Engineering of CH3 NH3 PbI3 Perovskite Crystals by Alloying Large Organic Cations for Enhanced Thermal Stability and Transport Properties

Phase Transitions and Dynamics in Mixed Three- and Low-Dimensional Lead Halide Perovskites

Lead halide perovskites are extensively investigated as efficient solution-processable materials for photovoltaic applications. The greatest stability and performance of these compounds are achieved by mixing different ions at all three sites of the APbX3 structure. Despite the extensive use of mixed lead halide perovskites in photovoltaic devices, a detailed and systematic understanding of the mixing-induced effects on the structural and dynamic aspects of these materials is still lacking. The goal of this review is to summarize the current state of knowledge on mixing effects on the structural phase transitions, crystal symmetry, cation and lattice dynamics, and phase diagrams of three- and low-dimensional lead halide perovskites. This review analyzes different mixing recipes and ingredients providing a comprehensive picture of mixing effects and their relation to the attractive properties of these materials.

Multifunctional Regulation of SnO2 Nanocrystals by Snail Mucus for Preparation of Rigid or Flexible Perovskite Solar Cells in Air

Tin oxide (SnO2) is widely used as an inorganic electron transport layer (ETL) for rigid and flexible perovskite solar cells (PSCs). In this work, an extract of snail shell, the sodium salt of polyaspartic acid (S-PASP), a water-soluble polypeptide polymer, has been used to multifunctionally regulate SnO2 nanograins. S-PASP has a strong chelating and dispersing effect; thus, chemically adsorbed SnO2 can inhibit agglomeration. The S-PASP:SnO2 ETL also improved the extraction and transferability of carriers, reducing body defects and interfacial charge. Moreover, the S-PASP:SnO2 ETL promotes the vertical growth of the perovskite crystals due to its bottom-up morphology, wettability, and strain release, which is conducive to improving the photoelectric performance of the device. The optimized rigid device prepared under open-air conditions obtained a PCE of 20.92%. In addition, due to the stress compensation of the S-PASP long chain, which prevented the cracking and displacement of the ETL, the optimal PCE of the flexible device was 17.96%, and the initial efficiency was maintained at 82.8% after 100 bends. This work introduces a molecular doping mechanism for organic-inorganic hybrid electronics.

Zero Threshold for Water Adsorption on MAPbBr3

Hybrid organic-inorganic perovskites (HOIPs) have shown great promise in a wide range of optoelectronic applications. However, this performance is inhibited by the sensitivity of HOIPs to various environmental factors, particularly high levels of relative humidity. This study uses X-ray photoelectron spectroscopy (XPS) to determine that there is essentially no threshold to water adsorption on the in situ cleaved MAPbBr3 (001) single crystal surface. Using scanning tunneling microscopy (STM), it shows that the initial surface restructuring upon exposure to water vapor occurs in isolated regions, which grow in area with increasing exposure, providing insight into the initial degradation mechanism of HOIPs. The electronic structure evolution of the surface was also monitored via ultraviolet photoemission spectroscopy (UPS), evidencing an increased bandgap state density following water vapor exposure, which is attributed to surface defect formation due to lattice swelling. This study will help to inform the surface engineering and designs of future perovskite-based optoelectronic devices.

Exchange-correlation functional's impact on structural, electronic, and optical properties of (N2H5)PbI3 perovskite

One of the most popular multifunctional materials in optoelectronic research domains is organometallic perovskites. In this research, DFT calculation on Hydrazinium Lead Iodide (N₂H₅PbI_3, HAPI) perovskite with orthorhombic phase has been studied with distinct exchange-correlation functionals. HAPI showed a slight structural deformation using the LDA CAPZ functionals, revealing the minimum total energy. A very slight change in Mulliken and Hirshfeld charges of each element was observed due to the variation of functionals. The GGA calculations resulted in a perfect orthorhombic phase of HAPI, whereas LDA functional showed slight deformation from the orthorhombic phase. The band gaps of 1.644, 1.633, 1.618, and 1.650 eV were obtained using GGA (PBE, PBEsol, PW91) and LDA (CAPZ) functionals, respectively. HAPI showed a high absorption coefficient of 10⁴ cm^-1 order with strong absorption of high energy visible wavelength. A maximum refractive index of 2.8 was observed in the visible wavelength region and a high optical conductivity of over 10¹⁵ s^-1 suggests that HAPI can be a potential material for numerous optoelectronic research.

