Shu-Chia Shiu

Revisit of series-SSHI with comparisons to other interfacing circuits in piezoelectric energy harvesting

SSHI (synchronized switch harvesting on inductor) techniques have been demonstrated to be capable of boosting power in vibration-based piezoelectric energy harvesters. However, the effect of frequency deviation from resonance on the electrical response of an SSHI system has not been taken into account from the original analysis. Here an improved analysis accounting for such an effect is proposed to investigate the electrical behavior of a series-SSHI system. The analytic expression of harvested power is proposed and validated numerically. Its performance evaluation is carried out and compared with the piezoelectric systems using either the standard or parallel-SSHI electronic interfaces. The result shows that the electrical response of an ideal series-SSHI system is in sharp contrast to that of an ideal parallel-SSHI system. The former is similar to a strongly coupled electromechanical standard system operated at the open circuit resonance, while the latter is analogous to that operated at the short circuit resonance with different magnitudes of matching impedance. In addition, the performance degradation due to non-ideal voltage inversion is also discussed. It shows that a series-SSHI system avails against the standard technique in the case of medium coupling, since its peak power is close to the ideal optimal power and the reduction in power is less sensitive to frequency deviation. However, the consideration of inevitable diode loss in practical devices favors the parallel-SSHI technique, since the frequency-insensitive feature is much more pronounced in parallel-SSHI systems than in series-SSHI systems.

Electroluminescence from ZnO/Si-Nanotips Light-Emitting Diodes

A new and general approach to achieving efficient electrically driven light emission from a Si-based nano p-n junction array is introduced. A wafer-scale array of p-type silicon nanotips were formed by a single-step self-masked dry etching process, which is compatible with current semiconductor technologies. On top of the silicon nanotip array, a layer of n-type ZnO film was grown by pulsed laser deposition. Both the narrow line width of 10 nm in cathodoluminescence spectra and the appearance of multiphonon Raman spectra up to the fourth order indicate the excellent quality of the ZnO film. The turn-on voltage of our ZnO/Si nanotip array is found to be approximately 2.4 V, which is 2 times smaller than its thin film counterpart. Moreover, electroluminescence (EL) from our ZnO/Si nanotips array light-emitting diode (LED) has been demonstrated. Our results could open up new possibilities to integrate silicon-based optoelectronic devices, such as highly efficient LEDs, with standard Si ultralarge-scale integrated technology.

GaAs nanowire/ poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) hybrid solar cells

In this paper, a new type of hybrid solar cell based on a heterojunction between poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and vertically aligned n-type GaAs nanowire (NW) arrays is investigated. The GaAs NW arrays are fabricated by directly performing the nano-etching of GaAs wafer with spun-on SiO(2) nanospheres as the etch mask through inductively coupled plasma reactive ion etching. The PEDOT:PSS adheres to the surface of the GaAs NW arrays to form a p-n junction. The morphology of GaAs NW arrays strongly influences the characteristics of the GaAs NW/PEDOT:PSS hybrid solar cells. The suppression of reflectance and the interpenetrating heterojunction interface of GaAs NW arrays offers great improvements in efficiency relative to a conventional planar cell. Compared to the planar GaAs/PEDOT:PSS cells, the power conversion efficiency under AM 1.5 global one sun illumination is improved from 0.29% to 5.8%.

Formation of self-organized platinum nanoparticles and their microphotoluminescence enhancement in the visible light region

Formation of Pt nanoparticles or nanoisland films as a function of annealing temperature, initial thickness, underlying substrates, and annealing process is investigated. Using microphotoluminescence (PL) measurement, we find great enhancement of self-emission in visible spectrum from Pt nanoparticles. The integral intensity of the micro-PL of the 49.38nm Pt nanoparticles is 38 times of that of the Pt thin film. In addition, the peak wavelength varies from 554to615nm as the surface morphology of Pt changes due to different annealing parameters. Spectral analyses suggest that this enhancement of micro-PL from Pt is due to the local field enhancement mechanism analogous to that of PL from noble metals.

Transfer of aligned single crystal silicon nanowires to transparent substrates

We demonstrate the method of transferring aligned single crystal silicon nanowires (SiNWs) to transparent substrate. The alignment of the transferred nanowires is almost identical to the original one. The density of the transferred SiNWs can achieve 3×107 nanowires/mm2. The low temperature fabrication processes are compatible for a wide range of substrates. The transmission coefficient below 10 % at a wide bandwidth, 400-1100 nm, was found in the transferred SiNWs. The high dense aligned SiNWs are promising for future photovoltaic applications.

