Long-lived electrets and lack of ferroelectricity in methylammonium lead bromide CH3NH3PbBr3 ferroelastic single crystals.

Hybrid lead halides CH3NH3PbX3 (X = I, Br, and Cl) have emerged as a new class of semiconductors for low-cost optoelectronic devices with superior performance. Since their perovskite crystal structure may have lattice instabilities against polar distortions, they are also being considered as potential photo-ferroelectrics. However, so far, research on their ferroelectricity has yielded inconclusive results and the subject is far from being settled. Here, we investigate, using a combined experimental and theoretical approach, the possible presence of electric polarization in tetragonal and orthorhombic CH3NH3PbBr3 (T-MAPB and O-MAPB). We found that T-MAPB does not sustain spontaneous polarization but, under an external electric field, it is projected into a metastable, ionic space-charge electret state. The electret can be frozen on cooling, producing a large and long-lasting polarization in O-MAPB. Molecular dynamics simulations show that the ferroelastic domain boundaries are able to trap charges and segregate ionic point defects, thus playing a favorable role in the stabilization of the electret. At lower temperatures, the lack of ferroelectric behavior is explained using first principles calculations as the result of the tight competition among many metastable states with randomly oriented polarization; this large configurational entropy does not allow a single polar state to dominate at any significant temperature range.

Investigation of Dimethylammonium Solubility in MAPbBr3 Hybrid Perovskite: Synthesis, Crystal Structure, and Optical Properties.

The possible existence of mixed methylammonium (MA)/dimethylammonium (DMA) lead bromide hybrid perovskites of general formula MA1- xDMA xPbBr3 (0 ≤ x ≤ 1) was investigated. A combined X-ray diffraction and solid-state nuclear magnetic resonance approach indicates that DMA can be incorporated up to about x = 0.30 while retaining the cubic lattice of MAPbBr3. By increasing the DMA content ( x), the absorption shows a progressive blue shift and the band gap moves from about 2.17 eV ( x = 0) to about 2.23 ( x = 0.30) with a concomitant slightly faster recombination in the mixed cation powders.

Excited State Properties of Hybrid Perovskites.

Metal halide perovskites have come to the attention of the scientific community for the progress achieved in solar light conversion. Energy sustainability is one of the priorities of our society, and materials advancements resulting in low-cost but efficient solar cells and large-area lighting devices represent a major goal for applied research. From a basic point of view, perovskites are an exotic class of hybrid materials combining some merits of organic and inorganic semiconductors: large optical absorption, large mobilities, and tunable band gap together with the possibility to be processed in solution. When a novel class of promising semiconductors comes into the limelight, lively discussions ensue on the photophysics of band-edge excitations, because just the states close to the band edge are entailed in energy/charge transport and light emission. This was the case several decades ago for III-V semiconductors, it has been up to 10 years ago for organics, and it is currently the case for perovskites. Our aim in this Account is to rationalize the body of experimental evidence on perovskite photophysics in a coherent theoretical framework, borrowing from the knowledge acquired over the years in materials optoelectronics. A crucial question is whether photon absorption leads to a population of unbound, conductive free charges or instead excitons, neutral and insulating bound states created by Coulomb interaction just below the energy of the band gap. We first focus on the experimental estimates of the exciton binding energy (Eb): at room temperature, Eb is comparable to the thermal energy kBT in MAPbI3 and increases up to values 2-3kBT in wide band gap MAPbBr3 and MAPbCl3. Statistical considerations predict that these values, even though comparable to or larger than thermal energy, let free carriers prevail over bound excitons for all levels of excitation densities relevant for devices. The analysis of photophysics evidence confirms that all hybrid halide perovskites behave as free-charge semiconductors. Thanks to such property, in combination with band gap energies covering the entire solar spectrum, perovskites represent a promising materials platform for highly efficient, single and multijunction solar cells. Concerning the use of perovskites as color-tunable materials in light emitting devices, free-charges are not the preferred species, as they recombine radiatively through a bimolecular process that is inefficient at the charge-injection levels typical of LED operation. Strategies to overcome this limit, and thus extend the use of perovskite materials beyond solar energy conversion, could be borrowed from inorganic semiconductor optoelectronics and include the fabrication of nanostructures with reduced dimensionality to alter the electronic density of states, as well as engineering composite materials.

