Hsueh-Chung Liao

Nitrogen-Doped Anatase Nanofibers Decorated with Noble Metal Nanoparticles for Photocatalytic Production of Hydrogen

We report the synthesis of N-doped TiO(2) nanofibers and high photocatalytic efficiency in generating hydrogen from ethanol-water mixtures under UV-A and UV-B irradiation. Titanate nanofibers synthesized by hydrothermal method are annealed in air and/or ammonia to achieve N-doped anatase fibers. Depending on the synthesis route, either interstitial N atoms or new N-Ti bonds appear in the lattice, resulting in slight lattice expansion as shown by XPS and HR-TEM analysis, respectively. These nanofibers were then used as support for Pd and Pt nanoparticles deposited with wet impregnation followed by calcination and reduction. In the hydrogen generation tests, the N-doped samples were clearly outperforming their undoped counterparts, showing remarkable efficiency not only under UV-B but also with UV-A illumination. When 100 mg of catalyst (N-doped TiO(2) nanofiber decorated with Pt nanoparticles) was applied to 1 L of water-ethanol mixture, the H(2) evolution rates were as high as 700 μmol/h (UV-A) and 2250 μmol/h (UV-B) corresponding to photo energy conversion percentages of ∼3.6 and ∼12.3%, respectively.

Quantitative Nanoorganized Structural Evolution for a High Efficiency Bulk Heterojunction Polymer Solar Cell

We have developed an improved small-angle X-ray scattering (SAXS) model and analysis methodology to quantitatively evaluate the nanostructures of a blend system. This method has been applied to resolve the various structures of self-organized poly(3-hexylthiophene)/C61-butyric acid methyl ester (P3HT/PCBM) thin active layer in a solar cell from the studies of both grazing-incidence small-angle X-ray scattering (GISAXS) and grazing-incidence X-ray diffraction (GIXRD). Tuning the various length scales of PCBM-related structures by a different annealing process can provide a flexible approach and better understanding to enhance the power conversion of the P3HT/PCBM solar cell. The quantitative structural characterization by this method includes (1) the mean size, volume fraction, and size distribution of aggregated PCBM clusters, (2) the specific interface area between PCBM and P3HT, (3) the local cluster agglomeration, and (4) the correlation length of the PCBM molecular network within the P3HT phase. The above terms are correlated well with the device performance. The various structural evolutions and transformations (growth and dissolution) between PCBM and P3HT with the variation of annealing history are demonstrated here. This work established a useful SAXS approach to present insight into the modeling of the morphology of P3HT/PCBM film. In situ GISAXS measurements were also conducted to provide informative details of thermal behavior and temporal evolution of PCBM-related structures during phase separation. The results of this investigation significantly extend the current knowledge of the relationship of bulk heterojunction morphology to device performance.

Nanoparticle-tuned self-organization of a bulk heterojunction hybrid solar cell with enhanced performance.

We demonstrate here that the nanostructure of poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PCBM) bulk heterojunction (BHJ) can be tuned by inorganic nanoparticles (INPs) for enhanced solar cell performance. The self-organized nanostructural evolution of P3HT/PCBM/INPs thin films was investigated by using simultaneous grazing-incidence small-angle X-ray scattering (GISAXS) and grazing-incidence wide-angle X-ray scattering (GIWAXS) technique. Including INPs into P3HT/PCBM leads to (1) diffusion of PCBM molecules into aggregated PCBM clusters and (2) formation of interpenetrating networks that contain INPs which interact with amorphous P3HT polymer chains that are intercalated with PCBM molecules. Both of the nanostructures provide efficient pathways for free electron transport. The distinctive INP-tuned nanostructures are thermally stable and exhibit significantly enhanced electron mobility, external quantum efficiency, and photovoltaic device performance. These gains over conventional P3HT/PCBM directly result from newly demonstrated nanostructure. This work provides an attractive strategy for manipulating the phase-separated BHJ layers and also increases insight into nanostructural evolution when INPs are incorporated into BHJs.

