Naphthalene-imide Self-assembled Monolayers As a Surface Modification of ITO for Improved Thermal Stability of Perovskite Solar Cells

Electron-transport-layer-free (ETL-free) perovskite solar cells (PSCs) show great promise for commercialization due to their simple design and ease of fabrication. However, the interface between the transparent conductive oxides such as indium doped tin oxide (ITO) and the perovskite is not optimal due to differences in their work functions, surface defects, and wettability of the substrates. Surface modification of ITO through self-assembled monolayers (SAMs) to get ITO/SAM charge selective layers has shown great improvement in device performance in recent years, but little emphasis has been put on the stability of these devices. Here, we address this gap by introducing a series of newly synthesized naphthalene-imide derivatives which self-assemble at the interface between ITO and the perovskite interface and study their impact on the thermal stability of triple-cation PSCs. The chemical and thermal stabilities of the naphthalene-imide SAMs help improve the thermal stability of the devices, reaching T80 lifetimes exceeding 800 h for devices containing a pyridine-functionalized naphthalene diimide carboxylic acid at 85 degrees C in air. In addition, all SAMs improve the stabilized power output of the devices with respect to ITO-only reference devices. Drift-diffusion simulations reveal the strong influence of the ITO work function on the efficiency in ETL-free devices, and a work function reduction of 0.2 eV could improve efficiencies by over 30%. The functional diversity of naphthalene imides coupled with the ease of SAM deposition opens a pathway for stable, PSCs based on electron selective monolayers.