abstract: The unprecedented increase of photovoltaic perovskites performance metrics has been obscured by the puzzling phenomenology underlying the device’s anomalous ionic/electronic landscape. Thus, the establishment of a reliable measurement protocol of efficiency measurements, unanimously adopted by the perovskite’s community, is indeed necessary to ensure comparability of device performance between different research laboratories. Here, we report a control strategy of ionic-electronic cooperative relaxation phenomena, eliminating the “hysteria around hysteresis”. The procedure, based on mixed engineering strategies, solves the problem that arises from the difficulty of providing steady-state device operation conditions, capturing in turn the anomalous capacitive behavior. From the determination of an optimal scan rate for J–V modeling, we provide an experimental visualization of the interplay between perovskites’ ionic and electronic responses. The results of this combined theoretical and experimental study identify key guidelines about assessment procedural of device performance that should be taken into account in future publications to further improve reproducibility in the research field.
B. Arredondo, J.C. Pérez-Martínez, L. Muñoz-Díaz, M.C. López-González, D. Martín-Martín, G. del Pozo, E. Hernández-Balaguera, J. Lamminaho, V. Turkovic and M. Madsen
Solar Energy, 232(7), 120-127, 2022, https://doi.org/10.1016/j.solener.2021.12.052
Abstract:The performance of organic solar cells has improved significantly in recent years due to the use of non-fullerene acceptors (NFA). While processing additives are typically added to the active layer blends to enhance device performance in NFA organic solar cells, their impact on device degradation remains unclear. In this work we have compared the performance, in pristine and degraded state, between air-processed slot-die coated NFA ITO-free organic solar cells with and without the processing additive DIO, using a structure of PET/Ag/ZnO/PBDB-T:ITIC/FHC PEDOT:PSS. We observed an improvement in the power conversion efficiency of the devices when adding DIO, from 4.03% up to 4.97%. The evolution of the performance for both devices under ISOS-L1 life testing protocol reveals that the drop in efficiency is mainly due to a decay of JSC for both cells. In the short time scale the efficiency of non-DIO cells decays faster than the DIO cells, whereas in the long time scale the efficiency of non-DIO cells tends to stabilize sooner. Carrier mobilities estimated from impedance measurements decrease with time at similar rate for both degraded samples. Besides, DIO devices present a steep increase of the series resistance with time causing a decrease of the FF and thus of the efficiency. Moreover, in both degraded devices, the open-circuit voltage saturates with increasing illumination intensity. Numerical simulations reveal that a reduced anode work function of 5 eV is needed to fit experimental data.
URL: https://www.sciencedirect.com/science/article/pii/S0038092X21011014
E. Hernández-Balaguera, B. Romero, M. Najafi and Y. Galagan
Analysis of Light‐Enhanced Capacitance Dispersion in Perovskite Solar Cells
Advanced Materials Interfaces, 9(9), 2102275, 2022, doi: 10.1002/admi.202102275
Abstract: Interface engineering has become one of the most important research strategies to harness the full potential of perovskite solar cells. Photovoltaic community seems to herald the imminent development of 30% efficient perovskite devices with competitive lifetimes, subject, among other aspects, to clarify “what happens under the interfaces.” One of the most powerful techniques to characterize the intermixing ionic-electronic processes that take place at perovskite interfaces is the Impedance Spectroscopy. It is a real fact that this methodology reveals the characteristic phenomenology attributed to interfacial phenomena, in form of complex spectra that interfere with a proper identification of dominant mechanisms. This article presents approaches to characterizing the non-ideal electrical properties associated with the appearance of an interfacial capacitance distribution, identifying major sources of intrinsic/extrinsic origin and so, providing further progress in the understanding of phenomenological mechanisms of photovoltaic perovskites. Importantly, the study addresses the evolution of dispersive capacitance’s characteristic parameters with light intensity (and bulk thickness), obtaining a useful alternative way of determining the ideality factor and thus, the recombination of charge carriers. The results are discussed in light of critical importance exhibited by the interactions between ionic and electronic species, with the aim of advancing in device efficiency and stability.