Highly efficient p-i-n perovskite solar cells that endure temperature variations

Daily temperature variations induce phase transitions and lattice strains in halide perovskites, challenging their stability in solar cells. We stabilized the perovskite black phase and improved solar cell performance using the ordered dipolar structure of β-poly(1,1-difluoroethylene) to control perovskite film crystallization and energy alignment. We demonstrated p-i-n perovskite solar cells with a record power conversion efficiency of 24.6% over 18 square millimeters and 23.1% over 1 square centimeter, which retained 96 and 88% of the efficiency after 1000 hours of 1-sun maximum power point tracking at 25° and 75°C, respectively. Devices under rapid thermal cycling between -60° and +80°C showed no sign of fatigue, demonstrating the impact of the ordered dipolar structure on the operational stability of perovskite solar cells.

Zwitterions in 3D Perovskites: Organosulfide-Halide Perovskites

Although sulfide perovskites usually require high-temperature syntheses, we demonstrate that organosulfides can be used in the milder syntheses of halide perovskites. The zwitterionic organosulfide, cysteamine (CYS; ⁺NH₃(CH₂)₂S^-), serves as both the X^- site and A⁺ site in the ABX₃ halide perovskites, yielding the first examples of 3D organosulfide-halide perovskites: (CYS)PbX₂ (X^- = Cl^- or Br^-). Notably, the band structures of (CYS)PbX₂ capture the direct bandgaps and dispersive bands of APbX₃ perovskites. The sulfur orbitals compose the top of the valence band in (CYS)PbX₂, affording unusually small direct bandgaps of 2.31 and 2.16 eV for X^- = Cl^- and Br^-, respectively, falling in the ideal range for the top absorber in a perovskite-based tandem solar cell. Measurements of the carrier dynamics in (CYS)PbCl₂ suggest carrier trapping due to defects or lattice distortions. The highly desirable bandgaps, band dispersion, and improved stability of the organosulfide perovskites demonstrated here motivate the continued expansion and exploration of this new family of materials, particularly with respect to extracting photocurrent. Our strategy of combining the A⁺ and X^- sites with zwitterions may offer more members in this family of mixed-anion 3D hybrid perovskites.

Mixology of MA1-x EA x PbI3 Hybrid Perovskites: Phase Transitions, Cation Dynamics, and Photoluminescence

Mixing molecular cations in hybrid lead halide perovskites is a highly effective approach to enhance the stability and performance of optoelectronic devices based on these compounds. In this work, we prepare and study novel mixed 3D methylammonium (MA)-ethylammonium (EA) MA_1-x EA _x PbI₃ (x < 0.4) hybrid perovskites. We use a suite of different techniques to determine the structural phase diagram, cation dynamics, and photoluminescence properties of these compounds. Upon introduction of EA, we observe a gradual lowering of the phase-transition temperatures, indicating stabilization of the cubic phase. For mixing levels higher than 30%, we obtain a complete suppression of the low-temperature phase transition and formation of a new tetragonal phase with a different symmetry. We use broad-band dielectric spectroscopy to study the dielectric response of the mixed compounds in an extensive frequency range, which allows us to distinguish and characterize three distinct dipolar relaxation processes related to the molecular cation dynamics. We observe that mixing increases the rotation barrier of the MA cations and tunes the dielectric permittivity values. For the highest mixing levels, we observe the signatures of the dipolar glass phase formation. Our findings are supported by density functional theory calculations. Our photoluminescence measurements reveal a small change of the band gap upon mixing, indicating the suitability of these compounds for optoelectronic applications.

The Ferroelectric-Ferroelastic Debate about Metal Halide Perovskites

Metal halide perovskites (MHPs) are solution-processed materials with exceptional photoconversion efficiencies that have brought a paradigm shift in photovoltaics. The nature of the peculiar optoelectronic properties underlying such astounding performance is still controversial. The existence of ferroelectricity in MHPs and its alleged impact on photovoltaic activity have fueled an intense debate, in which unanimous consensus is still far from being reached. Here we critically review recent experimental and theoretical results with a two-fold objective: we argue that the occurrence of ferroelectric domains is incompatible with the A-site cation dynamics in MHPs and propose an alternative interpretation of the experiments based on the concept of ferroelasticity. We further underline that ferroic behavior in MHPs would not be relevant at room temperature or higher for the physics of photogenerated charge carriers, since it would be overshadowed by competing effects like polaron formation and ion migration.

