Origin of Light-Induced Photophysical Effects in Organic Metal Halide Perovskites in the Presence of Oxygen

Composition Dictates Octahedral Tilt and Photostability in Halide Perovskites

Halide perovskites are excellent candidate materials for use in solar cell, LED, and detector devices, in part because their composition can be tuned to achieve ideal optoelectronic properties. Empirical efficiency optimization has led the field toward compositions rich in FA (formamidinium) on the A-site and I on the X-site, with additional small amounts of MA (methylammonium) or Cs A-site cations and Br X-site anions. However, it is not clear how and why the specific compositions of alloyed, that is, mixed component, halide perovskites relate to photo-stability of the materials. Here, this work combines synchrotron grazing incidence wide-angle X-ray scattering, photoluminescence, high-resolution scanning electron diffraction measurements and theoretical modelling to reveal the links between material structure and photostability. Namely, this work finds that increased octahedral titling leads to improved photo-stability that is correlated with lower densities of performance-harming hexagonal polytype impurities. These results uncover the structural signatures underpinning photo-stability and can therefore be used to make targeted changes to halide perovskites, bettering the commercial prospects of technologies based on these materials.

Light-enhanced oxygen degradation of MAPbBr3 single crystal

Organometal halide perovskites are promising materials for optoelectronic applications, whose commercial realization depends critically on their stability under multiple environmental factors. In this study, a methylammonium lead bromide (MAPbBr3) single crystal was cleaved and exposed to simultaneous oxygen and light illumination under ultrahigh vacuum (UHV). The exposure process was monitored using X-ray photoelectron spectroscopy (XPS) with precise control of the exposure time and oxygen pressure. It was found that the combination of oxygen and light accelerated the degradation of MAPbBr3, which could not be viewed as a simple addition of that by oxygen-only and light-only exposures. The XPS spectra showed significant loss of carbon, bromine, and nitrogen at an oxygen exposure of 1010 Langmuir with light illumination, approximately 17 times of the additive effects of oxygen-only and light-only exposures. It was also found that the photoluminescence (PL) emission was much weakened by oxygen and light co-exposure, while previous reports had shown that PL was substantially enhanced by oxygen-only exposure. Measurements using a scanning electron microscope (SEM) and focused ion beam (FIB) demonstrated that the crystal surface was much roughened by the co-exposure. Density functional theory (DFT) calculations revealed the formation of superoxide and oxygen induced gap state, suggesting the creation of oxygen radicals by light illumination as a possible microscopic driving force for enhanced degradation.

Ultrapure Green High Photoluminescence Quantum Yield from FAPbBr3 Nanocrystals Embedded in Transparent Porous Films

Achieving highly transparent and emissive films based on perovskite quantum dots (PQDs) is a challenging task since their photoluminescence quantum yield (PLQY) typically drops abruptly when they are used as building blocks to make a solid. In this work, we obtain highly transparent films containing FAPbBr₃ quantum dots that display a narrow green emission (λ = 530 nm, full width at half-maximum (FWHM) = 23 nm) with a PLQY as high as 86%. The method employed makes use of porous matrices that act as arrays of nanoreactors to synthesize the targeted quantum dots within their void space, providing both a means to keep them dispersed and a protective environment. Further infiltration with poly(methyl methacrylate) (PMMA) increases the mechanical and chemical stability of the ensemble and serves to passivate surface defects, boosting the emission of the embedded PQD and significantly reducing the width of the emission peak, which fulfills the requirements established by the Commission Internationale de l'Éclairage (CIE) to be considered an ultrapure green emitter. The versatility of this approach is demonstrated by fabricating a color-converting layer that can be easily transferred onto a light-emitting device surface to modify the spectral properties of the outgoing radiation.

