In semiconductors, exciton or charge carrier diffusivity is typically described as an inherent material property. Here, we show that the transport of excitons (i.e., bound electron-hole pairs) in CsPbBr3 perovskite nanocrystals (NCs) depends markedly on how recently those NCs were occupied by a previous exciton. Using fluence- and repetition-rate-dependent transient photoluminescence microscopy, we visualize the effect of excitation frequency on exciton transport in CsPbBr3 NC solids. Surprisingly, we observe a striking dependence of the apparent exciton diffusivity on excitation laser power that does not arise from nonlinear exciton-exciton interactions nor from thermal heating of the sample. We interpret our observations with a model in which excitons cause NCs to undergo a transition to a metastable configuration that admits faster exciton transport by roughly an order of magnitude. This metastable configuration persists for ~microseconds at room temperature, and does not depend on the identity of surface ligands or presence of an oxide shell, suggesting that it is an intrinsic response of the perovskite lattice to electronic excitation. The exciton diffusivity observed here (>0.15 cm2/s) is considerably higher than that observed in other NC systems on similar timescales, revealing unusually strong excitonic coupling in a NC material. The finding of a persistent enhancement in excitonic coupling between NCs may help explain other extraordinary photophysical behaviors observed in CsPbBr3 NC arrays, such as superfluorescence. Additionally, faster exciton diffusivity under higher photoexcitation intensity is likely to provide practical insights for optoelectronic device engineering.

Sub-nanosecond radiative decay time was first observed for weakly bound excitons in bulk semiconductors and was understood in terms of the phenomenon known as giant oscillator strength (GOS).¹ GOS is a quantum phenomenon connected with coherent excitation of excitons over the entire volume of exciton localization, and is counterintuitive because the radiative decay time is inversely proportional to this volume. Consequently, it was predicted theoretically² that an exciton weakly confined in a nanocrystal (NC), with radius a much larger than the exciton radius a_ex, is characterized by GOS with magnitude f_NC = f₀(a/a_ex)³ >> f₀, where f₀ is the exciton oscillator strength. Indeed ~100 ps radiative decay times were observed in large-size CsPbX₃ (X = Cl, Br, I) perovskite NCs.³

The development of organic-inorganic perovskite photovoltaics (PV) and radiation detectors (RD) has been particularly impressive. In the past 10 years, perovskite-based PV cells have reached a certified efficiency of 25.2% and perovskite-based RDs have demonstrated comparable progress by combining remarkable defect tolerance, large mobility-lifetime products, tunable band gaps, crystal growth from low-cost solution processes, and strong stopping power from Pb. We connect this progress with suppressed recombination in this material. Our analysis of the best PV cells, including perovskites, shows that their efficiencies are very close to ultimate PV limits in the absence of carrier recombination.⁴ The recent data on perovskite RDs⁵ also indicate, from our point of view, the advent of ultimate collection efficiency. For such RDs, we provide a theory that describes current collection efficiency in the absence of the carrier recombination.⁵

¹ E. I. Rashba and G. E. Gurgenishvili, Edge absorption theory in semiconductors. Sov. Phys. Solid State, 4, 759-760 (1962).

² Al. L Efros and A. L. Efros, Interband absorption of light in a semiconductor sphere, Sov. Phys.

³ M. A. Becker, et al. “Bright triplet excitons in caesium lead halide perovskites,” Nature, 553, 189-193 (2018).

⁴Al. L. Efros and V. G. Karpov “Electric power and current collection in semiconductor devices with suppressed electron−hole recombination”, ACS Energy Lett. 2022, 7, 3557−3563

⁵ M. Kovalenko, et al. Stable Near-to-Ideal Performance of a Solution grown Single-Crystal Perovskite X-Ray Detector. 2022, https://doi.org/10.21203/rs.3.rs-1117933/v1

Halide perovskites are exciting materials ushering in a new wave of photovoltaic and light-emitting technologies, with efficiencies already comparable to or beating more traditional semiconducting systems. However, a full understanding of carrier recombination and how it relates to performance losses and device operation in various device structures remains elusive. Here I will present results where we exploit luminescence from these materials to understand carrier recombination. I will give examples of different dimentionality systems and where carrier/exciton funnelling is critical for device operation. The luminescence approaches also allow tracking of device operation over time, and a decoupling of the impact of the active layer and contacts in understanding performance losses. These results allow us to draw generalised conclusions about carrier recombination and device performance and instability pathways driven by carrier trapping. Finally, I will show how we are exploiting this understanding to develop new solar cell and lighting devices, further pushing performances up.

