Limitations and Progresses in Carbon-Based Cesium Lead Halide Perovskite Solar Cells

Inorganic cesium lead halide perovskites (CsPbIxBr3-x, 0≤x≤3) are promising alternatives with great thermal stability. Additionally, the choice of moisture-resistive and dopant-free carbon as the electrode material can simultaneously solve the problems of stability and cost. Therefore, carbon electrode-based inorganic PSCs (C-IPSCs) represent a promising candidate for commercialization, yet both the efficiencies and stability of related devices demand further progress. This article reviews the recent advancement of C-IPSCs and then unravels the distinctive merits and limitations in this field. Subsequently, our perspective on various modification strategies is analyzed on a methodological level. Finally, this article outlooks the promising research contents and the remaining unresolved issues in this field. We believe that understanding and analyzing the related problems in this field are instructive to stimulate the future development of C-IPSCs.

Transparent Recombination Layers Design and Rational Characterizations for Efficient Two-Terminal Perovskite-Based Tandem Solar Cells

Two-terminal (2T) perovskite-based tandem solar cells (TSCs) arouse burgeoning interest in breaking the Shockley-Queisser (S-Q) limit of single-junction solar cells by combining two subcells with different bandgaps. However, the highest certified efficiency of 2T perovskite-based TSCs (33.9%) lags behind the theoretical limit (42-43%). A vital challenge limiting the development of 2T perovskite-based TSCs is the transparent recombination layers/interconnecting layers (RLs) design between two subcells. To improve the performance of 2T perovskite-based TSCs, RLs simultaneously fulfill the optical loss, contact resistance, carrier mobility, stress management, and conformal coverage requirements. In this review, the definition, functions, and requirements of RLs in 2T perovskite-based TSCs are presented. The insightful characterization methods applicable to RLs, which are inspiring for further research on the RLs both in 2T perovskite-based two-junction and multi-junction TSCs, are also highlighted. Finally, the key factors that currently limit the performance enhancement of RLs and the future directions that should be continuously focused on are summarized.

Challenges in Developing Perovskite Nanocrystals for Commercial Applications

Zero-dimensional lead halide perovskite nanocrystals (NCs) exhibit size-dependent bandgap and carrier confinement compared to bulk counterparts due to the quantum confinement effect, making them essential for achieving wide-color-gamut displays, studying excitonic spin relaxation, and constructing superlattices. Despite their promising potential, they face a variety of technical bottlenecks, such as insufficient color reproducibility, limited large-scale production, low stability, and toxicity. An outline of a research roadmap is provided in the review, which highlights key challenges in developing perovskite NCs for commercial applications.

Antisolvent-Free Heterogenous Nucleation Enabled by Employing 4-Tert-Butyl Pyridine Additive and Two-Step Annealing for Efficient CsPbI2Br Perovskite Solar Cells

All inorganic perovskite based on CsPbI2Br has attracted significant attention due to its relatively thermal stable structure compare to its hybrid counterparts. With a wide bandgap of 1.9 eV and excellent light absorption capability, it has been extensively explored for applications in indoor photovoltaics and as a front absorber in tandem devices. However, the uncontrollable crystallization process during solvent evaporation and thermal annealing leads to both macroscopic defects like cracks and microscopic defects such as voids. In this study, a metastable adduct with lead (II) halides by incorporating 4-tert-butyl pyridine as a volatile Lewis base monodentate ligand in the precursor solution is formed. The strategic preferential decomposition of the adduct during the early-stage low-temperature annealing facilitated the desorption of lead (II) halides, inducing antisolvent-free heterogenous nucleation. This, in turn, promoted crystal growth during subsequent high-temperature annealing, resulting in dense films with low defect density. As a result, a maximum open-circuit voltage of 1.30 V is achieved with the champion power conversion efficiency of 16.5% in CsPbI2Br-based perovskite solar cell. The work reveals a new mechanism of using Lewis acid-base adduct to obtain high quality perovskite films other than hindering crystallization in traditional way.

Understanding Defects in Perovskite Solar Cells through Computation: Current Knowledge and Future Challenge

Lead halide perovskites with superior optoelectrical properties are emerging as a class of excellent materials for applications in solar cells and light-emitting devices. However, perovskite films often exhibit abundant intrinsic defects, which can limit the efficiency of perovskite-based optoelectronic devices by acting as carrier recombination centers. Thus, an understanding of defect chemistry in lead halide perovskites assumes a prominent role in further advancing the exploitation of perovskites, which, to a large extent, is performed by relying on first-principles calculations. However, the complex defect structure, strong anharmonicity, and soft lattice of lead halide perovskites pose challenges to defect studies. In this perspective, on the basis of briefly reviewing the current knowledge concerning computational studies on defects, this work concentrates on addressing the unsolved problems and proposing possible research directions in future. This perspective particularly emphasizes the indispensability of developing advanced approaches for deeply understanding the nature of defects and conducting data-driven defect research for designing reasonable strategies to further improve the performance of perovskite applications. Finally, this work highlights that theoretical studies should pay more attention to establishing close and clear links with experimental investigations to provide useful insights to the scientific and industrial communities.

High-throughput screening of the transport behavior of tetragonal perovskites

Halide perovskites have attracted attention due to their low cost and excellent optoelectronic properties. Although their optical properties gained widespread consensus, there was still divergence in understanding carrier transport behavior. In this study, the mobility of tetragonal perovskites was investigated by empirical models, including longitudinal acoustic phonon (LAP) and polar optical phonon (POP) models. The results revealed that the mobility predicted from the LAP model was much higher than that from the POP model. A longitudinal optical phonon (LOP) was considered as the decisive scattering source for charge carriers in perovskites. Furthermore, the mobility was extremely sensitive to z-axis strain, and 8 types of perovskites with high carrier mobility were screened. Using the experimental lattice constants, the predicted mobility of CsSnI3 was μe,z = 1428 and μh,z = 2310 cm2 V-1 s-1, respectively. The tetragonal CsSnI3 has high mobility and moderate bandgaps, suggesting potential applications in high-efficiency solar cells.

