Unraveling the varied nature and roles of defects in hybrid halide perovskites with time-resolved photoemission electron microscopy

With rapidly growing photoconversion efficiencies, hybrid perovskite solar cells have emerged as promising contenders for next generation, low-cost photovoltaic technologies. Yet, the presence of nanoscale defect clusters, that form during the fabrication process, remains critical to overall device operation, including efficiency and long-term stability. To successfully deploy hybrid perovskites, we must understand the nature of the different types of defects, assess their potentially varied roles in device performance, and understand how they respond to passivation strategies. Here, by correlating photoemission and synchrotron-based scanning probe X-ray microscopies, we unveil three different types of defect clusters in state-of-the-art triple cation mixed halide perovskite thin films. Incorporating ultrafast time-resolution into our photoemission measurements, we show that defect clusters originating at grain boundaries are the most detrimental for photocarrier trapping, while lead iodide defect clusters are relatively benign. Hexagonal polytype defect clusters are only mildly detrimental individually, but can have a significant impact overall if abundant in occurrence. We also show that passivating defects with oxygen in the presence of light, a previously used approach to improve efficiency, has a varied impact on the different types of defects. Even with just mild oxygen treatment, the grain boundary defects are completely healed, while the lead iodide defects begin to show signs of chemical alteration. Our findings highlight the need for multi-pronged strategies tailored to selectively address the detrimental impact of the different defect types in hybrid perovskite solar cells.

Resonant Degenerate Four-Wave Mixing at the Defect Energy Levels of 2D Organic-Inorganic Hybrid Perovskite Crystals

Two-dimensional organic-inorganic lead halide perovskites are generating great interest due to their optoelectronic characteristics such as high solar energy conversion efficiency and a tunable direct band gap in the visible regime. However, the presence of defect states within the two-dimensional crystal structure can affect these properties, resulting in changes to their band gap emission as well as the emergence of nonlinear optical phenomena. Here, we have investigated the effects of the presence of defect states on the nonlinear optical phenomena of the 2D hybrid perovskite (BA)₂(MA)₂Pb₃Br₁₀. When two pulses, one narrowband pump pulse centered at 800 nm and one supercontinuum pulse with bandwidth from 800-1100 nm, are incident on a perovskite flake, degenerate four-wave mixing (FWM) occurs, with peaks corresponding to the energy levels of the defect states present within the crystal. The longer carrier lifetime of the defect state, in comparison to that of virtual transitions that take place in nonresonant FWM processes, allows for a larger population of electrons to be excited by the second pump photon, resulting in increased FWM signal at the defect energy levels. The quenching of the two-photon luminescence as flake thickness increases is also observed and attributed to the increased presence of defects within the flake at larger thicknesses. This technique shows the potential of detecting defect energy levels in crystals using FWM for a variety of optoelectronic applications.

Phthalide and 1-Iodooctadecane Synergistic Optimization for Highly Efficient and Stable Perovskite Solar Cells

The carrier non-radiative recombination and instability of device caused by the inherent defects are main factors limiting development of perovskite solar cells (PSCs). During the fabrication process of a PSC device, perovskite films often produce Pb⁰ and I⁰ defects. This paper reports a strategy for synergistic optimization of perovskite films by defects passivation and surface modification. The doping of phthalide (PT) in the Pb-rich (CH(NH₂ )₂ )_1-x (CH₃ NH₃ )_x PbI₃ film can passivate lead cation defects, and the modification of 1-iodooctadecane (1-IO) can reduce halogen anion defects and improve stability of PSCs owing to its hydrophobicity. The PT and 1-IO optimized device achieves a power conversion efficiency (PCE) of 22.27%. The optimized PSCs remain 93.2% of the initial PCE when placed in air environment (relative humidity of 10%, 25 °C) more than 70 days. The PT and 1-IO synergistic optimization provides a novel strategy for improving the performance and stability of PSCs.

Dynamics of photoconversion processes: the energetic cost of lifetime gain in photosynthetic and photovoltaic systems

The continued development of solar energy conversion technologies relies on an improved understanding of their limitations. In this review, we focus on a comparison of the charge carrier dynamics underlying the function of photovoltaic devices with those of both natural and artificial photosynthetic systems. The solar energy conversion efficiency is determined by the product of the rate of generation of high energy species (charges for solar cells, chemical fuels for photosynthesis) and the energy contained in these species. It is known that the underlying kinetics of the photophysical and charge transfer processes affect the production yield of high energy species. Comparatively little attention has been paid to how these kinetics are linked to the energy contained in the high energy species or the energy lost in driving the forward reactions. Here we review the operational parameters of both photovoltaic and photosynthetic systems to highlight the energy cost of extending the lifetime of charge carriers to levels that enable function. We show a strong correlation between the energy lost within the device and the necessary lifetime gain, even when considering natural photosynthesis alongside artificial systems. From consideration of experimental data across all these systems, the emprical energetic cost of each 10-fold increase in lifetime is 87 meV. This energetic cost of lifetime gain is approx. 50% greater than the 59 meV predicted from a simple kinetic model. Broadly speaking, photovoltaic devices show smaller energy losses compared to photosynthetic devices due to the smaller lifetime gains needed. This is because of faster charge extraction processes in photovoltaic devices compared to the complex multi-electron, multi-proton redox reactions that produce fuels in photosynthetic devices. The result is that in photosynthetic systems, larger energetic costs are paid to overcome unfavorable kinetic competition between the excited state lifetime and the rate of interfacial reactions. We apply this framework to leading examples of photovoltaic and photosynthetic devices to identify kinetic sources of energy loss and identify possible strategies to reduce this energy loss. The kinetic and energetic analyses undertaken are applicable to both photovoltaic and photosynthetic systems allowing for a holistic comparison of both types of solar energy conversion approaches.

