Zhaoxin Wu

Measurement of the collision time of dense electronic plasma induced by a femtosecond laser in fused silica

Electronic plasma induced by a focused femtosecond pulse (130 fs, 800 nm) in fused silica was investigated by use of pump-probe technology. Pump and probe shadow imaging and interferometric fringe imaging were combined to determine electronic collision time tau in the conduction band, and tau was measured to be 1.7 fs at an electron density near 5 x 10(19) cm(-3). The lifetime of the electronic plasma is also measured to be approximately 170 fs by use of the time-resolved shadow imaging technique.

Multifunctional perovskite capping layers in hybrid solar cells

In this study, the crucial role of perovskites capping layers in the TiO2/CH3NH3PbI3 hybrid solar cells is investigated. The capping layers are realized by controlling the concentration of PbI2 solutions in the sequential deposition process. The morphologies of the active layers are studied by high-resolution scanning electron microscopy (HR-SEM). The amount of perovskites in capping layers increases with the concentration of PbI2 solution, and the coverage of perovskite capping layers on TiO2 films is better developed. Except for the correlation between photocurrents and coverages of perovskite proposed by Snaith, we revealed a more detailed relationship between the photovoltaic performances and perovskite capping layers. It is noteworthy that UV-vis absorption increased with perovskites in capping layers. Moreover, according to the diffuse reflection spectra, light scattering, which is beneficial for the conversion efficiency of photons to electrons by directly preventing most of the incident light from transmitting out, is also enhanced due to both the emergence of larger-size particles in the capping layers and the higher effective dielectric coefficient. All of the aforementioned aspects result in high photocurrents up to 20.6 mA cm−2. Efficiency as high as 10.3% is ultimately achieved by a simple control of PbI2 concentration in the sequential deposition process.

Thiazole-based metallophosphors of iridium with balanced carrier injection/transporting features and their two-colour WOLEDs fabricated by both vacuum deposition and solution processing-vacuum deposition hybrid strategy

New phosphorescent iridium(III) cyclometallated complexes bearing thiazole-based ligands (IrTZ1 and IrTZ2) have been developed. The functionalized organic ligands derived by combining the thiazolyl moiety and triphenylamino group have conferred not only favorable hole-injection/hole-transporting (HI/HT) features but also more balanced charge carrier injection/transporting traits to the as-prepared iridium(III) metallophosphors. Owing to the unique electronic structures afforded by the ligand, the orange organic light-emitting devices (OLEDs) made from IrTZ1 can furnish peak external quantum efficiency (ηext) of 14.82%, luminance efficiency (ηL) of 39.97 cd A−1 and power efficiency (ηp) of 34.95 lm W−1. Inspired by its outstanding electroluminescence (EL) performance, the orange IrTZ1 phosphor complemented with a blue phosphor FIrpic was employed to fabricate highly efficient white organic light-emitting devices (WOLEDs) with a single emission layer. Despite their simple device configuration, the optimized WOLEDs can still maintain decent electroluminescence (EL) ability with ηext of 7.20%, ηL of 18.07 cd A−1 and ηp of 19.57 lm W−1. With the aim to simplify the fabrication process of multi-layered WOLEDs, two-component WOLEDs were obtained through a novel solution processing–vacuum deposition hybrid method with the doped blue fluorescent emission layer deposited by a solution process and the orange phosphorescent emission layer made by vacuum deposition. The WOLEDs prepared using such exploratory approach can show an attractive EL performance with ηext of 9.06%, ηL of 22.72 cd A−1 and ηp of 17.28 lm W−1. All these data have indicated not only the great potential of the orange phosphor in monochromatic and white OLEDs, but also the importance of the hybrid method for simplifying WOLED fabrication.

Multiple foci and a long filament observed with focused femtosecond pulse propagation in fused silica

Multiple foci and a long filament are observed when we focus a femtosecond laser pulse into a fused-silica sample. The dependences of the intensity distribution of the plasma luminescence on the pulse energy and the numerical aperture (NA) of the focusing objective are investigated. Multiple foci are observed when NA of </=0.65 . A long filament tail is formed instead of multiple foci when the NA is 0.85. A physical image of femtoseond pulse propagation is given by a model based on the nonlinear Schrödinger equation.

Investigation of the spectra of phosphorescent organic light-emitting devices in relation to emission zone

The dependence of the electroluminescent spectra on the emission zone of blue electrophosphorscent light emitting diodes (PHOLEDs) was investigated. The light emission of a PHOLED was tuned from blue to greenish blue by adjusting the position of the emission zone in the PHOLED. Experimental results agreed well with the numerical simulation based on the effect of the wide-angle optical interference by the metal cathode. The comparison of the numerical results and the electroluminescent spectra of the PHOLED was then extended to serve as the basis of another method to determine the location of the emission zones of PHOLEDs.