Quantum dots (QDs) offer unique physical properties and novel application possibilities like single-photon emitters for quantum technologies.^1,2 While strongly confined III–V and II–VI QDs have been studied extensively, their complex valence band structure often limits clear observations of individual transitions. In recently emerged lead-halide perovskites, band degeneracies are absent around the bandgap reducing the complexity of optical spectra. We show that for spherical-like CsPbBr₃ QDs with diameters >6 nm,³ excitons confine with respect to their center-of-mass motion leading to well-pronounced resonances in their absorption spectra. Optical pumping of the lowest-confined exciton with femtosecond laser pulses not only bleaches all excitons but also reveals a series of distinct induced absorption resonances which we attribute to exciton-to-biexciton transitions and are red-shifted by the biexciton binding energy (∼40 meV).⁵ The temporal dynamics of the bleached excitons further support our exciton confinement model. Our study provides the first insight into confined excitons in CsPbBr₃ QDs and gives a detailed understanding of their linear and nonlinear optical spectra.

All-inorganic lead-halide perovskite (CsPbX₃, X = Cl, Br, I) quantum dots (QDs) have emerged as a competitive platform for classical light emitting devices (in the weak light-matter interaction regime, e.g., LEDs and laser)^[1], as well as for devices exploiting strong light-matter interaction and operated at room-temperature.^[2] Many-body interactions and quantum correlations among photogenerated exciton complexes play an essential role, e.g., by determining the laser threshold, the overall brightness of LEDs, and the single-photon purity^{[3, 4]} in quantum light sources. Here, by combining single-QD optical spectroscopy performed at cryogenic temperatures in combination with configuration interaction (CI) calculations, we address the trion and biexciton binding energies and unveil their peculiar size dependence. We find that trion binding energies increase from 7 meV to 17 meV for QD sizes decreasing from 30 nm to 9 nm, while the biexciton binding energies increase from 15 meV to 30 meV, respectively. CI calculations quantitatively corroborate the experimental results and suggest that the effective dielectric constant for biexcitons slightly deviates from the one of the single excitons, potentially as a result of coupling to the lattice in the multiexciton regime. Our findings provide a deep insight into the multiexciton properties in all-inorganic lead-halide perovskite QDs, essential for classical and quantum optoelectronic devices.

Lead-based semiconductors are among the most explored compounds for the synthesis of colloidal nanomaterials, mainly due to the appealing optoelectronic properties demonstrated by lead halide perovskites in the UV-VIS and by lead chalcogenides in the IR spectral ranges. The mature research on both classes of materials has recently led to the exploration of systems where such properties can coexist and interact. One promising direction is that of heterostructures, which are composite materials formed by intimately connected domains of two different compounds. They are generally challenging to obtain, mostly because of the poor compatibility in terms of required synthetic conditions and crystal structures of the two materials. Another direction to investigate is represented by compounds that are chemically related to both lead halides and lead chalcogenides, namely the lead chalcohalides. These materials, with general formula PbaEbXc ,(E=S, Se, Te, X=F, Cl, Br) are expected to show intermediate properties in between those of lead halides and chalcogenides, and more importantly might feature the chemical and structural compatibility needed to interface with both. We pioneered the investigation of lead chalcohalides at the nanoscale, discovering additional colloidal nanocrystals (NCs): the new compounds are semiconductors with a band gap in between those of lead halide perovskites and of lead sulfide, were obtained in relatively mild reaction conditions and feature a remarkable chemical stability. Moreover, we achieved the synthesis of colloidal heterostructures formed by epitaxially connected domains of Pb₄S₃Br₂ sulfobromide and CsPbX₃ perovskite, thus demonstrating a synthetic and structural compatibility between lead halide perovskites and lead chalcohalides. Stability of the system is highly increased respect to CsPbX3 NCs We take advantage of the domains? compatibility and exploit CsPbCl₃ perovskite NCs as disposable and phase-selective epitaxial templates to drive the synthesis of lead sulfochloride NCs. As a result, we expand the family of lead chalcohalides with two new phases: Pb₃S₂Cl₂ and Pb₄S₃Cl₂.The perovskite domain is called a disposable template because, at a later stage, it can be etched from the heterostructures by exploiting the solubility of CsPbCl₃ in polar solvents, while leaving the Pb₄S₃Cl₂ domains intact. Hence, the full procedure delivered colloidally stable Pb₄S₃Cl₂ NCs that could not be obtained by direct synthesis due to the competitive nucleation. Our use of perovskite NCs as disposable and phase-selective epitaxial templates parallels that of reaction-directing groups in traditional organic chemistry and catalysis. Such an approach to a deterministic synthesis of NCs might be extended to other pairs of materials with known or predictable epitaxial relations, taking advantage of the vast library of already reported nanomaterials as starting templates. This approach could open new routes for the colloidal syntheses of materials which are now hindered by an excessive activation energy for the homogeneous nucleation, or by the competitive formation of undesired phases

Perovskite nanocrystals are highly advantageous semiconductor materials for tailored light applications [1]. Large quantum yields, narrow emission and broad spectral tunability are only some of the unique optoelectronic properties at the center of their beneficial performance. On the other hand, flat patterned microstructures have emerged as powerful platforms for controlled light-matter interactions [2]. When asymmetrically shaped nano-objects are chosen as the unit cell of the array structure, these near-field interactions get dependent on the circular polarization state. Particularly nonlinear optoelectronic devices governed by multi-photon processes could benefit from such a near-field control.

