Extremely Slow Photoconductivity Response of CH3NH3PbI3 Perovskites Suggesting Structural Changes under Working Conditions

Coupled Dynamic Characteristics of Mobile Ions in Halide Perovskites

Mixed perovskites show immense promise for diverse applications owing to their exceptional compositional flexibility and outstanding optoelectronic performance. Nevertheless, a significant hurdle in their widespread use is their susceptibility to compositional instability. Some mixed perovskites exhibit a tendency to segregate into regions with varying halide content, negatively impacting the material's electronic properties and impeding its overall advancement. This study focuses on investigating the lattice and A-site cation dynamics in mixed-halide perovskites as well as the relationship between the stability and dynamic properties of mixed-halide perovskites. Our findings reveal an intrinsic link between the kinetics of organic molecules and halogen ion migration. The stability of halide ions is linearly positively correlated with the radius, number of H atoms, and moment of inertia of the organic molecules. Organic molecules with lower rotational kinetics effectively suppress the overall cationic kinetic activity, enhancing lattice dynamic stability in mixed perovskite systems. This inhibition further impedes the migration of halogen ions and hinders the halide segregation process. The presence of dominant I/MA vacancies in perovskites accelerates the rotation of MA and the migration of halogen ions. The coupled dynamic behavior of varying vacancy concentrations in A-site cations/X-site anions within the inorganic framework significantly impacts the photovoltaic performance of these halide perovskites.

Ion Migration in Lead-Halide Perovskites: Cation Matters

Metal halide perovskites exhibit remarkable properties for optoelectronic applications, yet their susceptibility to ion migration poses challenges for device stability. Previous research has predominantly focused on the migration of the halide ions. However, the migration of cations, which also has a significant influence on the device performance, is largely overlooked. In this Perspective, we review the migration of cations and their impacts on perovskite materials and devices. Special attention shall be devoted to recent insights into the migration of L-site organic cations in 2D/3D perovskites. We outline inspirations and directions for further research into the cation migration of perovskites, highlighting new possibilities in advancing perovskite optoelectronics.

Inhibition of Ion Migration for Highly Efficient and Stable Perovskite Solar Cells

In recent years, organic-inorganic halide perovskites are now emerging as the most attractive alternatives for next-generation photovoltaic devices, due to their excellent optoelectronic characteristics and low manufacturing cost. However, the resultant perovskite solar cells (PVSCs) are intrinsically unstable owing to ion migration, which severely impedes performance enhancement, even with device encapsulation. There is no doubt that the investigation of ion migration and the summarization of recent advances in inhibition strategies are necessary to develop "state-of-the-art" PVSCs with high intrinsic stability for accelerated commercialization. This review systematically elaborates on the generation and fundamental mechanisms of ion migration in PVSCs, the impact of ion migration on hysteresis, phase segregation, and operational stability, and the characterizations for ion migration in PVSCs. Then, many related works on the strategies for inhibiting ion migration toward highly efficient and stable PVSCs are summarized. Finally, the perspectives on the current obstacles and prospective strategies for inhibition of ion migration in PVSCs to boost operational stability and meet all of the requirements for commercialization success are summarized.

Review on Chemical Stability of Lead Halide Perovskite Solar Cells

Lead halide perovskite solar cells (PSCs) have become a promising next-generation photovoltaic technology due to their skyrocketed power conversion efficiency. However, the device stability issues may restrict their commercial applications, which are dominated by various chemical reactions of perovskite layers. Hence, a comprehensive illustration on the stability of perovskite films in PSCs is urgently needed. In this review article, chemical reactions of perovskite films under different environmental conditions (e.g., moisture, oxygen, light) and with charge transfer materials and metal electrodes are systematically elucidated. Effective strategies for suppressing the degradation reactions of perovskites, such as buffer layer introduction and additives engineering, are specified. Finally, conclusions and outlooks for this field are proposed. The comprehensive review will provide a guideline on the material engineering and device design for PSCs.

Photoluminescence Intensity Enhancement in Tin Halide Perovskites

The prevalence of background hole doping in tin halide perovskites usually dominates their recombination dynamics. The addition of excess Sn halide source to the precursor solution is the most frequently used approach to reduce the hole doping and reveals photo-carrier dynamics related to defects activity. This study presents an experimental and theoretical investigation on defects under light irradiation in tin halide perovskites by combining measurements of photoluminescence with first principles computational modeling. It finds that tin perovskite thin films prepared with an excess of Sn halide sources exhibit an enhancement of the photoluminescence intensity over time under continuous excitation in inert atmosphere. The authors propose a model in which light irradiation promotes the annihilation of VSn 2- /Sni 2+ Frenkel pairs, reducing the deep carrier trapping centers associated with such defect and increasing the radiative recombination. Importantly, these observations can be traced in the open-circuit voltage dynamics of tin-based halide perovskite solar cells, implying the relevance of controlling the Sn photochemistry to stabilize tin perovskite devices.

