Hybrid Lead Iodide Perovskites Research Articles

Layer-by-Layer Structural Identification of 2D Ruddlesden–Popper Hybrid Lead Iodide Perovskites by Solid-State NMR Spectroscopy

Application of two-dimensional (2D) organic–inorganic hybrid halide perovskites in optoelectronic devices requires a detailed understanding of the local structural features including the Pb–I bondi...

Kinetic Molecular Cationic Control of Defect-Induced Broadband Light Emission in 2D Hybrid Lead Iodide Perovskites.

In this study, we examine the effects of changing organic cation concentrations on the efficiency and photophysical implications of exciton trapping in two-dimensional hybrid lead iodide self-assembled quantum wells (SAQWs). We show that increasing the concentration of alkyl and aryl ammonium cations causes the formation of SAQWs at a liquid-liquid interface to possess intense, broadband subgap photoluminescence (PL) spectra. Electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopic studies suggest that materials formed under these cation concentrations possess morphologies consistent with inhibited crystallization kinetics but exhibit qualitatively similar bulk chemical bonding to nonluminescent materials stabilized in the same structure from precursor solutions containing lower cation concentrations. Temperature- and power-dependent PL spectra suggest that the broadband subgap light emission stems from excitons self-trapped at defect sites, which we assign as edge-like, collective I vacancies using a simple model of the chemical equilibrium driving material self-assembly. These results suggest that changes to the availability of molecular cations can suitably control the light emission properties of self-assembled hybrid organic-inorganic materials in ways central to their applicability in lighting technologies.

Suppressed Lattice Disorder for Large Emission Enhancement and Structural Robustness in Hybrid Lead Iodide Perovskite Discovered by High‐Pressure Isotope Effect

AbstractThe soft nature of organic–inorganic halide perovskites renders their lattice particularly tunable to external stimuli such as pressure, undoubtedly offering an effective way to modify their structure for extraordinary optoelectronic properties. Here, using the methylammonium lead iodide as a representative exploratory platform, it is observed that the pressure‐driven lattice disorder can be significantly suppressed via hydrogen isotope effect, which is crucial for better optical and mechanical properties previously unattainable. By a comprehensive in situ neutron/synchrotron‐based analysis and optical characterizations, a remarkable photoluminescence (PL) enhancement by threefold is convinced in deuterated CD3ND3PbI3, which also shows much greater structural robustness with retainable PL after high peak‐pressure compression–decompression cycle. With the first‐principles calculations, an atomic level understanding of the strong correlation among the organic sublattice and lead iodide octahedral framework and structural photonics is proposed, where the less dynamic CD3ND3+ cations are vital to maintain the long‐range crystalline order through steric and Coulombic interactions. These results also show that CD3ND3PbI3‐based solar cell has comparable photovoltaic performance as CH3NH3PbI3‐based device but exhibits considerably slower degradation behavior, thus representing a paradigm by suggesting isotope‐functionalized perovskite materials for better materials‐by‐design and more stable photovoltaic application.

Hybrid Lead Iodide Perovskites with Mixed Cations of Thiourea and Methylamine, From One Dimension to Three Dimensions.

Hybrid halide perovskites featuring as new materials of high-performance solar cells have attracted great research interest. The temperature-dependent dimensional transition of halide perovskites is a crucial handle in the preparation of perovskite films. Only the small cations of methylammonium (MA) or formamidinium (FA) have been involved for most of the dimensional transition materials. In this work, thiourea (tu) is introduced into hybrid halide materials. A new series of 1D ribbonlike hybrid lead iodides with tu and MA cations are reported that were crystallographically characterized as MAn(Htu)n+1PbnI4n+1 (n = 1-4 denoted as 1-4, respectively; in 1, MA is replaced by tu). The width of the perovskite ribbon increases from one PbI6 octahedron to four corner-fused octahedra. Compounds 2 and 3 can be turned into a black 3D perovskite after annealing. This is an unusual mixed MA-tu hybrid halide perovskite system, in which the tu molecule plays an important role in manipulating the dimensions and their photoconductive properties. Scanning electron microscopy of the blackened sample shows that there are a lot of regular vent holes on the smooth crystal surface with sizes of hundreds of nanometers. The tunable structures and porous crystals might be advantageous in the sense of material modulation.

