Highly luminescent perovskite–aluminum oxide composites

In this communication we report on the preparation of CH3NH3PbBr3 perovskite/Al2O3 nanoparticle composites in a thin film configuration and demonstrate their high photoluminescence quantum yield. The composite material is solution-processed at low temperature, using stable alumina nanoparticle dispersions. There is a large influence of the alumina nanoparticle concentration on the perovskite morphology and on its photoluminescence

Luminescent perovskites: From materials to light-emitting devices

The thesis describes the preparation of efficient and simple bright halide perovskite materials and light-emitting devices. In the different chapters, several perovskite types are described and studied: Perovskites with different crystal sizes, from nanosized perovskites (Chapter 2, 3 and 4) to bulk perovskites (Chapter 5) all with a 3D crystal structure; and perovskites with different crystal structure dimensionalities, such as 3D or nanoplatelets with 2D (Chapter 3) and even quasi-2D structures (Chapter 2). Photoluminescence and electroluminescence with colours ranging from red to green and blue are reported. Chapter 2 focuses on obtaining stable and very PL efficient red-emitting hybrid quasi-2D perovskite NCs with a narrow linewidth. The work experiments with a controllable shift of the bandgap, accomplished by varying the chain length of the alkylammonium ligands employed in its synthesis. Chapter 3 describes the novel use of a bifunctional ligand for the synthesis of highly photoluminescent green-emitting MAPbBr3 NPs. Additionally, the ligand allows the nanostructures to effectively anchor on a variety of conducting polymers and inorganic semiconducting surfaces, which is used for the preparation of solution-based multilayer LEDs. Chapter 4 is about green-emitting LEDs with a remarkably high combination of EQE, luminance, power and current efficiency. The outstanding performance is described to be due to an energy cascade from hierarchical self-assembled structures. These structures, low-dimensional octylammonium lead bromide microplatelets (MPLs) and 3D FAPbBr3 NCs, form ultra-smooth films with a very high PLQY. Chapter 5 describes a mechanochemical synthesis of stable mixed-cation/mixed-halide lead perovskites powders (MA1-yCsyPb(BrnCl1-n)3) with emission spanning the green to blue region of the visible spectrum. Upon addition of amantadine hydrochloride, a strong enhancement in the PLQY can be obtained with only minor structural changes

Dry Mechanochemical Synthesis of Highly Luminescent, Blue and Green Hybrid Perovskite Solids

A simple method to obtain bright photoluminescent wide bandgap mixed‐halide 3D perovskites is reported. The materials are prepared by dry mechanochemical synthesis (ball‐milling) starting from neat binary precursors, and show enhanced photoluminescence upon the addition of an adamantane derivative in the precursors' mixture. The structural characterization suggests that the additive does not participate in the crystal structure of the perovskite, which remains unvaried even with high loading of amantadine hydrochloride. By simple stoichiometric control of the halide precursors, the photoluminescence can be finely tuned from the UV to the green part of the visible spectrum. Photoluminescence quantum yields as high as 29% and 5% have been obtained for green‐ and blue‐emitting perovskite solids, even at very low excitation densities

Molecular Iodine for a General Synthesis of Binary and Ternary Inorganic and Hybrid Organic-inorganic Iodide Nanocrystals

We report the synthesis of various binary and ternary inorganic and hybrid organic-inorganic iodide nanocrystals starting from molecular iodine (I2). The procedure utilizes a reaction between I2 and oleylamine that yields oleylammonium iodide -the iodide precursor for a subsequent preparation of nanocrystals. The syntheses are facile, carried out under air, in vials heated on a hotplate and deliver nanocrystals with narrow size distributions and, in the case of red and near infrared-emitting lead-based perovskites, with 70-80% photoluminescence quantum yields. The latter were used to fabricate red and infrared bright light-emitting diodes, with external quantum efficiencies (EQE) exceeding 3%

Peripheral halo-functionalization in [Cu(N^N)(P^P)]+ emitters: influence on the performances of light-emitting electrochemical cells

