Spectator Exciton Effects in Nanocrystals III: Unveiling the Stimulated Emission Cross Section in Quantum Confined CsPbBr Nanocrystals.

Quantifying stimulated emission in semiconductor nanocrystals (NCs) remains challenging due to masking of its effects on pump-probe spectra by excited state absorption and ground state bleaching signals. The absence of this defining photophysical parameter in turn impedes assignment of band edge electronic structure in many of these important fluorophores. Here we employ a generally applicable 3-pulse ultrafast spectroscopic method coined the "Spectator Exciton" (SX) approach to measure stimulated-emission efficiency in quantum confined inorganic perovskite CsPbBr NCs, the band edge electronic structure of which is the subject of lively ongoing debate.

Enhanced LED Performance by Ion Migration in Multiple Quantum Well Perovskite.

Here we study the effect of ion migration on the performance of perovskite light emitting diodes (PeLEDs). We compared aromatic and linear barrier molecules in Ruddlesden-Popper and Dion-Jacobson two-dimensional perovskites having multiple quantum well (MQW) structures. PeLED devices were fabricated by using the same conditions and architecture, while their electroluminescence properties and ion migration behavior were investigated.

Phase Segregation Mechanisms in Mixed-Halide CsPb(BrI) Nanocrystals in Dependence of Their Sizes and Their Initial [Br]:[I] Ratios.

Phase segregation in inorganic CsPb(BrI) nanoparticles (NPs) exhibiting originally a homogeneous [Br]:[I] mixture was investigated by means of in situ transmission electron microscopy (TEM) and evaluated by using multivariate analyses. The colloidal synthesis of the NPs offers good control of the halide ratios on the nanoscale. The spatially resolved TEM investigations were correlated with integral photoluminescence measurements.

A systematic discrepancy between the short circuit current and the integrated quantum efficiency in perovskite solar cells.

Halide perovskites solar cells are now approaching commercialisation. In this transition from academic research towards industrialisation, standardized testing protocols and reliable dissemination of performance metrics are crucial. In this study, we analyze data from over 16,000 publications in the Perovskite Database to investigate the assumed equality between the integrated external quantum efficiency and the short circuit current from JV measurements.

In Situ Intrinsic Self-Healing of Low Toxic Cs ZnX (X = Cl, Br) Metal Halide Nanoparticles.

This study reports on the intrinsic and fast self-healing ability of all inorganic, low-toxic Cs ZnX (X = Cl, Br) metal halide nanoparticles (NPs) when subjected to local heating by electron beam irradiation in high-resolution transmission electron microscopy (HR-TEM). The local heating induces the creation of nanoshells (NSs) following the template of the corresponding NPs, which are subsequently healed back to their original state within several minutes. Energy dispersive spectroscopy (EDS) and fast Fourier transform (FFT) analysis reveal that the composition, phase, and crystallographic structure of the original NPs are restored during the self-healing process, with a thin crystalline layer observed at the bottom of the NSs acting as the healing template.

Formamidinium Halide Perovskite and Carbon Nitride Thin Films Enhance Photoreactivity under Visible Light Excitation.

Photochemical and photocatalytic activity of adsorbates on surfaces is strongly dependent on the nature of a given substrate and its resonant absorption of the (visible) light excitation. An observation is reported here of the visible light photochemical response of formamidinium lead bromide (FAPbBr) halide perovskite and carbon nitride (CN) thin-film materials (deposited on a SiO/Si(100) substrate), both of which are known for their photovoltaic and photocatalytic properties. The goal of this study was to investigate the role of the substrate in the photochemical reactivity of an identical probe molecule, ethyl chloride (EC), when excited by pulsed 532 nm laser under ultrahigh vacuum (UHV) conditions.

Semitransparent Perovskite Solar Cells with > 13% Efficiency and 27% Transperancy Using Plasmonic Au Nanorods.

Semitransparent hybrid perovskites open up applications in windows and building-integrated photovoltaics. One way to achieve semitransparency is by thinning the perovskite film, which has several benefits such as cost efficiency and reduction of lead. However, this will result in a reduced light absorbance; therefore, to compromise this loss, it is possible to incorporate plasmonic metal nanostructures, which can trap incident light and locally amplify the electromagnetic field around the resonance peaks.

Critical Role of Removing Impurities in Nickel Oxide on High-Efficiency and Long-Term Stability of Inverted Perovskite Solar Cells.

