Thermal engineering of FAPbI3 perovskite material via radiative thermal annealing and in situ XRD

Band gap tuning of perovskite solar cells for enhancing the efficiency and stability: issues and prospects

The intriguing optoelectronic properties, diverse applications, and facile fabrication techniques of perovskite materials have garnered substantial research interest worldwide. Their outstanding performance in solar cell applications and excellent efficiency at the lab scale have already been proven. However, owing to their low stability, the widespread manufacturing of perovskite solar cells (PSCs) for commercialization is still far off. Several instability factors of PSCs, including the intrinsic and extrinsic instability of perovskite materials, have already been identified, and a variety of approaches have been adopted to improve the material quality, stability, and efficiency of PSCs. In this review, we have comprehensively presented the significance of band gap tuning in achieving both high-performance and high-stability PSCs in the presence of various degradation factors. By investigating the mechanisms of band gap engineering, we have highlighted its pivotal role in optimizing PSCs for improved efficiency and resilience.

Enhanced Charge Transfer in Quasi-2D Perovskite by Formamidinium Cation Gradient Incorporation for Efficient and Stable Solar Cells

Quasi-2D perovskites have attracted much attention in perovskite photovoltaics due to their excellent stability. However, their photoelectric conversion efficiency (PCE) still lags 3D counterparts, particularly with high short-circuit current (JSC) loss. The quantum confinement effect is pointed out to be the sole reason, which introduces widened bandgap and poor exciton dissociation, and undermines the light capture and charge transport. Here, the gradient incorporation of formamidinium (FA) cations into quasi-2D perovskite is proposed to address this issue. It is observed that FA prefers to incorporate into the larger n value phases near the film surface compared to the smaller n value phases in the bulk, resulting in a narrow bandgap and gradient structure within the film. Through charge dynamic analysis using in situ light-dark Kelvin probe force microscopy and transient absorption spectroscopy, it is demonstrated that incorporating 10% FA significantly facilitates efficient charge transfer between low n-value phases in the bulk and high n-value nearby film surface, leading to reduced charge accumulation. Ultimately, the device based on (AA)2(MA0.9FA0.1)4Pb5I16, where AA represents n-amylamine renowned for its exceptional environmental stability as a bulky organic ligand, achieves an impressive power conversion efficiency (PCE) of 18.58% and demonstrates enhanced illumination and thermal stability.

Single Crystal Seed Induced Epitaxial Growth Stabilizes α-FAPbI3 in Perovskite Solar Cells

FAPbI3 stands out as an ideal candidate for the photoabsorbing layer of perovskite solar cells (PSCs), showcasing outstanding photovoltaic properties. Nonetheless, stabilizing photoactive α-FAPbI3 remains a challenge due to the lower formation energy of the competitive photoinactive δ-phase. In this study, we employ tetraethylphosphonium lead tribromide (TEPPbBr3) single crystals as templates for the epitaxial growth of PbI2. The strategic use of TEPPbBr3 optimizes the evolution of intermediates and the crystallization kinetics of perovskites, leading to high-quality and phase-stable α-FAPbI3 films. The TEPPbBr3-modified perovskite exhibits optimized carrier dynamics, yielding a champion efficiency of 25.13% with a small voltage loss of 0.34 V. Furthermore, the target device maintains 90% of its initial PCE under maximum power point (MPP) tracking over 1000 h. This work establishes a promising pathway through single crystal seed based epitaxial growth for achieving satisfactory crystallization regulation and phase stabilization of α-FAPbI3 perovskites toward high-efficiency and stable PSCs.

A Catalyst-Like System Enables Efficient Perovskite Solar Cells

High-quality perovskite films are essential for achieving high performance of optoelectronic devices; However, solution-processed perovskite films are known to suffer from compositional and structural inhomogeneity due to lack of systematic control over the kinetics during the formation. Here, the microscopic homogeneity of perovskite films is successfully enhanced by modulating the conversion reaction kinetics using a catalyst-like system generated by a foaming agent. The chemical and structural evolution during this catalytic conversion is revealed by a multimodal synchrotron toolkit with spatial resolutions spanning many length scales. Combining these insights with computational investigations, a cyclic conversion pathway model is developed that yields exceptional perovskite homogeneity due to enhanced conversion, having a power conversion efficiency of 24.51% for photovoltaic devices. This work establishes a systematic link between processing of precursor and homogeneity of the perovskite films.

High-Quality Hybrid Perovskite Thin Films by Post-Treatment Technologies in Photovoltaic Applications

Incredible progress in photovoltaic devices based on hybrid perovskite materials has been made in the past few decades, and a record-certified power conversion efficiency (PCE) of over 26% has been achieved in single-junction perovskite solar cells (PSCs). In the fabrication of high-efficiency PSCs, the postprocessing procedures toward perovskites are essential for designing high-quality perovskite thin films; developing efficient and reliable post-treatment techniques is very important to promote the progress of PSCs. Here, recent post-treatment technological reforms toward perovskite thin films are summarized, and the principal functions of the post-treatment strategies on the design of high-quality perovskite films have been thoroughly analyzed by dividing into two categories in this review: thermal annealing (TA)-related technique and TA-free technique. The latest research progress of the above two types of post-treatment techniques is summarized and discussed, focusing on the optimization of postprocessing conditions, the regulation of perovskite qualities, and the enhancement of device performance. Finally, an outlook of the prospect trends and future challenges for the fabrication of the perovskite layer and the production of highly efficient PSCs is given.

Scaffold-Guided Crystallization of Oriented α-FAPbI3 Nanowire Arrays for Solar Cells

Perovskite nanowire arrays with large surface areas for efficient charge transfer and continuous highly crystalline domains for efficient charge transport exhibit ideal morphologies for solar-cell active layers. Here, we introduce a room temperature two-step method to grow dense, vertical nanowire arrays of formamidinium lead iodide (FAPbI3). PbI2 nanocrystals embedded in the cylindrical nanopores of anodized titanium dioxide scaffolds were converted to FAPbI3 by immersion in a FAI solution for a period of 0.5-30 min. During immersion, FAPbI3 crystals grew vertically from the scaffold surface as nanowires with diameters and densities determined by the underlying scaffold. The presence of butylammonium cations during nanowire growth stabilized the active α polymorph of FAPbI3, precluding the need for a thermal annealing step. Solar cells comprising α-FAPbI3 nanowire arrays exhibited maximum solar conversion efficiencies of >14%. Short-circuit current densities of 22-23 mA cm-2 were achieved, on par with those recorded for the best-performing FAPbI3 solar cells reported to date. Such large photocurrents are attributed to the single-crystalline, low-defect nature of the nanowires and increased interfacial area for photogenerated charge transfer compared with thin films.

