Disentangling Second Harmonic Generation from Multiphoton Photoluminescence in Halide Perovskites using Multidimensional Harmonic Generation.

Layered two-dimensional Ruddlesden-Popper (RP) halide perovskites are an intriguing class of semiconductors being explored for their linear and nonlinear optical and ferroelectric properties. Second harmonic generation (SHG) is commonly used to screen for noncentrosymmetric and ferroelectric materials. However, SHG measurements of perovskites can be obscured by their intense multiphoton photoluminescence (mPL). Here, we apply multidimensional harmonic generation as a method to eliminate the complications from mPL. By scanning and correlating both excitation and emission frequencies, we unambiguously assess whether a material supports SHG by examining if an emission feature scales as twice the excitation frequency. Measurements of a series of n = 2, 3 RP perovskites reveal that, contrary to previous belief, n-butylammonium (BA) RP perovskites are not SHG-active and thus centrosymmetric, but RP perovskites with phenylethylammonium (PEA) and 2-thiophenemethylammonium (TPMA) spacer cations display SHG. This work establishes multidimensional harmonic generation as a definitive method to measure SHG in halide perovskites.

Reversible luminescent humidity chromism of organic-inorganic hybrid PEA2MnBr4 single crystals.

Organic-inorganic hybrids have drawn great attention for gas sensors due to their high sensitivity, good selectivity and acceptable stability at room temperature. There are two main approaches by which organic-inorganic hybrids convert gas information to electric or optical signals (vapochromism). Here, we have reported a new organic-inorganic hybrid PEA2MnBr4 for humidity detection by luminescent visible chromism. PEA2MnBr4 single crystals were grown by the solution method and determined by single-crystal X-ray diffraction. Luminescent humidity chromism was found on PEA2MnBr4 from green emission at the water-desorption state to pink emission at the water-adsorption state within 18 s at a relative humidity of 38% RH. This obviously visible chromism was further used to check the water content in toluene with a low detection limit between 0.02 and 0.05 vol%.

Inorganic antimony halide hybrids with broad yellow emissions.

Lead halide perovskites exhibit unexceptionable photoelectric properties. However, these materials are unsatisfactory in terms of stability and toxicity. Herein, we report Rb7Sb3Cl16 as a new kind of lead free perovskite variants. This material can be easily obtained through hydrothermal reactions. The composition is determined through structure refinement, elemental analysis and X-ray photoelectron spectra. Rb7Sb3Cl16 exhibits a broad yellow emission at 560 nm, with a Stokes shift of 175 nm and a photoluminescence quantum yield (PLQY) around 26%. Rb7Sb3Cl16 also shows good thermal and water stability due to its inorganic composition. White light-emitting diodes (LEDs) are constructed by combining Rb7Sb3Cl16 as yellow phosphors, our previously reported Cs2SnCl6:2.75%Bi as blue phosphors, and commercial UV LED chips as the excitation source, producing a white light with the Commission Internationale de'Eclairage (CIE) color coordinates at (0.39, 0.38).

Efficient and stable emission of warm-white light from lead-free halide double perovskites.

Lighting accounts for one-fifth of global electricity consumption1. Single materials with efficient and stable white-light emission are ideal for lighting applications, but photon emission covering the entire visible spectrum is difficult to achieve using a single material. Metal halide perovskites have outstanding emission properties2,3; however, the best-performing materials of this type contain lead and have unsatisfactory stability. Here we report a lead-free double perovskite that exhibits efficient and stable white-light emission via self-trapped excitons that originate from the Jahn-Teller distortion of the AgCl6 octahedron in the excited state. By alloying sodium cations into Cs2AgInCl6, we break the dark transition (the inversion-symmetry-induced parity-forbidden transition) by manipulating the parity of the wavefunction of the self-trapped exciton and reduce the electronic dimensionality of the semiconductor4. This leads to an increase in photoluminescence efficiency by three orders of magnitude compared to pure Cs2AgInCl6. The optimally alloyed Cs2(Ag0.60Na0.40)InCl6 with 0.04 per cent bismuth doping emits warm-white light with 86 ± 5 per cent quantum efficiency and works for over 1,000 hours. We anticipate that these results will stimulate research on single-emitter-based white-light-emitting phosphors and diodes for next-generation lighting and display technologies.

Aqueous Synthesis of Lead Halide Perovskite Nanocrystals with High Water Stability and Bright Photoluminescence.

Lead halide perovskite nanocrystals (NCs) have attracted intense attention because of their excellent optoelectronic properties. The ionic nature of halide perovskites makes them highly vulnerable to water. Encapsulation of perovskite NCs with inorganic or organic materials has been reported to enhance their stability; however, they often suffer from large aggregation size, low water solubility, and difficulty for further surface functionalization. Here, we report a facile aqueous process to synthesize water-soluble CsPbBr3/Cs4PbBr6 NCs with the assistance of a fluorocarbon agent (FCA), which features a novel mechanism of the perovskite crystallization at the oil/water interface and direct perovskite NCs/FCA self-assembly in an aqueous environment. The products exhibit a high absolute photoluminescence quantum yield (PLQY) of ∼80% in water with the PL lasting for weeks. Through successive ionic layer adsorption and reaction, BaSO4 was further applied to encapsulate the NCs, which greatly enhanced their stability in phosphate-buffered saline solutions. The high stability in water and saline solution, high PLQY, and tunable emission wavelength, together with the successful demonstration of brain tissue labeling and PL under X-ray excitation, make our perovskite NCs a promising choice for X-ray fluorescent biolabels.

