Hsin-Ming Cheng

High-efficiency metal-free organic-dye-sensitized solar cells with hierarchical ZnO photoelectrode

Self-assembled ZnO secondary nanoparticles have been fabricated as an effective photoelectrode for dye-sensitized solar cells (DSCs). The hierarchical architecture, which manifested the significant light-scattering, can provide more photon harvesting. In addition, dye-molecule adsorption was sufficient due to enough internal surface area provided by the primary single nanocrystallites. Two indoline dyes, coded D149 and D205, were used as the sensitizers of ZnO DSCs with the optimal energy conversion efficiencies of 4.95% and 5.34%, respectively, under AM 1.5 full sunlight illumination (100 mW cm−2). The enhancement of the open-circuit photovoltage (Voc) and the short-circuit photocurrent density (Jsc) for D205-sensitized ZnO DSCs was ascribed to the effective suppression of electron recombination by extending the alkyl chain on the terminal rhodanine moiety from ethyl to octyl. Further evidence is obtained from the electrochemical impedance spectroscopy (EIS) which exhibits a longer electron lifetime for D205-sensitized ZnO DSC in comparison with the D149-sensitized one.

Efficient electron transport in tetrapod-like ZnO metal-free dye-sensitized solar cells

The tetrapod-like ZnO nanopowders are employed to construct an efficient electron transport network as the photoanode of the dye-sensitized solar cells (DSCs). Due to the high extinction coefficient of the metal-free D149 dye offering a high photocurrent through the ZnO network, the best performance of DSCs based on a 42 µm terapod-like ZnO film showed a high energy conversion efficiency of 4.9% with a high short-circuit photocurrent density of 12.3 mA cm−2, an open-circuit photovoltage of 0.6 V, and a fill factor of 0.65 under AM 1.5 irradiation. Highly efficient electron transport may also be ascribed by a long effective electron diffusion length of 46 µm determined from the electrochemical impedance spectroscopy which is consistent with thickness dependent JSC measurements.

Stress-mediated magnetic anisotropy and magnetoelastic coupling in epitaxial multiferroic PbTiO3-CoFe2O4 nanostructures

This study reports a self-assembled multiferroic nanostructure, composed of PbTiO3 (PTO) pillars embedded in a CoFe2O4 (CFO) matrix, deposited on MgO(001) by pulsed laser deposition. The epitaxial relationship in the PTO-CFO nanostructure is (100)[101]PTO∥(001)[101]CFO∥(001)[101]MgO, confirming the in-plane aligned polarization of PTO. The perpendicular magnetic anisotropy of this thin film results from the magnetoelastic anisotropy that exceeds the shape anisotropy. The increased frequency and the enhanced intensity of the tetrahedral (T-) site phonon modes by increasing the magnetic field indicate strong magnetoelastic coupling through magnetostriction in this multiferroic nanostructure. The anisotropic Raman strength enhancement of the T-site phonon along different directions suggests the magnetoelastic coupling is most efficient in the in-plane direction.

The substrate effect on the in-plane orientation of vertically well-aligned ZnO nanorods grown on ZnO buffer layers

Vertically well-aligned ZnO nanorods were synthesized without employing any metal catalysts on various substrates including glass, Si(111), 6H-SiC(0001) and sapphire (0001), which were pre-coated with c-oriented ZnO buffer layers, by simple physical vapour deposition. The alignments of the ZnO nanorods on the different substrates depend on the crystallographic alignments of the pre-coated ZnO buffer layers. The ZnO nanorods grown on glass and Si(111) are vertically aligned but randomly oriented in the in-plane direction. In contrast, the vertically aligned ZnO nanorods on 6H-SiC(0001) and sapphire (0001) show an in-plane alignment with azimuthally sixfold symmetry, which indicates the epitaxial relationship between ZnO and the substrate. Similarly, photoluminescence measurements show the distinct appearance of ZnO nanorods on different substrates. Besides the UV band, which was attributed to the recombination of free excitons near the band edge, defect-related visible emissions were also observed for the samples grown on both glass and Si(111) substrates. However, the ZnO nanorods exhibit only strong band edge emission peaks with no noticeable deep level emissions when grown on the 6H-SiC(0001) and sapphire (0001) substrates, which confirms the good crystalline and optical quality of the epitaxial ZnO nanorods.

Passive mode locking of ytterbium- and erbium-doped all-fiber lasers using graphene oxide saturable absorbers.

Broadband graphene oxide/PVA films were used as saturable absorbers (SAs) for mode locking erbium-doped fiber laser (EDFL) and ytterbium-doped fiber laser (YDFL) at 1.06 μm and 1.55 μm. They provide modulation depths of 3.15% and 6.2% for EDFL and YDFL, respectively. Stable self-starting mode-locked pulses are obtained for both lasers, confirming that the graphene oxide is cost-effective. We have generated mode-locked pulses with spectral width, repetition rate, and pulse duration of 0.75 nm, 9.5 MHz, and 2.7 ps. This is the shortest pulse duration directly obtained from an all-normal-dispersion YDFL with graphene-oxide saturable absorber.

