Nanoscale mapping of chemical composition in organic-inorganic hybrid perovskite films

Synchrotron infrared nanospectroscopy in fourth-generation storage rings

Fourth-generation synchrotron storage rings represent a significant milestone in synchrotron technology, offering outstandingly bright and tightly focused X-ray beams for a wide range of scientific applications. However, due to their inherently tight magnetic lattices, these storage rings have posed critical challenges for accessing lower-energy radiation, such as infrared (IR) and THz. Here the first-ever IR beamline to be installed and to operate at a fourth-generation synchrotron storage ring is introduced. This work encompasses several notable advancements, including a thorough examination of the new IR source at Sirius, a detailed description of the radiation extraction scheme, and the successful validation of our optical concept through both measurements and simulations. This optimal optical setup has enabled us to achieve an exceptionally wide frequency range for our nanospectroscopy experiments. Through the utilization of synchrotron IR nanospectroscopy on biological and hard matter samples, the practicality and effectiveness of this beamline has been successfully demonstrated. The advantages of fourth-generation synchrotron IR sources, which can now operate with unparalleled stability as a result of the stringent requirements for producing low-emittance X-rays, are emphasized.

Polycrystalline Lead-Free Perovskite Direct X-Ray Detectors with High Durability and Low Limit of Detection via Low-Temperature Coating

Direct X-ray detectors represent a transformative technology in the realm of radiography and imaging. The double halide-based perovskite cesium silver bismuth bromide (Cs2AgBiBr6) has emerged as a promising material for use in direct X-ray imaging, owing to its nontoxic composition, strong X-ray absorption, decent charge mobility lifetime product (μτ), and low-cost preparation. However, formidable issues related to scalability and ion migration, stemming from intrinsic factors such as halogen vacancies and grain boundaries, have presented significant impediments. These issues have been associated with substantial noise, baseline instability, and a curtailment of detection performance. In response to these multifaceted challenges, we propose a slurry-based in situ treatment technique for fabricating robust Cs2AgBiBr6 thick films. This novel approach adeptly mitigates halogen vacancies, actively passivates grain boundaries, and concurrently elevates the ion migration activation energy, thus effectively suppressing ion migration. Consequently, the obtained X-ray detector exhibits excellent operating stability with minimal signal drift of 8.5 × 10-9 nA cm-1 s-1 V-1 and achieves a remarkable 385% increase in sensitivity with a limit of detection as low as 7.8 nGyair s-1. These results mark a significant step toward the development of high-performance and long-lasting lead-free perovskite direct X-ray detectors.

Promoting Photocatalytic H2 Evolution through Retarded Charge Trapping and Recombination by Continuously Distributed Defects in Methylammonium Lead Iodide Perovskite

Inspired by its great success in the photovoltaic field, methylammonium lead iodide perovskite (MAPbI3 ) has recently been actively explored as photocatalysts in H2 evolution reactions. However, the practical application of MAPbI3 photocatalysts remains hampered by the intrinsically fast trapping and recombination of photogenerated charges. Herein, we propose a novel strategy of regulating the distribution of defective areas to promote charge-transfer dynamics of MAPbI3 photocatalysts. By deliberately designing and synthesizing the MAPbI3 photocatalysts featuring a unique continuation of defective areas, we demonstrate that such a feature enables retardation of charge trapping and recombination via lengthening the charge-transfer distance. As an outcome, such MAPbI3 photocatalysts turn out to achieve an impressive photocatalytic H2 evolution rate as high as 0.64 mmol ⋅ g-1  ⋅ h-1 , one order of magnitude higher than that of the conventional MAPbI3 photocatalysts. This work establishes a new paradigm for controlling charge-transfer dynamics in photocatalysis.

Disentangling Electronic Transport and Hysteresis at Individual Grain Boundaries in Hybrid Perovskites via Automated Scanning Probe Microscopy