All-Inorganic Perovskite Solar Cells: Defect Regulation and Emerging Applications in Extreme Environments

All-inorganic perovskite solar cells (PSCs), such as CsPbX3 , have garnered considerable attention recently, as they exhibit superior thermodynamic and optoelectronic stabilities compared to the organic-inorganic hybrid PSCs. However, the power conversion efficiency (PCE) of CsPbX3 PSCs is generally lower than that of organic-inorganic hybrid PSCs, as they contain higher defect densities at the interface and within the perovskite light-absorbing layers, resulting in higher non-radiative recombination and voltage loss. Consequently, defect regulation has been adopted as an important strategy to improve device performance and stability. This review aims to comprehensively summarize recent progresses on the defect regulation in CsPbX3 PSCs, as well as their cutting-edge applications in extreme scenarios. The underlying fundamental mechanisms leading to the defect formation in the crystal structure of CsPbX3 PSCs are firstly discussed, and an overview of literature-adopted defect regulation strategies in the context of interface, internal, and surface engineering is provided. Cutting-edge applications of CsPbX3 PSCs in extreme environments such as outer space and underwater situations are highlighted. Finally, a summary and outlook are presented on future directions for achieving higher efficiencies and superior stability in CsPbX3 PSCs.

Strain Engineering in Perovskites: Mutual Insight on Oxides and Halides

Perovskite materials have garnered significant attention over the past decades due to their applications, not only in electronic materials, such as dielectrics, piezoelectrics, ferroelectrics, and superconductors but also in optoelectronic devices like solar cells and light emitting diodes. This interest arises from their versatile combinations and physiochemical tunability. While strain engineering is a recognized powerful tool for tailoring material properties, its collaborative impact on both oxides and halides remains understudied. Herein, strain engineering in perovskites for energy conversion devices, providing mutual insight into both oxides and halides is discussed. The various experimental methods are presented for applying strain by using thermal mismatch, lattice mismatch, defects, doping, light illumination, and flexible substrates. In addition, the main factors that are influenced by strain, categorized as structure (e.g., symmetry breaking, octahedral distortion), bandgap, chemical reactivity, and defect formation energy are described. After that, recent progress in strain engineering for perovskite oxides and halides for energy conversion devices is introduced. Promising methods for enhancing the performance of energy conversion devices using perovskites through strain engineering are suggested.

Two-Stage Dynamic Transformation from δ- to α-CsPbI3

The phase transformation from δ- to α-CsPbI3has garnered extensive research interest. However, detailed understanding of this structural transformation at atomistic scale remains elusive. Here, we reported the full atomistic molecular dynamics simulation of this important phase transformation using an enhanced sampling method, Metadynamics (MetaD). Particularly, two-stage of dynamic transformation related to [PbI3]- chains' motions was observed, namely, the intrachain rearrangement followed by interchain connection. Moreover, the dynamic motion of Cs+ cations plays an important role in facilitating the interchain connection kinetically. The insights reported in this work will provide valuable guidance for further advancing the understanding of phase transformation of CsPbI3.

Spontaneous crystallization of strongly confined CsSnxPb1-xI3 perovskite colloidal quantum dots at room temperature

The scalable and low-cost room temperature (RT) synthesis for pure-iodine all-inorganic perovskite colloidal quantum dots (QDs) is a challenge due to the phase transition induced by thermal unequilibrium. Here, we introduce a direct RT strongly confined spontaneous crystallization strategy in a Cs-deficient reaction system without polar solvents for synthesizing stable pure-iodine all-inorganic tin-lead (Sn-Pb) alloyed perovskite colloidal QDs, which exhibit bright yellow luminescence. By tuning the ratio of Cs/Pb precursors, the size confinement effect and optical band gap of the resultant CsSnxPb1-xI3 perovskite QDs can be well controlled. This strongly confined RT approach is universal for wider bandgap bromine- and chlorine-based all-inorganic and iodine-based hybrid perovskite QDs. The alloyed CsSn0.09Pb0.91I3 QDs show superior yellow emission properties with prolonged carrier lifetime and significantly increased colloidal stability compared to the pristine CsPbI3 QDs, which is enabled by strong size confinement, Sn2+ passivation and enhanced formation energy. These findings provide a RT size-stabilized synthesis pathway to achieve high-performance pure-iodine all-inorganic Sn-Pb mixed perovskite colloidal QDs for optoelectronic applications.

Unveiling the Potential of Ambient Air Annealing for Highly Efficient Inorganic CsPbI3 Perovskite Solar Cells

Here, we report a detailed surface analysis of dry- and ambient air-annealed CsPbI3 films and their subsequent modified interfaces in perovskite solar cells. We revealed that annealing in ambient air does not adversely affect the optoelectronic properties of the semiconducting film; instead, ambient air-annealed samples undergo a surface modification, causing an enhancement of band bending, as determined by hard X-ray photoelectron spectroscopy measurements. We observe interface charge carrier dynamics changes, improving the charge carrier extraction in CsPbI3 perovskite solar cells. Optical spectroscopic measurements show that trap state density is decreased due to ambient air annealing. As a result, air-annealed CsPbI3-based n-i-p structure devices achieved a 19.8% power conversion efficiency with a 1.23 V open circuit voltage.

Stability Strategies and Applications of Iodide Perovskites

Iodide perovskites have demonstrated their unprecedented high efficiency and commercialization potential, and their superior optoelectronic properties, such as high absorption coefficient, high carrier mobility, and narrow direct bandgap, have attracted much attention, especially in solar cells, photodetectors, and light-emitting diodes (LEDs). However, whether it is organic iodide perovskite, organic-inorganic hybrid iodide perovskite or all-inorganic iodide perovskite the stability of these iodide perovskites is still poor and the contamination is high. In recent years, scholars have studied more iodide perovskites to improve their stability as well as optoelectronic properties from various angles. This paper systematically reviews the strategies (component engineering, additive engineering, dimensionality reduction engineering, and phase mixing engineering) used to improve the stability of iodide perovskites and their applications in recent years.

Pressure-Induced Topological Phase Transition and Large Rashba Effect in Halide Double Perovskite

In general, hydrostatic pressure can suppress ferroelectric polarization and further reduce Rashba spin-splitting, considering the spin-orbit coupling effect. Here, we present the design of ferroelectric double perovskite Cs2SnSiI6, which exhibits the anomalous enhancement of Rashba spin-splitting parameters by pressure-induced ferroelectric topological order. The Rashba effect is nonlinear with the decrease in polarization under pressure and reaches a maximum at the pressure-induced Weyl semimetal (WSM) state between the transition from a normal insulator (NI) to a topological insulator (TI). Furthermore, we discover that controlling ferroelectric polarization with an electric field can also induce the topological transition with a large Rashba spin-splitting but under a lower critical pressure. These discoveries show a tunable gaint Rashba effect and pressure-induced topological phase transition for Cs2SnSiI6, which can promote future research on the interaction between the Rashba effect and topological order, and its application to new electronic and spintronic devices.

