Takeshi Nojiri

A Crystalline Metallic Copper Network Application Film Produced by High-Temperature Atmospheric Sintering

We show the first production of a copper (Cu) application film (AF) consisting of a novel network of crystalline metallic Cu embedded with copper-phosphorus-oxygen glasses (Cu 2(PO4) and Cu 2P2O 7) to provide new feedstock materials for crystalline silicon (Si) photovoltaics (PVs). The Cu crystal network was preferentially grown in AF, and thus, a Cu AF with low-electrical resistivity was formed in air at elevated temperatures of ≥ 450°C by using a copper-phosphorus (Cu-P) alloy paste as a starting material for the sintering. The Cu-P alloy had the role that governed deoxidization of a cuprous oxide, which was formed on heating during the sintering, by virtue of a concurrent oxidation of the Cu phosphide at elevated temperatures. Our results may open the way to the widespread use of atmospherically sintered Cu AFs for mass-production of next-generation crystalline Si PVs.

Development of Innovative Application Films for Silicon Solar Cells Using a Copper–Phosphorus Alloy by an Atmospheric Sintering Process

In this paper we aim to develop copper (Cu)-based backside soldering tabs/pads for crystalline silicon (Si) solar cells using atmospheric sintering. In our previous study, we found that a Cu network can be formed in an application film (AF) by self-deoxidization when the AF consisting of copper-phosphorus (Cu-P) alloy paste is sintered in an atmospheric environment, and the Cu AF showed low electrical resistivity that satisfied the criteria for backside soldering tabs/pads. In this study, Si solar cells using Cu AF for front-side contacts were evaluated to confirm that Cu AF is applicable to Si solar cells in principle. The Cu-P alloy paste using Cu-7 mass%P particles was printed on an Si wafer and, finally, sintered at 640 °C in atmosphere. The resulting AF showed low electrical resistivity of 2.96 × 10 -5 Ωcm. The solar cell using the Cu AF and conductive glass layer had conversion efficiency (η) of 6.6%. We demonstrated the need for a barrier layer to prevent the formation of Cu 3Si compound on the Si wafer surface when applying it to Cu AF on the solar cells. Our results may improve the potential for the widespread use of an atmospheric sintered Cu AF as backside soldering tabs/pads for Si solar cells.

Effect of Glass Frits Amount on Atmospheric Sintering Behavior and Characteristics of Electrode Produced by Copper–Phosphorus Alloy

In this study, we evaluate the atmospheric sintering behavior and characteristics of electrodes produced by copper-phosphorus (Cu-P) alloy paste containing various amounts of B2O3-Bi2O3-SiO2 glass frits. We demonstrate the need for a certain extent of glass frits to expand the applicable temperature for use of Cu-P alloy paste shifted to the higher temperature side, where commercial pastes for crystalline silicon (Si) photovoltaics are sintered, although the minimum electrical resistivity of the Cu-P alloy electrode increased by increasing the amount of glass frits. Based on the characteristics of the Cu-P alloy electrode, the need to expand the Cu crystal network and prevent the oxidization of generated Cu has emerged to reduce electrical resistivity. A crystalline BiPO4 was formed in the sintered Cu-P alloy electrode, which suggests a reaction between partially crystallized glass frits and P2O5 generated from Cu-P-O glass derived from the Cu-P alloy that has occurred. Subsequently, the sintering behavior of Cu-P alloy paste, including the glass frits, was discussed based on the results of X-ray diffraction, transmission electron microscopy, scanning transmission electron microscopy energy-dispersive X-ray spectroscopy, and electron energy loss spectroscopy.