Deposition Of Perovskite Films Research Articles

Camphorsulfonic‐Salified Chitosan Allowing MACl‐Free Stabilization of Pure FAPbI3 α‐Phase via Gravure Printing in Ambient Air

Metal–halide perovskites have gained extreme interest in the photovoltaic field with formamidinium lead iodide (FAPbI3) currently being one of the best‐performing materials for single‐junction solar cells. Despite the outstanding record efficiencies, there are still several major issues hindering the large‐scale fabrication of perovskite solar cells. The vulnerability to environmental agents along with the need of controlled atmosphere and crystallization aids for the perovskite film deposition represents the major roadblocks. This is particularly true for FAPbI3 for which the thermodynamically stable phase at room temperature is photovoltaically inactive δ‐phase. To address those challenges, herein, a camphorsulfonic‐salified chitosan is specifically designed with the aid of DTF calculations to strongly interact with the perovskite and, as a result, improve the morphology and optoelectronic quality of the FAPbI3. Thanks to the numerous interactions and then the modulation of the solution viscosity, FAPbI3 devices are fabricated by gravure printing deposition without either antisolvent bath or inclusion of methylammonium chloride (MACl) as additive. The gravure‐printed devices with the chitosan feature an enhanced efficiency and stability in air, retaining 80% of the original efficiency after 1200 h in ambient air without any encapsulation.

Controlling Water for Enhanced Homogeneities in Perovskite Solar Cells with Remarkable Reproducibility

AbstractTwo‐step sequential deposition of perovskite films is proven efficient and compatible with large‐scale production. Nevertheless, its critically regulated environmental humidity often remains a burden, hindering its applications in reproducible fabrication of perovskite solar cells (PSCs). Herein, by introducing a water ethyl acetate (W‐EA) solution treatment before annealing, improved perovskite crystallinity and homogeneity in FA0.95‐xMAxCs0.05PbI3 films without additional composition regulation is achieved. In situ Grazing‐incidence Wide‐angle X‐ray Scattering experiments highlighted accelerated crystallization kinetics in W‐EA‐treated films, leading to high‐quality perovskite formation. PSCs based on this method reached an optimal power conversion efficiency of 25.01% and demonstrated enhanced moisture resistance, retaining 88% efficiency after 1000 h in ambient conditions. This W‐EA treatment simplifies the fabrication process, greatly reducing the need for controlled humidity, and offers insights into the role of water in perovskite film crystallization, with significant implications to produce high‐performance PSCs.

Synergistic Effect of Alkylammonium Chlorides to Trigger an Ultrafast Nucleation for Antisolvent‐Free Perovskite Solar Cells Processed from 2‐Methoxyethanol

Abstract2‐Methoxyethanol (2ME), as a more environmentally friendly solvent with a lower boiling point compared to dimethylformamide, is ideal for the fabrication of perovskite solar cells (PSCs). However, when 2ME is used for antisolvent‐free deposition of perovskite films, an uncontrolled nucleation process and an easy phase transition to the δ‐phase often occur. Herein, an ultrafast nucleation process is developed using methylamine chloride (MACl) and n‐butylammonium chloride (BACl) as dual additives in 2ME without further solvent addition. While MACl can rapidly induce MACl‐based nuclei to initiate the nucleation process for formamidinium lead iodide (FAPbI3), the addition of BACl to the precursor with MACl can further increase the nucleation rate and density of nuclei, and bypass the transition from δ‐ to α‐phase during crystal growth to obtain a highly crystalline and pinhole‐free perovskite film. As a result, the FAPbI3 PSCs achieve a power conversion efficiency (PCE) of 23.6%. This work provides a new inspiration for controlling the crystal quality of perovskite thin films via nucleation rate suitable for upscaling.