Super-Bright Green Perovskite Light-Emitting Diodes Using Ionic Liquid Additives

Halide perovskites have potential for use in next-generation low-cost, high-efficiency, and highly color-pure light-emitting diodes (LED) that can be used in various applications, such as flat and flexible displays and solid-state lighting. However, they still lag behind other mature technologies, such as organic LEDs and inorganic LEDs, in terms of performance, particularly brightness. This lag is partly due to the insulating nature of the long-chain organic ligands used to control the perovskite-film morphology. Herein, a 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid (IL) is incorporated as a potential additive with CsPbBr₃ perovskite precursors, which results in a super-bright green perovskite light emitting diode (PeLED) achieving a peak luminance of 3.28 × 10⁵  cd m^-2 only at a bias voltage of 6 V, with a peak external quantum efficiency of 13.75%. This achievement is the outcome of multirole support from IL that simultaneously enables superior control over the perovskite-film morphology, passivates defects, modifies the band energy levels, and prevents ion migration. Hence, this work demonstrates IL as a novel alternative additive with the potential to outperform conventional long-chain ligands in high-performance PeLED device fabrication.

Light-Induced Phase Segregation Evolution of All-Inorganic Mixed Halide Perovskites

Light-induced phase segregation in mixed halide perovskites is a major roadblock for commercialization of optoelectronics utilizing these materials. We investigate the phenomenon in a model material system consisting of only surfaces and the bulk of a single-crystalline-like microplate. We utilize environmental in-situ time-dependent photoluminescence spectroscopy to observe the bandgap evolution of phase segregation under illumination. This enables analysis of the evolution of the iodide-rich phase composition as a function of the environment (i.e., surface defects) and carrier concentration. Our study provides microscopic insights into the relationship among photocarrier generations, surface structural defects, and subsequently iodide ion migrations that result in the complex evolution of phase segregation. We elucidate the significance of surface defects with respect to the evolution of phase segregation, which may provide new perspectives for modulating ion migration by engineering of defects and carrier concentrations.

Responsive Optical Materials Based on Ligand-Free Perovskite Quantum Dots Embedded in Mesoporous Scaffolds

Herein we show that dispersing inorganic cesium lead bromide (CsPbBr₃) perovskite quantum dots (QDs) in optical quality films, possessing an accessible and controlled pore size distribution, gives rise to fluorescent materials with a controlled and highly sensitive response to ambient changes. A scaffold-based synthesis approach is employed to obtain ligand-free QDs, whose pristine surface endows them with high sensitivity to the presence of different vapors in their vicinity. At the same time, the void network of the host offers a means to gradually expose the embedded QDs to such vapors. Under these conditions, the luminescent response of the QDs is mediated by the mesostructure of the matrix, which determines the rate at which vapor molecules will adsorb onto the pore walls and, eventually, condensate, filling the void space. With luminescence quantum yields as high as 60%, scaffold-supported ligand-free perovskite nanocrystals display intense photoemission signals over the whole process, as well as high photo- and chemical stability, which allows illuminating them for long periods of time and recovering the original response upon desorption of the condensed phase. The results herein presented open a new route to explore the application of perovskite QD-based materials in sensing.

Self-Healing Ability of Perovskites Observed via Photoluminescence Response on Nanoscale Local Forces and Mechanical Damage

The photoluminescence (PL) of metal halide perovskites can recover after light or current-induced degradation. This self-healing ability is tested by acting mechanically on MAPbI3 polycrystalline microcrystals by an atomic force microscope tip (applying force, scratching, and cutting) while monitoring the PL. Although strain and crystal damage induce strong PL quenching, the initial balance between radiative and nonradiative processes in the microcrystals is restored within a few minutes. The stepwise quenching-recovery cycles induced by the mechanical action is interpreted as a modulation of the PL blinking behavior. This study proposes that the dynamic equilibrium between active and inactive states of the metastable nonradiative recombination centers causing blinking is perturbed by strain. Reversible stochastic transformation of several nonradiative centers per microcrystal under application/release of the local stress can lead to the observed PL quenching and recovery. Fitting the experimental PL trajectories by a phenomenological model based on viscoelasticity provides a characteristic time of strain relaxation in MAPbI3 on the order of 10-100 s. The key role of metastable defect states in nonradiative losses and in the self-healing properties of perovskites is suggested.

Synthesis and Properties of Stable Amino Metal Halide Molecular Clusters in the Solid State

Nanosized molecular clusters (MCs) composed of PbBr₂ and neutral ligand butylamine (BTYA) with unique optical properties in solution and solid states have been synthesized using ligand-assisted reprecipitation and spin-coating, separately. The studies of their optical properties using ultraviolet-visible (UV-vis) absorption and photoluminescence (PL) show the first electronic absorption and PL band of the MCs at 401 and 411 nm, respectively, for the solution and solid state samples that exhibit good stability under ambient conditions. Low-temperature PL spectra below 30 K show vibronic peaks indicative of a single size or a very narrow size distribution of the MCs. On the basis of Raman, X-ray diffraction, and transmission electron microscopy measurements, a layered structural model is proposed for the MCs with a BTYA ligand capping on the surface of the corner-shared tilted [PbBr₆]^4- octahedral framework. The stable and retained structure of MCs in the solid state is promising for photonics applications.