Ln3Q9 as a molecular framework for ion-size-driven assembly of heterolanthanide (Nd, Er, Yb) multiple near-infrared emitters.

A unique example of discrete molecular entity Nd(y)Er(x)Yb(3-(x+y))Q9 (1) (Q = quinolinolato) containing three different lanthanides simultaneously emitting in three different spectral regions in the NIR, ranging from 900 to 1600 nm, has been synthesized and fully chararacterized. A simple molecular strategy based on tuning metal composition in the Ln3Q9 framework, which contains inequivalent central and terminal coordination sites, has allowed a satisfactory ion-size-driven control of molecular speciation close to 90%. In 1 the central position of the larger Nd ion is well distinguished from the terminal ones of the smaller Yb(3+) and Er(3+), which are almost "vicariants" as found in the heterobimetallic Er(x)Yb(3-x)Q9 (2). The Ln3Q9 molecular architecture, which allows communication between the ions, has proved to afford multiple NIR emission in 1 and 2, and is promising to develop a variety of multifunctional materials through the variation of the Ln composition.

Interface properties of organic para-hexaphenyl/α-sexithiophene heterostructures deposited on highly oriented pyrolytic graphite.

It was recently reported, that heterostructures of para-hexaphenyl (p-6P) and α-sexithiophene (6T) deposited on muscovite mica exhibit the intriguing possibility to prepare lasing nanofibers of tunable emission wavelength. For p-6P/6T heterostructures, two different types of 6T emission have been observed, namely, the well-known red emission of bulk 6T crystals and additionally a green emission connected to the interface between p-6P and 6T. In this study, the origin of the green fluorescence is investigated by photoelectron spectroscopy (PES). As a prerequisite, it is necessary to prepare structurally similar organic crystals on a conductive surface, which leads to the choice of highly oriented pyrolytic graphite (HOPG) as a substrate. The similarity between p-6P/6T heterostructures on muscovite mica and on HOPG is evidenced by X-ray diffraction (XRD), scanning force microscopy (SFM), and optical spectroscopy. PES measurements show that the interface between p-6P and 6T crystals is sharp on a molecular level without any sign of interface dipole formation or chemical interaction between the molecules. We therefore conclude that the different emission colors of the two 6T phases are caused by different types of molecular aggregation.

Effects of the diphenyl ether additive in halogen-free processed non-fullerene acceptor organic solar cells.

The development of an environmentally friendly fabrication process for non-fullerene acceptor organic solar cells is an essential condition for their commercialization. However, devices fabricated by processing the active layer with green solvents still struggle to reach, in terms of efficiency, the same performance as those fabricated with halogenated solvents. The reason behind this is the non-optimal nanostructure of the active layer obtained with green solvents. Additives in solution have been used to fine-tune the nanostructure and improve the performance of organic solar cells. Therefore, the identification of non-halogenated additives and the study of their effects on the device performance and stability are of primary importance. In this work, we propose the use of diphenyl ether (DPE) as additive, in combination with the non-halogenated solvent o-xylene, to fabricate organic solar cells with a completely halogen-free process. Thanks to the addition of DPE, a best efficiency of 11.7% have been obtained for the system TPD-3F:IT-4F, an increase over 15% with respect to the efficiency of devices fabricated without additive. Remarkably, the stability under illumination of the solar cells is also improved when DPE is used. The addition of DPE has effects on the molecular organization in the active layer, with an enhancement in the donor polymer ordering, showing a higher domain purity. The resulting structure improves the charge carrier collection, leading to a superior short-circuit current and fill factor. Furthermore, a reduction of the non-radiative recombination losses and an improved exciton diffusion, are the results of the superior molecular ordering. With a comprehensive insight of the effects of DPE when used in combination with a non-halogenated solvent, our study provides an approach to make the fabrication of organic solar cell environmentally friendlier and more suitable for large scale production.

