Quantum dots are nanometer-sized crystallites of semiconductor that have a roughly spherical shape. Due to extensive research, quantum dots are now commercially used as a robust fluorescent material in displays and lighting. However, even with our best procedures, state-of-the-art samples still contain particles with a distribution in size and shape. Because this causes variations in their optical properties, their performance for applications is reduced. This leads to a fundamental question: can we achieve a sample of semiconductor nanocrystals in which all the particles are exactly the same? In this talk we will discuss this possibility by examining two classes of nanomaterials. First, we will consider thin rectangular particles known as semiconductor nanoplatelets. Amazingly, nanoplatelet samples can be synthesized in which all crystallites have the same atomic-scale thickness (e.g., 4 monolayers). This uniformity in one dimension suggests that routes to monodisperse samples might exist. After describing the underlying growth mechanism for nanoplatelets, we will then move to a much older nanomaterial—magic-sized clusters (MSCs). Such species are believed to be molecular-scale arrangements (i.e., clusters) of semiconductor atoms with a specific (“magic”) structure with enhanced stability compared to particles slightly smaller or larger. Their existence implies that MSC samples can in principle be the same size and shape. Unfortunately, despite three decades of research, the formation mechanism of MSCs remains unclear, especially considering recent experiments that track the evolution of MSCs to sizes well beyond the “cluster” regime. Again, we will discuss the underlying growth mechanism and its implications for nanocrystal synthesis. Finally, we will present an outlook if perfect nanomaterials can be obtained.

In the last 30 years, inorganic semiconductor nanocrystals have been extensively researched because of their size-dependent optoelectronic properties, thus successfully debuting in the commercial sector. The synthetic development of nanocrystals, however, has perplexed from the complex energy landscape of the reaction. Unlike intermediates that can be isolated and characterized with atomic precision in organic synthesis, the intermediates in nanocrystal synthesis are difficult to resolve because of the co-existence of metastable states with similar energy. If one can control the energy of the metastable intermediates originating from various factors including shapes and surface-ligand interaction and isolate the singular intermediate species, the nanocrystal growth with atomic precision could be achievable.

In this presentation, I will discuss the tetrapod InP nanocrystals as a crystalline “late intermediate” that warrants controlled colloidal nanocrystal growth. The use of the late intermediate with well-defined facets at the sub-10 nm scale for directional growth with atomic control and highlight the potential for the new directed approach of nanocrystal synthesis. Well-defined surface of tetrapod enable us to investigate the facet-dependent surface chemistry of InP. Finally, a geometry-driven transition from single to multi-photon emitting behavior in InP tetrapods will be discussed.

The search for metal halide perovskite nanocrystal (NC) materials alternative to Pb-based compositions has led to the discovery of several candidates with promising optical properties. Some of the most interesting compounds synthesized so far belong to the double-perovskite (DP) family, are characterized by a general Cs₂B⁺B³⁺Cl₆ stoichiometry and feature a perovskite structure composed of a 3D network of [B⁺Cl₆] and [B³⁺Cl₆] corner-sharing octahedra, with Cs⁺ ions filling the voids in between. Examples are Cs₂AgBiCl_6, Cs₂AgInCl₆, Cs₂NaInCl₆, Cs₂NaBiCl₆ and Cs₂AgSbCl₆ NCs. These materials, especially in the NC form, generally exhibit a weak photoluminesce (PL), as they feature either an indirect bandgap or a direct bandgap with a parity forbidden transition. However, when opportunely doped/alloyed, they become more efficient emitters.

In this work, I will provide an overview of all the development in the DP field with a particular focus on the computational modelling of these materials using density functional theory (DFT) and classical molecular dynamics. I will show how integrating experiments with calculations is helpful in improving the efficiency of these materials and make them strong candidates to replace Pb-based perovskites.

3D halide perovskites used in light emitters and PV systems exhibit an unusual anharmonicity, which is not accounted for in nowadays descriptions of phonons and electron-phonon coupling. Therefore, the low frequency lattice vibrations and relaxations were investigated in single crystals of the four 3D hybrid organolead perovskites, MAPbBr3, FAPbBr3, MAPbI3, and FAPbI3, at the Brillouin zone center using Raman and Brillouin scattering and at the zone boundary using inelastic neutron scattering. The temperature dependence of the PbX6 lattice modes in the four compounds can be renormalized into universal curves and no soft vibration is observed excluding a displacive-like transitional dynamics related to structural phase transitions. The reorientational (pseudospin) motions of the molecular cations exhibit a seemingly order-disorder character recalling that of plastic crystals, but attributed to a secondary order-parameter. At ultra-low frequency, a quasi-elastic component evidenced by Brillouin scattering and associated to the unresolved central peak observed in neutron scattering, is attributed to center of mass anharmonic motions and rattling of the molecular cations in the perovskite cavities. 3D halide perovskites are therefore in a very unusual situation where anharmonicity prevails. It is leading to partial, extensive and ultraslow critical and stochatic dynamics connected to structural instabilities. The extensive disorder of the lattice at normal operation temperatures for devices is similar to premelting where only long-range acoustic normal modes survive.  

Acknowledgments

The research leading to these results has received funding from the European Union’s Horizon 2020 program, through a FET Open research and innovation action under the grant agreement No 899141. (POLLOC).

Lead-halide perovskite nanocrystals (NCs) have emerged as attractive nano-building blocks for photovoltaics and optoelectronic devices. Optimization of perovskite NC-based devices relies on a better knowledge of the fundamental electronic and optical properties of the band-edge exciton, whose fine structure has long been debated. This talk will give an overview of our recent magneto-optical spectroscopic studies [1-5] revealing the entire excitonic fine structure and relaxation mechanisms in these materials, using a single-NC approach to get rid of the inhomogeneities in the NC morphologies and crystal structures. Moreover, it will highlight universal scaling laws relating the exciton fine structure splitting, the trion and biexciton binding energies to the band-edge exciton energy in lead-halide perovskite nanocrystals, regardless of their chemical composition (inorganic and hybrid organic-inorganic, as well as mixed halide alloys) and their spectral emission range (from the blue to the near infrared). These scaling laws offer a general predictive picture for the charge-carrier interactions in these emerging materials, from the bulk regime to the regime of highly confined carriers.

In the past few years, lead-halide perovskites have appeared as a new generation of promising semiconductor materials for photovoltaic and optoelectronic applications. Belonging to this family, CsPbCl₃ have the largest energy band gap showing an absorption threshold and photoluminescence in the blue spectral region.

As a representative of this new class, CsPbCl₃ has the largest energy band gap, hence showing an absorption threshold and a photoluminescence emission in the blue spectral region. The manifestation of polaritonic effects at room temperature makes it a privileged candidate for new photonic devices and suggests moreover that excitons are particularly robust [1]. 

Here we present the results of high magnetic field spectroscopy on bulk CsPbCl₃ thin films that allow to obtain the fundamental exciton parameters, as the exciton binding energy, the effective mass, dielectric constant and Landé factors. We confirm the strong value of the exciton binding energy that makes it very stable at room temperature. We compare our results with results obtained by other authors in a wide range of different halide perovskites and we provide evidence of universal laws according to the gap energy. In particular the binding energy law is obtained by comparing perovskite compounds and other more conventional semiconductor materials [2].

The electron-hole exchange interaction related to Coulomb interaction is responsible in semiconductors of the splitting of the lowest-energy degenerate exciton state leading to the so-called exciton fine structure(EFS). Taking into account the measured excitonic parameters in bulk CsPbCl₃ we will finally discuss the EFS of CsPbCl₃ nanocrystals considering symmetries, shape anisotropy and environmental parameters. 