Underlying the rapidly increasing photovoltaic efficiency and stability of metal halide perovskites (MHPs) is the advancement in the understanding of the microstructure of polycrystalline MHP thin film. Over the past decade, intense efforts have been aimed at understanding the effect of microstructures on MHP properties, including chemical heterogeneity, strain disorder, phase impurity, etc. It has been found that grain and grain boundary (GB) are tightly related to lots of microscale and nanoscale behavior in MHP thin films. Atomic force microscopy (AFM) is widely used to observe grain and boundary structures in topography and subsequently to study the correlative surface potential and conductivity of these structures. For now, most AFM measurements have been performed in imaging mode to study the static behavior; in contrast, AFM spectroscopy mode allows us to investigate the dynamic behavior of materials, e.g., conductivity under sweeping voltage. However, a major limitation of AFM spectroscopy measurements is that they require manual operation by human operators, and as such only limited data can be obtained, hindering systematic investigations of these microstructures. In this work, we designed a workflow combining the conductive AFM measurement with a machine learning (ML) algorithm to systematically investigate grain boundaries in MHPs. The trained ML model can extract GBs locations from the topography image, and the workflow drives the AFM probe to each GB location to perform a current-voltage (IV) curve automatically. Then, we are able to have IV curves at all GB locations, allowing us to systematically understand the property of GBs. Using this method, we discovered that the GB junction points are less conductive, potentially more photoactive, and can play critical roles in MHP stability, while most previous works only focused on the difference between GB and grains.

Exploring the Relationship of Microstructure and Conductivity in Metal Halide Perovskites via Active Learning-Driven Automated Scanning Probe Microscopy

Electronic transport and hysteresis in metal halide perovskites (MHPs) are key to the applications in photovoltaics, light emitting devices, and light and chemical sensors. These phenomena are strongly affected by the materials microstructure including grain boundaries, ferroic domain walls, and secondary phase inclusions. Here, we demonstrate an active machine learning framework for "driving" an automated scanning probe microscope (SPM) to discover the microstructures responsible for specific aspects of transport behavior in MHPs. In our setup, the microscope can discover the microstructural elements that maximize the onset of conduction, hysteresis, or any other characteristic that can be derived from a set of current-voltage spectra. This approach opens new opportunities for exploring the origins of materials functionality in complex materials by SPM and can be integrated with other characterization techniques either before (prior knowledge) or after (identification of locations of interest for detail studies) functional probing.

Inhibited Crack Development by Compressive Strain in Perovskite Solar Cells with Improved Mechanical Stability

Metal halide perovskites are promising as next-generation photovoltaic materials, but stability issues are still a huge obstacle to their commercialization. Here, the formation and evolution of cracks in perovskite films during thermal cycling, which affect their mechanical stability, are investigated. Compressive strain is employed to suppress cracks and delamination by in situ formed polymers with low elastic modulus during crystal growth. The resultant devices pass the thermal-cycling qualification (IEC61215:2016), retaining 95% of the initial power conversion efficiency (PCE) and compressive strain after 230 cycles. Meanwhile, the p-i-n devices deliver PCEs of 23.91% (0.0805 cm² ) and 23.27% (1 cm² ). The findings shed light on strain engineering with respect to their evolution, which enables mechanically stable perovskite solar cells.

Imaging the Terahertz Nanoscale Conductivity of Polycrystalline CsPbBr3 Perovskite Thin Films

Terahertz (THz) radiation is a valuable tool to investigate the electronic properties of lead halide perovskites (LHPs). However, attaining high-resolution information remains elusive, as the diffraction-limited spatial resolution (∼300 μm) of conventional THz methods prevents a direct analysis of microscopic effects. Here, we employ THz scattering scanning near-field optical microscopy (THz-sSNOM) for nanoscale imaging of cesium lead bromide (CsPbBr₃) thin films down to the single grain level at 600 GHz. Adopting a scattering model, we are able to derive the local THz nanoscale conductivity in a contact-free fashion. Increased THz near-field signals at CsPbBr₃ grain boundaries complemented by correlative transmission electron microscopy-energy-dispersive X-ray spectroscopy elemental analysis point to the formation of halide vacancies (V_Br) and Pb-Pb bonds, which induce charge carrier trapping and can lead to nonradiative recombination. Our study establishes THz-sSNOM as a powerful THz nanoscale analysis platform for thin-film semiconductors such as LHPs.