Exciton dissociation in 2D layered metal-halide perovskites.

Layered 2D perovskites are making inroads as materials for photovoltaics and light emitting diodes, but their photophysics is still lively debated. Although their large exciton binding energies should hinder charge separation, significant evidence has been uncovered for an abundance of free carriers among optical excitations. Several explanations have been proposed, like exciton dissociation at grain boundaries or polaron formation, without clarifying yet if excitons form and then dissociate, or if the formation is prevented by competing relaxation processes. Here we address exciton stability in layered Ruddlesden-Popper PEA2PbI4 (PEA stands for phenethylammonium) both in form of thin film and single crystal, by resonant injection of cold excitons, whose dissociation is then probed with femtosecond differential transmission. We show the intrinsic nature of exciton dissociation in 2D layered perovskites, demonstrating that both 2D and 3D perovskites are free carrier semiconductors and their photophysics is described by a unique and universal framework.

Tuning the energy transfer in Ruddlesden-Popper perovskites phases through isopropylammonium addition - towards efficient blue emitters.

Here we demonstrate blue LEDs with a peak wavelength of 481 nm, with outstanding colour purity of up to 88% (CIE coordinates (0.1092, 0.1738)), an external quantum yield of 5.2% and a luminance of 8260 cd m-2. These devices are based on quasi-2D PEA2(Cs0.75MA0.25)Pb2Br7, which is cast from solutions containing isopropylammonium (iPAm). iPAm as additive assist in supressing the formation of bulk-like phases, as pointed out by both photophysical and structural characterization. Additionally, the study of the excitation dynamics demonstrates a hindering of the energy transfer to domains of lower energy that generally undermines the performance and emission characteristics of blue-emitting LEDs based on quasi-2D perovskites. The achieved narrow distribution of quantum well sizes and the hindered energy transfer result in a thin film photoluminescence quantum yield exceeding 60%. Our work demonstrates the great potential to tailor the composition and the structure of thin films based on Ruddlesden-Popper phases to boost performance of optoelectronic devices - specifically blue perovskite LEDs.

Charged excitons, Auger recombination and optical gain in CdSe/CdS nanocrystals.

CdSe/CdS colloidal nanocrystals are members of a novel class of light-emitting nanoparticles with remarkable optical properties such as suppressed fluorescence blinking and enhanced emission from multiexciton states. These properties have been linked to the suppression of non-radiative Auger recombination. In this work we employ ultrafast spectroscopy techniques to identify optical signatures of neutral and charged excitonic and multiexcitonic states. We show that Auger recombination of biexcitons is not suppressed, while we observe optical gain and amplified spontaneous emission from multiexciton states and from long-lived charged-exciton states.

Pressure response of decylammonium-containing 2D iodide perovskites.

Manipulation by external pressure of the optical response of 2D Metal Halide Perovskites (MHPs) is a fascinating route to tune their properties and promote the emergence of novel features. We investigate here DA2PbI4 and DA2GeI4 (DA = decylammonium) perovskites in the pressure range up to ∼12 GPa by X-ray powder diffraction, absorption, and photoluminescence spectroscopy. Although the two systems share a similar structural evolution with pressure, the optical properties are rather different and influenced by Pb or Ge. DA2PbI4 shows a progressive red shift from 2.28 eV (P = 0 GPa) to 1.64 eV at 11.5 GPa, with a narrow PL emission, whereas DA2GeI4, changes from a non-PL system at ambient pressure to a clear broadband emitter centered around 730 nm with an intensity maximum at about 3.7 GPa. These results unveil the role of the central atom on the nature of emission under pressure in 2D MHPs containing a long alkyl chain.

Silicon-based fluorescent platforms for copper(ii) detection in water.

