Thermally Stimulated Electron–Hole and Ionic Processes in Zinc Oxide Crystals

The principal results of the investigation of thermally stimulated electron–hole and ionic processes in hydrothermal and gas-phase ZnO single crystals preexcited at low temperatures, based on simultaneous study of photo-EPR and thermoluminescence (TL), are presented. The nature of the traps determining the TL peaks at 17, 24, 40, 53, 90–110, 140–150, and 160–200 K is discussed. In particular, it has been established that the lithium paramagnetic centers (Li_Zn ⁺–O^I) play the role of hole traps in ZnO giving green and red TL in the temperature range 160–200 K and, in the case of association with small-sized donors, also TL in the temperature range 90–110 K. The other traps are electronic in character, and in the presence of acceptor lithium in the crystals, they form yellow-orange TL. Optical quenching of TL has been evaluated, and it has been found that there is a difference E 0.75 eV between the thermal and optical energies of ionization of lithium acceptors. Irreversible ionic processes associated with the healing of cationic vacancies at T 360–420 K have been revealed.     

Organic light emitting devices

The purpose of this paper is to provide general information about basic physical processes involved in organic electroluminescence and to present the main parameters and advantages of organic light emitting devices (OLEDs).     

氙激光等离子体的双电子复合过程研究(英文)

在自旋-轨道劈裂阵模型下,通过类铜的内壳层激发组态计算了类镍氙的双电子复合速率系数,其中考虑了共振和非共振辐射平衡跃迁对自电离能级的影响,而忽略了因碰撞跃迁引起的电子俘获,非共振辐射平衡跃迁在低电子温度条件下主要影响双电子复合过程;本文讨论了双电子复合系数及双电子伴线强度比随电子温度的变化.     

外加高压电场下空气中激光等离子体通道寿命研究

通过对飞秒激光在空气中产生的等离子体通道两端外加高压,来研究通道的寿命变化情况。实验得到,当在等离子体通道两端外加高压时(350 kV/m),等离子体通道寿命延长了近3倍。理论模拟和分析结果表明在外加电场条件下,碰撞电离得到增强,吸附作用相对减弱,解离复合系数随着电子平均能量的增加而下降的趋势更为剧烈,这进一步引起了等离子体通道寿命的延长。实验结果与理论分析共同表明了利用外加电场对空气中激光等离子体通道寿命进行延长的可行性。     

Interband transition matrix element and temperature dependence of the lasing threshold for GaN laser structures

By comparing experimental and theoretical radiative recombination spectra, we have determined the values of the square of the matrix element for interband optical transitions |M|² in epitaxial GaN at different temperatures T. The data obtained were used to analyze the mechanism for the temperature dependence of the lasing threshold for a GaN laser. In the experiments, we used epitaxial layers of GaN with a wurtzite crystal structure, grown on Al₂O₃ (0001) substrates and excited by a focused beam from a nitrogen laser. We have shown that the spontaneous emission spectra near the lasing threshold of a GaN laser are consistent with the model of optical interband transitions not obeying a selection rule for the electron wave vector. As we have established, the parameter |M|² practically does not vary for T = 300–470 K: 5.4·10⁻⁷³ kg²·m⁵/sec². Further increase in the temperature leads to an exponential fall-off in |M|² down to 3.4·10⁻⁷³ kg²·m⁵/sec² at T ≈ 520 K. Such behavior of |M|² as a function of temperature correlates with the sudden increase in the nonradiative recombination rate at T > 470 K, and may be connected with a change in the recombination mechanisms in the active layer of the GaN laser in the high temperature region. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 6, pp. 790–794, November–December, 2007.     

Interface configuration effects on excitation,exciton dissociation,and charge recombination in organic photovoltaic heterojunction

The morphology of donor-acceptor heterojunction interface significantly affects the electron/hole processes in organic solar cells, including charge transfer (CT), exciton dissociation (ED), and charge recombination (CR). Here, to investigate interface molecular configuration effects, the donor-acceptor complexes with face-on, edge-on, and end-on configurations were constructed as model systems for the p-SIDT(FBTTh₂)₂/C₆₀ heterojunction. The geometries, electronic structures, and excitation properties of monomers and the complexes with three configurations were studied based on density functional theory (DFT) and time-dependent DFT calculations with optimally tuned range separation parameters and solid polarization effects. In terms of Marcus theory, the rate constants of ED and CR processes were analyzed. The results show that most of the excited states for p-SIDT(FBTTh₂)₂ exhibit an intramolecular CT character, and the similarity of the excitation characters (CT and local excitation) and energies among three complexes with different configurations indicate that the electronic structure and excitation properties are insensitive to the interfacial molecular configurations. However, the rates of ED and CR processes heavily depend on it. These results underline the importance of controlling molecular configuration and then the morphology at the heterojunction interface in organic solar cells.     