Intensity Modulated Photocurrent Microspectrosopy for Next Generation Photovoltaics

In this report, a large-area laser beam induced current microscope that has been adapted to perform intensity modulated photocurrent spectroscopy (IMPS) in an imaging mode is described. Microscopy-based IMPS method provides a spatial resolution of the frequency domain response of the solar cell, allowing correlation of the optoelectronic response with a particular interface, bulk material, specific transport layer, or transport parameter. The system is applied to study degradation effects in back-contact perovskite cells where it is found to readily differentiate areas based on their markedly different frequency response. Using the diffusion-recombination model, the IMPS response is modeled for a sandwich structure and extended for the special case of lateral diffusion in a back-contact cell. In the low-frequency limit, the model is used to calculate spatial maps of the carrier ambipolar diffusion length. The observed frequency response of IMPS images is then discussed.

Recent Criterion on Stability Enhancement of Perovskite Solar Cells

Perovskite solar cells (PSCs) have captured the attention of the global energy research community in recent years by showing an exponential augmentation in their performance and stability. The supremacy of the light-harvesting efficiency and wider band gap of perovskite sensitizers have led to these devices being compared with the most outstanding rival silicon-based solar cells. Nevertheless, there are some issues such as their poor lifetime stability, considerable J–V hysteresis, and the toxicity of the conventional constituent materials which restrict their prevalence in the marketplace. The poor stability of PSCs with regard to humidity, UV radiation, oxygen and heat especially limits their industrial application. This review focuses on the in-depth studies of different direct and indirect parameters of PSC device instability. The mechanism for device degradation for several parameters and the complementary materials showing promising results are systematically analyzed. The main objective of this work is to review the effectual strategies of enhancing the stability of PSCs. Several important factors such as material engineering, novel device structure design, hole-transporting materials (HTMs), electron-transporting materials (ETMs), electrode materials preparation, and encapsulation methods that need to be taken care of in order to improve the stability of PSCs are discussed extensively. Conclusively, this review discusses some opportunities for the commercialization of PSCs with high efficiency and stability.

Nanostructured perovskites for nonvolatile memory devices

Perovskite materials have driven tremendous advances in constructing electronic devices owing to their low cost, facile synthesis, outstanding electric and optoelectronic properties, flexible dimensionality engineering, and so on. Particularly, emerging nonvolatile memory devices (eNVMs) based on perovskites give birth to numerous traditional paradigm terminators in the fields of storage and computation. Despite significant exploration efforts being devoted to perovskite-based high-density storage and neuromorphic electronic devices, research studies on materials' dimensionality that has dominant effects on perovskite electronics' performances are paid little attention; therefore, a review from the point of view of structural morphologies of perovskites is essential for constructing perovskite-based devices. Here, recent advances of perovskite-based eNVMs (memristors and field-effect-transistors) are reviewed in terms of the dimensionality of perovskite materials and their potentialities in storage or neuromorphic computing. The corresponding material preparation methods, device structures, working mechanisms, and unique features are showcased and evaluated in detail. Furthermore, a broad spectrum of advanced technologies (e.g., hardware-based neural networks, in-sensor computing, logic operation, physical unclonable functions, and true random number generator), which are successfully achieved for perovskite-based electronics, are investigated. It is obvious that this review will provide benchmarks for designing high-quality perovskite-based electronics for application in storage, neuromorphic computing, artificial intelligence, information security, etc.

Double Cascading Charge Transfer at Integrated Perovskite/Organic Bulk Heterojunctions for Extended Near-Infrared Photoresponse and Enhanced Photocurrent

Nowadays, nearly 48.7% near-infrared (NIR) irradiation (>800 nm) of the full solar spectrum has actually not been fully utilized since the state-of-the-art perovskite film usually can only absorb the most UV-vis sunlight radiation. Herein, high efficiency integrated Cs_0.15 FA_0.85 PbI₃ perovskite/organic bulk (PC₆₁ BM:D18:Y6) heterojunction solar cells with enhanced low energy photon harvest until 931 nm and a high maintained open circuit voltage of 1.04 V is successfully obtained. In particular, the favorable double cascading charge transfer paths pave an interesting possibility to spatially separate electrons upon visible light excitation and holes upon NIR photon absorption simultaneously at interfaces, significantly suppressing non-radiative bimolecular recombination and reaching the photocurrent density as high as 27.48 mA cm^-2 and power conversion efficiency of 20.31%. Besides, the strong hydrophobicity of the ternary organic film has effectively prevented ambient humidity penetration and improves the stability of the perovskite in the continuous aging test (humidity > 60%) compared with the control device. This work has opened a significantly new window to improve the NIR light harvest for next generation highly efficient solar cells with full spectrum response.