M.C. López-González, G. del Pozo, D. Martín-Martín, L. Muñoz-Díaz, J.C. Pérez-Martínez, E. Hernández-Balaguera, B. Arredondo, Y. Galagan, M. Najafi and B. Romero
Applied Sciences, 11(24), 11668, 2021, https://doi.org/10.3390/app112411668
Abstract: Perovskite solar cells (PSCs) have become very popular due to the high efficiencies achieved. Nevertheless, one of the main challenges for their commercialization is to solve their instability issues. A thorough understanding of the processes taking place in the device is key for the development of this technology. Herein, J-V measurements have been performed to characterize PSCs with different active layer thicknesses. The solar cells’ parameters in pristine devices show no significant dependence on the active layer thickness. However, the evolution of the solar cells’ efficiency under ISOS-L1 protocol reveals a dramatic burn-in degradation, more pronounced for thicker devices. Samples were also characterized using impedance spectroscopy (IS) at different degradation stages, and data were fitted to a three RC/RCPE circuit. The low frequency capacitance in the thickest samples suffers a strong increase with time, which suggests a significant growth in the mobile ion population. This increase in the ion density partially screens the electric field, which yields a reduction in the extracted current and, consequently, the efficiency. This paper has been validated with two-dimensional numerical simulations that corroborate (i) the decrease in the internal electric field in dark conditions in 650 nm devices, and (ii) the consequent reduction in the carrier drift and, therefore, of the effective current extraction and efficiency.
M. A. Amado-Briseño, O. J. Hernández-Ortíz , M. A. Veloz, K. Alemán, G. del Pozo, B. Romero, A. G. Hernández, A. Espinosa and R. A. Vázquez-García
Journal of Materials Science: Materials in Electronics, 2021, doi: 10.1007/s10854-021-07271-w
Abstract: In this work is report the synthesis by mechanochemical method of a small molecule of the oligophenylene (bisquinoline) design (OBM), excellent optical and electrical properties, and potential use as an electroluminescent material in the form of a nano-film in the manufacture of an organic light-emitting diode (OLED). OBM was synthesized by a Knoevenagel condensation reaction and was chemically characterized by 1H-NMR, 13C-NMR, FT-IR spectroscopy, and AccuTOF-DART mass spectrometry. UV–Vis and fluorescence spectroscopies were used to obtain the optical properties of OBM, both in solution and in film. Also, the OBM film was examined by atomic force microscopy and showed a high degree of homogeneity which allowed the manufacture of an OLED device with ITO/OBM/PEDOT: PSS/Al configuration with a luminance of 2350 cd/m2 when supplied with a fixed current of 15 mA and 10 volts.
Enrique Hernández-Balaguera,
Chaos, Solitons & Fractals, Volume 145, 2021, 110787, ISSN 0960-0779, https://doi.org/10.1016/j.chaos.2021.110787.
Abstract: In this paper, we analyze the electrical response of an electrode-tissue-electrode system to the application of a dc current for a sufficiently short time in order to obtain coulostatic conditions: A finite amount of charge is “instantaneously” and efficiently transferred to the capacitors formed by biological membranes at the tissue level and the electrode biointerfacial regions. To allow a more realistic study, the capacitances formed by the electrode-tissue interfaces and those of the cell membranes were modeled using constant phase elements (CPEs). The mathematical expressions for the current, voltage, and charge of the CPEs are obtained in response to the sudden injection of the controlled electric charge. It is predicted theoretically how, under certain conditions, the current path could be restricted to flow through the capacitors formed by the electrode-tissue interfaces and those of the cell membranes, and thus, the total charge injected is practically transferred to both types of capacitance (i.e., a coulostatic charge injection). Finally, we study the influence of the pulse shape (retaining the coulostatic nature) on the technique, from the theoretical perspective of the fractional calculus. The shape of the excitation signal is shown to play a dominant role in the coulostatic relaxation processes, in sharp contrast to the conventional approach. This methodology could be extended to include the membranes of organelles and also to implement a coulostatic test method involving electrical characterizations of biological tissues.
URL: https://www.sciencedirect.com/science/article/pii/S0960077921001399
Enrique Hernández-Balaguera,
Chaos, Solitons & Fractals, Volume 145, 2021, 110768, ISSN 0960-0779, https://doi.org/10.1016/j.chaos.2021.110768.
