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.