The performance enhancement of inverted perovskite solar cells applying nickel oxide (NiO ) as the hole transport layer (HTL) has been limited by impurity ions (such as nitrate ions). Herein, we have proposed a strategy to obtain high-quality NiO nanoparticles via an ionic liquid-assisted synthesis method (NiO -IL). Experimental and theoretical results illustrate that the cation of the ionic liquid can inhibit the adsorption of impurity ions on nickel hydroxide through a strong hydrogen bond and low adsorption energy, thereby obtaining NiO -IL HTL with high conductivity and strong hole-extraction ability.

Electrical and chemical properties of vacancy-ordered lead free layered double perovskite nanoparticles.

In this work we synthesized vacancy-ordered lead-free layered double perovskite (LDP) nanoparticles. This structure consists of two layers of trivalent metal halide octahedra [B(III)X] separated by a layer of divalent metal [B(II)X] (B is a divalent or trivalent metal). The chemical formula of this structure is based on AB(II)B(III)X where A is Cs, B(III) is Bi, X is Cl and B(II) is a different ratio between Mn and Cd.

Fabrication of Perovskite Solar Cells with Digital Control of Transparency by Inkjet Printing.

Semitransparency is an attractive and important property in solar cells since it opens new possibilities in a variety of applications such as tandem cell configuration and building-integrated photovoltaics. Metal halide perovskite has the optimal properties to function as the light harvester in solar cells and can be made as a thin film, while its chemical composition can change its band gap. However, achieving high transparency usually compromises the solar cell's efficiency.

Unusually Strong Biexciton Repulsion Detected in Quantum Confined CsPbBr Nanocrystals with Two and Three Pulse Femtosecond Spectroscopy.

Transient absorption measurements were conducted on pristine and monoexciton saturated CsPbBr nanocrystals varying in size within the regime of a strong quantum confinement. Once the difference spectra were translated to absolute transient changes in absorption cross section, a single exciton is shown to completely bleach the band edge absorption peak and induce a new absorption roughly two times weaker ∼100 meV to the blue. Difference spectra obtained during Auger recombination of biexciton demonstrate that the addition of a second exciton, rather than double the effect of a first, bleaches the blue-induced absorption band without producing a net stimulated emission at the band edge.

Green energy by recoverable triple-oxide mesostructured perovskite photovoltaics.

Perovskite solar cells have developed into a promising branch of renewable energy. A combination of feasible manufacturing and renewable modules can offer an attractive advancement to this field. Herein, a screen-printed three-layered all-nanoparticle network was developed as a rigid framework for a perovskite active layer.

Effect of Perovskite Thickness on Electroluminescence and Solar Cell Conversion Efficiency.

A hybrid organic-inorganic perovskite in a diode structure can lead to multifunctional device phenomena exhibiting both a high power conversion efficiency (PCE) of a solar cell and strong electroluminescence (EL) efficiency. Nonradiative losses in such multifunctional devices lead to an open circuit voltage () deficit, which is a limiting factor for pushing the efficiency toward the Shockley-Queisser limit. In this work, we analyze and quantify the radiative limit of in a perovskite solar cell as a function of its absorber thickness.

Electrical Characterization of Individual Cesium Lead Halide Perovskite Nanowires Using Conductive AFM.

Perovskite nanostructures have attracted much attention in recent years due to their suitability for a variety of applications such as photovoltaics, light-emitting diodes (LEDs), nanometer-size lasing, and more. These uses rely on the conductive properties of these nanostructures. However, electrical characterization of individual, thin perovskite nanowires has not yet been reported.

Formation of Semiconducting Supramolecular Fullerene Aggregates in a Dipeptide Organogel.

The formation of nanostructured fullerene aggregate inside supramolecular gels is a topic of great interest because of the potential applications of these nanostructures in photoelectronics. Gel phase facilitates the aggregation of fullerene by restricting the movement of solvent molecules, thereby increasing the local fullerene concentration. Herein, the supramolecular nanostructure formation of fullerene (C) in a minimalistic dipeptide (diphenylalanine, FF) organogel is reported.

Affecting an Ultra-High Work Function of Silver.

An ultra-high increase in the WF of silver, from 4.26 to 7.42 eV, that is, an increase of up to circa 3.

Dion-Jacobson Two-Dimensional Perovskite Solar Cells Based on Benzene Dimethanammonium Cation.

