The synthesis of perovskites

In this Focus issue, we examine synthetic routes to perovskites for a variety of applications.

Perovskites, an important class of materials with the general formula ABX₃, have wide ranging applications. In this issue, we present a selection of articles which showcase exciting, interesting or unusual synthetic routes to perovskites.

Featured on the cover of this issue, an Article by Li, Yang & co-workers reports the synthesis of halide perovskite chains which are a unit cell thick, confined in single-walled carbon nanotubes (SWCNTs). Two phases of solvents, one a good solvent for halide dissolution and the other a poor solvent for halide dissolution, are used to transport ions into the SWCNTs where they rapidly crystallize. Unconventional stoichiometries are achieved due to the low dimensions of the perovskite and the SWCNT confinement stabilizes the perovskite. direct X-ray detectors are fabricated from the materials and stable responses are seen, even under extreme working conditions. A News & Views by Jeremy Sloan discusses how the solvent system used enables a wide variety of lead- and non-lead-containing halide perovskites to be inserted into the SWCNTs.

Two-dimensional (2D) halide perovskites demonstrate interesting optoelectronic properties. Large organic cations confine the inorganic lattice and the optoelectronic properties of 2D halide perovskites are largely determined by the alignments and distortions of the inorganic octahedra, which in turn are influenced by the organic cations. An Article by Ahmadi and co-workers reports the high-throughput and autonomous investigation of the crystallization of 2D halide perovskites based on a bulky 3,3-diphenylpropylammonium organic cation. A moiré superlattice is observed indicating the formation of twisted stacks of 2D halide perovskites. A News & Views by Chen and Li discusses the possibilities of this synthetic approach for manipulating metal halide perovskite photophysical properties.

Perovskite solar cells are a promising photovoltaic technology and 2D surface passivation has been shown to help achieve state-of-the-art perovskite optoelectronics. An Article by Cai, Zhou and co-workers examines how a molecular interlayer can mediate the reaction of 2D passivators and perovskites in perovskite solar cell fabrication, leading to the formation of a purer-phase 2D perovskite capping layer. This capping layer contributes to improved stability and charge energetics, and inverted solar cells made with the interlayer achieve power conversion efficiencies of up to 25.97%. A News & Views by Grancini and Sutter-Fella examines how the molecular interlayer not only acts as an electron transport layer but also as an ion transport mediator for heterostructure interface control, addressing the challenges of phase heterogeneity commonly seen in conventional passivation methods.

The synthesis of perovskites for solar cell technologies is also discussed in a Comment in this issue. Wakamiya and Park discuss how, for commercial application, the stability and reproducibility of devices, and therefore perovskites, could be seen to be more important than achieving the highest reported power conversion efficiencies. They discuss how the purity of the perovskite precursors and the synthesis route itself can have a large impact on these properties.

Additionally, the single crystal growth of metal halide perovskites is examined in an Article in this issue. Huang and co-workers use an in situ microscope spectroscopy method to examine the solution temperature near the solid–liquid interface of MAPbI₃ (MA, methylammonium) single crystals. A lower temperature interfacial region is observed which serves as a protective layer, maintaining crystal facet orientation. Disruption of this layer results in polycrystals with irregular facets.

Tin is an attractive alternative to lead in halide perovskites but progress in developing tin perovskite nanocrystals remains limited due to complex defect chemistry and structural instability. In an Article in this issue, Yin, Sargent, Sun and co-workers report the computationally guided synthesis of hybrid tin perovskite nanocrystals. Calculations show that tin-rich conditions supress bulk defects whereas tin-poor conditions reduce surface defects. A synthesis method is developed where exogeneous monovalent cations are incorporated to produce defect-tolerant surfaces in tin-rich conditions. The resulting FASnI₃ (FA = formamidinium) nanocrystals have a photoluminescence quantum yield of 42.4%.

The ferroelectric properties of a series alkaline earth metal-based hybrid organic–inorganic perovskites are examined in a Research Highlight based on work by Zhang, Shi, Zhang and co-workers. Block-shaped single crystals are synthesized and (pyrrolidinium)Ba(ClO₄)₃ exhibits ferroelectricity which can be optimized by using a fluorination strategy. A Research Highlight based on work by Karunadasa and co-workers describes how a halide perovskite heterostructure, synthesized using 3-(methylthio)-propylammonium) as a bifunctional alkyl ammonium salt, features isolated magnetic sublattices.

Three research highlights in this issue examine various synthetic strategies for improving the performance of perovskite-based solar cells. A Research Highlight based on the work of Liang, Zhang, Song, Li and co-workers examines how the introduction of tyrosine into perovskite precursor solutions can modulate phase composition and optimize carrier transport pathways in quasi-2D perovskite solar cells. The power conversion efficiency of a perovskite solar cell module fabricated with the quasi-2D perovskites is 20.46% when the aperture area is 69 cm². Furthermore, a Research Highlight based on work by Wie, Zheng, Huang and co-workers examines how conjugated block copolymers can passivate interfacial defects and enhance charge transport in inverted perovskite solar cells. Block copolymers are synthesised using sequential Grignard metathesis polymerization with quaternary ammonium halides on the side chains that passivate perovskite surface defects and polyfluorene co-blocks which provide ordered electronic pathways. Finally, a Research Highlight based on the work by Hang, Lei, Yang and Yu describes how chiral aza-helicene phosphonic acids with helical backbones and phosphonic acid anchoring groups enable the formation of highly ordered monolayers at perovskite interfaces.

As exemplified by the articles in this issue, the chosen synthetic route of a perovskite impacts its properties and application performance. Synthetic routes to perovskites are constantly evolving and we look forward to seeing what comes next!