Search

The research on two-dimensional colloidal semiconductors has received a boost from the emergence of ultrathin lead halide perovskite nanoplatelets. While the optical properties of these materials have been widely investigated, their accurate structural and compositional characterization is still challenging. Here, we exploited the natural tendency of the platelets to stack into highly ordered films, which can be treated as single crystals made of alternating layers of organic ligands and inorganic nanoplatelets, to investigate their structure by multilayer diffraction. Using X-ray diffraction alone, this method allowed us to reveal the structure of ∼12 Å thick Cs-Pb-Br perovskite and ∼25 Å thick Cs-Pb-I-Cl Ruddlesden-Popper nanoplatelets by precisely measuring their thickness, stoichiometry, surface passivation type and coverage, as well as deviations from the crystal structures of the corresponding bulk materials. It is noteworthy that a single, readily available experimental technique, coupled with proper modeling, provides access to such detailed structural and compositional information.

The lead halide perovskite material shows its unique photoelectric properties, the resulting conversion efficiency of perovskite solar cells. However, the efficiency comes to a bottleneck owing to that mechanism research fall behind the device research. Multiple excitons dynamics play an important role, especially in the lifetime and binding energy of multiple excitons. A long multiexciton lifetime is beneficial to the application for light-emitting devices and photovoltaic devices. Large multiexciton binding energy means a large Stokes shift in exciton absorption, thus avoiding the loss of linear absorption. To conclude, discussions are presented regarding views of current multiple excitons research in terms of the biexciton lifetime and biexciton binding energy that should be considered for further advances in materials and devices.

Organic-inorganic lead halide perovskite (CH3NH3PbI3) solar cells have surpassed 25% power conversion efficiency, being ready for industrial-scale production of cheap photovoltaic (PV) panels. In this action, the major hurdle is its lead content, which in case of device failure, could be washed into the soil, entering the food chain. Since there is a zero tolerance on lead in the human organism, this health hazard is a critical obstacle for commercialization. Here, we propose a solution to this problem by incorporating phosphate salts (e.g., (NH4)(2)HPO4) in PV and other perovskite-based optoelectronic devices in various architectures. Phosphate salts do not react with CH3NH3PbI3 and do not alter its advantageous optoelectronic properties, but in a wet environment, they react immediately with lead, forming a highly insoluble compound, precluding this way the spread of lead into the environment. It is expected that this study will stimulate research, enabling lead halide perovskite solar cells to reach a similar environmental risk category as the commercially available, nonwater-soluble heavy metal-containing CdTe and gallium diselenide technologies.

The utilization of mixed cations is beneficial for taking the advantages of cations and achieving highly efficient perovskite solar cells (PSCs). Herein, the precisely small incorporation of CH(NH2)2 (FA) cations in methyl ammonium lead iodide (MAPbI3) enables the formation of compact, smooth perovskite film with high crystallinity. Consequently, the short-circuit current and the fill factor of the PSCs based on FAxMA1-xPbI3 perovskite are greatly improved, leading to the enhanced device efficiency. The champion PSC based on FA0.1MA0.9PbI3 exhibits a remarkably high efficiency of 22.02%. Furthermore, the PSCs based on FA0.1MA0.9PbI3 perovskite also show improved device stability. This work provides a simple approach to fabricate high-quality perovskite films and high-performance PSCs with better stability.

