Highly Efficient All-Inorganic Planar Heterojunction Perovskite Solar Cells Produced by Thermal Coevaporation of CsI and PbI2

In Situ Annealing Effect on Thermally Co-Evaporated CsPbI2Br Thin Films Studied via Spectroscopic Ellipsometry

All-inorganic cesium lead halide perovskites possess excellent thermal stability, a feature that renders them highly favorable for optoelectronic applications with an elevated thermal budget. Employing a coevaporation approach for their deposition holds promise for manufacturing at an industrial level, owing to improvements in device scalability and reproducibility. For unlocking the full potential of vacuum-evaporated perovskite thin films, it is crucial to delve deeper into their crystallization process, which, as a solid-state reaction, has been less investigated compared to the crystallization process of, most commonly used, solution-based methods. In this work, we employ spectroscopic ellipsometry, a nondestructive, high speed, and high accuracy characterization method, to study the real time annealing effect on thermally coevaporated CsPbI2Br thin films in a temperature range between 25 and 300 °C. We achieve this by developing a singular dynamic model that can be fitted in real time as a function of temperature, providing insights into how thermal annealing influences the perovskite film's morphology and optical constants. Based on the latter, we derive the temperature dependence of the thermo-optic coefficient and Urbach energy as well as analyze the interband transition energies via critical point analysis. We demonstrate that the γ- to β-phase transition can be identified through a pronounced shift in the bandgap energy, whereas the β- to α-phase transition can be discerned by a sharp increase in the film's roughness. We corroborate the obtained fit results with additional in- and ex situ measurements, such as in situ grazing incidence wide-angle X-ray scattering, atomic force microscopy, reflectance/transmittance, and profilometry.

Moisture is not always bad: H2O accelerates the conversion of DMAPbI3 intermediate to CsPbI3 for boosting the efficiency of carbon-based perovskite solar cells to over 16

Inorganic CsPbI3 perovskite has exhibited great application potential in perovskite solar cells (PSCs) due to its suitable optical bandgap and high chemical stability. However, the perovskite phases of CsPbI3 are not stable at room temperature, where they transition to non-perovskite phases. Humidity or water has been thought to be the primary factor inducing this phase transition, which should be avoided throughout the procedure of film and device processing. Surprisingly, the present study indicates that preparing a precursor solution in humid air is beneficial to the growth of high-quality CsPbI3 perovskite to enhance device performance. It is demonstrated that the incorporation of H2O in the precursor solution from humid air or by intentional addition significantly changes the composition of coordination compounds and increases the amount of low iodine coordination complexes. As a result, the crystallization of dimethylammonium lead iodide (DMAPbI3) intermediate is suppressed well, which accelerates its subsequent conversion to CsPbI3 perovskite. Consequently, an oriented CsPbI3 perovskite film with improved crystallinity and lower defect density is obtained. Most importantly, carbon-based PSCs (C-PSCs) based on the CsPbI3 perovskite film achieve an efficiency of 16.05%, a new record for inorganic C-PSCs.

Evaluating Pb-based and Pb-free Halide Perovskites for Solar-Cell Applications: A Simulation Study

Metal halide Pb-based and Pb-free perovskite crystal structures are an essential class of optoelectronic materials due to their significant optoelectronic properties, optical absorption and tuneable emission spectrum properties. However, the most efficient optoelectronic devices were based on the Pb as a monovalent cation, but its toxicity is a significant hurdle for commercial device applications. Thus, replacing the toxic Pb with Pb-free alternatives (such as tin (Sn)) for diverse photovoltaic and optoelectronic applications is essential. Moreover, replacing the volatile methylammonium (MA) with cesium (Cs) leads to the development of an efficient perovskite absorber layer with improved optical & thermal stability and stabilized photoconversion efficiency. This paper discusses the correlation between the experimental and theoretical work for the Pb-based and Pb-free perovskites synthesised using the hot-injection method at different temperatures. Here, simulation is also carried out using the help of SCAPS-1D software to study the effect of various parameters of CsSnI3 and CsPbI3 layers on solar cell performance. This experimental and theoretical comparative study of the Hot-injection method synthesised CsPbI3 and CsSnI3 perovskites is rarely investigated for optoelectronic applications.

Thermally Evaporated Metal Halide Perovskites and Their Analogues: Film Fabrication, Applications and Beyond

Metal halide perovskites emerge as promising semiconductors for optoelectronic devices due to ease of fabrication, attractive photophysical properties, their low cost, highly tunable material properties, and high performance. High-quality thin films of metal halide perovskites are the basis of most of these applications including solar cells, light-emitting diodes, photodetectors, and electronic memristors. A typical fabrication method for perovskite thin films is the solution method, which has several limitations in device reproducibility, adverse environmental impact, and utilization of raw materials. Thermal evaporation holds great promise in addressing these bottlenecks in fabricating high-quality halide perovskite thin films. It also has high compatibility with mass-production platforms that are well-established in industries. This review first introduces the basics of the thermal evaporation method with a particular focus on the critical parameters influencing the thin film deposition. The research progress of the fabrication of metal halide perovskite thin films is further summarized by different thermal evaporation approaches and their applications in solar cells and other optoelectronic devices. Finally, research challenges and future opportunities for both fundamental research and commercialization are discussed.

