Accelerated materials designing for light emission applications are in high demand. However, complex and interdependent structure-property-performance relations strongly limit the directed search and discovery of efficient materials. To tackle these challenges, we apply cutting-edge computational simulations that are invaluable for understanding and manipulating the functionalities and performances of a wide range of energy materials. In this talk, first I will discuss the details of charge carrier dynamics and recombination processes in layered metal halide perovskites that are leading contenders for next-generation optoelectronic devices. Using non-adiabatic molecular dynamics, I will illustrate the complex influences of dynamic structures on the excited-state carrier dynamics that strongly impact the optoelectronic performances of these materials.^[1-4] Following that, my talk will focus on data-driven approaches that substantially accelerate the materials selection process for excellent emitting layered halide perovskites.^[5] We will illustrate the significant influences of spacer inorganic cations on the carrier transport and recombination processes in these materials.    

References:

1.         Nayak et al. J. Mater. Chem. C, 2023, 11, 3521

2.         Ghosh et al. J. Mater. Chem. C, 2022, 10, 9563

3.         Ghosh et al. J. Mater. Chem. A 2020, 8, 22009

4.         Ghosh et al. J. Phys. Chem. Lett, 2020, 11, 2955

5.         Bhatt et al. Submitted, 2024

The introduction of chiral organic spacers in low-dimensional metal-halide perovskites triggers chiroptical activity, making these materials of great interest for spintronic applications. To enable such applications, it is necessary to develop a deep understanding of the structure formation of chiral two-dimensional (2D) perovskites and its impact on their optical properties. In the first part of the talk, I will discuss how changing the processing conditions impacts on the phase purity, microstructure, and chiroptical properties of chiral 2D perovskites. In the second part, I will focus on the optical properties of these materials, focusing in particular on the origin of the common observation of asymmetric shape of their photoluminescence. Finally, I will present the impact of chirality on exciton transport in these materials.

In this talk, I will demonstrate that the conventional time-resolved photoluminescence (TRPL) spectroscopy method does not work well for metal halide perovskites. This is because measurements of charge carrier lifetime and photoluminescence quantum yield can be misleading in the presence of hidden photoexcitations, such as charge carrier trapping. To address this, I will introduce a multi-pulse TRPL spectroscopy method and demonstrate it on cesium lead tribromide (CsPbBr₃) metal halide perovskite microplates. The multi-pulse TRPL allows scanning of the material luminescence state from nanoseconds to milliseconds. As a result, it allows direct extraction of the concentration of trapped charge carriers (over 10¹⁶ cm^-3) and the rate constant of trap depopulation (1.5×10^-10 cm³s^-1). At the end of my talk, I will show how the multi-pulse TRPL method can be used in photonic neuromorphic computing applications. This will be demonstrated with metal halide perovskite Memlumors - luminophores with memory. The multi-pulse TRPL method is already provided by setup producers, thus making it available for the researchers in advanced luminescent studies and applications.

Metal halide perovskites have attracted much attention for use in several optoelectronic applications, including photovoltaics and light emission.  Due to their superior stability and bright photoluminescence, quasi-2D or layered perovskites such as the Ruddlesden-Popper perovskite phenylethylammonium lead iodide (PEAPbI₄) have been a popular choice for many applications.  However, these materials typically have exciton binding energies of 100s of meV that can thus greatly alter optoelectronic properties due to a large population of excitons present at ambient temperatures [1].  Using both transient and steady-state spectroscopic methods, we have investigated the excitonic properties of quasi-2D perovskites including PEAPbI₄ and thiophenemethylammonium (ThMAPbI₄).  In PEAPbI₄, we separate contributions from free charge-carriers and excitons and observe ultrafast cooling of free charges followed by slower recombination of both excitons and a minority concentration of free charges [2].  In ThMAPbI₄, we investigate the temperature-dependent properties from room temperature to 77 K and characterize emission from a defect-bound exciton at low temperatures [3].  Together, these studies highlight the relationship between structure and optoelectronic properties of these materials.