Charge Carrier Relaxation in Colloidal FAPbI3 Nanostructures Using Global Analysis

We study the hot charge carrier relaxation process in weakly confined hybrid lead iodide perovskite colloidal nanostructures, FAPbI3 (FA = formaminidium), using femtosecond transient absorption (TA). We compare the conventional analysis method based on the extraction of the carrier temperature (Tc) by fitting the high-energy tail of the band-edge bleach with a global analysis method modeling the continuous evolution of the spectral lineshape in time using a simple sequential kinetic model. This practical approach results in a more accurate way to determine the charge carrier relaxation dynamics. At high excitation fluence (density of charge carriers above 1018 cm−3), the cooling time increases up to almost 1 ps in thick nanoplates (NPs) and cubic nanocrystals (NCs), indicating the hot phonon bottleneck effect. Furthermore, Auger heating resulting from the multi-charge carrier recombination process slows down the relaxation even further to tens and hundreds of picoseconds. These two processes could only be well disentangled by analyzing simultaneously the spectral lineshape and amplitude evolution.

Shallow Iodine Defects Accelerate the Degradation of α-Phase Formamidinium Perovskite

Summary Shallow defects are mostly benign in covalent semiconductors, such as silicon, given that they do not constitute non-radiative recombination sites. In contrast, the existence of shallow defects in ionic perovskite crystals might have significant repercussions on the long-term stability of perovskite solar cells (PSCs) because of the metastability of the ubiquitous formamidinium lead triiodide (FAPbI3) perovskite and the migration of charged point defects. Here, we show that shallow iodine interstitial defects ( I i ) can be generated unintentionally during commonly used post-fabrication treatments, which can lower the cubic-to-hexagonal transformation barrier of FAPbI3-based perovskites to accelerate its phase degradation. We demonstrate that concurrently avoiding the generation of I i and the more effective passivation of iodine vacancies ( V I ) can improve the thermodynamic stability of the films and operational stability of the PSCs. Our most stable PSC retained 92.1 % of its initial performance after nearly 1,000 h of continuous illumination operational stability testing.

Dynamical Imaging of Surface Photopotentials in Hybrid Lead Iodide Perovskite Films under High Optical Irradiance and the Role of Selective Contacts

AbstractTo leverage the outstanding photonic qualities of lead halide hybrid perovskites under high optical irradiance, their reliability and temporal stability must be assessed. Time‐resolved scanning electron microscopy unveils that when illuminating the free surface of methylammonium lead iodide (MAPbI3) films at 500 Sun in vacuum, a giant photopotential locally develops in tens of seconds, differently evolving depending on charge‐selective substrates. It is reversible on timescales of minutes in the case of hole‐transporting poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate, while more persistent effects occurs, on the timescale of hours, in the case of electron‐transporting TiO2. In addition, films grown on TiO2 show irreversible decay of photoluminescence measured in situ and photoinduced alteration of the work function at some grain boundaries. Different responses at high irradiance are ascribable to contact‐dependent and light‐induced spatial redistribution of charged defects, either ions or localized dipoles. It is also clear that photoexcited charges play different roles in the photochemistry of systems, depending on selective contacts and they are likely to mediate diverse photoassisted redox reaction paths. The TiO2 layer may act as a photocatalyst, leading to MAPbI3 degradation and to irreversible polarization, which will hinder high‐irradiance applications.

Direct Observation of Bandgap Oscillations Induced by Optical Phonons in Hybrid Lead Iodide Perovskites

AbstractHybrid organic–inorganic perovskites such as methylammonium lead iodide have emerged as promising semiconductors for energy‐relevant applications. The interactions between charge carriers and lattice vibrations, giving rise to polarons, have been invoked to explain some of their extraordinary optoelectronic properties. Here, time‐resolved optical spectroscopy is performed, with off‐resonant pumping and electronic probing, to examine several representative lead iodide perovskites. The temporal oscillations of electronic bandgaps induced by coherent lattice vibrations are reported, which is attributed to antiphase octahedral rotations that dominate in the examined 3D and 2D hybrid perovskites. The off‐resonant pumping scheme permits a simplified observation of changes in the bandgap owing to the Ag vibrational mode, which is qualitatively different from vibrational modes of other symmetries and without increased complexity of photogenerated electronic charges. The work demonstrates a strong correlation between the lead–iodide octahedral framework and electronic transitions, and provides further insights into the manipulation of coherent optical phonons and related properties in hybrid perovskites on ultrafast timescales.

Highly Efficient and Stable Pure Two-Dimensional Perovskite-Based Solar Cells with the 3-Aminopropionitrile Organic Cation.