Surface Passivation of Bismuth-Based Perovskite Variant Quantum Dots To Achieve Efficient Blue Emission.

Metal halide perovskite quantum dots (QDs) recently have attracted great research attentions. However, blue-emitting perovskite QDs generally suffer from low photoluminescence quantum yield (PLQY) because of easily formed defects and insufficient surface passivation. Replacement of lead with low toxicity elements is also preferred toward potential commercial applications. Here, we apply Cl-passivation to boost the PLQY of MA3Bi2Br9 QDs to 54.1% at the wavelength of 422 nm, a new PLQY record for blue emissive, lead-free perovskite QDs. Because of the incompatible crystal structures between MA3Bi2Br9 and MA3Bi2Cl9 and the careful kinetic control during the synthesis, Cl- anions are engineered to mainly locate on the surface of QDs acting as passivating ligands, which effectively suppress surface defects and enhance the PLQY. Our results highlight the potential of MA3Bi2Br9 QDs for applications of phosphors, scintillators, and light-emitting diodes.

Lead-Free, Blue Emitting Bismuth Halide Perovskite Quantum Dots.

Lead halide perovskite quantum dots (QDs) are promising candidates for future lighting applications, due to their high quantum yield, narrow full width at half maximum (FWHM), and wide color gamut. However, the toxicity of lead represents a potential obstacle to their utilization. Although tin(II) has been used to replace lead in films and QDs, the high intrinsic defect density and oxidation vulnerability typically leads to unsatisfactory material properties. Bismuth, with much lower toxicity than lead, is promising to constitute lead-free perovskite materials because Bi3+ is isoelectronic to Pb2+ and more stable than Sn2+ . Herein we report, for the first time, the synthesis and optical characterization of MA3 Bi2 Br9 perovskite QDs with photoluminescence quantum yield (PLQY) up to 12 %, which is much higher than Sn-based perovskite nanocrystals. Furthermore, the photoluminescence (PL) peaks of MA3 Bi2 X9 QDs could be easily tuned from 360 to 540 nm through anion exchange.

Hydrazine solution processed Sb2S3, Sb2Se3 and Sb2(S(1-x)Se(x))3 film: molecular precursor identification, film fabrication and band gap tuning.

Sb2(S(1-x)Se(x))3 (0 ≤ x ≤ 1) compounds have been proposed as promising light-absorbing materials for photovoltaic device applications. However, no systematic study on the synthesis and characterization of polycrystalline Sb2(S(1-x)Se(x))3 thin films has been reported. Here, using a hydrazine based solution process, single-phase Sb2(S(1-x)Se(x))3 films were successfully obtained. Through Raman spectroscopy, we have investigated the dissolution mechanism of Sb in hydrazine: 1) the reaction between Sb and S/Se yields [Sb4S7](2-)/[Sb4Se7](2-) ions within their respective solutions; 2) in the Sb-S-Se precursor solutions, Sb, S, and Se were mixed on a molecular level, facilitating the formation of highly uniform polycrystalline Sb2(S(1-x)Se(x))3 thin films at a relatively low temperature. UV-vis-NIR transmission spectroscopy revealed that the band gap of Sb2(S(1-x)Se(x))3 alloy films had a quadratical relationship with the Se concentration x and it followed the equation Eg(x) = 0.118x(2) - 0.662x + 1.621eV, where the bowing parameter was 0.118 eV. Our study provides a valuable guidance for the adjustment and optimization of the band gap in hydrazine solution processed Sb2(S(1-x)Se(x))3 alloy films for the future fabrication of improved photovoltaic devices.

Thermal evaporation and characterization of Sb2Se3 thin film for substrate Sb2Se3/CdS solar cells.

Sb2Se3 is a promising absorber material for photovoltaic cells because of its optimum band gap, strong optical absorption, simple phase and composition, and earth-abundant and nontoxic constituents. However, this material is rarely explored for photovoltaic application. Here we report Sb2Se3 solar cells fabricated from thermal evaporation. The rationale to choose thermal evaporation for Sb2Se3 film deposition was first discussed, followed by detailed characterization of Sb2Se3 film deposited onto FTO with different substrate temperatures. We then studied the optical absorption, photosensitivity, and band position of Sb2Se3 film, and finally a prototype photovoltaic device FTO/Sb2Se3/CdS/ZnO/ZnO:Al/Au was constructed, achieving an encouraging 2.1% solar conversion efficiency.