Recrystallizing Sputtered NiOx for Improved Hole Extraction in Perovskite/Silicon Tandem Solar Cells

Abstract

Sputtering nickel oxide (NiO_x) is a production-line-compatible route for depositing hole transport layers (HTL) in perovskite/silicon tandem solar cells. However, this technique often results in films with low crystallinity and structural flaws, which can impair electronic conductivity. Additionally, the complex surface chemistry and inadequate Ni³⁺/Ni²⁺ ratio impede the effective binding of self-assembled monolayers (SAMs), affecting hole extraction at the perovskite/HTL interface. Herein, these issues are addressed using a recrystallization strategy by treating sputtered NiO_x thin films with sodium periodate (NaIO₄), an industrially available oxidant. This treatment improved crystallinity and increased the Ni³⁺/Ni²⁺ ratio, resulting in a higher content of nickel oxyhydroxide. These enhancements strengthened the SAM's anchoring capability on NiO_x and improved the hole extraction at the perovskite/HTL interface. Moreover, the NaIO₄ treatment facilitated Na⁺ diffusion within the perovskite layer and minimized phase separation, thus improving device stability. As a result, single-junction perovskite solar cells with a 1.68 eV bandgap achieve a power conversion efficiency (PCE) of 23.22% for an area of 0.12 cm². Perovskite/silicon tandem cells with an area of 1 cm² reached a PCE of 30.48%. Encapsulated tandem devices retained 95% of their initial PCE after 300 h of maximum power point tracking under 1-sun illumination at 25 °C.