Size dependence of the electronic structures and electron-phonon coupling in ZnO quantum dots

The electronic structures and optical properties of various sizes of ZnO quantum dots (QDs) were studied using x-ray absorption, photoluminescence, and Raman spectroscopy. The increase in the intensity ratio of the second-order Raman spectra of longitudinal optical mode and its fundamental mode, which is related to the strength of the electron-phonon coupling (EPC), is found to increase with the size of QD. The trend of EPC also correlates with the increase of the intensity ratio of the O 2pπ (Iπ) and 2pσ (Iσ) orbital features in the O K-edge x-ray absorption near-edge structure (XANES) as the size of QD increases. The EPC and XANES results suggest that the crystal orientations of ZnO QDs are approximately aligned with the c axis parallel with the polarization of x-ray photons.

Spin and phonon anomalies in epitaxial self-assembled CoFe2O4-BaTiO3 multiferroic nanostructures

Temperature dependent magnetic and phonon anomalies in epitaxial self-assembled CoFe2O4 (CFO) rods embedded in BaTiO3 (BTO) matrix nanostructures were investigated. The temperature dependence of A1(2TO) phonon frequency of BTO indicates that the BTO matrix experiences structural transformations. The lattice strain produced during the structural transformations drives spin reorientation in CFO rods, resulting in anomalous changes of magnetization. Through correlating the phonon anomalies with the increase of in-plane spin ordering, we show the spin-phonon coupling induces the softening of A1g and A1(2TO) phonons. It suggests that spin strongly couples with lattice strain and phonons in this nanostructure.

Anisotropic strain, magnetic properties, and lattice dynamics in self-assembled multiferroic CoFe2O4-PbTiO3 nanostructures

We investigate the anisotropic stress dependent magnetic and phonon behaviors in self-assembled CoFe2O4-PbTiO3 (CFO-PTO) nanostructures deposited on SrRuO3 buffered SrTiO3 substrates of various thickness. The increased vertical compressed stress with increasing thickness enhances the vertical magnetic anisotropy of CFO while gradually reducing the vertical polarization of PTO. By applying the magnetic-field dependent Raman scattering, the CFO-A1g and T2g(1) phonon frequencies shift oppositely because of the magnetostriction. Moreover, the PTO-A1 mode intensities anomalously enhance and the A1 and E mode frequencies, respectively, increase and decrease, which prove the existence of the stress-mediated magnetoelectric (ME) effect. This study shows that the ME transmission process is dominated by interfacial stress, which is critical for enhancing the ME efficiency.

Functional graded fullerene derivatives for improving the fill factor and device stability of inverted-type perovskite solar cells

A graded fullerene derivative thin film was used as a dual-functional electron transport layer (ETL) in CH3NH3PbI3 (MAPbI3) solar cells, to improve the fill factor (FF) and device stability. The graded ETL was made by mixing phenyl-C61-butyric acid methyl ester (PCBM) molecules and C60-diphenylmethanofullerene-oligoether (C60-DPM-OE) molecules using the spin-coating method. The formation of the graded ETLs can be due to the phase separation between hydrophobic PCBM and hydrophilic C60-DPM-OE, which was confirmed by XPS depth-profile analysis and an electron energy-loss spectroscope. Comprehensive studies were carried out to explore the characteristics of the graded ETLs in MAPbI3 solar cells, including the surface properties, electronic energy levels, molecular packing properties and energy transfer dynamics. The elimination of the s-shape in the current density-voltage curves results in an increase in the FF, which originates from the smooth contact between the C60-DPM-OE and hydrophilic MAPbI3 and the formation of the more ordered ETL. There was an improvement in device stability mainly due to the decrease in the photothermal induced morphology change of the graded ETLs fabricated from two fullerene derivatives with distinct hydrophilicity. Consequently, such a graded ETL provides dual-functional capabilities for the realization of stable high-performance MAPbI3 solar cells.

High-quality and Large-size Topological Insulator Bi2Te3-Gold Saturable Absorber Mirror for Mode-Locking Fiber Laser.

A novel high-quality, large-size, reflection-type topological insulator Bi2Te3-Gold (BG) film-based nonlinear optical modulator has been successfully fabricated as a two-dimensional saturable absorber mirror (SAM) by pulsed laser deposition (PLD). This BG-SAM possesses saturation fluence of 108.3 μJ/cm2, modulation depth (ΔR) of 6.5%, non-saturable loss of 38.4%, high damage threshold above 1.354 mJ/cm2 and excellent uniformity providing for the generation of passive mode-locked (ML) pulses for erbium-doped fiber lasers (EDFLs) on a large sample area. Under 124 mW 976 nm pumping, We obtained 452-fs continuous-wave ML pulses with pulse energy of 91 pJ and full width at half-maximum (FWHM) of 6.72-nm from this EDFL. The results clearly evidence that the PLD is an efficient method for fabricating BG-SAM that is suitable for a compact ultrafast ML fiber laser system.