Monitoring Phase Separation and Dark Recovery in Mixed Halide Perovskite Clusters and Single Crystals Using In Situ Spectromicroscopy

Mixed halide perovskites (MHPs) are a group of semiconducting materials with promising applications in optoelectronics and photovoltaics, whose bandgap can be altered by adjusting the halide composition. However, the current challenge is to stabilize the light-induced halide separation, which undermines the device's performance. Herein we track down the phase separation dynamics of CsPbBr1.2I1.8 MHP single cubic nanocrystals (NCs) and clusters as a function of time by in situ fluorescence spectromicroscopy. The particles were sorted into groups 1 and 2 using initial photoluminescence intensities. The phase separation followed by recovery kinetics under dark and photo blinking analysis suggests that group 1 behaved more like single NCs and group 2 behaved like clusters. Under the 0.64 W/cm2 laser illumination, the phase shifts for single NCs are 3.4 ± 1.9 nm. The phase shifts are linearly correlated with the initial photoluminescence intensities of clusters, suggesting possible interparticle halide transportation.

Recent Progress of Low-Toxicity Poor-Lead All-Inorganic Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have achieved an impressive certified efficiency of 25.7%, which is comparatively higher than that of commercial silicon solar cells (23.3%), showing great potential toward commercialization. However, the low stability and high toxicity due to the presence of volatile organic components and toxic metal lead in the perovskites pose significant challenges. To obtain robust and low-toxicity PSCs, substituting organic cations with pure inorganic cations, and partially or fully replacing the toxic Pb with environmentally benign metals, is one of the promising methods. To date, continuous efforts have been made toward the construction of highly performed low-toxicity inorganic PSCs with astonishing breakthroughs. This review article provides an overview of recent progress in inorganic PSCs in terms of lead-reduced and lead-free compositions. The physical properties of poor-lead all-inorganic perovskites are discussed to unveil the major challenges in this field. Then, it reports notable achievements for the experimental studies to date to figure out feasible methods for efficient and stable poor-lead all-inorganic PSCs. Finally, a discussion of the challenges and prospects for poor-lead all-inorganic PSCs in the future is presented.

Dynamical control of nanoscale light-matter interactions in low-dimensional quantum materials

Tip-enhanced nano-spectroscopy and -imaging have significantly advanced our understanding of low-dimensional quantum materials and their interactions with light, providing a rich insight into the underlying physics at their natural length scale. Recently, various functionalities of the plasmonic tip expand the capabilities of the nanoscopy, enabling dynamic manipulation of light-matter interactions at the nanoscale. In this review, we focus on a new paradigm of the nanoscopy, shifting from the conventional role of imaging and spectroscopy to the dynamical control approach of the tip-induced light-matter interactions. We present three different approaches of tip-induced control of light-matter interactions, such as cavity-gap control, pressure control, and near-field polarization control. Specifically, we discuss the nanoscale modifications of radiative emissions for various emitters from weak to strong coupling regime, achieved by the precise engineering of the cavity-gap. Furthermore, we introduce recent works on light-matter interactions controlled by tip-pressure and near-field polarization, especially tunability of the bandgap, crystal structure, photoluminescence quantum yield, exciton density, and energy transfer in a wide range of quantum materials. We envision that this comprehensive review not only contributes to a deeper understanding of the physics of nanoscale light-matter interactions but also offers a valuable resource to nanophotonics, plasmonics, and materials science for future technological advancements.

Zirconium Doping to Enable High-Efficiency and Stable CsPbI2Br All-Inorganic Perovskite Solar Cells

All-inorganic wide-bandgap perovskite CsPbI2Br has attracted much attention because of its inherent thermal stability and ideal bandgap for the front subcell of tandem solar cells (TSCs). However, the low power conversion efficiency (PCE) and poor moisture stability of CsPbI2Br still restrict its future commercialization. Herein, zirconium tetrachloride (ZrCl4) was doped into CsPbI2Br films to modulate the crystal growth and improve the film quality. The partial substitution of the B-site Pb2+ of CsPbI2Br with Zr4+ suppresses the unwanted phase conversion from the crystallized black α-phase to the δ-phase, resulting in improved phase stability. Consequently, the humidity and thermal stability of the film are greatly improved. Additionally, the incorporation of ZrCl4 suppresses nonradiative recombination and forms a matched energy-level alignment with the hole-transport layer (Spiro-OMeTAD). Benefiting from these features, the ZrCl4-doped CsPbI2Br perovskite solar cell (PSC) shows an outstanding efficiency of 16.60% with a high open-circuit voltage of 1.29 V. The unencapsulated devices simultaneously show excellent humidity and thermal stability, retaining over 91% of PCEinitial after 1000 h of aging in ambient air conditions and 92% PCEinitial after 500 h of continuous heating at 85 °C in a nitrogen environment, respectively. Furthermore, ZrCl4-doped CsPbI2Br was employed as the front subcell of perovskite/organic TSCs and achieved a remarkable PCE of 19.42%, showing great potential for highly efficient and stable TSCs.

Interfacial alloying between lead halide perovskite crystals and hybrid glasses

The stellar optoelectronic properties of metal halide perovskites provide enormous promise for next-generation optical devices with excellent conversion efficiencies and lower manufacturing costs. However, there is a long-standing ambiguity as to whether the perovskite surface/interface (e.g. structure, charge transfer or source of off-target recombination) or bulk properties are the more determining factor in device performance. Here we fabricate an array of CsPbI3 crystal and hybrid glass composites by sintering and globally visualise the property-performance landscape. Our findings reveal that the interface is the primary determinant of the crystal phases, optoelectronic quality, and stability of CsPbI3. In particular, the presence of a diffusion "alloying" layer is discovered to be critical for passivating surface traps, and beneficially altering the energy landscape of crystal phases. However, high-temperature sintering results in the promotion of a non-stoichiometric perovskite and excess traps at the interface, despite the short-range structure of halide is retained within the alloying layer. By shedding light on functional hetero-interfaces, our research offers the key factors for engineering high-performance perovskite devices.

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

In situ imaging of the atomic phase transition dynamics in metal halide perovskites

Phase transition dynamics are an important concern in the wide applications of metal halide perovskites, which fundamentally determine the optoelectronic properties and stabilities of perovskite materials and devices. However, a more in-depth understanding of such a phase transition process with real atomic resolution is still limited by the immature low-dose electron microscopy and in situ imaging studies to date. Here, we apply an emergent low-dose imaging technique to identify different phase structures (α, β and γ) in CsPbI3 nanocrystals during an in-situ heating process. The rotation angles of PbI6 octahedrons can be measured in these images to quantitatively describe the thermal-induced phase distribution and phase transition. Then, the dynamics of such a phase transition are studied at a macro time scale by continuously imaging the phase distribution in a single nanocrystal. The structural evolution process of CsPbI3 nanocrystals at the particle level, including the changes in morphology and composition, is also visualized with increasing temperature. These results provide atomic insights into the transition dynamics of perovskite phases, indicating a long-time transition process with obvious intermediate states and spatial distribution that should be generally considered in the further study of structure-property relations and device performance.

