Inductive Loop in the Impedance Response of Perovskite Solar Cells Explained by Surface Polarization Model

The dual nature of metal halide perovskites

Metal halide perovskites have brought about a disruptive shift in the field of third-generation photovoltaics. Their potential as remarkably efficient solar cell absorbers was first demonstrated in the beginning of the 2010s. However, right from their inception, persistent challenges have impeded the smooth adoption of this technology in the industry. These challenges encompass issues such as the lack of reproducibility in fabrication, limited mid- and long-term stability, and concerns over toxicity. Despite achieving record efficiencies that have outperformed even well-established technologies, such as polycrystalline silicon, these hurdles have hindered the seamless transition of this technology into industrial applications. In this Perspective, we discuss which of these challenges are rooted in the unique dual nature of metal halide perovskites, which simultaneously function as electronic and ionic semiconductors. This duality results in the intermingling of processes occurring at vastly different timescales, still complicating both their comprehensive investigation and the development of robust and dependable devices. Our discussion here undertakes a critical analysis of the field, addressing the current status of knowledge for devices based on halide perovskites in view of electronic and ionic conduction, the underlying models, and the challenges encountered when these devices are optoelectronically characterized. We place a distinct emphasis on the positive contributions that this area of research has not only made to the advancement of photovoltaics but also to the broader progress of solid-state physics and photoelectrochemistry.

High-Performance and Stable Perovskite X-ray Detection and Imaging Based on a Ti Cathode

High-energy radiation detectors with a good imaging resolution, fast response, and high sensitivity are desired to operate at a high electric field. However, strong ion migration triggered by electrochemical reactions at the interface between a high-potential electrode and an organic-inorganic hybrid perovskite limits the stability of radiation detectors under a high electric field. Herein, we demonstrate that such ion migration could be effectively suppressed in devices with a Ti cathode, even at a high electric field of 50 V mm-1, through time-of-flight secondary-ion mass spectrometry. X-ray photoelectron spectroscopy illustrates that Ti-N bonds formed at the interface of MAPbBr3 perovskite single crystals/Ti electrode effectively inhibit the electrochemical reaction in organic-inorganic hybrid perovskite devices and ultimately improve the operating stability under a high electric field. The device with a Ti electrode reaches a high sensitivity of 96 ± 1 mC Gyair-1 cm-2 and a low detection limit of 2.8 ± 0.3 nGy s-1 under hard X-ray energy.

Accelerating the Assessment of Hysteresis in Perovskite Solar Cells

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.

Investigation of the Role of K2SO4 Electrolyte in Hole Transport Layer for Efficient Quasi-2D Perovskite Light-Emitting Diodes

Quasi-two-dimensional (2D) perovskite light-emitting diodes are promising light sources for color display and lighting. However, poor carrier injection and transport between the bottom hole transport layer (HTL) and perovskite limit the device performance. Here we demonstrate a simple and effective way to modify the HTL for enhancing the performance of perovskite light-emitting diodes (PeLEDs). An electrolyte K2SO4 is used to mix with poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as the hole transport layer. The K+ doping helped the quasi-2D perovskite phases grow vertically along the interface of the PEDOT:PSS, fine-modulate the phase distribution, and simultaneously reduce the defect density of quasi-2D perovskites. It also significantly reduced the exciton quenching and injection barrier at PEDOT:PSS and quasi-2D perovskite interface. The optimized green PeLEDs with the K2SO4 doped PEDOT:PSS HTL showed a maximum luminance of 17185 cd/m2 which is almost 4.7 times brighter than the control one, with a maximum external quantum efficiency of 18.64%.

Enhanced LED Performance by Ion Migration in Multiple Quantum Well Perovskite

Here we study the effect of ion migration on the performance of perovskite light emitting diodes (PeLEDs). We compared aromatic and linear barrier molecules in Ruddlesden-Popper and Dion-Jacobson two-dimensional perovskites having multiple quantum well (MQW) structures. PeLED devices were fabricated by using the same conditions and architecture, while their electroluminescence properties and ion migration behavior were investigated. Impedance spectroscopy measurements were used to analyze the PeLEDs, which found a direct link between the barrier molecule type, the device efficiency, and ion migration. The best performing LEDs were based on the aromatic barriers, which present dominant inductive impedance, indicating an earlier onset voltage of radiative recombination. These findings present an approach of how to control radiative emission in perovskite LEDs which opens the way for further improvement in PeLEDs and memristors.

