Recent progress in the development and understanding of silicon surface passivation by aluminum oxide for photovoltaics

In the recent years, considerable progress has been made in the understanding of the unique silicon surface passivation properties of aluminum oxide (Al2O3) films including its underlying mechanisms. Containing a high fixed negative charge density located close to the Si interface, Al2O3 provides a key merit over other available materials for the passivation of p-type surfaces and p+ emitters in silicon photovoltaics. The Al2O3 films also induce a high level of chemical passivation, which is vital to obtain excellent surface passivation properties. In this contribution, we highlight some recent work on the application and understanding of Al2O3 surface passivation films. We will discuss the material properties of Al2O3 including new data on the optical properties of ultrathin Al2O3 films (band gap is 6.4 ± 0.1 eV) for a large wavelength range. It will be shown that Al2O3 applied on top of SiO2 resulted in a firing stable double layer passivation scheme. The application of Al2O3 in solar cells will also be briefly addressed. The beneficial effect of enhanced rear reflectance and increased passivation will be discussed. The industrial feasibility will be assessed on the basis of excellent surface passivation properties (Seff <6 cm/s) and firing stability for ~2 cm p-type Si wafers with Al2O3 deposited in an inline thermal (H2O-based) ALD tool (Levitech’s Levitrack), and in an industrial ASM batch reactor with O3 as the oxidant. We furthermore demonstrate that "solar grade" Al(CH3)3, a precursor with reduced purity, resulted in Al2O3 films with similar passivation properties as semiconductor grade, high purity Al(CH3)3.