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 NiOx/active-layer buried interface for air-stable, high-efficiency organic photovoltaics (OPVs). A carbazole-based phosphonic-acid self-assembled material, pPhPADCB 4-([7H-dibenzo[c,hcarbazol-7-yl]phenyl)phosphonic acid], is introduced to mitigate the intrinsic roughness and energy-level mismatch of NiOx 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 NiOx. 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 NiOx, 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.