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Metal halide perovskites, including some of their related perovskitoid structures, form a semiconductor class of their own, which is arousing ever-growing interest from the scientific community. With halides being involved in the various structural arrangements, namely, pure corner-sharing MX6(M is metal and X is halide) octahedra, for perovskite networks, or alternatively a combination of corner-, edge-, and/or face-sharing for related perovskitoids, they represent the ideal probe for characterizing the way octahedra are linked together. Well known for their inherently large quadrupolar constants, which is detrimental to the resolution of nuclear magnetic resonance spectroscopy, most abundant halide isotopes (35/37Cl, 79/81Br, 127I) are in turn attractive for magnetic field-free nuclear quadrupolar resonance (NQR) spectroscopy. Here, we investigate the possibility of exploiting NQR spectroscopy of halides to distinctively characterize the various metal halide structural arrangements, using density functional theory simulations. Our calculations nicely match the available experimental results. Furthermore, they demonstrate that compounds with different connectivities of their MX6building blocks, including lower dimensionalities such as 2D networks, show distinct NQR signals in a broad spectral window. They finally provide a roadmap of the characteristic NQR frequency ranges for each octahedral connectivity, which may be a useful guide to experimentalists, considering the long acquisition procedures typical of NQR. We hope this work will encourage the incorporation of NQR spectroscopy to further our knowledge of the structural diversity of metal halides.

The 2D Ruddlesden–Popper (RP) perovskites Cs2PbI2Cl2(Pb-based, n= 1) and Cs2SnI2Cl2(Sn-based, n= 1) stand out as unique and rare instances of entirely inorganic constituents within the more expansive category of organic/inorganic 2D perovskites. These materials have recently garnered significant attention for their strong UV-light responsiveness, exceptional thermal stability, and theoretically predicted ultrahigh carrier mobility. In this study, we synthesized Pb and Sn-based n= 1 2D RP perovskite films covering millimeter-scale areas for the first time, utilizing a one-step chemical vapor deposition (CVD) method under atmospheric conditions. These films feature perovskite layers oriented horizontally relative to the substrate. Multilayered Cs3Pb2I3Cl4(Pb-based, n= 2) and Cs3Sn2I3Cl4(Sn-based, n= 2) films were also obtained for the first time, and their crystallographic structures were refined by combining X-ray diffraction (XRD) and density functional theory (DFT) calculations. DFT calculations and experimental optical spectroscopy support band-gap energy shifts related to the perovskite layer thickness. We demonstrate bias-free photodetectors using the Sn-based, n= 1 perovskite with reproducible photocurrent and a fast 84 ms response time. The present work not only demonstrates the growth of high-quality all-inorganic multilayered 2D perovskites via the CVD method but also suggests their potential as promising candidates for future optoelectronic applications.

One of the primary methods for band gap tuning in metal halide perovskites has been halide (I/Br) mixing. Despite widespread usage of this type of chemical substitution in perovskite photovoltaics, there is still little understanding of the structural impacts of halide alloying, with the assumption being the formation of ideal solid solutions. The FASnI3–xBrx(x= 0–3) family of compounds provides the first example where the assumption breaks down, as the composition space is broken into two unique regimes (x= 0–2.9; x= 2.9–3) based on their average structure with the former having a 3D and the latter having an extended 3D (pseudo 0D) structure. Pair distribution function (PDF) analyses further suggest a dynamic 5s2lone pair expression resulting in increasing levels of off-centering of the central Sn as the Br concentration is increased. These antiferroelectric distortions indicate that even the x= 0–2.9 phase space behaves as a nonideal solid-solution on a more local scale. Solid-state NMR confirms the difference in local structure yielding greater insight into the chemical nature and local distributions of the FA+cation. In contrast to the FAPbI3–xBrxseries, a drastic photoluminescence (PL) quenching is observed with x≥ 1.9 compounds having no observable PL. Our detailed studies attribute this quenching to structural transitions induced by the distortions of the [SnBr6] octahedra in response to stereochemically expressed lone pairs of electrons. This is confirmed through density functional theory, having a direct impact on the electronic structure.

