Zhigang Zang

Femtosecond laser direct writing of microholes on roughened ZnO for output power enhancement of InGaN light-emitting diodes.

A significant enhancement of light extraction efficiency from InGaN light-emitting diodes (LEDs) with microhole arrays and roughened ZnO was experimentally demonstrated. The roughened ZnO was fabricated using an Ar and H2 plasma treatment of ZnO films pre-coated on a p-GaN layer. When followed by a femtosecond laser direct writing technique, a periodic array of microholes could be added to the surface. The diameter of the microhole was varied by changing the output power of the femtosecond laser. Compared to conventional LEDs on the same wafer, the output power of LEDs with roughened ZnOs and a microhole (diameter of 2 μm) array was increased by 58.4% when operated with an injection current of 220 mA. Moreover, it was found that LEDs fabricated with roughened ZnO and the microhole array had similar current-voltage (I-V) characteristics to those of conventional LEDs and no degrading effect was observed.

Synthesis of MoS 2 /g-C 3 N 4 nanocomposites with enhanced visible-light photocatalytic activity for the removal of nitric oxide (NO)

Molybdenum disulfide and graphitic carbon nitride (MoS₂-g-C₃N₄) nanocomposites with visible-light induced photocatalytic activity were successfully synthesized by a facile ultrasonic dispersion method. The crystalline structure and morphology of the MoS₂-g-C₃N₄ nanocomposites were characterized by X-ray diffraction (XRD), transmission electron microcopy (TEM), high-resolution TEM (HRTEM) and scanning electron microscopy (SEM). The optical property of the as-prepared nanocomposites was studied by ultraviolet visible diffusion reflection (UV-vis) and photoluminescence(PL) spectrum. It could be observed from the TEM image that the MoS₂ nanosheets and g-C₃N₄ nanoparticles were well combined together. Moreover, the photocatalytic activity of MoS₂-g-C₃N₄ composites was evaluated by the removal of nitric oxide under visible light irradiation (>400nm). The experimental results demonstrated that the nanocomposites with the MoS₂ content of 1.5 wt% exhibited optimal photocatalytic activity and the corresponding removal rate of NO achieved 51.67%, higher than that of pure g-C₃N₄ nanoparticles. A possible photocatalytic mechanism for the MoS₂-g-C₃N₄ nanocomposites with enhanced photocatalytic activity could be ascribed to the hetero-structure of MoS₂ and g-C₃N₄.

Performance improvement of perovskite solar cells by employing a CdSe quantum dot/PCBM composite as an electron transport layer

Organic–inorganic hybrid perovskites have recently attracted considerable interest for application in solar cells due to their low cost, high absorption coefficient and high power conversion efficiency (PCE). Herein, we utilize a CdSe quantum dot/PCBM composite as an electron transport layer (ETL) to investigate the structure, stability and PCE of CH3NH3PbI3−xClx perovskite solar cells. It is found that adsorption of the CdSe/PCBM composite reduces the roughness of the perovskite, leading to a high-quality film with a compact morphology. Density functional theory (DFT) based first-principles calculations show that CdSe enhances the chemical stability of CH3NH3PbI3−xClx involving strong atomic orbital hybridization. Interestingly, an inorganic-terminated perovskite surface has much stronger interaction with CdSe compared to the surface with organic CH3NH3 termination, with noticeable interfacial charge redistribution. Experiments on solar cells incorporating the CdSe/PCBM composite as the ETL show enhanced photocurrent and fill factor, which is related to the in-built electric field between CH3NH3PbI3−xClx and CdSe that greatly facilitates the separation of electron and hole pairs. We show an improved PCE of 13.7% with enhanced device stability in a highly humid atmosphere. These joint theoretical–experimental results may provide a new aspect for improving the structural stability and operating performance of optoelectronic devices based on perovskite structures.

Flexible All-Inorganic Perovskite CsPbBr3 Nonvolatile Memory Device

All-inorganic perovskite CsPbX3 (X = Cl, Br, or I) is widely used in a variety of photoelectric devices such as solar cells, light-emitting diodes, lasers, and photodetectors. However, studies to understand the flexible CsPbX3 electrical application are relatively scarce, mainly due to the limitations of the low-temperature fabricating process. In this study, all-inorganic perovskite CsPbBr3 films were successfully fabricated at 75 °C through a two-step method. The highly crystallized films were first employed as a resistive switching layer in the Al/CsPbBr3/PEDOT:PSS/ITO/PET structure for flexible nonvolatile memory application. The resistive switching operations and endurance performance demonstrated the as-prepared flexible resistive random access memory devices possess reproducible and reliable memory characteristics. Electrical reliability and mechanical stability of the nonvolatile device were further tested by the robust current-voltage curves under different bending angles and consecutive flexing cycles. Moreover, a model of the formation and rupture of filaments through the CsPbBr3 layer was proposed to explain the resistive switching effect. It is believed that this study will offer a new setting to understand and design all-inorganic perovskite materials for future stable flexible electronic devices.

Enhanced Stability and Tunable Photoluminescence in Perovskite CsPbX3/ZnS Quantum Dot Heterostructure

All-inorganic perovskite CsPbX3 (X = Cl, Br, I) and related materials are promising candidates for potential solar cells, light emitting diodes, and photodetectors. Here, a novel architecture made of CsPbX3 /ZnS quantum dot heterodimers synthesized via a facile solution-phase process is reported. Microscopic measurements show that CsPbX3 /ZnS heterodimer has high crystalline quality with enhanced chemical stability, as also evidenced by systematic density functional theory based first-principles calculations. Remarkably, depending on the interface structure, ZnS induces either n-type or p-type doping in CsPbX3 and both type-I and type-II heterojunctions can be achieved, leading to rich electronic properties. Photoluminescence measurement results show a strong blue-shift and decrease of recombination lifetime with increasing sulfurization, which is beneficial for charge diffusion in solar cells and photovoltaic applications. These findings are expected to shed light on further understanding and design of novel perovskite heterostructures for stable, tunable optoelectronic devices.