In situ imaging of the atomic phase transition dynamics in metal halide perovskites

Phase transition dynamics are an important concern in the wide applications of metal halide perovskites, which fundamentally determine the optoelectronic properties and stabilities of perovskite materials and devices. However, a more in-depth understanding of such a phase transition process with real atomic resolution is still limited by the immature low-dose electron microscopy and in situ imaging studies to date. Here, we apply an emergent low-dose imaging technique to identify different phase structures (α, β and γ) in CsPbI3 nanocrystals during an in-situ heating process. The rotation angles of PbI6 octahedrons can be measured in these images to quantitatively describe the thermal-induced phase distribution and phase transition. Then, the dynamics of such a phase transition are studied at a macro time scale by continuously imaging the phase distribution in a single nanocrystal. The structural evolution process of CsPbI3 nanocrystals at the particle level, including the changes in morphology and composition, is also visualized with increasing temperature. These results provide atomic insights into the transition dynamics of perovskite phases, indicating a long-time transition process with obvious intermediate states and spatial distribution that should be generally considered in the further study of structure-property relations and device performance.

Thiocyanate-Stabilized Pseudo-cubic Perovskite CH(NH2)2PbI3 from Coincident Columnar Defect Lattices

α-FAPbI3 (FA+ = CH(NH2)2+) with a cubic perovskite structure is promising for photophysical applications. However, α-FAPbI3 is metastable at room temperature, and it transforms to the δ-phase at a certain period of time at room temperature. Herein, we report a thiocyanate-stabilized pseudo-cubic perovskite FAPbI3 with ordered columnar defects (α'-phase). This compound has a √5ap × √5ap × ap tetragonal unit cell (ap: cell parameter of primitive perovskite cell) with a band gap of 1.91 eV. It is stable at room temperature in a dry atmosphere. Furthermore, the presence of the α'-phase in a mixed sample with the δ-phase drastically reduces the δ-to-α transition temperature measured on heating, suggesting the reduction of the nucleation energy of the α-phase or thermodynamic stabilization of the α-phase through epitaxy. The defect-ordered pattern in the α'-phase forms a coincidence-site lattice at the twinned boundary of the single crystals, thus hinting at an epitaxy- or strain-based mechanism of α-phase formation and/or stabilization. In this study, we developed a new strategy to control defects in halide perovskites and provided new insight into the stabilization of α-FAPbI3 by pseudo-halide and grain boundary engineering.

Synergy of 3D and 2D Perovskites for Durable, Efficient Solar Cells and Beyond

Three-dimensional (3D) organic-inorganic lead halide perovskites have emerged in the past few years as a promising material for low-cost, high-efficiency optoelectronic devices. Spurred by this recent interest, several subclasses of halide perovskites such as two-dimensional (2D) halide perovskites have begun to play a significant role in advancing the fundamental understanding of the structural, chemical, and physical properties of halide perovskites, which are technologically relevant. While the chemistry of these 2D materials is similar to that of the 3D halide perovskites, their layered structure with a hybrid organic-inorganic interface induces new emergent properties that can significantly or sometimes subtly be important. Synergistic properties can be realized in systems that combine different materials exhibiting different dimensionalities by exploiting their intrinsic compatibility. In many cases, the weaknesses of each material can be alleviated in heteroarchitectures. For example, 3D-2D halide perovskites can demonstrate novel behavior that neither material would be capable of separately. This review describes how the structural differences between 3D halide perovskites and 2D halide perovskites give rise to their disparate materials properties, discusses strategies for realizing mixed-dimensional systems of various architectures through solution-processing techniques, and presents a comprehensive outlook for the use of 3D-2D systems in solar cells. Finally, we investigate applications of 3D-2D systems beyond photovoltaics and offer our perspective on mixed-dimensional perovskite systems as semiconductor materials with unrivaled tunability, efficiency, and technologically relevant durability.

Straight Manipulation Annealing in a Solvent Atmosphere for Quality-Improved Cs2AgBiBr6 Perovskites

As a new-generation photoelectric material, perovskites have attracted researchers' attention due to their excellent optoelectronic properties. However, the existence of defects inevitably causes structural degradation and restricts their performance, which need to be further improved by post-treatment. At present, post-treatments mostly focus on non-contact treatments, which may constrain the effect since the influence on the perovskites caused by the direct contact is much more straightly. Therefore, we proposed an annealing strategy of straight manipulation in a solvent atmosphere with the assistance of polyimide (PI) tape for the perovskite post-treatment, due to the high heat resistance and less glue residual of this tape. It casts an influence on the perovskite directly, proving the possibility of the straight manipulation by operators, promoting the recrystallization of the perovskite grains and removing the impurity substance. The optimized Pb-free perovskite film exhibits a better X-ray sensitivity of 7.5 × 104 μC Gyair-1 cm-2 and a great detection limit of 47 nGyair s-1, which is comparable to advanced Pb-based perovskite X-ray detectors and all commercial ones. The new annealing strategy provides a facile, effective, and simple method to improve the perovskite quality, exhibiting the potential and harmlessness of the direct contact post-treatment, which paves the way for a broader application of perovskites.