Charge Localization Induced by Reorientation of FA Cations Greatly Suppresses Nonradiative Electron-Hole Recombination in FAPbl3 Perovskites: a Time-Domain Ab Initio Study

Recent experiments report the rotation of FA (FA = HC[NH₂]₂\begin{document}$ ^+ $\end{document}) cations significantly influence the excited-state lifetime of FAPbI₃. However, the underlying mechanism remains unclear. Using ab initio nonadiabatic (NA) molecular dynamics combined with time-domain density functional simulations, we have demonstrated that reorientation of partial FA cations significantly inhibits nonradiative electron-hole recombination with respect to the pristine FAPbI₃ due to the decreased NA coupling by localizing electron and hole in different positions and the suppressed atomic motions. Slow nuclear motions simultaneously increase the decoherence time, which is overcome by the reduced NA coupling, extending electron-hole recombination time scales to several nanoseconds and being about 3.9 times longer than that in pristine FAPbI₃, which occurs within sub-nanosecond and agrees with experiment. Our study established the mechanism for the experimentally reported prolonged excited-state lifetime, providing a rational strategy for design of high performance of perovskite solar cells and optoelectronic devices.     

发射层厚度对透射式GaAs光电阴极表面光电压谱的影响

通过求解一维稳态少子扩散方程,推导了含有后界面复合速率和发射层厚度的透射式GaAs光电阴极表面光电压谱理论方程.通过对发射层厚度分别为1.6 μm和2.0 μm,掺杂浓度为1×1019 cm－3的GaAs透射式阴极样品测试,理论曲线和实验曲线基本一致.通过引入表面光电压谱积分灵敏度公式,仿真探讨了表面光电压谱在一定体材料参量条件下,积分灵敏度受发射层厚度的影响|发现在体材料参量一定条件下,透射式GaAs光电阴极具有最佳厚度,同时最佳厚度受后界面复合速率的影响更大,同时GaAlAs窗口层也能很好降低发射层后界面复合速率.     

Nuclear modification factor for J/ψ production in nucleus-nucleus collisions

The STAR Collaboration has offered an eminent nuclear modification factor of J/ψ at high p_T and midrapidity produced in Cu-Cu collisions at √s_NN=200 GeV. Recalling a prediction, we can understand that the feature of high-p_T nuclear modification factor is related to cc produced by 2 → 1 and 2 → 2 partonic processes in deconfined matter, particularly in the prethermal stage and to the recombination of c and c. The nuclear modification factor at high p_T is sensitive to the earliest form of deconfined matter that does not have a temperature.     

Enhanced performance of bi-layer Nb2O5 coated TiO2 nanoparticles/nanowires composite photoanode in dye-sensitized solar cells

Dye sensitized solar cells (DSSCs) were fabricated based on coumarin NKX-2700 dye sensitized bi-layer photoanode and quasi-solid state electrolyte sandwiched together with cobalt sulfide coated counter electrode. A novel bi-layer photoanode has been prepared using composite mixtures of 90 wt.% TiO₂ nanoparticles + 10 wt.% TiO₂ nanowires (TNPWs) as active layer and Nb₂O₅ is coated on the active layer, which acts as scattering layer. Hafnium oxide (HfO₂) was applied over the TNPWs/Nb₂O₅ photoanode film, as a blocking layer. TiO₂ nanoparticles (TNPs), TiO₂ nanowires (TNWs) and TNPWs/Nb₂O₅ were characterized by X-ray diffractometer (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The sensitizing organic dye coumarin NKX-2700 displayed maximum absorption wavelength (λ_max) at 525 nm, which could be observed from the UV–vis spectrum. DSSC-1 fabricated with composite bi-layer photoanode revealed enhanced photo-current efficiency (PCE) as compared to other DSSCs and illustrated photovoltaic parameters; short-circuit current J_SC = 18 mA/cm², open circuit voltage (V_OC) = 700 mV, fill factor (FF) = 64% and PCE (η) = 8.06%. The electron transport and charge recombination behaviors of DSSCs were investigated by electrochemical impedance spectra (EIS) and the results illustrated that the DSSC-1 showed the lowest charge transport resistance (R_tr) and the longest electron lifetime (τ_eff). Therefore, in the present investigation, it could be concluded that the novel bi-layer photoanode with blocking layer increased the short circuit current, electron transport and suppressed the recombination of charge carriers at the photoanode/dye/electrolyte interface in DSSC-1.