Defects in Hybrid Perovskites: The Secret of Efficient Charge Transport

The interaction of free carriers with defects and some critical defect properties are still unclear in methylammonium lead halide perovskites (MHPs). Here, a multi-method approach is used to quantify and characterize defects in single crystal MAPbI₃, giving a cross-checked overview of their properties. Time of flight current waveform spectroscopy reveals the interaction of carriers with five shallow and deep defects. Photo-Hall and thermoelectric effect spectroscopy assess the defect density, cross-section, and relative (to the valence band) energy. The detailed reconstruction of free carrier relaxation through Monte Carlo simulation allows for quantifying the lifetime, mobility, and diffusion length of holes and electrons separately. Here, it is demonstrated that the dominant part of defects releases free carriers after trapping; this happens without non-radiative recombination with consequent positive effects on the photoconversion and charge transport properties. On the other hand, shallow traps decrease drift mobility sensibly. The results are the key for the optimization of the charge transport properties and defects in MHP and contribute to the research aiming to improve perovskite stability. This study paves the way for doping and defect control, enhancing the scalability of perovskite devices with large diffusion lengths and lifetimes.     

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

Hybrid 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 A_g 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.     

The Relationship between Chemical Flexibility and Nanoscale Charge Collection in Hybrid Halide Perovskites

Hybrid organometal halide perovskites are known for their excellent optoelectronic functionality as well as their wide‐ranging chemical flexibility. The composition of hybrid perovskite devices has trended toward increasing complexity as fine‐tuned properties are pursued, including multielement mixing on the constituents A and B and halide sites. However, this tunability presents potential challenges for charge extraction in functional devices. Poor consistency and repeatability between devices may arise due to variations in composition and microstructure. Within a single device, spatial heterogeneity in composition and phase segregation may limit the device from achieving its performance potential. This review details how the nanoscale elemental distribution and charge collection in hybrid perovskite materials evolve as chemical complexity increases, highlighting recent results using nondestructive operando synchrotron‐based X‐ray nanoprobe techniques. The results reveal a strong link between local chemistry and charge collection that must be controlled to develop robust, high‐performance hybrid perovskite materials for optoelectronic devices.     

Highly Selective SAM–Nanowire Hybrid NO2 Sensor: Insight into Charge Transfer Dynamics and Alignment of Frontier Molecular Orbitals

Organic–inorganic hybrid gas sensors can offer outstanding performance in terms of selectivity and sensitivity towards single gas species. The enormous variety of organic functionalities enables novel flexibility of active sensor surfaces compared to commonly used pure inorganic materials, but goes along with an increase of system complexity that usually hinders a predictable sensor design. In this work, an ultra‐selective NO₂ sensor is realized based on self‐assembled monolayer (SAM)‐modified semiconductor nanowires (NWs). The crucial chemical and electronic parameters for an effective interaction between the sensor and different gas species are identified using density functional theory simulations. The theoretical findings are consistent with the experimentally observed extraordinary selectivity and sensitivity of the amine‐terminated SnO₂ NW towards NO₂. The energetic position of the SAM–gas frontier orbitals with respect to the NW Fermi level is the key to ensure or impede an efficient charge transfer between the NW and the gas. As this condition strongly depends on the gas species and the sensor system, these insights into the charge transfer mechanisms can have a substantial impact on the development of highly selective hybrid gas sensors.     

The Enabling Electronic Motif for Topological Insulation in ABO3 Perovskites

Stable oxide topological insulators (TIs) have been sought for years, but none have been found; whereas heavier (selenides, tellurides) chalcogenides can be TIs. The basic contradiction between topological insulation and thermodynamic stability is pointed out, offering a narrow window of opportunity. The electronic motif is first identified and can achieve topological band inversion in ABO₃ as a lone‐pair, electron‐rich B atom (e.g., Te, I, Bi) at the octahedral site. Then, twelve ABO₃ compounds are designed in the assumed cubic perovskite structure, which satisfy this electronic motif and are indeed found by density function theory calculations to be TIs. Next, it is illustrated that poorly screened ionic oxides with large inversion energies undergo energy‐lowering atomic distortions that destabilize the cubic TI phase and remove band inversion. The coexistence windows of topological band inversion and structure stability can nevertheless be expanded under moderate pressures (15 and 35 GPa, respectively, for BaTeO₃ and RbIO₃). This study traces the principles needed to design stable oxide topological insulators at ambient pressures as a) a search for oxides with small inversion energies; b) design of large inversion‐energy oxide TIs that can be stabilized by pressure; and c) a search for covalent oxides where TI‐removing atomic displacements can be effectively screened out.     

