TiO2-Assisted Halide Ion Segregation in Mixed Halide Perovskite Films

Hole Trapping in Lead Halide Perovskite Nanocrystal-Viologen Hybrids and Its Impact on Back Electron Transfer

Control of forward and back electron transfer processes in semiconductor nanocrystals is important to maximize charge separation for photocatalytic reduction/oxidation processes. By employing methyl viologen as the electron acceptor, we have succeeded in mapping the electron transfer from excited CsPbI3 nanocrystals to viologen as well as the hole trapping process. The electron transfer to viologen is an ultrafast process (ket = 2 × 1010 s-1) and results in the formation of extended charge separation as electrons are trapped at surface-bound viologen sites and holes at iodide sites. The I2─• formation, which is confirmed through the transient absorption at 750 nm, provides a convenient way to probe trapped holes and its participation in the back electron transfer process. By employing a series of mixed halide compositions, we were able to tune the bandgap and valence band energy of the perovskite donor. The back electron transfer rate constant (kbet = 1.3-2.6 × 107 s-1) is nearly three orders of magnitude smaller than that of forward electron transfer, thus extending the lifetime of the charge-separated state. The weak dependence of the back electron transfer rate constant on the valence band energy suggests that trapping of holes at halide (I or Br) sites is involved in the back electron transfer process. The ability to extend the lifetime of the charge-separated pair can offer new strategies to improve the redox properties of semiconductor-based photocatalytic systems.

Insights Into the Local Heating Effects in Triple-Cation Mixed-Halide Perovskite Cells: Charge Dynamics, Coherent Phonons, Anion Segregation

The behavior of triple-cation mixed halide perovskite solar cells (PSCs) under ultrashort laser pulse irradiation at varying fluences is investigated, with a focus on local heating effects observed in femtosecond transient absorption (TA) studies. The carrier cooling time constant is found to increase from 230 fs at 2 µJ cm⁻2 to 1.3 ps at 2 mJ cm⁻2 while the charge population decay accelerates from tens of nanoseconds to the picosecond range within the same fluence range. At fluences between 0.5 and 5 mJ cm2, distinct oscillations in the TA signal (≈1.1 MHz) reveal the presence of coherent longitudinal acoustic phonons (CLAP). These phonons induce lattice strain propagating at the speed of sound through the perovskite layer and exhibit relatively long damping times. TA spectra further reveal a partially reversible segregation of iodide and bromide anions under pulsed excitation. Interestingly, higher local heating at increased pump fluences slows the segregation process, with time constants extending from 40 min at low fluences to 110 min at high fluences. However, the continuous irradiation results in significantly smaller segregation effects compared to ultrashort pulse irradiation.

Solution-Processed Micro-Nanostructured Electron Transport Layer via Bubble-Assisted Assembly for Efficient Perovskite Photovoltaics

Organic-inorganic halide perovskite solar cells (PSCs) have attracted significant attention in photovoltaic research, owing to their superior optoelectronic properties and cost-effective manufacturing techniques. However, the unbalanced charge carrier diffusion length in perovskite materials leads to the recombination of photogenerated electrons and holes. The inefficient charge carrier collecting process severely affects the power conversion efficiency (PCE) of the PSCs. Herein, a solution-processed SnO2 array electron transport layer with precisely tunable micro-nanostructures is fabricated via a bubble-template-assisted approach, serving as both electron transport layers and scaffolds for the perovskite layer. Due to the optimized electron transporting pathway and enlarged perovskite grain size, the PSCs achieve a PCE of 25.35% (25.07% certificated PCE).

Inhibiting the Appearance of Green Emission in Mixed Lead Halide Perovskite Nanocrystals for Pure Red Emission

Mixed halide perovskites exhibit promising optoelectronic properties for next-generation light-emitting diodes due to their tunable emission wavelength that covers the entire visible light spectrum. However, these materials suffer from severe phase segregation under continuous illumination, making long-term stability for pure red emission a significant challenge. In this study, we present a comprehensive analysis of the role of halide oxidation in unbalanced ion migration (I/Br) within CsPbI2Br nanocrystals and thin films. We also introduce a new approach using cyclic olefin copolymer (COC) to encapsulate CsPbI2Br perovskite nanocrystals (PNCs), effectively suppressing ion migration by increasing the corresponding activation energy. Compared with that of unencapsulated samples, we observe a substantial reduction in phase separation under intense illumination in PNCs with a COC coating. Our findings show that COC enhances phase stability by passivating uncoordinated surface defects (Pb2+ and I-), increasing the formation energy of halide vacancies, improving the charge carrier lifetime, and reducing the nonradiative recombination density.