A common strategy for circumventing the poor transport properties of quasi-2D perovskites is to form heterostructures with 3D perovskites, where quasi-2D materials are incorporated into 3D perovskite thin films as either a mixture or a capping layer.  Using a combination of visible transient absorption spectroscopy (TAS) and optical pump/THz probe spectroscopy (OPTP), we have evaluated device-relevant 3D perovskite thin films which have been treated with phenylethylammonium salts in order to preferentially form RP phases at the surface of the films [4].   In all cases, we find that the surface is a complex mixture of 2D and 3D components.  In addition to observing that the charge-carrier dynamics are sensitive to the film preparation method, we distinguish between bulk and surface passivation effects and query charge transfer between RP and 3D species.

LHP NCs are of broad interest as classical light sources (LED/LCD displays) and as quantum light sources (quantum sensing and imaging, quantum communication, optical quantum computing). The current development in LHP NC surface chemistry, using designer phospholipid capping ligands, allows for their increased stability down to single particle level [1]. The brightness of such a quantum emitter is ultimately described by Fermi’s golden rule, where a radiative rate is proportional to its oscillator strength (intrinsic emitter property) and the local density of photonic states (photonic engineering, i.e., cavity). With perovskite NCs, we present a record-low sub-100 ps radiative decay time for CsPb(Br/Cl)₃, almost as short as the reported exciton coherence time, by the NC size increase to 30 nm [2]. The characteristic dependence of radiative rates on QD size, composition, and temperature suggests the formation of giant transition dipoles, as confirmed by effective-mass calculations for the case of the giant oscillator strength. Notably, the fast radiative rate is achieved along with the single-photon emission despite the NC size being ten times larger than the exciton Bohr radius. When such bright and coherent QDs are assembled into superlattices, collective properties emerge, such as superradiant emission from the inter-NC coupling [3]. In the most recent work [4], the functionality of the second SL component can give rise to the enhancement of the LHP NCs properties or the emergence of new collective effects. We present the formation of multicomponent SLs made from the CsPbBr₃ NCs of two different sizes. The diversity of obtained SLs encompassed the binary ABO₆-, ABO₃-, and NaCl-type structures, all of which contained orientationally and positionally confined NCs. For the selected model system, the ABO₆-type SL, we observed efficient NC coupling and Förster-like energy transfer from strongly confined 5.3 nm CsPbBr₃ NCs to weakly confined 17.6 nm CsPbBr₃ NCs. Exciton spatiotemporal dynamics measurements reveal that binary SLs exhibit enhanced exciton diffusivity compared to one-component SLs across the entire temperature range (from 5 K to 298 K). Observed incoherent NC coupling and controllable excitonic transport within the solid NC SLs hold promise for potential applications in optoelectronic devices.

[1] V. Morad, A. Stelmakh, M. Svyrydenko, L.G. Feld, S.C. Boehme, M. Aebli, J. Affolter, C.J. Kaul, N.J. Schrenker, S. Bals, Y. Sahin, D.N. Dirin, I. Cherniukh, G. Raino, A. Baumketner, M.V. Kovalenko Nature, 2024, 626, 542–548

[2] C. Zhu, S.C. Boehme, L.G. Feld, A. Moskalenko, D.N. Dirin, R.F. Mahrt, T. Stöferle, M.I. Bodnarchuk, A.L. Efros, P.C. Sercel, M.V. Kovalenko, G. Rainò. Nature, 2024, 626, 535–541

[3] I. Cherniukh, G. Rainò, T. Stöferle, M. Burian, A. Travesset, D. Naumenko, H. Amenitsch, R. Erni, R.F. Mahrt, M.I. Bodnarchuk & M.V. Kovalenko. Nature 2021, 593, 535–542

[4] T.V. Sekh, I. Cherniukh, E. Kobiyama, T.J. Sheehan, A. Manoli, C. Zhu, M. Athanasiou, M. Sergides, O. Ortikova, M.D. Rossell, F. Bertolotti, A. Guagliardi, N. Masciocchi, R. Erni, A. Othonos, G. Itskos, W.A. Tisdale, T. Stöferle, G. Rainò, M.I. Bodnarchuk, and M.V. Kovalenko. ACS Nano 2024, 8423–8436