Controllable Black-to-Yellow Phase Transition by Tuning the Lattice Symmetry in Perovskite Quantum Dots

The black-to-yellow phase transition in perovskite quantum dots (QDs) is more complex than in bulk perovskites, regarding the role of surface energy. Here, with the assistance of in situ grazing-incidence wide-angle and small-angle X-ray scattering (GIWAXS/GISAXS), distinct phase behaviors of cesium lead iodide (CsPbI3 ) QD films under two different temperature profiles-instant heating-up (IHU) and slow heating-up (SHU) is investigated. The IHU process can cause the phase transition from black phase to yellow phase, while under the SHU process, the majority remains in black phase. Detailed studies and structural refinement analysis reveal that the phase transition is triggered by the removal of surface ligands, which switches the energy landscape. The lattice symmetry determines the transition rate and the coexistence black-to-yellow phase ratio. The SHU process allows longer relaxation time for a more ordered QD packing, which helps sustain the lattice symmetry and stabilizes the black phase. Therefore, one can use the lattice symmetry as a general index to monitor the CsPbI3 QD phase transition and finetune the coexistence black-to-yellow phase ratio for niche applications.

Engineering Carrier Dynamics in Halide Perovskites by Dynamical Lattice Distortion

The electronic structure of halide perovskites is central to their carrier dynamics, enabling the excellent optoelectronic performance. However, the experimentally resolved transient absorption spectra exhibit large discrepancies from the commonly computed electronic structure by density functional theory. Using pseudocubic CsPbI3 as a prototype example, here, it is unveiled with both ab initio molecular dynamics simulations and transmission electron microscopy that there exists pronounced dynamical lattice distortion in the form of disordered instantaneous octahedral tilting. Rigorous first-principles calculations reveal that the lattice distortion substantially alters the electronic band structure through renormalizing the band dispersions and the interband transition energies. Most notably, the electron and hole effective masses increase by 65% and 88%, respectively; the transition energy between the two highest valence bands decreases by about one half, agreeing remarkably well with supercontinuum transient-absorption measurements. This study further demonstrates how the resulting electronic structure modulates various aspects of the carrier dynamics such as carrier transport, hot-carrier relaxation, Auger recombination, and carrier multiplication in halide perovskites. The insights provide a pathway to engineer carrier transport and relaxation via lattice distortion, enabling the promise to achieve ultrahigh-efficiency photovoltaic devices.

Nitrogen-Doped Graphene Quantum Dot-Passivated δ-Phase CsPbI3: A Water-Stable Photocatalytic Adjuvant to Degrade Rhodamine B

Inorganic halide perovskite CsPbI3 is highly promising in the photocatalytic field for its strong absorption of UV and visible light. Among the crystal phases of CsPbI3, the δ-phase as the most aqueous stability; however, directly using it in water is still not applicable, thus limiting its dye photodegradation applications in aqueous solutions. Via adopting nitrogen-doped graphene quantum dots (NGQDs) as surfactants to prepare δ-phase CsPbI3 nanocrystals, we obtained a water-stable material, NGQDs-CsPbI3. Such a material can be well dispersed in water for a month without obvious deterioration. High-resolution transmission electron microscopy and X-ray diffractometer characterizations showed that NGQDs-CsPbI3 is also a δ-phase CsPbI3 after NGQD coating. The ultraviolet-visible absorption spectra indicated that compared to δ-CsPbI3, NGQDs-CsPbI3 has an obvious absorption enhancement of visible light, especially near the wavelength around 521 nm. The good dispersity and improved visible-light absorption of NGQDs-CsPbI3 benefit their aqueous photocatalytic applications. NGQDs-CsPbI3 alone can photodegrade 67% rhodamine B (RhB) in water, while after compositing with TiO2, NGQDs-CsPbI3/TiO2 exhibits excellent visible-light photocatalytic ability, namely, it photodegraded 96% RhB in 4 h. The strong absorption of NGQDs-CsPbI3 in the visible region and effective transfer of photogenerated carriers from NGQDs-CsPbI3 to TiO2 play the key roles in dye photodegradation. We highlight NGQDs-CsPbI3 as a water-stable halide perovskite material and effective photocatalytic adjuvant.

Interface Modification for Energy Level Alignment and Charge Extraction in CsPbI3 Perovskite Solar Cells

In perovskite solar cells (PSCs) energy level alignment and charge extraction at the interfaces are the essential factors directly affecting the device performance. In this work, we present a modified interface between all-inorganic CsPbI3 perovskite and its hole-selective contact (spiro-OMeTAD), realized by the dipole molecule trioctylphosphine oxide (TOPO), to align the energy levels. On a passivated perovskite film, with n-octylammonium iodide (OAI), we created an upward surface band-bending at the interface by TOPO treatment. This improved interface by the dipole molecule induces a better energy level alignment and enhances the charge extraction of holes from the perovskite layer to the hole transport material. Consequently, a Voc of 1.2 V and a high-power conversion efficiency (PCE) of over 19% were achieved for inorganic CsPbI3 perovskite solar cells. Further, to demonstrate the effect of the TOPO dipole molecule, we present a layer-by-layer charge extraction study by a transient surface photovoltage (trSPV) technique accomplished by a charge transport simulation.

Multislip-enabled morphing of all-inorganic perovskites

All-inorganic lead halide perovskites (CsPbX3, X = Cl, Br or I) are becoming increasingly important for energy conversion and optoelectronics because of their outstanding performance and enhanced environmental stability. Morphing perovskites into specific shapes and geometries without damaging their intrinsic functional properties is attractive for designing devices and manufacturing. However, inorganic semiconductors are often intrinsically brittle at room temperature, except for some recently reported layered or van der Waals semiconductors. Here, by in situ compression, we demonstrate that single-crystal CsPbX3 micropillars can be substantially morphed into distinct shapes (cubic, L and Z shapes, rectangular arches and so on) without localized cleavage or cracks. Such exceptional plasticity is enabled by successive slips of partial dislocations on multiple [Formula: see text] systems, as evidenced by atomic-resolution transmission electron microscopy and first-principles and atomistic simulations. The optoelectronic performance and bandgap of the devices were unchanged. Thus, our results suggest that CsPbX3 perovskites, as potential deformable inorganic semiconductors, may have profound implications for the manufacture of advanced optoelectronics and energy systems.

