Structural and spectral dynamics of single-crystalline Ruddlesden-Popper phase halide perovskite blue light-emitting diodes.

Achieving perovskite-based high-color purity blue-emitting light-emitting diodes (LEDs) is still challenging. Here, we report successful synthesis of a series of blue-emissive two-dimensional Ruddlesden-Popper phase single crystals and their high-color purity blue-emitting LED demonstrations. Although this approach successfully achieves a series of bandgap emissions based on the different layer thicknesses, it still suffers from a conventional temperature-induced device degradation mechanism during high-voltage operations. To understand the underlying mechanism, we further elucidate temperature-induced device degradation by investigating the crystal structural and spectral evolution dynamics via in situ temperature-dependent single-crystal x-ray diffraction, photoluminescence (PL) characterization, and density functional theory calculation. The PL peak becomes asymmetrically broadened with a marked intensity decay, as temperature increases owing to [PbBr6]4- octahedra tilting and the organic chain disordering, which results in bandgap decrease. This study indicates that careful heat management under LED operation is a key factor to maintain the sharp and intense emission

Giant Light-Emission Enhancement in Lead Halide Perovskites by Surface Oxygen Passivation.

Surface condition plays an important role in the optical performance of semiconductor materials. As new types of semiconductors, the emerging metal-halide perovskites are promising for next-generation optoelectronic devices. We discover significantly improved light-emission efficiencies in lead halide perovskites due to surface oxygen passivation. The enhancement manifests close to 3 orders of magnitude as the perovskite dimensions decrease to the nanoscale, improving external quantum efficiencies from <0.02% to over 12%. Along with about a 4-fold increase in spontaneous carrier recombination lifetimes, we show that oxygen exposure enhances light emission by reducing the nonradiative recombination channel. Supported by X-ray surface characterization and theoretical modeling, we propose that excess lead atoms on the perovskite surface create deep-level trap states that can be passivated by oxygen adsorption

Direct determination of band-gap renormalization in degenerately doped ultrawide band gap β-Ga_{2}O_{3} semiconductor

Ga2O3 is emerging as a promising wide band-gap semiconductor for high-power electronics and deep ultraviolet optoelectronics. It is highly desirable to dope it with controllable carrier concentrations for different device applications. This work reports a combined photoemission spectroscopy and theoretical calculation study on the electronic structure of Si doped Ga_{2}O_{3} films with carrier concentration varying from 4.6×10^{18} cm^{−3} to 2.6×10^{20} cm^{−3}. Hard x-ray photoelectron spectroscopy was used to directly measure the widening of the band gap as a result of occupation of conduction band and band-gap renormalization associated with many-body interactions. A large band-gap renormalization of 0.3 eV was directly observed in heavily doped Ga_{2}O_{3}. Supplemented with hybrid density functional theory calculations, we demonstrated that the band-gap renormalization results from the decrease in energy of the conduction band edge driven by the mutual electrostatic interaction between added electrons. Moreover, our work reveals that Si is a superior dopant over Ge and Sn, because Si 3s forms a resonant donor state above the conduction band minimum, leaving the host conduction band mostly unperturbed and a high mobility is maintained though the doping level is high. Insights of the present work have significant implications in doping optimization of Ga_{2}O_{3} and realization of optoelectronic devices

Deep UV transparent conductive oxide thin films realized through degenerately doped wide-bandgap gallium oxide

Deep UV transparent thin films have recently attracted considerable attention owing to their potential in UV and organic-based optoelectronics. Here, we report the achievement of a deep UV transparent and highly conductive thin film based on Si-doped Ga_{2}O_{3} (SGO) with high conductivity of 2500 S/cm. The SGO thin films exhibit high transparency over a wide spectrum ranging from visible light to deep UV wavelength and, meanwhile, have a very low work-function of approximately 3.2 eV. A combination of photoemission spectroscopy and theoretical studies reveals that the delocalized conduction band derived from Ga 4s orbitals is responsible for the Ga_{2}O_{3} films’ high conductivity. Furthermore, Si is shown to act as an efficient shallow donor, yielding high mobility up to approximately 60 cm^{2}/Vs. The superior optoelectronic properties of SGO films make it a promising material for use as electrodes in high-power electronics and deep UV and organic-based optoelectronic devices