Ionic and electronic polarization effects in horizontal hybrid perovskite device structures close to equilibrium

The electrical response of hybrid perovskite devices carries a significant signature from mobile ionic defects, pointing to both opportunities and threats when it comes to functionality, performance and stability of these devices. Despite its importance, the interpretation of polarization effects due to the mixed ionic-electronic conducting nature of these materials and the quantification of their ionic conductivities still poses conceptual and practical challenges, even for the equilibrium situation. In this study, we address these questions and investigate the electrical response of horizontal devices based on methylammonium lead iodide (MAPI) close to equilibrium conditions. We discuss the interpretation of DC polarization and impedance spectroscopy measurements in the dark, based on calculated and fitted impedance spectra obtained using equivalent circuit models that account for the mixed conductivity of the perovskite and for the effect of device geometry. Our results show that, for horizontal structures with a gap width between the metal electrodes in the order of tens of microns, the polarization behavior of MAPI is well described by the charging of the mixed conductor/metal interface, suggesting a Debye length in the perovskite close to 1 nm. We highlight a signature in the impedance response at intermediate frequencies, which we assign to ionic diffusion in the plane parallel to the MAPI/contact interface. By comparing the experimental impedance results with calculated spectra for different circuit models, we discuss the potential role of multiple mobile ionic species and rule out a significant contribution from iodine exchange with the gas phase in the electrical response of MAPI close to equilibrium. This study helps to clarify the measurement and interpretation of mixed conductivity and polarization effects in hybrid perovskites with immediate relevance to the characterization and development of transistors, memristors and solar cells based on this class of materials as well as other mixed conductors.

Electrical Charge Coupling Dominates the Hysteresis Effect of Halide Perovskite Devices

Hysteresis effects in ionic-electronic devices are a valuable resource for the development of switching memory devices that can be used in information storage and brain-like computation. Halide perovskite devices show frequent hysteresis in current-voltage curves that can be harnessed to build effective memristors. These phenomena can be often described by a set of highly nonlinear differential equations that involve current, voltage, and internal state variables, in the style of the famous Hodgkin-Huxley model that accounts for the initiation and temporal response of action potentials in biological neurons. Here we extend the neuron-style models that lead to chemical inductors by introducing a capacitive coupling in the slow relaxation variable. The extended model is able to explain naturally previous observations concerning the transition from capacitor to inductor in impedance spectroscopy of MAPbBr solar cells and memristors in the dark. The model also generates new types of oscillating systems by the generation of a truly negative capacitance distinct from the usual inductive effect.

Correlation between hysteresis dynamics and inductance in hybrid perovskite solar cells: studying the dependency on ETL/perovskite interfaces

In this study, to elucidate the origin of inductance and its relationship with the phenomenon of hysteresis in hybrid perovskite solar cells (PSCs), two electron transport layer (ETL) structures have been utilized: (a) rutile titania nanorods grown over anatase titania (AR) and (b) anatase titania covering the rutile titania nanorods (RA). The rutile and anatase phases are prepared via hydrothermal synthesis and spray pyrolysis, respectively. PSCs based on an ETL with an RA structure attain higher short-circuit current density (J_SC) and open-circuit voltage (V_OC) while showing a slightly lower fill factor (FF) compared with their AR counterparts. Using electrochemical impedance spectroscopy (EIS) measurements, we show that the ETL plays a major role in setting the tone for ionic migration speed and consequent accumulation. Moreover, we consider the conductivity of transport layers as a determining factor in not only giving rise to inductive features but also dictating the bias region under which recombination takes place, ultimately influencing hysteresis locus.

Application of impedance spectroscopy in exploring electrical properties of dielectric materials under high pressure

Impedance spectroscopy (IS) is an indispensable method of exploring electrical properties of materials. In this review, we provide an overview on the specific applications of IS measurement in the investigations of various electrical properties of materials under high pressure, including electric conduction in bulk and grain boundary, dielectric properties, ionic conduction, and electrostrictive effect. Related studies are summarized to demonstrate the method of analyzing different electrical transport processes with various designed equivalent circuits of IS and reveal some interesting phenomena of electrical properties of materials under high pressure.

