Etch Characteristics and Morphology of Al 2 O 3 /TiO 2 Stacks for Silicon Surface Passivation

Chemical processes are very important for the development of high-efficiency crystalline solar cells, mainly for surface texturing to improve light absorption and cleaning processes to reduce surface recombination. Recently, research has been focusing on the impact of chemical polishing on the performance of a passivated emitter and rear cells (PERC), with particular emphasis on the dielectric passivation layers on the front side. This study examined the influence of etching on the passivation of Al 2 O 3 /TiO 2 stacks, where the films may each be deposited using a range of deposition and post-annealing parameters. Most TiO 2 films deposited at 300 °C were resistant to chemical etching, and higher temperature deposition and annealing produced more chemical-resistant films. TiO 2 films deposited at 100 °C were etched slightly by SC1 and SC2 solutions at room temperature, whereas they were etched at a relatively high rate in an HF solution, even when capped with a thick TiO 2 layer (up to 50 nm in thickness); blistering occurred in 20-nm-thick Al 2 O 3 films. In contrast to the as-deposited films, the annealed films showed a lower level of passivation as 1% HF etching proceeded. The implied open circuit voltage of the samples annealed at 300 °C after HF etching decreased more than those annealed at 400 °C. The dark area in the photoluminescence images was not resistant to the HF solution and showed more etch pits. The etching strategies developed in this study are expected to help setup integration processes and increase the applicability of this stack to solar cells.