Abstract:
This paper discusses the complexity of distributed relaxation processes in biological systems, particularly with regard to the slowest timescale phenomena that influence the modeling of physiological events. Specifically, our main interest is to determine the optimal excitation time at which the transient response, described in terms of relatively slow decays and memory effects, can be considered negligible. We estimate the time scale required for the Mittag-Leffler function to reach and stay within a range about the final value (dc “pseudo-steady state”). From numerical computations, we consider the problem of approximating holding times with common and rational (Padé-type) asymptotic approximations for comparative purposes. It is important to understand the physiological processes and to explore new mathematical models, based on efficient approximations, in order to design safe, controllable, and effective protocols for the electrical stimulation of excitable cells and the characterization of biological tissues.
URL: https://www.sciencedirect.com/science/article/pii/S096007792100120X
E. Hernández-Balaguera, G. del Pozo, B. Arredondo, B. Romero, C. Pereyra, H. Xie and M. Lira-Cantú
Solar RRL, 5(4), 2000707, 2021, https://doi.org/10.1002/solr.202000707
Selected as back cover of the issue: Solar RRL, 5(4), 202170044, 2021
Abstract: Capacitive response at long time scales seems to remain an elusive feature in the analysis of the electrical properties of perovskite-based solar cells. It belongs to one of the critical anomalous effects that arises from the characteristic phenomenology of this type of emerging photovoltaic devices. Thereby, accurately deducing key capacitance feature of new light harvesting perovskites from electrical measurements represents a significant challenge regarding the interpretation of physical processes and the control of undesired mechanisms, such as slow dynamic effects and/or current density–voltage (J–V) hysteresis. Herein, it is shown that long timescale mechanisms that give rise to hysteresis in stable and high-efficiency quadruple-cation perovskites are not due to a classical capacitive behavior in the sense of ideal charge accumulation processes. Instead, it is a phenomenological consequence of slow memory-based capacitive currents and the underlying cooperative relaxations. A fractional dynamics approach, based on the idea of capacitance distribution in perovskite devices, reliably models the slow transient phenomena and the consequent scan-rate- and bias-dependent hysteresis. Observable for a wide variety of photovoltaic halide perovskites, distributed capacitive effects are rather universal anomalous phenomena, which can be related to the long-time electrical response and hysteresis.
URL: https://onlinelibrary.wiley.com/doi/full/10.1002/solr.202000707
E. Hernández-Balaguera, B. Romero, B. Arredondo, G. del Pozo, M. Najafi and Y. Galagan
Nano Energy, 78, 105398, 2020, https://doi.org/10.1016/j.nanoen.2020.105398
Abstract:
The potential opportunities of perovskite-based technology to further advance in the photovoltaic field and implement an extensive industrial application, will mainly depend on whether some of its weaknesses, such as current density-voltage (J−V) hysteresis, can be overcome. In this sense, a critical aspect is the understanding and the subsequent accurate description of hysteresis in perovskite cells. Here, we present the theoretical underpinnings to interpret non-ideal transient dynamics in stepwise-JV measurements, using electrical analysis strategies and fractional calculus tools. Our model was validated with experimental measurements of CsFAPbIBr-based photovoltaic perovskites of different active layer thicknesses with persistent long-range time photocurrents. It is reported that hysteresis phenomena increase with more pronounced non-ideal capacitive effects suggesting that more trapping events limit ion motion and carrier transport. Our main interest is to provide valuable information about the memory-based slow timescale dynamics, which exhibits a significant impact on the appearance of hysteresis and, to a large extent, on the operation of the solar cell.
URL: https://www.sciencedirect.com/science/article/pii/S2211285520309757
B. Arredondo, G. del Pozo, E. Hernández-Balaguera, D. Martín-Martín, M. C. López-González, B. Romero, E. López-Fraguas, R. Vergaz, X. Quintana, J. Lamminaho, E. Destouesse, M. Ahmadpour, V. Turkovic and M. Madsen
ACS Applied Energy Materials, 3 (7), 6476–6485, 2020, https://doi.org/10.1021/acsaem.0c00711
Abstract: In this work, we have studied degradation mechanisms of nonfullerene-based organic solar cells with PET/Ag/ZnO/PBDTB-T:ITIC/PEDOT:PSS/CPP PEDOT:PSS device structure. We compare pristine and degraded samples that were subjected to outdoor degradation following the standard ISOS-O2 protocol. The ideality factors for different incident wavelengths obtained from open-circuit voltage vs irradiation level and current density–voltage (J–V) measurements at different temperatures indicate that for aged samples recombination is governed by the Shockley–Read–Hall mechanism occurring in a region near the anode. Samples were also characterized using impedance spectroscopy (IS) and fitted to an electrical model. Impedance parameters were used to obtain mobility, indicating a clear degradation of the active layer blend for aged samples. The change in the chemical capacitance also reveals a worsening in carrier extraction. Finally, two-dimensional (2D) numerical simulations and fits to experimental J–V curves confirm the existence of a layer near the anode contact with poorer mobility and a decrease in the anode work function (WF) for the degraded samples.
