22.1% Carbon-Electrode Perovskite Solar Cells by Spontaneous Passivation and Self-Assembly of Hole-Transport Bilayer

Low-temperature printable carbon-electrode perovskite solar cells (C-PSCs) promise commercially scalable and stable low-cost photovoltaic solutions. However, they suffer from low efficiency due to severe performance losses at the perovskite and carbon interface. Here, we propose a spontaneous interface assembly and passivation strategy based on a P3HT/NiO_x hole-transport bilayer by introducing quaternary ammonium bromide surfactants into NiO_x nanoparticles, and reveal the significant influences from their alkyl chains. Experimental and theoretical results demonstrate that hexyl trimethylammonium bromide (HTAB), with its optimal alkyl chain length, not only ensures the improved monodispersity and film quality of NiO_x nanoparticles but also matches and interacts strongly with P3HT side chains, significantly enhancing the molecular orientation of P3HT for superior electronic contact and efficient hole transport between P3HT and HTAB-NiO_x. In addition, the Br^- ions in HTAB-NiO_x spontaneously diffuse into a perovskite film, passivating uncoordinated Pb²⁺ or I vacancy defects and inhibiting the formation of metallic Pb⁰. Eventually, the low-temperature printable C-PSCs and modules achieve the highest reported efficiency of 22.1 and 18.0%, respectively; exhibit excellent stability at 80-90% high humidity without encapsulation; and demonstrate long-term operational stability for 500 h under maximum power point tracking conditions, maintaining 94% of the initial efficiency.