Internal Ultrathin Hydrophobic Self‐Assembled Capping Layers Enables Moisture‐Resistant All‐Perovskite Tandems

Operating stability is a critical challenge for all-perovskite tandem solar cells, with the degradation of wide-bandgap (WBG) perovskite films under high humidity posing a major obstacle to their commercial application. Herein, we demonstrate an internal encapsulation strategy in which the phosphonic acid-terminated hydrophobic 4-(3,6-dimethyl-9H-carbazol-9-yl)butylphosphonic acid molecules act not only as a buried hole-transporting layer in our devices, but also anchor to hydroxyl groups on the surface of the atomic layer deposition-grown tin oxide electron transport layer, forming an ultrathin and hydrophobic capping layer. This layer protects WBG perovskites from humidity-induced degradation while maintaining efficient interfacial charge transport, thereby preserving high device efficiencies and markedly improving stability. Consequently, encapsulated WBG (1.77 eV) perovskite devices with a maximum power conversion efficiency (PCE) of 20.58% retained 95% of their initial PCEs after 2500 h of storage at 65% relative humidity and 2000 h at 85% relative humidity. Furthermore, under ISOS-L-1 conditions, the encapsulated WBG and all-perovskite tandem (with a maximum steady-state PCE of 29.01%) devices maintained 90% of their initial efficiencies after 2000 and 750 h of continuous operation under 1-sun illumination, respectively. This strategy effectively enhances moisture and operational stability, providing a viable path for the commercialization of high-performance all-perovskite tandems.