Status and prospects of Al₂O₃-based surface passivation schemes for silicon solar cells

The reduction in electronic recombination losses by the passivation of silicon surfaces is a critical enabler for high-efficiency solar cells. In 2006, aluminum oxide (Al₂O₃) nanolayers synthesized by atomic layer deposition (ALD) emerged as a novel solution for the passivation of p- and n-type crystalline Si (c-Si) surfaces. Today, high efficiencies have been realized by the implementation of ultrathin Al₂O₃ films in laboratory-type and industrial solar cells. This article reviews and summarizes recent work concerning Al₂O₃ thin films in the context of Si photovoltaics. Topics range from fundamental aspects related to material, interface, and passivation properties to synthesis methods and the implementation of the films in solar cells. Al₂O₃ uniquely features a combination of field-effect passivation by negative fixed charges, a low interface defect density, an adequate stability during processing, and the ability to use ultrathin films down to a few nanometers in thickness. Although various methods can be used to synthesize Al₂O₃, this review focuses on ALD—a new technology in the field of c-Si photovoltaics. The authors discuss how the unique features of ALD can be exploited for interface engineering and tailoring the properties of nanolayer surface passivation schemes while also addressing its compatibility with high-throughput manufacturing. The recent progress achieved in the field of surface passivation allows for higher efficiencies of industrial solar cells, which is critical for realizing lower-cost solar electricity in the near future.