Understanding and Tailoring Grain Growth of Lead-Halide Perovskite for Solar Cell Application

Grain Boundary Elimination via Recrystallization-Assisted Vapor Deposition for Efficient and Stable Perovskite Solar Cells and Modules

Vapor deposition is a promising technology for the mass production of perovskite solar cells. However, the efficiencies of solar cells and modules based on vapor-deposited perovskites are significantly lower than those fabricated using the solution method. Emerging evidence suggests that large defects are generated during vapor deposition owing to a specific top-down crystallization mechanism. Herein, a hybrid vapor deposition method combined with solvent-assisted recrystallization for fabricating high-quality large-area perovskite films with low defect densities is presented. It is demonstrated that an intermediate phase can be formed at the grain boundaries, which induces the secondary growth of small grains into large ones. Consequently, perovskite films with substantially reduced grain boundaries and defect densities are fabricated. Results of temperature-dependent charge-carrier dynamics show that the proposed method successfully suppresses all recombination reactions. Champion efficiencies of 21.9% for small-area (0.16 cm2 ) cells and 19.9% for large-area (10.0 cm2 ) solar modules under AM 1.5 G irradiation are achieved. Moreover, the modules exhibit high operational stability, i.e., they retain >92% of their initial efficiencies after 200 h of continuous operation.

PbI2 -DMSO Assisted In Situ Growth of Perovskite Wafers for Sensitive Direct X-Ray Detection

Although perovskite wafers with a scalable size and thickness are suitable for direct X-ray detection, polycrystalline perovskite wafers have drawbacks such as the high defect density, defective grain boundaries, and low crystallinity. Herein, PbI2 -DMSO powders are introduced into the MAPbI3 wafer to facilitate crystal growth. The PbI2 powders absorb a certain amount of DMSO to form the PbI2 -DMSO powders and PbI2 -DMSO is converted back into PbI2 under heating while releasing DMSO vapor. During isostatic pressing of the MAPbI3 wafer with the PbI2 -DMSO solid additive, the released DMSO vapor facilitates in situ growth in the MAPbI3 wafer with enhanced crystallinity and reduced defect density. A dense and compact MAPbI3 wafer with a high mobility-lifetime (µτ) product of 8.70 × 10-4 cm2 V-1 is produced. The MAPbI3 -based direct X-ray detector fabricated for demonstration shows a high sensitivity of 1.58 × 104 µC Gyair-1  cm-2 and a low detection limit of 410 nGyair s-1 .

Impact of Precursor Concentration on Perovskite Crystallization for Efficient Wide-Bandgap Solar Cells

High-crystalline-quality wide-bandgap metal halide perovskite materials that achieve superior performance in perovskite solar cells (PSCs) have been widely explored. Precursor concentration plays a crucial role in the wide-bandgap perovskite crystallization process. Herein, we investigated the influence of precursor concentration on the morphology, crystallinity, optical property, and defect density of perovskite materials and the photoelectric performance of solar cells. We found that the precursor concentration was the key factor for accurately controlling the nucleation and crystal growth process, which determines the crystallization of perovskite materials. The precursor concentration based on Cs_0.05FA_0.8MA_0.15Pb(I_0.84Br_0.16)₃ perovskite was controlled from 0.8 M to 2.3 M. The perovskite grains grow larger with the increase in concentration, while the grain boundary and bulk defect decrease. After regulation and optimization, the champion PSC with the 2.0 M precursor concentration exhibits a power conversion efficiency (PCE) of 21.13%. The management of precursor concentration provides an effective way for obtaining high-crystalline-quality wide-bandgap perovskite materials and high-performance PSCs.

Water-Repellent Perovskites Induced by a Blend of Organic Halide Salts for Efficient and Stable Solar Cells

Despite tremendous progress in the power conversion efficiency (PCE) of perovskite solar cells (PeSCs), the long-term stability issue remains a significant challenge for commercialization. In this study, by blending organic halide salts, phenylethylammonium halide (PEAX, X = I, Br), with CH₃NH₃PbI₃ (MAPbI₃), we achieved remarkable enhancements in the water-repellency of perovskite films and long-term stability of PeSCs, together with enhanced PCE. The hydrophobic aromatic PEA⁺ group in PEAX protects the perovskite film from destruction by water. In addition, the smaller halide Br^- in PEABr restructures MAPbI₃ to form MAPbI_3-xBr_x during post-annealing, leading to lattice contraction with beneficial crystallization quality. The perovskite films modified by PEAX exhibited excellent water resistance. When the perovskite films were directly immersed in water, no obvious decompositions were observed, even after 60 s. The PEAX-decorated PeSCs exhibited considerable long-term stability. Under high-humidity conditions (60 ± 5%), the PEAX-decorated PeSCs held 80.5% for PEAI and 85.2% for PEABr of their original PCE after exposure for 100 h, whereas the pristine PeSC device lost more than 99% of its initial PCE after exposure for 60 h under the same conditions. Moreover, compared to the pristine device with a PCE of 13.28%, the PEAX-decorated PeSCs exhibited enhanced PCEs of 17.33% for the PEAI device and 17.18% for the PEABr device.

