Impact of Titanium Dioxide Surface Defects on the Interfacial Composition and Energetics of Evaporated Perovskite Active Layers

This investigation elucidates critical Brønsted and Lewis acid-base interactions at the titanium dioxide (TiO₂) surface that control the interfacial composition and, thus, the energetics of vacuum-processed methylammonium lead iodide (MAPbI₃) perovskite active layers (PALs). In situ photoelectron spectroscopy analysis shows that interfacial growth, chemical composition, and energetics of co-deposited methylammonium iodide (MAI)/PbI₂ thin films are significantly different on bare and (3-aminopropyl)triethoxysilane (APTES)-functionalized TiO₂ surfaces. Mass spectroscopy analysis indicates that MAI dissociates into hydrogen iodide and methylamine in the gas phase and suggests that MAPbI₃ nucleation is preceded by adsorption and coupling of these volatile MAI precursors. Prior to MAPbI₃ nucleation on the bare TiO₂ surface, we suggest that high coverages of methylamine adsorbed to surface defect sites (e.g., undercoordinated Ti atoms and hydroxyls) promote island-like growth of large, PbI₂-rich nuclei that inhibit nucleation and lead to a thick substoichiometric interface layer that impedes charge transport and collection energetics. APTES functional groups passivate TiO₂ surface defects and facilitate more conformal growth of small, PbI₂-rich nuclei that enhance MAPbI₃ nucleation and significantly improve interfacial energetics for charge transport and extraction. This work highlights the considerable influence of TiO₂ surface chemistry on PAL composition and energetics, which are critical factors that impact the performance and long stability of these materials in emerging photovoltaic device technologies.