Hsuen-Li Chen

Using colloidal lithography to fabricate and optimize sub-wavelength pyramidal and honeycomb structures in solar cells

The external quantum efficiency of solar cells can be improved by using texturing pyramid- and honeycomb-like structures with minimum reflection. In this study, we investigated the reflection properties of texturing structures through rigorous coupled-wave analysis and the three-dimensional finite-difference time domains (FDTD) method to analyze close-packed texturing structures. We also demonstrate a simple method-combining sub-wavelength-scale monolayer and bilayer polystyrene spheres with a one-step reactive ion etching process-to fabricate optimized pyramid- and honeycomb-shaped antireflection structures, respectively. Thus, sub-wavelength pyramidal and honeycomb-like structures displaying low reflectance were obtained readily without the need for any lithography equipment.

Top laminated graphene electrode in a semitransparent polymer solar cell by simultaneous thermal annealing/releasing method.

In this article, we demonstrate a semitransparent inverted-type polymer solar cell using a top laminated graphene electrode without damaging the underlying organic photoactive layer. The lamination process involves the simultaneous thermal releasing deposition of the graphene top electrode during thermal annealing of the photoactive layer. The resulting semitransparent polymer solar cell exhibits a promising power conversion efficiency of approximately 76% of that of the standard opaque device using an Ag metal electrode. The asymmetric photovoltaic performances of the semitransparent solar cell while illuminated from two respective sides were further analyzed using optical simulation and photocarrier recombination measurement. The devices consisting of the top laminated transparent graphene electrode enable the feasible roll-to-roll manufacturing of low-cost semitransparent polymer solar cells and can be utilized in new applications such as power-generated windows or multijunction or bifacial photovoltaic devices.

Robust Airlike Superhydrophobic Surfaces

Adv. Mater. 2010, 22, 597–601 2010 WILEY-VCH Verlag Gm T IO N Surfaces that exhibit superhydrophobic properties have attracted much attention for their use in a wide range of applications, such as electrowetting and self-cleaning. Wenzel and Cassie and Baxter have proposed models that consider the roles of the surface area and trapped air, respectively, to explain the effects of surface topography on the enhancement of hydrophobicity. In the Cassie state, the drop resembles a fakir resting on a bed of nails; therefore, this state is also known as the ‘‘fakir state’’. Although air trapped beneath the drop is conducive to lifting it from the surface (because a water drop has a contact angle of 1808 with air), increasing the air fraction of the surface may harm the sitting stability. Therefore, most Cassie states are unstable under external pressure, resulting in invasion and loss of water-repellent properties. This behavior has driven research into the design of nanostructures to investigate transitions between the Cassie and Wenzel states. Although a robust Cassie state could be achieved in theory by reducing the microstructure scale or increasing the Young contact angle, creating a nanostructured superhydrophobic state to improve the Young contact angle on other types of nanostructured surfaces remains a challenge. Most studies of such systems have involved the preparation of surfaces that exhibit roughness on two types of scales (micro and nano), but not on two nanometer scales. We present here a robust two-tier superhydrophobic Cassie state on a Si wafer surface – namely, a superhydrophobic Si nanograss constructed on a nanopillar array. The nanograss alone provided sufficient roughness to impart the large water contact angle required for superhydrophobicity, but it stuck quite strongly to the water drop. In contrast, the flat tops of the nanopillar array could not sustain the drop well when the space between the pillars reached 1mm. After fabricating the nanograss on the nanopillars, the two-tier topography became slippery. When we increased the spacing between the nanopillars to 5mm in the nanograss-on-nanopillar structure, over 99.9% of the surface was composed of air and, therefore, the water contact angle reached close to 1808 with a sliding angle close to 08. More interesting, a water drop that had been subjected to a pressure of 234 Pa readily rolled off the surface of this airlike nanograss-on-nanopillar material when tilted at an angle of 0.18. Our two-tier airlike structures consisted of nanopillars and nanograss. The nanopillars were fabricated using e-beam lithography and dry etching, followed by hydrogen plasma etching to form the nanograss on the surface. The designed pillar diameter was 100 nm, the pillar height was 1mm, and the space between the pillars ranged from 200nm to 50mm. The average diameter and length of the blades of nanograss were approx. 20 and 200 nm, respectively – smaller than the typical dimensions of the nanograss in ‘‘black silicon’’. After plasma etching, the surface was hydrophobized with CHF3 plasma for 9 s. Figure 1 presents scanning electron microscopy (SEM) images of a typical airlike sample possessing a pillar space of 5mm. The air fraction, calculated from the top-view image, was approx. 99.98%. Because the nanograss was created through anisotropic plasma etching, the sides of the nanopillars possessed smooth surfaces lacking fine roughness. Water contact angle measurements revealed that the nanograss, nanopillar, and two-tier surfaces were all superhydrophobic, with the nanograss surface being the stickiest. An 8mL water drop skated easily across the two-tier roughness pattern, but stuck to the edge of the pattern where the surface was decorated with nanograss only (see Supporting Information, Movie 1). Figure 2 presents the advancing, receding, and sliding contact angles for