STED microscopy reveals in-situ photoluminescence properties of single nanostructures in densely perovskite thin films

All-inorganic perovskite nanomaterials have attracted much attention recently due to their prominent optical performance and potential application for optoelectronic devices. The carriers dynamics of all-inorganic perovskites has been the research focus because the understanding of carriers dynamics process is of critical importance for improving the fluorescence conversion efficiency. While photophysical properties of excited carrier are usually measured at the macroscopic scale, it is necessary to probe the in-situ dynamics process at the nanometer scale and gain deep insights into the photophysical mechanisms and their localized dependence on the thin-film nanostructures. Stimulated emission depletion (STED) nanoscopy with super-resolution beyond the diffraction limit can directly provide explicit information at a single particle level or nanometer scale. Through this unique technique, we firstly study the in-situ dynamics process of single CsPbBr₃ nanocrystals(NCs) and nanostructures embedded inside high-dense samples. Our findings reveal the different physical mechanisms of PL blinking and antibunching for single CsPbBr₃ NCs and nanostructures that correlate with thin-film nanostructural features (e.g. defects, grain boundaries and carrier mobility). The insights gained into such nanostructure-localized physical mechanisms are critically important for further improving the material quality and its corresponding device performance.

Energy Resonance Transfer between Quantum Defects in Metal Halide Perovskites

Quantum defects have been shown to play an essential role in nonradiative recombination in metal halide perovskites (MHPs). Nonetheless, the processes of charge transfer assisted by defects are still ambiguous. Herein, we theoretically study the nonradiative multiphonon processes among different types of quantum defects in MHPs using Markvart's model for the induced mechanisms of electron-electron and electron-phonon interactions. We find that the charge carrier can transfer between the neighboring levels of the same type of shallow defects by multiphonon processes, but it will be distinctly suppressed with an increase in the defect depth. For the nonradiation multiphonon transitions between donor- and acceptor-like defects, the processes are very fast and not sensitive to the defect depth, which provides a possible explanation for the phenomenon of blinking of photoluminescence spectra. We also discuss the temperature dependence of these multiphonon processes and find that their variational trends depend on the comparison of the Huang-Rhys factor with the emitted phonon number. These theoretical results not only fill some of the gaps in defect-assisted nonradiative processes in the perovskite materials but also provide deeper physical insights into producing higher-performance perovskite-based devices.

Recent progress in metal sulfide-based electron transport layers in perovskite solar cells

High-quality electron transport layers (ETLs) are essential for stable and efficient perovskite solar cells (PSCs). Metal sulfides (MSs) are considered potential candidates for ETLs due to their high carrier mobility, low cost, and favorable chemical and physical stability. The quality of the MS films plays important role in the photovoltaic performance of PSCs. However, few reports focus on the relative preparation, characteristics, and corresponding mechanisms of MS-based ETLs. In this review, MS-based ETLs are summarized according to their preparation strategies and the mechanism. We hope that this review can help others understand the intrinsic phenomena of MS-based ETLs and motivate further investigations.

Plasmon-induced trap filling at grain boundaries in perovskite solar cells

The deep-level traps induced by charged defects at the grain boundaries (GBs) of polycrystalline organic-inorganic halide perovskite (OIHP) films serve as major recombination centres, which limit the device performance. Herein, we incorporate specially designed poly(3-aminothiophenol)-coated gold (Au@PAT) nanoparticles into the perovskite absorber, in order to examine the influence of plasmonic resonance on carrier dynamics in perovskite solar cells. Local changes in the photophysical properties of the OIHP films reveal that plasmon excitation could fill trap sites at the GB region through photo-brightening, whereas transient absorption spectroscopy and density functional theory calculations correlate this photo-brightening of trap states with plasmon-induced interfacial processes. As a result, the device achieved the best efficiency of 22.0% with robust operational stability. Our work provides unambiguous evidence for plasmon-induced trap occupation in OIHP and reveals that plasmonic nanostructures may be one type of efficient additives to overcome the recombination losses in perovskite solar cells and thin-film solar cells in general.

Nonlinear Optical Microscopy with Ultralow Quantum Light

Nonlinear optical (NLO) microscopy relies on multiple light-matter interactions to provide unique contrast mechanisms and imaging capabilities that are inaccessible to traditional linear optical imaging approaches, making them versatile tools to understand a wide range of complex systems. However, the strong excitation fields that are necessary to drive higher-order optical processes efficiently are often responsible for photobleaching, photodegradation, and interruption in many systems of interest. This is especially true for imaging living biological samples over prolonged periods of time or in accessing intrinsic dynamics of electronic excited-state processes in spatially heterogeneous materials. This perspective outlines some of the key limitations of two NLO imaging modalities implemented in our lab and highlights the unique potential afforded by the quantum properties of light, especially entangled two-photon absorption based NLO spectroscopy and microscopy. We further review some of the recent exciting advances in this emerging filed and highlight some major challenges facing the realization of quantum-light-enabled NLO imaging modalities.

Atomic Level Insights into Metal Halide Perovskite Materials by Scanning Tunneling Microscopy and Spectroscopy

Metal halide perovskite materials (MHPMs) have attracted significant attention because of their superior optoelectronic properties and versatile applications. The power conversion efficiency of MHPM solar cells (PSCs) has skyrocketed to 25.5 %. Although the performance of PSCs is already competitive, several important challenges still need to be solved to realize commercial applications. A thorough understanding of surface atomic structures and structure-property relationships is at the heart of these remaining issues. Scanning tunneling microscopy (STM) and spectroscopy (STS) can be used to characterize the surface properties of MHPMs, which can offer crucial insights into MHPMs at the atomic scale. This Review summarizes recent progress in STM and STS studies on MHPMs, with a focus on the surface properties. We provide understanding from the comparative perspective of several different MHPMs. We also highlight a series of novel phenomena observed by STM and STS. Finally, we outline a few research topics of primary importance for future studies.

