Improvement of CdSe/P3HT bulk hetero-junction solar cell performance due to ligand exchange from TOPO to pyridine

Cadmium selenide (CdSe) nanoparticles of 2–7 nm diameter were synthesized by a hot injection method and their TOPO ligands exchanged with pyridine by a liquid–liquid extraction process. The effects of the ligand exchange on the material properties of the nanoparticles and device performance of the bulk hetero-junction (BHJ) solar cell were systematically investigated. The wavelength shift to a shorter value observed in the PL spectra of the pyridine-capped CdSe indicates that the size of nanoparticles decreased due to the exchange of the surface ligands of the CdSe from TOPO to pyridine. Improvements in the power conversion efficiency of BHJ solar cells made with pyridine-capped CdSe are mainly attributed to the enhanced current collection, J_sc=0.43 mA/cm² (TOPO) to 1.4 mA/cm² (pyridine).     

High-brightness,high-power LED-based strobe illumination for double-frame micro particle image velocimetry

There is a need for more practical, cost-effective and adaptive flow measurement instruments. Unfortunately, most of the current equipment still has to employ bulky and expensive laser systems for flow illumination. In this work we explore the practicability of using high-brightness high-power light-emitting diodes (LED) as a strobe-light illuminator within a double-frame micro particle image velocimetry (microPIV) system. A compact, cost-effective, and controllable LED driver was designed and applied to study the optical and spectral behavior of green and blue family direct-color single chip LEDs, which have excitation wavelengths corresponding to the most widely used fluorescent micro particles. To achieve strobe illumination, LEDs were subjected to high current double-pulsed operating regimes (up to 35 A at 20 μs pulse duration), which are much beyond their physical damage thresholds. We have witnessed that the blue family LEDs are more suitable for the fluorescence based microPIV experiments as they exhibit more stable spectral behavior at the double-pulsed high currents compared to the green colored LEDs. Specifically, we have observed that the spectral wavelength shift of blue family LEDs due to the high current is less severe and still enough to excite particles than that of green LEDs. Also, we have studied the relation between the pulse duration (as a means of parameter controlling signal-to-noise ratio) and the motion blur effect that arises due to the prolonged pulse durations. As a result, adequate pulse durations for most widely used microPIV experimental conditions were calculated. The feasibility and robustness of the developed double-pulsed LED illumination system was validated through a series of microPIV experiments with real flows inside two different topology microchannels, where the measured velocity was in good agreement with the corresponding analytical solutions. Overall, we can conclude that the high-brightness LED devices are promising candidates to replace their laser counterparts in the microPIV systems.     

InGaN／AlGaN双异质结构蓝光LED的电学和光学性质

研究了ＩｎＧａＮ／ＡｌＧａＮ双异质结构（ＤＨ）蓝光发光二极管（ＬＥＤ）的电学和光学性质，实验表明，器件正向偏压下的Ｉ－Ｖ特性偏离了ｐｎ结二极管的肖克莱模型的结果，并且载流子的主要输运机制与载流子隧穿有关。通过对电致发光（ＥＬ）谱的测量，得到位于２．８ｅＶ的发射峰和位于３．２ｅＶ弱发射峰，随着电流增大而均出现蓝移。对大脉冲电流下ＬＥＤ的特性的退化作了研究。     

Pulsed-Laser-Induced Simple Synthetic Route for Tb3Al5O12:Ce3+ Colloidal Nanocrystals and Their Luminescent Properties

Cerium-doped Tb₃Al₅O₁₂ (TAG:Ce³⁺) colloidal nanocrystals were synthesized by pulsed laser ablation (PLA) in de-ionized water and lauryl dimethylaminoacetic acid betain (LDA) aqueous solution for luminescent bio-labeling application. The influence of LDA molecules on the crystallinity, crystal morphology, crystallite size, and luminescent properties of the prepared TAG:Ce³⁺ colloidal nanocrystals was investigated in detail. When the LDA solution was used, smaller average crystallite size, narrower size distribution, and enhanced luminescence were observed. These characteristics were explained by the effective role of occupying the oxygen defects on the surface of TAG:Ce³⁺ colloidal nanocrystal because the amphoteric LDA molecules were attached by positively charged TAG:Ce³⁺ colloidal nanocrystals. The blue-shifted phenomena found in luminescent spectra of the TAG:Ce³⁺ colloidal nanocrystals could not be explained by previous crystal field theory. We discuss the 5d energy level of Ce³⁺ with decreased crystal size with a phenomenological model that explains the relationship between bond distance with 5d energy level of Ce³⁺ based on the concept of crystal field theory modified by covalency contribution.     

Stable blue-green light-emitting electrochemical cells based on a cationic iridium complex with phenylpyrazole as the cyclometalated ligands

A novel blue-green light-emitting cationic iridium complex [Ir(ppz)₂pzpy]PF₆ (complex 1), where ppz is phenylpyrazole and pzpy is 2-(1-phenyl-1H-pyrazol-3-yl)pyridine, is developed. Its crystal, photophysical and electrochemical properties are characterized through experiments and quantum chemical calculations. The novel complex exhibits enlarged energy gap and blue-shifted emission spectrum at 77 K compared to a reference complex, [Ir(ppy)₂pzpy]PF₆ (complex 2), with phenylpyridine (ppy) as the cyclometalated ligands. Moreover, light-emitting electrochemical cells (LECs) fabricated with complex 1 show both improved color purity with CIE (Commission Internationale de L’Eclairage) coordinates of (0.25, 0.39) and elongated lifespan (t_1/2 = 218 min, E_tot = 0.55 mJ) compared to LECs based on complex 2.     

Optical Properties of Silicon—doped InGaN and GaN Layers

The optical properties of Silicon—doped InGaN and GaN grown on sapphire by MOCVD have been investigated by photoluminescence (PL) method. At room temperature, the band—gap peak of InGaN is 437.0 nm and its full width of half—maximum (FWHM) is about 14.3 nm. The band—gap peak and FWHM for GaN are 364.4 nm and 9.5 nm, respectively. By changing the temperature from 20 K to 293 K, it is found that the PL intensity of samples decreases but the FWHM broadens with the increasing of the temperature.GaN sample shows red—shift, InGaN sample shows red—blue—red—shift. The temperature dependence of peak energy shift is studied and explained.