Ultrasonic Al Bond on Mo Back-Contact Layer of CIGS Solar Panel Characterization Using Infinite Focus Microscope (IFM) and Micro-Ohmmeter

Abstract

The aim of this study is to identify the bonding mechanisms of the ultrasonic Al bond on the Mo back-contact layer (Al-Mo system) of a copper indium gallium (de) selenide (CIGS) thin-film photovoltaic (TFPV) solar panel. The bonding mechanism of the Al-Mo system was obtained from the identification of nonstick footprints of Al ribbon and the deformation of ultrasonic Al bonds on the Mo layer using an infinite focus microscope (IFM). The effect of Al bond deformation toward the bondability and electrical behavior of the bonds was evaluated by comparing the results obtained from IFM with the peel force, contact resistance from the transmission line method (TLM), and resistance from a micro-ohmmeter, respectively. It is observed that ultrasonic bonding parameters have a major impact on the deformation, bondability, and electrical properties of ultrasonic Al bonds on Mo layers. The bonding mechanism and failure mode can be identified through the IFM examination through the formation of footprints and the evolution of microwelds (remnant) of ultrasonic Al bonds and the occurrence of glass substrate cratering. It was found that arithmetical mean deviation of the assessed profile (R a) of more than 6 µm indicates that the ultrasonic Al bond has been formed on the Mo layer. The contact resistance values are more toward representing the variation of intimate contact area, while resistance is more toward representing the resistance of two bonds and the straightness of the Al ribbon.