Luminescent Intermediates and Humidity-Dependent Room-Temperature Conversion of the MAPbI3 Perovskite Precursor

Mechanism of Interaction of Water above the Methylammonium Lead Iodide Perovskite Nanocluster: Size Effect and Water-Induced Defective States

Water is often viewed as detrimental to organic halide perovskite stability. However, evidence highlights its efficacy as a solvent during organic perovskite liquid synthesis. This paradox prompts an investigation into water's influence on perovskite nanoclusters. Employing first principle calculations and ab initio molecular dynamics simulations, surprisingly, we discover some subsurface layers of methylammonium lead iodide (MAPbI3) nanoclusters exhibit stronger relaxation than surface layers. Moreover, a strong quantum confinement effect enhances the band gap of MAPbI3 as the nanocluster size decreases. Notably, the water molecules above MAPbI3 nanoclusters induce rich localized defect states, generating low-lying shallow states above the valence band for the small amounts of surface water molecules and band-like deep states across the whole gap for large nanoclusters. This work provides insights into water's role in the electronic structure and structural evolution of perovskite nanoclusters, aiding the design of water-resistant layers to protect perovskite quantum dots from ambient humidity.

Coordination Chemistry as a Universal Strategy for a Controlled Perovskite Crystallization

The most efficient and stable perovskite solar cells (PSCs) are made from a complex mixture of precursors. Typically, to then form a thin film, an extreme oversaturation of the perovskite precursor is initiated to trigger nucleation sites, e.g., by vacuum, an airstream, or a so-called antisolvent. Unfortunately, most oversaturation triggers do not expel the lingering (and highly coordinating) dimethyl sulfoxide (DMSO), which is used as a precursor solvent, from the thin films; this detrimentally affects long-term stability. In this work, (the green) dimethyl sulfide (DMS) is introduced as a novel nucleation trigger for perovskite films combining, uniquely, high coordination and high vapor pressure. This gives DMS a universal scope: DMS replaces other solvents by coordinating more strongly and removes itself once the film formation is finished. To demonstrate this novel coordination chemistry approach, MAPbI3 PSCs are processed, typically dissolved in hard-to-remove (and green) DMSO achieving 21.6% efficiency, among the highest reported efficiencies for this system. To confirm the universality of the strategy, DMS is tested for FAPbI3 as another composition, which shows higher efficiency of 23.5% compared to 20.9% for a device fabricated with chlorobenzene. This work provides a universal strategy to control perovskite crystallization using coordination chemistry, heralding the revival of perovskite compositions with pure DMSO.

Out-of-equilibrium processes in crystallization of organic-inorganic perovskites during spin coating

Complex phenomena are prevalent during the formation of materials, which affect their processing-structure-function relationships. Thin films of methylammonium lead iodide (CH₃NH₃PbI₃, MAPI) are processed by spin coating, antisolvent drop, and annealing of colloidal precursors. The structure and properties of transient and stable phases formed during the process are reported, and the mechanistic insights of the underlying transitions are revealed by combining in situ data from grazing-incidence wide-angle X-ray scattering and photoluminescence spectroscopy. Here, we report the detailed insights on the embryonic stages of organic-inorganic perovskite formation. The physicochemical evolution during the conversion proceeds in four steps: i) An instant nucleation of polydisperse MAPI nanocrystals on antisolvent drop, ii) the instantaneous partial conversion of metastable nanocrystals into orthorhombic solvent-complex by cluster coalescence, iii) the thermal decomposition (dissolution) of the stable solvent-complex into plumboiodide fragments upon evaporation of solvent from the complex and iv) the formation (recrystallization) of cubic MAPI crystals in thin film.

Complex phenomena are prevalent during the formation of materials, and they affect the processing structure-function relationship. Here the authors elucidate the stochastic transformation processes happening during the spin coating of perovskite colloidal precursors by multimodal characterization.

Using In Situ Optical Spectroscopy to Elucidate Film Formation of Metal Halide Perovskites

The research interest in halide perovskites has gained momentum enormously over the last recent years, also due to the demonstration of high-efficient perovskite-based optoelectronic devices. A prerequisite for such highly efficient devices is to realize high-quality perovskite layers, which requires a deep understanding about the perovskite formation and good process control. In that context, in situ optical spectroscopy during the processing of halide perovskites has become increasingly popular. Even though it is a relatively easily accessible yet powerful tool for studying perovskite formation, there exist some technical and analytical aspects that need to be considered to unfold its full potential. In this Perspective, we give an overview of the latest developments in the field of in situ optical spectroscopy to control and better understand the film processing of halide perovskites. We highlight possibilities and pitfalls regarding the analysis of measured optical data, discuss the development of technical concepts, and address future prospects of optical in situ spectroscopy.