Quantifying the Impact of Environmental Stimuli on the Structural Dynamics of Cesium Lead Iodide Perovskite

Metal halide perovskite is a new generation of semiconducting materials that providing promise for the development of efficient optoelectronic devices such as solar cells, light-emitting diodes (LEDs), and lasers due to its outstanding optical and electronic properties. In particular, all-inorganic cesium lead iodide (CsPbI3) presents great promise for photovoltaic applications due to its suitable bandgap for single and multi-junction solar cells, and its enhanced thermal stability compared to organic-based metal halide perovskite. However, exposure to environmental stimuli such as ambient moisture tends to structurally transform CsPbI3 perovskite (high-T) into a nonperovskite (low-T) phase that has a larger bandgap. Thorough understanding of the influence of the environmental stimuli is key in predicting solar cell stability and in designing more stable devices. By directly monitoring the high-T to low-T CsPbI3 phase transformation under controlled relative humidity, I am able to quantify the dependence of phase transformation processes on relative humidity, extract the associated nucleation barriers, and uncover the rate-limiting process. Furthermore, I find heating under general solar cell operating temperature to be a potential method of mitigating moisture-induced phase transformation. In addition, I demonstrate that illumination using above band-gap continuous-wave laser transforms high-T phase CsPbI3 to low-T phase in ambient conditions. In the presence of moisture, laser induces structural phase transformation at rates order of magnitude much faster than moisture-induced phase transformation. In the absence of moisture, laser does not trigger phase transformation, but introduces long-lasting defects that lower the photoluminescence emission and accelerate phase transformation upon exposure to moisture. Finally, I demonstrate direct visualization of phase growth in individual single-crystals utilizing a continuous-wave laser via photoluminescence imaging, as well as in situ heating in cathodoluminescence microscopy coupled with scanning electron microscopy. I find that initial growth direction and shape show correlation to the interface migration speed. These studies all contribute to the fundamental understanding of phase transformation energetics of CsPbI3, which can serve as references for device applications of CsPbI3 and for future designs of stable photovoltaics systems

An Extended Membrane System Based on Cell-like P Systems and Improved Particle Swarm Optimization for Image Segmentation

An extended membrane system with a dynamic nested membrane structure, which is integrated with the evolution-communication mechanism of a cell-like P system with evolutional symport/antiport rules and active membranes (ECP), and the evolutionary mechanisms of particle swarm optimization (PSO) and improved PSO inspired by starling flock behavior (SPSO), named DSPSO-ECP, is designed and developed to try to break application restrictions of P systems in this paper. The purpose of DSPSO-ECP is to enhance the performance of extended membrane system in solving optimization problems. In the proposed DSPSO-ECP, the updated model of velocity and position of standard PSO, as basic evolution rules, are adopted to evolve objects in elementary membranes. The modified updated model of the velocity of improved SPSO is used as local evolution rules to evolve objects in sub-membranes. A group of sub-membranes for elementary membranes are specially designed to avoid prematurity through membrane creation and dissolution rules with promoter/inhibitor. The exchange and sharing of information between different membranes are achieved by communication rules for objects based on evolutional symport rules of ECP. At last, computational results, which are made on numerical benchmark functions and classic test images, are discussed and analyzed to validate the efficiency of the proposed DSPSO-ECP

Controlling the Phase Transition in CsPbI3 Nanowires.

Cesium lead iodide (CsPbI3) is a promising semiconductor with a suitable band gap for optoelectronic devices. CsPbI3 has a metastable perovskite phase that undergoes a phase transition into an unfavorable nonperovskite phase in an ambient environment. This phase transition changes the optoelectronic properties of CsPbI3 and hinders its potential for device applications. Therefore, it is of central importance to understand the kinetics of such instability and develop strategies to control and stabilize the perovskite phase. Here, we use ultralong CsPbI3 nanowires as a model platform to investigate the phase transition kinetics. Our results depict the role of environmental stressors (moisture and temperature) in controlling the phase transition dynamics of CsPbI3, which can serve as guiding principles for future phase transition studies and the design of related photovoltaics. Furthermore, we demonstrate the controllability of phase propagation on individual nanowires by varying the moisture level and temperature