Strain Modulation for Light-Stable n-i-p Perovskite/Silicon Tandem Solar Cells

Perovskite/silicon tandem solar cells are promising to penetrate photovoltaic market. However, the wide-bandgap perovskite absorbers used in top-cell often suffer severe phase segregation under illumination, which restricts the operation lifetime of tandem solar cells. Here, a strain modulation strategy to fabricate light-stable perovskite/silicon tandem solar cells is reported. By employing adenosine triphosphate, the residual tensile strain in the wide-bandgap perovskite absorber is successfully converted to compressive strain, which mitigates light-induced ion migration and phase segregation. Based on the wide-bandgap perovskite with compressive strain, single-junction solar cells with the n-i-p layout yield a power conversion efficiency (PCE) of 20.53% with the smallest voltage deficits of 440 mV. These cells also maintain 83.60% of initial PCE after 2500 h operation at the maximum power point. Finally, these top cells are integrated with silicon bottom cells in a monolithic tandem device, which achieves a PCE of 26.95% and improved light stability at open-circuit.

Characterization of a New Low Temperature Encapsulation Method with Ethylene-Vinyl Acetate under UV Irradiation for Perovskite Solar Cells

In this work, the performance of a new ethylene-vinyl acetate-based low temperature encapsulation method, conceived to protect perovskite samples from UV irradiation in ambient conditions, has been analyzed. To this purpose, perovskite samples consisting of a set of MAPbI3 (CH3NH3PbI3) films and MAPbI3 with an ETL layer were deposited over glass substrates by spin-coating techniques and encapsulated using the new method. The samples were subjected to an UV lamp or to full solar irradiation in ambient conditions, with a relative humidity of 60–80%. Microscope imaging, spectroscopic ellipsometry and Fourier-transform infrared spectroscopy (FTIR) techniques were applied to analyze the samples. The obtained results indicate UV energy is responsible for the degradation of the perovskite layer. Thus, the cut-UV characteristics of the EVA encapsulate acts as an efficient barrier, allowing the laminated samples to remain stable above 350 h under full solar irradiation compared with non-encapsulated samples. In addition, the FTIR results reveal perovskite degradation caused by UV light. To extend the study to encompass whole PSCs, simulations were carried out using the software SCAPS-1D, where the non-encapsulated devices present a short-circuit current reduction after exposure to UV irradiation, while the encapsulated ones maintained their efficiency.

Nanoscale Visualization of Photodegradation Dynamics of MAPbI3 Perovskite Films

Herein, we report the nanoscale visualization of the photochemical degradation dynamics of MAPbI₃ (MA = CH₃NH₃⁺) using infrared scattering scanning near-field microscopy (IR s-SNOM) combined with a series of complementary analytical techniques such as UV-vis and FTIR-spectroscopy, XRD, and XPS. Light exposure of the MAPbI₃ films resulted in a gradual loss of MA⁺ cations starting from the grain boundaries at the film surface and slowly progressing toward the center of the grains and deeper into the bulk perovskite phase. The binary lead iodide PbI₂ was found to be the major perovskite photochemical degradation product under the experimental conditions used. Interestingly, the formation of the PbI₂ skin over the perovskite grains resulted in a largely enhanced photoluminescence, which resembles the effects observed for core-shell quantum dots. The obtained results demonstrate that IR s-SNOM represents a powerful technique for studying the spatially resolved degradation dynamics of perovskite absorbers and revealing the associated material aging pathways.

Ion-driven nanograin formation in early-stage degradation of tri-cation perovskite films

The operational stability of organic-inorganic halide perovskite based solar cells is a challenge for widespread commercial adoption. The mobility of ionic species is a key contributor to perovskite instability since ion migration can lead to unfavourable changes in the crystal lattice and ultimately destabilisation of the perovskite phase. Here we study the nanoscale early-stage degradation of mixed-halide mixed-cation perovskite films under operation-like conditions using electrical scanning probe microscopy to investigate the formation of surface nanograin defects. We identify the nanograins as lead iodide and study their formation in ambient and inert environments with various optical, thermal, and electrical stress conditions in order to elucidate the different underlying degradation mechanisms. We find that the intrinsic instability is related to the polycrystalline morphology, where electrical bias stress leads to the build-up of charge at grain boundaries and lateral space charge gradients that destabilise the local perovskite lattice facilitating escape of the organic cation. This mechanism is accelerated by enhanced ionic mobility under optical excitation. Our findings highlight the importance of inhibiting the formation of local charge imbalance, either through compositions preventing ionic redistribution or local grain boundary passivation, in order to extend operational stability in perovskite photovoltaics.