Kinetic-Controlled Crystallization of α-FAPbI3 Inducing Preferred Crystallographic Orientation Enhances Photovoltaic Performance

Crystallization kinetic controls the crystallographic orientation, inducing anisotropic properties of the materials. As a result, preferential orientation with advanced optoelectronic properties can enhance the photovoltaic devices' performance. Although incorporation of additives is one of the most studied methods to stabilize the photoactive α-phase of formamidinium lead tri-iodide (α-FAPbI₃ ), no studies focus on how the additives affect the crystallization kinetics. Along with the role of methylammonium chloride (MACl) as a "stabilizer" in the formation of α-FAPbI₃ , herein, the additional role as a "controller" in the crystallization kinetics is pointed out. With microscopic observations, for example, electron backscatter diffraction and selected area electron diffraction, it is examined that higher concentration of MACl induces slower crystallization kinetics, resulting in larger grain size and [100] preferred orientation. Optoelectronic properties of [100] preferentially oriented grains with less non-radiative recombination, a longer lifetime of charge carriers, and lower photocurrent deviations in between each grain induce higher short-circuit current density (J_sc ) and fill factor. Resulting MACl40 mol% attains the highest power conversion efficiency (PCE) of 24.1%. The results provide observations of a direct correlation between the crystallographic orientation and device performance as it highlights the importance of crystallization kinetics resulting in desirable microstructures for device engineering.

Photoexcitation of perovskite precursor solution to induce high-valent iodoplumbate species for wide bandgap perovskite solar cells with enhanced photocurrent

Solution-processed organic-inorganic hybrid perovskite solar cells are among the candidates to replace the traditional silicon solar cells due to their excellent power conversion efficiency (PCE). Despite this considerable progress, understanding the properties of the perovskite precursor solution is critical for perovskite solar cells (PSCs) to achieve high performance and reproducibility. However, the exploration of perovskite precursor chemistry and its effects on photovoltaic performances has been limited thus far. Herein, we modified the equilibrium of chemical species inside the precursor solution using different photoenergy and heat pathways to identify the corresponding perovskite film formation. The illuminated perovskite precursors exhibited a higher density of high-valent iodoplumbate species, resulting in the fabricated perovskite films with reduced defect density and uniform distribution. Conclusively, the perovskite solar cells prepared by the photoaged precursor solution had not only improved PCE but also enhanced current density, confirmed by device performance, conductive atomic force microscopy (C-AFM), and external quantum efficiency (EQE). This innovative precursor photoexcitation is a simple and effective physical process for boosting perovskite morphology and current density.

Controlling Phase Transition toward Future Low-Cost and Eco-friendly Printing of Perovskite Solar Cells

Perovskite solar cells (PSCs) have grown increasingly popular over the past few years and are considered to be game-changers in the energy conversion market. It has became vital to transfer the deep understanding of the perovskite film formation process during lab-scale fabrication to large-scale production. Complex phase transition during film formation has been revealed by in situ strategies. However, there is still debate which phase transition is the right route for a future scalable approach. Herein, we briefly summarize perovskite crystallization during scalable printing processes. The critical information about the intermediates involved in phase transition from precursors to perovskite crystals are discussed because it deeply impacts the morphology of printed films. Finally, important strategies to control phase transition and challenges toward future low-temperature and eco-friendly printing of perovskite solar cells are proposed. The information provided by this Perspective will assist the screening and development of the perovskite phase transition for future cost-efficient printed perovskite panels.

Controlling Intrinsic Quantum Confinement in Formamidinium Lead Triiodide Perovskite through Cs Substitution

Lead halide perovskites are leading candidates for photovoltaic and light-emitting devices, owing to their excellent and widely tunable optoelectronic properties. Nanostructure control has been central to their development, allowing for improvements in efficiency and stability, and changes in electronic dimensionality. Recently, formamidinium lead triiodide (FAPbI₃) has been shown to exhibit intrinsic quantum confinement effects in nominally bulk thin films, apparent through above-bandgap absorption peaks. Here, we show that such nanoscale electronic effects can be controlled through partial replacement of the FA cation with Cs. We find that Cs-cation exchange causes a weakening of quantum confinement in the perovskite, arising from changes in the bandstructure, the length scale of confinement, or the presence of δ_H-phase electronic barriers. We further observe photon emission from quantum-confined regions, highlighting their potential usefulness to light-emitting devices and single-photon sources. Overall, controlling this intriguing quantum phenomenon will allow for its suppression or enhancement according to need.

Full-Dimensional Grain Boundary Stress Release for Flexible Perovskite Indoor Photovoltaics

Perovskite photovoltaics are strong potential candidates to drive low-power off-grid electronics for indoor applications. Compared with rigid devices, flexible perovskite devices can provide a more suitable surface for indoor small electronic devices, enabling them have a broader indoor application prospect. However, the mechanical stability of flexible perovskite photovoltaics is an urgent issue solved. Herein, a kind of 3D crosslinking agent named borax is selected to carry out grain boundary penetration treatment on perovskite film to realize full-dimensional stress release. This strategy improves the mechanical and phase stabilities of perovskite films subjected to external forces or large temperature changes. The fabricated perovskite photovoltaics deliver a champion power conversion efficiency (PCE) of 21.63% under AM 1.5G illumination, which is the highest one to date. The merit of low trap states under weak light makes the devices present a superior indoor PCE of 31.85% under 1062 lux (LED, 2956 K), which is currently the best flexible perovskite indoor photovoltaic device. This work provides a full-dimensional grain boundary stress release strategy for highly stable flexible perovskite indoor photovoltaics.

Colloidal Quantum Dot Solar Cells: Progressive Deposition Techniques and Future Prospects on Large-Area Fabrication

Colloidally grown nanosized semiconductors yield extremely high-quality optoelectronic materials. Many examples have pointed to near perfect photoluminescence quantum yields, allowing for technology-leading materials such as high purity color centers in display technology. Furthermore, because of high chemical yield, and improved understanding of the surfaces, these materials, particularly colloidal quantum dots (QDs) can also be ideal candidates for other optoelectronic applications. Given the urgent necessity toward carbon neutrality, electricity from solar photovoltaics will play a large role in the power generation sector. QDs are developed and shown dramatic improvements over the past 15 years as photoactive materials in photovoltaics with various innovative deposition properties which can lead to exceptionally low-cost and high-performance devices. Once the key issues related to charge transport in optically thick arrays are addressed, QD-based photovoltaic technology can become a better candidate for practical application. In this article, the authors show how the possibilities of different deposition techniques can bring QD-based solar cells to the industrial level and discuss the challenges for perovskite QD solar cells in particular, to achieve large-area fabrication for further advancing technology to solve pivotal energy and environmental issues.

