Solution-Processed Multivalent Molybdenum Oxide Tailoring Band Alignment for Efficient Sb₂S₃ Solar Cells

Owing to its outstanding photoelectric characteristics, Sb₂S₃ 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 Sb₂S₃ 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 Sb₂S₃ 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 Sb₂S₃ device incorporating this novel MoO_x HTL achieves an impressive efficiency of 4.47%, representing the highest reported efficiency for Sb₂S₃ solar cells employing a green, and stable HTL. This work provides new insights for the development of green and non-toxic Sb₂S₃ devices.