Yi-Chung Wang

Wafer Scale Phase-Engineered 1T-and 2H-MoSe2/Mo Core-Shell 3D-Hierarchical Nanostructures toward Efficient Electrocatalytic Hydrogen Evolution Reaction

The necessity for new sources for greener and cleaner energy production to replace the existing ones has been increasingly growing in recent years. Of those new sources, the hydrogen evolution reaction has a large potential. In this work, for the first time, MoSe2 /Mo core-shell 3D-hierarchical nanostructures are created, which are derived from the Mo 3D-hierarchical nanostructures through a low-temperature plasma-assisted selenization process with controlled shapes grown by a glancing angle deposition system.

Resistive switching of Au/ZnO/Au resistive memory: an in situ observation of conductive bridge formation

A special chip for direct and real-time observation of resistive changes, including set and reset processes based on Au/ZnO/Au system inside a transmission electron microscope (TEM), was designed. A clear conducting bridge associated with the migration of Au nanoparticles (NPs) inside a defective ZnO film from anode to cathode could be clearly observed by taking a series of TEM images, enabling a dynamic observation of switching behaviors. A discontinuous region (broken region) nearby the cathode after reset process was observed, which limits the flow of current, thus a high resistance state, while it will be reconnected to switch the device from high to low resistance states through the migration of Au NPs after set process. Interestingly, the formed morphology of the conducting bridge, which is different from the typical formation of a conducting bridge, was observed. The difference can be attributed to the different diffusivities of cations transported inside the dielectric layer, thereby significantly influencing the morphology of the conducting path. The current TEM technique is quite unique and informative, which can be used to elucidate the dynamic processes in other devices in the future.

Non-antireflective Scheme for Efficiency Enhancement of Cu(In,Ga)Se2 Nanotip Array Solar Cells

We present systematic works in characterization of CIGS nanotip arrays (CIGS NTRs). CIGS NTRs are obtained by a one-step ion-milling process by a direct-sputtering process of CIGS thin films (CIGS TF) without a postselenization process. At the surface of CIGS NTRs, a region extending to 100 nm in depth with a lower copper concentration compared to that of CIGS TF has been discovered. After KCN washing, removal of secondary phases can be achieved and a layer with abundant copper vacancy (V(Cu)) was left. Such compositional changes can be a benefit for a CIGS solar cell by promoting formation of Cd-occupied Cu sites (Cd(Cu)) at the CdS/CIGS interface and creates a type-inversion layer to enhance interface passivation and carrier extraction. The raised V(Cu) concentration and enhanced Cd diffusion in CIGS NTRs have been verified by energy dispersive spectrometry. Strengthened adhesion of Al:ZnO (AZO) thin film on CIGS NTRs capped with CdS has also been observed in SEM images and can explain the suppressed series resistance of the device with CIGS NTRs. Those improvements in electrical characteristics are the main factors for efficiency enhancement rather than antireflection.

Recent developments in the synthesis of nanostructured chalcopyrite materials and their applications: a review

Chalcopyrite materials, in particular CuInS2 (CIS), CuInSe2 (CISe) and Cu(In,Ga)Se2 (CIGS), have drawn significant attention recently owing to their highly advantageous optoelectronic properties, making them well suited to their best known application in solar cells. In this review, we will introduce some of the recent advances in the field of chalcopyrite nanostructure synthesis and discuss the further benefits these nanostructured materials offer over their thin-film and bulk counterparts. We will highlight a number of synthesis methods that utilize both physical and chemical based techniques, encompassing vacuum, solvothermal and solution based approaches. The conclusion will briefly highlight some of the challenges that we are yet to overcome, whilst reiterating the benefits that nanostructured chalcopyrites have to offer.

Large-scale micro- and nanopatterns of Cu(In,Ga)Se2 thin film solar cells by mold-assisted chemical-etching process.

A reactive mold-assisted chemical etching (MACE) process through an easy-to-make agarose stamp soaked in bromine methanol etchant to rapidly imprint larger area micro- and nanoarrays on CIGS substrates was demonstrated. Interestingly, by using the agarose stamp during the MACE process with and without additive containing oil and triton, CIGS microdome and microhole arrays can be formed on the CIGS substrate. Detailed formation mechanisms of microstructures and the chemical composition variation after the etching process were investigated. In addition, various microand nanostructures were also demonstrated by this universal approach. The microstructure arrays integrated into standard CIGS solar cells with thinner thickness can still achieve an efficiency of 11.22%, yielding an enhanced efficiency of ∼18% compared with that of their planar counterpart due to an excellent absorption behavior confirmed by the simulation results, which opens up a promising way for the realization of high-efficiency micro- or nanostructured thin-film solar cells. Finally, the complete dissolution of agarose stamp into hot water demonstrates an environmentally friendly method by the mold-assisted chemical etching process through an easy-to-make agarose stamp.