Improving the conductivity of hole injection layer by heating PEDOT:PSS

Poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonic acid) (PEDOT:PSS) is a common material of hole injection layer used in polymer light emitting diodes (PLEDs) and organic solar cells. It can improve the efficiency of the charge collection at the anode. It has been reported that adding glycerol to PEDOT:PSS could increase the conductivity and improve the efficiency of PLEDs and organic solar cells. However, it is less noticed that the conductivity could be improved when the solution of PEDOT was heated before deposition. Here we experimented different concentrations of glycerol into PEDOT:PSS to make G-PEDOT:PSS solution, and heated the G-PEDOT:PSS solution at different temperatures before deposition. The solutions are then spin-coated on the glass and annealed at 140 °C. The conductivity was then measured and compared. The experiments showed that the conductivity of pure PEDOT:PSS slightly increased for 2-3 times, while the G-PEDOT:PSS increased over two orders of magnitudes. The conductivity increased with the heating temperature before deposition. The enhancement of the conductivity of the G-PEDOT:PSS film was higher than that of the pure PEDOT:PSS film. The overall conductivity increase for over three orders of magnitude. The reason is because the high temperature causes the glycerol and PEDOT:PSS to mix evenly. This is helpful for the swelling and aggregation of colloidal PEDOT-rich particles, forming a highly conductive network. When G-PEDOT:PSS resistance is reduced, it may not only increase the hole collection ability, but also replace ITO as the anode layer due to its advantages of low production cost and high work function.

Fabrication of Silicon Nanostructured Thin Film and Its Transfer from Bulk Wafers onto Alien Substrates

Various Si nanostructures can be fabricated using a metal-assisted etching technique, which must be applied on bulk Si wafers, limiting its applications and wasting a significant amount of material. Here, we report a technique to form a Si nanostructured thin film created by metal-assisted chemical etching from bulk Si wafers and to transfer it onto alien substrates. To detach the Si nanostructured thin films completely from bulk Si wafers, a second-step metal-assisted chemical etching made the root of the Si nanostructures become fragile. The transferred Si nanostructures are well-aligned along the normal direction of the receiver substrate. The X-ray diffraction spectrum reveals that the transferred Si nanostructured thin films exhibit good crystal orientation and morphology. A strong light trapping effect between the nanostructures causes such films of 16 μm thickness to exhibit nearly 99% absorption from 400 to 800 nm. This exceeds the theoretically calculated limits of planar Si.

Fabrication of crystalline Si spheres with atomic-scale surface smoothness using homogenized KrF excimer laser reformation system

A technique applying the homogenized KrF excimer laser reformation to fabricate Si spheres on the silicon on insulator platform is presented. High-power excimer laser was used to illuminate the Si rods which were fabricated using typical procedures. The Si rods were then melted and reshaped to spheres due to surface tension. This method is capable of fabricating submicrometer Si spheres with extremely smooth surface. Atomic force microscopy was used to reveal the atomic-scale surface smoothness of the fabricated Si spheres. It shows that root-mean-square roughness is smaller than 0.1nm. In addition, tunneling electron microscopy was used to investigate crystalline property of the Si spheres, showing that single-crystalline Si with lattice plane spacing of about 0.24nm was formed after the transformation of the Si rod into the sphere.

Effect of nanowire length to silicon nanowire/PEDOT:PSS solar cells

Silicon nanowire (SiNW)/Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solar cells were fabricated by solution process. The effect of nanowire length to cell performance was investigated for the nanowire length varying from 0.73 μm to 5.59 μm. The nanowire length was found to have negative effect on the power conversion efficiency (PCE), in the condition of the fixed thickness of PEDOTPSS. The highest PCE of 7.02% was obtained for the wire length of 0.73 μm.

Controlled formation of well-aligned GaAs nanowires with a high aspect ratio on transparent substrates

In this study, we present a facile way to fabricate large-scale arrays of GaAs nanowires (NWs) with a high aspect ratio on transparent substrates. It is demonstrated that a monolayer of SiO2 nanoparticles can be effectively used as etch masks for the inductively coupled plasma (ICP) etching process. To form the monolayer of SiO2 nanoparticles on a GaAs substrate, the concentration and temperature of a SiO2 colloidal dispersion solution as well as the interface wetting of the GaAs substrate were investigated. Afterward, by adjusting the ICP etching conditions, the high-aspect-ratio GaAs NWs with cross-sections of 70 nm and lengths of 4.3 ?m were successfully fabricated. Furthermore, the fabricated GaAs NWs were massively transferred onto the transparent substrate at low temperature. The x-ray diffraction spectrum and the scanning electron microscope observation reveal that the transferred GaAs NWs have vertically aligned morphology and good crystal property.