Grain boundary velocity and curvature are not correlated in Ni polycrystals

[Figure: see text].

MOFs based on the application and challenges of perovskite solar cells

In recent years, perovskite solar cells (PSCs) have attracted much attention because of their high energy conversion efficiency, low cost, and simple preparation process. Up to now, the photoelectric conversion efficiency of solar cells has been increased from 3.8% to 25.5%. Metal–organic skeleton-derived metal oxides and their composites (MOFs) are widely considered for application in PSCs due to their low and flat charge/discharge potential plateau, high capacity, and stable cycling performance. By combining MOFs and PSCs, based on the composition materials of perovskite film, electron transport layer, hole transport layer, and interfacial interlayer of PSCs, this article discusses the photovoltaic performance or structure optimization effect of MOFs in each function layer, which is of great significance to improve the photovoltaic performance of the cell. The problems faced by MOFs on perovskite solar cells are summarized, the next research directions are discussed, and the development of this crossover area of MOFs–PSC is foreseen to accelerate the comprehensive research and popularization of MOFs on PSCs.

Unnatural Hygroscopic Property of Nicotinic Acid by Restructuring Molecular Density: Self-Healing Halide Perovskites

An unnatural hygroscopic property of nonhygroscopic nicotinic acid (NA) is demonstrated by tuning the intermolecular distance. After addition of NA into methylammonium lead iodide, (MAPbI₃) NA molecules are preferentially aligned on the interface of the three-dimensional (3D) MAPbI₃ crystal structure by a hydrogen bond. This unique behavior allows NA to be used as a versatile additive to improve the water durability of MAPbI₃. After exposure under a high humidity atmosphere (RH 100%, 35 °C), MAPbI₃ films with NA exhibited self-healing phenomena against moisture while bare MAPbI₃ rapidly lost its own intrinsic property. Density functional theory (DFT) calculations were conducted to reveal how H₂O molecules can effectively be absorbed by NA according to its planar molecular density. Also, further optimization of photovoltaic device performances was carried out by investigating the relationship between NA concentration and additive alignment.

Benign Effects of Twin Boundaries on Charge Carrier Lifetime in Metal Halide Perovskites by a Time-Domain Study

Experiments show that two-dimensional twin boundaries (TBs) defects are benign to the excited-state lifetime of metal halide perovskites and solar cells performance. However, the mechanism remains unclear. By performing nonadiabatic (NA) molecular dynamics simulations on FAPbI₃ (FA= HC(NH₂)₂⁺), we demonstrate that TBs increase the bandgap without introducing midgap states, promote charge separation by localizing electrons and holes that reduce NA coupling and accelerate the loss of coherence, slowing nonradiative electron-hole recombination by a factor of 2.3 compared to pristine FAPbI₃, which occurs within sub-10 ns and agrees well with the experiment. Raising the temperature shortens the coherence time and reduces the NA coupling by increasing the charge localization due to the enhanced distortions of inorganic Pb-I lattice, making the recombination even slower. Our study rationalizes the positive influence of TBs and temperature on perovskite charge dynamics and emphasizes the roles played by the charge localization and quantum coherence.

Enhanced Efficiency and Stability of All-Inorganic CsPbI2 Br Perovskite Solar Cells by Organic and Ionic Mixed Passivation

All-inorganic perovskites have been intensively investigated as potential optoelectronic materials because of their excellent thermal stability, especially for CsPbI₂ Br. Herein, the authors studied the effects of mixed passivation utilizing organic phenylethylammonium bromide and inorganic ionic cesium bromide (PEABr + CsBr) on the all-inorganic perovskite (CsPbI₂ Br) solar cells for the first time. The treatment with different passivation mechanisms enhances the perovskite film quality, resulting in uniform surface morphology and compact film with low trap density. Besides, the passivation improves the energy level alignment, which benefits the hole extraction at the perovskite/HTL interface and drives the interface electron separation, suppressing the charge recombination and realizing a high open-circuit voltage (V_oc ). Finally, the device represents a high power conversion efficiency (PCE) of 16.70%, a V_oc of 1.30 V, and an excellent fill factor (FF) of 0.82. The V_oc loss and high FF should be among the best values for CsPbI₂ Br based devices. Furthermore, the treated devices exhibit remarkable long-term stability with only 8% PCE loss after storing in a glove box for more than 1000 h without encapsulation.

Superior photo-carrier diffusion dynamics in organic-inorganic hybrid perovskites revealed by spatiotemporal conductivity imaging

The outstanding performance of organic-inorganic metal trihalide solar cells benefits from the exceptional photo-physical properties of both electrons and holes in the material. Here, we directly probe the free-carrier dynamics in Cs-doped FAPbI₃ thin films by spatiotemporal photoconductivity imaging. Using charge transport layers to selectively quench one type of carriers, we show that the two relaxation times on the order of 1 μs and 10 μs correspond to the lifetimes of electrons and holes in FACsPbI₃, respectively. Strikingly, the diffusion mapping indicates that the difference in electron/hole lifetimes is largely compensated by their disparate mobility. Consequently, the long diffusion lengths (3~5 μm) of both carriers are comparable to each other, a feature closely related to the unique charge trapping and de-trapping processes in hybrid trihalide perovskites. Our results unveil the origin of superior diffusion dynamics in this material, crucially important for solar-cell applications.