Water-Accelerated Photooxidation of CH3NH3PbI3 Perovskite

Halide perovskites have the potential to disrupt the photovoltaics market based on their high performance and low cost. However, the decomposition of perovskites under moisture, oxygen, and light raises concerns about service lifetime, especially because degradation mechanisms and the corresponding rate laws that fit the observed data have thus far eluded researchers. Here, we report a water-accelerated photooxidation mechanism dominating the degradation kinetics of archetypal perovskite CH₃NH₃PbI₃ in air under >1% relative humidity at 25 °C. From this mechanism, we develop a kinetic model that quantitatively predicts the degradation rate as a function of temperature, ambient O₂ and H₂O levels, and illumination. Because water is a possible product of dry photooxidation, these results highlight the need for encapsulation schemes that rigorously block oxygen ingress, as product water may accumulate beneath the encapsulant and initiate the more rapid water-accelerated photooxidative decomposition.

Development of formamidinium lead iodide-based perovskite solar cells: efficiency and stability

Perovskite materials have been particularly eye-catching by virtue of their excellent properties such as high light absorption coefficient, long carrier lifetime, low exciton binding energy and ambipolar transmission (perovskites have the characteristics of transporting both electrons and holes). Limited by the wider band gap (1.55 eV), worse thermal stability and more defect states, the first widely used methylammonium lead iodide has been gradually replaced by formamidinium lead iodide (FAPbI3) with a narrower band gap of 1.48 eV and better thermal stability. However, FAPbI3 is stabilized as the yellow non-perovskite active phase at low temperatures, and the required black phase (α-FAPbI3) can only be obtained at high temperatures. In this perspective, we summarize the current efforts to stabilize α-FAPbI3, and propose that pure α-FAPbI3 is an ideal material for single-junction cells, and a triple-layer mesoporous architecture could help to stabilize pure α-FAPbI3. Furthermore, reducing the band gap and using tandem solar cells may ulteriorly approach the Shockley-Queisser limit efficiency. We also make a prospect that the enhancement of industrial applications as well as the lifetime of devices may help achieve commercialization of PSCs in the future.

Growth and Degradation Kinetics of Organic-Inorganic Hybrid Perovskite Films Determined by In Situ Grazing-Incidence X-Ray Scattering Techniques

Organic-inorganic halide perovskite (OIHP) solar cells hold a great promise for commercial breakthrough since their power conversion efficiency has been pushed beyond the mark of 25%, making them capable of competing with traditional crystalline silicon solar cells. The key to achieve efficient and stable perovskite solar cells is inherently related to the film morphology. The understanding of the kinetic processes of film formation and degradation opens up possibilities to tailor the film morphology via the regulation of precursor and processing parameters. In situ grazing-incidence X-ray scattering (GIXS) techniques allow for tracking the morphology evolution of thin films at different length scales and with high temporal resolution. In this review, the selected examples for application of in situ grazing-incidence wide-angle X-ray scattering and grazing-incidence small-angle X-ray scattering techniques to the growth and stability of OIHPs are summarized after a brief introduction to both techniques, highlighting particularly the morphological evolution of perovskite films over time. Then the correlated mathematical models are reviewed to give a toolbox for analyzing the mechanisms of film formation and degradation. Thus, an overview on the in situ GIXS methods is linked to the research of OIHP kinetics.

A Systematic Review of Metal Halide Perovskite Crystallization and Film Formation Mechanism Unveiled by In Situ GIWAXS

Metal halide perovskites are of fundamental interest in the research of modern thin-film optoelectronic devices, owing to their widely tunable optoelectronic properties and solution processability. To obtain high-quality perovskite films and ultimately high-performance perovskite devices, it is crucial to understand the film formation mechanisms, which, however, remains a great challenge, due to the complexity of perovskite composition, dimensionality, and processing conditions. Nevertheless, the state-of-the-art in situ grazing-incidence wide-angle X-ray scattering (GIWAXS) technique enables one to bridge the complex information with device performance by revealing the crystallization pathways during the perovskite film formation process. In this review, the authors illustrate how to obtain and understand in situ GIWAXS data, summarize and assess recent results of in situ GIWAXS studies on versatile perovskite photovoltaic systems, aiming at elucidating the distinct features and common ground of film formation mechanisms, and shedding light on future opportunities of employing in situ GIWAXS to study the fundamental working mechanisms of highly efficient and stable perovskite solar cells toward mass production.

Highly Stable Thin Films Based on Novel Hybrid 1D (PRSH)PbX3 Pseudo-Perovskites

In this study, the structure and morphology, as well as time, ultraviolet radiation, and humidity stability of thin films based on newly developed 1D (PRSH)PbX₃ (X = Br, I) pseudo-perovskite materials, containing 1D chains of face-sharing haloplumbate octahedra, are investigated. All films are strongly crystalline already at room temperature, and annealing does not promote further crystallization or film reorganization. The film microstructure is found to be strongly influenced by the anion type and, to a lesser extent, by the DMF/DMSO solvent volume ratio used during film deposition by spin-coating. Comparison of specular X-ray diffraction and complementary grazing incidence X-ray diffraction analysis indicates that the use of DMF/DMSO mixed solvents promotes the strengthening of a dominant 100 or 210 texturing, as compared the case of pure DMF, and that the haloplumbate chains always lie in a plane parallel to the substrate. Under specific DMF/DMSO solvent volume ratios, the prepared films are found to be highly stable in time (up to seven months under fluxing N₂ and in the dark) and to highly moist conditions (up to 25 days at 78% relative humidity). Furthermore, for representative (PRSH)PbX₃ films, resistance against ultraviolet exposure (λ = 380 nm) is investigated, showing complete stability after irradiation for up to 15 h at a power density of 600 mW/cm². These results make such thin films interesting for highly stable perovskite-based (opto)electronic devices.