Substrate Induced p-n Transition for Inverted Perovskite Solar Cells

The p- or n-type property of semiconductor materials directly determine the final performance of photoelectronic devices. Generally, perovskite deposited on p-type substrate tends to be p-type, while perovskite deposited on n-type substrate tends to be n-type. Motived by this, a substrate-induced re-growth strategy is reported to induce p- to n-transition of perovskite surface in inverted perovskite solar cells (PSCs). p-type perovskite film is obtained and crystallized on p-type substrate first. Then an n-type ITO/SnO2 substrate with saturated perovskite solution is pressed onto the perovskite film and annealed to induce the secondary re-growth of perovskite surface region. As a result, p- to n-type transition happens and induces an extra junction at perovskite surface region, thus enhancing the built-in potential and promoting carrier extraction in PSCs. Resulting inverted PSCs exhibit high efficiency of over 25% with good operational stability, retaining 90% of initial efficiency after maximum power point (MPP) tracking for 800 h at 65 °C with ISOS-L-2 protocol.

Charge Transfer Kinetics in Halide Perovskites: On the Constraints of Time-Resolved Spectroscopy Measurements

Understanding photophysical processes in lead halide perovskites is an important aspect of optimizing the performance of optoelectronic devices. The determination of exact charge carrier extraction rate constants remains elusive, as there is a large and persistent discrepancy in the reported absolute values. In this review, we concentrate on experimental procedures adopted in the literature to obtain kinetic estimates of charge transfer processes and limitations imposed by the spectroscopy technique employed. Time-resolved techniques (e.g., transient absorption-reflection and time-resolved photoluminescence spectroscopy) are commonly employed to probe charge transfer at perovskite/transport layer interfaces. The variation in sample preparation and measurement conditions can produce a wide dispersion of the measured kinetic parameters. The selected time window and the kinetic fitting model employed introduce additional uncertainty. We discuss here evaluation strategies that rely on multiexponential fitting protocols (regular or stretched) and show how the dispersion in the reported values for carrier transfer rate constants can be resolved.

The balancing act between high electronic and low ionic transport influenced by perovskite grain boundaries

A better understanding of the materials' fundamental physical processes is necessary to push hybrid perovskite photovoltaic devices towards their theoretical limits. The role of the perovskite grain boundaries is essential to optimise the system thoroughly. The influence of the perovskite grain size and crystal orientation on physical properties and their resulting photovoltaic performance is examined. We develop a novel, straightforward synthesis approach that yields crystals of a similar size but allows the tuning of their orientation to either the (200) or (002) facet alignment parallel to the substrate by manipulating dimethyl sulfoxide (DMSO) and tetrahydrothiophene-1-oxide (THTO) ratios. This decouples crystal orientation from grain size, allowing the study of charge carrier mobility, found to be improved with larger grain sizes, highlighting the importance of minimising crystal disorder to achieve efficient devices. However, devices incorporating crystals with the (200) facet exhibit an s-shape in the current density-voltage curve when standard scan rates are used, which typically signals an energetic interfacial barrier. Using the drift-diffusion simulations, we attribute this to slower-moving ions (mobility of 0.37 × 10-10 cm2 V-1 s-1) in combination with a lower density of mobile ions. This counterintuitive result highlights that reducing ion migration does not necessarily minimise hysteresis.

From Light to Dark: Dancing with Electrons in Colloidal 2D MoS2 Nanosheets

Extending the lifetime of photogenerated electrons in semiconductor systems is an important criterion for the conversion of light into storable energy. We have now succeeded in storing electrons in a photoirradiated colloidal molybdenum disulfide (MoS2) suspension, showcasing its unique reversible photoresponsive behavior. The dampened A and B excitonic peaks indicate the accumulation of photogenerated electrons and the minimization of interactions between MoS2 interlayers. The stored electrons were quantitatively extracted by titrating with a ferrocenium ion in the dark, giving ca. 0.2 electrons per MoS2 formula unit. The emergence of the photoinduced A1g* Raman mode and the decrease in zeta potential after irradiation suggest intercalation of counterions to maintain overall charge balance upon electron storage. These results provide insights into the mechanism of photogenerated electron storage in 2D materials and pave the way for the potential application of colloidal 2D materials in electron storage.

