Bimetallic nanoparticles of copper and indium by borohydride reduction

Abstract

This study investigated the preparation of copper–indium bimetallic nanoparticles through the borohydride reduction in an alcohol solution for application in ink-coating and sputtering target materials. Copper, indium metal and copper–indium intermetallic materials were synthesized by reacting CuCl₂ and InCl₃ with NaBH₄ in 2-propanol (IPA) and tetraethylene glycol (TEG) at room temperature. The Cu–In samples contained Cu₂In and CuIn phases with particle sizes of 10–100 nm and 30–200 nm in both the IPA and TEG solutions, respectively. The nanoparticulate Cu–In precursor layer was coated onto a soda-lime glass through spin-casting, where the Cu–In intermetallic phases of Cu₂In and Cu₁₁In₉ were produced through heat treatment in Ar gas and a microstructured CuInSe₂ layer was produced in a selenium atmosphere. Cu, In, Cu–In intermetallic nanoparticles and the films were characterized using powder X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray analyses.

Introduction

The chalcopyrite semiconductor, CuInSe₂ (CIS), and related quaternary compounds have emerged as promising light absorber layer materials in the large-scale production of low-cost, high-efficiency solar cell devices, and several techniques have been developed for the film and crystal growth of CIS [1], [2], [3], [4]. Especially, a two-stage process, which includes (i) the preparation of a Cu–In metallic precursor film, and (ii) the selenization of the precursor film at a high temperature, is a promising technique for the deposition of large-scale CIS films [3], [4]. Although Cu–In nanoparticles are favorable materials for the preparation of these metallic precursor films, Cu–In thin film phases have usually been synthesized using vacuum deposition processes, such as sputtering and co-evaporation [5], [6], [7], [8], [9], [10], which require high manufacturing costs.

Cu–In compounds can be synthesized through physical and chemical methods, and one of the representative physical methods is a melt atomization technique that was reported by Norsworthy et al. [11] This method was used to obtain a solar cell conversion efficiency of 10.5% through paste-coating. However, this approach requires a high-temperature over 900 °C under a hydrogen atmosphere and produces large, non-uniform Cu–In metal powders.

An alternative method for producing Cu–In nanoparticles is a chemical reduction that is the most common process for the synthesizing of homogeneous and nano-sized metallic powders with low-cost, fast and mild solution conditions. This chemical reduction requires the use of reducing agents and surfactants in order to protect the surfaces of the metal particles. While several reports have demonstrated the synthesis of copper and indium monometallic nanoparticles using chemical methods [12], [13], [14], [15], [16], [17], [18], [19], [20], no reports have described the low-cost synthesis of Cu–In bimetallic nanoparticles using the chemical reduction method for the growth of a CIS-type absorber layer for thin film solar cell applications. This work presented a systematic synthesis method for Cu–In intermetallic nanoparticles through the borohydride reduction in a solution at room temperature under an ambient pressure.