With the aim to equip perovskite nanocrystals with chiral effects, we have built a hybrid perovskite-metasurface system [3]. A suitable chiral z-shaped Si nanoantenna array was coated with a monolayer of cubic all-inorganic lead halide perovskite nanocrystals. The nanoantenna array exhibits pronounced chiral resonances in the visible to IR region which allow to confine the excitation light in the fabricated nanostructures. We demonstrate that the chiral near-field interactions can serve to induce polarization effects in the two-photon absorption process of the perovskite nanocrystals. By tuning the thickness of the perovskite film down to one monolayer, we restricted the interactions exclusively to the near-field regime.  We show that the layer’s two-photon excited luminescence is enhanced by up to one order of magnitude in this configuration. In particular, the enhancement is controllable by the excitation wavelength and by its polarization, revealing a pronounced fluorescence detected circular dichroism of the hybrid emitter-antenna system. Altogether, our findings present a pathway to control perovskite light emission via polarization sensitive near-field interactions, highlighting the potential of this hybrid system for sensing applications and display technologies.

In recent years, scintillating nanoparticles have been recognized as valid potential alternatives to inorganic and organic bulk scintillators, since they meet the needs and the demanding requirements of cutting-edge applications, such as nuclear and homeland security technologies, clinical and imaging devices. Nanoscintillators feature adaptable luminescence and scintillation properties, tuned by their physical and chemical characteristics, like the electronic structure, the dimensionality, and the defectiveness [1]. Most importantly, nanoscintillators can be embedded in suitable polymeric hosts to create composite materials and produce fast, efficient, and more sensitive detectors in a cost-effective way, thanks to reduced time-consuming synthesis procedures, to meet the specific demands of all up-to-date technological and medical applications [2].

The modern research is considering scintillating nanocomposites based on lead halide perovskite (LHP) nanocrystals (NCs) for the next generation of scintillation detectors [3,4]; when embedded in polymeric matrices, LHP NCs favors the enhancement of the interaction cross-section with ionizing radiation, thanks to their high atomic number [5], and retain exceptional levels of radiation hardness [6]. The implementation of this novel class of scintillating composites is imperative and aims at the achievements of scintillators with improved performances. This goal is essentially linked to the fundamental understanding of the correlation between the physical-chemical properties and the luminescence features and to the comprehension of the scintillation mechanism in nanosystems, from the primary interaction with the ionizing radiation, through energy transfer and trapping processes, to the emission of light. A fundamental stage in the scintillation process is the transport of free carriers generated upon the interaction between ionizing radiation and the scintillating material: it is often largely affected by the presence of trapping sites, which can capture migrating charge carriers and either delay their radiative recombination or decrease the overall scintillation efficiency, according to the characteristics of the traps involved.

In this work, we disclose the role of trapping defects and their interplay with the scintillation properties of LHP NCs and nanocomposites. Specifically, we present a thorough investigation of the tight correlation existing between delayed scintillation processes and defects acting as carrier traps, as well as of the competition between trapping sites and luminescent centers in free carrier capture. To these purposes, steady-state radio-luminescence as a function of both temperature and cumulated X-ray dose is combined with time-resolved photo-luminescence measurements and wavelength-resolved thermally stimulated luminescence (TSL) at cryogenic temperatures. The performances and defectiveness of CsPbBr₃ NCs prepared by hot-injection method are investigated and compared with those of CsPbBr₃ NCs produced by ligand-assisted reprecipitation synthetic approach. Our results suggest that shallow trap states, likely related to bromine vacancies, capture and slowly release electrons via a-thermal tunnelling to spatially correlated emissive centers responsible for delayed emission: because of the relatively large concentration of such defects, electron trapping in shallow defects is the main competitive channel to radiative exciton decay. The presence of surface-related defects is common in this class of materials, although advanced strategies for their passivation are continuously improved to enhance the emission efficiency. In addition, we prove that CsPbBr₃ NCs can be effectively embedded into polymethylmethacrylate matrix to obtain a high optical quality flexible and smooth scintillating nanocomposite film, whose defectiveness resembles that of LHP bulk crystals [7], featuring isolated energetically deep defects states that trap carriers which, upon heating, recombine in a specific intragap emission center, as revealed by TSL measurements. The effectiveness of this investigation approach coupling scintillation and TSL measurements, traditionally exploited only for classical single component bulk scintillators, is therefore here demonstrated also for nanosized materials.

