Cha-Hsin Chao

High Efficiency Flexible Polymer Solar Cells Based on PET Substrates with a Nonannealing Active Layer

The inverted bulk-heterojunction solar cell on the polyester (PET) substrate with a nonannealing active layer is investigated. The atomic force microscope images show that the morphology of the nonannealing active layer of the inverted plastic solar cell evolves with time, which improves the performance of the solar cell. Our investigations show that the grain size of the active layer increases with time, resulting in improvements in the fill factor (from 34.8 to 62.8%) and shunt resistance (from 107 to 505 Ω cm 2 ) as well as a reduction in the series resistance (from 4.82 to 0.96 Ω cm 2 ). The easily processed inverted device with a nonannealing active layer on the indium tin oxide-coated PET substrate exhibits a high power conversion efficiency of ~ 3.66%.

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.

High-density one-dimensional well-aligned germanium quantum dots on a nanoridge array

The selective growth of high-density one-dimensional well-aligned Ge quantum dots (QDs) on the top of nanoridges patterned on Si substrate is reported. The period of ridge array is 150 nm, the width of each ridge is 80 nm, and the depth of the trench is 20 nm. The areal density of QDs is about 5.4×109 cm−2. Simulations of the chemical potential show that a proper distribution of the surface curvature may give rise to a suitable chemical potential minimum helping positioning the QDs. These ridges can also be used to control the shape and the uniformity of QDs.

Blackbody radiation modified to enhance blue spectrum

Blackbody radiation is modified to enhance the blue spectrum with photonic boxes of ∼200 nm. The modified blackbody radiation has two temperature-independent features. First, the enhanced blue light has the peak intensity pinched at 390 nm with an enhancement factor of over 5000. This peak wavelength corresponds to the resonance wavelength of the largest-number boxes. Second, the spectral width is 90 nm and is governed by the variation of the box size. The physics can be easily explained by the significantly enhanced density of states at a certain spectrum as a result of photonic boxes.

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.

Enhance the blue-light emission of black-body radiation with metallic photonic boxes

Black-body radiation can be modified for blue-light enhancement. Using photonic boxes of about 200 nm, the enhanced blue light has peak intensity at 390 nm and a spectral width of 90 nm, which are both temperature-independent. The spectrum is also suppressed at long-wavelength region, so the enhanced peak intensity is over 5 times stronger than the background level at 470 nm for all measured temperatures. In contrast, for usual black-body radiation at those temperatures, the intensity is over 1000 times weaker at 390 nm than above 470 nm. Other visible spectra can also be enhanced by increasing the size of photonic boxes.

Compact ZnO nanorods composed film by re-growth of ZnO nanorods and Ar plasma treatment

Compact, film-like ZnO nanorods were fabricated by re-growth of ZnO nanorods on base ZnO nanorods using a simple low-temperature hydrothermal technique. The Ar plasma treatment was employed to blur the boundaries of re-grown ZnO nanorods. The morphology, crystalline and electrical properties of both re-grown and Ar plasma treated re-grown ZnO nanorods were studied by SEM, AFM, XRD, and four point probe measurement. The SEM images and AFM analysis revealed the increasing diameter and compact arrangement of re-grown ZnO nanorods. The XRD analysis showed highly uniform [002] orientation. The resistivity of re-grown ZnO nanorods composed film is 3.906Ω*cm while resistivity of re-grown ZnO nanorods composed film with Ar plasma treatment is 2.231Ω*cm which indicates that the Ar plasma treatment blurred the boundaries therefore decreasing the resistivity. The optical transmittance of as-grown and re-grown ZnO nanorods in the visible range was higher than 80%.

Controlled growth of well-aligned ZnO mirco/nanorod arrays on GaN substrates using a novel solution method

This work reports on the controlled growth of well-aligned ZnO micro/nanorod arrays at low temperature on GaN substrates. The influence of GaN surface morphology and doping on the growth of ZnO rods via hydrothermal method is studied. The structural properties of ZnO rods are investigated using field-emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD). The grown ZnO rods show preferred orientation along the c-axis and are well aligned with high aspect ratios. For precisely controlling the formation of well-aligned ZnO micro/nanorod arrays, a two-dimensional periodic polymer aperture arrays on top of GaN is also utilized for selective growth of ZnO rods. The size of ZnO rods is controlled over a considerably wide size range from 130 nm up to 3.5 μm in diameter by tuning the aperture size of the pattern and solution concentrations. It is observed that the ZnO rods are not grown directly through the aperture template. In addition, the size of ZnO rods is found to be dependent on the geometry of polymer aperture arrays. The detailed growth behavior is characterized and analyzed. This work provides a route to achieve the low-temperature heteroepitaxy of ZnO mirco/nanorod arrays on GaN, which can be very useful for many optoelectronics applications, especially for light emitting diodes.

Influence of Architecture-Controlled GaN Rod Arrays on the Output Power of GaN LEDs

We report the influence of controlled lengths and densities of GaN rod arrays on the output power of GaN light-emitting diodes (LEDs). The morphology-controlled GaN rod arrays are fabricated via using ZnO rod arrays as a dry etching mask. Our investigation indicates that the output power of GaN LEDs has a strong dependence on the lengths and densities of GaN rod arrays. The variation of output power of GaN LEDs with GaN rod arrays is caused by the Fabry-Pérot resonance of the film composed by GaN rod arrays. The theoretical analysis also shows a good agreement with the measurement results.