Bond polarizability as a probe of local crystal fields in hybrid lead-halide perovskites

A rotating organic cation and a dynamically disordered soft inorganic cage are the hallmark features of hybrid organic-inorganic lead-halide perovskites. Understanding the interplay between these two subsystems is a challenging problem but it is this coupling that is widely conjectured to be responsible for the unique behaviour of photo-carriers in these materials. In this work, we use the fact that the polarizability of the organic cation strongly depends on the ambient electrostatic environment to put the molecule forward as a sensitive probe of local crystal fields inside the lattice cell. We measure the average polarizability of the C/N--H bond stretching mode by means of infrared spectroscopy, which allows us to deduce the character of the motion of the cation molecule, find the magnitude of the local crystal field and place an estimate on the strength of the hydrogen bond between the hydrogen and halide atoms. Our results pave the way for understanding electric fields in lead-halide perovskites using infrared bond spectroscopy

Effective model for studying optical properties of lead-halide perovskites

We use general symmetry-based arguments to construct an effective model suitable for studying optical properties of lead-halide perovskites. To build the model, we identify an atomic-level interaction between electromagnetic fields and the spin degree of freedom that should be added to a minimally-coupled $\mathbf{k\cdot p}$ Hamiltonian. As an application, we study two basic optical characteristics of the material: the Verdet constant and the refractive index

Visualizing Buried Local Carrier Diffusion in Halide Perovskite Crystals via Two-Photon Microscopy.

Halide perovskites have shown great potential for light emission and photovoltaic applications due to their remarkable electronic properties. Although the device performances are promising, they are still limited by microscale heterogeneities in their photophysical properties. Here, we study the impact of these heterogeneities on the diffusion of charge carriers, which are processes crucial for efficient collection of charges in light-harvesting devices. A photoluminescence tomography technique is developed in a confocal microscope using one- and two-photon excitation to distinguish between local surface and bulk diffusion of charge carriers in methylammonium lead bromide single crystals. We observe a large dispersion of local diffusion coefficients with values between 0.3 and 2 cm2·s-1 depending on the trap density and the morphological environment-a distribution that would be missed from analogous macroscopic or surface measurements. This work reveals a new framework to understand diffusion pathways, which are extremely sensitive to local properties and buried defects

Visualizing Buried Local Carrier Diffusion in Halide Perovskite Crystals via Two-Photon Microscopy.

Halide perovskites have shown great potential for light emission and photovoltaic applications due to their remarkable electronic properties. Although the device performances are promising, they are still limited by microscale heterogeneities in their photophysical properties. Here, we study the impact of these heterogeneities on the diffusion of charge carriers, which are processes crucial for efficient collection of charges in light-harvesting devices. A photoluminescence tomography technique is developed in a confocal microscope using one- and two-photon excitation to distinguish between local surface and bulk diffusion of charge carriers in methylammonium lead bromide single crystals. We observe a large dispersion of local diffusion coefficients with values between 0.3 and 2 cm2·s-1 depending on the trap density and the morphological environment-a distribution that would be missed from analogous macroscopic or surface measurements. This work reveals a new framework to understand diffusion pathways, which are extremely sensitive to local properties and buried defects

Spin-Electric Coupling in Lead-Halide Perovskites

Lead-halide perovskites enjoy a number of remarkable optoelectronic properties. To explain their origin, it is necessary to question how electromagnetic fields interact with these systems. We address this question here by studying two classical quantities: Faraday rotation and the complex refractive index in a paradigmatic perovskite CH$_3$NH$_3$PbBr$_3$ in a broad wavelength range. We find that the minimal coupling of electromagnetic fields to the k$\cdot$p Hamiltonian is insufficient to describe the observed data even on the qualitative level. To amend this, we demonstrate that there exists a relevant atomic-level coupling between electromagnetic fields and the spin degree of freedom. This spin-electric coupling allows for quantitative description of a number of previous as well as present experimental data. In particular, we use it here to show that the Faraday effect in lead-halide perovskites is dominated by the Zeeman splitting of the energy levels, and has a substantial beyond-Becquerel contribution. Finally, we present general symmetry-based phenomenological arguments that in the low-energy limit our effective model includes all possible couplings to the electromagnetic field in the linear order

Probing buried recombination pathways in perovskite structures using 3D photoluminescence tomography

Perovskite solar cells and light-emission devices are yet to achieve their full potential owing in part to microscale inhomogeneities and defects that act as non-radiative loss pathways. These sites have been revealed using local photoluminescence mapping techniques but the short absorption depth of photons with energies above the bandgap means that conventional one-photon excitation primarily probes the surface recombination. Here, we use two-photon time-resolved confocal photoluminescence microscopy to explore the surface and bulk recombination properties of methylammonium lead halide perovskite structures. By acquiring 2D maps at different depths, we form 3D photoluminescence tomography images to visualise the charge carrier recombination kinetics. The technique unveils buried recombination pathways in both thin film and micro-crystal structures that aren't captured in conventional one-photon mapping experiments. Specifically, we reveal that light-induced passivation approaches are primarily surface-sensitive and that nominal single crystals still contain heterogeneous defects that impact charge-carrier recombination. Our work opens a new route to sensitively probe defects and associated non-radiative processes in perovskites, highlighting additional loss pathways in these materials that will need to be addressed through improved sample processing or passivation treatments. ©2018 The Royal Society of Chemistry.DOE (Contract: DE-AC02-05CH11231)EU Seventh Framework Programme REA (grant: PIOF-GA-2013-622630)EU Horizon 2020 research and innovation programme (grant: 756962)Royal Society and Tata Group (UF150033

Probing buried recombination pathways in perovskite structures using 3D photoluminescence tomography.