The Impact of Solvent Vapor on the Film Morphology and Crystallization Kinetics of Lead Halide Perovskites during Annealing

One of the key factors for the remarkable improvements of halide perovskite solar cells over the last few years is the increased control over perovskite crystallinity and its thin film morphology. Among various processing methods, solvent vapor-assisted annealing (SVAA) has proven to be promising in achieving high-quality perovskite films. However, a comprehensive understanding of the perovskite crystallization process during SVAA is still lacking. In this work, we use a home-built setup to precisely control the SVAA conditions to investigate in detail the perovskite crystallization kinetics. By changing the solvent vapor concentration during annealing, the perovskite grain size can be tuned from 200 nm to several micrometers. We monitor the crystallization kinetics during solvent-free annealing and SVAA using in situ grazing incidence wide-angle X-ray scattering, where we find a diminished perovskite growth rate and the formation of low dimensional perovskite at the top of the perovskite layer during SVAA. Scanning electron microscopy images of the final films further suggest that the perovskite growth follows an Ostwald ripening process at higher solvent concentrations. Thus, our results will contribute to achieve a more targeted processing of perovskite films.

MA Cation-Induced Diffusional Growth of Low-Bandgap FA-Cs Perovskites Driven by Natural Gradient Annealing

Low-bandgap formamidinium-cesium (FA-Cs) perovskites of FA_1-xCs_xPbI₃ (x < 0.1) are promising candidates for efficient and robust perovskite solar cells, but their black-phase crystallization is very sensitive to annealing temperature. Unfortunately, the low heat conductivity of the glass substrate builds up a temperature gradient within from bottom to top and makes the initial annealing temperature of the perovskite film lower than the black-phase crystallization point (~150°C). Herein, we take advantage of such temperature gradient for the diffusional growth of high-quality FA-Cs perovskites by introducing a thermally unstable MA⁺ cation, which would firstly form α-phase FA-MA-Cs mixed perovskites with low formation energy at the hot bottom of the perovskite films in the early annealing stage. The natural gradient annealing temperature and the thermally unstable MA⁺ cation then lead to the bottom-to-top diffusional growth of highly orientated α-phase FA-Cs perovskite, which exhibits 10-fold of enhanced crystallinity and reduced trap density (~3.85 × 10¹⁵ cm⁻³). Eventually, such FA-Cs perovskite films were fabricated into stable solar cell devices with champion efficiency up to 23.11%, among the highest efficiency of MA-free perovskite solar cells.

Insights into Microscopic Crystal Growth Dynamics of CH3NH3PbI3 under a Laser Deposition Process Revealed by In Situ X-ray Diffraction

The process dynamics for the vacuum deposition of methylammonium lead iodide (MAPbI₃) perovskite was analyzed by in situ X-ray diffraction using synchrotron radiation. MAPbI₃ was fabricated by alternatingly supplying PbI₂ and methylammonium iodide via a laser deposition system installed at the synchrotron beamline BL46XU at SPring-8, and in situ crystallization analysis was conducted. Microscopic insights into the crystallization were obtained, including observation of Laue oscillation during the PbI₂ growth and octahedral unit (PbI₆) rotation during the transformation into perovskite. On the basis of this analysis, conditions that favor the construction of atomically flat MAPbI₃ perovskite films were deduced.

Mechanism of Additive-Assisted Room-Temperature Processing of Metal Halide Perovskite Thin Films

Perovskite solar cells have received substantial attention due to their potential for low-cost photovoltaic devices on flexible or rigid substrates. Thiocyanate (SCN)-containing additives, such as MASCN (MA = methylammonium), have been shown to control perovskite film crystallization and the film microstructure to achieve effective room-temperature perovskite absorber processing. Nevertheless, the crystallization pathways and mechanisms of perovskite formation involved in MASCN additive processing are far from clear. Using in situ X-ray diffraction and photoluminescence, we investigate the crystallization pathways of MAPbI₃ and reveal the mechanisms of additive-assisted perovskite formation during spin coating and subsequent N₂ drying. We confirm that MASCN induces large precursor aggregates in solution and, during spin coating, promotes the formation of the perovskite phase with lower nucleation density and overall larger initial nuclei size, which forms upon reaching supersaturation in solution, in addition to intermediate solvent-complex phases. Finally, during the subsequent N₂ drying, MASCN facilitates the dissociation of these precursor aggregates and the solvate phases, leading to further growth of the perovskite crystals. Our results show that the nature of the intermediate phases and their formation/dissociation kinetics determine the nucleation and growth of the perovskite phase, which subsequently impact the film microstructure. These findings provide mechanistic insights underlying room-temperature, additive-assisted perovskite processing and help guide further development of such facile room-temperature synthesis routes.

Proton sponge lead halides containing 1D polyoctahedral chains

Hybrid organic/inorganic lead halides with the proton sponge moiety show face-sharing [PbX₆] octahedra forming linear 1D chains. These species exhibit complete (Br, I) miscibility and exceptional anisotropic thermal expansion.

Temperature-Assisted Crystal Growth of Photovoltaic α-Phase FAPbI3 Thin Films by Sequential Blade Coating

Formamidinium lead triiodide (FAPbI₃) exhibits the smallest band gap among lead halide perovskites, which is more desirable for solar cell applications compared to methylammonium-based counterparts. However, it remains a big challenge to prepare phase-pure α-FAPbI₃ in addition to controlling the crystal morphology during film formation. Herein, we developed a temperature-assisted crystal growth to prepare high-quality thin films of α-FAPbI₃ by sequential blade coating. It is found that depositing organic cation FAI at elevated temperatures facilitates the growth of α-FAPbI₃, which otherwise yields mainly a yellow δ-phase at room temperature. In parallel, the crystal morphology of the perovskite films can be effectively manipulated by taking advantage of the porous structure of PbI₂. Solar cells prepared with the blade-coated α-FAPbI₃ yield a champion efficiency of 18.41%, which is among the highest values for FAPbI₃-only solar devices. These results suggest that two-step sequential blade deposition offers a viable approach to fabricate high-quality α-FAPbI₃ films for optoelectronic applications.

Recent Advancements of N-Doped Graphene for Rechargeable Batteries: A Review

Graphene, a 2D carbon structure, due to its unique materials characteristics for energy storage applications has grasped the considerable attention of scientists. The highlighted properties of this material with a mechanically robust and highly conductive nature have opened new opportunities for different energy storage systems such as Li-S (lithium-sulfur), Li-ion batteries, and metal-air batteries. It is necessary to understand the intrinsic properties of graphene materials to widen its large-scale applications in energy storage systems. In this review, different routes of graphene synthesis were investigated using chemical, thermal, plasma, and other methods along with their advantages and disadvantages. Apart from this, the applications of N-doped graphene in energy storage devices were discussed.

