Visualizing Phase Segregation in Mixed-Halide Perovskite Single Crystals

Unraveling Halogen Role in Two-Step Solution Growth of Organic-Inorganic Hybrid Mixed-Halide Perovskites: Guidelines of Fabricating Single-Phase Perovskites with Predictable Stoichiometry

The challenge faced in optoelectronic applications of halide perovskites is their degradation. Minimizing material imperfections is critical to averting cascade degradation processes. Identifying causes of such imperfections is, however, hindered by mystified growth processes and is particularly urgent for mixed-halide perovskites because of inhomogeneity in growth and phase segregation under stresses. To unravel two-step solution growth of MAPbBr x I3-x , we monitored the evolution of Br composition and found that the construction of perovskite lattice is contributed by iodine from PbI2 substrate and Br from MABr solution with a 1:1 ratio rather than a 2:1 ratio originally thought. Kinetic analysis based on a derived three-stage model extracted activation energies of perovskite construction and anion exchange. This model is applicable to the growth of PbI2 reacting with a mixed solution of MABr and MAI. Two guidelines of fabricating single-phase MAPbBr x I3-x with predictable stoichiometry thus developed help strategizing protocols to reproducibly fabricate mixed-halide perovskite films tailored to specific optoelectronic applications.

Modulation of Photoinduced Phase Segregation and Stress-Driven Nanoscale Cracking in Hybrid Halide Perovskite Solar Cells

The negative effect of photoinduced halide segregation (PIHS) on the properties of hybrid halide perovskites poses a major obstacle for its future commercial application. Therefore, the in-depth understanding of halide-ion segregation and its causes is an urgent and intractable problem. When PIHS reaches a certain threshold, it will aggravate the deterioration of the film surface morphology and form nanoscale cracks. Herein, the formation mechanism and types of cracks are revealed by exploring the stress distribution in the film. Using the femtosecond time-resolved transient absorption spectroscopy, the ultrafast formation of the iodine rich phase is observed, which appears earlier than the bromine rich phase. In addition, the introduction of organic ligand didodecyldimethylammonium bromide can significantly inhibit PIHS and improve the surface morphology of the film, which can promote the device efficiency from 9.63 to 11.20%. This work provides a novel perspective for the exploration of the PIHS.

Formation of Iodide-Rich Domains During Halide Segregation in Lead-Halide Perovskite Nanocrystals

Mixed iodide-bromide methylammonium lead perovskite (MAPbIxBr3-x) nanocrystals (NCs) hold promise for use in light-emitting applications owing to the size- and composition-tunability of their bandgap. However, the segregation of halides during light exposure causes their band gaps to become unstable and narrow. Here, we use transient absorption spectroscopy to track excited-state dynamics during photoinduced halide segregation. The Auger recombination dynamics are observed to accelerate as the bandgap narrows, suggesting enhanced electron-hole overlap. We simulate the motion of iodide within the NC and estimate the evolving bandgap and electron-hole overlap during two possible mechanisms of halide segregation. Our results support a segregation mechanism in which iodide anions form a domain within the NC, rather than a mechanism in which iodide anions independently segregate toward the NC surface. Such mechanistic insight will contribute to future NC bandgap stabilization strategies.

Temperature-Dependent Reversal of Phase Segregation in Mixed-Halide Perovskites

Understanding the mechanism of light-induced halide segregation in mixed-halide perovskites is essential for their application in multijunction solar cells. Here, photoluminescence spectroscopy is used to uncover how both increases in temperature and light intensity can counteract the halide segregation process. It is observed that, with increasing temperature, halide segregation in CH₃ NH₃ Pb(Br_0.4 I_0.6 )₃ first accelerates toward ≈290 K, before slowing down again toward higher temperatures. Such reversal is attributed to the trade-off between the temperature activation of segregation, for example through enhanced ionic migration, and its inhibition by entropic factors. High light intensities meanwhile can also reverse halide segregation; however, this is found to be only a transient process that abates on the time scale of minutes. Overall, these observations pave the way for a more complete model of halide segregation and aid the development of highly efficient and stable perovskite multijunction and concentrator photovoltaics.