Lead halide perovskite nanocrystals (LHP NCs) are an emerging class of light-emitting semiconductors owing to their remarkable optoelectronic properties such as their high photoluminescence quantum yield (PLQY), tunable emission across the visible spectrum emission, and facile synthesis. The optical properties of LHP NCs are not only tunable by their halide composition, but also through doping with metal cations. Generally, doping not only improves the stability of LHP NCs but also reduces Pb-related toxicity by replacing them with non-toxic dopants. Among all, Mn2+-doped LHP NCs have received significant interest to understand the exciton-to-dopant energy or electron transfer process.1-4 The Mn2+-doping in CsPbCl3 NCs results in the transfer of the exciton energy from CsPbCl3 to the dopants leading to orange emission from a spin-forbidden Mn d–d transition. However, the energy transfer efficiency not only depends on the amount of the dopant and band alignment of the dopants with respect to the excitons but also on the surface traps. In this presentation, I will discuss our findings on the enhanced exciton-to-dopant energy transfer by post-synthetic surface treatment with didodecyldimethylammonium chloride (DDAC), a tightly binding ligand. The DDAC ligands have the ability to replace the weakly bound oleyalammonium cations and reduce the density of chlorine vacancies on the surface, resulting in an increase in the PLQY of Mn2+ ions. Ultrafast time pump-probe studies and time-resolved luminescence of dopants revealed that the DDAC ligands remove the surface traps and thus promote energy transfer as well as reduce the quenching of Mn2+ emission by the surface traps.

Perovskite material appeared to be perspective and promising in photovoltaic application, such as solar cells, photodetectors, light emitting diodes and etc. due to outrageous properties like high carrier mobility, effective light absorption, mechanical flexibility, cheapness and simplicity of production makes them worthy opponent to crystalline silicon in creation less environment polluted world for our children. Although perovskites are already started to be used in solar cell fabrication on the market level, the extrinsic and intrinsic stability of them can be greatly improved. The most discussed issue nowadays is halide movement under continuous light illumination for I-rich and Br-rich domains. I-rich domain act as recombination centers in halide perovskites what impedes the carrier generation in the bromine-rich domain and blocks the electron flow through the device and reduces the efficiency of solar cells.

Although broad consensus exists that photoirradiation of mixed-halide lead perovskites leads to anion segregation, no model today fully rationalizes all aspects of this near ubiquitous phenomenon. In this work, we quantitatively compare experimentally the variety of dimensionality materials (such as bulk thin films, 2D Ruddlesden-Popper and 0D nanocrystals (NCs)) terminal anion photosegregation stoichiometries and excitation intensity thresholds to a band gap-based, thermodynamic model of mixed-halide perovskite photosegregation. Mixed-halide NCs offer strict tests of theory given physical sizes, which dictate carrier diffusion lengths. Further highlighting the importance of these studies are prior results, which suggest increased stabilities of mixed-anion perovskite NCs to irradiation. Observed qualitative and quantitative agreement with theory support a band gap-based model for anion photosegregation. More importantly, they suggest that mixed-halide perovskite photostabilities can be predicted using local gradients of (empirical) Vegard’s law expressions of composition-dependent band gaps. We have recently tested this alternative photostability metric on a mixed-cation/mixed-anion system, MA_0.5Cs_0.5Pb(I_1−xBr_x)₃ compared to MAPb(I_1−xBr_x)₃ and predicted that smaller local band gap gradients indeed correspond to improved anion photostabilities. Thus, not only is the developed band gap-based photosegregation model predictive but future extensions may rationalize remaining unexplained phenomena in lead halide perovskites such as halide remixing under large excitation intensities.

High environmental stability and surprisingly high efficiency of solar cells based on 2D perovskites have renewed interest in these materials. These natural quantum wells consist of planes of metal-halide octahedra, separated by organic spacers.  Remarkably the organic spacers play crucial role in optoelectronic properties of these compounds. The characteristic for ionic crystal coupling of excitonic species to lattice vibration became particularly important in case of soft perovskite lattice. The nontrivial mutual dependencies between lattice dynamics, organic spacers and electronic excitation manifest in a complex absorption and emission spectrum which detailed origin is subject of ongoing controversy. First, I will discuss electronic properties of 2D perovskites with different thicknesses of the octahedral layers and two types of organic spacer.  I will demonstrate that the energy spacing of excitonic features depends on organic spacer but very weakly depends on octahedral layer thickness. This indicates the vibrionic progression scenario which is confirmed by high magnetic fields studies up to 67T. Next, I will show that in 2D perovskites, the distortion imposed by the organic spacers governs the effective mass of the carriers.  As a result, and unlike in any other semiconductor, the effective mass in 2D perovskites can be easily tailored. In the end, I will discuss exciton fine structure. The bright-dark splitting is also of paramount importance for light emitters which rely on the radiative recombination of excitons, since the excitons usually relax to the lowest lying dark state, which is detrimental for the device efficiency. I will discuss our optical spectroscopy measurements with an applied in-plane magnetic field to mix the bright and dark excitonic states of (PEA)₂PbI₄, providing the first direct measurement of the bright-dark splitting. The induced brightening of the dark state allows us to directly observe an enhancement of the absorption at the low-energy side of the spectrum related to the dark state. The evolution of the PL signal in the magnetic field, suggests that at low temperatures the exciton population is not fully thermalized due to the existence of a phonon bottleneck, which occurs due to the specific nature of the exciton-phonon coupling in soft perovskite materials.