Defect passivation using two-dimensional (2D)-layered perovskites with organic spacers on 3D bulk perovskites has been proposed as an effective strategy to improve perovskite solar cell stability and efficiency. Specifically, fluorination of the organic spacers has been employed due to the resulting hydrophobic nature and the defect passivation characteristics. In addition to the type of functional groups attached to the spacer molecules, conformational changes of fluorine isomers on layered perovskites can provide an extended strategy to control a variety of opto-electrical properties related to the interlayer spacing. As a model system for the structural isomer of fluorinated spacers, meta-CF3and para-CF3groups anchored to phenethylammonium iodide (PEAI) spacer molecules are employed to synthesize 2D perovskites and to investigate their full potential as an interfacial modifier for perovskite solar cells. The fluorination position change leads to altered opto-electrical characteristics in layered perovskites. Although they possess identical functional groups, the different orientations of the functional groups used in the perovskite layer deposited on the 3D perovskite absorber result in distinct electrical properties of 2D/3D heterostructures due to dissimilar intermolecular interactions. The 2D perovskite with meta-CF3-PEAI spacers exhibits an enhancement of the charge transport in the out-of-plane orientation and an improved suppression of the trap states of 3D perovskites while also providing a more favorable energy alignment for efficient charge transfers. Theoretical simulations are consistent with the experimental results. The structural isomers of fluorination anchoring to spacer cations alter the structural configuration of the spacer as well as the interlayer spacing that can improve the performance and the stability of 2D/3D perovskite solar cells.

Low electronic band gap 2D multilayered (n= 3,4) lead-iodide perovskites with formulas A′2An–1PbnI3n+1A″An–1PbnI3n+1are of great interest for photovoltaics, with recent demonstrations of stable solar cell operation based on 2D/3D bilayered heterostructures. Still, the difficulty in achieving optimal phase control, with potential formation of mixed n-domains, is a limiting factor for the photovoltaic performance of 2D/3D heterostructures, and the current choice for multi n-layered compounds is limited. Here, we report synthesis and XRD characterization of novel (I-EA)2MAn–1PbnI3n+1(n= 1–4) compound series, along with the (Br-EA)2PbBr4(n= 1) compound, incorporating iodo-ethylammonium (I-EA) and bromo-ethylammonium (Br-EA) spacers. These halide-featuring spacers lead to a small lattice mismatch between the inorganic and organic components, which explains the successful formation of multi n-layered compounds. The presence of bromine or iodine in the interlayer space impacts on the dielectric and electronic properties of these materials. Periodic DFT simulations predict vertical hole effective mass for n= 1 (I-EA)2PbI4as small as 1.8 me, comparable to popular organic semiconductors, like rubrene. UV–vis characterization sets the optical absorption onset of these materials around 1.71 eV for n= 3 and 4, hence suggesting that they can be successfully implemented in 2D/3D photovoltaic architectures.

Direct visualization of ultrafast coupling between charge carriers and lattice degrees of freedom in photoexcited semiconductors has remained a long-standing challenge and is critical for understanding the light-induced physical behaviour of materials under extreme non-equilibrium conditions. Here we obtain a direct visualization of the structural dynamics in monocrystalline 2D perovskites. We achieve this by monitoring the evolution of wavevector-resolved ultrafast electron diffraction intensity following above-bandgap high-density photoexcitation. Our analysis reveals a light-induced ultrafast reduction in antiferro-distortion resulting from a strong interaction between the electron–hole plasma and perovskite lattice, which induces an in-plane octahedra rotation towards a more symmetric phase. Correlated ultrafast spectroscopy performed at the same carrier density as ultrafast electron diffraction reveals that the creation of a dense electron–hole plasma triggers the relaxation of lattice distortion at shorter timescales by modulating the crystal cohesive energy. Finally, we show that the interaction between carrier gas and lattice can be altered by tailoring the rigidity of the 2D perovskite by choosing an appropriate organic spacer layer.