Morphology-Dependent Carrier Accumulation Dynamics in Mixed Halide Perovskite Thin Films Caused by Phase Segregation

The phase segregation in mixed halide perovskites is recently found to improve the photoluminescence quantum yield (PLQY) of the perovskites by concentrating the carriers. However, how phase segregation affects the photoinduced carrier dynamics is unclear. Herein, we find that the phase segregation in CH₃NH₃PbBr_xI_3-x mixed halide perovskite thin film is morphology-dependent by showing I-rich domains mainly along the grain boundaries. Ultrafast transient absorption (TA) and photoluminescence upconversion (PL-UC) spectroscopy measurements uncover that the carrier accumulation in the low energy I-rich domains includes two carrier transfer pathways. Carrier transfer from the Br-rich domain and the mixed phase to the I-rich domain is realized by consecutive hole (∼0.5 ps) and electron (<12.4 ps) transfer and energy transfer (<12.4 ps), respectively. The finding reveals the carrier funneling dynamic mechanism in phase-segregated halide perovskite films and provides a guideline for the applications of mixed halide perovskites in color-conversion devices or high-efficiency LEDs.

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.

Operando Imaging of Crystallinity-Dependent Multicolor Thermochromic Processes for Single Hydrated Hybrid Perovskite Particles

The thermochromic properties of hydrated metal halide perovskites (MHPs) are promising for applications in smart windows, solar cells, optical sensors, and information storage. Traditional ensemble characterization methods always study the averaged thermochromic activity, lacking the accurate structure-activity correlation. Here we utilize dark-field microscopy (DFM) to in situ image the thermochromic processes of single isolated hydrated hybrid perovskite (CH₃NH₃)₄PbI_6-xCl_x·2H₂O (MA₄PbI_6-xCl_x·2H₂O) microparticles. The thermal-induced dehydration transition is demonstrated to alter the color of single MA₄PbI_6-xCl_x·2H₂O particles. Operando single-particle mapping results reveal the significant intra- and interparticle variations of thermochromic behaviors, yielding unexpected single or multistep multicolor thermochromic processes. These phenomena are confirmed to be governed by the crystallinity of single MA₄PbI_6-xCl_x·2H₂O particles that results in distinct composition-dependent bandgaps and thermal decomposition pathways. The present work highlights the important role of single-particle imaging for resolving the intrinsic thermochromic characteristic of hydrated MHPs, therefore opening a way for rational design of stimuli-responsive materials.

Reversible Phase Segregation and Amorphization of Mixed-Halide Perovskite Nanocrystals in Glass Matrices

Mixed-halide perovskites have attracted great attention in applications of lighting and photovoltaic devices due to their excellent properties. Understanding the phase segregation mechanism of mixed-halide perovskite has significance for suppressing the performance degradation of optoelectronic devices. Herein, we investigate the mixed-halide perovskite nanocrystals (NCs) in isolation from the external factors (oxygen, moisture, and pressure) using glass encapsulation, which shows excellent photostability against phase segregation. By monitoring the structural evolution of the NCs in glass matrices, the coexisting phase segregation and amorphization of mixed-halide perovskites are observed in real-time. The results show that thermal-induced local temperature increase plays a dominant role in the phase segregation of mixed-halide perovskite NCs. The recovery process is driven by the spontaneous crystallization of the amorphous mixed-halide phase. The clarified dynamic equilibrium process between the compositional segregation (mixing) and structural disorder (order) gives us a better insight into the reversible phase segregation mechanism of mixed-halide perovskite.

Visualization of Ion Migration in an Inorganic Mixed Halide Perovskite by One-Photon and Multiphoton Absorption: Effect of Guanidinium A-Site Cation Incorporation

In this work, we present the ion migration of CsPbIBr₂ under illumination and impede it by incorporating the large cations of guanidinium (GA). A series of "probe-set-probe" operations are applied to assess the photoluminescence (PL) behavior spectrally and spatially, which is correlated to the ion migration-induced phase separation, of CsPbIBr₂ and GA_xCs_1-xPbIBr₂ perovskites. The local lattice distortion introduced by GA could reduce the strain gradient in GA_xCs_1-xPbIBr₂ to inhibit the ion migration, leading to a stable PL spectrum and enhanced device stability under light stimulation. A solar cell with an optimized stoichiometric composition of GA_0.1Cs_0.9PbIBr₂ delivers comparable photovoltaic performance and improved stability compared to those of CsPbIBr₂-based perovskite solar cells, retaining 80% of its initial power conversion efficiency after being continuously bathed in light for 8 h under ambient conditions without encapsulation, while the CsPbIBr₂ counterpart shows an efficiency that is <30% of its initial value under the same test condition.