Toward stabilization of formamidinium lead iodide perovskites by defect control and composition engineering

Phase instability poses a serious challenge to the commercialization of formamidinium lead iodide (FAPbI3)-based solar cells and optoelectronic devices. Here, we combine density functional theory and machine learning molecular dynamics simulations, to investigate the mechanism driving the undesired α-δ phase transition of FAPbI3. Prevalent iodine vacancies and interstitials can significantly expedite the structural transition kinetics by inducing robust covalency during transition states. Extrinsically, the detrimental roles of atmospheric moisture and oxygen in degrading the FAPbI3 perovskite phase are also rationalized. Significantly, we discover the compositional design principles by categorizing that A-site engineering primarily governs thermodynamics, whereas B-site doping can effectively manipulate the kinetics of the phase transition in FAPbI3, highlighting lanthanide ions as promising B-site substitutes. A-B mixed doping emerges as an efficient strategy to synergistically stabilize α-FAPbI3, as experimentally demonstrated by substantially higher initial optoelectronic characteristics and significantly enhanced phase stability in Cs-Eu doped FAPbI3 as compared to its Cs-doped counterpart. This study provides scientific guidance for the design and optimization of long-term stable FAPbI3-based solar cells and other optoelectronic devices through defect control and synergetic composition engineering.

Thin silica shell on Ag3PO4 nanoparticles augments stability and photocatalytic reusability

Semiconductor photocatalysts are promising cost-effective materials for degrading hazardous organic contaminants in water. Ag3PO4 is an efficient visible-light photocatalyst for the oxidation of water and dye degradation. The excited Ag3PO4 photocatalyst uses a hole to oxidise water or organic contaminants except the electron, which reduces Ag+ to Ag0. In the present study, the inherited disadvantage was overcome by a thin silica shell overcoating on Ag3PO4 nanoparticles. The silica-coated Ag3PO4 nanoparticles retain the photocatalytic activity even after five cycles of photodegradation, while the bare Ag3PO4 nanoparticles show a photocatalytic activity declined to half. The study demonstrates that the thin silica shell enhances the photostability, keeping the photocatalytic activity unaffected, even after several cycles of photodegradation of dyes. XPS analysis showed that the Ag0 formation on the surface of bare Ag3PO4 is greater than that on silica-coated Ag3PO4, which declines the photocatalytic activity of Ag3PO4 after five cycles of photodegradation. Electrochemical studies identified that the intermediates, such as OH˙ and O2-, formed during water oxidation play a crucial role in the photodegradation of dyes. This study can provide insights into the design of core-shell semiconductor nanostructures for reusable photocatalytic applications.

Carrier Modulation via Tunnel Oxide Passivating at Buried Perovskite Interface for Stable Carbon-Based Solar Cells

Carbon-based perovskite solar cells (C-PSCs) have the impressive characteristics of good stability and potential commercialization. The insulating layers play crucial roles in charge modulation at the buried perovskite interface in mesoporous C-PSCs. In this work, the effects of three different tunnel oxide layers on the performance of air-processed C-PSCs are scrutinized to unveil the passivating quality. Devices with ZrO2-passivated TiO2 electron contacts exhibit higher power conversion efficiencies (PCEs) than their Al2O3 and SiO2 counterparts. The porous feature and robust chemical properties of ZrO2 ensure the high quality of the perovskite absorber, thus ensuring the high repeatability of our devices. An efficiency level of 14.96% puts our device among the state-of-the-art hole-conductor-free C-PSCs, and our unencapsulated device maintains 88.9% of its initial performance after 11,520 h (480 days) of ambient storage. These results demonstrate that the function of tunnel oxides at the perovskite/electron contact interface is important to manipulate the charge transfer dynamics that critically affect the performance and stability of C-PSCs.