Surface Stabilization of a Formamidinium Perovskite Solar Cell Using Quaternary Ammonium Salt

Dimensionality engineering is an effective approach to improve the stability and power conversion efficiency (PCE) of perovskite solar cells (PSCs). A two-dimensional (2D) perovskite assembled from bulky organic cations to cover the surface of three-dimensional (3D) perovskite can repel ambient moisture and suppress ion migration across the perovskite film. This work demonstrates how the thermal stability of the bulky organic cation of a 2D perovskite affects the crystallinity of the perovskite and the optoelectrical properties of perovskite solar cells. Structural analysis of (FAPbI₃)_0.95(MAPbBr₃)_0.05 (FA = formamidinium ion, MA = methylammonium ion) mixed with a series of bulky cations shows a clear correlation between the structure of the bulky cations and the formation of surface defects in the resultant perovskite films. An organic cation with primary ammonium structure is vulnerable to a deprotonation reaction under typical perovskite-film processing conditions. Decomposition of the bulky cations results in structural defects such as iodide vacancies and metallic lead clusters at the surface of the perovskite film; these defects lead to a nonradiative recombination loss of charge carriers and to severe ion migration during operation of the device. In contrast, a bulky organic cation with a quaternary ammonium structure exhibits superior thermal stability and results in substantially fewer structural defects at the surface of the perovskite film. As a result, the corresponding PSC exhibits the PCE of 21.6% in a reverse current-voltage scan and a stabilized PCE of 20.1% with an excellent lifetime exceeding 1000 h for the encapsulated device under continuous illumination.

Simultaneous Enhanced Efficiency and Stability of Perovskite Solar Cells Using Adhesive Fluorinated Polymer Interfacial Material

For enhancing the performance and long-term stability of perovskite solar cell (PSC) devices, interfacial engineering between the perovskite and hole-transporting material (HTM) is important. We developed a fluorinated conjugated polymer PFPT3 and used it as an interfacial layer between the perovskite and HTM layers in normal-type PSCs. Interaction of perovskite and PFPT3 via Pb-F bonding effectively induces an interfacial dipole moment, which resulted in energy-level bending; this was favorable for charge transfer and hole extraction at the interface. The PSC device achieved an increased efficiency of 22.00% with an open-circuit voltage of 1.13 V, short-circuit current density of 24.34 mA/cm², and fill factor of 0.80 from a reverse scan and showed an averaged power conversion efficiency of 21.59%, which was averaged from forward and reverse scans. Furthermore, the device with PFPT3 showed much improved stability under an 85% RH condition because hydrophobic PFPT3 reduced water permeation into the perovskite layer, and more importantly, the enhanced contact adhesion at the PFPT3-mediated perovskite/HTM interface suppressed surface delamination and retarded water intrusion. The fluorinated conjugated polymeric interfacial material is effective for improving not only the efficiency but also the stability of the PSC devices.

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.

State of the Art and Prospects for Halide Perovskite Nanocrystals

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

Poly(catecholamine) coated CsPbBr3 perovskite microlasers: lasing in water and biofunctionalization

Lead halide perovskite (LHP) is a promising material for various optoelectronic applications. Surface coating on particles is a common strategy to improve their functionality and environmental stability, but LHP is not amenable to most coating chemistries because of its intrinsic weakness against polar solvents. Here, we describe a novel method of synthesizing LHP microlasers in a super-saturated polar solvent using sonochemistry and applying various functional coatings on individual microlasers in situ. We synthesize cesium lead bromine perovskite (CsPbBr3) microcrystals capped with organic poly-norepinephrine (pNE) layers. The catechol group of pNE coordinates to bromine-deficient lead atoms, forming a defect-passivating and diffusion-blocking shell. The pNE layer enhances the material lifetime of CsPbBr3 in water by 2,000-folds, enabling bright luminescence and lasing from single microcrystals in water. Furthermore, the pNE shell permits biofunctionalization with proteins, small molecules, and lipid bilayers. Luminescence from CsPbBr3 microcrystals is sustained in water over 1 hour and observed in live cells. The functionalization method may enable new applications of LHP laser particles in water-rich environments.

Origin, Influence, and Countermeasures of Defects in Perovskite Solar Cells

Defects are considered to be one of the most significant factors that compromise the power conversion efficiencies and long-term stability of perovskite solar cells. Therefore, it is urgent to have a profound understanding of their formation and influence mechanism, so as to take corresponding measures to suppress or even completely eliminate their adverse effects on device performance. Herein, the possible origins of the defects in metal halide perovskite films and their impacts on the device performance are analyzed, and then various methods to reduce defect density are introduced in detail. Starting from the internal and interfacial aspects of the metal halide perovskite films, several ways to improve device performance and long-term stability including additive engineering, surface passivation, and other physical treatments (annealing engineering), etc., are further elaborated. Finally, the further understanding of defects and the development trend of passivation strategies are prospected.

Robust Molecular Dipole-Enabled Defect Passivation and Control of Energy-Level Alignment for High-Efficiency Perovskite Solar Cells

The ability to passivate defects and modulate the interface energy-level alignment (IEA) is key to boost the performance of perovskite solar cells (PSCs). Herein, we report a robust route that simultaneously allows defect passivation and reduced energy difference between perovskite and hole transport layer (HTL) via the judicious placement of polar chlorine-terminated silane molecules at the interface. Density functional theory (DFT) points to effective passivation of the halide vacancies on perovskite surface by the silane chlorine atoms. An integrated experimental and DFT study demonstrates that the dipole layer formed by the silane molecules decreases the perovskite work function, imparting an Ohmic character to the perovskite/HTL contact. The corresponding PSCs manifest a nearly 20 % increase in power conversion efficiency over pristine devices and a markedly enhanced device stability. As such, the use of polar molecules to passivate defects and tailor the IEA in PSCs presents a promising platform to advance the performance of PSCs.