High-Performance and Stable Semi-Transparent Perovskite Solar Cells through Composition Engineering

Semi-transparent perovskite solar cells (ST-PeSCs) have tremendous potential as solar windows owing to their higher efficiency and visible transmittance. However, studies toward this application are still nascent, particularly in unraveling the interplay between how the perovskite composition impacts the achievable device performance and stability. Here, the role of A- and X-site modification in APbX₃ perovskites is studied to understand their influence on these factors. Through detailed experimental and simulation work, it is found that a perovskite composition consisting of cesium (Cs) and formamidinium (FA) at the A-site delivers the best device performance over a range of band gaps, which are tuned by changes to the X-site anion. Using this optimized perovskite composition, power conversion efficiencies of 15.5% and 4.1% are achieved for ST-PeSCs with average visible transmittance values between 20.7% and 52.4%, respectively. Furthermore, the CsFA-based ST-PeSCs show excellent long-term stability under continuous illumination and heating. The stability of the precursor solutions across each of the studied compositions has also been considered, showing dramatic differences in the structural properties of the perovskites and their device performance for all mixed A-site compositions possessing the archetypal methyl ammonium species, while also confirming the superior stability of the CsFA precursor solutions.

Toward Continuous-Wave Pumped Metal Halide Perovskite Lasers: Strategies and Challenges

Reliable and efficient continuous-wave (CW) lasers have been intensively pursued in the field of optoelectronic integrated circuits. Metal perovskites have emerged as promising gain materials for solution-processed laser diodes. Recently, the performance of CW perovskite lasers has been improved with the optimization of material and device levels. Nevertheless, the realization of CW pumped perovskite lasers is still hampered by thermal runaway, unwanted parasitic species, and poor long-term stability. This review starts with the charge carrier recombination dynamics and fundamentals of CW lasing in perovskites. We examine the potential strategies that can be used to improve the performance of perovskite CW lasers from the materials to device levels. We also propose the open challenges and future opportunities in developing high-performance and stable CW pumped perovskite lasers.

Mechanism of Photoinduced Phase Segregation in Mixed-Halide Perovskite Microplatelets and Its Application in Micropatterning

Photoinduced phase segregation (PPS) is considered as a dominant factor that greatly deteriorates the performances of mixed-halide perovskite devices. However, the mechanism of PPS is still under fierce debate. Herein, CsPb(Br_x/Cl_1-x)₃ microplatelets (MPs) with homogeneous and heterogeneous surfaces are obtained by controlling the growth conditions. Under continuous irradiation, a new photoluminescence (PL) band at 516 nm gradually appears in the heterogeneous MPs, accompanied with the decreased emission of the mixed phase at 480 nm, revealing the occurrence of PPS, while the photoirradiation only leads to slight PL dimming without PPS in the homogeneous MPs. The direct correlation between PPS and the structural heterogeneity indicates that the localized electric field-induced drift (LEFD) of halide ions/carriers is responsible for the PPS. In situ microfluorescence images evidence that the migration of halide ions is directed by the structural heterogeneity-induced localized electric field. Our refined model not only consolidates that PPS can be suppressed by eliminating the defects but also reveals that PPS can be directed by the distribution of defects. Therefore, a fluorescence micropatterning technique is developed based on PPS.

Halide Segregation in Mixed Halide Perovskites: Visualization and Mechanisms

Photoinduced halide segregation in mixed halide perovskites is an intriguing phenomenon and simultaneously a stability issue. In-depth probing this effect and unveiling the underpinning mechanisms are of great interest and significance. This article reviews the progress in visualized investigation of halide segregation, especially light-induced, by means of spatially-resolved imaging techniques. Furthermore, the current understanding of photoinduced phase separation based on several possible mechanisms is summarized and discussed. Finally, the remained open questions and future outlook in this field are outlined.

Stabilization of Mixed-Halide Lead Perovskites Under Light by Photothermal Effects

Mixed-halide lead perovskites (MHLPs) are semiconductor materials with bandgaps that are tunable across the visible spectrum and have seen promising applications in photovoltaics and optoelectronics. However, their segregation into phases with enriched halide components, under resonant light illumination and/or electric field, have hindered their practical applications. Herein, we demonstrate the stabilization of the MHLP photoluminescence (PL) peak as a function of their excitation intensities. This effect is associated with the phase segregation of MHLPs and their subsequent remixing by photothermal heating. We conclude that the balance between these opposing processes dictates the equilibrium PL peak of the MHLPs. The findings in this work could serve as a potential approach to obtain MHLP with stable emission peaks under operating conditions.