Stability evolution of ultrafine Ag nanoparticles prepared by laser ablation in liquids

Understanding the stability evolution of the silver nanoparticles (Ag NPs) in colloid has great benefits for its controllable preparation, storage and application. Herein, uncapped Ag NPs with diameter of 1.66 ± 0.37 nm are obtained by laser ablation of Ag target in deionized water, corresponding surface plasma resonance (SPR) bands, ζ potential and particle size distribution are monitored to investigate uncapped Ag NPs' stability evolution. Due to negatively charged surface, uncapped Ag NPs show an excellent dispersion stability in 70 days without any external disturbance. But its dispersion stability and structure stability are destroyed easily by an oscillation treatment, resulting in a tardy growth and the formation of one-dimensional Ag nanochain. In addition, the chemical stability of uncapped Ag NPs is dramatically varied by a displacement reaction with an inserted copper wire. As comparison, two typical cationic and anionic surfactant molecules, N-hexadecyl trimethyl ammonium chloride (CTAC) and sodium dodecyl benzene sulfonate (SDBS) are severally used to prepare surface capped Ag NPs. With same treatment of Ag colloid, both two kinds of capped Ag NPs display better dispersion stability and structure stability than uncapped Ag NPs. Moreover, CTAC capped Ag NPs keep a better chemical stability than SDBS capped Ag NPs.

Perovskite Crystallization Dynamics during Spin-Casting: An In Situ Wide-Angle X-ray Scattering Study

In situ wide-angle X-ray scattering (WAXS) has been measured during the spin coating process used to make the precursor films required for the formation of thin films of perovskite. A customized hollow axis spin coater was developed to permit the scattered X-rays to be collected in transmission geometry during the deposition process. Spin coating is the technique most commonly used in laboratories to make thin perovskite films. The dynamics of spin-casting MAPbI_3-x Cl _x and FAPbI_3-x Cl _x films have been investigated and compared to investigate the differences between the dynamics of MAPbI_3-x Cl _x and FAPbI_3-x Cl _x film formation. In particular, we focus on the crystallization dynamics of the precursor film formation. When casting MAPbI_3-x Cl _x , we observed relatively fast 1D crystallization of the intermediate product MA₂PbI₃Cl. There was an absence of the desired perovskite phase formed directly; it only appeared after an annealing step that converted the MA₂PbI₃Cl to MAPbI₃. In contrast, slower crystallization via a 3D precursor was observed for FAPbI_3-x Cl _x film formation compared to MAPbI_3-x Cl _x . Another important finding was that some FAPbI_3-x Cl _x perovskite was generated directly during spin-casting before annealing. These findings indicate that there are significant differences between the crystallization pathways for these two perovskite materials. These are likely to explain the differences in the lifetimes of the resulting perovskite solar cell devices produced using FA and MA cations.

Spontaneous low-temperature crystallization of α-FAPbI3 for highly efficient perovskite solar cells

Formamidinium lead triiodide (HC(NH₂)₂PbI₃ or FAPbI₃) is a promising light absorber for high-efficiency perovskite solar cells because of its superior light absorption range and thermal stability to CH₃NH₃PbI₃ (MAPbI₃). Unfortunately, it is difficult to fabricate high-quality FAPbI₃ thin films to surpass the MAPbI₃-based cells due to easily forming unwanted but more stable yellow δ-phase and thus requiring high annealing-temperature for wanted photovoltaic-active black α-phase. Herein, we reported a novel low-temperature fabrication of highly crystallized α-FAPbI₃ film exhibiting uniaxial-oriented nature with large grain sizes up to 2 μm. First-principles energetic calculations predicted that this novel deposition should be ascribed to the formation of a high-energy metastable two-dimensional (2D) intermediate of MAFAPbI₃Cl followed by a spontaneous conversion to α-FAPbI₃. The ions exchange reaction during this MAFAPbI₃Cl-FAPbI₃ conversion account for the perovskite film uniaxial-oriented grown along the (1 1 1) direction. This large-grain and uniaxial-oriented grown α-FAPbI₃ based solar cells exhibited an efficiency up to 20.4% accompanying with low density-voltage (J-V) hysteresis and high stability.

Template-Assisted Formation of High-Quality α-Phase HC(NH2)2PbI3 Perovskite Solar Cells

Formamidinium (FA) lead halide (α-FAPbI3) perovskites are promising materials for photovoltaic applications because of their excellent light harvesting capability (absorption edge 840 nm) and long carrier diffusion length. However, it is extremely difficult to prepare a pure α-FAPbI3 phase because of its easy transformation into a nondesirable δ-FAPbI3 phase. In the present study, a "perovskite" template (MAPbI3-FAI-PbI2-DMSO) structure is used to avoid and suppress the formation of δ-FAPbI3 phases. The perovskite structure is formed via postdeposition involving the treatment of colloidal MAI-PbI2-DMSO film with FAI before annealing. In situ X-ray diffraction in vacuum shows no detectable δ-FAPbI3 phase during the whole synthesis process when the sample is annealed from 100 to 180 °C. This method is found to reduce defects at grain boundaries and enhance the film quality as determined by means of photoluminescence mapping and Kelvin probe force microscopy. The perovskite solar cells (PSCs) fabricated by this method demonstrate a much-enhanced short-circuit current density (  J sc) of 24.99 mA cm-2 and a power conversion efficiency (PCE) of 21.24%, which is the highest efficiency reported for pure FAPbI3, with great stability under 800 h of thermal ageing and 500 h of light soaking in nitrogen.