Spin-Dependent Charge Transport in 1D Chiral Hybrid Lead-Bromide Perovskite with High Stability

Spin-dependent charge transport, along with the potential electronic applications, is investigated in chiral 2D iodide hybrid organic/inorganic perovskites (HOIPs) via the chirality-induced spin selectivity (CISS) effect, paving a new way in spintronics. Despite the high spin-polarized current enhancement, the intrinsic oxidation tendency of iodide ions brings about severe problems in the stability and lifetime of electronic devices. Here, spin-dependent charge transport properties in lead-bromide perovskites hybrid with chiral R/S-methylbenzylammonium (MBA), that is, (R/S-MBA)PbBr₃ are explored. Distinct from layered 2D iodide perovskites (R/S-MBA)₂PbI₄ which experience obvious crystal degradation along time, (R/S-MBA)PbBr₃ maintain good crystallinity even in the oxidative, humid, and high-temperature environment due to the lower Fermi level of bromide than iodide. Magnetic conductive atomic force microscopy displays a spin filtration efficiency as high as 90%, showing negligible decay after 1 month. This work expands the spin transport to chiral bromide perovskites with higher stability, and thus provides significant support for the practical application of HOIPs in spintronics.     

Distinct Optoelectronic Signatures for Charge Transfer and Energy Transfer in Quantum Dot–MoS2 Hybrid Photodetectors Revealed by Photocurrent Imaging Microscopy

Atomically thin transition metal dichalcogenides (TMDCs) have intriguing nanoscale properties like high charge mobility, photosensitivity, layer‐thickness‐dependent bandgap, and mechanical flexibility, which are all appealing for the development of next generation optoelectronic, catalytic, and sensory devices. Their atomically thin thickness, however, renders TMDCs poor absorptivity. Here, bilayer MoS₂ is combined with core‐only CdSe QDs and core/shell CdSe/ZnS QDs to obtain hybrids with increased light harvesting and exhibiting interfacial charge transfer (CT) and nonradiative energy transfer (NET), respectively. Field‐effect transistors based on these hybrids and their responses to varying laser power and applied gate voltage are investigated with scanning photocurrent microscopy (SPCM) in view of their potential utilization in light harvesting and photodetector applications. CdSe–MoS₂ hybrids are found to exhibit encouraging properties for photodetectors, like high responsivity and fast on/off response under low light exposure while CdSe/ZnS–MoS₂ hybrids show enhanced charge carrier generation with increased light exposure, thus suitable for photovoltaics. While distinguishing optically between CT and NET in QD–TMDCs is nontrivial, it is found that they can be differentiated by SPCM as these two processes exhibit distinctive light‐intensity dependencies: CT causes a photogating effect, decreasing the photocurrent response with increasing light power while NET increases the photocurrent response with increasing light power, opposite to CT case.     

Diffuson-Dominated and Ultra Defect-Tolerant Two-Channel Thermal Transport in Hybrid Halide Perovskites

Organic-inorganic hybrid halide perovskites show a unique two-channel thermal transport through propagons and diffusons, largely affecting other energy carriers for opto- and thermoelectric applications. Taking CH₃NH₃PbI₃ as a prototype, the impact of iodine vacancy point defects on the two-channel thermal transport is investigated using theoretical calculations and experimental validations. This work finds that iodine vacancies suppress the thermal transport in the propagon channel significantly, but less in the diffuson channel. This results in a weaker reduction of the total thermal conductivity (TC) than that predicted by the classical Klemens model. The TC reduction in the diffuson channel is mainly attributed to the declined vibrational density of states. Moreover, low-frequency diffusons transformed from propagons compensate the reduction of TC in the diffuson channel, resulting in a dominant contribution from the diffuson channel to the total TC, which is 55% to 85% for 0% to 6% vacancy concentration. CH₃NH₃PbI₃ also shows ultra-defect-tolerant diffusonic thermal transport, ≈1–2 orders of magnitude lower than diamond in the defect sensitivity factor. This work shows both scientific insights into the new two-channel thermal transport mechanism in complex material systems with disorder, and technological significance on halide perovskites for solar cell, light-emitting diode, thermoelectric, and memristor applications.     