Inverted Planar Perovskite Solar Cells with a High Fill Factor and Negligible Hysteresis by the Dual Effect of NaCl-Doped PEDOT:PSS

The performance of inverted perovskite solar cells is highly dependent on hole extraction and surface properties of hole transport layers. To highlight the important role of hole transport layers, a facile and simple method is developed by adding sodium chloride (NaCl) into poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The average power conversion efficiency of the perovskite solar cells prepared on NaCl-doped PEDOT:PSS is 17.1% with negligible hysteresis, compared favorably to the control devices (15.1%). Particularly, they exhibit markedly improved Voc and fill factor (FF), with the best FF as high as 81.9%. The enhancement of photovoltaic performance is ascribed to two effects. Better conductivity and hole extraction of PEDOT:PSS are observed after NaCl doping. More intriguingly, the perovskite polycrystalline film shows a preferred orientation along the (001) direction on NaCl-doped PEDOT:PSS, leading to a more uniform thin film. The comparison of the crystal structure between NaCl and MAPbCl3 indicates a lattice constant mismatch less than 2% and a matched chlorine atom arrangement on the (001) surface, which implies that the NaCl crystallites on the top surface of PEDOT:PSS might serve as seeds guiding the growth of perovskite crystals. This simple method is fully compatible with printing technologies to mass-produce perovskite solar cells with high efficiency and tunable crystal orientations.

Transient multiexponential signals analysis using Bayesian deconvolution

A new method based on Bayesian deconvolution is proposed for multiexponential transient signal analysis. The multiexponential signal is initially converted to a convolution model using logarithmic and differential transformation after which the Bayesian iteration is used to deconvolve the data. The numerical simulation is applied on four different multiexponential signals with different levels of noise. Thermal transient experiment data of the high power light emitting diodes are also analyzed using the proposed method. Simulation and experimental results indicate that the present method performs efficiently in accurately estimating the decay rates except at low SNR case.

Synthesis of Ag-In-Zn-S alloyed nanorods and their biological application

Monodisperse Ag-In-Zn-S (AIZS) nanorods with a length of 20 nm have been synthesized using a facile solution based route. These nanorods showed a wide range of fluorescence emissions from green to red, which was achieved by controlling the chemical composition. Moreover, the obtained AIZS nanorods showed high-quality photoluminescence, as well as attractive two-photon fluorescence properties, indicating their potential capability in biological tagging upon near-infrared excitation for deep tissue imaging. Furthermore, the AIZS nanorods presented in this report also show a promising perspective in applications such as solar cells and photocatalysts.

PEDOT:PSS monolayers to enhance the hole extraction and stability of perovskite solar cells

A hole transport layer (HTL) plays a key role in efficient hole extraction and transfer in inverted planar perovskite solar cells. A 10–20 nm thick poly (3,4-ethylenedioxythiphene):poly(styrenesulfonate) (PEDOT:PSS) layer is the most popular HTL in such a device structure. But is it essential to construct such a thick PEDOT:PSS layer? To address this question, herein self-assembled PEDOT:PSS monolayers are obtained on the indium tin oxide (ITO) surface through a facile water rinsing process. Perovskite solar cells with water rinsed PEDOT:PSS as a HTL yield improved power conversion efficiency (PCE) from 13.4% to 18.0%, compared with the control cells with as-cast PEDOT:PSS. The main contribution is from the open-circuit voltage (Voc) and fill factor (FF). Characterization indicates that the majority of PEDOT:PSS is washed away, but an ultra-thin layer of PEDOT:PSS can attach strongly onto ITO via In–O–S chemical bonds between the PSS chain and ITO. Subsequently, PEDOT and PSS form a bilayered structure due to Coulomb interaction. Such an arrangement induces an oriented electric field from positively charged PEDOT to negatively charged PSS, which can accelerate the process of hole extraction. Moreover, the oriented arrangement of PEDOT:PSS monolayers provides higher work function and stronger hydrophobicity, leading to the enhancement in Voc and stability in the ambient environment. This work suggests that there is still room for the efficiency improvement of perovskite solar cells by optimizing the traditional functional layers.

All-Inorganic Perovskite CsPb2Br5 Microsheets for Photodetector Application

Lead-halide perovskites have emerged as one kind of important optoelectronic materials with excellent performance in photovoltaic and light-emitting diode applications. Herein, we reported all-inorganic perovskite CsPb2Br5 microsheets prepared by a facile injection method. Through the X-ray diffraction (XRD) and Scanning Electron Microscope (SEM), it could be seen that the CsPb2Br5 microsheets showed single tetragonal crystalline phase and kept uniform square shape. Moreover, the as-synthesized CsPb2Br5 microsheets exhibited photoluminescence emission at 513 nm, and the UV–vis absorption spectrum further indicated the band gap of CsPb2Br5 microsheets was ≈ 2.50 eV. Additionally, the as-fabricated CsPb2Br5 microsheets based photodetector exhibited faster photoresponse characteristics of short rise time (0.71 s) and decay time (0.60 s), which demonstrated its promising application as high performance electronic and optoelectronic devices.