Instability Issues and Stabilization Strategies of Lead Halide Perovskites for Photo(electro)catalytic Solar Fuel Production

Photo(electro)catalysis is a promising route to utilizing solar energy to produce valuable chemical fuels. In recent years, lead halide perovskites (LHPs) as a class of high-performance semiconductor materials have been extensively used in photo(electro)catalytic solar fuel production because of their excellent photophysical properties. However, instability issues make it arduous for LHPs to achieve their full potential in photo(electro)catalysis. This Perspective discusses the instability issues and summarizes the stabilization strategies employed for prolonging the stability or durability of LHPs in photo(electro)catalytic solar fuel production. The strategies for particulate photocatalytic systems (including composition engineering, surface passivation, core-shell structures construction, and solvent selection) and for thin-film PEC systems (including physical protective coating, A site cation additive, and surface/interface passivation) are introduced. Finally, some challenges and opportunities regarding the development of stable and efficient LHPs for photo(electro)catalysis are proposed.

DC bias electric field effects on ac electrical conductivity of MAPbI3suggesting intrinsic changes on structure and charge carrier dynamics

Methylammonium lead iodide (MAPbI₃) emerges as a promising halide perovskite material for the next generation of solar cells due to its high efficiency and flexibility in material growth. Despite intensive studies of their optical and electronic properties in the past ten years, there are no reports on dc bias electric field effects on conductivity in a wide temperature range. In this work, we report the combined effects of frequency, temperature, and dc bias electric field on the ac conductivity of MAPbI₃. We found that the results of dc bias electric fields are very contrasting in the tetragonal and cubic phases. In the tetragonal phase, sufficiently high dc bias electric fields induce a conductivity peak appearance ∼290 K well evidenced at frequencies higher than 100 kHz. Excluding possible degradation and extrinsic factors, we propose that this peak suggests a ferroelectric-like transition. In the absence of a dc bias electric field, the ac conductivity in the tetragonal phase increases with temperature while decreases with temperature in the cubic phase. Also, ac activation energies for tetragonal and cubic phases were found to be inversely and directly proportional to the dc bias electric field, respectively. This behavior was attributed to the ionic conduction, possibly of MA⁺and I^-ions, for the tetragonal phase. As for the cubic phase, the ac conduction dynamics appear to be metallic-like, which seems to change to a polaronic-controlled charge transport to increased dc bias electric fields.

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.

Variational hysteresis and photoresponse behavior of MAPbX3(X= I, Br, Cl) perovskite single crystals

High-quality MAPbX₃(X= I, Br, Cl) single crystals with a desirable size were grown through an inverse temperature crystallization method. Systematically measurements of current-voltage (I-V) hysteresis show that the hysteresis is strongly dependent on the measuring protocol, including scan rate and light illumination condition, which reveals the competition of three main factors that influence the charge dynamics in different regimes, defect trap, MA⁺dipoles rotation, and ion migration. In the dark, defect trapping is the dominant charge transport dynamics at low bias in the MAPbI₃, while the MA⁺dipole rotation is significant in MAPbBr₃, and ion migration occurs in MAPbCl₃. However, as bias increases, MA⁺dipole rotation plays a crucial role in the conductivity either in the dark or under light illumination. The time-dependent photoresponse exhibits different tendencies under various biases. The slow rising dynamics of photoresponse in MAPbX₃is attributed to the slow rotation of MA⁺dipoles, while an immediate overshoot followed by a decay suggests significant ion migration contribution at high external bias. The results serve as comprehensive experimental support to understand the hysteresis behaviors and slow photoresponse in MAPbX₃, particularly in MAPbCl₃, and provide a guide for future work in MAPbX₃based optoelectronic devices.

Nanoscale light- and voltage-induced lattice strain in perovskite thin films

We report on localized nonlinear lattice deformation and nanoscale structural rearrangement in methylammonium lead triiodide films triggered by the combined action of light and voltage. These effects, revealed by second harmonic piezoresponse force microscopy, are connected with organic cation motion, implicating localized cation migration as a key contributor to perovskite optoelectronic device instability under operating conditions.