Nanoscale chemical heterogeneity dominates the optoelectronic response of alloyed perovskite solar cells

Halide perovskites perform remarkably in optoelectronic devices. However, this exceptional performance is striking given that perovskites exhibit deep charge-carrier traps and spatial compositional and structural heterogeneity, all of which should be detrimental to performance. Here, we resolve this long-standing paradox by providing a global visualization of the nanoscale chemical, structural and optoelectronic landscape in halide perovskite devices, made possible through the development of a new suite of correlative, multimodal microscopy measurements combining quantitative optical spectroscopic techniques and synchrotron nanoprobe measurements. We show that compositional disorder dominates the optoelectronic response over a weaker influence of nanoscale strain variations even of large magnitude. Nanoscale compositional gradients drive carrier funnelling onto local regions associated with low electronic disorder, drawing carrier recombination away from trap clusters associated with electronic disorder and leading to high local photoluminescence quantum efficiency. These measurements reveal a global picture of the competitive nanoscale landscape, which endows enhanced defect tolerance in devices through spatial chemical disorder that outcompetes both electronic and structural disorder.

Hierarchically encapsulating enzymes with multi-shelled metal-organic frameworks for tandem biocatalytic reactions

Biocatalytic transformations in living organisms, such as multi-enzyme catalytic cascades, proceed in different cellular membrane-compartmentalized organelles with high efficiency. Nevertheless, it remains challenging to mimicking biocatalytic cascade processes in natural systems. Herein, we demonstrate that multi-shelled metal-organic frameworks (MOFs) can be used as a hierarchical scaffold to spatially organize enzymes on nanoscale to enhance cascade catalytic efficiency. Encapsulating multi-enzymes with multi-shelled MOFs by epitaxial shell-by-shell overgrowth leads to 5.8~13.5-fold enhancements in catalytic efficiencies compared with free enzymes in solution. Importantly, multi-shelled MOFs can act as a multi-spatial-compartmental nanoreactor that allows physically compartmentalize multiple enzymes in a single MOF nanoparticle for operating incompatible tandem biocatalytic reaction in one pot. Additionally, we use nanoscale Fourier transform infrared (nano-FTIR) spectroscopy to resolve nanoscale heterogeneity of vibrational activity associated to enzymes encapsulated in multi-shelled MOFs. Furthermore, multi-shelled MOFs enable facile control of multi-enzyme positions according to specific tandem reaction routes, in which close positioning of enzyme-1-loaded and enzyme-2-loaded shells along the inner-to-outer shells could effectively facilitate mass transportation to promote efficient tandem biocatalytic reaction. This work is anticipated to shed new light on designing efficient multi-enzyme catalytic cascades to encourage applications in many chemical and pharmaceutical industrial processes.

Mimicking multi-enzyme catalytic cascades in natural systems with spatial organization in confined structures is gaining increasing attention in the emerging field of systems chemistry. Here, the authors demonstrate that multi-shelled metal-organic frameworks can be used as a hierarchical scaffold to spatially organize enzymes on nanoscale to enhance cascade catalytic efficiency.

Infrared spectroscopic monitoring of solid-state processes

We put a spotlight on IR spectroscopic investigations in materials science by providing a critical insight into the state of the art, covering both fundamental aspects, examples of its utilisation, and current challenges and perspectives focusing on the solid state.

Microstructural Evaluation of Phase Instability in Large Bandgap Metal Halide Perovskites

The optoelectronic performance of organic-inorganic halide perovskite (OIHP)-based devices has been improved in recent years. Particularly, solar cells fabricated using mixed-cations and mixed-halides have outperformed their single-cation and single-halide counterparts. Yet, a systematic evaluation of the microstructural behavior of mixed perovskites is missing despite their known composition-dependent photoinstability. Here, we explore microstructural inhomogeneity in (FAPbI₃)_x(MAPbBr₃)_1-x using advanced scanning probe microscopy techniques. Contact potential difference (CPD) maps measured by Kelvin probe force microscopy show an increased fraction of grains exhibiting a low CPD with flat topography as MAPbBr₃ concentration is increased. The higher portion of low CPD contributes to asymmetric CPD distribution curves. Chemical analysis reveals these grains being rich in MA, Pb, and I. The composition-dependent phase segregation upon illumination, reflected on the emergence of a low-energy peak emission in the original photoluminescence spectra, arises from the formation of such grains with flat topology. Bias-dependent piezo-response force microscopy measurements, in these grains, further confirm vigorous ion migration and cause a hysteretic piezo-response. Our results, therefore, provide insights into the microstructural evaluation of phase segregation and ion migration in OIHPs pointing toward process optimization as a mean to further enhance their optoelectronic performance.

