Solid-State Ionics of Hybrid Halide Perovskites

Space charge effects in mixed ionic-electronic conducting electrodes for solid-state batteries

Mixed ionic-electronic conductors are widely used as electroactive materials in energy applications. The contact of a mixed conductor with another phase plays a crucial role in charge storage and transport in energy devices. However, the interfacial chemistry at the heterojunctions comprising mixed conductors and its interplay with the bulk chemistry remains imperative yet inadequately understood. This study addresses the fundamentals of space charge effects by exploring the equilibrium situations for contacts consisting of mixed conductors. From the perspective of defect chemistry, and by unifying the bulk and interfacial conditions with the electrochemical potential, our treatment allows for predicting the built-in potential at heterojunctions, profiling the space charge distributions, and evaluating the resulting interfacial charge storage and transport. The treatment can be related to experimental characterization, including coulometric titration, conductivity, and capacitance measurements at electrochemical interfaces in all-solid-state batteries. Besides, our treatment also highlights the significance of size and doping effects in nanocrystalline electrodes. This work provides a comprehensive framework for understanding and engineering the heterojunctions in electrochemical devices.

Mixed ionic-electronic conduction in Ruddlesden-Popper and Dion-Jacobson layered hybrid perovskites with aromatic organic spacers

The understanding of mixed ionic-electronic conductivity in hybrid perovskites has enabled major advances in the development of optoelectronic devices based on this class of materials. While recent investigations revealed the potential of using dimensionality effects for various applications, the implication of this strategy on mixed conductivity is yet to be established. Here, we present a systematic analysis of mixed conduction in layered (2D) hybrid halide perovskite films based on 1,4-phenylenedimethylammonium (PDMA) and benzylammonium (BzA) organic spacers in (PDMA)PbI4 and (BzA)2PbI4 compositions, forming representative Dion-Jacobson (DJ) and Ruddleson-Popper (RP) phases, respectively. Electrochemical measurements of charge transport parallel to the layered structure reveal mixed ionic-electronic conduction with electronic transport mediated by electron holes in both DJ and RP phases. In comparison to the 3D perovskites, larger activation energies for both ionic and electronic conductivities are observed which result in lower absolute values. While the layered perovskites still allow for a relatively efficient exchange of iodine with the gas phase, the lower change of conductivity on the variation of the iodine partial pressure compared with 3D perovskites is consistent with the exchange affecting only a fraction of the film, with implications for the encapsulating efficacy of these materials. We complement the analysis with a demonstration of the superior thermal stability of DJ structures compared to their RP counterparts. This can guide future explorations of dimensionality and composition to control the transport and stabilization properties of 2D perovskite films.

Halide Perovskites Breathe Too: The Iodide-Iodine Equilibrium and Self-Doping in Cs2SnI6

The response of an oxide crystal to the atmosphere can be personified as breathing-a dynamic equilibrium between O2 gas and O2- anions in the solid. We characterize the analogous defect reaction in an iodide double-perovskite semiconductor, Cs2SnI6. Here, I2 gas is released from the crystal at room temperature, forming iodine vacancies. The iodine vacancy defect is a shallow electron donor and is therefore ionized at room temperature; thus, the loss of I2 is accompanied by spontaneous n-type self-doping. Conversely, at high I2 pressures, I2 gas is resorbed by the perovskite, consuming excess electrons as I2 is converted to 2I-. Halide mobility and irreversible halide loss or exchange reactions have been studied extensively in halide perovskites. However, the reversible exchange equilibrium between iodide and iodine [2I-(s) ↔ I2(g) + 2e-] described here has often been overlooked in prior studies, though it is likely general to halide perovskites and operative near room temperature, even in the dark. An analysis of the 2I-(s)/I2(g) equilibrium thermodynamics and related transport kinetics in single crystals of Cs2SnI6 therefore provides insight toward achieving stable composition and electronic properties in the large family of iodide perovskite semiconductors.