First-Principles Investigation into the Interaction of H2O with α-CsPbI3 and the Intrinsic Defects within It

CsPbI3 possesses three photoactive black phases (α, β, and γ) with perovskite structures and a non-photoactive yellow phase (δ) without a perovskite structure. Among these, α-CsPbI3 exhibits the best performance. However, it only exists at high temperatures and it tends to transform into the δ phase at room temperature, especially in humid environments. Therefore, the phase stability of CsPbI3, especially in humid environments, is the main obstacle to its further development. In this study, we studied the interaction of H2O with α-CsPbI3 and the intrinsic defects within it. It was found that the adsorption energy in the bulk is higher than that on the surface (-1.26 eV in the bulk in comparison with -0.60 eV on the surface); thus, H2O is expected to have a tendency to diffuse into the bulk once it adsorbs on the surface. Moreover, the intrinsic vacancy of VPb0 in the bulk phase can greatly promote H2O insertion due to the rearrangement of two I atoms in the two PbI6 octahedrons nearest to VPb0 and the resultant breaking of the Pb-I bond, which could promote the phase transition of α-CsPbI3 in a humid environment. Moreover, H2O adsorption onto VI+1 contributes to a further distortion in the vicinity of VI+1, which is expected to enhance the effect of VI+1 on the phase transition of α-CsPbI3. Clarifying the interaction of H2O with α-CsPbI3 and the intrinsic defects within it may provide guidance for further improvements in the stability of α-CsPbI3, especially in humid environments.

Impact of Ce doping on the optoelectronic and structural properties of a CsPbIBr2 perovskite solar cell

This paper provides a detailed analysis of pure CsPbIBr2 and 4% Ce-doped CsPbIBr2 perovskite films, emphasizing their structural, optical and photovoltaic properties. X-ray diffraction confirms a predominant cubic perovskite phase in both samples, with Ce doping leading to the increased crystal size (21 nm to 32 nm). UV-vis spectroscopy reveals a reduced bandgap energy (2.2 eV to 2.1 eV) with Ce doping. Dielectric constant analysis indicates the enhanced permittivity of the Ce-doped sample, crucial for solar-cell light trapping. Energy band structure analysis demonstrates improved photovoltaic cell performance with Ce doping, yielding higher open-circuit voltage, short-circuit current, and efficiency (9.71%) compared to pure CsPbIBr2 (8.02%). Ce doping mitigates electron-hole recombination, enhancing cell stability, electron affinity, and power output. This research underscores the potential of cost-effective, efficient, and stable CsPbIBr2 perovskite solar cells.

Strengthened Surface Modification for High-Performance Inorganic Perovskite Solar Cells with 21.3% Efficiency

Metal halide inorganic perovskites show excellent thermal stability compared to organic-inorganic perovskites. However, the performance of inorganic perovskite solar cells (PSCs) is far from theoretical values, together with unsatisfactory stability, mainly due to the poor interfacial properties. In this work, a facial but effective method is reported to realize high-performance inorganic PSCs by post-modifying the perovskite surface with 2-thiophene ethylamine (TEA). It is found that amine group from TEA can favorably interact with the undercoordinated Pb2+ via Lewis acid-based coordination, while thiophene ring with electron-rich sulfur assists such interaction by functioning as an electron donor. The synergetic interaction allows TEA to passivate perovskite film defects more efficiently, as compared to phenethylamine (PEA) with less electron-donating ability. Moreover, perovskite valence band is slightly upward shift to match with hole transport material and facilitate hole transfer. These combinations result in a reduced non-radiative charge recombination and improved charge carrier lifetime. Consequently, PSCs with TEA modification shows a drastic improvement of VOC by 54 mV, yielding a champion PCE of 21.3%, much higher than the control PSCs (19.3%), along with improved ambient stability. This work demonstrates that surface modifier with an electron-rich moiety is critical for achieving efficient and stable inorganic PSCs.

Optimization and Performance Analysis of a TiO2/i-CH3NH3SnBr3/CsPbI3/Al(BSF) Heterojunction Perovskite Solar Cell for Enhanced Efficiency

This paper reports the simulation and optimization of heterojunction perovskite solar cells (PSCs) with a proposed structure of TiO2/i-CH3NH3SnBr3/CsPbI3/Al(BSF) using SCAPS-1D software. The purpose of this study is to investigate the performance of the PSC with CsPbI3 perovskite active layers and i-CH3NH3SnBr3 as the permeable layer. Therefore, the thicknesses of the layers of the heterojunction perovskite are modified in order to find a better conversion efficiency of the solar cell, where the latter's performance is improved by optimizing the absorber's thickness, which is found to be 1 μm, with a permeable layer of 15 μm. The device efficiency of the i-CH3NH3SnBr3/CsPbI3 heterojunction is improved to 38.98%, and optimized parameters are Voc = 1.21 mV, Jsc = 35.63 mA/cm2, and FF = 89.84%. The acceptor concentration (Na), donor concentration (Nd), defect density, and series and shunt resistances are also investigated.

Recent progress in the stabilization of low band-gap black-phase iodide perovskite solar cells

All-inorganic and organic-inorganic hybrid perovskite solar cells (PSCs) have taken a quantum leap owing to their high performance and low-cost solution processability. Their efficiency has been dramatically increased up to ∼26%, matching the conventional inorganic photovoltaics like monocrystalline Si (26.1%), polycrystalline Si (21.6%), CdTe (22.1%), and CIGS (22.3%). Such outstanding performance has been achieved due to their excellent optoelectronic properties, such as a direct bandgap in the visible region, a very high absorption coefficient, a long charge-carrier diffusion length, and ambipolar carrier transport characteristics. FAPbI3 (FA = formamidinium) and CsPbI3 perovskites among the pool of perovskites are recommended for solar cell applications because they meet all the requirements for photovoltaic applications. However, the fundamental problem of these perovskites is that their photoactive black phase is highly unstable under ambient conditions due to small and large sizes of Cs+ and FA+ ions, respectively. The instability of the black phase of these perovskites hinders their applications in photovoltaic devices as a high-quality light absorber layer. Several approaches have been employed to prevent the formation of the photo-inactive yellow phase or to enhance the stability of the black phase of perovskites, such as dimensional and compositional engineering, the addition of external additives, and dimensional engineering. This perspective summarizes the various methods for stabilizing the black phase of CsPbI3 and FAPbI3 perovskites at room temperature as well as their application in photovoltaic devices.

