Zhonghua Deng

Dye-sensitized solar cells based on nanoparticle-decorated ZnO/TiO2 core/shell nanorod arrays

Nanoparticles (NPs) decorated ZnO/TiO2 core/shell nanorod arrays were fabricated on transparent conductive glass substrates by sequential plasma deposition and post-annealing processes for dye-sensitized solar cells (DSSCs) applications. The NPs decorated ZnO/TiO2 nanorods were composed of single-crystalline ZnO nanorods, homogeneously coated thin TiO2 shells and entirely covered anatase TiO2 NPs. The photocurrent density of the composite electrode was largely enhanced due to the enlarged surface area, the dark current was suppressed and the open-circuit voltage was increased because of the energy barrier formed at the interface between the ZnO core and the TiO2 shell. The increased photocurrent and open-circuit voltage led to an improvement of twice the energy conversion efficiency.

The effects of shell characteristics on the current-voltage behaviors of dye-sensitized solar cells based on ZnO/TiO2 core/shell arrays

The ZnO/TiO2 core/shell structure was formed through deposition of a TiO2 coating layer on the hydrothermally fabricated ZnO nanorod arrays through radio frequency magnetron sputtering. The effects of the TiO2 shell’s characteristics on the current-voltage behaviors of the core/shell-based dye-sensitized solar cells (CS-DSSC) were investigated. As the rates of injection, transfer, and recombination of electrons of such CS-DSSC were affected significantly by the crystallization, morphology, and continuity of the TiO2 shells, the photovoltaic efficiency was accordingly varied remarkably. In addition, the efficiency was further improved by enhancing the surface area in the core/shell electrode.

Introduction on the fabrication technique of phosphor in glass by tape-casting and investigation on the chromaticity property

We introduce a new fabrication technique of phosphor in glass (PiG) for light-emitting diodes (LEDs) employing the tape-casting. Through the detailed process described herein and the measurement results, it is clear that the PiG-on-glass not only share the same characteristic of those obtained from other techniques or the bulk PiG, but with more precisely controlled width from a few to hundreds micrometers. The samples are mounted on blue InGaN LED chips to test the color properties of the white light. Besides, we established an empirical model that could predict the final color properties of LEDs solely by the phosphor concentration of phosphor glass under certain conditions. This model would greatly facilitate the design of PiG-based LEDs.

Physical properties and growth kinetics of co-sputtered indium-zinc oxide films

Indium-doped zinc oxide (IZO) films were fabricated by radio-frequency (RF) magnetron sputtering through co-deposition of zinc oxide (ZnO) and indium (In) at ambient temperature. Transparency of the IZO films is higher than 80% in the visible range while the optical band gap decreases with increasing In dopant concentration. The optimal measured resistivity, Hall mobility and carrier concentration of the film are 1.39 × 10−3 Ω cm, 10.11 cm2 V−1 s−1 and 5.35 × 1020 cm−3, respectively. The In dopant favors smoothing the film surface. The In dopant concentration in the film decreases vertically from the surface to the interface due to the surface segregation during the kinetic deposition process. The misfit strains of the films are gradually relaxed through dislocation.

Observation of Fluorescence and Phosphorescence in Ca2MgSi2O7 : Eu2 + , Dy3 + Phosphors

This paper describes the experimental results of the fluorescence and phosphorescence observations in the Ca 2 MgSi 2 O 7 :Eu 2+ ,Dy 3+ phosphors. Discussions are made to elucidate the relationship between the photoluminescence of Eu 2+ and the crystal structure of the host, the luminescent quenching of the emission of Eu 2+ , and the role of Dy 3+ in the afterglow property. There exist two distinguishable Ca 2+ sites in the host, supported by two fluorescent emission bands located in the blue and green regions, respectively, originating from Eu 2+ doped into the host. The emission peak of Eu 2+ at about 474 nm quickly quenches as its concentration increases due to excitation energy transfer between the Eu 2+ ions doped into the two nonequivalent Ca 2+ sites. The answers to the question of why only one emission band was observed in some reports throughout the literature were supplied. Only the emission at 533 nm of Eu 2+ contributes to the long-lasting phosphorescence, whereas Dy 3+ ions do not act as the luminescent center. By investigating the decay time of the phosphorescence at 533 nm, preliminary conclusions were made that the co-doping of Dy 3+ obviously enhances the afterglow of Ca 2 MgSi 2 O 7 :Eu 2+ through electron trap-detrap and recombination processes by its electron trapping role.