The role of atmospheric conditions in the nonradiative recombination in individual CH3NH3PbI3 perovskite crystals

Organic-inorganic metal halide perovskites have been emerging as potential candidates for lightweight photovoltaic applications in space. However, fundamental physics concerning the effect of atmosphere on the radiative and nonradiative recombination in perovskites remains far from well understood. Here, we investigate the creation and annihilation of nonradiative recombination centers in individual CH3NH3PbI3 perovskite crystals by controlling the atmospheric conditions. We find that the photoluminescence (PL) of individual perovskite crystals can be quenched upon exposure from air to vacuum, while the subsequent PL enhancement in air shows a pressure dependence. Further analysis attributes the PL decline in vacuum to the activation of nonradiative trap sites, which is likely due to the lattice distortion caused by the variation of local strain on perovskites. With a gradual increase of the air pressure, the light-assisted chemisorption of oxygen on perovskite will passivate these nonradiative trap sites while simultaneously restoring the lattice imperfection, leading to PL enhancement. The present findings suggest that placing the perovskite in an environment with moderate oxygen content can protect the material from photophysical losses that can be pronounced under inert conditions.

Evaluation of surface passivating solvents for single and mixed halide perovskites

Surface passivation is one of the commonly used approaches to reduce the density of defects on the surfaces and interfaces hindering the performance and stability of perovskite optoelectronic devices. Although surface passivation leads to performance improvement for the targeted devices, details of the complex intermolecular interactions occurring between the molecules and perovskites are not entirely known. Here, we investigated a variety of commonly used solvents in the post-processing of perovskites by using photoluminescence (PL) spectroscopy on single and mixed halide perovskites (MAPbI₃, MAPbBr₃ and MAPb(Br_0.5I_0.5)₃). Our results show that solvents with medium and low Gutmann donor and acceptor numbers provide PL intensity increase for both single halide perovskites by passivating the surface defect sites. Among the single halide perovskites, MAPbBr₃ is more attracted to hydrogen bonding solvents, in contrast to MAPbI₃ that is preferred by Lewis bases. This halide selective attraction also has an influence on the mixed-halide composition. Identifying these interaction mechanisms provides new insights into passivating the surface of perovskites for future device design.

Degradation mechanisms of perovskite nanocrystals in color-converted InGaN micro-light-emitting diodes

The metal halide perovskite nanocrystals (NCs) have attracted much attention because of their excellent optical properties and potential for application in optoelectronic devices. However, their photo- and thermostability are still practical challenges and need further optimization. Here, we have studied the degradation behaviors of CsPbI₃ NCs utilized as optical conversion layer in InGaN based blue micro-LEDs in situ. Furthermore, the effects of temperature and light irradiation on perovskite NCs were investigated respectively. The results indicate that both blue light irradiation and high temperature can cause the increased nonradiative recombination rate, resulting in the degradation of perovskite NCs and reduction of the photoluminescence quantum yield (PLQY). Especially in high-temperature condition, both the single-exciton nonradiative recombination rate and the biexciton nonradiative recombination rate are increased, causing the significant reduction of PLQY of perovskite NCs in high temperature environment than blue light irradiation. Our work provides a detailed insight about the correlation between the light irradiation and temperature consequences for CsPbI₃ NCs and may help to pave the way toward optoelectronic device applications.

