Investigation of the Role of K2SO4 Electrolyte in Hole Transport Layer for Efficient Quasi-2D Perovskite Light-Emitting Diodes.

Quasi-two-dimensional (2D) perovskite light-emitting diodes are promising light sources for color display and lighting. However, poor carrier injection and transport between the bottom hole transport layer (HTL) and perovskite limit the device performance. Here we demonstrate a simple and effective way to modify the HTL for enhancing the performance of perovskite light-emitting diodes (PeLEDs). An electrolyte K2SO4 is used to mix with poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as the hole transport layer. The K+ doping helped the quasi-2D perovskite phases grow vertically along the interface of the PEDOT:PSS, fine-modulate the phase distribution, and simultaneously reduce the defect density of quasi-2D perovskites. It also significantly reduced the exciton quenching and injection barrier at PEDOT:PSS and quasi-2D perovskite interface. The optimized green PeLEDs with the K2SO4 doped PEDOT:PSS HTL showed a maximum luminance of 17185 cd/m2 which is almost 4.7 times brighter than the control one, with a maximum external quantum efficiency of 18.64%.

Impact of Bychkov-Rashba Spin Splitting on Dual Emissions for Lead Halide Perovskite Nanowires.

Bychkov-Rashba spin-orbit coupling (SOC) is decisive for photoinduced photoluminescence (PL) in terms of double emissions. It turns out to be remarkable for one-dimensional lead halide perovskite nanowires (PeNWs). This is primarily due to large surface to volume ratios and structural symmetry breaking fields in the reduced dimension. Systematic studies of the effect of Rashba SOC on PL and its discrimination with the self-trapped exciton in wide temperature and illumination intensity ranges are considerably important and, heretofore, have not been performed. Here, highly crystalline methylammonium lead triiodine (MAPbI3) PeNWs are demonstrated to be able to produce remarkable dual emissions at low temperatures. With extensive analyses by a photoelectrical device-based spin-photogalvanic effect and magnetophotoluminescence, the Rashba effect is proven to be the only factor that governs the dual emissions. We believe a complete understanding of the PL character of PeNWs is beneficial for the development of novel perovskite nanophotonic devices.

Packing-Shape Effects of Optical Properties in Amplified Spontaneous Emission through Dynamics of Orbit-Orbit Polarization Interaction in Hybrid Perovskite Quantum Dots Based on Self-Assembly.

This paper reports packing-shape effects of amplified spontaneous emission (ASE) through orbital polarization dynamics between light-emitting excitons by stacking perovskite (MAPbBr3) quantum dots (QDs sized between 10 nm and 14 nm) into rod-like and diamond-like aggregates. The rod-like packing shows a prolonged photoluminescence (PL) lifetime (184 ns) with 3 nm red-shifted peak (525 nm) as compared to the diamond-like packing (PL peak, 522 nm; lifetime, 19 ns). This indicates that the rod-like packing forms a stronger interaction between QDs with reduced surface-charged defects, leading to surface-to-inside property-tuning capability with an ASE. Interestingly, the ASE enabled by rod-like packing shows an orbit-orbit polarization interaction between light-emitting excitons, identified by linearly/circularly polarized pumping conditions. More importantly, the polarization dynamics is extended to the order of nanoseconds in the rod-like assembly, determined by the observation that within the ASE lifetime (2.54 ns) the rotating pumping beam polarization direction largely affects the coherent interaction between light-emitting excitons.

Over 17% Efficiency of Ternary Organic Photovoltaics Employing Two Acceptors with an Acceptor-Donor-Acceptor Configuration.

