Solution‐Processed Thickness‐Insensitive Molybdenum Oxide Hole‐Transporting Layer Regulated by Reductive Ionic Liquid for Stable and Efficient Organic Solar Cells

Abstract Developing solution‐processed, thickness‐insensitive hole‐transporting layers (HTLs) is a key challenge in scaling high‐performance organic solar cells (OSCs). Here, a simple and efficient method is presented to produce highly conductive molybdenum oxide (MoO_x) HTLs by n‐doping ammonium heptamolybdate with a reductive ionic liquid (IL). Owing to the n‐doping effect and inherent conductivity of IL, the conductivity of the 5% IL:MoO_x significantly increased to 8.06 × 10⁻³ S m⁻¹, surpassing traditional solution‐processed MoO_x HTLs. Moreover, the IL's multifunctional non‐covalent adsorption sites and high boiling point help reduce electronic disorder and passivate parasitic traps, enhancing the overall performance. As a result, 5% IL:MoO_x shows excellent versatility in commonly used photoactive systems and achieves a remarkable PCE of 19.55% in the D18:N3:L8‐BO ternary system. This outperforms neat MoO_x and PEDOT:PSS devices and represents as the highest reported value among single‐junction OSCs with solution‐processed MoO_x HTLs. Additionally, devices with 5% IL:MoO_x also exhibit superior stability compared to PEDOT:PSS devices. Furthermore, 5% IL:MoO_x shows impressive thickness insensitivity, maintaining 83.3% of the optimum PCE even at a thickness of 150 nm. The exceptional PCE, versatility, stability, and thickness insensitivity of the 5% IL:MoO_x HTL collectively highlight its potential as a substitute for PEDOT:PSS in scaling OSC production.