A Hybrid Hole Transport Layer for Perovskite-Based Solar Cells

This paper presents the effect of a composite poly(3,4-ethylenedioxythiophene) polystyrene sulfonate PEDOT:PSS and copper-doped nickel oxide (Cu:NiOx) hole transport layer (HTL) on the performance of perovskite solar cells (PSCs). Thin films of Cu:NiOx were spin-coated onto fluorine-doped tin oxide (FTO) glass substrates using a blend of nickel acetate tetrahydrate, 2-methoxyethanol and monoethanolamine (MEA) and copper acetate monohydrate. The prepared solution was stirred at 65 °C for 4 h and spin-coated onto the FTO substrates at 3000 rpm for 30 s in a nitrogen glovebox. The Cu:NiOx/FTO/glass structure was then annealed in air at 400 °C for 30 min. A mixture of PEDOT:PSS and isopropyl alcohol (IPA) (in 1:0.05 wt%) was spun onto the Cu:NiOx/FTO/glass substrate at 4000 rpm for 60 s. The multilayer structure was annealed at 130 °C for 15 min. Subsequently, the perovskite precursor (0.95 M) of methylammonium iodide (MAI) to lead acetate trihydrate (Pb(OAc)2·3H2O) was spin-coated at 4000 rpm for 200 s and thermally annealed at 80 °C for 12 min. The inverted planar perovskite solar cells were then fabricated by the deposition of a photoactive layer (CH3NH3PbI3), [6,6]-phenyl C61-butyric acid methyl ester (PCBM), and a Ag electrode. The mechanical behavior of the device during the fabrication of the Cu:NiOx HTL was modeled with finite element simulations using Abaqus/Complete Abaqus Environment CAE. The results show that incorporating Cu:NiOx into the PSC device improves its density–voltage (J–V) behavior, giving an enhanced photoconversion efficiency (PCE) of ~12.8% from ~9.8% and ~11.5% when PEDOT:PSS-only and Cu:NiOx-only are fabricated, respectively. The short circuit current density Jsc for the 0.1 M Cu:NiOx and 0.2 M Cu:NiOx-based devices increased by 18% and 9%, respectively, due to the increase in the electrical conductivity of the Cu:NiOx which provides room for more charges to be extracted out of the absorber layer. The increases in the PCEs were due to the copper-doped nickel oxide blend with the PEDOT:PSS which enhanced the exciton density and charge transport efficiency leading to higher electrical conductivity. The results indicate that the devices with the copper-doped nickel oxide hole transport layer (HTL) are slower to degrade compared with the PEDOT:PSS-only-based HTL. The finite element analyses show that the Cu:NiOx layer would not extensively deform the device, leading to improved stability and enhanced performance. The implications of the results are discussed for the design of low-temperature solution-processed PSCs with copper-doped nickel oxide composite HTLs.

Growth of two-dimensional formamidine lead halide perovskite single-crystalline sheets and their optoelectronic properties

Formamidine-based hybrid perovskite is an excellent optoelectronic material; however, its intrinsic non-layered crystalline structure makes it hard to isolate the corresponding 2D counterparts. In this work, a unique liquid-epitaxy technique was introduced to grow micro-sized two-dimensional FAPbX3 perovskite sheets. Such ultrathin sheets exhibited excellent photo-induced carrier properties with high crystalline quality, as well as provided new opportunities for next-generation optoelectronic devices.

Revealing Dynamic Effects of Mobile Ions in Halide Perovskite Solar Cells Using Time-Resolved Microspectroscopy

Halide perovskites are promising candidate materials for the next generation high-efficiency optoelectronic devices. Since perovskites are electronic-ionic mixed conductors, ion dynamics have a critical impact on the performance and stability of perovskite-based applications. However, comprehensively understanding ionic dynamics is challenging, particularly on nanoscale imaging of ionic dynamics in perovskites. In this review, mobile ion dynamics in halide perovskites investigated via luminescence spectroscopy combined with confocal microscopy are discussed, including mobile ion induced fluorescence quenching, phase segregation in mixed halide hybrid perovskite, and mobile ion accumulation at the interface in perovskite devices. Steady-state and time-resolved luminescence imaging techniques, combined with confocal microscopy, are unique tools for probing ionic dynamics in perovskites, providing invaluable insights on ionic dynamics in nanoscale resolution, along with a wide temporal range from picoseconds to hours. The works in this review are not only for understanding mobile ions to improve the design of perovskite-based devices but also foster the development of microspectroscopic methodologies in a broader solid-state physics context of investigating ionic transports in polycrystalline materials.