Linearly Manipulating Color Emission via Anion Exchange Technology for High Performance Amplified Spontaneous Emission of Perovskites

The most attractive advantages of all-inorganic cesium lead halide perovskites are their optical gain over broad spectral ranges through the visible spectrum, so are well suited to use in tunable lasers or broadband amplifiers. Most reported anion exchange reactions face a challenge to achieve the desired halogen-variable perovskites due to rapid and uncontrollable reactions and difficulty to synthesize directly. In this study, a simple vapor/solid anion exchange strategy is demonstrated for controlling the reaction process and realizing a wide range tuning of band gap and amplified spontaneous emission (ASE) wavelength, which exhibits a temperature-dependent anion exchange rate. By optimizing the reaction temperature at 90 °C, the ASE wavelength can be linearly manipulated by just controlling the reaction time. A clear quantitative relationship between ASE peak position and reaction time is achieved. Compares with the CsPbClBr2 film obtained via the liquid phase anion exchange method, the fabricated perovskite films obtained by vapor/solid anion exchange technology exhibit superior film quality and enhanced ASE performance. This work may have applications in the future using facile and controllable techniques to develop high-quality full-color visible lasers.

Photoinduced Current Transient Spectroscopy on Metal Halide Perovskites: Electron Trapping and Ion Drift

Metal halide perovskites (MHPs) are disruptive materials for a vast class of optoelectronic devices. The presence of electronic trap states has been a tough challenge in terms of characterization and thus mitigation. Many attempts based on electronic spectroscopies have been tested, but due to the mixed electronic-ionic nature of MHP conductivity, many experimental results retain a large ambiguity in resolving electronic and ionic charge contributions. Here we adapt a method, previously used in highly resistive inorganic semiconductors, called photoinduced current transient spectroscopy (PICTS) on lead bromide 2D-like ((PEA)2PbBr4) and standard "3D" (MAPbBr3) MHP single crystals. We present two conceptually different outcomes of the PICTS measurements, distinguishing the different electronic and ionic contributions to the photocurrents based on the different ion drift of the two materials. Our experiments unveil deep level trap states on the 2D, "ion-frozen" (PEA)2PbBr4 and set new boundaries for the applicability of PICTS on 3D MHPs.

On the Mechanism of Solvents Catalyzed Structural Transformation in Metal Halide Perovskites

Metal halide perovskites show the capability of performing structural transformation, allowing the formation of functional heterostructures. Unfortunately, the elusive mechanism governing these transformations limits their technological application. Herein, the mechanism of 2D-3D structural transformation is unraveled as catalyzed by solvents. By combining a spatial-temporal cation interdiffusivity simulation with experimental findings, it is validated that, protic solvents foster the dissociation degree of formadinium iodide (FAI) via dynamic hydrogen bond, then the stronger hydrogen bond of phenylethylamine (PEA) cation with selected solvents compared to dissociated FA cation facilitates 2D-3D transformation from (PEA)2 PbI4 to FAPbI3 . It is discovered that, the energy barrier of PEA out-diffusion and the lateral transition barrier of inorganic slab are diminished. For 2D films the protic solvents catalyze grain centers (GCs) and grain boundaries (GBs) transforme into 3D phases and quasi-2D phases, respectively. While in the solvent-free case, GCs transform into 3D-2D heterostructures along the direction perpendicular to the substrate, and most GBs evolve into 3D phases. Finally, memristor devices fabricated using the transformed films uncover that, GBs composed of 3D phases are more prone to ion migration. This work elucidates the fundamental mechanism of structural transformation in metal halide perovskites, allowing their use to fabricate complex heterostructures.

Ionic and electronic energy diagrams for hybrid perovskite solar cells

The development of photoelectrochemical devices based on mixed ionic-electronic conductors requires knowledge of transport, generation and reaction of electronic and ionic charge carriers. Thermodynamic representations can significantly help the understanding of these processes. They should be simple and reflect the necessity of dealing with ions and electrons. In this work, we discuss the extension of energy diagrams commonly used to describe electronic properties of semiconductors to the defect chemical treatment of electronic and ionic charge carriers in mixed conducting materials as introduced in the context of nanoionics. We focus on hybrid perovskites in relation to their use as the active layer material of solar cells. Owing to the presence of at least two ion types, a variety of native ionic disorder processes have to be dealt with in addition to the single fundamental electronic disorder process as well as potential frozen-in defects. Various situations are discussed that show how such generalized level diagrams can be usefully applied and appropriately simplified in the determination of the equilibrium behavior of bulk and interfaces in solar cell devices. This approach can serve as a basis for investigating the behavior of perovskite solar cells, but also other mixed-conducting devices operating under bias.

