Economical manufacturing of thin solar cells requires alternative concepts of passivating the cells backsides. The advantage of thin film passiviation using aluminum oxide over conventional passivation using screen printing aluminum paste for thin cells is that they will not bend during following heat treatment processes. Due to the high amount of negative carriers in aluminum oxide layers this material is best suited for the field-effect passivation of p-doped silicon surfaces.
Conventional methods for applying high-quality aluminum oxide on silicon are atomic layer deposition (ALD) and plasma-assisted chemical vapor deposition (PECVD). For applying aluminum oxide layers in nanometer range Fraunhofer IWS scientists developed an alternative atmospheric pressure method - the ultrasonic-spray-pyrolysis. Homogeneous layer deposition is possible on silicon wafers up to a size of 156 mm x 156 mm at substrate temperatures of about 340 °C. For sprayed 20 nm thick aluminum oxide layers on wafers carrier life times of 260 μs were measured. This means an effective surface recombination velocity of 113 cm s-1. In this case thermal after-treatment is not needed.