Self-aligned metallization and reactive ion etched buried base contact solar cells

Abstract

Damage-free reactive ion etch processes have been developed that allow the formation of grooves with intentional under-etching of the mask, e.g. photo resist. In a succeeding metal evaporation step this very same mask obstructs the deposition of metal such that the metal is only deposited on the bottom of the groove. In a lift-off process the surplus metal is removed from the wafer surface. This metallization technique allows simple fabrication of single-side contacted solar cells. It has been used to form a buried base contact through the homogeneous emitter of a rear contact solar cell. Only one mask was used to define emitter and base areas and to contact the base. After depositing the metal on the bottom of the groove, it was electroplated. No significant shunts, i.e. no short circuits between emitter and base, are observed even though during electroplating the metal has grown out of the grooves. This rear contact solar cell with buried-base contacts achieves an efficiency of 16·3% under front side illumination (neither n++ nor p++ doping under the contact fingers). The cell still lacks open-circuit voltage (595 mV) and fill factor (70·9%), probably due to the lack of side-wall passivation in the grooves and of a sealing of the open pn-junction. Copyright © 1999 John Wiley & Sons, Ltd.