Ionic and electronic polarization effects in horizontal hybrid perovskite device structures close to equilibrium

The electrical response of hybrid perovskite devices carries a significant signature from mobile ionic defects, pointing to both opportunities and threats when it comes to functionality, performance and stability of these devices. Despite its importance, the interpretation of polarization effects due to the mixed ionic-electronic conducting nature of these materials and the quantification of their ionic conductivities still poses conceptual and practical challenges, even for the equilibrium situation. In this study, we address these questions and investigate the electrical response of horizontal devices based on methylammonium lead iodide (MAPI) close to equilibrium conditions. We discuss the interpretation of DC polarization and impedance spectroscopy measurements in the dark, based on calculated and fitted impedance spectra obtained using equivalent circuit models that account for the mixed conductivity of the perovskite and for the effect of device geometry. Our results show that, for horizontal structures with a gap width between the metal electrodes in the order of tens of microns, the polarization behavior of MAPI is well described by the charging of the mixed conductor/metal interface, suggesting a Debye length in the perovskite close to 1 nm. We highlight a signature in the impedance response at intermediate frequencies, which we assign to ionic diffusion in the plane parallel to the MAPI/contact interface. By comparing the experimental impedance results with calculated spectra for different circuit models, we discuss the potential role of multiple mobile ionic species and rule out a significant contribution from iodine exchange with the gas phase in the electrical response of MAPI close to equilibrium. This study helps to clarify the measurement and interpretation of mixed conductivity and polarization effects in hybrid perovskites with immediate relevance to the characterization and development of transistors, memristors and solar cells based on this class of materials as well as other mixed conductors.

Mechanistic and Kinetic Analysis of Perovskite Memristors with Buffer Layers: The Case of a Two-Step Set Process

With the increasing demand for artificially intelligent hardware systems for brain-inspired in-memory and neuromorphic computing, understanding the underlying mechanisms in the resistive switching of memristor devices is of paramount importance. Here, we demonstrate a two-step resistive switching set process involving a complex interplay among mobile halide ions/vacancies (I^-/V_I⁺) and silver ions (Ag⁺) in perovskite-based memristors with thin undoped buffer layers. The resistive switching involves an initial gradual increase in current associated with a drift-related halide migration within the perovskite bulk layer followed by an abrupt resistive switching associated with diffusion of mobile Ag⁺ conductive filamentary formation. Furthermore, we develop a dynamical model that explains the characteristic I-V curve that helps to untangle and quantify the switching regimes consistent with the experimental memristive response. This further insight into the two-step set process provides another degree of freedom in device design for versatile applications with varying levels of complexity.

Chemical approaches for electronic doping in photovoltaic materials beyond crystalline silicon

Electronic doping is applied to tailor the electrical and optoelectronic properties of semiconductors, which have been widely adopted in information and clean energy technologies, like integrated circuit fabrication and PVs. Though this concept has prevailed in conventional PVs, it has achieved limited success in the new-generation PV materials, particularly in halide perovskites, owing to their soft lattice nature and self-compensation by intrinsic defects. In this review, we summarize the evolution of the theoretical understanding and strategies of electronic doping from Si-based photovoltaics to thin-film technologies, e.g., GaAs, CdTe and Cu(In,Ga)Se₂, and also cover the emerging PVs including halide perovskites and organic solar cells. We focus on the chemical approaches to electronic doping, emphasizing various chemical interactions/bonding throughout materials synthesis/modification to device fabrication/operation. Furthermore, we propose new classifications and models of electronic doping based on the physical and chemical properties of dopants, in the context of solid-state chemistry, which inspires further development of optoelectronics based on perovskites and other hybrid materials. Finally, we outline the effects of electronic doping in semiconducting materials and highlight the challenges that need to be overcome for reliable and controllable doping.

