Solution‐Processed Multivalent Molybdenum Oxide Tailoring Band Alignment for Efficient Sb ₂ S ₃ Solar Cells

Abstract Owing to its outstanding photoelectric characteristics, Sb ₂ S ₃ has witnessed rapid development as a light‐absorbing material for solar cells. However, the traditional utilization of Spiro‐OMeTAD and PbS as hole transport layers (HTLs) in Sb ₂ S ₃ solar cells poses significant challenges regarding stability and environmental toxicity. In this work, a solution‐processed high‐quality and smooth molybdenum oxide (MoO _x , 2 ≤ x ≤ 3) HTL is successfully applied in Sb ₂ S ₃ solar cells, boosting the fill factor from 15.37% to 53.00% and efficiency from 0.36% to 4.47% respectively, thereby presenting a cost‐effective, eco‐friendly, and non‐toxic alternative to conventional HTLs. To address the roll‐over effect observed in the J–V curves, attributed to back‐contact recombination in the device, sodium ascorbate, a reducing agent, is introduced into the precursor solution to modulate the valence states of molybdenum (+4, +5, +6), which generates diverse interstitial energy levels. These energy levels are anticipated to facilitate band alignment optimization and enhance hole extraction efficiency. Remarkably, the Sb ₂ S ₃ device incorporating this novel MoO _x HTL achieves an impressive efficiency of 4.47%, representing the highest reported efficiency for Sb ₂ S ₃ solar cells employing a green, and stable HTL. This work provides new insights for the development of green and non‐toxic Sb ₂ S ₃ devices.