MOF-Assisted Annealing-Free Crystallization Technology of Perovskites toward Efficient and Stable Perovskite Solar Cells

Although annealing is a commonly used crystallization method for perovskite films in perovskite solar cells (PSCs), the high thermal energy consumption and limitations on flexible devices hinder their further industrial application. We herein propose an annealing-free crystallization technology for perovskite films, assisted by the Zr-metal-organic framework (MOF) interface between SnO₂ and the perovskite. It is found that the Zr-MOF interface can accelerate the formation of perovskite intermediates and promote their conversion into perovskite crystals even without annealing. The trap density thus decreases by about one fold, accompanied by significant increases in electron and hole mobilities, resulting in enhanced carrier extraction and suppressed charge recombination. Therefore, the Zr-MOF-based PSC attains a power convention efficiency (PCE) of 20.24%, 2.2 times that (9.26%) of the pristine PSC. Furthermore, the Zr-MOF interface layer can significantly improve the air and thermal stabilities of PSCs. The Zr-MOF-based PSC exhibits 93% of its initial PCE versus 52% for the pristine PSC after 1018 h of storage in air. Additionally, after 360 h of continuous heating at 65 °C, the Zr-MOF-based PSC retains 91% of its initial PCE against 44% for the pristine PSC.

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.

Rapid hybrid perovskite film crystallization from solution

The use of a solution process to grow perovskite thin films allows to extend the material processability. It is known that the physicochemical properties of the perovskite material can be tuned by altering the solution precursors as well as by controlling the crystal growth of the film. This advancement necessarily implies the need for an understanding of the kinetic phenomena for the thin-film formation. Therefore, in this work we review the state of the art of perovskite hybrid crystal growth, starting from a comprehensive theoretical description towards broad experimental investigations. One part of the study focuses on rapid thermal annealing as a tool to control nucleation and crystal growth. We deduce that controlling crystal growth with high-precision photonic sintering simplifies the experimental framework required to understand perovskite crystallization. These types of synthesis methods open a new empirical parameter space. All this knowledge serves to improve the perovskite synthesis and the thin films' quality, which will result in higher device performances.

Formamidinium Incorporation into Compact Lead Iodide for Low Band Gap Perovskite Solar Cells with Open-Circuit Voltage Approaching the Radiative Limit

To bring hybrid lead halide perovskite solar cells toward the Shockley-Queisser limit requires lowering the band gap while simultaneously increasing the open-circuit voltage. This, to some extent divergent objective, may demand the use of large cations to obtain a perovskite with larger lattice parameter together with a large crystal size to minimize interface nonradiative recombination. When applying the two-step method for a better crystal control, it is rather challenging to fabricate perovskites with FA⁺ cations, given the small penetration depth of such large ions into a compact PbI₂ film. In here, to successfully incorporate such large cations, we used a high-concentration solution of the organic precursor containing small Cl^- anions achieving, via a solvent annealing-controlled dissolution-recrystallization, larger than 1 μm perovskite crystals in a solar cell. This solar cell, with a largely increased fluorescence quantum yield, exhibited an open-circuit voltage equivalent to 93% of the corresponding radiative limit one. This, together with the low band gap achieved (1.53 eV), makes the fabricated perovskite cell one of the closest to the Shockley-Queisser optimum.

Effect of intermediate phases on the optical properties of PbI2-rich CH3NH3PbI3 organic-inorganic hybrid perovskite

Methylammonium lead halide perovskite (CH3NH3PbI3) films, with high PbI2 concentration, were grown by the two-step spin coating method. The influence of the precursor concentration and annealing time on the optical and structural properties of the perovskite films was analyzed by optical absorption, photoluminescence, X-ray diffraction and scanning electron microscopy. The results showed that, in addition to the CH3NH3PbI3 and PbI2 phases, intermediate phases, such as (MA)2(DMF)2Pb3I8, were formed in the films, depending on the time and temperature of annealing, which can tune the optical absorption in the visible spectra. This intermediate phase induced the formation of perovskite nanowires, identified by SEM images, and their growth may be associated with the presence of the DMF solvent remaining in the PbI2 film.

A Review of the Role of Solvents in Formation of High-Quality Solution-Processed Perovskite Films

Recently, perovskite solar cells have attracted great attention because of their outstanding photovoltaic performance and ease of fabrication. High-quality perovskite films hold a key in getting highly efficient perovskite solar cells. Solution-processed fabrication technique is the most widely adopted for preparing perovskite films because of its low cost. In the solution-proceed perovskite films, solvents not only play the role of dissolving the solute but also participate in the crystallization of perovskite. In the one-step method, solvents play key roles in controlling morphology, widening process window, and achieving room-temperature crystallization of perovskite films. In addition, the solvents play important roles in controlling the nuclei/growth, suppressing volume expansion during the two-step method. Especially, the solvent can induce grain coarsening during the annealing process. A deep understanding of the multiplicity of roles during the formation of perovskite films will help understand the formation mechanism of perovskite films. Here, a systematic review on the progress in fabrication of high-quality perovskite films by making use of solvent to control the crystallization is presented. Meanwhile, we elucidate the key roles of solvent in the fabrication of high-quality perovskite films.