In this thesis, photovoltaic technologies were developed for space-based solar power. Two methods of realizing SBSP were introduced, namely concentrated photovoltaics (CPV) and radiation hard flat panel photovoltaics. Both techniques are instrumental to realizing SBSP as they are pathways to realizing high specific power and lower launch costs. Technologies developed to support these two forms of SBSP were then reported. In support of CPV, ultralight broadband mid-infrared coatings were developed for the concentrating mirrors used in our project. This was done to create radiative pathways for heat loss to ensure that the solar cells do not overheat. Using the rigorous coupled wave analysis technique, we optimized a backside single-layer coating using 2nm Cr/ 2μm CP1/ 500nm Ag that had an mIR emissivity of 0.6. Adding a second layer of this coating, we predicted that a 0.5nm Cr/ 1.9µm CP1/ 3nm Cr/ 2µm CP1/ 500nm Ag screen could achieve an emissivity of 0.8. We also optimized a 10nm ITO/ 2 μm CP1/ 500nm Ag frontside emitter which had a visible reflectivity of 0.896 and a mIR emissivity of 0.554. A backside emitter coating that was 0.927 emissive in the mIR with areal density 6.0 gm⁻² was successfully fabricated, as was a frontside mirror emitter coating with visible reflectivity of 0.896 and a mIR emissivity of 0.582 with areal density 4.1 gm⁻². In support of radiation hard photovoltaics, organo-lead halide perovskites (OHLP) were investigated. Challenges facing their fabrication were explored, with special focus on the electron transport layer PCBM as well as OHLP formulation. It was found that doping PCBM with a surfactant CTAB was beneficial, but did not work with all surfaces. An ITO/NiOx/MAPbI3/CTAB+PCBM/Cu device with in-house champion efficiency of 12.41% was achieved, and an ITO/NiOx/FA0.85Cs0.15PbI3/PCBM/Cu device with in-house champion efficiency of 11.81% was achieved. Time-dependant drift diffusion modelling was employed to account for the S-kink arising from poor PCBM carrier concentration. Finally, the proton degradation of OHLP devices and constituent transport layers were investigated to shed better light on how OHLP devices degrade under proton irradiation. Films of ITO, PEDOT, NiOx, PCBM, and PTCDi were found to degrade under 30keV and 75keV protons of up to 1.4 x 10¹⁴ p⁺cm⁻² fluence, but their electrical resistivity and optical transmissivity were not found to impact the cell as much as the OHLP absorber layer itself. Observing the light IV and EQE degradation of OHLP cells, it is evident that proton deposition in the OHLP layer itself causes the most damage, especially at 30keV and 75keV protons with fluences from 4.3 x 10¹³ p⁺cm⁻² to 1.7 x 10¹⁴ p⁺cm⁻². By considering the discrepancy in trends between Jsc and EQE, we concluded that the protons much accelerate intensity-based metastable photodegradation. Finally, by observing their anneal recovery, we concluded that it was temperature dependant and that maximum irrecoverable damage occurs at the OHLP/HTL interface.

Lead halide perovskites are receiving large attention due to their potential use as active materials in a wide range of electronic, optoelectronic and photonic devices. Among this wide family of materials quasi-two-dimensional (2D) lead halide perovskites show improved stability and higher photoluminescence (PL) quantum yield with respect to their 3D counterparts. They are formed by quantum well (QW) structures with inorganic lead halide layers comprised between organic bulky spacers, which determine environmental and operational stability to the perovskite. In particular, the recent evidence of optical gain, Amplified Spontaneous Emission (ASE) and optically pumped lasing propose these materials as potentially interesting innovative active materials for laser applications. In this work we review the main results on the ASE properties of quasi-2D perovskites and on the understanding of their basic photophysics, in order to describe the current state of the art of the research in this field.

Metal-halide perovskite semiconductors have attracted intense interest over the past decade, particularly for applications in photovoltaics. Low-energy optical phonons combined with significant crystal anharmonicity play an important role in charge-carrier cooling and scattering in these materials, strongly affecting their optoelectronic properties. We have observed optical phonons associated with Pb–I stretching in both MAPbI3 single crystals and polycrystalline thin films as a function of temperature by measuring their terahertz conductivity spectra with and without photoexcitation. An anomalous bond hardening was observed under above-bandgap illumination for both single-crystal and polycrystalline MAPbI3. First-principles calculations reproduced this photo-induced bond hardening and identified a related lattice contraction (photostriction), with the mechanism revealed as Pauli blocking. For single-crystal MAPbI3, phonon lifetimes were significantly longer and phonon frequencies shifted less with temperature, compared with polycrystalline MAPbI3. We attribute these differences to increased crystalline disorder, associated with grain boundaries and strain in the polycrystalline MAPbI3. Thus we provide fundamental insight into the photoexcitation and electron–phonon coupling in MAPbI3.

Highlights Oxyacid anions (NO3−, SO42−, CO32−, PO43− and SiO32−) were investigated both theoretically and experimentally about their passivation effect on CsPbI2Br perovskite interface.Adjustment of oxysalt layer thickness can optimize the surface band position that could be beneficial for electronic band alignment at perovskite/transport layer interface.Using silicate as a passivator, the CsPbI2Br solar cells achieved a PCE of 17.26% with an open-circuit voltage of 1.36 V. This strategy is also effective for organic-inorganic perovskite solar cells. Electronic supplementary material The online version of this article (10.1007/s40820-021-00683-7) contains supplementary material, which is available to authorized users., Passivation, as a classical surface treatment technique, has been widely accepted in start-of-the-art perovskite solar cells (PSCs) that can effectively modulate the electronic and chemical property of defective perovskite surface. The discovery of inorganic passivation compounds, such as oxysalts, has largely advanced the efficiency and lifetime of PSCs on account of its favorable electrical property and remarkable inherent stability, but a lack of deep understanding of how its local configuration affects the passivation effectiveness is a huge impediment for future interfacial molecular engineering. Here, we demonstrate the central-atom-dependent-passivation of oxysalt on perovskite surface, in which the central atoms of oxyacid anions dominate the interfacial oxygen-bridge strength. We revealed that the balance of local interactions between the central atoms of oxyacid anions (e.g., N, C, S, P, Si) and the metal cations on perovskite surface (e.g., Pb) generally determines the bond formation at oxysalt/perovskite interface, which can be understood by the bond order conservation principle. Silicate with less electronegative Si central atoms provides strong O-Pb motif and improved passivation effect, delivering a champion efficiency of 17.26% for CsPbI2Br solar cells. Our strategy is also universally effective in improving the device performance of several commonly used perovskite compositions. Electronic supplementary material The online version of this article (10.1007/s40820-021-00683-7) contains supplementary material, which is available to authorized users.