Extended red light harvesting in a poly(3-hexylthiophene)/iron disulfide nanocrystal hybrid solar cell

A polymer solar cell based on poly(3-hexylthiophene) (P3HT)/iron disulfide (FeS2) nanocrystal (NC) hybrid is presented. The FeS2 NCs of 10 nm in diameter were homogeneously blended with P3HT to form an active layer of a solar cell. An extended red light harvesting up to 900 nm resulting from the NCs in the device has been demonstrated, compared to a typical absorption edge of 650 nm of a pristine P3HT. The environmentally friendly and low-cost FeS2 NCs can be used as a promising candidate for an acceptor in the polymer solar cell device application with an enhanced photovoltaic response in the extended red light region.

Silicon-based broadband antenna for high responsivity and polarization-insensitive photodetection at telecommunication wavelengths

Although the concept of using local surface plasmon resonance based nanoantenna for photodetection well below the semiconductor band edge has been demonstrated previously, the nature of local surface plasmon resonance based devices cannot meet many requirements of photodetection applications. Here we propose the concept of deep-trench/thin-metal (DTTM) active antenna that take advantage of surface plasmon resonance phenomena, three-dimensional cavity effects, and large-area metal/semiconductor junctions to effectively generate and collect hot electrons arising from plasmon decay and, thereby, increase photocurrent. The DTTM-based devices exhibited superior photoelectron conversion ability and high degrees of detection linearity under infrared light of both low and high intensity. Therefore, these DTTM-based devices have the attractive properties of high responsivity, extremely low power consumption, and polarization-insensitive detection over a broad bandwidth, suggesting great potential for use in photodetection and on-chip Si photonics in many applications of telecommunication fields.

Eco-friendly plasmonic sensors: using the photothermal effect to prepare metal nanoparticle-containing test papers for highly sensitive colorimetric detection.

Convenient, rapid, and accurate detection of chemical and biomolecules would be a great benefit to medical, pharmaceutical, and environmental sciences. Many chemical and biosensors based on metal nanoparticles (NPs) have been developed. However, as a result of the inconvenience and complexity of most of the current preparation techniques, surface plasmon-based test papers are not as common as, for example, litmus paper, which finds daily use. In this paper, we propose a convenient and practical technique, based on the photothermal effect, to fabricate the plasmonic test paper. This technique is superior to other reported methods for its rapid fabrication time (a few seconds), large-area throughput, selectivity in the positioning of the NPs, and the capability of preparing NP arrays in high density on various paper substrates. In addition to their low cost, portability, flexibility, and biodegradability, plasmonic test paper can be burned after detecting contagious biomolecules, making them safe and eco-friendly.

