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Register a new userThe impact of the halide cage on the electronic properties of fully inorganic caesium lead halide perovskites
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Paulina Plochocka Dr
Publication date
2017
fields
Physics
and research's language is
English
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Perovskite solar cells with record power conversion efficiency are fabricated by alloying both hybrid and fully inorganic compounds. While the basic electronic properties of the hybrid perovskites are now well understood, key electronic parameters for solar cell performance, such as the exciton binding energy of fully inorganic perovskites, are still unknown. By performing magneto transmission measurements, we determine with high accuracy the exciton binding energy and reduced mass of fully inorganic CsPbX$_3$ perovskites (X=I, Br, and an alloy of these). The well behaved (continuous) evolution of the band gap with temperature in the range $4-270$,K suggests that fully inorganic perovskites do not undergo structural phase transitions like their hybrid counterparts. The experimentally determined dielectric constants indicate that at low temperature, when the motion of the organic cation is frozen, the dielectric screening mechanism is essentially the same both for hybrid and inorganic perovskites, and is dominated by the relative motion of atoms within the lead-halide cage.
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2D Ruddlesden Popper perovskites have been extensively studied for their exceptional optical and electronic characteristics while only a few studies have shed light on their mechanical properties. The existing literature mainly discusses the mechanical strength of single crystal perovskites, however a systematic study towards structure tunability of 2D perovskite thin films is still missing. In this study, we report the effect of number of inorganic layers `n on elastic modulus of Butylammonium based 2D, quasi-2D perovskites and 3D perovskite using nanoindentation technique. The experimental results have also been substantiated using first principle density functional theory calculations. Understanding the mechanical behaviour of 2D Ruddlesden Popper perovskites thin films in comparison with conventional 3D perovskite offers intriguing insights into the atomic layer dependent properties and paves the path for next generation mechanically durable novel devices.
The electronic structure evolution of deficient halide perovskites with a general formula $(A,A)_{1+x}M_{1-x}X_{3-x}$ was investigated using the density functional theory. The focus is placed on characterization of changes in the band gap, band alignment, effective mass, and optical properties of deficient perovskites at various concentrations of defects. We uncover unusual electronic properties of the defect corresponding to a $M!-!X$ vacancy filled with an $A$ cation. This defect repels electrons and holes producing no trap states and, in moderate quantities ($xle0.1$), does not hinder charge transport properties of the material. This behavior is rationalized using a confinement model and provides an additional insight to the defect tolerance of halide perovskites.
Excitonic/electronic coupling and cooperative interactions in self-assembled lead halide perovskite nanocrystals were reported to give rise to a collective low energy emission peak with accelerated dynamics. Here we report that similar spectroscopic features could appear as a result of the nanocrystal reactivity within the self-assembled superlattices. This is demonstrated by using CsPbBr3 nanocrystal superlattices under room temperature and cryogenic micro-photoluminescence spectroscopy. It is shown that keeping such structures under vacuum, a gradual contraction of the superlattices and subsequent coalescence of the nanocrystals occurs over several days. As a result, a narrow, low energy emission peak is observed at 4 K with a concomitant shortening of the photoluminescence lifetime due to the energy transfer between nanocrystals. When exposed to air, self-assembled CsPbBr3 nanocrystals develop bulk-like CsPbBr3 particles on top of the superlattices. At 4 K, these particles produce a distribution of narrow, low energy emission peaks with short lifetimes and excitation fluence-dependent, oscillatory decays, resembling the features of superfluorescence. Overall, the reactivity of CsPbBr3 nanocrystals dramatically alters the emission of their assemblies, which should not be overlooked when studying collective optoelectronic properties nor confused with superfluorescence effects.
Behaving like atomically-precise two-dimensional quantum wells with non-negligible dielectric contrast, the layered HOIPs have strong electronic interactions leading to tightly bound excitons with binding energies on the order of 500 meV. These strong interactions suggest the possibility of larger excitonic complexes like trions and biexcitons, which are hard to study numerically due to the complexity of the layered HOIPs. Here, we propose and parameterize a model Hamiltonian for excitonic complexes in layered HOIPs and we study the correlated eigenfunctions of trions and biexcitons using a combination of diffusion Monte Carlo and very large variational calculations with explicitly correlated Gaussian basis functions. Binding energies and spatial structures of these complexes are presented as a function of the layer thickness. The trion and biexciton of the thinnest layered HOIP have binding energies of 35 meV and 44 meV, respectively, whereas a single exfoliated layer is predicted to have trions and biexcitons with equal binding enegies of 48 meV. We compare our findings to available experimental data and to that of other quasi-two-dimensional materials.
Lead halide perovskites are a remarkable class of materials that have emerged over the past decade as being suitable for application in a broad range of devices, such as solar cells, light-emitting diodes, lasers, transistors, and memory devices, among others. While they are often solution-processed semiconductors deposited at low temperatures, perovskites exhibit properties one would only expect from highly pure inorganic crystals that are grown at high temperatures. This unique phenomenon has resulted in fast-paced progress toward record device performance; unfortunately, the basic science behind the remarkable nature of these materials is still not well understood. This review assesses the current understanding of the photoluminescence (PL) properties of metal halide perovskite materials and highlights key areas that require further research. Furthermore, the need to standardize the methods for characterization of PL in order to improve comparability, reliability and reproducibility of results is emphasized.
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