Reduced Defects and Enhanced Performance of (FAPbI3)0.97(MAPbBr3)0.03-Based Perovskite Solar Cells by Trimesic Acid Additives

A high-quality organolead trihalide perovskite film with large-sized crystalline grains and smooth surfaces is required to obtain efficient perovskite solar cells (PSCs). Herein, high-quality (FAPbI₃)_0.97(MAPbBr₃)_0.03 perovskite films were fabricated using trimesic acid (TMA) additives in a halide perovskite precursor solution to obtain efficient PSCs. The X-ray diffraction analysis and scanning electron microscopy of the films revealed that the TMA had a significant effect on the roughness of the films by acting as a surface link, thus reducing the surface defects and recombination at the grain boundaries. In addition, with the addition of the TMA additive, a smooth perovskite film with a flat surface and no pinholes was obtained. The perovskite film was used to fabricate a PSC device, and the device exhibited a high power conversion efficiency of 17.26%, which was higher than that of the control device (15.15%) under the same conditions. This study demonstrates a facile method to passivate defects on the perovskite layer via surface modification.

Morphology-Dependent Ambient-Condition Growth of Perovskite Nanocrystals for Enhanced Stability in Photoconversion Device

CsPbBr₃ perovskite nanocrystals with two different dimensionalities were synthesized at different temperatures and then integrated as optoelectronic transducers into transistor-type photoconversion devices. Postsynthesis transformation was observed for two-dimensional (2D) nanoplatelets, while the transformation was rarely found in 3D nanocubes. At ambient temperature and pressure, neighboring nanoplatelets made facet-to-facet contact and then fused into larger 2D nanoplatelets (2-5 times) without defects. The coalescence of 2D nanoplatelets at the ambient condition lowered the density of defects at the surface of the nanocrystals and thus could facilitate effective and stable photoconversion behavior in the nanocrystal film integrated into the device. Consequently, the ambient-condition aging of 2D nanoplatelets on device substrate led to 3 times higher retention in photoconversion performance. Importantly, these results provide a new concept of how perovskite nanocrystals can be integrated into a device for enhanced stability in device performance.

Multimodal Microscale Imaging of Textured Perovskite-Silicon Tandem Solar Cells

Halide perovskite/crystalline silicon (c-Si) tandem solar cells promise power conversion efficiencies beyond the limits of single-junction cells. However, the local light-matter interactions of the perovskite material embedded in this pyramidal multijunction configuration, and the effect on device performance, are not well understood. Here, we characterize the microscale optoelectronic properties of the perovskite semiconductor deposited on different c-Si texturing schemes. We find a strong spatial and spectral dependence of the photoluminescence (PL) on the geometrical surface constructs, which dominates the underlying grain-to-grain PL variation found in halide perovskite films. The PL response is dependent upon the texturing design, with larger pyramids inducing distinct PL spectra for valleys and pyramids, an effect which is mitigated with small pyramids. Further, optimized quasi-Fermi level splittings and PL quantum efficiencies occur when the c-Si large pyramids have had a secondary smoothing etch. Our results suggest that a holistic optimization of the texturing is required to maximize light in- and out-coupling of both absorber layers and there is a fine balance between the optimal geometrical configuration and optoelectronic performance that will guide future device designs.

Durable Defect Passivation of the Grain Surface in Perovskite Solar Cells with π-Conjugated Sulfamic Acid Additives

Defect passivation has shown an essential role in improving the efficiency and stability of perovskite solar cells (PSCs). Herein, an efficient and low-cost π-conjugated sulfamic acid additive, 4-aminobenzenesulfonic acid (4-ABSA), is used to realize durable defect passivation of PSCs. The incorporation of 4-ABSA not only constructs a compact and smooth perovskite film but is also capable of passivating both negative- and positive-charged defects derived from under-coordinated lead and halogen ions. Besides, the π-conjugated system in 4-ABSA can induce preferred perovskite crystal orientation and stabilize the coordination effect between 4-ABSA and perovskite grains. As a result, the inverted planar PSC incorporated with 4-ABSA additives demonstrates an improved power conversion efficiency (PCE) from 18.25 to 20.32%. Moreover, this 4-ABSA passivation agent also enhances the stability of devices, which retains 83.5% of its initial efficiency under ambient condition at 60 °C after 27 days. This work provides a π-conjugated sulfamic acid for durable defect passivation of perovskite optoelectronic devices.

Thermal and Humidity Stability of Mixed Spacer Cations 2D Perovskite Solar Cells

In this article, two different types of spacer cations, 1,4-butanediamonium (BDA2+) and 2-phenylethylammonium (PEA+) are co-used to prepare the perovskite precursor solutions with the formula of (BDA)1- a (PEA2) a MA4Pb5X16. By simply mixing the two spacer cations, the self-assembled polycrystalline films of (BDA)0.8(PEA2)0.2MA4Pb5X16 are obtained, and BDA2+ is located in the crystal grains and PEA+ is distributed on the surface. The films display a small exciton binding energy, uniformly distributed quantum wells and improved carrier transport. Besides, utilizing mixed spacer cations also induces better crystallinity and vertical orientation of 2D perovskite (BDA)0.8(PEA2)0.2MA4Pb5X16 films. Thus, a power conversion efficiency (PCE) of 17.21% is achieved in the optimized perovskite solar cells with the device structure of ITO/PEDOT:PSS/Perovskite/PCBM/BCP/Ag. In addition, the complementary humidity and thermal stability are obtained, which are ascribed to the enhanced interlayer interaction by BDA2+ and improved moisture resistance by the hydrophobic group of PEA+. The encapsulated devices are retained over 95% or 75% of the initial efficiency after storing 500 h in ambient air under 40 ± 5% relative humidity or 100 h in nitrogen at 60 °C.

