Phosphates Modulated NiO_x HTL toward a Lower <i>V</i>_oc Loss in Wide Bandgap Perovskite Solar Cells

Abstract The combination of p‐type NiO x and self‐assembled monolayers (SAMs) has recently emerged as an optimal structure for hole transport layer (HTL) structure in wide‐bandgap perovskite solar cells (WBG PSCs). However, the unique requirements for NiO x in this cascade HTL system differ significantly from those of neat NiO x . Specifically, the tendency of NiO x to agglomerate can lead to poor film morphology and inadequate interfacial contact with SAMs, resulting in significant open‐circuit voltage ( V oc ) loss in PSCs. Herein, these issues are addressed by incorporating sodium hexametaphosphate (SHMP) into NiO x ink. This approach enhances the dispersibility of NiO x nanoparticles, improving the morphology and conductivity of the NiO x films through interactions between the P = O and P‐O groups and Ni ions. Additionally, SHMP promotes better contact between the NiO x and Me‐4PACz interface by increasing the number of hydroxyl groups on the uniform surface of NiO x films. Consequently, a high power conversion efficiency (PCE) of 21.02% is achieved for WBG (1.79 eV) PSCs with the smallest relative V oc loss of 24.69%. The encapsulated devices exhibit excellent stability under high humidity and elevated temperatures. Furthermore, when combined with Sn‐Pb narrow‐bandgap perovskite, a PCE of 27.66% is attained for the 2‐terminal tandem solar cells (TSCs).