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Tin based halide perovskites have drawn attention in the last years because of their unique combination of interesting physical properties and low toxicity, showing a great potential for solar cells, photodetectors, sensors, lasers, and light emitting diodes applications ^[1,7]. However, the development of electronic devices has been hindered, in part, for the poor stability of Sn (II) against atmospheric conditions (moisture and oxygen) ^[2]. Currently, 2D- hybrid perovskite materials were proposed to overcome these issues owing to the hydrophobic nature of large organic cations, isolating tin halide octahedrons from the moisture ^[3]. Among the organic cations that have been tested for 2D perovskite structures, 2- thiophene ethyl ammonium tin (II) iodide (TEA₂SnI₄) perovskite has shown high exciton energy binding, and low exciton- phonon interactions ^[4]. Although there are some published papers about TEA₂SnI₄ thin films ^[5] and nanoplates^[6,8], the development of nano- and microcrystals have been limited due to fast crystallization rate of tin-based perovskites and stability issues ^[7]. Moreover, in the literature there are few optical and structural information about TEA₂SnBr₄ and TEA₂SnCl₄. In this work we report by first time the synthesis of TEA₂SnX₄ (X= Cl, Br, I) microcrystals by mean of hot injection method. We observed that the optical and structural properties of TEA₂SnX₄ (X= Cl, Br, I) microcrystals are dependent of the molar ratio of 2-thiophene ethyl amine and SnX₂-Trioctyl phosphine (TOP) precursor solutions. In all of cases we observe a X ray diffraction pattern characterized for strong (00l; l= 2,4,6 and 8) reflections which is consistent with the quantum well like structure of 2D hybrid tin halide perovskites. However, for the synthesis of TEA₂SnBr₄ microcrystals, d- spacing decrease from 1.8658 nm to 1.5782 nm when the rTEA/Sn molar ratio of TEA/Sn(II) precursors decrease from 2 to 0.25. We hypothesized that at high r_TEA/Sn, TEA molecules might dimerize by oxidative polymerization reaction, obtaining a long chain diammonium cation between [SnBr₆]^4- sheets. Evidence from ¹³ C nuclear magnetic resonance experiments might confirm this mechanism. TEA₂SnCl₄ and TEA₂SnBr₄ show a photoluminescence spectrum characterized by a broad band emission centred at 595 nm and 608 nm respectively. Owing to the Stoke shift between PL excitation and emission spectra of TEA₂SnCl₄ and TEA₂SnBr₄ are 318 nm and 335 nm, PL emission of this materials might be related to self- confined excitons. In the case of TEA₂SnI₄ microcrystals, PL emission and excitation spectra is equal to previous works ^[6,8], represented by a sharp band emission centred at 641 nm with a FWHM = 40 nm, and a low PLQY = 1%. We measured a high PLQY= 50% for TEA₂SnBr₄ microcrystals and PLQY= 20% for TEA₂SnCl₄ microcrystals synthesized at a high molar ratio of TEA/Sn (II) of 2. By first time, we have also demonstrated photoconductivity effect for TEA₂SnI₄ films containing microcrystals synthesized with a molar ratio TEA/Sn(II)= 0.5.

State-of-the-art first-principles calculations for cubic halide perovskites are performed by assuming that the potential energy felt by electrons is described with the nuclei clamped at their crystallographic positions. This assumption inevitably misses the effect of structural disorder which is ubiquitous in this class of materials, affecting their light-absorbing and emitting properties. Here, we demonstrate the important role of disorder in the electron-phonon physics of cubic perovskites, starting first from its distinct effect on the electronic structure and lattice dynamics. We show that ground-state structural deformations yield large renormalization of the bandgap, dielectric constant, and effective charges, as well as lead to dynamically stable phonons. We include electron-phonon dynamics in our calculations via the special displacement method and show that disorder is critical in the evaluation of phonon-induced bandgap renormalization. We also demonstrate that the effect of disorder is central to diffuse scattering, opening the way to interpret time-resolved phenomena manifested in X-ray or electron diffraction experiments.

To cope with the toxicity of Pb-based perovskites and achieve smaller electronic band gaps, replacing Pb with Sn has gained growing attention recently ^1,2. Importantly, the low-bandgap Pb-Sn alloyed perovskite allows the building of all-perovskite tandem solar cells that shall overcome the performances of single-junction perovskite cells ^3,4. In fact, of all possible means to improve perovskite film quality and suppress nonradiative recombination in optoelectronic devices for a high photo conversion efficiency purpose, the surface and interface functionalizations after the assembly with charge transport layer (CTL) are one of the most critical parameters ^5,6. In view of the sophisticated chemical and physical properties of Sn-based perovskites, theoretical calculations based on density functional theory (DFT) provide a useful insight into the interplay between absorbers and CTLs. Here, FASnI₃ is chosen as a benchmark material. We thoroughly investigate the influence of its surface termination on structural and electronic properties when interfacing with organic C₆₀ (100) and inorganic SnO₂ (100) and (110), including the intermediate work function calculations on the free-standing slabs. Based on the theoretical methodology developed in our team⁷, we have evidenced the proportionality between work function shifts and surface dipoles, making the optimization of interfacial charge transport possible by surface dipole tuning. Our findings on the charge transport properties of the two targeted CTLs, contribute to analyzing promising alternatives as CTLs for Pb-free perovskites thanks to the surface and interface engineering.

Versatile surface functionalization of highly ionic surfaces, ubiquitous among inorganic nanomaterials, remains a formidable challenge in the view of inherently non-covalent surface bonding.[1] Colloidal lead halide perovskite nanocrystals (NCs), which are of interest for classical and quantum light generation,[2,3] are one of the examples. Despite some recent empirical progress in surface chemistry of lead halide perovskite NCs, the general strategy towards their robust surface functionalization still remains a challenge. In this study we present the first structural investigation of perovskite surfaces capped with zwitterionic phospholipid molecules. In line with molecular dynamics simulations and solid-state NMR, zwitterionic phospholipid ligands bind to the perovskite surfaces with both head-groups, thus hindering their desorption. Furthermore, the ligand head-group affinity to the surface is primarily governed by geometric fitness of its cationic and anionic moieties into the crystal lattice. As a result, stable and colloidally robust nanocrystals of inherently soft and chemically labile lead halide perovskites – FAPbX₃ and MAPbX₃ (X – Br, I) – can be obtained with a lattice-matched phosphoethanolamine head-group. Stable surface passivation is also reflected in excellent optical properties of the NCs. As an example, alkylphospholipid-capped FAPbBr₃ NCs display a stable emission with near unity photoluminescence quantum yield in a broad concentration range, as well as in thick films. Ligand tail engineering, on the other hand, allows diverse surface functionalization of the NCs, broadening the scope of their potential applications.

Halide perovskite nanocrystals (PNCs) are prominent and interesting nanomaterials for actively studying photovoltaics, optoelectronics, photonics, and solar-driven chemistry. Valuable features including easy preparation, narrow width at half-maximum photoluminescence (PL) peak, adjustable/modifiable surface chemistry, a notable PL quantum yield (PLQY) of up to 100%, and a modulable band gap have been exploited to fabricate highly efficient devices such as PNC solar cells and multiple color light-emitting diodes (LEDs). Despite of above benefits of the PNCs, the PLQY and material quality are limited by their defective structure, making them prone to degradation. Factors such as (i) synthetic protocols for the PNCs formation and (ii) the loss of capping ligands from PNCs surface are pivotal to create a defective structure. In both cases, halide deficiency is the main reason to cause the appearance of a high density of nonradiative recombination sites, reducing the stability of the final product, and hindering the effective extraction of charge carriers. To overcome these drawbacks, we have analyzed the influence of the synthesis temperature, ligand concentration and metal doping engineering on the enhancement of the photophysical properties, stability, and suppression of non-radiative carrier traps of red-emitting APbI₃ (A= Cs⁺, FA⁺) PNCs. By establishing an efficient surface passivation through ligand coverage [1] and the incorporation of Sr to substitute Pb during the PNCs synthesis [2], less defective PNCs are formed, with long-term stability around 15 months, providing a pure deep red tonality. Therefore, it is possible to produce excellent candidates to improve the performance of optoelectronic devices and to conduct lead solar driven processes more efficiently.

The potential applicability of random lasers is largely limited because of chaotic fluctuations of spectral positions of lasing modes. In the present work we disclosed that spectrally stable narrow random lasing (RL) lines with quality factor Q as high as Q ≈ 10^4 at 20 K and the Amplified Spontaneous Emission (ASE) threshold as low as 2 microJ/cm2 are generated in thin polycrystalline films of formamidinium tin triiodide (FASnI3) perovskite chemically stabilized against Sn2+ to Sn4+ oxidation. The spectral positions of the RL lines are stable not only under unchanged excitation fluence, but also when the fluence alters by more than one order of magnitude. We found that different mechanisms are responsible for the formation of broad ASE contour and narrow RL lines and that ASE can be essentially suppressed by means of a decrease of the excitation spot linear size from ≈ 400 to ≈ 100 micrometers. Statistical analysis of RL line spacing suggests that a set of strongly localized modes is responsible for the observed set of narrow RL lines. We suggest also that high refractive index as well as high density of grain package are the main factors which determine the RL spectral stability and high Q and differ FASnI3 perovskite polycrystalline films from their widely studied Pb-based perovskite counterparts where chaotic RL is observed.