Enhanced stability of triple-halide perovskites CsPbI3-x-yBrxCly (x and y = 0-0.024): understanding the role of Cl doping from ab initio calculations

Doping of chloride in mixed iodide-bromide perovskites has been shown experimentally to suppress the photo-induced halide-ion segregation and enhance the stability of triple-halide perovskites (THP). However, a fundamental understanding of the effects of Cl doping is yet to be achieved especially when the doping concentration is low. Here we report the results of a state-of-the-art ab initio study of the atomic structure of THP by considering small doping concentrations of Br and Cl in CsPbI3. We find a reduction in the Pb-I bond lengths and tilting of PbI6 octahedra with Cl doping which lead to exothermic heat of mixing and therefore higher stability of THP. Moreover, using quasi-chemical approximation, our results show that there is a very small contribution of configurational entropy to Gibbs free energy at such low doping concentrations and at the operational temperature of 50 °C. This suggests that the favorable heat of mixing value is more important for the stability at low doping concentrations of Cl while a higher concentration of Cl increases the risk of halide segregation. Further calculations on Frenkel defect formation energy of I or Br-interstitial shows that the doping of Cl in I/Br mixed binary-compounds hinders the formation of Frenkel defects. These results support experiments and help to understand the role of chloride in suppressing the halide ion mobility with only a slight increase in the band gap. Accordingly, the THPs manifest a promising pathway for developing single-phase perovskites for solar cells and light-emitting diodes with improved performance and enhanced stability.

The Role of SnO2 Processing on Ionic Distribution in Double-Cation-Double Halide Perovskites

Moving toward a future of efficient, accessible, and less carbon-reliant energy devices has been at the forefront of energy research innovations for the past 30 years. Metal-halide perovskite (MHP) thin films have gained significant attention due to their flexibility of device applications and tunable capabilities for improving power conversion efficiency. Serving as a gateway to optimize device performance, consideration must be given to chemical synthesis processing techniques. Therefore, how does common substrate processing techniques influence the behavior of MHP phenomena such as ion migration and strain? Here, we demonstrate how a hybrid approach of chemical bath deposition (CBD) and nanoparticle SnO₂ substrate processing significantly improves the performance of (FAPbI₃)_0.97(MAPbBr₃)_0.03 by reducing micro-strain in the SnO₂ lattice, allowing distribution of K⁺ from K-Cl treatment of substrates to passivate defects formed at the interface and produce higher current in light and dark environments. X-ray diffraction reveals differences in lattice strain behavior with respect to SnO₂ substrate processing methods. Through use of conductive atomic force microscopy (c-AFM), conductivity is measured spatially with MHP morphology, showing higher generation of current in both light and dark conditions for films with hybrid processing. Additionally, time-of-flight secondary ionization mass spectrometry (ToF-SIMS) observed the distribution of K⁺ at the perovskite/SnO₂ interface, indicating K⁺ passivation of defects to improve the power conversion efficiency (PCE) and device stability. We show how understanding the role of ion distribution at the SnO₂ and perovskite interface can help reduce the creating of defects and promote a more efficient MHP device.

Origins of Photoluminescence Instabilities at Halide Perovskite/Organic Hole Transport Layer Interfaces

Metal halide perovskites are promising for optoelectronic device applications; however, their poor stability under solar illumination remains a primary concern. While the intrinsic photostability of isolated neat perovskite samples has been widely discussed, it is important to explore how charge transport layers─employed in most devices─impact photostability. Herein, we study the effect of organic hole transport layers (HTLs) on light-induced halide segregation and photoluminescence (PL) quenching at perovskite/organic HTL interfaces. By employing a series of organic HTLs, we demonstrate that the HTL's highest occupied molecular orbital energy dictates behavior; furthermore, we reveal the key role of halogen loss from the perovskite and subsequent permeation into organic HTLs, where it acts as a PL quencher at the interface and introduces additional mass transport pathways to facilitate halide phase separation. In doing so, we both reveal the microscopic mechanism of non-radiative recombination at perovskite/organic HTL interfaces and detail the chemical rationale for closely matching the perovskite/organic HTL energetics to maximize solar cell efficiency and stability.

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

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

A Universal Microscopic Patterned Doping Method for Perovskite Enables Ultrafast, Self-Powered, Ultrasmall Perovskite Photodiodes

Novel metal halide perovskite is proven to be a promising optoelectronic material. However, fabricating microscopic perovskite devices is still challenging because the perovskite is soluble with the photoresist, which conflicts with conventional microfabrication technology. The size of presently reported perovskite devices is about 50 µm. Limited by the large size of perovskite optoelectronic devices, they cannot be readily adopted in the fields of imaging, display, etc. Herein a universal microscopic patterned doping method is proposed, which can realize microscale perovskite devices. Rather than by the conventional doping method, in this study the local Fermi level of perovskite is modulated by the redistributing intrinsic ion defects via a polling voltage. A satisfactorily stable polarized ion distribution can be achieved by optimization of the perovskite material and polling voltage, resulting in ultrafast (40 µs), self-powered microscale (2 µm) photodiodes. This work sheds light on a route to fabricate integrated perovskite optoelectronic chips.

