Publication date: 15th December 2025
Precise chemical modification of the nickel oxide (NiOx) interface is essential to maximize the charge extraction efficiency and stability of p–i–n structured perovskite solar cells (PSCs). In this study, we report an interface control mechanism that achieves high efficiencies exceeding 23% power conversion efficiency (PCE) through the physicochemical synergy of the standard MeO-2PACz, novel functional self-assembled monolayers (SAMs: V1782, V1779, V1626), and the bifunctional additive 6dPA[1-4].
MeO-2PACz bonds firmly with the surface hydroxyl groups (–OH) of NiOx via its strong phosphonic acid anchoring group, providing fundamental energy level alignment for hole extraction. The novel functional SAM (V-series) molecules possess extended aromatic cores and potent dipole functional groups, such as cyano (–CN) and carboxyl (–COOH) groups. These structural specificities enable precise passivation of coordinatively unsaturated Ni ions and oxygen vacancies on the NiOx surface and allow fine-tuning of HOMO energy levels to minimize interfacial recombination losses.
Particularly, 6dPA (1,6-hexylenediphosphonic acid) acts as a bifunctional molecule that integrates into the SAM layer and exposes a second phosphonic acid group on the surface, dramatically increasing surface polarity through "polarity supercharging." This mechanism increases the interface energy by over 10%, inducing uniform wettability of the perovskite precursor solution and resulting in a significant reduction of defective pixels. Experimental results show that devices employing the optimized mixed SAM interface recorded a peak PCE exceeding 23%, surpassing the control group (≈20.5%). However, when these multi-functional interfacial layers exceed a critical thickness, they act as an "insulating barrier," causing fill factor (FF) losses. To address this, we propose an ultra-thin capping strategy through high-dilution treatment[5], proving it to be a key process for achieving simultaneous defect passivation and improved wettability without increasing series resistance.
The authors acknowledge financial support from the École Polytechnique Fédérale de Lausanne (EPFL) and the Swiss Federal Government Excellence Scholarships (ESKAS 2025-2026). B.G.K. acknowledges support from the ESKAS postdoctoral fellowship program. D.H.W. acknowledges support from Chung-Ang University Research Foundation. We thank Prof. Michael Grätzel for laboratory access and scientific guidance.
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