Merits and Challenges of Ruddlesden–Popper Soft Halide Perovskites in Electro‐Optics and Optoelectronics

Abstract

Following the rejuvenation of 3D organic–inorganic hybrid perovskites, like CH3NH3PbI3, (quasi)‐2D Ruddlesden–Popper soft halide perovskites R2An−1PbnX3n+1 have recently become another focus in the optoelectronic and photovoltaic device community. Although quasi‐2D perovskites were first introduced to stabilize optoelectronic/photovoltaic devices against moisture, more interesting properties and device applications, such as solar cells, light‐emitting diodes, white‐light emitters, lasers, and polaritonic emission, have followed. While delicate engineering design has pushed the performance of various devices forward remarkably, understanding of the fundamental properties, especially the charge‐transfer process, electron–phonon interactions, and the growth mechanism in (quasi)‐2D halide perovskites, remains limited and even controversial. Here, after reviewing the current understanding and the nexus between optoelectronic/photovoltaic properties of 2D and 3D halide perovskites, the growth mechanisms, charge‐transfer processes, vibrational properties, and electron–phonon interactions of soft halide perovskites, mainly in quasi‐2D systems, are discussed. It is suggested that single‐crystal‐based studies are needed to deepen the understanding of the aforementioned fundamental properties, and will eventually contribute to device performance.

Oriented and Uniform Distribution of Dion–Jacobson Phase Perovskites Controlled by Quantum Well Barrier Thickness

“…The structure of the 2D layered perovskites is defined by the general formula A 2 A′ n − 1 M n X 3 n + 1 or AA′ n − 1 M n X 3 n + 1 , wherein the former, having monoammonium spacer cations, is named the Ruddlesden–Popper (RP) phase, and the latter, with diammonium spacer cations, is called the Dion–Jacobson (DJ) phase . At present, the research on RP phase perovskites is relatively mature, and various methods have been used to control crystal orientation and QW distribution, which are two critical factors influencing the performance. Hot casting was first reported to achieve vertical orientation, followed by the use of additives, shorter spacers, and the introduction of small amounts of FA + and Cs + .…”

Mixed Spacer Cation Stabilization of Blue‐Emitting n = 2 Ruddlesden–Popper Organic–Inorganic Halide Perovskite Films

“…As a consequence, e–ph coupling for n = 2 and n = 3 is first order of magnitude larger than the coupling for n = 1. This indicates that previously reported tremendous difference in photoluminescence quantum yield (PLQY) for PEA perovskites compared to BA (17% for BA‐based and 79% for PEA‐based 2D perovskites) would only hold for n = 1 (excluding any contributions of the energy funneling effect to the emission in films with multiple n phases).…”

“…Halide perovskite light‐emitting diodes (LEDs), including those based on Ruddlesden–Popper perovskites (RPPs), have been attracting increasing attention . RPPs have a general formula R 2 A n −1 B n X 3 n +1 , where R + is the bulky amine spacer cation, A + is an organic cation, B 2+ is a metal cation, X − is a halide anion, and n is the number of perovskite layers between the spacer cations.…”

“…Emission tuning by changing n instead of using halide mixtures enables improved color stability compared to mixed halide LEDs which commonly exhibit peak shifts with time and/or bias due to phase segregation . However, while exfoliated RPP single crystals exhibit clear dependence of properties on the number of layers n , thin films typically consist of a mixture of phases with different n and the phase pure films (achieved for one spacer cation via melt‐processing instead of commonly used spin‐coating) have been scarce . The existence of multiple n phases is undesirable for LED applications since it can lead to emission peak shift with increasing bias .…”

“…Despite these efforts, the differences in behavior (processability, stability, and emission efficiency) of the RPP with BA and PEA spacer cations are not well understood, and the blue‐emitting RPPs typically contain multiple peaks in either absorption or emission spectra, indicating that phase purity has not been achieved. Furthermore, energy funneling phenomenon, which involves energy transfer from lower n (higher energy) to higher n (lower energy) domains resulting in a brighter, but redshifted emission, represents further difficulty in obtaining pure blue emission from n = 2 RPP emitters. Therefore, green emission or at best sky blue (≈490 nm) emission is usually obtained in LEDs even for predominant n = 2 composition of the film and suppressed formation of n = 1 phase using processing additives .…”