Manipulating the Mixed-Perovskite Crystallization Pathway Unveiled by In Situ GIWAXS

Mixed perovskites have achieved substantial successes in boosting solar cell efficiency, but the complicated perovskite crystal formation pathway remains mysterious. Here, the detailed crystallization process of mixed perovskites (FA_0.83 MA_0.17 Pb(I_0.83 Br_0.17 )₃ ) during spin-coating is revealed by in situ grazing-incidence wide-angle X-ray scattering measurements, and three phase-formation stages are identified: I) precursor solution; II) hexagonal δ-phase (2H); and III) complex phases including hexagonal polytypes (4H, 6H), MAI-PbI₂ -DMSO intermediate phases, and perovskite α-phase. The correlated device performance and ex situ characterizations suggest the existence of an "annealing window" covering the duration of stage II. The spin-coated film should be annealed within the annealing window to avoid the formation of hexagonal polytypes during the perovskite crystallization process, thus achieving a good device performance. Remarkably, the crystallization pathway can be manipulated by incorporating Cs⁺ ions in mixed perovskites. Combined with density functional theory calculations, the perovskite system with sufficient Cs⁺ will bypass the formation of secondary phases in stage III by promoting the formation of α-phase both kinetically and thermodynamically, thereby significantly extending the annealing window. This study provides underlying reasons of the time sensitivity of fabricating mixed-perovskite devices and insightful guidelines for manipulating the perovskite crystallization pathways toward higher performance.

Improvement of perovskite crystallinity by omnidirectional heat transfer via radiative thermal annealing

As promising photo-absorbing materials for photovoltaics, organic-inorganic hybrid perovskite materials such as methylammonium lead iodide and formamidinium lead iodide, have attracted lots of attention from many researchers. Among the various factors to be considered for high power conversion efficiency (PCE) in perovskite solar cells (PSCs), increasing the grain size of perovskite is most important. However, it is difficult to obtain a highly crystalline perovskite film with large grain size by using the conventional hot-plate annealing method because heat is transferred unidirectionally from the bottom to the top. In this work, we presented radiative thermal annealing (RTA) to improve the structural and electrical properties of perovskite films. Owing to the omnidirectional heat transfer, swift and uniform nuclei formation was possible within the perovskite film. An average grain size of 500 nm was obtained, which is 5 times larger than that of the perovskite film annealed on a hot-plate. This perovskite film led to an enhancement of photovoltaic performance of PSCs. Both short-circuit current density and PCE of the PSCs prepared by RTA were improved by 10%, compared to those of PSCs prepared by hot-plate annealing.

Perovskite Quantum Dot Photovoltaic Materials beyond the Reach of Thin Films: Full-Range Tuning of A-Site Cation Composition

We present a cation-exchange approach for tunable A-site alloys of cesium (Cs⁺) and formamidinium (FA⁺) lead triiodide perovskite nanocrystals that enables the formation of compositions spanning the complete range of Cs_{1- x}FA _xPbI₃, unlike thin-film alloys or the direct synthesis of alloyed perovskite nanocrystals. These materials show bright and finely tunable emission in the red and near-infrared range between 650 and 800 nm. The activation energy for the miscibility between Cs⁺ and FA⁺ is measured (∼0.65 eV) and is shown to be higher than reported for X-site exchange in lead halide perovskites. We use these alloyed colloidal perovskite quantum dots to fabricate photovoltaic devices. In addition to the expanded compositional range for Cs_{1- x}FA _xPbI₃ materials, the quantum dot solar cells exhibit high open-circuit voltage ( V_OC) with a lower loss than the thin-film perovskite devices of similar compositions.

Pressure-Induced Phase Transformation and Band-Gap Engineering of Formamidinium Lead Iodide Perovskite Nanocrystals

Formamidinium lead halide (FAPbX₃, X = Cl, Br, I) perovskite materials have recently drawn an increased amount of attention owing to their superior optoelectronic properties and enhanced material stability as compared with their methylammonium-based (MA-based) analogues. Herein, we report a study of the pressure-induced structural and optical evolutions of FAPbI₃ hybrid organic-inorganic perovskite nanocrystals (NCs) using a synchrotron-based X-ray scattering technique coupled to in situ absorption and photoluminescence spectroscopies. As a result of their unique structural stability and soft nature, FAPbI₃ NCs exhibit a wide range of band-gap tunability (1.44-2.17 eV) as a function of pressure (0-13.4 GPa). The study presented here not only provides an efficient and chemically orthogonal means to controllably engineer the band gap of FAPbI₃ NCs using pressure but more importantly sheds light on how to strategically design the band gaps of FA-based hybrid organic-inorganic perovskites for various optoelectronic applications.

Composition-Tunable Formamidinium Lead Mixed Halide Perovskites via Solvent-Free Mechanochemical Synthesis: Decoding the Pb Environments Using Solid-State NMR Spectroscopy

Mixed-halide lead perovskites are becoming of paramount interest in the optoelectronic and photovoltaic research fields, offering band gap tunability, improved efficiency, and enhanced stability compared to their single halide counterparts. Formamidinium-based mixed halide perovskites (FA-MHPs) are critical to obtaining optimum solar cell performance. Here, we report a solvent-free mechanochemical synthesis (MCS) method to prepare FA-MHPs, starting with their parent compounds (FAPbX₃; X = Cl, Br, I), achieving compositions not previously accessible through the solvent synthesis (SS) technique. By probing local Pb environments in MCS FA-MHPs using solid-state nuclear magnetic resonance spectroscopy, along with powder X-ray diffraction for long-range crystallinity and reflectance measurements to determine the optical band gap, we show that MCS FA-MHPs form atomic-level solid solutions between Cl/Br and Br/I MHPs. Our results pave the way for advanced methods in atomic-level structural understanding while offering a one-pot synthetic approach to prepare MHPs with superior control of stoichiometry.

In Situ Real-Time Study of the Dynamic Formation and Conversion Processes of Metal Halide Perovskite Films

Metal halide perovskite solar cells (PSCs) have advanced to the forefront of solution-processed photovoltaic techniques and made stunning progress in power conversion efficiency (PCE). Further improvements in device performances rely on perfecting the structure and morphology of perovskite films. However, undesirable defects such as pinholes and grain boundaries are often created in film preparations due to lack of knowledge of the precise reaction mechanism. Here, in situ grazing-incidence X-ray diffraction (GI-XRD) investigations are performed, facilitated by other techniques, on the formation of the widely adopted MAPbI₃ (MA = methylammonium) perovskite films from their intermediate adduct (IA) phases. The influences of solvent vapor atmospheres on MAPbI₃ films are also systematically investigated, where the dynamic conversion processes between different phases are visualized in real time. Further in situ GI-XRD and infrared spectroscopy measurements reveal that the IA phases contain both N,N-dimethylformamide and dimethyl sulfoxide (DMSO) as coordinating molecules. By tuning the DMSO concentration in perovskite precursors, the ideal perovskite film is formed and the best PCE is achieved for the planar MAPbI₃ -based PSCs. These findings highlight the role of IA phases and the effect of solvent atmospheres on the quality of perovskite films, providing direct insights into their growth mechanism.