Colloidal semiconductor nanocrystals (NCs) emitting in the near-infrared (NIR) are of particular interest as they can find application in telecommunications to night vision, photovoltaics, lasing, and in vivo biological imaging.^[1] Since the most developed NIR emitting materials are very toxic, being Pb- or Hg-based chalcogenides, the current trend is to move toward compounds being RoHS (European “Restriction of Hazardous Substances”) compliant.^[2] In this context, the most promising NIR candidates are InAs NCs whose bandgap can be tuned from the visible up the the whole NIR range. Currently, the most advanced colloidal strategies for InAs NCs rely on pyrophoric, toxic and not commercially available tris-trimethylsilyl arsine (or derivatives).^[3] Therefore, in recent years alternative synthesis routes based on less toxic and commercially available As precursors, such as tris(dimethylamino)arsine (amino-As), have been explored.^[4] Such procedures deliver InAs NCs that need to be further optimized in terms of size distribution and optical properties, in order to meet the standard reached with tris-trimethylsilyl arsine. To this aim, in this work we show that the use of ZnCl₂ as an additive in the synthesis of such NCs (based on amino-As  and alane N,N-dimethylethylamine as the reducing agent) leads to two important achievements: i) helps to improve the size distribution of InAs NCs; ii) allows for the in-situ formation of bright emissive InAs@ZnSe core@shell NCs, with photoluminescence quantum yield ≥40%.^[5] The results of this study open new perspectives for the production of efficient InAs-based NC systems with amino-As and, consenquently, for their exploitation in optoelectronic devices.

The integration of triplet-triplet annihilation (TTA) components as electrically and optically active elements in vertically-structured photoactive device architectures is a challenging task to achieve. Herein we present a simple methodology for incorporating a photon absorbing layer of the (2,3,7,8,12,13,17,18-octaethyl-porphyrinato) platinum^II (PtOEP) metallorganic complex, as a self-TTA annihilator medium in a vertically stacked photodiode device structure. At low-power illumination, the PtOEP photodiode exhibits photocurrent generation via the fusion of optically-induced PtOEP excited states and it develops an open-circuit voltage as high as 1.15 V. The structural and spectroscopic characterization of the nanostructured PtOEP photoactive layer in combination with electronic structure calculations identify emissive PtOEP dimer species as the annihilating excited state responsible for the formation of charges. The participation of the fusion process in the mechanism of charge photogeneration manifests in the supralinear dependence of the short-circuit current density on the incoming photoexcitation intensity, both when incoherent and coherent light is used for illuminating the PtOEP photodiode. The photoresponse of the PtOEP device allows for the highly selective and sensitive photodetection within the 500 - 560 nm narrow spectral range. At short-circuit conditions a power-law is observed in the dependence of the device responsivity on fluence. Based on a scrutinized device engineering methodology the PtOEP annihilator is further utilized for stimulating the response of UV-only organic photodetector with visible light. In overall, these findings propose that triplet-excited annihilator species with appropriately selected frontier orbital energetics are valuable photoactive components, capable to extend the photosensitivity spectral window of electronic devices with a vertically-configured device structure.

Narrow-band line-emitter phosphors have enormous potential for use in LEDs, as they offer improved luminous efficacy (lm/W) for white LEDs, while maintaining high color rendering (CRI>90). Especially trivalent Eu-doped phosphors are ideal from an emitter perspective, due to their efficient and extremely narrow-band emission in the red spectral region. However, the major bottleneck for implementation of these phosphors in LEDs, is the lack of absorption in the blue spectral region. To solve this problem, we have been exploring nano-engineered interparticle energy transfer (IFRET), an innovative approach to decouple emission and excitation properties.¹ Even though IFRET would enable the use of trivalent lanthanide-doped phosphors for solid state lighting applications in general, the nanoscale components of IFRET materials are more broadly of interest for next generations LEDs, as cutting-edge LED technology is moving towards micro-devices.

IFRET technology allows for almost independent engineering of the emission and excitation properties to harness the best properties of each phosphor material, as you can have two different materials acting as a sensitizer and emitter. This does, for example, enable blue-sensitization of Eu³⁺-doped materials. Harnessing IFRET and successful implementation in commercial devices require nanoscale control over very high quality nanomaterials. Especially Ce-doped nano-YAG is of interest for LED applications, due to its very stable and efficient emission, and strong absorption in the blue spectral range. However, this material is particularly challenging to make at the nanoscale.² We have been developing high-quality (i.e. high absorption and quantum yield) nanophosphors, including nano-YAG, that could be implemented in IFRET applications and/or small-size LEDs.

These innovations could find applications beyond lanthanide doped phosphors, by combining these more conventional materials with new cutting-edge technologies like quantum dots. Furthermore the applications could go well beyond LEDs, including sensing, medical imaging, solar and displays. In this talk we will present our latest technological developments by discussing the underlying physics, progress and challenges in achieving nano-engineered phosphors for practical LEDs.

The last decade has witnessed a huge progress in thermally activated delayed fluorescence (TADF) emitters employed for fabrication of the latest generation OLEDs.[1,2] TADF allows harvesting of up to 100% triplet excitons through reverse intersystem crossing (RISC) process in organic materials without employing precious metals, thus rivalling the efficiencies of state-of-the-art phosphorescent OLEDs.[3,4] Even though TADF-OLEDs have demonstrated an upper limit of internal quantum efficiency (100%) across the whole visible range, their short operational lifetime in the blue range restricts the development and commercialization of these devices.[5] Half-life of blue-emitting devices typically does not exceed a few hours at a practical luminance of 1000 cd/m².[6] Among the key factors affecting the device stability are the degradation of TADF emitter and host material.

The design of efficient blue emitter–host combinations is one of the greatest challenges in organic light-emitting diode (OLED) research. The choice of the host material crucially affects device efficiency, roll-off and lifetime. High triplet energy hosts are in high demand, especially for thermally activated delayed fluorescence (TADF) emitters, due to their high exciton energies. The hosts are far less investigated than the emitters and require further progress.[7]

In this work, we introduce two novel hosts based on acridine donor and triazine acceptor (ATRZ) exhibiting good thermal and morphological properties and high triplet energies 3.07-3.27 eV. The donor and acceptor combination enables transport of both electrons and holes. Application of these bipolar hosts in blue TADF-OLEDs resulted in a blue electroluminescence peaking at 480-488 nm, a maximum EQE of 10%, and a low efficiency roll-off. The device based on ATRZ host was found to express the highest stability, i.e. the longest operational lifetime, which even surpassed that of OLED based on the benchmark mCBP host.

Colloidal nanoplatelets (NPLs) offer the unique opportunity to investigate at room temperature excitonic light-matter interactions in two-dimensional (2D) materials. Recently, we showed that center-of-mass localization accounts for the experimental exciton radiative lifetime at room temperature, which is constant for NPLs of different area S and far slower than expected based on the giant oscillator strength f_X,tot of exciton absorption.[1] On the other hand, the biexciton oscillator strength f_BX in 2D materials is uniquely linked to the biexciton extension S_BX, which is the area covered by the average interdistance a_BX of the interacting excitons. Since a_BX is an internal coordinate, f_BX should thus be intrinsically independent from S and not be diminished by center-of-mass related localizations. Here, we assess this predicted scaling on a set of 4.5 ML CdSe NPLs of increasing S using circularly-polarized transient absorption (TA) spectroscopy. We confirm that the direct exciton-to-biexciton transition yields a photo-induced absorption band in the TA spectrum. Moreover, using quantitative TA spectroscopy, we demonstrate that f_BX  is indeed area-independent and is approximately smaller than f_X,tot by a factor a_BX²/S. Interestingly, we find that S_BX is comparable to the room-temperature exciton coherence area, suggesting that the radiative recombination rates of the biexciton and the localized exciton are similar. Since f_BX governs absorption, emission and stimulated emission through the exciton/biexciton transition, these results underscore the relevance of this transition for the opto-electronic characteristics of 2D NPLs. Moreover, the approach to quantify the exciton/biexciton transition introduced here provides a means to rank such transitions in different 2D materials and to verify biexciton descriptors predicted from theory.