Controlling Molecular Orientation of Small Molecular Dopant-Free Hole-Transport Materials: Toward Efficient and Stable Perovskite Solar Cells

Perovskite solar cells (PSCs) have great potential for future application. However, the commercialization of PSCs is limited by the prohibitively expensive and doped hole-transport materials (HTMs). In this regard, small molecular dopant-free HTMs are promising alternatives because of their low cost and high efficiency. However, these HTMs still have a lot of space for making further progress in both efficiency and stability. This review firstly provides outlining analyses about the important roles of molecular orientation when further enhancements in device efficiency and stability are concerned. Then, currently studied strategies to control molecular orientation in small molecular HTMs are presented. Finally, we propose an outlook aiming to obtain optimized molecular orientation in a cost-effective way.

Degradation mechanisms of perovskite light-emitting diodes under electrical bias

Metal-halide perovskite light-emitting diodes (PeLEDs) are considered as new-generation highly efficient luminescent materials for application in displays and solid-state lighting. Since the first successful demonstration of PeLEDs in 2014, the research on the development of efficient PeLEDs has progressed significantly. Although the device efficiency has significantly improved over a short period of time, their overall performance has not yet reached the levels of mature technologies for practical applications. Various degradation processes are the major impediment to improving the performance and stability of PeLED devices. In this review, we discuss various analysis techniques that are necessary to gain insights into the effects of various degradation mechanisms on the performance and stability of PeLEDs. Based on the causes and effects of external and internal factors, the degradation processes and associated mechanisms are examined in terms of critical physical and chemical parameters. Further, according to the progress of the current research, the challenges faced in studying degradation mechanisms are also elucidated. Given the universality of the degradation behavior, an in-depth understanding of the device degradation may promote the development of optimization strategies and further improve the performance and stability of PeLEDs.

How Hole Injection Accelerates Both Ion Migration and Nonradiative Recombination in Metal Halide Perovskites

Ion migration, hole trapping, and electron-hole recombination are common processes in metal halide perovskites. We demonstrate using ab initio non-adiabatic molecular dynamics and time-domain density functional theory that they are intricately related and strongly influence each other. The hole injection accelerates ion migration by decreasing the diffusion barrier and shortening the migration length. The injected hole also promotes the nonradiative charge recombination by strengthening electron-phonon interactions in the low-frequency region and prolonging the quantum coherence time. The synergy stems from the soft perovskite lattice and response of the valence band maximum to the Pb-I lattice distortion induced by the hole. This work provides important insights into the influence of ion mobility and hole injection on the performance of perovskite solar cells and suggests that high concentration of holes should be avoided.

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.

Beyond the Phase Segregation: Probing the Irreversible Phase Reconstruction of Mixed-Halide Perovskites

Mixed-halide perovskites can undergo a photoinduced phase segregation. Even though many reports have claimed that such a phase segregation process is reversible, what happens after phase segregation and its impact on the performance of perovskite-based devices are still open questions. Here, the phase transformation of MAPb(I1- x Brx )3 after phase segregation and probe an irreversible phase reconstruction of MAPbBr3 is investigated. The photoluminescence imaging microscopy technique is introduced to in situ record the whole process. It is proposed that the type-I band alignment of segregated I-rich and Br-rich domains can enhance the emission of the I-rich domains by suppressing the nonradiative recombination channels. At the same time, the charge injection from Br-rich to I-rich domains drives the expulsion of iodide from the lattice, and thus triggers the reconstruction of MAPbBr3 . The work highlights the significance of ion movements in mixed-halide perovskites and provides new perspectives to understand the property evolution.