Impact of Vertical Inhomogeneity on the Charge Extraction in Perovskite Solar Cells: A Study by Depth-Dependent Photoluminescence

In perovskite solar cells (PSCs), the vertical inhomogeneities which include uneven grains, voids, and grain boundaries are closely linked to the underlying charge transport layer which controls the nucleation and grain growth in the perovskite film. Herein, the vertical inhomogeneity of perovskite films in the device structure is analyzed by depth-dependent photoluminescence (PL) achieved with different excitation wavelengths. An analytical representation between vertical inhomogeneity and depth-dependent PL, parametrized with a factor, b, is introduced to understand the relation between inhomogeneity and charge recombination. Lower values of b correlate to lower vertical inhomogeneity and hence reduced recombination. The analytical representation is validated in two sets of devices that show remarkable differences in perovskite film morphology, device based on mesoporous TiO₂ and planar SnO₂. By exploring the morphological properties and the PL emission from different depths across the device structures, we show that the lower vertical inhomogeneity leads to more efficient charge carrier extraction in planar SnO₂-based devices. Moreover, the SnO₂-based devices exhibit lower Urbach energy, which concurs with the slow transient photovoltage decay, suggesting less defects and recombination losses. This work provides a broader understanding of the impact of vertical inhomogeneity on the charge extraction efficiency and presents a methodology to study quantitatively the inhomogeneity of perovskite films in device structures.

Good or evil: what is the role of water in crystallization of organometal halide perovskites?

Perovskite solar cells (PSCs) have the potential to become one of the most cost-efficient photovoltaic devices. However, current fabrication methods of PSCs still require strict environment control and ultrahigh purity chemicals, which could prevent their large-scale commercialization. To tackle this challenge, the role of water is the first to be thoroughly understood in a perovskite formation process. Until now, there is still controversy about whether water is harmful or beneficial for perovskite formation, not to mention exactly what role water plays therein. In this Focus article, we review recent studies on water involved chemical reactions, solvent interaction, intermediates, and crystal growth in the perovskite film formation process, in order to bring out a full picture about what water does in the perovskite formation process. As our current understanding stands, a suitable amount of water could be of help for growing high quality perovskite films due to the resultant formation of intermediates, such as MAPbI3·H2O, which facilitates the conversion from precursors to perovskites. However, too much water would induce the formation of relatively stable components, such as (MA)4PbI6·2H2O, which are left in the product-films as impurities resulting in degraded device performance. Continual efforts should be made to further understand and develop water-involved strategies for consistent PSC fabrication under ambient conditions.

Relating Defect Luminescence and Nonradiative Charge Recombination in MAPbI3 Perovskite Films

Nonradiative losses in semiconductors are related to defects. At cryogenic temperatures, defect-related photoluminescence (PL) at energies lower than the band-edge PL is observed in methylammonium lead triiodide perovskite. We applied multispectral PL imaging to samples prepared by two different procedures and exhibiting 1 order of magnitude different PL quantum yield (PLQY). The high-PLQY sample showed concentration of the emitting defect sites around 10¹²-10¹³ cm^-3. No correlation between PLQY and the relative intensity of the defect emission was found when micrometer-sized local regions of the same sample were compared. However, a clear positive correlation between the lower PLQY and higher defect emission was observed when two preparation methods were contrasted. Therefore, although the emissive defects are not connected directly with the nonradiative centers and may be spatially separated at the nano scale, chemical processes during the perovskite synthesis promote/prevent formation of both types of defects at the same time.

A prenucleation strategy for ambient fabrication of perovskite solar cells with high device performance uniformity

Humidity is known to be inimical to the halide perovskites and thus typically avoided during fabrication. The poor fundamental understanding of chemical interactions between water and the precursors hampers the further development of perovskite fabrication in ambient atmosphere. Here, we disclose a key finding that the ambient water could promote the formation of lead complexes, which when uncontrolled would make their way into large intermediate fibrillar crystallites and thus discontinuous perovskite films unfavorable for photovoltaics among others. To counter this effect, a prenucleation strategy is proposed, which embodies the controlled burst of profuse intermediate nuclei. Consequently, we are able to obtain a compact and uniform perovskite layer, which affords high efficiency perovskite solar cells. More excitingly, the solar cells show high performance uniformity, demonstrating the distinctive advantages of our prenucleation strategy. This work sheds light on developing reliable and cost-effective fabrication methods for industrial production of perovskite solar cells.

Ambient processing of perovskite solar cells is desired but the resulting cell performance is poor due to the negative effects of moisture on film fabrication. Here Zhang et al. propose a prenucleation strategy to overcome the moisture effect, achieving good film quality and high and uniform cell performance.