Phosphates Modulated NiO_x HTL toward a Lower V_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).