Redox-Mediated Stabilization of the Hole Transport Layer and Buried Interface Toward Stable Perovskite Solar Cells

Achieving uniform self-assembled monolayer (SAM) deposition on nickel oxide (NiO_x) and suppressing interfacial defects caused by high-oxidation-state nickel species remains a challenge for inverted perovskite solar cells (PSCs). Here, we develop a surface modification strategy using cesium oxalate (CsOA) to synergistically regulate the NiO_x/SAM buried interface. The CsOA treatment suppresses detrimental Ni⁴⁺ content and chelates with Ni³⁺ to form the complex [Ni(C₂O₄)₃]^3-, which maintains a stable oxidation state of Ni³⁺ and inhibits its continuing redox reactions as a result of the enhanced conductivity and p-type characteristics. Moreover, as a buffer layer, CsOA can prevent high-oxidation-state nickel species (Ni^≥3+) from reacting directly with the perovskite in uncovered regions and passivate buried perovskite defects through the interaction of the oxalate ion and under-coordinated Pb²⁺. Additionally, the enhanced anchoring between SAM and NiO_x/CsOA promotes uniform SAM assembly, thereby improving film quality and stability. As a result, the optimized NiO_x/CsOA/SAM HTL enables inverted PSCs with efficiencies of 22.89% (1.67 eV) and 26.48% (1.54 eV), retaining 85.7% of the initial efficiency after 1560 h under AM1.5G illumination at 65°C. A scalable mini-module (an active area of 11.0 cm²) achieves an efficiency of 23.45%, highlighting the approach's potential for high-performance, stable, and industrially viable PSCs.