Achieving a high fill factor for organic solar cells

Organic photovoltaics (OPVs) have developed rapidly in the last decades due to their potential for providing cost-efficient, low-energy consumption, and environmentally friendly renewable energy sources. Some research reports have focused on the device physics of organic photovoltaics that governs open circuit voltage (Voc) and short circuit current (Jsc) to improve their performance. In this review, we focus on the third parameter, fill factor (FF), that is equally important in determining the power conversion efficiency. We discuss the mathematical calculation of the FF and the relationship between the FF and equivalent circuit model elements, namely, shunt resistance, series resistance, and diode ideal factor. In order to provide a strategy toward a high FF for OPVs from the viewpoints of device design and material synthesis, we review important device features and BHJ features that have a large impact on the device FF, including preventing shorting, buffer layer design, domain size or purity, gradated BHJ structures, π–π stacking distance or direction, etc. We hope this article can provide a comprehensive insight into elements controlling the FF of OPVs and give a valuable direction for better device and material design.

Facile synthesis of wurtzite copper–zinc–tin sulfide nanocrystals from plasmonic djurleite nuclei

The present research demonstrates a facile one-pot heating process without injection to synthesize an important light harvesting quaternary nanocrystal: wurtzite copper–zinc–tin sulfide (w-CZTS). High quality w-CZTS nanocrystals can be easily obtained by mixing all the precursors and simply heating to the reaction temperature. The nano-crystal formation mechanism is thoroughly investigated and resolved by X-ray diffraction spectroscopy (XRD), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). It starts with the nucleation of plasmonic djurleite Cu1.94S, subsequent growth of CZTS–Cu1.94S heterostructures and inter-diffusion of cations and then finally leads to single phase and single crystal w-CZTS nanocrystals. The mechanism of nanocrystal formation can be applied universally regardless of the type of zinc and tin precursor for high quality w-CZTS nanocrystals. The in-depth interpretations of the reaction mechanism of this process significantly advance the current knowledge of multi-component nanocrystal formation. The developed method is scalable for high throughput and low cost w-CZTS suspensions which await practical photovoltaic applications.

Bi-hierarchical nanostructures of donor–acceptor copolymer and fullerene for high efficient bulk heterojunction solar cells

Solvent additive processing has become the most effective method to tune the nanostructure of donor–acceptor (D–A) type copolymer/fullerene bulk heterojunctions (BHJs) solar cells for improving power conversion efficiencies. However, to date qualitative microscopic observations reveal discrepant results on the effects of solvent additives. Here, we present quantitative evolution of bi-hierarchical nanostructure of D–A copolymers and fullerenes by employing grazing-incidence small/wide angle X-ray scattering (GISAXS/GIWAXS) techniques and [2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]/[6,6]-phenyl-C71-butyric acid methyl ester (PCPDTBT/PCBM) BHJ as model materials. An accurate GISAXS model analysis is established herein for revealing the distinctive bi-hierarchical nanostructures from molecular level to a scale of hundreds of nanometers. The mechanisms of hierarchical formation and mutual influence between PCPDTBT and PCBM domains are proposed to correlate with photovoltaic properties. These results provide a comprehensive interpretation in respect to previous studies on the nanostructures of D–A copolymer/fullerene BHJs. It is helpful for optimum structural design and associated synthesis improvement for achieving high efficiency BHJ solar cells.

Diketopyrrolopyrrole-based oligomer modified TiO2 nanorods for air-stable and all solution processed poly(3-hexylthiophene):TiO2 bulk heterojunction inverted solar cell

Diketopyrrolopyrrole-based oligomer was synthesized and used to modified TiO2 nanorods. The surface modified TiO2 was employed in the fabrication of air-stable and all solution processed poly(3-hexylthiophene):titanium dioxide nanorods (P3HT:TiO2 nanorods) bulk heterojunction (BHJ) inverted solar cells. The oligomer (copolymerized 4,5-diaza-9,9′-spirobifluorene with diketopyrrolopyrrole (PZFDPP)) was synthesized by Stille coupling reaction. The PZFDPP was coated on TiO2 nanorods by refluxing the TiO2 nanorods in oligomer containing solution at low temperature (70 °C). A concentration gradient profile of polymer/nanocrystals (P3HT/TiO2 nanorods) BHJ was observed for the first time by X-ray photoelectron spectroscopy (XPS) technique together with in situ ion sputtering, showing that the TiO2-rich region and P3HT rich region are aggregated adjacent to electron transport layer (ETL) and hole transport layer (HTL) respectively. The obtained depth profile indicates the inverted device structure is more suitable for polymer/inorganic nanocrystals BHJ solar cells. Furthermore, instead of using an energy consuming process for ETL layer deposition, the PZFDPP modified TiO2 nanorods were used to deposit the ETL layer by spin coating. The surface features and properties of deposited TiO2 ETL that was coated by PZFDPP were systematically investigated. The developed photovoltaic device shows a promising power conversion efficiency (PCE) of 1.2% benefited from improved electron mobility in P3HT:TiO2 BHJ film and across the ETL/active layer interfaces. Moreover, the device is extremely stable stored in ambient condition without encapsulation (less than 10% loss over 1000 h test). The results of this work demonstrate the successful development of highly efficient and air-stable polymer/inorganic nanocrystal hybrid BHJ inverted solar cells based on chemically modified nanocrystals which significantly extend the current knowledge of device fabrication.