Influence of catalyst choices on transport behaviors of InAs NWs for high-performance nanoscale transistors

The influence of the catalyst materials on the electron transport behaviors of InAs nanowires (NWs) grown by a conventional vapor transport technique is investigated. Utilizing the NW field-effect transistor (FET) device structure, ~20% and ~80% of Au-catalyzed InAs NWs exhibit strong and weak gate dependence characteristics, respectively. In contrast, ~98% of Ni-catalyzed InAs NWs demonstrate a uniform n-type behavior with strong gate dependence, resulting in an average OFF current of ~10(-10) A and a high I(ON)/I(OFF) ratio of >10(4). The non-uniform device performance of Au-catalyzed NWs is mainly attributed to the non-stoichiometric composition of the NWs grown from a different segregation behavior as compared to the Ni case, which is further supported by the in situ TEM studies. These distinct electrical characteristics associated with different catalysts were further investigated by the first principles calculation. Moreover, top-gated and large-scale parallel-array FETs were fabricated with Ni-catalyzed NWs by contact printing and channel metallization techniques, which yield excellent electrical performance. The results shed light on the direct correlation of the device performance with the catalyst choice.

Facile Growth of Cu2ZnSnS4 Thin-Film by One-Step Pulsed Hybrid Electrophoretic and Electroplating Deposition

The use of costly and rare metals such as indium and gallium in Cu(In,Ga)Se2 (CIGS) based solar cells has motivated research into the use of Cu2ZnSnS4 (CZTS) as a suitable replacement due to its non-toxicity, abundance of compositional elements and excellent optical properties (1.5 eV direct band gap and absorption coefficient of ~104 cm−1). In this study, we demonstrate a one-step pulsed hybrid electrodeposition method (PHED), which combines electrophoretic and electroplating deposition to deposit uniform CZTS thin-films. Through careful analysis and optimization, we are able to demonstrate CZTS solar cells with the VOC, JSC, FF and η of 350 mV, 3.90 mA/cm2, 0.43 and 0.59%, respectively.

Graphene-coated copper nanowire networks as a highly stable transparent electrode in harsh environments toward efficient electrocatalytic hydrogen evolution reactions

Copper nanowire networks (NWs) coated with a graphene layer through a carbon-enclosed chemical vapor deposition technique at a low temperature of 400 °C with a low sheet resistance of 23.2 Ω sq−1 and a high transmittance of 83.4%, which is comparable to the typical values of tin-doped indium oxide (ITO), as a transparent conducting electrode were demonstrated. The graphene-coated copper NW networks retain a low sheet resistance of less than 25 Ω sq−1 even after annealing at a temperature of 240 °C in a pure oxygen environment for 1 h, while a sheet resistance less than 100 Ω sq−1 can still be maintained in natural sea water, and acidic and basic solutions. Their highly stable features in harsh environments make these graphene-coated copper nanowire networks suitable as a catalyst toward high-efficiency hydrogen evolution reactions (HERs) with a low overpotential of 252 mV at 10 mA Cm−2 and a low Tafel slope of 67 mV dec−1. The non-corrosive and anti-oxidant graphene-coated copper nanowire networks could be used as an alternative transparent conducting electrode in harsh environments, such as in tandem photocatalytic water splitting.

Enhanced solar performance of chemical bath deposited-Zn(O,S)/Cu(In,Ga)Se2 solar cells via interface engineering by a wet soaking process

A facile wet soaking process by immersing a CIGS thin film in a mixed aqueous solution, containing gallium trichloride and thioacetamide at 80 °C for a few tens of seconds, was proposed to reduce the existence of defects in the CIGS absorption layer which can be confirmed by the temperature dependence of the open-circuit voltage (Voc). The depth profiles of X-ray photoelectron spectroscopy (XPS) results indicate that the gallium (Ga) concentration increases during the short wet soaking time, resulting in a widening of the band gap near the surface region. The enhanced carrier lifetime attributed to the Ga-induced defect reduction during thermal treatment of device fabrication was evaluated by time-resolved photoluminescence (TRPL) spectroscopy. With wet and light soaking processes, Voc, short circuit current (Jsc) and fill factor (F.F.) can be increased, yielding a significant enhancement in cell efficiency from ∼1% to ∼6.4%. We believe that this fast, simple and effective method can further stimulate the development of CBD-Zn(O,S)/post-selenization CIGS solar cells toward commercialized thin film photovoltaics.

Vertical Al2Se3/MoSe2 heterojunction on sapphire synthesized using ion beam

The vertical Al2Se3/MoSe2 heterojunction on sapphire was first fabricated via an ion beam-assisted process. The MoSe2 was formed via Mo selenization, while Al2Se3 was formed via Se substitution for O in sapphire. The applications of this heterojunction will be developed in the future.