Understanding equivalent circuits in perovskite solar cells. Insights from drift-diffusion simulation

Perovskite solar cells (PSCs) have reached impressively high efficiencies in a short period of time; however, the optoelectronic properties of halide perovskites are surprisingly complex owing to the coupled ionic-electronic charge carrier dynamics. Electrical impedance spectroscopy (EIS) is a widely used characterization tool to elucidate the mechanisms and kinetics governing the performance of PSCs, as well as of many other semiconductor devices. In general, equivalent circuits are used to evaluate EIS results. Oftentimes these are justified via empirical constructions and the real physical meaning of the elements remains disputed. In this perspective, we use drift-diffusion numerical simulations of typical thin-film, planar PSCs to generate impedance spectra avoiding intrinsic experimental difficulties such as instability and low reproducibility. The ionic and electronic properties of the device, such as ion vacancy density, diffusion coefficients, recombination mechanism, etc., can be changed individually in the simulations, so their effects can be directly observed. We evaluate the resulting EIS spectra by comparing two commonly used equivalent circuits with series and parallel connections respectively, which result in two signals with significantly different time constants. Both circuits can fit the EIS spectra and by extracting the values of the elements of one of the circuits, the values of the elements of the other circuit can be unequivocally obtained. Consequently, both can be used to analyse the EIS of a PSC. However, the physical meaning of each element in each circuit could differ. EIS can produce a broad range of physical information. We analyse the physical interpretation of the elements of each circuit and how to correlate the elements of one circuit with the elements of the other in order to have a direct picture of the physical processes occurring in the device.

Evaluating the Capacitive Response in Metal Halide Perovskite Solar Cells

The perovskites solar cells (PSCs) is composed of multifaceted device architecture and involve complex charge extraction (both electronic and ionic), this makes the task demanding to unlock the origin of the different physical process that occurs in a PSC. The capacitance in PSCs depends on several external perturbations including frequency, illumination, temperature, applied bias, and importantly on the interface modification of perovskites/charge selective contact. Arguably, different features including interfacial and bulk; ionic, and electronic charge transport in PSCs occur at different time scales. Capacitance spectroscopy is a prevailing technique to unravel the various physical phenomenon that occurs in a PSC at different time scales. A deeper knowledge of the capacitive response of a PSCs is essential to understand the charge carrier kinetics and unlock the device physics. This work highlights the capacitive response of PSCs and its application to unlock the device physics which is essential for the further optimization and improvement of the device performance.

Impedance Spectroscopy of Metal Halide Perovskite Solar Cells from the Perspective of Equivalent Circuits

Impedance spectroscopy (IS) provides a detailed understanding of the dynamic phenomena underlying the operation of photovoltaic and optoelectronic devices. Here we provide a broad summary of the application of IS to metal halide perovskite materials, solar cells, electrooptic and memory devices. IS has been widely used to characterize perovskite solar cells, but the variability of samples and the presence of coupled ionic-electronic effects form a complex problem that has not been fully solved yet. We summarize the understanding that has been obtained so far, the basic methods and models, as well as the challenging points still present in this research field. Our approach emphasizes the importance of the equivalent circuit for monitoring the parameters that describe the response and providing a physical interpretation. We discuss the possibilities of models from the general perspective of solar cell behavior, and we describe the specific aspects and properties of the metal halide perovskites. We analyze the impact of the ionic effects and the memory effects, and we describe the combination of light-modulated techniques such as intensity modulated photocurrent spectroscopy (IMPS) for obtaining more detailed information in complex cases. The transformation of the frequency to time domain is discussed for the consistent interpretation of time transient techniques and the prediction of features of current-voltage hysteresis. We discuss in detail the stability issues and the occurrence of transformations of the sample coupled to the measurements.

Technology agnostic frequency characterization methodology for memristors

Over the past decade, memristors have been extensively studied for a number of applications, almost exclusively with DC characterization techniques. Studies of memristors in AC circuits are sparse, with only a few examples found in the literature, and characterization methods with an AC input are also sparingly used. However, publications concerning the usage of memristors in this working regime are currently on the rise. Here we propose a "technology agnostic" methodology for memristor testing in certain frequency bands. A measurement process is initially proposed, with specific instructions on sample preparation, followed by an equipment calibration and measurement protocol. This article is structured in a way which aims to facilitate the usage of any available measurement equipment and it can be applied on any type of memristive technology. The second half of this work is centered around the representation of data received from following this process. Bode plot and Nyquist plot representations are considered and the information received from them is evaluated. Finally, examples of expected behaviors are given, characterizing simulated scenarios which represent different internal device models and different switching behaviors, such as capacitive or inductive switching. This study aims at providing a cohesive way for memristor characterization, to be used as a good starting point for frequency applications, and for understanding physical processes inside the devices, by streamlining the measuring process and providing a frame in which data representation and comparison will be facilitated.