In Situ Probing the Crystallization Kinetics in Gas‐Quenching‐Assisted Coating of Perovskite Films

AbstractThe pursuit of commercializing perovskite photovoltaics is driving the development of various scalable perovskite crystallization techniques. Among them, gas quenching is a promising crystallization approach for high‐throughput deposition of perovskite films. However, the perovskite films prepared by gas‐quenching assisted blade coating are susceptible to the formation of pinholes and frequently show inferior crystallinity if the interplay between film coating, film drying, and crystallization kinetics is not fully optimized. That arguably requires a thorough understanding of how single processing steps influence the crystallization kinetics of printed perovskite films. Here, in situ optical spectroscopies are integrated into a doctor‐blading setup that allows to real‐time monitor film formation during the gas‐quenching process. It is found that the essential role of gas quenching treatment is in achieving a smooth and compact perovskite film by controlling the nucleation rate. Moreover, with the assistance of phase‐field simulations, the role of excessive methylammonium iodide is revealed to increase grain size by accelerating the crystal growth rate. These results show a tailored control of crystal growth rate is critical to achieving optimal film quality, leading to fully printed solar cells with a champion power conversion efficiency of 19.50% and mini solar modules with 15.28% efficiency are achieved.

Post-hot-cast annealing deposition of perovskite films with infused multifunctional organic molecules to enhance the performance of large-area light-emitting devices.

All-inorganic perovskites show great promise as an emission layer in perovskite light-emitting diodes (PeLEDs) owing to their easy solution processing, low manufacturing cost, and excellent optoelectronic properties. However, there is still an immense performance gap from small-area devices to large-area PeLED devices. The inhomogeneity of large-area high-quality perovskite films inevitably leads to vast defects and electroluminescence performance losses. Herein, a post-hot-cast annealing deposition scheme and the introduction of the multifunctional molecule 2-amino-1,3-propanediol (APDO) were proposed to regulate the crystallization of the perovskite film. As a result, uniform APDO:CsPbBr2.5Cl0.5 perovskite films with high crystallinity and lower defect density were deposited by post-hot-cast annealing. A decent maximum brightness of 2659 cd m-2 was achieved for the large-area cyan PeLEDs with an emitting area of 400 mm2.

Effect of Chloride Incorporation on the Intermediate Phase and Film Morphology of Methylammonium Lead Halide Perovskites.

The influence of chloride integration on perovskite film deposition, encompassing both the structures of intermediate phases and the properties of the final films, was explored. Our methodology involved the fabrication of perovskite intermediate-phase films with varying concentrations of methylammonium chloride (MACl). Subsequently, we conducted an analysis employing X-ray diffraction and Rietveld refinement, incorporating the March-Dollase correction, to gain insights into how chloride-induced intermediate phases impact film morphology. Remarkably, a distinct preferred orientation was observed in the (020) lattice plane perpendicular to the substrate surface, and this orientation was found to be directly correlated to the MACl concentration. This distinctive arrangement of chloride-induced intermediate-phase complexes facilitated controlled crystallization, leading to highly oriented crystals and an improved film morphology. As a consequence, perovskite solar cell devices incorporating chloride-containing methylammonium lead iodide achieved a power conversion efficiency exceeding 20%. These findings suggest a crucial link between the preferred orientation observed in the final chlorine-derived perovskite films and the intermediate-phase structure formed during the initial stages of perovskite formation. These results suggest a profound impact of intermediate phase compositions and crystal structures on perovskite formation, emphasizing the importance of a comprehensive understanding of these factors to enable precise control over ideal structures and the subsequent transformation into high-quality perovskite films.

Fabrication of high-efficiency perovskite solar cells and mini-modules by expanding the processing window with KSCN additive

Perovskite solar cells (PSCs) have garnered significant attention due to their high efficiency and low cost, making them a promising contender for the future of photovoltaic (PV) technology. Stable and controllable preparation of high-efficiency PSCs is crucial for the advancement of perovskite PV technology's industrialization. Herein, we demonstrate an additive-assisted blade-coating technique that boasts a wide air knife pressure window processing, making it ideal for the scalable deposition of perovskite films. KSCN was added to the perovskite precursor to induce an extra endothermic reaction, which effectively reduced the solvent evaporation rate, broadened the processing window, and slowed down the nucleation and crystallization process of the perovskite film. Moreover, the introduction of K+ led to the suppression of the formation of metal Pb0 and iodine vacancies in the film. Therefore, the high-quality perovskite films with full coverage, larger grains, higher crystallinity, fewer defects, as well as large-scale uniformity can be obtained under variable air knife pressure. As a result, power conversion efficiencies of 20.40% for small-area (0.07 cm2) blade-coated PSCs and 17.62% for perovskite solar modules with an active area of 10.00 cm2 were achieved. The proposed blade-coating technique with a wide processing window holds great potential for the commercialization of perovskite PV technology.