Study on the correlations between color rendering indices and the spectral power distribution

The intrinsic spectrally resolved sensitivity (ISRS) of color rendering indices (CRIs) is investigated by using spectral loss simulations. It is demonstrated that R(a) exhibits large sensitivities around 444, 480, 564, and 622 nm, while for R(9) the sensitivity peaks are around 461, 581 and 630 nm, which all shift slightly with the correlated color temperature. If considering the ISRS as a bridge between the spectral power distribution of LED and its CRI, one could obtain a high CRI by minimizing the deviation between the shapes of the illuminant spectrum and the reference spectrum, both after modulations by the ISRS as a weighting function. This approach, recommended as a guideline for the spectra design aiming at a high CRI, is described and justified in depth via a mathematical model. This method is spectra-oriented and could largely facilitate the spectra design.

Optical enhancement brought by doping Gd(3+) ions into Ce: YAG ceramics for indoor white light-emitting diodes.

We dope Gd(3+) ions into Ce: YAG ceramics to induce red-shift in the photoluminescence, of which the degree is proportional to the Gd(3+) doping concentration. This kind of ceramic, when collaborating with InGaN blue chips, proves to be a promising fluorescent material of white light-emitting diodes, for not only its high in-line transmittance and decent quantum yield, but also the improvement in color rendering ability brought by the red-shift, which enhances the ratio of red portion in spectra. We demonstrate that 10% is the optimized value of Gd(3+) doping concentration, as it provides the maximum color rendering index of ~78 with luminous efficiency as high as 128 lm/W.

Crystal Structure and Luminescent Properties of Eu2 + -Doped Li2BaSiO4 with a Polymorph for White LEDs

The crystal structure and photoluminescence of Li 2 BaSiO 4 :Eu 2+ phosphors were investigated. A preliminary conclusion was reached through X-ray diffraction, Fourier transform infrared absorption, photoluminescent excitation and emission investigations that a Li 2 BaSiO 4 polymorph was found. The Li 2 BaSiO 4 :Eu 2+ characterizes an intense blue emission with a peak at 465 nm and a broad excitation band in the UV/visible range. Warm white light with CIE coordinates (0.334, 0.347) and color temperature of about 5500 K is generated by mixing this blue phosphor with the Li 2 SrSiO 4 :Eu 2+ yellow phosphor. These results indicate that Li 2 BaSiO 4 :Eu 2+ with the polymorph of the host would be a promising blue phosphor candidate for UV-chip-based multiphosphor converted white light-emitting diodes (LEDs).

Low content indium-doped zinc oxide films with tunable work function fabricated through magnetron sputtering

Low content indium-doped zinc oxide (LC-IZO) films have been prepared through radio-frequency (RF) magnetron sputtering. The work functions of the LC-IZO films were successfully tuned up by modulating the deposited conditions, such as indium content, substrate temperature, bias voltage and film thickness. The work functions of the films may be modulated in a large range between 4.59 eV and 5.56 eV. The change of the work function may be the results induced by the change in the surface state. The results may be helpful for widening the application of LC-IZO films.

Defects dynamics during ageing cycles of InGaN blue light-emitting diodes revealed by evolution of external quantum efficiency--current dependence.

We report in detail the defect dynamics in the active region by monitoring the external quantum efficiency (EQE) - injection current curves, I-V curves, and electroluminescence spectra during the ageing test, under a forward current of 850 mA (85 A/cm2), room temperature. We apply a two-level model to analyze the EQE curves and the electroluminescence spectra. The results suggest that high injection density during the ageing may reduce the density of the Shockley-Reed-Hall nonradiative recombination centers and enhance the carrier mobility and diffusion length. The former effect would directly lead to initial surge of EQE, whereas the latter would enhance the effect of extended defects which leads to reduction in peak EQE and increase in EQE droop rate.