Pure two-dimensional (2D) perovskite (n = 1)-based perovskite solar cells (PSCs) have been proven to have excellent stability against humidity, but the photovoltaic performance is very poor due to the parallel orientation to the substrate and mismatched energy alignment in the PSC device. We report herein a novel bulky organic cation of 3-aminopropionitrile (3-APN) for constructing a pure 2D hybrid lead-iodide perovskite. The crystal structure of (3-APN)2PbI4 features a stable layered and undistorted PbI6 octahedral geometry (∠Pb-I-Pb = 180°) with a small I···I distance (4.66 Å), and the crystals grow in a dominant out-of-plane direction to the substrate. In addition, the existence of an intramolecular H bond between cyano groups and ammonium heads result in an appropriate valence band level of (3-APN)2PbI4 for a well-matched energy level alignment in the device, benefitting the interfacial charge transfer and hence a better photovoltaic performance. As a result, the PSC with the pure 2D (3-APN)2PbI4 perovskite-based PSC achieves a power conversion efficiency of 3.39%, which is the highest value thus far for the pure 2D lead-iodide perovskite family, to the best of our knowledge. More importantly, this pure 2D (3-APN)2PbI4 perovskite-based PSC demonstrates excellent stability against humidity. This work demonstrates that there is great potential to realize efficient and stable pure 2D perovskite-based PSCs through the wise design of organic cations.

Origin of Rashba Spin-Orbit Coupling in 2D and 3D Lead Iodide Perovskites

We studied spin dynamics of charge carriers in the superlattice-like Ruddlesden-Popper hybrid lead iodide perovskite semiconductors, 2D (BA)2(MA)Pb2I7 (with MA = CH3NH3, and BA = CH3(CH2)3NH3), and 3D MAPbI3 using the magnetic field effect (MFE) on conductivity and electroluminescence in their light emitting diodes (LEDs) at cryogenic temperatures. The semiconductors with distinct structural/bulk inversion symmetry breaking, when combined with colossal intrinsic spin–orbit coupling (SOC), theoretically give rise to giant Rashba-type SOC. We found that the magneto-conductance (MC) magnitude increases monotonically with the emission intensity and saturates at ≈0.05% and 0.11% for the MAPbI3 and (BA)2(MA)Pb2I7, respectively. The magneto-electroluminescence (MEL) response with similar line shapes as the MC response has a significantly larger magnitude, and essentially stays constant at ≈0.22% and ≈0.20% for MAPbI3 and (BA)2(MA)Pb2I7, respectively. The sign and magnitude of the MC and MEL responses can be quantitatively explained in the framework of the Δg-based excitonic model using rate equations. Remarkably, the width of the MEL response in those materials linearly increases with increasing the applied electric field, where the Rashba coefficient in (BA)2(MA)Pb2I7 is estimated to be about 7 times larger than that in MAPbI3. Our studies might have significant impact on future development of electrically-controlled spin logic devices via Rashba-like effects.

Influence of Charge Transport Layers on Capacitance Measured in Halide Perovskite Solar Cells

Summary Capacitance-based techniques have been used to measure the electrical properties of halide perovskite solar cells (PSCs) such as defect activation energy and density, carrier concentration, and dielectric constant, which provide key information for evaluating the device performance. Here, we show that capacitance-based techniques cannot be used to reliably analyze the properties of defects in the perovskite layer or at its interface, because the high-frequency capacitance signature is due to the response of charge carriers in the hole-transport layer (HTL). For HTL-free PSCs, high-frequency capacitance can be considered as the geometric capacitance for analyzing the dielectric constant of the perovskite layer because there is no trapping and de-trapping of charge carriers in the perovskite layer. We further find that the low-frequency capacitance signature can be used to calculate the activation energy of the ionic conductivity of the perovskite layer, but the overlapping effects with charge transport materials must be avoided.