The conventional strategy of synthetic colloidal chemistry for bright and stable quantum dots has been the production of epitaxially matched core/shell heterostructures to mitigate the presence of deep trap states. This mindset has been shown to be incompatible with lead halide perovskite nanocrystals (LHP NCs) due to their dynamic surface and low melting point. Nevertheless, enhancements to their chemical stability are still in great demand for the deployment of LHP NCs in light-emitting devices. Rather than contend with their attributes, we propose a method in which we can utilize their dynamic, ionic lattice and uniquely defect-tolerant band structure to prepare non-epitaxial salt-shelled heterostructures that are able to stabilize these materials against their environment, while maintaining their excellent optical properties and increasing scattering to improve out-coupling efficiency. To do so, anchored LHP NCs are first synthesized through the heterogeneous nucleation of LHPs onto the surface of microcrystalline carriers, such as alkali halides. This first step stabilizes the LHP NCs against further merging, and this allows them to be coated with an additional inorganic shell through the surface-mediated reaction of amphiphilic Na and Br precursors in apolar media. These inorganically shelled NC@carrier composites offer significantly improved chemical stability toward polar organic solvents, such as γ-butyrolactone, acetonitrile, N-methylpyrrolidone, and trimethylamine, demonstrate high thermal stability with photoluminescence intensity reversibly dropping by no more than 40% at temperatures up to 120 °C, and improve compatibility with various UV-curable resins. This mindset for LHP NCs creates opportunities for their successful integration into next-generation light-emitting devices., ACS Nano, 13 (10), ISSN:1936-0851, ISSN:1936-086X

Because of the interesting features of the organic/inorganic halide perovskite (e.g., CH3NH3PbX 3, X = Cl, Br) materials, such as long electron–hole diffusion length, high absorption coefficient and adjustable band gap, they have attracted a great deal of attention. In this research, CH3NH3PbX 3 (X = Br, Cl) are synthesized and perovskite-based single-layer and perovskite-based bilayer diodes are fabricated. Using electron beam deposition technique, thin films of CH3NH3PbX 3 (X = Br, Cl) hybrids with excellent homogeneity are manufactured. Absorption spectrum, X-ray spectrum and photoluminescence spectrum of organic/inorganic halide perovskites along with electroluminescence spectra, SEM micrographs and current–voltage characteristic of perovskite-based light-emitting diodes are investigated. Perovskite-based bilayer diodes are evaluated for the first time. The difference between the emission wavelengths of single-layer and bilayer diodes is the main characteristic of bilayer diodes.

The vast majority of lead halide perovskite (LHP) nanocrystals (NCs) are currently based on either a single halide composition (CsPbCl3, CsPbBr3, and CsPbI3) or an alloyed mixture of bromide with either Cl–or I–[i.e., CsPb(Br:Cl)3or CsPb(Br:I)3]. In this work, we present the synthesis as well as a detailed optical and structural study of two halide alloying cases that have not previously been reported for LHP NCs: Cs2PbI2Cl2NCs and triple halide CsPb(Cl:Br:I)3NCs. In the case of Cs2PbI2Cl2, we observe for the first time NCs with a fully inorganic Ruddlesden–Popper phase (RPP) crystal structure. Unlike the well-explored organic–inorganic RPP, here, the RPP formation is triggered by the size difference between the halide ions. These NCs exhibit a strong excitonic absorption, albeit with a weak photoluminescence quantum yield (PLQY). In the case of the triple halide CsPb(Cl:Br:I)3composition, the NCs comprise a CsPbBr2Cl perovskite crystal lattice with only a small amount of incorporated iodide, which segregates at RPP planes’ interfaces within the CsPb(Cl:Br:I)3NCs. Supported by density functional theory calculations and postsynthetic surface treatments to enhance the PLQY, we show that the combination of iodide segregation and defective RPP interfaces are most likely linked to the strong PL quenching observed in these nanostructures. In summary, this work demonstrates the limits of halide alloying in LHP NCs because a mixture that contains halide ions of very different sizes leads to the formation of defective RPP interfaces and a severe quenching of LHP NC’s optical properties.