Halide perovskites possess outstanding optoelectronic properties favorable for applications as highly efficient absorbers in perovskite photovoltaics and as bright emitters in perovskite light emitting devices, lasers as well as quantum emitters. Underpinning this spectacular rise are their exceptional properties such as large absorption cross-sections, defect tolerance, large spin-orbit coupling, long balanced charge diffusion lengths, slow hot carrier cooling, ion migration, radiation tolerance etc. Hence, there has been a rapid proliferation of their applications beyond solar cells and light-emitting devices to fields such as spintronics, radiation detectors, memristors, bioimaging etc. Their versatile structures and diverse dimensionalities afford new levers for tunning their photophysical properties. For instance, the vast library of large organic cations allows one to tune the energy landscape in layered perovskites. In this talk, I will distill some of the underpinning photophysical mechanisms in low-dimensional emitters such as layered Ruddlesden Popper perovskites and colloidal perovskite nanocrystals. I will also take the opportunity to highlight some of our latest works on this topic.

Organic-inorganic metal halide perovskites have emerged as attractive materials for solar cells with power-conversion efficiencies of single-junction devices now exceeding 25%.

On issue that still limits the implementation of silicon-perovskite tandem cells in particular, is the peculiar mechanisms underlying detrimental halide segregation in mixed iodide-bromide lead perovskites with desirable electronic band gaps near 1.75eV.[1,2,3,4] We reveal that, surprisingly, halide segregation results in negligible impact to the THz charge-carrier mobilities.[2] However, remarkably fast, picosecond charge funnelling into the narrow-bandgap I-rich domains leads to enhanced radiative recombination.[3] Performance losses in photovoltaic devices may therefore potentially be mitigated by deployment of careful light management strategies. We further demonstrate[3] how a combination of simultaneous in-situ photoluminescence and X-ray diffraction measurements is able to demonstrate clear differences in compositional and optoelectronic changes associated with halide segregation in MAPb(Br0.5I0.5)3 and FA0.83Cs0.17Pb(Br0.4I0.6)3 films. While MAPb(Br0.5I0.5)3 exhibits rearrangement of halide ions only in localized volumes of perovskite, FA0.83Cs0.17Pb(Br0.4I0.6)3 lacks such low-barrier ionic pathways and is, consequently, more stable against halide segregation. However, under prolonged illumination, it exhibits a considerable ionic rearrangement throughout the bulk material, which may be triggered by an initial demixing of A-site cations, altering the composition of the bulk perovskite and reducing its stability against halide segregation.[4] We further explore the influence of a hole-transport layer, necessary for a full device. We show that top coating FA0.83Cs0.17Pb(Br0.4I0.6)3 perovskite films with a poly(triaryl)amine (PTAA) hole-extraction layer surprisingly leads to suppression of halide segregation because photogenerated charge carriers are rapidly trapped at interfacial defects that do not drive halide segregation.[4]

We also discuss the charge-carrier dynamics in layered, 2D perovskites that have been found to improve the stability of metal halide perovskite thin films and devices.[5] We show that the 2D perovskite PEA2PbI4 exhibits an excellent long-range mobility of 8.0 cm2 (V s)–1, ten times greater than the long-range mobility determined for a comparable 3D material FA0.9Cs0.1PbI3. These values shows that the polycrystalline 2D thin films already have single-crystal-like qualities. We further demonstrate that these materials exhibit unexpectedly high densities of sustained populations of free charge carriers.

[1] A. J. Knight and L. M. Herz, Energy Environmental Science 13, 2024 (2020).

[2] S. G. Motti, J. B. Patel, R. D. J. Oliver, H. J. Snaith, M. B. Johnston, L. M. Herz, Nature Communications 12, 6955 (2021).

[3] A. J. Knight, J. Borchert, R. D. J. Oliver, J. B. Patel, P. G. Radaelli, H. J. Snaith, M. B. Johnston, and L. M. Herz, ACS Energy Letters 6, 799 (2021).

[4] Impact of hole-transport layer and interface passivation on halide segregation in mixed-halide perovskites, V. J.-Y. Lim, A. J. Knight, R. D. J. Oliver, H. J. Snaith, M. B. Johnston, and L. M. Herz, Advanced Functional Materials 32, 2204825 (2022).

[5] Excellent long-range charge-carrier mobility in 2D perovskites, M. Kober-Czerny, S. G. Motti, P. Holzhey, B. Wenger, J. Lim, L. M. Herz, and H. J. Snaith, Advanced Functional Materials 32, 2203064 (2022).

Lead halide perovskite nanocrystals are promising candidates for applications in light-emitting devices due to their high exciton binding energy, exceptionally high quantum efficiency, and good stability. Depending on crystal symmetry and shape anisotropy, a distinct exciton fine structure controls their emission properties [1]. Here, we demonstrate the strength of polarization-resolved micro-photoluminescence (PL) spectroscopy on a single nanocrystal level for getting insight into exciton fine structure states and their relation to crystal symmetry and shape anisotropy.

In nearly cubic FAPbBr₃ nanocrystals, the degeneracy of the bright exciton triplet is lifted leading to three bright states with transition dipoles oriented along the orthorhombic crystal symmetry at cryogenic temperatures. Depending on the orientation of the nanocrystals with respect to the optical axis between one and three polarized emission lines are visible [2]. Magneto-PL and time-resolved-PL experiments on single nanocrystals demonstrate that the dark singlet exciton is energetically below the bright one, shifted by about 2.6 meV.