Spectral Stable Blue-Light-Emitting Diodes via Asymmetric Organic Diamine Based Dion-Jacobson Perovskites

The spectral instability issue is a challenge in blue perovskite light-emitting diodes (PeLEDs). Dion–Jacobson (DJ) phase perovskites are promising alternatives to achieve high-quality blue PeLEDs. However, the current exploration of DJ phase perovskites is focused on symmetric divalent cations, and the corresponding efficiency of blue PeLEDs is still inferior to that of green and red ones. In this work, we report a new type of DJ phase CsPb(Br/Cl)₃ perovskite via introduction of an asymmetric molecular configuration as the organic spacer cation in perovskites. The primary and tertiary ammonium groups on the asymmetric cations bridge with the lead halide octahedra forming the DJ phase structures. Stable photoluminescence spectra were demonstrated in perovskite films owing to the suppressed halide segregation. Meanwhile, the radiative recombination efficiency of charges is improved significantly as a result of the confinement effects and passivation of charge traps. Finally, we achieved an external quantum efficiency of 2.65% in blue PeLEDs with stable spectra emission under applied bias voltages. To our best knowledge, this is the first report of asymmetric cations used in PeLEDs, which provides a facile solution to the halide segregation issue in PeLEDs.

Photoinduced Halide Segregation in Ruddlesden-Popper 2D Mixed Halide Perovskite Films

2D lead halide perovskites, which exhibit bandgap tunability and increased chemical stability, have been found to be useful for designing optoelectronic devices. Reducing dimensionality with decreasing number of layers (n = 10-1) also imparts resistance to light-induced ion migration as seen from the halide ion segregation and dark recovery in mixed halide (Br:I = 50:50) perovskite films. The light-induced halide ion segregation efficiency, as determined from difference absorbance spectra, decreases from 20% to <1% as the dimensionality is decreased for 2D perovskite film from n = 10 to 1. The segregation rate constant (k_segregation ), which decreases from 5.9 × 10^-3  s^-1 (n = 10) to 3.6 × 10^-4  s^-1 (n = 1), correlates well with nearly an order of magnitude decrease observed in charge-carrier lifetime (τ_average  = 233 ps for n = 10 vs τ_avg  = 27 ps for n = 1). The tightly bound excitons in 2D perovskites make charge separation less probable, which in turn decreases the halide mobility and resulting phase segregation. The importance of controlling the dimensionality of the 2D architecture in suppressing halide ion mobility is discussed.

Mixed or Segregated: Toward Efficient and Stable Mixed Halide Perovskite-Based Devices

Convenient modulation of bandgap for the mixed halide perovskites (MHPs) (e.g., CsPbBr _x I_1-x ) through varying the halide composition (i.e., the ratio of bromide to iodide) allows for optimizing the light-harvesting properties in perovskite solar cells (PSCs) and emission color in perovskite light-emitting diodes (PeLEDs). Such MHPs, yet, severely suffered from the instability under light irradiation and electrical bias as a result of an intrinsic soft, ionic lattice and a high halide ion mobility. Understanding the halide ion migration (mediated through halide vacancies) and suppressing the halide ion segregation, thus, remain a significant challenge both in the field of PSCs and PeLEDs since it is directly linked to the long-term stability and performances of the corresponding devices. In this Mini-Review, we discuss the intrinsic instability of the MHPs arising from the ionic nature of perovskites. The liquid crystalline properties with the low formation energy of halide ion defects facilitate the defect-mediated halide ion migration. Several different mechanistic models are provided to explain the fundamental origin of the photo- or electric field-driven halide ion segregation based upon thermodynamics and kinetics. These reflect that lattice strains (internal or polaron-induced) and bandgap energy differences between parent mixed halide and iodide-rich domain serve as the thermodynamic driving forces for halide segregation. On the basis of the deeper understanding of the underpinning segregation mechanism mediated through hole trapping and accumulation at the iodide-rich sites, we further discuss the strategies to mitigate the detrimental halide segregation through composition-, defect-, dimension-, and interface-engineering. Finally, we provide a fundamental insight into designing perovskite-based photovoltaic and optoelectronic devices for the long-term operational stability.