Lead halide perovskite CsPbBr3 is a wide-gap semiconductor material potentially very attractive for next generations of real-time monitors and particle detectors in high-energy physics. Here, we present the first characterization of crystalline CsPbBr3 point dosimeters with submillimeter size, under 6 MV X-photon beams used in clinical radiotherapy. Current response of the devices proved to be promising in terms of fast rise and decay times, of the same order of the X-ray beam onset and offset ones; absence of polarization effects; reproducibility to repeated irradiations; and linearity of the collected charge as a function of the absorbed dose. Comparing the measured sensitivity with the theoretical one, a charge collection distance of about 100 μm has been evaluated, of the same order of the linear dimensions of crystallites within the samples, suggesting that recombination centers are mainly placed at grain boundaries. A much higher sensitivity per unit area measured with crystalline CsPbBr3 as compared with drop-casted ones can be explained in terms of a less disordered crystalline structure. This work opens the way to CsPbBr3 point dosimeters, with linear dimensions meeting the strict spatial resolution constraints for bidimensional dose mapping required in clinical radiotherapy.

Lead halide perovskites are currently widely investigated as active materials in photonic and optoelectronic devices. While the lack of long term stability actually limits their application to commercial devices, several experiments demonstrated that beyond the irreversible variation of the material properties due to degradation, several possibilities exist to reversibly modulate the perovskite characteristics by acting on the environmental conditions. These results clear the way to possible applications of lead halide perovskites to resistive and optical sensors. In this review we will describe the current state of the art of the comprehension of the environmental effects on the optical and electronic properties of lead halide perovskites, and of the exploitation of these results for the development of perovskite-based sensors.

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

The preparation of strongly emissive CsPbBr3perovskite nanocrystals with robust surface passivation is a challenge in the field of lead halide perovskite nanomaterials. We report an approach to prepare polymer-capped CsPbBr3perovskite nanocrystals by reacting oleylammonium/oleate-capped Cs4PbBr6nanocrystals with poly(maleic anhydride-alt-1-octadecene) (PMAO). PMAO contains succinic anhydride units that are reactive towards the oleylamine species present on the Cs4PbBr6nanocrystals' surface and produces polysuccinamic acid, which, in turn, triggers the Cs4PbBr6to CsPbBr3conversion. The transformation occurs through the formation of Cs4PbBr6–CsPbBr3heterostructures as intermediates, which are captured because of the mild reactivity of PMAO and are investigated by high-resolution electron microscopy. The Cs4PbBr6–CsPbBr3heterostructures demonstrate a dual emission at cryogenic temperature with an indication of the energy transfer from Cs4PbBr6to CsPbBr3. The fully-transformed CsPbBr3NCs have high photoluminescence quantum yield and enhanced colloidal stability, which we attribute to the adhesion of polysuccinamic acid to the NC surface through its multiple functional groups in place of oleate and alkylammonium ligands. The PMAO-induced transformation of Cs4PbBr6NCs opens up a strategy for the chemical modification of metal halide NCs initially passivated with nucleophilic amines.

Two-dimensional (2D) layered perovskites recently emerged as one of the most promising materials for optoelectronics. These not only behave like a quantum well structures, but also on doping Mn(II) ions at the Pb site result as a highly emissive doped material. These structures can also be used as a starting material for obtaining 3D perovskites. Thermal treatment of Mn(II)-incorporated layered perovskites in presence of Cs(I) ions led to Mn(II)-doped 2D platelet–shaped perovskites, which also tune their dimensions with varying dopant ion concentrations. Keeping the importance of the 2D-layered perovskites, this review focused on the basic structures to the recently developed synthetic strategies and their optical properties of layered structured perovskites, Mn(II)-doped layered perovskites, and Mn(II)-doped 2D platelet–shaped perovskites.

Summary Perovskite solar cells (PSCs) are one of the emerging solar cell technologies with high conversion efficiency. Several deposition methods had been applied for preparing their hole transport layer (HTL). However, there are few direct evidences to demonstrate whether HTL and its interfaces in PSCs have been influenced by the deposition methods. In this study, the 3D morphology of PSCs has been reconstructed by focused ion beam-scanning electron microscopy from the PSCs in which HTLs are deposited by different methods. The compositional distribution of HTLs is unveiled as well. All these associated layers and interfaces display obvious morphological and compositional differences that are attributed to the HTL components’ solubility differences in the precursor solvent. Our investigation demonstrates the PSCs that HTL fabricated by dynamic spin-coating method have higher efficiency, better film uniformity, and less interfacial roughness than the static spin-coating-based devices.