Sputtered PbI2 with Post‐Processing for Perovskite Solar Cells

Conformal deposition of perovskite films on textured silicon surfaces using a dry process is crucial for producing high‐performance perovskite/silicon tandem solar cells. Herein, a radio frequency magnetron sputtering process is used with a PbI2 target to deposit precursor films. Iodination, thermal annealing, and dimethyl sulfoxide treatment are employed as posttreatment processes to improve the stoichiometry, crystallinity, and surface morphology of the PbI2 precursor. The precursor films are converted into perovskite using direct contact conversion process, and the interfacial and bulk properties are enhanced by methylamine vapor annealing to fabricate perovskite solar cells with a power conversion efficiency of 12.2%. Also, 18.3% efficiency is confirmed at a wider voltage sweep range, which suggests that further efficiency improvement is possible by removing defects inside the perovskite. Finally, uniform perovskite films are conformally deposited on a 25 cm2 textured silicon surface. With such high‐efficiency potential and conformality, the method of sputtering PbI2 can open a new way to fabricate perovskite/silicon tandem solar cells.

Buried Interface Engineering Enables Efficient, Scalable, and Stable Inverted Perovskite Solar Cells

Poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine] (PTAA), as an extensively used hole transport material in inverted perovskite solar cells (PSCs), has given reason for concern due to its hydrophobicity for a long time. Herein, buried interface engineering is applied for the scalable deposition of perovskite films by introducing an amphiphilic molecule hexadecyltrimethylammonium chloride on the PTAA surface, which improves the interfacial wettability of the perovskite precursor solution on the organic hole transport layer (HTL), facilitates the nucleation and growth of perovskites, and reduces the nonradiative recombination at the perovskite/HTL interface. As a result, all photovoltaic parameters of the inverted PSCs are improved significantly. The champion devices demonstrate power conversion efficiencies (PCEs) of 22.04% and 20.47% with aperture areas of 0.148 and 1.0 cm2, respectively. Moreover, the encapsulated 1.0 cm2 device exhibits excellent stability and maintains over 70% of its initial PCE after 1200 h under continuous 1 sun illumination at 65 °C in a nitrogen environment.

Molten Salt Strategy for Reproducible Evaporation of Efficient Perovskite Solar Cells

AbstractVacuum evaporation is promising for the scalable fabrication of perovskite solar cells (PSCs). Nevertheless, the poor thermal conductivity of metal halide powder leads to unfavorable temperature inhomogeneity, which destabilizes the evaporation rate, posing a major challenge to the reproducible deposition of perovskite films, particularly by coevaporation. Herein, a molten salt strategy is reported for sequentially vacuum evaporation of PSCs. The molten salt increases the thermal conductivity of metal halides and greatly homogenizes the temperature, which stabilizes the evaporation rate and the composition of the resulting perovskite films. The PSCs yield power conversion efficiencies (PCEs) of ≈24% with exceptional reproducibility. The unencapsulated PSCs maintain 85% of the initial PCE after 3600 h of maximum power point tracking and maintain 85% of the initial PCE after being heated at 60 °C for 3000 h. The molten salt strategy opens a new avenue for the application of evaporation in perovskite optoelectronics.

(Digital Presentation) Preparation of Metal Halide Perovskite Thin Films Via Laser Deposition