Critical Intermediate Structure That Directs the Crystalline Texture and Surface Morphology of Organo-Lead Trihalide Perovskite

We have identified an often observed yet unresolved intermediate structure in a popular processing with dimethylformamide solutions of lead chloride and methylammonium iodide for perovskite solar cells. With subsecond time-resolved grazing-incidence X-ray scattering and X-ray photoemission spectroscopy, supplemental with ab initio calculation, the resolved intermediate structure (CH₃NH₃)₂PbI₂Cl₂·CH₃NH₃I features two-dimensional (2D) perovskite bilayers of zigzagged lead-halide octahedra and sandwiched CH₃NH₃I layers. Such intermediate structure reveals a hidden correlation between the intermediate phase and the composition of the processing solution. Most importantly, the 2D perovskite lattice of the intermediate phase is largely crystallographically aligned with the [110] planes of the three-dimensional perovskite cubic phase; consequently, with sublimation of Cl ions from the organo-lead octahedral terminal corners in prolonged annealing, the zigzagged octahedral layers of the intermediate phase can merge with the intercalated methylammonium iodide layers for templated growth of perovskite crystals. Regulated by annealing temperature and the activation energies of the intermediate and perovskite, deduced from analysis of temperature-dependent structural kinetics, the intermediate phase is found to selectively mature first and then melt along the layering direction for epitaxial conversion into perovskite crystals. The unveiled epitaxial conversion under growth kinetics controls might be general for solution-processed and intermediate-templated perovskite formation.

Progress on Perovskite Materials and Solar Cells with Mixed Cations and Halide Anions

Organic-inorganic halide perovskite materials (e.g., MAPbI₃, FAPbI₃, etc.; where MA = CH₃NH₃⁺, FA = CH(NH₂)₂⁺) have been studied intensively for photovoltaic applications. Major concerns for the commercialization of perovskite photovoltaic technology to take off include lead toxicity, long-term stability, hysteresis, and optimal bandgap. Therefore, there is still need for further exploration of alternative candidates. Elemental composition engineering of MAPbI₃ and FAPbI₃ has been proposed to address the above concerns. Among the best six certified power conversion efficiencies reported by National Renewable Energy Laboratory on perovskite-based solar cells, five are based on mixed perovskites (e.g., MAPbI_1-xBr_x, FA_0.85MA_0.15PbI_2.55Br_0.45, Cs_0.1FA_0.75MA_0.15PbI_2.49Br_0.51). In this paper, we review the recent progress on the synthesis and fundamental aspects of mixed cation and halide perovskites correlating with device performance, long-term stability, and hysteresis. In the outlook, we outline the future research directions based on the reported results as well as related topics that warrant further investigation.

Direct Laser Writing of δ- to α-Phase Transformation in Formamidinium Lead Iodide

Organolead halide perovskites are increasingly considered for applications well beyond photovoltaics, for example, as the active regions within photonic devices. Herein, we report the direct laser writing (DLW: 458 nm cw-laser) of the formamidinium lead iodide (FAPbI₃) yellow δ-phase into its high-temperature luminescent black α-phase, a remarkably easy and scalable approach that takes advantage of the material's susceptibility to transition under ambient conditions. Through the DLW of α-FAPbI₃ tracks on δ-FAPbI₃ single-crystal surfaces, the controlled and rapid microfabrication of highly luminescent structures exhibiting long-term phase stability is detailed, offering an avenue toward the prototyping of complex perovskite-based optical devices. The dynamics and kinetics of laser-induced δ- to α-phase transformations are investigated in situ by Raman microprobe analysis, as a function of irradiation power, time, temperature, and atmospheric conditions, revealing an interesting connection between oxygen intercalation at the surface and the δ- to α-phase transformation dynamics, an insight that will find application within the wider context of FAPbI₃ thermal phase relations.

Accelerated Lifetime Testing of Organic-Inorganic Perovskite Solar Cells Encapsulated by Polyisobutylene

Metal halide perovskite solar cells (PSCs) have undergone rapid progress. However, unstable performance caused by sensitivity to environmental moisture and high temperature is a major impediment to commercialization of PSCs. In the present work, a low-temperature, glass-glass encapsulation technique using high performance polyisobutylene (PIB) as the moisture barrier is investigated on planar glass/FTO/TiO₂/FAPbI₃/PTAA/gold perovskite solar cells. PIB was applied as either an edge seal or blanket layer. Electrical connections to the encapsulated PSCs were provided by either the FTO or Au layers. Results of a "calcium test" demonstrated that a PIB edge-seal effectively prevents moisture ingress. A shelf life test was performed and the PIB-sealed PSC was stable for at least 200 days. Damp heat and thermal cycling tests, in compliance with IEC61215:2016, were used to evaluate different encapsulation methods. Current-voltage measurements were performed regularly under simulated AM1.5G sunlight to monitor changes in PCE. The best results we have achieved to date maintained the initial efficiency after 540 h of damp heat testing and 200 thermal cycles. To the best of the authors' knowledge, these are among the best damp heat and thermal cycle test results for perovskite solar cells published to date. Given the modest performance of the cells (8% averaged from forward and reverse scans) especially with the more challenging FAPbI₃ perovskite material tested in this work, it is envisaged that better stability results can be further achieved when higher performance perovskite solar cells are encapsulated using the PIB packaging techniques developed in this work. We propose that heat rather than moisture was the main cause of our PSC degradation. Furthermore, we propose that preventing the escape of volatile decomposition products from the perovskite solar cell materials is the key for stability. PIB encapsulation is a very promising packaging solution for perovskite solar cells, given its demonstrated effectiveness, ease of application, low application temperature, and low cost.