Nowadays, one of the most urgent societal challenges is focused on the progressive reduction of the global energetic consumption and pollution. Considering that lighting accounts for the 15 percent of the global electricity consumption and 5 percent of the worldwide greenhouse gas emission, new technological strategies are required in order to tackle and revert the current energetic troublesome. To reach that goals, not only the development of new materials and light-emitting technologies are required, but also a complete understanding of their working principles and mechanisms involved in the charge-to-photon conversion is a mandatory field. In this talk, I will discuss about the performance of a series of light-emitting diodes (LEDs) based on different semiconductors, e.g. lead-based and lead-free halide perovskites, II-VI and IV-VI quantum dots (QDs), fabricated through different approaches aimed at improving their performance and/or inducing synergies between the light-active materials. In addition, I will show the experimental tools and techniques employed to elucidate the photo-electrochemical mechanisms behind their functioning, which in turn determine their efficiency, brightness, durability and other relevant parameters.

In semiconductors, exciton or charge carrier diffusivity is typically described as an inherent material property. Here, we show that the transport of excitons, which are 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 (TPLM), 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. Upon photoexcitation, NCs transition to a transient configuration exhibiting an order-of-magnitude faster exciton transport rate. The transient 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 finding of a transient 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.

In the current context of rising popularity of perovskites as excellent materials for optoelectronics, inorganic lead halide perovskite (CsPbX₃, X=Cl, Br, I) quantum dots constitute a promising platform for light emitting devices thanks to their emission spectra having extensive coverage of frequencies with narrow band as well as their photoluminescence (PL) quantum yield approaching 100% [1]. Understanding light-matter coupling and the various many-body effects is at the core of realizing the full potential of perovskite quantum dots as both classical and quantum light sources [2,3]. Many-body interaction is notably important not only for single-exciton PL [4] but also for the emission energies of trion and biexciton systems [5]. For this purpose, we utilize the Configuration Interaction (CI) approach to study the binding energies of these multiexcitonic complexes. Corroborating the experimental findings from single-dot spectroscopy, our calculations quantitatively reproduce the measured redshifts of the trion and biexciton relative to the single exciton emission from CsPbX₃ quantum dots [6]. Additionally, the same theoretical method can also be employed to explain the sub-nanosecond exciton radiative lifetime in these fascinating materials [4].

[1] “Light Generation in Lead Halide Perovskite Nanocrystals: LEDs, Color Converters, Lasers, and Other Applications”, Yan, F. et al., Small 2019, https://onlinelibrary.wiley.com/doi/full/10.1002/smll.201902079

[2] “Coherent single-photon emission from colloidal lead halide perovskite quantum dots”, Utzat, H. et al., Science 2019, https://www.science.org/doi/10.1126/science.aau7392

[3] “The dark exciton ground state promotes photon-pair emission in individual perovskite nanocrystals”, Tamarat, P. et al., Nature Communications 2020, https://www.nature.com/articles/s41467-020-19740-7

[4] “Bright triplet excitons in caesium lead halide perovskites”, Becker, M. A. et al., Nature 2018, https://www.nature.com/articles/nature25147

[5] “Calculation of the biexciton shift in nanocrystals of inorganic perovskites”, Nguyen, T. P. T. et al., Phys. Rev. B 2020, https://journals.aps.org/prb/abstract/10.1103/PhysRevB.101.125424

[6] “Many-body Correlations and Bound Exciton Complexes in CsPbX3 (X=Br, Br/Cl) Quantum Dots”, Zhu, C. et al., in preparation

The past decade, metal halide perovskites have attracted great scientific and technological interest due to their promising application in diverse fields. Despite the great success of organic-inorganic metal halide perovskite materials mainly in photovoltaics, several challenges remain to be addressed such as their moisture, oxygen, light and heat intrinsic sensitivity. In this talk, we will focus on the synthesis with low-temperature re-precipitation-based colloidal methods of centrosymmetric or non-centrosymmetric all-inorganic metal halide perovskites in nano- and micro-particles form. Morphological and structural features as well as their physicochemical properties of these materials will be discussed and their potential utilization as energy storage and gas sensing materials. Furthermore, photo-induced processes in liquid dispersions will be presented as efficient methods to induce shape and crystal structure transformations of all-inorganic metal halide perovskites or even to conjugate them with 2D materials to improve their properties for these specific applications.The past decade, metal halide perovskites have attracted great scientific and technological interest due to their promising application in diverse fields. Despite the great success of organic-inorganic metal halide perovskite materials mainly in photovoltaics, several challenges remain to be addressed such as their moisture, oxygen, light and heat intrinsic sensitivity. In this talk, we will focus on the synthesis with low-temperature re-precipitation-based colloidal methods of centrosymmetric or non-centrosymmetric all-inorganic metal halide perovskites in nano- and micro-particles form. Morphological and structural features as well as their physicochemical properties of these materials will be discussed and their potential utilization as energy storage and gas sensing materials. Furthermore, photo-induced processes in liquid dispersions will be presented as efficient methods to induce shape and crystal structure transformations of all-inorganic metal halide perovskites or even to conjugate them with 2D materials to improve their properties for these specific applications.

Colloidal quantum dots (QDs) are attractive materials for realizing solution processable laser diodes that could benefit from the unique features of these zero-dimensional structures such as size-controlled emission wavelengths, low optical-gain thresholds, and ease of integration with photonic and electronic circuits. However, the implementation of such devices has been hampered by fast Auger recombination of gain-active multicarrier states, poor stability of QD films at high current densities, and the difficulties in obtaining net optical gain in a complex device stack wherein a thin electroluminescent QD layer is combined with optically lossy charge-transport layers. Here we resolve these challenges and achieve lasing regime in electrically pumped devices that employ compact, continuously graded QDs with strongly suppressed Auger recombination incorporated into a low-loss photonic waveguide integrated into a pulsed, high-current density light-emitting diode. These prototype colloidal QD laser diodes exhibit strong, broad-band optical gain and demonstrate low-threshold, room-temperature laser action at both the band-edge (637 nm) and the excited-state (586 nm) transitions.

Cesium lead halide nanocrystals can be arranged in various kinds of superlattices by drying-mediated self-assembly using mono, binary and ternary nanocrystal compounds. Due to their small inhomogeneous emission energy spread, large oscillator strength and long dephasing times, we observe characteristic signatures of coherent, cooperative light emission, so-called superfluorescence [1]. The versatility of the material platform allows us to control the superfluorescent properties by changing the superlattice geometry [2].

Furthermore, in order to harness even more enhanced oscillator strength and narrower ensemble linewidth, we study giant nanocrystals of 50 – 200 nm size that are synthesized using ligand-assisted precipitation (LARP). With thin films of these bulk-like crystals where the excitons are clearly in the weak confinement regime, we observe signatures of superfluorescence at about an order of magnitude lower excitation power. Moreover, we investigate the emission properties in different excitation geometries and explore the cross-over to amplified spontaneous emission (ASE) at elevated temperatures.

Nanostructured semiconductors, or quantum dots (QDs), are heavily investigated for their applications in light emission such as light emitting diodes and lasers. The premise of cost-effective solution processing of such devices based on nanocrystals has recently driven research towards electrically pumped population inversion in laser diode structures. Challenges however remain to achieve net light amplification in the cavity due to a balance between limited material gains and lossy electrical contacts. Further reductions in threshold current densities, mainly limited by the non-radiative cap of ca. 1 nanosecond on the gain lifetime, are also required to achieve stable operation. Finally, color tunability is limited to the red by the gain bandwidth of the red-emitting CdSe/CdS QDs or even core/shell nanoplatelets used.

Here, we show that weakly confined charge carriers in giant CdS quantum dots display disruptive optical gain metrics that could alleviate these remaining issues. Being active in the green part of the spectrum, their properties match and even outcompeting state-of-the-art colloidal materials in the red. Material gain coefficients up to 50.000 cm-1 combined with a broad gain window of 160 nm. Also, a very promising gain lifetimes close to 3 ns is found. Invoking a model of stimulated emission based on bulk semiconductor physics, we are able to explain all of these remarkable gain metrics, yet only if a large band gap renormalization effect is invoked. Our results show that weakly confined nanomaterials are excellent gain materials, combining straightforward wet chemical synthesis and the promise of solution processability with beyond state-of-the-art gain metrics.

