MXenes for perovskite solar cells: Progress and prospects

Perovskite solar cells (PSCs) have been developed over the past decade as the forefront of the state-of-the-art photovoltaic technologies owing to their high efficiency and low cost, where nanostructured functional materials play key roles in performance optimization. As a versatile class of two-dimensional (2D) materials, transition metal carbides/nitrides MXenes have gained enormous attentions in PSCs since 2018 due to their superior properties such as excellent metallic conductivity, abundant surface functional groups, tunable work functions, high optical transparency, and mechanical robustness. The explorations of MXenes are of significance in performance promotion and commercialization expansion of devices. As such, this review focuses on the diversified advantages of MXenes, comprehensively summarizing their applications and developments in PSCs as additives, electron/hole transporting layers, interfacial engineering layers, and electrodes in sequence and explaining the relevant mechanisms behind. Simultaneously, the problems emerged from the related studies are considered and the corresponding suggestions like opening up the type of MXenes usage, taking further insight of the modulation of surface termination groups on Fermi levels, understanding the effect on energy level structures of perovskite or other functional layers, and realizing commercialization, etc. are provided for the future in-depth explorations. This review is intended to provide overall perspective of the current status of MXenes and highlight the direction for the future advancements in MXenes design and processes towards efficient, stable, large-area, and low-cost PSCs.