Interaction of the Cation and Vacancy in Hybrid Perovskites Induced by Light Illumination

Mixed A-site engineering is an emerging strategy to overcome the difficulties in realizing high-quality perovskite films together with high ambient stability. Particularly, the α-FACsPbI₃-based hybrid perovskites have been considered as a promising candidate for solar cell applications. However, the degradation mechanism of α-FACsPbI₃ hybrid perovskites induced by light illumination remains unclear. Here, the illumination-caused instability of α-FACsPbI₃ hybrid perovskites is investigated using various surface detection technologies, including photoelectron spectroscopy, scanning electron microscopy, and grazing incidence X-ray diffraction. The experimental findings reveal that the A-site vacancies arise from the migration of Cs⁺ cations from the perovskite surface into the bulk under light illumination, while their content is dependent on the light energy. The visible light enlarges the crystal lattice on the perovskite surface, leading to the Cs⁺ cation migration along with the lattice distortion of the PbI₆^4- octahedron and phase separation. However, the ultraviolet light further causes a stronger interaction between FA⁺ and [PbI₆]^4-, leading to the partial decomposition of [PbI₆]^4- into Pb⁰ and I^-. These results enrich the photodegradation mechanism, guiding the design of efficient and stable perovskite solar cells through surface passivation to suppress the Cs⁺ cation migration and to increase the octahedron dissociation energy.

Optomechanical Effects Occurring in a Hybrid Metal-Halide Perovskite Single Crystal Based on Photoinduced Resonant Ultrasound Spectroscopy

This letter reports optomechanical effects occurring in a hybrid metal-halide perovskite single crystal (MAPbBr₃) based on resonant ultrasound spectroscopy (RUS) measurements under continuous wave (CW) laser illumination. The optomechanical effects are a new phenomenon in hybrid perovskite single crystals where the elastic constant of a single crystal is measured by RUS probed under varying excitation conditions. Our studies show that applying a CW laser (405 nm) to the single-crystal face shifts the RUS peaks to higher frequencies by about 1-4% in the perovskite single crystal at room temperature. The light-induced shift of the RUS peaks can be observed only when photoexcitation is occurring, rather than during heating, by positioning the laser wavelength within the optical absorption spectrum. In contrast, positioning the laser wavelength outside of the optical absorption spectrum leads to an absence of RUS peak shifting. Clearly, the laser-light-induced RUS peak shifts shows that the crystal elastic moduli can be changed by photoexcitation, leading to an optomechanical phenomenon via excited states. Essentially, the observed optomechanical phenomenon reflects the fact that the mechanical properties can be optically changed through internal repulsive and attractive force constants by external photoexcitation in a hybrid perovskite single crystal.

Progress in Materials Development for the Rapid Efficiency Advancement of Perovskite Solar Cells

The efficiency of perovskite solar cells (PSCs) has undergone rapid advancement due to great progress in materials development over the past decade and is under extensive study. Despite the significant challenges (e.g., recombination and hysteresis), both the single-junction and tandem cells have gradually approached the theoretical efficiency limit. Herein, an overview is given of how passivation and crystallization reduce recombination and thus improve the device performance; how the materials of dominant layers (hole transporting layer (HTL), electron transporting layer (ETL), and absorber layer) affect the quality and optoelectronic properties of single-junction PSCs; and how the materials development contributes to rapid efficiency enhancement of perovskite/Si tandem devices with monolithic and mechanically stacked configurations. The interface optimization, novel materials development, mixture strategy, and bandgap tuning are reviewed and analyzed. This is a review of the major factors determining efficiency, and how further improvements can be made on the performance of PSCs.

Competing Dissolution Pathways and Ligand Passivation-Enhanced Interfacial Stability of Hybrid Perovskites with Liquid Water

Material instability issues, especially moisture degradation in ambient operating environments, limit the practical application of hybrid perovskite in photovoltaic and light-emitting devices. Very recent experiments demonstrate that ligand passivation can effectively improve the surface moisture tolerance of hybrid perovskites. In this work, the interfacial stability of as-synthesized pristine and alkylammonium-passivated methylammonium lead iodide (MAPbI₃) with liquid water is systematically investigated using molecular dynamics simulations and reaction kinetics models. Interestingly, the more hydrophilic [PbI₂]⁰ surface is more stable than the less hydrophilic [MAI]⁰ surface because of the higher polarity of the former surface. Linear alkylammoniums significantly stabilize the [MAI]⁰ surface with highly reduced (by 1-2 orders of magnitude) dissociation rates of both MA⁺ and ligands themselves, while branched ligands, surprisingly, lead to higher dissociation rates as the surface coverage increases. Such anomalous behavior is attributed to the aggregation-assisted dissolution of surfactant-like ligands as micelles during the degradation process. Short-chain linear alkylammonium at the full surface coverage is found to be the optimal ligand to stabilize the [MAI]⁰ surface. This work not only provides fundamental insights into the ionic dissolution pathways and mechanisms of hybrid perovskites in water but also inspires the design of highly stable hybrid perovskites with ligand passivation layers. The computational framework developed here is also transferrable to the investigation of surface passivation chemistry for weak ionic materials in general.