We have been preparing metal halide perovskite (hereinafter referred to as perovskite) thin films by laser deposition. In this study, we report the results of our investigation of the effects of laser wavelength and fluence on the deposition process. One of the most important features of perovskites, which have been rapidly developed in recent years as materials for high-efficiency and low-cost thin-film solar cells, is the applicability of a simple process using solution coating under atmospheric pressure [1]. Crystal growth of perovskite by the solution method occurs through equilibrium chemical reactions, enabling the formation of thin films on the order of μm thickness in less than a few seconds; however, it is difficult to systematically control the crystal growth process at the molecular layer level by the solution method. On the other hand, in the physical vapor deposition (PVD) method, although the deposition rate is relatively low, ranging from sub-nanometer to several nm/s, the deposition can be controlled at the molecular layer level, as well as allowing different atoms to be readily introduced into the film. Therefore, the PVD method allows the addition of elements that are difficult to achieve with solution reactions and is suitable for studies that investigate the effects of such elements on physical properties.In laser deposition (LD), one of the PVD methods, materials are irradiated with an infrared (IR) or an ultraviolet (UV) laser beam, and thin films are formed by evaporation through IR heating or ablation with plasma formation through UV electron excitation. The advantage of LD is that it can cause instantaneous heating or ablation by switching on and off laser irradiation only on the material surface, thus enabling the deposition of molecular layers of organic materials with high vapor pressure or high melting point materials such as inorganic oxides with minimal cross-contamination caused by evaporation. We have prepared perovskite thin films by alternate deposition of PbI2 and CH3NH3I (MAI) using an IR laser and solid-phase reaction at room temperature aiming at the detailed investigation of the crystal growth process and introduction of various dopants in perovskite materials. [2, 3]. In this study, we have also performed perovskite film deposition by UV pulsed laser and investigated the difference in the physical properties of the perovskite films compared to those deposited by IR laser.Fig. 1 shows a schematic structure and appearance of the laser deposition system used in this experiment. The experimental method is described below. A synthetic quartz substrate was introduced into an ultra-high vacuum chamber with a base vacuum of 2×10-5 Pa., Then a semiconductor continuous IR laser (wavelength: 808 nm) or a UV pulse laser beam generated by the fourth harmonic of an Nd:YAG Q-Switch laser (wavelength: 266 nm, 10 Hz) was irradiated alternately onto PbI2 and MAI targets to form CH3NH3PbI3.The optical band gap of the thin films deposited by continuous infrared laser deposition shows a change in optical band gap depending on the thickness of the alternately deposited PbI2 and MAI, which can be attributed to the different content ratios of the tetragonal and orthorhombic phases. Interestingly, in UV pulsed laser deposition, the deposition rate was about 0.35 nm/s under the high laser fluence condition with lens focusing, while a high deposition rate of 20 nm/s was obtained under the low laser fluence condition without lens focusing. This result suggests that the excess kinetic energy of the deposition precursor in the case of lens-focused deposition causes re-sputtering on the surface of the already deposited film, resulting in a reduction of the effective deposition rate.In the presentation, we will demonstrate and discuss the results of crystallinity and optical absorption properties.[1] T. Miyasaka, A. Kulkarni, G. M. Kim, S. Öz and A. K. Jena, Adv. Energy Mater. 2020, 11902500 (2020).[2] K. Kawashima, Y. Okamoto, O. Annayev, N. Toyokura, R. Takahashi, M. Lippmaa, K. Itaka, Y. Suzuki, N. Matsuki and H. Koinuma, Science and Technology of Advanced Materials 18, 307 (2017).[3] N. Matsuki, Y. Iida, T. Shimada, Yuta Abeand T. Sato, ECS Meeting abst. MA2020-02, 1860 (2020). Figure 1

A route towards the fabrication of large-scale and high-quality perovskite films for optoelectronic devices.

Halide perovskite materials have been extensively explored for their unique electrical, optical, magnetic, and catalytic properties. Most notably, solar cells based on perovskite thin films have improved their power conversion efficiency from 3.8% to over 25% during the last 12 years. However, it is still a challenge to develop a perovskite-based ink, suitable for upscaling the fabrication process of high-quality perovskite films with extreme purity, good crystallinity, and complete coverage over the deposition area. This is particularly important if the perovskite films are to be used for the scaled production of optoelectronic devices. Therefore, to make halide perovskites commercially available for various applications, it is vital to develop a reliable and highly robust deposition method, which can then be transferred to industry. Herein, the development of perovskite precursor inks suitable for use at low-temperature and vacuum-free solution-based deposition processes is reported. These inks can be further tailored according to the requirements of the deposition method, i.e., we propose their use with the industrially viable deposition technique called “slot-die coating”. Furthermore, a route for the preparation of low-cost and high-volume manufacturing of perovskite films on both rigid and flexible substrates is suggested in this paper. The presented approach is suitable for the fabrication of any functional layers of perovskites, that can be employed in various scaled applications, and it seeks the potential and the methodology for perovskite film deposition that is scalable to industrial standards.