Two-dimensional (2D) halide perovskites exhibit extraordinary stability and structural diversity because they can incorporate different organic cations (both cage A-site cation and the spacers). Here, we report on the behavior of Tz = 1,2,4-triazolium as an A-site cation as well as a spacer cation in new perovskite compounds. We describe the synthesis and structure–property relationships of a new family of hydrogen-bonding stabilized 2D perovskites (IPA)2(Tz)n−1PbnBr3n+1(n= 1–3) (IPA = isopropylammonium, Tz = 1,2,4-triazolium), which represents the rare example of 2D perovskites incorporating the large A-site cation (Tz) that can form both the n= 2 and 3 phases. However, excess of Tz cations can split the n= 2 layers to act as a spacer and combine with another spacer PA (PA = propylammonium) or BA (BA = butylammonium) to form n= 1 structures (PA)(Tz)PbBr4and (BA)(Tz)PbBr4. These ordered mixed-spacer perovskites with different interlayer distances are unusual because the different spacers tend to stay in the same interlayer. (IPA)2(Tz)n−1PbnBr3n+1all exhibit broad photoluminescence emission but for different reasons. For the n= 1 phase, the large octahedral tilting (small Pb–Br–Pb angle) gives rise to the broad emission, while for the n= 2 and 3 phases, the individual octahedral distortion (large distortion index) plays a more important role. For the mixed-spacer (PA)(Tz)PbBr4and (BA)(Tz)PbBr4, their band gaps are in between the parent n= 1 compounds [(BA)2PbBr4or (PA)2PbBr4, and (Tz)2PbBr4]. Density functional theory calculations suggest that the band structures of (PA)(Tz)PbBr4and (BA)(Tz)PbBr4combine features from both parent compounds yet differ from a simple superposition. This work demonstrates the dual role of Tz molecules as the A-site cation in multilayer 2D perovskites and as the spacer in ordered mixed-spacer 2D phases.

Three-dimensional (3D) halide perovskites have attracted enormous research interest, but the choice of the A-site cations is limited by the Goldschmidt tolerance factor. In order to accommodate cations that lie outside the acceptable range of the tolerance factor, low-dimensional structures usually form. To maintain the favorable 3D connection, the links among the metal-halide octahedra need to be rearranged to fit the large cations. This can result in a departure from the proper corner-sharing perovskite architectures and lead to distinctly different perovskitoid motifs with edge- and face-sharing. In this work, we report four new 3D bromide perovskitoids incorporating linear organic diammonium cations, A′Pb2Br6(A′ is a +2 cation). We propose a rule that can guide the further expansion of this class of compounds, analogous to the notion of Goldschmidt tolerance factor widely adopted for 3D AMX3perovskites. The fundamental building blocks in A′Pb2Br6consist of two edge-shared octahedra, which are then connected by corner-sharing to form a 3D network. Different compounds adopt different structural motifs, which can be transformed from one to another by symmetry operations. Electronic structure calculations suggest that they are direct bandgap semiconductors, with relatively large band dispersions created by octahedra connected by corner-sharing. They exhibit similar electronic band structures and dynamic lattice characteristics to the regular 3D AMX3perovskites. Structures with smaller Pb–Br–Pb angles and larger octahedra distortion exhibit broad photoluminescence at room temperature. The emerging structure–property relationships in these 3D perovskitoids set the foundation for designing and investigating these compounds for a variety of optoelectronic applications.

Understanding and tailoring the physical behaviour of halide perovskites under practical environments is critical for designing efficient and durable optoelectronic devices. Here, we report that continuous light illumination leads to >1% contraction in the out-of-plane direction in two-dimensional hybrid perovskites, which is reversible and strongly dependent on the specific superlattice packing. X-ray photoelectron spectroscopy measurements show that constant light illumination results in the accumulation of positive charges in the terminal iodine atoms, thereby enhancing the bonding character of inter-slab I–I interactions across the organic barrier and activating out-of-plane contraction. Correlated charge transport, structural and photovoltaic measurements confirm that the onset of the light-induced contraction is synchronized to a threefold increase in carrier mobility and conductivity, which is consistent with an increase in the electronic band dispersion predicted by first-principles calculations. Flux-dependent space-charge-limited current measurement reveals that light-induced interlayer contraction activates interlayer charge transport. The enhanced charge transport boosts the photovoltaic efficiency of two-dimensional perovskite solar cells up to 18.3% by increasing the device’s fill factor and open-circuit voltage.

