Reconstruction of the Indium Tin Oxide Surface Enhances the Adsorption of High-Density Self-Assembled Monolayer for Perovskite/Silicon Tandem Solar Cells

Self-assembled monolayers (SAMs) are widely used as carrier transport interlayers for enabling high-efficiency perovskite solar cells (PSCs). However, achieving uniform and pinhole-free monolayers on metal oxide (e.g., indium tin oxide, ITO) surfaces is still challenging due to the sensitivity of SAM adsorption to the complex oxide's surface chemistry. Here, the hydrofluoric acid and the subsequent UV-ozone treatment are employed to reconstruct the ITO surface by selectively removing the undesired terminal hydroxyl and hydrolysis product. This can significantly increase the ITO surface activity and area, thus facilitating the adsorption of high-density SAMs. The resultant fluorinated surface can also prevent the direct contact of ITO with the perovskite active layer and passivate the perovskite bottom interface. Benefiting from the synergistically improved perovskite film formation, charge extraction, energy level alignment, and interfacial chemical stability, the corresponding PSC achieves a greatly enhanced power conversion efficiency of 21.3%, along with an enhanced long-term stability as compared to the control counterpart. Furthermore, a semitransparent PSC with a certified efficiency of 19.0% (with a record fill factor of 84.1%) and a four-terminal perovskite/silicon tandem with an efficiency of 28.4% are also demonstrated.