Hydroquinone Isomer-Directed n-Doping Modulation of MoO <i> _x </i> Enables Thickness-Insensitive Hole Transport Layers for High-Efficiency Organic Solar Cells

Developing solution-processed, thickness-insensitive hole-transporting layers (HTLs) remains a key challenge for the scalable fabrication of organic solar cells (OSCs). Herein, three dihydroxybenzene isomers─catechol (CT), resorcinol (RS), and hydroquinone (HQ)─were employed as mild reducing agents to synthesize highly conductive MoO_<i>x</i> HTLs, enabling a systematic investigation of the structure-property-performance relationship linking the reducing agent's molecular structure, the resulting HTL functionality, and the final OSC device performance. Systematic characterization revealed a clear hierarchy in reduction capability (HQ > CT > RS), consistent with their molecular structures. Among the three, HQ-modified MoO<i>_x</i> exhibits the highest conductivity, excellent optical transparency, smooth surface morphology, and well-preserved high work function. When incorporated as HTLs, OSCs based on 5% HQ:MoO_<i>x</i> deliver outstanding performance with power conversion efficiencies (PCEs) of 18.52% (PM6:L8-BO) and 19.80% (D18:L8-BO:BTP-eC9)─the highest reported values for single-junction OSCs employing solution-processed MoO_<i>x</i>. Moreover, the HQ:MoO_<i>x</i>-based devices show superior thermal stability and remarkable thickness tolerance, maintaining over 82% of their peak efficiency even at a 150 nm HTL thickness, whereas neat MoO_<i>x</i> devices degrade sharply to 0.72%. This work elucidates a clear structure-property-device performance correlation and demonstrates HQ-modulated MoO_<i>x</i> as a scalable, robust HTL platform for high-efficiency and thickness-tolerant OSCs.