New structures with favorable band structures and optical properties are of broad interest to the halide perovskite community. Recently, lead-free two-dimensional (2D) double perovskites have emerged as dimensionally reduced counterparts of their three-dimensional analogues. In addition to the structural diversity provided by the organic cation, the achievement of 2D lead-free iodide double perovskites has prompted researchers to explore more structures in this new material family. Here, we report the synthesis and structures of a series of 2D iodide double perovskites based on cyclic diammonium cations (aminomethyl)piperidinium (AMP) and (aminomethyl)pyridinium (AMPY), (4AMP)2AgBiI8and (3AMPY)2AgBiI8, and compare them with one-dimensional (1D) structures with Bi only (x-AMP)BiI5and (x-AMPY)BiI5(x= 3 and 4). The crystallographic structures of the double perovskite phases are highly distorted, because of the inability of Ag to form regular octahedral coordination with iodine. The experimental bandgaps of the double perovskite phases are surprisingly similar to ((4AMP)2AgBiI8) or even larger than ((3AMPY)2AgBiI8) those in the 1D structures with the same cations ((4AMP)BiI5and (3AMPY)BiI5). Density functional theory calculations suggest that the effective electronic dimensionality of the double perovskites is on a par with or lower than that of the 1D structures. The reduced electronic dimension of the 2D compounds originates from the weak electronic coupling between the corner-sharing Ag and Bi octahedra. The band structures of the 1D compounds are dispersive in the chain direction, suggesting that their electronic and structural dimensions are similar. Low-frequency Raman spectra exhibit distinct peaks at room temperature for all compounds reported here, suggesting rigid lattices.

Taking advantage of an innovative design concept for layered halide perovskites with active chromophores acting as organic spacers, we present here the synthesis of two novel two-dimensional (2D) hybrid organic–inorganic halide perovskites incorporating for the first time 100% of a photoactive tetrazine derivative as the organic component. Namely, the use of a heterocyclic ring containing a nitrogen proportion imparts a unique electronic structure to the organic component, with the lowest energy optical absorption in the blue region. The present compound, a tetrazine, presents several resonances between the organic and inorganic components, both in terms of single particle electronic levels and exciton states, providing the ideal playground to discuss charge and energy transfer mechanisms at the organic/inorganic interface. Photophysical studies along with hybrid time-dependent DFT simulations demonstrate partial energy transfer and rationalise the suppressed emission from the perovskite frame in terms of different energy-transfer diversion channels, potentially involving both singlet and triplet states of the organic spacer. Periodic DFT simulations also support the feasibility of electron transfer from the conduction band of the inorganic component to the LUMO of the spacer as a potential quenching mechanism, suggesting the coexistence and competition of charge and energy transfer mechanisms in these heterostructures. Our work proves the feasibility of inserting photoactive small rings in a 2D perovskite structure, meanwhile providing a robust frame to rationalize the electronic interactions between the semiconducting inorganic layer and organic chromophores, with the prospects of optimizing the organic moiety according to the envisaged application.

Organic–inorganic hybrid halide perovskites are promising semiconductors with tailorable optical and electronic properties. The choice of A-site cation to support a three-dimensional (3D) perovskite structure AMX3(where M is a metal and X is a halide) is limited by the geometric Goldschmidt tolerance factor. However, this geometric constraint can be relaxed in two-dimensional (2D) perovskites, providing us an opportunity to understand how various A-site cations modulate the structural properties and thereby the optoelectronic properties. Here, we report the synthesis and structures of single-crystal (BA)2(A)Pb2I7where BA = butylammonium and A = methylammonium (MA), formamidinium (FA), dimethylammonium (DMA), or guanidinium (GA), with a series of A-site cations varying in size. Single-crystal X-ray diffraction reveals that the MA, FA, and GA structures crystallize in the same Cmcmspace group, while the DMA imposes the Ccmbspace group. We observe that as the A-site cation becomes larger, the Pb–I bond continuously elongates, expanding the volume of the perovskite cage, equivalent to exerting “negative pressure” on the perovskite structures. Optical studies and DFT calculations show that the Pb–I bond length elongation reduces the overlap of the Pb s- and I p-orbitals and increases the optical bandgap, while Pb–I–Pb tilting angles play a secondary role. Raman spectra show lattice softening with increasing size of the A-site cation. These structural changes with enlarged A cations result in significant decreases in photoluminescence intensity and lifetime, consistent with a more pronounced nonradiative decay. Transient absorption microscopy results suggest that the PL drop may derive from a higher concentration of traps or phonon-assisted nonradiative recombination. The results highlight that extending the range of Goldschmidt tolerance factors for 2D perovskites is achievable, enabling further tuning of the structure–property relationships in 2D perovskites.