Organic-inorganic perovskite structured compounds have recently emerged as attractive materials in the fields of photovoltaic due to their exciting optical properties and easy syntheses, as well as exceptional structural and optical tunability. This work presents a Dion-Jacobson two-dimensional (2D) perovskite using diammonium as the barrier molecule. We show that the diammonium barrier molecule is responsible for the perovskite layers' orientation supported by Hall Effect measurements, which results in a high efficiency solar cell for 2D perovskite without the need for additives or any additional treatment.

Cell refinement of CsPbBr perovskite nanoparticles and thin films.

In this work, we performed a detailed study of the phase transformations and structural unit cell parameters of CsPbBr nanoparticles (NPs) and thin films. X-ray diffraction patterns were acquired as a function of temperature, where the positions and widths of the diffraction peaks were systematically tracked upon heating and cooling down to room temperature (RT). Scanning electron microscopy provides physical insight on the CsPbBr thin films upon annealing and transmission electron microscopy gives physical and crystallographic information for the CsPbBr NPs using electron diffraction.

Fine-tuning of the metal work function by molecular doping.

A new approach for fine tuning of the metal work function (WF) in the range of 1 eV is described. WF control is achieved by 3D molecular doping of the metal rather than the classical 2D adsorption. Both small molecules (Congo red, thionine) and polymers (Nafion, poly(vinylbenzyltrimethylammonium)chloride) were shown to affect the work function of gold and silver.

CsPbBr and CHNHPbBr promote visible-light photo-reactivity.

Inorganic and organic lead halide perovskite materials attract great interest in the scientific community because of their potential for low-cost, high efficiency solar cells. In this report we add a new property of these materials, namely their photochemical activity in the visible light range. Both inorganic (CsPbBr3) and organic (CH3NH3PbBr3-MAPbBr3) perovskite thin films were demonstrated to promote photo-dissociation of adsorbed ethyl chloride (EC), employing 532 nm pulsed laser irradiation under ultra-high vacuum (UHV) conditions.

Reflectivity Effects on Pump-Probe Spectra of Lead Halide Perovskites: Comparing Thin Films versus Nanocrystals.

Due to the sizable refractive index of lead halide perovskites, reflectivity off their interface with air exceeds 15%. This has prompted a number of investigations into the prominence of photoreflective contributions to pump-probe data in these materials, with conflicting results. Here we report experiments aimed at assessing this by comparing transient transmission from lead halide perovskite films and weakly quantum confined nanocrystals of cesium lead iodide (CsPbI) perovskite.

Near ultra-violet to mid-visible band gap tuning of mixed cation RbCsPbX (X = Cl or Br) perovskite nanoparticles.

One of the most attractive features of perovskite materials is their chemical flexibility. Due to innovative chemical compositions of perovskites, their optical and structural properties, and functionalities have become more advanced, enabling better solar performance in photovoltaics, as well as robustness and excellent properties in the nanoscale for optoelectronics. The quest for novel perovskite compositions in the nano-scale is significantly important.

Free Carrier Emergence and Onset of Electron-Phonon Coupling in Methylammonium Lead Halide Perovskite Films.

Sub-10 fs resolution pump-probe experiments on methylammonium lead halide perovskite films are described. Initial response to photoexcitation is assigned to localized hot excitons which dissociate to free carriers. This is attested to by band integrals of the pump-probe spectra where photoinduced bleaching rises abruptly 20 fs after photoexcitation.

Low-Dimensional Organic-Inorganic Halide Perovskite: Structure, Properties, and Applications.

Three-dimensional (3 D) perovskite has attracted a lot of attention owing to its success in photovoltaic (PV) solar cells. However, one of its major crucial issues lies in its stability, which has limited its commercialization. An important property of organic-inorganic perovskite is the possibility of forming a layered material by using long organic cations that do not fit into the octahedral cage.

Methylammonium-Mediated Evolution of Mixed-Organic-Cation Perovskite Thin Films: A Dynamic Composition-Tuning Process.

Methylammonium-mediated phase-evolution behavior of FA MA PbI mixed-organic-cation perovskite (MOCP) is studied. It is found that by simply enriching the MOCP precursor solutions with excess methylammonium cations, the MOCPs form via a dynamic composition-tuning process that is key to obtaining MOCP thin films with superior properties. This simple chemical approach addresses several key challenges, such as control over phase purity, uniformity, grain size, composition, etc.