Particle-Size Distribution and Surface Properties Enable Air-Stable Organic Photovoltaics via Self-Assembled Layers

We report a particle-size-distribution (PSD) and surface-property-guided strategy to decode and optimize self-assembled layers at the NiO<i>_x</i>/active-layer buried interface for air-stable, high-efficiency organic photovoltaics (OPVs). A carbazole-based phosphonic-acid self-assembled material, pPhPADCB [4-([7H-dibenzo[<i>c</i>,<i>h</i>]carbazol-7-yl]phenyl)phosphonic acid], is introduced to mitigate the intrinsic roughness and energy-level mismatch of NiO<i>_x</i> and to function as an efficient hole-selective interfacial layer. In contrast to flexible alkyl spacers, a phenyl spacer acts as a rigid, π-conjugated bridge that can enhance packing order and electronic delocalization, potentially enabling more efficient charge tunneling across the SAM. PSD analysis reveals that pPhPADCB forms a narrower distribution with higher effective surface coverage than the structurally related control at lower concentration, indicating a more uniform and reproducible self-assembly on NiO<i>_x</i>. Consistently, Kelvin probe force microscopy (KPFM) shows an increased and spatially homogenized surface potential after pPhPADCB treatment, evidencing a favorable work-function shift and suppressed local potential fluctuations. The phenyl spacer enhances molecular rigidity and ordering, strengthening interfacial dipoles and improving interfacial compatibility with NiO<i>_x</i>, thereby promoting optimized contact formation at the buried interface. As a result, pPhPADCB-enabled devices outperform the 4PADCB-based counterparts, delivering power conversion efficiencies (PCEs) of 19.96% for PM6:L8-BO and 19.65% for D18:L8-BO under AM 1.5G (100 mW cm^-2). Importantly, the devices exhibit excellent air stability, retaining over 80% of the initial PCE after 1000 h of ambient storage without encapsulation. Overall, this work establishes quantitative links between self-assembly metrics (PSD and coverage) and key surface properties (surface potential/work function and interfacial compatibility), providing practical design rules for self-assembled interlayers toward efficient and environmentally durable OPVs.