Steric Mixed-Cation 2D Perovskite as a Methylammonium Locker to Stabilize MAPbI3

The reduced dimension perovskite including 2D perovskites are one of the most promising strategies to stabilize lead halide perovskite. A mixed-cation 2D perovskite based on a steric phenyltrimethylammonium (PTA) cation is presented. The PTA-MA mixed-cation 2D perovskite of PTAMAPbI₄ can be formed on the surface of MAPbI₃ (PTAI-MAPbI₃) by controllable PTAI intercalation by either spin coating or soaking. The PTAMAPbI₄ capping layer can not only passivate PTAI-MAPbI₃ perovskite but also act as MA⁺ locker to inhibit MAI extraction and significantly enhance the stability. The highly stable PTAI-MAPbI₃ based perovskite solar cells exhibit a reproducible photovoltaic performance with a champion PCE of 21.16 %. Such unencapsulated devices retain 93 % of initial efficiency after 500 h continuous illumination. This steric mixed-cation 2D perovskite as MA⁺ locker to stabilize the MAPbI₃ is a promising strategy to design stable and high-performance hybrid lead halide perovskites.     

Enhanced Charge Transport by Incorporating Formamidinium and Cesium Cations into Two‐Dimensional Perovskite Solar Cells

Organic‐inorganic hybrid two‐dimensional (2D) perovskites (n≤5) have recently attracted significant attention because of their promising stability and optoelectronic properties. Normally, 2D perovskites contain a monocation [e.g., methylammonium (MA⁺) or formamidinium (FA⁺)]. Reported here for the first time is the fabrication of 2D perovskites (n=5) with mixed cations of MA⁺, FA⁺, and cesium (Cs⁺). The use of these triple cations leads to the formation of a smooth, compact surface morphology with larger grain size and fewer grain boundaries compared to the conventional MA‐based counterpart. The resulting perovskite also exhibits longer carrier lifetime and higher conductivity in triple cation 2D perovskite solar cells (PSCs). The power conversion efficiency (PCE) of 2D PSCs with triple cations was enhanced by more than 80 % (from 7.80 to 14.23 %) compared to PSCs fabricated with a monocation. The PCE is also higher than that of PSCs based on binary cation (MA⁺‐FA⁺ or MA⁺‐Cs⁺) 2D structures.     

Ion‐Exchange‐Induced 2D–3D Conversion of HMA1−xFAxPbI3Cl Perovskite into a High‐Quality MA1−xFAxPbI3 Perovskite

High‐quality phase‐pure MA_1?xFA_xPbI₃ planar films (MA=methylammonium, FA=formamidinium) with extended absorption and enhanced thermal stability are difficult to deposit by regular simple solution chemistry approaches owing to crystallization competition between the easy‐to‐crystallize but unwanted δ‐FAPbI₃/MAPbI₃ and FA_xMA_1?xPbI₃ requiring rigid crystallization conditions. Here A 2D–3D conversion to transform compact 2D mixed composition HMA_1?xFA_xPbI₃Cl perovskite precursor films into 3D MA_1?xFA_xPbI₃ (x=0.1–0.9) perovskites is presented. The designed Cl/I and H/FA(MA) ion exchange reaction induced fast transformation of compact 2D perovskite film, helping to form the phase‐pure and high quality MA_1?xFA_xPbI₃ without δ‐FAPbI₃ and MAPbI₃ impurity. In all, we successfully developed a facile one‐step method to fabricate high quality phase‐pure MA_1?xFA_xPbI₃ (x=0.1–0.9) perovskite films by 2D–3D conversion of HMA_1?xFA_xPbI₃Cl perovskite. This 2D–3D conversion is a promising strategy for lead halide perovskite fabrication.     

Stabilization of Highly Efficient and Stable Phase‐Pure FAPbI3 Perovskite Solar Cells by Molecularly Tailored 2D‐Overlayers

As a result of their attractive optoelectronic properties, metal halide APbI₃ perovskites employing formamidinium (FA⁺) as the A cation are the focus of research. The superior chemical and thermal stability of FA⁺ cations makes α‐FAPbI₃ more suitable for solar‐cell applications than methylammonium lead iodide (MAPbI₃). However, its spontaneous conversion into the yellow non‐perovskite phase (δ‐FAPbI₃) under ambient conditions poses a serious challenge for practical applications. Herein, we report on the stabilization of the desired α‐FAPbI₃ perovskite phase by protecting it with a two‐dimensional (2D) IBA₂FAPb₂I₇ (IBA=iso‐butylammonium overlayer, formed via stepwise annealing. The α‐FAPbI₃/IBA₂FAPb₂I₇ based perovskite solar cell (PSC) reached a high power conversion efficiency (PCE) of close to 23 %. In addition, it showed excellent operational stability, retaining around 85 % of its initial efficiency under severe combined heat and light stress, that is, simultaneous exposure with maximum power tracking to full simulated sunlight at 80 °C over 500 h.     