High-temperature inverted annealing for efficient perovskite photovoltaics

High-quality perovskite films with large grains and reduced surface defects were obtained via an inverted annealing process. Corresponding photovoltaic devices achieved a highest efficiency of 20.4% with a stabilized power conversion efficiency (PCE) of 19.8%.

A Direct Mapping Approach to Understand Carrier Relaxation Dynamics in Varied Regions of a Polycrystalline Perovskite Film

We developed a direct mapping approach to overlay the image of a polycrystalline perovskite film obtained from the transient absorption microscope (TAM) with that from the scanning electron microscope (SEM). By mapping these imaging data pixel by pixel, we are able to observe the relaxation dynamics of the photo-generated charge carriers on varied regions of the film. The carrier relaxation dynamics contain a dominated single-exponential decay component owing to the recombination of charge carriers. The lifetime distribution of charge recombination shows a bimodal feature, for which the rapid and slow distributions are assigned as free and trapped carriers, respectively. The charge recombination was slower in the grain boundary (GB) region than in the grain interior (GI) region. The small grains have longer lifetimes than the large grains for the crystal size smaller than 500 nm. Therefore, GB with retarded charge recombination might play a positive role in a perovskite solar cell.

MAPbI3 Quasi-Single-Crystal Films Composed of Large-Sized Grains with Deep Boundary Fusion for Sensitive Vis-NIR Photodetectors

Perovskite single-crystal (SC) or quasi-single-crystal (QSC) films are promising candidates for excellent performance of photoelectric devices. However, it is still a great challenge to fabricate large-area continuous SC or QSC films with proper thickness. Herein, we propose a pressure-assisted high-temperature solvent-engineer (PTS) strategy to grow large-area continuous MAPbI₃ QSC films with uniformly thin thickness and orientation. Dramatic grain growth (∼100 μm in the lateral dimension) and adequate boundary fusion are realized in them, vastly eliminating the grain boundaries. Thus, remarkable diminution of the trap density (n_trap: 7.43 × 10¹¹ cm^-3) determines a long carrier lifetime (τ₂: 1.7 μs) and superior photoelectric performance of MAPbI₃-based lateral photodetectors; for instance, an ultrahigh on/off ratio (>2.4 × 10⁶, 2 V), great stability, fast response (283/306 μs), and high detectivity (1.41 × 10¹³) are achieved. The combination properties and performance of the QSC films surpass most of the reported MAPbI₃. This effective approach in growing perovskite QSC films points out a novel way for perovskite-based optoelectronic devices with superior performance.

Off-Stoichiometric Methylammonium Iodide Passivated Large-Grain Perovskite Film in Ambient Air for Efficient Inverted Solar Cells

Hot-casting is a promising technique of depositing high-quality organic-inorganic hybrid perovskite thin films with large crystal grain size. Here, we reported the that the crystallinity and grain size of perovskite films could be systematically tailored by modulating the stoichiometry of the precursor solution in the hot-casting process under ambient condition with a relative humidity of 40%. It was found that a slight excess of methylammonium iodide (MAI) in the precursor solution could effectively compensate the MAI loss due to the high substrate temperature. A significant increase in the grain size and crystallinity of the perovskite film was observed together with a decrease in defect density and a carrier concentration enhancement in MAI-rich samples. The corresponding devices exhibited a notable increase in fill factor (up to 80.70%) and short-circuit current density. In addition, in MAI-deficient samples, an enrichment of PbI₂ at the grain boundaries was directly observed by optical microscopy and laser confocal microscopy. Time-resolved photoluminescence spectroscopy revealed an increase in the charge carrier lifetime in the MAI-deficient samples, which was in line with the previous results with a small amount of excess PbI₂ in the perovskite film. This work highlights a new strategy to prepare high-quality perovskite thin films with excellent crystal quality under ambient condition.

Nuclei position-control and crystal growth-guidance on frozen substrates for high-performance perovskite solar cells

Nucleation and crystal growth are key stages for high-quality perovskite films that dominate the performance of perovskite solar cells. However, the position of nuclei in the films and the orientation of the crystal growth have not yet been intendedly controlled during their fabrication. In this study, we developed a method of spin-coating perovskite films on frozen substrates to control the position of the nuclei and the direction of the crystal growth at the same time. In this way, the position of the crystal nuclei and the growth orientation of the perovskite crystals in the perovskite films can be simultaneously controlled. A high-quality perovskite film with grains spanning vertically the entire film thickness has been obtained using this new method. And an efficient inverted planar solar cell (ITO/PEDOT:PSS/CH₃NH₃PbI₃/PC61BM/BCP/Ag) with the highest power conversion efficiency of 17.14% and open-circuit voltage of 1.14 V has been achieved by using this technique.