Quantitative nanoscale monitoring the effect of annealing process on the morphology and optical properties of poly(3-hexylthiophene)/[6,6]-phenyl C61-butyric acid methyl ester thin film used in photovoltaic devices

We have studied the nanoscale changes in morphology and optical properties during annealing for bulk-heterojunction poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) composite film. Thermal atomic force microscopy was used to monitor the morphology evolution of the film in situ quantitatively, which showed a migration and aggregation of PCBM with increasing temperature. Scanning near-field microscopy was used to investigate the quantitative changes in absorption behavior of the film in nanoscale with increasing annealing time at 140 °C, which indicated that the extent of absorption of the film was increased with increasing annealing time. However, a large PCBM aggregate (1 μm) was formed after the film annealed at 140 °C for 1 h. The aggregate interrupted the bicontinous morphology of the film and further affected the absorption behavior in nanoscale. Furthermore, the refractive index and extinction coefficient of the films increased after annealed 30 min at 140 °C, but d...

Using Spectroscopic Ellipsometry to Characterize and Apply the Optical Constants of Hollow Gold Nanoparticles

In this paper, we report the optical constants (refractive index, extinction coefficient) of self-assembled hollow gold nanoparticle (HGN) monolayers determined through spectroscopic ellipsometry (SE). We prepared a series of HGNs exhibiting various morphologies and surface plasmon resonance (SPR) properties. The extinction coefficient (k) curves of the HGN monolayers exhibited strong SPR peaks located at wavelengths that followed similar trends to those of the SPR positions of the HGNs in solution. The refractive index (n) curves exhibited an abnormal dispersion that was due to the strong SPR extinction. The values of Deltan and kmax both correlated linearly with the particle number densities. From a comparison of the optical constant values of HGNs with those of solid Au nanoparticles (NPs), we used SE measurements to demonstrate a highly sensitive Si-based chemical sensor. HGNs display a slightly lower value of k at the SPR peak but a much higher sensitivity to changes in the surrounding medium than do solid Au NPs.

Regioregularity effects in the chain orientation and optical anisotropy of composite polymer/fullerene films for high-efficiency, large-area organic solar cells

In this paper, we demonstrate the strong influence of the regioregularity (RR) of poly(3-hexylthiophene) (P3HT) on the optical anisotropy of hybrid P3HT/fullerene films before and after thermal annealing. We determined the conversion efficiency and characterized the optical anisotropy of P3HT/6,6-phenyl-C61-butyric acid methyl ester (PCBM) blends featuring various values of RR. Unlike grazing-incidence X-ray diffraction analysis, optical anisotropic measurement provides a clear and convenient view of the polymer orientation and the device anisotropic absorption at the same time. By calculating the in-plane and out-of-plane optical constants (extinction coefficients and refractive indices), we determined that the optical anisotropy of P3HT/PCBM films was improved in both orientations upon increasing the RR. Upon increasing the thermal annealing temperature, the main chains of high-RR P3HT were converted from an amorphous structure to an alignment parallel to the substrate, resulting in higher optical anisotropy. The degree of anisotropy of the high-RR P3HT/PCBM blend was up to six times higher than that of the low-RR sample. This strong RR effect on optical anisotropy was also evident in the power conversion efficiency of large-area P3HT/PCBM-based organic solar cells.

Romantic Story or Raman Scattering? Rose Petals as Ecofriendly, Low-Cost Substrates for Ultrasensitive Surface-Enhanced Raman Scattering

In this Article, we present a facile approach for the preparation of ecofriendly substrates, based on common rose petals, for ultrasensitive surface-enhanced Raman scattering (SERS). The hydrophobic concentrating effect of the rose petals allows us to concentrate metal nanoparticle (NP) aggregates and analytes onto their surfaces. From a systematic investigation of the SERS performance when using upper and lower epidermises as substrates, we find that the lower epidermis, with its quasi-three-dimensional (quasi-3D) nanofold structure, is the superior biotemplate for SERS applications. The metal NPs and analytes are both closely packed in the quasi-3D structure of the lower epidermis, thereby enhancing the Raman signals dramatically within the depth of focus (DOF) of the Raman optical system. We have also found the effect of the pigment of the petals on the SERS performance. With the novel petal-based substrate, the SERS measurements reveal a detection limit for rhodamine 6G below the femtomolar regime (10(-15) M), with high reproducibility. Moreover, when we employ an upside-down drying process, the unique effect of the Wenzal state of the hydrophobic petal surface further concentrate the analytes and enhanced the SERS signals. Rose petals are green, natural materials that appear to have great potential for use in biosensors and biophotonics.