Lead‐ and Iodide‐Deficient (CH3NH3)PbI3 (d‐MAPI): The Bridge between 2D and 3D Hybrid Perovskites

3D and 2D hybrid perovskites, which have been known for more than 20 years, have emerged recently as promising materials for optoelectronic applications, particularly the 3D compound (CH₃NH₃)PbI₃ (MAPI). The discovery of a new family of hybrid perovskites called d ‐MAPI is reported: the association of PbI₂ with both methyl ammonium (MA⁺) and hydroxyethyl ammonium (HEA⁺) cations leads to a series of five compounds with general formulation (MA)_1−2.48x(HEA)_3.48x[Pb_1−xI_3−x]. These materials, which are lead‐ and iodide‐deficient compared to MAPI while retaining 3D architecture, can be considered as a bridge between the 2D and 3D materials. Moreover, they can be prepared as crystallized thin films by spin‐coating. These new 3D materials appear very promising for optoelectronic applications, not only because of their reduced lead content, but also in account of the large flexibility of their chemical composition through potential substitutions of MA⁺, HEA⁺, Pb²⁺ and I⁻ ions.     

环境友好型无铅卤化物钙钛矿太阳能电池研究进展

ABX_3(A为甲胺、甲脒等有机离子或铯离子,B为铅或锡等金属离子,X为溴、碘等卤化物离子)卤化物钙钛矿材料具有优异的光电特性,是当前太阳能电池研究的前沿和热点之一。然而,这类太阳能电池普遍面临含毒性元素铅和稳定性差等问题,极大地阻碍了钙钛矿太阳能电池商业化应用进程。因此,发展新型高效无铅钙钛矿太阳能电池势在必行。本文评述了环境友好型无铅卤化物钙钛矿太阳能电池的最新研究动态和进展,探讨了该类太阳能电池的制备、性能及其稳定性等问题,展望了其未来发展趋势。     

Cesium Methylammonium Lead Iodide (CsxMA1−xPbI3) Nanocrystals with Wide Range Cation Composition Tuning and Enhanced Thermal Stability of the Perovskite Phase

Cesium methylammonium lead iodide (Cs_xMA_1−xPbI₃) nanocrystals were obtained with a wide range of A-site Cs-MA compositions by post-synthetic, room temperature cation exchange between CsPbI₃ nanocrystals and MAPbI₃ nanocrystals. The alloyed Cs_xMA_1−xPbI₃ nanocrystals retain their photoactive perovskite phase with incorporated Cs content, x, as high as 0.74 and the expected composition-tunable photoluminescence (PL). Excess methylammonium oleate from the reaction mixture in the MAPbI₃ nanocrystal dispersions was necessary to obtain fast Cs-MA cation exchange. The phase transformation and degradation kinetics of films of Cs_xMA_1−xPbI₃ nanocrystals were measured and modeled using an Avrami expression. The transformation kinetics were significantly slower than those of the parent CsPbI₃ and MAPbI₃ nanocrystals, with Avrami rate constants, k, at least an order of magnitude smaller. These results affirm that A-site cation alloying is a promising strategy for stabilizing iodide-based perovskites.     

All‐Inorganic Perovskite CsSnBr3 as a Thermally Stable,Free‐Carrier Semiconductor

Hybrid organic‐inorganic perovskites, especially methylammonium lead triiodide (MAPbI₃), are intensely studied for their optoelectronic properties. The organic MA⁺ cation is held responsible for the superior performance of MAPbI₃ but also its instability toward moisture and heat. To explore compositions beyond MAPbI₃, we performed experiments and calculations on two isomorphous perovskites CsSnBr₃ and MASnBr₃. CsSnBr₃ is slightly smaller than MASnBr₃ in cell dimension, but outperforms MASnBr₃ in band gap energy, charge‐carrier reduced effective mass, and optical dielectric constant all by ≈19 %. These merits accumulate to drastically cut the exciton binding energy from 33 meV for MASnBr₃ to 19.6 meV for CsSnBr₃, making CsSnBr₃ a black, free‐carrier semiconductor. CsSnBr₃ also exhibits distinctly higher stability toward moisture and heat than its organic counterparts. These advantages suggest ecofriendly applications for CsSnBr₃, such as tandem solar cells and direct X‐ray detectors.     

