Investigating the link between LeTID and hydrogen induced contact resistance in PERC devices

Abstract

In recent years academia and industry have made significant efforts to mitigate the problem of LeTID in silicon solar cells, particularly in p-type multi crystalline PERC cells. Many of these approaches involve a post-firing thermal anneal between 300-500 °C after metal contact firing. This paper investigates observed increases in the front contact resistance of PERC cells in this temperature range. Changes in contact resistance have been primarily attributed to a hydrogen passivation effect, which might then be used to observe and study hydrogen kinetics. A new sample structure is developed to allow a more direct measurement of the current-voltage characteristics of the front contacts, without the contribution from resistance elsewhere in the cell. A careful analysis of such measurements leads to three key findings: (i) It is experimentally shown that there is no increase in resistance for any region of the device other than the front contact. (ii) It is shown that the contact resistance change is affected by the frequency of in-situ measurements and becomes highly unstable once resistance change reaches a high value, and (iii) Annealing prior I-V measurements can act to significantly increase the rate at which contact resistance changes, likely through an increase in the mobile hydrogen concentration throughout the cell. These findings are crucial to the understanding and future study of hydrogen in silicon and its relation to degradation in solar cells.