Lead halide perovskites are considered to be most promising photovoltaic materials. Highest efficiency and improved stability of perovskite solar cells have been achieved by using cation and anion mixtures. Experimental information on electronic and ionic charge carriers is key to evaluate device performance, as well as processes of photo‐decomposition and photo‐demixing which are observed in these materials. Here, we measure ionic and electronic transport properties and investigate various cation and anion substitutions with a special eye on their photo‐ionic effect, following our previous study on CH3NH3PbI3, where we found that light enhances not only electronic but also ionic conductivities. We find that this phenomenon is very sensitive to the nature of the halide, while the cationic substitutions are less relevant. Based on the observation that the ionic conductivity enhancement found for iodide perovskites is significantly weakened by bromide substitution, we provide a chemical rationale for the photo‐demixing in mixed halide compositions., The photo‐effect on ion conduction in mixed cation and halide perovskites is studied. Unlike A‐site substitution, anion replacement is of great influence. In I‐Br mixtures the differences in hole localization and defect formation favor (reversible) photo‐demixing (the situation in the right part is simplified as the interstitial neutral iodine is further stabilized by ionic rearrangement, and the hole in the bromide is delocalized over several regular anions).

Lead halide perovskites of APbX_3 type [A = Cs, formamidinium (FA), methylammonium; X = Br, I] in the form of ligand-capped colloidal nanocrystals (NCs) are widely studied as versatile photonic sources. FAPbBr_3 and CsPbBr_3 NCs have become promising as spectrally narrow green primary emitters in backlighting of liquid-crystal displays (peak at 520–530 nm, full width at half-maximum of 22–30 nm). Herein, we report that wet ball milling of bulk APbBr_3 (A = Cs, FA) mixed with solvents and capping ligands yields green luminescent colloidal NCs with a high overall reaction yield and optoelectronic quality on par with that of NCs of the same composition obtained by hot-injection method. We emphasize the superiority of oleylammonium bromide as a capping ligand used for this procedure over the standard oleic acid and oleylamine. We also show a mechanically induced anion-exchange reaction for the formation of orange-emissive CsPb(Br/I)_3 NCs., ACS Applied Nano Materials, 1 (3), ISSN:2574-0970

Lead halide perovskites are currently receiving increasing attention due to their potential to combine easy active layers fabrication, tunable electronic and optical properties with promising performance of optoelectronic and photonic device prototypes. In this paper, we review the main development steps and the current state of the art of the research on lead halide perovskites amplified spontaneous emission and on optically pumped lasers exploiting them as active materials.

[EN] This paper explores the gas sensing properties of graphene nanolayers decorated with lead halide perovskite (CH3NH3PbBr3) nanocrystals to detect toxic gases such as ammonia (NH3) and nitrogen dioxide (NO2). A chemical-sensitive semiconductor film based on graphene has been achieved, being decorated with CH3NH3PbBr3 perovskite (MAPbBr3) nanocrystals (NCs) synthesized, and characterized by several techniques, such as field emission scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. Reversible responses were obtained towards NO2 and NH3 at room temperature, demonstrating an enhanced sensitivity when the graphene is decorated by MAPbBr3 NCs. Furthermore, the effect of ambient moisture was extensively studied, showing that the use of perovskite NCs in gas sensors can become a promising alternative to other gas sensitive materials, due to the protective character of graphene, resulting from its high hydrophobicity. Besides, a gas sensing mechanism is proposed to understand the effects of MAPbBr3 sensing properties, This work was funded in part by MINECO, MICINN and FEDER via grants no. RTI2018-101580-B-I00, by AGAUR under grant. 2017SGR 418 J.C.C gratefully acknowledges a doctoral fellowship from URV under the Marti i Franques fellowship program. E.L. is supported by the Catalan institution for Research and Advanced Studies via the 2012 and 2018 Editions of the ICREA Academia Award. P.A. acknowledges the financial support from the Spanish Government through 'Severo Ochoa"(SEV-2016-0683, MINECO) and PGC2018-099744-B-I00 (MCIU/AEI/FEDER, UE), and R.G.A. acknowledges FPI scholarship the Spanish Government-MINECO for a (TEC2015-74405-JIN), MAT2015-69669-P.