Thermal Transport Properties of Phonons in Halide Perovskites

Halide perovskites have emerged as promising candidates for various applications, such as photovoltaic, optoelectronic and thermoelectric applications. The knowledge of the thermal transport of halide perovskites is essential for enhancing the device performance for these applications and improving the understanding of heat transport in complicated material systems with atomic disorders. In this work, the current understanding of the experimentally and theoretically obtained thermal transport properties of halide perovskites is reviewed. This study comprehensively examines the reported thermal conductivity of methylammonium lead iodide, which is a prototype material, and provides theoretical frameworks for its lattice vibrational properties. The frameworks and discussions are extended to other halide perovskites and derivative structures. The implications for device applications, such as solar cells and thermoelectrics, are discussed.

Optical Memory, Switching, and Neuromorphic Functionality in Metal Halide Perovskite Materials and Devices

Metal halide perovskite based materials have emerged over the past few decades as remarkable solution-processable optoelectronic materials with many intriguing properties and potential applications. These emerging materials have recently been considered for their promise in low-energy memory and information processing applications. In particular, their large optical cross-sections, high photoconductance contrast, large carrier-diffusion lengths, and mixed electronic/ionic transport mechanisms are attractive for enabling memory elements and neuromorphic devices that are written and/or read in the optical domain. Here, recent progress toward memory and neuromorphic functionality in metal halide perovskite materials and devices where photons are used as a critical degree of freedom for switching, memory, and neuromorphic functionality is reviewed.

Recent progress in the stabilization of low band-gap black-phase iodide perovskite solar cells

All-inorganic and organic-inorganic hybrid perovskite solar cells (PSCs) have taken a quantum leap owing to their high performance and low-cost solution processability. Their efficiency has been dramatically increased up to ∼26%, matching the conventional inorganic photovoltaics like monocrystalline Si (26.1%), polycrystalline Si (21.6%), CdTe (22.1%), and CIGS (22.3%). Such outstanding performance has been achieved due to their excellent optoelectronic properties, such as a direct bandgap in the visible region, a very high absorption coefficient, a long charge-carrier diffusion length, and ambipolar carrier transport characteristics. FAPbI3 (FA = formamidinium) and CsPbI3 perovskites among the pool of perovskites are recommended for solar cell applications because they meet all the requirements for photovoltaic applications. However, the fundamental problem of these perovskites is that their photoactive black phase is highly unstable under ambient conditions due to small and large sizes of Cs+ and FA+ ions, respectively. The instability of the black phase of these perovskites hinders their applications in photovoltaic devices as a high-quality light absorber layer. Several approaches have been employed to prevent the formation of the photo-inactive yellow phase or to enhance the stability of the black phase of perovskites, such as dimensional and compositional engineering, the addition of external additives, and dimensional engineering. This perspective summarizes the various methods for stabilizing the black phase of CsPbI3 and FAPbI3 perovskites at room temperature as well as their application in photovoltaic devices.

Synergistic Halide- and Ligand-Exchanges of All-Inorganic Perovskite Nanocrystals for Near-Unity and Spectrally Stable Red Emission

All-inorganic perovskite nanocrystals (NCs) of CsPbX3 (X = Cl, Br, I) are promising for displays due to wide color gamut, narrow emission bandwidth, and high photoluminescence quantum yield (PLQY). However, pure red perovskite NCs prepared by mixing halide ions often result in defects and spectral instabilities. We demonstrate a method to prepare stable pure red emission and high-PLQY-mixed-halide perovskite NCs through simultaneous halide-exchange and ligand-exchange. CsPbBr3 NCs with surface organic ligands are first synthesized using the ligand-assisted reprecipitation (LARP) method, and then ZnI2 is introduced for anion exchange to transform CsPbBr3 to CsPbBrxI3-x NCs. ZnI2 not only provides iodine ions but also acts as an inorganic ligand to passivate surface defects and prevent ion migration, suppressing non-radiative losses and halide segregation. The luminescence properties of CsPbBrxI3-x NCs depend on the ZnI2 content. By regulating the ZnI2 exchange process, red CsPbBrxI3-x NCs with organic/inorganic hybrid ligands achieve near-unity PLQY with a stable emission peak at 640 nm. The CsPbBrxI3-x NCs can be combined with green CsPbBr3 NCs to construct white light-emitting diodes with high-color gamut. Our work presents a facile ion exchange strategy for preparing spectrally stable mixed-halide perovskite NCs with high PLQY, approaching the efficiency limit for display or lighting applications.

Surface Modification in CsPb0.5Sn0.5I2Br Inorganic Perovskite Solar Cells: Effects of Bifunctional Dipolar Molecules on Photovoltaic Performance

Inorganic tin-lead binary perovskites have piqued the interest of researchers as effective absorbers for thermally stable solar cells. However, the nonradiative recombination originating from the surface undercoordinated Sn²⁺ cations and the energetic offsets between different layers cause an excessive energy loss and deteriorate the perovskite device's performance. In this study, we investigated two thioamide derivatives that differ only in the polar part connected to their common benzene ring, namely, benzenecarbothioamide and 4-fluorophenylcarbothioamide (F-TBA). These two molecules were implemented as modifiers onto the inorganic tin-lead perovskite (CsPb_0.5Sn_0.5I₂Br) surface in the perovskite solar cells. Modifiers that carry C═S and NH₂ functional groups, equipped with lone electron pairs, can autonomously associate with surface Sn²⁺ through coordination and electrostatic attraction mechanisms. This interaction serves effectively to passivate the surface. In addition, due to the permanent dipole moment of the intermediate layer, an interfacial dipole field appears at the PCBM/CsPb_0.5Sn_0.5I₂Br interface, reducing the electron extraction potential barrier. Consequently, the planar solar cell with an ITO/PEDOT:PSS/CsPb_0.5Sn_0.5I₂Br/PCBM/BCP/Ag layered structure featuring an F-TBA surface post-treatment demonstrated a noteworthy power conversion efficiency of 14.01%. Simultaneously, after being stored for 1000 h in an inert atmosphere glovebox, the non-encapsulated CsPb_0.5Sn_0.5I₂Br solar cells managed to preserve 94% of their original efficiency.