Impedance Spectroscopy Dynamics of Biological Neural Elements: From Memristors to Neurons and Synapses

Understanding the operation of neurons and synapses is essential to reproducing biological computation. Building artificial neuromorphic networks opens the door to a new generation of faster and low-energy-consuming electronic circuits for computation. The main candidates to imitate the natural biocomputation processes, such as the generation of action potentials and spiking, are memristors. Generally, the study of the performance of material neuromorphic elements is done by the analysis of time transient signals. Here, we present an analysis of neural systems in the frequency domain by small-amplitude ac impedance spectroscopy. We start from the constitutive equations for the conductance and memory effect, and we derive and classify the impedance spectroscopy spectra. We first provide a general analysis of a memristor and demonstrate that this element can be expressed as a combination of simple parts. In particular, we derive a basic equivalent circuit where the memory effect is represented by an RL branch. We show that this ac model is quite general and describes the inductive/negative capacitance response in many systems such as halide perovskites and organic LEDs. Thereafter, we derive the impedance response of the integrate-and-fire exponential adaptative neuron model that introduces a negative differential resistance and a richer set of spectra. On the basis of these insights, we provide an interpretation of the varied spectra that appear in the more general Hodgkin-Huxley neuron model. Our work provides important criteria to determine the properties that must be found in material realizations of neuronal elements. This approach has the great advantage that the analysis of highly complex phenomena can be based purely on the shape of experimental impedance spectra, avoiding the need for specific modeling of rather involved material processes that produce the required response.

Exploring Transport Behavior in Hybrid Perovskites Solar Cells via Machine Learning Analysis of Environmental-Dependent Impedance Spectroscopy

Hybrid organic-inorganic perovskites are one of the promising candidates for the next-generation semiconductors due to their superlative optoelectronic properties. However, one of the limiting factors for potential applications is their chemical and structural instability in different environments. Herein, the stability of (FAPbI₃ )_0.85 (MAPbBr₃ )_0.15 perovskite solar cell is explored in different atmospheres using impedance spectroscopy. An equivalent circuit model and distribution of relaxation times (DRTs) are used to effectively analyze impedance spectra. DRT is further analyzed via machine learning workflow based on the non-negative matrix factorization of reconstructed relaxation time spectra. This exploration provides the interplay of charge transport dynamics and recombination processes under environment stimuli and illumination. The results reveal that in the dark, oxygen atmosphere induces an increased hole concentration with less ionic character while ionic motion is dominant under ambient air. Under 1 Sun illumination, the environment-dependent impedance responses show a more striking effect compared with dark conditions. In this case, the increased transport resistance observed under oxygen atmosphere in equivalent circuit analysis arises due to interruption of photogenerated hole carriers. The results not only shed light on elucidating transport mechanisms of perovskite solar cells in different environments but also offer an effective interpretation of impedance responses.

Theory of Hysteresis in Halide Perovskites by Integration of the Equivalent Circuit

Perovskite solar cells show a number of internal electronic-ionic effects that produce hysteresis in the current-voltage curves and a dependence of the temporal response on the conditions of the previous stimulus applied to the sample. There are many models and explanations in the literature, but predictive methods that may lead to an assessment of the solar cell behavior based on independent measurements are needed. Here, we develop a method to predict time domain response starting from the frequency domain response measured by impedance spectroscopy over a collection of steady states. The rationale of the method is to convert the impedance response into a set of differential equations, in which the internal state variables emerge naturally and need not be predefined in terms of a physical (drift/diffusion/interfaces) model. Then, one solves (integrates) the evolution for a required external perturbation such as voltage sweep at a constant rate (cyclic voltammetry). Using this method, we solve two elementary but relevant equivalent circuit models for perovskite solar cells and memristors, and we show the emergence of hysteresis in terms of the relevant time and energy constants that can be fully obtained from impedance spectroscopy. We demonstrate quantitatively a central insight in agreement with many observations: regular hysteresis is capacitive, and inverted hysteresis is inductive. Analysis of several types of perovskite solar cells shows excellent correlation of the type of equivalent circuit and the observed hysteresis. A new phenomenon of transformation from capacitive to inductive hysteresis in the course of the current-voltage curve is reported.