Tin-halide perovskites are deemed as a more sustainable alternative to lead-halide perovskites. However, a clear understanding and control of their chemistry remains slender compared to the lead counterparts in bulk and nano owing to their inability to provide a stable oxidation state (+2) to maintain a pristine crystal structure phase in ambient conditions. Being ionic, halide-perovskites (ASnX₃) exhibit dimensional dynamics depending on the size of the intercalated cation (A⁺). Thus, it becomes important to develop a rational chemical design for directing the reaction towards a desired 3D or 2D perovskite structure. Our investigations uncover that a sub-stoichiometric amount of ligands and a high concentration of SnX₂ salt are paramount to achieving stable, tunable, and monodisperse CsSnX₃ perovskite nanocrystals (X – I, Br) with defined optical features. Pertaining to their comparable formation energies, the 2D perovskite phase ([R-NH₃]₂SnX₄) can be easily converted to 3D nanocrystals via a cation exchange reaction in solution as well as in thin-films, and can undergo facile anion exchange reactions with benzoyl halides. The ligand coordination effects in the formation of FASnI₃ nanocrystals reveal the presence of [SnI₃]^- complex as an intermediate species to form perovskite nanostructures. Finally, we demonstrate the hybrid organic-inorganic encapsulation by depositing a thin layer of PMMA followed by a 40 nm alumina layer via ALD improves the ambient stability of CsSnX₃ NCs thin-film for up to record 15 days, to help facilitate their photophysical studies. This research highlights the insistent necessity for a comprehensive understanding of the synthesis and photophysical properties of tin-halide perovskite nanostructures to unlock their full potential.

Phosphor converted light emitting diodes (pc-LEDs) have revolutionized solid-state white lighting by replacing energy-inefficient filament-based incandescent lamps. However, such energy inefficient tungsten halogen lamps still remain the source for ultrabroad near-infrared (NIR) radiation. It is primarily because of lack of rational approach to design ultrabroad NIR emitting phosphors hence their scarcity. To address this issue, we rationale a host halide perovskite with self-trapped exciton (STE) emission that can merge with dopant emission due to d-d transitions. The host STE emission covers NIR-1 and also provides the π-donor ligand Cl⁻ that significantly reduces the energy of dopant d-d transitions emitting in the NIR-II. Thereby, we developed W⁴⁺-doped and Mo⁴⁺-doped Cs₂(NaAg)BiCl₆ perovskites emitting ultrabroad NIR radiation with unprecedented spectral widths of 434 and 468 nm, respectively. Upon band-edge excitation, the soft lattice of the host exhibits broad STE emission covering NIR-I (680 nm), which then nonradiatively excites the dopants leading NIR-II emission with a peak at ~950 nm via d-d transitions. Vibronic coupling broadens the dopant emission. A combination of large spin-orbit coupling, Jahn-Teller distortion (d2 electronic configuration), and the distortion due to the heterovalent replacement of Bi³⁺ by W⁴⁺ or Mo⁴⁺ lead to intense NIR photoluminescence with quantum yield ~40%. The composite of our ultrabroad NIR phosphors with biodegradable polymer could be processed into free-standing films and 3D printed structures. Large (170 × 170 mm²), robust, and thermally stable 3D printed pc-LED panels emit ultrabroad NIR radiation, demonstrating NIR imaging applications.

In this presentation, we will discuss the operational principles of the perovskite light-emitting metatransistor, a new device concept that integrates monolithic, functional dielectric metamaterials into the active region of perovskite light-emitting transistors. Using methylammonium lead iodide (MAPbI3) films patterned by focused ion beam or nanoimprint lithography as an example, we will show how the unique combination of optical, luminescence, and charge transport properties of halide perovskites, combined with appropriate metamaterial design, enables a wide range of optical and electro-optical functionalities. These include enhanced, polarized, and directional photoluminescence, the optical Rashba effect, topological laser emission with optical bistability, and spin-polarized, electrically driven polaritonic emission in the strong coupling regime. Although these concepts are currently at the proof-of-principle stage, they may find application in next generation immersive visual technologies, which will require light emitting devices with unprecedented control over electroluminescence parameters at the subwavelength regime, including spectrum, polarization, luminosity, wavefront, and directionality.