Stabilized tilted-octahedra halide perovskites inhibit local formation of performance-limiting phases

[Figure: see text].

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.

Intermediate Phase-Free Process for Methylammonium Lead Iodide Thin Film for High-Efficiency Perovskite Solar Cells

Solvent engineering by Lewis-base solvent and anti-solvent is well known for forming uniform and stable perovskite thin films. The perovskite phase crystallizes from an intermediate Lewis-adduct upon annealing-induced crystallization. Herein, it is explored the effects of trimethyl phosphate (TMP), as a novel aprotic Lewis-base solvent with a low donor number for the perovskite film formation and photovoltaic characteristics of perovskite solar cells (PSCs). As compared to dimethylsulfoxide (DMSO) or dimethylformamide (DMF), the usage of TMP directly crystallizes the perovskite phase, i.e., reduces the intermediate phase to a negligible degree, right after the spin-coating, owing to the high miscibility of TMP with the anti-solvent and weak bonding in the Lewis adduct. Interestingly, the PSCs based on methylammonium lead iodide (MAPbI3 ) derived from TMP/DMF-mixed solvent exhibit a higher average power conversion efficiency of 19.68% (the best: 20.02%) with a smaller hysteresis in the current-voltage curve, compared to the PSCs that are fabricated using DMSO/DMF-mixed (19.14%) or DMF-only (18.55%) solvents. The superior photovoltaic properties are attributed to the lower defect density of the TMP/DMF-derived perovskite film. The results indicate that a high-performance PSC can be achieved by combining a weak Lewis base with a well-established solvent engineering process.

Strain analysis and engineering in halide perovskite photovoltaics

Halide perovskites are a compelling candidate for the next generation of clean-energy-harvesting technologies owing to their low cost, facile fabrication and outstanding semiconductor properties. However, photovoltaic device efficiencies are still below practical limits and long-term stability challenges hinder their practical application. Current evidence suggests that strain in halide perovskites is a key factor in dictating device efficiency and stability. Here we outline the fundamentals of strain within halide perovskites relevant to photovoltaic applications and rationalize approaches to characterize the phenomenon. We examine recent breakthroughs in eliminating the adverse impacts of strain, enhancing both device efficiencies and operational stabilities. Finally, we discuss further challenges and outline future research directions for placing stress and strain studies at the forefront of halide perovskite research. An extensive understanding of strain in halide perovskites is needed, which would allow effective strain management and drive further enhancements in efficiencies and stabilities of perovskite photovoltaics.

Integrated Tapping Mode Kelvin Probe Force Microscopy with Photoinduced Force Microscopy for Correlative Chemical and Surface Potential Mapping

Kelvin probe force microscopy (KPFM) is a popular technique for mapping the surface potential at the nanoscale through measurement of the Coulombic force between an atomic force microscopy (AFM) tip and sample. The lateral resolution of conventional KPFM variants is limited to between ≈35 and 100 nm in ambient conditions due to the long-range nature of the Coulombic force. In this article, a novel way of generating the Coulombic force in tapping mode KPFM without the need for an external AC driving voltage is presented. A field-effect transistor (FET) is used to directly switch the electrical connectivity of the tip and sample on and off periodically. The resulting Coulomb force induced by Fermi level alignment of the tip and sample results in a detectable change of the cantilever oscillation at the FET-switching frequency. The resulting FET-switched KPFM delivers a spatial resolution of ≈25 nm and inherits the high operational speed of the AFM tapping mode. Moreover, the FET-switched KPFM is integrated with photoinduced force microscopy (PiFM), enabling simultaneous acquisitions of high spatial resolution chemical distributions and surface potential maps. The integrated FET-switched KPFM with PiFM is expected to facilitate characterizations of nanoscale electrical properties of photoactive materials, semiconductors, and ferroelectric materials.