Anomalous and colossal thermal expansion, photoluminescence, and dielectric properties in lead halide-layered perovskites with cyclohexylammonium and cyclopentylammonium cations

A detailed study of lead halide-layered perovskites with general formula A2PbX4 (where A is cyclohexylammonium (CHA) or cyclopentylammonium (CPA) cation and X is Cl- or Br- anion) is presented. Using variable temperature synchrotron X-ray powder diffraction, we observe that these compounds exhibit diverse crystal structures above room temperature. Very interestingly, we report some unconventional thermomechanical responses such as uniaxial negative thermal expansion and colossal positive thermal expansion in a perpendicular direction. For the polymorphs of (CHA)2PbBr4, the volumetric thermal expansion coefficient is among the highest reported for any extended inorganic crystalline solid, reaching 480 MK-1. The phase transitions are confirmed by calorimetry and dielectric measurements, where the dielectric versus temperature curves show anomalies related with the order-disorder phase transitions. In addition, these compounds exhibit a broad photoluminescence (PL) emission with a large Stokes shift, which is related with an exciton PL emission.

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

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

Nanosegregation in arene-perfluoroarene π-systems for hybrid layered Dion-Jacobson perovskites

Layered hybrid perovskites are based on organic spacers separating hybrid perovskite slabs. We employ arene and perfluoroarene moieties based on 1,4-phenylenedimethylammonium (PDMA) and its perfluorinated analogue (F-PDMA) in the assembly of hybrid layered Dion-Jacobson perovskite phases. The resulting materials are investigated by X-ray diffraction, UV-vis absorption, photoluminescence, and solid-state NMR spectroscopy to demonstrate the formation of layered perovskite phases. Moreover, their behaviour was probed in humid environments to reveal nanoscale segregation of layered perovskite species based on PDMA and F-PDMA components, along with enhanced stabilities of perfluoroarene systems, which is relevant to their application.

Ultrasensitive and Robust 120 keV Hard X-Ray Imaging Detector based on Mixed-Halide Perovskite CsPbBr3- n In Single Crystals

The relatively low resistivity and severe ion migration in CsPbBr₃ significantly degrade the performance of X-ray detectors due to their high detection limit and current drift. The electrical properties and X-ray detection performances of CsPbBr₃ -nIn single crystals are investigated by doping the iodine atoms into the melt-grown CsPbBr₃ . The resistivity of CsPbBr₃ -nIn single crystals increases from 3.6 × 10⁹ (CsPbBr₃ ) to 2.2 × 10¹¹ (CsPbBr₂ I) Ω cm, restraining the leak current and decreasing the detection limit of the detector. Additionally, CsPbBr₃ -nIn single crystals exhibit stable dark currents, arising from their high ion migration activation energy. A record sensitivity of 6.3 × 104 µC Gy^-1 cm^-2 (CsPbBr_2.9 I_0.1 ) and a low detection limit of 54 nGy s^-1 (CsPbBr₂ I) are achieved by CsPbBr₃ -nIn single crystals for the 120 keV hard X-ray detection under a 5000 V cm^-1 electrical field. The CsPbBr_2.9 I_0.1 detector shows a stable current response with a dark current density of 0.58 µA cm^-2 for 30 days and clear imaging for 120 keV Xrays at ambient conditions. The effective iodine atom doping strategy makes the CsPbBr₃ -nIn single crystals promising for reproducible high-energy hard X-ray imaging systems.

Electro-chemo-mechanical charge carrier equilibrium at interfaces

Electrochemical interfaces involving solids enable charge transfer, electrical transport, and mass storage in energy devices. One central concept that determines the interfacial charge carrier concentration is the space-charge field. The classical theory accounts for electrochemical equilibrium in the absence of mechanical effects; such effects have recently been found critical in many solids, such as materials for lithium-ion and solid-state batteries, perovskite solar cells, and fuel cells. Towards elucidating the interplay between charge carriers and mechanics, we establish a generalized electro-chemo-mechanical space-charge model and categorize the carriers into physically-meaningful four types, based on the signs of the charge number (i.e., polarity) and the partial molar volume (i.e., expansion coefficient). Beyond the electrostatic effects discussed in the literature, our work reveals the importance of elastic effects, as demonstrated by simulations of a composite beam bending experiment. The analysis highlights opportunities to systematically tune the interfacial electrical conductivity and the reaction kinetics of solids through mechanics. Our treatment provides a rational basis for understanding stress-driven phenomena at interfaces in a wide range of solids.