Engineering the Morphology and Component via Multistep Deposition of CsPbBr3Films toward High Detectivity and Stable Self‐Powered Photodetectors

AbstractSelf‐powered perovskite photodetectors (PDs) have attracted tremendous interest for their promising advantages such as fast response speed and high detectivity. However, they usually suffer from severe operating instability issues stemming from the poor film quality, impurity phase formation, and numerous surface defects. Herein, the high performance and stable self‐powered PD based on CsPbBr3film is demonstrated using a multistep spin‐coating method. The annealing temperature of lead bromide (PbBr2) films and the spin‐cycles of cesium bromide (CsBr) are prioritized, which are two key factors for realizing efficient devices during multistep deposition of perovskite films. Results illustrate that these two factors affect the crystallization, covering properties, and the composition of the final film. The CsPbBr3film prepared from PbBr2treatment at 100 °C and CsBr spin‐coating for 8‐cycles has full coverage, higher phase purity, larger grain size, higher crystallinity, and fewer defects compared with the other counterparts. Therefore, the self‐powered PD based on optimized CsPbBr3film achieves an excellent specific detectivity of 7.01 × 1012Jones at 520 nm at 0 V bias voltage. In addition, 96.6% of the original photocurrent can be maintained after operating for 3600 s with 93% long‐term storage (8 weeks) in ambient air, indicating its potential applications in optoelectronics field.

Low‐temperature crystallization and growth of CsPbIBr 2 films through PbX 2 ‐DMSO adduct towards stable and efficient carbon‐based all‐inorganic perovskite solar cells

In the realm of solar cells, currently, the carbon-electrode-based, all-inorganic perovskite solar cells (C-IPSCs) based on cesium lead mixed halide perovskites have drawn much attention. Band-gap tunability and environmental stability of these perovskites (CsPbIBr2) have been gaining attraction as a suitable absorber material for device fabrication. Fabrication of a high-performing and stable C-IPSC based on CsPbIBr2 is a challenging process due to high-temperature fabrication, poor solution deposition of perovskite film, and high recombination at perovskite/carbon interface. In this work, a simple single-step deposition process was adopted to engineer a good quality perovskite film by replacing conventional precursors with DMSO-adducts of lead halide (PbX2). Incorporation of the adduct-based precursor enables crystallization of CsPbIBr2 at a low temperature (~150°C). This enhances the crystallinity of the film, improves the grain size and defects passivation which imparts endurance to the perovskite against phase change and thermal stress. Thereby, enhancing the light absorption and suppressing the non-radiative recombination in CsPbIBr2 film. As a result, the open-circuit voltage is enhanced up to 1.18 V and fill factor up to 57%, further improving the power conversion efficiency by 59.4% leading to a champion device efficiency of 5.9%. Adduct-based C-IPSC displayed substantial enhancement of ambient stability and the adduct-based perovskite exhibited excellent thermal stability.

Mechanochemistry Advances High-Performance Perovskite Solar Cells.

A prerequisite for commercializing perovskite photovoltaics is to develop a swift and eco-friendly synthesis route, which guarantees the mass production of halide perovskites in the industry. Herein, a green-solvent-assisted mechanochemical strategy is developed for fast synthesizing a stoichiometric δ-phase formamidinium lead iodide (δ-FAPbI3 ) powder, which serves as a high-purity precursor for perovskite film deposition with low defects. The presynthesized δ-FAPbI3 precursor possesses high concentration of micrometer-sized colloids, which are in favor of preferable crystallization by spontaneous nucleation. The resultant perovskite films own preferred crystal orientations of cubic (100) plane, which is beneficial for superior carrier transport compared to that of the films with isotropic crystal orientations using "mixture of PbI2 and FAI" as precursors. As a result, high-performance perovskite solar cells with a maximum power conversion efficiency of 24.2% are obtained. Moreover, the δ-FAPbI3 powder shows superior storage stability for more than 10 months in ambient environment (40± 10% relative humidity), being conducive to a facile and practical storage for further commercialization.

Luminescence and Structural Characteristics of Lead Halide Perovskite Films Deposited In Situ by a Versatile Multisource Aerosol Assisted Chemical Vapor Deposition (AACVD) Method

