Internal bridging engineering of NiO _<i>x</i> /Me-4PACz <i>via</i> selective guanidine-based hydrochlorides for efficient and stable inverted perovskite solar cells

The introduction of self-assembled molecular layers (SAMs) has recently brought breakthroughs to inverted perovskite solar cells (PSCs). Among them, the NiO_<i>x</i>/Me-4PACz hole transport layer (HTL) system exhibits remarkable potential for enhancing device performance. However, Me-4PACz suffers from inherent limitations, including uneven distribution, poor wettability with perovskite solution and limited stability. In this study, we introduce metformin hydrochloride (MFCl) and triaminoguanidine hydrochloride (TGCl) as internal molecular "bridges" between NiO_<i>x</i> and Me-4PACz to address these issues. This strategy aims to enhance charge transport, strengthen the interfacial anchoring between NiO_<i>x</i> and Me-4PACz, regulate the molecular organization of Me-4PACz, and optimize the buried interface. As a result, the crystallinity and quality of the perovskite layer are significantly improved, leading to enhanced device performance. The power conversion efficiency (PCE) of the modified device increases notably from 20.45% to 23.11% (MFCl) and 23.06% (TGCl), along with substantial improvements in operational stability.