Cesium lead halide perovskite nanocrystals, owing to high oscillator strength of bright triplet excitons, long coherence times and minimal inhomogeneous broadening of emission lines, are promising building blocks for creating superlattice structures that exhibit collective phenomena in their optical spectra. Thus far, only single-component superlattices with the simple cubic packing have been devised from these nanocrystals, which have been shown to exhibit superfluorescence – a collective emission resulting in a burst of photons with ultrafast radiative decay (ca. 20 ps) that could be tailored for use in ultrabright (quantum) light sources [1]. However, far broader structural engineerability of superlattices, required for programmable tuning of the collective emission and for building a theoretical framework can be envisioned from the recent advancements in colloidal science. We show that co-assembly of cubic and spherical steric-stabilized nanocrystals is experimentally possible and that the cubic shape of perovskite nanocrystals leads to a vastly different outcome compared to all-spherical systems. Five superlattice structures have been obtained: novel AB₂, ABO₃, ABO₆, besides expected NaCl or common to all-sphere assemblies AlB₂ superlattices [2,3]. In binary ABO₃ and ABO₆ superlattices, larger spherical nanocrystals occupy the A sites and smaller cubic CsPbBr₃ nanocrystals reside on the B and O sites. Targeted substitution of B-site nanocubes by truncated cuboid PbS nanocrystals leads to the exclusive formation of ternary perovskite-type ABO₃ superlattice. Truncated cuboid PbS NCs can occupy A-sites in binary ABO₃, NaCl, and AlB₂ SLs with smaller CsPbBr₃ nanocubes. All synthesized superlattices exhibit a high degree of orientational ordering of the CsPbBr₃ nanocubes. We also demonstrate the effect of superlattice structure on the collective optical properties.

Cesium lead halide perovskites are great candidates for lighting applications such as LEDs or screens, due to their bright photoluminescence with narrow bandwidth, originating from a tunable band gap. However, before applications can be realized, important challenges that are inherent to the materials need to be addressed. Lead halide perovskites are unstable under operation conditions - they degrade easily, for example upon contact with water, heat, or strong irradiation. Encapsulating perovskites in silica spheres stabilizes the perovskite emitters, protecting them from these influences. By controlling the shape and size of the silica, the material can be patterned and the optical properties can be tuned. In this talk, I will show the enhanced stability of perovskite emitters in silica spheres. Futhermore, I will provide an optical characterization of individual perovskite-filled silica spheres in the size range in the order of hundreds of nanometers, and methods to pattern these spheres for applications.

Research into semiconductor lead halide perovskite (LHP) nanocrystals (NCs) has rapidly developed for solution-processed light emitters and photovoltaics since their hot-injection synthesis in 2015 [1, 2]. LHP NCs are currently synthesized with narrow size distributions, which naturally leads to investigations on their self-assembly capabilities. Ordered NC arrays, or supercrystals (SCs) [3], are the forms of solid-state LHP materials alternative to polycrystalline films and single crystals, which can behave very differently from their individual constituents when they interact coherently with each other. Superradiance have been recently observed for LHP SCs, identified by its spectral features [3]. In this study we show superradiance emission from CsPbBr3 supercrystal structures, with NCs lateral dimensions between 4 and 11 nm. We performed statistical analysis of the spectral characteristics, comparing our results with a theoretical model, and extract an estimation of the number of single emitters contributing to the superradiance emission. Following the photoluminiscence spectra and lifetime evolution as a function of temperature we identified two main channels for thermal decoherence of the superradiance. Our work represents an important step to understand how the supercrystal emission enhancement factor depends on thermal dephasing processes and size distribution. The future ability to manipulate N photon coherent light states with high photon homogeneity at room temperature (RT) represents a great goal for the development of new quantum technology applications [4, 5].

The success of the colloidal semiconductor quantum dot (QD) 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 drive diverse fields of applications. While the exploitation of the strong confinement regime has been driving commercial and scientific interest in more traditional (e.g. InP or CdSe) QDs, a thorough exploration of such a regime has still not been achieved for lead-halide perovskite QDs. Here, we overcome previous challenges of insufficient chemical stability and monodispersity of small perovskite QDs via a post-synthetic treatment employing didodecyldimethylamonium bromide ligands. The achieved high monodispersity in both size and shape enables us to prepare self-assembled QD superlattices of exceptional long-range order and uniform thickness, with an unusual rhombic packing, and with narrow-band cyan emission. Their enhanced chemical stability allows us to explore strongly confined perovskite QDs at the single-particle/single-photon level.

Attaining pure single-photon emission is key for many quantum technologies,^[1] from optical quantum computing^[2] to quantum key distribution^[3] and quantum imaging.^[4] The past 20 years have seen the development of several solid-state quantum emitters, but most of them require highly sophisticated techniques (e.g., ultra-high vacuum growth methods and cryostats for low-temperature operation). The system complexity may be significantly reduced by employing quantum emitters capable of working at room temperature. Lead-halide perovskite APbX₃ (A=Cs or organic cation; X=Cl, Br, I) quantum dots (QDs) are one of the desired materials, of particular interest due to their low-cost synthesis, solution processability, tunability of the emission wavelength via size and composition, narrow-band emission, short radiative lifetime (~ns at RT) as well as high photoluminescence quantum yield (QY).^[5,6] Here, we present a systematic study across ∼ 170 photostable single CsPbX₃ (X: Br and I) colloidal QDs of different sizes and compositions, unveiling that increasing quantum confinement is an effective strategy for maximizing single-photon purity due to the suppressed biexciton quantum yield. Leveraging the latter, we achieve 98% single-photon purity (g⁽²⁾(0) as low as 2%) from a cavity-free, non-resonantly excited single 6.6 nm CsPbI₃ QDs, showcasing the great potential of CsPbX₃ QDs as room-temperature highly pure single-photon sources for quantum technologies.

Although studied since the 1980’s, research on two-dimensional metal halide perovskites (2D HaPs) has only recently exploded in the shadow of their 3D counterparts. Now, these compounds are studied for improving both the performance and the stability of solar cells, as well as for use in light-emitting devices, scintillators, gas detectors, and many more applications.

As a key difference compared to 3D HaPs, their two-dimensional nature leads charge carriers to form strongly bound excitons, making them excellent testbeds for exciton physics in confined systems, and for applications that require bright luminescence.

Curiously, some of these compounds exhibit narrow emission lines from the band edge (often with a complex sub-structure), whereas others give rise to broad and strongly red-shifted luminescence – or a combination of both.

In my talk, I shall address the different concepts invoked to explain the luminescence of 2D HaPs, how different propositions can be distinguished experimentally, and how variation of the constituent metal and halides affect the overall optoelectronics of this family of materials. A combination of photoluminescence spectroscopy and density functional theory computation is used to reveal the intricate origin of broad emission bands in a family of <100> oriented compounds.

Lead halide perovskites successfully advance towards applications in solar cells, light-emitting devices, and high-energy radiation detectors. Recent progress in understanding their uniqueness highlights the role of optoelectronic tolerance to intrinsic defects, particularly long diffusion lengths of carriers, and highly dynamic 3d inorganic framework. This picture indicates that finding an analogous material among non-group-14 metal halides can be very challenging, if possible at all. On the other hand, Sn (II) iodide perovskites exhibit comparably good performance in photovoltaics when synthesized with a low number of trap states. The main challenge with this material originates from the easiness of the trap states generation, which are typically ascribed to the oxidation of Sn(II) to Sn(IV). In this work, we describe the synthesis of colloidal monodisperse FASnI3 NCs, wherein thorough care on the purity of precursors and redox chemistry reduces the concentration of Sn(IV) to an insignificant level, to probe the intrinsic structural and optical properties of these NCs.

The advantages of Solution Processed Organic and Hybrid Perovskite Photovoltaics, such as their light weight, mechanical flexibility in addition to the small energy demand, and low-cost equipment requirements for roll-to-roll printing mass production, characterize them as interested candidate sources for future electrical power [1]. The presentation aims in covering a range of scientific and engineering issues needed to bring organic and hybrid perovskite solar cells to commercial viability in terms of efficiency, lifetime and cost [2-4]. A systematic understanding of the relationship between photovoltaic materials, interfaces and electrodes relevant to printed photovoltaics product development targets will be presented [5,6].

References:

[1] Printing Highly Efficient Organic Solar Cells, Hoth, C.N., Schilinsky, P., Choulis, S.A., Brabec, C.J., Nano Letters, Vol. 8 (9), 2008, 2806-2813, 2008.

