Impact of extended contact co-firing on multicrystalline silicon solar cell parameters

Abstract

During the temperature spike of the contact co-firing step in a solar cell process, it has been shown that the concentration of lifetime-killer dissolved metallic impurities increases, while adding an annealing after the spike getters most of the dissolved impurities towards the phosphorus emitter, where they are less detrimental. The contact co-firing temperature profile including the after-spike annealing has been called extended contact co-firing, and it has also been proposed as a means to decrease the emitter saturation current density of highly doped emitters, benefiting thus a wide range of materials in terms of detrimental impurity content. The aim of the present work is to determine the effect of performing this additional annealing on contact quality and solar cell performance, looking for an optimal temperature profile for reduction of bulk and emitter recombination without affecting contact quality. It presents the effect of the extended co-firing step on fill factor, series resistance and contact resistance of solar cells manufactured with different extended co-firing temperature profiles. Fill factor decreases when extended co-firing is performed. Series resistance and contact resistance increase during annealing, more dramatically when the temperature peak is decreased. Scanning Electron Microscopy (SEM) images show silver crystallites in contact with silver bulk before the annealing that allow a direct current path, and silver crystallites totally surrounded by glass layer (>100 nmthick) after annealing. Glass layer redistribution and thickening at low temperatures at the semiconductor-metal interface can be related to the series resistance increase. Degradation of series resistance during the temperature spike, when it is below the optimum one, can be also attributed to an incomplete silicon nitride etching and silver crystallite formation. To make full use of the beneficial effects of annealing, screen-printing metallic paste development supporting lower temperatures without thick glass layer growth is needed.