Lifespan optimization and quantification of bonds on thin-film solar modules

Abstract

The research group Energy Systems Engineering (ESE) is part of imo-imomec, an affiliated lab of imec. ESE examines the reliability and efficiency of photovoltaic systems. One focus point of the research group is the integration of thin-film photovoltaics (TPFV) in building-integrated photovoltaics. The aim of this master's thesis is to optimize the lifespan and efficiency of the connection between molybdenum back contacted CIGS TFPV and the conductor to the junction box. To optimize the connection, three methods are compared: ultrasonic soldering (USS), ultrasonic welding (USW) and electrically conductive tapes (ECT). The connections were subjected to accelerated stress tests: damp heat (DH) and thermal cycling (TC) tests based on the IEC 61646 norm. While doing these tests, the electrical contact resistance is characterized on fixed intervals. Next, scanning electron microscope (SEM) images are made of pristine samples as well as stressed samples to do a visual inspection of the degradation. The first measurements show that the resistance of the ECT was about 1000 times higher than USW and USS. TC did not cause degradation of the samples. Meanwhile, DH tests did cause degradation. After 300 hours multiple ribbons, which were attached with USS, started to detach and the contact resistance increased. The contact resistance of USW bonds increased after 440 hours but the ribbons did not detach from the substrate. The conclusion is that USW is the best method to create bonds on molybdenum back contacted CIGS TFPV.