Bias-Dependent Normal and Inverted J- V Hysteresis in Perovskite Solar Cells

Nanocarriers address intracellular barriers for efficient drug delivery, overcoming drug resistance, subcellular targeting and controlled release

The cellular barriers are major bottlenecks for bioactive compounds entering into cells to accomplish their biological functions, which limits their biomedical applications. Nanocarriers have demonstrated high potential and benefits for encapsulating bioactive compounds and efficiently delivering them into target cells by overcoming a cascade of intracellular barriers to achieve desirable therapeutic and diagnostic effects. In this review, we introduce the cellular barriers ahead of drug delivery and nanocarriers, as well as summarize recent advances and strategies of nanocarriers for increasing internalization with cells, promoting intracellular trafficking, overcoming drug resistance, targeting subcellular locations and controlled drug release. Lastly, the future perspectives of nanocarriers for intracellular drug delivery are discussed, which mainly focus on potential challenges and future directions. Our review presents an overview of intracellular drug delivery by nanocarriers, which may encourage the future development of nanocarriers for efficient and precision drug delivery into a wide range of cells and subcellular targets.

Unveiling heterogeneity of hysteresis in perovskite thin films

The phenomenon of current-voltage hysteresis observed in perovskite-based optoelectronic devices is a critical issue that complicates the accurate assessment of device parameters, thereby impacting performance and applicability. Despite extensive research efforts aimed at deciphering the origins of hysteresis, its underlying causes remain a subject of considerable debate. By employing nanoscale investigations to elucidate the relationship between hysteresis and morphological characteristics, this study offers a detailed exploration of photocurrent-voltage hysteresis at the nanoscale within perovskite optoelectronic devices. Through the meticulous analysis of localized I-V curve arrays, our research identifies two principal hysteresis descriptors, uncovering a predominantly inverted hysteresis pattern in 87% of the locations examined. This pattern is primarily attributed to the energetic barrier encountered at the interface between the probe and the perovskite material. Our findings underscore the pronounced heterogeneity and grain-dependent variability inherent in hysteresis behavior, evidenced by an average Hysteresis Index value of 0.24. The investigation suggests that the localized hysteresis phenomena cannot be exclusively attributed to either photocharge collection processes or organic cation migration at grain boundaries. Instead, it appears significantly influenced by localized surface trap states, which play a pivotal role in modulating electron and hole current dynamics. By identifying the key factors contributing to hysteresis, such as localized surface trap states and their influence on electron and hole current dynamics, our findings pave the way for targeted strategies to mitigate these effects. This includes the development of novel materials and device architectures designed to minimize energy barriers and enhance charge carrier mobility, thereby improving device performance and longevity. This breakthrough in understanding the microscale mechanisms of hysteresis underscores the critical importance of surface/interface defect trap passivation in mitigating hysteretic effects, offering new pathways for enhancing the performance of perovskite solar cells.

Hysteresis and Its Correlation to Ionic Defects in Perovskite Solar Cells

Ion migration has been reported to be one of the main reasons for hysteresis in the current-voltage (J-V) characteristics of perovskite solar cells. We investigate the interplay between ionic conduction and hysteresis types by studying Cs0.05(FA0.83MA0.17)0.95Pb(I0.9Br0.1)3 triple-cation perovskite solar cells through a combination of impedance spectroscopy (IS) and sweep-rate-dependent J-V curves. By comparing polycrystalline devices to single-crystal MAPbI3 devices, we separate two defects, β and γ, both originating from long-range ionic conduction in the bulk. Defect β is associated with a dielectric relaxation, while the migration of γ is influenced by the perovskite/hole transport layer interface. These conduction types are the causes of different types of hysteresis in J-V curves. The accumulation of ionic defects at the transport layer is the dominant cause for observing tunnel-diode-like characteristics in the J-V curves. By comparing devices with interface modifications at the electron and hole transport layers, we discuss the species and polarity of involved defects.

Synergy of 3D and 2D Perovskites for Durable, Efficient Solar Cells and Beyond

Three-dimensional (3D) organic-inorganic lead halide perovskites have emerged in the past few years as a promising material for low-cost, high-efficiency optoelectronic devices. Spurred by this recent interest, several subclasses of halide perovskites such as two-dimensional (2D) halide perovskites have begun to play a significant role in advancing the fundamental understanding of the structural, chemical, and physical properties of halide perovskites, which are technologically relevant. While the chemistry of these 2D materials is similar to that of the 3D halide perovskites, their layered structure with a hybrid organic-inorganic interface induces new emergent properties that can significantly or sometimes subtly be important. Synergistic properties can be realized in systems that combine different materials exhibiting different dimensionalities by exploiting their intrinsic compatibility. In many cases, the weaknesses of each material can be alleviated in heteroarchitectures. For example, 3D-2D halide perovskites can demonstrate novel behavior that neither material would be capable of separately. This review describes how the structural differences between 3D halide perovskites and 2D halide perovskites give rise to their disparate materials properties, discusses strategies for realizing mixed-dimensional systems of various architectures through solution-processing techniques, and presents a comprehensive outlook for the use of 3D-2D systems in solar cells. Finally, we investigate applications of 3D-2D systems beyond photovoltaics and offer our perspective on mixed-dimensional perovskite systems as semiconductor materials with unrivaled tunability, efficiency, and technologically relevant durability.