Self-Healing and Light-Soaking in MAPbI3 : The Effect of H2 O

The future of halide perovskites (HaPs) is beclouded by limited understanding of their long-term stability. While HaPs can be altered by radiation that induces multiple processes, they can also return to their original state by "self-healing." Here two-photon (2P) absorption is used to effect light-induced modifications within MAPbI₃ single crystals. Then the changes in the photodamaged region are followed by measuring the photoluminescence, from 2P absorption with 2.5 orders of magnitude lower intensity than that used for photodamaging the MAPbI₃ . After photodamage, two brightening and one darkening process are found, all of which recover but on different timescales. The first two are attributed to trap-filling (the fastest) and to proton-amine-related chemistry (the slowest), while photodamage is attributed to the lead-iodide sublattice. Surprisingly, while after 2P-irradiation of crystals that are stored in dry, inert ambient, photobrightening (or "light-soaking") occurs, mostly photodarkening is seen after photodamage in humid ambient, showing an important connection between the self-healing of a HaP and the presence of H₂ O, for long-term steady-state illumination, practically no difference remains between samples kept in dry or humid environments. This result suggests that photobrightening requires a chemical-reservoir that is sensitive to the presence of H₂ O, or possibly other proton-related, particularly amine, chemistry.

Quantitative Predictions of Moisture-Driven Photoemission Dynamics in Metal Halide Perovskites via Machine Learning

Metal halide perovskite (MHP) photovoltaics may become a viable alternative to standard Si-based technologies, but the current lack of long-term stability precludes their commercial adoption. Exposure to standard operational stressors (light, temperature, bias, oxygen, and water) often instigate optical and electronic dynamics, calling for a systematic investigation into MHP photophysical processes and the development of quantitative models for their prediction. We resolve the moisture-driven light emission dynamics for both methylammonium lead tribromide and triiodide thin films as a function of relative humidity (rH). With the humidity and photoluminescence time series, we train recurrent neural networks and establish their ability to quantitatively predict the path of future light emission with 18% error over 4 h. Together, our in situ rH-PL measurements and machine learning forecasting models provide a framework for the rational design of future stable perovskite devices and, thus, a faster transition toward commercial applications.

Reversible Photochromism in ⟨110⟩ Oriented Layered Halide Perovskite

Extending halide perovskites' optoelectronic properties to stimuli-responsive chromism enables switchable optoelectronics, information display, and smart window applications. Here, we demonstrate a band gap tunability (chromism) via crystal structure transformation from three-dimensional FAPbBr₃ to a ⟨110⟩ oriented FA_n+2Pb_nBr_3n+2 structure using a mono-halide/cation composition (FA/Pb) tuning. Furthermore, we illustrate reversible photochromism in halide perovskite by modulating the intermediate n phase in the FA_n+2Pb_nBr_3n+2 structure, enabling greater control of the optical band gap and luminescence of a ⟨110⟩ oriented mono-halide/cation perovskite. Proton transfer reaction-mass spectroscopy carried out to precisely quantify the decomposition product reveals that the organic solvent in the film is a key contributor to the structural transformation and, therefore, the chromism in the ⟨110⟩ structure. These intermediate n phases (2 ≤ n ≤ ∞) stabilize in metastable states in the FA_n+2Pb_nBr_3n+2 system, which is accessible via strain or optical or thermal input. The structure reversibility in the ⟨110⟩ perovskite allowed us to demonstrate a class of photochromic sensors capable of self-adaptation to lighting.

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.

Boosting Long-Term Stability of Pure Formamidinium Perovskite Solar Cells by Ambient Air Additive Assisted Fabrication

Due to the high industrial interest for perovskite-based photovoltaic devices, there is an urgent need to fabricate them under ambient atmosphere, not limited to low relative humidity (RH) conditions. The formamidinium lead iodide (FAPI) perovskite α-black phase is not stable at room temperature and is challenging to stabilize in an ambient environment. In this work, we show that pure FAPI perovskite solar cells (PSCs) have a dramatic increase of device long-term stability when prepared under ambient air compared to FAPI PSCs made under nitrogen, both fabricated with N-methylpyrrolidone (NMP). The T 80 parameter, the time in which the efficiency drops to 80% of the initial value, increases from 21 (in N2) to 112 days (in ambient) to 145 days if PbS quantum dots (QDs) are introduced as additives in air-prepared FAPI PSCs. Furthermore, by adding methylammonium chloride (MACl) the power conversion efficiency (PCE) reaches 19.4% and devices maintain 100% of the original performance for at least 53 days. The presence of Pb-O bonds only in the FAPI films prepared in ambient conditions blocks the propagation of α- to δ-FAPI phase conversion. Thus, these results open the way to a new strategy for the stabilization in ambient air toward perovskite solar cells commercialization.