[2] Long Thermal Stability of Inverted Perovskite Photovoltaics Incorporating Fullerene‐Based Diffusion Blocking Layer, F. Galatopoulos, I. T Papadas, G. S. Armatas, S. A Choulis, 5, 1800280, 2018.

[3] Implementing Inkjet‐Printed Transparent Conductive Electrodes in Solution‐Processed Organic Electronics, F. Hermerschmidt, S.A Choulis, E.J.W List‐Kratochvil, Advanced Materials Technologies 4 (5), 1800474, 7, 2019.

[4] Highly Efficient Indium Tin Oxide-Free Inverted Organic Photovoltaics using Laser Induced Forward Transfer Silver Nanoparticle Embedded Metal Grids, S. M. Pozov, K. Andritsos, I. Theodorakos, E. Georgiou, A. Ioakeimidis, A. Kabla, S. Melamed, F. de la Vega, I. Zergioti and S. A Choulis, ACS Applied Electronic Materials, 4, 6, 2689–2698, 2022.

Surface Treatment of Cu: NiOx Hole-Transporting Layer Using β-Alanine for Hysteresis-Free and Thermally Stable Inverted Perovskite Solar Cells. F. Galatopoulos, I.T Papadas, A Ioakeimidis, P. Eleftheriou, S.A Choulis, Nanomaterials 10 (10), 1961, 2020.

[6] High-performance non-fullerene acceptor inverted organic photovoltaics incorporating solution processed doped metal oxide hole selective contact, A. Ioakeimidis, A. Hauser, M. Rossier, F. Linardi, S.A. Choulis, Applied Physics Letters 120 (23), 233301, 2022.

Metal halides perovskites with nanoscale geometries have revolutionized the field of solution-processed photovoltaics and light-emitting devices due to their strong absorption and exceptional photoluminescence properties combined with a remarkable tolerance to structural defects. However, the further development of these materials to practical commercialization is hindered by their toxic components like lead and their inherent structural lability. Moreover, we still have little understanding of their crystallographic structures, chemical and physical interactions, and surface chemistry at a fundamental level.

The chemical design of metal halide perovskites proved to be the key to addressing those issues. Our recent findings confirm that a direct transition of the synthetic approach at nanoscale from Pb to Sn halide perovskites is not feasible. For Sn-based perovskites nanostructures, the structural dynamics between 3D cuboids and 2D Ruddlesden-Popper nanosheets colloids are highly dependent on the synthetic strategy. The presence of both species has a direct impact on the optoelectronic properties. Moreover, heterostructures containing halide perovskites nanocrystals are the next step towards obtaining accessible materials for industry. With the help of polymers or metal-organic frameworks,  we can access air stable materials or ultra-small quantum dots.

The optical absorption and emission properties of a semiconductor or molecular materials coupled to an optical cavity differ strongly from those of the bare, uncoupled, compounds.[1] For instance, it may intensely absorb light at frequencies for which its intrinsic absorption is very low, or emit light at spectral ranges at which the uncoupled system barely shows luminescence.[2,3] The integration of lead halide perovskites into photonic structures have demonstrated the great potential this approach bears to controllably modify and enhance the photophysical properties of these semiconductors. In this talk, different ways to controllably modify the optical response of lead halide perovskites by tailoring their optical environment will be overviewed. Recent advances in the improvement of the optical quality of perovskite quantum dot solids, which has opened the door to their integration in optical cavities, will be discussed in detail, as well as their implications in the field of photovoltaics, light emitting devices and photodetection.

Hybrid halide perovskites are a novel class of semiconductor materials with promising and versatile optoelectronic properties, enabled by their chemically adjustable structures and dimensionality. The diversity in the metal ions, halide anions, and organic spacers enables a wide range of materials with highly tunable properties and variable dimensionalities. These materials are studied for various applications such as solar cells, detectors, and light-emitting diodes. The ability to control and adjust the optical properties for a required application is significant. Thus, an improved understanding of the structure and optical mechanisms is crucial.

Specific low-dimensionality hybrid halide perovskites exhibit white-light emission at room temperature, associated with self-trapped excitons (STE), making them ideal candidates for illumination applications. We study the correlation between structural motifs of low dimensionality (2D, 1D) halide perovskites and their STE emission. We further study how the composition and specifically exchanging the halide anions while maintaining the structure affect the STE properties.

Scintillators are critical for high-energy radiation detection across a wide array of potential applications, from medical radiography and safety inspections all the way to space exploration. However, constrained by their current shortcomings, including high-temperature and complex fabrication, high cost, and inherent brittleness and fragility among thick films and bulk crystals, traditional scintillators are finding it difficult to meet the rising demand for cost-effective, eco-friendly, and flexible X-ray detection. Here, we describe the development of highly efficient and flexible X-ray scintillators based on films of Cu-doped Cs₂AgI₃, which exhibit ultrahigh X-ray sensitivity. The materials exhibit a high scintillation light yield of up to 82900 photons/MeV and a low detection limit of 77.8 nGy_air/s, which is approximately 70 times lower than the dosage for a standard medical examination. Moreover, richly detailed X-ray images of biological tissue and electronic components with a high spatial resolution of 16.2 lp/mm were obtained using flexible large-area solution-processed scintillation screens. These findings demonstrate that Cu-doped Cs₂AgI₃ films can serve as extraordinary scintillation screens for high-performance X-ray imaging devices.

Over the last 7-8 years, lead halide perovskite nanocrystals (LHP NCs) have emerged as extremely efficient light sources with color-tunable capability by simple chemistry.¹ LHP NCs exhibit very high photoluminescence quantum yields regardless of the synthesis conditions and the quality of the precursors owing to their defect tolerance nature (especially the Br and I-based). The light emission of LHP NCs is not only tunable by their dimensions and composition² but also through self-assembly into ordered architectures.³ The tunability of halide composition has been greatly exploited to tune the optical properties of LHP NCs, while the A-site and B-site cation tunability has been relatively less explored. Recently, our group showed that the A-site cation exchange in LHP NCs is as fast as halide exchange in the presence of excess ligands.⁴ The ligands act as transporters of cations from one NC to the other.  This talk will be focused on the latest results of our group regarding the A-cation tuning and the interesting properties that are observed from the mixed A-cation perovskites in comparison with single A-cation LHP NCs. In addition, some of the latest results on confined self-assembly into chiral architectures emitting chiral light.

References

1.         (a) Dey et al., State of the Art and Prospects for Halide Perovskite Nanocrystals. ACS Nano 2021, 15 (7), 10775-10981; (b) Akkerman et al., Genesis, challenges and opportunities for colloidal lead halide perovskite nanocrystals. Nat. Mater. 2018, 17 (5), 394-405; (c) Shamsi  et al., Post-Synthesis Modifications, and Their Optical Properties. Chem. Rev. 2019, 119 (5), 3296-3348; (d) Protesescu et al., Nanocrystals of Cesium Lead Halide Perovskites (CsPbX₃, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 2015, 15 (6), 3692-3696.

2.         Otero-Martínez et al., Dimensionality Control of Inorganic and Hybrid Perovskite Nanocrystals by Reaction Temperature: From No-Confinement to 3D and 1D Quantum Confinement. Angew. Chem. Int. Ed. 2021, 60 (51), 26677-26684.

3.         (a) Rainò et al., Superfluorescence From Lead Halide Perovskite Quantum Dot Superlattices. Nature 2018, 563 (7733), 671-675; (b) Vila-Liarte et al., Templated-Assembly of CsPbBr3 Perovskite Nanocrystals into 2D Photonic Supercrystals with Amplified Spontaneous Emission. Angew. Chem. Int. Ed. 2020, 59 (40), 17750-17756.

4.           Otero-Martínez et al. Fast A-Site Cation Cross-Exchange at Room Temperature: Single-to Double- and Triple-Cation Halide Perovskite Nanocrystals. Angew. Chem. Int. Ed. 2022, 61 (34), e202205617.

Lead halide perovskite nanocrystals (LHP NCs) are attractive for optically-pumped, solution-processed lasers in the visible, owing to their wide spectral tunability, their outstanding optical gain properties and the suppressed non-radiative recombination losses stemming from their defect-tolerant nature. In this work, we demonstrate flexible waveguides combining the transparent, bioplastic, polymer cellulose acetate (CA) with green-emitting CsPbBr₃ or red-emitting CsPb(Br,I)₃ NCs in solution-processed architectures based on polymer-NC multilayers deposited on CA micro-slabs.