Surface Termination on Unstable Methylammonium-based Perovskite Using a Steric Barrier for Improved Perovskite Solar Cells

Compared to widely adopted low-dimensional/three-dimensional (LD/3D) heterostructure, functional organic cation based surface termination on perovskite can not only realize advantage of defect passivation but also prevent potential disadvantage of the heterostructure induced intercalation into 3D perovskite. However, it is still very challenging to controllably construct surface termination on organic–inorganic hybrid perovskite because the functional organic cations’ substitution reaction is easy to form LD/3D heterostructure. Here, we report using a novel benzyltrimethylammonium (BTA) functional cation with rational designed steric hindrance to effectively surface terminate onto methylammonium lead triiodide (MAPbI₃) perovskite, which is composed of the most unstable MA cations. The BTA cation is difficult to form a specific 1.5-dimensional perovskite of BTA₄Pb₃I₁₀ by cation substitution with MA cation, which then provides a wide processing window (around 10 minutes) for surface terminating on MAPbI₃ films. Moreover, the BTAI surface terminated BTAI-MAPbI₃ shows better passivation effect than BTA₄Pb₃I₁₀-MAPbI₃ heterojunction. Finally, BTAI surface terminated solar cell (0.085 cm²) and mini-module (11.52 cm²) obtained the efficiencies of 22.03 % and 18.57 %, which are among the highest efficiencies for MAPbI₃ based ones.     

Room‐Temperature Ferroelectric Material Composed of a Two‐Dimensional Metal Halide Double Perovskite for X‐ray Detection

Although two‐dimensional (2D) metal–halide double perovskites display versatile physical properties due to their huge structural compatibility, room‐temperature ferroelectric behavior has not yet been reported for this fascinating family. Here, we designed a room‐temperature ferroelectric material composed of 2D halide double perovskites, (chloropropylammonium)₄AgBiBr₈, using an organic asymmetric dipolar ligand. It exhibits concrete ferroelectricity, including a Curie temperature of 305 K and a notable spontaneous polarization of ≈3.2 μC cm^?2, triggered by dynamic ordering of the organic cation and the tilting motion of heterometallic AgBr₆/BiBr₆ octahedra. Besides, the alternating array of inorganic perovskite sheets and organic cations endows large mobility‐lifetime product (μτ=1.0×10^?3 cm² V^?1) for detecting X‐ray photons, which is almost tenfold higher than that of CH₃NH₃PbI₃ wafers. As far as we know, this is the first study on an X‐ray‐sensitive ferroelectric material composed of 2D halide double perovskites. Our findings afford a promising platform for exploring new ferroelectric materials toward further device applications.     

How Methylammonium Iodide Reactant Size Affects Morphology and Defect Properties of Mechanochemically Synthesized MAPbI3 Powder

Here, we investigate in detail the impact of the size of the methylammonium iodide (MAI) reactants in the mechanochemical powder synthesis of the halide perovskite methylammonium lead iodide (MAPbI₃). Morphology and structural characterizations by scanning electron microscopy and X-ray diffraction reveal that with increasing MAI reactant size, the particle size of the perovskite powder increases, while its defect density decreases, as suggested by nuclear quadrupole resonance spectroscopy and photoluminescence investigations. The reason for this behavior seems to be associated to the sensitive influence of the MAI size on the time durations of MAPbI₃ synthesis and delayed MAPbI₃ crushing stage during ball milling. Thus, our results emphasize the high importance the reactant properties have on the mechanochemical synthesis of halide perovskites and will contribute to enhance the reproducibility and control of the fabrication of halide perovskites in powder form.     

Using steric hindrance to manipulate and stabilize metal halide perovskites for optoelectronics

The chemical instability of metal halide perovskite materials can be ascribed to their unique properties of softness, in which the chemical bonding between metal halide octahedral frameworks and cations is the weak ionic and hydrogen bonding as in most perovskite structures. Therefore, various strategies have been developed to stabilize the cations and metal halide frameworks, which include incorporating additives, developing two-dimensional perovskites and perovskite nanocrystals, etc. Recently, the important role of utilizing steric hindrance for stabilizing and passivating perovskites has been demonstrated. In this perspective, we summarize the applications of steric hindrance in manipulating and stabilizing perovskites. We will also discuss how steric hindrance influences the fundamental kinetics of perovskite crystallization and film formation processes. The similarities and differences of the steric hindrance between perovskite solar cells and perovskite light emission diodes are also discussed. In all, utilizing steric hindrance is a promising strategy to manipulate and stabilize metal halide perovskites for optoelectronics.