Exploring Transport Behavior in Hybrid Perovskites Solar Cells via Machine Learning Analysis of Environmental-Dependent Impedance Spectroscopy

Hybrid organic-inorganic perovskites are one of the promising candidates for the next-generation semiconductors due to their superlative optoelectronic properties. However, one of the limiting factors for potential applications is their chemical and structural instability in different environments. Herein, the stability of (FAPbI3 )0.85 (MAPbBr3 )0.15 perovskite solar cell is explored in different atmospheres using impedance spectroscopy. An equivalent circuit model and distribution of relaxation times (DRTs) are used to effectively analyze impedance spectra. DRT is further analyzed via machine learning workflow based on the non-negative matrix factorization of reconstructed relaxation time spectra. This exploration provides the interplay of charge transport dynamics and recombination processes under environment stimuli and illumination. The results reveal that in the dark, oxygen atmosphere induces an increased hole concentration with less ionic character while ionic motion is dominant under ambient air. Under 1 Sun illumination, the environment-dependent impedance responses show a more striking effect compared with dark conditions. In this case, the increased transport resistance observed under oxygen atmosphere in equivalent circuit analysis arises due to interruption of photogenerated hole carriers. The results not only shed light on elucidating transport mechanisms of perovskite solar cells in different environments but also offer an effective interpretation of impedance responses.

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

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

Mechanism and Timescales of Reversible p-Doping of Methylammonium Lead Triiodide by Oxygen

Understanding and controlling the energy level alignment at interfaces with metal halide perovskites (MHPs) is essential for realizing the full potential of these materials for use in optoelectronic devices. To date, however, the basic electronic properties of MHPs are still under debate. Particularly, reported Fermi level positions in the energy gap vary from indicating strong n- to strong p-type character for nominally identical materials, raising serious questions about intrinsic and extrinsic defects as dopants. ​In this work, photoemission experiments demonstrate that thin films of the prototypical methylammonium lead triiodide (MAPbI₃ ) behave like an intrinsic semiconductor in the absence of oxygen. Oxygen is then shown to be able to reversibly diffuse into and out of the MAPbI₃ bulk, requiring rather long saturation timescales of ≈1 h (in: ambient air) and over 10 h (out: ultrahigh vacuum), for few 100 nm thick films. Oxygen in the bulk leads to pronounced p-doping, positioning the Fermi level universally ≈0.55 eV above the valence band maximum. The key doping mechanism is suggested to be molecular oxygen substitution of iodine vacancies, supported by density functional theory calculations. This insight rationalizes previous and future electronic property studies of MHPs and calls for meticulous oxygen exposure protocols.

Photoemission Studies on the Environmental Stability of Thermal Evaporated MAPbI3 Thin Films and MAPbBr3 Single Crystals

Hybrid organic inorganic perovskites have been considered as a potential candidate for the next generational solar cell due to their outstanding optoelectronic properties and rapid development in recent years. However, the biggest challenge to prevent them from massive commercial use is their long-term stability. Photoemission spectroscopy has been widely used to investigate properties of the perovskites, which provide critical insights to better understand the degradation mechanisms. In this article, we review mainly our photoemission studies on the degradation processes of perovskite thin films and single crystals with different environmental factors, such as gases, water, and light by monitoring changes of chemical composition and electronic structure. These studies on the effects by different environmental parameters are discussed for the understanding of the stability issues and the possible solutions.

Ion migration drives self-passivation in perovskite solar cells and is enhanced by light soaking

Perovskite solar cells have rapidly become the most promising emerging photovoltaic technology. This is largely due to excellent self-passivating properties of the perovskite absorber material, allowing for a remarkable ease of fabrication. However, the field is plagued by poor reproducibility and conflicting results. This study finds that dynamic processes (ion migration) taking place after fabrication (without external stimuli) have a large influence on materials properties and need to be controlled to achieve reproducible results. The morphological and optoelectronic properties of triple cation perovskites with varying halide ratios are studied as they evolve over time. It is found that ion migration is essential for self-passivation, but can be impeded by low ion mobility or a low number of mobile species. Restricted ion movement can lead to crack formation in strained films, with disastrous consequences for device performance. However, a short light soaking treatment after fabrication helps to mobilize ions and achieve self-passivation regardless of composition. The community should adopt this treatment as standard practice to increase device performance and reproducibility.