Stabilization of photoactive phases for perovskite photovoltaics

Interest in photovoltaics (PVs) based on Earth-abundant halide perovskites has increased markedly in recent years owing to the remarkable properties of these materials and their suitability for energy-efficient and scalable solution processing. Formamidinium lead triiodide (FAPbI₃)-rich perovskite absorbers have emerged as the frontrunners for commercialization, but commercial success is reliant on the stability meeting the highest industrial standards and the photoactive FAPbI₃ phase suffers from instabilities that lead to degradation - an effect that is accelerated under working conditions. Here, we critically assess the current understanding of these phase instabilities and summarize the approaches for stabilizing the desired phases, covering aspects from fundamental research to device engineering. We subsequently analyse the remaining challenges for state-of-the-art perovskite PVs and demonstrate the opportunities to enhance phase stability with ongoing materials discovery and in operando analysis. Finally, we propose future directions towards upscaling perovskite modules, multijunction PVs and other potential applications.

Highly Stable and Efficient Light-Emitting Diodes Based on Orthorhombic γ-CsPbI3 Nanocrystals

Orthorhombic γ-CsPbI₃ possesses the highest structural stability among the optically active (light-emissive) CsPbI₃ perovskites. Here, we make use of a seed-assisted heteroepitaxial growth to fabricate seed/core/shell CaI_x/γ-CsPbI₃/CaI₂ nanocrystals. Ultrasmall CaI_x nanoparticles serve as seeds to template the Pb-centered octahedral arrangement which enables the formation of the γ-CsPbI₃ phase and at the same time inhibit lattice strain by blocking the force transfer that otherwise leads to an octahedral twist and so improve the structural stability of the resulting nanocrystals. An outer shell composed from the same material, CaI₂, isolates the formed γ-CsPbI₃ nanocrystals from the environment, which also significantly improves their stability under ambient conditions. Optical and electrical studies indicate that the seed/core/shell CaI_x/γ-CsPbI₃/CaI₂ structure possesses a shallower set of trap states as compared to cubic α-CsPbI₃ nanocrystals. Light-emitting diodes utilizing these γ-CsPbI₃ nanocrystals show a record high external quantum efficiency of 25.3%, high brightness of over 13600 cd/m², and an operational lifetime of ∼14 h before reaching 50% of their initial luminance. These devices can repeatedly be illuminated over 650 times at ∼500 cd/m² with no decline of brightness, which indicates their great commercial potential.

Increasing the Performance and Stability of Red-Light-Emitting Diodes Using Guanidinium Mixed-Cation Perovskite Nanocrystals

Halide perovskite nanocrystals (PNCs) exhibit growing attention in optoelectronics due to their fascinating color purity and improved intrinsic properties. However, structural defects emerging in PNCs progressively hinder the radiative recombination and carrier transfer dynamics, limiting the performance of light-emitting devices. In this work, we explored the introduction of guanidinium (GA⁺) during the synthesis of high-quality Cs_1-xGA_xPbI₃ PNCs as a promising approach for the fabrication of efficient bright-red light-emitting diodes (R-LEDs). The substitution of Cs by 10 mol % GA allows the preparation of mixed-cation PNCs with PLQY up to 100% and long-term stability for 180 days, stored under air atmosphere and refrigerated condition (4 °C). Here, GA⁺ cations fill/replace Cs⁺ positions into the PNCs, compensating intrinsic defect sites and suppressing the nonradiative recombination pathway. LEDs fabricated with this optimum material show an external quantum efficiency (EQE) near to 19%, at an operational voltage of 5 V (50-100 cd/m²) and an operational half-time (t₅₀) increased 67% respect CsPbI₃ R-LEDs. Our findings show the possibility to compensate the deficiency through A-site cation addition during the material synthesis, obtaining less defective PNCs for efficient and stable optoelectronic devices.

Long-chain anionic surfactants enabling stable perovskite/silicon tandems with greatly suppressed stress corrosion

Despite the remarkable rise in the efficiency of perovskite-based solar cells, the stress-induced intrinsic instability of perovskite active layers is widely identified as a critical hurdle for upcoming commercialization. Herein, a long-alkyl-chain anionic surfactant additive is introduced to chemically ameliorate the perovskite crystallization kinetics via surface segregation and micellization, and physically construct a glue-like scaffold to eliminate the residual stresses. As a result, benefiting from the reduced defects, suppressed ion migration and improved energy level alignment, the corresponding unencapsulated perovskite single-junction and perovskite/silicon tandem devices exhibit impressive operational stability with 85.7% and 93.6% of their performance after 3000 h and 450 h at maximum power point tracking under continuous light illumination, providing one of the best stabilities to date under similar test conditions, respectively.

Light-Induced Wavelength Dependent Self Assembly Process for Targeted Synthesis of Phase Stable 1D Nanobelts and 2D Nanoplatelets of CsPbI3 Perovskites

Despite black cubic phase α-CsPbI₃ nanocrystals having an ideal bandgap of 1.73 eV for optoelectronic applications, the phase transition from α-CsPbI₃ to non-perovskite yellow δ-CsPbI₃ phase at room temperature remains a major obstacle for commercial applications. Since γ-CsPbI₃ is thermodynamically stable with a bandgap of 1.75 eV, which has great potential for photovoltaic applications, herein we report a conceptually new method for the targeted design of phase stable and near unity photoluminescence quantum yield (PLQY) two-dimensional (2D) γ-CsPbI₃ nanoplatelets (NPLs) and one-dimensional (1D) γ-CsPbI₃ nanobelts (NBs) by wavelength dependent light-induced assembly of CsPbI₃ cubic nanocrystals. This article demonstrates for the first time that by varying the excitation wavelengths, one can design air stable desired 2D nanoplatelets or 1D nanobelts selectively. Our experimental finding indicates that 532 nm green light-driven self-assembly produces phase stable and highly luminescent γ-CsPbI₃ NBs from CsPbI₃ nanocrystals. Moreover, we show that a 670 nm red light-driven self-assembly process produces stable and near unity PLQY γ-CsPbI₃ NPLs. Systematic time-dependent microscopy and spectroscopy studies on the morphological evolution indicates that the electromagnetic field of light triggered the desorption of surface ligands from the nanocrystal surface and transformation of crystallographic phase from α to γ. Detached ligands played an important role in determining the morphologies of final structures of NBs and NPLs from nanocrystals via oriented attachment along the [110] direction initially and then the [001] direction. In addition, XRD and fluorescence imaging data indicates that both NBs and NPLs exhibit phase stability for more than 60 days in ambient conditions, whereas the cubic phase α-CsPbI₃ nanocrystals are not stable for even 3 days. The reported light driven synthesis provides a simple and versatile approach to obtain phase pure CsPbI₃ for possible optoelectronic applications.