Manipulation on steric hindrance can influence the fundamental kinetics of perovskite crystallization and film formation, therefore stabilizing and passivating perovskite structures, and promoting the commercialization of stable perovskite devices.     

Tailoring Component Interaction for Air‐Processed Efficient and Stable All‐Inorganic Perovskite Photovoltaic

All‐inorganic lead halide perovskites are promising candidates for optoelectronic applications. However, fundamental questions remain over the component interaction in the perovskite precursor solution due to the limitation of the most commonly used solvents of N,N‐dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). Here, we report an interaction tailoring strategy for all‐inorganic CsPbI_3?xBr_x perovskites by involving the ionic liquid solvent methylammonium acetate (MAAc). C=O shows strong interaction with lead (Pb²⁺) and N?H???I hydrogen bond formation is observed. The interactions stabilize the perovskite precursor solution and allow production of the high‐quality perovskite films by retarding the crystallization. Without the necessity for antisolvent treatment, the one‐step air‐processing approach delivers photovoltaic cells regardless of humidity, with a high efficiency of 17.10 % along with long operation stability over 1500 h under continuous light illumination.     

Enhanced Stability and Optical Absorption in the Perovskite-Based Compounds MA1−xCsxPbI3−yBry

Organometal halide perovskites have been outstanding from enormous amount of functional materials thanks to their highly cost-effective processability and prominent light harvesting capacity. Unfortunately, poor long-term stability seriously hinders their further development. The recent experimental observations suggest that Cesium is a promising candidate to enhance the stability of MAPbI₃. To explore the inherent mechanism, a first-principles investigation based on density functional theory, including hybrid functional, has been performed to analyze the electronic and optical properties of perovskite series MA_0.75Cs_0.25PbI_3−yBr_y. The results indicate that perovskite compound MA_0.75Cs_0.25PbI₂Br is significantly superior to the other doped series in terms of optical absorption within the visible-light range. In the meanwhile, both Bader charge analysis and charge density distribution show that the compound of MA_0.75Cs_0.25PbI₂Br is the most stable among all the doped perovskite series. Moreover, it is clearly manifested that the impact of cesium is mainly embodied in the enhancement of the stability rather than in the improvement of optical absorption. Our study sheds a new light on screening new-type light harvesting materials, and provides theoretical insight into the rationale design of highly efficient and stable photovoltaic devices based on these functional materials.     

All‐Inorganic Perovskite CsSnBr3 as a Thermally Stable,Free‐Carrier Semiconductor

Hybrid organic‐inorganic perovskites, especially methylammonium lead triiodide (MAPbI₃), are intensely studied for their optoelectronic properties. The organic MA⁺ cation is held responsible for the superior performance of MAPbI₃ but also its instability toward moisture and heat. To explore compositions beyond MAPbI₃, we performed experiments and calculations on two isomorphous perovskites CsSnBr₃ and MASnBr₃. CsSnBr₃ is slightly smaller than MASnBr₃ in cell dimension, but outperforms MASnBr₃ in band gap energy, charge‐carrier reduced effective mass, and optical dielectric constant all by ≈19 %. These merits accumulate to drastically cut the exciton binding energy from 33 meV for MASnBr₃ to 19.6 meV for CsSnBr₃, making CsSnBr₃ a black, free‐carrier semiconductor. CsSnBr₃ also exhibits distinctly higher stability toward moisture and heat than its organic counterparts. These advantages suggest ecofriendly applications for CsSnBr₃, such as tandem solar cells and direct X‐ray detectors.     

Halide Double Perovskite Ferroelectrics

Halide double perovskites have recently emerged as a promising environmentally friendly optoelectronic and photovoltaic material for their inherent thermodynamic stability, high defect tolerance, and appropriate band gaps. However, to date, no ferroelectric material based on halide double perovskites has been discovered. Herein, by hetero‐substitution of lead and cation intercalation of n‐propylamine, the first halide double perovskite ferroelectric, (n‐propylammonium)₂CsAgBiBr₇ ( 1 ), is reported and it exhibits distinct ferroelectricity with a notable saturation polarization of about 1.5 μC cm^?2. More importantly, single‐crystal photodetectors of 1 exhibit extraordinary performance with containing high on/off ratios of about 10⁴, fast response rates of 141 μs, and detectivity as high as 5.3×10¹¹ Jones. This finding opens a new way to design high‐performance perovskite ferroelectrics, and provides a viable approach in the search for stable and lead‐free optoelectronic materials as an alternative to the lead‐containing system.     