Determination of Mobile Ion Densities in Halide Perovskites via Low-Frequency Capacitance and Charge Extraction Techniques

Mobile ions in perovskite photovoltaic devices can hinder performance and cause degradation by impeding charge extraction and screening the internal field. Accurately quantifying mobile ion densities remains a challenge and is a highly debated topic. We assess the suitability of several experimental methodologies for determining mobile ion densities by using drift-diffusion simulations. We found that charge extraction by linearly increasing voltage (CELIV) underestimates ion density, but bias-assisted charge extraction (BACE) can accurately reproduce ionic lower than the electrode charge. A modified Mott-Schottky (MS) analysis at low frequencies can provide ion density values for high excess ionic densities, typical for perovskites. The most significant contribution to capacitance originates from the ionic depletion layer rather than the accumulation layer. Using low-frequency MS analysis, we also demonstrate light-induced generation of mobile ions. These methods enable accurate tracking of ionic densities during device aging and a deeper understanding of ionic losses.

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.

Photoelectrochemical Application and Charge Transport Dynamics of a Water-Stable Organic-Inorganic Halide (C6H4NH2CuCl2I) Film in Aqueous Solution

A water-stable thin film composed of C₆H₄NH₂CuCl₂I was fabricated using spin-coating precursor solutions that dissolved equimolar amounts of C₆H₄NH₂I and CuCl₂ in N,N-dimethylformamide. Photoelectrochemical characteristics show that the C₆H₄NH₂CuCl₂I film demonstrated a stable photocurrent (∼1 μA/cm²) in an aqueous solution under white light (11.5 mW/cm²) even after 3000 s, while exhibiting a photon-to-current efficiency of 0.093% under AM1.5 (100 mW/cm²) illumination. However, these values were significantly lower than those of the CH₃NH₃PbX₃ (X = I, Cl) film in solid devices. The electron diffusion length L(e^-) (373 nm) and hole diffusion length L(h⁺) (177 nm) in the C₆H₄NH₂CuCl₂I photoelectrode were significantly lower than those of CH₃NH₃PbX₃, limiting the photoelectrochemical and photocatalysis performances. Moreover, L(h⁺) was shorter than L(e^-) in the C₆H₄NH₂CuCl₂I photoelectrode, resulting in the hole-collecting efficiency [η_c(h⁺)] being lower than the electron-collecting efficiency [η_c(e^-)]. A CuO interlayer was introduced as a hole transport layer for the C₆H₄NH₂CuCl₂I photoelectrode, which improved L(h⁺) and η_c(h⁺).

Fundamentals of Hysteresis in Perovskite Solar Cells: From Structure-Property Relationship to Neoteric Breakthroughs

Perovskite solar cells (PSC) have shown a rapid increase in efficiency than other photovoltaic technology. Despite its success in terms of efficiency, this technology is inundated with numerous challenges hindering the progress towards commercial viability. The crucial one is the anomalous hysteresis observed in the photocurrent density-voltage (J-V) response in PSC. The hysteresis phenomenon in the solar cell presents a challenge for determining the accurate power conversion efficiency of the device. A detailed investigation of the fundamental origin of hysteresis behavior in the device and its associated mechanisms is highly crucial. Though numerous theories have been proposed to explain the causes of hysteresis, its origin includes slow transient capacitive current, trapping, and de-trapping process, ion migrations, and ferroelectric polarization. The remaining issues and future research required toward the understanding of hysteresis in PSC device is also discussed.

Performance improvement of P3HT nanowire-based organic solar cells by interfacial morphology engineering

Poly (3-hexylthiophene-2,5-diyl) nanowires (nw-P3HT) have been a great interest for organic electronics, including organic field-effect transistors, organic photodetectors, organic photovoltaics, etc due to easy formation in the solution process. Thus both explanations of charge transport dynamics and morphology are crucial for device performance. Here we demonstrated the optoelectronic properties of the P3HT nanowires where the polymer backbones were parallel to the nanowire axis. The nanowires tended to form a bundle due to van der Waals interactions. Nanowire bundles were separated by 1,8-diiodooctane (DIO) additive for photovoltaic fabrication. The bundle separation was visualized by atomic force microscopy. The charge transfer mechanism was evaluated by electrochemical impedance spectroscopy. The electrical analysis showed that short-circuit current density (J _sc) increases to 10.74 mA cm^-2 after the bundle separation. According to impedance analysis, there is a correlation between effective lifetime and DIO ratio. These findings were considered as promising results for improving the transport by forming pathways for charge carriers.