Synergistic Halide- and Ligand-Exchanges of All-Inorganic Perovskite Nanocrystals for Near-Unity and Spectrally Stable Red Emission

All-inorganic perovskite nanocrystals (NCs) of CsPbX3 (X = Cl, Br, I) are promising for displays due to wide color gamut, narrow emission bandwidth, and high photoluminescence quantum yield (PLQY). However, pure red perovskite NCs prepared by mixing halide ions often result in defects and spectral instabilities. We demonstrate a method to prepare stable pure red emission and high-PLQY-mixed-halide perovskite NCs through simultaneous halide-exchange and ligand-exchange. CsPbBr3 NCs with surface organic ligands are first synthesized using the ligand-assisted reprecipitation (LARP) method, and then ZnI2 is introduced for anion exchange to transform CsPbBr3 to CsPbBrxI3-x NCs. ZnI2 not only provides iodine ions but also acts as an inorganic ligand to passivate surface defects and prevent ion migration, suppressing non-radiative losses and halide segregation. The luminescence properties of CsPbBrxI3-x NCs depend on the ZnI2 content. By regulating the ZnI2 exchange process, red CsPbBrxI3-x NCs with organic/inorganic hybrid ligands achieve near-unity PLQY with a stable emission peak at 640 nm. The CsPbBrxI3-x NCs can be combined with green CsPbBr3 NCs to construct white light-emitting diodes with high-color gamut. Our work presents a facile ion exchange strategy for preparing spectrally stable mixed-halide perovskite NCs with high PLQY, approaching the efficiency limit for display or lighting applications.

Advances in the Application of Perovskite Materials

Nowadays, the soar of photovoltaic performance of perovskite solar cells has set off a fever in the study of metal halide perovskite materials. The excellent optoelectronic properties and defect tolerance feature allow metal halide perovskite to be employed in a wide variety of applications. This article provides a holistic review over the current progress and future prospects of metal halide perovskite materials in representative promising applications, including traditional optoelectronic devices (solar cells, light-emitting diodes, photodetectors, lasers), and cutting-edge technologies in terms of neuromorphic devices (artificial synapses and memristors) and pressure-induced emission. This review highlights the fundamentals, the current progress and the remaining challenges for each application, aiming to provide a comprehensive overview of the development status and a navigation of future research for metal halide perovskite materials and devices.

Synchronous Modulation of Energy Level Gradient and Defects for High-Efficiency HTL-Free Carbon-Based All-Inorganic Perovskite Solar Cells

In order to improve the thermal stability of perovskite solar cells (PSCs) and reduce production costs, hole transport layer (HTL)-free carbon-based CsPbI₃ PSCs (C-PSCs) have attracted the attention of researchers. However, the power conversion efficiency (PCE) of HTL-free CsPbI₃ C-PSCs is still lower than that of PSCs with HTL/ metal electrodes. This is because the direct contact between the carbon electrode and the perovskite layer has a higher requirement on the crystal quality of perovskite layer and matched energy level at perovskite/carbon interface. Herein, the acyl chloride group and its derivative trichloroacetyl chloride are used to passivate CsPbI₃ C-PSCs for the first time. The results show that the carbonyl group of trichloroacetyl chloride can effectively passivate the uncoordinated Pb²⁺ ions in perovskite. At the same time, leaving group Cl^- ions can increase the grain size of perovskite and improve the crystallization quality of perovskite layer. In addition, the trichloroacetyl chloride tends to generate cesium chloride acetate, which acts as an electron blocking layer, reduces charge recombination, promotes gradient energy level arrangement, and effectively improves the separation and extraction ability of carriers. The PCE of CsPbI₃ HTL-free C-PSCs is successfully increased from 13.40% to 14.82%.

Electronic, Optical, Thermoelectric and Elastic Properties of RbxCs1-xPbBr3 Perovskite

Inorganic halide perovskites of the type AMX₃, where A is an inorganic cation, M is a metal cation, and X is a halide anion, have attracted attention for optoelectronics applications due to their better optical and electronic properties, and stability, under a moist and elevated temperature environment. In this contribution, the electronic, optical, thermoelectric, and elastic properties of cesium lead bromide, CsPbBr₃, and Rb-doped CsPbBr₃, were evaluated using the density functional theory (DFT). The generalized gradient approximation (GGA) in the scheme of Perdew, Burke, and Ernzerhof (PBE) was employed for the exchange-correlation potential. The calculated value of the lattice parameter is in agreement with the available experimental and theoretical results. According to the electronic property results, as the doping content increases, so does the energy bandgap, which decreases after doping 0.75. These compounds undergo a direct band gap and present an energies gap values of about 1.70 eV (x = 0), 3.76 eV (x = 0.75), and 1.71 eV (x = 1). The optical properties, such as the real and imaginary parts of the dielectric function, the absorption coefficient, optical conductivity, refractive index, and extinction coefficient, were studied. The thermoelectric results show that after raising the temperature to 800 K, the thermal and electrical conductivities of the compound RbxCs_1-xPbBr₃ increases (x = 0, 0.25, 0.50 and 1). Rb_0.75Cs_0.25PbBr₃ (x = 0.75), which has a large band gap, can work well for applications in the ultraviolet region of the spectrum, such as UV detectors, are potential candidates for solar cells; whereas, CsPbBr₃ (x = 0) and RbPbBr₃ (x = 1), have a narrow and direct band gap and outstanding absorption power in the visible ultraviolet energy range.

Fabrication of High-Quality CsPbI3 Perovskite Films with Phosphorus Pentachloride Additive for Highly Stable Solar Cells

Fully inorganic perovskite cesium lead triiodide (CsPbI₃ ) has garnered much attention from researcher for photovoltaic application because of its excellent thermal stability compared with the inorganic-organic hybrid counterparts, along with the potential to serve as the top cell in tandem devices with silicon solar cell. However, the active α-phase cubic CsPbI₃ spontaneously tends to transform into the non-perovskite δ-CsPbI₃ when subjected to ambient condition. This work proposes an effective method to fabricate high-quality and stable α-phase cubic CsPbI₃ films by introducing phosphorus pentachloride (PCl₅ ) as an additive. PCl₅ acts as colloidal binder for modulating crystallization dynamics of perovskites, resulting in high-quality film and a significantly suppressed phase transition. Finally, highly stable CsPbI₃ perovskite solar cells can be achieved with a power conversion efficiency up to 17.85 %, and a long-term stability in N₂ filled glove box.