Experiments and simulations indicate that the employment of the thin, free-standing CA membranes results in confined electrical fields, enhanced by two orders of magnitude compared to identical multilayer stacks deposited on conventional, thick and rigid quartz substrates. As a result, the polymer structures exhibit improved amplified spontaneous emission (ASE) characteristics under nanosecond excitation, with ASE thresholds down to 95 μJ cm^-2 in the green and 70 μJ cm^-2 in the red and high net modal gain up to 450 cm^-1 and 630 cm^-1, respectively. The optimized gain properties are accompanied by a notable improvement of the ASE operational stability due to the low thermal resistance of the substrate-less membranes and the intimate thermal contact between the polymer and the NCs. Their application potential is further highlighted by the membrane ability to sustain dual-color ASE in the green and red through excitation by a single UV source and the activation of underwater stimulated emission

Lead halide perovskite nanocrystals (LHP NCs) are attractive for optically-pumped, solution-processed lasers in the visible, owing to their wide spectral tunability, their outstanding optical gain properties and the suppressed non-radiative recombination losses stemming from their defect-tolerant nature. In this work, we demonstrate flexible waveguides combining the transparent, bioplastic, polymer cellulose acetate (CA) with green-emitting CsPbBr₃ or red-emitting CsPb(Br,I)₃ NCs in solution-processed architectures based on polymer-NC multilayers deposited on CA micro-slabs.

Experiments and simulations indicate that the employment of the thin, free-standing CA membranes results in confined electrical fields, enhanced by two orders of magnitude compared to identical multilayer stacks deposited on conventional, thick and rigid quartz substrates. As a result, the polymer structures exhibit improved amplified spontaneous emission (ASE) characteristics under nanosecond excitation, with ASE thresholds down to 95 μJ cm^-2 in the green and 70 μJ cm^-2 in the red and high net modal gain up to 450 cm^-1 and 630 cm^-1, respectively. The optimized gain properties are accompanied by a notable improvement of the ASE operational stability due to the low thermal resistance of the substrate-less membranes and the intimate thermal contact between the polymer and the NCs. Their application potential is further highlighted by the membrane ability to sustain dual-color ASE in the green and red through excitation by a single UV source and the activation of underwater stimulated emission

Photon recycling (PR) is an outstanding physical process that consists on the reabsorption and re-emission of radiatively emitted photons in semiconductor thin films. These multiple reabsorption/reemission cycles can significantly increase the photon and carrier densities in solar cells or light-emitting sources, hence it has important implications towards the enhancement of the performances of these devices (Ansari-Rad & Bisquert, 2018). In this context, perovskite materials present optimum characteristics to favor the PR effect (Pazos-Outón et al., 2016); namely large absorption coefficient and sharp edge, together with relatively narrow photoluminescence (PL) spectra, short Stokes-shift (SS) between exciton PL and absorption, and high PL quantum yield. In this scenario, we have recently demonstrated a record of PR effect by integrating a close packed layer of CsPbBr3 NCs on a planar waveguide configuration (PMMA/perovskite/PMMA). The advantage of this geometry, compared to the case of a standard thin film, is the high confinement of the electromagnetic field at the semiconductor and, with it, the important enhancement of the exciton absorption and emission processes. As a consequence, this photonic device extended the PR effect over mm-distances, as revealed by the SS of the collected PL up to 32.5 meV and the increase of the decay time from 3 to 9 ns (see figure). In this talk we will compare and discuss the PR effect observed in optical waveguides incorporating films of CsPbBr3 nanocrystals (3D carrier confinement) and PEA2SnI4 (1D carrier confinement). The PR effect was appropriately characterized by a frequency-domain fluorescence spectroscopy technique, and modelled with rate equations and stochastic Monte Carlo simulations in the case of perovskite nanocrystals (J. Navarro et al., 2020), where carrier diffusion is absent. In the case of the 2D tin-perovskite, we evaluate the effect of diffusion in the waveguided PR mechanism. Our results provide new insights towards a deeper understanding of the PR phenomenon, whose significance could be extended for designing and optimizing the PR effect in solar cells and light-emitting devices.

The development of metal-halide perovskite-based optoelectronic devices continues to reach new record efficiencies and operation lifetimes. While most of the device fabrication is still done by spin-coating of a previous-prepared precursor solution, inkjet printing was already used for the fabrication of solar cells and light emitting diodes.[1] Beyond scalable, high‑throughput and large‑area coating, inkjet printing also enables variation of the deposited material during processing.
Using combinatorial inkjet printing, multiple precursor inks can be deposited in a single printing step to achieve compositionally mixed perovskite thin films and gradients. The composition of the resulting film can thereby be precisely controlled by adjusting the droplet ratio of both inks during the printing process. A pre-made image, fed into the printer, allows thereby to produce arbitrary patterns of metal halide perovskite film with compositional gradients.[2] Beyond being a powerful tool for high-throughput material discovery, we demonstrate the direct use of combinatorial inkjet printing for device fabrication. Utilizing methylammonium lead iodide, bromide and chloride inks, we fabricated a series of MAPb(IxBr1-x)3 and MAPb(BrxCl1-x)3 thin films. The well-known gradient of the bandgap energy in this series allowed us to produce a series of wavelength-selective photodetectors, covering the whole spectrum of visible light from 1.6 eV (MAPbI3) to 3.0 eV (MAPbCl3) in a single printing step.[3]

Lead halide perovskites are intriguing candidates for the display industry. Lead-free perovskites are of interest due to their nontoxicity. However, engineering their optical properties is still in its infancy.

Controlling the optical properties is a fundamental goal for improving device performances. Here, we focus on tuning the optical properties of silver-based double perovskite nanocrystals via two iso-valent dopants, Bi and Sb. The PL quantum yield of the intrinsic Cs₂Ag_1-yNa_yInCl₆ perovskite increases dramatically upon doping. The two dopants affect the optical properties differently. We hypothesize that this effect is related to exciton-phonon coupling in double perovskites. The mechanism allows broad white light emission wavelength and no overlap between the absorption and the emission, which is compatible with luminescent solar concentrators. Luminescent solar concentrators are light-harvesting devices that redirect the light to a photovoltaic cell. We test the exciton-phonon coupling through absorption and photoluminescence measurements using the Haung-Rhys factor calculation. Using colloidal open-air synthesis, we control the perovskite matrix and the doping concentration. We can differentiate between contributions from Bi and Sb levels marking different electronic considerations. This exemplifies the essence of the resulting practices for tuning and optimizing the optical properties of double perovskite nanocrystals.

Achieving high-quality cesium-formamidinium lead iodide (CsxFA1-xPbI3) perovskites with widely tuneable bandgaps
is highly desired for their optoelectronic applications yet remains a challenge because of the different phase transition
behaviour of FAPbI3 and CsPbI3 perovskite during film depositions.1,2 Herein, by utilizing an alkaline-interface-assisted
cation-exchange method, we fabricate a set of highly emissive CsxFA1-xPbI3 perovskite films with fine-tuning Cs-FA
alloying ratio for emission-tuneable near-infrared light-emitting diodes (NIR-LEDs). We reveal that the deprotonation
of FA+ cations and the formation of hydrogen-bonded gels consisting of CsI and FA facilitated by zinc oxide underneath
effectively removes the Cs-FA ion-exchange barrier, promoting the formation of phase-pure CsxFA1-xPbI3 films with
emission filling the gap between that of pure Cs- and FA-based perovskites.3 The obtained NIR-LEDs with narrow
electroluminescence peaking from 715 to 780 nm simultaneously demonstrate high peak external quantum efficiencies
of over 15%, maximum radiances exceeding 300 W sr-1 m-2, and high power conversion efficiencies above 10 % at 100
mA cm-2, representing the best-performing devices based on solution-processed wavelength-tuneable NIR emitters in
the similar region4.

1 Li, Z., et.al. Chem. Mater. 28, 284-292 (2016)
2 Lee, J.-W., et.al. Adv. Energy Mater. 5, 1501310 (2015)
3 Z, Yuan., et.al. Nat. Commun 10, 2818 (2019)
4 Z, Yuan., et.al. In revision.