Metal halide perovskites have exhibited great potential in next-generation display applications owing to their high colour purity and high photoluminescence quantum yields. They are known to be highly defect-tolerant compared with conventional semiconductors. However, multiple evidence shows that the effect of defects has significant impacts on the performance of light-emitting diodes. Additionally, we can not ignore the impact of carrier injection on the perovskite light-emitting diode performance. Based on the fundamental principles of perovskite passivation, we introduced multifunctional phosphine-containing molecules, including diphenylphosphinamide and phosphonic acid, as the additive to passivate surface defects, modulate the phase distribution, and smooth the energy transfer of the quasi-2D perovskite, giving rise to higher optoelectronic properties of perovskites and better performance. Meanwhile, we designed the tris(4-trifluoromethylphenyl)phosphine oxide to modulate the energy level and hole mobility of originally commercial poly(9-vinylcarbazole) layer, resulting efficient blue perovskite light-emitting diodes. demonstrate that phosphine-containing molecules not only serve as passivation agents for perovskites but also modify the properties of the charge transport layer. We believe that phosphine-containing molecules hold significant potential for enhancing the performance of perovskite LEDs.

Environmentally friendly tin (Sn) perovskites have received considerable attention due to their great potential for replacing their toxic lead counterparts in applications of photovoltaics and light-emitting diodes (LEDs). However, the device performance of Sn perovskites lags far behind that of lead perovskites. The poor performance stems mainly from the numerous defects within Sn perovskite crystallites and grain boundaries, leading to serious non-radiative recombination. Various epitaxy methods have been introduced to obtain high-quality perovskites, although their sophisticated processes limit the scalable fabrication of functional devices. In this talk, I will present our effort to boost the efficiency of tin based perovskite LEDs. We find that epitaxial heterodimensional Sn perovskite films can be fabricated using a spin-coating process, and efficient LEDs with an external quantum efficiency of 11.6% can be achieved based on these films. The film is composed of a two-dimensional perovskite layer and a three-dimensional perovskite layer, which is highly ordered and has a well-defined interface with minimal interfacial areas between the different dimensional perovskites. This unique nanostructure is formed through direct spin coating of the perovskite precursor solution with tryptophan and SnF₂ additives onto indium tin oxide glass.

Use of additives to achieve careful control over perovskite morphology and interfaces can provide high-performance LEDs in the infrared and visible. I will discuss some recent examples where high efficiencies and high brightness can be obtained. Why, though, are efficiencies generally so high? Where extraction barriers are large, electron and hole injection must be well-balanced, and I will show how this is achieved through redistribution of the field across the two transport layers.  I will also show that light emission is rather uniform throughout the thickness of the devices despite the fact that electron and hole distributions are highly non-uniform.  Finally I will discuss optical outcoupling efficiency, highlighting the role of photon recycling where waveguided photons are absorbed and remitted, giving additional chances to escape. I will demonstrate a simple technique to enhance photon recycling by reducing parasitic optical absorption in the device, giving a significant boost to the overall device efficiency.

Halide perovskite solar cells have revolutionized the photovoltaic field in the last decade, due to its direct bandgap and high absorption coefficient, with a benign defect physic, decreasing non-radiative recombination even in polycrystalline films. However, these properties are not just optimum for solar cell but for any optoelectronic device, causing that currently the research with these materials widespread to different optoelectronic fields. Among these fields the one that likely has taken more attention has been in light emitting diodes (LEDs), promoted by the high photoluminescence quantum yield (PLQY), especially for perovskite nanoparticles, narrow electroluminescence (EL) peak, providing purer colors, and versatility to tune the bandgap and consequently the emitting color. In this talk, I discuss different strategies to increase the performance, considering both the external quantum efficiency (EQE) and stability), but also the reproducibility. After an intensive work in perovskite LEDs field, it has been a huge improvement in the performance of these devices, however, the two main drawbacks of this system, the use of hazardous Pb and the long term stability, still to be open questions that have not been fully addressed. Preparation and optimization of Sn-based LEDs will be discussed as well as the possibilities of fabrication with industrially friendly methods as inkjet printing. We report as the use of additives and alternative synthesis of microcrystal precursor play a key role in the increase of LED luminance, EQE and stability of Sn-based perovskite LEDs.