The potential of silicon-based fluorescent platforms for the detection of trace toxic metal ions was investigated in an aqueous environment. To this aim, silicon chips were first functionalized with amino groups, and fluorescein organic dyes, used as sensing molecules, were then covalently linked to the surface via formation of thiourea groups. The obtained hybrid heterostructures exhibited high sensitivity and selectivity towards copper(ii), a limit of detection compatible with the recommended upper limits for copper in drinking water, and good reversibility using a standard metal-chelating agent. The fluorophore-analyte interaction mechanism at the basis of the reported fluorescence quenching, as well as the potential of performance improvement, were also studied. The herein presented sensing architecture allows, in principle, tailoring of the selectivity towards other metal ions by proper fluorophore selection, and provides a favorable outlook for integration of fluorescent chemosensors with silicon photonics technology.

Versatile vapor phase deposition approach to cesium tin bromide materials CsSnBr3, CsSn2Br5 and Cs2SnBr6.

We report on the successful application of RF-magnetron sputtering to deposit, by using a single type of target, three different materials in the form of thin films within the Cs-Sn-Br compositional range, namely, CsSnBr3, CsSn2Br5 and Cs2SnBr6. It is shown that, by playing with the deposition parameters and post-deposition treatments, it is possible to stabilize these three perovskites or perovskite related compounds by exploiting the versatility of vapor phase deposition. Full characterization in terms of crystal structure, optical properties and morphology is reported. The power of vapor phase methods in growing all-inorganic materials of interest for photovoltaic and optoelectronic applications is demonstrated here, indicating the advantageous use of sputtering for these complex materials.

Simple Processing Additive-Driven 20% Efficiency for Inverted Planar Heterojunction Perovskite Solar Cells.

Compositional engineering has been a strong tool to improve the quality of the perovskite materials and, in turn, the reproducibility of the solar cells. However, the control over the active layer uniformity, one of the most important requirements for the obtainment of efficient devices, is still a weak point of perovskite solar cells (PSCs) manufacturing. Here, we develop an approach to grow a uniform mixed cation perovskite layer, foreseeing its implementation in inverted solar cells endowing organic transporting layers, through the addition of a stoiochiometric amount of tropolone as chelating agent for the lead. Thanks to low melting and boiling temperatures, tropolone is present in the system only during the colloidal liquid phase, leaving the film during its formation; this unique characteristic promotes the obtainment of ideal perovskite surface morphologies and an increased short circuit current of photovoltaic devices. A maximum power conversion efficiency of 20% was obtained, with a 25% increase with respect to the reference.

Exciton-Exciton Interaction and Optical Gain in Colloidal CdSe/CdS Dot/Rod Nanocrystals.

Exciton-exciton interaction in dot/rod CdSe/CdS nanocrystals has proved to be very sensitive to the shape of nanocrystals, due to the unique band alignment between CdSe and CdS. Repulsive exciton-exciton interaction is demonstrated, which makes CdSe/CdS dot/rods promising gain media for solution-processable lasers, with projected pump threshold densities below 1 kW cm(-2) for continuous wave lasing.

Highly emissive nanostructured thin films of organic host-guests for energy conversion.

All-organic nanostructured host-guest systems, based on dyes inserted in the nanochannels of perhydrotriphenylene (PHTP) and deoxycholic acid (DCA), show enhanced fluorescence properties with quantum yields even higher than those of the dyes in solution, thanks to the high concentration of emissive molecules with controlled spatial and geometrical organization that prevents aggregation quenching. Both host molecules crystallize, growing with the long axis oriented along the direction of the nanochannels where the linear-chain dyes are inserted, to yield crystals emitting well-polarized light. For the DCA-based host-guests, homogeneous thin films suitable for several applications are obtained. Colour emission in such films can be tuned by co-inclusion of two or three dyes due to resonant energy-transfer processes. We show that films obtained by low-cost techniques, such as solution casting and spin-coating, convert UV light into visible light with an efficiency much higher than that of the standard polymeric blends.

Spatial control of 3D energy transfer in supramolecular nanostructured host-guest architectures.

Systematic control of 3D energy transfer (ET) dynamics is achieved in supramolecular nanostructured host-guest systems using spacer-functionalized guest chromophores. Quantum chemistry-based Monte Carlo simulations reveal the strong impact of the spacer length on the ET dynamics, efficiency, and dimensionality. Remarkably high exciton diffusion lengths demonstrate that there is ample scope for optimizing oligomeric or polymeric optoelectronic devices.