Enhancing the efficiency of low bandgap conducting polymer bulk heterojunction solar cells using P3HT as a morphology control agent

The development of low bandgap conducting polymers has made bulk heterojunction solar cells a viable low cost renewable energy source. The high boiling point of 1,8-diiodooctane (DIO) is usually used to control the morphology of the active layer consisting of a conducting polymer and PCBM, so that a high power conversion solar cell can be achieved. We report here an alternative approach using nonvolatile, crystalline and conducting P3HT as an effective morphology control agent. A model system of PCPDTBT/PC61BM was selected for this study. The change of optoelectronic properties with the introduction of P3HT was monitored by measuring the absorption spectra and charge carrier mobility, and the morphology change with the introduction of P3HT in the active layer was monitored by AFM, TEM, and GIXRD. The results indicate that favorable bi-continuous phase separation and appropriate domain size of each phase can be achieved to facilitate fast charge transport, and thus improve the power conversion efficiency of the solar cell. By adding 1 wt% P3HT into the blend of PCPDTBT/PC61BM, the power conversion efficiency can be improved by 20%. Moreover, with the incorporation of 1 wt% P3HT to the blend of PCPDTBT/PC61BM with DIO, the power conversion efficiency can be further increased by 17%. The strategy of this study can be expanded to other low bandgap conducting polymers for high efficiency bulk heterojunction solar cells.

Synthesis, optical and photovoltaic properties of bismuth sulfide nanorods

Bismuth sulfide (Bi2S3) nanorods exhibit a low band gap, a high absorbance coefficient and good dispersity. In this study, the synthesis conditions of Bi2S3 nanorods were systematically investigated to obtain nanorods of a desired dimension, with high aspect ratios and good crystallinity. The as synthesized Bi2S3 nanorods, 37.2 nm in length and 6.1 nm in width, have a low band gap of ∼1.4 eV with a conduction band and valence band of −3.8 eV and −5.2 eV, respectively. The nanorods were blended with poly(3-hexylthiophene) (P3HT) at a weight ratio of 1:1 to form a light harvesting P3HT:Bi2S3 hybrid film. The incorporated Bi2S3 nanorods can not only contribute light harvesting but also lead to a more ordered structure of the P3HT phase and a more efficient π–π* transition. Surface potential mapping of the hybrid film, measured by Kelvin probe force microscope (KPFM), shows a significantly negative shift (−34 mV) under white light illumination, which indicates carrier dissociation and the accumulation of negative charge on top of the hybrid film. The photovoltaic characteristics of the devices were also observed for those based on the P3HT:Bi2S3 hybrid film. This novel P3HT:Bi2S3 hybrid material provides a new candidate for the fabrication of low-cost and environmentally friendly polymer/inorganic hybrid solar cells.

A novel polyurethane/cellulose fibrous scaffold for cardiac tissue engineering

The present work demonstrates a biomimetic electrospun scaffold based on polyurethane (PU) and ethyl cellulose (EC), featuring uniform fibrous nanostructures and three dimensional porous networks. The relationship between processing conditions and fibrous nanostructures is established which guides the rational processing with tunable fiber diameters. Additionally, the developed scaffold template reveals biocompatibility in retention and proliferation of cardiac myoblast H9C2 cells. The high mechanical strength of the PU/EC scaffolds enables the processing and handling of an ultrathin patch. Their elastomeric characteristics revealed the compatibility between the patch and contractile tissues. Furthermore, anisotropic PU/EC scaffolds with aligned nanofibers were successfully fabricated, exhibiting higher mechanical strength and essential characteristics for the survival and function of cardiac cells with native anisotropy. This work demonstrates a bioengineered PU/EC fibrous scaffold with uniform nanostructural webs and provides insight into the relationships between processing control, nanostructures and associated properties, with promising potential in cardiac tissue engineering.