Hsien-Ping Feng

High-performance flat-panel solar thermoelectric generators with high thermal concentration

The conversion of sunlight into electricity has been dominated by photovoltaic and solar thermal power generation. Photovoltaic cells are deployed widely, mostly as flat panels, whereas solar thermal electricity generation relying on optical concentrators and mechanical heat engines is only seen in large-scale power plants. Here we demonstrate a promising flat-panel solar thermal to electric power conversion technology based on the Seebeck effect and high thermal concentration, thus enabling wider applications. The developed solar thermoelectric generators (STEGs) achieved a peak efficiency of 4.6% under AM1.5G (1 kW m(-2)) conditions. The efficiency is 7-8 times higher than the previously reported best value for a flat-panel STEG, and is enabled by the use of high-performance nanostructured thermoelectric materials and spectrally-selective solar absorbers in an innovative design that exploits high thermal concentration in an evacuated environment. Our work opens up a promising new approach which has the potential to achieve cost-effective conversion of solar energy into electricity.

Effect of plating current density and annealing on impurities in electroplated Cu film

This study uses secondary-ion-mass spectrometry to examine the effects of plating current density and annealing temperature on the nature of electroplated copper (Cu) films. The experimental results reveal that high levels of impurities, such as C, O, S, and Cl, are incorporated into Cu deposits at the lower current density region while superfilling occurs. The C and O impurities can be released from the plated films by thermal annealing, while S and Cl cannot. This work proposes a possible mechanism based on bond strength to explain the phenomena. Rapid C and O desorption is observed when the films are first cycled to 220°C immediately after electroplating. The activation energy of C desorption is found to be approximately 9.8kJ∕mol. For Cu electroplating, this investigation suggests that high plating current density and an adequate annealing temperature are required to reduce impurities.

Nanoparticle‐Enabled Selective Electrodeposition

Electroplating is a common process used in a wide range of industries. For example, electroplated copper and copper-based alloys are used in ultralarge-scale integration devices requiring multilevel metallization. [ 1 , 2 ] Electroplating has more recently been used in optoelectronic components, such as transparent thin-fi lm transistors, fl at panel displays, light-emitting diodes, photovoltaic cells, and electrochromic windows, where substrates are typically semiconductors (GaAs) or transparent conductive oxides (TCOs, e.g., zinc oxide, indium tin oxide, and fl uorine-doped tin oxide). [ 3 , 4 ] However, metallization directly on glass and other low-roughness ceramics is diffi cult because the smooth interface does not provide opportunities to interlock at the interface between the substrate and the materials to be plated. [ 5 ] Accordingly, prior to electroplating, etching is commonly used to increase surface roughness, followed by sputtering a thin adhesive layer (such as titanium) to improve adhesion. For electroplating metallization, the key is the nucleation process, which is determined by the formation energy, excess energy, and internal strain energy. [ 6–9 ] In general, a smooth and hydrophobic semiconductor surface, such as silicon, gallium arsenide, or transparent conductive glass, has low surface energy and poor wettability, leading to a relatively high excess energy for electroplating nucleation. As a consequence, scattered and irregular grains grow on a small number of nucleation sites, causing poor interface adhesion and large surface roughness. Strain energy, originating from the different atomic arrangement of the two adjacent layers, increases with increasing fi lm thickness and can sometimes cause the fi lm to spontaneously peel off. In many applications, it is relatively common to electroplate metal over an entire surface of a base conductor even though only small areas of the metal are needed on the surface. The use of electroplating in this context typically consists of a patterning process followed by a metallization process. Photolithography is the most popular method to create such patterns where a photoresist is used as the patterning layer. [ 10 , 11 ] The metallization process typically consists of sputtering, electroplating, and chemical mechanical polishing. Metal is fi rst sputtered onto the patterned regions, which improves both the adhesion and electrical conductivity of the primary structure,

Thermoelectric energy conversion using nanostructured materials

High performance thermoelectric materials in a wide range of temperatures are essential to broaden the application spectrum of thermoelectric devices. This paper presents experiments on the power and efficiency characteristics of lowand mid-temperature thermoelectric materials. We show that as long as an appreciable temperature difference can be created over a short thermoelectric leg, good power output can be achieved. For a mid-temperature n-type doped skutterudite material an efficiency of over 11% at a temperature difference of 600 °C could be achieved. Besides the improvement of thermoelectric materials, device optimization is a crucial factor for efficient heat-to-electric power conversion and one of the key challenges is how to create a large temperature across a thermoelectric generator especially in the case of a dilute incident heat flux. For the solar application of thermoelectrics we investigated the concept of large thermal heat flux concentration to optimize the operating temperature for highest solar thermoelectric generator efficiency. A solar-to-electric power conversion efficiency of ~5% could be demonstrated. Solar thermoelectric generators with a large thermal concentration which minimizes the amount of thermoelectric nanostrucutured bulk material shows great potential to enable cost-effective electrical power generation from the sun.