Mechanochemical synthesis of MAPbBr3/carbon sphere composites for boosting carrier-involved superoxide species

Lead halide perovskites MAPbX₃ (MA = CH₃NH₃ or Cs; X = I, Br, Cl) are well considered to be potential candidates for photocatalytic reaction due to its excellent photoelectrical properties, but they still suffer from the low charge separation efficiency and slow catalytic reaction dynamics. To tackle the drawbacks, herein, MAPbBr₃/carbon sphere (CS) composite photocatalysts using glucose as the carbon source were elaborately designed and fabricated via a dry mechanochemical grinding process. The interfacial interaction Pb-O-C chemical bonds were constructed between MAPbBr₃ and the carbon sphere surface containing organic functional groups. By optimizing the content of CSs, the enhanced photocatalytic degradation kinetic rate of Malachite Green (MG) pollutants (92% within 20 min) for MAPbBr₃/CS_x (x = 17 wt.%) is about 3.6-fold of that for pristine MAPbBr₃, which is attributed to the corporative adsorption and enhanced carrier transportation and separation of MAPbBr₃/CS_x. Furthermore, the possible degradation mechanism was proposed on basis of the electrochemical, mass spectrometry and optical characterization results. Owing to the robust interfacial interaction, effective electron extraction rate (k_et = 4.6 × 10⁷ sec^-1) from MAPbBr₃ to CS can be established, which driven oxygen activation where superoxide radicals (•O₂^-) played an important role in MG degradation. It is expected that mechanochemistry strategy may provide a new route to design efficient lead halide perovskite-carbon or metal oxide or sulfide composite photocatalysts.

Modulation of Photoinduced Iodine Expulsion in Mixed Halide Perovskites with Electrochemical Bias

Hole trapping at iodine (I) sites in MAPbBr_1.5I_1.5 mixed halide perovskites (MHP) is responsible for iodine migration and its eventual expulsion into solution. We have now modulated the photoinduced iodine expulsion in MHP through an externally applied electrochemical bias. At positive potentials, electron extraction at TiO₂/MHP interfaces becomes efficient, leading to hole buildup within MHP films. This improved charge separation, in turn, favors iodine migration as evident from the increased apparent rate constant of iodine expulsion (k_expulsion = 0.0030 s^-1). Conversely, at negative potentials (-0.3 V vs Ag/AgCl) electron-hole recombination is facilitated within MHP, slowing down iodine expulsion by an order of magnitude (k_expulsion = 0.00018 s^-1). The tuning of the E_Fermi level through external bias modulates electron extraction at the TiO₂/MHP interface and indirectly controls the buildup of holes, ultimately inducing iodine migration/expulsion. Suppressing iodine migration in perovskite solar cells is important for attaining greater stability since they operate under internal electrical bias.

Halide Ion Migration in Perovskite Nanocrystals and Nanostructures

ConspectusThe optical and electronic properties of metal halide perovskites provide insight into the operation of solar cells as well as their long-term operational stability. Halide mobility in perovskite films is an important factor influencing solar cell performance. One can visualize halide ion migration through halide exchange between two nanocrystal suspensions or between physically paired films of two different metal halide perovskites. The ability to tune band gap by varying halide ratios (Cl:Br or Br:I) allows the synthesis of mixed halide perovskites with tailored absorption and emission across the entire visible spectrum. Interestingly, mixed halide (e.g., MAPb(Br_0.5I_0.5)₃) films undergo phase segregation to form Br-rich and I-rich sites under steady state illumination. Upon halting illumination, segregated phases mix to restore original mixed halide compositions. Introducing multiple cations (Cs, formamidinium) at the A site or alloying with Cl greatly suppresses halide mobilities. Long-term irradiation of MAPb(Br_0.5I_0.5)₃ films also cause expulsion of iodide leaving behind Br-rich phases. Hole trapping at I-rich sites in MAPb(Br_0.5I_0.5)₃ is considered to be an important step in inducing halide mobility in photoirradiated films. This Account focuses on halide ion migration in nanocrystals and nanostructured films driven by entropy of mixing in dark and phase segregation under light irradiation.