Ternary organic photovoltaics (OPVs) were constructed with one wide-band-gap donor PM6 and two A-D-A-type acceptors (M-series M36 and MQ5) with similar chemical structures. Power conversion efficiency (PCE) of the optimal ternary OPVs reaches 17.24% with 20 wt % MQ5 content, arising from a simultaneously increased short circuit current density (JSC) of 25.36 mA cm-2 and a fill factor (FF) of 76.02% as compared to those of two binary OPVs. The photon harvesting of ternary active layers can be maximized by adjusting the MQ5 content by reason of the complementary absorption spectra of M36 and MQ5. The molecular arrangement of PM6 and M36 can be collectively optimized by introducing an appropriate amount of MQ5 as a morphology regulator for facilitating effective charge transportation in ternary active layers. The improved photon harvesting and charge transport in active layers should be two important factors responsible for JSC and FF improvement of optimal ternary OPVs, respectively. More than an 8.8% improvement of PCE is achieved in ternary OPVs with an appropriate amount of MQ5 as the photon-harvesting enhancer and morphology regulator. The huge potential of A-D-A-type materials in constructing highly efficient OPVs can be further exploited based on a ternary strategy.

Room-Temperature Magnetic Field Effect on Excitonic Photoluminescence in Perovskite Nanocrystals.

Magnetic-field-enhanced spin-polarized electronic/optical properties in semiconductors are crucial for fabricating various spintronic devices. However, this spin polarization is governed by weak spin exchange interactions and easily randomized by thermal fluctuations; therefore, it is only produced at cryogenic temperatures, which severely limits the applications. Herein, a room-temperature intrinsic magnetic field effect (MFE) on excitonic photoluminescence is achieved in CsPbX3 :Mn (X = Cl, Br) perovskite nanocrystals. Through moderate Mn doping, the MFE is enhanced by exciton-Mn interactions, and through partial Br substitution, the MFE is stabilized at room temperature by exciton orbital ordering. The orbital ordering significantly enhances the g-factor difference between electrons and holes, which is evidenced by a parallel orbit-orbit interaction among excitons generated by circular polarized laser excitation. This study provides a clear avenue for engineering spintronic materials based on orbital interactions in perovskites.

Establishing charge-transfer excitons in 2D perovskite heterostructures.

Charge-transfer excitons (CTEs) immensely enrich property-tuning capabilities of semiconducting materials. However, such concept has been remaining as unexplored topic within halide perovskite structures. Here, we report that CTEs can be effectively formed in heterostructured 2D perovskites prepared by mixing PEA2PbI4:PEA2SnI4, functioning as host and guest components. Remarkably, a broad emission can be demonstrated with quick formation of 3 ps but prolonged lifetime of ~0.5 µs. This broad PL presents the hypothesis of CTEs, verified by the exclusion of lattice distortion and doping effects through demonstrating double-layered PEA2PbI4/PEA2SnI4 heterostructure when shearing-away PEA2SnI4 film onto the surface of PEA2PbI4 film by using hand-finger pressing method. The below-bandgap photocurrent indicates that CTEs are vital states formed at PEA2PbI4:PEA2SnI4 interfaces in 2D perovskite heterostructures. Electroluminescence shows that CTEs can be directly formed with electrically injected carriers in perovskite LEDs. Clearly, the CTEs presents a new mechanism to advance the multifunctionalities in 2D perovskites.

Two-Photon Up-Conversion Photoluminescence Realized through Spatially Extended Gap States in Quasi-2D Perovskite Films.

A new approach to generate a two-photon up-conversion photoluminescence (PL) by directly exciting the gap states with continuous-wave (CW) infrared photoexcitation in solution-processing quasi-2D perovskite films [(PEA)2 (MA)4 Pb5 Br16 with n = 5] is reported. Specifically, a visible PL peaked at 520 nm is observed with the quadratic power dependence by exciting the gap states with CW 980 nm laser excitation, indicating a two-photon up-conversion PL occurring in quasi-2D perovskite films. Decreasing the gap states by reducing the n value leads to a dramatic decrease in the two-photon up-conversion PL signal. This confirms that the gap states are indeed responsible for generating the two-photon up-conversion PL in quasi-2D perovskites. Furthermore, mechanical scratching indicates that the different-n-value nanoplates are essentially uniformly formed in the quasi-2D perovskite films toward generating multi-photon up-conversion light emission. More importantly, the two-photon up-conversion PL is found to be sensitive to an external magnetic field, indicating that the gap states are essentially formed as spatially extended states ready for multi-photon excitation. Polarization-dependent up-conversion PL studies reveal that the gap states experience the orbit-orbit interaction through Coulomb polarization to form spatially extended states toward developing multi-photon up-conversion light emission in quasi-2D perovskites.

