Detailed analysis of annealing silver front side contacts on silicon solar cells

Abstract

New concepts for high efficient solar cells require a post processing annealing step for passivation quality improvement. For cutting costs, thick film metallization is used for the front side. In this paper annealing steps of different duration and temperature are applied to standard industrial silicon solar cells to probe the sensitivity for such a front side metallization towards annealing. This paper focuses on five minute annealing under nitrogen atmosphere and determines favourable annealing temperatures and metallization pastes. In our investigations I-V curve measurements showed that from a certain threshold onwards, an increasing thermal budget decreases cell's performance. Series and contact resistance measurements were determined to be significantly affected by annealing. Scanning electron microscopy (SEM) revealed that silver crystallites at the silver silicon interface are transformed in shape for annealing at high temperatures which is most probable the microscopic reason for an increased contact resistance

Physical understanding of the behavior of silver thick-film contacts on n-type silicon under annealing conditions

“…If the thermal budget exceeds a certain limit, the nitrogen PMA leads to a lower fill factor due to an increased series resistance. Early hints, that the increase in series resistance is caused by an increased contact resistance, were found in [11]. The aim of this paper is to understand what mechanism deteriorates the contact resistance of screen printed front side metallization, to design a contact, that supports nitrogen PMA.…”

Optimizing annealing steps for crystalline silicon solar cells with screen printed front side metallization and an oxide‐passivated rear surface with local contacts

“…12 PMA in nitrogen above 4258C was shown to deteriorate the fill factor due to an increased contact resistance. 6,15 The increase in contact resistance is assigned to a smaller contact area between the silver crystallites and the silicon bulk at the silver-silicon interface of the front contact as well as to a redistributed glass layer isolating crystals from the silver bulk. During PMA a gap forms between the crystallites and the surrounding Si bulk, 15 which cuts the current transport and thus increases the contact resistance.…”

“…6,15 The increase in contact resistance is assigned to a smaller contact area between the silver crystallites and the silicon bulk at the silver-silicon interface of the front contact as well as to a redistributed glass layer isolating crystals from the silver bulk. During PMA a gap forms between the crystallites and the surrounding Si bulk, 15 which cuts the current transport and thus increases the contact resistance. The gap most likely originates from diffusion of silicon 16,17 and silver 18 at the contact interface.…”