DC bias electric field effects on ac electrical conductivity of MAPbI3suggesting intrinsic changes on structure and charge carrier dynamics

Methylammonium lead iodide (MAPbI₃) emerges as a promising halide perovskite material for the next generation of solar cells due to its high efficiency and flexibility in material growth. Despite intensive studies of their optical and electronic properties in the past ten years, there are no reports on dc bias electric field effects on conductivity in a wide temperature range. In this work, we report the combined effects of frequency, temperature, and dc bias electric field on the ac conductivity of MAPbI₃. We found that the results of dc bias electric fields are very contrasting in the tetragonal and cubic phases. In the tetragonal phase, sufficiently high dc bias electric fields induce a conductivity peak appearance ∼290 K well evidenced at frequencies higher than 100 kHz. Excluding possible degradation and extrinsic factors, we propose that this peak suggests a ferroelectric-like transition. In the absence of a dc bias electric field, the ac conductivity in the tetragonal phase increases with temperature while decreases with temperature in the cubic phase. Also, ac activation energies for tetragonal and cubic phases were found to be inversely and directly proportional to the dc bias electric field, respectively. This behavior was attributed to the ionic conduction, possibly of MA⁺and I^-ions, for the tetragonal phase. As for the cubic phase, the ac conduction dynamics appear to be metallic-like, which seems to change to a polaronic-controlled charge transport to increased dc bias electric fields.

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.

Gold-Cage Perovskites: A Three-Dimensional AuIII-X Framework Encasing Isolated MX63- Octahedra (MIII = In, Sb, Bi; X = Cl-, Br-, I-)

The Cs₈Au^III₄M^IIIX₂₃ (M = In³⁺, Sb³⁺, Bi³⁺; X = Cl^-, Br^-, I^-) perovskites are composed of corner-sharing Au-X octahedra that trace the edges of a cube containing an isolated M-X octahedron at its body center. This structure, unique within the halide perovskite family, may be derived from the doubled cubic perovskite unit cell by removing the metals at the cube faces. To our knowledge, these are the only halide perovskites where the octahedral sites do not bear an average 2+ charge. Charge compensation in these materials requires a stoichiometric halide vacancy, which is disordered around the Au atom at the unit-cell corner and orders when the crystallization is slowed. Using X-ray crystallography, X-ray absorption spectroscopy, and pair distribution function analysis, we elucidate the structure of this unusual perovskite. Metal-site alloying produces further intricacies in this structure, which our model explains. Compared to other halide perovskites, this class of materials shows unusually low absorption onset energies ranging between ca. 1.0 and 2.4 eV. Partial reduction of Au³⁺ to Au⁺ affords an intervalence charge-transfer band, which redshifts the absorption onset of Cs₈Au₄InCl₂₃ from 2.4 to 1.5 eV. With connected Au-X octahedra and isolated M-X octahedra, this structure type combines zero- and three-dimensional metal-halide sublattices in a single material and stands out among halide perovskites for its ordering of homovalent metals, ordering of halide vacancies, and incorporation of purely trivalent metals at the octahedral sites.

Probing the ionic defect landscape in halide perovskite solar cells

Point defects in metal halide perovskites play a critical role in determining their properties and optoelectronic performance; however, many open questions remain unanswered. In this work, we apply impedance spectroscopy and deep-level transient spectroscopy to characterize the ionic defect landscape in methylammonium lead triiodide (MAPbI₃) perovskites in which defects were purposely introduced by fractionally changing the precursor stoichiometry. Our results highlight the profound influence of defects on the electronic landscape, exemplified by their impact on the device built-in potential, and consequently, the open-circuit voltage. Even low ion densities can have an impact on the electronic landscape when both cations and anions are considered as mobile. Moreover, we find that all measured ionic defects fulfil the Meyer–Neldel rule with a characteristic energy connected to the underlying ion hopping process. These findings support a general categorization of defects in halide perovskite compounds.

Defects in perovskite affect the properties and performance in optoelectronic devices, yet the nature of ionic defects remains elusive. Here, the authors investigate the ionic defect landscape in perovskite introduced by varying precursor stoichiometry, and find the defects fulfill the Meyer-Neldel rule.