Dimethyl sulfoxide: a promising solvent for inorganic CsPbI3 perovskite

Inorganic CsPbI₃ perovskite is an important photovoltaic material due to its suitable band gap and high chemical stability. However, it is a challenge to grow high-quality CsPbI₃ perovskite because the stability of perovskite phase is low and is sensitive to solvent. So far, most of CsPbI₃ perovskites in high-performance perovskite solar cells (PSCs) were prepared from N,N-dimethylformamide, a highly toxic solvent, and no successful case has been reported for dimethyl sulfoxide (DMSO), which is environmentally-friendly with considerably higher complexation capability. Herein, we reveal that forming DMSO-based adduct is the main cause for limiting the quality of CsPbI₃ perovskite from DMSO-based solutions, which would inhibit the formation of DMAPbI₃ (DMA = dimethylammonium, (CH₃)₂NH₂⁺) intermediate. Then, by introducing a vacuum treatment, DMSO molecules could be efficiently extracted from the adduct to induce the formation of DMAPbI₃ intermediate. After annealing, the intermediate is transitioned to the CsPbI₃ perovskite with enhanced crystallinity, high orientation, low defect density, and high uniformity. By using the CsPbI₃ perovskite as a light absorber, the PSCs based on carbon electrode (C-PSCs) achieve an efficiency of 16.7%, a new record for inorganic C-PSCs.

Trimethylammonium Sn(IV) and Pb(IV) Chlorometalate Complexes with Incorporated Dichlorine

Supramolecular dichloro-chlorostannate(IV) and -plumbate(IV) complexes (Me3NH)2{[MCl6]Cl2} (M = Sn (1), Pb (2)) feature dichlorine units incorporated into a halometalate framework. Both compounds were characterized by X-ray diffractometry and Raman spectroscopy.

Effect of Various Electron and Hole Transport Layers on the Performance of CsPbI3-Based Perovskite Solar Cells: A Numerical Investigation in DFT, SCAPS-1D, and wxAMPS Frameworks

CsPbI₃ has recently received tremendous attention as a possible absorber of perovskite solar cells (PSCs). However, CsPbI₃-based PSCs have yet to achieve the high performance of the hybrid PSCs. In this work, we performed a density functional theory (DFT) study using the Cambridge Serial Total Energy Package (CASTEP) code for the cubic CsPbI₃ absorber to compare and evaluate its structural, electronic, and optical properties. The calculated electronic band gap (E _g) using the GGA-PBE approach of CASTEP was 1.483 eV for this CsPbI₃ absorber. Moreover, the computed density of states (DOS) exhibited the dominant contribution from the Pb-5d orbital, and most charges also accumulated for the Pb atom as seen from the electronic charge density map. Fermi surface calculation showed multiband character, and optical properties were computed to investigate the optical response of CsPbI₃. Furthermore, we used IGZO, SnO₂, WS₂, CeO₂, PCBM, TiO₂, ZnO, and C₆₀ as the electron transport layers (ETLs) and Cu₂O, CuSCN, CuSbS₂, Spiro-MeOTAD, V₂O₅, CBTS, CFTS, P3HT, PEDOT:PSS, NiO, CuO, and CuI as the hole transport layers (HTLs) to identify the best HTL/CsPbI₃/ETL combinations using the SCAPS-1D solar cell simulation software. Among 96 device structures, the best-optimized device structure, ITO/TiO₂/CsPbI₃/CBTS/Au, was identified, which exhibited an efficiency of 17.9%. The effect of the absorber and ETL thickness, series resistance, shunt resistance, and operating temperature was also evaluated for the six best devices along with their corresponding generation rate, recombination rate, capacitance-voltage, current density-voltage, and quantum efficiency characteristics. The obtained results from SCAPS-1D were also compared with wxAMPS simulation results.

Surface Versus Bulk State Transitions in Inkjet-Printed All-Inorganic Perovskite Quantum Dot Films

The anion exchange of the halides, Br and I, is demonstrated through the direct mixing of two pure perovskite quantum dot solutions, CsPbBr3 and CsPbI3, and is shown to be both facile and result in a completely alloyed single phase mixed halide perovskite. Anion exchange is also observed in an interlayer printing method utilizing the pure, unalloyed perovskite solutions and a commercial inkjet printer. The halide exchange was confirmed by optical absorption spectroscopy, photoluminescent spectroscopy, X-ray diffraction, and X-ray photoemission spectroscopy characterization and indicates that alloying is thermodynamically favorable, while the formation of a clustered alloy is not favored. Additionally, a surface-to-bulk photoemission core level transition is observed for the Cs 4d photoemission feature, which indicates that the electronic structure of the surface is different from the bulk. Time resolved photoluminescence spectroscopy indicates the presence of multiple excitonic decay features, which is argued to originate from states residing at surface and bulk environments.

Encapsulation of Dual-Passivated Perovskite Quantum Dots for Bio-Imaging

Due to their marvelous electrical and optical properties, perovskite nanocrystals have reached remarkable landmarks in solar cells, light-emitting diodes, and photodetectors. However, the intrinsic instability of ionic perovskites, which would undergo an undesirable phase transition and decompose rapidly in ambient humidity, limits their long-term practical deployment. To address this challenge, halogenated trimethoxysilane as the passivation additive is chosen, which utilizes simultaneous halide and silica passivation to enhance the stability of perovskite nanoparticles via a dual-passivation mechanism. The processable nanoparticles show high photoluminescence quantum yield, tunable fluorescence wavelength, and excellent resistance against air and water, highlighting great potential as green to deep-red bio-labels after further phospholipid encapsulation. This work demonstrates that the dual-passivation mechanism could be used to maintain the long-term stability of ionic crystals, which sheds light on the opportunity of halide perovskite nanoparticles for usage in a humid environment.