Interphases, Interfaces, and Surfaces of Active Materials in Rechargeable Batteries and Perovskite Solar Cells

The ever-increasing demand for clean sustainable energy has driven tremendous worldwide investment in the design and exploration of new active materials for energy conversion and energy-storage devices. Tailoring the surfaces of and interfaces between different materials is one of the surest and best studied paths to enable high-energy-density batteries and high-efficiency solar cells. Metal-halide perovskite solar cells (PSCs) are one of the most promising photovoltaic materials due to their unprecedented development, with their record power conversion efficiency (PCE) rocketing beyond 25% in less than 10 years. Such progress is achieved largely through the control of crystallinity and surface/interface defects. Rechargeable batteries (RBs) reversibly convert electrical and chemical potential energy through redox reactions at the interfaces between the electrodes and electrolyte. The (electro)chemical and optoelectronic compatibility between active components are essential design considerations to optimize power conversion and energy storage performance. A focused discussion and critical analysis on the formation and functions of the interfaces and interphases of the active materials in these devices is provided, and prospective strategies used to overcome current challenges are described. These strategies revolve around manipulating the chemical compositions, defects, stability, and passivation of the various interfaces of RBs and PSCs.

Prospects of Z-Scheme Photocatalytic Systems Based on Metal Halide Perovskites

Considering the attractive optoelectronic properties of metal halide perovskites (MHPs), their introduction to the field of photocatalysis was only a matter of time. Thus far, MHPs have been explored for the photocatalytic generation of hydrogen, carbon dioxide reduction, organic synthesis, and pollutant degradation applications. Of growing research interest and possible applied significance are the currently emerging developments of MHP-based Z-scheme heterostructures, which can potentially enable efficient photocatalysis of highly energy-demanding redox processes. In this Perspective, we discuss the advantages and limitations of MHPs compared to traditional semiconductor materials for applications as photocatalysts and describe emerging examples in the construction of MHP-based Z-scheme systems. We discuss the principles and material properties that are required for the development of such Z-scheme heterostructure photocatalysts and consider the ongoing challenges and opportunities in this emerging field.

Unraveling the Charge Transport Mechanism in Mechanochemically Processed Hybrid Perovskite Solar Cell

The long-term operation of organic-inorganic hybrid perovskite solar cells is hampered by the microscopic strain introduced by the multiple thermal cycles during the synthesis of the material via a solution process route. This setback can be eliminated by a room temperature synthesis scheme. In this work, a mechanochemical synthesis technique at room temperature is employed to process CH₃NH₃PbI₂Br films for fabricating perovskite solar cell devices. The solar cell device has produced a 957 mV V_oc, a 16.92 mA/cm² short circuit current density, and a 10.5% efficiency. These values are higher than the published values on mechanochemically synthesized CH₃NH₃PbI₃. The charge transport properties of the devices are studied using DC conductivity and AC impedance spectroscopy, which show a multichannel transport mechanism having both ionic and electronic contributions. A much smaller defect density in the mechanochemically synthesized hybrid perovskite material is confirmed. A polarization assisted recombination mechanism is observed to have a dominant effect on the overall charge transport mechanism. However, no obvious grain boundary and intralayer lattice defect related responses are found in the perovskite layer. Interfacial charge transport and recombination are found to show major effects on both the temperature dependent and illumination dependent impedance spectra.

Recent advances toward environment-friendly photodetectors based on lead-free metal halide perovskites and perovskite derivatives

Recently, metal-halide perovskites have emerged as promising materials for photodetector (PD) applications owing to their superior optoelectronic properties, such as ambipolar charge transport characteristics, high carrier mobility, and so on. In the past few years, rapid progress in lead-based perovskite PDs has been witnessed. However, the critical environmental instability and lead-toxicity seriously hinder their further applications and commercialization. Therefore, searching for environmentally stable and lead-free halide perovskites (LFHPs) to address the above hurdles is certainly a worthwhile subject. In this review, we present a comprehensive overview of currently explored LFHPs with an emphasis on their crystal structures, optoelectronic properties, synthesis and modification methods, as well as the PD applications. LFHPs are classified into four categories according to the replacement strategies of Pb²⁺, including AB(ii)X₃, A₃B(iii)₂X₉, A₂B(i)B(iii)'X₆, and newly-emerging perovskite derivatives. Then, we give a demonstration of the preliminary achievements and limitations in environment-friendly PDs based on such LFHPs and perovskite derivatives, and also discuss their applications in biological synapses, imaging, and X-ray detection. With the perspective of their properties and current challenges, we provide an outlook for future directions in this rapidly evolving field to achieve high-quality LFHPs and perovskite derivatives for a broader range of fundamental research and practical applications.