Synchronized-pressing fabrication of cost-efficient crystalline perovskite solar cells via intermediate engineering

A cost-effective fabrication method that can produce a remarkable enhancement in the device efficiency along with a reduction in the fabrication cost is one of the crucial requirements for the commercialization of perovskite-based solar cells. Here, we report a low-cost, printable, and highly effective synchronized-pressing annealing (SPA) method for inverted planar perovskite solar cells. In this method, two films are combined face-to-face for annealing, and separated as in a roll-to-roll process. Consequently, the SPA method provides two homogeneous highly crystalline MAPbI3 films with monolithic millimeter-scale crystalline grains by intermediate-induced crystallization engineering. The grains present a tendency of oriented growth along the <110> direction, parallel to the substrate, which leads to efficient charge transport. The IPSCs fabricated by the SPA method demonstrate a high efficiency of ∼17% with significantly enhanced photocurrents and fill factors. Moreover, the characteristics of both top and bottom devices are very similar, with nearly identical J-V curves and photoresponse spectra. As the SPA method is compatible with the printing technology for mass production, and as it can produce twin devices of high quality via one fabrication process, it can provide a remarkable reduction in the fabrication cost.

Dependence of Acetate-Based Antisolvents for High Humidity Fabrication of CH3NH3PbI3 Perovskite Devices in Ambient Atmosphere

High-efficiency perovskite solar cells (PSCs) need to be fabricated in the nitrogen-filled glovebox by the atmosphere-controlled crystallization process. However, the use of the glovebox process is of great concern for mass level production of PSCs. In this work, notable efficient CH₃NH₃PbI₃ solar cells can be obtained in high humidity ambient atmosphere (60-70% relative humidity) by using acetate as the antisolvent, in which dependence of methyl, ethyl, propyl, and butyl acetate on the crystal growth mechanism is discussed. It is explored that acetate screens the sensitive perovskite intermediate phases from water molecules during perovskite film formation and annealing. It is revealed that relatively high vapor pressure and high water solubility of methyl acetate (MA) leads to the formation of highly dense and pinhole free perovskite films guiding to the best power conversion efficiency (PCE) of 16.3% with a reduced hysteresis. The devices prepared using MA showed remarkable shelf life stability of more than 80% for 360 h in ambient air condition, when compared to the devices fabricated using other antisolvents with low vapor pressure and low water solubility. Moreover, the PCE was still kept at 15.6% even though 2 vol % deionized water was added in the MA for preparing the perovskite layer.

Solvent-Assisted Thermal-Pressure Strategy for Constructing High-Quality CH3NH3PbI3- xCl x Films as High-Performance Perovskite Photodetectors

High-quality CH₃NH₃PbI_3-xCl _x films have attracted research interests in photoelectric devices because of their improved carrier diffusion length and charge mobility. Herein, a solvent-assisted thermal-pressure strategy is developed to promote the secondary growth of perovskite grains in the films. Highly oriented perovskite films are then obtained with large-sized grains (5-10 μm). As a consequence, the photodetectors based on the high-quality CH₃NH₃PbI_{3- x}Cl _x films exhibit enhanced ophtoelectrical performance, including high on/off ratio (>2.1 × 10⁴), fast response time (54/63 μs), and high detectivity (∼1.3 × 10¹²). This work suggests an effective approach for high-quality perovskite films, which will be promising candidates for other high-efficiency photoelectric devices.

High-Quality CH3NH3PbI3 Films Obtained via a Pressure-Assisted Space-Confined Solvent-Engineering Strategy for Ultrasensitive Photodetectors

High-quality organic-inorganic hybrid perovskite films are crucial for excellent performance of photoelectric devices. Herein, we demonstrate a pressure-assisted space-confined solvent-engineering strategy to grow highly oriented, pinhole-free thin films of CH₃NH₃PbI₃ with large-scale crystalline grains, high smoothness, and crystalline fusion on grain boundaries. These single-crystalline grains vertically span the entire film thickness. Such a film feature dramatically reduces recombination loss and then improves the transport property of charge carriers in the films. Consequently, the photodetector devices, based on the high-quality CH₃NH₃PbI₃ films, exhibit high photocurrent (105 μA under 671 nm laser with a power density of 20.6 mW/cm² at 10 V), good stability, and, especially, an ultrahigh on/off ratio (I_light/I_dark > 2.2 × 10⁴ under an incident light of 20.6 mW/cm²). These excellent performances indicate that the high-quality films will be potential candidates in other CH₃NH₃PbI₃-based photoelectric devices.