Optoelectronic Properties and Defect Physics of Lead-Free Photovoltaic Absorbers Cs2AuIAuIIIX6 ( X=I,Br )

Stability and toxicity issues with the hybrid lead iodide perovskite MAPbI$_3$ necessitate a hunt for potential alternatives. Here, we shed light on promising photovoltaic properties of gold mixed-valence halide perovskites Cs$_2$Au$_2$X$_6$ (X = I, Br, Cl). They satisfy fundamental requirements such as nontoxicity, better stability, a band gap in the visible range, and a low excitonic binding energy. Our study shows a favorable electronic structure, resulting in a high optical-transition strength, and thus a sharp rise in the absorption spectrum near the band gap. This, in turn, yields a very high short-circuit current density and hence higher simulated efficiency compared with MAPbI$_3$. However, careful investigation of defect physics reveals the possibility of deep-level defects (such as V$_X$ , V$_{Cs}$, X$_{Au}$, X$_{Cs}$, Au$_i$, and Au$_X$ , X = I, Br), depending on the growth conditions. These can act as carrier traps and become detrimental to photovoltaic performance. The present study should help in taking necessary precautions in synthesizing these compounds in a controlled chemical environment, which should minimize performance limiting defects and pave the way for future studies on this class of materials.

Water-resistant 2D lead(ii) iodide perovskites: correlation between optical properties and phase transitions

Two-dimensional hybrid lead(II) iodide perovskites with up to 13 days stability when directly immersed in water. The materials may lead to stable perovskite LEDs and solar cells.

Origin of Broad-Band Emission and Impact of Structural Dimensionality in Tin-Alloyed Ruddlesden–Popper Hybrid Lead Iodide Perovskites

Hybrid organic–inorganic lead halide perovskites have shown promising results as active layers in light-emitting diodes, typically utilizing the near-monochromatic, free exciton emission. Some perovskite compounds, however, show broad-band emission that is more intense than the free exciton counterpart. In this study, we show that the light emission properties of Ruddlesden–Popper hybrid perovskites PEA2MAn–1PbnI3n+1 (PEA = phenethylammonium, MA = methylammonium) can be tuned by Sn alloying and are highly sensitive to Sn %. With increasing dimensionality, the broad-band emission quantum yield decreases drastically, from 23% in n = 1 to <1% for the n = 3 compound. Using density functional theory calculations and transient reflectance spectroscopy, the broad emission is identified as originating from self-trapped excitons. A dynamic picture of the formation process is also presented, for which ultrafast (<5 ps) hole-trapping at the Sn site is the first step, followed by electron localization from Coulombic ...

Spin-dependent charge transport through 2D chiral hybrid lead-iodide perovskites.

Chiral-induced spin selectivity (CISS) occurs when the chirality of the transporting medium selects one of the two spin ½ states to transport through the media while blocking the other. Monolayers of chiral organic molecules demonstrate CISS but are limited in their efficiency and utility by the requirement of a monolayer to preserve the spin selectivity. We demonstrate CISS in a system that integrates an inorganic framework with a chiral organic sublattice inducing chirality to the hybrid system. Using magnetic conductive-probe atomic force microscopy, we find that oriented chiral 2D-layered Pb-iodide organic/inorganic hybrid perovskite systems exhibit CISS. Electron transport through the perovskite films depends on the magnetization of the probe tip and the handedness of the chiral molecule. The films achieve a highest spin-polarization transport of up to 86%. Magnetoresistance studies in modified spin-valve devices having only one ferromagnet electrode confirm the occurrence of spin-dependent charge transport through the organic/inorganic layers.

Influence of the Organic Chain on the Optical Properties of Two-Dimensional Organic–Inorganic Hybrid Lead Iodide Perovskites

Two-dimensional (2D) organic–inorganic hybrid perovskites (OIHPs) have recently drawn great interest due to their easy synthesis and excellent photoelectric properties. With tunable bandgaps, 2D OI...

High-Temperature Antiferroelectric of Lead Iodide Hybrid Perovskites.

Antiferroelectrics, characterized by the natural polarization-electric field (P-E) double hysteresis loops, has been developed as a promising branch for energy storage. Here, we present the first antiferroelectric in the booming family of lead iodide hybrid perovskites, (BA)2(EA)2Pb3I10 (1, where BA = n-butylammonium and EA = ethylammonium), which exhibits one of the highest Curie temperatures (∼363 K) for the majority of known molecular systems. Strikingly, its high-temperature antiferroelectricity, triggered by an antipolar alignment of adjacent dipoles, is confirmed by the characteristic double P-E hysteresis loops, thus enabling remarkable energy storage efficiencies in the range of 65%-83%. This merit is almost comparable to those of many inorganic counterparts, suggesting the great potential of 1 for energy storage. Another fascinating attribute is that 1 also acts as a room-temperature biaxial ferroelectric with spontaneous polarization of 5.6 μC·cm-2. As far as we know, this study on the high-temperature antiferroelectric, along with room-temperature biaxial ferroelectricity, is unprecedented for the versatile lead iodide hybrid perovskites, which sheds light on the design of new electric-ordered materials and facilitates their application of high-performance devices.