1,10-Phenanthroline as an Efficient Bifunctional Passivating Agent for MAPbI3 Perovskite Solar Cells

Passivation is one of the most promising concepts to heal defects created at the surface and grain boundaries of polycrystalline perovskite thin films, which significantly deteriorate the photovoltaic performance and stability of corresponding devices. Here, 1,10-phenanthroline, known as a bidentate chelating ligand, is implemented between the methylammonium lead iodide (MAPbI₃) film and the hole-transport layer for both passivating the lead-based surface defects (undercoordinated lead ions) and converting the excess/unreacted lead iodide (PbI₂) buried at interfaces, which is problematic for the long-term stability, into "neutralized" and beneficial species (PbI₂(1,10-phen)_x, x = 1, 2) for efficient hole transfer at the modified interface. The defect healing ability of 1,10-phenanthroline is verified with a set of complementary techniques including photoluminescence (steady-state and time-resolved), space-charge-limited current (SCLC) measurements, light intensity dependent JV measurements, and Fourier-transform photocurrent spectroscopy (FTPS). In addition to these analytical methods, we employ advanced X-ray scattering techniques, nano-Fourier transform infrared (nano-FTIR) spectroscopy, and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) to further analyze the structure and chemical composition at the perovskite surface after treatment at nanoscale spatial resolution. On the basis of our experimental results, we conclude that 1,10-phenanthroline treatment induces the formation of different morphologies with distinct chemical compositions on the surface of the perovskite film such that surface defects are effectively passivated, and excess/unreacted PbI₂ is converted into beneficial complex species at the modified interface. As a result, an improved power conversion efficiency (20.16%) and significantly more stable unencapsulated perovskite solar cells are obtained with the 1,10-phenanthroline treatment compared to the MAPbI₃ reference device (18.03%).

Quantification of electron accumulation at grain boundaries in perovskite polycrystalline films by correlative infrared-spectroscopic nanoimaging and Kelvin probe force microscopy

Organic-inorganic halide perovskites are emerging materials for photovoltaic applications with certified power conversion efficiencies (PCEs) over 25%. Generally, the microstructures of the perovskite materials are critical to the performances of PCEs. However, the role of the nanometer-sized grain boundaries (GBs) that universally existing in polycrystalline perovskite films could be benign or detrimental to solar cell performance, still remains controversial. Thus, nanometer-resolved quantification of charge carrier distribution to elucidate the role of GBs is highly desirable. Here, we employ correlative infrared-spectroscopic nanoimaging by the scattering-type scanning near-field optical microscopy with 20 nm spatial resolution and Kelvin probe force microscopy to quantify the density of electrons accumulated at the GBs in perovskite polycrystalline thin films. It is found that the electron accumulations are enhanced at the GBs and the electron density is increased from 6 × 10¹⁹ cm^-3 in the dark to 8 × 10¹⁹ cm^-3 under 10 min illumination with 532 nm light. Our results reveal that the electron accumulations are enhanced at the GBs especially under light illumination, featuring downward band bending toward the GBs, which would assist in electron-hole separation and thus be benign to the solar cell performance. Correlative infrared-spectroscopic nanoimaging by the scattering-type scanning near-field optical microscopy and Kelvin probe force microscopy quantitatively reveal the accumulated electrons at GBs in perovskite polycrystalline thin films.

Dual-Protected Metal Halide Perovskite Nanosheets with an Enhanced Set of Stabilities

Approaches to achieve stable perovskite nanocrystals (PNCs) of interest, in particular those with large structural anisotropy, through protective coating of the inorganic shell at a single-nanocrystal (NC) level are comparatively few and limited in scope. Reported here is a robust amphiphilic-diblock-copolymer-enabled strategy for crafting highly-stable anisotropic CsPbBr₃ nanosheets (NSs) by in situ formation of a uniform inorganic shell (1st shielding) that is intimately ligated with hydrophobic polymers (2nd shielding). The dual-protected NSs display an array of remarkable stabilities (i.e., thermal, photostability, moisture, polar solvent, aliphatic amine, etc.) and find application in white-light-emitting diodes. In principle, by anchoring other multidentate amphiphilic polymer ligands on the surface of PNCs, followed by templated-growth of shell materials of interest, a rich variety of dual-shelled, multifunctional PNCs with markedly improved stabilities can be created for use in optics, optoelectronics, and sensory devices.