Bifacial Diffuse Absorptance of Semitransparent Microstructured Perovskite Solar Cells.

An optical radiometry technique enabling simultaneous transmittance and reflectance measurements from both sides of a device was used to investigate bifacial diffuse absorptance of neutral-colored semitransparent perovskite solar cells based on a thin film of microsized perovskite islands. In such microstructured solar cells, diffuse irradiance was more effectively absorbed than direct irradiance at near-normal incidence, in contrast to reference solar cells comprising a continuous perovskite thin film. Experimental findings were discussed in ray-optic approximation in relation to the surface texture of the active layer, highlighting the role of light trapping. This absorptance spectroscopy technique is envisaged to find wide applicability to bifacial solar cells for building-integrated photovoltaics and other bifacial light-harvesting systems.

Temporary dense seismic network during the 2016 Central Italy seismic emergency for microzonation studies.

In August 2016, a magnitude 6.0 earthquake struck Central Italy, starting a devastating seismic sequence, aggravated by other two events of magnitude 5.9 and 6.5, respectively. After the first mainshock, four Italian institutions installed a dense temporary network of 50 seismic stations in an area of 260 km2. The network was registered in the International Federation of Digital Seismograph Networks with the code 3A and quoted with a Digital Object Identifier ( https://doi.org/10.13127/SD/ku7Xm12Yy9 ). Raw data were converted into the standard binary miniSEED format, and organized in a structured archive. Then, data quality and completeness were checked, and all the relevant information was used for creating the metadata volumes. Finally, the 99 Gb of continuous seismic data and metadata were uploaded into the INGV node of the European Integrated Data Archive repository. Their use was regulated by a Memorandum of Understanding between the institutions. After an embargo period, the data are now available for many different seismological studies.

Novel Physical Vapor Deposition Approach to Hybrid Perovskites: Growth of MAPbI3 Thin Films by RF-Magnetron Sputtering.

Solution-based methods represent the most widespread approach used to deposit hybrid organic-inorganic perovskite films for low-cost but efficient solar cells. However, solution-process techniques offer limited control over film morphology and crystallinity, and most importantly do not allow sequential film deposition to produce perovskite-perovskite heterostructures. Here the successful deposition of CH3NH3PbI3 (MAPI) thin films by RF-magnetron sputtering is reported, an industry-tested method to grow large area devices with precisely controlled stoichiometry. MAPI films are grown starting from a single-target made of CH3NH3I (MAI) and PbI2. Films are single-phase, with a barely detectable content of unreacted PbI2, full surface coverage and thickness ranging from less than 200 nm to more than 3 µm. Light absorption and emission properties of the deposited films are comparable to as-grown solution-processed MAPI films. The development of vapor-phase deposition methods is of interest to advance perovskite photovoltaic devices with the possibility of fabricating perovskite multijunction solar cells or multicolor bright light-emitting devices in the whole visible spectrum.

Ultra-Bright Near-Infrared Perovskite Light-Emitting Diodes with Reduced Efficiency Roll-off.

Herein, an insulating biopolymer is exploited to guide the controlled formation of micro/nano-structure and physical confinement of α-δ mixed phase crystalline grains of formamidinium lead iodide (FAPbI3) perovskite, functioning as charge carrier concentrators and ensuring improved radiative recombination and photoluminescence quantum yield (PLQY). This composite material is used to build highly efficient near-infrared (NIR) FAPbI3 Perovskite light-emitting diodes (PeLEDs) that exhibit a high radiance of 206.7 W/sr*m2, among the highest reported for NIR-PeLEDs, obtained at a very high current density of 1000 mA/cm2, while importantly avoiding the efficiency roll-off effect. In depth photophysical characterization allows to identify the possible role of the biopolymer in i) enhancing the radiative recombination coefficient, improving light extraction by reducing the refractive index, or ii) enhancing the effective optical absorption because of dielectric scattering at the polymer-perovskite interfaces. Our study reveals how the use of insulating matrixes for the growth of perovskites represents a step towards high power applications of PeLEDs.