Perovskite Light-Emitting Diodes with External Quantum Efficiency Exceeding 22% via Small-Molecule Passivation

Perovskite light-emitting diodes (PeLEDs) are considered as particularly attractive candidates for high-quality lighting and displays, due to possessing the features of wide gamut and real color expression. However, most PeLEDs are made from polycrystalline perovskite films that contain a high concentration of defects, including point and extended imperfections. Reducing and mitigating non-radiative recombination defects in perovskite materials are still crucial prerequisites for achieving high performance in light-emitting applications. Here, ethoxylated trimethylolpropane triacrylate (ETPTA) is introduced as a functional additive dissolved in antisolvent to passivate surface and bulk defects during the spinning process. The ETPTA can effectively decrease the charge trapping states by passivation and/or suppression of defects. Eventually, the perovskite films that are sufficiently passivated by ETPTA make the devices achieve a maximum external quantum efficiency (EQE) of 22.49%. To our knowledge, these are the most efficient green PeLEDs up to now. In addition, a threefold increase in the T₅₀ operational time of the devices was observed, compared to control samples. These findings provide a simple and effective strategy to make highly efficient perovskite polycrystalline films and their optoelectronics devices.

High-Performance Photodetectors Based on Nanostructured Perovskites

In recent years, high-performance photodetectors have attracted wide attention because of their important applications including imaging, spectroscopy, fiber-optic communications, remote control, chemical/biological sensing and so on. Nanostructured perovskites are extremely suitable for detective applications with their long carrier lifetime, high carrier mobility, facile synthesis, and beneficial to device miniaturization. Because the structure of the device and the dimension of nanostructured perovskite have a profound impact on the performance of photodetector, we divide nanostructured perovskite into 2D, 1D, and 0D, and review their applications in photodetector (including photoconductor, phototransistor, and photodiode), respectively. The devices exhibit high performance with high photoresponsivity, large external quantum efficiency (EQE), large gain, high detectivity, and fast response time. The intriguing properties suggest that nanostructured perovskites have a great potential in photodetection.

Dopant-Free All-Organic Small-Molecule HTMs for Perovskite Solar Cells: Concepts and Structure–Property Relationships

Since the introduction of Perovskite Solar Cells, their photovoltaic efficiencies have grown impressively, reaching over 25%. Besides the exceptional efficiencies, those solar cells need to be improved to overcome some concerns, such as their intrinsic instability when exposed to humidity. In this respect, the development of new and stable Hole Transporting Materials (HTMs) rose as a new hot topic. Since the doping agents for common HTM are hygroscopic, they bring water in contact with the perovskite layer, thus deteriorating it. In the last years, the research focused on “dopant-free” HTMs, which are inherently conductive without any addition of dopants. Dopant-free HTMs, being small molecules or polymers, have still been a relatively small set of compounds until now. This review collects almost all the relevant organic dopant-free small-molecule HTMs known so far. A general classification of HTMs is proposed, and structure analysis is used to identify structure–property relationships, to help researchers to build better-performing materials.

Mapping Trap Dynamics in a CsPbBr3 Single-Crystal Microplate by Ultrafast Photoemission Electron Microscopy

For versatile lead-halide perovskite materials, their trap states, both in the bulk and at the surface, significantly influence optoelectronic behaviors and the performance of the materials and devices. Direct observation of the trap dynamics at the nanoscale is necessary to understand and improve the device design. In this report, we combined the femtosecond pump-probe technique and photoemission electron microscopy (PEEM) to investigate the trap states of an inorganic perovskite CsPbBr₃ single-crystal microplate with spatial-temporal-energetic resolving capabilities. Several shallow trap sites were identified within the microplate, while the deep traps were resolved throughout the surface. The results revealed high-defect tolerance to the shallow traps, while the surface dynamics were dominated by the surface deep traps. The ultrafast PEEM disclosed a full landscape of fast electron transfer and accumulation of the surface trap states. These discoveries proved the excellent electronic properties of perovskite materials and the importance of surface optimization.

Materials, photophysics and device engineering of perovskite light-emitting diodes

Here we provide a comprehensive review of a newly developed lighting technology based on metal halide perovskites (i.e. perovskite light-emitting diodes) encompassing the research endeavours into materials, photophysics and device engineering. At the outset we survey the basic perovskite structures and their various dimensions (namely three-, two- and zero-dimensional perovskites), and demonstrate how the compositional engineering of these structures affects the perovskite light-emitting properties. Next, we turn to the physics underpinning photo- and electroluminescence in these materials through their connection to the fundamental excited states, energy/charge transport processes and radiative and non-radiative decay mechanisms. In the remainder of the review, we focus on the engineering of perovskite light-emitting diodes, including the history of their development as well as an extensive analysis of contemporary strategies for boosting device performance. Key concepts include balancing the electron/hole injection, suppression of parasitic carrier losses, improvement of the photoluminescence quantum yield and enhancement of the light extraction. Overall, this review reflects the current paradigm for perovskite lighting, and is intended to serve as a foundation to materials and device scientists newly working in this field.

Perovskite Light-Emitting Diodes with Near Unit Internal Quantum Efficiency at Low Temperatures

Room-temperature-high-efficiency light-emitting diodes based on metal halide perovskite FAPbI₃ are shown to be able to work perfectly at low temperatures. A peak external quantum efficiency (EQE) of 32.8%, corresponding to an internal quantum efficiency of 100%, is achieved at 45 K. Importantly, the devices show almost no degradation after working at a constant current density of 200 mA m^-2 for 330 h. The enhanced EQEs at low temperatures result from the increased photoluminescence quantum efficiencies of the perovskite, which is caused by the increased radiative recombination rate. Spectroscopic and calculation results suggest that the phase transitions of the FAPbI₃ play an important role for the enhancement of exciton binding energy, which increases the recombination rate.