Hysteresis Analysis of Hole-Transport-Material-Free Monolithic Perovskite Solar Cells with Carbon Counter Electrode by Current Density-Voltage and Impedance Spectra Measurements

Due to the tremendous increase in power conversion efficiency (PCE) of organic–inorganic perovskite solar cells (PSCs), this technology has attracted much attention. Despite being the fastest-growing photovoltaic technology to date, bottlenecks such as current density–voltage (J–V) hysteresis have significantly limited further development. Current density measurements performed with different sweep scan speeds exhibit hysteresis and the photovoltaic parameters extracted from the current density–voltage measurements for both scan directions become questionable. A current density–voltage measurement protocol needs to be established which can be used to achieve reproducible results and to compare devices made in different laboratories. In this work, we report a hysteresis analysis of a hole-transport-material-free (HTM-free) carbon-counter-electrode-based PSC conducted by current density–voltage and impedance spectra measurements. The effect of sweep scan direction and time delay was examined on the J–V characteristics of the device. The hysteresis was observed to be strongly sweep scan direction and time delay dependent and decreased as the delay increased. The J–V analysis conducted in the reverse sweep scan direction at a lower sweep time delay of 0.2 s revealed very large increases in the short circuit current density and the power conversion efficiency of 57.7% and 56.1%, respectively, compared with the values obtained during the forward scan under the same conditions. Impedance spectroscopy (IS) investigations were carried out and the effects of sweep scan speed, time delay, and frequency were analyzed. The hysteresis was observed to be strongly sweep scan direction, sweep time delay, and frequency dependent. The correlation between J–V and IS data is provided. The wealth of photovoltaic and impendence spectroscopic data reported in this work on the hysteresis study of the HTM-free PSC may help in establishing a current density–voltage measurement protocol, identifying components and interfaces causing the hysteresis, and modeling of PSCs, eventually benefiting device performance and long-term stability.

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.

Facile Formation of 2D-3D Heterojunctions on Perovskite Thin Film Surfaces for Efficient Solar Cells

The interfaces between perovskite and charge transport layers greatly impact the device efficiency and stability of perovskite solar cells (PSCs). Inserting an ultrathin wide-band-gap layer between perovskite and hole transport layers (HTLs) has recently been shown as an effective strategy to enhance device performance. Herein, a small amount of an organic halide salt, N,N'-dimethylethylene-1,2-diammonium iodide, is used to create two-dimensional (2D)-three-dimensional (3D) heterojunctions on MAPbI₃ thin film surfaces by facile solution processing. The formation of an ultrathin wide-band-gap 2D perovskite layer on top of 3D MAPbI₃ changes the morphological and photophysical properties of perovskite thin films, effectively reduces the surface defects, and suppresses the charge recombination in the interfaces between perovskite and HTL. As a result, a power conversion efficiency of ∼20.2%, with an open-circuit voltage of 1.14 V, a short-circuit current density of 22.57 mA cm^-2, and a fill factor of 0.78, is achieved for PSCs with enhanced stability.

Efficient air-stable perovskite solar cells with a (FAI)0.46(MAI)0.40(MABr)0.14(PbI2)0.86(PbBr2)0.14 active layer fabricated via a vacuum flash-assisted method under RH > 50

In this work, we present a new kind of perovskite, (FAI)0.46(MAI)0.40(MABr)0.14(PbI2)0.86(PbBr2)0.14, the vacuum flash-assisted solution processing (VASP) of which can be carried out under relative humidity (RH) higher than 50% in ambient air. The smooth and highly crystalline perovskite showed a maximum PCE of 18.8% in perovskite solar cells. This kind of perovskite was demonstrated to be of good stability in ambient air. Holes and electrons have larger and more balanced diffusion lengths (643.7/621.9 nm) than those in the MAPbI3 perovskite (105.0/129.0 nm) according to the PL quenching experiment. The role of incorporating a large amount of MA+ cations to stabilize the intermediate phase via VASP under high RH is attributed to their better ability to intercalate into the sharing face of the one-dimensional face-sharing [PbI6] octahedra, forming the three-dimensional corner-sharing form. Moreover, hole/electron transfer times at the perovskite/Spiro-OMeTAD (PCBM) interfaces (8.90/9.20 ns) were found to be much larger than those in the MAPbI3 perovskite (0.75/0.40 ns), indicating that there still is enormous potential in further improving the performance of this kind of perovskite solar cell by interfacial engineering.

Effect of Crystal Grain Orientation on the Rate of Ionic Transport in Perovskite Polycrystalline Thin Films

In this work, we examine the effect of microstructure on ion-migration-induced photoluminescence (PL) quenching in methylammonium lead iodide perovskite films. Thin films were fabricated by two methods: spin-coating, which results in randomly oriented perovskite grains, and zone-casting, which results in aligned grains. As an external bias is applied to these films, migration of ions causes a quenching of the PL signal in the vicinity of the anode. The evolution of this PL-quenched zone is less uniform in the spin-coated devices than in the zone-cast ones, suggesting that the relative orientation of the crystal grains plays a significant role in the migration of ions within polycrystalline perovskite. We simulate this effect via a simple Ising model of ionic motion across grains in the perovskite thin film. The results of this simulation align closely with the observed experimental results, further solidifying the correlation between crystal grain orientation and the rate of ionic transport.

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).