Redox mediator-modified self-assembled monolayer stabilizes a buried interface in efficient inverted perovskite solar cells

Abstract

Utilizing a self-assembled monolayer (SAM) of [4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid as the interfacial layer on NiO_x (Me-4PACz) has been proven to be a feasible approach to improve the photovoltaic performance of inverted perovskite solar cells (PSCs). However, considering the chemical reactivity of the NiO_x surface and inhomogeneous SAM, interface stability is still an urgent issue that needs to be addressed. Herein, methylene blue, a redox molecule, was co-absorbed with Me-4PACz to serve as a functionalized interfacial layer, which effectively enhanced interface stability. Methylene blue could serve as a redox mediator to selectively reduce the high-valence state of Ni species and oxidize metallic Pb⁰, suppressing the undesirable interfacial reaction and reducing interfacial defects. Besides, methylene blue could mitigate the uneven dispersion of Me-4PACz through π–π interaction, which reduced the generation of voids and aggregates, providing a flattened and compact interfacial layer. These effects enabled inverted 0.08 cm²-PSCs and 1 cm²-PSCs to achieve efficiencies of 26.39% and 24.89%, respectively. Encouragingly, the device possessed enhanced stability even under harsh damp-heat conditions, retaining 91% of its initial efficiency after 1500 hours of UV irradiation and 90% of its initial efficiency after 1500 hours of operation under 1-sun illumination.