A Heat-Liquefiable Solid Precursor for Ambient Growth of Perovskites with High Tunability, Performance and Stability

Halide perovskites are intensively studied for applications in optoelectronic devices because of their outstanding properties and relatively low cost. However, the common precursor solutions for perovskite fabrication are rather unstable in the presence of moisture and oxygen, limiting the large-scale low-cost production of perovskite. Herein, water is used counterintuitively to formulate an ambient stable perovskite precursor, which is peculiar in that it is solid at room temperature but becomes a liquid at 75 °C. The non-fluidity of the precursor stemmed from the water-assisted intermediate fiber assembly, conferring high damp air stability. Yet the heat-liquefiability made the precursor highly processible for perovskite growth, and when guided by polyvinyl pyrrolidone coordination with Pb²⁺ , the perovskite can preferentially grow along the [200] direction, significantly improving the film quality. To demonstrate the utility of the precursor, it has been used to fabricate self-driven halide perovskite photodetectors, which exhibited a low noise current of 2.0 × 10^-14  A Hz^-1/2 , a high specific detectivity up to 1.4 × 10¹³ Jones, and high stability of 20 days of operation with only < 5% external quantum efficiency decay. This type of solid-liquid convertible precursor opens up new opportunities for wider applications of perovskites.

Precise design and preparation of two 3D organic-inorganic perovskite ferroelectrics (1,5-diazabicyclo[3.2.2]nonane)RbX3 (X = Br, I)

Compared with the spherical molecule 1,4-diazoniabicyclo[2.2.2]-octane (2.2.2-dabco), 1,5-diazabicyclo[3.2.2]nonane (3.2.2-dabcn) bears a lower symmetry and larger size. As expected, reactions of 3.2.2-dabcn with rubidium halides gave two 3D molecular ferroelectrics [3.2.2-H₂dabcn]RbX₃ (X = Br for 1; X = I for 2) with T_c at 342 K (1) and 293 K (2).

The Chemistry of the Passivation Mechanism of Perovskite Films with Ionic Liquids

Passivation of perovskite films by ionic liquids (ILs) improves the performance (efficiency and stability) of perovskite solar cells (PSCs). However, the role of ILs in the passivation of perovskite films is not fully understood. Here, we report the reactions of commonly used ILs with the components of perovskites. The reaction of ILs with perovskite precursors (PbI₂ and methylammonium iodide or formamidinium iodide) in a 1:1:1 molar ratio affords one-dimensional (1D) salts composed of the IL cation interspersed along infinite 1D polymeric [PbI₃]^-_n chains. If the IL is applied in excess, the resulting crystal is composed of six cations surrounding a discrete [Pb₃I₁₂]^6- cluster. All the isolated salts were unambiguously characterized by single-crystal X-ray diffraction analysis, which also reveals extensive hydrogen-bonding interactions.

Synergistic Regulation Effect of Nitrate and Calcium Ions for Highly Luminescent and Robust α-CsPbI3 Perovskite

The α-CsPbI₃ nanocrystals (NCs) easily transform into yellow non-perovskite, accompanying with declining photoelectric properties that restricting their practical applications in diverse fields. Herein, the highly luminescent and robust α-CsPbI₃ NCs is achieved through engineering the lattice symmetry of perovskite, enabled by the synergistic effect of NO₃^- ion passivation and Ca²⁺ ion doping. The introduced NO₃^- ions enhance the phase-change energy barrier and the surface steric hindrance, thus promoting the formation of α-CsPbI₃ NCs with hyper-symmetric crystal structure, while the Ca²⁺ ion doping contributes to improving their lattice symmetry by significant regulation of the tolerance factor. As a result, the obtained α-CsPbI₃ NCs display an outstanding photoluminescence quantum yield of 96.6%, together with the reduced defect state density and eminent conductivity. Most importantly, the as-engineered α-CsPbI₃ NCs exhibit excellent stability under ambient conditions for 9 months and UV illumination for 32 h. It displays brilliant thermal stability, maintaining luminous intensity for 15 min under 140 °C, and performing desired durability and reversibility, evidenced by 160 °C cyclic test and 120 °C reversibility test. Given enhanced robustness, the as-engineered α-CsPbI₃ NCs based light-emitting-diode devices are constructed, exhibiting a power efficiency of 105.3 lm W^-1 and the excellent working stability for 18 h.