Water Triggered Synthesis of Highly Stable and Biocompatible 1D Nanowire, 2D Nanoplatelet, and 3D Nanocube CsPbBr3 Perovskites for Multicolor Two-Photon Cell Imaging

Two-photon imaging in the near-infrared window holds huge promise for real life biological imaging due to the increased penetration depth. All-inorganic CsPbX3 nanocrystals with bright luminescence and broad spectral tunability are excellent smart probes for two-photon bioimaging. But, the poor stability in water is a well-documented issue for limiting their practical use. Herein, we present the development of specific antibody attached water-resistant one-dimensional (1D) CsPbBr3 nanowires, two-dimensional (2D) CsPbBr3 nanoplatelets, and three-dimensional (3D) CsPbBr3 nanocubes which can be used for selective and simultaneous two-photon imaging of heterogeneous breast cancer cells in the near IR biological window. The current manuscript reports the design of excellent photoluminescence quantum yield (PLQY), biocompatible and photostable 1D CsPbBr3 nanowires, 2D CsPbBr3 nanoplatelets, and 3D CsPbBr3 nanocubes through an interfacial conversion from zero-dimensional (0D) Cs4PbBr6 nanocrystals via a water triggered strategy. Reported data show that just by varying the amount of water, one can control the dimension of CsPbBr3 perovskite crystals. Time-dependent transition electron microscopy and emission spectra have been reported to find the possible pathway for the formation of 1D, 2D, and 3D CsPbBr3 nanocrystals from 0D Cs4PbBr6 nanocrystals. Biocompatible 1D, 2D, and 3D CsPbBr3 nanocrystals were developed by coating with amine-poly(ethylene glycol)-propionic acid. Experimental data show the water-driven design of 1D, 2D, and 3D CsPbBr3 nanocrystals exhibits strong single-photon PLQY of ∼66-88% as well as excellent two-photon absorption properties (σ2) of ∼8.3 × 105-7.1 × 104 GM. Furthermore, reported data show more than 86% of PL intensity remains for 1D, 2D, and 3D CsPbBr3 nanocrystals after 35 days under water, and they exhibit excellent photostability of keeping 99% PL intensity after 3 h under UV light. The current report demonstrates for the first time that antibody attached 1D and 2D perovskites have capability for simultaneous two-photon imaging of triple negative breast cancer cells and human epidermal growth factor receptor 2 positive breast cancer cells. CsPbBr3 nanocrystals exhibit very high two-photon absorption cross-section and good photostability in water, which are superior to those of commonly used organic probes (σ2 = 11 GM for fluorescein), and therefore, they have capability to be a better probe for bioimaging applications.

Electrochemically induced iodine migration in mixed halide perovskites: suppression through chloride insertion

The role of chloride in improving the stability of mixed halide perovskites (MAPbCl_xBr_0.5(1-x)I_0.5(1-x))₃ is probed using spectroelectrochemistry. The injection of holes into mixed halide perovskite films through applied anodic bias results in the selective migration of iodine with ultimate expulsion into the electrolyte. Increasing the Cl content (x = 0 to 0.1) in the mixed halide perovskite suppresses the iodine mobility and thus decreases the rate of its expulsion into the solution. Implications of iodine mobility induced by hole accumulation and its impact on overall stability is discussed.

Impact of improvements in mesoporous titania layers on ultrafast electron transfer dynamics in perovskite and dye-sensitized solar cells

Improvement in the performance of perovskite solar cells (PSC) and dye-sensitized solar cells (DSSC) upon modifications of mesoporous titania layers has been studied. For PSC with triple cation perovskite (FA0.76 MA0.19 Cs0.05 Pb (I0.81 Br0.19)3) about 40% higher photocurrent (up to ∼24 mA cm-2) was found for more homogenous, made of larger particles (30 nm) and thinner (150-200 nm) titania layer. For DSSC (both with liquid cobalt-based electrolyte as well as with solid state hole transporter - spiro-OMeTAD), a greater dye loading, rise in photovoltage, and the enhancement in relative photocurrent were observed for the cells prepared from the diluted titania paste (2 : 1 w/w ratio) with respect to those prepared from undiluted one. The impact of these improvements in titania layers on charge transfer dynamics in the complete solar cells as well as in pristine TiO2 layers was investigated by femtosecond transient absorption. Shorter photocarriers lifetime in perovskite material observed in better PSC, indicated that faster electron transfer at the titania interface was responsible for the higher photocurrent. Moreover, the photoinduced changes close to TiO2 interface were revealed in better PSC, which may indicate that in the efficient devices halide segregation takes place in perovskite material. In liquid DSSC, the fast component of unwanted recombination was slower in the samples with the diluted titania paste than in those made with undiluted ones. In solid state DSSC, hole injection from MK2 dye to spiro-OMeTAD takes place on the very fast ps time scale (comparable to that of electron injection) and the evidence of better penetration of spiro-OMeTAD into thinner and more porous titania layers was provided.