E. Hernández-Balaguera, B. Arredondo, G. del Pozo and B. Romero
Communications in Nonlinear Science and Numerical Simulation, 90, 105371, 2020, https://doi.org/10.1016/j.cnsns.2020.105371
Abstract: Perovskite-based solar cells are devices of increasing interest among new generation of photovoltaic technologies due to the outstanding power conversion efficiency (PCE) achieved in the last few years, and the favorable fabrication conditions. Among all these excellent photovoltaic properties, perovskite light-harvesting devices also show relevant issues such as the hysteresis mechanisms in the current density-voltage (J–V) characteristics, whose underlying origins have not yet been clarified from a single and universally accepted point of view. Here, we focus our attention on the ideal capacitive mechanisms which, on the other hand, are often modeled using fractional-order capacitors in the equivalent circuits used to fit impedance measurements. In particular, we provide the theoretical framework for the transient-photocurrent analysis described by fractional-order responses which involve the generalized Mittag-Leffler function. Importantly, the model introduces a connection between perovskite traps and defects, memory processes, fractional dynamics, and Cole-Cole behavior. Crucial dependences of non-ideal capacitive dynamics on fractional-order α and the selected scan rate were found in the J–V hysteresis behavior from the perspective of the non-integer order calculus. In this sense, it is necessary to point out that our numerical simulations of dark J–V curves by considering non-ideal capacitive effects reveal more prominent distortions than those of the ideal case, leading from free-hysteretic influences (α = 1) to significant hysteresis distortions (0<α<1) under certain voltage rates. Thus, this study can help to advance in the origin and the understanding of the experimental hysteresis mechanisms under pristine conditions or during degradation processes (α value decreases indicating a highly disordered morphology).
URL: https://www.sciencedirect.com/science/article/pii/S1007570420302033
E. Hernández-Balaguera, J.L. Polo
Enabling selective absorption in perovskite solar cells for refractometric sensing of gases
Volume 856,, 2020, 113631, ISSN 1572-6657,, doi: https://doi.org/10.1016/j.jelechem.2019.113631.
Abstract: Perovskite solar cells are currently considered a promising technology for solar energy harvesting. Their capability to deliver an electrical signal when illuminated can sense changes in environmental parameters. We have numerically analyzed the variation of the current delivered by a perovskite cell as a function of the index of refraction of air, that is in contact with the front surface of the cell. This calculation identifies which geometrical and material structures enhance this behavior. After replacing the top transparent electrode of a solar cell by an optimized subwavelength metallic grating, we find a large variation in the responsivity of the cell with respect to the change in the index of refraction of the surrounding medium. Such a refractometric sensor can be interrogated electronically, avoiding the cumbersome set-ups of spectral or angular interrogation methods. We present an adaptation of the performance parameters of refractometric sensors (sensitivity and figure of merit) to the case of opto-electronic interrogation methods. The values of sensitivity and Figure of Merit are promising for the development of refractometric perovskite-based sensors.
URL: https://www.sciencedirect.com/science/article/pii/S1572665719308999
M. H. Elshorbagy, A. Cuadrado, B. Romero and J. Alda
On the potential-step hold time when the transient-current response exhibits a Mittag-Leffler decay
Scientific Reports, 10, 7761, 2020.
We present the theoretical underpinnings for the use of a constant phase element (CPE), along with the potential-step test method (PSM), to determine the parameter values of a CPE-RP Randles-type circuit model from the transient-current response. Specifically, we analyze the potential-step hold time covering the fractional-order dynamic transition between the sufficiently short times, in which the current measured is practically due to the non-ideal interfacial capacitance, and the longer times corresponding to a current practically faradaic. An interface of biomedical interest consisting of a gold electrode immersed in a 0.9% NaClphysiological saline solution was analyzed using both PSM and impedance measurements.