The curious case of ion migration in solid-state and liquid electrolyte-based perovskite devices: unveiling the role of charge accumulation and extraction at the interfaces

Electrochemical impedance spectroscopy (EIS) has been extensively used for the detailed investigation and understanding of the plethora of physical properties of variegated electrochemical and solid-state systems. Over the past few years, EIS has revealed many significant findings in hybrid halide perovskite (HHP)-based optoelectronic devices too. Photoinduced ion-migration, negative capacitance, anomalous mid-frequency capacitance, hysteresis, and instability to heat, light and moisture in HHP-based devices are among the few issues addressed by the IS technique. However, performing EIS in perovskite devices presents new challenges related to multilayer solid-state device geometry and complicated material properties. The ions in the perovskite behave in a specified manner, which is dictated by the energy-levels of the transport layer. Electronic-ionic coupling is one of the major challenges to understand ion transport kinetics in solid-state devices. In this work, we have performed impedance measurements in both solid-state (S-S) and liquid-electrolyte (L-E) device geometry to unfold the effect of charge transport layers on the ac ionic conductivity in perovskite materials. We have modelled the impedance spectra using the electrical equivalent circuit (EEC) and compared the behaviour of ions in different controlling environments. It was concluded that the AC as well as dc ionic conductivity and the accumulation of ions in the perovskite material are highly influenced by the nature of the interface in different device geometry. Charge accumulation in the S-S device gives rise to large polarisation, thereby negative capacitance or any inductive loop can be observed in the Nyquist plot while in the L-E device the presence of an electric double layer at the perovskite/electrolyte interface reduces the surface polarisation effect. Ionic conductivity is hopping limited in the low field regime and diffusion limited in the high field regime in the S-S device. Moreover, the perovskite/electrolyte based devices are promising candidates for electrolyte gated field-effect transistors, perovskite-based supercapacitors and electrochemical cells for water splitting or CO₂ reduction.

Mechanistic origin and unlocking of negative capacitance in perovskites solar cells

We have unlocked the mechanistic behavior of negative capacitance in perovskite solar cells (PSCs) by analyzing impedance spectra at variable photovoltage and applied bias, temperature-dependent capacitance versus frequency (C-f) spectra, and current-voltage (J-V) characteristics. We noted that p-i-n type PSCs having PEDOT:PSS or PTAA as hole transport layer display negative capacitance feature at low and intermediate frequencies. The activation energies (E_a) for the observance of negative capacitance were found to be in a similar order of magnitude required for the ionic migration. Moreover, the kinetic relaxation time (τ_kin) estimated to be in the same order of magnitude required to activate the halide ion migration. Our investigation suggests that the primary reason for the appearance of negative capacitance in PSCs with a p-i-n configuration is associated with the migration of halide ions and vacancies in the perovskite layers.

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Negative capacitance in p-i-n device was unraveled from immittance spectroscopy

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Under external bias, halide ions/vacancies migrate toward HTL/perovskites interface

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Charge carriers discharge in trap states leading to the negative capacitance

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In p-i-n devices PTAA-based HTL display improved charge transport compared with PEDOT:PSS

Beyond Impedance Spectroscopy of Perovskite Solar Cells: Insights from the Spectral Correlation of the Electrooptical Frequency Techniques

Small perturbation techniques have proven to be useful tools for the investigation of perovskite solar cells. A correct interpretation of the spectra given by impedance spectroscopy (IS), intensity-modulated photocurrent spectroscopy (IMPS), and intensity-modulated photovoltage spectroscopy (IMVS) is key for the understanding of device operation. The utilization of a correct equivalent circuit to extract real parameters is essential to make this good interpretation. In this work, we present an equivalent circuit, which is able to reproduce the general and the exotic behaviors found in impedance spectra. From the measurements, we demonstrate that the midfrequency features that may appear to depend on the active layer thickness, and we also prove the spectral correlation of the three techniques that has been suggested theoretically.