Looking beyond the Surface: The Band Gap of Bulk Methylammonium Lead Iodide

Despite the intense research on photovoltaic lead halide perovskites, reported optical properties as basic as the absorption onset and the optical band gap vary significantly. To unambiguously answer the question whether the discrepancies are a result of differences between bulk and "near-surface" material, we perform two nonlinear spectroscopies with drastically different information depths on single crystals of the prototypical (CH₃NH₃)PbI₃ methylammonium lead iodide. Two-photon absorption, detected via the resulting generation of carriers and photocurrents (2PI-PC), probes the interband transitions with an information depth in the millimeter range relevant for bulk (single-crystal) material. In contrast, the transient magneto-optical Kerr effect (trMOKE) measured in a reflection geometry determines the excitonic transition energies in the region near (hundreds of nm) the surface which also determine the optical properties in typical thin films. To identify differences between structural phases, we sweep the sample temperature across the orthorhombic-tetragonal phase transition temperature. In the application-relevant room-temperature tetragonal phase (at 170 K), we find a bulk band gap of 1.55 ± 0.01 eV, whereas in the near-surface region excitonic transitions occur at 1.59 ± 0.01 eV. The latter value is consistent with previous reflectance measurements by other groups and considerably higher than the bulk band gap. The small band gap of the bulk material explains the extended infrared absorption of crystalline perovskite solar cells, the low-energy bands which carry optically driven spin-polarized currents, and the narrow bandwidth of crystalline perovskite photodetectors making use of the spectral filtering at the surface.

The engineering of stilbazolium/iodocuprate hybrids with optical/electrical performances by modulating inter-molecular charge transfer among H-aggregated chromophores

Good electrical bistability performances in stilbazolium/iodocuprate hybrids stem from the better face-to-face π⋯π stacking interactions induced by the substituents with appropriate lengths and electronic natures.

A Small Lattice Change Induces Significant Dynamic Changes of CH3NH3+ Caged in Hybrid Perovskite Crystals: Toward Understanding the Interplay between Host Lattices and Guest Molecules

Two perovskite-type compounds, (MA)₂[B'Co(CN)₆] (MA = methylammonium, B' = K(I) and Na(I)), have very similar structures, but exhibit marked differences in the phase and dielectric transitions. Solid state ²H NMR studies reveal the detailed dynamic changes of the caged methylammonium (MA) cations before and after the phase transitions, which are correlated with the different dielectric states of the compounds. Using solid state ⁵⁹Co NMR, the dynamic changes of the host lattices before and after the transitions, which accompany the changes in the dynamics of the caged MA cations, are unveiled, demonstrating the intriguing interplay between the MA cations and the host lattices. On the basis of these observations, the molecular origins of the dielectric transitions are discussed in detail.

Perovskites for Next-Generation Optical Sources

Next-generation displays and lighting technologies require efficient optical sources that combine brightness, color purity, stability, substrate flexibility. Metal halide perovskites have potential use in a wide range of applications, for they possess excellent charge transport, bandgap tunability and, in the most promising recent optical source materials, intense and efficient luminescence. This review links metal halide perovskites' performance as efficient light emitters with their underlying materials electronic and photophysical attributes.

Phase segregation due to ion migration in all-inorganic mixed-halide perovskite nanocrystals

Semiconductor mixed-halide perovskites featured with a tunable energy bandgap are ideal candidates for light absorbers in tandem solar cells as well as fluorescent materials in light-emitting diodes and nanoscale lasers. These device advancements are currently hindered by the light-induced phase segregation effect, whereby ion migration would yield smaller-bandgap domains with red-shifted photoluminescence. Here we show that upon laser excitation all-inorganic mixed-halide nanocrystals unexpectedly exhibit a blue shift in the photoluminescence peak that can revert back in the dark, thus depicting the processes of ion migration out of and back to the originally excited nanocrystals. Interestingly, this reversible photoluminescence shift can also be induced by electrical biasing of mixed-halide nanocrystals without the injection of charge carriers. The above findings suggest that it is the local electric field that breaks the ionic bonds in mixed-halide nanocrystals, which could be a universal origin for light-induced phase segregation observed in other mixed-halide perovskite materials.

Mixed-halide perovskites possess excellent semiconductor properties but suffer severely from notorious light-induced phase segregation effect. Here Zhang et al. employ simple photoluminescence measurements to link the effect to the local electric field induced ion migration process.