A series of heteroleptic [Cu(N^N)(P^P)][PF6] complexes is described in which P^P = bis(2-(diphenylphosphino)phenyl)ether (POP) or 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos) and N^N = 4,4′-diphenyl-6,6′-dimethyl-2,2′-bipyridine substituted in the 4-position of the phenyl groups with atom X (N^N = 1 has X = F, 2 has X = Cl, 3 has X = Br, 4 has X = I; the benchmark N^N ligand with X = H is 5). These complexes have been characterized by multinuclear NMR spectroscopy, mass spectrometry, elemental analyses and cyclic voltammetry; representative single crystal structures are also reported. The solution absorption spectra are characterized by high energy bands (arising from ligand-centred transitions) which are red-shifted on going from X = H to X = I, and a broad metal-to-ligand charge transfer band with λmax in the range 387–395 nm. The ten complexes are yellow emitters in solution and yellow or yellow-orange emitters in the solid-state. For a given N^N ligand, the solution photoluminescence (PL) spectra show no significant change on going from [Cu(N^N)(POP)]+ to [Cu(N^N)(xantphos)]+; introducing the iodo-functionality into the N^N domain leads to a red-shift in λmaxem compared to the complexes with the benchmark N^N ligand 5. In the solid state, [Cu(1)(POP)][PF6] and [Cu(1)(xantphos)][PF6] (fluoro-substituent) exhibit the highest PL quantum yields (74 and 25%, respectively) with values of τ1/2 = 11.1 and 5.8 μs, respectively. Light-emitting electrochemical cells (LECs) with [Cu(N^N)(P^P)][PF6] complexes in the emissive layer have been tested. Using a block-wave pulsed current driving mode, the best performing device employed [Cu(1)(xantphos)]+ and this showed a maximum luminance (Lummax) of 129 cd m−2 and a device lifetime (t1/2) of 54 h; however, the turn-on time (time to reach Lummax) was 4.1 h. Trends in performance data reveal that the introduction of fluoro-groups is beneficial, but that the incorporation of heavier halo-substituents leads to poor devices, probably due to a detrimental effect on charge transport; LECs with the iodo-functionalized N^N ligand 4 failed to show any electroluminescence after 50 h

Dipole reorientation and local density of optical states influence the emission of light-emitting electrochemical cells

Herein, we analyze the temporal evolution of the electroluminescence of light-emitting electrochemical cells (LECs), a thin-film light-emitting device, in order to maximize the luminous power radiated by these devices. A careful analysis of the spectral and angular distribution of the emission of LECs fabricated under the same experimental conditions allows describing the dynamics of the spatial region from which LECs emit, i.e. the generation zone, as bias is applied. This effect is mediated by dipole reorientation within such an emissive region and its optical environment, since its spatial drift yields a different interplay between the intrinsic emission of the emitters and the local density of optical states of the system. Our results demonstrate that engineering the optical environment in thin-film light-emitting devices is key to maximize their brightness

Exploring mobility in Italian Neolithic and Copper Age communities

As a means for investigating human mobility during late the Neolithic to the Copper Age in central and southern Italy, this study presents a novel dataset of enamel oxygen and carbon isotope values (δ18Oca and δ13Cca) from the carbonate fraction of biogenic apatite for one hundred and twenty-six individual teeth coming from two Neolithic and eight Copper Age communities. The measured δ18Oca values suggest a significant role of local sources in the water inputs to the body water, whereas δ13Cca values indicate food resources, principally based on C3 plants. Both δ13Cca and δ18Oca ranges vary substantially when samples are broken down into local populations. Statistically defined thresholds, accounting for intra-site variability, allow the identification of only a few outliers in the eight Copper Age communities, suggesting that sedentary lifestyle rather than extensive mobility characterized the investigated populations. This seems to be also typical of the two studied Neolithic communities. Overall, this research shows that the investigated periods in peninsular Italy differed in mobility pattern from the following Bronze Age communities from more northern areas

Comprehensive defect suppression in perovskite nanocrystals for high-efficiency light-emitting diodes

Electroluminescence efficiencies of metal halide perovskite nanocrystals (PNCs) are limited by a lack of material strategies that can both suppress the formation of defects and enhance the charge carrier confinement. Here we report a one-dopant alloying strategy that generates smaller, monodisperse colloidal particles (confining electrons and holes, and boosting radiative recombination) with fewer surface defects (reducing non-radiative recombination). Doping of guanidinium into formamidinium lead bromide PNCs yields limited bulk solubility while creating an entropy-stabilized phase in the PNCs and leading to smaller PNCs with more carrier confinement. The extra guanidinium segregates to the surface and stabilizes the undercoordinated sites. Furthermore, a surface-stabilizing 1,3,5-tris(bromomethyl)-2,4,6-triethylbenzene was applied as a bromide vacancy healing agent. The result is highly efficient PNC-based light-emitting diodes that have current efficiency of 108 cd A−1 (external quantum efficiency of 23.4%), which rises to 205 cd A−1 (external quantum efficiency of 45.5%) with a hemispherical len

Low-dimensional iodide perovskite nanocrystals enable efficient red emission

We report herein a simple ligand-assisted reprecipitation method at room temperature to synthesize mixed-cation hybrid organic-inorganic perovskite nanocrystals with low structural dimensionality. The emission wavelength of iodide-based perovskites is thus tuned from the near-infrared to the red part of the visible spectrum. While this is mostly achieved in the literature by addition of bromide, we demonstrate here a controllable blueshift of the band gap by varying the chain length of the alkylammonium ligands. Furthermore, an antisolvent washing step was found to be crucial to purify the samples and obtain single-peak photoluminescence with a narrow linewidth. The so-formed nanocrystals exhibit high and stable photoluminescence quantum yields exceeding 90% over 500 hours, making these materials ideal for light-emitting applications