Effect of surface recombination in high performance white-light CH3NH3PbI3 single crystal photodetectors.

Methylammonium lead iodide (CH3NH3PbI3), with the organic-inorganic hybrid perovskite (OIHP) structure, has gained tremendous research interest due to its excellent photo-electron conversion ability in the application of photovoltaics. Despite its solution processed polycrystalline thin film form in solar cells, the single crystalline counterpart may offer some incredibly novel optoelectronic functionalities. In this work, a sizable (>5 mm) and high quality CH3NH3PbI3 single crystal has been synthesized by the inverse temperature crystallization method, and a white-light photodetector of the structure glass/ITO/Ga/ CH3NH3PbI3/Au was fabricated. Overbroad photo-excitation intensities ranging from 0.1 mW/cm2 to 100 mW/cm2 using a sun-light simulator, the on-off ratio is tunable in a wide-range from 65 to 2250 at zero bias voltage. The responsivity (R) and detectivity (D*) are 36.2 mA/W and 2.68×1011 Jones respectively at a weak white-light intensity such as 0.1 mW/cm2. Both the photodetective parameters decrease with the increase of the illumination intensity. Based on impedance spectra obtained at working condition and light intensity dependent Jsc measurements, the surface trap-assist recombination may play a dominating role. The corresponding lifetime (τsurf) and resistance (Rsurf_trap) exhibit fast decays at higher illumination intensities. This fundamental study may pave the way for exploring the contribution of the surface trap-assist recombination in the CH3NH3PbI3 single crystal based photodetector. We believe it is applicable for integration in micro-photonics for sensitive and weak white-light photo-detection.

Beam wander of a partially coherent Airy beam in oceanic turbulence.

The beam wander of a partially coherent Airy beam in oceanic turbulence is investigated with the help of the extended Huygens-Fresnel integral formula. Analytical expression for the second-order moment and the beam wander of a partially coherent Airy beam propagating in oceanic turbulence is derived. From the numerical results based on the analytical formula, we find that increasing the dissipation rate of turbulent kinetic energy or decreasing the dissipation rate of mean-square temperature and relative strength of temperature and salinity fluctuations of oceanic turbulence tends to decrease the wander effect of a partially coherent Airy beam. Moreover, it is found that increasing the transverse scale factor and wavelength or decreasing the coherent length and exponential truncation factor of a partially coherent Airy beam decreases beam wander in oceanic turbulence. Our results will be useful in optical underwater communications and laser defense.

Major variables of zinc homeostasis in Chinese toddlers.

BACKGROUND: Measurement of the major variables of zinc homeostasis is an essential prerequisite for estimating human zinc requirements, which currently require a factorial approach. The data required for this approach have not been available for toddlers, whose requirements have been estimated by extrapolation from other age groups. OBJECTIVE: The objective of the study was to measure key variables of zinc homeostasis in rural and small-town Chinese toddlers. DESIGN: Zinc stable-isotope tracers were administered intravenously and orally with all meals for 1 d to 43 toddlers. Subsequent metabolic collections in the homes included duplicate diets, quantitative fecal collections, and spot urine sampling. Fractional absorption of zinc (FAZ) was measured by a dual-isotope tracer ratio technique, and endogenous fecal zinc (EFZ) was measured by an isotope dilution technique. RESULTS: No group or sex differences were found. Therefore, results were combined for 43 toddlers aged 19-25 mo whose major food staple was white rice. Selected results (x+/- SD) were 1.86 +/- 0.55 mg total dietary Zn/d; 0.35 +/- 0.12 FAZ; 0.63 +/- 0.24 mg total absorbed Zn/d; 0.67 +/- 0.23 mg EFZ/d; and 65.0 +/- 8.3 microg plasma Zn/dL. The molar ratio of dietary phytate to zinc was 2.7:1. CONCLUSIONS: The mean intake and absorption of zinc in this population are low in comparison with estimated average dietary and physiologic requirements for zinc, and plasma zinc values are consistent with zinc deficiency. Intestinal losses of endogenous zinc exceed previous estimates for toddlers, and only modest evidence exists of conservation in response to low zinc intake and absorption.