Controlled Assembly and Anomalous Thermal Expansion of Ultrathin Cesium Lead Bromide Nanoplatelets

Quantum confined lead halide perovskite nanoplatelets are anisotropic materials displaying strongly bound excitons with spectrally pure photoluminescence. We report the controlled assembly of CsPbBr₃ nanoplatelets through varying the evaporation rate of the dispersion solvent. We confirm the assembly of superlattices in the face-down and edge-up configurations by electron microscopy, as well as X-ray scattering and diffraction. Polarization-resolved spectroscopy shows that superlattices in the edge-up configuration display significantly polarized emission compared to face-down counterparts. Variable-temperature X-ray diffraction of both face-down and edge-up superlattices uncovers a uniaxial negative thermal expansion in ultrathin nanoplatelets, which reconciles the anomalous temperature dependence of the emission energy. Additional structural aspects are investigated by multilayer diffraction fitting, revealing a significant decrease in superlattice order with decreasing temperature, with a concomitant expansion of the organic sublattice and increase of lead halide octahedral tilt.

Ultrafast One-Step Deposition Route to Fabricate Single-Crystal CsPbX3 (X = Cl, Cl/Br, Br, and Br/I) Photodetectors

Inorganic perovskites have received much attention due to their stability and high performance in luminescence, photoelectric conversion, and photodetection. However, perovskite optoelectronic devices prepared by the solution technique are still suffering from time-consuming and complex operations. In this paper, a single-crystal perovskite-based photodetector (PD) is prepared by very fast one-step deposition of synthesizing microplatelets (MPs) on the electrode directly. The saturated precursor is carefully optimized by adding appropriate antisolvent chlorobenzene (CB) to fabricate the MPs with their PL wavelength ranging from 418 to 600 nm. Furthermore, the PDs with a low dark current on order of nanoangstroms, high responsivity and detectivity of up to 10.7 A W^-1 and 10¹² Jones, respectively, and an ultrafast response rate featured by 278/287 μs (rise/decay time) are achieved. These all-inorganic perovskite PDs with a simple fabricating process and tunable detection wavelength meet the evolution tendency of PDs toward low cost and high performance, which is a high-profile strategy to realize high-performance perovskite PDs.

Approaching the Fill Factor Limit in Dopant-Free Hole Transporting Layer-Based All-Inorganic Perovskite Solar Cells

As an important part of perovskite solar cells (PSCs), hole transporting layer (HTL) has a critical impact on the performance and stability of the devices. In an attempt to alleviate the moisture and thermal stability issues from the commonly used HTL Spiro-OMeTAD with dopant, it is urgent to develop novel HTLs with high stability. In this study, a new class of polymers D18 and D18-Cl are applied as undoped HTL for CsPbI₂Br-based PSCs. In addition to the excellent hole transporting properties, we unveil that D18 and D18-Cl with larger thermal expansion coefficient than that of CsPbI₂Br could impose a compressive stress onto the CsPbI₂Br film upon thermal treatment, which could release the residual tensile stress in the film. As a result, the efficiency of CsPbI₂Br-based PSCs with D18-Cl as HTL reaches 16.73%, and the fill factor (FF) exceeds 85%, which is one of the highest FF records for the conventional-structured device to date. The devices also show impressive thermal stability with over 80% of the initial PCE retained after 85 °C heating for 1500 h.

21.15%-Efficiency and Stable γ -CsPbI3 Perovskite Solar Cells Enabled by an Acyloin Ligand

Cesium lead triiodide (CsPbI₃ ) is a promising light-absorbing material for constructing perovskite solar cells (PSCs) owing to its favorable bandgap and thermal tolerance. However, the high density of defects in the CsPbI₃ film not only act as recombination centers, but also facilitate ion migration, leading to lower PCE and inferior stability compared with the state-of-the-art organic-inorganic hybrid PSC counterpart. Theoretical analyses suggest that the effective suppression of defects in CsPbI₃ film is helpful for improving the device performance. Herein, the stable and efficient γ -CsPbI₃ PSCs are demonstrated by developing an acyloin ligand (1,2-di(thiophen-2-yl)ethane-1,2-dione (DED)) as a phase stabilizer and defect passivator. The experiment and calculation results confirm that carbonyl and thienyl in DED can synergistically interact with CsPbI₃ by forming a chelate to effectively passivate Pb-related defects and further suppress ion migration. Consequently, DED-treated CsPbI₃ PSCs yield a champion PCE of 21.15%, which is one of the highest PCE among all the reported CsPbI₃ PSCs to date. In addition, the unencapsulated DED-CsPbI₃ PSC can retain 94.9% of itsinitial PCE when stored under ambient conditions for 1000 h and 92.8% of its initial PCE under constant illumination for 250 h.

Au-Seeded CsPbI3 Nanowire Optoelectronics via Exothermic Nucleation

Converting vapor precursors to solid nanostructures via a liquid noble-metal seed is a common vapor deposition principle. However, such a noble-metal-seeded process is excluded from the crystalline halide perovskite synthesis, mainly hindered by the growth mechanism shortness. Herein, powered by a spontaneous exothermic nucleation process (ΔH < 0), the Au-seeded CsPbI₃ nanowires (NWs) growth is realized based on a vapor-liquid-solid (VLS) growth mode. It is energetically favored that the Au seeds are reacted with a Pb vapor precursor to form molten Au-Pb droplets at temperatures down to 212 °C, further triggering the low-temperature VLS growth of CsPbI₃ NWs. More importantly, this Au-seeded process reduces in-bandgap trap states and consequently avoids Shockley-Read-Hall recombination, contributing to outstanding photodetector performances. Our work extends the powerful Au-seeded VLS growth mode to the emerging halide perovskites, which will facilitate their nanostructures with tailored material properties.

Stable Mott Polaron State Limits the Charge Density in Lead Halide Perovskites

Large polarons are known to form in lead halide perovskites (LHPs). Photoinduced isolated polarons at low densities have been well-researched, but many-body interactions at elevated polaron densities, exceeding the Mott criterion (i.e., Mott polaron density), have remained elusive. Here, employing ultrafast terahertz spectroscopy, we identify a stable Mott polaron state in LHPs at which the polaron wavefunctions start to overlap. The Mott polaron density is determined to be ∼10¹⁸ cm^-3, in good agreement with theoretical calculations based on the Feynman polaron model. The electronic phase transition across the Mott density is found to be universal in LHPs and independent of the constituent ions. Exceeding the Mott polaron density, excess photoinjected charge carriers annihilate quickly within tens to hundreds of picoseconds, before reaching the stable and long-lived Mott state. These results have considerable implications for LHP-based devices and for understanding exotic phenomena reported in LHPs.