Perovskite solar cells and light-emission devices are yet to achieve their full potential owing in part to microscale inhomogeneities and defects that act as non-radiative loss pathways. These sites have been revealed using local photoluminescence mapping techniques but the short absorption depth of photons with energies above the bandgap means that conventional one-photon excitation primarily probes the surface recombination. Here, we use two-photon time-resolved confocal photoluminescence microscopy to explore the surface and bulk recombination properties of methylammonium lead halide perovskite structures. By acquiring 2D maps at different depths, we form 3D photoluminescence tomography images to visualise the charge carrier recombination kinetics. The technique unveils buried recombination pathways in both thin film and micro-crystal structures that aren't captured in conventional one-photon mapping experiments. Specifically, we reveal that light-induced passivation approaches are primarily surface-sensitive and that nominal single crystals still contain heterogeneous defects that impact charge-carrier recombination. Our work opens a new route to sensitively probe defects and associated non-radiative processes in perovskites, highlighting additional loss pathways in these materials that will need to be addressed through improved sample processing or passivation treatments.EPSRC (Nano-Doctoral Training Centre) U.S. Department of Energy Winton Graduate Exchange Scholarship European Union’s Seventh Framework Programme (PIOF-GA-2013-622630) European Union’s Horizon 2020 research and innovation programme (ERC 756962) Royal Society and Tata Group (UF150033) EPSRC (EP/M005143/1

Solvent-Templated Methylammonium-Based Ruddlesden–Popper Perovskites with Short Interlayer Distances

Two-dimensional (2D) halide perovskites are exquisite semiconductors with great structural tunability. They can incorporate a rich variety of organic species that not only template their layered structures but also add new functionalities to their optoelectronic characteristics. Here, we present a series of new methylammonium (CH3NH3+ or MA)-based 2D Ruddlesden–Popper perovskites templated by dimethyl carbonate (CH3OCOOCH3 or DMC) solvent molecules. We report the synthesis, detailed structural analysis, and characterization of four new compounds: MA2(DMC)PbI4 (n = 1), MA3(DMC)Pb2I7 (n = 2), MA4(DMC)Pb3I10 (n = 3), and MA3(DMC)Pb2Br7 (n = 2). Notably, these compounds represent unique structures with MA as the sole organic cation both within and between the perovskite sheets, while DMC molecules occupy a tight space between the MA cations in the interlayer. They form hydrogen-bonded [MA···DMC···MA]2+ complexes that act as spacers, preventing the perovskite sheets from condensing into each other. We report one of the shortest interlayer distances (∼5.7–5.9 Å) in solvent-incorporated 2D halide perovskites. Furthermore, the synthesized crystals exhibit similar optical characteristics to other 2D perovskite systems, including narrow photoluminescence (PL) signals. The density functional theory (DFT) calculations confirm their direct-band-gap nature. Meanwhile, the phase stability of these systems was found to correlate with the H-bond distances and their strengths, decreasing in the order MA3(DMC)Pb2I7 > MA4(DMC)Pb3I10 > MA2(DMC)PbI4 ∼ MA3(DMC)Pb2Br7. The relatively loosely bound nature of DMC molecules enables us to design a thermochromic cell that can withstand 25 cycles of switching between two colored states. This work exemplifies the unconventional role of the noncharged solvent molecule in templating the 2D perovskite structure

Ultralong Radiative States in Hybrid Perovskite Crystals: Compositions for Submillimeter Diffusion Lengths

Organic–inorganic hybrid perovskite materials have recently evolved into the leading candidate solution-processed semiconductor for solar cells due to their combination of desirable optical and charge transport properties. Chief among these properties is the long carrier diffusion length, which is essential to optimizing the device architecture and performance. Herein, we used time-resolved photoluminescence (at low excitation fluence, 10.59 μJ·cm^–2 upon two-photon excitation), which is the most accurate and direct approach to measure the radiative charge carrier lifetime and diffusion lengths. Lifetimes of about 72 and 4.3 μs for FAPbBr₃ and FAPbI₃ perovskite single crystals have been recorded, presenting the longest radiative carrier lifetimes reported to date for perovskite materials. Subsequently, carrier diffusion lengths of 107.2 and 19.7 μm are obtained. In addition, we demonstrate the key role of the organic cation units in modulating the carrier lifetime and its diffusion lengths, in which the defect formation energies for FA cations are much higher than those with the MA ones