Ion migration can assist self-passivation and strain relaxation in lead halide perovskite films, while restriction of ion migration can lead to crack formation. Light soaking increases ion migration, allowing self-passivation and strain relaxation.

Environment-Induced Reversible Modulation of Optical and Electronic Properties of Lead Halide Perovskites and Possible Applications to Sensor Development: A Review

Lead halide perovskites are currently widely investigated as active materials in photonic and optoelectronic devices. While the lack of long term stability actually limits their application to commercial devices, several experiments demonstrated that beyond the irreversible variation of the material properties due to degradation, several possibilities exist to reversibly modulate the perovskite characteristics by acting on the environmental conditions. These results clear the way to possible applications of lead halide perovskites to resistive and optical sensors. In this review we will describe the current state of the art of the comprehension of the environmental effects on the optical and electronic properties of lead halide perovskites, and of the exploitation of these results for the development of perovskite-based sensors.

Photon-induced deactivations of multiple traps in CH3NH3PbI3 perovskite films by different photon energies

Photon-induced trap deactivation is commonly observed in organometal halide perovskites. Trap deactivation is characterized by an obvious photoluminescence (PL) enhancement. In this work, the properties of traps in CH3NH3PbI3 perovskite films were studied based on the PL enhancement excited by lasers of different wavelengths (633 nm and 405 nm). Two types of electron traps were identified; one can be deactivated by both 633 nm and 405 nm illuminations, whereas the other one can only be deactivated by 405 nm illumination. The energy levels of both types of traps were beneath the conduction band minimum. The expressions of the PL enhancement kinetics due to the trap deactivations by lasers of different wavelengths were derived. The ratio of the constants of the radiative recombination rate and the initial capture rates for both traps was determined from the PL enhancement. The trap deactivation was a photon-related process rather than a photocarrier-related process, and the deactivation time was inversely proportional to the photon flux density.

Light-Induced Passivation in Triple Cation Mixed Halide Perovskites: Interplay between Transport Properties and Surface Chemistry

Mixed halide perovskites have attracted a strong interest in the photovoltaic community as a result of their high power conversion efficiency and the solid opportunity to realize low-cost and industry-scalable technology. Light soaking represents one of the most promising approaches to reduce non-radiative recombination processes and thus to optimize device performances. Here, we investigate the effects of 1 sun illumination on state-of-the-art triple cation halide perovskite thin films Cs_0.05(MA_0.14, FA_0.86)_0.95 Pb (I_0.84, Br_0.16)₃ by a combined optical and chemical characterization. Competitive passivation and degradation effects on perovskite transport properties have been analyzed by spectrally and time-resolved quantitative imaging luminescence analysis and by X-ray photoemission spectroscopy (XPS). We notice a clear improvement of the optoelectronic properties of the material, with a increase of the quasi fermi level splitting and a corresponding decrease of methylammonium MA⁺ for short (up to 1 h) light soaking time. However, after 5 h of light soaking, phase segregation and in-depth oxygen penetration lead to a decrease of the charge mobility.

Prolonged Lifetime of Perovskite Solar Cells Using a Moisture-Blocked and Temperature-Controlled Encapsulation System Comprising a Phase Change Material as a Cooling Agent

Although the power conversion efficiency of perovskite solar cells (PSCs) reached up to 25% that made them comparable to the commercial solar cells, they are facing issues toward commercialization, especially their short lifetime. Remarkably, the most important key factors that regulate the durability of the devices are moisture, light, and heat. In this work, prolonging the device lifetime is focused by designing a flexible moisture-blocked and temperature-controlled encapsulation system. In this regard, a thermally adjusted phase change material is embedded in a polymer encapsulation layer to avoid the moisture diffusion, rapid temperature fluctuation, and undesired crystalline phase change of the perovskite layer in the PSCs under the operation condition. As a result, a 2 year stable device is achieved, whereas the reference device loses more than 50% of its performance after 10 days. Surprisingly, the charge transport resistance and recombination rate show no significant change during 450 days of storage, which confirms no increase in the defect density.