In the recent decade, optical microcavities have attracted a great deal of interest in a broad range of research fields [1]. The monolithic structure of most microcavities, such as Fabry-Perot (FP) or whispering-gallery-mode (WGM) resonators, lack the capability of large-range spectral and spatial tuning. As an alternative, an open-access microcavity system combines several advantages [2]: realization of small mode volume, large-range tunability, flexible structural engineering, and easiness of integration with external emitters. Thus, this fiber-based cavity system results from an outstanding scheme to implement a new kind of semiconductor devices which are promising candidates for the future development of the new and exciting field of quantum polaritonics [3,4].

From the material perspective, tin-based perovskites have become attractive materials for light-matter interaction. Their advantages include straightforward fabrication by chemical synthesis, and Tin-based perovskites have accentuated because of their low toxicity and the optimal optical bandgap (1.2-1.4 eV) close to the Shockley-Queisser (SQ) limit under 1 sun illumination. tin (II) halide perovskite has drawn attention because of the hybrid structure. The hybrid term refers to the combination of organic cations (in our case 2-tiopheneethylammonium, TEA⁺) and an inorganic layer of tin iodide octahedral layer. Despite this, TEA₂SnI₄ perovskite exhibits high exciton binding energy and reduced exciton-phonon interactions. In our work, the cavity consists of a first planar DBR mirror and a second microscale concave DBR mirror embedded at the tip of an optical fibre. These two mirrors are facing each other with a micron-sized gap, building a semi-concave Fabry-Perot cavity. The relative position between each mirrors can be tunned independently by nano-positioners. Here, we study the coupling of the open cavity modes with the TEA₂SnI₄ exciton optical emission at room temperature (RT). We observe an avoided crossing between the bulk exciton and the cavity mode levels, which is the principal signature of the strong coupling regime of the light-matter interaction, and hence of the formation of new dressed states at RT, i.e excitons-polaritons (Upper Polaritons- UP & Lower Polaritons-LP). The use of fibre based open cavity facilitates the tuning of the cavity mode as a function of the cavity length, thus obtaining indepent control of the cavity resonance with respect to the exciton light emission. As it is shown in Figure, the avoided crossing is characterized by a vacuum Rabi splitting of 15 meV. The development of lead-free perovskite polaritons are new candidates to be included in the quantum material database for the new exciting polariton condensates and quantum topology applications at RT.

Layered metal-dielectric-metal (MDM) cavities sustain resonances with vanishing dielectric permittivity that can couple to the light emission of fluorophores positioned either on their surface [1] or within the cavity [2]. The wavelength of the cavity resonance depends on the polarization, the angle, and the thickness of the dielectric layer.[3] In the weak coupling regime, the polarization and angular properties of the MDM cavity resonances are transferred to the emission of the fluorophores inside the dielectric layer of the cavity. [2] In the strong coupling regime, the interaction of the cavity modes with the excitonic emission of the fluorophores leads to the formation of polaritons and Rabi splitting of the resonance modes.[4] Typically, the dispersion of the Rabi splitting is mapped by varying the k-vector that is in-plane with the emitter and mirror layers, for example by imaging the angle-dependent signal in the back focal plane of the objective. [4] Another option to tune the cavity resonance is to vary the length of the photonic cavity (thereby varying the out-of-plane k-vector), however, this is difficult to achieve, for example via the tedious fabrication of several samples with different dimensions.

Here we present an elegant method to vary the thickness of the dielectric layer (that corresponds to the length of the cavity) in situ by using a surface-forces apparatus (SFA). [5] In this experiment, two cylindrical and semitransparent mirrors are separated by a dielectric liquid in which the light emitting nanocrystals or molecules are dispersed. The distance of the mirrors can be controlled by a cantilever spring over the range from large (mm) separation to almost close contact. Here a crossed configuration of the cylindrical mirrors results in a well-defined contact region with sub-millimeter dimension. Such a SFA therefore acts as a tunable MDM cavity that allows to detect the strong coupling of multiple cavity harmonics with the dye emission in a single, rapid, and continuous sweep of cavity thickness. We observed Rabi splitting exceeding 100 meV, and we analyze the Rabi splitting of the different cavity harmonics, finding a square root dependence with respect to the number of the involved emitters.

Furthermore, we prepared planar MDM cavities with fixed dimension and investigated the Rabi splitting via the angle-dependence with spectroscopic ellipsometry. These experiments demonstrated that the polaritons inherit the ENZ character of the cavity modes. The complementarity of this experiment with the SFA study allows to investigate the impact of the in-plane and out-of-plane k vector on the Rabi splitting.

References

[1] V. Caligiuri, M. Palei, M. Imran, L. Manna, R. Krahne, ACS Photonics 2018, 5, 2287-2294.

[2] V. Caligiuri, G. Biffi, M. Palei, B. Martín-García, R. D. Pothuraju, Y. Bretonnière, R. Krahne, Adv. Opt. Mater. 2020, 8, 1901215.

[3] V. Caligiuri, M. Palei, G. Biffi, S. Artyukhin, R. Krahne, Nano Lett. 2019, 19, 3151-3160.

[4] A. Patra, V. Caligiuri, R. Krahne, A. De Luca, Adv. Opt. Mater. 2021, 9, 2101076

[5] A. Fieramosca, et al., Sci Adv. 2019;5(5):eaav9967.

[6] B. Zappone, V. Caligiuri, A. Patra, R. Krahne, A. De Luca, ACS Photonics 2021.

Short-wave infrared (SWIR) image sensors based on colloidal quantum dots (QDs) are characterized by low cost, small pixel pitch, and spectral tunability. Adoption of QD-SWIR imagers is, however, hampered by a reliance on restricted elements such as Pb and Hg. Here, QD photodiodes, the central element of a QD image sensor, made from non-restricted In(As,P) QDs that operate at wavelengths up to 1400 nm are demonstrated. Three different In(As,P) QD batches that are made using a scalable, one-size-one-batch reaction and feature a band-edge absorption at 1140, 1270, and 1400 nm are implemented. These QDs are post-processed to obtain In(As,P) nanocolloids stabilized by short-chain ligands, from which semiconducting films of n-In(As,P) are formed through spincoating. For all three sizes, sandwiching such films between p-NiO as the hole transport layer and Nb:TiO2 as the electron transport layer yields In(As,P) QD photodiodes that exhibit best internal quantum efficiencies at the QD band gap of 46±5% and are sensitive for SWIR light up to 1400 nm.

Near-infrared Light-emitting diodes (NIR-LEDs) based on colloidal semiconductor quantum dots (QDs) are of interest for a variety of optoelectronic applications such as hyperspectral and biomedical imaging, night vision and telecommunication systems. Importantly, all efficient NIR-LEDs based on QDs employ toxic heavy metals compositions [1] while The European Union’s “Restriction of Hazardous Substances” (RoHS) directive limits the use of such compounds. Colloidal indium arsenide QDs are emerging as a promising candidate for NIR applications[2] thanks to their low toxicity and recent progresses in material synthesis leading to stable and highly efficient QDs [3]. Here, we fabricated NIR-LEDs emitting in the range of 800-1000 nm based on heavy metal-free efficient InAs/ZnSe core-shell QDs (PLQY >40%) [3]. The resulting external quantum efficiency of NIR-LEDs benefited from nanoengineering at both material and device levels. This study demonstrates that InAs based QDs are a promising nanomaterial for the fabrication of optoelectronic devices operating in the NIR.

The use of solution‐based materials is the key to making new devices accessible within the field of organic and hybrid electronics. It also enables manufacturing at a high-throughput industrial scale, by being compatible with scalable fabrication techniques, such as inkjet printing. This technique offers both the capabilities of coating as well as precise control of the deposited material during processing. Expanding these capabilities to combinatorial inkjet printing allows the deposition of multiple inks in a single printing step – again, with precise control of the droplet ratio during printing – to produce films with tunable composition.

One class of solution-processed materials utilisable in this way are metal halide perovskites, which have contributed to significant performance increases when employed as active material in solar cells and hybrid light-emitting diodes (LEDs) during the past decade. However, no active material can function alone in an optoelectronic device, but must be sandwiched between two electrodes, of which at least one must be transparent. By moving away from the ubiquitous material indium tin oxide (ITO), a wide variety of transparent conducting electrodes can be utilised that overcome some of the inherent limitations of ITO, such as its brittleness.

This contribution highlights our work on these two main avenues of optoelectronic device research: the processing of active materials in hybrid and organic optoelectronics [1-3], and the implementation of ITO-free transparent conducting electrodes [4-7]. The findings presented address the importance of continuing work in organic and hybrid (opto)electronic devices, in order to move towards high performance flexible electronics.