Evaluation of Solder Resists for Module Integration of MWT Solar Cells

Back contact solar cells reach higher conversion efficiencies in comparison to conventional H-pattern solar cells as shading from front contacts is reduced or omitted. Besides Sunpowers interdigitated back contact cells, metal wrap through (MWT) solar cells are currently the back contact cells closest to large scale market integration. One of the major challenges is to isolate the different electrodes since both emitter and base contacts are on the rear side of the cell. In this work we examine the use of screen printed solder resists as isolation layer. Therefore eight different epoxy based solder resists, which are cured either thermally or via illumination with ultraviolet radiation, are evaluated in terms of the suitability for MWT modules. Our qualification procedure consists of investigations on thermal stability, electric breakdown, mechanical adhesion and printability of the solder resists. We find six out of eight solder resists to meet our requirements. The failure of two solder resists is caused by insufficient isolation properties or poor resistance to temperature and to scratching. Due to their curing time between two and three seconds and the higher electrical breakdown voltage with respect to the applied coating weight, we consider UV cured solder resists as preferable.