Design of a CH3NH3PbI3/CsPbI3-based bilayer solar cell using device simulation

With lead-based light harvesters, perovskite solar cells (PSCs) have an efficiency of approximately 25.5%, making them a viable photovoltaic technology. The selection of the absorber materials for PSC in this work are (i) Cesium lead iodide (CsPbI₃) with a 1.73eV bandgap as the first absorber layer, this halide imparts higher stability to perovskite solar cells (ii) CH₃NH₃PbI₃ (MAPbI₃) with a bandgap of 1.55eV is selected as the second absorber layer as this material provides better efficiency to the perovskite solar cells. SCAPS-1D simulation software is used to perform an efficiency analysis of perovskite-perovskite CsPbI₃/MAPbI₃ bilayer solar cell. For efficiency optimization of the perovskite-perovskite bilayer solar cell, we have tried to calibrate seven parameters of the cell. These parameters are (i & ii) selection of the electron and hole transport material (iii, iv & v) variation in the: defect density of bulk material, doping concentration and the thickness of absorber layers, (vi) variation in work function of front electrode (vii) varying interface defect density. After optimization, the efficiency (η) of bilayer PSC is estimated to be 33.54%. The other PV parameters observed in optimal efficiency condition are open-circuit voltage (V_OC) = 1.34V, short-circuit current density (J_SC) = 27.45 mA/cm² and fill factor (FF) = 90.49%. The CsPbI₃/MAPbI₃ bilayer perovskite solar cell efficiency is roughly double the efficiency of single junction CsPbI₃ or MAPbI₃ PSC. Our analysis observed that the variation in the doping and defect density of narrow bandgap material profoundly impacts the efficiency of perovskite-perovskite bilayer solar cells compared to the wide bandgap material.

Annealing Engineering in the Growth of Perovskite Grains

Perovskite solar cells (PSCs) are a promising and fast-growing type of photovoltaic cell due to their low cost and high conversion efficiency. The high efficiency of PSCs is closely related to the quality of the photosensitive layer, and the high-quality light absorbing layer depends on the growth condition of the crystals. In the formation of high-quality crystals, annealing is an indispensable and crucial part, which serves to evaporate the solvent and drive the crystallization of the film. Various annealing methods have different effects on the promotion of the film growth process owing to the way they work. Here, this review will present a discussion of the growth puzzles and quality of perovskite crystals under different driving forces, and then explain the relationship between the annealing driving force and crystal growth. We divided the main current annealing methods into physical and chemical annealing, which has never been summarized before. The main annealing methods currently reported for crystal growth are summarized to visualize the impact of annealing design strategies on photovoltaic performance, while the growth mechanisms of thin films under multiple annealing methods are also discussed. Finally, we suggest future perspectives and trends in the industrial fabrication of PSCs in the future. The review promises industrial manufacturing of annealed PSCs. The review is expected to facilitate the industrial fabrication of PSCs.

All-Inorganic Perovskite Solar Cells: Recent Advancements and Challenges

Organic–inorganic metal-halide-based hybrid perovskite solar cells (SCs) have attracted a great deal of attention from researchers around the globe with their certified power conversion efficiencies (PCEs) having now increased to 25.2%. Nevertheless, organic–inorganic hybrid halide perovskite SCs suffer the serious drawback of instability with respect to moisture and heat. However, all-inorganic perovskite SCs have emerged as promising candidates to tackle the thermal instability problem. Since the introduction of all-inorganic perovskite materials to the field of perovskite photovoltaics in 2014, a plethora of research articles has been published focusing on this research topic. The PCE of all-inorganic PSCs has climbed to a record 18.4% and research is underway to enhance this. In this review, I survey the gradual progress of all-inorganic perovskites, their material design, the fabrication of high-quality perovskite films, energetics, major challenges and schemes opening new horizons toward commercialization. Furthermore, techniques to stabilize cubically phased low-bandgap inorganic perovskites are highlighted, as this is an indispensable requirement for stable and highly efficient SCs. In addition, I explain the various energy loss mechanisms at the interface and in the bulk of perovskite and charge-selective layers, and recap previously published reports on the curtailment of charge-carrier recombination losses.

Recent Advances in Colloidal Quantum Dots or Perovskite Quantum Dots as a Luminescent Downshifting Layer Embedded on Solar Cells

The solar cell has a poor spectral response in the UV region, which affects its power conversion efficiency (PCE). The utilization of a luminescent downshifting (LDS) layer has been suggested to improve the spectral response of the photovoltaics in the short wavelength region through photoluminescence (PL) conversion and antireflection effects, which then enhance the PCE of the solar cell. Recently, colloidal quantum dots (CQDs) or perovskite quantum dots (PQDs) have been gaining prime importance as an LDS material due to their eminent optical characteristics, such as their wide absorption band, adjustable visible emission, short PL lifetime, and near-unity quantum yields. However, the instability of QDs that occurs under certain air, heat, and moisture conditions limits its commercialization. Thus, in this review, we will focus on the physical and optical characteristics of QDs. Further, we will discuss different synthesis approaches and the stability issues of QDs. Different approaches to improve the stability of QDs will be discussed in detail alongside the recent breakthroughs in QD-based solar cells for various applications and their current challenges. We expect that this review will provide an effective gateway for researchers to fabricate LDS-layer-based solar cells.

Remarkable Black-Phase Robustness of CsPbI3 Nanocrystals Sealed in Solid SiO2 /AlOx Sub-Micron Particles

This work combines the high-temperature sintering method and atomic layer deposition (ALD) technique, and yields SiO₂ /AlO_x -sealed γ-CsPbI₃ nanocrystals (NCs). The black-phase CsPbI₃ NCs, scattered and encapsulated firmly in solid SiO₂ sub-micron particles, maintain in black phases against water soaking, ultraviolet irradiation, and heating, exhibiting remarkable phase stability. A new phase-transition route, from γ via β to α phase without transferring into δ phase, has been discovered upon temperature increasing. The phase stability is ascribed to the high pressure exerted by the rigid SiO₂ encapsulations, and its condensed amorphous structures that prevent the permeation of H₂ O molecules. Nanoscale coating of Al₂ O₃ thin films, which are deposited on the surface of the CsPbI₃ -SiO₂ by ALD, enhances the protection against O₂ infiltration, greatly elevating the high-temperature stability of CsPbI₃ NCs sealed inside, as the samples remain bright after 1-h annealing in air at 400 °C. These fabrication and encapsulation techniques effectively prevent the formation of δ-CsPbI₃ under harsh environment, bringing the high-pressure preservation of black-phase CsPbI₃ from laboratory to industry toward potential applications in both photovoltaic and fluorescent areas.