Organic-Inorganic Hybrid Halide Perovskites for Memories, Transistors, and Artificial Synapses

Fascinating characteristics of halide perovskites (HPs), which cannot be seen in conventional semiconductors and metal oxides, have boosted the application of HPs in electronic devices beyond optoelectronics such as solar cells, photodetectors, and light-emitting diodes. Here, recent advances in HP-based memory and logic devices such as resistive-switching memories (i.e., resistive random access memory (RRAM) or memristors), transistors, and artificial synapses are reviewed, focusing on inherently exotic properties of HPs: i) tunable bandgap, ii) facile majority carrier control, iii) fast ion migration, and iv) superflexibility. Various fabrication techniques of HP thin films from solution-based methods to vacuum processes are introduced. Up-to-date work in the field, emphasizing the compositional flexibility of HPs, suggest that HPs are promising candidates for next-generation electronic devices. Taking advantages of their unique electrical properties, low-cost and low-temperature synthesis, and compositional and mechanical flexibility, HPs have enormous potential to provide a new platform for future electronic devices and explosively intensive studies will pave the way in finding new HP materials beyond conventional silicon-based semiconductors to keep up with "More-than-Moore" times.

PEOz-PEDOT:PSS Composite Layer: A Route to Suppressed Hysteresis and Enhanced Open-Circuit Voltage in a Planar Perovskite Solar Cell

The successful commercialization of perovskite solar cells (Pvs-SCs) calls for the need to find low-temperature processable interlayers with outstanding charge-transport features. In this work, we strategically blend poly(2-ethyl-2-oxazoline) (PEOz) with PEDOT:PSS as the modified hole transport layer (HTL) to achieve high-efficiency P-I-N CH₃NH₃PbI₃ Pvs-SCs. The PEOz-PEDOT:PSS HTL exhibits enhanced features over the conventional layer including the following: (1) promoting perovskite with enlarged grain sizes to decrease the perovskite layer's recombination, (2) increasing the work function of the HTL, and (3) decreasing the noncapacitive current in Pvs-SCs. Remarkably, we demonstrate a 17.39% power conversion efficiency with very low hysteresis and high V_oc values of 1.075 V for Pvs-SCs with PEOz-PEDOT:PSS.

Critical Role of Water in Defect Aggregation and Chemical Degradation of Perovskite Solar Cells

The chemical stability of methylammonium lead iodide (MAPbI₃) under humid conditions remains the primary challenge facing halide perovskite solar cells. We investigate defect processes in the water-intercalated iodide perovskite (MAPbI₃_H₂O) and monohydrated phase (MAPbI₃·H₂O) within a first-principles thermodynamic framework. We consider the formation energies of isolated and aggregated vacancy defects with different charge states under I-rich and I-poor conditions. It is found that a PbI₂ (partial Schottky) vacancy complex can be formed readily, while the MAI vacancy complex is difficult to form in the hydrous compounds. Vacancies in the hydrous phases create deep charge transition levels, indicating the degradation of the lead halide perovskite upon exposure to moisture. Electronic structure analysis supports a mechanism of water-mediated vacancy pair formation.

First-principles investigation of the Lewis acid-base adduct formation at the methylammonium lead iodide surface

We have here performed a campaign of ab initio calculations focusing on the anchoring mechanism and adduct formation of some Lewis bases, both aliphatic and aromatic, on a PbI2-rich flat (001) methylammonium lead iodide (MAPI) surface. Our goal is to provide theoretical support to the recently reported experimental techniques of MAPI surface passivation via Lewis acid-base neutralization and similarly of MAI·PbI2·(Lewis base) adduct formation. We tested several X-donor bases (X = :N, :O, :S), paying attention to the thermodynamic stability of the final MAPI·base adducts and to their electronic properties. Factors that impact on the passivation mechanism are the directionality of the Lewis base lone pair and its enhanced/reduced overlap with MAPI Pb p orbitals, the dipole moment of the base and, similarly, the electronegativity of the X donor atom. Also non-covalent interactions, both at the surface side (intra, MAPI) and at the very interface (inter, MAPI·Lewis base), seem to contribute to the stability of the final adducts. Here we show that the thermodynamic stability does not necessarily correspond to the most effective base → acid dative bond formation. Starting from a low coverage (12.5% of the undercoordinated Pb atoms available at the surface are passivated) this paper paves the way towards the study of cooperative and steric effects among Lewis bases at higher coverages representing, to the best of our knowledge, one of the very first studies focusing on the molecular anchoring on the surfaces of this very important class of materials.

Light-induced lattice expansion leads to high-efficiency perovskite solar cells

Light-induced structural dynamics plays a vital role in the physical properties, device performance, and stability of hybrid perovskite-based optoelectronic devices. We report that continuous light illumination leads to a uniform lattice expansion in hybrid perovskite thin films, which is critical for obtaining high-efficiency photovoltaic devices. Correlated, in situ structural and device characterizations reveal that light-induced lattice expansion benefits the performances of a mixed-cation pure-halide planar device, boosting the power conversion efficiency from 18.5 to 20.5%. The lattice expansion leads to the relaxation of local lattice strain, which lowers the energetic barriers at the perovskite-contact interfaces, thus improving the open circuit voltage and fill factor. The light-induced lattice expansion did not compromise the stability of these high-efficiency photovoltaic devices under continuous operation at full-spectrum 1-sun (100 milliwatts per square centimeter) illumination for more than 1500 hours.

