Paper PS+TF-ThA4
Influence of Plasma Power on the Si Solar Cell Passivation Properties of Al₂O₃ Thin Films deposited by Atomic Layer Deposition at 90 °C

Thursday, November 2, 2017, 3:20 pm, Room 23

In the recent years ALD Al₂O₃ surface passivation for Si solar cells has gained increasing popularity. The excellent passivation properties of ALD Al₂O₃ are based on a combined effect of chemical passivation and fixed high negative charge density. A bulk of the work has concentrated on thermal ALD, but also plasma enhanced ALD (PEALD) has been considered. However, the effect of plasma parameters, particularly plasma power, on the passivation properties remain uncharted.

In this work, we have studied the effect of plasma power on the properties of PEALD Al₂O₃ deposited at low temperatures with focus on the Si solar cell passivation. The Al₂O₃ was grown from TMA and O₂ plasma. All depositions were done with a Beneq TFS 200 ALD reactor at 90 °C. The lifetime was studied for as deposited and post-annealed samples.

The plasma power significantly affected the film properties. Low plasma power appeared to lead to the lowest film quality in regards of purity, density and refractive index. When the power was increased from 50 to 100–300 W the density increased from 2.6 to 2.8 g/cm^-3. Similarly, the refractive index increased from 1.61 to 1.62 (at 628 nm). The higher plasma power appeared to increase the amount of available O radicals, leading to more efficient reaction completion and improved film optical and structural properties. The same trend was also clear for the passivation properties for Si solar cells. For a 25 nm Al₂O₃ deposited with 50 W plasma power and annealed at 400 °C the lifetime at 10¹⁵ cm^-3 injection level was 1.1 ms, while for Al₂O₃ deposited with 100 or 180 W it was 2.0 ms. The interface properties were also influenced. The 50 W sample had the lowest density of negative charge and the highest interface defect density, which agreed with the lower lifetime of the sample. Interestingly, the 100 W sample had the lowest level of defect density. This can be related to the more moderate level of ultraviolet radiation from the O₂ plasma that the 100 W sample was exposed to as compared to the 180 W sample. Nevertheless, the 180 W sample had the best passivation properties due to its highest negative charge density. In fact, the negative charge density plays a major role in surface passivation when the magnitude of the negative charge density is much greater than that of the defect density.

High quality surface passivation of Si solar cells was achieved with PEALD Al₂O₃ grown at 90 °C. The passivation properties were shown to significantly improve with increasing plasma power.