Colloidal cesium lead halide perovskite nanocrystals exhibit unique photophysical properties including high quantum yields, tunable emission colors, and narrow photoluminescence spectra that have marked them as promising light emitters for applications in diverse photonic devices. Randomly oriented transition dipole moments have limited the light outcoupling efficiency of all isotropic light sources, including perovskites. In this report we design and synthesize deep blue emitting, quantum confined, perovskite nanoplates and analyze their optical properties by combining angular emission measurements with back focal plane imaging and correlating the results with physical characterization. By reducing the dimensions of the nanocrystals and depositing them face down onto a substrate by spin coating, we orient the average transition dipole moment of films into the plane of the substrate and improve the emission properties for light emitting applications. We then exploit the sensitivity of the perovskite electronic transitions to the dielectric environment at the interface between the crystal and their surroundings to reduce the angle between the average transition dipole moment and the surface to only 14° and maximize potential light emission efficiency. This tunability of the electronic transition that governs light emission in perovskites is unique and, coupled with their excellent photophysical properties, introduces a valuable method to extend the efficiencies and applications of perovskite based photonic devices beyond those based on current materials.

The synthesis of hydroxylammonium lead iodide NH3OHPbI3 was accomplished by means of the reaction between water solutions of HI and NH2OH with PbI2 in sulfolane in conjunction with either crystallization by CH2Cl2 vapor diffusion or sulfolane extraction with toluene. The appropriate choice of the solvent was found to be crucial in order to attain the desired material. The synthesized compound was extensively characterized by single crystal and powder X-ray diffraction, UV-Vis diffuse reflectance spectroscopy, FT-IR spectroscopy, 1H-NMR spectroscopy, TG-DTA-QMS EGA (Evolved Gas Analysis), ESI-MS, and CHNS analysis. NH3OHPbI3 is an extremely reactive, deliquescent solid that easily oxidizes in air releasing iodine. Furthermore, it is the first reported perovskite to melt (m.p. around 80 °C) before decomposing exothermally at 103 °C. Such a chemical behavior, together with its optical absorption properties (i.e. yellow-colored perovskite), renders this material totally unsuitable for photovoltaic applications. The deliquescence of the material is to be ascribed to the strong hydrophilicity of hydroxylammonium ion. On the other hand, the relatively high Bronsted acidity of hydroxylammonium (pKa = 5.97) compared to other ammonium cations, promotes the reduction of atmospheric oxygen to water and the NH3OHPbI3 oxidation. The crystal structure, determined by single crystal X-ray diffraction with synchrotron radiation, is orthorhombic, but differs from the prototypal perovskite structure. Indeed it comprises infinite chains of face-sharing PbI6 octahedra along the c-axis direction with hydroxylammonium cations positioned between the columns, forming layers on the ac plane. The solvent intercalates easily between the layers. The crystal structure is apparently anomalous considering that the expected Goldschmidt's tolerance factor for the system (0.909) lies in the range of a stable prototypal perovskite structure. Therefore, the strong hydrogen bond forming tendency of hydroxylamine is likely to account for the apparent structural anomaly.

Hybrid lead halide perovskite (PS) derivatives have emerged as very promising materials for the development of optoelectronic devices in the last few years. At the same time, inorganic nanocrystals with quantum confinement (QDs) possess unique properties that make them suitable materials for the development of photovoltaics, imaging and lighting applications, among others. In this work, we report on a new methodology for the deposition of high quality, large grain size and pinhole free PS films (CH3NH3PbI3) with embedded PbS and PbS/CdS core/shell Quantum Dots (QDs). The strong interaction between both semiconductors is revealed by the formation of an exciplex state, which is monitored by photoluminescence and electroluminescence experiments. The radiative exciplex relaxation is centered in the near infrared region (NIR), ≈1200 nm, which corresponds to lower energies than the corresponding band gap of both perovskite (PS) and QDs. Our approach allows the fabrication of multi-wavelength light emitting diodes (LEDs) based on a PS matrix with embedded QDs, which show considerably low turn-on potentials. The presence of the exciplex state of PS and QDs opens up a broad range of possibilities with important implications in both LEDs and solar cells. Generalitat Valenciana PROMETEOII/2014/020 PROMETEOII/2014/059 ISIC/2012/008 Spanish MINECO (Ministry of Economy and Competitiveness) MAT2013-47192-C3-1-R MAT2015-70611-ERC TEC2014-53727-C2-1-R Spanish MINECO

