Editorial on the Special Issue “Advances of Low-Dimensional Metal Halide Perovskite Materials”

Due to their outstanding performance in optoelectronic applications, lead-based halide perovskites (LHPs) have attained significant attention from scientists worldwide. These kinds of semiconductors exhibit very attractive photoelectric properties, exemplified by halide/layer-dependent tunable emission covering the whole visible range, a high photoluminescence (PL) quantum yield (QY) up to unity, a long carrier diffusion length and lifetime, etc. To overcome the poor stability and toxicity of LHPs, other metal-based halide perovskites with similar photoelectrics are desired. By replacing them with larger A-site organic countercations or B-site metal ions, the connected modes and dimensionality can be modified, thereby resulting in a wide structural and compositional tunability of metal halide hybrids (MHHs). Since the photoelectric properties and applications of MHHs are highly relative to the dimensionality and electronic structures of B-site metals, it is of great importance to obtain a deep understanding of their key role in constructing MHHs with specific metal species and dimensionalities.

In this Special Issue, eight peer-reviewed original research articles shed light on the construction of new low-dimensional MHHs, the PL and stability improvement of perovskite nanocrystals (PNCs), rare earth metal doping in double halide perovskites, and their applications in solar cells. These studies provide deeper insights into the structural design, the structure–property relationships, and the commercial applications of MHHs.

As a result of the strong steric hindrance, hybridizing branched organic countercations with metal halides will result in 1D or 0D MHHs, which usually exhibit strong exciton binding energies and broadband emission due to the self-trapping of excitons. As reported by Nelyubina et al. [1], the derivatives of polyaromatic and conjugated molecules, including anthracene, pyrene, and (E)-stilbene, have been adopted to construct 0D and 1D MHHs. Although charge transfer complexes were not obtained, the successful synthesis of low-dimensional MHHs with unusual mini-rods of four face- and edge-shared octahedra demonstrates the great potential of combining specific organic and metal halides together for desired optical and electronic properties. For example, tetraethylammonium has been hybridized with copper (I) bromide to increase the reaction rate and X-ray scintillation performance [2]. Due to the large Stokes shift originating from self-trapped excitons, a light yield of 7623 photons MeV⁻¹ is achieved. Moreover, inorganic A-site cations can also be adopted to tune the electronic structures of low-dimensional metal halides. Zeng’s group predicted the presence of (CsK₂)BiCl₆ based on density functional theory (DFT) results and successfully synthesized (CsK₂)BiCl₆ through a solvothermal method [3]. Although (CsK₂)BiCl₆ exhibits an indirect bandgap and poor luminescence, it functions as a good host for Mn²⁺ doping due to the efficient energy transfer from self-trapped excitons to the d-state of the Mn emission center. Similarly, lead-free double perovskite Cs₂Ag_0.3Na_0.7InCl₆ can also function as a host for the co-doping of Eu³⁺, Ho³⁺, and Yb³⁺ to achieve both downconversion and upconversion luminescence [4]. Such triple emissions of warm yellow, red, and green have great potential in anti-counterfeiting applications.

Due to the presence of hydrophobic organic cations, 0D MHHs usually exhibit good stability, while this is a major obstacle for LHP-based devices. Although many approaches have been proposed to improve the stability of LHPs, the strategy of constructing a core shell is the most efficient. For example, porous SiO₂ nanoparticles with entropy ligand functionalization have been adopted as a nanoreactor to produce CsPbBr₃ PNCs [5]. The obtained CsPbBr₃@SiO₂ nanocomposites not only retain the excellent optical properties of CsPbBr₃, but also showed outstanding stability and colloidal dispersity in various solvents. Polymers are also good candidates for encapsulating CsPbBr₃ PNCs. Through controlling the reaction temperature, Lin et al. successfully tuned the size of CsPbBr₃ PNCs from 4.4 to 10 nm, concurrent with a shift in emission from 510 to 460 nm due to the effect of quantum confinement [6]. Moreover, such CsPbBr₃ PNCs with tunable bandgaps and emission exhibit long-term air and water stability through encapsulation of polydimethylsiloxane. Additionally, surface treatment is a common and effective approach to remove the surface defects and improve the colloidality and stability. As reported by Pang’s group [7], the strong binding force of tridentate cysteine with lead ions results in the partial replacement of passivated ligands during post-processing, which decreases the density of surface defects, thereby enhancing the photoluminescence and air stability. Finally, Sun’s group conducted a systematic study on the aggregation of lithium salts in 2,2,7,7-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobi-fluorene (Spiro-OMeTAD) [8], which is usually used as a hole-transport material in photovoltaic applications. To overcome the problem of voids/pinholes caused by the accumulation and hydrolysis of Li-TFSI, CsI was used as an additive. This is because the complex formed from CsI and 4-tert-butylpyridine (TBP) can prevent the rapid evaporation of TBP and the formation of cracks in Spiro-OMeTAD.

In summary, the papers in this Special Issue provide interesting insights into the structural design and improved stability and PL of MHPs and related MHHs. These contributions will deepen our understanding of the physiochemical properties of low-dimensional MHHs and guide the direction of future investigations.