Advanced electroless metallisation of high efficiency laser doped and inkjet printed silicon solar cells

摘要

This thesis investigates the difficulties of metallising grooves created using inkjet printing and laser doping in high efficiency solar cell structures. Poor metallisation of these grooves has been a persistent problem due to the shallowness of the created grooves and, in the case of inkjet printing, the smoothness of the groove surface. In this thesis we select a suitable metallisation technique for plating double sided or interdigitated contact solar cells; identify and understand the current problems faced using this metallisation technique; establish techniques to overcome these limitations; and evaluate the performance of the proposed solutions.The thesis begins with a review of buried contact solar cell structures, laser doping and inkjet printing technologies and the suitability of various wet chemical plating processes. Having identified electroless plating as the most suitable metallisation technique, we continue with an in-depth study of the chemistry behind electroless metal deposition and the effect of different metal solution constituents in the deposition reaction. Experiments in metallising inkjet printed and laser doped grooves find that the metal adhesion of heavily doped phosphorous grooves is the limiting factor in the application of these technologies, and that palladium activation is the cause of this metal-silicon adhesion failure.Electroless plating using nickel boron reducing agent, combined with surface roughening techniques in the case of inkjet printed cells, is investigated as an alternative to palladium activation in both laser doped and inkjet printed cells. We show that excellent adhesion can be achieved using the novel metallisation scheme. An in-depth study identifies the optimal conditions for forming silicide and removing unreacted nickel, therefore improving the adhesion and obtaining negligible contact resistance. We continue by presenting a detailed investigation into the different available contact resistance measurement techniques. The thesis concludes by showing that negligible ohmic contact resistance on both heavily doped phosphorous and boron grooves can be achieved through the use of the proposed novel metallisation scheme.