Carrier Modulation of Ambipolar Few‐Layer MoTe2 Transistors by MgO Surface Charge Transfer Doping

Semiconducting molybdenum ditelluride (2H‐MoTe₂), a fast‐emerging 2D material with an appropriate band gap and decent carrier mobility, is configured as field‐effect transistors and is the focus of substantial research interest, showing hole‐dominated ambipolar characteristics. Here, carrier modulation of ambipolar few‐layer MoTe₂ transistors is demonstrated utilizing magnesium oxide (MgO) surface charge transfer doping. By carefully adjusting the thickness of MgO film and the number of MoTe₂ layers, the carrier polarity of MoTe₂ transistors from p‐type to n‐type can be reversely controlled. The electron mobility of MoTe₂ is significantly enhanced from 0.1 to 20 cm² V^?1 s^?1 after 37 nm MgO film doping, indicating a greatly improved electron transport. The effective carrier modulation enables to achieve high‐performance complementary inverters with high DC gain of >25 and photodetectors based on few‐layer MoTe₂ flakes. The results present an important advance toward the realization of electronic and optoelectronic devices based on 2D transition‐metal dichalcogenide semiconductors.     

Compositional and Dimensional Control of 2D and Quasi‐2D Lead Halide Perovskites in Water

Blue light emitting two dimensional (2D) and quasi‐2D layered halide perovskites (LHPs) are gaining attention in solid‐state lighting applications but their fragile stability in humid condition is one of the most pressing issues for their practical applications. Though water is much greener and cost effective, organic solvents must be used during synthesis as well as the device fabrication process for these LHPs due to their water‐sensitivity/instability and consequently, water‐stable blue‐light emitting 2D and quasi‐2D LHPs have not been documented yet. Here, water‐mediated facile and cost‐effective syntheses, characterizations, and optical properties of 16 organic–inorganic hybrid compounds are reported including 2D (A′)₂PbX₄ (A′ = butylammonium, X = Cl/Br/I) (8 compounds), 3D perovskites (4), and quasi‐2D (A′)_pA_x?1B_xX_3x+1 LHPs (A = methylammonium) (4) in water. Here, both composition and dimension of LHPs are tuned in water, which has never been explored yet. Furthermore, the dual emissive nature is observed in quasi‐2D perovskites, where the intensity of two photoluminescence (PL) peaks are governed by 2D and 3D inorganic layers. The Pb(OH)₂‐coated 2D and quasi‐2D perovskites are highly stable in water even after several months. In addition, single particle imaging is performed to correlate structural–optical property of these LHPs.     

2D Antiferroelectric Hybrid Perovskite with a Large Breakdown Electric Field And Energy Storage Density

Energy conversion and storage devices are highly desirable for the sustainable development of human society. Hybrid organic–inorganic perovskites have shown great potential in energy conversion devices including solar cells and photodetectors. However, its potential in energy storage has seldom been explored. Here the crystal structure and electrical properties of the 2D hybrid perovskite (benzylammonium)₂PbBr₄ (PVK-Br) are investigated, and the consecutive ferroelectric-I (FE1) to ferroelectric-II (FE2) then to antiferroelectric (AFE) transitions that are driven by the orderly alignment of benzylamine and the distortion of [PbBr₆] octahedra are found. Furthermore, accompanied by field-induced AFE to FE transition near room temperature, a large energy storage density of ≈1.7 J cm⁻³ and a wide working temperature span of ≈70 K are obtained; both of which are among the best in hybrid AFEs. This good energy storage performance is attributed to the large polarization of ≈7.6 µC cm⁻² and the high maximum electric field of over 1000 kV cm⁻¹, which, as revealed by theoretical calculations, originate from the cooperative coupling between the [PbBr₆] octahedral framework and the benzylamine molecules. The research clarifies the discrepancy in the phase transition character of PVK-Br and shed light on developing high-performance energy storage devices based on 2D hybrid perovskite.     

Effect of Charge Density on Energy‐Transfer Properties of Cationic Conjugated Polymers

Cationic conjugated polymers (CCPs) with different charge densities are synthesized via Suzuki polymerization. The CCPs show similar optical properties in aqueous solutions but obvious difference in fluorescence resonance energy transfer (FRET) to Texas Red‐labeled single‐stranded DNA (ssDNA‐TR). Both CCP and TR fluorescence quenching are revealed to influence the energy‐transfer process. The difference in quantum yields of CCP/ssDNA complexes highlights the importance of polymer side‐chain structures and charge density. A CCP with a high charge density and ethylene oxide as the side chain provides the highest quantum yield for CCP/ssDNA complexes, which favors FRET. TR quenching within the CCP/ssDNA complexes is predominantly determined by the CCP charge density. In contrast to the other two polymers, the CCP with low charge density provides the most‐intense polymer‐sensitized TR emission, which is due to the collective response of more optically active polymer units around TR and the minimized TR self‐quenching within the CCP/ssDNA‐TR complexes. These studies provide a new guideline for improving the signal amplification of conjugated‐polymer‐based optical sensors.     

