Sputter-resistant aluminium oxide layer enables robust perovskite tandem solar cells

Protective buffer layers are a key strategy to facilitate the sputter deposition of transparent front electrodes in semi-transparent perovskite and highly efficient perovskite tandem solar cells. Here, we explore a thin aluminium oxide (AlO x ) layer, deposited through atomic layer deposition (ALD), as a buffer layer between the established C 60 electron transport layer and sputtered indium tin oxide (ITO) contact in a “p-i-n” (inverted) device architecture. By using multimodal spectroscopic methods, we compare AlO x to the more widely employed SnO x . Although ALD is known as a conformal process, we find that the growth of SnO x (<20 nm) on the C 60 layer frequently results in pinholes, whereas ∼ 3–5 nm-thin layers of AlO x allow for a more homogeneous coverage. The presence of in-gap states revealed in the wide-bandgap alumina buffer layer enables efficient charge extraction. Under low-power magnetron sputtering conditions, a 3 nm AlO x buffer layer effectively suppresses sputtering-induced damage, whereas a 1.5 nm AlO x layer does not. Devices with 3 nm AlO x perform similarly to those with 20 nm SnO x , with power conversion efficiencies exceeding 17% for semi-transparent perovskite and 26% for perovskite/silicon tandem devices. Thus, we conclude the suitability of thin AlO x as a sputter-resistant buffer layer offering the advantage of a dense and uniform ALD process, reduced material consumption, and a faster fabrication process for future perovskite tandem and semi-transparent devices.