Hybrid layered halide perovskites have achieved impressive performance in optoelectronics. New structural types in the two-dimensional (2D) halide system such as the Dion–Jacobson phases have attracted wide research attention due to the short interlayer distance and unique layer orientation that facilitate better charge-transport and higher stability in optoelectronic devices. Here, we report the first solid solution series incorporating both A and A’ cations in the 2D Dion–Jacobson family, with the general formula (A’)(A)Pb2Br7((A’ = 3-(aminomethyl)piperidinium (3AMP) and 4-(aminomethyl)piperidinium) (4AMP); A = methylammonium (MA) and formamidinium (FA)). Mixing the spacing A’ cations and perovskitizer A cations generates the new (3AMP)a(4AMP)1–a(FA)b(MA)1–bPb2Br7perovskites. The crystallographically refined crystal structures using single-crystal X-ray diffraction data reveal that the distortion of the inorganic framework is heavily influenced by the degree of A’ and A alloying. A rising fraction of 4AMP in the structure, decreases the Pb–Br–Pb angles, making the framework more distorted. On the contrary, higher FA fractions increase the Pb–Br–Pb angles. This structural evolution fine-tunes the optical properties where the larger the Pb–Br–Pb angle, the narrower the band gap. The photoluminescence emission energy mirrors this trend. Raman spectroscopy reveals a highly dynamical lattice similar to MAPbBr3and consistent with the local distortion environment of the [Pb2Br7] framework. Density functional theory (DFT) calculations of the electronic structures reveal the same trend as the experimental results where (3AMP)(FA)Pb2Br7has the smallest band gap while (4AMP)(MA)Pb2Br7has the largest band gap. The structural effects from solely the organic cations in the 2D system highlight the importance of understanding the high sensitivity of the optoelectronic properties on the structural tuning in this broad class of materials.

Large organic A cations cannot stabilize the 3D perovskite AMX3structure because they cannot be accommodated in the cubo-octhedral cage (do not follow the Goldschmidt tolerance factor rule), and they generally template low-dimensional structures. Here we report that the large dication aminomethylpyridinium (AMPY) can template novel 3D structures which resemble conventional perovskites. They have the formula (xAMPY)M2I6(x= 3 or 4, M = Sn2+or Pb2+) which is double of the AMX3formula. However, because of the steric requirement of the Goldschmidt tolerance factor rule, it is impossible for (xAMPY)M2I6to form proper perovskite structures. Instead, a combination of corner-sharing and edge-sharing connectivity is adopted in these compounds leading to the new 3D structures. DFT calculations reveal that the compounds are indirect band gap semiconductors with direct band gaps presenting at slightly higher energies and dispersive electronic bands. The indirect band gaps of the Sn and Pb compounds are ∼1.7 and 2.0 eV, respectively, which is slightly higher than the corresponding AMI33D perovskites. The Raman spectra for the compounds are diffuse, with a broad rising central peak at very low frequencies around 0 cm–1, a feature that is characteristic of dynamical lattices, high anharmonicity, and dissipative vibrations very similar to the 3D AMX3perovskites. Devices of (3AMPY)Pb2I6crystals exhibit clear photoresponse under ambient light without applied bias, reflecting a high carrier mobility (μ) and long carrier lifetime (τ). The devices also exhibit sizable X-ray generated photocurrent with a high μτ product of ∼1.2 × 10–4cm2/V and an X-ray sensitivity of 207 μC·Gy–1·cm–2.

Layered hybrid metal-halide perovskites with non-centrosymmetric crystal structure are predicted to show spin-selective band splitting from Rashba effects. Thus, fabrication of metal-halide perovskites with defined crystal symmetry is desired to control the spin-splitting in their electronic states. Here, we report the influence of halogen para-substituents on the crystal structure of benzylammonium lead iodide perovskites (4-XC6H4CH2NH3)2PbI4(X= H, F, Cl, Br). Using X-ray diffraction and second-harmonic generation, we study structure and symmetry of single-crystal and thin-film samples. We report that introduction of a halogen atom lowers the crystal symmetry such that the chlorine- and bromine-substituted structures are non-centrosymmetric. The differences can be attributed to the nature of the intermolecular interactions between the organic molecules. We calculate electronic band structures and find good control of Rashba splittings. Our results present a facile approach to tailor hybrid layered metal halide perovskites with potential for spintronic and nonlinear optical applications.