Negative Capacitance and Inverted Hysteresis: Matching Features in Perovskite Solar Cells

Negative capacitance in the low-frequency domain and inverted hysteresis are familiar features in perovskite solar cells, which origin is still under discussion. Here we use impedance spectroscopy to analyze these responses in methylammonium lead bromide cells treated with lithium cations at the electron-selective layer/perovskite interface and in iodide devices exposed to different relative humidity conditions. Employing the surface polarization model, we obtain a time constant associated with the kinetics of the interaction of ions/vacancies with the surface, τ_kin, in the range of 10⁰-10² s for all the cases exhibiting both features. These interactions lead to a decrease in the overall recombination resistance, modifying the low-frequency perovskite response and yielding a flattening of the cyclic voltammetry. As a consequence of these results we find that negative capacitance and inverted hysteresis lead to a decrease in the fill factor and photovoltage values.

Self-Aggregation-Controlled Rapid Chemical Bath Deposition of SnO2 Layers and Stable Dark Depolarization Process for Highly Efficient Planar Perovskite Solar Cells

Planar perovskite solar cells (PSCs) incorporating n-type SnO₂ have attracted significant interest because of their excellent photovoltaic performance. However, the film fabrication of SnO₂ is limited by self-aggregation and inhomogeneous growth of the intermediate phase, which produces poor morphology and properties. In this study, a self-controlled SnO₂ layer is fabricated directly on a fluorine-doped tin oxide (FTO) surface through simple and rapid chemical bath deposition. The PSCs based on this hydrolyzed SnO₂ layer exhibit an excellent power conversion efficiency of 20.21 % with negligible hysteresis. Analysis of the electrochemical impedance spectroscopy on the charge transport dynamics indicates that the bias voltage influences both interfacial charge transportation and the ionic double layer under illumination. The hydrolyzed SnO₂ -based PSCs demonstrate a faster ionic charge response time of 2.5 ms in comparison with 100.5 ms for the hydrolyzed TiO₂ -based hysteretic PSCs. The results of quasi-steady-state carrier transportation indicate that a dynamic hysteresis in the J-V curves can be explained by complex ionic-electronic kinetics owing to the slow ionic charge redistribution and hole accumulation caused by electrode polarization, which causes an increase in charge recombination. This study reveals that SnO₂ -based PSCs lead to a stabilized dark depolarization process compared with TiO₂ -based PSCs, which is relevant to the charge transport dynamics in the high-performing planar SnO₂ -based PSCs.

Unveiling the Morphology Effect on the Negative Capacitance and Large Ideality Factor in Perovskite Light-Emitting Diodes

Perovskite light-emitting diodes have almost reached the threshold for potential commercialization within a few years of research. However, there are still some unsolved puzzles such as large ideality factor and the presence of large negative capacitance especially at the low-frequency regime yet to be addressed. Here, we have fabricated a methylammonium lead tri-bromide perovskite n-i-p structure for light-emitting diodes from a smooth and textured emissive layer and demonstrated for the first time that these two factors are strongly dependent on the perovskite film morphology. Bias-dependent capacitance measurement also reveals the transition between negative to positive capacitance in textured films at the low-frequency regime. We have observed an anomalous capacitive behavior at the mid-frequency regime in smooth perovskite films but not in textured films. The relatively large ideality factor and anomalous capacitive behavior observed in perovskite light-emitting diodes are due to the presence of strong coupling between ions and electrons near the electrode interface. Therefore, the ideality factor and anomalous capacitance at the mid-frequency regime can be decreased by minimizing electronic-ionic coupling in textured perovskite films, while light outcoupling can be improved significantly.

Low-Frequency Carrier Kinetics in Perovskite Solar Cells

Hybrid organic-inorganic halide perovskite solar cells have emerged as leading candidates for third-generation photovoltaic technology. Despite the rapid improvement in power conversion efficiency (PCE) for perovskite solar cells in recent years, the low-frequency carrier kinetics that underlie practical roadblocks such as hysteresis and degradation remain relatively poorly understood. In an effort to bridge this knowledge gap, we perform here correlated low-frequency noise (LFN) and impedance spectroscopy (IS) characterization that elucidates carrier kinetics in operating perovskite solar cells. Specifically, we focus on planar cell geometries with a SnO₂ electron transport layer and two different hole transport layers-namely, poly(triarylamine) (PTAA) and spiro-OMeTAD. PTAA and spiro-OMeTAD cells with moderate PCEs of 5-12% possess a Lorentzian feature at ∼200 Hz in LFN measurements that corresponds to a crossover from electrode to dielectric polarization. In comparison, spiro-OMeTAD cells with high PCEs (>15%) show 4 orders of magnitude lower LFN amplitude and are accompanied by a cyclostationary process. Through a systematic study of more than a dozen solar cells, we establish a correlation with noise amplitude, PCE, and fill factor. Overall, this work establishes correlated LFN and IS as an effective methodology for quantifying low-frequency carrier kinetics in perovskite solar cells, thereby providing new physical insights that can rationally guide ongoing efforts to improve device performance, reproducibility, and stability.

