Fabricating high efficiency solar cells with high sheet resistance emitters by ion implantation and contact resistance modeling

We present improvements in c-Si solar cell performance for high sheet resistance (R_sheet) emitters fabricated by ion implantation. We have investigated the effect of sheet resistance (60-115 Ω/sq) on cell efficiency (CE) and also evaluated the effect of dopant profile shape on the contact resistance for the ion implanted emitters. High efficiency cells, with average CE>19.3%, can be achieved with ion implanted high R_sheet emitters (60-90 Ω/sq) using commercially available screen printed Ag paste. It is to be noted that the best results were obtained for those cells with emitter R_sheet ~ 70-75 Ω/sq, as the cell performance is limited by the FF, namely front contact resistance (R_c) for emitters with R_sheet > 75 Ω/sq. To better understand the effect of emitter R_sheet and the dopant profile on contact resistance we have used VSE´s bottom-up physics-based Technology Computer-Aided Design (TCAD) model to simulate these experimental results. We found that the traditional model of evaluating R_c using the phosphorus surface concentration (N_s) does not accurately predict the increase in R_c and consequently the loss in FF for high R_sheet emitters. We propose an alternative approach to model R_c where the contact depth and its associated dopant concentration (N_d) is employed to calculate R_c. This contact depth is not necessarily zero and may lie below the original Si surface. Our simulated results show that the use of N_d at a depth of the order of 10´s of nm below the Si surface leads to better agreement between the experimental and simulated R_series, FF and CE than assuming that the contact is made with Si at the original wafer surface. The implications of these findings with regards to emitter profile engineering via ion implantation and formulation of new pastes to lower - _c of high R_sheet emitters are discussed.