Integration of spatial ALD aluminum oxide for rear side passivation of p-type PERC/PERL solar cells

Abstract

In this paper we present an overview of the integration of amorphous aluminum oxide (Al2O3) as rear side passivation layer for large area industrial PERC solar cells (i-PERC). The technique used for the deposition of the Al2O3 layers is spatial Atomic Layer Deposition (ALD), for its industrial relevance due to the high deposition speed and low Trimethyl-Aluminum (TMA) consumption. After analyzing the surface passivation properties, we will describe how Al2O3 can be integrated in an i-PERC process flow in a simple and cost-effective way, while maintaining high levels of solar cell efficiency. In particular, a special effort is devoted in finding solutions for process simplification and in the reduction of the cost per wafer (€/W) of the spatial ALD process step. Top efficiencies of 20.1 % and 20.6 % have been reached (Cz-Si, 6 inch cells) when coupling a spatial ALD Al2O3 rear passivation to a standard i-PERC integration flow with front Ag-screen printed or Ni/Cu/Ag plated contacts, respectively. Besides passivation, ALD Al2O3 can also be used as doping source of aluminum to form localized p back surface field (BSF) regions at the rear contact by means of laser processing. In this way, dielectric opening and BSF are formed simultaneously and firing step is not needed anymore, leading both to a suppression of Al2O3 blistering and optical enhancement. This process is a suitable complement of front contact formation based on plating schemes. Ni/Cu/Ag-plated solar cells with rear laser doping from ALD Al2O3 layers have been fabricated with efficiencies topping 20.4 % (FF = 79.7 %) and outperforming the control group featuring local BSF formed via a firing step.