In highly anisotropic CsPbBr₃ nanoplatelets (NPLs) either one, two or three resolvable emission lines (case I, II and III, respectively) with significantly different polarization patterns are found. In case a single peak is observed, the emission is mostly unpolarized and linewidths generally exceed 1 meV. In contrast, the polarization of the two emission lines of case II NPLs are oriented orthogonally with respect to each other. In case III NPLs, the lowest and highest energy peaks are polarized collinearly, while the central emission line is polarized in a direction orthogonal to the former two. This quite characteristic polarization pattern can be explained by the occurrence of orthorhombic CsPbBr₃ NPLs with two different orientations of the crystal axes [3]. In case II NPLs, one of the orthorhombic crystal axes is aligned with the NPL thickness and the observation direction and the radiation emitted by the corresponding dipole cannot be detected. In case III NPLs, in contrast, all orthorhombic crystal axes have a finite projection perpendicular to the observation direction allowing the observation of all emission lines of the fine structure split triplet. The negligible fine structure splitting in case I NPLs is consistent with the coexistence of these different lattice polymorphs within a single NPL. Our findings not only allow the unambiguous identification of the crystal configuration of individual CsPbBr₃ nanoplatelets from pure optical measurements, but in addition facilitate the determination of a nanoplatelets’ absolute spatial orientation with respect to the lab coordinates via its characteristic polarization pattern.[4]

2D perovskites are often used to stabilize perovskite solar cells. By adding a thin layer of 2D perovskite on top of a 3D bulk solar cell, stability can be massively improved. This effect has lead to the impression that 2D perovskites are stable materials. However, we show that these materials are extremely unstable under illumination, especially when combined with air. 

We quantify the decrease in photoluminescence after illumination. The photoluminescence first increases (photobrightens) and then decays dramatically. We show that this decay in luminescence coincides with a loss of material and a decomposition into gasses and precursors. We also show some initial mechanistic explanation for a better stability under nitrogen atmosphere.

Our results indicate that 2D perovskites are not intrinsically stable, but rather protect the underlying 3D perovskite from ambient influences, presumably by forming a hydrophobic barrier. Our results further illustrate the caution required when studying 2D perovskites because they continuously change during the measurement.

Recently, microcavity exciton polariton research has attracted considerable interests in a number of excellent optical gain materials that demonstrate unique properties compared with conventional III-V or II-VI semiconductor quantum wells and organic semiconductors. Those materials include transition metal dichalcogenides (TMDs), and certain halide perovskite semiconductors. Particularly, those materials exhibit large exciton binding energies (much larger than thermal fluctuation energy ~ 26 meV), large oscillator strength and peculiar electronic band structures such as valley polarization or encoded chiroptical responses. In this talk, we will discuss our recent effort in manipulating exciton polariton condensates in halide perovskite semiconductors microcavities, for instance using artificial lattices to engineer the strong optical responses including topological properties, and their ultrafast propagation. Finally, we will briefly discuss the nonlinear optical properties in polariton condensate trapped in artificial potential landscapes, by a pump-probe transient spectroscopy at momentum space. Our results demonstrate a promising perspective of polaritonics in a wide range of ultrafast optical and photonic applications at room temperature.

1. S. Ghosh, R. Su*, J.X. Zhao, A. Fieramosca, J.Q. Wu, T.F. Li, Q. Zhang, F. Li, Z.H. Chen, T.C.H. Liew, D. Sanvitto, Qihua Xiong*, “Microcavity exciton polaritons at room temperature”, Photonics Insights 1, R04 (2022) (review)
2. J.Q. Wu^#, S. Ghosh^#, R. Su*, A. Fieramosca, T.C.H. Liew*, and Qihua Xiong*, “Nonlinear parametric scattering of exciton polaritons in perovskite microcavities”, Nano Lett. 21, 3120–3126 (2021)
3. J.G. Feng et al., “All-optical switching based on interacting exciton polaritons in self-assembled perovskite microwires”, Science Advances 7, eabj6627 (2021)
4. R. Su et al., “Optical control of topological polariton phase in a perovskite lattice”, Science Advances 7, eabf8049 (2021)
5. R. Su et al., “Observation of exciton polariton condensation in a perovskite lattice at room temperature”, Nature Physics 16, 301-306 (2020)
6. R. Su et al., “Room temperature long-range coherent exciton polariton condensate flow in lead halide perovskites”, Science Advances 4, eaau0244 (2018)
7. R. Su et al., “Room-temperature polariton lasing in all-inorganic perovskite nanoplatelets”, Nano Lett. 17, 3982–3988 (2017)

Increasing the efficiency of light-emitting devices is currently a hot topic in the field of semiconductor research. Here, perovskites are of special interest due to their excellent optical properties. Additionally, the possibility of sample preparation by vacuum deposition makes this class of material suitable for future large-scale industrial production. It is known that light-emission efficiency can benefit from an island-type active layer structure. If the lateral dimension of the structures is sufficiently small, excitons are confined and thus are far less likely to dissociate prior to radiative recombination.