Defects in CsPbX3 Perovskite: From Understanding to Effective Manipulation for High-Performance Solar Cells

The rapid development of all inorganic metal perovskite (CsPbX₃ , X represents halogen) materials holds great promise for top-cells in tandem junctions due to their glorious thermal stability and continuous adjustable band gap in a wide range. Due to the presence of defects, the power conversion efficiency (PCE) of CsPbX₃ perovskite solar cells (PSCs) is still substantially below the Shockley-Queisser (SQ) limit. Therefore, it is imperative to have an in-depth understanding of the defects in PSCs, thus to evaluate their impact on device performances and to develop corresponding strategies to manipulate defects in PSCs for further promoting their photoelectric properties. In this review, the latest progress in defect passivation in the CsPbX₃ PSCs field is summarized. Starting from the effect of non-radiative recombination on open circuit voltage (V_oc ) losses, the defect physics, tolerance, self-healing, and the effect of defects on the photovoltaic properties are discussed. Some techniques to identify defects are compared based on quantitative and qualitative analysis. Then, passivation manipulation is discussed in detail, the defect passivation mechanisms are proposed, and the passivation agents in CsPbX₃ thin films are classified. Finally, directions for future research about defect manipulation that will push the field to progress forward are outlined.

Formation and Near-Infrared Emission of CsPbI3 Nanoparticles Embedded in Cs4PbI6 Crystals

Cs₄PbI₆, as a rarely investigated member of the Cs₄PbX₆ (X is a halogen element) family, has been successfully synthesized at low temperatures, and the synthetic conditions have been optimized. Metal iodides such as LiI, KI, NiI₂, CoI₂, and ZnI₂, as additives, play an important role in enhancing the formation of the Cs₄PbI₆ microcrystals. ZnI₂ with the lowest dissociation energy is the most efficient additive to supply iodide ions, and its amount of addition has also been optimized. Strong red to near-infrared (NIR) emission properties have been detected, and its optical emission centers have been identified to be numerous embedded perovskite-type α-CsPbI₃ nanocrystallites (∼5 nm in diameter) based on investigations of temperature- and pressure-dependent photoluminescent properties. High-resolution transmission electron microscopy was used to detect these hidden nanoparticles, although the material was highly beam-sensitive and confirmed a "raisin bread"-like structure of the Cs₄PbI₆ crystals. A NIR mini-LED for the biological application has been successfully fabricated using as-synthesized Cs₄PbI₆ crystals. This work provides information for the future development of infrared fluorescent nanoscale perovskite materials.

All-Evaporated, All-Inorganic CsPbI3 Perovskite-Based Devices for Broad-Band Photodetector and Solar Cell Applications

Following the rapid increase of organic metal halide perovskites toward commercial application in thin-film solar cells, inorganic alternatives attracted great interest with their potential of longer device lifetime due to the stability improvement under increased temperatures and moisture ingress. Among them, cesium lead iodide (CsPbI₃) has gained significant attention due to similar electronic and optical properties to methylammonium lead iodide (MAPbI₃), with a band gap of 1.7 eV, high absorption coefficient, and large diffusion length, while also offering the advantage of being completely inorganic, providing a higher thermal stability and preventing material degradation. On a device level, however, it seems also essential to replace organic transport layers by inorganic counterparts to further prevent degradation. In addition, devices are mostly fabricated by spin coating, limiting their reproducibility and scalability; in this case, exploring all-evaporated devices allows us to improve the quality of the layers and to increase their reproducibility. In this work, we focus on the deposition of CsPbI₃ by CsI and PbI₂ co-evaporation. We fabricate devices with an all-inorganic, all-evaporated structure, employing NiO and TiO₂ as transport layers, and evaluate these devices for both photodetector and solar cell applications. As a photodetector, low leakage current, high external quantum efficiency (EQE) and detectivity, and fast rise and decay times were obtained, while as a solar cell, acceptable efficiencies were achieved. These all-inorganic, all-evaporated devices represent one step forward toward higher stability and reproducibility while enabling large area compatibility and easier integration with other circuitry and, in future, the possible commercialization of perovskite-based technology.

Layered Perovskite Oxyiodide with Narrow Band Gap and Long Lifetime Carriers for Water Splitting Photocatalysis

The development of semiconductors with narrow band gap and high stability is crucial for achieving solar to chemical energy conversion. Compounds with iodine, which has a high polarizability, have attracted attention because of their narrow band gap and long carrier lifetime, as typified by halide perovskite solar cells; however, they have been regarded as unsuitable for harsh photocatalytic water splitting because iodine is prone to self-oxidation. Here, we demonstrate that Ba₂Bi₃Nb₂O₁₁I, a layered Sillén-Aurivillius oxyiodide, not only has access to a wider range of visible light than its chloride and bromide counterparts, but also functions as a stable photocatalyst, efficiently oxidizing water. Density functional theory calculations reveal that the oxygen 2p orbitals in the perovskite block, rather than the fluorite Bi₂O₂ block as previously pointed out, anomalously push up the valence band maximum, which can be explained by a modified Madelung potential analysis that takes into account the high polarizability of iodine. In addition, the highly polarizable iodide contributes to longer carrier lifetime of Ba₂Bi₃Nb₂O₁₁I, allowing for a significantly higher quantum efficiency than its chloride and bromide counterparts. Visible-light-driven Z-scheme water splitting was achieved for the first time in an iodine-based system using Ba₂Bi₃Nb₂O₁₁I as an oxygen-evolution photocatalyst. The present study provides a novel approach for incorporating polarizable "soft" anions into building blocks of layered materials to manipulate the band structure and improve the carrier dynamics for visible-light responsive functions.