Efficient and Stable Red Perovskite Light-Emitting Diodes with Operational Stability >300 h

The long-term operational stability of perovskite light-emitting diodes (PeLEDs), especially red PeLEDs with only several hours typically, has always faced great challenges. Stable β-CsPbI₃ nanocrystals (NCs) are demonstrated for highly efficient and stable red-emitting PeLEDs through incorporation of poly(maleic anhydride-alt-1-octadecene) (PMA) in synthesizing the NCs. The PMA can chemically interact with PbI₂ in the precursors via the coupling effect between O groups in PMA and Pb²⁺ to favor crystallization of stable β-CsPbI₃ NCs. Meanwhile, the cross-linked PMA significantly reduces the Pb_Cs anti-site defect on the surface of the β-CsPbI₃ NCs. Benefiting from the improved crystal phase quality, the photoluminescence quantum yield for β-CsPbI₃ NCs films remarkably increases from 34% to 89%. The corresponding red-emitting PeLEDs achieves a high external quantum efficiency of 17.8% and superior operational stability with the lifetime, the time to half the initial electroluminescence intensity (T₅₀ ) reaching 317 h at a constant current density of 30 mA cm^-2 .

Colorful Efficient Moiré-Perovskite Solar Cells

Light harvesting is crucial for thin-film solar cells. To substantially reduce optical loss in perovskite solar cells (PSCs), hierarchical light-trapping nano-architectures enable absorption enhancement to exceed the conventional upper limit and have great potential for achieving state-of-the art optoelectronic performances. However, it remains a great challenge to design and fabricate a superior hierarchical light-trapping nano-architecture, which exhibits extraordinary light-harvesting ability and simultaneously avoids deteriorating the electrical performance of PSCs. Herein, colorful efficient moiré-PSCs are designed and fabricated incorporating moiré interference structures by the imprinting method with the aid of a commercial DVD disc. It is experimentally and theoretically demonstrated that the light harvesting ability of the moiré interference structure can be well manipulated through changing the rotation angle (0°-90°). The boosted short-circuit current is credited to augment light diffraction channels, leading to elongated optical paths, and fold sunlight into the perovskite layer. Moreover, the imprinting process suppresses the trap sites and voids at the active-layer interfaces with eliminated hysteresis. The moiré-PSC with an optimized 30° rotation angle achieves the best enhancement of light harvesting (28.5% higher than the pristine), resulting in efficiencies over 20.17% (MAPbI₃ ) and 21.76% ((FAPbI₃ )_{1- x} (MAPbBr₃ )_x ).

Cementitious grain-boundary passivation for flexible perovskite solar cells with superior environmental stability and mechanical robustness

The power conversion effciency (PCE) of flexible perovskite solar cells (PSCs) has increased rapidly, while the mechanical flexibility and environmental stability are still far from satisfactory. Previous studies show the environmental degradation and ductile cracks of perovskite films usually begin at the grain boundaries (GBs). Herein, sulfonated graphene oxide (s-GO) is employed to construct a cementitious GBs by interacting with the [PbI₆]^4- at GBs. The resultant s-GO-[PbI₆]^4- complex can effectively passivate the defects of vacant iodine, and the devices with s-GO exhibit remarkable waterproofness and flexibility due to the tough and water-insoluble GBs. The champion PCE of 20.56% (1.01 cm²) in a device treated with s-GO is achieved. This device retains 90% of its original PCE after 180 d stored in the ambient condition, as well as over 80% retention after 10,000 bending cycles at a curvature radius of 3 mm.

Energetic and electronic properties of CsPbBr3 surfaces: a first-principles study

Surface properties of all-inorganic halide perovskites play a crucial role in determining optoelectronic performance of these materials. We investigate the surface energies and electronic structures of cubic CsPbBr₃ surfaces systematically using density functional theory (DFT) methods. We calculate the surface phase diagrams of low-index surfaces of CsPbBr₃, i.e., (100), (110), (111) surfaces. We found that nonpolar (100) surfaces are more stable than polar (110) and (111) surfaces. The nonpolar CsBr-terminated (100) surface shows the best stability, which is attributed to the effect of surface relaxation and high ionicity of the surface layer. The electronic structures reveal that charge transfer to compensate the polarity raises the energy of polar surfaces, which makes polar surfaces unstable. Furthermore, we found that the modulation of surface chemical composition provides an effective way to compensate polarity and thus make polar surfaces of CsPbBr₃ stable. Our results provide physical insights into understanding and further enhancing the surface stability of all-inorganic halide perovskites. This would be helpful in promoting the advancement of all-inorganic halide perovskite-based materials and devices.

Frenkel defects promote polaronic exciton dissociation in methylammonium lead iodide perovskites

Hybrid organic-inorganic perovskite materials, such as CH₃NH₃PbI₃, exhibit substantial potential in a variety of optoelectronic applications. Nevertheless, the interplay between the photoinduced excitations and iodine Frenkel defects which are abundant in CH₃NH₃PbI₃ films remains poorly understood. Here we study the light-triggered electronic and excitonic properties in the presence of iodine Frenkel defects in CH₃NH₃PbI₃ by using a combination of density functional theory (DFT) and time-dependent DFT approaches, the latter of which treats electron-hole and electron-nucleus interactions on the same footing. For isolated Frenkel defects, electrons are trapped close to the iodine vacancies and the electron-hole correlation brings the holes in close vicinity to the electrons, yielding tightly bound polaronic excitons. However, in the presence of multiple interactive Frenkel defects, the holes are pulled out from an electron-hole Coulomb well by the iodine interstitials, leading to spatially separated electron-hole pairs. The X-ray photoelectron spectra are then simulated, unravelling the light-triggered charge transfer induced by Frenkel defects at the atomistic level. We also find that the energy and spatial distributions of polaronic excitons at the Frenkel defects can be controlled by the dynamical rotation of organic cations.