Origin of apparent light-enhanced and negative capacitance in perovskite solar cells

So-called negative capacitance seems to remain an obscure feature in the analysis of the frequency-dependent impedance of perovskite solar cells. It belongs to one of the puzzling peculiarities arising from the mixed ionic-electronic conductivity of this class of semiconductor. Here we show that apparently high capacitances in general (positive and negative) are not related to any capacitive feature in the sense of a corresponding charge accumulation. Instead, they are a natural consequence of slow transients mainly in forward current of the diode upon ion displacement when changing voltage. The transient current leads to a positive or negative ‘capacitance’ dependent on the sign of its gradient. The ‘capacitance’ appears so large because the associated resistance, when thinking of a resistor-capacitor element, results from another physical process, namely modified electronic charge injection and transport. Observable for a variety of devices, it is a rather universal phenomenon related to the hysteresis in the current–voltage curve.

The apparent negative capacitance remains elusive in the impedance analysis of metal halide perovskite solar cells. Here Ebadi et al. show that it can be attributed to slow transients in the injection current instead of classical capacitive effect, i.e. charge accumulation.

The Influence of Embedded Plasmonic Nanostructures on the Optical Absorption of Perovskite Solar Cells

The interaction of light with plasmonic nanostructures can induce electric field intensity either around or at the surface of the nanostructures. The enhanced intensity of the electric field can increase the probability of light absorption in the active layer of solar cells. The absorption edge of perovskite solar cells (PSCs), which is almost 800 nm, can be raised to higher wavelengths with the help of plasmonic nanostructures due to their perfect photovoltaic characteristics. We placed plasmonic nanoparticles (NPs) with different radii (20–60 nm) within the bulk of the perovskite solar cell and found that the Au nanoparticles with a radius of 60 nm increased the absorption of the cell by 20% compared to the bare one without Au nanoparticles. By increasing the radius of the nanoparticles, the total absorption of the cell will increase because of the scattering enhancement. The results reveal that the best case is the PSC with the NP radius of 60 nm.

Detecting and identifying reversible changes in perovskite solar cells by electrochemical impedance spectroscopy

Reversible changes in perovskite solar cells (PSC) are detected and analysed by electrochemical impedance spectroscopy (EIS).

Photocarrier dynamics in perovskite-based solar cells revealed by intensity-modulated photovoltage spectroscopy

We studied perovskite photovoltaic devices with intensity-modulated photovoltage spectroscopy. Two coexisting relaxation times are found in accordance with the results of previous impedance spectroscopy (IS) measurements. The slower time constant is independent of the light power while the faster one is inversely proportional to the light power. We employed the surface polarization picture used in the IS analysis augmented by a plausible assumption that the surface polarization is proportional to the light intensity to explain the inverse power dependence of the fast time constant. Because the surface polarization results from the surface accumulated charges, its lateral (parallel to the electrode) distribution and dynamics should be known. We present evidence that the surface accumulated charges indeed form a two-dimensional layer, and have a finite binding energy and a diffusion length.

Effect of Cs-Incorporated NiO x on the Performance of Perovskite Solar Cells

The effect of Cs-incorporated NiO _x on perovskite solar cells with an inverted structure was investigated, where NiO _x and PCBM were used as selective contacts for holes and electrons, respectively. It was found that the generation of an Ni phase in an NiO _x layer was significantly suppressed by employing cesium. Furthermore, Cs-incorporated NiO _x enabled holes to be efficiently separated at the interface, showing the improved photoluminescent quenching and thus generating higher short-circuit current. The effect of Cs incorporation was also prominent in the inhibition of recombination. The recombination resistance of Cs-incorporated NiO _x was noticeably increased by more than three-fold near the maximum power point, leading to a higher fill factor of 0.78 and consequently a higher power conversion efficiency of 17.2% for the device employing Cs-incorporated NiO _x .