Visualization and Studies of Ion-Diffusion Kinetics in Cesium Lead Bromide Perovskite Nanowires

The facile chemical transformation of metal halide perovskites via ion exchange has been attributed to their "soft" crystal lattices that enable fast ion migration. Kinetic studies of such processes could provide mechanistic insights on the ion migration dynamics. Herein, by using aligned single-crystal nanowires of cesium lead bromide (CsPbBr₃) perovskite on epitaxial substrates as platforms, we visualize and investigate the cation or anion interdiffusion kinetics via spatially resolved photoluminescence measurement on heterostructures fabricated by stacking CsPbCl₃, MAPbI₃, or MAPbBr₃ microplates on top of CsPbBr₃ nanowires. Time-dependent confocal photoluminescence microscopy and energy-dispersive X-ray spectroscopy showed the solid-state anion interdiffusion readily occurs to result in halide concentration gradients along CsPbBr_{3-3 x}Cl_{3 x} ( x = 0-1) nanowires. Quantitative analysis of such composition profiles using Fick's law allowed us, for the first time, to extract interdiffusion coefficients of the chloride-bromide couple and an activation energy of 0.44 ± 0.02 eV for ion diffusion from temperature-dependent studies. In contrast, iodide-bromide interdiffusion is limited, likely due to the complex phase behaviors of mixed alloys of CsPb(Br,I)₃. In contrast to the relatively mobile anions, A-site cation interdiffusion across the MAPbBr₃/CsPbBr₃ junctions was barely observed at room temperature. Our results present a general method to investigate the kinetics of the solid-state ion migration, and the gained insights on ion diffusion can provide guidelines for rationally designing perovskite heterostructures that could lead to new properties for fundamental studies and technological applications.

Superfast Room-Temperature Activation of SnO2 Thin Films via Atmospheric Plasma Oxidation and their Application in Planar Perovskite Photovoltaics

The power conversion efficiency (PCE) of perovskite solar cells (PSCs) has now exceeded 20%; thus, research focus has shifted to establishing the foundations for commercialization. One of the pivotal themes is to curtail the overall fabrication time, to reduce unit cost, and mass-produce PSCs. Additionally, energy dissipation during the thermal annealing (TA) stage must be minimized by realizing a genuine low-temperature (LT) process. Here, tin oxide (SnO₂ ) thin films (TFs) are formulated at extremely high speed, within 5 min, under an almost room-temperature environment (<50 °C), using atmospheric Ar/O₂ plasma energy (P-SnO₂ ) and are applied as an electron transport layer of a "n-i-p"-type planar PSC. Compared with a thermally annealed SnO₂ TF (T-SnO₂ ), the P-SnO₂ TF yields a more even surface but also outstanding electrical conductivity with higher electron mobility and a lower number of charge trap sites, consequently achieving a superior PCE of 19.56% in P-SnO₂ -based PSCs. These findings motivate the use of a plasma strategy to fabricate various metal oxide TFs using the sol-gel route.

High-Quality CH3NH3PbI3 Films Obtained via a Pressure-Assisted Space-Confined Solvent-Engineering Strategy for Ultrasensitive Photodetectors

High-quality organic-inorganic hybrid perovskite films are crucial for excellent performance of photoelectric devices. Herein, we demonstrate a pressure-assisted space-confined solvent-engineering strategy to grow highly oriented, pinhole-free thin films of CH₃NH₃PbI₃ with large-scale crystalline grains, high smoothness, and crystalline fusion on grain boundaries. These single-crystalline grains vertically span the entire film thickness. Such a film feature dramatically reduces recombination loss and then improves the transport property of charge carriers in the films. Consequently, the photodetector devices, based on the high-quality CH₃NH₃PbI₃ films, exhibit high photocurrent (105 μA under 671 nm laser with a power density of 20.6 mW/cm² at 10 V), good stability, and, especially, an ultrahigh on/off ratio (I_light/I_dark > 2.2 × 10⁴ under an incident light of 20.6 mW/cm²). These excellent performances indicate that the high-quality films will be potential candidates in other CH₃NH₃PbI₃-based photoelectric devices.

