The photophysics of Ruddlesden-Popper perovskites: A tale of energy, charges, and spins

Abstract

Quasi two-dimensional halide perovskites (also known as Ruddlesden-Popper or RPs) are the most recent and exciting evolution in the perovskite field. Possessing a unique combination of enhanced moisture and material stability, whilst retaining the excellent optoelectronic properties, RPs are poised to be a game changer in the perovskite field. Spurred by their recent achievements in solar cells, light-emitting diodes and spintronic devices, these materials have garnered a mounting interest. Herein, we critically review the photophysics of RPs and distil the science behind their structure-property relations. We first focus on their structure and morphology by highlighting the crucial role of large cations: dictating the RPs' layered structure and the statistical distribution of thicknesses (i.e., n-phases). Next, we discuss how optoelectronic properties of RPs differ from conventional halide perovskites. Structural disorder, stronger excitonic and polaronic interaction shape the nature of photo-excitations and their fate. For example, faster recombinations and hindered transport are expected for charge carriers in thinner n-phases. However, the complex energetic landscape of RPs, which originates from the coexistence of different n-phases, allows for funnelling of energy and charges. Presently, the photophysics of RPs is still nascent, with many recent exciting discoveries from coherence effects in the above-mentioned funnelling cascade to spin effects.Giant Rashba spin-orbit coupling, also observed in RPs, dictates their spin dynamics and provides exciting spintronics opportunities. To leverage these propitious RPs, future research must entail a cross-disciplinary approach. While materials engineering will unlock new chiral RPs and Dion-Jacobson variants, novel characterization techniques such as in situ synchrotronbased X-ray diffraction, ultrafast electron microscopy, and multi-dimensional electronic spectroscopy etc. are essential in unravelling their secrets and unleashing their full potential.

Tailoring the Energy Manifold of Quasi‐Two‐Dimensional Perovskites for Efficient Carrier Extraction

“…[21] Such energetic disorder is proposed as a possible cause of the differences in the reported dynamics of the energy cascade. [16] In the context of average excess energy loss of carriers, the coherent process will facilitate sub-ps localization of carriers into high-n QWs (out-competing extraction, that we estimated previously to be of the order of tens of ps) where BCP can no longer extract them. We performed 2DES for the two RPP systems to probe such ultrafast dynamics.…”

“…RPP are layered materials, where alternate octahedron perovskite and large organic cation layers form QWs with strong confinement in the z-dimension. [16] The unique structure of mixed-phase RPP films presents another lever for tailoring Harvesting the excess energy from absorbed above bandgap photons is a promising approach to overcome the detailed balance limit for higher solar cell efficiencies. However, this remains very challenging for 2D layered halide perovskites as the fast excess energy loss competes effectively with charge extraction.…”

Energy Funneling in Quasi‐2D Ruddlesden–Popper Perovskites: Charge Transfer versus Resonant Energy Transfer

“…It is well accepted that the domain distribution is a key factor determining the energy funneling degree. [53] The amplitude of 734 nm PB band is weak due to the low content of n ¼ 8 domains. Nevertheless, the 734 nm band can be distinguished unambiguously.…”

“…However, the exact mechanism in details for the funneling process is yet unclear until now. [10,[46][47][48][49][50][51][52][53] Förster resonance energy transfer (FRET) and Dexter-type charge transfer (CT) via carrier diffusion are proposed to explain the energy funneling. [38,[54][55][56] The FRET model suggests that the energy resonantly transfers from large-bandgap grains to grains with narrower bandgaps through dipole-dipole coupling.…”

In situ, seed-free formation of a Ruddlesden–Popper perovskite Cs2PbI2Cl2 nanowires/PbI2 heterojunction for a high-responsivity, self-powered photodetector

“…Two-dimensional (2D) Ruddlesden-Popper (RP) perovskites have recently drawn ever-increasing attention for their unique characteristics of great quantum confinement effect, large exciton binding energy, and improved stability. [25][26][27] Some more recent studies have revealed that Cs 2 PbI 2 Cl 2 is an inorganic 2D perovskite with the typical RP phase. 28,29 Furthermore, many appealing optoelectronic properties, including high carrier mobility, a direct bandgap, small carrier effective masses, a low exciton binding energy, and excellent ambient/thermal stability have been identified for it.…”