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

Abstract

ABSTRACT 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)butyl]phosphonic 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.