Structural and spectroscopic studies of a nanostructured silicon-perovskite interface

While extensively investigated in thin film form for energy materials applications, this work investigates the formation of APbBr3 structures (A = CH3NH3+ (MA), Cs+) in silicon and oxidized silicon nanotubes (SiNTs) with varying inner diameter. We carefully control the extent of oxidation of the nanotube host and correlate the relative Si/Si oxide content in a given nanotube host with the photoluminescence quantum efficiency (PLQE) of the perovskite. Complementing these measurements is an evaluation of average PL lifetimes of a given APbBr3 nanostructure, as evaluated by time-resolved confocal photoluminescence measurements. Increasing Si (decreasing oxide) content in the nanotube host results in a sensitive reduction of MAPbBr3 PLQE, with a concomitant decrease in average lifetime (τave). We interpret these observations in terms of decreased defect passivation by a lower concentration of oxide species surrounding the perovskite. In addition, we show that the use of selected nanotube templates leads to more stable perovskite PL in air over time (weeks). Taken in concert, such fundamental observations have implications for interfacial carrier interactions in tandem Si/perovskite photovoltaics.

Unravelling the Photocatalytic Behavior of All-Inorganic Mixed Halide Perovskites: The Role of Surface Chemical States

Within the most mesmerizing materials in the world of optoelectronics, mixed halide perovskites (MHPs) have been distinguished because of the tunability of their optoelectronic properties, balancing both the light-harvesting efficiency and the charge extraction into highly efficient solar devices. This feature has drawn the attention of analogous hot topics as photocatalysis for carrying out more efficiently the degradation of organic compounds. However, the photo-oxidation ability of perovskite depends not only on its excellent light-harvesting properties but also on the surface chemical environment provided during its synthesis. Accordingly, we studied the role of surface chemical states of MHP-based nanocrystals (NCs) synthesized by hot-injection (H-I) and anion-exchange (A-E) approaches on their photocatalytic (PC) activity for the oxidation of β-naphthol as a model system. We concluded that iodide vacancies are the main surface chemical states that facilitate the formation of superoxide ions, O₂^●-, which are responsible for the PC activity in A-E-MHP. Conversely, the PC performance of H-I-MHP is related to the appropriate balance between band gap and a highly oxidizing valence band. This work offers new insights on the surface properties of MHP related to their catalytic activity in photochemical applications.

Investigation of Oxygen Passivation for High-Performance All-Inorganic Perovskite Solar Cells

Defect passivation using oxygen has been identified as an efficient and convenient approach to suppress nonradiative recombination and improve the photovoltaic performance of hybrid organic-inorganic halide perovskites (HHPs). However, oxygen can seriously undermine the chemical stability of HHPs due to the reaction of superoxide with protonated organic cations such as CH₃NH₃⁺ and [(NH₂)₂CH]⁺, thus hindering the deep understanding of how oxygen affects their defect properties. Here we substitute free-proton inorganic Cs⁺ for organic moiety to avoid the negative effect of oxygen and then systematically investigate the oxygen passivation mechanism in all-inorganic halide perovskites (IHPs) from theory to experiment. We find that, in contrast to conventional oxygen molecule passivation just through physisorption on the surface of perovskites, the oxygen atom can provide a better passivation effect due to its stronger interaction with perovskites. The key point to achieve O-passivated perovskites rather than O₂ is the dry-air processing condition, which can dissociate the O₂ into O during the annealing process. O-passivated IHP solar cells exhibit enhanced power conversion efficiency (PCE) and better air stability than O₂-passivated cells. These results not only provide deep insights into the passivation effect of oxygen on perovskites but also demonstrate the great potential of IHPs for high photovoltaic performance with simplified ambient processing.

Photoinduced photoluminescence enhancement in self-assembled clusters of formamidinium lead bromide perovskite nanocrystals

Nanocrystals of formamidinium lead bromide perovskite (FAPbBr3) self-assemble into clusters in powder and film samples and provide a prolonged photoluminescence lifetime, which is attributed to the diffusion of charge carriers through interparticle states formed among nanocrystals. Interestingly, the photoluminescence lifetime decreases and the emission intensity increases for the clusters, which is with the increase in the intensity of excitation light. By doping the nanocrystal clusters with C60, we successfully harvested the photogenerated charge carriers. Nonetheless, at high intensities of excitation, the rate of radiative recombination becomes comparable to that of the electron transfer to C60. Thus, the optimum rate of electron transfer to C60 is accomplished by minimally exciting the self-assembled nanocrystals.