A-Site Mixing to Adjust the Photovoltaic Performance of a Double-Cation Perovskite: It Is Not Always the Simple Way

Considerable progress has been made in improving the performance of optoelectronic devices based on hybrid organic-inorganic perovskites of the form ABX₃. However, the influences of A-site doping on the structure and dynamics of the inorganic perovskite crystal lattice and, in turn, on the optoelectronic performance of the resulting devices remain poorly understood at an atomic level. This work addresses this issue by combining the results of several experimental characterization methods for three-dimensional MA_1-xDMA_xPbBr₃ perovskite single crystals (MA, methylammonium; DMA, dimethylammonium). The results reveal a two-stage change in lattice with an increase in DMA content, which has completely opposite effects on the optoelectronic performance of the double-cation perovskite. At low DMA concentrations, fast reorientation of incorporated DMA cations strengthens the interaction between MA cations and the lattice without significant lattice distortion, which could suppress lattice fluctuation and thus improve the photovoltaic performance. At high DMA concentrations, the lattice get a severe distortion, leading to poorer photovoltaic performance.

Cyanamide Passivation Enables Robust Elemental Imaging of Metal Halide Perovskites at Atomic Resolution

Lead halide perovskites (LHPs) have attracted a tremendous amount of attention because of their applications in solar cells, lighting, and optoelectronics. However, the atomistic principles underlying their decomposition processes remain in large part obscure, likely due to the lack of precise information about their local structures and composition along regions with dimensions on the angstrom scale, such as crystal interfaces. Aberration-corrected scanning transmission electron microscopy combined with X-ray energy dispersive spectroscopy (EDS) is an ideal tool, in principle, for probing such information. However, atomic-resolution EDS has not been achieved for LHPs because of their instability under electron-beam irradiation. We report the fabrication of CsPbBr₃ nanoplates with high beam stability through an interface-assisted regrowth strategy using cyanamide. The ultrahigh stability of the nanoplates primarily stems from two contributions: defect-healing self-assembly/regrowth processes and surface modulation by strong electron-withdrawing cyanamide molecules. The ultrahigh stability of as-prepared CsPbBr₃ nanoplates enabled atomic-resolution EDS elemental mapping, which revealed atomically and elementally resolved details of the LHP nanostructures at an unprecedented level. While improving the stability of LHPs is critical for device applications, this work illustrates how improving the beam stability of LHPs is essential for addressing fundamental questions on structure-property relations in LHPs.

Hole-Transporting Low-Dimensional Perovskite for Enhancing Photovoltaic Performance

Halide perovskites with low-dimensionalities (2D or quasi-2D) have demonstrated outstanding stabilities compared to their 3D counterparts. Nevertheless, poor charge-transporting abilities of organic components in 2D perovskites lead to relatively low power conversion efficiency (PCE) and thus limit their applications in photovoltaics. Here, we report a novel hole-transporting low-dimensional (HT2D) perovskite, which can form a hole-transporting channel on the top surface of 3D perovskite due to self-assembly effects of metal halide frameworks. This HT2D perovskite can significantly reduce interface trap densities and enhance hole-extracting abilities of a heterojunction region between the 3D perovskite and hole-transporting layer. Furthermore, the posttreatment by HT2D can also reduce the crystal defects of perovskite and improve film morphology. As a result, perovskite solar cells (PSCs) can effectively suppress nonradiative recombination, leading to an increasement on photovoltage to >1.20 V and thus achieving >20% power conversion efficiency and >500 h continuous illumination stability. This work provides a pathway to overcome charge-transporting limitations in low-dimensional perovskites and delivers significant enhancements on performance of PSCs.

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.

Polyhydroxy Ester Stabilized Perovskite for Low Noise and Large Linear Dynamic Range of Self-Powered Photodetectors

Solution-processed perovskites as emerging semiconductors have achieved unprecedented milestones in sensor optoelectric devices. Stability along with the device noise issues are the major obstacle for photodetectors to compete with the traditional devices. Here, we demonstrated that l-ascorbic acid (l-AA) as a polyhydroxy ester can coordinate with the amino group of formamidine cations (FA⁺) through multiple hydrogen bond interactions to stabilize the perovskite, which protect the FA⁺ ions from nucleophile attack and effectively suppress the degradation of FA⁺ ions, improving the perovskite stability and suppressing the device noise to below 0.3 pA Hz^-1/2 with a large linear dynamic range of 239 dB. The dual functions of l-AA enable the perovskite photodetector to have a high detectivity of 10¹² Jones. The self-powered device works with no energy consumption and maintains an undegraded performance over 1200 h of inspection at ambient conditions, which is promising for infrastructure construction, signal sensing, and real-time information delivery.