AbstractMost solution‐based synthetic routes for perovskite film deposition take place under highly controlled atmospheres and cannot be easily escalated for mass production. Here, the control in situ over luminescent, morphological, and structural characteristics of hybrid and inorganic perovskite films deposited on top of hot glass substrates by multisource AACVD is reported. By simply adjusting the methylammonium chloride (MACl) to lead bromide (PbBr2) molar ratio in the N‐N dimethylformamide (DMF) precursor solution, a gradual transformation from orthorhombic PbBr2 to cubic MAPbBr2Cl hybrid perovskite films is observed, and the generation of highly luminescent MAPbBr3−xClx nanoparticulate domains in a PbBr2 matrix is easily achieved when a specific ratio is used. Additionally, the crystallization of the films is performed in situ modified by the implementation of an antisolvent stream source during deposition which in turn yields films with superior luminescence properties. Finally, an NIR emitting CsPbBr2Cl:Yb inorganic perovskite film is deposited to probe the versatility of the multisource AACVD system, by the simultaneous deposition of separated PbBr2/YbCl3·6H2O/DMF and CsCl/methanol solutions. The high degree of control on the final properties of the films indicates that AACVD could be a breakthrough for low‐cost, scalable, high‐throughput metal‐halide perovskite film deposition.

Solvent Engineering as a Vehicle for High Quality Thin Films of Perovskites and Their Device Fabrication.

Organic-inorganic halide perovskite solar cells (PSCs) have shown a significant growth in power conversion efficiencies (PCEs) during last decade. Progress in device architecture and high-quality perovskite film fabrication has led to an incredible efficiency over 25% in close to a decade. Developments in solution-based thin film deposition techniques for perovskite layer preparation in PSCs provide low cost and ease of process for their manufacturing, making them a potential contender in future solar energy harvesting technologies. From small area single solar cells to large area perovskite solar modules, solvents play crucial roles in thin film quality and therefore, the device performance and stability. A comprehensive overview of solvent engineering toward achieving the highest qualities for perovskite light absorbing layers with various compositions and based on different fabrication processes is provided in this review. The mechanisms indicating the essential roles a solvent, or a solvent mixture can play to improve the crystallinity, uniformity, coverage and surface roughness of the perovskite films, are discussed. Finally, the role of solvent engineering in transferring from small area laboratory scale PSC fabrication to large area perovskite film deposition processes is explored.

A general approach to high-efficiency perovskite solar cells by any antisolvent

Deposition of perovskite films by antisolvent engineering is a highly common method employed in perovskite photovoltaics research. Herein, we report on a general method that allows for the fabrication of highly efficient perovskite solar cells by any antisolvent via manipulation of the antisolvent application rate. Through detailed structural, compositional, and microstructural characterization of perovskite layers fabricated by 14 different antisolvents, we identify two key factors that influence the quality of the perovskite layer: the solubility of the organic precursors in the antisolvent and its miscibility with the host solvent(s) of the perovskite precursor solution, which combine to produce rate-dependent behavior during the antisolvent application step. Leveraging this, we produce devices with power conversion efficiencies (PCEs) that exceed 21% using a wide range of antisolvents. Moreover, we demonstrate that employing the optimal antisolvent application procedure allows for highly efficient solar cells to be fabricated from a broad range of precursor stoichiometries.

Highly crystalline methylammonium lead iodide films: Phase transition from tetragonal to cubic structure by thermal annealing

Herein, a very simple, solvent free, scalable, and single-step approach to prepare organometal halide perovskite powders via mechanochemical synthesis followed by the deposition of perovskite films by spin coating is reported. This work particularly deals with various parameters that influence the crystallization process and morphology (hyperbranched) of methylammonium lead iodide films. Moreover, the influence of growth temperature on the morphology and the transition from tetragonal to cubic structure are investigated. The mechanosynthesized perovskite provides hyperbranched morphology and crystalline films in a hexagonal shape and serves as a better precursor for the absorber layer in perovskite solar cells.

Crystallization in one-step solution deposition of perovskite films: Upward or downward?

Despite the fast progress of perovskite photovoltaic performances, understanding the crystallization and growth of perovskite films is still lagging. One unanswered fundamental question is whether the perovskite films are grown from top (air side) to bottom (substrate side) or from bottom to top despite 10 years of development. Here, by using grazing incidence x-ray diffraction and morphology characterizations, we unveil that the perovskite films prepared by one-step solution processes, including antisolvent-assisted spin coating and blade coating, follow the downward growth from intermediate phase during thermal annealing. Such a top-to-bottom downward growth is initialized by the evaporation of residual solvent from the top surface of "wet" films and is less sensitive to perovskite compositions and the wettability of underlying substrates. Addressing this fundamental question is important to understand the heterogeneity of perovskite films along the vertical direction, which markedly affects the efficiency and stability of perovskite solar cells.