Unveiling and Overcoming Instabilities in Perovskite Solar Cells Induced by Atomic‐Layer‐Deposition Tin Oxide

Atomic layer deposition of tin oxide (ALD–SnO x ) has emerged as a promising buffer/protection layer, often replacing bathocuproine (BCP) in applications such as semitransparent and tandem devices. However, the long‐term stability and underlying degradation mechanisms of perovskite solar cells (PSC) incorporating ALD–SnO x remain elusive. Herein, the long‐term stability of PSCs featuring an ALD–SnO x buffer layer is systematically investigated. Intriguingly, it is observed that cells with ALD–SnO x exhibit heightened susceptibility to severe degradation, surpassing even the degradation levels observed with BCP under humid conditions. Through an extensive analysis employing X‐ray photoelectron spectroscopy and X‐ray diffraction, it is unveiled that ALD–SnO x triggers a phase transition in the perovskite when exposed to moisture, transitioning from the black cubic phase to the yellow delta phase, despite the presence of a thin layer of fullerene between the SnO x and the perovskite. Replacing ALD–SnO x with ALD–AlO x as a buffer layer emerges as a transformative strategy, effectively bolstering the humidity and thermal stability of the cells, without affecting device efficiency. The optimized ALD–AlO x ‐buffered device exhibits a high efficiency of 24.61% and maintains 88% of its initial efficiency after maximum power point tracking under 1 sun illumination for 1350 h at 65 °C in ambient air when encapsulated.