Controlled anion mixing in halide perovskites has been shown to be an effective route to precisely tune optoelectronic properties in order to achieve efficient photovoltaic, light emission, and radiation detection devices. However, an atomistic understanding behind the precise mechanism impacting the performances of mixed halide perovskite devices, particularly as a radiation detector, is still missing. Combining high-level computational methods and multiple experiments, here we systematically investigate the effect of chlorine (Cl) incorporation on the optical and electronic properties, structural stability, ion migration, and the γ-ray radiation detection ability of MAPbBr3–xClx. We observe that precise halide mixing suppresses bromide ion migration and consequently reduces the dark current by close to a factor of two, which significantly increases the resistance of the mixed anion devices. Furthermore, reduced carrier effective masses and mostly unchanged exciton binding energies indicate enhanced charge carrier transport for these perovskite alloys. At the atomistic level, modifications to ion migration and charge carrier transport properties improve electronic properties and predominantly contribute to the better response and resolution in high-energy γ-ray detection with MAPbBr3–xClxas compared to MAPbBr3. This study provides a systematic approach to enhance the high-energy radiation detection ability of MAPbBr3–xClx-based devices by understanding the atomistic properties underpinning performance.

The Goldschmidt tolerance factor in halide perovskites limits the number of cations that can enter their cages without destabilizing their overall structure. Here, we have explored the limits of this geometric factor and found that the ethylammonium (EA) cations which lie outside the tolerance factor range can still enter the cages of the 2D halide perovskites by stretching them. The new perovskites allow us to study how these large cations occupying the perovskite cages affect the structural, optical, and electronic properties. We report a series of cation engineered 2D Ruddlesden–Popper lead iodide perovskites (BA)2(EAxMA1–x)2Pb3I10(x= 0–1, BA is n-butylammonium, MA is methylammonium) by the incorporation of a large EA cation in the cage. An analysis of the single-crystal structures reveals that the incorporation of EA in the cage significantly stretches Pb–I bonds, expands the cage, and induces a larger octahedral distortion in the inorganic framework. Spectroscopic and theoretical studies show that such structural deformation leads to a blue-shifted bandgap, sub-bandgap trap states with wider energetic distribution, and stronger photoluminescence quenching. These results enrich the family of 2D perovskites and provide new insights for understanding the structure–property relationship in perovskite materials.

Nickel oxide (NiO) is a commonly used contact material for a variety of thin-film optoelectronic technologies based on organic or hybrid materials. In such setups, interfaces play a crucial role as they can reduce, if not kill, the device performances by bringing additional traps or energy barriers, hindering the extraction of charge carriers from the active layer. Here, we computationally examine a prototype halide perovskite architecture, NiO/MAPbI3(MA = CH3NH3+), that has shown excellent photovoltaic performance and, in particular, a large open-circuit voltage. We show that efficient hole collection is achieved only when considering the role of vacancies induced by standard material deposition techniques. Specifically, Ni vacancies lead to nearly perfect valence band energy level alignment between the active layer and the contact material. Finally, we show how Li doping greatly improves the performances of the device and further propose alternative dopants. Our results suggest the high tunability of NiO interfaces for the design of optimized optoelectronic devices far beyond that of halide perovskites.

