Current transport in thick film Ag metallization: Direct contacts at Silicon pyramid tips?

Abstract

Good progress in the front silver metallization paste development has lead to significant efficiency improvements for standard crystalline silicon solar cells in the past years. In order to gain more insight into the contact mechanism, recent silver pastes capable of contacting higher resistive emitters – without selective emitter and without subsequent plating – were studied in this work. Within solar cell silver thick film contact investigations, two different theories predominate to explain the current transport from the silicon into the silver finger's bulk. The first one establishes that the current mainly flows through the Ag-crystallites grown into the silicon emitter that are either directly connected to the silver bulk or separated from the silver finger by a thin glass layer. The second one suggests that at optimum firing conditions nano-Ag-colloids are formed inside the glass layer allowing the current to flow into the silver finger through multi-step tunnelling. By sequential selective silver and glass etch-back and reconnection via liquid conductive silver, different combinations of conductivity path systems were investigated. On pyramidal textured Si, contacts from recent silver pastes reveal Ag-crystallites at the pyramid tops which are likely directly connected to the body of the Ag-finger. Even on flat Si we observe these likely direct contacts with advanced silver pastes. Our liquid conductive silver experiments indicate that the largest current contribution is flowing through these Ag-crystallites.