Air‐Stable,Lead‐Free Zero‐Dimensional Mixed Bismuth‐Antimony Perovskite Single Crystals with Ultra‐broadband Emission

Lead‐free zero‐dimensional (0D) organic‐inorganic metal halide perovskites have recently attracted increasing attention for their excellent photoluminescence properties and chemical stability. Here, we report the synthesis and characterization of an air‐stable 0D mixed metal halide perovskite (C₈NH₁₂)₄Bi_0.57Sb_0.43Br₇?H₂O, in which individual [BiBr₆]^3? and [SbBr₆]^3? octahedral units are completely isolated and surrounded by the large organic cation C₈H₁₂N⁺. Upon photoexcitation, the bulk crystals exhibit ultra‐broadband emission ranging from 400 to 850 nm, which originates from both free excitons and self‐trapped excitons. This is the first example of 0D perovskites with broadband emission spanning the entire visible spectrum. In addition, (C₈NH₁₂)₄Bi_0.57Sb_0.43Br₇?H₂O exhibits excellent humidity and light stability. These findings present a new direction towards the design of environmentally‐friendly, high‐performance 0D perovskite light emitters.     

Air‐Stable,Lead‐Free Zero‐Dimensional Mixed Bismuth‐Antimony Perovskite Single Crystals with Ultra‐broadband Emission

Lead‐free zero‐dimensional (0D) organic‐inorganic metal halide perovskites have recently attracted increasing attention for their excellent photoluminescence properties and chemical stability. Here, we report the synthesis and characterization of an air‐stable 0D mixed metal halide perovskite (C₈NH₁₂)₄Bi_0.57Sb_0.43Br₇?H₂O, in which individual [BiBr₆]^3? and [SbBr₆]^3? octahedral units are completely isolated and surrounded by the large organic cation C₈H₁₂N⁺. Upon photoexcitation, the bulk crystals exhibit ultra‐broadband emission ranging from 400 to 850 nm, which originates from both free excitons and self‐trapped excitons. This is the first example of 0D perovskites with broadband emission spanning the entire visible spectrum. In addition, (C₈NH₁₂)₄Bi_0.57Sb_0.43Br₇?H₂O exhibits excellent humidity and light stability. These findings present a new direction towards the design of environmentally‐friendly, high‐performance 0D perovskite light emitters.     

Lead‐Free,Blue Emitting Bismuth Halide Perovskite Quantum Dots

Lead halide perovskite quantum dots (QDs) are promising candidates for future lighting applications, due to their high quantum yield, narrow full width at half maximum (FWHM), and wide color gamut. However, the toxicity of lead represents a potential obstacle to their utilization. Although tin(II) has been used to replace lead in films and QDs, the high intrinsic defect density and oxidation vulnerability typically leads to unsatisfactory material properties. Bismuth, with much lower toxicity than lead, is promising to constitute lead‐free perovskite materials because Bi³⁺ is isoelectronic to Pb²⁺ and more stable than Sn²⁺. Herein we report, for the first time, the synthesis and optical characterization of MA₃Bi₂Br₉ perovskite QDs with photoluminescence quantum yield (PLQY) up to 12 %, which is much higher than Sn‐based perovskite nanocrystals. Furthermore, the photoluminescence (PL) peaks of MA₃Bi₂X₉ QDs could be easily tuned from 360 to 540 nm through anion exchange.     

Lead‐Free,Blue Emitting Bismuth Halide Perovskite Quantum Dots

Lead halide perovskite quantum dots (QDs) are promising candidates for future lighting applications, due to their high quantum yield, narrow full width at half maximum (FWHM), and wide color gamut. However, the toxicity of lead represents a potential obstacle to their utilization. Although tin(II) has been used to replace lead in films and QDs, the high intrinsic defect density and oxidation vulnerability typically leads to unsatisfactory material properties. Bismuth, with much lower toxicity than lead, is promising to constitute lead‐free perovskite materials because Bi³⁺ is isoelectronic to Pb²⁺ and more stable than Sn²⁺. Herein we report, for the first time, the synthesis and optical characterization of MA₃Bi₂Br₉ perovskite QDs with photoluminescence quantum yield (PLQY) up to 12 %, which is much higher than Sn‐based perovskite nanocrystals. Furthermore, the photoluminescence (PL) peaks of MA₃Bi₂X₉ QDs could be easily tuned from 360 to 540 nm through anion exchange.