Atomic and electronic structure of cesium lead triiodide surfaces

The (001) surface of the emerging photovoltaic material cesium lead triiodide (CsPbI₃) is studied. Using first-principles methods, we investigate the atomic and electronic structure of cubic (α) and orthorhombic (γ) CsPbI₃. For both phases, we find that CsI-termination is more stable than PbI₂-termination. For the CsI-terminated surface, we then compute and analyze the surface phase diagram. We observe that surfaces with added or removed units of nonpolar CsI and PbI₂ are most stable. The corresponding band structures reveal that the α phase exhibits surface states that derive from the conduction band. The surface reconstructions do not introduce new states in the bandgap of CsPbI₃, but for the α phase, we find additional surface states at the conduction band edge.

Unexpected Polymorphism in Bromoantimonate(III) Complexes and Its Effect on Optical Properties

Reactions of [SbBr₆]^3- containing HBr solutions with bromide salts of 1,1'-(1,2-ethanediyl)bis(pyridine) (PyC₂²⁺) or 1,1'-(1,2-ethanediyl)bis(3,5-dimethylpyridine) (3,5-MePyC₂²⁺) initially result in the formation of the deep orange complexes Cat[SbBr₅] (1 and 2), featuring unusual Sb···Br interactions in the solid state. In the mother liquor, 1 transforms into discrete binuclear (C₂Py)₂[Sb₂Br₁₀], which demonstrates polymorphism (triclinic 3 and monoclinic 4), while 2 transforms into polymeric (3,5-MePy){[SbBr₄]} (5). DFT calculations reveal that the system of noncovalent Sb···Br contacts may be responsible for the appearance of the observed optical properties (unusual deep orange coloring).

Recent Advances and Challenges in Halide Perovskite Crystals in Optoelectronic Devices from Solar Cells to Other Applications

Organic-inorganic hybrid perovskite materials have attracted tremendous attention as a key material in various optoelectronic devices. Distinctive optoelectronic properties, such as a tunable energy band position, long carrier diffusion lengths, and high charge carrier mobility, have allowed rapid progress in various perovskite-based optoelectronic devices (solar cells, photodetectors, light emitting diodes (LEDs), and lasers). Interestingly, the developments of each field are based on different characteristics of perovskite materials which are suitable for their own applications. In this review, we provide the fundamental properties of perovskite materials and categorize the usages in various optoelectronic applications. In addition, the prerequisite factors for those applications are suggested to understand the recent progress of perovskite-based optoelectronic devices and the challenges that need to be solved for commercialization.

One-Dimensional Diiodine-Iodobismuthate(III) Hybrids Cat3{[Bi2I9](I2)3}: Syntheses, Stability, and Optical Properties

One-dimensional iodine-rich iodobismuthates(III), Cat₃{[Bi₂I₉](I₂)₃} [Cat = 1,4-MePy (1) and 1-EtBMAP (2)], feature the highest amount of "trapped" diiodine units in polyhalogen-halometalates of p-block elements. Both complexes have narrow optical band gaps (1.55 and 1.63 eV, respectively) and moderate thermal stability.

Choose Your Own Adventure: Fabrication of Monolithic All-Perovskite Tandem Photovoltaics

Metal halide perovskites (MHPs) have transfixed the photovoltaic (PV) community due to their outstanding and tunable optoelectronic properties coupled to demonstrations of high-power conversion efficiencies (PCE) at a range of bandgaps. This has motivated the field to push perovskites to reach the highest possible performance. One way to increase the efficiency is by fabricating multijunction solar cells, which can split the solar spectrum, reducing thermalization loss. Low-cost all-perovskite tandems have a real chance to soon exceed 30% PCE, which could transform the PV industry. Achieving this goal requires the identification of perovskite sub-cells that are both highly efficient and can be effectively integrated. Herein, it is discussed how to navigate the multiple-choice adventure in choosing between the myriad of options and considerations present when deciding what perovskite materials, contact layers, and processing tools to use. Some of the potential fabrication pitfalls often encountered in MHP based tandem PVs are highlighted, so that they can hopefully be avoided in the future.

Chemically Stable Black Phase CsPbI3 Inorganic Perovskites for High-Efficiency Photovoltaics

Research on chemically stable inorganic perovskites has achieved rapid progress in terms of high efficiency exceeding 19% and high thermal stabilities, making it one of the most promising candidates for thermodynamically stable and high-efficiency perovskite solar cells. Among those inorganic perovskites, CsPbI₃ with good chemical components stability possesses the suitable bandgap (≈1.7 eV) for single-junction and tandem solar cells. Comparing to the anisotropic organic cations, the isotropic cesium cation without hydrogen bond and cation orientation renders CsPbI₃ exhibit unique optoelectronic properties. However, the unideal tolerance factor of CsPbI₃ induces the challenges of different crystal phase competition and room temperature phase stability. Herein, the latest important developments regarding understanding of the crystal structure and phase of CsPbI₃ perovskite are presented. The development of various solution chemistry approaches for depositing high-quality phase-pure CsPbI₃ perovskite is summarized. Furthermore, some important phase stabilization strategies for black phase CsPbI₃ are discussed. The latest experimental and theoretical studies on the fundamental physical properties of photoactive phase CsPbI₃ have deepened the understanding of inorganic perovskites. The future development and research directions toward achieving highly stable CsPbI₃ materials will further advance inorganic perovskite for highly stable and efficient photovoltaics.

