Ultrathin Plasma Oxide for Passivation of Phosphorus-Diffused Silicon Solar Cell Emitters

Different passivation layers on phosphorous diffused emitters have been studied. A comparison between silicon nitride (SiNx) single layer at different deposition temperatures, stacks of outgassing silicon oxide (that was grown during the moving in/out of the furnace in ambient air during the outgassing process) and an ultrathin SiOx layer formed by plasma oxidation capped with SiNx layers have been investigated. Optimization of plasma enhanced chemical vapor deposition (PECVD) SiNx at different deposition temperatures showed good passivation quality depending on the hydrogen content. Plasma oxidation process by a PECVD tool without utilizing a silicon source in the process gas flow is an obvious choice to form a thin SiOx as it allows the formation of a SiOx/SiNx stack in a sequence of deposition processes in the same inline PECVD tool. Such an ultrathin SiOx layer was found to not only improve the surface passivation after a fast firing process but also to maintain the excellent anti-reflection coating (ARC) property of the SiNx. Low emitter saturation current density J(0)(e) values of 15 fA/cm(2) for planar and 24 fA/cm(2) for textured surfaces for an emitter with a sheet resistivity of 161 omega/ sq has been reached after firing at 850 degrees C, which is comparable with the outgassing oxide. These stacks gave an improvement of the J(0)(e) values with a factor of 6-9 for emitters with low surface concentration comparing to single SiNx layer.