Light-induced reversal of ion segregation in mixed-halide perovskites

Bandgap instability due to light-induced phase segregation in mixed-halide perovskites presents a major challenge for their future commercial use. Here we demonstrate that photoinduced halide-ion segregation can be completely reversed at sufficiently high illumination intensities, enabling control of the optical bandgap of a mixed-halide perovskite single crystal by optimizing the input photogenerated carrier density. We develop a polaron-based two-dimensional lattice model that rationalizes the experimentally observed phenomena by assuming that the driving force for photoinduced halide segregation is dependent on carrier-induced strain gradients that vanish at high carrier densities. Using illumination sources with different excitation intensities, we demonstrate write-read-erase experiments showing that it is possible to store information in the form of latent images over several minutes. The ability to control the local halide-ion composition with light intensity opens opportunities for the use of mixed-halide perovskites in concentrator and tandem solar cells, as well as in high-power light-emissive devices and optical memory applications.

Strain-activated light-induced halide segregation in mixed-halide perovskite solids

Light-induced halide segregation limits the bandgap tunability of mixed-halide perovskites for tandem photovoltaics. Here we report that light-induced halide segregation is strain-activated in MAPb(I_1−xBr_x)₃ with Br concentration below approximately 50%, while it is intrinsic for Br concentration over approximately 50%. Free-standing single crystals of CH₃NH₃Pb(I_0.65Br_0.35)₃ (35%Br) do not show halide segregation until uniaxial pressure is applied. Besides, 35%Br single crystals grown on lattice-mismatched substrates (e.g. single-crystal CaF₂) show inhomogeneous segregation due to heterogenous strain distribution. Through scanning probe microscopy, the above findings are successfully translated to polycrystalline thin films. For 35%Br thin films, halide segregation selectively occurs at grain boundaries due to localized strain at the boundaries; yet for 65%Br films, halide segregation occurs in the whole layer. We close by demonstrating that only the strain-activated halide segregation (35%Br/45%Br thin films) could be suppressed if the strain is properly released via additives (e.g. KI) or ideal substrates (e.g. SiO₂).

Mixed-halide perovskites are of interest for photovoltaic devices, but light-induced halide segregation obstructs bandgap tuning and is not fully understood. Here the authors study the effects of strain and iodide/bromide ratio on light-induced halide segregation in mixed-halide perovskites.

Photoinduced Phase Segregation Leading to Evident Open-Circuit Voltage Loss in Efficient Inorganic CsPbIBr2 Solar Cells

The photoinduced phase segregation (PIPS) of mixed-halide perovskites (MHPs), due to halogen migration, has reaped considerable attention for its retroaction on film photostability and photovoltaic output. Nevertheless, the original mechanism is still unclear. Herein, taking the representative CsPbIBr₂ material as an example, a confocal laser scanning microscope (CLSM) technique was adopted to track the PIPS and dark recovery procedures. Besides the aggregation of iodide-rich (I-rich) domains at grain boundaries (GBs), some sporadic iodide "islands" with a swifter light response also appear throughout the polycrystalline films. It illustrates again that GBs are not essential for iodide aggregation. Furthermore, the iodide "islands" have substantial influence on a device's open-circuit voltage (V_oc), resulting in an obvious plunge in the first tens of seconds. Results reveal the internal reason for the failure to reach the larger V_oc outputs expected from wide-bandgap perovskites. Importantly, this finding can help promote the exploration of an efficient means to stabilize MHPs.

Photoinduced phase separation in the lead halides is a polaronic effect

We present a perspective on recent observations of the photoinduced phase separation of halides in multi-component lead-halide perovskites. The spontaneous phase separation of an initial homogeneous solid solution under steady-state illumination conditions is found experimentally to be reversible, stochastic, weakly dependent on morphology, yet strongly dependent on composition and thermodynamic state. Regions enriched in a specific halide species that form upon phase separation are self-limiting in size, pinned to specific compositions, and grow in number in proportion to the steady-state carrier concentration until saturation. These empirical observations of robustness rule out explanations based on specific defect structures and point to the local modulation of an existing miscibility phase transition in the presence of excess charge carriers. A model for rationalizing existing observations based on the coupling between composition, strain, and charge density fluctuations through the formation of polarons is reviewed.