Double Charge Transfer Dominates in Carrier Localization in Low Bandgap Sites of Heterogeneous Lead Halide Perovskites

Heterogeneous organic-inorganic halide perovskites possess inherent non-uniformities in bandgap that are sometimes engineered and exploited on purpose, like in quasi-2D perovskites. In these systems, charge carrier and excitation energy migration to lower-bandgap sites are key processes governing luminescence. The question, which of them dominates in particular materials and under specific experimental conditions, still remains unanswered, especially when charge carriers comprise excitons. In this study transient absorption (TA) and transient photoluminescence (PL) techniques are combined to address the excited state dynamics in quasi-2D and other heterogeneous perovskite structures in broad temperature range, from room temperature down to 15 K. The data provide clear evidence that charge carrier transfer rather than energy migration dominates in heterogeneous quasi-2D perovskite films.     

High-Performing Quasi-2D Perovskite Photodetectors with Efficient Charge Transport Network Built from Vertically Orientated and Evenly Distributed 3D-Like Phases

Quasi-two-dimensional (Q-2D) perovskites are emerging as one of the most promising materials for photodetectors. However, a significant challenge to Q-2D perovskites for photodetection is their insufficient charge transport ability, which is mainly attributed to their hybrid low-dimensional n-phase structure. This study demonstrates that evenly-distributed 3D-like phases with vertical orientation throughout the film can greatly facilitate charge transport and suppress charge recombination, outperforming the prevalent phase structure with a vertical dimension gradient. Based on such a phase structure, a Q-2D Ruddlesden−Popper perovskite self-powered photodetector achieving a combination of exceptional figures-of-merit is realized, including a responsivity of 0.45 AW⁻¹, a peak specific detectivity of 2.3 × 10¹³ Jones, a 156 dB linear dynamic range, and a rise/fall time of 2.89 µs/1.93 µs. The desired phase structure is obtained by utilizing a double-hole transport layer (HTL), combining hydrophobic PTAA and hydrophilic PEDOT: PSS. Besides, the dependence of the hybrid low-dimensional phase structure is also identified on the surface energy of the buried HTL substrate. This study gives insight into the correlation between Q-2D perovskites’ phase structure and performance, providing a valuable design guide for Q-2D perovskite-based photodetectors.     

Band Edge Control of Quasi-2D Metal Halide Perovskites for Blue Light-Emitting Diodes with Enhanced Performance

Perovskite light-emitting diodes (PeLEDs) have received great attention for their potential as next-generation display technology. While remarkable progress has been achieved in green, red, and near-infrared PeLEDs with external quantum efficiencies (EQEs) exceeding 20%, obtaining high performance blue PeLEDs remains a challenge. Poor charge balance due to large charge injection barriers in blue PeLEDs has been identified as one of the major roadblocks to achieve high efficiency. Here band edge control of perovskite emitting layers for blue PeLEDs with enhanced charge balance and device performance is reported. By using organic spacer cations with different dipole moments, that is, phenethyl ammonium (PEA), methoxy phenethyl ammonium (MePEA), and 4-fluoro phenethyl ammonium (4FPEA), the band edges of quasi-2D perovskites are tuned without affecting their band gaps. Detailed characterization and computational studies have confirmed the effect of dipole moment modification to be mostly electrostatic, resulting in changes in the ionization energies of ≈0.45 eV for MePEA and ≈ −0.65 eV for 4FPEA based thin films relative to PEA-based thin films. With improved charge balance, blue PeLEDs based on MePEA quasi-2D perovskites show twofold increase of the EQE as compared to the control PEA based devices.     

Spatial Charge Separation as the Origin of Anomalous Stark Effect in Fluorous 2D Hybrid Perovskites

2D hybrid perovskites (2DP) are versatile materials, whose electronic and optical properties can be tuned through the nature of the organic cations (even when those are seemingly electronically inert). Here, it is demonstrated that fluorination of the organic ligands yields glassy 2DP materials featuring long‐lived correlated electron–hole pairs. Such states have a marked charge‐transfer character, as revealed by the persistent Stark effect in the form of a second derivative in electroabsorption. Modeling shows that electrostatic effects associated with fluorination, combined with the steric hindrance due to the bulky side groups, drive the formation of spatially dislocated charge pairs with reduced recombination rates. This work enriches and broadens the current knowledge of the photophysics of 2DP, which will hopefully guide synthesis efforts toward novel materials with improved functionalities.