~23% rear side poly-Si/SiO2 passivated silicon solar cell with optimized ion-implanted boron emitter and screen-printed contacts

Abstract

We report on the understanding and optimization of ion-implanted boron emitters in combination with screen-printed contacts to produce very low recombination current density and high-efficiency cells with rear poly-Si/SiO2 passivated contact. Due to high bulk lifetime in n-base Si and very low recombination in the rear-side n+ passivated contact, recombination in the emitter limits the efficiency potential of manufacturable single-side passivated contact cells. Emitter recombination is governed by recombination under metal contacts (J0e,metal) and in the passivated regions in between (J0e,pass). Emitter profile engineering and dielectric passivation can lower the J0e,pass while paste chemistry and firing conditions can reduce J0e,metal by altering the extent of screen-printing induced emitter surface etching and the percentage of unetched dielectric islands under the metal contacts. We optimized implanted boron profiles, surface concentration and passivation, and junction depth in the sheet resistance range of 48-200 Ω/□ and achieved very low J0e,pass (<15 fA/cm2) for textured boron emitters with Rsheet≥140 Ω/□. Additionally, screen-printing paste and firing optimization reduced J0e,metal from 1172 to 707 fA/cm2 on 170 Ω/□ boron emitter by virtually eliminating emitter surface etching and creating distributed local contacts with unetched dielectric islands (~40% area coverage) underneath the grid lines. This resulted in very low total metallized J0e of 30 fA/cm2 with 3% metal coverage. Our device modeling showed the optimum boron sheet resistance is in the range of 120-170 Ω/□. The simulated results were validated by fabricating fully screen-printed 100 cm2 22.9% efficient bifacial cell with n+ poly-Si/SiO2 passivated rear contact.