Researchers developed a 3 nm aluminum oxide buffer layer to protect perovskite cells during transparent-electrode deposition
The layer provided more uniform coverage than a conventional 20 nm tin oxide layer while using less material
Semi-transparent cells exceeded 17% efficiency, while a 2T perovskite-silicon tandem cell reached 26.9%
Indium tin oxide (ITO) is a preferred transparent electrode for semi-transparent perovskite and perovskite-silicon tandem solar cells. However, the magnetron sputtering process used to deposit ITO can damage the fullerene (C₆₀) electron transport layer and perovskite absorber underneath. This damage increases charge-carrier recombination and reduces cell performance.
Researchers from the University of Stuttgart, Forschungszentrum Jülich, the International Iberian Nanotechnology Laboratory and AIKO Solar’s European research unit investigated an ultrathin aluminum oxide (AlOₓ) buffer deposited by atomic layer deposition (ALD). Published in Materials Today, the study is titled Sputter-resistant aluminium oxide layer enables robust perovskite tandem solar cells.
Tin oxide (SnOₓ) is commonly used to protect the cell layers during ITO sputtering. However, thin SnOₓ layers did not form a complete and even coating over the C₆₀ surface, leaving small gaps called pinholes. A thickness of around 20 nm was needed to improve this protective coating. In comparison, a much thinner 3 nm AlOₓ layer formed a more even coating.
The team identified 3 nm as the optimum AlOₓ thickness under the tested conditions. A 1.5 nm layer did not sufficiently limit sputtering-induced damage. Meanwhile, thicker layers restricted charge extraction because AlOₓ is electrically insulating. Although AlOₓ is an insulating material, the ultrathin 3 nm layer still allowed electrical charges to pass through. The 3 nm AlOₓ layer delivered similar semi-transparent cell efficiency to the 20 nm SnOₓ layer while using considerably less material.
Semi-transparent perovskite cells with the 3 nm AlOₓ layer achieved efficiencies above 17%, an open-circuit voltage (Voc) of around 1.14 V, and a fill factor (FF) of up to 82%. By comparison, cells without a buffer layer recorded around 1.0 V and an FF of approximately 55%, showing the buffer layer's protective effect.
The researchers then used the AlOₓ layer in a monolithic 2-terminal (2T) perovskite-silicon tandem cell. The device achieved 26.9% efficiency and an FF of 77%, with stable performance in both voltage-scan directions. The SnOₓ reference achieved 27.2% efficiency in the forward voltage scan and 26.6% in the reverse scan, showing a difference between the two measurements.
During a 1,000-second maximum power point test, the unencapsulated AlOₓ-based tandem retained 99% of its initial efficiency. The SnOₓ reference retained 93% over the same period. These results represent short-term performance and do not establish long-term operational stability.
ALD produces thin and uniform coatings but is generally a slow process. Since fewer deposition cycles are required for a thinner layer, reducing the buffer thickness from 20 nm to 3 nm could lower material consumption and shorten processing time. According to the researchers, this could support the future production of semi-transparent perovskite and tandem solar cells.
