The 2D Halide Perovskite Rulebook: How the Spacer Influences Everything from the Structure to Optoelectronic Device Efficiency

Abstract

halide perovskites have emerged as outstanding semiconducting materials thanks to their superior stability and structural diversity. However, the ever-growing field of optoelectronic device research using 2D perovskites requires systematic understanding of the effects of the spacer on the structure, properties, and device performance. So far, many studies are based on trial-and-error tests of random spacers with limited ability to predict the resulting structure of these synthetic experiments, hindering the discovery of novel 2D materials to be incorporated into high-performance devices. In this review, we provide guidelines on successfully choosing spacers and incorporating them into crystalline materials and optoelectronic devices. We first provide a summary of various synthetic methods to act as a tutorial for groups interested in pursuing synthesis of novel 2D perovskites. Second, we provide our insights on what kind of spacer cations can stabilize 2D perovskites followed by an extensive review of the spacer cations, which have been shown to stabilize 2D perovskites with an emphasis on the effects of the spacer on the structure and optical properties. Next, we provide a similar explanation for the methods used to fabricate films and their desired properties. Like the synthesis section, we will then focus on various spacers that have been used in devices and how they influence the film structure and device performance. With a comprehensive understanding of these effects, a rational selection of novel spacers can be made, accelerating this already exciting field.

Interfaces in metal halide perovskites probed by solid-state NMR spectroscopy

“…Layered 2D MHP phases can form when large ammonium or diammonium cations are introduced that exceed the tolerance and octahedral factors of perovskites. [3][4][5][6][7][8] The layered phases that form bear intrinsic interfaces between spacer molecule and octahedral metal halide substructures. 2D metal halide phases have been explored for decades as promising anisotropic optoelectronic materials.…”

“…2c). 4,5,[220][221][222] Organic ammonium spacer cations containing long-chain alkyl-, aryl-, adamantantyl-and alkylphenyl-and heterocyclic moieties can be readily incorporated to generate layered structures of different compositions. 5 The lattice plane along which this layering occurs depends on the composition of the MHP.…”

“…Structures with similar octahedral bonding, coordination and chemical composition are also commonly described as MHPs, 2 such as layered homologues. [3][4][5][6][7][8] Halide perovskites have been heralded as next-generation materials for photovoltaics (PV), [9][10][11][12] with power conversion efficiencies (PCE) currently exceeding 25%. 13,14 MHPs have also been explored for applications including light-emitting diodes (LEDs), [15][16][17][18][19][20][21][22][23] phosphors for display technology, 24,25 lasers, [26][27][28][29][30] photodetectors, [31][32][33][34][35] ionizing radiation detectors 30,[36][37][38][39][40] and as quantum emitters.…”

“…For example, low-dimensional Ruddlesden-Popper (RP), Dion-Jacobson (DJ) phases are composed of 2D slabs of MHP sub-phases separated by insulating 'spacer' cations. [3][4][5][6][7][8] Characterizing amorphous, disordered and interfacial structures in MHPs remains an experimental challenge. Multi-technique approaches to bridge different length-and timescales, such as diffraction, electron microscopy and ssNMR spectroscopy, are well suited to correlate bulk behaviors with interfacial and atomic-scale structures in MHPs.…”

Using steric hindrance to manipulate and stabilize metal halide perovskites for optoelectronics

“…When larger or bulky cations are the main cations in the perovskite component, the perovskite structure cannot maintain a 3D framework and a low-dimensional perovskite may be formed. 82 2D perovskites with a corner-sharing inorganic layer structure, especially quasi-2D perovskites, have similar optical and electrical properties but much higher stability than 3D perovskites. This is because long chain cations can lock the unstable octahedral frame and inhibit the penetration of water molecules owing to the steric effect.…”