Mechanism of Photoluminescence Intermittency in Organic-Inorganic Perovskite Nanocrystals

Lead halide perovskite nanocrystals have demonstrated their potential as active materials for optoelectronic applications over the past few years. Nevertheless, one issue that hampers their applicability has to do with the observation of photoluminescence intermittency, commonly referred to as "blinking", as in their inorganic counterparts. Such behavior, reported for structures well above the quantum confinement regime, has been discussed to be strongly related to the presence of charge carrier traps. In this work, we analyze the characteristics of this intermittency and explore the dependence on the surrounding atmosphere, showing evidence for the critical role played by the presence of oxygen. We discuss a possible mechanism in which a constant creation/annihilation of halide-related carrier traps takes place under light irradiation, with the dominant rate being determined by the atmosphere.

Highly Efficient and Environmentally Stable Flexible Color Converters Based on Confined CH3NH3PbBr3 Nanocrystals

In this work, we demonstrate a synthetic route to attain methylammonium lead bromide (CH₃NH₃PbBr₃) perovskite nanocrystals (nc-MAPbBr₃, 1.5 nm < size < 3 nm) and provide them with functionality as highly efficient flexible, transparent, environmentally stable, and adaptable color-converting films. We use nanoparticle metal oxide (MOx) thin films as porous scaffolds of controlled nanopores size distribution to synthesize nc-MAPbBr₃ through the infiltration of perovskite liquid precursors. We find that the control over the reaction volume imposed by the nanoporous scaffold gives rise to a strict control of the nanocrystal size, which allows us to observe well-defined quantum confinement effects on the photo-emission, being the luminescence maximum tunable with precision between λ = 530 nm (green) and λ = 490 nm (blue). This hybrid nc-MAPbBr₃/MOx structure presents high mechanical stability and permits subsequent infiltration with an elastomer to achieve a self-standing flexible film, which not only maintains the photo-emission efficiency of the nc-MAPbBr₃ unaltered but also prevents their environmental degradation. Applications as adaptable color-converting layers for light-emitting devices are envisaged and demonstrated.

Impact of Moisture on Photoexcited Charge Carrier Dynamics in Methylammonium Lead Halide Perovskites

Organic-inorganic metal halide perovskites are notoriously unstable in humid environments. While many studies have revealed the morphology and crystal structure changes that accompany exposure to humidity, little is known about changes to the photophysics that accompany the degradation process. By combining in situ steady-state and time-resolved photoluminescence with Hall effect measurements, we examined the changes in the photoexcited carrier dynamics for methylammonium lead iodide (MAPbI₃) and bromide (MAPbBr₃) films exposed to nitrogen gas containing water vapor at 80% relative humidity. The changes in the photophysics of MAPbI₃ interacting with water follow a four-stage process, consisting of surface passivation, free electron doping, interfacial hydration, and bulk hydration. In contrast, MAPbBr₃ exhibits only features associated with the first two stages, which occur at a faster rate. Our results elucidate the degradation mechanisms of perovskite films in high humidity from the perspective of the photophysics, providing insights for how humidity affects the stability of the perovskite materials.

Interfacial Charge Transfer between Excited CsPbBr3 Nanocrystals and TiO2: Charge Injection versus Photodegradation

Record-breaking efficiency achieved with quantum dot solar cells made of perovskite nanocrystals demands understanding of the excited-state interactions between perovskite nanocrystals and metal oxide electron transport layers. The interfacial electron transfer between excited CsPbBr₃ perovskite nanocrystals and metal oxides (TiO₂, SnO₂, and ZnO) was elucidated using transient absorption spectroscopy and found to occur with a rate constant in the range of 2-4 × 10¹⁰ s^-1. In an inert atmosphere, back electron transfer helps to maintain the stability of the perovskite nanocrystals. However, the presence of oxygen introduces instability as it scavenges away transferred electrons from the electron-transporting metal oxide, leaving behind holes to accumulate at CsPbBr₃ nanocrystals, which in turn induce anodic corrosion. X-ray photoelectron spectroscopy measurements have enabled us to identify PbO as the major photodegraded product. The importance of the surrounding atmosphere and the supporting metal oxide in governing the stability of perovskite nanocrystals is discussed.