Development of Halide Perovskite Single Crystal for Radiation Detection Applications

Preface: Recently, low-cost perovskite single crystals have attracted intensive attention due to their excellent optoelectronic properties and improved stability when compared to polycrystalline films for various applications, such as solar cells (Kojima et al., 2009; Lee et al., 2012; Tsai et al., 2016; Sahli et al., 2018), lasers (Gu et al., 2016; Veldhuis et al., 2016), radiation detection (Kim et al., 2017), and so on. The unique optoelectronic properties and low-cost growing processes for large-sized single crystals also make them greatly suitable for radiation detection. In this review, we summarize various synthesis methods of perovskite single crystals and introduced the high radiation detection performance of the perovskite single crystal. The advantages and limitations of halide perovskite single crystals as radiation detector candidates will be discussed in detail, and corresponding future development trends can be expected by overcoming current obstacles (Leijtens et al., 2018; Boyd et al., 2019), such as ion migration (Eames et al., 2015), stability, etc.

MACl-Induced Intermediate Engineering for High-Performance Mixed-Cation Perovskite Solar Cells

Recently, mixed-cation perovskites have been extensively used for high-performance solar cells. Nevertheless, the mixed-cation perovskite based on formamidinium methylammonium lead tri-iodide (FA_xMA_1-xPbI₃) fabricated through the existing methods often suffers from phase stability and trap density. Herein, we demonstrate a facile intermediate engineering approach to improve the quality of the mixed-cation perovskite based on FA_xMA_1-xPbI₃. Varying concentrations of methylammonium chloride (MACl) are used to treat the FA-MA-PbI₃-solvent intermediate. It is noted that MACl has a strong impact on the crystallization kinetics and charge carrier dynamics as well as the defect density of the obtained perovskite. The mixed-cation perovskite treated with 20 mg mL^-1 MACl yields a large grain size, highly uniform morphology, and better crystalline stability. Subsequently, the device with an acquired high-quality mixed-cation perovskite shows a high efficiency of 20.40%, which is obviously higher than that obtained from the traditional nontreated method. Moreover, the device prepared through the developed method could retain over 85% of the initial efficiency after 860 h at room temperature.

Rapid Decoherence Induced by Light Expansion Suppresses Charge Recombination in Mixed Cation Perovskites: Time-Domain ab Initio Analysis

Using time-domain density functional theory combined with nonadiabatic molecular dynamics, we have investigated the effect of light-induced lattice expansion on the nonradiative electron-hole recombination in the mixed-cation perovskite FA_0.75MA_0.25PbI₃. We demonstrate that charge carrier lifetime extends by a factor of 1.5 within 1% lattice expansion; the bandgap grows only by 0.04 eV; the electron-phonon coupling increases by 13%; and the decoherence time shortens by 37%. The small bandgap change has negligible influence on recombination times. Lattice expansion enhances atomic fluctuations that lead to the enhancement of electron-phonon coupling and acceleration of decoherence. By creating several high-frequency phonon modes, the lattice expansion shortens the decoherence time further. As a result, rapid decoherence beats an enhanced electron-phonon coupling, playing the dominant role in suppressing the nonradiative electron-hole recombination. The light-induced lattice expansion or strain effects provide a rational route to improve the perovskite photovoltaic and photoelectronic device performance.

Facile Formation of 2D-3D Heterojunctions on Perovskite Thin Film Surfaces for Efficient Solar Cells

The interfaces between perovskite and charge transport layers greatly impact the device efficiency and stability of perovskite solar cells (PSCs). Inserting an ultrathin wide-band-gap layer between perovskite and hole transport layers (HTLs) has recently been shown as an effective strategy to enhance device performance. Herein, a small amount of an organic halide salt, N,N'-dimethylethylene-1,2-diammonium iodide, is used to create two-dimensional (2D)-three-dimensional (3D) heterojunctions on MAPbI₃ thin film surfaces by facile solution processing. The formation of an ultrathin wide-band-gap 2D perovskite layer on top of 3D MAPbI₃ changes the morphological and photophysical properties of perovskite thin films, effectively reduces the surface defects, and suppresses the charge recombination in the interfaces between perovskite and HTL. As a result, a power conversion efficiency of ∼20.2%, with an open-circuit voltage of 1.14 V, a short-circuit current density of 22.57 mA cm^-2, and a fill factor of 0.78, is achieved for PSCs with enhanced stability.

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.

Cation substitution enables the complete conversion of 1D perovskites to 3D perovskites for photovoltaic application

More versatile and effective strategies are needed to prepare high-quality perovskites and their precursors for promoting the efficiency of perovskite solar cells (PSCs). Herein, the strategy of amine (CH3NH2, MA) gas treatment is successfully exploited to synthesize high-quality MAI precursors and MAPbI3 perovskites. During MA gas treatment, ethylammonium (EA) cations in EAI are completely substituted with the MA cations from MA gas to generate pure MAI. Furthermore, cation substitution induced by MA gas treatment is further extended to convert 1D EAPbI3 perovskites to 3D MAPbI3 perovskites. Compared with traditional perovskites, 1D-to-3D perovskites show a much smoother surface and lower defect density. Consequently, the performance and stability of carbon-based PSCs without hole transport materials are considerably enhanced. Therefore, amine gas treatment has been proved to be a versatile and promising strategy for preparing high-quality perovskites and their precursors.