In this work, we investigate the impact of the environment on the island formation in thermally deposited bilayers of CsPbBr₃/LiBr. We demonstrate that the island formation occurs only in humid environments, leading to an enhancement of the photoluminescence quantum yield by a factor of 350. Time-resolved grazing-incidence wide-angle X-ray scattering (GIWAXS) experiments document the island growth process and reveal that the LiBr has already changed the perovskite crystal orientation before the island formation.

Despite the relatively early level of development, Perovskite light-emitting diodes (PeLEDs) have reached outstanding luminance and radiative efficiency levels that roughly graze the maximum theoretical efficiency limits. Unfortunately, the complete understanding of the working principles and the photo-electrochemical mechanisms involved in the charge carrier generation/recombination dynamics is still a conundrum. Additionally, the strong ionic character of MHPs enables the migration of ions and the gradual formation of crystalline defects upon exposing to light and/or to an external electric field, which aggravate the complexity of these systems. In fact, these ionic processes are apparently coupled with those electrical involved in the generation of light, and seem to be connected with the widely reported limited long-term stability of the devices. Here, I will discuss on the exploitation of a new methodology based on the combination of two frequency-domain modulated techniques, i.e. electrochemical impedance spectroscopy (EIS) and light emission voltage-modulated spectroscopy (LEVS), aimed at reaching a full understanding of the working principles of perovskite LEDs. Particularly important is the deconvolution of the electrical, optical and ionic processes that are involved in the current-to-photon conversion, heat generation and/or degradation of the materials employed. We propose a new theoretical model and an equivalent circuit that considers both the non-radiative and radiative contributions, as a new tool for the advanced characterization of perovskite-based optoelectronic devices. 

Metal-Halide perovskite materials are known to be of high potential for solar and LED applications thanks to their very high absorption coefficients, remarkably long carrier-lifetime and high photoluminescence quantum yield in comparison to traditional semiconductor absorbers [1], [2]. This is particularly remarkable given that high materials quality can be obtained even when samples are grown from solutions. It seems that not only their defect tolerance but, even more so, the self-healing ability of halide perovskites are a fundamental reason for their excellent opto-electronic properties. Nevertheless, dynamic effects also give rise to a number of meta-stabilities. Materials can be degraded and temporarily perturbed under external stressors, such as light and bias [3],[4],[5]. In this work, we focus on methyl-ammonium lead tri-bromide microplatelets, prepared following the work from Mao et. al. [6]. We use these microplatelets to study fundamental dynamics of charge carriers interacting with physical or light-induced defects generated in these crystals by external perturbation. The samples can be considered an idealized model-system that exclude typical intra-grain boundaries of polycrystalline thin film samples, thus removing the different phenomena that could occur at these local sites.

Within these model systems, we show a correlation between time-resolved photoluminescence decay dynamics and sample properties, perturbed by the history of the micro-sized sample to illumination, generating light-induced defects; or mechanical stress, generating structural defects. We utilized excitation-intensity and repetition rate-dependent photoluminescence measurements [7] to provide a "finger print" of samples exposed to different degrading conditions to investigate the characteristic difference between light- and mechanically-induced defects affecting the charge carrier dynamics within the crystal.

Finally, we refined a numerical model to fit photoluminescence lifetime data based on population rate equations, including trapped populations [8],[9]. A fit of the presented data is used to see how material properties, such as trap densities, are affected by illumination intensity and/or repetition rate.

Photoexcitation of semiconductors by photons of higher energy than the bandgap creates ‘hot-charge carriers’ (i.e. different thermal energy w.r.t. lattice), which further cool down to the band-edge state by releasing their excess energy as a waste form of heat. Harvesting the excess energy of these hot carriers (HC) could eventually boost the performance of solar cells by manifold, while practical realization of this is still limited owing to the rapid cooling of these HC [1, 2]. On the contrary, in light-emitting applications, rapid HC cooling is highly desired to enable efficient radiative recombination by preventing carrier trapping. Therefore, detailed understanding of the HC cooling mechanisms and further controlling their dynamical pathways is prerequisite for engineering the semiconductor optoelectronics.

Herein I will explore this key area on CsPbX3 perovskite nanostructures of various dimensions and compositions by employing novel pump-push-probe based transient spectroscopy [3], which provides direct access to selectively control the hot carrier density and study their influence. This experimental finding unravels the role of carrier-carrier, carrier-phonon interactions to carrier-impurity (defect) scatterings in tailoring the hot carrier cooling dynamics in perovskite nanosystems. Our results of HC cooling dynamics among halide-composition space with controlled defect densities reveals that this dynamics is defect-tolerant for pure-iodide based perovskites unlike pure-Br and mixed (Br/I)-systems. Importantly, we also examine the slowing down pattern of HC cooling dynamics under high HC-density (termed as hot-phonon bottleneck effect), whose effect is significantly suppressed with the increase in quantum confinement (from 3D to 2D systems) due to reduced screening by phonons and enhanced Coulombic interaction between electron and holes for the latter systems. Although halide-vacancy related defects accelerate the HC cooling dynamics for pure Br-and mixed (Br/I)-systems, but they do not induce any such effect on the hot-phonon bottleneck process possibly due to saturation of the trap-states. This detailed understanding of the key routes that control the HC cooling dynamics would be instrumental for prospective applications of the next-generation perovskite optoelectronics.