Organo-lead halide perovskites have recently attracted great interest for potential applications in thin-film photovoltaics and optoelectronics. Herein, we present a protocol for the fabrication of this material via the low-pressure vapor assisted solution process (LP-VASP) method, which yields ~19% power conversion efficiency in planar heterojunction perovskite solar cells. First, we report the synthesis of methylammonium iodide (CH(3)NH(3)I) and methylammonium bromide (CH(3)NH(3)Br) from methylamine and the corresponding halide acid (HI or HBr). Then, we describe the fabrication of pinhole-free, continuous methylammonium-lead halide perovskite (CH(3)NH(3)PbX(3) with X = I, Br, Cl and their mixture) films with the LP-VASP. This process is based on two steps: i) spin-coating of a homogenous layer of lead halide precursor onto a substrate, and ii) conversion of this layer to CH(3)NH(3)PbI(3-x)Br(x) by exposing the substrate to vapors of a mixture of CH(3)NH(3)I and CH(3)NH(3)Br at reduced pressure and 120 °C. Through slow diffusion of the methylammonium halide vapor into the lead halide precursor, we achieve slow and controlled growth of a continuous, pinhole-free perovskite film. The LP-VASP allows synthetic access to the full halide composition space in CH(3)NH(3)PbI(3-x)Br(x) with 0 ≤ x ≤ 3. Depending on the composition of the vapor phase, the bandgap can be tuned between 1.6 eV ≤ E(g )≤ 2.3 eV. In addition, by varying the composition of the halide precursor and of the vapor phase, we can also obtain CH(3)NH(3)PbI(3-x)Cl(x). Films obtained from the LP-VASP are reproducible, phase pure as confirmed by X-ray diffraction measurements, and show high photoluminescence quantum yield. The process does not require the use of a glovebox.

International audience; Revealing the crystal structure of lead halide perovskite nanocrystals is essential for the optimization of stability of these emerging materials in applications such as solar cells, photodetectors and light emitting devices. We use magneto-photoluminescence spectroscopy of individual perovskite CsPbBr3 nanocrystals as a unique tool to determine their crystal structure, which imprints distinct signatures in the excitonic sublevels of charge complexes at low temperatures. At zero magnetic field, the identification of two classes of photoluminescence spectra, displaying either two or three sublevels in their exciton fine structure, shows evidence for the existence of two crystalline structures, namely tetragonal D4h and orthorhombic D2h phases. Magnetic field shifts, splitting and coupling of the sublevels provide a determination of the diamagnetic coefficient and valuable information on the exciton g-factor and its anisotropic character. Moreover, this spectroscopic study reveals the optical properties of charged excitons and allows the extraction of the electron and hole g-factors for perovskite systems.

Several aspects on the photophysical characterization of lead halide hybrid organic inorganic perovskite solar cells remain unsolved. It has been observed that ionic transport and polarization of the interfaces cause very slow changes that interfere with transient measurements with effects that cannot be separated from recombination kinetics. Here we establish a protocol of initial measurement of the solar cell that provides information on recombination characteristics prior to applying any voltage cycling. The photovoltaic device is measured by several methods (photovoltage versus light intensity, open-circuit voltage decay, and impedance spectroscopy) while minimizing the exposure to external voltage stimulus to avoid ionic migration to the contacts. Results are independently confirmed by the analysis of samples with interdigitated electrodes. We show that the high-efficiency perovskite solar cells behave very closely to a bulk recombination ideal photovoltaic model. However, when voltage is scanned to determine current density voltage curves and impedance spectroscopy at fixed illumination intensity, the cell undergoes significant changes, which we attribute to a dominance of recombination at contacts that have been modified by ionic polarization. Our method provides an effective approach for determining quantitatively the rather significant changes that occur to perovskite solar cells during standard measurements, such as current voltage curves.

Lead halide perovskites have recently been used as light absorbers in hybrid organic–inorganic solid-state solar cells, with efficiencies as high as 15% and open-circuit voltages of 1 V. However, a detailed explanation of the mechanisms of operation within this photovoltaic system is still lacking. Here, we investigate the photoinduced charge transfer processes at the surface of the perovskite using time-resolved techniques. Transient laser spectroscopy and microwave photoconductivity measurements were applied to TiO2 and Al2O3 mesoporous films impregnated with CH3NH3PbI3 perovskite and the organic hole-transporting material spiro-OMeTAD. We show that primary charge separation occurs at both junctions, with TiO2 and the hole-transporting material, simultaneously, with ultrafast electron and hole injection taking place from the photoexcited perovskite over similar timescales. Charge recombination is shown to be significantly slower on TiO2 than on Al2O3 films.

Organic–inorganic metal halide perovskites have recently demonstrated outstanding efficiencies in photovoltaics as well as highly promising performances for a wide range of optoelectronic applications such as lasing, light emission, optical detectors, and even for radiation detection. Key to the realization of functional perovskite micro/nanosystems on the ubiquitous silicon optoelectronics platform is through sophisticated lithography. Despite the rapid progress made in halide perovskite lasing, direct lithographic patterning of perovskite films to form optical cavities on conventional substrates remains extremely challenging. This study realizes room‐temperature high‐quality factor whispering‐gallery‐mode lasing (Q ≈ 1210) from patterned lead halide perovskite microplatelets fabricated in periodic arrays on silicon substrate with micropatterned BN film as the buffer layer. By varying the size of the platelets, modal selectivity for single mode lasing can be achieved with different cavity sizes or by simply breaking the structural symmetry of the cavity through designing the pattern. Importantly, this work demonstrates a straightforward, versatile bottom‐up scalable strategy to realize high‐quality periodic perovskite arrays with variable cavity sizes for large‐area light‐emitting and optical gain applications.