Efficient Planar Heterojunction FA1- xCs xPbI3 Perovskite Solar Cells with Suppressed Carrier Recombination and Enhanced Open Circuit Voltage via Anion-Exchange Process

Introduction of Cs into FAPbI₃ displayed great potential to stabilize the black perovskite phase by forming FA_{1- x}Cs _xPbI₃, which has been investigated widely based on solution process. During solution processing, the over-rapid intercalating reaction rate between PbI₂ and A cations (FA⁺ and Cs⁺) can bring some undesirable structural transitions. However, in vapor-assisted solution process (VASP), the over-rapid intercalating reaction rate can be reduced effectively. In addition, the formation process can be regulated significantly by the intermediate perovskite phase. In this study, FACl was employed together with FAI to improve the FA_0.9Cs_0.1PbI₃ films by VASP. In the vapor deposition process, the FACl and FAI vapor coreacted with the PbI₂ solid films, preferentially forming the intermediate perovskite phase FA_0.9Cs_0.1PbI _xCl _y. The intermediate perovskite phase FA_0.9Cs_0.1PbI _xCl _y supplied a plenty of seeds for rapid nucleation of perovskite, which prolonged the crystallization time of FA_0.9Cs_0.1PbI₃, and thus, a smooth FA_0.9Cs_0.1PbI₃ film with suppressed nonradiative recombination, prolonged carrier lifetime and decreased trap state density was acquired. Corresponding planar heterojunction perovskite solar cells achieved a champion power conversion efficiency (PCE) of 16.39% with a V_oc of 0.99 V, J_sc of 22.87 mA/cm², and fill factor of 74.82% under reverse scanning. Meanwhile, a hysteresis index of the FACl-10 device was decreased to 0.024 compared with 0.075 of the control device. Moreover, under the condition of nitrogen atmosphere, the normalized PCE of FACl-10 device diminished only 4.9% which was more stable comparing with 31.88% diminishing of the control device after 30 days.

A Eu3+-Eu2+ ion redox shuttle imparts operational durability to Pb-I perovskite solar cells

The components with soft nature in the metal halide perovskite absorber usually generate lead (Pb)0 and iodine (I)0 defects during device fabrication and operation. These defects serve as not only recombination centers to deteriorate device efficiency but also degradation initiators to hamper device lifetimes. We show that the europium ion pair Eu3+-Eu2+ acts as the “redox shuttle” that selectively oxidized Pb0 and reduced I0 defects simultaneously in a cyclical transition. The resultant device achieves a power conversion efficiency (PCE) of 21.52% (certified 20.52%) with substantially improved long-term durability. The devices retained 92% and 89% of the peak PCE under 1-sun continuous illumination or heating at 85°C for 1500 hours and 91% of the original stable PCE after maximum power point tracking for 500 hours, respectively.

Enhanced Photovoltaic Performance and Thermal Stability of CH3NH3PbI3 Perovskite through Lattice Symmetrization

The organic-inorganic lead halide perovskites are attractive materials for photovoltaic application. The most widely studied perovskites based on methyl ammonium organic cation are less likely to form an ideal high-symmetry configuration at room temperature, leading to the appearance of local lattice strain. Herein, this study reports a strategy for the construction of thermally stable cubic perovskites at room temperature through the incorporation of the larger organic cation dimethyl ammonium. Detailed characterization on the single crystals and thin films reveals the formation of cubic phase with the addition of a certain amount of dimethyl ammonium at room temperature. With the presence of dimethyl ammonium, the nonradiative recombination in perovskite is suppressed, showing a longer PL lifetime and hole diffusion length. The more efficient charge extraction leads to an improvement in the photocurrent density, and then the device efficiency from 17.1% to 18.6%, together with an enhanced thermal stability at 85 °C. The influence of incorporating a larger organic cation on the structural configuration, optical properties, charge extraction, as well as the photovoltaic performance is systematically investigated, which offers an alternative way to improve the intrinsic stability of hybrid perovskites.