The role of the A-cations in the polymorphic stability and optoelectronic properties of lead-free ASnI3 perovskites

Tin-based ASnI3 perovskites have been considered excellent candidates for lead-free perovskite solar cell applications; however, our atomistic understanding of the role of the A-cations, namely, CH3NH3 (methylammonium, MA), CH3PH3 (methylphosphonium, MP) and CH(NH2)2 (formamidinium, FA), in the physical chemistry properties is far from satisfactory. For the first time, we report a density functional theory investigation of the MPSnI3 perovskite and non-perovskite phases as well as their comparison with the MASnI3 and FASnI3 phases, where we considered the role of the A-cation orientations in the structural stability of the ASnI3 phases. The orthorhombic structure is the most stable studied phase, which agrees with experimentally reported phase-transition trends. In contrast with the cation size and the weak hydrogen bonding interactions, which contribute to structural cohesion between the inorganic framework and A-cation, the dipole-dipole interactions play an important role to drive the structures to the lowest energy configurations. From our analysis, the inorganic framework dominates the optical properties, band structure, and density of states around the band edges. Broader absorption and smaller band gap energies occur for the perovskite structures compared to the low-dimensional hexagonal/pseudo-hexagonal non-perovskites.

Incorporation of Lithium Fluoride Restraining Thermal Degradation and Photodegradation of Organometal Halide Perovskite Solar Cells

Because of the facile formation of defects in organometal halide perovskites, the defect passivation has become an important prerequisite for the stable and efficient perovskite solar cell (PSC). Regarding that ionic defects of the perovskites play a significant role on the performance and stability of PSCs, we introduce lithium fluorides as effective passivators based on their strong ionic characteristics and small ionic radii. Both Li⁺ and F^- are observed to successfully incorporate within the perovskite layer, improving the device performances with the best efficiency over 20%, while the hysteresis effects are significantly reduced, confirming the passivation of perovskite defects. Moreover, LiF restrains both thermal degradation and photodegradation of PSCs, where over 90% of the initial efficiencies have been retained by LiF-incorporated devices for more than 1000 h under either 1 sun illumination or 85 °C thermal condition. As the trap density of states is analyzed before and after the thermal stress, not only the mitigation of electronic traps as fabricated but also the dramatic relaxation of traps during the postannealing step is observed with the LiF incorporation. From this work, LiF has shown its potential as a promising ionic passivator, and the phenomenal achievement of device stability by LiF provides a clear insight to overcome the stability issues of PSCs, a key to the commercialization of next-generation photovoltaics.

CuCrO2 Nanoparticles Incorporated into PTAA as a Hole Transport Layer for 85 °C and Light Stabilities in Perovskite Solar Cells

High-mobility inorganic CuCrO2 nanoparticles are co-utilized with conventional poly(bis(4-phenyl)(2,5,6-trimethylphenyl)amine) (PTAA) as a hole transport layer (HTL) for perovskite solar cells to improve device performance and long-term stability. Even though CuCrO2 nanoparticles can be readily synthesized by hydrothermal reaction, it is difficult to form a uniform HTL with CuCrO2 alone due to the severe agglomeration of nanoparticles. Herein, both CuCrO2 nanoparticles and PTAA are sequentially deposited on perovskite by a simple spin-coating process, forming uniform HTL with excellent coverage. Due to the presence of high-mobility CuCrO2 nanoparticles, CuCrO2/PTAA HTL demonstrates better carrier extraction and transport. A reduction in trap density is also observed by trap-filled limited voltages and capacitance analyses. Incorporation of stable CuCrO2 also contributes to the improved device stability under heat and light. Encapsulated perovskite solar cells with CuCrO2/PTAA HTL retain their efficiency over 90% after ~900-h storage in 85 °C/85% relative humidity and under continuous 1-sun illumination at maximum-power point.

Strong Linear Correlation between CH3NH2 Molecular Defect and THz-Wave Absorption in CH3NH3PbI3 Hybrid Perovskite Thin Film

To control the density of a CH₃NH₂ molecular defect, which strongly contributed to a significant THz-wave absorption property in the CH₃NH₃PbI₃ hybrid perovskite thin film formed by the sequential vacuum evaporation method, we performed post-annealing processes with various temperatures and times. In the thin film after post-annealing at 110 °C for 45 min, the density of the CH₃NH₂ molecular defect was minimized, and CH₃NH₃I and PbI₂ disappeared in the thin film after the post-annealing process at 150 °C for 30 min. However, the density of the CH₃NH₂ molecular defect increased. Moreover, the THz-wave absorption property for each thin film was obtained using a THz time-domain spectroscopy to understand the correlation between the density of a molecular defect and the THz-wave oscillation strength at 1.6 THz, which originated in the molecular defect-incorporated hybrid perovskite structure. There is a strong linear correlation between the oscillator strength of a significant THz-wave absorption at 1.6 THz and the CH₃NH₂ molecular defect density.