Local Rearrangement of the Iodide Defect Structure Determines the Phase Segregation Effect in Mixed-Halide Perovskites

Mixed-halide perovskites represent a particularly relevant case within the family of lead-halide perovskites. Beyond their technological relevance for a variety of optoelectronic devices, photoinduced structural changes characteristic of this type of material lead to extreme photophysical changes that are currently the subject of intense debate. Herein we show that the conspicuous photoinduced phase segregation characteristic of these materials is primarily the result of the local and metastable rearrangement of the iodide sublattice. A local photophysical study comprising spectrally resolved laser scanning confocal microscopy is employed to find a correlation between the defect density and the dynamics of photoinduced changes, which extend far from the illuminated region. We observe that iodide-rich regions evolve much faster from highly defective regions. Also, by altering the material composition, we find evidence for the interplay between the iodide-related defect distribution and the intra- and interdomain migration dynamics giving rise to the complexity of this process.

Phase Segregation and Photothermal Remixing of Mixed-Halide Lead Perovskites

Mixed-halide lead perovskites (MHPs) are promising materials for photovoltaics and optoelectronics due to their highly tunable band gaps. However, they phase segregate under continuous illumination or an electric field, the mechanism of which is still under debate. Herein we systematically measure the phase segregation behavior of polymer-encapsulated CH₃NH₃Pb(Br_xI_1-x)₃ MHPs as a function of excitation intensity and the nominal halide ratio by in situ photoluminescence microspectroscopy and observe surprising phase dynamics at the beginning of the illumination. The initial phase segregation to I-rich and Br-rich phases is observed followed by the formation of a new mixed-halide phase within several seconds that has not been reported before. We propose that the photothermal effect is amplified at the small-size I-rich domains, which significantly changes the local phase segregation in the otherwise uniform film within milliseconds after illumination.

Laser induced ion migration in all-inorganic mixed halide perovskite micro-platelets

Despite intensive research on ion migration (IM) in organic-inorganic hybrid metal halide perovskites, much less is known about the irradiation effect on IM in all-inorganic perovskites, especially for those single crystals lacking complicated grain boundaries. Herein, the real-time IM process and the corresponding photoluminescence (PL) spectra induced by laser irradiation in all-inorganic CsPbBr _x I_(3-x) single crystals prepared by chemical vapor deposition (CVD) were investigated. We proposed that a local electric field acts as a driving force for IM and confirmed this by applying a bias to an indium tin oxide (ITO)/perovskite/ITO configuration. According to the control experiments on CsPbBr _x I_(3-x) micro-platelets with and without polymethyl methacrylate (PMMA) coating, it is concluded that the vacancy defect on the single crystal surface is the main pathway for IM. Our work is important for understanding and controlling light induced IM in all-inorganic perovskites.

Electrochemical Hole Injection Selectively Expels Iodide from Mixed Halide Perovskite Films

Halide ion mobility in metal halide perovskites remains an intriguing phenomenon, influencing their optical and photovoltaic properties. Selective injection of holes through electrochemical anodic bias has allowed us to probe the effect of hole trapping at iodide (0.9 V) and bromide (1.15 V) in mixed halide perovskite (CH₃NH₃PbBr_1.5I_1.5) films. Upon trapping holes at the iodide site, the iodide gradually gets expelled from the mixed halide film (as iodine and/or triiodide ion), leaving behind re-formed CH₃NH₃PbBr₃ domains. The weakening of the Pb-I bond following the hole trapping (oxidation of the iodide site) and its expulsion from the lattice in the form of iodine provided further insight into the photoinduced segregation of halide ions in mixed halide perovskite films. Transient absorption spectroscopy revealed that the iodide expulsion process leaves a defect-rich perovskite lattice behind as charge carrier recombination in the re-formed lattice is greatly accelerated. The selective mobility of iodide species provides insight into the photoinduced phase segregation and its implication in the stable operation of perovskite solar cells.

Optimizing optoelectronic performances by controlling halide compositions of MAPb(ClxI1−x)3 single crystals

This article aims at investigating the structures of iodide/chloride mixed-halide perovskites and correlating the I : Cl ratio with their optoelectronic performances.