Efficient air-stable perovskite solar cells with a (FAI)0.46(MAI)0.40(MABr)0.14(PbI2)0.86(PbBr2)0.14 active layer fabricated via a vacuum flash-assisted method under RH > 50%

In this work, we present a new kind of perovskite, (FAI)_0.46(MAI)_0.40(MABr)_0.14(PbI₂)_0.86(PbBr₂)_0.14, the vacuum flash-assisted solution processing (VASP) of which can be carried out under relative humidity (RH) higher than 50% in ambient air. The smooth and highly crystalline perovskite showed a maximum PCE of 18.8% in perovskite solar cells. This kind of perovskite was demonstrated to be of good stability in ambient air. Holes and electrons have larger and more balanced diffusion lengths (643.7/621.9 nm) than those in the MAPbI₃ perovskite (105.0/129.0 nm) according to the PL quenching experiment. The role of incorporating a large amount of MA⁺ cations to stabilize the intermediate phase via VASP under high RH is attributed to their better ability to intercalate into the sharing face of the one-dimensional face-sharing [PbI₆] octahedra, forming the three-dimensional corner-sharing form. Moreover, hole/electron transfer times at the perovskite/Spiro-OMeTAD (PCBM) interfaces (8.90/9.20 ns) were found to be much larger than those in the MAPbI₃ perovskite (0.75/0.40 ns), indicating that there still is enormous potential in further improving the performance of this kind of perovskite solar cell by interfacial engineering.

In this work, we present a new kind of perovskite, (FAI)_0.46(MAI)_0.40(MABr)_0.14(PbI₂)_0.86(PbBr₂)_0.14, the vacuum flash-assisted solution processing (VASP) of which can be carried out under relative humidity (RH) higher than 50% in ambient air.

Dopant-induced localized light absorption in CsPbX3 (X = Cl, Br, I) perovskite quantum dots

Doping is known to play an important role in the optoelectronic properties of semiconducting materials.

Ethylammonium as an alternative cation for efficient perovskite solar cells from first-principles calculations

Mixed-cation lead halide perovskites have emerged as a new class of promising photovoltaic materials for perovskite solar cells. Formamidinium (FA), methylammonium (MA), and Cs cations are widely studied in the field of mixed-cation hybrid halide perovskites. In this work, we have investigated ethylammonium (CH₃CH₂NH₃, EA) as an alternative cation to explore the stabilities and electronic properties of mixed MA_1−xEA_xPbI₃ perovskites. The results indicate that replacing MA with EA is a more effective way to improve the stabilities of the mixed MA_1−xEA_xPbI₃ perovskites except for MA_0.75EA_0.25PbI₃. The band gap of MA_1-xEA_xPbI₃ slightly increases with x from 0.25 to 1.00, which is quite different from the MA–FA mixed-cation perovskites. The results indicate that the c axis distortion of the Pb–I–Pb bond angles can play a greater role in tuning the band gap. Moreover, the mixed MA_1−xEA_xPbI₃ perovskites show comparable absorption abilities in the visible light region to the pure MAPbI₃ structure. We hope that our study will be greatly helpful for further experiments to find more efficient perovskite materials in the future.

Mixed-cation lead halide perovskites have emerged as a new class of promising photovoltaic materials for perovskite solar cells.

Design of an Inorganic Mesoporous Hole-Transporting Layer for Highly Efficient and Stable Inverted Perovskite Solar Cells

The unstable feature of the widely employed organic hole-transporting materials (HTMs) (e.g., spiro-MeOTAD) significantly limits the practical application of perovskite solar cells (PSCs). Therefore, it is desirable to design new structured PSCs with stable HTMs presenting excellent carrier extraction and transfer properties. This work demonstrates a new inverted PSC configuration. The new PSC has a graded band alignment and bilayered inorganic HTMs (i.e., compact NiO_x and mesoporous CuGaO₂ ). In comparison with planar-structured PSCs, the mesoporous CuGaO₂ can effectively extract holes from perovskite due to the increased contact area of the perovskite/HTM. The graded energy alignment constructed in the ultrathin compact NiO_x , mesoporous CuGaO₂ , and perovskite can facilitate carrier transfer and depress charge recombination. As a result, the champion device based on the newly designed mesoscopic PSCs yields a stabilized efficiency of ≈20%, which is considered one of the best results for inverted PSCs with inorganic HTMs. Additionally, the unencapsulated PSC device retains more than 80% of its original efficiency when subjected to thermal aging at 85 °C for 1000 h in a nitrogen atmosphere, thus demonstrating superior thermal stability of the device. This study may pave a new avenue to rational design of highly efficient and stable PSCs.