Two-Dimensional Dion-Jacobson Hybrid Lead Iodide Perovskites with Aromatic Diammonium Cations.

Two-dimensional (2D) halide perovskites have extraordinary optoelectronic properties and structural tunability. Among them, the Dion-Jacobson phases with the inorganic layers stacking exactly on top of each other are less explored. Herein, we present the new series of 2D Dion-Jacobson halide perovskites, which adopt the general formula of A'An-1PbnI3n+1 (A' = 4-(aminomethyl)pyridinium (4AMPY), A = methylammonium (MA), n = 1-4). By modifying the position of the CH2NH3+ group from 4AMPY to 3AMPY (3AMPY = 3-(aminomethyl)pyridinium), the stacking of the inorganic layers changes from exactly eclipsed to slightly offset. The perovskite octahedra tilts are also different between the two series, with the 3AMPY series exhibiting smaller bandgaps than the 4AMPY series. Compared to the aliphatic cation of the same size (AMP = (aminomethyl)piperidinium), the aromatic spacers increase the rigidity of the cation, reduce the interlayer spacing, and decrease the dielectric mismatch between inorganic layer and the organic spacer, showing the indirect but powerful influence of the organic cations on the structure and consequently on the optical properties of the perovskite materials. All A'An-1PbnI3n+1 compounds exhibit strong photoluminescence (PL) at room temperature. Preliminary solar cell devices based on the n = 4 perovskites as absorbers of both series exhibit promising performances, with a champion power conversion efficiency (PCE) of 9.20% for (3AMPY)(MA)3Pb4I13-based devices, which is higher than the (4AMPY)(MA)3Pb4I13 and the corresponding aliphatic analogue (3AMP)(MA)3Pb4I13-based ones.

Huge Photostability Enhancement in Bismuth-Doped Methylammonium Lead Iodide Hybrid Perovskites by Light-Induced Transformation

The doping strategy of hybrid perovskites is being extensively explored not only for higher efficiency but also to overcome issues in photovoltaic materials such as self-degradation pathways in an ambient atmosphere or under visible irradiation. Here, BiI3 is introduced in the synthesis of MAPbI3 films (MA: CH3–NH3+) to stabilize the material. Around 25% of nominal Bi3+ is accommodated in the perovskite structure, producing a shrinking of the unit cell and a small increase of the band gap. The presence of empty Bi gap states quenches the 770 nm red interband emission and results in a near-infrared emission at 1100 nm. However, high enough visible irradiation density induces a progressive segregation of Bi3+ out of the perovskite lattice and promotes the re-emergence of the red emission. This emission is blue-shifted, and its intensity increases strongly with time until it reaches a saturation value which remains stable in the transformed films for extremely high power densities, around 1000 times higher t...

Oriented Growth of Ultrathin Single Crystals of 2D Ruddlesden-Popper Hybrid Lead Iodide Perovskites for High-Performance Photodetectors.

Compared with their 3D counterparts, layered 2D Ruddlesden-Popper perovskites (2D RPPs) with the formula of (A')2A n-1Pb nX3 n+1 exhibit improved photo and moisture stability. To develop flexible single-crystalline electronics and wearable devices, cost-effective growth of large-area single crystals of 2D RPPs with large lateral size, ultrathin thickness, parallel orientation, and well-defined and controlled n values are highly desirable, but it remains still a challenge. Here, we modified the space-confined aqueous solution growth method to fabricate single-crystalline films of (BA)2(MA) n-1Pb nX3 n+1 and n = 1, 2, 3, respectively, with the lateral size reaching millimeter and thickness about 400-1000 nm. Moreover, the quantum well layers are found to be parallel to the substrate, well suitable for lateral transport requirement. We successfully integrated ultrathin single crystals of (BA)2(MA) n-1Pb nX3 n+1 ( n = 1-3) into a metal-semiconductor-metal two-terminal photodetector. The photoresponsive wavelength range was extended from 525 nm for n = 1 and 590 nm for n = 2 to 630 nm for n = 3 as a result of the reduced exciton band gap. The device of n = 2 single crystals shows the highest responsivity of 2.96 A/W and a detectivity of 1013 jones, while the device of n = 3 shows the lowest dark current 10-14 A, and therefore, its on/off ratio reaches 2862 at the bias voltage of 1 V.