Lead halide perovskites (LHP) are rapidly emerging as efficient, low-cost, solution-processable scintillators for radiation detection [1], [2]. Carrier trapping is arguably the most critical limitation to the scintillation performance [3], [4]. Nonetheless, no clear picture of the trapping and detrapping mechanisms to/from shallow and deep trap states involved in the scintillation process has been reported to date, as well as on the role of the material dimensionality. In this talk, this issue is addressed by performing, for the first time, a comprehensive study using radioluminescence and photoluminescence measurements side-by-side to thermally-stimulated luminescence (TSL) and afterglow experiments on CsPbBr₃ with increasing dimensionality, namely nanocubes, nanowires, nanosheets, and bulk crystals. All systems are found to be affected by shallow defects resulting in delayed intragap emission following detrapping via a-thermal tunneling. TSL further reveals the existence of additional temperature-activated detrapping pathways from deeper trap states, whose effect grows with the material dimensionality, becoming the dominant process in bulk crystals. These results highlight that, compared to massive solids where the suppression of both deep and shallow defects is critical, low dimensional nanostructures are more promising active materials for LHP scintillators, provided that their integration in functional devices meets efficient surface engineering.

Organic-inorganic hybrid low-dimensional perovskites are attracting significant attention for optoelectronic applications due to their higher stability, uncomplicated manufacturing process, and wide tunability of the optical and electronic properties. The use of various organic cations leads to the formation of two-dimensional (2D) layers, one-dimensional (1D) chains, or isolated zero-dimensional (0D) clusters depending on how the metal halide octahedra are connected. Even minimal differences in the structure of the organic cations can cause remarkable changes in the structural arrangement and, as a consequence, a modification of the optoelectronic properties of the perovskite system.

The incorporation of functionalized ammonium cations in perovskites allows the design of a variety of new organic-inorganic hybrid materials, in which functionalized and even semiconducting organic layers are assembled with a semiconducting inorganic layer at the molecular scale. In recent works devoted to low-dimensional inorganic-organic halide perovskites much attention has been paid to the broadband emission properties primarily originating from the self-trapped excitons (STEs) [1,2]. High structural distortion of low-dimensional perovskite structures induces electron-phonon coupling and enhances the STE process, resulting in an enhanced broadband emission. In addition, it has been demonstrated that perovskites grown in a form of atomically thin sheets possess unique features compared to their bulk counterparts [3]. For example, hybrid perovskite sheets exhibit an unusual structural relaxation, which leads to a band gap shift. Consequently, the controllable synthesis of ultrathin perovskite sheets has gained a great research interest for implementation in high-performance optoelectronic devices.

In this work, a series of low-dimensional hybrid organic-inorganic metal halides, based on multiple-ring aromatic ammonium cations and lead iodide were fabricated and their structures and properties were investigated. Examination of the influence of organic cations on the structural properties reveals that the number and position of amino groups at the aromatic ring affect the dimensionality of the perovskite, resulting in 2D and 1D corner- and face-sharing structures with different optical behavior. Highly distorted 1D perovskites show broadband emissions originating from STEs. Theoretical results reveal, that the organic cations in the 1D compounds strongly contribute to the band structure resulting in strong orbitals hybridization. Using a facile and fast crystallization method we also synthesized organic-inorganic hybrid perovskite few-layer free-standing nanosheets containing the naphthalene diammonium-based linker. Varying the synthetic conditions, we can modulate the thickness and lateral sizes of the nanosheets.

The success of the colloidal semiconductor quantum dots (QDs) field is rooted in the precise synthetic control of QD size, shape, and composition, enabling electronically well-defined functional nanomaterials that foster fundamental science and motivate diverse fields of applications. While the exploitation of the strong confinement regime has been driving commercial and scientific interest in InP or CdSe QDs, such a regime has still not been thoroughly explored and exploited for lead-halide perovskite QDs, mainly due to a so far insufficient chemical stability and size monodispersity of perovskite QDs smaller than about 7 nm. Here, we demonstrate chemically stable strongly confined 5 nm CsPbBr₃ colloidal quantum dots via a post-synthetic treatment employing didodecyldimethylammonium bromide ligands. The achieved high size monodispersity (7.5%±2.0%) and shape-uniformity enables the self-assembly of QD superlattices with exceptional long-range order, uniform thickness, an unusual rhombic packing with an obtuse angle of 104º, and narrow-band cyan emission. The enhanced chemical stability indicates the promise of strongly confined perovskite QDs for solution-processed single-photon sources, with single QDs showcasing a high single-photon purity of 73% and minimal blinking (78% “on” fraction), both at room temperature.