Impedance Analysis of Thin Films of Organic-Inorganic Perovskites CH3NH3PbI3 with Control of Microstructure

The effect of starting reagents (PbI2:{CH₃NH₃I + CH₃NH₃Cl}) with different ratios in raw solutions on the microstructure of films of organic-inorganic perovskites CH₃NH₃PbI_3-xCl_x, as well as on the electrical properties, has been investigated. It was found that the crystallinity is increased sharply when the ratio of the starting reagents increases from 1:1 to 1:2 and is changed slightly with a further increase of ratio to 1:3. It is shown that when the ratio of starting reagents varies, the morphology of the films changes; at a ratio of 1:1, the films consist of needle-like particles, and when the ratio is increased, particles become roundish and then faceted. Additionally, the average grain size is decreased. Complex impedance curves and I-V curves have been investigated for samples with different ratios of the starting reagents. With increasing this ratio, the concentration of charge carriers remains unchanged, the mobility of charge carriers decreases, and conductivity passes through a maximum at a ratio of 1:2. The electrical properties of film are the highest at the ratio of starting reagents 1:2 due to the effect of two competing factors: the growth of crystallinity and the decrease of grain size.

Heterojunction Engineering for High Efficiency Cesium Formamidinium Double-Cation Lead Halide Perovskite Solar Cells

It is essential to minimize the interfacial trap states and improve the carrier collection for high efficiency perovskite solar cells (PSCs). Herein, we present a facile method to construct a p-type graded heterojunction (GHJ) in normal PSCs by deploying a gradient distribution of hole-transporting materials (poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine], PTAA, in this case) in the shallow perovskite layer. The formation of the GHJ structure facilitates charge transfer and collection, and passivates interfacial trap states, thus delivering a power conversion efficiency (PCE) of 20.05 % along with steady output efficiency of 19.3 %, which is among the highest efficiencies for cesium formamidinium (Cs-FA) lead halide PSCs. Moreover, the unencapsulated devices based on these (Cs-FA) lead halide perovskites show excellent long-term stability; more than 95 % of their initial PCE can be retained after 1440 h storage under ambient conditions. This study may provide an effective strategy to fabricate high-efficiency PSCs with great stability.

Cations substitution tuning phase stability in hybrid perovskite single crystals by strain relaxation

Methylammonium (MA) and formamidinium (FA) are two typical A site cations in lead halide perovskites. Instability of synthesised crystals will degrade the properties of the photoelectrical device constructed by such perovskites. MAPbI₃ and FAPbI₃ in cubic crystal structure have been demonstrated to be the most stable at room temperature. Herein we synthesised MA(EA)PbI₃ and FA(MA)PbI₃ single crystals using an inverse-temperature crystallization strategy by partially substituting the methylammonium (MA) with ethylammonium (EA) and the formamidinium (FA) with methylammonium (MA) respectively. The XRD results show that both crystal structures are cubic, which means organic incorporation can stabilize the crystal structure of lead halide perovskites. The lattice distortion decrease and strain relaxation in single crystals were considered to be the reason leading to higher stability. The single crystals of MA(EA)PbI₃ and FA(MA)PbI₃ with low trap state density exhibit excellent light-absorbing properties, indicating their potential applications in photoelectric devices.

Cations size induced phase tuning in hybrid perovskite single crystals: interplay of lattice distortion and strain relaxation.

Highly (100)-oriented CH3NH3PbI3(Cl) perovskite solar cells prepared with NH4Cl using an air blow method

The effects of adding NH₄Cl via an air blow process on CH₃NH₃PbI₃(Cl) perovskite solar cells were investigated. CH₃NH₃PbI₃(Cl) solar cells containing various amounts of NH₄Cl were fabricated by spin-coating. The microstructures of the resulting cells were investigated by X-ray diffraction, optical microscopy, and scanning electron microscopy. The current density–voltage characteristics of the cell were improved by adding an appropriate amount of NH₄Cl and air blowing, which increased the photoconversion efficiency to 14%. Microstructure analysis indicated that the perovskite layer contained dense grains with strong (100) orientation, as a result of NH₄Cl addition and air blowing. The ratio of the (100)/(210) reflection intensities for the perovskite crystals was 2000 times higher than that of randomly oriented grains. The devices were stable when stored in ambient air for two weeks.

Perovskite solar cells with dense grains with strong (100) orientation were developed by adding NH₄Cl and air blowing.

Surface-enhanced Raman scattering on organic–inorganic hybrid perovskites

For the first time SERS on organic–inorganic hybrid perovskites is explored. The enhancement mechanism is discussed according to charge transfer.