Recent advances in interface engineering of all-inorganic perovskite solar cells

All-inorganic perovskite solar cells (PSCs) have become one of the most attractive research fields in recent years due to their excellent thermal stability and light stability as compared with their organic-inorganic hybrid counterparts. However, there is still a long way to go for their commercial application due to their low efficiency and poor stability under humidity conditions. Herein, an overview of the recent progress of all-inorganic PSCs based on interface engineering is provided. The main roles of interface engineering, adjusting energy-level alignment, enhancing charge transport capacity, passivating interface defects, modulating morphology of perovskite films, stabilizing perovskite phase, broadening spectral absorption, eliminating electrical hysteresis and enhancing operational stability, are summarized with examples, which paves the way for highly efficient and stable all-inorganic PSCs. Some of the latest progress in incorporating dopants to charge transport materials and modifying interface properties in all-inorganic PSCs are also covered.

Versatile vapor phase deposition approach to cesium tin bromide materials CsSnBr3, CsSn2Br5 and Cs2SnBr6

We report on the successful application of RF-magnetron sputtering to deposit, by using a single type of target, three different materials in the form of thin films within the Cs-Sn-Br compositional range, namely, CsSnBr₃, CsSn₂Br₅ and Cs₂SnBr₆. It is shown that, by playing with the deposition parameters and post-deposition treatments, it is possible to stabilize these three perovskites or perovskite related compounds by exploiting the versatility of vapor phase deposition. Full characterization in terms of crystal structure, optical properties and morphology is reported. The power of vapor phase methods in growing all-inorganic materials of interest for photovoltaic and optoelectronic applications is demonstrated here, indicating the advantageous use of sputtering for these complex materials.

Incorporation of Vanadium(V) Oxide in Hybrid Hole Transport Layer Enables Long-term Operational Stability of Perovskite Solar Cells

Recent studies have shown that charge transport interlayers with low gas permeability can increase the operational lifetime of perovskite solar cells serving as a barrier for migration of volatile decomposition products from the photoactive layer. Herein we present a hybrid hole transport layer (HTL) comprised of p-type polytriarylamine (PTAA) polymer and vanadium(V) oxide (VO_x). Devices with PTAA/VO_x top HTL reach up to 20% efficiency and demonstrate negligible degradation after 4500 h of light soaking, whereas reference cells using PTAA/MoO_x as HTL lose ∼50% of their initial efficiency under the same aging conditions. It was shown that the main origin of the enhanced device stability lies in the higher tolerance of VO_x toward MAPbI₃ compared to the MoO_x interlayer, which tends to facilitate perovskite decomposition. Our results demonstrate that the application of PTAA/VO_x hybrid HTL enables long-term operational stability of perovskite solar cells, thus bringing them closer to commercial applications.

Structural, Hirshfeld Surface Analysis, Morphological Approach, and Spectroscopic Study of New Hybrid Iodobismuthate Containing Tetranuclear 0D Cluster Bi4I16·4(C6H9N2) 2(H2O)

The Bi4I16·4(C6H9N2) 2(H2O) compound was synthesized by slow evaporation at room temperature. It exhibits a zero-dimensional (0D) tetrameric structure, comprising [Bi4I16]4− distorted octahedra, with strong I⋯I interactions among adjacent anionic clusters. We used Hirshfeld surface analysis to discuss the strength of hydrogen bonds and to quantify the inter-contacts (two-dimensional (2D) fingerprint plots). It revealed that the hydrogen bonding interactions H⋯I (56.3%), π–π stacking (11.7%), and I⋯I interactions (5.9%) play the major role in the stability of the crystal structure. The crystal morphology was simulated using Bravais–Friedel, Donnay–Harker (BFDH) and growth morphology (GM) methods. The experimental habit of the title compound was adequately reproduced by the two models. The calculated results show that the crystal morphology of the title compound in a vacuum is dominated by five facets: (020), (011), (110), (10−1), and (11−1). The (020) facet is the largest among all the facets calculated. Projection of the facet showed that there are a few polar groups on the (020) facet. In the 50–400 and 400–4000 cm−1 frequency regions, we measured the Raman and infrared spectra, respectively, of the title compound, and we assigned the observed vibration modes.

Optimizing Band Gap of Inorganic Halide Perovskites by Donor-Acceptor Pair Codoping

Inorganic halide perovskites (IHPs) are promising candidates for applications in solar cell devices. However, the band gaps of most IHPs are too large, so that the energy conversion efficiency is limited. In this work, we proposed a donor-acceptor pair codoping scheme to reduce the band gaps Sn- and Pb-based IHPs, based on first-principles calculations. Interestingly, the donor-acceptor pair codoping in CsSnBr₃ and CsPbI₃ can produce band gaps of 1.2 and 1.1 eV, respectively, both of which are close to the optimal band gap for solar cell materials. The absorption coefficient of donor-acceptor pair codoped CsSnBr₃ and CsPbI₃ in the visible light region are large, which indicates that they are good light absorbers for applications in solar cell devices.

All-Inorganic CsPbBr3 Perovskite Films Prepared by Single Source Thermal Ablation

Hybrid organo-lead halide perovskites are becoming the benchmark material for next generation photovoltaics and a very important player for other applications such as photodetectors and light emitting diodes. Nevertheless, the most important issue hindering the large-scale application of these materials remains their intrinsic instability due to the organic cation. Although the substitution with inorganic cesium (Cs) enhances stability, in most cases solution deposition methods of fully inorganic perovskites result in high surface roughness and poor surface coverage. This work reports on the evaporation of the CsPbBr₃ precursor by Single Source Thermal Ablation, showing that just after deposition films consist of a mixture of CsPbBr₃, CsPb₂Br₅, and Cs₄PbBr₆ due to a vertical composition gradient. We point out that mild post deposition treatments lead to the conversion of CsPb₂Br₅ and Cs₄PbBr₆ into CsPbBr₃ due to its higher thermodynamic stability. Conversion results into smooth and pinhole-free CsPbBr₃ films with good light absorption and emission properties. We demonstrate the suitability of obtained films for planar devices by preparing perovskite-based pure-green light emitting diodes, thus promoting Single Source Thermal Ablation as a promising alternative deposition technique for all-inorganic perovskite-based devices.