Physical Mechanism and Chemical Trends in the Thermal Expansion of Inorganic Halide Perovskites

The considerable thermal expansion of halide perovskites is one of the challenges to device stability, yet the physical origin and modulation strategy remain unclear. Herein, we report first-principles calculations of the thermal properties of halide perovskites at 300 K using oxides as a reference. We found that the large thermal expansion of halide perovskites can mainly be attributed to their low bulk modulus and volumetric heat capacity because of the soft crystal lattice, whereas composition-dependent anharmonicity emerges as the most important factor in determining thermal expansion with the same structure. We discovered that thermal expansion of halide perovskites can be decreased by weakening the B-X bond to promote the octahedral anharmonicity. We further proposed an effective thermal expansion coefficient descriptor of halide perovskites with a Pearson correlation coefficient of nearly -80%. Our findings provide insights into the underlying mechanisms and chemical trends in the thermal expansion behavior of halide perovskites.

Recent progress of crystal orientation engineering in halide perovskite photovoltaics

Manipulating the crystallographic orientation of semiconductor crystals plays a vital role in fine-tuning their facet-dependent properties, such as surface properties, charge transfer properties, trap state density, and lattice strain. The success in crystal orientation engineering enables the preferential growth orientation of perovskite thin films with favorable crystal planes by precise nucleation manipulation and growth condition optimization, rendering the films with the unique optoelectronic properties to further improve the efficiency of perovskite solar cells (PSCs). However, the origin and impact of preferential crystallographic orientation of perovskite thin films on the corresponding photovoltaic performance of PSCs are still far from being well understood. Herein, we explore the crystal orientation-dependent optoelectronic properties of halide perovskites and their influence on the photovoltaic performance of PSCs. We summarize the basic strategies for crystal facet engineering in the fabrication of preferentially oriented perovskite thin films, with a focus on the oriented growth mechanism during thin film formation. Based on the above knowledge and the recent research progress in terms of crystal orientation engineering in PSCs, a brief outlook on the remaining challenges and perspectives are provided.

An embedded interfacial network stabilizes inorganic CsPbI3 perovskite thin films

The black perovskite phase of CsPbI₃ is promising for optoelectronic applications; however, it is unstable under ambient conditions, transforming within minutes into an optically inactive yellow phase, a fact that has so far prevented its widespread adoption. Here we use coarse photolithography to embed a PbI₂-based interfacial microstructure into otherwise-unstable CsPbI₃ perovskite thin films and devices. Films fitted with a tessellating microgrid are rendered resistant to moisture-triggered decay and exhibit enhanced long-term stability of the black phase (beyond 2.5 years in a dry environment), due to increasing the phase transition energy barrier and limiting the spread of potential yellow phase formation to structurally isolated domains of the grid. This stabilizing effect is readily achieved at the device level, where unencapsulated CsPbI₃ perovskite photodetectors display ambient-stable operation. These findings provide insights into the nature of phase destabilization in emerging CsPbI₃ perovskite devices and demonstrate an effective stabilization procedure which is entirely orthogonal to existing approaches.

Lewis Acid-Base Adducts for Efficient and Stable Cesium-Based Lead Iodide-Rich Perovskite Solar Cells

All-inorganic cesium-lead-iodide (CsPbI₃ Br_{3- x} (2 < x < 3)) perovskite presents preeminent photovoltaic performance and chemical stability. Unfortunately, this kind of material suffers from phase transition to a nonperovskite phase under oxidative chemical stresses. Herein, the introduction of a low concentration of Lewis acid-base adducts (LABAs) is reported to synergistically reduce defect density, optimize interfacial energy alignment, and improve device stability of CsPbI_2.75 Br_0.24 Cl_0.01 (CsPbTh₃ ) solar cells. Both theoretical simulations and experimental measurements reveal that the noncoordinating anions, PF₆ ^- , as a Lewis base can more effectively bind with undercoordinated Pb²⁺ to passivate iodide vacancy defects than the BF₄ ^- and absorbed I^- , and thus the point defects are well suppressed. In addition, N-propyl-methyl piperidinium (NPMP⁺ ) is selected to assemble with PF₆ ^- in CsPbTh₃ film. The NPMP⁺ can regulate the crystal growth and finally homogenize the grain size and decrease the trap density. Consequently, the LABAs strategy can improve the power conversion efficiency of CsPbTh₃ solar cells to 19.02% under 1-sun illumination (100 mW cm^-2 ). Fortunately, the NPMP⁺ and PF₆ ^- -treated CsPbTh₃ film shows great phase stability after storage in ambient air for 250 days, and the power conversion efficiency of corresponding solar cells is almost 76% of the initial value after 60 days aging under ambient conditions.

Lattice distortion inducing exciton splitting and coherent quantum beating in CsPbI3 perovskite quantum dots

Anisotropic exchange splitting in semiconductor quantum dots results in bright-exciton fine-structure splitting important for quantum information processing. Direct measurement of fine-structure splitting usually requires single/few quantum dots at liquid-helium temperature because of its sensitivity to quantum dot size and shape, whereas measuring and controlling fine-structure splitting at an ensemble level seem to be impossible unless all the dots are made to be nearly identical. Here we report strong bright-exciton fine-structure splitting up to 1.6 meV in solution-processed CsPbI₃ perovskite quantum dots, manifested as quantum beats in ensemble-level transient absorption at liquid-nitrogen to room temperature. The splitting is robust to quantum dot size and shape heterogeneity, and increases with decreasing temperature, pointing towards a mechanism associated with orthorhombic distortion of the perovskite lattice. Effective-mass-approximation calculations reveal an intrinsic 'fine-structure gap' that agrees well with the observed fine-structure splitting. This gap stems from an avoided crossing of bright excitons confined in orthorhombically distorted quantum dots that are bounded by the pseudocubic {100} family of planes.

Impact of Strain Relaxation on 2D Ruddlesden-Popper Perovskite Solar Cells

Although the photovoltaic performance of perovskite solar cells (PSCs) has reached the commercial standards, the unsatisfactory stability limits their further application. Hydrophobic interface and encapsulation can block the damage of water and oxygen, while the instability induced by intrinsic residual strain remains inevitable. Here, the residual strain in a two-dimensional (2D) Ruddlesden-Popper (RP) perovskite film is investigated by X-ray diffraction and atomic force microscopy. It's found that the spacer cations contribute to the residual strain even though they are not in the inorganic cages. Benefited from strain relaxation, the film quality is improved, leading to suppressed recombination, promoted charge transport and enhanced efficiency. More significantly, the strain-released devices maintain 86 % of the initial efficiency after being kept in air with 85 % relative humidity (RH) for 1080 h, 82 % under maximum power point (MPP) tracking at 50 °C for 804 h and 86 % after continuous heating at 85 °C for 1080 h.