Solid-State NMR and NQR Spectroscopy of Lead-Halide Perovskite Materials

Two- and three-dimensional lead-halide perovskite (LHP) materials are novel semiconductors that have generated broad interest owing to their outstanding optical and electronic properties. Characterization and understanding of their atomic structure and structure-property relationships are often nontrivial as a result of the vast structural and compositional tunability of LHPs as well as the enhanced structure dynamics as compared with oxide perovskites or more conventional semiconductors. Nuclear magnetic resonance (NMR) spectroscopy contributes to this thrust through its unique capability of sampling chemical bonding element-specifically (1/2H, 13C, 14/15N, 35/37Cl, 39K, 79/81Br, 87Rb, 127I, 133Cs, and 207Pb nuclei) and locally and shedding light onto the connectivity, geometry, topology, and dynamics of bonding. NMR can therefore readily observe phase transitions, evaluate phase purity and compositional and structural disorder, and probe molecular dynamics and ionic motion in diverse forms of LHPs, in which they can be used practically, ranging from bulk single crystals (e.g., in gamma and X-ray detectors) to polycrystalline films (e.g., in photovoltaics, photodetectors, and light-emitting diodes) and colloidal nanocrystals (e.g., in liquid crystal displays and future quantum light sources). Herein we also outline the immense practical potential of nuclear quadrupolar resonance (NQR) spectroscopy for characterizing LHPs, owing to the strong quadrupole moments, good sensitivity, and high natural abundance of several halide nuclei (79/81Br and 127I) combined with the enhanced electric field gradients around these nuclei existing in LHPs as well as the instrumental simplicity. Strong quadrupole interactions, on one side, make 79/81Br and 127I NMR rather impractical but turn NQR into a high-resolution probe of the local structure around halide ions.

Single Ensemble Non-exponential Photoluminescent Population Decays from a Broadband White-Light-Emitting Perovskite

The mechanism of white-light emission from layered Pb-X (X = Cl or Br) perovskites following UV excitation has generated considerable interest. Prior time-dependent studies indicated that the broadband photoluminescence (PL) from (110) perovskites arises from a distribution of self-trapped excitonic sites emitting in different regions of the visible spectrum with different decay dynamics. Here, using time-correlated single photon counting to study single crystals, we show that the white-light emission decay from the (110) perovskite (EDBE)PbBr₄ (EDBE = 2,2'-(ethylenedioxy)bis(ethylammonium)) behaves as a single ensemble. Following the rapid decay (0.6 ns) of a small spectral side band, the broad emission line shape is constant to 100 ns. We propose that rapid local structural fluctuations cause the self-trapped excitons (STEs) to experience a wide range of energies, resulting in the very broad PL. The STEs sample fluctuating local environments on time scales fast compared to the PL, which averages the PL decay at all emission wavelengths, yielding single ensemble PL dynamics. Although emission occurs from a very wide, inhomogeneously broadened spectral line with time-averaged single ensemble luminescence dynamics, the decay is tri-exponential. Two heuristic models for the tri-exponential decay involving defects are discussed. Spin-coated films show faster non-exponential decays with the slowest component of the crystal PL absent. Like the crystals, the film PL decays as a single ensemble. These results demonstrate that the broadband emission decay of (EDBE)PbBr₄ arises from a time-averaged single ensemble and not from a set of excited states emitting with distinct luminescence decays at different wavelengths.

Improved Carrier Collection and Hot Electron Extraction Across Perovskite, C60, and TiO2 Interfaces

The use of C₆₀ as an interfacial layer between TiO₂ and methylammonium lead iodide perovskite is probed to reduce the current-voltage hysteresis in perovskite solar cells (PSCs) and, in turn, to impact the interfacial carrier injection and recombination processes that limit solar cell efficiencies. Detailed kinetic analyses across different time scales, that is, from the femtoseconds to the seconds, reveal that the charge carrier lifetimes as well as the charge injection and charge recombination dynamics depend largely on the presence or absence of C₆₀. In addition, we corroborate that C₆₀ is applicable in hot carrier PSCs as it is capable of extracting hot carriers generated throughout the early time scales following photoexcitation.

First-principles comparative study of perfect and defective CsPbX3 (X = Br, I) crystals

This paper presents first principles Density Functional Theory hybrid functional calculations of the atomic and electronic structure of perfect CsPbI₃, CsPbBr₃ and CsPbCl₃ crystals, as well as defective CsPbI₃ and CsPbBr₃ crystals.