The use of a solution process to grow perovskite thin films allows to extend the material processability. It is known that the physicochemical properties of the perovskite material can be tuned by altering the solution precursors as well as by controlling the crystal growth of the film. This advancement necessarily implies the need for an understanding of the kinetic phenomena for the thin-film formation. Therefore, in this work we review the state of the art of perovskite hybrid crystal growth, starting from a comprehensive theoretical description towards broad experimental investigations. One part of the study focuses on rapid thermal annealing as a tool to control nucleation and crystal growth. We deduce that controlling crystal growth with high-precision photonic sintering simplifies the experimental framework required to understand perovskite crystallization. These types of synthesis methods open a new empirical parameter space. All this knowledge serves to improve the perovskite synthesis and the thin films' quality, which will result in higher device performances.

Highly photoluminescent hybrid lead halide perovskite nanoparticles have recently attracted wide interest in the context of high-stake applications, such as light emitting diodes (LEDs), light emitting transistors and lasers. In addition, they constitute ideal model systems to explore energy and charge transport phenomena occurring at the boundaries of nanocrystalline grains forming thin films in high-efficiency perovskite solar cells (PSCs). Here we report a complete photophysical study of CH3NH3PbBr3 perovskite nanoparticles suspended in chlorobenzene and highlight some important interaction properties. Colloidal suspensions under study were constituted of dispersed aggregates of quasi-2D platelets of a range of thicknesses, decorated with 3D-like spherical nanoparticles. These types of nanostructures possess different optical properties that afford a handle for probing them individually. The photophysics of the colloidal particles was studied by femtosecond pump-probe spectroscopy and time-correlated single-photon counting. We show here that a cascade of energy and exciton-mediated charge transfer occurs between nanostructures: upon photoexcitation, localized excitons within one nanostructure can either recombine on a ps timescale, yielding a short-lived emission, or form charge-transfer states (CTSs) across adjacent domains, resulting in longer-lived photoluminescence in the millisecond timescale. Furthermore, CTSs exhibit a clear signature in the form of a strong photoinduced electroabsorption evidenced in femtosecond transient absorption measurements. Charge transfer dynamics at the surface of the nanoparticles have been studied with various quenchers in solution. Efficient hole transfer to N,N,N′,N′-tetrakis(4-methoxyphenyl)benzidine (MeO-TPD) and 1,4-bis(diphenyl-amino)benzene (BDB) donors was attested by the quenching of the nanoparticles emission. The charge transfer rate was limited by the organic layer used to stabilize the nanoparticles, which acted as a wide spacer between reactants. The forward charge transfer was found to take place in the sub-microsecond time-scale in competition with slow carrier recombination, while back transfer was shown to occur with a time-constant τ = 25 ms., Chemical Science, 8 (6), ISSN:2041-6520, ISSN:2041-6539

The high power conversion efficiency of perovskite solar cells reaching 25.5% today [1], raises the question on the relation between the photovoltaic performance and physical nature of these materials. To get a better understanding of the properties, good quality single crystals are highly demanded. In this paper we report on large single crystals of formamidinium lead halide grown by a combination of inverse temperature crystallization and seed growth. Structure, phase transitions, dielectric and optical properties of the single crystals are shown. published

Lead halide perovskites are intriguing semiconductors for lasers due to high quantum yield, tunable bandgaps and facile solution-process ability. However, limited by the weak optical confinement, continuous-wave (CW) pumped lasing, as one prerequisite for the electrically pumped lasing, is still challenging in bare lead halide perovskites without high-quality factor (Q) artificial optical cavity. Herein, we report the lasing emission in methylammonium lead tribromide (MAPbBr3) incorporated with a vertical microcavity under continuous pumping at 80 K. The single-crystalline MAPbBr3 perovskite nanoplates were fabricated by the two-step solution method. The MAPbBr3 based vertical cavity surface emitting laser (VCSEL) presents a low threshold of 55.2 W cm-2 and a high Q-factor of 1140 at low temperature. The low threshold lasing emission can be attributed to strong optical confinement in the high-Q cavity and great PL enhancement at 80 K, which is induced by a transition from tetragonal to orthorhombic phase, demonstrated by in-situ temperature Raman spectroscopy. These findings envisage the prospective applications of single-crystalline metal halide perovskites in practicable laser devices. Ministry of Education (MOE) National Research Foundation (NRF)