Two-dimensional layered halide organic perovskites (LHOPs) are promising candidates for many optoelectronic applications due to their interesting tunable properties. They provide a unique opportunity to control energy and charge dynamics via the independent tunability of the energy levels within the perovskite and the organic spacer for various optoelectronic applications. In the perovskite layer alone, one can replace the Pb (Sn), the halide (X = I, Br, Cl), the organic component, and the number of layers between the organic spacer layers. In addition, there are many possibilities for organic spacer layers between the perovskite layers, making it difficult for experimental methods to comprehensively explore such an extensive combinatorial space. Of particular technological interest is alignment of electronic levels between the perovskite layer and the organic spacer layer, leading to desired transfer of energy or charge carriers between perovskite and organic components. For example, as band edge absorption is almost entirely attributed to the perovskite layer, one way to demonstrate energy transfer is to observe triplet emission from organic spacers. State-of-the-art computational chemistry tools can be used to predict the properties of many stoichiometries in search for LHOPs that have the most promising electronic-structure features. In this first-principles study, we survey a group of π-conjugated organic spacer candidates for use in triplet light-emitting LHOPs. Utilizing density functional theory (DFT) and time-dependent DFT, we calculate the first singlet (S1) and triplet (T1) excitation energy in the ground-state geometry and the first triplet excitation energy in the excited-triplet-state relaxed geometry (T1*). By comparing these energies to the known lowest exciton energy level of PbnX3n+1perovskite layers (X = I, Br, Cl), we can identify organic spacer and perovskite layer pairings for possible transfer of Wannier excitons from the inorganic perovskite lattice to spin-triplet Frenkel excitons located on the organic cation. We successfully identify ten organic spacer candidates for possible pairing with perovskite layers of specific halide composition to achieve triplet light emission across the visible energy range. Molecular dynamics simulations predict that finite temperatures and perovskite environment have little influence on the average excitation energies of the two common organic spacers naphthylethylammonium (NEA) and phenelethylammonium (PEA). We find significant thermal broadening up to 0.5 eV of the optical excitation energies appearing due to finite temperature effects. The findings herein provide insights into alignment of electronic levels of the conjugated organic spacer with the layer.

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+1compounds 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)3Pb4I13and the corresponding aliphatic analogue (3AMP)(MA)3Pb4I13-based ones.

Two-dimensional (2D) hybrid halide perovskites are promising in optoelectronic applications, particularly solar cells and light-emitting devices (LEDs), and for their increased stability as compared to 3D perovskites. Here, we report a new series of structures using propylammonium (PA+), which results in a series of Ruddlesden–Popper (RP) structures with the formula (PA)2(MA)n−1PbnI3n+1(n= 3, 4) and a new homologous series of “step-like” (SL) structures where the PbI6octahedra connect in a corner- and face-sharing motif with the general formula (PA)2m+4(MA)m−2Pb2m+1I7m+4(m= 2, 3, 4). The RP structures show a blue-shift in bandgap for decreasing n(1.90 eV for n= 4 and 2.03 eV for n= 3), while the SL structures have an even greater blue-shift (2.53 eV for m= 4, 2.74 eV for m= 3, and 2.93 eV for m= 2). DFT calculations show that, while the RP structures are electronically 2D quantum wells, the SL structures are electronically 1D quantum wires with chains of corner-sharing octahedra “insulated” by blocks of face-sharing octahedra. Dark measurements for RP crystals show high resistivity perpendicular to the layers (1011Ω cm) but a lower resistivity parallel to them (107Ω cm). The SL crystals have varying resistivity in all three directions, confirming both RP and SL crystals’ utility as anisotropic electronic materials. The RP structures show strong photoresponse, whereas the SL materials exhibit resistivity trends that are dominated by ionic transport and no photoresponse. Solar cells were made with n= 3 giving an efficiency of 7.04% (average 6.28 ± 0.65%) with negligible hysteresis.

We report room-temperature synthesis of lead- and iodide-deficient α-[HC(NH2)2]PbI3perovskites (abbreviated d-α-FAPI, FA+= formamidinium), with the general formula (A′,FA)1+x[Pb1–xI3–x] (with A′ = hydroxyethylammonium (HEA+) or thioethylammonium (TEA+) cations, 0.04 ≤ x≤ 0.15). These materials retain a 3D character of their perovskite network despite incorporation of large HEA+or TEA+cations, demonstrating that the Goldschmidt tolerance factor can be bypassed. We found that thin films of (TEA,FA)1+x[Pb1–xI3–x] (x= 0.04 and 0.13) show exceptional α-phase stability under ambient conditions, 1 order of magnitude higher than α-FAPI and α-(Cs,FA)PbI3thin films. d-α-FAPI phases are shown to maintain a direct band gap, which increases monotonously for xranging from 0 up to 0.20, with characteristics of a p-type semiconductor for low concentrations of vacancies (x≤ 0.13) and n-type for larger ones. They offer alternatives to reach the methylammonium- and bromine-free stable α-FAPI-type phase and open new avenues in the field of perovskite solar cells, up to band gap tuning desirable for tandem solar cells.