Highly Transparent, Conductive, Flexible Resin Films Embedded with Silver Nanowires

In this article, a low sheet resistance and highly transparent silver nanowire (AgNW) resin composite film was demonstrated, which was prepared by a simple and efficacious two-step spin-coating method. By burying the AgNWs below the surface of the transparent resin matrix which was cured at 150 °C in air, we achieved a uniform, highly transparent, conductive, flexible film. Compared to the reported transparent electrodes, this composite transparent and conductive film showed 10 Ω/□ sheet resistance and nearly 90% mean optical transmittance over the UV-visible range simultaneously. Undergoing hundreds of cycles of tensile and compression folding, the composite film slightly increased its sheet resistance by less than 5%, displaying good electromechanical flexibility. These characteristics of the composite AgNW-resin films were expected to be used in applications of flexible optoelectronics.

Construction of Compact Methylammonium Bismuth Iodide Film Promoting Lead-Free Inverted Planar Heterojunction Organohalide Solar Cells with Open-Circuit Voltage over 0.8 V

A bismuth-based organohalide material, methylammonium bismuth iodide (MA3Bi2I9), has been recently explored as an efficient lead-free light absorber in photovoltaic applications. However, the poor surface morphology of the MA3Bi2I9 film fabricated via conventional one-step spin-coating methods has limited the performance of the device. In this work, a smooth, uniform, and compact MA3Bi2I9 thin film was realized by a novel two-step evaporation-spin-coating film fabrication strategy for the first time. Taking advantage of the superior MA3Bi2I9 thin film, the best-performing inverted planar heterojuncion photovoltaic device exhibited a power conversion efficiency of 0.39% with open-circuit voltage as high as 0.83 V, which demonstrated the lowest loss-in-potential to date in MA3Bi2I9-based solar cells. Moreover, the facile film fabrication strategy utilized in this work paves the way for high reproducibility of lead-free organohalide films and devices.

Ag-encapsulated Au plasmonic nanorods for enhanced dye-sensitized solar cell performance

In this article, Ag-encapsulated Au nanorods (Au@Ag NRs) are prepared and introduced into dye-sensitized solar cells (DSSCs). As a unique plasmonic nanostructure, this composite exhibits the superiorities of enhanced light-harvesting as well as restrained charge recombination of DSSCs. Remarkably, the enhanced light absorption of the photoanode can be obtained via the surface plasmon resonance (SPR) effect of the Au@Ag NRs, whereas a broadened absorption in the red and near-infrared (NIR) region ensures the full utilization of the solar energy. Beyond the dominated optical utility, the presence of the Au@Ag NRs promotes the suppression of the charge recombination, further enhancing the photochemical catalysis of DSSCs. An optimized Au@Ag NR modified DSSC is achieved with a power conversion efficiency of 8.43%, which is significantly superior to that of the pure TiO2 DSSC with a PCE of 5.91%.

Formation of ultrasmooth perovskite films toward highly efficient inverted planar heterojunction solar cells by micro-flowing anti-solvent deposition in air

Ultrasmooth perovskite thin films are prepared by a solution-based one-step micro-flowing anti-solvent deposition (MAD) method carried out in air with simplicity and practicability. Engaging inert gas blow and anti-solvent drips as accelerators, ultrafast crystallizing, thickness controllable, and high quality methylammonium lead iodide films are prepared with a least root mean square roughness of 1.43 nm (1.95 nm on average), achieving the smoothest surface morphology to the best of our knowledge, as well as a rather compact perovskite layer with a high coverage ratio. Perovskite films formed from MAD require no annealing procedure to ultimately crystallize, realizing a very fast crystallizing procedure within few seconds. By controlling the thickness of perovskite films, superior photovoltaic performance of solar cells with a large fill factor of 0.8 and a PCE of 15.98% is achieved without a glovebox. MAD technology will benefit not only highly efficient photovoltaic devices, but also perovskite-based hybrid optoelectronic devices with field effect transistors and light emitting diodes as well.

Phosphorescent Iridium(III) Complexes Bearing Fluorinated Aromatic Sulfonyl Group with Nearly Unity Phosphorescent Quantum Yields and Outstanding Electroluminescent Properties

A series of heteroleptic functional Ir(III) complexes bearing different fluorinated aromatic sulfonyl groups has been synthesized. Their photophysical features, electrochemical behaviors, and electroluminescent (EL) properties have been characterized in detail. These complexes emit intense yellow phosphorescence with exceptionally high quantum yields (ΦP > 0.9) at room temperature, and the emission maxima of these complexes can be finely tuned depending upon the number of the fluorine substituents on the pendant phenyl ring of the sulfonyl group. Furthermore, the electrochemical properties and electron injection/transporting (EI/ET) abilities of these Ir(III) phosphors can also be effectively tuned by the fluorinated aromatic sulfonyl group to furnish some desired characters for enhancing the EL performance. Hence, the maximum luminance efficiency (ηL) of 81.2 cd A(-1), corresponding to power efficiency (ηP) of 64.5 lm W(-1) and external quantum efficiency (ηext) of 19.3%, has been achieved, indicating the great potential of these novel phosphors in the field of organic light-emitting diodes (OLEDs). Furthermore, a clear picture has been drawn for the relationship between their optoelectronic properties and chemical structures. These results should provide important information for developing highly efficient phosphors.