The high demand for ultralow detection limits of ionizing radiation in aerospace radiographic testing, medical radiography, high-energy physics, and security screening has driven extensive research on X-ray imaging scintillators. While high-performance scintillators in the current X-ray imaging market made of ceramics that require harsh and costly preparation and engineering conditions, perovskites and their related structures, heavy-atom engineered thermally activated delayed fluorescence (TADF) and copper nanoclusters with their unique optical behaviors and high X-ray absorption cross section are now promising competitors if not alternatives. In this talk, I will present the engineering of perovskite nanosheets with excellent scintillation performance due to efficient energy transfer processes between stacked thin and thick nanosheet. Additionally, I will talk about the efficient and ultrafast energy transfer strategies between perovskite nanosheets and TADF that successfully  produced a reabsorption-free organic X-ray imaging scintillator with an ultralow detection limits and outstanding X-ray imaging resolution. Similarly, I will talk about perovskite related Cu and Ag halides as well as Cu-based halide nanostructures that showed outstanding X-ray imaging performance.⁷ Moreover, we will discuss the fabrication of a thick pixelated needle-like array scintillator capable of micrometer resolution via waveguide structure engineering that lead to ultra-high spatial resolutions of 60.8 lp mm^-1, representing a laboratory-scale record for extensively studied metal halide scintillators. The talk also discusses a novel top-filter-bottom sandwich structure scintillator for high-performance dual-energy X-ray imaging within a single exposure. Finally, our innovation  of true-color multi-energy X-ray imaging technology centered around multiple scintillator architecture with a six-layer ∆E-E telescope configuration to achieve powerful material-specific capability, surpassing what is offered by traditional X-ray imaging technologies will also be discussed in this talk. This breakthrough research enables clear resolution of different biological tissues and materials objects based on their corresponding colors and paves the way for the development of new imaging scintillator architectures with potential applications in medical imaging, industrial monitoring and security checks.

In recent years, metal halide perovskites have emerged as a new class of semiconductors with remarkable optoelectronic properties. Owing to their low cost, tuneable bandgaps, high luminescence quantum yields and narrow emission bandwidth, perovskite light-emitting diodes (PeLEDs) become one of the most promising candidates for next-generation display technology. Molecular and ionic additives have been widely used as effective strategies to enhance the external quantum efficiencies (EQEs) and operational stability of PeLEDs. Here we present the latest results from our group on efficient and stable PeLEDs enabled by different types of additives. Understanding the effects of additives is of great importance for developing high-performance PeLEDs. The roles of additives have been identified as passivators to eliminate deep-level defects in the early stages of development, as stabilizers to suppress ion migration, and more recently as electronic dopants to control the semiconducting properties of perovskites. Dipolar molecular stabilizers have been demonstrated to be able to form strong bonds or interactions with cations and anions at the grain boundaries in perovskites, enabling high-performance near-infrared PeLEDs with record-long operational lifetimes (T₅₀, extrapolated) of 11,539 h (~1.3 years) and 32,675 h (~3.7 years) for initial radiance (or current densities) of 3.7 W sr⁻¹ m⁻² (~5.0 mA cm⁻²) and 2.1 W sr⁻¹ m⁻² (~3.2 mA cm⁻²), respectively, with even longer lifetimes forecasted for lower radiance^[1]. Stability of red^[2] and green^[3] PeLEDs is also enhanced to a certain extent with similar strategies. Further, a phosphonic acid molecular dopant with strong electron-withdrawing abilities was found to be able to adjust the p- and n-type characteristics in a perovskite emitter, unlocking the direction of controllable electronic doping in perovskite semiconductors. The controllable doping in the emissive perovskite semiconductor enables the demonstration of ultrahigh brightness (1.16 × 10⁶ cd m⁻²) with an exceptional EQE of 28.4% in PeLEDs.