Elucidating the Key Role of a Lewis Base Solvent in the Formation of Perovskite Films Fabricated from the Lewis Adduct Approach

High-quality perovskite films can be fabricated from Lewis acid-base adducts through molecule exchange. Substantial work is needed to fully understand the formation mechanism of the perovskite films, which helps to further improve their quality. Here, we study the formation of CH₃NH₃PbI₃ perovskite films by introducing some dimethylacetamide into the PbI₂/N,N-dimethylformamide solution. We reveal that there are three key processes during the formation of perovskite films through the Lewis acid-base adduct approach: molecule intercalation of solvent into the PbI₂ lattice, molecule exchange between the solvent and CH₃NH₃I, and dissolution-recrystallization of the perovskite grains during annealing. The Lewis base solvents play multiple functions in the above processes. The properties of the solvent, including Lewis basicity and boiling point, play key roles in forming smooth perovskite films with large grains. We also provide some rules for choosing Lewis base additives to prepare high-quality perovskite films through the Lewis adduct approach.

Fully Ambient-Processed Perovskite Film for Perovskite Solar Cells: Effect of Solvent Polarity on Lead Iodide

Fully ambient-processed and highly efficient methylammonium lead iodide (MAPbI₃) perovskite films are very desirable for industrial manufacturing of perovskite solar cells (PSCs). To date, most reported highly efficient MAPbI₃ PSCs rely on the fabrication of lead iodide (PbI₂) films inside the glovebox. Here we report a simple fabrication method using extra dry isopropanol (IPA100) for obtaining uniform and loosely packed PbI₂ film, which leads to a uniform and highly crystalline MAPbI₃ film under ambient conditions. Compared with recently reported results (10%-15%) using IPA treatment in the glovebox, we achieved over 16% efficiency of PSCs while fabricating perovskite films in fully ambient conditions. We have found the removal of even trace amounts of water from IPA to be a key factor for the successful ambient fabrication of PbI₂ films, as the high polarity of water negatively influences the crystallinity and morphology of the PbI₂ film.

Enhanced performance of perovskite solar cells by strengthening a self-embedded solvent annealing effect in perovskite precursor films

Smooth perovskite films with large grains are fabricated by strengthening the self-embedded solvent annealing effect in the perovskite precursor film via pre-depositing a protective layer.

Facile preparation of high-quality perovskites for efficient solar cells via a fast conversion of wet PbI2precursor films

Facile preparation of high-quality CH₃NH₃PbI₃films with excellent photovoltaic performance by using an annealing-free method and wet PbI₂precursor films.

Solvent annealing of PbI2 for the high-quality crystallization of perovskite films for solar cells with efficiencies exceeding 18

While most work carried out to date has focused on the solvent annealing of perovskite, in the present work, we focused on the solvent annealing of lead iodide. Based on the two-step spin-coating method, we designed a screening method to search for an effective solvent annealing process for PbI₂. PbI₂ films were annealed in diverse solvent atmospheres, including DMF, DMSO, acetone, and isopropanol (IPA). We found that the solvent annealing of PbI₂ in the DMF, acetone, and IPA atmospheres resulted in dense PbI₂ films, which impeded the complete conversion of PbI₂ to CH₃NH₃PbI₃. Surprisingly, employing the DMSO solvent annealing process for PbI₂ led to porous PbI₂, which facilitated the complete conversion of PbI₂ to perovskite with larger grain sizes. Solar cells fabricated using the DMSO solvent annealing process exhibited the best efficiency of 18.5%, with a fill factor of 76.5%. This unique solvent annealing method presents a new way of controlling the perovskite film quality for highly efficient solar cells.

Tailoring Mixed-Halide, Wide-Gap Perovskites via Multistep Conversion Process

Wide-band-gap mixed-halide CH3NH3PbI3-XBrX-based solar cells have been prepared by means of a sequential spin-coating process. The spin-rate for PbI2 as well as its repetitive deposition are important in determining the cross-sectional shape and surface morphology of perovskite, and, consequently, J-V performance. A perovskite solar cell converted from PbI2 with a dense bottom layer and porous top layer achieved higher device performance than those of analogue cells with a dense PbI2 top layer. This work demonstrates a facile way to control PbI2 film configuration and morphology simply by modification of spin-coating parameters without any additional chemical or thermal post-treatment.

Tuning PbI2layers by n-butanol additive for improving CH3NH3PbI3light harvesters of perovskite solar cells

Modifying the morphology of PbI₂layers withn-butanol additive leads to efficient CH₃NH₃PbI₃layers for perovskite solar cells.

Preferential (100)-oriented CH3NH3PbI3 perovskite film formation by flash drying and elucidation of formation mechanism

It was found that the amount of thermal energy delivered during annealing and the amount of residual solvent remaining after spin coating play critical roles in determining the growth properties of (100)-oriented perovskite films.

High quality perovskite films fabricated from Lewis acid–base adduct through molecular exchange

High quality CH₃NH₃PbI₃ perovskite films without residual PbI₂ are fabricated from the Lewis adduct of PbI₂·xDMF through molecular exchange. The photovoltaic performances of the perovskite solar cells are thus improved significantly.