Perovskite photodetectors with both visible-infrared dual-mode response and super-narrowband characteristics towards photo-communication encryption application

Photo-communication has attracted great attention because of the rapid development of wireless information transmission technology. However, it is still a great challenge in cryptography communications, where it is greatly weakened by the openness of the light channels. Here, visible-infrared dual-mode narrowband perovskite photodetectors were fabricated and a new photo-communication encryption technique was proposed. For the first time, highly narrowband and two-photon absorption (TPA) resultant photoresponses within a single photodetector are demonstrated. The full width at half maximum (FWHM) of the photoresponse is as narrow as 13.6 nm in the visible range, which is superior to state-of-the-art narrowband photodetectors. Furthermore, these two merits of narrowband and TPA characteristics are utilized to encrypt the photo-communication based on the above photodetectors. When sending information and noise signals with 532 and 442 nm laser light simultaneously, the perovskite photodetectors only receive the main information, while the commercial Si photodetector responds to both lights, losing the main information completely. The final data are determined by the secret key through the TPA process as preset. Such narrowband and TPA detection abilities endow the perovskite photodetectors with great potential in future security communication and also provide new opportunities and platforms for encryption techniques.

Metal-Halide Perovskite Transistors for Printed Electronics: Challenges and Opportunities

Following the unprecedented rise in photovoltaic power conversion efficiencies during the past five years, metal-halide perovskites (MHPs) have emerged as a new and highly promising class of solar-energy materials. Their extraordinary electrical and optical properties combined with the abundance of the raw materials, the simplicity of synthetic routes, and processing versatility make MHPs ideal for cost-efficient, large-volume manufacturing of a plethora of optoelectronic devices that span far beyond photovoltaics. Herein looks beyond current applications in the field of energy, to the area of large-area electronics using MHPs as the semiconductor material. A comprehensive overview of the relevant fundamental material properties of MHPs, including crystal structure, electronic states, and charge transport, is provided first. Thereafter, recent demonstrations of MHP-based thin-film transistors and their application in logic circuits, as well as bi-functional devices such as light-sensing and light-emitting transistors, are discussed. Finally, the challenges and opportunities in the area of MHPs-based electronics, with particular emphasis on manufacturing, stability, and health and environmental concerns, are highlighted.

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

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

Optical detection of charge dynamics in CH3NH3PbI3/carbon nanotube composites

We have investigated the optical absorption of metallic and semiconducting carbon nanotubes/CH₃NH₃PbI₃ micro- and nanowire composites. Upon visible light illumination semiconducting carbon nanotube based samples show a photo-induced doping, originating from the charge carriers created in the perovskite while this kind of change is absent in the composites containing metallic nanotubes, due to their strikingly different electronic structure. The response in the nanotubes shows, beside a fast diffusion of photo-generated charges, a slow component similar to that observed in pristine CH₃NH₃PbI₃ attributed to structural rearrangement, and leading to slight, light induced changes of the optical gap of the perovskite. This charge transfer from the illuminated perovskite confirms that carbon nanotubes (especially semiconducting ones) can form efficient charge-transporting layers in the novel organometallic perovskite based optoelectronic devices.

Enhanced Performance and Stability of Perovskite Solar Cells Using NH4I Interfacial Modifier

Despite organic-inorganic hybrid perovskite solar cells have rapid advances in power conversion efficiency in recent years, the serious instability of the device under practical working conditions is the current main challenge for commercialization. In this study, we have successfully inserted NH₄I as an interfacial modifier between the TiO₂ electron transport layer and perovskite layer. The result shows that it can significantly improve the quality of the perovskite films and electron extraction efficiency between the perovskite and electron transport layer. The devices with NH₄I are obtained an improved power conversion efficiency of 18.31% under AM 1.5G illumination (100 mW cm^-2). More importantly, the humidity and UV light stability of the devices are greatly improved after adding NH₄I layer. The uncoated devices only decrease by less than 15% of its original efficiency during 700-h stability tests in a humidity chamber (with a relative humidity of 80%) and the efficiency almost maintains 70% of its initial value over 20 h under UV light stress tests. This work provides a potential way by interfacial modification to significantly improve photovoltaic performance and stability of perovskite solar cells.

A room temperature reversible phase transition containing dielectric switching of a host-guest supramolecular metal-halide compound

Following our recent findings on dielectric materials, we synthesized a new host-guest supramolecular metal-halide compound, [(2-AMPD)(18-crown-6)]CuCl₄ (1, 2-AMPD = 2-aminomethylpiperidinium). Systematic characterization techniques such as variable-temperature crystal structure analyses, differential scanning calorimetry (DSC) measurements, temperature-dependent dielectric measurements and powder X-ray diffraction (PXRD) measurements demonstrate that 1 undergoes a reversible phase transition at room temperature, accompanied by switchable dielectric responses and remarkable anisotropy along three different crystallographic axes. The structural phase transition mechanism is triggered by the order-disorder transition of the 18-crown-6 molecules. We believe that these findings might further promote the application of a host-guest inclusion compound in the field of switchable dielectric materials.